Wydarzenia

Seminarium Fizyki Politechniki Wrocławskiej

Magnetic ordering and magnetocaloric effect in structurally disordered melt-spun YCo_2- based compounds

prof. Bogdan Idzikowski

Instytut Fizyki Molekularnej PAN, Poznań

The YCo₂ intermetallic compound is an exchange-enhanced Pauli paramagnet. In fine crystalline form it becomes ferromagnetic in the surface of the grains. Also in amorphous state the magnetic ordering appears. Samples obtained by melt-spinning process exhibit magnetic ordering induced by chemical and topological disorder. Experimentally observed ferrimagnetic ground states for YCo₂ systems with Y atoms substituted by Ti and Nb [1] find theoretical explanation on the basis of the electronic band structure calculations. Site preference energies for the Nb and Ti substitutions in YCo₂ were analyzed by systematic study of YCo₂ structures with point defects. The considered defects are vacancies and Nb and Ti substitutions on Y or Co sites. Calculations show that the Ti and Nb impurities strongly prefer to occupy the Y sites. Magnetocaloric effect (MCE) is a property of all magnetic materials, which in an isothermal process (adiabatic) leads to magnetic entropy change ΔSM (adiabatic temperature change ΔTad) due to variation of applied magnetic field H [1]. MCE is utilized in magnetic refrigeration, which is becoming a new alternative for conventional cooling methods. Our research is focused on the investigation of the MCE in structurally metastable alloys in crystalline states with quenched-in topological and chemical disorder in Y_1-xTb_xCo₂ (0 ≤ x ≤ 1). Maximum value of magnetic entropy changes ΔSMpk = 4.55 J/kgK, full width at the half maximum of entropy changes peak determined from its temperature dependence δTFWHM = 68 K and refrigerant capacity RC is equal to 222 J/kg for TbCo₂ compound in as-quenched state. Rather high value of δTFWHM may be associated with relatively high topological and/or chemical disorder [2], as observed also for Gd-based compounds [3]. At low temperatures, in the range up to ca 30 K, inverse MCE is observed due to the presence of spin glass type behavior specified from χAC(T) for x = 0.4, 0.2, 0 or due to antiferromagnetic coupling.

1. Z. Śniadecki, M. Werwiński, A. Szajek, U.K. Rößler and B. Idzikowski, J. Appl. Phys. 115 (2014) 17E129.
2. N. Pierunek, Z. Śniadecki, J. Marcin, I. Skorvanek and B. Idzikowski, IEEE Trans. Magn. 50 (2014) 2506603.
3. N. Pierunek et al., in preparation.