Study on the Magnetism of Phase Separation and Magnetocaloric Effect of Rare-earth Manganites
|School||University of Science and Technology of China|
|Course||Condensed Matter Physics|
|Keywords||Manganese oxide Phase separation Exchange bias Interface Spin Magnetocaloric effect|
Manganese oxide perovskite structure has been a focus for researchers, not only because of the kind of material has a wealth of physical connotation, more importantly, in the magnetic storage, magnetic sensor and magnetic refrigeration has broad application prospects. Manganese oxide as a strongly correlated electronic materials, spin, orbital, charge, and lattice degrees of freedom between the existence of strong coupling, resulting in the system showing a the complex electromagnetic phase diagram and charge ordering phase separation of interesting physical phenomena. Directly or indirectly by means of various experiments confirmed the presence of manganese oxide phase separation was also investigated based on the phase separation of the exchange bias effect. As the manganese oxide has a higher saturation magnetization, the smaller coercive force and the Curie temperature of the material points can be adjusted within a very wide temperature range by the type of doping, and thus has a short magnetic refrigeration application value. In this thesis, based on the exchange bias the phase separation La , 1-x Ca x MnO 3 (0.80 ≤ x ≤ 0.95) effect and its impact on the transport properties of the system; La 2/3 Sr 1/3 MnO 3 single crystal in different directions. magnetocaloric effect. In addition, explore La 2/3 Ca 1/3 Mn 1-x Fe x O 3 (x = 0, 0.1, 0.2) magnetic transport properties and electron spin resonance behavior. Specific content is divided into four chapters and summarized as follows: the first chapter of manganese oxide progress, its crystal structure, the main magnetic physical mechanism, La 1-x Ca x MnO 3 and other electromagnetic phase diagram of the system, based on the phase separation of the exchange bias effect, and manganese oxide material magnetocaloric effect progress and electron paramagnetic resonance (EPR) findings. Chapter II study in detail the the La 1-x Ca x the MnO 3 (0.80 ≤ x ≤ 0.95) the structure and magnetic properties of. The structural analysis that are orthogonal to the crystal structure of the sample at room temperature; transition with decreasing temperature the system partially orthogonal structure to a monoclinic structure, coexist in the system in a certain temperature range and doping interval orthorhombic and monoclinic structure. Magnetic measurements showed that the La 1-x the MnO of the Ca x 3 (0.80 ≤ x lt; 0.95) exists in the exchange bias effect, and the system exchange bias field in Ca doping x = 0.90 when reaches the maximum value in the system for (x = 0.95), the exchange bias effect disappeared when the G-type antiferromagnetic spin tilt. The analysis shows that by the C-type antiferromagnetic electronic phase separation as a result of the exchange bias effect and C-type antiferromagnetic spin tilt G-type antiferromagnetic pinning exchange bias effect on the system have contributed to this. Magneto-transport measurements show that the the interface spin system charge transport has an important impact. The third chapter explores the growth of single crystal optical floating zone method, discussed in detail the annealing La 2/3 Sr 1/3 MnO 3 sub > magnetocaloric effect of the single crystal structure and single crystal in different directions. No single crystal after the annealing treatment, there is a certain degree of disordered arrangement in the ac plane, i.e., a, c is formed of two shaft disorder; improve sample after the annealing treatment, the degree of crystallinity within the ac plane, but in the b direction monocrystalline structure structure between dissociated into flaky crystals. The La 2/3 Sr 1/3 the MnO 3 single crystals exhibit relatively large magnetocaloric effect in the low-field and certain to the opposite sex. The measurement results show that the potential applications in high-temperature magnetic refrigeration. The fourth chapter discusses La 2/3 Ca 1/3 Mn 1-x Fe x O 3 (x = 0, 0.1, 0.2) magnetic transport properties and ESR experimental results. Magneto-transport results show that Fe doped destroyed the system double exchange interaction, resulting in the generation of the giant magnetoresistance effect; ESR measurement results show that the presence of the non-uniformity of the magnetic system, mainly due to the Fe 3 sup> antiferromagnetic coupling of Mn ions around the the system antiferromagnetic insulating phase appears.