Antiferromagnetic-ferromagnetic Phase Transition Research Articles

Magnetic-field-controlled growth of magnetoelastic phase domains in FeRh

Magnetic phase transition materials are relevant building blocks for developing green technologies such as magnetocaloric devices for solid-state refrigeration. Their integration into applications requires a good understanding and controllability of their properties at the micro- and nanoscale. Here, we present an optical microscopy study of the phase domains in FeRh across its antiferromagnetic–ferromagnetic phase transition. By tracking the phase-dependent optical reflectivity, we establish that phase domains have typical sizes of a few microns for relatively thick epitaxial films (200 nm), thus enabling visualization of domain nucleation, growth, and percolation processes in great detail. Phase domain growth preferentially occurs along the principal crystallographic axes of FeRh, which is a consequence of the elastic adaptation to both the substrate-induced stress and laterally heterogeneous strain distributions arising from the different unit cell volumes of the two coexisting phases. Furthermore, we demonstrate a magnetic-field-controlled directional growth of phase domains during both heating and cooling, which is predominantly linked to the local effect of magnetic dipolar fields created by the alignment of magnetic moments in the emerging (disappearing) FM phase fraction during heating (cooling). These findings highlight the importance of the magnetoelastic character of phase domains for enabling the local control of micro- and nanoscale phase separation patterns using magnetic fields or elastic stresses.

Towards shaping picosecond strain pulses via magnetostrictive transducers

Using time-resolved x-ray diffraction, we demonstrate the manipulation of the picosecond strain response of a metallic heterostructure consisting of a dysprosium (Dy) transducer and a niobium (Nb) detection layer by an external magnetic field. We utilize the first-order ferromagnetic–antiferromagnetic phase transition of the Dy layer, which provides an additional large contractive stress upon laser excitation compared to its zero-field response. This enhances the laser-induced contraction of the transducer and changes the shape of the picosecond strain pulses driven in Dy and detected within the buried Nb layer. Based on our experiment with rare-earth metals we discuss required properties for functional transducers, which may allow for novel field-control of the emitted picosecond strain pulses.

Layer-independent ferromagnetic insulators in a new structural phase of Cr2S3

Ferromagnetic insulators (FMIs) are crucial for sensing and storage, but they are currently rare. The magnetism of the widely studied ${\mathrm{CrI}}_{3}$ and ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ is layer dependent; thus, obtaining FMIs from these materials is challenging because the film thickness must be carefully controlled. Based on the generalized gradient approximation (GGA), $\mathrm{GGA}+U$, and HSE06 hybrid functional calculations, this work predicts a new structural phase of ${\mathrm{Cr}}_{2}{\mathrm{S}}_{3}$ $(\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Cr}}_{2}{\mathrm{S}}_{3})$ and further predicts it as an intrinsic and layer-independent FMI. The material is dynamically metastable. It crystallizes a rhombohedral lattice, stacking the ${\mathrm{Cr}}_{2}{\mathrm{S}}_{3}$ quintuple layers (QLs) along the [111] direction through van der Waals interactions. The in-plane lattice constant is 3.4 \AA{}, and each Cr atom carries a magnetic moment of $3.0{\ensuremath{\mu}}_{B}$. $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Cr}}_{2}{\mathrm{S}}_{3}$ behaves as an intrinsic FMI, irrespective of the layer number, but its indirect energy gap can be tuned from 1.23 to 1.97 eV by changing the film thickness according to the HSE06 calculations. In-plane compressive strain facilitates the ferromagnetism and decreases the energy gap, while tensile strain leads to antiferromagnetism and decreases the energy gap. In the 1QL case, 0.5% in-plane tensile strain causes the ferromagnetic-antiferromagnetic phase transition. The origin of the ferromagnetism and the strain-induced magnetic transition of $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Cr}}_{2}{\mathrm{S}}_{3}$ are discussed and attributed to the dominant ferromagnetic part of the superexchange.

Enhanced Performance of ΔTad upon Frequent Alternating Magnetic Fields in FeRh Alloys by Introducing Second Phases.

