Novel perovskite–spinel composite approach to enhance the magnetization of LaFeO₃

Abstract

In the present work, the perovkite–spinel interface effect on the bulk magnetic behavior of lanthanum ferrite (LaFeO₃) based composite systems is under investigation in view of the enhancement of the magnetization of LaFeO₃. By using LaFeO₃ as the perovskite phase and NiFe₂O₄ as the spinel phase with the compositions x = 0, 20, 30, 40 and 100 wt%, a composite system is developed by mechanical mixing. The structures are confirmed using X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). Physical interaction at the interface of the LaFeO₃ and NiFe₂O₄ phases is realized by XRD peak broadening and shifting. The M–T and M–H curves are closely monitored to investigate the perovskite–spinel interface effect on the bulk magnetic behavior of the composites. Significant enhancement in the magnetization of the perovskite–spinel composite phase with a 60% LaFeO₃–40% NiFe₂O₄ composition over individual phases is detected. The composition effect up to 60 : 40 of LaFeO₃–NiFe₂O₄ is considered to preserve the dominance of the LaFeO₃ phase. The spin coupling mechanism across the interface is speculated for the enhancement of the magnetization in the composite. Mössbauer spectroscopic investigation confirms the co-existence of magnetization in the composites.

---

Introduction

Current trends in materials research intensively focus on the development of complex oxide composites to create new functional materials and improve the performance of existing ones. Composite interfaces play a key role in modulating effective material properties and thus improving functionality.¹ The role of the interface is primarily important because enhancement in the properties of composites results from the interaction of component phases at the interface.² Several phenomena occur at the interface i.e. rearrangement of chemical bonding;³ spin, charge and orbital reconstruction;⁴ modifications to the electronic structure;⁵ etc.

The combination of a perovskite and a spinel seems to be very promising for making perovskite–spinel systems with multifunctional or multiferroic properties.⁶ Elastic coupling between them plays a major role in the observation of magnetoelectric coupling.⁷ The perovskite–spinel interface is also of great interest in complex oxide based magnetic tunnel junctions.² Several combinations have been extensively studied by taking BiFeO₃ and BaTiO₃ as the perovskites and exploiting their magnetic and magnetoelectric responses.^8–15 Using LaFeO₃ (LFO) as the perovskite, significant magnetization was observed in its superlattices.^16,17 But, the combination of the perovskite LFO with a spinel system is very rare. In this paper, we have made an attempt to study the enhancement in magnetization when the perovskite LFO combines with a spinel phase. LFO is a member of the rare earth orthoferrite family. It has an orthorhombic perovskite structure (space group: Pbnm) and exhibits a phase transition from orthorhombic to rhombohedral at T ∼ 1260 K.¹⁸ LFO possesses G-type antiferromagnetism (T_N ∼ 738 K) in which inter and intralayer spin couplings are antiparallel. It also shows an Fe³⁺–O–Fe³⁺ superexchange interaction. Fe³⁺ is in its high spin state (t_2g³, e_g²).^19,20 LFO has various promising applications in solid oxide fuel cells, catalysts, chemical sensors, etc.²¹ LFO can be exploited as a room temperature multiferroic as an alternative to BiFeO₃, but its functionality is limited by low magnetization.

Bulk NiFe₂O₄ (NFO) is a well known soft ferrimagnetic insulator with an inverse spinel structure. Its ferrimagnetic ordering temperature is around T_c ∼ 850 K.^22–24 Within an inverse spinel structure, Fe³⁺ ions occupy the tetrahedral sites in sublattice A. Ni²⁺ and Fe³⁺ ions are located at the octahedral sites in sublattice B. It means, Fe³⁺ ions are equally distributed across tetrahedral as well as octahedral sites inside the inverse spinel structure. The magnetic structure of NFO consists of antiferromagnetic coupling between the two sublattices A and B. With this ordering, the moment of the Fe³⁺ ion at the A and B sites cancels out leaving a 2 μ_B per f.u. magnetic moment which only arises due to Ni²⁺.^22–25 Magnetic interaction (Fe–Fe superexchange interaction) plays an important role in determining its utility in disk recording or in the fabrication of magnetic cores of read/write heads for high speed digital tape.^26,27 This background motivates us to develop LFO–NFO composite systems to investigate the interface effect on magnetic behavior. The main purpose of this study is the enhancement of the magnetization of the LFO system, so the composite effect in compositions up to 60% LFO–40% NFO is studied so that the dominance of the LFO phase could be preserved.

