Reorientation Transition Research Articles

Large magnetocaloric effect near liquid hydrogen temperatures in Er1-xTmxGa materials

Low-temperature magnetocaloric materials are of great importance for potential applications of gas liquefaction such as nitrogen, hydrogen and helium for their low liquidation temperatures (∼4 K for helium, ∼20 K for hydrogen and ∼77 K for nitrogen respectively), of which the working temperature, the maximal magnetic entropy change ((-ΔSM)max), the maximal adiabatic temperature change ((ΔTad)max), and the temperature average entropy change (TEC) are the key assessment parameters. Herein, we designed and synthesized Er1-xTmxGa series compounds based on the optimization of the spin quantum number (Spin) with their magnetic ordering temperature successfully adjusted from 31.0 K to 15.0 K, which covers the liquid hydrogen temperature range. Particularly, Er0.8Tm0.2Ga shows outstanding (-ΔSM)max, TEC(20), and (ΔTad)max values of 13.6 J/kg K, 10.1 J/kg K, and 4.3 K under the field change of 0–2 T, respectively, which are increased by 32.0 %, 36.4 %, and 48.2 % compared with the parent ErGa compound. It should be noted that the refrigerant capacity (RC) of Er0.8Tm0.2Ga is not only larger than ErGa but also larger than TmGa. Furthermore, neutron powder diffraction (NPD) was employed on Er0.8Tm0.2Ga to reveal the physical mechanism of its enhanced magnetocaloric effect (MCE). It is found that for Er0.8Tm0.2Ga the more pronounced order-to-disorder transition than the spin reorientation (SR) transition, the characteristic second order phase transition, and the existence of the short-range magnetic ordering above the magnetic ordering temperature should be jointly responsible for its large magnetocaloric effect.

Multiple magnetic transitions and anomalous magnetocaloric effect in an arc melting TbMn2 compound

Magnetic properties of Mn-base alloys are obviously influenced by Mn-content. In this work, multiple magnetic transitions and anomalous magnetocaloric effect of an arc melting TbMn2 intermetallic compound were measured and discussed. TbMn2 shows paramagnetic properties at room temperature. The small bulge at ∼ 230.3 K suggests the rise of a Griffiths phase. The long-range ferromagnetic ordering is established at TC ∼ 60.0 K. The observed antiferromagnetic transition at TN ∼ 45.0 K is caused by the collapse of Mn moments. At ∼ 20.2 K, an unexpected spin reorientation transition is probably due to the arrangement of the sublattice of rare earth. The field induced metamagnetic transition below TN results in an anomalous magnetocaloric effect.

Néel Vector Auto-Oscillations and Reorientations Induced by Spin-Polarized Electric Currents in Antiferromagnetic Mn2Au Nanolayer

The electrical excitation of spin dynamics in antiferromagnets is important for the development of high-speed spintronic devices with a low power consumption. Here, we present a theoretical study of the spin dynamics generated in the Ni/Cu/Mn2Au/Cu/Ni nanostructure by a perpendicular-to-plane electric current. Our investigation includes atomistic simulations of spin reorientations in a few monolayer antiferromagnetic Mn2Au film and numerical calculations of nonequilibrium spin accumulation in the nanostructure comprising ferromagnetic Ni polarizers with antiparallel magnetizations. This combined approach enables us to quantify the dynamics of Mn magnetic moments induced by the spin-transfer torque created by the spin-polarized charge flow. The calculations show that the direct electric current with the density exceeding some temperature-dependent threshold value gives rise to steady-state auto-oscillations of the Néel vector in the [Formula: see text]-oriented Mn2Au nanolayer. As the precession of Mn magnetic moments occurs around an out-of-plane direction strongly deflected from their initial in-plane orientations, the emergence of auto-oscillations should be regarded as the realization of a dynamic spin reorientation transition in the antiferromagnetic crystal. Remarkably, the precession frequency rises with increasing current density J and exceeds 1[Formula: see text]THz at [Formula: see text][Formula: see text]A[Formula: see text]m[Formula: see text], which makes the described antiferromagnetic spin transfer nano-oscillator attractive for device applications. Furthermore, our simulations demonstrate that a picosecond current pulse injected into the Ni/Cu/Mn2Au/Cu/Ni nanostructure can induce a precessional switching of the Néel vector. Depending on the current density and pulse duration, the Néel vector rotates by either 90° or 180° and attains stable orientation along the corresponding [Formula: see text] easy axis in the plane of the Mn2Au nanolayer. This feature shows that the Ni/Cu/Mn2Au/Cu/Ni nanostructure could be used as a nonvolatile memory cell with the electrical writing and readout.

