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Ferromagnetic intercalated compounds CrHfxTe2 with a magnetocaloric effect and negative magnetoresistance

Kunqi Li ab, Xueyang Tu ab, Xuzhou Sun ab, Hui Bi *a, Yuqiang Fang *c and Fuqiang Huang *c
aThe State Key Laboratory of High-Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China. E-mail: bihui@mail.sic.ac.cn
bCenter of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
cState Key Lab of Metal Matrix Composites, School of Materials Science and Engineering, Zhangjiang Institute for Advanced Study, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail: fangyuqiang@sjtu.edu.cn; huangfq@sjtu.edu.cn

Received 3rd June 2025 , Accepted 26th August 2025

First published on 4th September 2025


Abstract

Intercalation in van der Waals materials enables novel structures and exotic properties. Herein, we have successfully synthesized two intercalated compounds CrHfxTe2 (x = 0.1, 1/3) via a flux method. CrHf0.1Te2 and CrHf1/3Te2 exhibit ferromagnetism with Curie temperatures of 234.9 K and 247.2 K under out-of-plane fields. The maximum magnetic entropy variation of CrHf0.1Te2 attains 2.85 J kg−1 K near 185 K, and the relative cooling power (216.6 J kg−1) exceeds that of multiple van der Waals ferromagnets. Furthermore, negative magnetoresistances of −3.8% and −4.7% are respectively observed in CrHf0.1Te2 and CrHf1/3Te2. This work provides a strategy for designing new intercalation compounds promising for electronic and magnetic applications.


Introduction

Magnetic van der Waals (vdW) materials hold particular promise for nano-scale spintronics devices, magnetic sensors and magnetic refrigeration due to their inherent spin polarization, potential spin–orbit torque effect or intrinsic magnetocaloric effects.1–4 In recent years, the discovery of novel vdW ferromagnets has provided an ideal platform for exploring exotic quantum states such as large tunneling magnetoresistances and magnetic skyrmions. For instance, ferromagnetic CrI3 is characterized by an extremely robust tunnelling magnetoresistance (as large as 10[thin space (1/6-em)]000%).5 Moreover, chiral magnetic phases and Moire Skyrmions observed in twisted bilayer CrI3 demonstrate the possibility of engineering nontrivial magnetic ground states.6 However, the low TC (61 K) of CrI3 weakens its utilization potential in large-scale spintronic devices. Room-temperature ferromagnetic CrTe2 is still gaining considerable research attention owing to its colossal anomalous Hall conductivity,7 room-temperature in-plane anisotropic magnetoresistance8 and many other unique physical properties.

The intercalation of atoms or molecules between the layers of vdW crystals can effectively adjust the electronic structure and interlayer interaction, thereby modifying the electronic and magnetic properties of the matrix. For TaS2, magnetic atom intercalated Fe0.25TaS2 was confirmed to have long-range ferromagnetic order, a √3 × √3 superstructure, as well as an anomalous Hall effect.9 In the Cr1/3TaS2 single crystal, the coupling of the strong spin–orbit interaction from TaS2 and the chiral arrangement of Cr ions evokes a robust Dzyaloshinskii–Moriya interaction, hence forming chiral helimagnetism.10 In terms of the intercalated CrTe2 compound, Cr1.25Te2 was reported to present orthogonal ferromagnetism, featured by alternating in-plane and out-of-plane ferromagnetic layers coupled via antiferromagnetic exchange coupling.11 As one of the 5d transition metals, the Hf atom possesses significantly strong spin–orbit coupling (SOC) effects. The intercalation of Hf atoms within vdW layered crystals probably enhances the SOC between the layers, thereby modifying the interlayer exchange interaction and the electronic bands, which is conducive to inducing novel electronic and magnetic properties.