Experimental

Polycrystalline LFO and NFO compounds were synthesized separately by gel combustion and hydrothermal routes, respectively. Both of the systems were mechanically ground to form (1 − x)LaFeO₃–xNiFe₂O₄ (x = 20, 30, & 40%) powder composites. Lanthanum nitrate [La(NO₃)₃·6H₂O, 99.9% pure, Sigma-Aldrich], iron nitrate [Fe(NO₃)₃·9H₂O, 99.9% pure, Sigma-Aldrich], nickel nitrate [Ni(NO₃)₂·6H₂O, 99% pure, Sigma-Aldrich], sodium hydroxide flakes [NaOH, 90% Merck], and glycine were used as precursors. LFO was synthesized by a microwave-assisted sol gel combustion route. A Ragatech microwave synthesis assembly was used to synthesize LFO. It consists of a microwave processor with the dimensions 360 mm × 210 mm × 430 mm, and a 2.45 GHz frequency multimode source with a maximum deliverable power output of 700 W. Lanthanum nitrate and iron nitrate in stoichiometric (1 : 1) proportions were dissolved in double distilled water and stirred for 15 minutes. Then, 4 moles of glycine were added into the prepared solution. The resultant solution was evaporated using the microwave synthesis assembly at 210 W power until gelification occurred. The resultant gel was combusted at the same power level. After combustion, a yellowish powder was formed within a few seconds. The as-synthesized powder was calcined at 600 °C for 3 hours (optimized condition) to obtain the final product as polycrystalline LFO.

NFO was synthesized by a hydrothermal synthesis route. Stoichiometric amounts of nickel nitrate and iron nitrate were dissolved in 100 ml of double distilled water. This solution was stirred for 30 minutes. Then, the pH of the solution was adjusted to 13 by dropwise addition of 2 M NaOH solution into it. After adjusting the pH, the solution was transferred into a sealed Teflon stainless steel autoclave and then placed in a hot air oven at 160 °C for 18 h. After the hydrothermal reaction, the autoclave was allowed to cool naturally. The resultant precipitate (insoluble in water) was separated by centrifugation and washed with distilled water in sequence to remove water soluble salts. Then, the precipitate was dried at 80 °C in an oven to get the final polycrystalline NFO powder. Both of these compounds were mechanically ground in appropriate proportions to form (1 − x)LFO–xNFO (x = 10, 20, 30, & 40 wt%) powder composites.

X-ray powder diffraction measurements of the composites were performed using a PANalytical Empyrean X-ray diffractometer equipped with a copper target (CuKα ∼ 1.5406 Å). Diffraction data was collected in the range of 20–80° with a step size of 0.01973° for a step time of 46.5 ms. The obtained diffraction data was refined using the Rietveld refinement method with the help of the Full Prof suite. Fourier Transform Infrared Spectroscopy was carried out using a Bruker spectrometer (Germany, Model: Vertex70) with a resolution of ∼0.5 cm⁻¹ and a wave no. accuracy of ∼ 0.01 cm⁻¹. FTIR measurements were done within the far IR range, from 50 to 680 cm⁻¹, at room temperature. Field and temperature dependent magnetization measurements were performed using a Quantum Design MPMS Superconducting Quantum Interference Device (SQUID), Vibrating Sample Magnetometer [specification: magnetic field up to 70 kOe; temperature range: 350–1000 K; field charging rate: 4 to 700 Oe s⁻¹]. ⁵⁷Fe Mössbauer spectra of LFO, NFO and their composites were recorded using a Mössbauer spectrometer operated in constant acceleration mode in transmission geometry at RT. A ⁵⁷Co source was employed which further decayed into ⁵⁷Fe. The calibration of the velocity scale was done using enriched ⁵⁷Fe metal foil.

Results and discussion

A. Structural confirmation

X-ray diffraction patterns for pure LFO, NFO and LFO–NFO composites are shown in Fig. 1. The XRD pattern of pure LFO is indexed as an orthorhombic phase (JCPDS file no. 74-2203) while that of NFO is matched to a cubic spinel phase (JCPDS file no.89-4927) without any traces of impurities. LFO and NFO each retain their respective structure and phase in LFO–NFO composite form. This is as expected because both of the phases were synthesized separately. However, broadening and shifting of the LFO peaks [see Fig. 1b] with increasing NFO concentration are evidence for physical interaction at the interface of LFO–NFO. In addition to that, the peak corresponding to the spinel phase along the (311) direction that appears stronger with increasing concentration of NFO in the composites [see Fig. 1c] is a clear indication of the formation of a perovskite–spinel mixed structure.^5,8 Rietveld refinement of the X-ray diffraction patterns reveals an orthorhombic (space group: Pbnm) symmetry for LFO and an inverse spinel cubic (space group: Fd3m) symmetry for NFO [see Fig. 2]. The refined lattice parameters a = 5.5581(7) Å, b = 5.5722(8) Å and c = 7.8601(11) Å for LFO and a = b = c = 8.349(6) Å for NFO are comparable with literature.²⁸

Fig. 1 (a) Combined X-ray diffraction patterns of LFO, NFO and LFO–NFO composites, (b) broadening of the LFO peaks with increasing NFO concentration, and (c) the appearance of a ferrite phase with increasing NFO percentage in the composites.

