Magnetoresistances and magnetic entropy changes associated with negative lattice expansions in NaZn 13 -type compounds LaFeCoSi

Magnetoresistances and magnetic entropy changes in NaZn 13 -type compounds La(Fe 1− x Co x ) 11.9 Si 1.1 ( x =0.04, 0.06 and 0.08) with Curie temperatures of 243 K, 274 K and 301 K, respectively, are studied. The ferromagnetic ordering is accompanied by a negative lattice expansion. Large magnetic entropy changes in a wide temperature range from ~230 K to ~320 K are achieved. Raising Co content increases the Curie temperature but weakens the magnetovolume effect, thereby causing a decrease in magnetic entropy change. These materials exhibit a metallic character below T C , whereas the electrical resistance decreases abruptly and then recovers the metal-like behaviour above T C . Application of a magnetic field retains the transitions via increasing the ferromagnetic ordering temperature. An isothermal increase in magnetic field leads to an increase in electrical resistance at temperatures near but above T C , which is a consequence of the field-induced metamagnetic transition from a paramagnetic state to a ferromagnetic state.