Magnetic hybrid transition metal halides

Abstract

Metal halide semiconductors (MHS) have gained significant attention in recent years due to their exceptional optoelectronic properties. While most MHS systems that are currently under investigation are nonmagnetic, the diverse chemical and structural properties of MHS offer a unique opportunity to incorporate magnetic spins, such as transition-metal ions, into the metal halide lattice. This enables the exploration of novel magnetic phenomena and exchange mechanisms, making MHS a promising platform for multifunctional magnetic materials. In this review, we provide an overview of the historic and recent developments in magnetic transition metal halides (TMHs) with different chemical and structural diversities, highlighting the structure–magnetic property relationships in terms of different structure dimensions and metal ions. We also discuss the potential for multifunctional magnetic TMHs, where magnetism co-exists or is coupled with a secondary property of interest, such as conductivity, ferroelectricity, optical absorption, and luminescence. These coupling effects enable the TMHs to be promising candidates for multifunctional materials.

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1. Introduction

Metal halide semiconductors (MHS) that adopt the perovskite crystal structure (AMX₃, A = monovalent organic or inorganic cation, M = divalent metal, X = halide) have emerged as a revolutionary solution-processable semiconductor system because of their excellent optical and electronic properties as demonstrated in solar cells and light-emitting devices.^1–3 Current studies focus mainly on group IV metal halides (e.g., M = Pb²⁺, Sn²⁺, Ge²⁺), which are, however, non–magnetic systems. Unlike transition-metal oxides whose cooperative magnetism has been intensively studied for decades in various crystal structures (e.g., spinel, garnet, and perovskite),^4,5 hybrid transition-metal halides that contain magnetic spins remain relatively unexplored. These materials possess rich chemical and structural diversity and thus offer a great opportunity for the search of new magnetic materials and novel magnetic phenomena.^6–8

The framework formed by metal halide octahedra (MX₆) is normally stabilized by another inorganic metal cation or a library of organic cations, forming inorganic or organic–inorganic hybrid MHS. MX₆ octahedra can be connected by single or multiple octahedral connectivity mode(s) (i.e., corner-, edge-, and face-sharing) in these metal halide materials forming a three-dimensional (3D) structure or condensate into two-dimensional (2D), one-dimensional (1D), or cluster (0D) networks. 3D metal halide perovskites or double-perovskites can be stabilized within the constraints of the Goldschmidt tolerance factor while lower dimensional structures can be formed with larger organic/inorganic cations. This versatile structural platform not only provides tunability for electronic properties, but also modulates magnetic properties through geometry, bond distance, and bond angles.

Research on the magnetic properties of transition metal halides (TMHs) can be traced back to the 1960s by de Jongh and Drumheller, when a number of hybrid 2D TMHs were studied and the structure–magnetic property relationships were revealed.^9,10 Their unique 2D lattice with tunable magnetic anisotropy attracts a lot of experimental interest to examine the theory of the 2D magnetic-ordering model, namely, the Heisenberg, Ising, or X–Y model.¹¹ A recent study on the monolayer transition metal halides has shown their potential in modulating spin properties.^12–14 Meanwhile, renewed interest in these transition-metal halides has emerged along with the rise of MHS (extensive reviews on the MHS can be seen in ref. 1–3). New crystal structures with interesting magnetic properties including spin frustration,¹⁵ magnetic anisotropy,¹⁶ hybrid magnonic,¹⁷ and chiral ferromagnetic¹⁸ were reported.

Compared to traditional magnetic semiconductors or metals, the main advantages of these hybrid TMHs are their structural diversity and solution processability. There is a large opportunity to modulate their structure and magnetic interactions, and solution processability provides convenience for studying the magneto-optical properties and fabricating devices. Moreover, TMHs exhibit excellent semiconductor properties, such as a high absorption coefficient, tunable bandgap, and high carrier mobility. It makes them an ideal platform to explore the coupling effects between the magnetism and optoelectronic properties, giving rise to multi-functional materials, where control of light and charge carriers is coupled with magnetic spins.¹⁹ These materials show potential applications in spintronic devices, information storage, and quantum technology. Herein, we summarize the research progress of magnetic hybrid metal halides, highlighting the structure–magnetic property relationships.

2. Structure diversity and synthetic methods of TMHs

In a typical perovskite (AMX₃) structure, metal halide octahedra are corner-shared forming a 3D framework, with the A-site cations occupying voids within the cage. Similar to the well-studied hybrid MHS based on group IV metal ions, TMHs display a large chemical and structural diversity. Depending on the manner of the inorganic octahedral linkage, TMHs display 3D, 2D, 1D, and 0D crystal structures (Fig. 1). Typical 3D TMHs are limited to metal fluorides, such as KM^IIF₃ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺),²⁰ with the AMX₃ perovskite structures. Other 3D transition-metal halides are rare, not because of the constraint from the Tolerance factor, but due to the limitation of the octahedral factor (cation to anion ratio, μ_oct > 0.414)²¹ as transition metals normally have a smaller ionic radius. In the 3D framework, the perovskite structure can be doubled to form a cation-ordered double perovskite structure, A₂M^IM^IIIX₆, where monovalent M^I or trivalent M^III are transition metal ions. Another category of 3D TMHs is the dilute magnetic perovskites, which are composed of diamagnetic perovskite doped with transition metal ions (Section 5.2), such as MA(Pb:Mn)I₃ (MA⁺ = methylammonium).¹⁹

Fig. 1 Different crystal structures of TMHs. (a) 3D TMHs, (b) 2D TMHs, (c) 1D TMHs, and (d) 0D TMHs.

2D structures can be derived by sliding the 3D perovskite structures from different crystallographic orientations, including (100), (110), and (111) orientations, with the general formula of A^I₂M^IIX₄, A^IIM^IIX₄, and A^I₄M^IIM^III₂X₁₂ (where A is a monovalent or divalent organic cation), respectively. The space between the inorganic layers is then filled by the organic cations through electrostatic interactions. The (100) orientated 2D perovskites can be divided into a Ruddlesden–Popper (RP) phase and a Dion–Jacobson (DJ) phase. When incorporated with the monoammonium cation, the structure will be staggered in the RP phase with the formula A₂M^IIX₄, such as EA₂CuCl₄ (EA = ethylammonium).²² The diammonium cation will lead to the eclipsed DJ phase with the formula of A^IIM^IIX₄, such as EDACuCl₄ (EDA = ethylenediammonium).²³ (100) orientated structures are the most common among the 2D TMHs. The various structure anisotropy in 2D perovskites will affect the related magnetic properties, giving rise to magnetic anisotropy.