Fig. 2 Rietveld refinement of XRD data of (a) LFO and (b) NFO.

The local structure of the composite systems was probed using an FTIR technique. FTIR spectra of the pure LFO, NFO and LFO–NFO composites taken between 50–680 cm⁻¹ are shown in Fig. 3. According to symmetry calculations, the orthorhombic Pbnm phase (for LFO) should possess 25 IR active optical phonon modes [9B_1u + 7B_2u + 9B_3u] in the range 115–645 cm⁻¹.^28,30 In the present study, from the FTIR spectra of LFO (Fig. 3a), 8 broad vibrational modes are identified. Modes below 200 cm⁻¹ (ν₁ & ν₂) can be assigned to La³⁺ vibrations. The strong absorption peak at 170 cm⁻¹ (ν₂) can be assigned to an ‘external’ phonon mode, which arises due to vibration of La³⁺ ions against FeO₆ octahedra.³¹ The absorption bands between 200–300 cm⁻¹ (ν₃ & ν₄) can be referred to as oxygen octahedral tilting modes.³² The broad absorption peak (ν₅) near 350 cm⁻¹ is correlated with Fe³⁺–O²⁻ bending vibrations.³³ The transverse optic (TO) mode B_1u (ν₆) observed at 441 cm⁻¹ is associated with the O–Fe–O deformation vibration of the perovskite LFO and the band (TO, B_3u) at 540 cm⁻¹ (ν₇) is related to iron–oxygen (Fe–O) stretching vibrations.^30,32,33 A small shoulder (ν₈) appears at 600 cm⁻¹ that is a characteristic feature of rare earth orthoferrites.³³

Fig. 3 FTIR spectra of LFO, NFO and LFO–NFO composites.

Fig. 3b exhibits FTIR spectra of NFO. In spinel structures, lower energy bands can be assigned to the intrinsic stretching vibrations of metal–oxygen bonds at tetrahedral sites whereas higher energy bands can be correlated to metal–oxygen bonds at octahedral sites.³⁴ For NFO, a characteristic absorption peak at 603 cm⁻¹ (ν^′₆) can be associated with the stretching vibrations of the (Fe–O)_tetra bond which is the main feature of a spinel ferrite. A band in the form of a shoulder at 550 cm⁻¹ (ν^′₅) is also a sign of the stretching vibrations of a tetrahedrally coordinated Fe³⁺–O²⁻ bond. A broad band observed near 400 cm⁻¹ (ν^′₄) is related to the stretching vibrations of Ni–O bonds at octahedral sites.^34–39 Weak absorption bands observed at 110, 191 and 270 cm⁻¹ arise due to the vibration of Fe³⁺ cations at octahedral sites.^34,40 The higher energy modes of the spinel structure (NFO) and the lower energy modes of the perovskite (LFO) both appear in the FTIR spectra of the composites (Fig. 3c and d). No new vibrational modes are detected due to the interface effect. This means that there is no structural modulation across the interface after mixing individual phases of LFO and NFO. LFO and NFO retain their identity in the prepared composites.

B. Magnetic field and temperature dependent magnetization

Magnetic field dependence magnetizations (M–H curve) of LFO, NFO and LFO–NFO composites at room temperature (R.T. ∼ 300 K) are shown in Fig. 4a and b. A magnified view of the M–H curve for LFO [Fig. 4b] shows a weak ferromagnetic nature with very little remanent magnetization (M_r ∼ 0.0005 emu g⁻¹). Actually, bulk LFO possesses G-type AFM with canted Fe³⁺ spins.^41,42 The weak ferromagnetism is due to partial alignment of the canted Fe³⁺ spins. The same magnetic behaviour for LFO was previously observed in ref. 43. Magnetic hysteresis of NFO shows a large saturation magnetization (M_s ∼ 48.80 emu g⁻¹). The nature of the magnetic ordering may be either ferrimagnetic or ferromagnetic. It is very difficult to distinguish between ferro and ferrimagnetic from SQUID-VSM measurements. This difficulty has been sorted out by measuring the Mössbauer spectra of NFO. This will be discussed later on. For composite systems, the remanent (M_r) and saturation (M_s) magnetizations are higher than those measured for the LFO-phase and continuously increase with the increasing weight percentage of NFO. The M_r, M_s and squareness ratio (R) values for all of the systems are tabulated in Table 1. The squareness ratio (R) for LFO and the composites is calculated by dividing M_r by M_s. The significance of R is to determine the type of the intergrain exchange.⁴² Non zero values of R for LFO–NFO composites [see Fig. 4c] experimentally prove that there are intergrain magnetostatic interactions across the LFO and NFO interface. On the other hand, the coercivity (H_c) of the composite systems is found to be lower than that of pure LFO indicating the soft magnetic behavior of the composites as compared to LFO.

