Magnetic and Magnetocaloric Properties of LaFe } (Si } Al ) Alloys and Their Hydrides

The influence of aluminum on the magnetic and magnetocaloric properties of LaFe 13-x Si x alloys is investigated. For this purpose the magnetic phase diagrams of three quasi-ternary alloy series of LaFe 13-x (Si 1-w Al w ) x with x = 1.14, 1.43, and 1.71 are presented. Samples with high iron-content ( x = 1.14) and high aluminum-content show antiferromagnetic (AFM) ordering which changes over to ferromagnetic (FM) ordering as the aluminum-content is decreased. For intermediate Si- and Al-contents samples show both low temperature FM ordering and higher temperature AFM ordering. As the overall iron-content is decreased ( x = 1.43, and 1.71) FM ordering dominates the phase diagram. Upon hydrogenation all samples become ferromagnetic and transition temperatures are increased by about 150 K. A detailed analysis of the hydrogen content shows that aluminum-rich samples take up more hydrogen compared to the silicon-rich samples. This can be linked to larger interstitial voids resulting from the lattice expansion when Si is replaced with Al. The isothermal entropy change of aluminum-substituted alloys is always lower than the one of the silicon-variants at the same iron concentration.