Percolation scale effects in metal-insulator thin films

Thin metal films near their continuity threshold and metal-insulator mixture films near their metal-insulator transition are well described by the percolation theory. Here we demonstrate that statement by reviewing some geometrical measurements done on both types of thin films. We then comment on the measurements of the other physical quantities.     

Anisotropic metal-insulator transition in epitaxial thin films

By comparing the properties of In and Pb quantum wells in a scanning tunneling microscopy subsurface imaging experiment, we found the existence of lateral bound states, a 2D Mott-Hubbard correlation gap, induced by transverse confinement. Its formation is attributed to spin or charge overscreening of quasi-2D excitations. The signature of the 2D confinement-deconfinement transition is also experimentally observed, with the correlation gap being pinned in the middle of the conduction band. A self-organized 2D Anderson lattice is suggested as a new ground state.     

Micron-sized atom traps made from magneto-optical thin films

We have produced magnetic patterns suitable for trapping and manipulating neutral atoms on a 1-m length scale. The required patterns are made in Co/Pt thin films on a silicon substrate, using the heat from a focused laser beam to induce controlled domain reversal. In this way we draw lines and paint shaped areas of reversed magnetisation with sub-micron resolution. These structures produce magnetic microtraps above the surface that are suitable for holding rubidium atoms with trap frequencies as high as 1 MHz. PACS 39.25.+k; 03.75.Be; 75.50.Ss; 75.70.-i     

Magnetic and magneto-optical properties of FeRh thin films

The magnetic and magneto-optical properties of FeRh thin films epitaxially deposited onto MgO(1 0 0) substrates by RF sputter-deposition system have been investigated in conjunction with the structure. An intriguing virgin effect has been found in the M–T curves of the as-deposited FeRh thin films, which is presumably interpreted in term of a change in structural phase when heating. Also, a (negative) maximum peak of Kerr rotation at around 3.8 eV has been observed when FeRh thin films are in ferromagnetic state. The polar Kerr rotation angle is found to increase at temperatures above 100 °C, which corresponds to the antiferromagnet (AF)–ferromagnet (FM) transition of FeRh thin films.     

Electrically controlled metal-insulator transition process in VO2 thin films

We report the evolution process of VO(2) thin films from the insulating phase to the metallic phase under current injection for the two-electrode-based thin-film devices. Based on electrical characterization and Raman microscopic detection, it was found that there exist two critical current densities, based on which the insulator-to-metal transition process can be divided into three stages. In stage I with low current injection, the VO(2) film in the insulating (semiconducting) phase acts as a resistor until the first critical current density, above which the insulator-metal transition is a percolation process with metallic rutile and insulating monoclinic phases coexisting (stage II); while beyond a second critical current density, a filamentary current path with pure metallic phase is formed with the remaining part outside of the current path receding back to the pure insulating phase (stage III). We confirm that a critical current density is required for the onset of electrically induced insulator-to-metal transition in VO(2) thin films.     

Thermal analysis of magneto-optical thin films under laser irradiation

A two-dimensional finite-difference method was applied to study the thermal behavior of thin films irradiated by a pulsed Gaussian laser beam. In particular, the method was applied to rare-earth/transition-metal alloyed films for magneto-optic recording. The effects of film parameters (thermal conductivity, specific heat, absorption coefficient, and film thickness) and laser parameters (output power and pulse duration) on healing and cooling behavior were investigated extensively. The temperature profile of a film was sensitively dependent on these parameters. The results might be used to predict the recorded bit size and to optimize the laser parameters. For a given energy density of the laser beam, a large output power with a short pulse width was desirable for effective heating of a film. Heating by radial thermal diffusion became important with increasing thermal conductivity of a film. Therefore, to achieve high storage density, a film must have low thermal conductivity.     

Structure and magneto-optical properties of fine-grain garnet thin films

The structure and magneto-optical properties of fine-grain garnet thin films crystallized by the rapid recurrent thermal annealing (RRTA) method have been studied. The RRAT method has been used to crystallize BiGaDyIG garnet single-layer or BiGaDyIG/AI double-layer films and to get nanometer grain size (about 30–50 nm), which results in a large Faraday rotation angle, a smoother surface and fewer voids in the films. Meanwhile we have discovered that the Faraday rotation angle increases with the number of recurrences during the rapid annealing and quenching. With the more recurrent annealing one can not only get a strong Faraday effect, but it suppresses the appearance of DyFeO₃ phase in garnet films, which has been explained very well. By applying the new method, the as-deposited films have been succesfully crystallized to the (BiDy)₃(FeGa)₅O₁₂ garnet phase. They exhibited excellent magneto-optical properties with a coercivity of about 1500 Oe and effective Faraday rotation angle of 1.5°. The composition, magnetic and magneto-optical properties of the crystallized garnet films have been examined.     

Memory effects and slow dynamics in ultra thin magnetic films

Monte Carlo studies of a uniaxial spin system on a square lattice with both ferromagnetic exchange interactions and dipolar interactions reveal three distinct dynamical regimes at low temperatures. In the first regime the magnetization decays through nucleation and growth of spin islands, in the second regime, despite the magnetization being effectively zero, there is a persistent memory effect most clearly revealed by the asymmetry between the numbers of up and down islands. The third regime is characterized by slow crystallization of the equilibrium smectic phase from the quenched polycrystalline state.     

