Volume 95, Issue 11, 01 June 2004
Index of content:
- The 9th Joint MMM/Intermag Conference
- Magnetic Semiconductors I: III-V Compounds
95(2004); http://dx.doi.org/10.1063/1.1683031View Description Hide Description
Magnetic properties of (Ga,Mn)As digital ferromagneticheterostructures have been investigated by polarized neutron reflectometry and magnetometry. of three samples with 20, 50, and 100 ML GaAs spacers ranges from 30 to 40 K. The saturationmagnetization of three samples exhibits a pronounced tail extending over 50 K above in addition to a temperature-independent background. For the 50 ML sample, PNR measurements show a similar tail but no background. These behaviors can be explained by a two-step ordering process. In the tail region, two-dimensional islands first individually become ferromagnetic. Long-range order develops as the temperature is decreased below
95(2004); http://dx.doi.org/10.1063/1.1669337View Description Hide Description
We report on measurements of the hole density,ferromagnetictransition temperature and magnetization in a series of as-grown and annealed (Ga,Mn)As samples. Estimating the fraction of incorporated Mn occupying interstitial and substitutional sites allows a direct comparison of the predictions of mean field theory with experiment, and a determination of the magnetic moment per substitutional Mn. The saturation of at high Mn concentration is consistent with the mean field prediction. The estimated magnetic moment per Mn is close to the expected for all samples studied if an antiferromagnetic coupling between interstitial and substitutional Mn is assumed.
95(2004); http://dx.doi.org/10.1063/1.1688591View Description Hide Description
Magnetic MnGeAsP films with the nominal composition were grown on GaAs(100) by solid sourcemolecular beam epitaxy. The films were grown keeping the Mn, Ge, and As fluxes constant while varying the P flux via its cell temperature. A streaky reflection high energy electron diffraction pattern with intensity variations along each reflection line was observed, suggesting small domain sizes in the films. The incorporation of P in the films was examined by x-ray photoelectron spectroscopy, and an increase of the P/As peak ratio was observed with increasing phosphorous cell temperature. Superconducting quantum interference device magnetic measurements revealed a clear change in the magnetic properties as phosphorous was introduced into the films. The coercive field decreases with increasing P/As ratio. The measured Curie temperatures were over 380 K for P/As ratios larger than 2, which is higher than that of a nominal and film.
95(2004); http://dx.doi.org/10.1063/1.1687553View Description Hide Description
We investigate the properties of interfaces between highly spin-polarized zinc-blende (ZB) CrAs and GaAs using first-principles density functional calculations. It is found that the local spin polarization at the Fermi level is very high even at ZB-CrAs/GaAs interface and then exhibits gradual decay from the interface to the inside of GaAs, reflecting their ideal structural matching. We further study the properties of ZB-CrAs/GaAs multilayer where two-monolayer ZB-CrAs and two-monolayer GaAs stack alternately. The multilayer shows high spin polarization throughout its entire region. Since the spin polarization is found insensitive to Cr-Ga substitutional disorder, both ZB-CrAs and ZB-CrAs/GaAs multilayer may work as efficient filter for spin-polarized current injection into GaAs.
- Magnetic Microscopy and Imaging I
95(2004); http://dx.doi.org/10.1063/1.1669346View Description Hide Description
Magnetic domain structures of nanocrystallinemagnetic materials were extensively investigated by electron holography with a change in temperature or magnetic field applied. In both soft and hard magnetic materials, the distribution of lines of magnetic flux clarified in situ by electron holography was found to correspond well to their magnetic properties. An attempt to produce a strong magnetic field using a sharp needle made of a permanent magnet, whose movement is controlled by piezo drives has been presented. This article demonstrates that the attempt is promising to investigate the magnetization process of hard magnetic materials by electron holography.
Magnetic domain structures of focused ion beam-patterned cobalt films using scanning ion microscopy with polarization analysis95(2004); http://dx.doi.org/10.1063/1.1689433View Description Hide Description
Studies of magnetic domain distributions in patterned magnetic materials are of pivotal importance in the areas of ultrahigh density magnetic recording, MRAM design, and miniaturized magnetic sensor arrays. Scanning ion microscopy with polarizationanalysis (SIMPA) is used to perform in situ topographic and magnetic domain imaging and focused ion beam(FIB) patterning. For FIB-patterned 30 nm thick Co films, it is found that rectangular Co bars of sizes between 10–30 μm exhibit S type, whereas circular shaped magnetic elements show C type micromagnetic magnetization patterns. It is shown that SIMPA provides a simple way to directly identify different micromagnetic domain patterns.
