Magnetic properties of epitaxial Co nanodisk arrays packed on atomically smooth and vicinal Si substrates

Arrays of circular nanodisks have been formed from epitaxial Co films deposited on atomically smooth and vicinal Si(111) single crystals by using a focused Ga⁺ ion beam. The surface roughness has been determined by scanning tunneling microscopy. The coercive force and the processes of magnetization reversal in films and arrays of epitaxial Co nanodisks have been studied. It has been shown that the coercive force of the Co nanodisk arrays on the atomically smooth Si(111) surface is larger and that on the vicinal Si(111) surface is smaller than that in the epitaxial films. The studies of the magnetic structure of the nanodisks by magnetic-force microscopy in combination with the micromagnetic simulation have shown that the processes of magnetization reversal in the nanodisks on the atomically smooth substrates occur through the vortex-like states and, on the vicinal substrates, through the C-type state.     

Magnetic properties of Co/Pd multilayered films on porous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> templates with developed cell substructure

We studied the structure and magnetic properties of porous multilayered Co/Pd films deposited on the templates of anodized Al₂O₃ with a specific surface morphology that is characterized by a cellular–porous structure with several pores inside each cell. X-ray diffraction analysis and reflectometry are used to study the peculiarities of the formation of phases in deposited films. The effect of morphological features of porous Co/Pd films on their magnetoanisotropic properties and magnetization reversal processes (magnetization reversal mechanisms, domain structure of films, and coercive field H _c ) is revealed by SQUID magnetometry and magnetic force microscopy.     

Experimental observation of disorder-driven hysteresis-loop criticality

We have studied the effect of magnetic disorder on the magnetization reversal process in thin Co/CoO films. The antiferromagnetic CoO layer allows a reversible tuning of the magnetic disorder by simple temperature variation. For temperatures above a critical temperature T(c), we observe a discontinuous magnetization reversal, whereas smooth magnetization loops occur for T相似文献   

Compressed exponential form for disordered domain wall motion in ultra-thin Au/Co/Au ferromagnetic films

Magnetization reversal in ultra-thin Au/Co/Au films deposited on single crystal silicon (1 0 0) was investigated using Kerr microscopy. In the considered ultra-thin Co films, with a thickness between 0.7 and 1 nm, the coercivity and magnetic anisotropy decrease with decrease in cobalt layer thickness and the magnetization reversal dynamics is dominated by disordered domain wall motion. An analysis of the observed magnetization reversal dynamics is proposed, starting from the Fatuzzo-Labrune model. We show that the relaxation curves of these samples are well described by a function obtained by a technical transformation of Fatuzzo-Labrune model in the regime dominated by domain wall motion.     

Magnetization reversal dynamics in Au/Co/Au(111) ultrathin films: Effect of roughness of the buffer layer

We present a study of the magnetization reversal dynamics in ultrathin Au/Co/Au films with perpendicular magnetic anisotropy, for a Co thickness of 0.5, 0.7 and 1 nm. In these films, the magnetization reversal is dominated by domain nucleation for t_Co=0.5, 0.7 nm and by domain wall propagation for t_Co=1 nm. The prevalence of domain nucleation for the thickness range 0.5-0.7 nm is different from results reported in the literature, for the same system and for the same thickness range, where the magnetization reversal took place mainly by domain wall motion. We attribute this difference to the effect of roughness of the Au buffer layer on the morphology of the magnetic layer.     

Electronic structures of magnetic ultrathin films Co/Au(1 1 1) studied by spin-resolved photoelectron spectroscopy

We have measured the spin-resolved photoemission spectra of the Co thin films grown epitaxially on Au(1 1 1) substrate in order to investigate their valence band structures. It is proved that the electronic structures of Co thin films are pretty different from that of bulk hcp-Co. It is observed that as the films grow thicker, the electronic structures become closer to those of the bulk Co with the magnetic anisotropy turning into in-plane magnetization from out-of-plane magnetization.     

Magnetization reversal in exchange biased Co/CoO patterns

The influence of patterning on exchange bias has been investigated using arrays of micron-sized Co/CoO dots with different lateral confinement and length-to-width ratio. The patterned samples show higher coercive and exchange bias fields than a continuous Co/CoO bilayer. As in unpatterned film, magnetization reversal mechanisms on opposite sides of the hysteresis loops of the microstructured samples are different. However, with the increase of lateral confinement and shape anisotropy of the dots, the asymmetry in the magnetization reversal starts to differ from that observed in continuous Co/CoO films.     

