Colossal magnetoresistance and the giant magnetocaloric effect in transition metal compounds.

摘要

The magnetic properties of several doped manganites displaying colossal magnetoresistance (CMR) and of Ni-Mn-Ga alloys exhibiting a giant magnetocaloric effect (GME) have been investigated.; An analysis of the magnetic critical behaviour of a single crystal La 0.73Ba0.27MnO3, based on the use of modified Arrott plots, reveals that the 3D Heisenberg model best describes the critical behaviour of both exponent and critical amplitude values. Around 200 K, some 45 K below Tc, a structural phase transition from a high moment/temperature rhombohedral (R) phase to a lower moment/temperature orthorhombic (O) phase is observed in both the ac susceptibility and zero field cooled/field cooled (ZFC/FC) data. Similar studies on single crystal La0.73Ba0.27MnO3 reveal for the first time a phase transition with simultaneous characteristics of both first-order and second-order phase change. These two features are coincident in the field and temperature plane, a previously unreported feature of the magnetic behaviour of manganites.; Detailed studies on a series of polycrystalline (La1-xNd x)0.67Pb0.33MnO3 (0 ≤ x ≤ 1) samples reveal that this system displays a phase transition from a ferromagnetic metal to a paramagnetic insulator for all x, characteristics typical of double exchange systems. Substitution of Nd3+ on the rare-earth site significantly changes the corresponding average A site radius and its variance. The disorder arising from ion size mismatch and site distribution plays a key role. 3D Heisenberg exponents have been extracted in highly doped samples (x = 1, 0.8). Variable range hopping processes has been found predominately in the high-temperature regime, especially at intermediate doping levels, a result which is consistent with a distribution of allowed electronic energy levels arising from both spin and site disorder.; Studies of the magnetocaloric effect were carried out on the ferromagnetic shape memory alloys system---Ni-Mn-Ga. This system typically possesses two types of phase transitions: a first-order structural and magnetic (order-order) phase transition and a higher temperature second-order ferromagnetic to paramagnetic (order-disorder) phase transition. The temperatures of both phase transitions are very sensitive to composition. By a careful compositional tuning, a maximum magnetic entropy change of -20.4 J kg-1K -1 has been produced in a field of 50 kOe in Ni55.2Mn 18.6Ga26.2. This enhanced magnetic entropy change has been traced to the coincidence of first-order/metamagnetic structural transition with a second-order phase transition. The larger magnetocaloric effect and ease of preparation make this system a promising candidate for magnetic refrigeration.