FeRh Alloy Research Articles

Reliable thermodynamic estimators for screening caloric materials

Reversible, diffusionless, first-order solid-solid phase transitions accompanied by caloric effects are critical for applications in the solid-state cooling and heat-pumping devices. Accelerated discovery of caloric materials requires reliable but faster estimators for predictions and high-throughput screening of system-specific dominant caloric contributions. We assess reliability of the computational methods that provide thermodynamic properties in relevant solid phases at or near a phase transition. We test the methods using the well-studied B2 FeRh alloy as a “fruit fly” in such a materials genome discovery, as it exhibits a metamagnetic transition which generates multicaloric (magneto-, elasto-, and baro-caloric) responses. For lattice entropy contributions, we find that the commonly-used linear-response and small-displacement phonon methods are invalid near instabilities that arise from the anharmonicity of atomic potentials, and we offer a more reliable and precise method for calculating lattice entropy at a fixed temperature. Then, we apply a set of reliable methods and estimators to the metamagnetic transition in FeRh (predicted 346±12 K, observed 353±1 K) and calculate the associated caloric properties, such as isothermal entropy and isentropic temperature changes.

Atomic‐Scale Observation of the Metal–Promoter Interaction in Rh‐Based Syngas‐Upgrading Catalysts

AbstractThe direct conversion of syngas to ethanol, typically using promoted Rh catalysts, is a cornerstone reaction in CO2 utilization and hydrogen storage technologies. A rational catalyst development requires a detailed structural understanding of the activated catalyst and the role of promoters in driving chemoselectivity. Herein, we report a comprehensive atomic‐scale study of metal–promoter interactions in silica‐supported Rh, Rh–Mn, and Rh–Mn–Fe catalysts by aberration‐corrected (AC) TEM. While the catalytic reaction leads to the formation of a Rh carbide phase in the Rh–Mn/SiO2 catalyst, the addition of Fe results in the formation of bimetallic Rh–Fe alloys, which further improves the selectivity and prevents the carbide formation. In all promoted catalysts, Mn is present as an oxide decorating the metal particles. Based on the atomic insight obtained, structural and electronic modifications induced by promoters are revealed and a basis for refined theoretical models is provided.

Atomic-Scale Observation of the Metal-Promoter Interaction in Rh-Based Syngas-Upgrading Catalysts.

The direct conversion of syngas to ethanol, typically using promoted Rh catalysts, is a cornerstone reaction in CO2 utilization and hydrogen storage technologies. A rational catalyst development requires a detailed structural understanding of the activated catalyst and the role of promoters in driving chemoselectivity. Herein, we report a comprehensive atomic-scale study of metal-promoter interactions in silica-supported Rh, Rh-Mn, and Rh-Mn-Fe catalysts by aberration-corrected (AC) TEM. While the catalytic reaction leads to the formation of a Rh carbide phase in the Rh-Mn/SiO2 catalyst, the addition of Fe results in the formation of bimetallic Rh-Fe alloys, which further improves the selectivity and prevents the carbide formation. In all promoted catalysts, Mn is present as an oxide decorating the metal particles. Based on the atomic insight obtained, structural and electronic modifications induced by promoters are revealed and a basis for refined theoretical models is provided.

Application of the exchange-striction model for the calculation of the FeRh alloys magnetic properties

Application of the exchange-striction model for the calculation of the magnetic properties of antiferro (AFM) - and ferromagnetic (FM) phases of the iron-rhodium alloy is proposed. The following properties of the antiferromagnetic phase have been calculated and compared with the experimental data: the temperature dependences of the sublattice magnetization, the relative change in volume and entropy, and the pressure dependence of the Néel temperature. For the ferromagnetic phase, the temperature dependence of the magnetization, the dependence of the volume and entropy on temperature, and the dependence of the Curie temperature on pressure were calculated. A satisfactory agreement between the results of calculations and experiment has been obtained. The reasons for the AFM-FM phase transition in the iron-rhodium alloy are discussed.

Magnetothermodynamic Properties and Anomalous Magnetic Phase Transition in FeRh Nanowires

Thermal properties of submicrometer wires of Pd-doped FeRh alloys are evaluated from measurements of transport properties around the first-order antiferromagnetic to ferromagnetic phase transition. Recently, in a submicrometer wire of FeRh alloy, an asymmetric structure between heating and cooling processes was reported in the vicinity of the phase transition temperature. A system where finite-size effects appear is expected to show different magnetic and thermal properties from those in bulk and thin film. However, thermal or magnetization measurements are difficult to perform on samples with such a tiny volume. Here, we demonstrate a quantitative evaluation of thermal properties in thin films and submicrometer wires of B2-ordered Pd-doped FeRh alloy grown on MgO (001). Resistivity measurements on a series of wires with different widths reveal that the anomaly in the resistance change is enhanced in narrow wires. As the transport properties in submicrometer wires sensitively reflect those magnetic states, the entropy change and the irreversible energy loss during the first-order phase transition have been determined via resistance measurements. We find a larger energy loss in smaller samples where wider hysteresis loops appear in temperature-driven measurements. This method can be a probe to evaluate the finite-size effect induced by a restricted magnetic domain nucleation process during the phase transition.

