Magnetic Properties of Dilute Alloys: Equations for Magnetization and Its Structural Fluctuations (original) (raw)
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Physica B: Condensed Matter, 2002
In this work we present a theoretical study of the magnetic behavior of disordered Fe-Al alloys on the basis of a simple-diluted quantum Heisenberg spin model with the assumption that the exchange interaction J depends on the Al concentration q: We calculated the critical temperature and exponents through the mean field renormalization group method. An acceptable fit to the experimental phase diagram for Al concentration in the range 0:30pqp0:45 is obtained. r
Le Journal de Physique Colloques
Rksumk.-Les mesures magnetiques permettent une etude detaillee de la transition du nonmagnetisme au magnetisme d'un alliage. L'Btude d'alliages ou cette transition a lieu pour de faibles concentrations perrnet de deduire un modele simple reliant le comportement d'une impurete aux interactions avec les impuretks voisines. Pour les alliages oh cette transition a lieu dans une gamme plus Blevke de concentration, il n'est plus possible de dkduire un modele de la seule analyse des resultats experimentaux. L'extension du modkle trouve pour les alliages dilues permet une comprehension correcte des principaux phenomknes caracteristiques des alliages concentres (superparamagnetisme de moments gCants notamment). Les effets de polarisation de sites passifs par des sites actifs, mis en evidence sur des alliages ternaires, complktent un modele du comportement des alliages au cours de la transition.
Magnetic properties of Fe0.9-qMn0.1Alq disordered alloys: Theory
instname: Universidad de Antioquia, 2000
By using the free-energy variational method based on the Bogoliubov inequality and a diluted and randombond Ising model with nearest-neighbors interactions, we investigate the magnetic phase diagram and some of the magnetic properties of the disordered Fe 0.9Ϫq Mn 0.1 Al q alloys system. Thus the mean magnetization per site, and hence the average hyperfine magnetic field, are computed. The so-obtained results are compared with room-temperature experimental data obtained by 57 Fe Mössbauer spectroscopy and vibrating sample magnetometry, from which a very good agreement is achieved. Here, the occurrence of a ''critical concentration'' at 40.0 at. % Al, for which the system passes from a ferromagnetically ordered state to a paramagnetic one, is evidenced. The model allows obtaining an estimate of the exchange energy between Fe-Fe pairs. How this energy depends on the Al concentration and the role of the manganese atoms is also presented and discussed.
The magnetic properties of disordered Fe-Al alloy system
Physica A: Statistical …, 2010
Using the effective field theory with a probability distribution technique that accounts for the self-spin correlation functions, the magnetic properties of disordered Fe-Al alloys on the basis of a site-diluted quantum Heisenberg spin model are examined. We calculated the critical temperature and the hysteresis loops for this system. We find a number of characteristic phenomena. In particular, the effect of concentration c of magnetic atoms and the reduced exchange anisotropic parameter η on both the critical temperature and magnetization profiles are clarified.
Effects of Disorder on Ferromagnetism in Diluted Magnetic Semiconductors
Physical Review Letters, 2001
We present results of a numerical mean-field treatment of interacting spins and carriers in doped diluted magnetic semiconductors, which takes into account the positional disorder present in these alloy systems. Within our mean-field approximation, disorder enhances the ferromagnetic transition temperature for metallic densities not too far from the metal-insulator transition. Concurrently, the ferromagnetic phase is found to have very unusual temperature dependence of the magnetization as well as specific heat as a result of disorder. Unusual spin and charge transport is implied.
Fluctuations in the Diamagnetic Response of Disordered Metals
Europhysics Letters (EPL), 1990
We study fluctuations in the diamagnetic response of disordered mesoscopic samples. For weak magnetic fields, we derive a convenient expression for the susceptibility x of any specific sample and show the existence of large anomalous fluctuations in x, in agreement with other recent calculations. For strong magnetic fields, we show that the fluctuations are considerably suppressed.
Ising model for disordered ferromagnetic Fe-Al alloys
Physical Review B, 2000
In this work we present a theoretical study of the magnetic behavior of disordered Fe-Al alloys on the basis of a simple-diluted quantum Heisenberg spin model with the assumption that the exchange interaction J depends on the Al concentration q: We calculated the critical temperature and exponents through the mean field renormalization group method. An acceptable fit to the experimental phase diagram for Al concentration in the range 0:30pqp0:45 is obtained. r
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Solid-Solid Phase Transformations in Inorganic Materials, Pts 1-2, 2011
Vacancy-solute interactions play a crucial role in diffusion-controlled phase transformations, such as ordering or decomposition, which occur in alloys under heat treatment or under irradiation. The knowledge of these interactions is important for predicting long-term behavior of nuclear materials (such as reactor steels and nuclear-waste containers) under irradiation, as well as for advancing our general understanding of kinetic processes in alloys. Using firstprinciples calculations based on density functional theory and employing the locally self-consistent Green's function technique, we develop a database of vacancy-solute interactions in dilute alloys of bcc Fe with 3p (Al, Si, P, S), 3d (Ti -Cu), and 4d (Nb -Ag) elements. Interactions within the first two coordination shells have been computed in the ferromagnetic state as well as in the paramagnetic (disordered local moment) state of the iron matrix. Magnetism is found to have a very strong effect on the vacancy-solute interactions.
Influence of disorder on ferromagnetism in diluted magnetic semiconductors
Physical Review B, 2002
Influence of disorder in the concentration of magnetic Mn 2+ ions on the ferromagnetic phase transition in diluted (III,Mn)V semiconductors is investigated analytically. The regime of small disorder is addressed, and the enhancement of the critical temperature by disorder is found both in the mean field approximation and from the analysis of the zero temperature spin stiffness. Due to disorder, the spin wave fluctuations around the ferromagnetically ordered state acquire a finite mass. At large charge carrier band width, the spin wave mass squared becomes negative, signaling the breakdown of the ferromagnetic ground state.