Magnetic circular dichroism of f-f electron transitions in Ho3+ and Nd3+ ions in antiferromagnetic crystal Ho0.75Nd0.25Fe3(BO3)4 in the region of spin-reorientation transition

Absorption and magnetic circular dichroism (MCD) spectra have been studied in the region of 5I8 → 5F2, 5S2, 5F3, and 5F5 absorption bands of the Ho3+ ion and in the region of 4I9/2→4G7/2, 2K13/2, 4S3/2, 4F7/2, 4F5/2 and 2H9/2 absorption bands of the Nd3+ ion in an antiferromagnetic trigonal crystal Ho0.75Nd0.25Fe3(BO3)4 in the temperature range from 5 to 90 K. A qualitative change in the shape of the MCD spectra of Ho3+ ion was revealed during the spin-reorientation transition at TR = 6.9 К. Below TR, in the easy-axis state of the crystal, the MCD spectrum has a shape typical for paramagnets, i.e., it consists of diamagnetic and paramagnetic parts. Just above TR, in the easy-plane state of the crystal, MCD of Ho3+ ion has a spectrum similar to the absorption spectrum, which is typical for the paramagnetic part of the MCD. It was shown, that in the easy-plane state the exchange field of iron, being perpendicular to the external field directed along the C3 axis, quenches the usual paramagnetic and diamagnetic MCD in Ho3+ ion, but creates a condition for the appearance of a large paramagnetic MCD of mixing (B-term). With temperature increasing, the MCD spectrum of Ho3+ ion gradually turns into a spectrum typical for paramagnets with the predominance of the sign-changing diamagnetic part, because the influence of the exchange field decreases. The shape of the MCD spectra of Nd3+ ions are typical for paramagnets but does not change during the reorientation transition in the rest of the crystal. This means that the magnetic moment of the Nd3+ ion does not make a reorientation transition simultaneously with the Fe3+ and Ho3+ ions of the rest of the crystal. This phenomenon is accounted for by a strong local magnetic anisotropy of the Nd3+ ion and by a weak exchange interaction Fe-Nd, which is not enough for rotation of the Nd3+ ion moment synchronously with that of the Fe3+ ion. The magneto-optical properties of neodymium are controlled by the easy-axis anisotropy of neodymium.

Tailoring the Spin Reorientation Transition of Co Films by Pd Monolayer Capping.

We have characterized the magnetization easy-axis of ultra-thin Co films (2-5 atomic layers, AL) grown on Ru(0001) when they are capped with a monolayer of Pd. The addition of a Pd monolayer turns the magnetization of 3 and 4 AL-thick Co films from an in-plane to an out-of-plane alignment, but not that of a 5 AL-thick film. These observations are explained in terms of an enhancement of the surface anisotropy. The exposure of the sample to hydrogen, CO or a combination of both gases does not overcome this effect.

Mn doping-induced twofold spin reorientation and multi-step spin switching in NdFeO3 single crystal

Magnetization reversal and magnetic state regulation are the basis of application for magnetic materials in information processing. Here, twofold spin reorientation transitions (SRT) and field-induced multi-step spin switching phenomena were observed in NdMn0.2Fe0.8O3 (NMFO) single crystals grown via the optical floating zone method. Magnetic anisotropy was systematically investigated in both Zero-Field-Cooled (ZFC) and Field-Cooled (FC) modes. The NMFO single crystal exhibited twofold SRTs: from Γ4 (Gx, Ay, Fz) to Γ1 (Ax, Gy, Cz) and from Γ1 (Ax, Gy, Cz) to Γ2 (Fx, Cy, Gz). Furthermore, a magnetic field-induced spin reorientation transition was identified along the c-axis. Additionally, closer proximity to the SRT temperature corresponded to a reduced magnetic field requirement for inducing the spin reorientation transition. These findings contribute to our comprehension of spin dynamics in NMFO, presenting possibilities for future device applications.

