Spin Reorientation Temperature Research Articles

Enhancement of rotating magnetocaloric effect by Fe substitution in NdCo5-xFex alloys

The effect of Fe substitution for Co on microstructure, phase constitution, and magnetocaloric properties was investigated in NdCo5-xFex (x = 0.2–1.0) alloys. The annealed NdCo5-xFex alloys are almost composed of the Nd(Co,Fe)5 matrix with a dispersed Nd2(Co,Fe)7 phase for x ≤ 0.6, and show entirely different microstructure of the Nd(Co,Fe)5 matrix surrounded by Nd2(Co,Fe)7, Nd(Co,Fe)3, and Nd(Co,Fe)2 phases for x ≥ 0.8. With increasing Fe content x from 0.2 to 1.0, two spin-reorientation transition temperatures are elevated from 262 to 310 K and 286 to 327 K, respectively. These field-oriented NdCo5-xFex alloys exhibit good rotating magnetocaloric properties, e.g., entropy changes of 2.25–3.66 J/kg K and refrigerant capacity of 188–233 J/kg in a magnetic field of 2 T. In addition, the rotating adiabatic temperature change reaches up to 2.5 K for NdCo4.2Fe0.8 in 2 T. The good combination of enhanced rotating magnetocaloric properties and adjustable working temperature of these alloys makes them suitable for use as refrigerants in room-temperature rotating magnetic refrigeration.

Spin-reorientation transition and temperature dependence of high-frequency properties in [Co/Pd]6/Permalloy multilayer thin films

High-frequency studies, of exchange-coupled multilayer films, are the key to advance the development of fast switching spintronic devices. In this paper, we utilize the first-order reversal curve (FORC) to improve the understanding of the spin-reorientation transition process when the permalloy (Py) thickness increases. We also investigate the temperature dependence of the high-frequency properties of exchange-coupled [Co/Pd]6/Py (t nm) multilayer films during the spin-reorientation transition (SRT). The results show that the phase state of the magnetic domain changes, from the perpendicular phase to the multi-domain phase as the Py layer increases during the SRT. The temperature-dependent high frequency results indicate that the resonance linewidth ΔHres increases gradually when the temperature decreases from 300 K to 50 K. Our experimental study offers new insight into the SRT process in exchange-coupled multilayer films and can help to promote the development of fast switching spintronic devices.

Near room temperature spin reorientation and temperature evolution of magnetic structure in Mn substituted HoFeO3

Magnetic structure and spin reorientation transition of 45% Mn substituted HoFeO3 have been investigated using bulk magnetization, neutron powder diffraction and specific heat capacity techniques. These studies confirm the presence of antiferromagnetic ordering of Fe/Mn sublattice below 336 K. The spin reorientation (SR) transition (TSR ~ 290 K) where the magnetic structure changes from Γ4(Ax, Fy, Gz) to Γ1(Gx, Cy, Az) is confirmed by Rietveld analysis of neutron diffraction patterns taken at various temperatures. Though the spin reorientation (SR) transition is happening at 290 K, the system is found to be in mixed Γ4(Ax, Fy, Gz) + Γ1(Gx, Cy, Az) phase from room temperature to 275 K. Below 275 K, the sample exhibits a pure Γ1(Gx, Cy, Az) structure in which the weak ferromagnetic component is absent. Bulk magnetization and specific heat capacity measurements also confirm the Néel temperature and spin reorientation transitions and support the neutron diffraction analysis.

Terahertz spectroscopy study of doping and magnetic field induced effects on spin reorientation in Ho<sub>1–<i>x</i></sub>Y<sub><i>x</i></sub>FeO<sub>3</sub> single crystals