In 1D hybrid TMHs, such as MANiCl₃,²⁴ the inorganic 1D chains are formed by face-sharing octahedra, charge compensated by organic spacers. In 0D hybrid TMHs, inorganic metal halide units are isolated, either as individual octahedron or dimers. For example, Cs₃Fe₂Br₉ are composed of face-sharing [Fe₂Br₉]³⁻ octahedral dimers.²⁵ Another interesting 0D TMHs system is the vacancy-ordered double perovskite with the formula of A₂M^IVX₆, in which the octahedra are isolated. As the magnetic property is closely related to the crystal structure, below we will discuss the magnetic exchange mechanism, magnetic property, and structure–property relationships of hybrid transition-metal halides based on their crystal structures.

The solution-processable synthetic methods of TMHs show obvious advantages compared to traditional semiconductors, i.e., convenient implementation and free of harsh reaction conditions. To grow the single crystals of TMHs, the two most commonly used techniques are the cooling method and the antisolvent method. In the cooling method, the metal salts and organic ammonium salts are dissolved under heating, to form a supersaturated solution. With the temperature slowly cooling to room temperature, crystals gradually precipitated. By slowly diffusing antisolvent into the precursor solution, the antisolvent method enables the preparation of TMHs single crystals at room temperature. To prepare polycrystalline thin films, spin coating is the most used technique, which allows for fast and convenient operations but is only suitable for substrates with small areas. Polycrystalline thin films are the main format for the study of magneto-optical properties and the fabrication of related devices.^26,27

3. Magnetic exchange mechanism of TMHs

Any transition metal ions with unpaired electrons will exhibit paramagnetism, in which the spins tend to align in the direction of the external magnetic field. Below the critical temperature, the spin fluctuation induced by thermal energy is removed, and long-range magnetic order will emerge in the magnetically correlated systems, which includes ferromagnetism and antiferromagnetism. In ferromagnetic (FM) materials, the spin will align parallelly below the ordering temperature, known as the Curie temperature (T_c). In antiferromagnetic materials, the spins will align antiparallelly below the Néel temperature (T_N). In the structures of TMHs, the transition metal ions are bridged through the halide ions in single or multiple M–X–M linkages depending on the connectivity of inorganic building blocks. Therefore, the magnetic order in TMHs is generally realized through the M–X–M superexchange path, which can be understood by the Goodenough–Kanamori rules.²⁸

For those symmetric MX₆ octahedra (e.g., Mn²⁺, Fe²⁺) without Jahn–Teller distortion (Fig. 2a), the nearest-neighbor metal ions exchange via the same d orbital (e.g., d_x²−y²), and there will be two situations.^29,30 When the M–X–M bond angle is 180°, the linear M–X–M superexchange path will result in the AFM order according to the Pauli exclusion principle. When the M–X–M bond angle approaches 90°, both the p_x and p_y orbitals in the halide ions participate in the superexchange path. The spin alignment on the two orthogonal 2p orbitals ensures the maximum spin multiplicity, therefore leading to a ferromagnetic exchange between the nearest-neighbor metal ions. For the Jahn–Teller distorted MX₆ octahedra (Cr²⁺, Cu²⁺), the neighboring octahedra are arranged in the antiferrodistortive structure.^31–33 For the d⁴ Cr²⁺ ions, two bonds will be elongated along the z direction in the octahedra, lowering the energy level of d_z². In the distorted layered octahedral plane, the elongated Cr–X bond will be located orthogonally in the nearest octahedra (Fig. 2b). Therefore, the two d_z² orbitals on the neighboring octahedra will be orthogonal, leading to a zero-integral of the orbitals and FM superexchange interaction.^34–36 In the Jahn–Teller distorted Cu²⁺ octahedra, the one unpaired electron is located on the d_x²−y² orbital, resulting in the d_x²−y²–d_z² FM superexchange path.^37–41 The exchange energy between magnetic spins can be described by the Heisenberg–Dirac–Van Vleck (HDVV) Hamiltonian⁴² (1).

S_i is the spin operator, and J_ij is the exchange constant between the magnetic ions at sites i and j. The value of exchange constant J_ij suggests the coupling strength between the magnetic ions, which is positive for FM exchange and negative for AFM exchange.

Fig. 2 Illustration of the superexchange mechanism in Mn²⁺/Fe²⁺ systems (a) and Jahn–Teller distorted Cr²⁺/Cu²⁺ systems (b).

To characterize the magnetic properties of TMHs, temperature-dependent magnetization measurement is usually conducted, where the material is cooled with (field cooled, FC) or without (zero field cooled, ZFC) an external magnetic field. The bifurcation between ZFC and FC is induced by the thermal barrier in domain reorientation. Based on the mean field theory, the magnetic susceptibility (χ) of TMHs obeys the Curie–Weiss law (2) above the ordering temperature.

C is the Curie constant, χ₀ is the diamagnetic component, and θ_cw is the Curie–Weiss temperature that indicates the magnetic correlation. Ferromagnetic materials will have a positive θ_cw value, while antiferromagnets have a negative θ_cw value. As the paramagnet transforms to a ferromagnet below T_c, the spins align parallelly spontaneously, inducing a significant increase in the magnetic moment. For an antiferromagnet, the magnetization will show a maximum at Néel temperature. For detailed methodologies to analyze the experimental magnetic data refer to the protocol from Seshadri's group.⁴³

4. Magnetic properties of different TMHs

The magnetic properties of TMHs are strongly related to the crystal structures. In 2D structures, the interlayer structure anisotropy gives rise to magnetic anisotropy. With the structure dimension lowered to 1D and 0D, the magnetic coupling between metal ions will be weakened. Below, we will discuss the magnetic properties of TMHs in terms of different dimensions and different transition metal ions.

4.1 3D magnetic TMHs

Typical 3D magnetic TMHs are in the 3D perovskite structure, such as the series of KMF₃ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺)¹⁷ and KMnCl₃,⁴⁴ which are strong antiferromagnets with high Néel temperatures from 88 to 275 K (Table 1). In the cubic structure of KMF₃ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺), T_N is higher with higher d electron metal ions, which might be induced by the enhanced orbital overlap. Another 3D magnetic MHS that has been found recently is the CsEuX₃ (X = Cl⁻, Br⁻).⁴⁵ These compounds order antiferromagnetically with a Néel temperature of ∼2 K, originating from the nearly 180° bond angle of Eu–X–Eu. The contracted 4f-orbitals of the lanthanide centers result in weaker magnetic exchange interactions compared to 3d metal ions.⁴⁶ Interestingly, nanocrystals and thermally evaporated thin films of CsEuCl₃ were reported to be ferromagnetic with Curie temperatures of 2.5–3.0 K, which indicates that the type and strength of magnetic coupling can be tuned from antiferromagnetic to ferromagnetic through quantum confinement or substrate film interactions.

