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viii ABSTRACT Study of the Magnetic Properties on Molecular Nanomagnets: Mn 12 Ac and Mn 6 salao Fernanda Farias Advisor: Miguel A. Novak Co-advisor: Monica Bahiana Abstract of Master thesis submitted to Programa de Pós-graduação em Física do Instituto de Física da Universidade Federal do Rio de Janeiro, as a partial fulfillment of the requirements for the degree of Master of Science – M.Sc. – (Physics). This work presents some aspects of the magnetic properties on the molecular nanomagnets Mn 12 Ac and Mn 6 salao. The first one has been studied since the 90s, and is considered an ideal superparamagnet at low temperatures, presenting hysteresis at temperatures below the blocking temperature (T b = 3K), characterized by steps in the magnetization, associated to thermally activated quantum tunneling. Its study is focused on comparisons between the magnetization curves acquired in the SQUID magnetometer and the ones simulated through the Monte Carlo method. The simulated results show a good agreement with the experimental data above T b , for the different directions in which the magnetic field was applied in relation to the easy axis of magnetization, and also point out a path towards reproducing the steps related to quantum tunneling. The Mn 6 compounds, on the other hand, have been gaining attention in the last two years for being the molecular nanomagnet with the highest blocking temperature observed (T b = 4.5K). In its more common form, it presents in the fundamental spin state S = 4, and AC susceptibilities measurements show that this nanomagnet can have its spin state altered according to the changes in the quantity of solvent in the sample, which is related to alterations in the angles formed between the directions of the ion’s positions in the molecule. The possibility to control the spin state of this cluster has not only an academic interest but also a strong technological appeal, as it may contribute to the development of more powerful devices to store information. Keywords: Molecular Magnetism, SQUID Magnetometer, Monte Carlo Simulation. Rio de Janeiro February 2009
Índice 1 Introdução 1 2 Revisão Teórica 4 2.1 Nanomagnetos Moleculares . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1.1 Mn 12 Ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1.2 Família Mn 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2 Magnetização e suscetibilidade DC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.3 Superparamagnetismo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.4 Susceptibilidade AC e processos de relaxação . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.5 Tunelamento Quântico da Magnetização . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2.6 Método Monte Carlo . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.6.1 Implementação do Método . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3 Técnicas Experimentais de Magnetometria 26 3.1 Supercondutividade . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.2 Quantização do fluxo magnético . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.3 Efeito Josephson . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.4 SQUID dc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.5 Transformador de fluxo e bobinas de detecção . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.6 SQUID como uma caixa preta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.7 Magnetômetro e suscetômetro de SQUID . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 3.8 PPMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 4 Resultados e Discussões 38 4.1 Mn 12 Ac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 4.2 Mn 6 salao . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.2.1 Mn 6 PG195/2007 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 4.2.2 Mn 6 PG195 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 4.2.3 Mn 6 PG118 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 5 Conclusões 91
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Page 6 and 7: vi Agradecimentos Aos professores M
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Referências [1] R. L. Carlin, Magn
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REFERÊNCIAS 96 [32] W. H. Press. B
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