Fig. 4 (a) Field dependence magnetisation of LFO and LFO–NFO composites at RT, (b) a magnified view of the M–H curves of LFO and (c) the 60% LFO–40% LFO composite.

Table 1 Table showing the various magnetic parameters of LFO, NFO and LFO–NFO

	Ms (emu g^−1)	Mr (emu g^−1)	Hc (Oe)	R = Mr/Ms
LFO	0.0012	0.0005	250	0.416
80% LFO–20% NFO	5.86	0.37	20	0.063
70% LFO–30% NFO	15.58	0.57	10	0.036
60% LFO–40% NFO	23.15	4.49	60	0.193
NFO	48.80	7.65	101	0.157

---

The role of the interface magnetization in the M_r and M_s values of the composites can also be confirmed by estimating their values, using conventional Vegard's law approximation,⁵ as M_cal = M_LFO(1 − x) + M_NFO(x), where M_cal, is the estimated remanent magnetization of the composite, while M_LFO and M_NFO are the observed remanent magnetizations of the LFO and NFO single phases, respectively. The observed and calculated comparative plots of remanent magnetization with the wt% composition of NFO are shown in the inset of Fig. 4a. Surprisingly, the observed remanent magnetization values for the initial two compositions (20% NFO and 30% NFO) are found to be lower than calculated ones. However for the 40% NFO composition, the observed remanent magnetization (M_r ∼ 4.49 emu g⁻¹) is found to be higher than the calculated one (M_r ∼ 3.06 emu g⁻¹). Almost the same trend is also observed for the saturation magnetization. The magnetic coupled interactions at the interface of the LFO–NFO composite are exclusively responsible for the enhancement in this composition. This confirms the modulation of spin-alignment at the interface for the LFO–NFO composites as compared to their individual phases.

To get a clear understanding of the modulation of the spin-coupling at the interface; the mechanism of the spin alignments across the interface are speculated as shown in Fig. 5. Individually, LFO has a weak magnetization due to the canting of Fe³⁺ spin magnetic moments. NFO has an inverse spinel structure with ferrimagnetic ordering. Spin magnetic moments of Fe³⁺ positioned at octahedral lattice sites are anti-parallel with Fe³⁺ at tetrahedral lattice sites. Therefore, the net contribution of the magnetization due to Fe³⁺ spins is canceled out (as shown in Fig. 5). The saturation magnetization of ferrimagnetic NFO is only due to Ni²⁺ spins^22–25 which are aligned in parallel at octahedral lattice sites. At the bulk interface of LFO–NFO composites, spin reconstruction of LFO and NFO domains takes place. The spins of Fe³⁺ from LFO and the spins of Ni²⁺ from NFO may attribute to the enhancement at the bulk interface. The estimated magnetic contributions of LFO and NFO grains in the composite by Vegard's law are different than the observed ones. This is clear evidence of spin reconstruction at the interface. The mechanism is visualized as (Fig. 5) an uncompensated spin alignment at the interface, which gives rise to the enhancement, while compensation leads to a decrement, which is tuned by the LFO–NFO interface ratio and respective grain distribution across the interface.

Fig. 5 Proposed interface mechanism of spin alignment in LFO–NFO composites.

