Surface Anisotropy Ks Research Articles

In situ preparation and magnetic properties of Fe3O4/wood composite

A Fe3O4/wood composite, a magnetic material, was prepared by in situ chemosynthesis at room temperature. X-ray diffraction (XRD), vibrating sample magnetometry (VSM) and Fourier transform infrared (FTIR) spectroscopy were applied to characterise the resulting products. The XRD results show that the average grain size of Fe3O4 was ∼14 nm. The VSM shows that the composites have saturation magnetisation Ms values from 4·7 to 25·3 emu g−1 with the increase in weight per cent gains of the wood for the composites, but the coercive forces Hc are invariable, which is different from the magnetic materials reported before. It may be due to the fact that the interaction between wood and Fe3O4 becomes stronger when less Fe3O4 particles are introduced in the composition, and this also changes the surface anisotropy Ks of the magnetism. Structural characterisation by FTIR proves the interaction between Fe3O4 particles and the wood matrix, and it also illustrates that this interaction influences the coercive force of the composite.

Anisotropy Enhancement in Co Granular Multilayers by Capping

The effect of capping with Cu, Au and Pt of an array of Co nanoparticles is revised. The magnetic surface anisotropy KS was found to be the dominant contribution to the effective anisotropy Keff of the particles. Recent X-ray Magnetic Circular Dichroism (XMCD) measurements show that there is hybridization between the 3d Co electrons and the d and 4p electrons of the capping metal. By comparison to the mechanisms which give rise to the surface anisotropy in thin films, it is argued that this hybridization governs the modification of KS, and hence, of Keff.

Locally resolved ferromagnetic resonance in Co stripes

Microwave excitations of Co stripes of 100×1.5×0.025μm3 were investigated by angular dependent ferromagnetic resonance (FMR) and by locally resolved scanning thermal microscopy based (SThM) FMR, offering a lateral resolution of <100nm and a sensitivity of 106 spins. Besides the uniform excitation, backward volume modes and a rim resonance were identified by SThM-FMR imaging. Micromagnetic simulations (OOMMF) confirm the experimentally observed lateral confinement of these modes. The magnetic parameters of the Co stripes correspond to the ones of Co bulk with a surface anisotropy Ks=0.5mJ∕m2.

Ferromagnetic resonance study of polycrystalline cobalt ultrathin films

We present room-temperature ferromagnetic resonance (FMR) studies of polycrystalline ∥Pt∕10nmCu∕tCo∕10nmCu∕Pt∥ films as a function of Co layer thickness (1⩽t⩽10nm) grown by evaporation and magnetron sputtering. FMR was studied with a high-frequency broadband coplanar waveguide (up to 25 GHz) using a flip-chip method. The resonance field and the linewidth were measured as functions of the ferromagnetic layer thickness. The evaporated films exhibit a lower magnetization density (Ms=1131emu∕cm3) compared to the sputtered films (Ms=1333emu∕cm3), with practically equal perpendicular surface anisotropy (Ks≃−0.5erg∕cm2). For both series of films, a strong increase of the linewidth was observed for Co layer thickness below 3 nm. For films with a ferromagnetic layer thinner than 4 nm, the damping of the sputtered films is larger than that of the evaporated films. The dependence of the linewidth can be understood in terms of the spin-pumping effect, from which the interface spin-mixing conductance is deduced.

Role of the Surface Anisotropy in Magnetization Reversal of a Spherical Nanoparticle

Numerical micromagnetics is used to investigate the influence of the surface anisotropy on the magnetic behaviour of single domain spherical nanoparticles. Two kinds of surface anisotropy are considered: easy axis type (radial) and easy plane type (tangential). It is shown that the coercivity increases whereas the remanence decreases with the increase of the surface anisotropy and this tendency is more pronounced for radial surface anisotropy (Ks < 0) than for tangential one (Ks > 0). It is additionally shown that the enhancement of the coercivity depends on the particle size and is the strongest for the smallest particles.

Particle size effects and surface anisotropy in Fe-based granular films

Structure and magnetic properties of Fe/SiO2 and Fe/Al2O3 granular films were studied in the particle size (d) range of 1–50 nm. The coercivity (Hc) shows a maximum around 600 Oe at the particle size dm≈18 nm, and then decreases in the larger particle size range for both Fe/SiO2 and Fe/Al2O3 systems at room temperature. A relation of coercivity, particle size, and temperature was built by using the Néel–Brown theory and the Stoner–Wohlfarth model basically. The experimental results were successfully fitted by assuming that the α-Fe particles possess uniaxial surface anisotropy Ks≈0.4 erg/cm2.

