Micromagnetic Properties Research Articles

Analyzing micromagnetic properties with FORCIT Software

A primary method of characterizing the magnetic properties of a sample is to measure its magnetization while gradually varying an applied ambient magnetic field between large positive values and large negative values. The change in magnetization with field can be used to map the magnetic hysteresis loop of the sample, which defines some of its bulk magnetic properties (Figure 1).Each sample, however, is actually composed of a population of magnetic grains and domains, which may vary in their magnetic properties and which may interact with their neighbors. The micromagnetic properties of these populations, such as the distribution of micro‐coercivities and the magnetic interactions or biases, can be mapped by probing the interior of a hysteresis loop with a series of partial hysteresis curves referred to as first‐order reversal curves (FORCs) [Mayergoyz, 1986; Pike et al., 1999] (Figures 1 and 2). These properties can then be used in various ways ranging from characterization of the magnetic composition, grain size, and concentration of samples to understanding the mechanisms of magnetic acquisition [e.g., Pike et al., 1999,2001; Roberts et al., 2006; Carvallo et al., 2006; Muxworthy et al., 2005].

Reversible susceptibility studies of magnetization switching in FeCoB synthetic antiferromagnets

In this paper we present a study of switching characteristics of a series of synthetic antiferromagnet (SAF) structures using reversible susceptibility experiments. Three series of SAF samples were considered in our study with (t1, t2), the thickness of the FeCoB layers of (80nm, 80nm), (50nm, 50nm), and (80nm, 20nm) and with the interlayer of Ru ranging from 0to2nm. A vector vibrating sample magnetometer was used to measure the hysteresis loops along the different directions in the plane of the samples. The reversible susceptibility experiments were performed using a resonant method based on a tunnel diode oscillator. We showed that the switching peaks in the susceptibility versus field plots obtained for different orientations of the applied dc field can be used to construct the switching diagram of the SAF structure. The critical curve constitutes the fingerprint of the switching behavior and provides information about micromagnetic and structural properties of SAF which is an essential component of modern magnetic random access memories.

Micromagnetic properties of epitaxial MnAs films on GaAs surfaces

We present a systematic study of the micromagnetic properties of MnAs deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. In epitaxial MnAs films, the strain state in MnAs-on-GaAs(001) (anisotropic) and MnAs-on-GaAs(111)B (isotropic) has a strong influence on the magnetostructural phase transition and thus the micromagnetic properties. The ferromagnetic α and the β phase coexist over a wide temperature range exhibiting self-organized, magnetically coupled nanostructures. Independent of the substrate orientation, magnetic flux-closure domain patterns are formed in the basal plane of MnAs. The spatial distribution of the phases in equilibrium (stripes and quasi-hexagonal islands, respectively) stabilizes various magnetic states, which were found experimentally and confirmed by micromagnetic simulations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Micromagnetic properties of epitaxial MnAs films on GaAs surfaces

We present a systematic study of the micromagnetic properties of MnAs deposited by molecular-beam epitaxy on GaAs(001) and GaAs(111)B surfaces. In epitaxial MnAs films, the strain state in MnAs-on-GaAs(001) (anisotropic) and MnAs-on-GaAs(111)B (isotropic) has a strong influence on the magnetostructural phase transition and thus the micromagnetic properties. The ferromagnetic α and the β phase coexist over a wide temperature range exhibiting self-organized, magnetically coupled nanostructures. Independent of the substrate orientation, magnetic flux-closure domain patterns are formed in the basal plane of MnAs. The spatial distribution of the phases in equilibrium (stripes and quasi-hexagonal islands, respectively) stabilizes various magnetic states, which were found experimentally and confirmed by micromagnetic simulations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation of the micromagnetic properties of sub-micron permalloy dot arrays

This paper examines the static micromagnetic properties of several configurations of sub-micron permalloy circular dots. Brief details of the two-dimensional (2-D) finite-element model used for the simulations are presented. The model is then applied to a 2 × 2 array of dots, showing that there is a single ground state in which the four dots assume an overall circular magnetisation distribution. It is then shown that introducing an additional central dot to the 2 × 2 array of dots yields a system with two ground states. Physical explanations for these states are given, and a phase diagram presented which shows the combinations of array parameters (dot diameter and separation) required for the system to adopt these two states. The final system examined combines two of the five dot arrays above side by side, and it is shown that this system has two ground states, one the inverse of the other. Potential applications for magnetic data processing of these arrays are discussed.

