FeRh Alloy Research Articles

Ta cap-induced stabilization of interfacial ferromagnetism and enhanced magnetoelectricity in ultrathin FeRh films

We report ab initio electronic structure calculations to study the effect of Ta cap on the magnetic properties and the magnetoelectric response of Ta/FeRh/MgO nanojunctions. The calculations reveal that the Ta cap reverses the magnetic phase stability in ultrathin FeRh films leading to the stabilization of the ferromagnetic phase and interfacial reconstructed ferromagnetism. We demonstrate that the Ta cap induces a large charge transfer which in turn reduces dramatically the magnetic moments of the interfacial Fe atoms regardless the magnetic configuration and enhances the in-plane (out-of-plane) magnetization orientation of the antiferromagnetic (AFM) (ferromagnetic (FM)) phase compared to the uncapped bilayer. The Ta cap modifies substantially and enhances the magnetoelectric response under an external electric field, where the voltage controlled magnetic anisotropy (VCMA) changes from ∨-shape in the AFM to linear behavior in the FM phase with large VCMA efficiency. These findings demonstrate the manipulation of magnetic ordering of FeRh films with heavy metal capping and electric field which can promote the application of FeRh alloy in MeRAM.

Magnetoelastic transition and magnetocaloric effect in induction melted Fe100−xRhx bulk alloys with x = 50, 51

Magnetoelastic transitions (METs) in bulk in nearly equiatomic Fe-Rh alloys produced by arc melting may show poor reproducibility related to insufficient chemical homogeneity and presence of impurity phases in variable concentrations. To better understand the synthesis conditions that reliably yield bulk FeRh materials with reproducible MET characteristics, Fe100-xRhx alloys with x = 50, 50.5 and 51 at. % were prepared by induction melting and thermal annealing under identical conditions. The fabricated samples were cut into several slices, followed by characterization of METs in each of the slices using isothermal and isofield magnetization measurements, differential scanning calorimetry, and direct measurements of the magnetocaloric effect. All of the slices exhibit METs between the AFM and FM states, but the transitions are abrupt with nearly the same change of magnetization, ΔM, when x = 50.5 and 51, whereas for the x = 50 alloy the transition spreads over a wide temperature interval and ΔM may fluctuate by as much as 10 % from one specimen to another. A comparison of the magnetocaloric responses of x = 50 and 51 materials is presented. The clearly different effect of the magnetic field on the transition in both directions leads to significant differences in the reversibility and maximum values of the magnetic field-induced entropy and adiabatic temperature changes, as well as average hysteresis losses. In terms of reproducibility, our results suggest that induction melting is a more appropriate technique to prepare these binary alloys.

Phase domain boundary motion and memristance in gradient-doped FeRh nanopillars induced by spin injection

The B2-ordered alloy FeRh shows a metamagnetic phase transition, transforming from antiferromagnetic to ferromagnetic order at a temperature Tt∼380 K in bulk. In addition to temperature, the phase transition can be triggered by many means such as strain, chemical doping, or magnetic or electric fields. Its first-order nature means that phase coexistence is possible. Here, we show that a phase boundary in a 300-nm-diameter nanopillar, controlled by a doping gradient during film growth, is moved by an electrical current in the direction of electron flow. We attribute this to spin injection from one magnetically ordered phase region into the other driving the phase transition in a region just next to the phase boundary. The associated change in resistance of the nanopillar shows memristive properties, suggesting potential applications as memory cells or artificial synapses in neuromorphic computing schemes.

