Surface Magnetic Anisotropy Research Articles

A facile method to syntheses monodisperse γ-Fe2O3 nanocubes with high magnetic anisotropy density

The performance of iron oxides, as a clinical tool for hyperthermia application, is strongly depending on their size and structural morphology. The precise control of the iron oxide shape and morphology offers a unique strategy to modify the strength of the dipolar interactions between iron oxide nanoparticles through the engineering of the magnetic surface anisotropy density. This article presents a novel recipe to synthesize γ-Fe2O3 nanocrystals with cubic morphology. The γ-Fe2O3 nanocubes were prepared through microwave assisted solvothermal technique. The use of 2,3-oxidosqualene results in iron oxide with cubic shape. The γ-Fe2O3 nanocubes were analyzed by X-ray diffractometer (XRD), transmission electron microscopy (TEM) and electron diffraction (SAED). The magnetic analysis revealed that the γ-Fe2O3 nanocubes have a saturation magnetization of 62emu/g and magnetic surface anisotropy density Keff=2×105erg/cm3 compared to bulk iron oxide.

Magneto-Electric Control of Surface Anisotropy and Nucleation Modes in L1<inline-formula><tex-math> $_{0}$</tex-math></inline-formula>-CoPt Thin Films

The interplay between electric field-controlled surface magnetic anisotropy and micromagnetic nucleation modes for L1 0 -CoPt thin films is investigated with density-functional and micromagnetic model calculations. The electric field redistributes electron states near the Fermi level, which has a fairly strong effect on the surface anisotropy, but due to inversion symmetry, the net anisotropy of the films with odd numbers of layers remains unchanged. By contrast, the micromagnetic nucleation mode is spatially asymmetric even for symmetric thin films with odd numbers of layers. This leads to a reduction of the nucleation field (coercivity) and-for suitably chosen nanostructures-to substantial changes in the hysteretic behavior. In the lowest order, the coercivity reduction is independent of the total film thickness. This counterintuitive feature can potentially be exploited in magneto-electric switching devices.

Shape-induced anisotropy in antiferromagnetic nanoparticles

High fraction of the surface atoms considerably enhances the influence of size and shape on the magnetic and electronic properties of nanoparticles. Shape effects in ferromagnetic nanoparticles are well understood and allow us to set and control the parameters of a sample that affect its magnetic anisotropy during production. In the present paper we study the shape effects in the other widely used magnetic materials – antiferromagnets, – which possess vanishingly small or zero macroscopic magnetization. We take into account the difference between the surface and bulk magnetic anisotropy of a nanoparticle and show that the effective magnetic anisotropy depends on the particle shape and crystallographic orientation of its faces. The corresponding shape-induced contribution to the magnetic anisotropy energy is proportional to the particle volume, depends on magnetostriction, and can cause formation of equilibrium domain structure. Crystallographic orientation of the nanoparticle surface determines the type of domain structure. The proposed model allows us to predict the magnetic properties of antiferromagnetic nanoparticles depending on their shape and treatment.

Surface and interface effects in magnetic core–shell nanoparticles

Abstract

Preparation and giant magneto-impedance behavior of Co-based amorphous wires

The giant magneto-impedance (GMI) in Co69.5Fe4.35Cr1Si8B17 microwires is studied. Wires with different diameter were fabricated by a modified glass-coated melt spinning technique, and then the glass was removed before the metallic nucleus was cold drawn. A transition from single-peak (SP) to double-peak (DP) GMI behavior was observed for the microwires after cold drawing. This could be explained by the induced circumferential stress during cold drawing, which changes the surface magnetic anisotropy of the wire from helical domains to circular domains. GMI ratio as well as field sensitivity were improved significantly after cold drawing, and that would be significant for application.

Magnetic anisotropy of ultrathin Fe films grown on vicinal Si (111)

We have investigated magnetic anisotropy of ultrathin Fe films grown on vicinal Si (111) with 4° miscut towards [11-2] direction. Spin reorientation transition (SRT) from out-of-plane to in-plane proceeds in a wider thickness range than on flat substrates. Meanwhile, the easy axis of in-plane uniaxial magnetic anisotropy varies from [11-2] to [-110] with an intermediate state of approximate four-fold symmetry. The evolution of magnetic anisotropy is attributed to competition of surface magnetic anisotropy, first-order magnetocrystalline anisotropy, and step induced magnetic anisotropy from symmetry breaking and dipolar interactions.