Table 1 Néel temperatures of several 3D TMHs

Compounds	T N (K)
KMnCl3	98
KMnF3	88
KFeF3	113
KCoF3	114
KNiF3	275
KCuF3	243
Cs2AgFeCl6	18
Cs2NaFeCl6	3
CsEuX3 (X = Cl, Br)	2

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We have studied the magnetic properties of 3D Cs₂AgFeCl₆ and Cs₂NaFeCl₆ double perovskites, revealing their geometrically frustrated AFM systems.¹⁵ The Néel temperatures of these double perovskites are much lower than that of the typical 3D perovskite KFeF₃, as the magnetic ions (Fe³⁺) are not directly connected, but are interconnected through a diamagnetic ion (Na⁺ or Ag⁺), resulting in weaker superexchange interactions. A recent study on perovskites with trivalent lanthanide, the Cs₂NaLnI₆ (Ln = Ce³⁺, Nd³⁺, Gd³⁺, Tb³⁺, Dy³⁺) series,⁴⁷ reveals the weakened coupling between magnetic ions due to the long Ln–Ln distance and contracted f orbitals.

4.2 2D magnetic TMHs

4.2.1 2D Hybrid manganese (d⁵) halides. The study of 2D layered Mn²⁺ halides started in 1970. Epstein et al. revealed that Rb₂MnCl₄ and Cs₂MnCl₄ order antiferromagnetically with two-dimensional spin correlations.⁴⁸ A later study of MA₂MnCl₄ by van Amstel et al. found magnetic anisotropy in this 2D antiferromagnet.²⁹ This indicates that the octahedral tilting and the anisotropic layered structure can lead to the spin canting phenomenon in 2D Mn²⁺ halides, in which spins are tilted by a small angle about their axis rather than being co-parallel.

Recently, the García group systematically studied the spin canting phenomenon in a series of 2D Mn²⁺ halides.¹⁶ PEA₂MnCl₄ (PEA = (C₆H₅CH₂CH₂NH₃)⁺) and EA₂MnCl₄ crystallize in staggered RP phases (Fig. 3(a) and (b)), and EDAMnCl₄ belongs to the DJ phase (Fig. 3(c)). In PEA₂MnCl₄, the out-of-plane temperature dependent magnetization curve, M(T), shows a typical AFM order. The steep increase of magnetic moment in the in-plane direction indicates a weak FM component in the [MnCl₆] octahedral plane (Fig. 3(d)), which suggests that the out-of-plane direction is the easy axis of the AFM order. This is consistent with the isothermal magnetization curve in these two directions (Fig. 3(e)). The weak FM component in the in-plane direction is attributed to the Dzyaloshinskii–Moriya interaction, which refers to the antisymmetric magnetic exchange in a system that lacks inversion symmetry. In EA₂MnCl₄, the magnetic order along the out-of-plane direction is much more complex, exhibiting a series of metamagnetism transitions, i.e., AFM → AFM with spin-canting → AFM with FM contributions (Fig. 3(f) and (g)). When looking at the DJ phase of EDAMnCl₄, the magnetic properties are different from the RP structures. Both the in-plane and out-of-plane M(T) curves show a typical AFM order (Fig. 3(h)) and are absent from spin canting, indicating a much smaller magnetic anisotropy in DJ structures compared to RP structures.

Fig. 3 Crystal structure of PEA₂MnCl₄ (a), EA₂MnCl₄ (b), and EDAMnCl₄ (c). Temperature (T) dependent magnetization (M) at 500 Oe parallel (‖, red lines) and perpendicular (⊥, blue lines) to the MnCl₆ octahedral layers for PEA₂MnCl₄ (d), EA₂MnCl₄ (f), and EDAMnCl₄ (h). Field (H) dependent magnetization (M) at 5 K parallel and perpendicular to the MnCl₆ octahedral layers for PEA₂MnCl₄ (e), EA₂MnCl₄ (g), and EDAMnCl₄ (i). The insets show the zoom-in of the M(H) curves at low magnetic fields. Reproduced from ref. 16 with permission from Wiley-VCH, Copyright 2022.

The origin of the spin-canting in Mn²⁺ halides is the antisymmetric Dzyaloshinskii–Moriya interaction, which is related to both the octahedral tilting and the interlayer anisotropy. In the RP structure, the layer of [MnCl₆] octahedra is staggered with the adjacent layer, which results in anisotropy in the out-of-plane direction. While in DJ structures, the inorganic layers are vertically aligned. Both the structure symmetry and shorter interlayer distance are responsible for the absence of spin canting in the DJ structure. Other layered Mn²⁺ halides with RP structures also show spin canting behavior with different canting angles, as summarized in Table 2. This structure–property relationship provides us with a strategy to modulate the magnetic anisotropy in layered manganese halides by changing the organic cations.

Table 2 Magnetic properties of different layered manganese halides

Compound	Néel temperature TN (K)	J/KB (K)	Curie–Weiss temperature θCW (K)	Spin canting degree (°)	Ref.
PA = C6H5NH3^+, PMA = C6H5CH2NH3^+, PPA = C6H5CH2CH2CH2NH3^+. ‖ (parallel) and ⊥ (perpendicular) is the direction of magnetic field to the MnCl6 octahedral layers.
PEA2MnCl4	45	−9.9	(‖) −173	0.06	16, 49, 50
			(⊥) −449
EA2MnCl4	43	−8.8	(‖) −195	0.03	16
			(⊥) −256
EDAMnCl4	80	−10.2	(‖) −190		16
			(⊥) −180
PA2MnCl4	42.6		−126	0.036	51
PMA2MnCl4	44.6		−135		51
PPA2MnCl4	41.8		−148	0.029	51
MA2MnCl4	44		−23		29,52

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4.2.2 2D Hybrid iron (d⁶) halides. Layered Fe²⁺ halides show similar magnetic properties as 2D Mn²⁺ halides, where AFM interaction plays a dominant role. Kurmoo's group studied several RP-type layered Fe²⁺ compounds, MA₂FeCl₄, EA₂FeCl₄ and PEA₂FeCl₄ (Table 3).^53–55 MA₂FeCl₄ only exhibits antiferromagnetism with hidden canting and a low metamagnetic critical field of 200 Oe. By changing the organic cation to C₂H₅NH₃⁺, the extra CH₂ introduces more degrees of freedom in EA₂FeCl₄ compared to MA₂FeCl₄ (Fig. 4a), giving rise to a series of temperature-dependent phase transitions and ferroelectric and ferroelastic properties. EA₂FeCl₄ also shows canted antiferromagnetism along the c axis with a spin canting angle of 0.64° (Fig. 4(b) and (c)). PEA₂FeCl₄ is more stable than the former two compounds, benefiting from the hydrophobic 2-phenylethyl ammonium cation, (C₆H₅C₂H₄NH₃)⁺. It exhibits canted antiferromagnetism along the a axis with an obvious hysteresis loop (Fig. 4(d) and (e)). Due to the ferroelastic structure in PEA₂FeCl₄, the temperature and mechanical stress induced structural transition between single- and multi-ferroelastic domain states can effectively modulate the magnetic properties. Uniaxial stress can even switch the axes in the crystal. The hysteresis loop along the a axis in the virgin sample will vanish after applying stress along the b axis (Fig. 4(e)).