Effect of strong metal-oxide interaction on low-temperature ethanol reforming over Fe-promoted Rh/SiO2 catalyst

An Fe-promoted Rh/SiO2 catalyst (Fe + Rh/SiO2) was investigated in terms of the strong metal-oxide interaction (SMOI) between Rh metal and Fe oxide (FeOx) and its application to low-temperature ethanol reforming. Fe + Rh/SiO2 was prepared by a conventional co-impregnation method followed by high-temperature calcination in air at 973 K. For low-temperature ethanol reforming, Fe + Rh/SiO2 was subjected to low-temperature reduction with hydrogen at 573 K, and exhibited enhanced dehydrogenation of ethanol. The generation of carbon monoxide and methane byproducts was suppressed by the SMOI between Rh and FeOx, which inhibited acetaldehyde decomposition. Moreover, high-temperature reduction of Fe + Rh/SiO2 with hydrogen at 773 K (HTR) significantly enhanced steam reforming of ethanol due to interaction between the Fe-Rh alloy and FeOx. These results will lead to efficient utilization of rare and active precious metals by the addition of cheap iron oxide, and promise efficient production of hydrogen and acetaldehyde as basic chemicals from bioethanol.

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.

Влияние амплитуды СВЧ-воздействия на спиновый ток границы платина/железоиттриевый гранат

A numerical study of the features of the propagation of spin waves in a waveguide made of yttrium iron garnet (YIG) and Fe-Rh alloy in the form of a plate, located on top of the central part of the YIG, is carried out. Based on the simulation results, the possibilities of controlling the dynamics of spin waves in the structure under study were also revealed. Micromagnetic numerical simulation was used to study the transfer of a spin-wave signal in a multimode mode by numerically solving the Landau – Lifshitz – Hilbert equation. Transformation of the transmission spectra of spin waves shows that the proposed structure will make it possible to control the propagation of spin-wave modes due to a sharp change in the Fe-Rh magnetization in the region of the magnetic phase transition temperature close to room temperature. In addition, the spin wave signal can be controlled by a small temperature change in the Fe-Rh plate generated by the laser radiation. The two-layer structure of YIG / Fe-Rh, from an applied point of view, can be used as a functional unit in planar magnon networks performing space-frequency demultiplexing and filtering of spin-wave modes.

Модовая фильтрация поверхностных магнитостатических волн в YIG/FeRh

A numerical study of the features of the propagation of spin waves in a waveguide made of yttrium-ron garnet (YIG) and Fe-Rh alloy in the form of a plate, located on top of the central part of the YIG, has been carried out. Based on the simulation results, the possibilities of controlling the dynamics of spin waves in the structure under study were also revealed. Micromagnetic numerical simulation was used to study the transfer of a spin-wave signal in a multimode mode by numerically solving the Landau – Lifshitz – Hilbert equation. Transformation of the transmission spectra of spin waves shows that the proposed structure will make it possible to control the propagation of spin-wave modes due to a sharp change in the Fe-Rh magnetization in the region of the magnetic phase transition temperature close to room temperature. In addition, the spin wave signal can be controlled by a small temperature change in the Fe-Rh plate generated by laser radiation. The two-layer structure of YIG/Fe-Rh, from an applied point of view, can be used as a functional unit in planar magnon networks performing space-frequency demultiplexing and filtering of spin-wave modes.

Sputter-engineering a first-order magnetic phase transition in sub-15-nm-thick single-crystal FeRh films

Equiatomic FeRh alloys undergo a fascinating first-order metamagnetic phase transition (FOMPT) just above room temperature, which has attracted reinvigorated interest for applications in spintronics. Until now, all attempts to grow nanothin FeRh alloy films have consistently shown that FeRh layers tend to grow in the Volmer-Weber growth mode. Here we show that sputter-grown sub-15-nm-thick FeRh alloy films deposited at low sputter-gas pressure, typically $\ensuremath{\sim}0.1$ Pa, onto (001)-oriented MgO substrates, grow in a peening-induced Frank-van der Merwe growth mode for FeRh film thicknesses above 5 nm, circumventing this major drawback. The bombardment of high-energy sputtered atoms, the atom-peening effect, induces a rebalancing between adsorbate-surface and adsorbate-adsorbate interactions, leading to the formation of a smooth continuous nanothin FeRh film. Chemical order in the films increases with the FeRh thickness, ${t}_{\text{FeRh}}$, and varies monotonically from 0.75 up to 0.9. Specular x-ray diffraction scans around Bragg peaks show Pendell\"osung fringes for films with ${t}_{\text{FeRh}}\ensuremath{\ge}5.2$ nm, which reflects in smooth well-ordered densified single-crystal FeRh layers. The nanothin film's roughness varies from 0.6 down to about 0.1 nm as ${t}_{\text{FeRh}}$ increases, and scales linearly with the integral breadth of the rocking curve, proving its microstructured origin. Magnetometry shows that the FOMPT in the nanothin films is qualitatively similar to that of the bulk alloy, except for the thinnest film of 3.7 nm.