In Situ Formation of FeRh Nanoalloys for Oxygenate Synthesis

Early and late transition metals are often combined as a strategy to tune the selectivity of catalysts for the conversion of syngas (CO/H2) to C2+ oxygenates, such as ethanol. Here we show how the use of a highly reducible Fe2O3 support for Rh leads to the in situ formation of supported FeRh nanoalloy catalysts that exhibit high selectivity for ethanol synthesis. In situ characterizations by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) reveal the coexistence of iron oxide, iron carbide, metallic iron, and FeRh alloy phases depending on reaction conditions and Rh loading. Structural analysis coupled with catalytic testing indicates that oxygenate formation is correlated to the presence of FeRh alloys, while the iron oxide and carbide phases lead mainly to hydrocarbons. The formation of nanoalloys by in situ reduction of a metal oxide support under working conditions represents a simple approach for the preparation bimetallic catalysts with enhanced catalytic properties.

Phase Coexistence and Kinetic Arrest in the Magnetostructural Transition of the Ordered Alloy FeRh

In materials where two or more ordering degrees of freedom are closely matched in their free energies, coupling between them, or multiferroic behavior can occur. These phenomena can produce a very rich phase behavior, as well as emergent phases that offer useful properties and opportunities to reveal novel phenomena in phase transitions. The ordered alloy FeRh undergoes an antiferromagnetic to ferromagnetic phase transition at ~375 K, which illustrates the interplay between structural and magnetic order mediated by a delicate energy balance between two configurations. We have examined this transition using a combination of high-resolution x-ray structural and magnetic imaging and comprehensive x-ray magnetic circular dichroism spectroscopy. We find that the transition proceeds via a defect-driven domain nucleation and growth mechanism, with significant return point memory in both the structural and magnetic domain configurations. The domains show evidence of inhibited growth after nucleation, resulting in a quasi-2nd order temperature behavior.

Peculiarities of the magnetocaloric effect in FeRh-based alloys in the vicinity of the first order magnetic phase transition

Medical applications of magnetocaloric effect (MCE) require possibility for precision shift of a temperature of the magnetic phase transition at the same MCE value and minimize irreversibility. Thus, detail dynamic MCE investigation of such alloys with non-toxic biocompatible dopants need to be done. In present work, the giant magnetocaloric effect, which is observed in the whole family of Fe-Rh alloys, has been investigated in Pd-doped samples in slowly cycled magnetic fields of up to 1.8 T in magnitude for a range of temperatures, 250 K < T < 350 K. The shift of the ferromagnetic/antiferromagnetic transition temperature down towards room temperature and the decrease in the MCE have been observed in these alloys in comparison with a quasi-equiatomic FeRh alloy. The measurements have also shown an asymmetric behaviour of the first order magnetic phase transition with respect to whether the transition is traversed by heating from lower temperatures or cooling from above. These peculiarities have been explained in the framework of the ab-initio density functional theory-based disordered local moment theory of the MCE. The results have been compared with the those for the non-doped FeRh alloy. Thus features of the first order magnetic phase transition that these alloys have in common have been revealed which enable some predictions to be made appropriate for practical applications.

Spin torque in FeRh alloy measured by spin-torque ferromagnetic resonance

Spin injection into 3d transition metals and their alloys has been investigated intensively in recent years. In this study, we focus on the FeRh alloy, which is often used in the context of antiferromagnetic spintronics. We explore the interaction between the spin current and magnetic moments in FeRh by means of spin-torque ferromagnetic resonance. We obtain a spin Hall efficiency of ξSH = 24 ± 3%, which indicates that the spin current from Pt interacts with the magnetic moments in FeRh.

Modeling of the structural and magnetic properties of Fe-Rh-(Z) (Z = Mn, Pt) alloys by first principles methods

The structural and magnetic properties of Mn- and Pt-doped Fe-Rh alloys are investigated by first principles calculations. The 16-atom supercell (Fe8Rh8-xZx) with different initial spin configurations including ferromagnetic and two types of antiferromagnetic orders are considered. It is shown that in a case of Fe8Rh8-xMnx, the ferromagnetic spin configuration in a cubic cell is energetically favorable compared to other magnetic configurations. The addition of Mn content stimulates the martensitic transformation from ferromagnetic cubic phase to antiferromagnetic tetragonal one. Moreover, an increase in Mn content leads to increase in the energy difference between cubic and tetragonal phases which is equivalent of increase in a temperature of structural transformation. Contrary, for Fe8Rh8-xPtx, an increase in Pt content results to appearance of stable tetragonal state with the checkerboard-like antiferromagnetic configuration.