The temperature profile of the magnetization (M–T curve) of LFO, NFO and LFO–NFO composites are studied for fields H ∼ 500 and 1000 Oe, respectively. Fig. 6 shows the M–T curves at a magnetic field of 500 Oe. The magnified view of the M–T curve of LFO [see Fig. 6b] clearly exhibits that magnetization decreases with temperature. A rapid drop in the magnetization is observed near the anti-ferromagnetic transition temperature (T_N ∼ 744 K). This behavior of the M–T curve shows the presence of the weak ferromagnetic component, which is associated with Fe³⁺ AFM ordering.⁴⁴ The composite effect on the transition temperature of LFO is predicted from the derivative plot of the magnetization with respect to temperature [see Fig. 6c]. The derivative plot of the magnetization of LFO [inset Fig. 6b] clearly shows that the T_N of LFO is at ∼744 K. However, the T_N is found to shift to a lower temperature at about 600 K for the 80% LFO–20% NFO composite. For the 70% LFO–30% NFO and 60% LFO–40% NFO composites, the T_N values are found at around 617 and 667, respectively [see Fig. 6c]. The decrease in the T_N of the LFO–NFO composites can be assigned to (i) a lattice parameter mismatch, which induces mechanical strain at the interface between the LFO and NFO component phases as reported elsewhere for composite systems⁴⁵ and (ii) the superexchange interaction between AFM coupled Fe³⁺ ions (Fe³⁺–O²⁻–Fe³⁺) in LFO that are weakened due to the NFO phase. However, with increasing NFO concentration in the composite, a higher T_N shift may be due to the higher magnetic transition temperature of NFO, which is around ∼850 K. Hence, with an incremental increase in the NFO concentration, the transition temperature increases but not higher than the T_N of LFO because the LFO phase is still dominant in the composite samples [Fig. 6c].

Fig. 6 (a) Temperature dependence magnetization (M–T) of LFO and LFO–NFO composites at field 500 Oe. (b) Enlarged view of M–T curve for LFO at field 500 Oe with inset showing derivative plot for the evaluation of Neel temperature. (c) dM/dT vs. temperature plot for exact finding of transition temperature. (d) M–T curve for NFO at field 500 Oe with inset showing derivative plot for the evaluation of Curie temperature.

In addition to that, a broadening of the peaks corresponding to the Neel temperature (T_N) is observed with the inclusion of NFO into LFO. Broadening at T_N has been previously observed in the case of composite and doping effects and was found to be very sensitive to composition as well as to chemical ordering.^46–49 The broadening observed in the current work can be assigned to inhomogeneity arising in the LFO matrix due to the presence of NFO. With respect to the NFO composition, the inhomogeneity increases and a much broader peak is detected for the 60% LFO–40% LFO composition. The M–T curve [Fig. 6d] for NFO is nearly same as that for LFO. The magnetization is found to reduce with increasing temperature and approaches a lower value at the Curie temperature (T_c). This is a feature of ferromagnetic or ferrimagnetic materials. A derivative plot of the magnetization for NFO [inset of Fig. 6d] gives the Curie temperature ∼882 K i.e. the transition temperature from a ferrimagnet to a paramagnet. The same behavior is observed in the temperature dependent magnetization of LFO, NFO and LFO–NFO composites at H = 1000 Oe also [see Fig. S1 of ESI†].²⁹

C. Mössbauer spectroscopy

To probe the local magnetic behavior of the composites, room temperature Mössbauer spectra were studied. Raw Mössbauer experimental data are fitted using the NORMOS-SITE DOS based program. Fig. 7 shows the ⁵⁷Fe Mössbauer spectrum of LFO. The raw data are fitted with one sextet (which is the feature of AFM ordering in LFO) along with a singlet.⁵⁰ Various hyperfine parameters are obtained, with respect to natural iron, from the fitted spectra. The isomer shift (δ) provides direct information about the electron density at the nucleus. δ is found to be ∼0.2405 which indicates that Fe is in the +3 oxidation state.⁵¹ Quadrupole splitting (Δ) reflects the interaction between the nuclear quadrupole and the surrounding electric field. The hyperfine magnetic field (B_hf) is the effective magnetic field which gives information about the interaction between the nucleus and the surrounding magnetic field. Obtained hyperfine parameters corresponding to a sextet and a singlet for LFO, are as shown in Table 2. All of these values are characteristic of the presence of octahedrally coordinated high spin Fe³⁺.^32,52,53 Along with a highly intense sextet, a weak singlet is also detected. The relative area of that singlet is less than 1%. In literature, such an isomer shift (0.049 mm s⁻¹) observed for a singlet has been correlated to the superparamagnetic state of Fe by Saverio Braccini et al. in 2013.⁵⁴ In our work, the isomer shift of this single line is found to correspond exactly to the superparamagnetic state of Fe. Therefore the singlet can be assigned to the superparamagnetic state of Fe. As the line is much less intense, it means that very little Fe (less than 1%) is in a superparamagnetic state. The state of Fe is not very prominent in our case. It is well known that LFO is superparamagnetic at room temperature (T. Fujii et al., 2011).⁵⁵ Fig. 7b shows the Mössbauer spectrum of spinel NFO. This spectrum should have been fitted as ferrimagnetic with two sextets (one for tetrahedral Fe and the other for octahedral), but due to broadening, only one ferrimagnetic sextet is fitted using our raw data. Various hyperfine parameters corresponding to NFO are tabulated in Table 2 and are found to be close to the values reported elsewhere.^56–58 The Mössbauer spectrum of 60% LFO–40% NFO is fitted with two sextets, one corresponding to LFO and the other for NFO. The relative areas for LFO and NFO phases (from Table 2) are found to be 59.61 and 40.39%, respectively. This confirms the exact phase proportion in our prepared composite. Hyperfine parameters for both of the sextets are shown in Table 2.

