Continuous spin-reorientation phase transition in the surface region of an inhomogeneous magnetic film

A continuous spin-reorientation transition from a uniform magnetic state with the in-plane orientation of the moments of all atomic layers to a nonuniform canted state in the surface region is considered. This transition was discovered in experiments on the divergence of magnetic susceptibility in a perpendicular magnetic field at a temperature of about 240 K, which is lower than the Curie point of gadolinium, equal to 292.5 K. These experiments were carried out on an ultrathin iron magnetic film deposited on the (0001) surface of a thin gadolinium film. It is shown that, in the vicinity of the spin-reorientation transition, the thermodynamic potential has a form characteristic of the Landau theory of second-order phase transitions. The orientation angle of the moment of the surface atomic layer with respect to the plane of the film, which is chosen as an order parameter, exhibits anomalous behavior and increases with temperature. Expressions are derived for the magnetic susceptibility of each atomic layer. It is shown that, in the vicinity of the transition, the irregular part of the magnetic susceptibility of each atomic layer exhibits behavior characteristic of the susceptibility in the Landau theory: it is less by a factor of two in the low-symmetry phase and diverges at the transition point. The regular part of the magnetic susceptibility of each atomic layer makes an additional contribution to the asymmetry of the total susceptibility in the vicinity of the transition point; this result follows from the fact that the inhomogeneous magnetic system considered is semi-infinite.     

Surface distortions of the magnetic structure of a uniaxial antiferromagnet: The “magnetic field-roughness” phase diagram

Magnetic-structure distortions induced by an external magnetic field near a rough surface of a uniaxial collinear antiferromagnet are analyzed using mathematical modeling. The “magnetic field-roughness” phase diagram of these distortions is constructed. It is demonstrated that, under specific conditions, the presence of atomic steps leads to the formation of domains in the surface layer of the antiferromagnet. In the case of a strong roughness, there arise static vortices at the surface and a 90° domain wall aligned parallel to the surface of the antiferromagnet.     

Corroboration of a multiscale approach with all atom calculations in analysis of dislocation nucleation from surface steps

We present a comparative study of the influence of atomic-scale surface steps on dislocation nucleation at crystal surfaces based on an all atom method and a hierarchal multiscale approach. The multiscale approach is based on the variational boundary integral formulation of the Peiersl–Nabarro dislocation model in which interatomic layer potentials derived from atomic calculations of generalized stacking fault energy surfaces are incorporated. We have studied nucleation of screw dislocations in two bcc material systems, molybdenum and tantalum, subjected to simple shear stress. Compared to dislocation nucleation from perfectly flat surfaces, the presence of atomic scale surface steps rapidly reduces the critical stress for dislocation nucleation by almost an order of magnitude as the step height increases. In addition, they may influence the slip planes on which dislocation nucleation occurs. The results of the all atom method and the multiscale approach are in good agreement, even for steps with height of only a single atomic layer. Such corroboration supports the further use of the multiscale approach to study dislocation nucleation phenomena in more realistic geometries of technological importance, which are beyond the reach of all current atom simulations.     

Magnetorotational instability in a nonuniform magnetic field

It is shown that the characteristics of the magnetorotational instability noticeably change in the presence of the inhomogeneity of a magnetic field in which rotation occurs. A decrease in the magnetic field with an increase in the distance from the rotation center, which is typical for astrophysical objects, can lead to a significant decrease in the threshold velocity of the object medium rotation, as well as to mitigation of the requirements on a rotation velocity profile that are necessary for the development of instability. Other examples that demonstrate the indicated effects are given. Original Russian Text ? V.I. Ilgisonis, I.V. Khalzov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 11, pp. 815–818.     

