Evidence for noncollinearity between surface and bulk magnetization in ultrathin Co films

The magnetization reversal of ultrathin Co films on Cu(001) has been investigated by grazing ion scattering and magneto-optical Kerr effect. Differences in the behavior of surface and bulk magnetization are found and attributed to the reduced coordination and site symmetry at the surface. The reversal behavior of the surface magnetization depends on the chemical surface composition. For pure Co films, the reversal of the bulk magnetization is preceded by a complete reversal of the surface magnetization. A particular magnetic state of the surface is suggested as a precursor for magnetization reversal.     

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.     

Magnetization and magnetic circular dichroism of La0.7Sr0.3MnO3/YSZ polycrystalline films

The magnetic and magneto-optical properties as well as the surface morphology and structure of La_0.7Sr_0.3MnO₃ polycrystalline films of different thicknesses have been investigated. It has been shown that films have a rough surface with a characteristic in-plane linear dimension of ～30 nm, and crystallites with linear dimensions of 10–20 nm are randomly oriented in the plane. The temperature and field dependences of the magnetization of the films indicate their magnetic inhomogeneity. The magnetic circular dichroism spectra contain a number of bands, one of which at 2.4 eV is explained by the contribution of conduction electrons.     

Observation of reversed hysteresis loops and negative coercivity in CoFeAlO magnetic thin films

The magnetic and magneto-optical properties of CoFeAlO thin films fabricated by the RF magnetron reactive sputtering technique have been studied via vibrating sample magnetometer and the magneto-optical Kerr effect. It is found that, along the hard magnetization direction, the magnetic hysteresis loop is reversed and the coercive force is negative. This phenomenon can qualitatively be interpreted within the framework of the proposed two-layers model. It was also found that when as-deposited CoFeAlO thin films were annealed in vacuum (2×10⁻⁷ Torr) at a temperature of 180 °C for 2 h, the negative coercivity disappeared. The relationship between the magnetic and microstructural behaviors of the thin films is discussed.     

Structure and the magnetic and magneto-optical properties of Co-Sm-O nanogranular films

The magnetic properties and magneto-optical effects in nanocomposites based on Co-Sm-O films prepared through pulsed plasma sputtering of a SmCo₅ target are investigated. It is shown that, depending on the technological conditions and regimes of subsequent annealing, the films can have different structures from cobalt nanoparticles distributed in the dielectric samarium oxide matrix with a magnetic phase volume of more than 60% to a continuous polycrystalline cobalt film with embedded samarium oxide nanoparticles. The evolution of the spectra of the magneto-optical Kerr effect and the field dependences of the magnetization is studied as a function of the film structure.     

Influence of induced anisotropy on the processes of magnetization reversal of cobalt circular nanodots

The magnetic structure and the processes of magnetization reversal of individual cobalt nanodots and arrays of cobalt nanodots have been studied using the magneto-optical Kerr effect and magnetic force microscopy. Arrays of nanodots have been prepared by ion etching from a continuous cobalt film. Magnetic anisotropy is induced during deposition of the cobalt films. The nanodots have the diameter d = 600 nm and the period varying from 1.5d to 3.0d. Magnetic force microscopy images have shown that the induced magnetic anisotropy affects the orientation of magnetization of noninteracting nanodots and the direction of displacement of the magnetic vortex center in the nanodots coupled by the dipole-dipole interaction.     

Magneto-optical effects for detection of in-plane magnetization in plasmonic crystals

Methods for magneto-optical detection of the in-plane magnetization in a magnetic film due to the deposition of a one-dimensional metallic diffraction grating on the film have been considered. This structure is a magnetoplasmonic crystal, in which the excitation of the waveguide and plasmon modes leads to the appearance of five resonant magneto-optical effects that consist in a change of the intensity, the polarization, and the phase of the transmitted and reflected waves. The conditions responsible for the origin of these effects and their magnitude are determined by the configuration of the incident light, the parameters of the metallic grating, and the chemical composition of the magnetic layer. It has been found that the magnetophotonic intensity effects are the most optimal for the detection of the in-plane magnetization. The influence of the parameters of the metallic grating on the magneto-optical effects has been analyzed and the most optimal conditions for the observation of these effects have been determined. It has also been found that an increase in the concentration of bismuth in a magnetic dielectric material can lead to a weakening of the optical and magneto-optical responses.     