In this work, we synthesized CrHfxTe2 (x = 0.1, 1/3) via a simple flux method. Similar to 1T CrTe2, CrHf0.1Te2 crystallizes in a trigonal phase where each Cr atom is coordinated in a regular octahedron with Te atoms. In terms of magnetic properties, ferromagnetic CrHf0.1Te2 and CrHf1/3Te2 both exhibit perpendicular magnetic anisotropy due to significant SOC that retains the spin in an out-of-plane orientation. CrHf0.1Te2 demonstrates high Curie temperature (234.9 K) and large coercivity (691 Oe at 3 K), exceeding those of multiple Cr-based vdW ferromagnets. Along the easy magnetization axis, CrHf0.1Te2 displays the maximum magnetic entropy change of 2.85 J kg−1 K at approximately 185 K, and the relative cooling power (216.6 J kg−1) exceeds that of multiple vdW ferromagnets. Furthermore, intrinsic negative magnetoresistance exists in CrHfxTe2, rendering it a candidate for magnetic sensors. This study unveils a novel materials design strategy to synthesize intercalation compounds for advanced electronic and magnetic applications.

Results and discussion

CrHfxTe2 (x = 0.1, 1/3) crystals were prepared using CsI as the flux agent. CsI flux controls Hf intercalation in CrTe2 as follows: first, Cs+ ions temporarily intercalate into the interlayers in the molten state, enlarging the interlayer spacing and creating more diffusion channels for Hf atoms. Moreover, Hf dissolves in CsI and diffuses more rapidly than the solid-state diffusion, so that it is rapidly transported to the crystal surface. While CrTe2 layers bear a weak negative charge, Hf4+ is positively charged. The ionic environment of CsI enhances electrostatic interactions, hence driving the migration of Hf4+ toward the interlayers. CrHf0.1Te2 crystallizes in the trigonal space group P[3 with combining macron] (No. 147). As shown in Fig. 1a, each Cr atom is coordinated by six Te atoms in a regular octahedron, and 10% Hf atoms are intercalated in the CrTe2 layers. Fig. S1 (SI) reveals the lamellar morphologies of CrHf0.1Te2 and CrHf1/3Te2. Cr, Hf and Te elements are distributed homogeneously in the energy-dispersive X-ray spectroscopy (EDS) elemental mapping images, and Table S1 (SI) shows that the atomic proportion of Cr, Hf and Te is close to the stoichiometric ratio in CrHfxTe2.
image file: d5tc02160j-f1.tif
Fig. 1 (a) Crystal structure of trigonal CrHf0.1Te2 viewed from the c-axis and a-axis. (b) PXRD patterns. (c) Raman spectra. (d) and (e) High-resolution XPS spectra of Cr 2p, Te 3d and Hf 4f for CrHfxTe2 (x = 0.1, 1/3).

The powder X-ray diffraction (XRD) pattern of CrHf0.1Te2 aligns well with the simulated diffraction peaks from the resolved single crystal structure through single-crystal X-ray diffraction (SC-XRD) (Fig. 1b). In contrast to CrHf0.1Te2, the diffraction peak of the (0 0 2) plane within CrHf1/3Te2 shifts from 2θ = 14.6° to 14.2°, indicating a slight increase in the crystal plane spacing (d). The crystallographic parameters of trigonal CrHf0.1Te2 are detailed in Tables S2–S6 (a = b = 3.8907(6) Å, c = 6.1053(15) Å). For trigonal CrHfxTe2, the increase of d(002) means bigger c, the perpendicular parameters for CrHf1/3Te2. In addition, Raman spectroscopy was employed to probe diverse vibrational modes of chemical bonds. In Fig. 1c, two typical in-plane (Eg) and out-of-plane vibration modes (A1g) are clearly found in the Raman spectra of CrHfxTe2. As the ratio of Hf element increases, both vibration peaks exhibit a slight blue shift (A1g shifts from 121.3 cm−1 to 123.1 cm−1, and Eg shifts from 139.6 cm−1 to 141.4 cm−1). Compared with the A1g (100.0 cm−1) and Eg (134.1 cm−1) modes of pure CrTe2,8 this kind of blue shift occurs due to increased interlayer coupling arising from intercalated atoms.1 As the intercalated ratio of Hf elevates, the interlayer interaction, especially the SOC, gets enhanced, thereby modifying the magnetic or electric properties.

