K. Hyomi

Micromagnetic study of CoPt–SiO2 granular films by spin-wave Brillouin scattering

A spin-wave Brillouin scattering study of a CoPt–SiO2 granular magnetic recording medium was made. This film contains ferromagnetic CoPt particles in a SiO2 matrix, and has an extremely low medium noise property due to little exchange coupling between magnetic grains. Spin waves of both the propagating surface mode and standing wave mode were found to be excited in granular magnetic films with various microstructures. A possible origin of the spin wave is a magnetostatic coupling between regularly ordered CoPt grains, as reported for artificially patterned magnetic thin films. This result shows two promising features of the CoPt–SiO2 granular film for high density recording medium: It is an ordered media obtained in a self-organizing manner, and it is less influenced by the thermal fluctuation effect, which is a serious problem for current high density magnetic recording.

Submicron scale hybrid structures of diluted magnetic semiconductor quantum wells with ferromagnetic Co wires

Submicron scale hybrid structures of diluted magnetic semiconductor (DMS) quantum wells (QWs) with ferromagnetic Co wires have been developed for the purpose of applying microscopic magnetic fields to the DMS quantum structures. The Zn1−x−yCdxMnySe QW was made into wires with the width down to 0.1 μm and each wire was sandwiched between the Co wires. The magneto-optical properties of the hybrid structures were studied by spin–flip light scattering of paramagnetic Mn ions in the DMS QW in addition to ferromagnetic surface spin-wave scattering of the Co wires, where the spin–flip energy is highly sensitive to the magnetic field HCo flux applied from the Co wires. The application of uniform magnetic fields HCo flux⩾0.25 T from the Co wires to the DMS QW has been attained with the Co-wire spacing of 0.2 μm. In addition, the distribution of HCo flux in the DMS QW is quantitatively evaluated from the spectral shape of the spin–flip scattering. Moreover, the application of HCo flux to the DMS QW is switched on a...

SPIN-WAVE BRILLOUIN STUDY OF ULTRA-THIN CO/AU(111) FILMS WITH PERPENDICULAR MAGNETIC ANISOTROPY

We have studied ultra-thin Co/Au(111) films with perpendicular magnetic anisotropy by means of spin-wave Brillouin scattering. The ultra-thin Co films with thicknesses from 1 to 7 monolayer (ML) were grown by molecular beam epitaxy on 1 ML Au underlayers without intentional substrate heating. With 2 or more MLs of Co, spin-wave excitations are clearly observed. We find the magnetic field dependence of the spin-wave energy can be quantitatively explained by a calculation including out-of-plane magnetization due to uniaxial perpendicular magnetic anisotropy. The first-order perpendicular anisotropy constant systematically increases as the Co thickness decreases from 7 to 3 ML. The second-order perpendicular anisotropy constant is much smaller than that expected in Co(0001) bulk and also increases as the Co thickness decreases. The physical origin of the thickness dependence of the perpendicular anisotropy is discussed. We find the field dependence of the scattering intensity is different between Stokes and anti-Stokes processes. Finally, with 2 ML of Co, significant broadening of the spin-wave spectrum is observed, as well as the degradation of the perpendicular anisotropy. We believe this is the result of the effect of magnetic inhomogeneities due to the structural imperfections.

Low-frequency excitations of spin-wave Brillouin scattering in ultrathin Co/Au(1 1 1) films

We have observed low-frequency excitations of spin-wave Brillouin scattering in ultrathin Co/Au(1 1 1) films with a large perpendicular magnetic anisotropy. With several monolayers (ML) of Co, we observe diffuse scattering under an external field just below a critical field at which the magnetization starts to incline toward out-of-plane. With Co films thicker than 6 ML, we find a new mode at fields lower than the critical field, in addition to a surface spin wave.

Giant Zeeman effect of excitons in a hybrid nanostructure of diluted magnetic semiconductor quantum well sandwiched in Co wires

Giant Zeeman effects of excitons in a hybrid nanostructure of diluted magnetic semiconductor (DMS) quantum well (QW) with ferromagnetic Co have been demonstrated, where the DMS-QW wire with the width of 200 nm is sandwiched in the Co wires and magnetic fields nearly perpendicular to the well plane can efficiently be applied from the Co wires to all of the DMS QW. The spectral shape of excitonic photoluminescence and the field dependence show clearly that the microscopic perpendicular fields generated from the Co wires induce the giant Zeeman shifts of excitons in the DMS QW.

