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| Author(s): |
Zulmara Virgínia de Carvalho
Total Authors: 1
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| Document type: | Doctoral Thesis |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto de Física (IF/SBI) |
| Defense date: | 2006-03-17 |
| Examining board members: |
Carlos Castilla Becerra;
Artur da Silva Carriço;
Renato de Figueiredo Jardim;
Frederico Cavalcanti Montenegro;
Mario Jose de Oliveira
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| Advisor: | Carlos Castilla Becerra |
| Abstract | |
In the context of low anisotropy, the magnetization measurements to find out the magnetic effects induced by the substitution of Mn+2 by Cu+2 íons in the quaseone-dimensional Heisenberg-like antiferromagnets CsMn1-xCuxA3.2H2O (A = Cl,Br) were made. In the diluted samples of the Br derivative, we observe the appearance of a remanent magnetization (Mr) below TN when they are cooled in a small axial magnetic field applied along the easy axis. This does not occur in the diluted samples of the Cl derivative. The intra-chain exchange both in Cl and Br compounds is antiferromagnetic, however the inter-chain exhange along the easy axis is antiferromagnetic in the chloride compound and ferromagnetic in the bromide. This fact seems to be deterministic in the appearance of the net moments below TN in the bromide. Moreover, the magnetization measurements on single crystals of the sitediluted antiferromagnet A2Fe1-xInxCl5.H2O (A = Cs) were carried out at low magnetic fields (H) applied along the easy axis. The data revealed that a Mr develops below the Néel temperature TN. This Mr(T) is parallel to the easy axis , saturates for H ~ 1 Oe and it increases with decreasing T. It has also temperature dependence as another diluted systems of the same family (A = K, Rb). For all these systems the normalized Mr(t)/Mr(t = 0,3), where t = T/TN is the reduced temperature, is independet of x and follow a universal curve. In the context of high anisotropy, the temperature dependence of the excess magnetization at low and high fields was investigated for the diluted antiferromagnet FexZn1-xF2 (x = 0.72; 0.46 and 0.31) and pure system FeF2 as well. It was found that Mr is either along the easy axis or perpendicular to it. The size of Mr for very low fields (H < 1 Oe) depends on H but it sature for fields of the order of few Oersteds. The expected random field (RF) behaivor is observed when H is applied along the easy axis at higher fields. (AU) | |