Normal and inverse magnetocaloric effects in La_(0:5)Ca _(0:5)Mn_(1-x)Ni_xO_3

摘要

We have investigated magnetic and magnetocaloric properties of La _(0.5)Ca_(0.5)Mn_(1-x)Ni_xO_3 (x=0, 0.02, 0.04, 0.06, 0.08). It is shown that charge-ordered antiferromagnetic ground state of x=0 is destabilized and ferromagnetism is induced by just 2 Ni substitution. The ferromagnetic Curie temperature (TC) decreases from TC=220K for x=0 to 85K (x=0.08). Unusual field-induced metamagnetic transition is found above TC for x=0.02-0.06 and even below TC in the parent compound (x=0). Magnetic entropy change (ΔSm) was estimated from isothermal magnetization data and it is found that the parent compound (x=0) exhibits both normal (negative ΔSm) and inverse (positive ΔSm) magnetocaloric effects at TC and TN (Neel temperature), respectively. The ΔSm=+6.5Jkg-1K-1 at TN is twice larger than that at TC(ΔSm=-3Jkg-1K-1) for a field change (ΔH) of 5T. However, all the Ni doped samples in La_(0.5)Ca _(0.5)Mn_(1-x)Ni_xO_3 system show only normal magnetocaloric effect at TC. 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.