The cyclability and frequency dependence of the adiabatic temperature change (ΔTad) under an alternating magnetic field (AMF) are significantly important from the viewpoint of refrigeration application. Our studies demonstrated, by direct measurements, that the cyclability and low-magnetic-field performance of ΔTad in FeRh alloys can be largely enhanced by introducing second phases. The ΔTad under a 1.8 T, 0.13 Hz AMF is reduced by 14%, which is much better than that (40-50%) of monophase FeRh previously reported. More importantly, the introduction of second phases enables the antiferromagnetic-ferromagnetic phase transition to be driven by a lower magnetic field. Thus, ΔTad is significantly enhanced under a 0.62 T, 1 Hz AMF, and its value is 70% larger than that of monophase FeRh previously reported. Although frequency dependence of ΔTad occurs, the specific cooling power largely increases by 11 times from 0.17 to 1.9 W/g, as the frequency increases from 1 to 18.4 Hz under an AMF of 0.62 T. Our analysis of the phase transition dynamics based on magnetic relaxation measurements indicates that the activation energy barrier is lowered owing to the existence of second phases in FeRh alloys, which should be responsible for the reduction of the driving field. This work provides an effective way to enhance the cyclability and low-magnetic-field performance of ΔTad under an AMF in FeRh alloys by introducing second phases.

Phase-transition-induced giant Thomson effect for thermoelectric cooling

The Seebeck and Peltier effects have been widely studied and used in various thermoelectric technologies, including thermal energy harvesting and solid-state heat pumps. However, basic and applied studies on the Thomson effect, another fundamental thermoelectric effect in conductors, are limited despite the fact that the Thomson effect allows electronic cooling through the application of a temperature gradient bias rather than the construction of junction structures. In this article, we report the observation of a giant Thomson effect that appears owing to magnetic phase transitions. The Thomson coefficient of FeRh-based alloys reaches large values approaching –1000 μV K−1 around room temperature because of the steep temperature dependence of the Seebeck coefficient associated with the antiferromagnetic–ferromagnetic phase transition. The Thomson coefficient is several orders of magnitude larger than the Seebeck coefficient of the alloys. Using the active thermography technique, we demonstrate that the Thomson cooling can be much larger than Joule heating in the same material even in a nearly steady state. The operation temperature of the giant Thomson effect in the FeRh-based alloys can be tuned over a wide range by applying an external magnetic field or by slightly changing the composition. Our findings provide a new direction in the materials science of thermoelectrics and pave the way for thermal management applications using the Thomson effect.

Bias-dependent tunneling anisotropic magnetoresistance in antiferromagnetic Pd-doped FeRh-based junctions

We report the bias-dependent tunneling anisotropic magnetoresistance (TAMR) in antiferromagnetic α′-Fe(Rh0.98Pd0.02)/MgO/γ-Fe(Rh0.98Pd0.02) junctions. The TAMR effect is driven by the antiferromagnetic-ferromagnetic phase transition of α′-Fe(Rh0.98Pd0.02) and concomitantly large variation of the density of states (DOS) near the Fermi level. It exhibits polarity reversion behavior with increasing bias voltage, i.e., negative and positive polarities for low and high bias voltages, respectively. Such bias-dependent TAMR is comprehended by first-principle calculations, where a crossing point and subsequent magnitude-reversion emerge between the DOS of antiferromagnetic and ferromagnetic phases of α′-Fe(Rh0.98Pd0.02). Harnessing the tunneling behavior by a feasible bias voltage in an antiferromagnet-based junction is a frontier of great promise in antiferromagnet spintronics.

Correlation between magnetic and crystal structural sublattices in palladium-doped FeRh alloys: Analysis of the metamagnetic phase transition driving forces

FeRh alloys doped with the third element exhibit a change in the lattice and magnetic subsystems, which are manifested in antiferromagnetic- ferromagnetic (AFM-FM) first-order phase transition temperature, the shrinkage of the temperate hysteresis under transition, and the reduction of the saturation magnetization. All aforementioned parameters are crucial for practical applications. To control them it is quite important to determine the driving forces of the metamagnetic transition and its origins. In this manuscript ab initio calculations and experimental studies results are presented, which demonstrate the correlation between the structural and magnetic properties of the Fe50Rh50−xPdx alloys. The qualitative analysis of the metamagnetic phase transition driving forces in palladium-doped FeRh alloys was performed to determine their contribution to the evolution of magnetic and lattice subsystems. In addition, the impact of the impurities phases together with its magnetic behavior on the AFM-FM phase transition was considered.