Fig. 7 Mössbauer spectra of (a) LFO (b) NFO and (c) the 60% LFO–40% NFO composite.

Table 2 Hyperfine parameters obtained from the Mössbauer spectra of LFO, NFO and LFO–NFO composites

Sample		Isomer shift (mm s^−1)	Quadrupole splitting (mm s^−1)	Hyperfine field (T)	Area (%)
LaFeO3 (LFO)	Sextet	0.2405 ± 0.0029	0.1039 ± 0.0056	52.289 ± 0.019	99.04
	Singlet	0.0492 ± 0.0051	—	—	0.96
80% LaFeO3–20% NiFe2O4	Sextet I (LFO)	0.2168 ± 0.0122	0.1077 ± 0.0273	52.373 ± 0.101	84.87
	Sextet II (NFO)	0.0624 ± 0.0101	0.1554 ± 0.1027	47.428 ± 0.312	15.13
70% LaFeO3–30% NiFe2O4	Sextet I (LFO)	0.2488 ± 0.0054	0.0732 ± 0.0167	52.124 ± 0.071	76.12
	Sextet II (NFO)	0.1562 ± 0.0460	0.1742 ± 0.0746	49.310 ± 0.434	23.88
60% LaFeO3–40% NiFe2O4	Sextet I (LFO)	0.2379 ± 0.0073	0.1085 ± 0.0138	52.140 ± 0.054	59.61
	Sextet II (NFO)	0.1852 ± 0.0261	0.0032 ± 0.0051	48.839 ± 0.257	40.39
NiFe2O4 (NFO)	Sextet	0.1868 ± 0.0179	0.0032 ± 0.0041	49.904 ± 0.112	100

---

Conclusions

In summary, we have developed perovskite oxide (LaFeO₃) and spinel (NiFe₂O₄) composites to study the interface effect on the bulk magnetic behavior in view of the enhancement of the magnetization of LFO. The formation of the composites is confirmed using X-ray diffraction and from FTIR studies. The perovskite–spinel interface effect on the magnetic behaviour of the LFO–NFO bulk composites is confirmed from M–H and M–T behaviours. A noticeable enhancement in the magnetization of the LFO dominant composite system is observed. A spin-coupling mechanism at the interface of the perovskite–spinel composite is proposed to understand the effect. A shift in the magnetic transition temperature upon increasing the amount of NFO in the LFO is explained on the basis of a lattice parameter mismatch and a weakening of the superexchange interaction in LFO by NFO. Local magnetic behavior and phase proportions are confirmed using Mössbauer spectroscopy. This work demonstrates that the perovskite–spinel interface approach has the potential to enhance the magnetization of LFO.

Acknowledgements

We acknowledge UGC-DAE Consortium for Scientific Research, Indore (India), for providing magnetic, Mössbauer and FTIR characterization facilities and also for financial support through travel assistance and local hospitality. We are grateful to Dr R. J. Choudhary and Mr Pankaj Pandey for their help with the magnetic measurements. We would like to thank Mr U. P. Deshpande, Scientist, UGC-DAE CSR, Indore for his help in recording FTIR spectra. We express our sincere thanks to Dr V. R. Reddy and Mr Anil for their help in recording Mössbauer spectra and their analysis. One of the authors VMG wants to acknowledge UGC, New Delhi for providing financial assistance through RGNF fellowship.

