Influence of hydrogenation on the electronic structure and the itinerant-electron metamagnetic transition in strong magnetocaloric compound La(Fe0.88Si0.12)13

The influence of interstitial hydrogen on the electronic structure and the itinerant-electron metamagnetic (IEM) transition in strong magnetocaloric compound La(Fe0.88Si0.12)13H1.6 has been investigated by Mössbauer spectroscopy. A slight change in the average hyperfine field at 4.2 K was observed after hydrogen absorption. In contrast, the thermally induced first-order transition related to the IEM transition for y=1.6 appears at the Curie temperature TC=330 K, much higher than TC=195 K for y=0.0. The increase of isomer shift δIS at 4.2 K indicates that the valence electron transfer from hydrogen to Fe is negligibly small, hence the change in the magnetic state is closely associated with a volume expansion after hydrogen absorption. No change in shape by hydrogenation for the Mössbauer spectra in the paramagnetic state has been observed except for a difference in only δIS, indicating the volume expansion by hydrogenation is isotropic. Accordingly, the significant increase of TC by hydrogen absorption is attributed to the magnetovolume effect associated with characteristic feature in IEM compounds. A discontinuous change of ferromagnetic moment, ΔM, around TC has been observed by Mössbauer spectra, as expected from the magnetization measurement. The value of ΔM is slightly decreased by increase of TC after hydrogenation but its magnitude is almost the same due to the stabilization of ferromagnetic moment. As a result, strong magnetocaloric effect is maintained up to room temperature after hydrogenation.