Application of longitudinal generalized magneto-optical ellipsometry in magnetic ultrathin films

The longitudinal generalized magneto-optical ellipsometry(GME) method is extended to the measurement of threelayer ultrathin magnetic films. In this work, the theory of the reflection matrix is introduced into the GME measurement to obtain the reflective matrix parameters of ultrathin multilayer magnetic films with different thicknesses. After that, a spectroscopic ellipsometry is used to determine the optical parameter and the thickness of every layer of these samples, then the magneto-optical coupling constant of the multilayer magnetic ultrathin film can be obtained. After measurements of a series of ultrathin Fe films, the results show that the magneto-optical coupling constant Q is independent of the thickness of the magnetic film. The magneto-optical Kerr rotations and ellipticity are measured to confirm the validity of this experiment. Combined with the optical constants and the Q constant, the Kerr rotations and ellipticity are calculated in theory. The results show that the theoretical curve fits very well with the experimental data.     

Surface magnetic anisotropy effects in the elastoexchange spin dynamics of thin magnetic films

Anomalies arising in the elastoexchange spin dynamics of a bounded magnet when exchange boundary conditions are imposed are studied for the example of a uniformly magnetized film of a rhombic antiferromagnetic. Zh. Tekh. Fiz. 68, 110–115 (August 1998)     

Size effects in thin antiferromagnetic layers and “ferromagnet-nonmagnetic metal” multilayer magnetic structures

The specific features of the spin-flop and spin-flip transitions in thin antiferromagnetic layers and “ ferromagnet-nonmagnetic metal” multilayer magnetic structures are considered. The dependence of the magnetic fields corresponding to these phase transitions on the thickness of the antiferromagnet or on the number of layers in the multilayer is determined.     

Linear and quadratic magneto-optical effects observed in La0.7Sr0.3MnO3 thin films in transmitted light

Magnetic linear dichroism (MLD) is observed in polycrystalline La_0.7Sr_0.3MnO₃ films in transmitted light. The spectral and temperature dependences of the MLD are determined for the energy E of the light wave ranging from 1.5 to 3.5 eV. The experimental spectra are fitted by a sum of Lorentzian curves. The MLD spectrum exhibits more features in comparison to the spectra of magnetic circular dichroism (MCD), which we measured earlier using the same film samples. Various temperature behaviors of the intensities of the Lorentzian components centered at different E values have been revealed. This can be related to the effect of the surface states on the magneto-optical characteristics of the samples.     

c轴垂直取向FePt薄膜的磁和磁光性能研究

采用直流磁控溅射的方法在氧化镁(100)单晶基板上生长了一系列c轴垂直取向的FePt薄膜,通过改变沉积时的基板温度,薄膜从Fe,Pt原子无序排列的面心立方结构逐渐变化到有序排列的L10相面心四方结构.在此基础上,系统研究了FePt薄膜的化学有序度对磁和磁光性能的影响.随着有序度的增加,FePt薄膜的磁晶各向异性能,以及沿垂直方向的矫顽力、剩磁比均增加,在基板温度高于530℃时制备的薄膜中的磁晶各向异性能超过1J/cm3.同时,还观察到有序FePt合金薄膜的磁光克尔光谱(克尔转角的大小和极值所对应的跃迁光子能量)随化学有序度的显著变化.     

An investigation of time-dependent domain wall pinning effects in Tb/Fe multilayer thin films

Reverse domain nucleation time measurements have been performed on two Tb/Fe multilayer magneto-optic films exhibiting different degrees of domain wall pinning. A linear relationship between ln (reverse domain nucleation time) and the applied field has been predicted and observed for a sample exhibiting weak domain wall pinning. This is in agreement with theoretical work presented which addresses time dependence in systems possessing weak domain wall pinning. A non-linear relationship applicable over a restricted field range has been derived for a sample exhibiting strong domain wall pinning. Experimental results have indicated that this relationship is also valid.     

Giant magneto-optical faraday effect in HgTe thin films in the terahertz spectral range

We report the observation of a giant Faraday effect, using terahertz (THz) spectroscopy on epitaxial HgTe thin films at room temperature. The effect is caused by the combination of the unique band structure and the very high electron mobility of HgTe. Our observations suggest that HgTe is a high-potential material for applications as optical isolator and modulator in the THz spectral range.     

Enhanced magneto-optical Kerr effects in nanocrystalline Sc-doped CoFe2O4 thin films

Nanocrystalline CoFe_2−xSc_xO₄ (x=0-0.4) thin films were prepared on silicon substrates at reduced temperature by a sol-gel process, and the doping effects of scandium on the microstructure, magnetism and polar magneto-optical Kerr effects of the as-deposited films were examined. It was shown that the intensities of both of the Kerr rotation peaks increase with the doping content x of Sc³⁺. The increase for the peak at 540 nm is due to the decrease of the electrostatic polarization of O²⁻ resulting from the relatively large radius of Sc³⁺, and that for the peak at 620 nm was a result of the migration of Co²⁺ from octahedral to tetrahedral sites in the presence of the dopant of Sc³⁺.     

Quantum size effects for the extraordinary Hall effect in thin magnetic films

A theory for the extraordinary Hall effect in thin films is derived using the Kubo formalism. We calculate the skew-scattering contribution to the Hall resistivity. Oscillations of the resistivity with the thickness of the magnetic layers are obtained similar to the diagonal resistivity, but as the Hall current is due to d-electrons, the period of these oscillations is connected with the Fermi wave vector of the d-electrons.