Ab initio calculations of interfacial magnetism in Fe/Mo superlattices

Ab initio calculations have been performed on Fe/Mo(1 0 0) superlattices in order to study the interfacial magnetic properties and layer thickness effect on the magnetic moments. In most cases, the magnetic moments of interfacial Fe monolayers are always smaller than those of the inner layers, and the induced magnetic moments of interfacial Mo monolayers oriented in the opposite direction. Calculation results show that the Fe layers are ferromagnetic when n = 3. As the thickness of the Mo layers increases, the influence of the Mo layer increases and the magnetic state of the Fe layer gradually changes into an antiferromagnetic or non-magnetic state. The change of magnetic moments of Fe/Mo superlattices is in agreement with the experimentally observed oscillation periods.     

Effect of Bi surfactant on the heteroepitaxial growth in Fe/Cr(100) multilayers

This study was to investigate the surfactant effect of Bi on the heteroepitaxial growth of Fe/Cr(100) multilayers by reflection high-energy electron diffraction (RHEED) measurements. With predeposition of submonolayer Bi on Fe(100) prior to evaporation of Fe/Cr multilayer, more long-lasting RHEED intensity oscillations were observed. This implies that the layer-by-layer growth of Fe/Cr multilayer is enhanced. The observations of grazing incidence X-ray reflectivity confirmed that the interface structures of Fe/Cr multilayer with Bi were sharper than that of multilayer without Bi. The study was also to investigate the magnetotransport properties between Bi surfactant-mediated multilayers and normal ones. The magnetoresistance (MR) ratio of the multilayers was enhanced by predeposition of Bi.     

Onset of ferromagnetism in ultrathin Fe films on semiconductors

Very thin Fe films have been grown by molecular beam epitaxy on Ge(001), GaAs(001) and ZnSe(001) substrates, under identical preparation conditions. The electronic and magnetic properties of such interfaces have been studied, as a function of the Fe thickness, by means of spin resolved inverse photoemission. From the spin dependence of Fe empty states, we observe the onset of room temperature ferromagnetism to occur at a Fe thickness as low as three monolayers (ML) for Fe/Ge, while 5 and 8 ML have been found for Fe/GaAs and Fe/ZnSe, respectively.     

First-principles study of spin-density waves in Fe/Cr and V/Cr multilayers

A first-principles electronic structure calculation for Fe/Cr and V/Cr multilayers is performed, and spin-density wave order in the Cr layer is investigated. It is found that for a sufficiently large spacer thickness of the Cr layer the spin-density wave order grows spontaneously and becomes similar to that in bulk Cr in the vicinity of the middle of the Cr layer.     

Spin reorientation transition in Fe/CeH2 multilayers probed by soft X-ray resonant magnetic scattering

The magnetic domain configurations of Fe 3d spins in Fe/CeH₂ multilayers were measured by soft X-ray resonant magnetic scattering. The interface region could be probed by setting up X-ray standing waves due to the multilayer periodicity. By resolving first- and second-order magnetic scattering contributions, we show that the latter probe directly the magneto-crystalline anisotropy which is dominated by the Fe interface layers causing a spin reorientation transition when the temperature is lowered. Received: 30 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Effect of Co layer thickness on the magnetic properties of Tb30Co70/SiNx/Co thin films

The SiN_x (20 nm)/Tb₃₀Co₇₀ (90 nm)/SiN_x (5 nm)/Co (3–37 nm)/SiN_x (10 nm)/Si multilayer films are deposited on naturally oxidized Si wafer by magnetron sputtering. The saturation magnetization (M_s) of the multilayer films is increased with the thickness of high M_s ferromagnetic Co layer. The perpendicular coercivity (_Hc⊥$H_{{\mathrm{c}}^{\perp }}$) value is increased with Co layer thickness as the thickness of the Co layer is lower than 15 nm and then decreases drastically when the thickness of the Co layer further increased. The increase of the _Hc⊥$H_{{\mathrm{c}}^{\perp }}$ value is owing to the interlayer exchange effect [Li Zhang, Physica B 390 (2007) 373] between TbCo and Co layers. Co under-layer with in-plane magnetic anisotropy would pin the magnetic moment of the TbCo layer near by the Co layer and cause the value of _Hc⊥$H_{{\mathrm{c}}^{\perp }}$ to increase. However, as the Co layer is thicker than a critical thickness, the _Hc⊥$H_{{\mathrm{c}}^{\perp }}$ value of the multilayer film would decrease. Therefore, the Co layer with in-plane magnetic anisotropy and soft magnetic properties is expected to dominate the magnetic properties of the multilayer films.     