Simulation of transverse and longitudinal magnetic ripple structures induced by surface anisotropy

Micromagnetic ripple structures on the surfaces of thick specimens of ultra-soft magnetic material having strong surface anisotropy Ks favouring out-of-surface magnetization have been calculated. These ripples have wavelengths of the order of 0.1 μm and extend to a depth ∼ √A/Ms, where A is the exchange constant and Ms is the saturation magnetization. The wave-vectors of the ripple structures are either transverse or parallel to the bulk magnetization. Both structures have lower energy than the one-dimensional structure discussed by O'Handley and Woods, and they exhibit stronger normal magnetization. The transverse structure requires a surface anisotropy Ks ≥ 0.80K0, where K0 = (2πA)12Ms is that required for the one-dimensional structure. The threshold for longitudinal ripples is 0.84K0. It is suggested that the transverse structure probably constitutes the ground state. The magnitudes of Ks and A should be obtainable from measurements of the ripple wavelength and amplitude, and Ms.

Magnetic and magneto-optical properties of CoxNi1−x/Au multilayers

We have prepared CoxNi1-x/Au multilayers and studied their magnetic and magneto-optical properties. The surface anisotropy (Ks) for Ni/Au is negative but it becomes positive with the addition of Co and increases linearly with Co concentration. For x = 0.7 the perpendicular loops are rectangular when the magnetic layer is 6 Å thick.

Conventional and photothermally modulated ferromagnetic resonance investigations of anisotropy fields in an epitaxial Fe(001) film

Conventional angle-dependent ferromagnetic resonance (FMR) measurements on an epitaxial (001) Fe film grown on a GaAs substrate were performed as a function of the orientation of the external magnetic field Bext for three configurations: One with Bext in the film plane and two with Bext out of plane starting in plane in different crystallographic orientations. From these measurements the magnetization M, the crystalline anisotropy constants K1 and K2, the surface anisotropy Ks and an additional uniaxial anisotropy Ku were deduced self-consistently. The temperature dependence of K1 and M was investigated for the in-plane [110] orientation in the range from 100 to 570 K. In addition, locally resolved photothermally modulated FMR measurements were carried out to study the homogeneity of the magnetization and of the crystalline anisotropy across the film. Disturbances of both magnetic parameters as a function of position were observed which are related to local inhomogeneities in the semiconducting substrate.

Magnetic properties of CoxNi1−x/Pt multilayers

We have prepared CoxNi1−x/Pt multilayers by evaporation under ultrahigh vacuum conditions and studied their magnetic and magneto-optical properties. The addition of Co to Ni leads to an increase in both the surface anisotropy and TC. It is found that the surface anisotropy Ks of CoxNi1−x/Pt multilayers can be expressed as the sum of Ks of Ni/Pt and of Co/Pt times their respective concentration. In Co0.3Ni0.7/Pt multilayers, for t(Co,Ni)=0.45 nm, one observes a perpendicular M-H loop with a good rectangularity and a coercivity of 1 kOe and TC of 180 °C. These characteristics are very interesting for application in magneto-optic storage.

Ｃｏ／Ｐｔ多層膜の磁気特性に及ぼす熱処理効果

Co/Pt multilayers sputter-deposited with and without substrate bias VB of -30 V were annealed in vacuum at 300 and 400°C. With annealing. the saturation magnetization Ms and the saturation Kerr rotaion angle θK increased. On the other hand, the perpendicular anisotropy energy Ku decreased manotonically. The decrease of Ku was larger for -30 V films than for 0 V films because of the larger decrease of surface anisotropy Ks for -30 V films. This larger decrease of Ks was discussed by taking account of relaxation in the lattice strain due to annealing.

Observability of a quasilinear temperature dependence of the magnetization in a semi-infinite ferromagnet (abstract)

Current theory [G. T. Rado, Phys. Rev. B 40, 407 (1989)] shows that the existence of magnetic surface anisotropy leads to surface spin waves which can provide a mechanism for a quasilinear temperature dependence of the magnetization (MT) near the surface of a semi-infinite ferromagnet. Since volume spin waves as well as surface spin waves are excited thermally, we now generalize this theory in two ways. First, we include in the calculation the effects of both surface and volume spin waves, and second, we include the effects of both of the surface anisotropy constants Kss and Ks. The fractional magnetization deviation (M0−MT)/M0 caused by the volume spin waves competes, of course, with that caused by the surface spin waves. We find that, for all allowed values of Ks and Kss, it is mostly at very low temperatures that the contribution to (M0−MT)/M0 arising from surface spin waves is comparable to the contribution to (M0−MT)/M0 arising from volume spin waves. This result depends on the assumption that the material parameters are independent of position throughout the sample volume. A full account of this work will be published elsewhere.