Strain tunable intrinsic ferromagnetic in 2D square CrBr2

Two-dimensional intrinsic magnetic materials with high Curie temperature (Tc) coexisting with 100% spin polarization are highly desirable for realizing promising spintronic devices. In the present work, the intrinsic magnetism of monolayer square CrBr2 is predicted by using first-principles calculations. The monolayer CrBr2 is an intrinsic ferromagnetic half-metal with the half-metallic gap of 1.58 eV. Monte Carlo simulations based on the Heisenberg model estimate Tc as 212 K. Furthermore, the large compressive strain makes CrBr2 undergo ferromagnetic–antiferromagnetic phase transition when the biaxial tensile strain larger than 9.3% leads to the emergence of semiconducting electronic structures. Our results show that the intrinsic half-metal with a high Tc and controllable magnetic properties endow monolayer square CrBr2 as a potential material for spintronic applications.

Prediction of two-dimensional M2As (M = Mn, Fe) with high Curie temperature and large perpendicular magnetic anisotropy

Recently, with the miniaturization of electronic devices, there is an urgent need to use two-dimensional (2D) ferromagnetic materials with outstanding properties (such as high conductivity, better stability, etc.) and high Curie temperature (TC) in spintronic devices. By first-principles calculations based on density functional theory, we predict two new ferromagnetic metal monolayers M2As (M = Mn, Fe) with hight dynamic and thermal stabilities. Caculation results show that M2As monolayers possess high spin polarization ratio (27.3% for Mn2As and 52.6% for Fe2As), obvious out-of-plane magnetic anisotropy and large magnetic perpendicular anisotropy (101 μeV/Mn for Mn2As and 84 μeV/Fe for Fe2As). In addition, we found that the Mn2As monolayer exhibit high temperature ferromagnetism (TC = 450 K); while for the Fe2As monolayer, a high Curie temperature of 251 K was predicted. Moreover, we applied biaxial strain to the material to examine the changes in its magnetic properties and found that under a certain biaxial strain, Mn2As monolayer will have a ferromagnetic-antiferromagnetic phase transition, while Fe2As monolayer may exhibit a higher TC. Our findings render M2As (M = Mn, Fe) monolayers potential candidates for future spintronics and pave the way for exploring novel 2D magnetism in arsenic-based magnetic materials.

Temperature dependence of the effective Gilbert damping constant of FeRh thin films

Antiferromagnetic (AFM) materials have attracted attention for device applications due to the absence of the stray field and high-frequency response. To integrate AFM materials into magnetic devices, the understanding of the interfacial effect between AFM and ferromagnetic (FM) materials is required. In particular, magnetization dynamics and magnetic damping are critical phenomena to be elucidated since they govern magnetization switching, spin-wave propagation, etc. Although a conventional method for studying the interfacial effects is stacking materials, the approach may cause unfavorable factors. To get insight into the dynamic properties at the AFM and FM interfaces, we have focused on B2-ordered FeRh, showing the first-order phase transition from the AFM to FM states, since the coexistence of AFM and FM domains occurs during transitions, which is an ideal platform for studying interfacial effects. For this study, we have studied ferromagnetic resonance (FMR) of FeRh thin films during the AFM–FM phase transition as a function of temperature. From the FMR measurements, we characterize the temperature dependence of the effective Gilbert damping constant αeff. We find that αeff decreases with increasing temperature, indicating that the temperature variation of the effective Gilbert damping constant originates from the exchange interaction between the AFM and FM domains in the film and/or AFM domains as a spin sink.

Direct and Inverse Magnetocaloric Effects in FeRh Alloy: A Theoretical Study

There are two types of magnetocaloric effects that are the so-called direct and inverse effects originating from ferromagnetic-paramagnetic (FM-PM) and antiferromagnetic--ferromagnetic (AFM-FM) phase transitions, respectively. Here, the magnetocaloric effects of FeRh alloy, which exhibit both the direct and inverse effects, have been studied by combining first-principles calculations and Monte-Carlo simulations. The magnetic exchange coupling constants in the AFM and FM phases are evaluated by first-principles calculations based on the Liechtenstein formula. The Monte-Carlo calculations considering the two-phase mixtures of AFM and FM well reproduce the experimental magnetization curves at zero external magnetic field. It is also shown that the isothermal magnetic entropy changes near the AFM-FM transition temperature under magnetic fields are successfully reproduced based on the Maxwell relation. The entropy changes in a wide range of the magnetic fields near the AFM-FM and FM-PM transition temperatures are investigated and the direct and inverse magnetocaloric effects of FeRh are discussed. The giant relative cooling power of FeRh alloy is achieved due to the large saturation magnetizations and the first-order AFM-FM phase transition.