Peculiarities of the phase transformation dynamics in bulk FeRh based alloys from magnetic and structural measurements

We analyze coexistence of antiferromagnetic and ferromagnetic phases in bulk iron-rhodium and its alloys with palladium, Fe50,4Rh49,6, Fe49,7Rh47,4Pd2,9 and Fe48,3Rh46,8Pd4,9, using neutron diffraction, magnetization and scanning Hall probe imaging. Temperature dependencies of the lattice parameters, AFM and FM phase weight fractions, and Fe magnetic moment values were obtained on cooling and heating across the AFM-FM transition. Substantial thermomagnetic hysteresis for the phases’ weight fractions and a relatively narrow one for the unit cell volume has been observed on cooling-heating. A clear dependence of hysteretic behavior on Pd concentration has been traced. Additional direct magnetic measurements of the spatial distribution of the phase transition are acquired using scanning Hall probe microscopy, which reveals the length scale of the phase coexistence and the spatial progression of the transition in the presence of external magnetic field. Also, the magnetic phase diagram has been constructed for a series of Pd-doped FeRh alloys.

Asymmetric magnetic relaxation behavior of domains and domain walls observed through the FeRh first-order metamagnetic phase transition

The phase coexistence present through a first-order phase transition means there will be finite regions between the two phases where the structure of the system will vary from one phase to the other, known as a phase boundary wall. This region is said to play an important but unknown role in the dynamics of the first-order phase transitions. Here, by using both x-ray photon correlation spectroscopy and magnetometry techniques to measure the temporal isothermal development at various points through the thermally activated first-order metamagnetic phase transition present in the near-equiatomic FeRh alloy, we are able to isolate the dynamic behavior of the domain walls in this system. These investigations reveal that relaxation behavior of the domain walls changes when phase coexistence is introduced into the system and that the domain wall dynamics is different to the macroscale behavior. We attribute this to the effect of the exchange coupling between regions of either magnetic phase changing the dynamic properties of domain walls relative to bulk regions of either phase. We also believe this behavior comes from the influence of the phase boundary wall on other magnetic objects in the system.

Electric-field control of spin dynamics during magnetic phase transitions.

Controlling magnetization dynamics is imperative for developing ultrafast spintronics and tunable microwave devices. However, the previous research has demonstrated limited electric-field modulation of the effective magnetic damping, a parameter that governs the magnetization dynamics. Here, we propose an approach to manipulate the damping by using the large damping enhancement induced by the two-magnon scattering and a nonlocal spin relaxation process in which spin currents are resonantly transported from antiferromagnetic domains to ferromagnetic matrix in a mixed-phased metallic alloy FeRh. This damping enhancement in FeRh is sensitive to its fraction of antiferromagnetic and ferromagnetic phases, which can be dynamically tuned by electric fields through a strain-mediated magnetoelectric coupling. In a heterostructure of FeRh and piezoelectric PMN-PT, we demonstrated a more than 120% modulation of the effective damping by electric fields during the antiferromagnetic-to-ferromagnetic phase transition. Our results demonstrate an efficient approach to controlling the magnetization dynamics, thus enabling low-power tunable electronics.