Investigating magnetization dynamics in permalloy microstructures using time-resolved x-ray photoemission electron microscope95(2004); http://dx.doi.org/10.1063/1.1687531View Description Hide Description
We present results of a direct imaging approach to visualize the dynamics of magnetic domains on the nanosecond scale. The experiments are carried out by means of an x-rayphotoemissionelectron microscope (X-PEEM) in a stroboscopic mode and exploit the intrinsic time structure of the synchrotron radiation delivered by the storage ring facility ESRF (Grenoble). In this way we combine the high lateral resolution of a PEEM with a subnanosecond time resolution.
Time and layer resolved magnetic domain imagig of FeNi/Cu/Co trilayers using x-ray photoelectron emission microscopy (invited)95(2004); http://dx.doi.org/10.1063/1.1676027View Description Hide Description
We have performed magnetic domain imaging with spatial, temporal, and layer resolution using x-ray photoelectron emission microscopy. The element selectivity of x-raymagnetic circular dichroism allows the magnetization dynamics of the different magnetic layers in spin-valve-like FeNi/Cu/Co trilayers to be studied separately, using the time structure of synchrotron radiation. The unique possibilities of this technique have been used to study the influence of the intrinsic magnetic properties of the different layers on the magnetization dynamics and the interlayer magnetic coupling.
95(2004); http://dx.doi.org/10.1063/1.1688252View Description Hide Description
We have used circularly polarized x rays to image the spiral magnetic domain structure in a single crystal of Ho metal. In these structures, the magnetization direction rotates between successive atomic layers forming a helix. At magnetic Bragg diffraction peaks, circularly polarized x rays are sensitive to the handedness of such a helix (i.e., either right or left handed). By reversing the helicity of the incident beam with phase-retarding optics and measuring the difference in the Bragg scattering, contrast between domains of opposing handedness can be obtained.
95(2004); http://dx.doi.org/10.1063/1.1687254View Description Hide Description
We have investigated the highly localized magnetic dynamics of mesoscopic thin film elements by time-resolved scanning Kerr microscopy. When a magnetic element is excited by applying a short magnetic field pulse, it is found that the magnetization evolves into a well-developed, labyrinth-shaped domain structure. The emergent domain pattern is little dependent on element shape. We also observed that large amplitude precessional oscillations of closure domain magnetization are actively involved in the switching process far from equilibrium. Pursuant to these oscillations, the magnetization reversal continues by means of the precessional motion of the magnetization vector in other localized regions.
- Magnetic Thin Films I
95(2004); http://dx.doi.org/10.1063/1.1682787View Description Hide Description
We have used x-ray magnetic circular dichroism (XMCD) to determine the spin and orbital magnetic moments of Fe atoms at the ferromagnetic/superparamagnetic boundary, superparamagnetic thickness, and submonolayer thickness at the interface. The samples, grown by molecular beam epitaxy at room temperature, were 5, 4, and 0.5 ML (monolayer) Fe on GaAs (100)-4×6 capped with 7 ML Cr, preceded in the case of the 0.5 ML Fe by 9 ML Co. The XMCD results show orbital moments enhanced by almost 300% with respect to the bulk and bulklike spin moments for all three thicknesses demonstrating unambiguously that the Fe/GaAs interface is magnetic.
95(2004); http://dx.doi.org/10.1063/1.1667434View Description Hide Description
We report the epitaxialgrowth of high quality Fe thin films on both n-type GaAs(001) and GaAs(011) substrates using electrochemicaldeposition.X-ray diffraction shows Fe(001)//GaAs(001) and Fe(011)//GaAs(011) as the primary epitaxial relations similarly to Fe filmsgrown by molecular-beam epitaxy. The structural quality depends on the composition of the plating solution. The values of these films are around 30–100 Oe. The value is about 20–21 kG. In-plane angular ferromagnetic resonance measurements showed crystalline anisotropy for both the Fe(001) and Fe(011) films which is superimposed by a substrate induced uniaxial anisotropy for the Fe(011) layers.
95(2004); http://dx.doi.org/10.1063/1.1667418View Description Hide Description
We demonstrate how epitaxial FM (Ni)films can be grown by ECD directly onto GaAs(001) without outdiffusion or surface segregation of As or Ga. The thickness dependence of electrical and magnetic properties (saturation magnetic moment) verify the good quality of the layers and indicate that electrochemical deposition is a suitable candidate for the growth of epitaxialNifilms with sharp interfaces on GaAs(001). X-ray photoelectron spectroscopy(XPS) analysis on electroplated (epitaxial)Nifilms showed no interdiffusion for annealing up to 250 °C. Annealing at higher temperatures, up to 350 °C, significantly increases both the As outdiffusion and the Schottky barrier heights, which indicates the correlation between intermixing and the diode quality.