In this paper, the effects of magnetic field and nonmagnetic Y<sup>3+</sup> doping on spin state and spin reorientation in HoFeO<sub>3</sub> single crystal are systematically studied by the self-developed terahertz time-domain spectroscopy (THz-TDS) under magnetic field. By doping nonmagnetic Y<sup>3+</sup>, we find that the spin reorientation temperature range decreases. Meanwhile, we also find the type of spin reorientation of HoFeO<sub>3</sub> does not change with Y<sup>3+</sup> doping, indicating that the Y<sup>3+</sup> doping can exchange the interaction energy of Ho<sup>3+</sup>-Fe<sup>3+</sup> without introducing any new magnetic structure. Moreover, the resonance frequency of quasi-ferromagnetic mode (q-FM) decreases with temperature increasing in the low temperature range, while the resonance frequency of quasi-antiferromagnetic mode (q-AFM) increases with temperature increasing in the high temperature range in Ho<sub>1–<i>x</i></sub>Y<sub><i>x</i></sub>FeO<sub>3</sub> single crystals. With the external magnetic field (<i> <b>H</b> </i><sub>DC</sub>) applied along the (110) axis, on the one hand the magnetic field can not only tune the resonant frequency of q-FM but also induce the spin reorientation in Ho<sub>1–<i>x</i></sub>Y<sub><i>x</i></sub>FeO<sub>3</sub> single crystals, and on the other hand this magnetic field induced spin reorientation phenomenon can happen more easily if the temperature approaches to the intrinsic spin reorientation temperature range of the single crystals. Besides, the critical magnetic field induced spin reorientation increases with the doping of Y<sup>3+</sup> increasing. Our research shows that THz spectroscopy data can be used to detect the doping concentration of Y<sup>3+</sup> ions in HoFeO<sub>3</sub>; in addition, Y<sup>3+</sup> doping can make the spin state in HoFeO<sub>3</sub> crystal more stable and not easily affected by external magnetic fields. We anticipate that the role of doping and magnetic field in spin reorientation transition will trigger great interest in understanding the mechanism of the spin exchange interaction and the mechanism of external field tuning effect in the vast family of rare earth orthoferrites.

Spin phonon coupling in Mn doped HoFeO3 compounds exhibiting spin reorientation behaviour

An investigation has been carried out on the spin phonon coupling in a series of isostructural polycrystalline orthorhombic (Space group Pnma) compounds HoFe1−XMnXO3 (x ⩽ 0.6) exhibiting spin reorientation below Néel temperature (TN), using magnetization, neutron diffraction, and Raman scattering techniques. Mn doping leads to an anomalous increase in the spin reorientation temperature (TSR), shifting it close to room temperature from TSR ~ 60 K for x = 0 sample, and concomitant lowering of TN. The TSR is absent in samples for x ⩾ 0.5. The magnetic structure undergoes a transition at TSR from Γ4 → Γ1 in the Mn doped compounds as against Γ4 → Γ2 observed in HoFeO3 sample. In the region T < TN an anomalous softening of Raman phonon modes viz., B2g(5) and B3g(3), identified with the stretching motion and breathing mode, respectively, of Fe/Mn–O6 octahedra, is observed in compounds exhibiting spin-reorientation behaviour, indicating a spin-phonon coupling in these compounds. A quadratic correlation between the deviation of phonon frequency and variation of antiferromagnetic moment (Δω M2) is observed in these compounds. The temperature evolution of the M2+ mode obtained from the analysis of neutron diffraction data based on symmetry adapted mode decomposition of the Pnma structure further corroborates the mode softening observation.

Magnetocaloric effect and sign reversal of magnetic entropy change across the spin reorientation temperature in R3Fe5O12 (R = Gd, Dy)

The cubic structure with space group Ia3‾d for Gd3Fe5O12 and Dy3Fe5O12 are successfully synthesized and the magnetic behaviors are studied in details. The dc magnetization measurements show that an obvious negative magnetization phenomenon is observed in Gd3Fe5O12 and the compensation temperature is as high as 287 K. An adjustable magnetic switching effect with good cyclicity can be realized near the compensation temperature. Besides, sign reversal of magnetic entropy change across the spin reorientation temperature has been observed in both of Gd3Fe5O12 and Dy3Fe5O12, which is ascribed to the transition of the ordered collinear structure to the distorted non-collinear structure with lower order as the temperature decreases. Moreover, the magnetocaloric effect at the lower temperatures is also studied and the maximum values of magnetic entropy change are 3.40 J kg−1 K−1 at 40 K and 3.51 J kg−1 K−1 at 58 K for Gd3Fe5O12 and Dy3Fe5O12, respectively.

Temperature-dependent magnetocrystalline anisotropy of rare earth/transition metal permanent magnets from first principles: The light RCo5 (R=Y, La-Gd) intermetallics

Computational design of more efficient rare earth/transition metal (RE-TM) permanent magnets requires accurately calculating the magnetocrystalline anisotropy (MCA) at finite temperature, since this property places an upper bound on the coercivity. Here, we present a first-principles methodology to calculate the MCA of RE-TM magnets which fully accounts for the effects of temperature on the underlying electrons. The itinerant electron TM magnetism is described within the disordered local moment picture, and the localized RE-4f magnetism is described within crystal field theory. We use our model, which is free of adjustable parameters, to calculate the MCA of the RCo$_5$ (R=Y, La-Gd) magnet family for temperatures 0--600 K. We correctly find a huge uniaxial anisotropy for SmCo$_5$ (21.3 MJm$^{-3}$ at 300 K) and two finite temperature spin reorientation transitions for NdCo$_5$. The calculations also demonstrate dramatic valency effects in CeCo$_5$ and PrCo$_5$. Our calculations provide quantitative, first-principles insight into several decades of RE-TM experimental studies.