Onsager reaction field theory applied to the phase diagram of Heisenberg chain with ferromagnetic long-range interaction and antiferromagnetic nearest-neighbor interaction

Onsager reaction field theory is used to investigate the one-dimensional ferromagnetic long-range interacting spin chain with the antiferromagnetic nearest-neighbor interaction (NNI) [Formula: see text]. The ferromagnetic long-range interactions considered in this paper decay as [Formula: see text] with the distance [Formula: see text] between lattice sites. It is found that both the zero temperature and finite-temperature phase diagrams of the system are strongly affected by the interplay between ferromagnetic long-range and antiferromagnetic NNIs. The critical temperature and the uniform susceptibility are obtained as a function of [Formula: see text] and [Formula: see text]. At finite temperatures and in the region [Formula: see text] in which [Formula: see text] is dependent of [Formula: see text], the ferromagnetic-paramagnetic phase transition survives for [Formula: see text] and no phase transition exists for [Formula: see text]. At [Formula: see text], the ferromagnetic-antiferromagnetic phase transition happens at zero temperature for [Formula: see text]. The ground state of the system keeps ferromagnetic when [Formula: see text]. But for [Formula: see text], the system becomes antiferromagnetic at all temperatures.

Specific heat, electrical resistivity, and magnetocaloric study of phase transition in Fe48Rh52 alloy

In this paper, we focus on understanding the magnetic field and temperature dependences of the heat capacity CP, electrical resistivity ρ, and magnetocaloric effect ΔTad in Fe48Rh52 alloy near room temperatures. The phase diagram is constructed according to the CP(H, T) and ρ(H, T) data, and the field shift of the critical temperature is found to be 9.6 K/T. The experimental results on the heat capacity do not confirm the existing assumption about the electronic nature of the antiferromagnet–ferromagnet phase transition in the FeRh alloy. An increase in resistance through the ferromagnetic–antiferromagnetic phase transition is explained by a sharp decrease of the carrier density and simultaneously the appearance of an additional conducting channel. The adiabatic temperature change ΔTad at a field change of 1.8 T equals −9.8 K, and the maximum value of the entropy change ΔS estimated from CP(H, T) and ΔTad(H, T) data is equal to 12.8 J/kg K. The differences in the magnetocaloric effect values in the heating and cooling run in weak magnetic fields are explained based on the thermal expansion data. An almost reversible degradation of the magnetocaloric effect was discovered upon the continuous application of a cyclic magnetic field at temperatures near the magnetostructural transition.

Thickness-dependent magnetic order and phase transition in V5S8**Project supported by the National Natural Science Foundation of China (Grant Nos. 51922011 and 61888102), the National Key Research & Development Project of China (Grant Nos. 2016YFA0202300 and 2019YFA0308500), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant Nos. XDB30000000 and XDB28000000). A portion of the research was performed in

V5S8 is an ideal candidate to explore the magnetism at the two-dimensional (2D) limit. A recent experiment has shown that the V5S8 thin films exhibit an antiferromagnetic (AFM) to ferromagnetic (FM) phase transition with reducing thickness. Here, for the first time, using density functional theory calculations, we report the antiferromagnetic order of bulk V5S8, which is consistent with the previous experiments. The specific antiferromagnetic order is reproduced when Ueff = 2 eV is applied on the intercalated vanadium atoms within LDA. We find that the origin of the magnetic ordering is from superexchange interaction. We also investigate the thickness-dependent magnetic order in V5S8 thin films. It is found that there is an antiferromagnetic to ferromagnetic phase transition when V5S8 is thinned down to 2.2 nm. The main magnetic moments of the antiferromagnetic and ferromagnetic states of the thin films are located on the interlayered vanadium atoms, which is the same as that in the bulk. Meanwhile, the strain in the thin films also influences the AFM–FM phase transition. Our results not only reveal the magnetic order and origin in bulk V5S8 and thin films, but also provide a set of parameters which can be used in future calculations.

Phase boundary exchange coupling in the mixed magnetic phase regime of a Pd-doped FeRh epilayer

Spin-wave resonance measurements were performed in the mixed magnetic phase regime of a Pd-doped FeRh epilayer that appears as the first-order ferromagnetic-antiferromagnetic phase transition takes place. It is seen that the measured value of the exchange stiffness is suppressed throughout the measurement range when compared to the expected value of the fully ferromagnetic regime, extracted via the independent means of a measurement of the Curie point, for only slight changes in the ferromagnetic volume fraction. This behavior is attributed to the influence of the antiferromagnetic phase: inspired by previous experiments that show ferromagnetism to be most persistent at the surfaces and interfaces of FeRh thin films, we modelled the antiferromagnetic phase as forming a thin layer in the middle of the epilayer through which the two ferromagnetic layers are coupled up to a certain critical thickness. The development of this exchange stiffness is then consistent with that expected from the development of an exchange coupling across the magnetic phase boundary, as a consequence of a thickness dependent phase transition taking place in the antiferromagnetic regions and is supported by complimentary computer simulations of atomistic spin-dynamics. The development of the Gilbert damping parameter extracted from the ferromagnetic resonance investigations is consistent with this picture.