Effect of magnetic surface anisotropy on domain wall structure in magnetic triaxial films

The influence of the presence of surface anisotropy on the micromagnetic structure of the domain wall in magnetic-triaxial films has been investigated. The investigation method is the numerical minimization of the energy functional of the domain wall used in the two-dimensional model of the distribution of magnetization. Films with different orientations of crystallographic axes with respect to the film surface have been considered. The effect of the surface anisotropy type and surface anisotropy constant K S value on the stability of different types of domain walls has been found.

Polymer-inorganic composite with ultradisperse gadolinium particles

The objects of investigation are polystyrene-based composites with ultradisperse particles (including nanoparticles) of metallic Gd and SiO2. The composites prepared by milling starting materials in a barrel mill at room temperature are studied by the ferromagnetic resonance method, transmission electron microscopy (TEM), and reflection X-ray diffraction (RXD). It is found that the magnetic subsystem of the composites is formed by magnetic nanoclusters, Gd crystallites 30 ± 10 nm across, which possess volume and surface magnetic anisotropy and pass into the superparamagnetic state at 210 ± 10 K. It is also found that the Landau-Lifshitz equation with the damping term in the Landau-Lifshitz form provides the best quantitative fit to experimental data for the ferromagnetic resonance of superparamagnetic metal nanoparticles.

Магнитоэлектрический эффект и типы магнитного упорядочения в пленках мультиферроиков

Space – modulated structures of cycloidal type in a (111) – oriented bismuth ferrite type multiferroic films have been investigated. It has been shown that spin cycloids with a definite direction of propagation vector are realized in the films which is connected with the presence of surface magnetic anisotropy attributed to magnetoelastic interactions. The influence of magnetic field applied in principal crystal axis direction on the character of magnetic ordering in multiferroic films has been considered. The boundaries of transition of space – modulated structure into homogeneous magnetic state has been determined.

Thickness-driven spin reorientation transition in ultrathin films

We review recent studies by different experimental means of ultrathin films, exhibiting thickness-driven spin reorientation transitions (SRTs). The stage is set by determining, via phenomenological thermodynamic description, of the relevant phase diagrams for the possible types of SRT with and without applied magnetic field. Suitable representation may be chosen such that best use is made of the linear character (under thickness variation) of the system’s path in anisotropy space. The latter involves higher-order bulk and surface anisotropies in a substantial way. We examine sensitive experimental techniques for the detection and quantification of SRTs, such as hysteresis measurements with magneto-optical Kerr effect (MOKE), micromagnetic studies utilizing scanning electron microscopy with polarization analysis (SEMPA), photoemission electron microscopy (PEEM) and spin-polarized low-energy electron microscopy (SPLEEM) as well as ac magnetic susceptibility measurements via MOKE. Key issues are conclusively discussed including the identification of reliable experimental fingerprints about whether a given SRT proceeds via a phase of coexistence or via a cone (canted) phase. We demonstrate how the application of the general theoretical ideas to carefully designed measurements leads to the determination of the most important material parameters in any ultrathin-film SRT, namely, the surface (interface) magnetic anisotropy constants. The review concludes by our personal outline for future promising work on SRTs.

Magnetoresistive Memory with Ultralow Critical Current for Magnetization Switching

AbstractThe data writing and thermal stability of information storage are studied theoretically for a magnetic random access memory (MRAM) composed of a magnetic tunnel junction or multilayer exhibiting giant magnetoresistance. The theoretical analysis focuses on the magnetization switching in the “free” layer of a MRAM cell, which is induced by a spin‐polarized current imposing a spin‐transfer torque (STT) on the magnetization. It is shown that the writing current in such an STT‐MRAM reduces dramatically near a spin reorientation transition (SRT) driven by lattice strains and/or surface magnetic anisotropy and even tends to zero under certain conditions. In particular, at the size‐driven SRT in the perpendicular‐anisotropy CoFeB‐MgO tunnel junctions, the critical current densities for magnetization reorientations between the parallel and antiparallel states are expected to fall to low values of about 1.3 × 105 and −3.3 × 104 A cm−2. Remarkably, STT‐MRAMs may combine low writing current with very high thermal stability of information storage (retention over 10 years) even at a high density ≈500 Gbit inch−2.