References

1. J. P. Zhou, L. Lv, Q. Liu, Y. X. Zhang and P. Liu, Sci. Technol. Adv. Mater., 2012, 13, 045001.
2. Q. Zhan, R. Yu, S. P. Crane, H. Zheng, C. Kisielowski and R. Ramesh, Appl. Phys. Lett., 2006, 89, 172902.
3. A. P. Sutton and R. W. Ballufi, Interfaces in Crystalline Materials, Clarendon, Oxford, 1995, p. 240.
4. R. Ramesh, Curr. Sci., 2013, 105, 1107–1114.
5. S. Pillai, D. Bhuwal, A. Banerjee and V. Shelke, Appl. Phys. Lett., 2013, 102, 072907.
6. J. Hoffmann, et al., J. Mater. Res., 2012, 27, 11.
7. R. Muralidharan, N. Dix, V. Skumryev, M. Varela, F. Sánchez and J. Fontcuberta, J. Appl. Phys., 2008, 103, 07E301.
8. X.-M. Liu, S.-Y. Fu and C.-J. Huang, Mater. Sci. Eng., B, 2005, 121, 255–260.
9. C. H. Yang, F. Yildiz, S. H. Lee, Y. H. Jeong, U. Chon and T. Y. Koo, Appl. Phys. Lett., 2007, 90, 163116.
10. R. Y. Zheng, J. Wang and S. Ramakrishna, J. Appl. Phys., 2008, 104, 034106.
11. S. P. Crane, C. Bihler, M. S. Brandt, S. T. B. Goennenwein, M. Gajek and R. Ramesh, J. Magn. Magn. Mater., 2009, 321, L5–L9.
12. K. Sone, S. Sekiguchi, H. Naganuma, T. Miyazaki, T. Nakajima and S. Okamura, J. Appl. Phys., 2012, 111, 124101.
13. C. M. Kanamadi, J. S. Kim, H. K. Yang, B. K. Moon, B. C. Choi and J. H. Jeong, Appl. Phys. A: Mater. Sci. Process., 2009, 97, 575–580.
14. Y. Zhang, C. Deng, J. Ma, Y. Lin and C.-W. Nan, Appl. Phys. Lett., 2008, 92, 062911.
15. R. Liu, Y. Zhao, R. Huang, Y. Zhao and H. Zhou, J. Mater. Chem., 2010, 20, 10665–10670.
16. K. Ueda, H. Tabata and T. Kawai, Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 60, R12561.
17. Y. B. Chen, J. Zhou, S.-T. Zhang, F.-X. Wu, S.-H. Yao, Z.-B. Gu, D. Wu and Y.-F. Chen, Appl. Phys. Lett., 2013, 102, 042403.
18. R. Koferstein and S. G. Ebbinghaus, Solid State Ionics, 2013, 231, 43–48.
19. D. Wang and M. Gong, J. Appl. Phys., 2011, 109, 114304.
20. M. Idrees, M. Nadeem, M. Mehmood, M. Atif, K. H. Chae and M. M. Hassan, J. Phys. D: Appl. Phys., 2011, 44, 105401.
21. D. Wang, X. Chu and M. Gong, Nanotechnology, 2006, 17, 5501–5505.
22. U. Luders, A. Barthelemy and M. Bibes, et al., Adv. Mater., 2006, 18, 1733–1736.
23. S. E. Shirsath, S. S. Jadhav, B. G. Toksha, S. M. Patange and K. M. Jadhav, J. Appl. Phys., 2011, 110, 013914.
24. M. Younas, M. Nadeem, M. Atif and R. Grossinger, J. Appl. Phys., 2011, 109, 093704.
25. S. M. Patange, S. E. Shirsath, G. S. Jangam, K. S. Lohar, S. S. Jadhav and K. M. Jadhav, J. Appl. Phys., 2011, 109, 053909.
26. K. V. P. M. Shafi, Y. Koltypin, A. Gedanken, R. Prozorov, J. Balogh, J. Lendvai and J. Felner, J. Phys. Chem. B, 1997, 101, 6409–6414.
27. Y. Shi, J. Ding, Z. X. Shen, W. X. Sun and L. Wang, Solid State Commun., 2000, 115, 237–241.
28. I. S. Smirnova, A. V. Bazhenov, T. N. Fursova, A. F. Dubovitskii, L. S. Uspenskaya and M. Y. Maksimuk, Phys. B, 2008, 403, 3896–3902.
29. ESI, Fig. S1† Comparative temperature dependence magnetization at field 1000 Oe for LFO–NFO composites.
30. M. Romero, R. W. Gomez, V. Morquina, J. L. Perez-Mazariego and R. Escamilla, Phys. B, 2014, 443, 90–94.
31. R. A. Lewis, A. D. Martin, X. L. Wang and S. X. Dou, Aust. J. Phys., 1999, 52, 197–203.
