Study On Critical Behavior And Magnetocaloric Effect Of Rare Earth Manganese Oxides

The magnetic properties and magnetocaloric effect of La_0.8Ca_0.2Mn_1-xNi_xO₃(x=0,0.1,0.2,0.3)and La_0.75Sr_0.25Mn_1-xCo_xO₃(x=0,0.1,0.15,0.2)rare earth manganese oxide series polycrystalline samples were investigated.All the samples were prepared by high temperature solid-phase reaction method.The structure of the samples was obtained by X-ray diffraction(XRD),and the magnetic properties of the samples were obtained by a multifunctional physical property measurement system(PPMS).The main findings of this paper are divided into the following two points:(1)The X-ray diffraction patterns of the La_0.8Ca_0.2Mn_1-xNi_xO₃(x=0,0.1,0.2,0.3)perovskite manganese oxide polycrystalline samples indicate that the series of samples have good single-phase properties.With the increase of Ni²⁺ion doping amount,the parameters of unit cell parameters a and c,unit cell volume,Curie temperature(T_C)and magnetic entropy change of the four samples gradually decreased with the Ni²⁺doping amount increased.The partially replacing Mn³⁺with Ni²⁺results in a change in the Mn³⁺/Mn⁴⁺ratio,leading to a double exchange(DE)decrease.In the temperature range of 10-400 K,as the temperature decreases,the magnetic properties of the series of samples change from a paramagnetic(PM)to a paramagnetic(PM)-ferromagnetic(FM)coexistence.The critical behaviors of the samples are all consistent with the Tricritical model.The Arrott curve and the recalibration curve were analyzed.The parent phase samples belonged to the transition stage from the first–order(FOPT)to the second-order phase transition(SOFT),and the phase transitions of the samples after doping were all SOPT.At a applied magnetic field of 7 T,the maximum value of the magnetic entropy changes for series samples are 6.25 J/kg·K,3.59 J/kg·K,3.69 J/kg·K and 2.56J/kg·K,respectively,and the refrigerant capacity(RCP)is 375 J/kg,315.92 J/kg,333J/kg and 291.84J/kg.(2)The unit cell volume of La_0.75Sr_0.25Mn_1-xCo_xO₃(x=0,0.1,0.15,0.2)series polycrystalline samples decrease slightly with the increase of doping content.All samples appear PM–FM transition near the Tc,in the lower temperature region,the zero-field cooling(ZFC)curve and the field cooling(FC)curve of the sample appear bifurcation phenomenon,which is caused by the competition between FM and antiferromagnetic(AFM),and the series samples show possible clusters spin glass.In particular,the sample with x=0.1 has a T_C of 292 K,which has a large magnetic entropy change near room temperature.After the Curie-Weiss fitting of the series samples,the series samples show a possible preformed cluster phase.When the Co doping amount is x=0,the samples fit the 3D-ising model better,the samples with x=0.1 fit the mean-field model better,and the samples with x=0.15 and 0.2 all fit the 3D-heisenberg model best.And the samples are belong to the SOPT.When the applied magnetic field is 7 T,the maximum value of the magnetic entropy change of the series of samples is 5.8J/kg·K,4.96 J/kg·K,4.4 J/kg·K and 4.2 J/kg·K.After calculation,the RCP of La_0.75Sr_0.25Mn_1-xCo_xO₃(x=0,0.1,0.15,0.2)series samples are 382.8 J/kg,396.8 J/kg,431.2 J/kg and 361.2 J/kg,respectively.After doping,the full width half maximum(FWHM)widened and the RCP increased.