Spin-flop coupling induced large coercivity enhancement in Fe/FeRh/W(110) bilayers across ferromagnetic–antiferromagnetic phase transition of FeRh alloy

We report on the large coercivity enhancement in epitaxial Fe/FeRh bilayer grown on W(110). It is shown that the coercive field increases by almost an order of magnitude when the FeRh alloy undergoes a phase transition from the ferromagnetic to antiferromagnetic state. Analysis of temperature spin structure evolution of the FeRh alloy across the magnetic transition reveals that the observed coercivity enhancement is caused by the interfacial spin-flop coupling between Fe and the antiferromagnetic FeRh layer.

Perpendicular magnetic anisotropy and residual magnetic phases in gold-capped FeRh film on MgO(0 0 1)

We studied the magnetic properties of 10 and 20 nm-thick, gold-capped FeRh layers grown on MgO(0 0 1). Whereas the majority of the FeRh film undergoes ferromagnetic–antiferromagnetic phase transition upon cooling, a small fraction of the FeRh system remains in the ferromagnetic state at low temperatures. This residual ferromagnetic phase located at the FeRh/MgO interface exhibits perpendicular magnetic anisotropy. Moreover, in the 10 nm-thick film, a small amount of the antiferromagnetic phase survives to high temperatures above the phase transition and probably originates from the epitaxial strains generated at the FeRh/Au interface.

Magnetic phase transition and magnetoelectric coupling in FeRh/PZT film composite

Abstract The Fe48Rh52 magnetic film with a thickness of 50 nm was grown on a piezoelectric substrate by magnetron sputtering. The chemical composition and crystalline structure were examined by energy dispersive X-ray spectroscopy and X-ray diffraction, respectively. Two types of structural phases were detected: a bcc phase chemically ordered to the CsCl-type (B2 or α′) and an fcc γ-phase. Magnetic measurements of the FeRh film helped to observe an antiferromagnetic-ferromagnetic phase transition around 305 K (B = 0.1 T) in heating with a 27 K hysteresis, which is broader than in bulk alloys. A maximum of direct and converse magnetoelectric coefficients around the temperature of the antiferromagnetic-ferromagnetic transition was found during magnetoelectric measurements; they correlate with the results of magnetic measurements.

Modification of the order of the magnetic phase transition in cobaltites without changing their crystal space group

It has been found that GdBa1-xSrxCo2O6-δ can exhibit consecutive magnetic transitions: antiferromagnetic-ferromagnetic (AFM-FM) transition followed by ferromagnetic-paramagnetic transition (FM-PM), which give rise to a coexistence of inverse and conventional magnetocaloric effect (MCE), respectively. In the pristine compound (x = 0), its AFM-FM transition is shown to belong to a first-order phase transition and the FM-PM to a second-order type. Despite it is widely known that the properties of cobaltites are highly influenced by their oxygen content and type of doping carriers, in this work, further evaluation using magnetocaloric analysis (universal curve method and a quantitative criterion using magnetic field dependence of the magnetic entropy change) reveals that the first-order AFM-FM phase transition converts into a second-order character with just Sr doping of x = 0.1 (despite of having the same space group at room temperature and type of dopant carrier as x = 0), severely affecting its thermomagnetic properties. Moreover, the peaks of these two MCE span over a temperature range that is larger than those reported for cobaltite-type materials, making it closer to room temperature applications.

Switching of Co Magnetization Driven by Antiferromagnetic-Ferromagnetic Phase Transition of FeRh Alloy in Co/FeRh Bilayers

We show that Co spins in Co/FeRh epitaxial bilayers grown on W(110) switch reversibly between the two orthogonal in-plane directions as the FeRh layer undergoes temperature driven antiferromagnetic-ferromagnetic (AFM-FM) phase transition. Switching of Co magnetization is characterized by a hysteretic behavior owing to a temperature hysteresis of the AFM-FM transition in FeRh. The spin reorientation of Co is driven by the evolution interfacial exchange coupling to FeRh system across the AFM-FM process. Our results provide a new method of writing information purely by a local temperature change.