Specific heat study of the magnetocaloric effect in the Haldane-gap S=1 spin-chain material [Ni(C2H8N2)(2)NO2](BF4)
2017
Abstract
Magnetocaloric effect in the Haldane-gap S = 1 spin-chain [Ni(C2H8N2)(2)NO2](BF4) was investigated at low temperatures (k(B)T << J) and magnetic fields up to nominally 0.1B(sat), where J and B-sat denote an exchange interaction and saturation magnetic field, respectively. Magnetic entropy was found to scale as exp(Delta/k(B)T) in zero magnetic field and T-0.67 for B = B-c, in which the Haldane gap should be closed, in reasonable agreement with the theoretical predictions.
The existence of the inverse magnetocaloric effect for B < B-c was confirmed. The occurrence of normal magnetocaloric response at lowest temperatures and magnetic fields was attributed to S = 1/2 degrees of freedom arising from the ends of chain segments.