Table 3 Magnetic properties of different layered iron halides

Compound	Néel temperature TN (K)	Curie–Weiss temperature θCW (K)	Spin canting degree (°)	Ref.
PEA2FeCl4	98	−298	0.53	55
EA2FeCl4	99	H ‖ a or b, −300 K
		H ‖ c, −530 K	0.64	54
MA2FeCl4	95	H ‖ a or b, −250 K
		H ‖ c, −330 K	1.4	53

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Fig. 4 (a) Crystal structure of EA₂FeCl₄. (b) Temperature dependence of the zero-field-cool and field-cool magnetizations of EA₂FeCl₄ in different applied fields H ‖ c-axis. (c) Isothermal magnetization at different temperatures for EA₂FeCl₄ after zero-field-cooling with H ‖ c-axis. (b) and (c) are reproduced from ref. 54 with permission from American Chemical Society, Copyright 2015. (d) Overlay of the crystal phase regions with the temperature dependence of the magnetization of PEA₂FeCl₄ (H = 10 Oe). (e) Isothermal magnetizations of a PEA₂FeCl₄ single crystal for the same orientation at 25 K; (left) the virgin sample and (right) the sample after applying stress along the b-axis. (d) and (e) are reproduced from ref. 55 with permission from Wiley-VCH, Copyright 2017.

This study reveals that the antisymmetric Dzyaloshinskii–Moriya interaction is coupled to ferroelasticity in PEA₂FeCl₄. Furthermore, the designed combination of functional organic molecules and transition metal ions can make molecule-based multiferroics, which are expected to realize the coupling of ferroelectric, ferroelastic, and AFM/FM properties within one material.

4.2.3 2D Hybrid copper (d⁹) halides. Unlike the Mn²⁺ and Fe²⁺ MHS, the Jahn–Teller distortion of [CuX₆] octahedra gives rise to intralayer ferromagnetism in 2D hybrid Cu²⁺ halide perovskites with a Curie temperature in the range of 4–16 K. The intralayer FM exchange can be modulated with the halide composition, with Br showing larger FM interactions than Cl due to higher covalency (Fig. 5a). Organic cations show a weak impact on the intralayer FM interaction (Fig. 5a). However, the interlayer magnetic exchange interaction is quite different from the intralayer FM interaction, which generally gives a weak AFM in the out-of-plane direction.^9,10,33 Interestingly, the interlayer magnetic interaction can be modulated by changing the interlayer distance and halide-halide distance.^33,56 A systematic study of interlayer and intralayer interactions in the alkylammonium Cu²⁺ metal halides (C_nH_2n+1NH₃)₂CuX₄ (X = Cl, Br, n = 1–10) was done by de Jongh et al., and is now summarized in Fig. 5(a) and (b).^57–60 Due to the much longer interlayer distance compared to the Cu–X bonds, the interlayer exchange constant J′ is significantly smaller than the intralayer exchange constant J (Fig. 5b). The Br compounds show a larger absolute value of J′ compared to Cl compounds because of the larger orbital radius of Br ions, which enhances the interlayer interaction.³⁸ In the staggered RP structures of (C_nH_2n+1NH₃)₂CuX₄, the interlayer interaction is reduced due to the offset between the inorganic layers and the larger interlayer distance. The interlayer distance is increased with longer organic cations, leading to a reduced J′ with a larger n value. The n = 1 (X = Cl) and n = 2 (X = Br) are two exceptions to this trend. (CH₃NH₃)₂CuCl₄ even shows a positive value of J′, indicating an FM interaction in the out-of-plane direction.⁶¹ The exact mechanism remains to be further understood.

Fig. 5 (a) The plot of J versus the n value in the RP structures of (C_nH_2n+1NH₃)₂CuX₄ (X = Cl, Br). (b) The plot of J′ versus the n value in the RP structures of (C_nH_2n+1NH₃)₂CuX₄ (X = Cl, Br). (c) The plot of J′ versus the n value in the DJ structures of (NH₃(CH₂)_nNH₃)CuBr₄ (X = Cl, Br). J′ is the interlayer interaction. J is the intralayer interaction. (d) Powder magnetic susceptibility versus temperature data for (NH₃(CH₂)_nNH₃)CuCl₄ with n = 6–10. (e) Powder magnetic susceptibility versus temperature data for (NH₃(CH₂)_nNH₃)CuBr₄ with n = 5–10.⁶² Solid data points are referenced to the left scale and open data points to the right scale. (d) and (e) are reproduced from ref. 62 with permission from Elsevier, Copyright 1984. (f) Antiferromagnetic resonance in (CH₃CH₂NH₃)₂CuCl₄ single crystals with a strong magnon–magnon coupling. (f) is reproduced from ref. 17 with permission from Nature Springer, Copyright 2023.

When incorporating the divalent alkyldiammonium cations, the structure of layered Cu²⁺ halides transforms into the eclipsed DJ phase. The magnetic properties of (NH₃(CH₂)_nNH₃)CuX₄ (X = Cl, Br) were studied by Rubenacker et al., and are summarized in Fig. 5(c).^62–67 As the inorganic layers are arranged vertically in the out-of-plane direction, the X–X halide distance is shorter and the absolute value of J′ is larger compared to the RP structure (Fig. 5c). Similar to the RP structure, the interlayer exchange interactions decay rapidly with the increasing length of organic spacers. In (NH₃(CH₂)₂NH₃)CuCl₄, the interlayer interaction is even stronger than the intralayer interaction, resulting in an overall AFM order.⁶⁸ The interlayer interaction J′ is also decreased as the interlayer distance increases, leading to an overall FM order in n = 10 (Fig. 5(d)). More interestingly, in the series of (NH₃(CH₂)_nNH₃)CuBr₄, an unexpected alternation of J′ is observed. J′ is positive when n = 7, 9, and 10 and negative when n = 6 and 8 (Fig. 5(e)), which might be related to the structural geometry of the Cu–X⋯X–Cu linkage under layered structures.⁶² These results indicate that the magnetic order and anisotropic exchange interactions in layered Cu²⁺ halides can be effectively modulated by changing the halides and organic spacers. To raise the Curie temperatures of copper halides, which is crucial for the development of spintronics, it's necessary to enhance the intralayer FM coupling and weaken the interlayer AFM coupling. Therefore, Br/I halides and long organic spacers are promising for making 2D copper halides with high Curie temperatures.

