The scientists of the MSU Faculty of Physics (Department of General Physics and Magneto-Ordered Matter, and Low Temperatures Physics and Superconductivity Department) in collaboration with colleagues from the universities of Hirosaki (Japan) and Warwick (UK) have studied the peculiarities of the magnetocaloric effect in alloys based on Fe-Rh in antiferromagnetic – ferromagnetic phase transitions and have developed a theoretical model explaining them.
The magnetocaloric effect (MCE) is manifested in the adiabatic change of temperature ΔT and the isothermal change of entropy ΔSM of the magnet material when the value of the external magnetic field changes. The materials having high values of MCE are very promising in the technologies of magnet freezing and medicine. Besides being applicable in particular areas of technology, magnetocaloric materials present a theoretical challenge in studying the mechanism of magnet phase transitions.
Among the materials currently known, Fe-Rh alloys reach the greatest ΔT (in compositions close to equiatomic: 50- 50) in the area of magnet phase transition of the first kind, antiferromagnetic (AFM) – ferromagnetic (FM). Using alloys in magnet refrigerators is limited by the high cost of rhodium, whereas medical requirements are met more easily in this respect, as phase transitions occur in Fe-Rh alloys at temperatures close to those of the human body, and can be controlled with a high degree of accuracy by adding platinum or palladium.
Together with scientists from Hirosaki University (Japan) and Warwick University (UK), the physicists of the Faculty of Physics experimentally showed the impact of small defects in the crystal structure of the Fe-Rh alloy on the MCE and the temperature of the phase transition AFM – FM. The effect of the terminal temperature failing to return to the initial one was discovered as a result of numerous repeated direct measurements of the MCE in the dynamic mode with the magnetic field being constantly altered after a full cycle of changing the external magnetic field. The experimental results were theoretically explained using the model in which part of the rhodium atoms (nearly 1%) were replaced by ferrum atoms and vice versa. The model applied implies a strong impact of the replacement value upon the MCE, and also upon the temperature of the phase transition in the Fe-Rh alloy. As a result of these experiments, a double decrease of the MCE was found in alloys based on Fe-Rh in the second and following cycles of changing the magnetic field. The peculiarities detected can influence the technological application of these alloys and require being analyzed in detail in future research.
The results of the research are published in the article: V. I. Zverev, A. M. Saletsky, R. R. Gimaev, A. M. Tishin, T. Miyanaga, and J. B. Staunton. “Influence of structural defects on the magnetocaloric effect in the vicinity of the first order magnetic transition in Fe50.4Rh49.6.” Applied Physics Letters, 108(19): 192405 (2016).