Impact of magnetic surface anisotropy on the precessional switching of magnetization in Pt-alloy nanofilms

Precessional switching of magnetization in CoPt and FePt nanofilms is investigated by solving the Landau–Lifshitz–Gilbert (LLG) equation analytically and numerically. Switching in these films occurs only above a critical value of the magnetic field, and it further depends on the magnetocrystalline anisotropy and saturation magnetization of the film. The presence of magnetic surface anisotropy in these films reduces the switching time significantly. Also, the switching time in the case of Pt-alloys of Co and Fe is low compared to that in the case of pure Co and Fe films.

Adsorption of Doxorubicin onto Citrate-Stabilized Magnetic Nanoparticles

We have synthesized citric-acid-stabilized magnetic nanoparticles with very good magnetization behavior and relatively low contribution from shape and surface magnetic anisotropies. In this work, w...

Electric field control of interface magnetic anisotropy

We present a model for determining the Rashba spin-orbit coupling (RSOC) at magnetic surfaces or interfaces by explicitly taking into account the interaction between the inversion-symmetry-broken potential and the spin-dependent electric dipoles of the Bloch states. We show that the RSOC alone can generate a perpendicular surface magnetic anisotropy comparable to the observed values in transition metals. When an external electric field is applied across the interface, the induced screening potential modifies the RSOC and thus controls the direction of the magnetization. Our results are consistent with the existing experiments.

Effect of postgrowth oxygen exposure on the magnetic properties of Ni on the Cu-CuO stripe phase

The magnetism and morphology of thin Ni films deposited on the Cu(110)-Cu(110)-$(2\ifmmode\times\else\texttimes\fi{}1)$O surface (Cu-CuO stripe phase) have been studied by scanning tunneling microscopy (STM) and reflectance difference magneto-optical Kerr spectroscopy (RD-MOKE). The magnetic easy axis of the Ni films lies completely in plane up to a coverage of 22.5 ML. Exceeding this coverage, a small remanent magnetization component pointing out of plane evolves. Upon postgrowth oxygen exposure the Ni film becomes completely out-of-plane magnetized and the out-of-plane remanence and coercive field strongly increase during exposure. STM images reveal a fully $(2\ifmmode\times\else\texttimes\fi{}1)$O reconstructed topmost Ni layer after the oxygen exposure, but no morphological changes of the Ni film. We thus conclude that the oxygen-induced surface reconstruction strongly modifies the magnetic properties of the Ni film by enhancing the surface magnetic anisotropy.

Potential explanation of charge response of magnetization in nanoporous systems

An example of technologically promising magnetoelectric effects in nanostructures is the reversible tuning of the magnetic properties of nanoporous systems by charging them in an electrolyte. So far this effect, which has been observed experimentally in various metallic nanoporous materials, has been tentatively related to two different microscopic mechanisms: the filling of the electronic $d$ band upon charging, or the magnetoelastic response to charge-induced structural changes of the nanoparticles which form the nanoporous systems. In the present paper a potential mechanism is introduced which is related to a change in the magnetic surface anisotropy of the nanoparticle upon charging. An essential assumption of the mechanism is that only those surfaces of the nanoparticle which are not in the grain-boundary contact with the neighboring nanoparticles of the nanoporous material are charged in the electrolyte. This is underpinned by computer simulations on the atomic level with information on the charge response of the magnetic surface anisotropy from ab initio electron density functional calculations.

