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- W2274409429 abstract "This work is concerned with the simulation of magnetovolume effects inthe Fe-Ni and FeRh transition-metal alloysas well as in the Laves-phase compound YMn 2 and the closely relatedpseudobinary compounds Y x Sc 1-x Mn 2 and Y(Mn-Al) 2 .These materials show a strongly reduced (or vanishing) thermalexpansion coefficient ( Invar effect ) combined with an enhancedcompressibility. The opposite effect of a strongly enhanced thermalexpansion ( anti-Invar effect) can also be observed in thesematerials. Ab initio calculations suggest, that these anomalies are caused by the magnetic instability of one of the transition metalatoms. In this work this instability is modeled by a generalized Blume-CapelHamiltonian. The use of a spin-1 Ising model allows for a treatment of the distinct magnetic states of the transition metal atomswithin the framework of a localized spin model.The different magnetic states are represented by the spinstates 0 and ±1, respectively. Unlike quantum mechanical approaches,this model can be easily evaluated at finite temperatures.The coupling between magnetic and spatial degrees of freedom have been taken into account in two different ways.In the case of Fe-Ni and YMn 2 this is achieved by spin dependentpair potentials. In analogy to the 2-γ-states model by Weiss,a change of the spin state is connected with an alteration of theequilibrium lattice constant. In the case of Fe-Rh, themagnetovolume-effects are caused by exchange parameters depending on theinter-atomic distance.The model parameters are determined by comparison withexperimental quantities and the results of ab initio calculations.In the case of Y x Sc 1-x Mn 2 andY(Mn-Al) 2 KKR-CPA calculations were carried out for this purpose.The computation of thermodynamical quantities at finite temperatures wasperformed using the Monte Carlo method. A separate program was developedfor this purpose, which can be used efficiently on massive parallel computers. The calculations show that the most important magnetovolume anomaliesof Fe-Ni alloys - and also of the above mentioned Y-Mn compounds -can be reproduced with the Weiss-like model proposed in this work.The change of the behavior, which shows up when physical pressure is applied or the composition is altered, is reproduced qualitatively.This approach was extended to the simulation of Fe-Ni nanoparticles.The transition from bulk to small clusters changes the magnetic andelastic properties of the material in a peculiar manner.The simulations concerning FeRh demonstrate, that asimple statistical model can explain the occurrence of a temperaturedriven metamagnetic phase transition. The magnetovolume effects and thelarge change of entropy connected with the phase transition are describedby an exchange parameter depending linearly on the interatomic distance." @default.
- W2274409429 created "2016-06-24" @default.
- W2274409429 creator A5043659159 @default.
- W2274409429 date "2003-12-22" @default.
- W2274409429 modified "2023-09-27" @default.
- W2274409429 title "Monte-Simulationen von Magnetovolumeneffekten in Festkörpern und Nanopartikeln" @default.
- W2274409429 hasPublicationYear "2003" @default.
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