Magnetic and Magnetothermal Properties of Hydrogenated Materials Based on Rare Earths and Iron

Iron-rich rare-earth compounds are the basis for high-energy permanent magnets. These include the R(Fe,T)₁₂, R₂Fe₁₇, and R₂FeB intermetallics (R = rare earth; T = Ti, V, Mo, Si) that readily absorb hydrogen, which changes their fundamental and functional characteristics. In this letter, we investigate the stability of the magnetic properties of some hydrides with maximum hydrogen content (namely, TmFe₁₁TiH₁ and Tm₂Fe₁₇H_5.5) in a wide temperature range using high magnetic fields. The magnetic phase transition from the ferrimagnetic to the ferromagnetic state was studied immediately after hydrides were obtained, as well as after one year of storage of the samples at ambient temperature. Only the TmFe₁₁TiH hydride is stable over time. The effect of hydrogen on the magnetothermal properties of the Nd₂Fe₁₄B and Nd₁Pr₁Fe₁₄B compounds and a range of magnetic phase transitions were also investigated. Hydrogenation leads to a decrease in the magnetocaloric effect in all investigated compounds as a result of an increase in the distance between magnetically active ions due to the lattice expansion. A magnetic phase diagram is constructed.