Rare earth-transition metal compound-based MOSLM for the visible spectral range

We have demonstrated a magneto-optical spatial light modulator in which functionality is realized by (i) heating up to Curie temperature (T_c) magneto-optical elements (pixels) with a semiconductor laser and (ii) application of a switching magnetic field. The pixels were made of films of amorphous rare earth-transition metal compounds (TbFe films with T_c=403 K and DyFe films with T_c=343 K) having good magneto-optical responses for wavelengths from the visible spectral range. We have found that the magnetization direction of pixels can be modulated with a laser radiation density of 5 mJ/cm² and in a switching magnetic field of 15 Oe.     

Perpendicular magnetic anisotropy induced by tetragonal distortion of FeCo alloy films grown on Pd(001)

We grew tetragonally distorted FexCo1-x alloy films on Pd(001). Theoretical first-principles calculations for such films predicted a high saturation magnetization and a high uniaxial magnetic anisotropy energy for specific values of the lattice distortion c/a and the alloy composition x. The magnetic anisotropy was investigated using the magneto-optical Kerr effect. An out-of-plane easy axis of magnetization was observed for Fe0.5Co0.5 films in the thickness range of 4 to 14 monolayers. The magnetic anisotropy energy induced by the tetragonal distortion is estimated to be almost 2 orders of magnitude larger than the value for bulk FeCo alloys. Using LEED Kikuchi patterns, a change of the easy axis of magnetization can be related to a decrease of the tetragonal distortion with thickness.     

Theoretical and experimental study of Fe/Cr nanometric quasiperiodic multilayers

An experimental and theoretical study of magnetization curves of Fe/Cr nanometric magnetic films grown with the structure of the quasiperiodic Fibonacci sequence is presented. Fe ferromagnetic films with interfilm exchange coupling provided by intervening Cr non-ferromagnetic layers were grown on MgO (100) by dc magnetron sputtering at 300 ^°C. The magnetization curves were investigated using the magneto-optical Kerr effect with the external field applied along the easy axis. The theoretical approach for this system is based on a realistic phenomenological model that includes the following contributions to free magnetic energy: Zeeman, cubic magneto-crystalline anisotropy, as well as bilinear and biquadratic exchange energies. Our numerical results are in very good agreement with the experimental data.     

Magnetic and magneto-optical properties of ion-synthesized cobalt nanoparticles in silicon oxide

The magnetic and magneto-optical properties of ion-synthesized cobalt nanoparticles in the amorphous silicon oxide matrix are investigated as a function of the implantation dose. The analysis of the field dependences of the magnetization and the magneto-optical Faraday and Kerr effects demonstrates that, as the ion implantation dose increases, the superparamagnetic behavior of an ensemble of cobalt nanoparticles at room temperature gives way to a ferromagnetic response with the anisotropy characteristic of a thin magnetic film. The magnetization curves for the superparamagnetic and ferromagnetic ensembles of cobalt nanoparticles are simulated to determine their average sizes and the filling density in the irradiated layer of the silicon dioxide matrix. It is revealed that the spectral dependences of the Faraday and Kerr effects for ion-synthesized cobalt nanoparticles differ substantially from those for continuous cobalt films due to the localized excitations of free electrons in the nanoparticles.     

Domain and wall structures in films with helical magnetization profile

We study soft magnetic bilayers having orthogonal, in-plane easy axes. The layers are thicker than the Bloch wall width linked to the anisotropy, so that a helical magnetization with a large angle exists across the sample thickness. The magnetic domains structure has been investigated at both sample surfaces, using magneto-optical microscopy. The domain structure is found to be similar to that of double films with biquadratic coupling. Two kinds of domain walls are identified, namely with a 90° and 180° rotation of the average magnetization. The detailed structure and energy of these walls are studied by micromagnetic calculations.     

Nongyrotropic magneto-optical effects in metal-insulator magnetic multilayer thin films

Nongyrotropic magneto-optical effects are investigated in metal-insulator magnetic multilayer thin films. These effects manifest themselves in changes in the coefficients of transmission and reflection of electromagnetic radiation from the surface of a multilayer film due to the crossover of the magnetic structure from an antiferromagnetic configuration to a ferromagnetic configuration. The nongyrotropic magneto-optical effect observed in reflected light is analyzed theoretically. It is assumed that the multilayer structure is exposed to radiation of a monochromatic plane wave polarized along the direction of magnetization of the film. The magneto-optical effect is described in terms of the permittivity tensor of the multilayer medium, which depends only on the light frequency. The Boltzmann kinetic equation is treated with allowance made for spin-dependent electron scattering both inside conducting layers and at rough interfaces. Using an Fe/C multilayer as an example, it is demonstrated that the nongyrotropic magneto-optical effect is equal in order of magnitude to the equatorial Kerr effect or other strong magneto-optical effects.     