Composition and magnetic properties of Fe/Fe x N thin film multilayers

The structure, composition, and magnetic characteristics of thin films multilayers of iron-iron nitride with high magnetic moment and very low coercive field are investigated. The present study together with previous ones that described in detail the magnetic properties lead to the establishment of certain requirements to obtain iron-iron nitride multilayers with outstanding magnetic characteristics.Supported in part by Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brazil)     

Magnetic property and magnetoresistance in Fe/ITO multilayers

Giant magnetoresistance was found in DC magnetron sputtering Fe/ITO multilayers. The magnetic properties, electrical properties and magnetoresistance were investigated. A critical temperature is found around 50 K where the temperature dependence of resistivity and magnetoresistance ratio exhibit an abruptly change. The temperature dependence of resistance is found to obey Mott's 1/4 law for low temperature. The max magnetoresistance ratio of 2.0% and 6.7% is found at room temperature and 12.5 K, respectively. The increase of magnetoresistance ratio at low temperature is due to the decrease of spin-mixing effect.     

First-principle study on the electronic structure and magnetic properties in Fe/MgO/Fe magnetic tunnel junction with Mg insertion layer

The effect of a Mg insertion layer between the Fe electrode and the MgO barrier layer on the electronic structure and magnetic properties of Fe/MgO/Fe magnetic tunnel junction has been studied by first-principle method. Two models of (a) Fe(1 0 0)/MgO(1 0 0)/Fe(1 0 0) and (b) Fe(1 0 0)/Mg/MgO(1 0 0)/Mg/Fe(1 0 0) were established. Our calculation results show that the Mg insertion layer has enhanced both the spin polarization and the magnetic moment of its adjacent Fe layer. The results have been discussed in terms of the variation in the DOS features and charge transfer with the Mg insertion layer.     

Underlayer diffusion-induced enhancement of coercivity in high anisotropy FePt thin films

The L1₀ ordered FePt films have been prepared at 300 °C with a basic structure of CrRu/MgO/FePt, followed by a post-annealing process at temperatures from 200 to 350 °C. The magnetic properties and the microstructure of the films were investigated. It is found that coercivity of FePt films increases greatly from 3.57 to 9.1 kOe with the increasing annealing temperature from 200 to 350 °C. The loop slope of the M–H curves decreases with the increasing annealing temperature, which is due to the grain isolation induced by MgO underlayer diffusion during the annealing process. The underlayer diffusion could be a useful approach to prepare the FePt-based composite films for high-density recording media.     

Pulsed laser deposition and characterization of multilayer metal-carbon thin films

Cobalt-DLC multilayer films were deposited with increasing content of cobalt, keeping carbon content constant by pulsed laser deposition technique. A cobalt free carbon film was also deposited for comparison. Excimer laser was employed to ablate the materials onto silicon substrate, kept at 250 °C, while post-deposition annealing at 400 °C was also performed in situ. The formation of cobalt grains within the carbon matrix in Co-DLC films can be seen through scanning electron and atomic force micrographs while no grains on the surface of the cobalt-free DLC film were observed. Raman spectra of all the films show D- and G-bands, which is a confirmation that the films are DLC in nature. According to Vibrating sample magnetometer (VSM) measurements, the DLC films with cobalt revealed ferromagnetic behaviour whereas the cobalt free DLC film exhibited diamagnetic behaviour. The pure DLC film also shows ferromagnetic nature when diamagnetic background is subtracted. Spectroscopic Ellipsometry (SE) analysis showed that the optical band gaps, refractive indices and extinction coefficients of Co-DLC films can be effectively tuned with increasing content of cobalt.     

Structural and Magnetic Properties of [Fe/Ni]N Multilayers

[Fe/Ni]N multilayered structure grows epitaxially on the single crystalline MgO substrate. Due to the different directions of magnetic easy axes of Fe and Ni and the strong strain, large anisotropy dispersion is assumed. According to the layer model, the magnetization of Fe and Ni layers cannot follow each easy axis because of exchange coupling, and then the anisotropies are averaged out. The reduction of the effective anisotropy enhances with the decrease of periodic thickness. Thus, the coercivity of [Fe/Ni]N multilayers reduces with decreasing periodic thickness.     

Out-of-plane exchange bias and magnetic anisotropy in MnPd/Co multilayers

Magnetic and structural properties in [MnPd/Co]₁₀ multilayers deposited onto Si(1 1 1) substrates have been investigated. The dependences of anisotropy and exchange bias on the thicknesses of both MnPd and Co layers have been studied. In most of the samples, the out-of-plane magnetic anisotropy and both large out-of-plane and in-plane exchange biases have been observed at cryogenic temperature below the blocking temperature T_B≈240 K. With appropriate MnPd and Co thicknesses, we have obtained samples with a large out-of-plane exchange bias along with a large out-of-plane magnetic anisotropy. The origin of the out-of-plane magnetic anisotropy in the samples has been suggested to be due to the formation of CoPd interfacial alloys which have tensile in-plane strains, while the spin structure of the antiferromagnetic layer at the interface which is believed to be responsible for exchange bias may be the same as that of the bulk material. Also, the present study shows that the interplay between the out-of-plane magnetic anisotropy and exchange bias is evident in our multilayers and plays an important role in the out-of-plane exchange-bias mechanism.     