Effect of spacer material on the magnetic surface anisotropy in ultrathin Fe70B30 multilayer films

It has been found recently that the magnetic surface anisotropy Ks in Fe70B30/Ag multilayer films decreases monotonically with magnetic layer thickness (2L) for 2L&amp;lt;16.5 Å. In order to determine possible effects of the spacer material on the surface anisotropy in the aforementioned system, Ag has been replaced with Al2O3 and ferromagnetic resonance (FMR) measurements have been made on these films. These Fe70B30/Al2O3 films were fabricated by magnetron sputtering and were characterized by x-ray-diffraction and vibrating sample magnetometer (VSM) measurements in addition to FMR. In the region where Ks depends upon 2L, the data is insufficient to confirm the thickness dependence of Ks that was observed in Fe70B30/Ag, while in the region where Ks is independent of 2L, the values of Ks deduced for Fe70B30/Ag and Fe70B30/Al2O3 are in good agreement. The latter is particularly interesting in light of the enormous difference in conductivity between Ag and Al2O3.

PREPARATION AND PROPERTIES OF Co/Pt MULTILAYERS

We have prepared Co/Pt multilayers by evaporation under UHV conditions and studied the properties as a function of the nature of the substrate and the deposition temperature. For t(Co) < 8 A the perpendicular M-H loops are rectangular with coercivity of about 1.9 kOe. The surface anisotropy KS is found to be 0.6 erg.cm-2 and 0.35 erg.cm-2 for ML deposited on glass and MgO(100) substrates respectively. The growth parameters such, as the deposition temperature and a Pt buffer layer, influence the anisotropyonly for t(Co) < 10 A. However the loop charcteristcs are sensitive to the above parameters. The results are discussed in the context of those published in the literature.

Magnetic Properties of Co/Pt Multilayers on MgO (100) Substrates

ABSTRACTWe have prepared Co/Pt multilayers by evaporation in UHV on MgO (100) substrates kept at various temperatures. Most of the samples were grown on a Pt buffer layer. Magnetic and magneto-optical properties have been studied. The surface anisotropy Ks for MgO substrates is 0.35 erg.cm−2 which is smaller than 0.6 erg.cm−2 obtained for glass substrates. The effect of the growth conditions on Ks is observed only for t(Co)&lt;10 Å where the crossover to Keff&gt;O occurs. Deposition at higher temperature leads to an increase in the coercivity. The magnetization and the anisotropy is practically temperature independent in the range 5 to 295 K. However the coercivity increases sharply below 100K.

Static magnetization orientation under perpendicular surface anisotropy (abstract)

Secondary electron-spin polarization measurements1 on single-crystal Fe surfaces in ultrahigh vacuum suggest a perpendicular magnetization at the surface but in-plane magnetization at 10 Å or more beneath the surface. We consider this problem with a simple model. The depth dependence of magnetization tilt angle θ(z) (measured from the surface normal) is solved analytically for a semi-infinite, exchange-coupled ferromagnet with dipole energy and perpendicular surface anisotropy Ks. The surface magnetization is found to remain in plane for Ks≤(πM2A)1/2=Kc, a critical surface anisotropy. For Fe, Kc is 4 ergs/cm2. The analytic solution of the magnetization rotation θ(z)=tan−1 [sinh(a+bz)] is identical to the bulk domain wall solution but the length scale of the rotation, b−1, is reduced by more than an order of magnitude compared to bulk domain walls in soft magnets (5 nm vs 100 nm). The magnetization configuration is, in fact, a partial domain wall at the surface.

Confirmation by SQUID magnetometry of FMR measurements of magnetization in ultrathin Fe-B amorphous films

Direct measurements were made by means of SQUID magnetometry of the magnetization of ultrathin Fe50B50 and Fe70B30 amorphous films at room temperature. The film thicknesses used included most of those investigated in the authors’ recent ferromagnetic resonance (FMR) determinations of the surface anisotropy Ks and the magnetization M of such films. The directly measured values of M confirm adequately the M values obtained by computer fitting of a theory to experimental FMR data. Thus, the present experiments offer substantial support for the reliability of the FMR method for measurements of Ks and M in ultrathin films.

Ferromagnetic resonance and surface anisotropy in iron single crystals

Ferromagnetic resonance and antiresonance experiments have been performed on (100) planes of iron whiskers in order to determine the value of surface anisotropy Ks. From the measurements one obtains Ks = (0.1 ± 0.05) erg cm-2 with the easy axis normal to the surface. This result is compared with experimental data on thin films and with the conclusions of Néel's theory.

Ferromagnetic resonance in thin single crystal platelets of iron

Ferromagnetic resonance measurements were performed in single crystals of α-iron in the form of thin platelets. These platelets were grown from the vapour phase; their thickness was several microns. The frequency used in the measurements was in the region of 20 to 72 GHz and the samples were statically magnetized parallel to the plane of the platelet. By comparing the line-widths obtained from measurements with a macroscopic theory of the line-shape the values of the frequency-independent Landau-Lifsic damping constant (λ=4·2×107 rad/sec) and surface anisotropyKs= 0·03 erg/cm2) were evaluated.