The chemical valences of Cr, Hf, and Te elements were investigated through X-ray photoelectron spectroscopy (XPS). Fig. 1d and e present the high-resolution XPS spectra for the Cr 2p, Te 3d and Hf 4f orbitals. The binding energies of the Cr 2p1/2 and Cr 2p3/2 peaks match well with the reference value for Cr3+ in Cr2O3 (Cr 2p1/2: 586.3 eV, Cr 2p3/2: 576.2 eV), confirming the +3 valence of Cr in the CrHfxTe2 systems.12 The Hf 4f spectra both exhibit two pairs of characteristic peaks (18.8 eV and 17 eV for CrHf0.1Te2, 18.6 eV and 16.9 eV for CrHf1/3Te2), which correspond to the 4f5/2 and 4f7/2 orbitals of Hf4+, respectively.13

The magnetic properties of the CrHfxTe2 crystals were examined through the vibrating sample magnetometer (VSM) module of a PPMS. Both measurement modes of zero-field-cooling (ZFC) and field-cooling (FC) were applied to obtain the temperature (T)-dependent magnetic susceptibilities (χ).

The χT curves demonstrate that the susceptibility values of both crystals are of 10−3 to 10−2 emu per (Oe·g) orders of magnitude under a 0.1 T magnetic field (Fig. 2a–d). For CrHf1/3Te2, the value of χ along the c-axis is approximately 8 times greater than that along the ab-plane. Based on the FC inverse susceptibility versus temperature (χ−1T) plots (Fig. S2, SI) and the Curie–Weiss law (χ = χ0 + C/(TθCW), where θCW is equivalent to the Curie temperature (TC) for ferromagnetic system), TC was determined for these tested samples (Fig. 2e): when exerting the in-plane magnetic field (H//ab-plane), TC of CrHf0.1Te2 and CrHf1/3Te2 attain 204.3 K and 217.4 K, respectively; under the out-of-plane field (H//c-axis), both CrHf0.1Te2 and CrHf1/3Te2 possess higher TC (234.9 K and 247.2 K, respectively). Different magnetization intensities and Curie temperatures along two distinct orientations demonstrate magnetic anisotropy in the CrHfxTe2 crystals. Notably, the magnetization separations between the ZFC and FC processes below ∼150 K are probably attributed to spin-glass-like irreversible thermomagnetic behavior under cryogenic conditions.14 Furthermore, an abnormal AFM-like kink occurs in the in-plane χT curves of CrHf1/3Te2 (Fig. 2c). This phenomenon stems from the crossover from the pinning to the depinning of the magnetic moments.15 In the ZFC process, the magnetic domains start to be pinned below the crossover temperature. As the applied field increases, the gained energy shifts the crossover temperature to a lower value.16


image file: d5tc02160j-f2.tif
Fig. 2 (a) and (b) Temperature-dependent magnetization of CrHf0.1Te2 under a magnetic field parallel to the ab-plane and c-axis, respectively. (c) and (d) Temperature-dependent magnetization of CrHf1/3Te2 under a magnetic field parallel to the ab-plane and c-axis, respectively. (e) Calculated Curie temperatures of CrHfxTe2 along two different orientations. (f)–(i) Magnetic hysteresis loops of CrHfxTe2 for the H//ab-plane and H//c-axis.

