Critical behavior of ferromagnetic pure and random diluted nanoparticles with competing interactions: variational and Monte Carlo approaches

The magnetic properties and critical behavior of both ferromagnetic pure and metallic nanoparticles having concurrently atomic disorder, dilution and competing interactions, are studied in the framework of an Ising model. We have used both the free energy variational principle based on the Bogoliubov inequality and Monte Carlo simulation. As a case of study for random diluted nanoparticles we have considered the Fe$_{0.5}$Mn$_{0.1}$Al$_{0.4}$ alloy characterized for exhibiting, under bulk conditions, low temperature reentrant spin glass (RSG) behavior and for which experimental and simulation results are available. Our results allow concluding that the variational model is successful in reproducing features of the particle size dependence of the Curie temperature for both pure and random diluted particles. In this last case, low temperature magnetization reduction was consistent with the same type of RSG behavior observed in bulk in accordance with the Almeida-Thouless line at low fields and a linear dependence of the freezing temperature with the reciprocal of the particle diameter was also obtained. Computation of the correlation length critical exponent yielded the values $\nu=0.926\pm 0.004$ via Bogoliubov and$\nu =0.71\pm 0.04$ via Monte Carlo. This fact indicates that even though thermodynamical models can be indeed used in the study of nanostructures and they can reproduce experimental features, special attention must be paid regarding critical behavior. From both approaches, differences in the $\nu$ exponent with respect to the pure Ising model agree with Harris and Fisher arguments.Comment: 11 pages, 11 figures. Submitted to Phys. Rev.

Polar Spinel-Perovskite Interfaces: an atomistic study of Fe3O4(111)/SrTiO3(111) structure and functionality

Atomic resolution scanning transmission electron microscopy and electron energy loss spectroscopy combined with ab initio electronic calculations are used to determine the structure and properties of the Fe3O4(111)/SrTiO3(111) polar interface. The interfacial structure and chemical composition are shown to be atomically sharp and of an octahedral Fe/SrO3 nature. Band alignment across the interface pins the Fermi level in the vicinity of the conduction band of SrTiO3. Density functional theory calculations demonstrate very high spin-polarization of Fe3O4 in the interface vicinity which suggests that this system may be an excellent candidate for spintronic applications

Origin of reduced magnetization and domain formation in small magnetite nanoparticles

The structural, chemical, and magnetic properties of magnetite nanoparticles are compared. Aberration corrected scanning transmission electron microscopy reveals the prevalence of antiphase boundaries in nanoparticles that have significantly reduced magnetization, relative to the bulk. Atomistic magnetic modelling of nanoparticles with and without these defects reveals the origin of the reduced moment. Strong antiferromagnetic interactions across antiphase boundaries support multiple magnetic domains even in particles as small as 12–14 nm

Atomic and electronic structure of twin growth defects in magnetite

We report the existence of a stable twin defect in Fe3O4 thin films. By using aberration corrected scanning transmission electron microscopy and spectroscopy the atomic structure of the twin boundary has been determined. The boundary is confined to the (111) growth plane and it is non-stoichiometric due to a missing Fe octahedral plane. By first principles calculations we show that the local atomic structural configuration of the twin boundary does not change the nature of the superexchange interactions between the two Fe sublattices across the twin grain boundary. Besides decreasing the half-metallic band gap at the boundary the altered atomic stacking at the boundary does not change the overall ferromagnetic (FM) coupling between the grains

Estudio monte carlo del comportamiento magnético e histerético de la magnetita

Utilizando un modelo de Ising-Monte Carlo 3D con interacciones a primeros vecinos magnéticos, estudiamos la evolución térmica de la magnetización y el comportamiento histerético de magnetita Fe3O4 estequiométrica monocristal por encima de la temperatura de Verwey. En el modelo, los io-nes de hierro se distribuyen en una estructura espinela inversa y las interacciones de superinter-cambio, para las cuales proponemos una función de distribución, se consideran antiferromagnéti-cas para enlaces Fe3+A-Fe3+A, Fe3+A-Fe3+B y Fe3+A-Fe2+B y ferromagnéticas para Fe3+B-Fe3+B, Fe3+B-Fe2+B, Fe2+B-Fe2+B, donde los rótulos A y B se refieren a sitios tetraedrales y octaedrales respecti-vamente. Los resultados de la histéresis revelan la existencia de dos campos críticos diferentes en la dinámica de inversión de la magnetización, adscritos a la influencia de las integrales de superin-tercambio, los diferentes números de coordinación en ambas subredes y a la interacción Zeeman

Look at the study and the technological and biomedical applications of magnetite

La investigación orientada al conocimiento de las propiedades magnéticas de los materiales se mantiene como un tema de actualidad y alta importancia en ciencia, tecnología e ingeniería; con mayor razón en las últimas décadas, dado el desarrollo de los materiales nanoestructurados. Con el presente trabajo se pretende ilustrar la importancia de la ferrita de hierro o magnetita (Fe3O4), en los campos de la ingeniería y la nanotecnología. Se examinan algunos de los aspectos más relevantes de las aplicaciones de materiales nanoestructurados a la tecnología moderna, y se revisa con detalle la bibliografía sobre estudios científicos de la magnetita en su aspecto biológico, técnico y teórico-computacional. Se espera ofrecer así un panorama amplio, aunque no acabado, de las formas de utilización de este interesante material en la apasionante área del magnetismo y la nanotecnología.In the last decades, research oriented towards the development of knowledge regarding magnetic properties of materials remains as a relevant field of study in science, technology and engineering due to its implications on nanostructured materials. The present review has the aim to illustrate the importance of iron ferrite (Fe3O4), also called magnetite, in the context of engineering and nanotechnology. Aspects related to the applications of nanostructured materials in modern technology are discussed. Specifically, an extensive literature review about previous scientific work on the biological, technical, and computational aspects associated to magnetite is presented in detail. By portraying a general overview of the potential uses of this material, it is intended to state its wide possibilities and implications in the advance of magnetism and nanotechnology areas

Simulación Monte Carlo de películas delgadas ferromagnéticas

Con base en el método Monte Carlo se estudian las propiedades magnéticas y el comportamiento crítico de películas delgadas ferromagnéticas de dimensión LxLxd, con estructura cúbica simple. Se implementó una dinámica de Metropolis, un modelo de Ising con interacciones a primeros vecinos y condiciones de frontera periódicas en dirección transversal y condiciones de frontera libres en la dirección d perpendicular al plano de la película. Para diferentes espesores de película, definidos por el número de monocapas, se calculan la magnetización por espín, la susceptibilidad magnética y el cumulante de cuarto orden de la magnetización en función de la temperatura. Se calculan también en forma diferenciada las contribuciones del bulk y la superficie, y se estiman los ex-ponentes críticos ?, ß y ?. Finalmente se presenta y discute la dependencia de la temperatura crítica con el espesor de las películas