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Direct measurements of the magnetocaloric effect (MCE) in different materials (Gd, Fe48Rh52, Ni43Mn37.9In12.1Co7 and Ni2.07Co0.09Mn0.84Ga) in alternating magnetic fields with frequencies f ≤ 22 Hz and an amplitude ∆H = 6.2 kOe are carried out. The MCE in Gd shows inconsiderable changes with field frequency. Near paramagnetic-ferromagnetic phase transition in Ni43Mn37.9In12.1Co7 Heusler alloy a slight reduction of MCE with frequency is observed. In weak alternating fields in materials with AFM-FM magneto-structural phase transitions (Fe48Rh52, Ni43Mn37.9In12.1Co7) it is not possible to get a structural contribution to overall MCE because of irreversibility of the transitions in these fields. Near magnetostructural phase transitions the MCE in these alloys has only magnetic contribution, and does not show a significant dependence on the magnetic field frequency. In Ni2.07Co0.09Mn0.84Ga Heusler alloy the MCE vanishes at frequencies about 20 Hz. The obtained results show the increase of frequencies of operating cycles is one of the powerful methods to improve the efficiency of magnetic refrigerators in case of Gd as a refrigerant.

The magnetic properties and 57Fe Mössbauer spectra of the compounds YFe11TiHx (x = 0, 1) were investigated. The magnetocrystalline anisotropy of YFe11Ti and its hydride was studied by analysing the hard and easy magnetization curves of single crystal samples in the temperature range 4.2–300 K. The spontaneous magnetization was determined in wide temperature range 4.2–650 K. It is established that at T = 4.2 K magnetic anisotropy constants K1 and K2 for YFe11TiH1 single crystal reach values of 25.8 K f.u.−1 and 0.24 K f.u.−1, respectively. The Mössbauer effect spectra of YFe11Ti and its hydride were analysed in terms of a model which takes into account the local environment of Fe atoms on three crystallographic sites (8f, 8j and 8i) and an influence of the random distribution of titanium on the 8i site. Upon hydrogenation both the hyperfine fields and the isomer shifts increase. These results are discussed in terms of the hydrogen-induced unit cell expansion and the electron charge transfer from the conduction band onto the H atoms.

An upturn in the high-field magnetization curve of a GdCo 5 single crystal is observed at about 46 T, which is interpreted as the onset of a process of spin reorientation from the ferrimagnetic towards ferromagnetic structure via an intermediate canted phase. This enables us to evaluate the Co-Gd exchange field in GdCo 5 as B TR = 233± 12 T. This value is very close to the total exchange field on Gd, B R = 236± 8 T, known from earlier inelastic neutron experiments. The difference B RR = B R − B TR is attributed to the Gd-Gd exchange interaction. Its smallness provides solid experimental justification for neglecting the 4f-4f exchange in 3d-4f hard magnetic materials. A full-potential density-functional calculation of B TR has been carried out using an approach within the fixed-spin-moment (FSM) technique. The calculated value B TR = 258 T is 11% too high, most likely because the FSM formalism disregards orbital magnetic moments.

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