Optical and magnetic properties of Gd1−xSrxCrO3 (0 ⩽ x ⩽ 0.15)

Optical and magnetic properties of Gd1−xSrxCrO3 (0 ⩽ x ⩽ 0.15) polycrystalline samples have been investigated to identify the role of Sr2+ ion in GdCrO3 compound. Quantitative structural analysis using Rietveld refinement suggests the formation of single orthorhombic phase up to x = 0.1 composition, and in addition, a secondary monazite-type structure of SrCrO4 arises for x = 0.15 composition. Raman and Fourier transform infrared spectra also confirm the presence of SrCrO4 component for higher concentration of Sr2+ ion. Mixed valence state of Cr such as Cr3+ and Cr4+ is obtained from x-ray photoelectron spectroscopy. The presence of Cr4+ ion induces a reduction of optical band gap with increase of Sr content. The unoccupied states originate from Cr4+ modify d–d transitions. Larger cell volume and breaking of magnetic exchange pathways through nonmagnetic element (Sr) weaken the strength of interaction and consequently lower Néel and spin reorientation temperatures.

Temperature and field evolution of site-dependent magnetism in ε−Fe2O3 nanoparticles

8~nm epsilon-Fe2O3 nanoparticles exhibit a spin reorientation transition that begins at 150 K which is a hallmark of this unique iron-oxide polymorph. We find that the change from the high to low temperature magnetic structures has been suppressed by ~50 K. At the spin reorientation temperature, a change of the field-dependent response of the tetrahedral sites in intermediate field strengths (0.25 - 1.5 T) indicates that a collective tetrahedral distortion occurs to which the octahedral sites adjust, altering the magnetic anisotropy. An abrupt step in the hyperfine parameters' temperature dependencies, especially at 125 K for the hyperfine field associated with the Fe4 tetrahedral sites, suggests strongly a change in the superexchange pathways are responsible for the spin reorientation.

High temperature spin reorientation, magnetization reversal and magnetocaloric effect in 50% Mn substituted polycrystalline ErFeO3

The structural, magnetic and magnetocaloric properties of 50% Mn substituted ErFeO3 are studied by X-ray diffraction, magnetization and specific heat measurements. ErFe0.5Mn0.5O3 crystallizes in an orthorhombic structure (Pbnm). DC magnetization and specific heat studies point to two magnetic transitions: (i) Fe3+/Mn3+ ordering, TN ~ 245 K and (ii) spin re-orientation transition, TSR ~ 218 K. Below TSR, ErFe0.5Mn0.5O3 exhibits a reduction in magnetization and coercivity which could be due to a Γ4-Γ1 magnetic transition. Interestingly, we have observed a negative magnetization in field cooled curves at lower temperatures under lower applied fields (<50 Oe) as a result of the polarization of paramagnetic Er3+ moments with the internal field created by Mn3+ ordering from the Mn-rich regions. A significant magnetic entropy change (-ΔSm) of 13.4 J/kg.K is also observed at 9.5 K for an applied field of 7 T.

Microstructure and magnetic properties of NdFeB alloys by co-doping alnico elements

Elements of alnico 8 are added into Nd-Fe-B alloys fabricated by rapid solidification method. It is observed that the magnetic properties at high temperature improved by small addition of alnico elements. The Curie temperature of the alloys increased from 580 K for standard alloy to 639 K by 20% addition of alnico 8 elements. The spin reorientation temperature decreased from 133 K to 104 K. The TEM analysis showed that elements of alnico 8 refine the microstructure of Nd-Fe-B ribbons. The STEM analysis confirmed the heterogeneous distribution of Nd, Fe, Cu, Al, Ni and homogeneous distribution of Ti, Nb and Co. The boundaries of nano grains contain more than 70% ferromagnetic elements, ensuring strong inter-grain coupling among the grains.