Origin of ferromagnetism and nano-scale phase separations in diluted magnetic semiconductors

This paper reviews various origins of ferromagnetic response that has been detected in diluted magnetic semiconductors (DMS). Particular attention is paid to those ferromagnetic DMS in which no precipitation of other crystallographic phases has been observed. It is argued that these materials can be divided into three categories. The first consists of (Ga,Mn)As and related compounds. In these solid solutions the theory built on p–d Zener's model of hole-mediated ferromagnetism and the Kohn–Luttinger kp theory of semiconductors describes quantitatively thermodynamic, micromagnetic, optical, and transport properties. Moreover, the understanding of these materials has provided a basis for the development of novel methods enabling magnetisation manipulation and switching. To the second group belong compounds, in which a competition between long-range ferromagnetic and short-range antiferromagnetic interactions and/or the proximity of the localisation boundary lead to an electronic nano-scale phase separation that results in characteristics similar to colossal magnetoresistance oxides. Finally, in a number of compounds a chemical nano-scale phase separation into the regions with small and large concentrations of the magnetic constituent is present. It has recently been suggested that this spinodal decomposition can be controlled by the charge state of relevant magnetic impurities. This constitutes a new perspective method for 3D self-organised growth of coherent magnetic nanocrystals embedded by the semiconductor matrix.

Micromagnetics of Ni-nanowires filled in nanochannels of porous silicon

An array of ferromagnetic nanowires embedded in silicon is fabricated using an anodizing and electroplating process. During the former one oriented porous silicon channels are fabricated whereas by the latter process Ni is incorporated into these channels. The electrodeposited Ni-wires have a great pore-length to pore-diameter (aspect) ratio up to 1000 : 1 (typical diameters between 10 and 60 nm, pore-length from 10 to 30 μm). Due to this property the samples exhibit a magnetic perpendicular anisotropy. The micromagnetic properties of Ni (e.g. Bloch-wall thickness) are responsible for the peculiar magnetic behaviour of this ferromagnet/silicon nanocomposite. Structural as well as magnetic investigations like scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), and SQUID-magnetometry display this nanocomposite as a bimodal ferromagnetic structure consisting of perpendicularly oriented Ni-wires and of single domain Ni-granules embedded in the silicon based matrix. The hysteresis curve outlines a two-fold magnetic switching, one at low fields (∼0.05 T) and the second at high fields (> 3 T). The micromagnetic magnetization reversal is treated in terms of an analytic formulation.

Micromagnetic properties of MnAs(0001)∕GaAs(111) epitaxial films

The micromagnetic properties of MnAs thin films grown on the (111)B-oriented GaAs surface have been investigated. Compared to films grown on (001) surfaces, these films exhibit completely different domain patterns, as the c axis of the hexagonal unit cell is oriented normal to the surface. In the course of the first order phase transition, ferromagnetic α-MnAs forms a network of quasihexagonal areas separated by β-MnAs. We present an analysis of the micromagnetic properties based on imaging and simulations. We observe closure domains that either appear as a vortex-like state or a stripe structure.

Magnetic properties of (Ga,Mn)As

A summary of experimental findings and theoretical modelling of micromagnetic properties of zinc-blende ferromagnetic semiconductor (Ga,Mn)As is presented. It is shown that the Zener p–d model explains quantitatively observed Curie temperatures in compensation free samples and that major strain-related effects are correctly accounted for, including the presence of the magnetization reorientation transition, observed as a function of hole concentration and temperature. It is evidenced that a presence of a small trigonal distortion could account for both the presence and properties of uniaxial in-plane magnetic anisotropy.