95(2004); http://dx.doi.org/10.1063/1.1682891View Description Hide Description
We used Brillouinlight scattering (BLS) to study spin dynamics in annealed ultra-thin (0.4–2.0 nm) Fe layers buried by 6.0–8.0 nm of Al. All samples reported here were grown by molecular beam epitaxy on GaAs(001). We correlated our BLS results with film structure obtained from reflection high energy electron diffraction low angle x-ray reflectometry, and in situ scanning tunneling microscopy. Our results find that annealing of room temperature deposited Fe layers to 300 °C has a significant effect on both the magnetic properties and surface structure of the Fe.
95(2004); http://dx.doi.org/10.1063/1.1676023View Description Hide Description
We report an experimental and theoretical study of the influence of uniaxial strain on the magnetic properties of Fe/GaAs(100) ultrathin film systems. In-plane tensile strains have been applied along different directions relative to the intrinsic uniaxial magnetic anisotropy axes of the Fe/GaAs(100) films, and the loops were measured. In an attempt to interpret the observed behavior, a phenomenological energy model is considered, which incorporates both intrinsic and strain-induced uniaxial anisotropy terms. Theoretical magnetization reversal loops were obtained from the model, assuming that the magnetoelastic coefficients of the thin films are same as the bulk bcc Fe. The model qualitatively predicted the magnetization loops for all the experimental data, assuming a positive uniaxial anisotropy constant.
Effect of postgrowth annealing on the morphology and magnetic properties of MnAs thin films grown on GaAs(001) substrates95(2004); http://dx.doi.org/10.1063/1.1688254View Description Hide Description
We have investigated the effect of postgrowth annealing on the surface morphology and magnetic properties of monocrystalline MnAs thin films epitaxially grown on GaAs(001) substrates by molecular beam epitaxy. After the growth of MnAs, the postgrowth annealing was carried out in the temperature range between 330 and 400 °C, with keeping the MnAs surface exposed to the As flux. The surface morphology and crystallinity of MnAs thin films were drastically improved by optimizing the annealing temperature. The magnetic properties, such as saturation magnetization, coercivity, and the magnetoresistance characteristics of the MnAs films were sensitive to their crystallinity, which was determined by the annealing temperature.
95(2004); http://dx.doi.org/10.1063/1.1667837View Description Hide Description
MnAs/Ge multilayer structures successfully fabricated using molecular beam epitaxy were grown on (001) GaAs substrates at a growth temperature of The multilayer with a 100 Å period thickness exhibited ferromagnetism up to 345 K with a coercive field of 147 Oe at 300 K and a vanishingly small in-plane magnetic anisotropy, as determined from temperature-dependent magnetization and hysteresis loop measurements. These results indicate the formation of novel ferromagneticmultilayers, which may have spintronic applications.
Magnetic anisotropies of ultrathin Fe films on Au(111) as function of iron thickness and gold capping95(2004); http://dx.doi.org/10.1063/1.1689231View Description Hide Description
Ironthin films between 1.5 and 3.75 ML grown on a reconstructed Au(111) surface have been studied by in situ magneto-optic Kerr effect in the polar and longitudinal geometries. In films with Fe coverage less than 2.8 ML, we observe a perpendicular magnetic anisotropy tendency at room temperature. With further iron deposition, the film exhibits a continuous spin reorientation from out-of-plane to in-plane. Distinct changes in the magnetic properties as a function of substrate quality and gold capping layer thickness may be related to different state of the filmstructures.
95(2004); http://dx.doi.org/10.1063/1.1683011View Description Hide Description
Magnetic properties of one monolayer (ML) Ni/1 ML Co/Pt(111) film upon thermal annealing were investigated. Increases in polar Kerr rotation correspond to the topmost Ni layer incorporated with the second Co layer on Pt, and the further alloying of Co-Pt during the annealing. Interestingly, Curie temperature shifted dramatically to 325 K when the film was annealed at 830 K. The alloy formation of Ni-Pt and top-layer enrichment of Pt may be the main reason causing the great shift of Curie temperature.
95(2004); http://dx.doi.org/10.1063/1.1688653View Description Hide Description
Magnetic circular dichroism was measured as function of temperature for the and absorption edges in single crystal EuO thin films. The edge is resolved into two major features, one with the same helicity dependence as the edge, and one with opposite helicity dependence. While the ratio of the absorption peaks with similar helicity dependence remains constant as a function of temperature, the ratio of those with opposite dependence is found to change linearly with temperature below 75 K. The physical mechanism for this linear behavior in the absorption peak intensity is not well understood.