Effect of CoSi2 buffer layer on structure and magnetic properties of Co films grown on Si（001） substrate

Buffer layer provides an opportunity to enhance the quality of ultrathin magnetic films.In this paper,Co films with different thickness of Co Si2buffer layers were grown on Si(001)substrates.In order to investigate morphology,structure,and magnetic properties of films,scanning tunneling microscope(STM),low energy electron diffraction(LEED),high resolution transmission electron microscopy(HRTEM),and surface magneto-optical Kerr effect(SMOKE)were used.The results show that the crystal quality and magnetic anisotropies of the Co films are strongly affected by the thickness of Co Si2buffer layers.Few Co Si2monolayers can prevent the interdiffusion of Si substrate and Co film and enhance the Co film quality.Furthermore,the in-plane magnetic anisotropy of Co film with optimal buffer layer shows four-fold symmetry and exhibits the two-jumps of magnetization reversal process,which is the typical phenomenon in cubic(001)films.     

Asymmetric domain nucleation and unusual magnetization reversal in ultrathin Co films with perpendicular anisotropy

We report unexpected phenomena during magnetization reversal in ultrathin Co films and Co/Pt multilayers with perpendicular anisotropy. Using magneto-optical Kerr microscopy and magnetic force microscopy we have observed asymmetrical nucleation centers where the reversal begins for one direction of the field only and is characterized by an acute asymmetry of domain-wall mobility. We have also observed magnetic domains with a continuously varying average magnetization, which can be explained in terms of the coexistence of three magnetic phases: up, down, and striped.     

Interface magnetization reversal and anisotropy in Fe/AlGaAs(001)

The reversal process of the Fe interface layer magnetization in Fe/AlGaAs heterostructures is measured directly using magnetization-induced second-harmonic generation, and is compared with the reversal of the bulk magnetization as obtained from magneto-optic Kerr effect. The switching characteristics are distinctly different due to interface-derived anisotropy--single step switching occurs at the interface layer, while two-jump switching occurs in the bulk Fe for the magnetic field orientations employed. The angle between the interface and bulk magnetization may be as large as 40-85 degrees. Such interface switching will dominate the behavior of nanoscale structures.     

Fractal model of magnetization reversal in a strained garnet ferrite film

The domain structure in strained garnet ferrite films and its behavior in an external magnetic field are studied using the Faraday effect. Based on the experimental results, a model of magnetization reversal in thin polycrystalline layers is proposed that describes the process of remagnetization as the development of fractal clusters. The model proposed is verified using a computer simulation of magnetization reversal.     

Giant moments of Fe and Co on and in rubidium aaand potassium films

Thin quench-condensed films of Rb and K are covered with 1/100 of a mono-layer of Fe or Co. Then the impurities are covered with several atomic layers of the host. The magnetization of the films is measured by means of the anomalous Hall effect (AHE). The magnetization follows a Brillouin function with a magnetic moment of more than 10 Bohr magnetons for bulk Co and Fe impurities. These moments are much larger than the moments of the atomic configurations of Fe and Co and suggest enhanced magnetic moments of the impurities. Received 22 September 2001 and Received in final form 11 December 2001     

Magnetization reversal in bulk and thin films of the ferrimagnetic ErCo0.50Mn0.50O3 perovskite

We present herein a comparison of the magnetic properties of bulk ceramics and thin films of the ferrimagnetic ErCo_0.50Mn_0.50O₃ compound. Epitaxial thin films were deposited onto (1 0 0) SrTiO₃ substrates by pulsed-laser ablation while bulk ceramics were prepared by solid state reaction. When cooling under low applied fields, a spin reversal is observed in both thin film and bulk due to the competition between two magnetic sublattices (Co/Mn and Er) coupled by a negative exchange interaction. Original features are observed in the M(H) loops for bulk materials: abrupt jumps at 4 T due to a reorientation of domains, while in the low field region, the increasing and decreasing branches of the magnetization intersect each other. In the thin film, the ordering temperature increased from 69 to 75 K, and the ZFC anomaly (AF transition) became sharper, compared to the bulk specimen. The oxygen content and the microstructure are crucial to observe the intersection of the magnetization branches.     

Structural,magnetic, and electrical properties of thin film Pd/Co layered structures

We report on properties of layered coherent structures of Pd and Co, prepared by RF sputtering. X-ray diffraction analysis characterizes these films as having a well-ordered periodic structure (periods λ in the range 10A < λ < 80A) of stacked (111) planes of fcc Co and Pd. Room temperature magnetic properties were measured with a vibrating sample magnetometer. All films are ferromagnetic, with a magnetic moment in excess of that attributable to Co. This excess, which increases as λ decreases, is interpreted as induced ferromagnetism in the Pd layers. The in-plane magnetization is harder for smaller values of λ and appears to depend mainly on the thickness of the Co layers. The in-plane electrical resistivity was measured in the range 2K–300K by a four-electrode method. Below 40K, the resistivity is dominated by residual resistivity; above this temperature, its rise is attributed mainly to the resistivities of bulk Pd and Co. The λ-dependence of the resistivity is described by a model of interfacial scattering of electrons. Evidence for the presence of coherency strains at small λ is present in the x-ray data, the magnetization behavior, as well as in the interfacial scattering mechanism deduced from the analysis of the resistivity.     