Lorentz microscopy and giant magnetoresistance of nanostructured heterogeneous Au–Co alloys

A detailed Lorentz transmission electron microscopy study of melt-spun Au78Co22 and Au86Co14, and microstructural changes upon annealing, was conducted. The microstructure of the as-cast alloys consisted of small grains with fcc Co precipitates dispersed at the grain boundaries. The interior structures of these grains were Au∕Co lamellar eutectic or very small Co precipitates dispersed in Au matrix, depending on the composition. Up to 10±1at.% Co were dissolved in the Au matrix. This large amount of Co in the Au matrix reduced the mean free path, probably leading to low magnetoresistance values [MR(H=1.45T)=2.4%–2.6%]. Annealing caused precipitation of Co out of the Au matrix and growth of some Co precipitates, which transformed to multidomain particles when exceeded 160–200nm. Annealing at 673K for 1h caused a reduction of MR(H=1.45T) to 0.7%–0.8%, mainly due to growth of Co precipitates. The magnetic and micromagnetic properties were correlated with the microstructural analyses using computer simulations. The Co lamellae and precipitates were single-domain particles and single-domain vortices, respectively.

Magnetic Properties of (Ga,Mn)As

AbstractFor Abstract see ChemInform Abstract in Full Text.

Evidence of native oxides on the capping and substrate of Permalloy gratings by magneto-optical spectroscopy in the zeroth- and first-diffraction orders

Magneto-optical Kerr effect (MOKE) spectroscopy in the zeroth- and first-diffraction orders at polar magnetization is applied to Permalloy wire gratings deposited on Si substrates and protected by Cr capping. The experimental MOKE data are compared with data simulated using the local modes method. The extensive simulations of the MOKE spectroscopic parameters exhibit significant sensitivity to t(Cr2O3) and t(SiO2), the thicknesses of native oxide layers developed on the capping and the substrate, respectively. The approach may be useful for monitoring the basic micromagnetic properties of small elements with nanometer-scale resolution, as well as for monitoring the deposition processes and aging of magnetic nanostructures in magnetic recording and magnetic random access memory technologies.

Micromagnetic studies of iron microbars prepared by nanoimprint lithography and electrodeposition

Micromagnetic and macromagnetic switching properties of iron microbars produced by electrodeposition into voids patterned by nanoimprint lithography were investigated. The study was focused on microbars with six different aspect ratios. Magnetization along the long axis produced single-domain states in all microbars regardless of their aspect ratio. In the remanent state, magnetization in microbars with smaller aspect ratios resumed the closed magnetic flux configuration. Closed magnetic flux configuration was also found in demagnetized microbars. Individual, as well as the net remanence curves, were extracted from magnetic force microscopy images to verify their agreement with the macroscopic magnetization measurements.

Spatially resolved ferromagnetic resonance: Imaging of ferromagnetic eigenmodes

Fast magnetization dynamics of ferromagnetic elements on sub-micron length scales is currently attracting substantial scientific interest. Studying the ferromagnetic eigenmodes in such systems provides valuable information in order to trace back the dynamical response to the underlying micromagnetic properties. The inherent time structure of third generation synchrotron sources allows for time-resolved imaging (time resolution: 70–100 ps) of magnetization dynamics at soft x-ray microscopes (lateral resolution down to 20 nm). Stroboscopic pump-and-probe experiments were performed on micron-sized Permalloy samples at a full-field magnetic transmission x-ray microscope (XM-1, beamline 6.1.2) at the ALS at Berkeley, CA. Complementary to these time-domain experiments a frequency-domain “spatially resolved ferromagnetic resonance” (SR-FMR) technique was applied to magnetic x-ray microscopy. In contrast to time-domain measurements which reflect a broadband excitation of the magnetization, the frequency-domain SR-FMR technique allows for detailed studies of specific ferromagnetic eigenmodes. First SR-FMR experiments at a scanning x-ray transmission microscope (STXM, ALS, BL 11.0.2) are reported. The sample, a 1×1μm2 Permalloy pattern, was excited by an alternating magnetic field with a frequency of 250 MHz. By varying the phase relation between the sine excitation and the x-ray flashes of the synchrotron, the dynamics of a vortex motion eigenmode was investigated in time and space.

High-Resolution Optical Imaging of Magnetic-Domain Structures

High-resolution optical techniques for imaging magnetic domains in ferromagnetic materials such as confocal microscopy and scanning near-field optical microscopy (SNOM) are reviewed. The imaging capabilities of different techniques and image formation are discussed in the case of in-plane as well as out-of-plane magnetic anisotropy in different illumination configurations. It is shown that the magnetooptical resolution of near-field measurements depends on the film thickness and is limited by the diffraction on magnetic domains throughout the film. For thin magnetic films, subwavelength resolution can be attained. In addition to well-known near-field magnetooptical effects (out-of plane magnetization sensitivity of linear near-field microscopy and in-plane magnetization sensitivity of nonlinear near-field measurements), linear SNOM imaging of in-plane magnetization in thin magnetic films as well as nonlinear imaging of out-of-plane domains has been demonstrated. Thus, linear and second-harmonic near-field imaging of both in-plane and out-of-plane oriented magnetic domains can be achieved in transparent and opaque specimens. This enables applications of SNOM for studies of all kinds of magnetic materials. High-resolution optical imaging is required for characterization of the micro-magnetic and magnetooptical properties of novel magnetic structures in order to adopt a bottom-up approach in the search for new materials with improved characteristics.

