Magnetic Properties,Electrical Properties And Magnetocaloric Effect In Pyrochlore Iridum Oxides

Geometrical frustration is supported in the pyrochlore structure and was found to be origin of many novel phenomena,such as the anomalous Hall effect,superconductivity and spin ice,etc.which has been extensively researched.With strong spin-orbit-coupling?SOC?and low-spin S=1/2 configuration in 5d electrons of tetravalent Ir.Pyrochlore iridates exhibit qualitatively different properties from compounds rely on S=1/2 configuration,which has been an especially appealing system among the pyrochlore oxides due to Giant Magnetoresistance effect,highly conductive magnetic domain walls in a magnetic insulator,metal-insulator transiton and Hall effect etc.The powder neutron diffraction and ?SR experiment have been certified that there is a magnetic transition accompanied by the metal-insulator transition that involves short-range order of Ir⁴⁺,and with temperature decreasing,the long-range order of magnetic moment of rare earth ion.It also has revealed that the ground magnetic structure of R₂Ir₂O₇ was all-in-all-out type.Supposed that there is a change from disorder to short-range order,finally to long-range order,we think there is a big magnetic entropy change based on magnetocaloric effect?MCE?.Magnetic refrigeration has been a hotspot in area of refrigeration because of environmentally-friendly and high-efficiency advantages.But we don't sure if the change of order can lead to a big magnetic entropy change,and we also want to certify whether the pyrochlore iridium can be functional materials.Here,we firstly have systematically investigated the transport properties,magnetism,and heat capacity of R₂Ir₂O₇?R=Nd,Dy and Ho?.Finally we research the magnetocaloric effect of R₂Ir₂O₇?R=Nd,Dy and Ho?.Firstly,according to resitivity versus temperature of the pyrochlore iridates,we are sure that there is a metal-insulator transition in Nd₂Ir₂O₇ at 34 K,and there also were semiconductor-insulator transitions in Dy₂Ir₂O₇ and Ho₂Ir₂O₇ at 125 K,137 K respectively.??T?can expressed in terms of variable range hopping at region of paramagnetism at high temperature for Dy₂Ir₂O₇ and in Ho₂Ir₂O₇,the maximum values of Eg for Nd₂Ir₂O₇,Dy₂Ir₂O₇ and Ho₂Ir₂O₇ are found to be 4 K,10 K,11 K.There also were bifurcates for zero-field-cooling and field-cooling thermal magnetization curves at TMI,and the magnetization rises at low temperature was due to the ferromagnetism.Meanwhile,the Curie-Weiss fitting denotes existence of anti-ferromagnetism.We also measured heat capacity at zero field and 70 k Oe field,a Schottky anomaly occur at low temperature for Nd₂Ir₂O₇,Dy₂Ir₂O₇ and Ho₂Ir₂O₇,it means this system has been long-range order.Secondly,we have calculated the value of magnetic entropy change from isothermal magnetization curves,and heat capacity respectively,both results show that there is a peak value of magnetic entropy change at low temperature due to long-rang order of magnetic moment.the value of adiabatic temperature rise is calculated by magnetic entropy change and heat capacity,although we have got a small value of magnetic entropy change in contrast to the that of alloys,and the adiabatic temperature rise we got as same as the value of alloys.Finally,the origin of magnetic order of R₂Ir₂O₇?R=Nd,Dy and Ho?is change of magnetic structure,and we are sure R₂Ir₂O₇?R=Nd,Dy and Ho?are good functional materials for magnetic refrigeration based on Carnot cycling at low temperature.