Modulation of ultrafast laser-induced magnetization precession in BiFeO_3-coated La_(0.67)Sr_(0.33)MnO_3 thin films

The ultrafast laser-excited magnetization dynamics of ferromagnetic(FM) La_(0.67)Sr_(0.33)MnO_3(LSMO) thin films with BiFeO_3(BFO) coating layers grown by laser molecular beam epitaxy are investigated using the optical pump-probe technique.Uniform magnetization precessions are observed in the films under an applied external magnetic field by measuring the time-resolved magneto-optical Kerr effect.The magnetization precession frequencies of the LSMO thin films with the BFO coating layers are lower than those of uncoated LSMO films,which is attributed to the suppression of the anisotropy field induced by the exchange interaction at the interface between the antiferromagnetic order of BFO and the FM order of LSMO.     

Magnetic properties of soft layer/FePt--MgO exchange coupled composite perpendicular recording media

The magnetic properties of exchange coupled composite （ECC） media that are composed of perpendicular magnetic recording media FePt MgO and two kinds of soft layers have been studied by using an x-ray diffractometer, a polar Kerr magneto-optical system （PMOKE） and a vibrating sample magnetometer （VSM）. The results show that ECC media can reduce the coercivities of perpendicular magnetic recording media FePt-MgO. The ECC media with granular-type soft layers have weaker exchange couplings between magnetic grains and the magnetization process, for ECC media of this kind mainly follow the Stoner Wohlfarth model.     

Parameters of the transition layer in the exchange-biased NiFe/DyCo film structure

The parameters of the transition layer in exchange-biased film structures are necessary agents to understand the mechanism of formation of unidirectional anisotropy. The layer thickness in NiFe/DyCo films has been determined by comparison of signals of the polar magneto-optical Kerr effect from a reference DyCo film and a hard magnetic layer of the exchange-biased structure. The layer thickness obtained is one order of magnitude larger than that characteristic of ferromagnet-antiferromagnet bilayer films. The mechanism of magnetization reversal of the structure under study has been explained within the model suggesting the formation of 180° boundaries in the interface.     

Magnetic-Field-Induced Optical Second-Harmonic Generation Study of Co/Pt and Co/Ta Interfaces

Magnetic properties of an interface between cobalt and platinum or tantalum nanolayers have been studied by the optical second-harmonic generation and nonlinear magneto-optical Kerr effect methods. It has been shown that a high sensitivity of the second-harmonic generation method makes it possible to determine the orientation of the easy magnetization axis in the plane of a polycrystalline structure without measurement of the magnetic field dependence of second-harmonic generation. The comparison of the field dependences of magnetic-field-induced second-harmonic generation with the linear magneto-optical effect indicates the difference in the processes of magnetization reversal in Co/Pt and Co/Ta interfaces and the bulk of the cobalt film. In particular, a new linear in magnetization effect has been observed in the second harmonic that is symmetry-forbidden for uniformly magnetized structures.     

Interface effect on the magnetic properties of Ni-Ge bilayer films

Magnetic and magneto-optical properties of the Ni-Ge bilayer films are studied. The unusual temperature behavior of the magnetization curves is revealed: the hysteresis loops at room temperature have a near-rectangular shape; when the films are cooled down to 4.2 K, the coercive force increases by more than an order of magnitude and the asymmetry and displacement of the loop along the field axis are observed. These effects are stronger in thinner Ni layers. The observed features are attributed to the effect of the interlayer between the Ni and Ge layers, which has a complex magnetic structure.     

Preisach modeling of aftereffect in a magneto-optical medium with perpendicular magnetization

The aftereffect of Co/Pt multilayer films with perpendicular magnetization has been measured with a magneto-optical Kerr effect (MOKE) magnetometer and calculated with a newly developed Preisach model. Compared to materials such as traditional magnetic recording media, Co/Pt multilayer films show a more complete picture of the progression of aftereffect because the magnetization of this material decays from saturation almost all the way to a ground state in a reasonable length of time. The magnetization measurements for times equal to negative and positive infinity are asymptotically horizontal, with a transition region that is linear on a logarithmic time scale. In contrast, typical published aftereffect analyses exhibit only a very small percentage of the total aftereffect that could be observed if time were not a factor in making measurements. A Preisach–Arrhenius model is used to calculate the magnetic aftereffect in the Co/Pt multilayer. Comparisons of the model to experimental results show not only the validity of the model, but also its value in predicting very short-time and long-time aftereffect behavior, and low levels of aftereffect occurring in noisy data, all of which are difficult to observe experimentally.