Structural, electronic, and magnetic properties of bcc iron surfaces

Trends in atomic multilayer relaxations, surface energy, electronic work function, and magnetic structure of several low-Miller-index surfaces of iron are investigated employing density functional theory total energy calculations. The calculated topmost layer relaxations reproduce well the experimental contractions and their variation with the surface crystallographic orientation, and surface roughness. The multilayer relaxation sequences correlate with the reduced coordination in surface layers and can be explained in terms of a simple electrostatic picture. The surface energies scale almost linearly with the surface roughness. They agree well with the experimental surface tensions and show a small anisotropy in agreement with predictions based on measurements for other metals. The equilibrium shape of a bcc Fe crystal is determined and discussed. The work function anisotropy is calculated and rationalized in terms of changes in the valence charge distribution. Significantly increased local magnetic moments of atoms in the surface region are determined. The correlation between the anisotropy of the surface magnetic moments and atomic coordination in the outermost layers is demonstrated to follow a simple rule.     

LEED studies of adsorption on vicinal copper surfaces

As a means for studying the role of atomic steps in adsorption phenomena LEED has been used to investigate the properties of vicinal copper surfaces. Single crystalline surfaces were cut at angles up to 20° from the (100) pole along [001] and [011̄] zones. The diffraction patterns obtained for the clean surfaces and after adsorption of oxygen, nitrogen ions, carbon and sulphur are described. The emphasis of the paper is on the method of interpretation of the geometry of the patterns, which may be done by straightforward kinematic analyses. In the case of nitrogen it is found that if the steps are widely separated the structure of the layer adsorbed on the terrace is the same as that on the low index surface. When the step spacing is small, and comparable with the crystalline parameter of the adsorbed layer, modifications occur which give rise to different superlattices which extend over several terraces. Adsorption of sulphur on 〈11〉 steps can produce a change in the periodicity of the adsorbed layer parallel to the step direction. The study of diffraction patterns for vicinal surfaces with different step spacings may provide an interesting technique for verifying the interpretation of patterns for low index surfaces.     

In plane to out of plane magnetic reorientation transition in partially ordered FePd thin films

It is demonstrated that perpendicular magnetic anisotropy may be obtained with a room temperature growth process in ordered (FePd) alloys. Indeed, using atomic layer by atomic layer epitaxy, a partial chemical ordering into the L1₀ structure is obtained, with a corresponding intermediate perpendicular anisotropy (). These samples provide an appropriate template for the study of the magnetic reorientation from in plane to out of plane magnetization upon layer's thickness increase. VSM, transverse Kerr measurements and magnetic force microscopy have been used in order to determine the relevant magnetic parameters and the occurrence of the reorientation transition. Received 13 October 1998 and Received in final form 5 February 1999     

Structure and magnetism of Fe mono‐ and multi‐layer systems as seen by conversion electron Mössbauer spectroscopy at atomic scale

The characteristic magnetic phenomena of ultrathin films are attributed to their reduced dimensionality and increased importance of the interfacial properties originated at their boundaries. The loss of nearest neighbor interactions at the interfaces, band hybridization, expansion or contraction of the atomic spacing occur, resulting in local changes of the energy band structure. Recent technical developments make it now possible to grow ultrathin films in a strictly layer‐by‐layer mode and to produce large areas of flat surfaces. Nevertheless, small structural perturbations in the local atomic configuration can still exist and result in significant changes of the global magnetic properties. Conversion Electron Mössbauer Spectroscopy (CEMS) determines the hyperfine interaction parameters which are sensitive to the arrangement at the atomic scale. In particular, depth selectivity at a monolayer level has been achieved in Fe films with one atomic layer replaced by the Mössbauer isotope ⁵⁷Fe. This contribution reviews the experimental work on magnetic phenomena of bcc, fcc and hcp Fe ultrathin films (including monolayer and multilayer structures), epitaxially grown by condensation from molecular beam under ultrahigh vacuum conditions. Since the structural and magnetic information can be achieved by using one method only, Mössbauer spectroscopy is pointed out as being an extremely effective and convenient tool for such purposes.     

Distribution of the magnetic scattering amplitudes in the Fe/Cu multilayer investigated by resonant magnetic diffraction with circularly polarized hard X-rays

Resonant X-ray magnetic diffraction profiles were measured for an epitaxial Fe/Cu multilayer using circularly polarized X-rays near the Fe and Cu K-edges. Diffraction intensities were compared with those obtained from the theoretical and empirical models. It is found that the interface Fe moment is reduced to 70% of the inner-layer moment. Concerning the Cu layer, the observed energy dependence of the magnetic diffraction intensities is consistent with that derived from the first-principle band calculation, indicating that magnetic proximity effect in the Cu layer is confined within a few atomic layers near the interface.     

