Disordered Magnets Alloys

Researching magnetic properties of disordered alloys is necessary for both the knowledge of phase transformations in alloys and from the perspective of fundamental concerns of magnetism in solids. In fact Disorder in a magnetic system can result in unconventional magnetic structures, for example spin glass, that have somewhat unusual features. In this Thesis, an extremely common technique to researching disordered magnetic alloys from first principles is offered. We have considered Phase transformations and magnetic behavior of crystalline substitutional alloys. This approach is exemplified by calculations of an archetypical spin-glass substance: the CuMn alloy. Initially, a general theoretical framework for the outline of the thermodynamics of disordered magnetic alloys is provided. We have shown that under certain circumstances, an intricate magnetic system can be reduced to an efficient system that contains no magnetic levels of freedom. This significantly makes easier the analysis of phase transformations in magnetic alloys. The model is explained with regards to material-specific interaction parameters. We have demonstrated that interaction parameters could be acquired from the ground-state property of a disordered alloy which are thus computed from first principles by way of extremely accurate up-to-date numerical methods in accordance with the Green’s function method. The interaction parameters can eventually be implemented in thermodynamic Monte-Carlo simulations to generate the atomic and magnetic structures of an alloy. A good example of calculations for the Cu-rich CuMn alloy is offered. We have shown that the atomic and magnetic structure of the alloy acquired by the introduced method agrees perfectly with the outcomes of neutron-scattering experiments for this system.


1 Introduction
2 Disordered alloys
2.1 General description of alloys
2.1.1 Basic definitions and properties
2.1.2 Cluster expansion
2.2 Thermodynamics of a magnetic disordered alloy
3 First-principles calculations of materials properties
3.1 The basic method
3.1.1 The density functional theory approach
3.1.2 Green’s function method
3.1.3 Korringa-Kohn-Rostocker method
3.1.4 Exact muffin-tin orbital method
3.1.5 Coherent potential approximation
3.2 Interatomic interactions
3.2.1 Magnetic force theorem
3.2.2 Generalized perturbation method
3.3 Thermodynamic calculations
3.4 The structure of the CuMn alloy
3.4.1 Interactions in the CuMn alloy
4 Spin glasses
4.1 Theoretical description
4.2 Edward-Anderson model
4.2.1 The replica method
4.2.2 Order parameters
4.3 Critical behavior of finite-size systems
4.4 Advanced Monte-Carlo methods
4.4.1 Heat bath algorithm
4.4.2 Overrelaxation………

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Source: Uppsala University Library

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