Novel applications of magnetic materials and technologies for medicine

We analyze the applications of the magnetocaloric effect (MCE) achieved with advanced magnetic materials and technologies. New results such as the use of MCE-based composite material for ‘smart’ coating of implants, which allows for a controlled release of drugs over time. The material is comprised of two layers: the one manifesting a large MCE and the other made of a temperature-responsive polymer containing the drug. The latter is released by the polymer if the temperature induced by magnetic field (due to MCE) is changed. The polymer undergoes sharp phase transition in response to the slight temperature drop of ∼1–3 °C (down to 34–36 °C in the body). Local cooling is achieved with FeRh alloy, one of the most perspective MCE materials with the first order phase transition. It is critically important to take into account several peculiarities of phase transition and MCE behavior in these series of alloys. Recent studies on high-purity samples showed an ‘irreversible’ effect of MCE indicating that the temperature of FeRh does not return to its initial value after the full cycle of the magnetic field during dynamic MCE measurements. A theoretical explanation based on ab initio calculations has been provided. Magnetic hyperthermia in oncology is rapidly developing therapeutic modality. Recent experiments have shown that Mn-Zn ferrite emerges as a cost-effective material for this method compared with currently used magnetite nanoparticles. We present our achievements on magnetically guided capsule endoscopy. The advances in magnetically guided capsule endoscopy are also discussed. We provide the experimental setup for the computer-controlled system of magnetic field sources that create necessary forces to control the position of the capsule in five degrees of freedom in a given area. The system has the feedback on the position and is designed to monitor the endoscopic capsule during gastrointestinal tract examination.

Rh-Fe alloy derived from YRh0.5Fe0.5O3/ZrO2 for higher alcohols synthesis from syngas

Traditional Rh-Fe catalysts usually exhibit high alkane or methanol selectivity due to the existence of mono iron and rhodium in the catalysts, and thus the selectivity to higher alcohols is low. To overcome this problem, in this work, YRh0.5Fe0.5O3 with perovskite structure was loaded on ZrO2 by using the citrate complexing method and used for higher alcohols synthesis. Compared with RhOx/ZrO2, YRhO3/ZrO2 and Rh-FeOx/ZrO2, the prepared YRh0.5Fe0.5O3/ZrO2 exhibited better activity and higher selectivity to higher alcohols, the mass fraction of higher alcohols in the total alcohols is more than 80%, especially ethanol occupied 74%. The high selectivity and activity of YRh0.5Fe0.5O3/ZrO2 are attributed to the formation of the homogeneous and highly dispersed Rh-Fe alloys. YRh0.5Fe0.5O3/ZrO2 also showed very good stability. More importantly, this preparation scheme can be extended for the preparation of other multi-component nano alloy catalysts.

The effect of the microstructure on the antiferromagnetic to ferromagnetic transition in FeRh alloys

A detailed study of Fe48Rh52 samples heat treated at different temperatures (1123 K, 1273 K and 1423 K) has been performed using scanning and transmission electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction and magnetic measurements. The microstructure of all samples consisted of α′ (B2) and γ (A1) phases. Although the chemical composition, chemical ordering and the lattice parameter of the magnetic α′ phase in all three samples did not vary with heat treatment temperature, the temperature of the metamagnetic transition between the antiferromagnetic (AFM) and ferromagnetic (FM) states varied by 50 K. No gradients in chemical composition on a length scale larger than approximately 10 nm exist at the α′/γ phase boundaries. Due to the 1% volume expansion of the α′ phase, which occurs during the AFM-FM transition, stress fields around the α′/γ phase boundaries are expected. The composition, lattice parameter, size, shape and distribution of the γ phase was shown to change with heat treatment temperature. Finite element models indicated that these microstructural parameters influence the magnitude and extent of the stress fields at the phase boundaries. The variation in the transition temperature is therefore ascribed to the differences in the stress fields.

First-principles calculations on elastic and entropy properties in FeRh alloys

First-principles calculations were employed to investigate elastic and entropy properties of B2 ferromagnetic and antiferromagnetic FeRh phases. We calculated lattice parameters, elastic constants, isotropic moduli like bulk modulus B, shear modulus G, Young’s modulus E, and Poisson’s ratio of two phases and compared these with other calculations and experiments. At finite temperatures, the values of vibrational (from both Debye model and phonon dispersion) and thermal electronic contributions to the total entropy change between the two phases are calculated about -50J/kg/K and 7.8J/kg/K comparable to the experimental results (−33±9J/kg/K and 8±1J/kg/K).