Thermal expansion, quasi-metamagnetism and spin-reorientation in Fe7Se8 substituted with chromium

The properties of the selenide compound Fe7Se8 with a layered crystal structure of the NiAs type are strongly influenced by substitutions and the distribution of vacancies. The Cr-substituted compound Fe6.5Cr0.5Se8 was obtained in single-crystalline form and studied by x-ray diffraction, energy-dispersive x-ray spectroscopy, thermal expansion and magnetization measurements. It was observed that the partial replacement of iron with chromium led to a twofold decrease in spontaneous volume magnetostriction due to changes in competing magnetoelastic contributions to thermal expansion along and perpendicular to the c axis of the crystal. The replacement of iron with chromium slightly decreases the Néel temperature (from 440 to 435 K) and significantly enhances the critical temperature of spin reorientation transition Tsr (from 115 to 160 K), apparently due to a change in the crystal electric field. Below 160 K, the Fe6.5Cr0.5Se crystal is found to exhibit metamagnetic-like behavior of the magnetization when the magnetic field is applied along the c axis. A jump-like change of the magnetization at a critical field up to ∼10 kOe is attributed to the presence of pinning centers of domain walls presumably due the ordering of chromium atoms substituting iron in cationic layers.

Evolution of static and dynamic magnetic properties of Re/Co/Pt and Pt/Co/Re trilayers with enhanced Dzyaloshinskii-Moriya interaction

The influence of Re layer insertion either at the bottom or top interface of epitaxially grown Pt/Co/Pt heterostructures on their static and dynamic magnetic properties is discussed in terms of their crystalline structure. Magnetic properties were studied as a function of both Re and Co layer thicknesses (dRe and dCo, respectively) in the matrix-like samples with a double-wedge structure, in which the layer thickness gradients were oriented orthogonally. The comprehensive investigations of the changes in coercivity, perpendicular magnetic anisotropy, spin reorientation transition, interfacial Dzyaloshinskii-Moriya interaction (iDMI) and spin wave damping are reported. Two different magnetic phases depending on the Co layer thickness with volume anisotropies (determined without demagnetization term) of KV∼0.25 (low) and KV∼0.75 MJ/m3 (high) were observed. The relation between these phases depends on the stack sequence and thickness of Re inserted layer. For dRe = 0.2 ÷ 0.7 nm deposited as the bottom interface, dCo induced transition at dtr ∼ 2 nm from low to high volume anisotropy phases was observed. Low and high volume anisotropy phases are associated to Co fcc and hcp structural phases. The insertion of half atomic layer of Re had no influence on surface anisotropy but significantly enhanced iDMI above 1 pJ/m and reduced spin wave damping.

Evidence for spin reorientation transition in antiferromagnetic FeRh

FeRh is a unique intermetallic compound known for its temperature-induced phase transition from an antiferromagnetic (AFM) to a ferromagnetic (FM) state, making it a promising candidate for AFM memory applications. In this study, we discovered that FeRh exhibits a temperature-driven spin reorientation transition in the AFM phase. Our investigation using magnetoresistance (MR) measurements reveals that the Néel vector changes from an in-plane direction at high temperatures to an out-of-plane direction at low temperatures. This spin reorientation can be effectively detected through MR, offering a more practical approach for applications. Our findings are crucial for the emerging field of AFM memory, as controlling and identifying the Néel vector is vital for defining the memory state in AFM-based data storage technologies.

Spin reorientation transition in epitaxial nanometric Nd-Fe-B films with large perpendicular magnetic anisotropy

The development of hard magnetic properties in Nd-Fe-B films with a thickness below 50 nm has been studied and the effect of the film strain on the spin reorientation transition temperature (TSR) evaluated. A value of 35 nm is found to be the lowest thickness threshold in the films grown by DC magnetron sputtering in this study for the full development of both the Nd2Fe14B phase and the broadening of the out of plane hysteresis loops. Large perpendicular magnetic anisotropy is obtained for a film of 40 nm in thickness, reaching a coercivity of 8 kOe at room temperature. A detailed characterization of epitaxial Nd2Fe14B with high magnetic anisotropy has been done by Grazing Incidence X-Ray Diffraction, lattice parameters are reported and the chemical states of Nd 3d and Fe 2p have been stablished by the analysis of Hard X-Rays Photoelectron Spectroscopy spectra. The spin reorientation transition temperature has been estimated to a value of 124.2 K. Based on experimental results, lattice strain is attributed as the main reason for the deviation of TSR by comparison with the theoretical value (135 K). No evidence of tetragonal distortion in the lattice is found below TSR in this system in contrast to previous considerations.