To further explore the field-dependent magnetization along the easy and hard orientations, we performed field-dependent magnetization curves from 3 K to 300 K, as shown in Fig. 2f–i. The hysteresis loops from −9 to 9 T at 3–200 K illustrate the typical ferromagnetic behavior in the CrHfxTe2 systems. In terms of CrHf0.1Te2 and CrHf1/3Te2, the easy magnetization axes are both parallel to the c-axis: magnetic saturation occurs at about 3 T and 1 T for CrHf0.1Te2 and CrHf1/3Te2 along the c-axis, which are smaller than 6 T and 8 T along the H//ab-plane direction. The measured saturated moments of CrHf0.1Te2 and CrHf1/3Te2 at 3 K along the c-axis reach 33.8 emu g−1 and 18.1 emu g−1, respectively. Furthermore, large coercivities for both directions, as shown in Table S7 (SI), indicate the retentive ferromagnetism in both CrHfxTe2 crystals. In comparison with other reported Cr-based ferromagnetic materials (e.g. CrBr3, CrI3, Cr2Ge2Te6, Cr2Te3 and Cr5Te8),17–24 CrHf0.1Te2 demonstrates superior Curie temperature and coercivity (691 Oe at 3 K) at the easy magnetization orientation (Table S8, SI). Such a novel ferromagnetic crystal exhibits an excellent anti-demagnetization capability.

The perpendicular magnetic anisotropy in CrHfxTe2 arises from the SOC effect, which exists in both Te and Hf atoms. The p orbitals of Te tend to hybridize with the d orbitals of Cr, forming a strongly anisotropic electronic structure. While Cr 3d electrons form a ferromagnetic order through the itinerant mechanism, combined effects from SOC and the crystal field lead to the minimization of the out-of-plane exchange energy. Although pure CrTe2 has an in-plane easy magnetization direction, the intercalation of Hf rotates the easy magnetization axis to the out-of-plane orientation, probably because such significant SOC from Hf atoms restricts the spin to the out-of-plane direction and minimizes the perpendicular magnetization energy.

The magnetocaloric effect in CrHfxTe2 systems is analyzed by the change of isothermal magnetic entropy (ΔSM(T, H)) which can be calculated from the isothermal field-dependent magnetization with a 4 K variation of temperature (Fig. 3a, b and Fig. S3a, b). ΔSM(T, H) is defined as25

 
image file: d5tc02160j-t1.tif(1)


image file: d5tc02160j-f3.tif
Fig. 3 (a) and (b) Isothermal field-dependent magnetization curves along the H//c-axis direction for CrHf0.1Te2 and CrHf1/3Te2, respectively. (c) and (d) Curves of −ΔSM with temperature under a magnetic field parallel to the c-axis. (e) and (f) Rotating magnetic entropy changes of CrHfxTe2 obtained by rotating from the c-axis to the ab-plane.

According to the Maxwell equation ∂S(T, H)/∂H = ∂M(T, H)/∂T, ΔSM(T, H) can be expressed as25

 
image file: d5tc02160j-t2.tif(2)

For isothermal magnetization examined at small temperature intervals, ΔSM(T, H) is approximately equivalent to25

 
image file: d5tc02160j-t3.tif(3)

Calculated −ΔSM as a function of temperature under 1–9 T magnetic fields are shown in Fig. 3c, d and Fig. S3c, d. It is clear that the value of −ΔSM at the same temperature elevates gradually with the external magnetic field increasing from 1 T to 9 T, and the maximum of −ΔSM (−ΔSmaxM) for the H//c-axis surpasses that for the H//ab-plane in CrHfxTe2 systems. For CrHf0.1Te2 and CrHf1/3Te2, −ΔSmaxM along the H//c-axis orientation reaches 2.85 J kg−1 K and 1.77 J kg−1 K, respectively. The relative cooling power (RCP), which determines the cooling efficiency, is expressed as RCP = ΔSmaxM × δTFWHMTFWHM refers to the full width at half-maximum of the −ΔSM(T, H) curve). Along the H//c-axis orientation, RCP is determined as 216.6 J kg−1 for CrHf0.1Te2 and 233.6 J kg−1 for CrHf1/3Te2. Although the magnetic entropy change of CrHfxTe2 is inferior to that of rare earth compounds (e.g. TbScO3, ErNi, EuCl2, Er1−xTmxGa, Zr-doped EuTiO3, etc.), the peaks of −ΔSM are found only near the liquid hydrogen temperature (∼20 K) for these compounds.4,26–29 Table S9 reveals that the ΔSM of CrHf0.1Te2 is comparable to that of reported vdW materials like VI3, Fe3GeTe2 and Cr1.15Te2, but the RCP value significantly exceeds that of multiple vdW ferromagnets.30–34 Notably, the peak of ΔSM is located near 185 K, surpassing that of multiple vdW ferromagnets and rare-earth compounds.

