The interplay between the lattice and magnetism in La(Fe_11.4Al_1.6)C_0.02 studied by powder neutron diffraction

The crystallographic structure and magnetic properties of La(Fe_11.4Al_1.6)C_0.02 are studied by magnetic measurement and powder neutron diffraction with temperature and applied magnetic field. Rietveld refinement shows that La(Fe_11.4Al_1.6)C_0.02 crystallizes into the cubic NaZn₁₃-type with two different Fe sites: Fe^I (8b) and Fe^II (96i), and that Al atoms preferentially occupy the Fe^II site. A ferromagnetic state can be induced at a medial temperature of 39 K-139 K by an external magnetic field of 0.7 T, and a large lattice is correspondingly found at 100 K and 0.7 T. In all other conditions, La(Fe_11.4Al_1.6)C_0.02 has no net magnetization in the paramagnetic (T>T_N=182 K) or antiferromagnetic states, and thus keeps its small lattice. Analysis of the Fe-Fe bond length indicates that the ferromagnetic state prefers longer Fe-Fe distances.