Study Of Magnentocaloric Effect In MnAs_1-xP_x Magnetic Refrigerant Materials

In this paper, the MnAs_1-xP_x（x=0.03,0.1） compounds were prepared bymechanical milling plus sintering method and the MnAs_0.97P_0.03thin films wereprepared by means of pulsed laser deposition （PLD）. Crystalline structure, microstrainand microstructure of the samples were characterized by means of X-ray diffraction（XRD） and Scanning Electron Microscopy （SEM）. The magnetic properties weremeasured using a superconducting quantum interference device （SQUID）. Effect ofstructure, particle size and microstress of these compounds on the magnetic phasetransitions, and magnetic-entropy change especially on the hysteresis loss has beensystematically studied.MnAs_0.97P_0.03nanocrystallines, prepared by mechanical milling, exhibit theorthorhombic MnP structure. XRD quantitative analysis shows that the average grainsize of the sample is23nm and there exists a large microstrain of0.68%caused by theball milling process. The microstrain induces a recoverable helimagnetic state at lowtemperatures and suppresses the temperature/field-induced orthorhombic-hexagonalphase transition, which leads to a remarkable reduction of both the thermal and themagnetic hysteresis at the Curie temperature, T_C=258K. Corresponding to thehelimagnetic-ferromagnetic and ferromagnetic-paramagnetic transition, a inversemagnetocaloric effect （MCE） with magnetic entropy change ΔS_mof5.6J/kg·K at208Kand a normal MCE with ΔS_mof4.4J/kg·K at253K for a5T field change are observed.After annealing treatment at850°C, the Curie temperature T_Cof MnAs_0.97P_0.03compound was increased to273K, the compound exhibits a large MCE with ΔS_mof14J/kg·K for a field change from0to5T.The compound MnAs_0.9P_0.1, prepared by mechanical milling plus sintering method,exhibits a multistep magnetic order-order transition from a helimagnetic γ-phase withHα-type magnetic order to a ferromagnetic β-phase at80K and then to a helimagneticα-phase at203K The γ-β transition exhibits the characteristics of a first-order transition with a thermal hysteresis as large as6K, while the β-α transition without obviousmagnetic hysteresis which proved to be a second-order transition by Arrott curve. Withthese two successive helimagnetism-related transitions, magnetic-entropy changes of-2.1J/kg·K at203K for a field change from0to5T and0.1J/kg·K at83K for a fieldchange from0to1T are obtained.The MnAs_0.97P_0.03thin film was grown on single-crystalline silicon （001） subatratesby means of PLD. The XRD patterns show the hexagonal NiAs structure of the thefilms and the crystallinity of the thin film deposited at500°C was obviously improvedcompared with the film deposited at260°C. After annealing at700°C, the Braggdiffraction peak of the thin film obviously shift to low angles, which can be ascribed tothe increase of the particle size. The SEM imges reveal that the crystalline grains of thethin films are relatively homogeneous and the energy spectrum analysis shows thatelement compositions of the thin films are approximately consistent with those of thetarget.