We have further studied the Dzyaloshinskii–Moriya interaction in layered AFM (CH₃CH₂NH₃)₂CuCl₄ through ferromagnetic resonance measurements.¹⁷ This interaction can control the magnetic dynamics particularly in AFM materials, such as collective spin excitations (magnons). The mixed magnon in this AFM material has a simple planar anisotropy. In principle, due to the opposite parity, two orthogonal magnon modes exist independently and cannot interact with each other, allowing for a degeneracy to occur. However, due to the presence of Dzyaloshinskii–Moriya interaction, the symmetry of even parity high-frequency optical magnon (under a twofold rotation) and odd parity low-frequency acoustic magnon is broken (Fig. 5(f)). These two magnon modes couple, lifting the degeneracy and forming new hybrid states. This coupling between the optical and acoustic magnons allows for the spin information transfer between these two quasi-particles.

Recently, Zhang et al. reported two new layered Cu²⁺ halides with aromatic organic spacers, (PED)CuCl₄ (PED = N-phenylethylenediammonium) and (BED)₂CuCl₆ and (BED = N-benzylethylenediammonium), where the former one features a (110) structure and the latter a (100) structure (Fig. 6(a) and (b)).⁶⁹ The Curie temperatures of these two compounds are 12 K (Fig. 6(a) and (d)). The value of saturated magnetization in (PED)CuCl₄ is much smaller than that in (BED)₂CuCl₆ (Fig. 6(e) and (f)), which is ascribed to a more distorted octahedra in (PED)CuCl₄ ((110) structure) than that in (BED)₂CuCl₆. More distorted octahedra and corrugated [CuCl₆] layers suppress the superexchange interaction between Cu²⁺ ions in (PED)CuCl₄, leading to weaker ferromagnetism. Similar to (PEA)₂FeCl₄, (PEA)₂CuCl₄ also exhibits multiferroic behavior involving the coexistence of ferroelectricity induced by the organic spacers and ferromagnetism induced by the corner-shared [CuCl₆] octahedral layer, giving rise to large magnetoelectric coupling.⁷⁰

Fig. 6 Crystal structures of (PED)CuCl₄ (a), and (BED)₂CuCl₆ (b). (c) and (d) Temperature-dependent magnetization of (PED)CuCl₄, (BED)₂CuCl₆ under the field of 100 Oe. (e) and (f) M–H curves of (PED)CuCl₄ and (BED)₂CuCl₆ at 2 K, 5 K, and 10 K. Reproduced from ref. 69 with permission from Wiley-VCH, Copyright 2019.

4.2.4 2D Hybrid chromium (d⁴) halides. 2D layered Cr²⁺ halides show a similar crystal structure to layered hybrid Cu²⁺ halides with a significant Jahn–Teller distortion. The high-spin configuration (d⁴) of the Cr²⁺ ion results in strong FM coupling and relatively high Curie temperature compared to the copper compounds.^71–73 A series of layered Cr²⁺ halides (C_nH_2n+1NH₃)₂CrCl₄ was studied by Day et al. in 1986, showing an FM order with a Curie temperature in the range of 37–52 K (Table 4).^74–76 (C₆H₅CH₂NH₃)₂CrBr₄ shows a relatively high Curie temperature of 52 K, owing to the larger interlayer distance.⁷⁷ Recently, Xiong's group reported multifunctional Cr²⁺ halides (DFCBA)₂CrCl₄ (DFCBA = 3,3-difluorocyclobutylammonium), which show the co-existence of ferroelectricity (transition temp = 387 K) and ferromagnetism (T_c = 32.6 K) (Fig. 7(a) and (b)).⁷⁸ At 2 K, [DFCBA]₂CrCl₄ shows a hysteresis loop with a coercive field (H_C) of 85 Oe (Fig. 7(c)). The relatively high Curie temperature of Cr²⁺ halide perovskites combined with ferroelectricity is of great potential in multifunctional smart devices.

Table 4 Magnetic properties of layered chromium halides

Compound	T c/K	(J/KB)/K	θ CW (K)	d/Å	Ref.
(CH3NH3)2CrCl4	42	13.0	59	9.44	71
(C2H5NH3)2CrCl4	41	10.1	58	10.71	71
(C3H7NH3)2CrCl4	39.5	9.3	57	12.35	74
(C5H11NH3)2CrCl4		9.3	57	17.81	74
(C6H5CH2NH3)2CrCl4	37	10.6	58	15.71	77
(C6H5CH2NH3)2CrBr3.3Cl0.7	49	12.5	62	16.10	79
(C6H5CH2NH3)2CrBr4	52	13.1	77	16.24	77
(C6H5CH2NH3)2CrBr3.3I0.7	51	11.0	69	16.15	72
[DFCBA]2CrCl4	32.6	7.15	45	5.4	78

---

Fig. 7 (a) Crystal structure of [DFCBA]₂CrCl₄. (b) The temperature dependent ZFC/FC plots at 10 Oe field. (c) The isothermal magnetization curve at 2 K, the inset is the low field region. Reproduced from ref. 78 with permission from Wiley-VCH, Copyright 2022.

4.2.5 2D Halide double perovskites. Halide double perovskites provide a platform to incorporate transition metal ions and to tune the magnetic properties through composition modulation. The formula of typical 3D double perovskite is (A₂M^IM^IIIX₆). The combination of diamagnetic metal and transition metal ions (e.g. Ag^I/Na^I–Fe^III) will give an M^III–X–M^I–X–M^III linkage, different from the traditional M–X–M superexchange pathway. We have studied the magnetic properties of a series of Ag^I/Na^I–Fe^III double perovskites, including 3D structures and their 2D analogues.¹⁵ As depicted in Fig. 8(a), three different lengths of organic ammoniums were inserted into the 3D perovskite, resulting in a 2D (100) structure. Magnetic measurements were then conducted on Ag–Fe double perovskites with three different organic cations (phenylethylammonium (PEA), R-(+)-β-methylphenethylammonium (R-MPA) and butanediammonium (BDA)). The impact of monovalent metal ions on magnetic properties is also examined in PEA₄NaFeCl₄ (Fig. 8(b) and (c)). In the Ag–Fe–Cl series, we found a linear relationship between the Curie–Weiss temperature and the nearest Fe–Fe distance represented by the red line in Fig. 8(c), indicating that the longer distance between Fe–Fe leads to weaker magnetic coupling. However, when replacing Ag⁺ with Na⁺, we observed that the antiferromagnetic coupling did not increase, despite the further decrease in Fe–Fe distance, suggesting that bridging metal is also involved in the magnetic exchange interaction. It further indicates that the antiferromagnetism comes from Fe^III–Cl–M^I–Cl–Fe^III long-range superexchange rather than a direct magnetic exchange pathway. Compared to the geometrically frustrated 3D Cs₂AgFeCl₆ and Cs₂NaFeCl₆, the frustration factors of 2D analogues are dramatically reduced due to the square lattice. Furthermore, AFM coupling in the Ag^I–Mo^III and Ag–Ru^III layered double perovskites have been studied by different groups.^80,81 The experiment results are consistent with the M^III–X–M^I–X–M^III long-range superexchange pathway.