Effect of an ultrathin Au interlayer at a Co/Cu(111) interface on the magnetic properties

We have found from spin-wave Brillouin spectra that a half-monatomic thick Au interlayer at a Co/Cu(111) interface causes a significant improvement in magnetic uniformity in the Co ultrathin overlayer film deposited. The field-independent Brillouin spectrum width abruptly decreases with the insertion of the Au-interlayer, and an inhomogeneous transition at a critical field between out-of-plane and in-plane magnetizations also is suppressed. From the field dependence of the spin-wave energy, we find that the second-order uniaxial perpendicular anisotropy increases with the Au interlayer.

GRAIN STRUCTURE AND MAGNETIC CLUSTERING IN SIO2 ADDED CONIPT GRANULAR FILMS

Grain structure and its relation to the magnetic properties in SiO2 added CoNiPt granular films have been studied, with the SiO2 content varied from 0 to 8 mol %. With SiO2 more than 4 mol %, significant grain boundaries with a width of a few nm are formed by the segregated SiO2. The average grain size in the film plane decreases from 16 to 9 nm with the SiO2 increased up to 8 mol %. The media noise significantly decreases with increasing SiO2 and remanent magnetized domains observed by magnetic force microscopy (MFM) also become finer. With 8 mol % SiO2, MFM shows that weak magnetic clustering spreads over the grains, with a diameter of about 100 nm. This clustering indicates the collaborative behavior of the magnetization between grains is responsible both for the steep magnetization reversal as well as the slight increase in the media noise as the recording density is increased.

Perpendicular magnetic anisotropy in Au/Co/Au(1 1 1) films: interface anisotropy and effect of strain

We have studied perpendicular anisotropy in ultrathin Au/Co/Au(1 1 1) films. Misfit strain in Co grown on the Au-underlayer increases monotonically with increasing Au thicknesses from 0 to 5 monolayer (ML). We find that the interface anisotropy shows an abrupt increase for 2 ML of Au-underlayer. The volume anisotropy is larger than the magneto-crystalline anisotropy.

Effect of an Au underlayer on perpendicular magnetic anisotropy in Au/Co/Au(111) sandwich films

We have used spin-wave Brillouin scattering to study perpendicular magnetic anisotropy in Au/Co/Au(111) sandwiched films with varying thicknesses of Au-underlayers. By using 5-monolayer (ML)-thick Co films, the transition of spin-wave frequency between out-of-plane and in-plane magnetizations can be observed by varying the in-plane field. We find that the field-dependence of the spin-wave energy is explained by a calculation including both the first-order (second power) and the second-order uniaxial perpendicular anisotropies depending on the Au-underlayer. We observe saturation of the anisotropy at 5 ML of Au-underlayer thickness, which corresponds to the saturation of in-plane lattice expansion of Co on that Au-underlayer. The contribution of interface anisotropy is evaluated from the Co-thickness dependence of the anisotropy observed. We also have observed a field-dependent broadening of the spin-wave Brillouin spectra at a critical field between out-of-plane and in-plane magnetizations, which we attribute to a distribution of the first-order anisotropy.

Effects of misfit strain due to epitaxial growth on interface perpendicular magnetic anisotropy in ultrathin Co/Au/Cu(111) films

We observe a systematic increase in interface perpendicular magnetic anisotropy (PMA) with increasing Au-interlayer thickness tAu ranging from 1 to 5 monolayers (MLs) in Cu/Co/Au/Cu(111), where misfit strain in Co due to the epitaxial growth increases with increasing tAu. This result can be understood within the framework of the magneto-elastic contribution to the interface PMA. With a Cu overlayer, a constant contribution of interface PMA as well as of volume anisotropy is observed when the Co thicknesses are greater than 1.3 ML. However, we have found an unexpected suppression of this variation of interface PMA with the use of Au overlayers instead of Cu ones. With Au or Pd overlayers, the PMA starts to degrade as the Co thickness is reduced below 2.5 ML.