Magnetoresistances and magnetic entropy changes associated with negative lattice expansions in NaZn₁₃-type compounds LaFeCoSi

Magnetoresistances and magnetic entropy changes in NaZn₁₃-type compounds La(Fe_1-xCo_x)_11.9Si_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 gnetovolume 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.