34 research outputs found
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 FeMnAl 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 via Bogoliubov
and 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
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
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 crÃticos 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
Estudio de la relajación y comportamiento crÃtico en pelÃculas delgadas de magnetita
En el presente trabajo se estudian las propiedades magnéticas de pelÃculas delgadas de magnetita con base en un modelo Monte Carlo-Ising con dinámica de Metropolis. Se simulan pelÃculas de magnetita de base cuadrada LxL y espesor d (medidos en celdas unidad), aplicando condiciones de frontera periódicas en dirección transversal y condiciones de frontera libres en la dirección d perpendicular al plano base. En el modelo consideramos la estructura espinela inversa 3D como el es-quema de interacciones entre sitios tetraédricos y octaédricos de manera realista. Se estudia el pro-ceso de relajación partiendo de dos configuraciones diferentes para varias temperaturas, se calculan las funciones de relajación no lineal para la magnetización y la energÃa y los tiempos de relaja-ción integral en cada caso. Adicionalmente, se estima el exponente crÃtico de la magnetización (ß) y se hace una comparación con el obtenido para la magnetita en bulk
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, tecnologÃa e ingenierÃa; 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 ingenierÃa y la nanotecnologÃa. Se examinan algunos de los aspectos más relevantes de las aplicaciones de materiales nanoestructurados a la tecnologÃa moderna, y se revisa con detalle la bibliografÃa sobre estudios cientÃficos de la magnetita en su aspecto biológico, técnico y teórico-computacional. Se espera ofrecer asà un panorama amplio, aunque no acabado, de las formas de utilización de este interesante material en la apasionante área del magnetismo y la nanotecnologÃa.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
Critical behavior of magnetite: a mössbauer and Monte Carlo study
Por medio de la espectrometrÃa Mössbauer se estudió una muestra de magnetita en el rango de temperaturas de 170 a 900 K. Los resultados revelan un colapso de los espectros Mössbauer a una única lÃnea de absorción a una temperatura crÃtica TC alrededor de los 870 K caracterizado por un exponente crÃtico ß =0.28(2) para el campo magnético hiperfino. Para describir la dependencia del campo hiperfino con la temperatura, se llevó a cabo una simulación Monte Carlo-Metropolis de una magnetita estequiométrica con base en un modelo de Ising. Finalmente se presenta una discusión sobre el comportamiento crÃtico de la magnetita y una comparación entre el campo hiperfino obtenido por la técnica Mössbauer y la magnetización calculada a partir de la simulación computacional