Study on the magnetothermal properties of the [formula omitted] series of compounds

In this work, we developed a theoretical model Hamiltonian, in the mean field approximation, to describe the magnetic and magnetocaloric behavior of the series of compounds Dy1-xTbxAl2(x=0.00,0.15,0.25,0.30,0.35,0.40 and 0.75). We adjusted the exchange parameters λDyDy,λTbTb, and λDyTb to obtain the spin reorientation temperatures TSR and the critical temperature TC for each compound in the series. The results obtained by the Hamiltonian model agree satisfactorily with the experimental results. The heat capacity curves with and without an applied magnetic field, adiabatic temperature variation and isothermal entropy variation were modeled and compared with the experimental data. As the experimental results show, our model was also able to reproduce the change in the spin reorientation process: a first order spin reorientation transition appears for concentrations x = 0.15, 0.25, 0.30, 0.35, and no spin reorientation transitions after x = 0.40.

Influence of chalcopyrite nanoplatelets on nematic phases of bend-shaped dimeric molecules: Phase diagram, birefringence, and reorientation transition

We investigated the influence of chalcopyrite (CuFeS2) nanoplatelets on the ordering of uniform and twist-bend nematic phases of achiral bend-shaped dimers, employing polarized optical microscopy, optical and electro-optical measurements. Specifically, aliphatic amine surface-functionalized hydrophobic chalcopyrite nanoplatelets were synthesized and characterized by XRD, TEM, and IR-vis-UV spectroscopy. Nanocomposites of the liquid crystal compound 1”,9”-bis(4-cyanobiphenyl-4'-yl)nonane (CB9CB) with the nanoplatelets were prepared. The study encompasses an assessment of the thermodynamic stability of the nanocomposites, constructing the corresponding phase diagram as a function of temperature and nanoplatelet mass fraction. Large phase temperature shifts were observed for minute mass fractions of NPs. Birefringence measurements were performed to investigate the orientational order parameter and the conical tilt angle's dependence on the mass fraction of the nanoplatelets and the temperature. Additionally, we measured a Fréedericksz-like reorientation transition voltage threshold and switching times as functions of the mass fraction and temperature. A huge increase of the voltage threshold was measured in the twist-bend nematic phase. The dependence of the viscoelastic coefficient ζ on NPls' mass fraction was investigated. The splay elastic constant of the helicoidal structure was estimated, using a coarse grain approximation, to be two orders of magnitude higher than in the uniform nematic phase. Finally, in the twist-bend nematic phase, the impact of temperature and nanoplatelets mass fraction on the hysteresis loop of the reorientation transition was investigated.

Theoretical description of the thermodynamic properties and the magnetocaloric effect in R2Cu2Cd (R = Dy,Ho)

In this paper, we present a theoretical discussion of the magnetocaloric effect in the compounds Dy2Cu2Cd and Ho2Cu2Cd whose curves of the heat capacity and isothermal entropy change exhibit two peaks. For this purpose, we use a model Hamiltonian of local magnetic moments with two anisotropic magnetic sublattices. In the model, it is considered that exchange interactions between magnetic moments depend on their xyz components. Besides, it is also included in the model a single ion uniaxial anisotropy and the Dzyaloshinskii-Moriya interaction to account for the canted magnetic moments. Our theoretical calculations show that the existence of these two peaks is associated with a spin reorientation transition at a lower temperature and a ferromagnetic-paramagnetic one at a higher temperature. The results of our theoretical calculations are in reasonable agreement with the existing experimental data of heat capacity and the isothermal entropy change.