32. M. Sivakumar, A. Gedanken, W. Zhong, Y. H. Jiang, Y. W. Du, I. Brukental, D. Bhattacharya, Y. Yeshurun and I. Nowik, J. Mater. Chem., 2004, 14, 764–769.
33. M. A. Ahmed, R. Seoudi and S. I. El-dek, J. Mol. Struct., 2005, 754, 41–44.
34. B. Senthilkumar, R. K. Selvan, P. Vinothbabu, I. Perelshtein and A. Gedanken, Mater. Chem. Phys., 2011 DOI:10.1016/j.matchemphys.2011.06.043.
35. Z. H. Zhou, J. M. Xue, J. Wang, H. S. O. Chan, T. Yu and Z. X. Shen, J. Appl. Phys., 2002, 91, 6015.
36. N. Kasapoglu, A. Baykal, M. S. Toprak, Y. Koseoglu and H. Bayrakdar, Turk. J. Chem., 2007, 31, 659–666.
37. S. Rana, A. Gallo, R. S. Srivastava and R. D. K. Misra, Acta Biomater., 2007, 3, 233–242.
38. X. Li, G. Tan, W. Chen, B. Zhou, D. Xue, Y. Peng, F. Li and N. J. Mellors, J. Nanopart. Res., 2012, 14, 751.
39. S. Singh, M. Singh, N. K. Ralhan, R. K. Kotnala and K. C. Verma, Adv. Mater. Lett., 2012, 3(6), 504–506.
40. H. M. Zaki and S. F. Mansour, J. Phys. Chem. Solids, 2006, 67, 1643–1648.
41. M. A. Ahmed and S. I. El-Dek, Mater. Sci. Eng., B, 2006, 128, 30–33.
42. M. A. Ahemed, N. Okasha and B. Hussein, J. Magn. Magn. Mater., 2012, 324, 2349–2354.
43. M. B. Bellakki, V. Manivannan, P. McCurdy and S. Kohli, J. Rare Earths, 2009, 27, 691.
44. R. Rajagukguk, D. G. Shin and B. W. Lee, Journal of Magnetics, 2011, 16, 101–103.
45. M. Murakami, K.-S. Chang, M. A. Aronova, C.-L. Lin, M. H. Yu, J. H. Simpers, M. Wuttig, I. Takeuchi, C. Gao, B. Hu, S. E. Lofland, L. A. Knauss and L. A. Bendersky, Appl. Phys. Lett., 2005, 87, 112901.
46. A. Sharoni, O. Millo, G. Leitus and S. Reich, J. Phys.: Condens. Matter, 2001, 13, L503.
47. R. Mazumder, S. Ghosh, P. Mondal, D. Bhattacharya, S. Dasgupta, N. Das, A. Sen, A. K. Tyagi, M. Sivakumar, T. Takami and H. Ikuta, J. Appl. Phys., 2006, 100, 033908.
48. P. Kushwaha, P. Bag, R. Rawat and P. Chaddah, J. Phys.: Condens. Matter, 2012, 24, 096005.
49. M. Halder, S. M. Yusuf and M. D. Mukadam, Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 81, 1744022.
50. T. Fujii, I. Matsusue, M. Nakanishi and J. Takada, Hyperfine Interact., 2012, 205, 97.
51. M. Sorescu, T. Xu, J. D. Burnett and J. A. Aitken, J. Mater. Sci., 2011, 46, 6709–6717.
52. D. Kothari, V. R. Reddy, A. Gupta, D. M. Phase, N. Lakshmi, S. K. Deshpande and A. M. Awasthi, J. Phys.: Condens. Matter, 2007, 19, 136202.
53. M. Eibschut, S. Shtrikma and D. Treves, Phys. Rev., 1967, 156, 562.
54. S. Braccini, O. Pellegrinelli and K. Krämer, World J. Nucl. Sci. Technol., 2013, 3, 91–95.
55. T. Fujii, I. Matsusue, M. Nakanishi and J. Takada, Hyperfine Interact., 2012, 205, 97–100.
56. A. S. Albuquerque, J. D. Ardisson, W. A. A. Macedo, J. L. Lòpez, R. Paniago and A. I. C. Persiano, J. Magn. Magn. Mater., 2001, 226–230, 1379–1381.
57. A. S. Albuquerque, J. D. Ardisson, W. A. A. Macedo, T. S. Plivelic, I. L. Torriani, J. Larrea and E. B. Saitovitch, J. Magn. Magn. Mater., 2004, 272–276, 2211–2213.
58. M. I. Oshtrakh, M. V. Ushakov, B. Senthilkumar, R. K. Selvan, C. Sanjeeviraja, I. Felner and V. A. Semionkin, Hyperfine Interact., 2013, 219, 7–12.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra11619d

---