The rotational magnetic entropy change (ΔSRM) is determined by rotating the field from the c-axis to the ab-plane:

 
image file: d5tc02160j-t4.tif(4)

As depicted in Fig. 3e and f, all positive −ΔSRM further demonstrate the perpendicular magnetic anisotropy in CrHfxTe2. When applying a 9 T field, the peak of −ΔSRM attains 0.72 J kg−1 K for CrHf0.1Te2, and 0.9 J kg−1 K for CrHf1/3Te2. The higher concentration of intercalated Hf atoms probably strengthens the interlayer spin-orbital coupling, which tends to constrain the spin perpendicular to the ab-plane in CrHfxTe2, thereby enhancing the perpendicular magnetic anisotropy. As a result, the −ΔSRM peak value of CrHf1/3Te2 gets larger than that of CrHf0.1Te2 at a certain temperature and field.

To investigate the electrical transport characteristics, CrHfxTe2 flakes were connected into the measurement channels based on a standard four-probe configuration, ensuring that the current aligned parallel and the magnetic field maintained perpendicular to the ab-plane of each crystal sample. Fig. 4a and b show that the resistivity decreases monotonously while cooling from 300 K to 2 K, indicating a metallic behavior in both CrHfxTe2 crystals. As illustrated in Fig. S4 (SI), the resistivity at cryogenic temperatures can be fitted with the equation for Fermi liquids: ρ(T) = ρ0 + AT2. This result implies that the electron–electron scattering occupies a prominent position under cryogenic conditions.


image file: d5tc02160j-f4.tif
Fig. 4 (a) and (b) Temperature-dependent electrical resistivity (ρxxT) curves. (c) and (d) Field-dependent magnetoresistance (MR–μ0H) curves at 2 K and 10 K.

Magnetoresistance (MR) was evaluated at 2 K and 10 K, with the exerted magnetic field aligning vertically to the ab-plane. Based on the field-dependent resistivity, magnetoresistance was quantitatively determined using the following formula: MR = (RHR0)/R0 × 100%. Fig. 4c and d reveals intrinsic negative magnetoresistances (nMR) that enlarge monotonously with increasing field in the CrHfxTe2 systems. As the Hf concentration increases from 0.1 to 1/3, the resistivity of CrHfxTe2 at 2 K reduces from 1.4 × 10−3 to 1.5 × 10−4 Ω cm and simultaneously the nMR maximum rises from −3.8% to −4.7%. The larger nMR of CrHf1/3Te2 is possibly attributed to less electron scattering and enhanced carrier mobility. Such an nMR phenomenon was also discovered in ferromagnetic Cr1.2Te2, Cr2Ge2Te6, and anti-ferromagnetic FeNbTe2.19,35,36 For ferromagnetic crystals, the negative magnetoresistance probably arises from the ordering of the magnetic moments and spin-dependent electron scattering.35,37 With no external magnetic field, CrHfxTe2 has a large number of magnetic domains and disorganized magnetic moments. Electrons encounter localized magnetic moments along various directions during transport and experience strong spin scattering, leading to an increase in resistivity. When a sufficiently strong magnetic field is applied, magnetic moments are highly ordered and the spin scattering of electrons is significantly suppressed. Such a robust field also reduces the number of domain walls and the probability of electron scattering is minimized to a great extent. As a result, the nMR effect occurs in the whole field range. However, this characteristic is in contrast to CrTe2: The MR curves exhibit humps (positive MR under low magnetic field) at 2 K, which originates from the spin-reorientation.37 During the spin-reorientation process, the noncoplanar spin state strengthens the spin scattering of electrons, resulting in positive MR under low field conditions.35 To sum up, the intrinsic nMR performance in CrHfxTe2 makes it promising for magnetic sensors, electromagnetic protection and electronic information storage devices.