Investigation on structural, electrical and magnetic properties: Dy0.4Sm0.6FeO3 orthoferrite with Mn doping at Fe site

This paper delves the structural and magnetic properties of Mn-doped Dy0.4Sm0.6Fe1-xMnxO3 (x = 0, 0.1, 0.2 and 0.3) orthoferrite. With an increase in Mn concentration, the a and c lattice parameters show a decreasing trend, while the b parameter shows a linear increase and an variance is observed in Fe–O–Fe and Fe–O-Sm bond angles and bond lengths, indicating a distortion in the FeO6 octahedra. FESEM images show that as the Mn content increases, the grain size increases and porosity decreases. EDX analysis confirms that, the prepared samples are high in purity and the elemental composition. The Dy0.4Sm0.6FeO3 (i.e., x = 0.0) orthoferrite demonstrates a spin reorientation transition at 234K. With an increase in Mn doping at the Fe site induces a complex magnetic structure, Mn/Fe3+-O-Mn/Fe3+ interactions weaken the antiferromagnetic nature thereby decreasing spin reorientation transition (SRT) temperature. The moderate Mn doping at the Fe site leads to a canted-like antiferromagnetic structure, playing a crucial role in tuning the magneto-electric properties at room temperature.

Negative magnetization and magnetic ordering of rare earth and transition metal sublattices in NdFe0.5Cr0.5O3

We investigate the effect of alloying at the 3d transition metal site of a rare-earth-transition metal oxide, by considering NdFe0.5Cr0.5O3 mixed perovskite with two equal and random distribution of 3d ions, Cr and Fe, interacting with an early 4f rare earth ion, Nd. Employing temperature- and field- dependent magnetization measurements, temperature-dependent x-ray diffraction, neutron powder diffraction, and Raman spectroscopy, we characterize its structural and magnetic properties. Our study reveals bipolar magnetic switching (arising from negative magnetization) and magnetocaloric effect which underline the potential of the studied mixed perovskite in device application. The neutron diffraction study shows the absence of spin reorientation transition over the entire temperature range of 1.5–320 K, although both parent compounds exhibit spin orientation transition. We discuss the microscopic origin of this curious behavior. The neutron diffraction results also reveal the ordering of Nd spins at an unusually high temperature of about 40 K, which is corroborated by Raman measurements.

Temperature-driven spin reorientation transition in van der Waals Cr1.7Te2 ferromagnet

The phenomenon of spin reorientation transition in magnetic materials is truly captivating, as it involves a fascinating change in the direction of magnetic moments. However, the research on spin reorientation transition in two-dimensional (2D) magnetic materials has received limited attention, thus hindering its immense potential for significant advancements in various device applications. In this study, we present a discovery of a spin reorientation transition from an in-plane to an out-of-plane direction in the van der Waals ferromagnet Cr1.7Te2 (Tc = 300 K). This transition occurs at 70 K when the temperature ranges from 3 to 300 K, which is evidenced by the temperature-dependent Hall effect and magnetic anisotropy energy measurements. Notably, the anisotropic evolution observed reveals that the shape anisotropy effect surpasses the magnetocrystalline anisotropy in van der Waals ferromagnet at low temperatures, which is distinct from reported ferromagnetic materials. Furthermore, temperature-dependent x-ray diffraction characterizations confirm that no structural phase transition occurs during this intriguing spin reorientation transition process. These findings establish a strong and solid foundation, offering a promising platform for the design and development of cutting-edge 2D spintronic devices.

Spin switching and magnetocaloric effect of high-entropy orthoferrite single crystal

The concept of high-entropy oxides/alloys was introduced into the rare-earth orthoferrite (RFeO3, R represents a rare-earth ion) system. Doping five different rare-earth ions with equal molar ratios at the R-site, a single crystal of high-entropy orthoferrite (Y0.2Sm0.2Eu0.2Gd0.2Er0.2)FeO3 (HERFO) was synthesized using optical floating zone method. At high temperatures, the weak ferromagnetic (wFM) moments induced by the distortion of the Fe sublattices align along the c axis, with a spin configuration of Γ4. As the temperature decreases, a spin reorientation transition (SRT) occurs from Γ4 to Γ2 due to the Fe3+-R3+ super-exchange interactions, with an immediate state Γ24. The wFM moments gradually rotate from the c axis to the a axis within the ac plane. In the zero-field-cooling (ZFC) mode, both the type-I and type-II spin switching (SSW) are observable along the a axis. Under low magnetic fields, the type-II SSW is magnetically controlled. The trigger temperature and variation in net moment of the type-II SSW exhibit systematic changes concerning the applied magnetic field. The rich magnetic phase transitions in HERFO make it a promising candidate for spintronic applications aimed at spin manipulation. Additionally, HERFO single crystal exhibits a large magnetocaloric effect (MCE) and magnetic entropy anisotropy, with the negative magnetic entropy change (‐ΔSm) along the c axis (14.45 J/(kg·K)) being greater than that along the a and b axes (9.57 J/(kg·K) and 10.32 J/(kg·K)). Consequently, HERFO holds great promise as a novel material for magnetic refrigeration applications aimed at achieving rotational magnetic cooling.