Atomic-layer resolved magnetic and electronic structure analysis of ni thin film on a Cu(001) surface by diffraction spectroscopy

Up until now there has been no direct method for detecting the electronic and magnetic structure of each atomic layer at the surface, which is an essential analysis technique for nanotechnology. For this purpose, we have developed a new method, diffraction spectroscopy, based on the photon energy dependence of the angular distribution of Auger electron emission. We have applied this method to analyze the magnetic structure of a Ni ultrathin film on a Cu(001) surface around the spin reorientation transition. Atomic-layer resolved x-ray absorption and magnetic circular dichroism spectra were obtained. Surface and interior core-level shifts and magnetic moments are determined for each atomic layer individually.     

Study of the influence of native oxide layers on atomic force microscopy imaging of semiconductor surfaces

We have investigated the influence of the native oxide layer on semiconductor surfaces on the imaging properties of the atomic force microscope operated under ambient conditions by using epitaxial In_1–x Ga _x As layers grown by Metal-Organic Chemical Vapour Deposition (MOCVD) on (001) oriented InP substrates which have been kept under ambient conditions for two years. The thickness and composition of the native oxide layers were studied with ellipsometry and X-ray photoelectron spectroscopy, respectively. Subsequently, the sample surfaces were imaged by means of atomic force microscopy operated in air which revealed terrace structures separated by monoatomic steps. The obtained data were compared with the surface morphology which can be expected from the MOCVD growth process. The results suggest that an accurate study of semiconductor layer growth by atomic force microscopy in air is possible.     

Effect of roughness on the magnetic structure of ferro/antiferromagnetic interface

Spin structures at the ferro/antiferromagnetic interfaces perturbed by defects such as atomic high steps are analytically investigated. A two-dimensional model is proposed to describe the spin distribution formed on the interfacial step at the domain wall. A criterion of the domain wall configuration relative to the interface is found, defined by the magnetic and geometrical characteristics of the interface and the magnet.     

Modification of local magnetic properties of steel surfaces by a Q-switched Nd:YAG laser beam

Using a Q-switched Nd:YAG laser, we have successfully obtained laser-induced ripple patterns on various ferric metal surfaces. Changes of magnetic properties associated with the rippled areas were then investigated using a simple magnetic detecting system, which consisted of an audio recorder head, an audio amplifier and an oscilloscope. Significant magnetic changes associated with the formation of the ripples were found on a steel substrate pre-coated with a layer of chromium. This technique can be used as the basis of a machine readable laser marking system on steel surfaces, or even other surfaces that could be cotated with a steel/chromium film.     

Coordination-number dependence of magnetic hyperfine fields at (111)Cd on Ni surfaces

Ferromagnetic Ni surfaces were investigated on an atomic scale using the perturbed angular correlation spectroscopy probe (111)Cd. A comprehensive set of data for magnetic hyperfine fields (B(hf)) at various probe sites is presented. A field variation from -7 T in Ni bulk to the surprisingly large value of 16 T at the adatom position on Ni(111) is observed. A continuous nonlinear dependence is found, correlating the experimental B(hf) values with the number of their nearest Ni neighbors. The data are discussed on the basis of recent calculations on B(hf) values at sp-element impurities on ferromagnetic surfaces.     

Compressive surface stress in magnetic transition metals

Because of the increased electron density within the surface layer, metal surfaces are generally expected to have tensile surface stress. Here, using first-principles density functional calculations, we demonstrate that in magnetic 3d metals surface magnetism can alter this commonly accepted picture. We find that the thermodynamically stable surfaces of chromium and manganese possess compressive surface stress. The revealed negative surface stress is shown to be ascribed to the enhanced magnetic moments within the surface layer relative to the bulk values.     

