Research Of Magnetocaloric Effect On A And B Site Substitution In The Perovskite Manganites

Rare earth oxides of the perovskite structure system due to their strongcoupling between the intrinsic spin, lattice, orbital and charge degrees of freedomshows unique optical, electrical, magnetic transport properties, excellentmagnetoresistance effect, catalytic performance and magnetocaloric effect, whichhave been widely used in many fields such as, magnetic refrigeration, heated at hightemperature materials, solid resistance and oxidation-reduction catalysis. In recentyears, because of the rapid development of room temperature magnetic refrigerationmaterials, the perovskite manginates have been widely concern due to their obviousadvantages: relative large magnetic entropy change in low magnetic field, chemicalstability, the variable Curie temperature, high resistivity, low cost, light weight,non-toxic and easy miniaturization. However, there are also some defects inPerovskite manginates, such as magnetic entropy compared with the other alloys orcompounds is still small and magnetic properties intensely affected by substitutions.In this thesis we have addressed detailed discussion on material structure, magneticproperties and magnetocaloric effect of A and B site substitution. Seek and explorethe method and mechanism of enhancement of magnetic entropy change in the lowmagnetic field, meanwhile, tailor the Curie temperature of the materials, so that theycan become the ideal room temperature magnetic refrigeration materials.The main study includes:（1） In this paper, based on substrate of Pr_0.5Sr_0.5MnO₃, the monovalentdoping on the A site of Pr_0.5Sr_0.5-x（Li, Na, K）xMnO₃（0≤x≤0.3） series ofsamples were prepared by standard solid-state sinter method. The change ofphase transition, the Curie temperature and the magnetic entropy change ofthe samples were researched; With increase of the monovalent elementcontent, the samples undergoing a second order phase transition changes to afirst order phase transition, long-range ferromagnetic order were enhanced,the highest Curie temperature was around room temperature; Using Landautheory discussed that the magnetic entropy change of the material undergoing second order phase transition was the result of rearrangement of spinmagnetic moment parallel to the external magnetic field; the effect of different concentrations of substitution on exchange interaction of thematerial and different elements on the magnetic entropy change of thematerials were also studied. Maximum magnetic entropy changes in the lowmagnetic field were found to be-2.64J kg^-1K^-1,-2.70J kg^-1K^-1and-3.37Jkg^-1K^-1for Li⁺, Na⁺and K⁺ions for x=0.3, respectively. At the same time,with small hysteresis the sample could be a potential ideal magneticrefrigerant.（2） The B-site substitution of La_0.7Sr_0.3Mn_1-xCo_xO₃（x=0,0.05and0.1）and La_0.7Ca_0.3Mn_1-xNi_xO₃（x=0,0.05and0.1） perovskite manganites wereprepared by standard solid-state sinter method. The effect of B-sitesubstitution on the structure, magnetic properties and magnetocaloric effectin perovskite manganites were studied. With increasing doping content, thecell volume decreases, phase transition spread a wider temperature and theCurie temperature rapidly changed. The Co ions take part in super exchangeinteraction leading to coexistence of double exchange interaction and superexchange interaction. The competition between mixed exchange interactionresult in breakdown of long range ferromagnetic order and enhancement ofshort range of ferromagnetic order.From the comparisons of effective magnetic moment between theoreticalcalculations by Curie-Weiss law and experimental data, the results shown that,with a small amount of doped Co ions, Co³⁺was in a favor, as the dopingcontent increased, Co4+content gradually increased and antiferromagneticinteraction was enhanced. The B site substitution could directly affectferromagnetic double exchange interaction of manganites, so we couldachieve the purpose of tailoring material Curie temperature.