Normal and inverse magnetocaloric effects in La0.5Ca0.5Mn1−xNixO3

We have investigated magnetic and magnetocaloric properties of La0.5Ca0.5Mn1−xNixO3 (x=0x=0, 0.02, 0.04, 0.06, & 0.08). It is shown that charge-ordered antiferromagnetic ground state of x=0x=0 is destabilized and ferromagnetism is induced by just 2% Ni substitution. The ferromagnetic Curie temperature (TC)(TC) decreases from TC=220K for x=0x=0 to 85 K (x=0.08)(x=0.08). Unusual field-induced metamagnetic transition is found above TCTC for x=0.02–0.06x=0.02–0.06 and even below TCTC in the parent compound (x=0)(x=0). Magnetic entropy change (ΔSm)(ΔSm) was estimated from isothermal magnetization data and it is found that the parent compound (x=0)(x=0) exhibits both normal (negative ΔSmΔSm) and inverse (positive ΔSmΔSm) magnetocaloric effects at TCTC and TNTN (Neel temperature), respectively. The ΔSm=+6.5Jkg−1K−1 at TNTN is twice larger than that at TC(ΔSm=−3Jkg−1K−1) for a field change (ΔH)(ΔH) of 5 T. However, all the Ni doped samples in La0.5Ca0.5Mn1−xNixO3 system show only normal magnetocaloric effect at TCTC. The largest MCE in the Ni doped series occurs for x=0.04(ΔSm=−3.9Jkg−1K−1,ΔH=5T) which also has the largest relative cooling power (RCP=235J/kg,ΔH=5T) in the series. We discuss our results in the scenario of phase separation induced by Ni substitution.