Inelastic tunnel transport of electrons through an anisotropic magnetic structure in an external magnetic field

Quantum transport of electrons through a magnetic impurity located in an external magnetic field and affected by a substrate is considered using the Keldysh diagram technique for the Fermi and Hubbard operators. It is shown that in a strongly nonequilibrium state induced by multiple reflections of electrons from the impurity, the current-voltage (I–V) characteristic of the system contains segments with a negative conductivity. This effect can be controlled by varying the anisotropy parameter of the impurity center as well as the parameters of coupling between the magnetic impurity and metal contacts. The application of the magnetic field is accompanied by an increase in the number of Coulomb steps in the I–V curve of the impurity. The effect of appreciable magnetoresistance appears in this case. We demonstrate the possibility of switching between magnetic impurity states with different total spin projection values in the regime of asymmetric coupling of this impurity with the contacts.     

Monte Carlo study of magnetization reversal in the model of a hard/soft magnetic bilayer

Magnetization reversal in the model of a hard/soft magnetic bilayer under the action of an external magnetic field has been investigated by the Monte Carlo method. Calculations have been performed for three systems: (i) the model without a soft-magnetic layer (hard-magnetic layer), (ii) the model with a soft-magnetic layer of thickness 25 atomic layers (predominantly exchange-coupled system), and (iii) with 50 (weak exchange coupling) atomic layers. The effect of a soft-magnetic phase on the magnetization reversal of the magnetic bilayer and on the formation of a 1D spin spring in the magnetic bilayer has been demonstrated. An inf lection that has been detected on the arch of the hysteresis loop only for the system with weak exchange coupling is completely determined by the behavior of the soft layer in the external magnetic field. The critical fields of magnetization reversal decrease with increasing thickness of the soft phase.     

Manipulating magnetic anisotropy and ultrafast spin dynamics of magnetic nanostructures

We present our extensive research into magnetic anisotropy. We tuned the terrace width of Si(111) substrate by a novel method: varying the direction of heating current and consequently manipulating the magnetic anisotropy of magnetic structures on the stepped substrate by decorating its atomic steps. Laser-induced ultrafast demagnetization of a Co Fe B/Mg O/Co Fe B magnetic tunneling junction was explored by the time-resolved magneto-optical Kerr effect(TRMOKE) for both the parallel state(P state) and the antiparallel state(AP state) of the magnetizations between two magnetic layers. It was observed that the demagnetization time is shorter and the magnitude of demagnetization is larger in the AP state than those in the P state. These behaviors are attributed to the ultrafast spin transfer between two Co Fe B layers via the tunneling of hot electrons through the Mg O barrier. Our observation indicates that ultrafast demagnetization can be engineered by the hot electron tunneling current. This opens the door to manipulate the ultrafast spin current in magnetic tunneling junctions. Furthermore, an all-optical TR-MOKE technique provides the flexibility for exploring the nonlinear magnetization dynamics in ferromagnetic materials, especially with metallic materials.     

Dual immobilization and magnetic manipulation of magnetic nanoparticles

By suitably bio-functionalizing the surfaces, magnetic nanoparticles are able to bind specific biomolecules, and may serve as vectors for delivering bio-entities to target tissues. In this work, the synthesis of bio-functionalized magnetic nanoparticles with two kinds of bio-probes is developed. Here, the stem cell is selected as a to-be-delivered bio-entity and infarcted myocardium is the target issue. Thus, cluster designation-34 (CD-34) on stem cell and creatine kinase-MB (CK-MB) (or troponin I) on infarcted myocardium are the specific biomolecules to be bound with bio-functionalized magnetic nanoparticles. In addition to demonstrating the co-coating of two kinds of bio-probes on a magnetic nanoparticle, the feasibility of manipulation on bio-functionalized magnetic nanoparticles by external magnetic fields is investigated.     

Atomic hydrogen cooling under adiabatic expansion in a magnetic trap

A new method for cooling atomic hydrogen down to 100 μK or below is suggested. The method exploits the unique properties of atomic hydrogen, which are the following: atomic hydrogen does not condense at temperatures as low as 20 μK and cannot be heated by IR radiation in the absence of atom-wall collisions. Therefore, the most efficient and well-known gas cooling technique can be employed, namely, the adiabatic expansion of the volume occupied by the gas (this approach is used in a gas-expansion machine). It is suggested to adiabatically expand the volume of a magnetic trap containing atomic hydrogen.