Tuning of spin reorientation temperature of SmFeO3 by doping of Tm3+ ion: Role of exchange interaction between 4f & 3d electrons

Emergent technologies based on spintronics demand the unprecedented control on magnetic anisotropy. Here we present the control on magnetic anisotropy of SmFeO3 by the means of Tm3+doping at Sm3+site. A series of nano particles of Sm1-xTmxFeO3 (X = 0, 0.3, 0.5, 0.7, 1) orthoferrite have been synthesized by sol-gel auto-combustion method and phase purity was confirmed by the X-ray diffraction pattern. The crystallographic information obtained from Rietveld refinement and Raman spectroscopy confirmed the changes in oxygen octahedra with the change in doping percentage of Tm3+ ion in SmFeO3. Temperature dependent dc magnetization measurement implies the tuning of spin reorientation temperature from 480 K to 80 K as a function of Tm3+ ion doping concentration. The room temperature magnetic isotherm confirms the canted antiferromagnetic character of nano particles. Sm0.7Tm0.3FeO3 shows the spin reorientation at room temperature which might be used in room temperature spintronics devices.

Structural and magnetic phase transitions along with optical properties in GdMn1-xFexO3 perovskite

Herein, we report the structural evolution and the rich sequence of magnetic transitions in GdMn1–xFexO3 (x = 0, 0.3, and 0.5) synthesized through the sol-gel technique. Rietveld refinement of X-ray diffraction patterns reveals a structural transformation from O′ to O type orthorhombic one accompanied with a decrease in lattice volume when x increases from 0 to 0.5. The decrease in lattice volume is due to the presence of Mn4+ confirmed from X-ray photoelectron spectroscopy and photoluminescence (PL) studies. Such a structural transformation considerably reduces the Jahn-Teller distortion factor as indicated by Raman and PL spectra. The temperature dependent magnetization shows an increase in Néel temperature (TN) from ∼42 K for pristine GdMnO3 to almost room temperature for x = 0.5. Interestingly, we observe a spin reorientation temperature (TSR) at ∼270 and ∼253 K for x = 0.3 and 0.5, respectively. The mixed valency of Mn, i.e., Mn3+ and Mn4+ not only demonstrates a spin-glass (SG) behavior but also contributes toward the strong emission spectra irrespective of Fe concentration. The structural and magnetic properties of GdMnO3 after doping Fe suggest a possible way to achieve a better magnetoelectric coupling and modify the multiferroic property.

Dielectric and magnetocaloric study of TmCrO3

Dielectric and magnetocaloric measurements are carried out for the chromite TmCrO3. This oxide was reported to be multiferroic below the Néel temperature (TN) of ∼125 K, likely due to a structural transformation. The dielectric response shows large dielectric constants below 300 K. However, from the analyses of loss tangent, AC conductivity and dielectric modulus, this behavior is rooted in hopping of charge carriers rather than electric dipoles, as proposed for some other chromites. No dielectric anomaly is found at TN. The magnetocaloric effect shows that the magnetic transitions at TN as well as the spin reorientation temperature are of a second order. This result strongly suggests the absence of magnetostructural transition at TN in accord with no observation of ferroelectric transition at this temperature.

Crystal structure, spin reorientation, and rotating magnetocaloric properties of NdCo5-xSix compounds

The structure, spin-reorientation transitions, and the rotating magnetocaloric effect of field-oriented polycrystalline NdCo5-xSix (x = 0.0, 0.2, and 0.4) compounds were investigated. These compounds crystallize in the CaCu5-type structure with the space group P6/mmm, and the doping element Si prefers to occupy the 2c site. All these compounds show two spin-reorientation temperatures (TSR1 and TSR2). When Si content x reaching up to 0.4, TSR1 of the compounds monotonously increases from 260 K to 285 K and TSR2 increases from 285 K to 317 K, while the rotating entropy change value slightly decreases from 3.23 J/kg K to 2.60 J/kg K under a magnetic field of 2 T. These materials will be suitable refrigerants for application in room-temperature rotating magnetic refrigeration technology.

Magnetic order in ErFeO3 single crystals studied by mean-field theory

Recent experiments showed magnetic moment reversal and pronounced exchange-bias (EB) effect in the vicinity of the compensation temperature ${T}_{\mathrm{comp}}$ of orthoferrites $R\mathrm{Fe}{\mathrm{O}}_{3}$ ($R=\mathrm{Nd}$, Sm, Er). Although different orthoferrites exhibit diverse $R$-Fe interactions, ${T}_{\mathrm{comp}}$ values, and spin-reorientation temperatures, the EB field in the like manner emerges and diverges upon approaching ${T}_{\mathrm{comp}}$ and changes its sign with crossing ${T}_{\mathrm{comp}}$. In order to explain these observations, a mean-field theory approach for the representative $\mathrm{ErFe}{\mathrm{O}}_{3}$ orthoferrite is proposed. The general case of two sublattices, antiferromagnet with exchange anisotropy and rare-earth--iron interactions, is considered. A small applied magnetic field appears to be a source of additional anisotropy, resulting from canting of sublattice moments. This anisotropy leads to an imbalance of free energy for two different types of magnetic domains, causing a spin jump near the ${T}_{\mathrm{comp}}$. The suggested approach allows reproduction of magnetization reversal and main features of the coercive and exchange-bias fields observed in $\mathrm{ErFe}{\mathrm{O}}_{3}$.