First principles study of the structural and magnetic properties of Fe(Rh, Pd) and Fe(Rh, Ni) alloys

In this work, the structural and magnetic properties of Ni- and Pd-doped Fe-Rh alloys are investigated by using the density functional theory calculations. The ab initio calculations have been carried out by using the 16-atom supercell approach with different initial spin configurations. It was shown that the checkerboard-like antiferromagnetic configuration of Fe-Rh is more energetically favorable compared with other configurations. The addition of third element into Fe-Rh system slightly changes the optimized lattice parameter and stimulates the martensitic phase transformation.

Epitaxial strain-engineered self-assembly of magnetic nanostructures in FeRh thin films

In this paper we introduce an innovative bottom–up approach for engineering self-assembled magnetic nanostructures using epitaxial strain-induced twinning and phase separation. X-ray diffraction, 57Fe Mössbauer spectroscopy, scanning tunneling microscopy, and transmission electron microscopy show that epitaxial films of a near-equiatomic FeRh alloy respond to the applied epitaxial strain by laterally splitting into two structural phases on the nanometer length scale. Most importantly, these two structural phases differ with respect to their magnetic properties, one being paramagnetic and the other ferromagnetic, thus leading to the formation of a patterned magnetic nanostructure. It is argued that the phase separation directly results from the different strain-dependence of the total energy of the two competing phases. This straightforward relation directly enables further tailoring and optimization of the nanostructures’ properties.

Cubic chemically ordered FeRh and FeCo nanomagnets prepared by mass-selected low-energy cluster-beam deposition: a comparative study.

Near the point of equiatomic composition, both FeRh and FeCo bulk alloys exhibit a CsCl-type (B2) chemically ordered phase that is related to specific magnetic properties, namely a metamagnetic anti-ferromagnetic/ferromagnetic transition near room temperature for FeRh and a huge magnetic moment for the FeCo soft alloy. In this paper, we present the magnetic and structural properties of nanoparticles of less than 5 nm diameter embedded in an inert carbon matrix prepared by mass-selected low-energy cluster-beam deposition technique. We obtained a CsCl-type (B2) chemically ordered phase for annealed nanoalloys. Using different experimental measurements, we show how decreasing the size affects the magnetic properties. FeRh nanoparticles keep the ferromagnetic order at low temperature due to surface relaxation affecting the cell parameter. In the case of FeCo clusters, the environment drastically affects the intrinsic properties of this system by reducing the magnetization in comparison to the bulk.

Magnetism and morphology in faceted B2-ordered FeRh nanoparticles

Whereas the bulk equiatomic FeRh alloy with B2 structure is antiferromagnetic (AFM) below 370 K, we demonstrate that the surface configuration can stabilize the low-temperature ferromagnetic (FM) state in FeRh nanoparticles in the 6–10 nm range. The most stable configuration for FM nanoparticles, predicted through first-principles calculations, is obtained in magnetron sputtering synthesized nanoparticles. The structure, morphology and Rh-(100) surface termination are confirmed by aberration-corrected (scanning) transmission electron microscopy. The FM magnetic state is verified by vibrating sample magnetometry experiments. This combined theoretical and experimental study emphasizes the strong interplay between surface configuration, morphology and magnetic state in magnetic nanoparticles.

Acetonitrile via CO hydrogenation in the presence of NH3

We are presenting the use of an alumina supported FeRh alloy catalyst for the formation of nitrogen containing compounds via the CO hydrogenation in the presence of ammonia. In contrast to previous studies on either similar catalyst systems or on an iron-based catalyst, the prepared FeRh material displays a high selectivity to a single nitrogen-containing compound, acetonitrile. The formation of acetonitrile occurs at the expense of oxygenates, mostly ethanol, which form in the absence of ammonia.

Strain-induced structural instability in FeRh

We perform density functional calculations to investigate the structure of the inter-metallic alloy FeRh under epitaxial strain. Bulk FeRh exhibits a metamagnetic transition from a low-temperature antiferromagnetic (AFM) phase to a ferromagnetic (FM) phase at 350K, and its strain dependence is of interest for tuning the transition temperature to the room-temperature operating conditions of typical memory devices. We find an unusually strong dependence of the structural energetics on the choice of exchange-correlation functional, with the usual local density approximation (LDA) yielding the wrong ground-state structure, and generalized gradient (GGA) extensions being in better agreement with the bulk experimental structure. Using the GGA we show the existence of a metastable face-centered-cubic (fcc)-like AFM structure that is reached from the ground state body-centered-cubic (bcc) AFM structure by following the epitaxial Bain path. We predict that this metastable fcc-like structure has a significantly higher conductivity than the bcc AFM phase. We show that the behavior is well described using non-linear elasticity theory, which captures the softening and eventual sign change of the orthorhombic shear modulus under compressive strain, consistent with this structural instability. Finally, we predict the existence of an additional unit-cell-doubling lattice instability, which should be observable at low temperature.