The mechanisms by which magnetisation changes occur in magnetic materials may be investigated by a variety of hysteresis measurements. During this study both alternating and rotational hysteresis measurements were used to characterise the mechanisms of magnetisation change in a number of permanent magnet materials. Studies of the time dependence of magnetisation, remanent magnetisations and the dependence of the reversible magnetisation on the irreversible magnetisation were undertaken. These studies revealed that in sintered rare-earth iron magnets the magnetisation change is predominately controlled by domain nucleation, with a lesser contribution from domain wall pinning within the boundary regions of the grains. Similar mechanisms control the magnetisation change in the larger grains of melt-quenched rare-earth iron magnets. In the single domain grains of the melt-quenched materials incoherent rotation mechanisms control the changes of magnetisation. Magnetisation change in MnAlC and sintered AlNiCo was found to be controlled by domain wall pinning within the interior of the grains of the materials. Two devices were constructed for the measurement of rotational hysteresis. The first measures the angular acceleration of a sample set spinning in a magnetic field, from which the rotational hysteresis loss may be determined. The second employs rotating search coils to make direct measurements of the component of magnetisation that contributes to rotational hysteresis loss during the rotation of a sample in a field. Both devices were found to produce data consistent with that in the literature and to be useful for the characterisation of rotational hysteresis in permanent magnet materials. A simple model was used to examine the dependence of rotational hysteresis loss on various material parameters. It was found that the value of the rotational hysteresis integral is dependent on interactions and to a lesser extent distributions in anisotropy. This is contrary to assumptions commonly made in the literature but consistent with published experimental data, which has been reinterpreted. Measurements of rotational hysteresis losses in the materials studied were found to be effected by geometric demagnetisation effects. A method by which such data may be corrected for these effects is proposed. Following correction and consideration of the interactions within the materials, the rotational hysteresis data was found to be consistent with the characterisations performed in linearly alternating fields.

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Made available in DSpace on 2014-10-09T12:52:04Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:58:08Z (GMT). No. of bitstreams: 0 / Nesse trabalho estudou-se a influência da adição de 0,5 % at. de zircônio (Zr) e gálio (Ga) nos ímãs permanentes à base de praseodímio (Pr) HD sinterizados obtidos por uma mistura de ligas. As ligas utilizadas foram: Pr12,6Fe68,3Co11,6B6Zr0,5Ga1, Pr16Fe75,5B8Zr0,5 e Pr13Fe80,5B6Zr0,5. A investigação foi realizada a partir das medidas das propriedades magnéticas e da microestrutura. Inicialmente, compararam-se as fases das ligas com as fases obtidas nos ímãs sinterizados. Posteriormente, os ímãs foram submetidos a um ciclo de tratamento térmico (de duas horas a 1000º C com resfriamento rápido até 10 horas, em seguida, de cinco horas nas mesmas condições, até 35 horas). Para o entendimento do papel de cada aditivo na fase magneticamente dura (fase Φ), foram analisadas as mudanças na microestrutura e relacionadas com as mudanças nas propriedades magnéticas. O Ga e Zr atuam como refinadores da fase Φ (matriz). O Ga atua no grão da fase Φ possibilitando a estabilidade da sua forma e melhorias das propriedades magnéticas. Para o ímã Pr14,3Fe71,9Co5,8B7Zr0,5Ga0,5 a evolução das propriedades magnéticas, após 15 h de tratamento térmico, foi: remanência de (1,25±0,02) T para (1,30±0,02) T, coercividade intrínseca de (1,11±0,02) T para (0,87±0,02) T, fator de quadratura de (0,68±0,02) para (0,89±0,02) e produto de energia de (285±5) kJ/m3 para (317±5) kJ/m3. O Zr tem uma atuação dual. Inibe o crescimento desordenado do grão e aumenta a anisotropia, mas, ao alojar-se no contorno de grão, gera domínios reversos prejudicando a coercividade intrínseca. Para o ímã Pr14.5Fe78B7Zr0.5 a evolução das propriedades magnéticas, após 15 h de tratamento térmico, foi: remanência (1,19±0,02) T para (1,25±0,02) T, coercividade de (0,74±0,02) T para (0,94±0,02) T, fator de quadratura de (0,88±0,02) para (0,85±0,02) e produto de energia (258±5) kJ/m para (291±5) kJ/m. Para o ímã Pr3316Fe75.5B8Zr0.5 a evolução das propriedades magnéticas, após 20 h de tratamento térmico, foi: remanência (1,17±0,02) T para (1,24±0,02) T, coercividade de (0,90±0,02) T para (1,22±0,02) T, fator de quadratura de (0,93±0,02) para (0,66±0,02) e produto de energia (258±5) kJ/m3 para (272±5) kJ/m3. / Dissertação (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP