Influence of hydrogenation on the electronic structure and the itinerant-electron metamagnetic transition in strong magnetocaloric compound La(Fe_(0.88)Si_(0.12))_(13)

摘要

The influence of interstitial hydrogen on the electronic structure and the itinerant-electron metamagnetic (IEM) transition in strong magnetocaloric compound La(Fe_(0.88)Si_(0.12))_(13)H_(1.6) has been investigated by Mossbauer 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 T_C = 330 K, much higher than T_C = 195 K for y - 0.0. The increase of isomer shift δ_(IS) 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 Moessbauer spectra in the paramagnetic state has been observed except for a difference in only δ_(IS), indicating the volume expansion by hydrogenation is isotropic. Accordingly, the significant increase of T_C by hydrogen absorption is attributed to the magnetovolume effect associated with characteristic feature in IEM compounds. A discontinuous change of ferromagnetic moment, AM, around T_C has been observed by Moessbauer spectra, as expected from the magnetization measurement. The value of AM is slightly decreased by increase of T_C 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.