Propriedades magneticas de manganitas, fitas amorfas e filmes finos com anisotropia unidirecional

AUTOR(ES)

Alberto Einstein Pereira de Araujo

DATA DE PUBLICAÇÃO

2002

RESUMO

In this work we will present a study of the magnetic properties of three systems: manganites, amorphous ribbons and bilayers with exchange bias. Whole the magnetization measurements were made in a SQUID magnetometer developed during the period of this thesis. We studied several films and buck manganitas samples. All samples present the effect of the colossal magneto-resistance. Transport properties were fitting using the conduction channels model. The measurements in the Ho-doped sample presented a peculiar behavior with a minimum in the resistance at low temperatures. We also found termoremanent effect, histerese and irreversibility at the resistance. From the magnetization measurements we made phase diagrams with showed spin glass behavior. Low temperatures properties were associated with electronic scatter in magnetic domain walls and mixture of magnetic phases. The minimum in the resistivity was explained which a two-dimensional scattering. Showed a new evidence for a two-dimensional transport in these systems. After we studied samples of amorphous ribbons with high permeability. In this theme, the giant magneto-impedance experiments was extended up to frequencies in the microwaves range, and shown its equivalence with the ferromagnetic resonance. Finally, we were made a study of the magnetic properties in films composed by two layers, one ferromagnetic layer and other anti-ferromagnetic. These films present exchange bias with the cycle moved from zero field in the hysteresis measurements. The results demonstrated effects of magnetic disorder with both spin glass and random field properties. For the first time those behaviors were found in same system. The results were compared with simulations using medium field formalism, applied to bi-layers model with roughness. Then we obtained definitive evidence that the roughness in the interface is source of magnetic disorder in those systems

ASSUNTO(S)

fisica exchange bias magnetismo multi-camadas squid cmr magnetos desordenados

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