Аннотация:Earlier in [1] it was shown that the giant magnetocaloric effect is observed in materials near the temperature of the metamagnetic transition. The interaction between the crystal lattice and the spin subsystem manifests itself in a change in magnetization during the phase transition. Such phase transition is responsible for the remarkable thermal properties of materials. Thus magnetocaloric materials to be used in practice need the temperature of the first-order magnetic-structural phase transition to be determined at thestage of material synthesis, and the temperature hysteresis of this transition to be minimal. At the moment, there is no ab-initio theory that could be used to predict the magnetocaloric properties of the synthesized material. Therefore, the problem of determining the mechanisms that are responsible for the magnetic structural changes in magnetocaloric materials is worth investigating.Fe49Rh51 alloy was chosen as the object of this study, since it reveals a giant negative magnetocaloric effect of -7÷8 K [2,3] in a magnetic field of up to 2 T near room temperature, it has a binary elemental composition, and its crystal structure does not change symmetry during a phase transition. These peculiarities make it possible to construct a simple descriptive model of a first-order magnetic phase transition. Iron-rhodium alloy has been studied for a long time and its static properties are well described [4]. The authors of this work were interested in the relaxation processes that are observed in this alloy and aredescribed only in several papers [5, Ref. in]. Long-term relaxation of magnetization is associated with a change in the crystal lattice parameter and can provide additional information on the dynamics of the phase transition in such systems. In this work, the structural and magnetic properties of bulk Fe49Rh51 alloys were investigated. The samples were synthesized using arc melting and after that they were annealed at 1000 ºC for 48 hours. One of the samples was biphasic. The second phase is paramagnetic and occupies up to 35% of the sample volume (according to EDX data). Elemental analysis was performed with the use of SEM Tescan Vega 3 with EDX. The temperature, field, and time dependencies of magnetization were measured using a Lake Shore 7407 Series vibration magnetometer.In this work, the mechanisms of the ferromagnetic phase growth were considered. It has been analyzedfrom the field, temperature (including hysteresis values determination) and time dependencies (timerelaxation) of magnetization at different field and temperature values. The relaxation curves of magnetization have clearly visible steps. The presence of steps in the relaxation dependence of magnetization for a bulk alloy is associated with the collective effects of the ferromagnetic phase nucleation and its growth on the alloy surface. To confirm this hypothesis, a numerical simulation of the phase growth process was carried out. The experimental and modeling results are in qualitative agreement. Additional comparison of relaxation dependences of the magnetization for single-phase and two-phase samples makes it possible to separate the process of merging of ferromagnetic clusters from the processes of nucleation and growth.