Structure and magnetic properties of Fe4/Cun (n=2,4) superlattices from first-principles study

The structures and magnetic properties of Fe4/Cun (n=2, 4) superlattices have been investigated by the first-principles pseudopotential plane-wave method based on spin density approximation. Compared with the ideal fcc-Cu bulk structure, for the optimized Fe4/Cu2 model, obvious contraction of interlayer distances occurs on the interior Fe layers, whereas the interlayer distances of Fe layers in Fe4/Cu4 are expanded. The anti-parallel alignment magnetic moment and negative polarization of the interior Fe layer have been found in the Fe4/Cu2 model. This can be explained in terms of the magnetic-volume effect, and the moment of anti-parallel alignment attributes to the contracted interlayer distances between the interior Fe layers. The MR ratio has also been evaluated by means of the two-current model. The MR ratio of the Fe4/Cu2 model (4.89%) is much small than that of the Fe4/Cu4 one (23.65%).     

Dependence of magnetic properties on ferromagnetic layer thickness in trilayer Co/Ge/Co films with granular semiconducting spacer

We have investigated the magnetic properties of trilayer films of Co–Ge–Co. At a fixed thickness of germanium of 3.5 nm, the formation and distribution of metastable amorphous and cubic phases depends on the thickness of the ferromagnetic layer. The portion of the stable hexagonal phase is affected, too. Possible mechanisms for forming the observed magnetic structure are discussed.     

Magnetic properties and structure of FePt/FeMn multilayers

A systematic study of the magnetic properties by ion beam sputter-deposition system, was conducted in conjunction with the structure of FePt/FeMn multilayers fabricated onto MgO(0 0 1) substrates. Both parallel and perpendicular exchange biases were observed in the multilayers and were found to decrease drastically, as the deposition temperature is higher than 350 °C, which is evidently due to the interdiffusion at the interface. The thickness dependence study shows that the perpendicular magnetic anisotropy observed in the multilayers originates from surface anisotropy, being consistent with the decrease of perpendicular magnetic anisotropy as the deposition temperature is increased. The difference between parallel and perpendicular blocking temperatures that was clearly observed, is possibly due to the spin canting out of plane at the interface.     

Numerical Simulation and Experimental Analysis of Photonic Band Gap in Hollow-Core Photonic Crystal Fibres

Based on the full-vector plane-wave method （FVPWM）, a hollow-core photonic crystal fibre （HC-PCF） fabricated by using the improved stack-and-draw technique is simulated. Under given propagation constants β, several effective photonie band gaps with different sizes emerge within the visible wavelength range from 575 to 720 nm. The fundamental mode and second-order mode lying in a part of PBGs are investigated. In the transmission spectrum tested, the positions of PBGs are discovered to be shifting to shorter wavelengths. The main reason is the existence of interstitial holes at nodes in the cladding region. In the later experiment, green light is observed propagating in the air-core region, and the result is more consistent with our theoretical simulation.     

Two-dimensional electron-hole pair diffusivities in thin GaAs/AlGaAs Quantum Wells

Diffusivities of two-dimensional electron-hole pairs in thin GaAs/AlGaAs Quantum Wells (QWs) are studied experimentally and theoretically as functions of temperature and well-width. With growing well-widths, increasing diffusivities are observed for fixed Al-contents. Experimental diffusivities for the lateral carrier motion in continuously as well as in interrupted-grown thin QWs of different barrier Al-content are presented for T>150 K. Increasing diffusivities are observed for rising temperatures in the range T190 K. A comparison of the experimental data and results of theoretical model calculations indicates that the increase is partly related to thermal dissociation of excitons into free carrier pairs. The effective diffusivity of this two-component system is calculated using a system of rate equations and considering acoustic-deformation-potential scattering, polar-optical scattering and barrier-alloy-disorder scattering.The experimental data were obtained at: 4. Physikalisches Institut, Universität Stuttgart, Pfaffenwaldring 57, W-7000 Stuttgart 80, Germany     

High-frequency dynamics of Co/Fe multilayers with stripe domains

Within the framework of two-dimensional (2D) numerical micromagnetic simulations, the equilibrium magnetization configuration and the high-frequency (0.1–30 GHz) linear response of Co/Fe multilayers have been investigated in detail. Due to the perpendicular anisotropy of Co layers, a stripe domain pattern can develop through the whole multilayer, the characteristics of which depend on the magnitude of the perpendicular anisotropy, the respective thicknesses of Co and Fe layers and the number of Co/Fe bilayers in the stack. One of the most striking features associated with the layering effect is the ripening aspect of the static magnetization configuration across the multilayers which induces complicated dynamic susceptibility spectra including surface modes and volume modes strongly confined within the inner Fe layers. The effect of the cubic magnetocrystalline anisotropy of Fe layers and the influence of a nonuniform perpendicular magnetic anisotropy within the Co layers on the static and dynamic magnetic properties of Co/Fe multilayers are then analyzed quantitatively.