Fig. 8 (a) Schematic illustration of the evolution from 3D double perovskite to 2D double perovskite by different organic amines. (b) Magnetic susceptibility vs. temperature for PEA₄AgFeCl₈. (c) Correspondence between Curie–Weiss temperature and nearest Fe–Fe distance in the hybrid layered Fe–Cl double-perovskite series. The orange dot represents PEA₄NaFeCl₈, and black dots represent the Ag–Fe–Cl series. The red line is the linear fit for the Ag–Fe–Cl series. (PEA = phenylethylammonium, R-MPA = R-(+)-β-methylphenethylammonium, BDA = butanediammonium). (a)–(c) are reproduced from ref. 15 with permission from the American Chemical Society, Copyright 2022. (d) Photographs of Cs₄Mn_1−xCu_xSb₂Cl₁₂ (x = 0–1) and the corresponding absorption spectra. (e) Temperature dependence of the inverse magnetic susceptibility-temperature of Cs₄Mn_1−xCu_xSb₂Cl₁₂, the solid lines are the best fit of the data. (d) and (e) Are reproduced from ref. 82 with permission from American Chemical Society, Copyright 2018.

Another series of layered double perovskite is the [111] oriented structure, which is achieved through the incorporation of vacancies rather than the large organic cations found in the [100] perovskite. In the [111] layered double perovskite, one vacancy and one bivalent metal ion replace two monovalent metal ions of [100] perovskites, resulting in a general formula of A₄M^IIM₂^IIIX₁₂.⁸³ Bivalent metals, such as Cu^II or Mn^II, often provide unpaired electrons, whereas trivalent metals are typically diamagnetic (e.g., Sb^III or Bi^III). In 2017, the first [111] layered double perovskite, Cs₄CuSb₂Cl₁₂, was reported.⁸⁴ Follow-up research on the Cs₄Mn_1−xCu_xSb₂Cl₁₂ series (Fig. 8(d)) demonstrated that Mn alloying can modulate the antiferromagnetic behavior (Fig. 8(e)).⁸² As x values increase, the temperature-independent paramagnetism and the Pauli paramagnetism state gradually increase, originating from the rising concentrations of movable charge carriers. In addition, DFT calculation showed that intralayer antiferromagnetic behavior occurs within each [MCl₆] (M = Cu²⁺, Mn²⁺) octahedral chain along the [010] direction.

4.3 1D and 0D Hybrid magnetic metal halides

In 1D and 0D TMHs, the magnetic coupling is expected to be weakened owing to the reduced connectivity. In the 1D structures, the [MX₆] octahedra are usually connected in a face-sharing manner. Magnetic interactions can also be divided into two parts in 1D TMHs, namely, the intrachain interaction, and interchain interaction. Since the intrachain interaction plays a dominant role and is working through the superexchange via the bridging halides. The M–X–M angle will be crucial to determine the overall magnetic order.

Recently, the Cava group studied the magnetic properties of 1D ANiCl₃ (A = MA⁺, Gu⁺, FA⁺, DMA⁺, TMA⁺) compounds with different organic spacers.⁸⁵ These Ni halides crystallize in a hexagonal-perovskite-type structure. The [NiCl₆] octahedra are face-sharing connected to form 1D chains, which are separated by organic cations. From the Curie–Weiss fitting, TMANiCl₃ shows a positive Curie–Weiss temperature, while the other four compounds have negative Curie–Weiss temperatures. TMANiCl₃ has the largest Ni–Cl–Ni angle of 78.75° among the above five compounds (Fig. 9). The intrachain ferromagnetic coupling increases as the Ni–Cl–Ni angle approaches 90°, which is consistent with the superexchange model.

Fig. 9 The structure (a) and magnetic properties (b) of the 1D ANiCl3 (A = MA⁺, Gu⁺, FA⁺, DMA⁺, TMA⁺) compound. TMA = (N(CH₃)₄⁺), MA = (CH₃NH₃⁺), DMA = ((CH₃)₂NH₂⁺), Gu = (C(NH₂)₃⁺), and FA = (CH(NH₂)₂⁺). Reproduced from ref. 85 with permission from American Chemical Society, Copyright 2022.

In the 1D series of MACoCl₃, FACoCl₃, and GuCoCl₃, the structure-magnetic property relationship is more complex.⁸⁶ The one-dimensional intrachain coupling is antiferromagnetic (Fig. 10(a)). However, there is no direct relationship between the Co–Cl–Co angle and the corresponding intrachain coupling constant because the unpaired t_2g electron in high-spin Co²⁺ contributes to the complexity of magnetic exchange within the system. Instead, there is a direct relationship between the interchain Co–Co distance and the critical temperature of the long-range magnetic order. With the organic cation changing from MA⁺ to FA⁺, and Gu⁺, the size of organics is larger, resulting in a longer interchain Co-Co distance and lower critical long-range ordering temperatures (Fig. 10(b)).

Fig. 10 (a) Temperature-dependent magnetic susceptibility of MACoCl₃, FACoCl₃, and GuCoCl₃. (b) Field-dependent magnetization of MACoCl₃, FACoCl₃, and GuCoCl₃ at the critical long-range ordering temperature. Reproduced from ref. 86 with permission from American Chemical Society, Copyright 2023.

The magnetic properties of a series of low-dimensional Ru metal halides were studied by the Cheetham group.⁸⁷ MA₂RuCl₆ crystallizes as a vacancy-ordered halide perovskite, and the [RuCl₆]²⁻ octahedra are isolated and undistorted (Fig. 11(a)). Their magnetic properties are expected to show isolated spin-ion behavior without the exchange interactions between the [RuCl₆]²⁻ octahedron. The theoretical value of m_eff can be predicted by the octahedral crystal field with spin–orbit coupling. The experimental data of MA₂RuCl₆ coincides well with the Kotani plot calculated from a spin–orbit coupling constant ξ of 1400 cm⁻¹, which is consistent with the d⁴ Ru⁴⁺ ions (Fig. 11(b)). As the temperature decreases, the effective moment falls to the ground state J = 0, indicating the absence of coupling between the isolated [RuCl₆]²⁻ octahedra. MA₂NaRuCl₆ crystallizes in a 1D structure with the infinite face-sharing chains of [RuCl₆]³⁻ octahedra. Intrachain interaction plays a dominant role in the overall magnetic order. Therefore, the experimental m_eff value is varied from the Kotani prediction for isolated d⁵ Ru³⁺ ions. Different from the MA₂RuCl₆, at lower temperatures, the effective moment of Ru³⁺ is not zero, as the ground state is J = 1/2. The moment is always lower than the ideal Kotani prediction for isolated [RuX₆]³⁻ ions, which indicates the AFM exchange interactions inside the inorganic chains. The MA₃Ru₂Br₉ crystallizes in the 0D structure with the basic structural units of face-shared [Ru₂Br₉]³⁻ dimers. Due to the exchange interaction inside the [Ru₂Br₉]³⁻ dimer, the magnetic moment of MA₃Ru₂Br₉ is much lower than the ideal d⁵ Ru³⁺ behavior. They have also studied another two series of vacancy-ordered Ru⁴⁺ halides, A₂RuCl₆ and A₂RuBr₆, where A is K⁺, NH₄⁺, Rb⁺ or Cs⁺.⁸⁸ It shows that Ru–X covalency, and the delocalization of metal d-electrons, can modulate the spin–orbit coupling constants systematically by changing the A cation and the halide anion. The study on the vacancy-ordered Os⁴⁺ (5d⁴) halides, K₂OsCl₆, K₂OsBr₆, and Na₂OsBr₆, suggests that they also display a Kotani-like behavior consistent with a J = 0 ground state.⁸⁹