Effect of surface anisotropy on the dynamic properties of Néel domain walls in the films with planar anisotropy

This work is devoted to the investigation into the effect of surface magnetic anisotropy on the motion of domain walls in magnetic uniaxial films with an in-plane anisotropy in the range of thicknesses corresponding to the stability of one-dimensional Neel walls. It has been established that the surface anisotropy with a constant K s > 0 (K s < 0) increases (decreases) the velocity of both stationary and nonstationary motion of the domain wall, as well as the period of oscillations of the velocity of nonstationary motion. The magnitude of the critical field as a function of K s has been determined. Effect of surface anisotropy with K s < 0 on the scenario of the dynamic transformation of the domain-wall structure has been revealed.

Synthesis, characterization, magnetic and electrical properties of the novel conductive and magnetic Polyaniline/MgFe 2O 4 nanocomposite having the core–shell structure

Conductive and magnetic Polyaniline/MgFe 2O 4 nanocomposite was successfully synthesized in the form of core–shell via in situ chemical polymerization of aniline in the presence of MgFe 2O 4 nano-particles. X-ray powder diffraction of ferrites indicated that the structure of the core material is having the spinel structure, and demonstrated the formation of PAni/MgFe 2O 4 nanocomposite. XRD and TEM photographs showed that the particle's size of the MgFe 2O 4 core-material were around 30–35 nm before coating with Polyaniline, and grown up to 45 nm in the core–shell nanocomposite after coating. Although PAni has a relatively smaller electrical field coefficient than the core–shell nanocomposite, the resistivity of the core–shell material decreased, and hence its conductivity increased after a certain threshold voltage of 0.98 V equivalent to threshold electric field value equals 5.5 V cm −1. The magnetic hysteresis loops investigated with VSM indicated that coating MgFe 2O 4 with Polyaniline has an healing effect which covers the ferrite surface defects, thus decreasing the magnetic surface anisotropy of ferrite particles leading to a decrease of the saturated magnetization (Ms) from 21.33 emu/g to 5.905 emu/g and a decrease of the coercivity (Hc) from 88.66 Oe to 81.6 Oe for MgFe 2O 4 and the core–shell nanocomposite respectively due to the amount of Polyaniline added. TGA and DTA revealed improved thermal stability of the core–shell nanocomposite with respect to that of Polyaniline due to the incorporation of ferrites. Raman spectroscopy confirmed TGA, DTA and XRD studies, and revealed that pure PAni is less stable than the corresponding core–shell nanocomposite with respect to molecular changes which might occur during heating at elevated temperatures. Moreover, Raman study confirmed the interfacial interaction between the core and the shell materials, and lead to an assumption about the presence of different conjugation chain lengths and types, such as the presence of the semi-quinones aside the quinone rings in the polymer chain, which showed different response upon heating the sample.

Effects of a perpendicular magnetic field in the dipolar Heisenberg model with dominantexchange interaction

The effects of a uniform magnetic field on the phase diagram of the dipolar Heisenbergmodel with a dominant antiferromagnetic exchange interaction have beeninvestigated. The model consists of a square lattice of classical spin vectors, wherethe spins interact through an antiferromagnetic exchange interaction of strengthJ and a dipole–dipoleinteraction of strength g. The spins couple to a magnetic surface anisotropy of strengthκ and to an applied external magnetic field of strengthH.The external field is applied perpendicular to the plane of the lattice. From extensive Monte Carlosimulations, representative magnetic phase diagrams have been determined as a function of the ratiosκ/g andT/g, whereT is temperature, and at three different ratios ofH/g (H/g = 10, 20, 27). These results are compared to the previously investigated case ofH/g = 0 and to analytic calculations for the ground state energies. The nature of the equilibriumphases and order of the phase boundaries separating them are considered and changes dueto the strength of the applied field are highlighted.

Surface morphology and magnetic anisotropy in (Ga,Mn)As

Atomic force microscopy and grazing incidence x-ray diffraction measurements have revealed the presence of ripples, aligned along the [11¯0] direction on the surface of (Ga,Mn)As layers grown on GaAs(001) substrates and buffer layers, with periodicity of about 50 nm in all samples that have been studied. These samples show the strong symmetry breaking uniaxial magnetic anisotropy normally observed in such materials. We observe a clear correlation between the amplitude of the surface ripples and the strength of the uniaxial magnetic anisotropy component suggesting that these ripples might be the source of such anisotropy.