Role of Dy 4f electrons on magnetic coupling and reorientation in DyFeO3

Spin reorientation transition is an ubiquitous phenomenon observed in magnetic rare earth orthferrites RFeO3, which has garnered significant attention in recent years due to its potential applications in spintronics or magnetoelectric devices. Although a plenty of experimental works suggest that the magnetic interaction between R3+ and Fe3+ spins is at the heart of the spin reorientation, but a direct and conclusive theoretical support has been lacking thus far, primarily due to the challenging nature of handling R 4f electrons. In this paper, we explored DyFeO3 as an example by means of comprehensive first principles calculations, and compared two different approaches, where the Dy 4f electrons were treated separately as core or valence states, aiming to elucidate the role of Dy 4f electrons, particularly in the context of the spin reorientation transition. The comparison provides a solid piece of evidence for the experimental argument that the Dy3+−Fe3+ magnetic interactions play a vital role in triggering spin reorientation of Fe3+ moments at low temperatures. The findings revealed here not only extend our understanding on the underlying mechanism for spin reorientation transition in RFeO3, but also highlight the importance of explicit description of R 4f electrons in rationally reproducing their structural, electronic and magnetic properties.

Disentangling the Unusual Magnetic Anisotropy of the Near‐Room‐Temperature Ferromagnet Fe4GeTe2

AbstractIn the quest for 2D conducting materials with high ferromagnetic ordering temperature the new family of the layered FenGeTe2 compounds, especially the near‐room‐temperature ferromagnet Fe4GeTe2, receives a significant attention. Fe4GeTe2 features a peculiar spin reorientation transition at TSR ≈ 110 K suggesting a non‐trivial temperature evolution of the magnetic anisotropy (MA)—one of the main contributors to the stabilization of the magnetic order in the low‐dimensional systems. An electron spin resonance (ESR) spectroscopic study reported here provides quantitative insights into the unusual magnetic anisotropy of Fe4GeTe2. At high temperatures the total MA is mostly given by the demagnetization effect with a small contribution of the counteracting intrinsic magnetic anisotropy of an easy‐axis type, whose growth below a characteristic temperature Tshape ≈ 150 K renders the sample seemingly isotropic at TSR. Below one further temperature Td ≈ 50 K the intrinsic MA becomes even more complex. Importantly, all the characteristic temperatures found in the ESR experiment match those observed in transport measurements, suggesting an inherent coupling between magnetic and electronic degrees of freedom in Fe4GeTe2. This finding together with the observed signatures of the intrinsic two‐dimensionality should facilitate optimization routes for the use of Fe4GeTe2 in the magneto‐electronic devices, potentially even in the monolayer limit.

In-situ HRTEM observations of intermediate phase transformation in lattice reorientation in HCP rhenium

In-situ high-resolution transmission electron microscopy (HRTEM) is performed to investigate the deformation behavior of hexagonal close-packed rhenium (Re) which is compressed along the 〈11¯00〉 direction. Atomistic simulations are also conducted to better understand the deformation mechanisms. Two types of lattice reorientation are observed during compression. The first type involves the reorientation of one lattice by ∼90° around 〈112¯0〉, which is accomplished by the formation of an intermediate face-center-cubic (FCC) phase at the interface. This transformation sequence can be described as {11¯00}matrix→{111}FCC→(0001)twin. In the second type, a new grain is formed but does not satisfy any known twin relationship with the matrix, and an intermediate FCC phase is also formed. The transformation sequence can be described as {11¯01}matrix→{111}FCC→(0001)grain. Mechanisms responsible for the observed lattice reorientation and sequential phase transitions are analyzed by conducting lattice correspondence analyses on the simulation results. Strain accommodation is also analyzed to explain the mechanisms for lattice reorientation and the intermediate phase transformations. The results provide new insight into the deformation behavior of HCP metals.