Effect of pressure on the magnetic and structural properties of Fe5SiB2 compound

Magnetic and structural parameters of tetragonal Fe5SiB2 compound was studied in applied magnetic field up to 7 T and hydrostatic pressure up to 6 GPa in temperature ranging from 2 K up to TC = 844 K. The temperature dependence of saturated magnetization was measured and described by analytical equation. The decrease of saturated magnetization of dlnM/dp = −1.5 10−2 GPa−1 was observed. The bulk modulus of 124 GPa was determined by means of high pressure X-ray diffraction. Low temperature spin-reorientation transition was observed and small negative dependence of the transition temperature with respect to pressure was established.

Magnetic and Dielectric Properties of LiFePO₄

A LiFePO₄ was prepared using the ball milling. The X-ray diffraction patterns of the sample were measured, and the results confirmed that the sample’s structure was orthorhombic with space group Pnma. The particle size and morphology of the sample prepared by the ball mill method confirmed by FE-SEM. The magnetization curves of the sample were measured using a vibrating sample magnetometer at temperatures of 4.2 to 295 K at 1000 Oe. The Néel temperature (TN) and spin-reorientation temperature (TS) were found to be 51.5 and 25 K, respectively. We investigated the magnetic hyperfine interaction using Mössbauer spectrometry at various temperatures between 4.2 and 295 K. At temperatures below TN, the Mössbauer spectra of the sample were exhibited eight absorption lines resulting from the magnetic dipole and electric quadruple interaction. The Fe ions state of sample at all temperatures were found to be ferrous state (Fe2+) ions. Debye temperature (θD) of 433 ± 5 K was obtained for the sample. The permeability and permittivity were obtained using a Network analyzer.

Tailoring magnetic properties with interfacial spins interaction for bundled cobalt ferrites nanowires

Abstract Bundled cobalt ferrites (CoFe2O4) nanowires (BNWs) were successfully prepared by nanocasting pathway with ordered mesoporous SiO2 (SBA-15) templates. The well-dispersed CoFe2O4 nanowires (NWS) were separated from CoFe2O4 BNWs by centrifugal technology. Microstructure characterization indicated that CoFe2O4 BNWs and NWS possessed the same diameter and similar length. Thus, the influence of surface spins interaction between neighboring nanowires on the magnetic properties could be distinguished and discussed excluding the difference of morphology. Both samples presented ferromagnetism with the wasp-waisted hysteresis loops and the exchange bias effect was observed above 100 K. The surface spins interactions between the neighboring nanowires not only recombined the mismatch spins at the surface to form antiferromagnetic surface, but also inhibited the shape anisotropy of nanowires, leading to the decrease of saturation magnetization and coercivity of CoFe2O4 BNWs. The wasp-waisted hysteresis loops was attributed to the low temperature spin reorientation caused by the inclined spin on the surface of nanowires.

All Electrical Access to Topological Transport Features in Mn1.8PtSn Films.

The presence of nontrivial magnetic topology can give rise to nonvanishing scalar spin chirality and consequently a topological Hall or Nernst effect. In turn, topological transport signals can serve as indicators for topological spin structures. This is particularly important in thin films or nanopatterned materials where the spin structure is not readily accessible. Conventionally, the topological response is determined by combining magnetotransport data with an independent magnetometry experiment. This approach is prone to introduce measurement artifacts. In this study, we report the observation of large topological Hall and Nernst effects in micropatterned thin films of Mn1.8PtSn below the spin reorientation temperature TSR ≈ 190 K. The magnitude of the topological Hall effect ρ xyT = 8 nΩm is close to the value reported in bulk Mn2PtSn, and the topological Nernst effect S xyT = 115 nV K-1 measured in the same microstructure has a similar magnitude as reported for bulk MnGe ( S xyT ∼ 150 nV K-1), the only other material where a topological Nernst was reported. We use our data as a model system to introduce a topological quantity, which allows one to detect the presence of topological transport effects without the need for independent magnetometry data. Our approach thus enables the study of topological transport also in nanopatterned materials without detrimental magnetization related limitations.