Fig. 11 (a) The structure of MA₂RuCl₆, MA₂NaRuCl₆, and MA₃Ru₂Br₉. (b) Experimental Kotani plots for MA₂RuCl₆, MA₂NaRuCl₆, and MA₃Ru₂Br₉. The theoretical Kotani plots are given as black lines. Reproduced from ref. 87 with permission from Wiley-VCH, Copyright 2020.

Zheng et al. systematically studied a series of manganese halides of 2D (4-fluorobenzylamine)₂MnCl₄, 1D ((R)-3-fluoropyrrolidinium)MnCl₃, and 0D (pyrrolidinium)₂MnCl₄ and demonstrated the effect of structure dimensions on spin decoherence processes.⁹⁰ The spin decoherence time follows the trend 2D > 1D > 0D due to the reduced connectivity and larger distance between adjacent Mn²⁺–Mn²⁺ ions in lower structure dimensions. It indicates that the structure dimensions can strongly affect the magnetic interaction.

The 1D [(S)/(R)-MPA]₂[MnCl₄(H₂O)] is found to be a weak chiral ferromagnet below 3.2 K, which is attributed to a spin canting AFM state induced by Dzyaloshinskii–Moriya interactions owing to spatial inversion symmetry breaking in a chiral MHS.⁹¹ (C₅H₁₄N₂)[CuCl₄] and (C₅H₈N₃)[CuCl₃] are 1D structures, constructed from a double edge-sharing Cu₂Cl₆ chain.^92,93 Their magnetic properties are correlated with the long Cu–Cl distance and the bridging Cu–Cl–Cu angle, and obey the magnetostructural correlations in Cu dimer structure discovered by Hatfield and Hodgson.⁹⁴ 0D Mn²⁺ halides with isolated MnX₆ octahedra or MnX₄ tetrahedra are paramagnet, owing to the absence of magnetic coupling.⁹⁵ Cs₃Fe₂Cl₉ and Cs₃Fe₂Br₉ crystallize in buckled honeycomb networks and contain isolated face-sharing [Fe₂X₉]³⁻ dimers, which order antiferromagnetically through the superexchange inside the dimer. The easy-axis anisotropy and complex magnetic coupling between the dimers contribute to the complicated magnetic phase diagram, which remains to be further understood.^25,96

5. Emerging properties and potential applications of magnetic metal halides

5.1 Chiral ferromagnetic TMHs

The interesting magnetic properties of metal halides are expected to couple with their semiconducting property, giving rise to multifunctional materials. For example, in chiral ferromagnetic TMHs, chirality breaks the space inversion and mirror symmetry and contributes to novel coupling effects, such as magneto circular dichroism (MCD) and the magneto-electric coupling effect. In 2020, Zhang's group first reported a pair of 2D chiral ferromagnetic perovskite, (R-MPEA)₂CuCl₄ and (S-MPEA)₂CuCl₄ (MPEA is β-methylphenethylamine) (Fig. 12(a) and (b)).¹⁸ The temperature dependent magnetization curve of the chiral and racemic compounds indicates the transition from paramagnet to ferromagnet at the Curie temperature of 6 K (Fig. 12(b)). It should be noted that (R-MPEA)₂CuCl₄ and (S-MPEA)₂CuCl₄ have almost the same magnetic moment values, while the racemic compound has a smaller moment value at low temperatures, which might be attributed to the symmetric structure of the racemic one. The MCD effect is present when a chiral compound absorbs unpolarized light under an external magnetic field. The structure origin of the MCD effect is the magneto-chiral anisotropy. MCD spectra (B = 1 T) of (R-MPEA)₂CuCl₄ and (S-MPEA)₂CuCl₄ at 2 K are shown in Fig. 12(d). The R and S compounds exhibit symmetric and opposite optical signal ΔA, suggesting the co-existence of the chirality and magnetism in the structures.

Fig. 12 Single crystal structures of (R-MPEA)₂CuCl₄ (a) and (S-MPEA)₂CuCl₄ (b). (c) Temperature dependence of the magnetizations of (R-MPEA)₂CuCl₄, (S-MPEA)₂CuCl₄, and (rac-MPEA)₂CuCl₄ in applied fields of 100 Oe. (d) MCD spectrum recorded on (R-MPEA)₂CuCl₄ and (S-MPEA)₂CuCl₄ at 2 K. Reproduced from ref. 18 with permission from American Chemical Society, Copyright 2020.

Taniguchi et al. demonstrated the magneto-electric correlation in (R/S-MPEA)₂CuCl₄.⁹⁷ The chiral-polar molecules were intercalated between ferromagnetic Cu–Cl layers to afford a non-centrosymmetric ferromagnet. Electric polarization occurs not only in the diamagnetic organic spacers but also in the magnetic inorganic layers. The magneto-electric correlation was clearly observed by measuring the magneto-electric directional anisotropy, in which an optical absorption coefficient changes with reversal of the light propagating direction under a low magnetic field of about 50 mT. This work suggests a new strategy to design functional magneto-electric multiferroics. Researchers also found spin-dependent charge transport with a high filtration efficiency (90%) in chiral ferromagnetic (R/S-MBA)₂CuX₄ (MBA = methylbenzylammonium, X = Cl, Br).⁹⁸ The lead-free and air-stable chiral TMHs exhibit potential applications in spintronic devices.