Conclusion

In summary, a novel intercalation crystal has been successfully synthesized, designated as CrHfxTe2. CrHf0.1Te2 is characterized with [CrTe6] octahedral coordination and a trigonal crystal structure. The hysteresis loops illustrate ferromagnetic behaviors in CrHfxTe2, and the easy magnetization direction remains parallel to the c-axis. CrHf0.1Te2 displays higher Curie temperature and coercivity than multiple Cr-based vdW ferromagnetic materials like CrI3, Cr2Ge2Te6 and Cr5Te8. In addition, the maximum magnetic entropy variation of CrHf0.1Te2 attains 2.85 J kg−1 K at about 185 K, and the RCP value (216.6 J kg−1) exceeds that of various vdW ferromagnets. The intrinsic negative magnetoresistance in both CrHfxTe2 crystals makes them suitable for magnetic sensors. This work provides a promising approach for synthesizing new intercalation compounds with unique properties for advanced electronic and magnetic functions.

Experimental section

Synthesis of CrHfxTe2 crystals

Both CrHf0.1Te2 and CrHf1/3Te2 were synthesized via the flux method, using CsI as the flux agent. Firstly, Cr (Adamas-beta, 99.9%), Hf (Adamas-beta, 99.9%), and Te (Adamas-beta, 99.9%) powders were uniformly mixed in a molar ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)]x[thin space (1/6-em)]:[thin space (1/6-em)]2 with anhydrous CsI (Innochem, 99.9%). All mixed raw materials and flux were then sealed in evacuated silica tubes by oxyhydrogen flame. Sealed silica tubes were placed in a muffle furnace, which was heated up to 1473 K at a rate of 2 K min−1, equilibrated for two days, and then slowly cooled for three days to 1173 K. The production in silica tubes was finally washed with deionized water to remove the remnants of CsI flux.

Structure characterizations and analyses

Powder X-ray diffraction (PXRD) patterns were obtained through a Bruker D8QUEST diffractometer (Cu Kα radiation). Single-crystal X-ray diffraction (SC-XRD) data were collected on a Bruker D8QUEST diffractometer (Mo Kα radiation), and the crystal structure refinement was analyzed through the APEX3 program. The elemental composition was determined by energy-dispersive X-ray spectroscopy (EDS) coupled with a JEOL (JSM6510) scanning electron microscope (SEM). Chemical valences were analyzed through X-ray photoelectron spectroscopy (XPS). Raman spectra were obtained through a Jobin-Yvon LabRAM HR-800 spectrometer with a laser of 532 nm excitation wavelength.

Electrical transport and magnetic susceptibility measurements

Electrical resistivity and magnetoresistance were tested in a physical property measurement system (PPMS, Quantum Design). Temperature-dependent and field-dependent magnetization characteristics were examined in the VSM module of the PPMS, with external magnetic fields parallel to the ab-plane or the c-axis of each crystal flake.

Author contributions

Kunqi Li: writing, data assessment, investigation, formal analysis. Xueyang Tu & Xuzhou Sun: software, formal analysis. Hui Bi: writing – review & editing. Yuqiang Fang: writing – review & editing, supervision. Fuqiang Huang: conceptualization, funding acquisition, writing – review & editing, supervision.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this work are available within the article and the SI. See DOI: https://doi.org/10.1039/d5tc02160j

Acknowledgements

This work was financially supported by the Shanghai Rising-Star Program (23QA1410700), Science and Technology Commission of Shanghai Municipality (Grant no. 24LZ1401000), National Natural Science Foundation of China (Grants no. 12104307, no. 12174062, and no. 12241402), Innovation Program for Quantum Science and Technology (2024ZD0300102) and the Program of Shanghai Academic Research Leader (22XD1424300).

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