Magnetization reversal in a novel gradient nanomaterial

The deposition of Co/Pd multilayer films onto self-assembled particle arrays with particle sizes down to 50 nm leads to pronounced curvature-induced physical properties. Unlike in classical nanosystems, the so-formed single caps on top of the spherical particles exhibit a radial symmetric anisotropy orientation across their surface. Its impact on the magnetization reversal process was analyzed experimentally for different particle sizes and compared to micromagnetic simulations, offering new opportunities in the functionalization of magnetic nanostructures.     

FMR studies in compositionally modulated Co-Nb and Co films

Compositionally modulated (CM) films of Co-Nb have been prepared by sequential evaporation onto Si substrates using two E-guns fitted in an UHV system. The thickness of the Co sublayer was varied in the range 50 to 105 Å and that of Nb in the range 18 to 90 Å. A single layer Co film of about 850 Å was also prepared and studied. Magnetization was measured using a VSM at 290 K. FMR was observed at about 9.8 GHz in the range 3 to 290 K. For Co a layer thickness of 50 Å the magnetization in CM film is about 12% lower at 4.2 K, as compared to the bulk value. The FMR spectrum of single layer Co film shows a single absorption in the perpendicular configuration with a narrow line width of 40 Oe. Other properties of this film agree with those published in the literature. In CM films, multiple absorption modes are observed. A small perpendicular anisoropy is also measured. The magnetization in all the samples varies at T^3/2. The Curie temperature of CM films is lower than that of bulk Co. The resonance line width in CM films at lower temperatures, increases much faster than in Co film.     

Growth-induced perpendicular anisotropy of grains in Co-Al-O nanogranular ferromagnetic films

This paper reports on the results of the magnetostatic measurements for Co-Al-O nanogranular films over a wide range of concentrations of the ferromagnetic component x. It has been revealed that grains in the films are characterized by the growth-induced anisotropy with easy axes directed perpendicular to the film plane. The maximum field of the single-grain perpendicular anisotropy reaches ∼2.5 kOe for samples in the vicinity of the percolation threshold (x ≈ 61 at % Co). It has been established that the characteristic features of the superparamagnetic behavior of an ensemble of oriented Stoner-Wohlfarth particles are retained for the sample with x ≈ 61 at % Co in the presence of the demagnetization field associated with the net magnetization of the film. The influence of the demagnetization field of the film on the shape of the magnetization reversal curves, the coercivity, and the blocking temperature has been investigated and simulated. The results of the simulation are consistent with the experimental data.     

Thermally activated magnetization reversal in magnetic tunnel junctions

In this paper, the magnetization reversal of the ferromagnetic layers in the IrMn/CoFe/AlO_x/CoFe magnetic tunnel junction has been investigated using bulk magnetometry. The films exhibit very complex magnetization processes and reversal mechanism. Thermal activation phenomena such as the training effect, the asymmetry of reversal, the loop broadening and the decrease of exchange field while holding the film at negative saturation have been observed on the hysteresis loops of the pinned ferromagnetic layer while not on those of the free ferromagnetic layer. The thermal activation phenomena observed can be explained by the model of two energy barrier distributions with different time constants.     

Behavior of domain structure in a Co/Ag superlattice with dispersion of magnetic anisotropy

It is found that the magnetization reversal of an array of superthin Co films coupled by the ferromagnetic exchange interaction through the Ag layers may result in a domain structure of an unexpected new type. Due to the incoherent different-sense spin rotation upon lowering the field perpendicular to the easy axis, the specific macrodomains first form in a sample. They are separated not by the Neél domain wall but by a wide transition region containing high-density microdomains of sizes correlating with the grain sizes in the films. Further magnetization reversal proceeds through the formation of standard domain walls in the macrodomain in a magnetostatic field at the plate edge and through their shifting toward the transition region. These processes are explained with taking into account the character of the revealed magnetic anisotropy dispersion.     

Magnetization reversal of individual Co nanoislands

We investigate the magnetization reversal of individual Co islands on Cu(111) in the size range of N=700 to 18,000 atoms by spin-polarized scanning tunneling microscopy at 8 K. The switching field H(sw) changes with island size in a nonmonotonic manner: it increases with island size and reaches a maximum value of 2.4 T at N=5500 atoms, and it decreases for larger islands. We extract the energy barrier for magnetization reversal as a function of island size. The maximum H(sw) corresponds to an energy barrier of 1 eV. Our results elucidate a crossover of the magnetization reversal from an exchange-spring behavior to domain wall formation with increasing size at around 7500 atoms.