Peculiarities of Magnetic and Magnetocaloric Properties of Fe–Rh Alloys in the Range of Antiferromagnet–Ferromagnet Transition

Abstract—The present work surveys literature data related to the study of iron–rhodium-based (Fe–Rh) alloys. The crystal, magnetic, and electronic properties of the FeRh alloy and FeRh-based materials in the form of both bulk, thin-film, and nano-structured objects are considered. Peculiarities of the first-order antiferromagnet–ferromagnet transition are analyzed, and various explanations of its nature are discussed. Different approaches to the preparation of the iron–rhodium-based alloys are reported; an analysis of the effect of heat treatment conditions on the properties of the material and their reproducibility in measuring the magnetocaloric properties is performed. Causes for the record values of the magnetocaloric effect (MCE) observed for the material are shown, and prospects of the application of this alloy in magnetic refrigeration technology, medicine, electronics, and magnetic data recording technology are discussed.

Inverse magnetocaloric effect in the solids undergoing ferromagnetic – antiferromagnetic phase transition: Landau theory applied to Fe-Rh alloys

In the present article a magnetic solid exhibiting the phase transition from the high-temperature ferromagnetic (FM) to the low-temperature antiferromagnetic (AFM) phase is considered. The Landau theory of phase transitions is used for the analysis of inverse magnetocaloric effect (MCE) observed below the phase transition temperature. The magnetic-field-induced shift of the phase transition temperature has been calculated and the dependence of the phase transition field on the temperature of solid has been computed; the quantitative agreement between the theoretical results and the experimental data obtained in the recent experiments with Fe49Rh51 alloy has been achieved. The magnetic-field-induced entropy change and refrigerant capacity of Fe49Rh51 alloy have been calculated. Reasonable agreement between theoretical results and experimental data has been demonstrated. The dependence of characteristics of MCE on the spin-exchange parameter has been shown. The MCE in the artificial AFM structures has been discussed.

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.

Phase transition and magnetocaloric effect in particulate Fe-Rh alloys

Micro-sized particles of FexRh100−x (x = 50, 48, 45, 40, and 35) binary alloys were synthesized by a solid-phase reduction method and their structural properties, magnetic properties and cytotoxicity have been characterized. From X-ray diffraction results, the Fe50Rh50 alloy is in a single chemically ordered α′ phase while the face-centered cubic disordered γ phase arises and increases with Rh content when x is smaller than 50. The magnetization behavior of the alloys demonstrates that the α′ phase consists of ferromagnetic and antiferromagnetic components due to the nonuniformity of elements in the particulate alloys. The maximal magnetic entropy change and the relative refrigeration capacity of the Fe50Rh50 particles reach 9.7 J/kg K and 230 J/kg, respectively, with a 0–3 T magnetic field change. Despite its moderate magnetic entropy among the first-order transition materials, Fe50Rh50 microparticles are promising for biomedical applications due to its large relative cooling power and low cytotoxicity.

Topography dependence of the metamagnetic phase transition in FeRh thin films

The equiatomic alloy FeRh is of great scientific and technological interest due its highly unusual first-order antiferromagnetic (AF) to ferromagnetic (FM) phase transition. Here we report an exploration of the interplay between topography and phase evolution with a comprehensive magnetic force microscopy study of nominal 50 nm thick FeRh thin films and subtractively patterned wires of width 0.2 µm–2 µm. In continuous films where the surface morphology had not been optimised for smoothness, the topographical variation was observed to dominate the distribution of the magnetic transition temperatures and dictates the nucleation and growth of the magnetic phases. This observation was repeated for patterned elements, where the effects of surface morphology were more significant than those of spatial confinement. These results have clear implications for future studies of low-dimensional FeRh films, as surface topography must be considered when analysing and comparing the transition behaviour of FeRh thin films.

Ab initio study of FeRh multilayers supported on MgO(0 0 1)

