Magnetocaloric properties and microstructure of FeRh-based alloys

The metamagnetic transition from an antiferromagnetic (AF) to the ferromagnetic (FM) state in FeRh alloys and the accompanying magnetocaloric effect (MCE) have been investigated with a particular attention to the sample preparation routes. Direct measurements of the adiabatic temperature change show that the MCE in FeRh remains partly reversible despite the hysteresis and exceeds the effect in the benchmark material Gd by 15 %. The AF−FM transition is strongly affected by the microstructure that is formed depending on the heat treatment parameters. This can explain the discrepancy in the reported data over 80 years of research. The effect on the magnetic properties is found to originate from the interaction of the major α'-phase with the secondary γ-phase that has been typically ignored for its negligible magnetic contribution. The nominal composition of the magnetic α'-phase is found to differ from the actual one for binary and substituted FeRh alloys. The elements can be redistributed within the two phases in such a way, that the actual amount of the doping element in the α'-phase that experiences the AF−FM transition is greatly reduced. This demonstrates the significance of microstructural studies, especially when comparing experimental results with theoretical calculations and developing routes to tune and optimize the magnetocaloric properties of materials

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.Fil: Komlev, Aleksei S.. Lomonosov Moscow State University; RusiaFil: Karpenkov, Dmitriy Y.. National University of Science and Technology; Rusia. Lomonosov Moscow State University; RusiaFil: Gimaev, Radel R.. Lomonosov Moscow State University; RusiaFil: Chirkova, Alisa. Institute for Materials Science; AlemaniaFil: Akiyama, Ayaka. Hirosaki University; JapónFil: Miyanaga, Takafumi. Hirosaki University; JapónFil: Hupalo, Marcio Ferreira. Universidade Estadual do Ponta Grossa; BrasilFil: Aguiar, D.J.M.. Universidade Federal do Paraná; BrasilFil: Carvalho, Alexandre Magnus G.. Universidade Estadual de Maringá; Brasil. Universidade Federal de Sao Paulo; BrasilFil: Jiménez, María Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Cabeza, Gabriela Fernanda. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Zverev, Vladimir I.. Lomonosov Moscow State University; RusiaFil: Perov, Nikolai S.. Lomonosov Moscow State University; Rusi

Influence of Structural Disorder on the Magnetic Order in FeRhCr Alloys

Magnetic phase transitions in alloys are highly influenced by the sample preparation techniques. In the present research, electronic and magnetic properties of Fe48Cr3Rh49 alloys with varying cooling rates were studied, both experimentally and theoretically. The degree of crystalline ordering was found to depend on the cooling rate employed after annealing the alloy. Modeling of alloy structures with different degrees of crystalline ordering was carried out via strategic selection of substitution positions and distances between chromium atoms. Theoretical calculations revealed significant changes in magnetic and electronic properties of the alloy with different substitutions. A comprehensive analysis of the calculated and experimental data established correlations between structural characteristics and parameters governing the magnetic phase transition. In this study, we also developed a method for evaluating the magnetic properties of the alloys obtained under different heat treatments. The proposed approach integrates atom substitution and heat treatment parameters, offering precise control over alloy manufacturing to effectively tune their essential magnetic properties

Magnetocaloric properties and microstructure of FeRh-based alloys

The metamagnetic transition from an antiferromagnetic (AF) to the ferromagnetic (FM) state in FeRh alloys and the accompanying magnetocaloric effect (MCE) have been investigated with a particular attention to the sample preparation routes. Direct measurements of the adiabatic temperature change show that the MCE in FeRh remains partly reversible despite the hysteresis and exceeds the effect in the benchmark material Gd by 15 %. The AF−FM transition is strongly affected by the microstructure that is formed depending on the heat treatment parameters. This can explain the discrepancy in the reported data over 80 years of research. The effect on the magnetic properties is found to originate from the interaction of the major α'-phase with the secondary γ-phase that has been typically ignored for its negligible magnetic contribution. The nominal composition of the magnetic α'-phase is found to differ from the actual one for binary and substituted FeRh alloys. The elements can be redistributed within the two phases in such a way, that the actual amount of the doping element in the α'-phase that experiences the AF−FM transition is greatly reduced. This demonstrates the significance of microstructural studies, especially when comparing experimental results with theoretical calculations and developing routes to tune and optimize the magnetocaloric properties of materials

Electronic entropy change in Ni-doped FeRh

The net entropy change corresponding to the free charge carriers in a Ni-doped FeRh bulk polycrystal was experimentally evaluated in a single sample using low-temperature heat capacity experiments with applied magnetic field and using Seebeck effect and Hall coefficient measurements at high temperatures across the first-order phase transition. From the heat capacity data, a value for the electronic entropy change dSel=8.9 J/(kg K) was extracted. The analysis of the Seebeck coefficient allows tracing the change of the electronic entropy jump with applied magnetic field directly across the transition. The difference in electronic entropy contribution obtained is as high as 10% from 0.1 to 6 T

Simultaneous Multi-Property Probing During Magneto-Structural Phase Transitions: An Element-Specific and Macroscopic Hysteresis Characterization at ID12 of the ESRF

We present a new instrument for advanced magnetic studies based on the high field X-ray magnetic circular dichroism (XMCD) end-station developed at the beamline ID12 of the European Synchrotron Radiation Facility (ESRF, Grenoble, France). It offers a unique possibility to measure simultaneously element-specific and macroscopic properties related to magnetic, electronic, and structural degrees of freedom of magnetic materials. Under strictly the same experimental conditions, one can measure the XMCD response, macroscopic magnetization, volume changes, and caloric properties of a magnetic material as a function of magnetic field (up to 17 T) and temperature (5–325 K). To illustrate the performance of this new instrument, we present a case study of an equiatomic FeRh alloy across the first-order magneto-structural transition. This development is the first step toward a new fully dedicated end-station based on a 7 T split-coil superconducting magnet with an additional capability to perform X-ray diffraction experiments

Martensitic Phase Transformation in Short-Range Ordered Fe50Rh50 System Induced by Thermal Stress and Mechanical Deformation

Metallic/intermetallic materials with BCC structures hold an intrinsic instability due to phonon softening along [110] direction, causing BCC to lower-symmetry phases transformation when the BCC structures are thermally or mechanically stressed. Fe50Rh50 binary system is one of the exceptional BCC structures (ordered-B2) that has not been yet showing such transformation upon application of thermal stress, although mechanical deformation results in B2 to disordered FCC (γ) and L10 phases transformation. Here, a comprehensive transmission electron microscopy (TEM) study is conducted on thermally-stressed samples of Fe50Rh50 induced by quenching in water and liquid nitrogen from 1150°C and 1250°C. We demonstrated that samples quenched from 1150°C into water and liquid nitrogen show the presence of 1/4{110} and 1/2{110} satellite reflections, the latter of which is expected from phonon dispersion curves obtained by density functional theory calculation. Therefore, it is proposed that Fe50Rh50 maintains the B2 structure that is in premartensite state. Once Fe50Rh50 is quenched from 1250°C into liquid nitrogen, formation of two short-range ordered tetragonal phases with various c/a ratios (∼1.15 and 1.4) is observed in line with phases formed from mechanically deformed (30%) sample. According to our observations, an accurate atomistic shear model ({110}) is presented that describes the martensitic transformation of B2 to these tetragonal phases