Magnetic anisotropy and domain structure in carrier-controlled ferromagneticsemiconductors

A review is given of the experimental situation and theoretical modelling of micromagneticproperties of zinc-blende ferromagnetic semiconductors, such as (Ga, Mn)As and p-(Cd,Mn)Te. It is emphasized that the Zener p–d model explains quantitatively the effect ofstrain on the easy axis direction as well as predicting correctly the presence of thereorientation transition, observed as a function of hole concentration and temperature.Possible suggestions put forward to explain the existence of in-plane uniaxialmagnetocrystalline anisotropy are then quoted. Finally, magnetic stiffness computed withinthe same model of ferromagnetism is presented, and is shown to explain the domain widthin perpendicular films. It is emphasized that a rather large magnitude of the stiffnessaccounts for both the excellent micromagnetic properties of ferromagnetic (III, Mn)Vfilms and the quantitative applicability of the mean-field approximation to thesematerials.

High-resolution magnetic force microscopy study of high-density transitions in perpendicular recording media

High-resolution magnetic force microscopy (hrMFM) has been used to investigate the write characteristics of forward and reverse flying heads on perpendicular recording media with soft underlayers. hrMFM has the advantage that it provides much better resolution than the read element of a head, particularly in the cross-track direction. Using a quantitative analysis method, many parameters, such as transition position jitter, transition width and signal-to-noise ratio are estimated from the hrMFM image. Furthermore, these parameters, which can also be measured using a spinstand, are compared to the micromagnetic properties of the transitions, such as track curvature, transition width and the local noise associated with transitions. The results give insights into the recording physics and, in particular, in the write process of shielded and un-shielded write heads in perpendicular recording.

Micromagnetic studies of cobalt microbars fabricated by nanoimprint lithography and electrodeposition

Micromagnetic and macromagnetic switching properties of cobalt bars with six different aspect ratios produced by electrodeposition of cobalt into patterns defined by nanoimprint lithography were studied. It was found that microbars in the demagnetized state formed configurations with closed magnetic flux. Magnetization along the long axis of the microbars resulted in a single-domain state of magnetization for microbars of all aspect ratios. In the remanent state, magnetization in microbars with smaller aspect ratios resumed the vortex configuration. Individual remanent curves, as well as the net remanent curve, were extracted from magnetic force microscopy images for microbars with all aspect ratios, and their agreement with the macroscopic magnetization measurements is discussed.

Magnetic Properties of (Ga,Mn)As

Abstract A summary of experimental findings and theoretical modelling of micromagnetic properties of zinc-blende ferromagnetic semiconductor (Ga,Mn)As is presented. It is shown that the Zener p–d model explains quantitatively observed Curie temperatures in compensation free samples and that major strain-related effects are correctly accounted for, including the presence of the magnetization reorientation transition, observed as a function of hole concentration and temperature. It is evidenced that a presence of a small trigonal distortion could account for both the presence and properties of uniaxial in-plane magnetic anisotropy.

The effect of diameter on micro-magnetic properties of Fe 0.68Ni 0.32 nanowire arrays

Nanowire arrays of the alloy, Fe 0.68Ni 0.32, with different nanowire diameters have been fabricated by the electrochemical deposition method. X-ray diffraction and Mössbauer spectroscopy were employed to characterize the size effect on the structure and micro-magnetic properties of the nanowire array, respectively. The Fe 0.68Ni 0.32 arrays with different diameters are stabilized in a BCC structure with a [1 1 0] texture oriented along the long axis of the nanowires. The Mössbauer spectra of the nanowire arrays with smaller diameters show an absence of the second and fifth peaks, which then emerge with increasing diameter. The size effects on the hyperfine parameters are discussed.