Compositional control of magnetocaloric performance and energy losses in LaFe11.5−( x + y )Co x Ni y Si1.5 alloys

Alloys of La(Fe,Si)13 with transition metal substitutions are promising magnetocaloric materials. Here, we systematically investigate Co and Ni substitution in LaFe11.5−(x+y)CoxNiySi1.5 to tune the magnetic and structural properties and assess the resulting impacts on magnetocaloric performance. Co substitution produces a strong, nearly linear increase in the Curie temperature (TC), reaching 360 K at x = 1.5, while largely preserving refrigerant capacity (RC). In contrast, Ni substitution suppresses the abrupt first-order character of the transition more strongly than Co substitution, significantly reducing isothermal entropy change (ΔSM) and RC, but also lowering lattice strain and magnetic hysteretic energy losses. Combining Co and Ni produces an intermediate regime that illustrates the trade-offs between elevated TC, reduced magnetostructural losses, and peak magnetocaloric response. We map the compositional dependence of TC, ΔSM, RC, phase constitution, magnetic hysteresis, and field-induced lattice strain, revealing the distinct and complementary roles of Co and Ni. These results establish a tunable design space for La(Fe,Si)13-based alloys.