Low dimensional films are materials of interest for the changes of the structural, electronic and magnetic properties they undergo, especially when they form interfaces with a substrate. The iron-rhodium (FeRh) alloy is an excellent example. Experimentally, FeRh films are deposited on single crystal substrates like MgO, sapphire or silicon. Particularly, MgO (0 0 1) is an excellent support for FeRh because it is possible to deposit high quality epitaxial films on it. MgO is also highly stable at high temperatures and tensile in-plane strain favors the FM over the AFM state.In order to improve the knowledge of the supported bimetallic systems properties, theoretical calculations using density functional theory (DFT) have been carried out. The different thicknesses considered for the multilayers are 0.6 nm, 0.9 nm and 1.2 nm for both, terminated in Fe or Rh. To complete the study, we present the results obtained on the influence of the termination of the surfaces, the number of alloy layers, the different magnetic configurations (FM – AFM), the charge transfer and the adhesion of the films to the MgO substrate.The analysis of the results shows that as the thickness of the film grows, the adhesion energy tends to an average value of the order of 1.5 J/m2; on the other hand, the AFM coupling facilitates the takeoff compared to the FM coupling. Moreover, comparing the difference between FM and AFM results for different thicknesses, the percentages are slightly lower for.–Rh terminated films.Regarding the out-of-plane relaxation percentages, they depend on the thickness of the film and the magnetic coupling. For the AFM coupling of films -Rh terminated, lower values are observed compared to those obtained for the FM coupling and this behavior is maintained for all three thicknesses studied. However, for the -Fe terminated films as the film grows the relaxations are practically twice those corresponding to the cases of films terminated in Rh tending to tetragonal structures.With respect to magnetic properties, in the three presented systems for -Rh terminated multilayers, the tendency of magnetic moments values for the FM coupling is maintained in around 3.2 μB/at for the case of Fe and 1.0 μB/at for the case of Rh. For AFM the most noticeable difference is the cancellation of the magnetic moments of Rh. However, the situation is different for the Fe atoms in the -Fe terminated multilayers. For 0.9 nm the Fe atoms in contact with the substrate undergo the least relaxation and their magnetic moments are parallel and equal to −1 μB/at. The Fe atoms of the other layers recovered their values of ±3 μB/at. But when the film grows the Rh atoms acquire a small magnetization in increasing order as it approaches to the free surface. The presence of residual ferromagnetism in the interfaces of the FeRh films deposited on MgO (0 0 1) has also been observed experimentally by other groups. In summary, we can conclude that the obtained results are mostly influenced by the surface termination.(-Rh or -Fe).

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.

Ab initio study of FeRh alloy properties

Abstract In this work the structural, energetic, thermodynamic, magnetic and thermoelectric properties of FeRh alloy have been studied using first-principles calculations. The structural and magnetic results obtained for the FM phase are in agreement with experimental and other theoretical calculated values (local magnetization of about 3 μB and 1 μB for Fe and Rh atoms respectively). The transition AFM to the FM state is accompanied by a 2.4% increase in the volume of the cell and by 9.6% in the Fe spin magnetic moment. The characterization of the AFM phase shows a distortion of the cubic and tetragonal cell to the orthorhombic solving the instabilities observed in the phonon band structures curves of the corresponding to cubic structure. The stability of the AFM configurations was corroborated with molecular dynamics (MD) simulations. The magnitude of the Seebeck coefficient increases with the temperature at a certain chemical potential. While the magnitude of the electrical conductivity does not present meaningful changes with the temperature in the selected values, the thermal conductivity increases with the temperature. Another consequence of the AFM to FM transition is the drop of the resistivity ρ in the FM phase as compared to the AFM state. Based on the results obtained using MD simulations from stable AFM configurations and the results of entropy changes, the configuration AFM-B (type I) could be identified as the most favorable in achieving the metamagnetic phase transition.

The exchange bias effect on single layer of Fe-rich FeRh thin film

Thin films of Fe rich FeRh were prepared by using DC magnetron sputtering from a Fe67Rh33 target onto MgO (0 0 1) substrate. In particular, here we identify a unique exchange-bias phenomenon. This is one of the homogenous system to have an exchange bias phenomena without the presence of two layer system with well-defined interface of ferromagnetic (FM)/antiferromagnetic (AFM) layers. Meanwhile, annealing at higher temperatures resulted in ordering of FeRh alloy which in turn gives rise to a higher exchange bias strength. Possible scenario for the exchange bias should have occurred from the boundaries between AFM/FM domains which could be due to the formation of AFM domain inside of FM matrix or vice versa in our single layer thin film.