5.2 Dilute magnetic metal halides

Apart from the above-mentioned magnetic metal halides, which consist of the periodic lattice of transition metal ions, there is another kind of magnetic metal halides, dilute magnetic metal halides. They are formed by doping with a small number of paramagnetic transition metal ions into the diamagnetic perovskite lattice. Dilute magnetic semiconductors (DMS) combine the properties of semiconductors and ferromagnets. The mechanism of ferromagnetism in typical DMS including metal oxides, III–V and II–VI semiconductors has been demonstrated. The ferromagnetism in Mn-doped semiconductors can be explained using the p–d Zener model, which attributes it to the exchange between band carriers and localized magnetic spins.⁹⁹ Considering the Friedel oscillations of spin density, Ruderman, Kittel, Kasuya, and Yosida proposed the (Ruderman–Kittel–Kasuya–Yosida) RKKY theory,¹⁰⁰ which shows that magnetic spins from impurity dopants can interact with charge carriers or lattice polarons and results in long-range FM coupling. Metal halide perovskites possess both excellent defect tolerance and convenient solution processability, enabling them to be a tunable platform to incorporate paramagnetic transition metal ions. The coupling between long-range ferromagnetic order and excellent optoelectronic properties in dilute magnetic metal halides can induce emerging properties and applications. The modulation of charge carriers in dilute magnetic metal halides can potentially tune the magnetic properties in a new way.

The Horvath group reported the ferromagnetic dilute perovskite and revealed that the photo-excited electrons rapidly melt the local magnetic order through the RKKY interactions.¹⁹ They synthesized the Mn-doped CH₃NH₃(Mn:Pb)I₃ crystals with a Mn doping ratio of 10% (Fig. 13(a)). The temperature dependent magnetization curve indicates the ferromagnetic order below the Curie temperature T_c = 25 K. The spontaneous ferromagnetic order results in the rapid shift of the resonance field B₀ and broadening of the electron spin resonance (ESR) linewidth below T_c (Fig. 13(b)). Through the in situ ESR measurement under illumination, the light-on ESR signal is weaker and narrower than the dark signal (Fig. 13(c)). As the intensity of the ESR signal is proportional to the ferromagnetic volume, the decreased intensity under illumination suggests a decrease of the ferromagnetic order. This phenomenon is attributed to the light-induced RKKY interactions, in which the photo-excited conduction electrons melt the aligned spin order in the ferromagnetic ground state. The illumination melted ferromagnetism provides a fast and convenient method to manipulate the spin order. By using the light to melt the ferromagnetic ground state, only a small magnetic field is needed to rewrite the spin order in the sample (Fig. 13(d)). Ren et al. also found that the photocurrent of Mn-doped MAPbI₃ perovskite solar cells was slightly increased under a magnetic field of 1 T as the FM coupling enhanced the electron hopping and decreased the photoresistance.¹⁰¹ It suggests the potential of TMHs in modulating the photovoltaic performance.

Fig. 13 (a) Sketch of the crystal structure of the CH₃NH₃(Mn:Pb)I₃ crystal. (b) ESR linewidth (red dots) and resonant field (blue dots, offset by a reference value B₀) as a function of temperature recorded at 9.4 GHz. (c) ESR spectra at 157 GHz and 5 K of pristine CH₃NH₃PbI₃ (green line–no signal), of CH₃NH₃(Mn:Pb)I₃ in dark (blue line) and its reduction (red line) on visible light illumination. (d) Schematic illustration of rewriting a magnetic bit. Reproduced from ref. 19 with permission from Nature Springer, Copyright 2016.

Neumann et al. studied the magneto-optical properties of Mn-doped Ruddlesden–Popper hybrid perovskite thin-film Mn:(PEA)₂PbI₄ (Fig. 14(a)) and found that Mn doping will affect the spin polarization.¹⁰² The introduction of Mn²⁺ can induce the paramagnetic order (Fig. 14(b)) and Zeeman effect in the perovskite structure and increase the energy degeneracy, which gives rise to the imbalanced population of the exciton spin states. A dark exciton is populated in the presence of a magnetic field. The spin polarization reaches a saturation value of 13% at 4 K under a magnetic field of 6 T (Fig. 14(c)). This is induced by the spin-dependent exciton dynamics, indicating an Mn-mediated spin–flip process (Fig. 14(d)), which enables the direct control of exciton spin physics via a magnetic field. Subsequent study of Co²⁺ doped (PEA)₂PbI₄ also confirmed the above findings.¹⁰³ These studies highlight the potential of TMHs in the application of magneto-optics.”

Fig. 14 (a) Sketch of the Ruddlesden–Popper quantum well structure of Mn:(PEA)₂PbI₄. (b) The isothermal magnetization curves at different temperatures. (c) Circular polarization of the PL and magnetization curve of Mn:(PEA)₂PbI₄ at 4 K and Brillouin function for a non-interacting J = 5/2 spin system. (d) Proposed interaction mechanism between excitons and Mn²⁺ spins in an external magnetic field. X_B and X_D denote bright and dark excitons, respectively. Reproduced from ref. 102 with permission from Nature Springer, Copyright 2021.

These findings revealed that the doped magnetic ions in perovskite could induce various cross effects, which enable the manipulation of spin order and exciton spin physics via external stimulate. Although various dilute magnetic perovskites have been synthesized to date,^19,102–104 there is an absence of strong ferromagnetism. This might be attributed to the lack of manipulation of carrier concentration. It is thus critical to realize the simultaneous control of localized magnetic moments and carrier concentration to tune the magnetic properties in dilute magnetic perovskites.

Conclusion

In summary, we have summarized the chemical and structure diversity and magnetic properties of transition metal halides in terms of different structure dimensions and metal ions. The magnetic order in metal halides is determined by the structure geometry, which enables tuning magnetic properties through structural engineering. The abundant composition in metal halides provides a splendid platform to design functional magnetic properties. We expect that future research in this field will be focused on the following directions.

(1) Design of functional organic spacers in hybrid transition metal halides. The magnetic order in various transition metal halides has been extensively studied. There remains a sizeable exploration space for the design of organic spacers. For example, chiral-polar organics are expected to bring chiral ferromagnetic metal halides with multiferroic behavior. Incorporating the abundant magnetic and electric properties in a single material can expand their application and induce distinct cross coupling effects.

(2) One of the advantages of MHS is the solution-processable structure feature, which is convenient to make thin films for device fabrication. Most studies on the magnetic properties of TMHs are focused on single crystals. Expanding the study to thin films and nanocrystals is expected to bring new magnetic phenomenon, which is of huge potential in spintronic devices.

(3) Weak FM coupling and low Curie temperature are generally observed in TMHs, due to the M–X–M superexchange path. Dilute magnetic metal halides provide a new platform to modulate the magnetic properties. The charge carrier concentration in MHS can be easily modulated by photo-excitation and electrical doping, which may induce dramatic modification in the long-range magnetic order.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 22205186), the Research Grants Council of Hong Kong via the Early Career Scheme (ECS, 26300721), and Fei Chi En Education and Research Fund from Y-Lot Foundation. We acknowledge the start-up funding support from the Hong Kong University of Science and Technology (HKUST) School of Science (SSCI) and the Department of Chemistry (R9270).

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