In-plane Easy Direction Research Articles

Acoustically driven ferromagnetic resonance in YIG thin films

Acoustically driven ferromagnetic resonance (ADFMR) is a platform that enables efficient generation and detection of spin waves via magnetoelastic coupling with surface acoustic waves (SAWs). While previous studies successfully achieved ADFMR in ferromagnetic metals, there are only few reports on ADFMR in magnetic insulators such as yttrium iron garnet (Y3Fe5O12, YIG) despite more favorable spin wave properties, including low damping and long coherence length. The growth of high-quality YIG films for ADFMR devices is a major challenge due to poor lattice-matching and thermal degradation of the piezoelectric substrates during film crystallization. In this work, we demonstrate ADFMR of YIG thin films on LiNbO3 (LNO) substrates. We employed a SiOx buffer layer and rapid thermal annealing for crystallization of YIG films with minimal thermal degradation of LNO substrates. Optimized ADFMR device designs and time-gating measurements were used to enhance the ADFMR signal and overcome the intrinsically low magnetoelastic coupling of YIG. YIG films have a polycrystalline structure with an in-plane easy direction due to biaxial stresses induced during cooling after crystallization. The YIG device shows clear ADFMR patterns with maximum absorption for H ≈ 160 mT parallel to SAW propagation, which is consistent with our simulation results based on existing theoretical models. These results expand possibilities for developing efficient spin wave devices with magnetic insulators.

Zero-field dynamics stabilized by in-plane shape anisotropy in MgO-based spin-torque oscillators

Here, we demonstrate numerically that shape anisotropy in MgO-based spin-torque nano-oscillators consisting of an out-of-plane magnetized free layer and an in-plane polarizer is necessary to stabilize out-of-plane magnetization precession without the need of external magnetic fields. As the in-plane anisotropy is increased, a gradual tilting of the magnetization towards the in-plane easy direction is introduced, favouring zero-field dynamics over static in-plane states. Above a critical value, zero-field dynamics are no longer observed. The optimum ratio of in-plane shape to out-of-plane uniaxial anisotropy, for which large angle out-of-plane zero-field dynamics occur within the widest current range, is reported.

Damping constant in a free layer in nanoscale CoFeB/MgO magnetic tunnel junctions investigated by homodyne-detected ferromagnetic resonance

We investigate the damping constant of a free layer with a perpendicular magnetic easy axis in nanoscale CoFeB/MgO magnetic tunnel junctions (MTJs) with a reference layer with an in-plane easy direction. The built-in noncollinear magnetization configuration in the MTJs allows us to measure homodyne-detected ferromagnetic resonance without tilting the magnetization direction of the free layer from the device normal. The damping constants determined from the spectral linewidths after the subtraction of the inhomogeneous broadening are nearly independent of the device diameter ranging from 70 to 100 nm, and take values similar to those reported for blanket CoFeB films.

Magnetically Anisotropic Ni<sub>2</sub>MnGa Thin Films: Coating Glass and Si Micro-Cantilevers Substrates

Ni2MnGa thin films, with thickness between 30 and 60 nm, were pulsed-laser deposited at room temperature on Si micro-cantilevers and glass substrates. Two different deposition processes were performed: normal deposition and off¬-normal. After annealing in an inert atmosphere, in-plane isotropic magnetic hysteresis loops were measured for the normal deposited films. In contrast, in-plane anisotropic hysteresis loops were obtained from the off-normal deposited ones. An in-plane easy direction for the magnetisation, perpendicular to the incidence plane of the plasma during deposition, was measured with an anisotropy field of ≈100 Oe and an easy coercive field of ≈24 Oe. The mechanical behaviour of the magnetically anisotropic coated micro-cantilevers and their response to a decreasing temperature permitted observing the martensitic transformation of the Ni2MnGa thin films.

Origin of transverse magnetization in epitaxial Cu/Ni/Cu nanowire arrays

The patterning-induced changes in the magnetic anisotropy and hysteresis of epitaxial (100)-oriented Cu/Ni(9, 10, 15 nm)/Cu planar nanowires have been quantified. When the Ni films are patterned into lines, strain relaxation leads to a thickness-dependent net in-plane anisotropy transverse to the lines. The magnetoelastic anisotropy was found from the three-dimensional strain state measured directly by synchrotron x-ray diffraction and has a value of $\ensuremath{-}21\text{ }\text{kJ}/{\text{m}}^{3}$ for 10-nm-thick nanowires. The angular dependence of the remanence of the nanowires indicates that the in-plane easy direction is the result of the competition between the cubic magnetocrystalline anisotropy and a uniaxial anisotropy that includes shape and magnetoelastic effects. The patterning-induced changes in magnetoelastic anisotropy, combined with the shape and magnetocrystalline anisotropies, quantitatively explain the net anisotropy of the nanowires. Thus by controlling the film thickness and wire orientation, the easy axis direction of the nanowires may be controlled.

Thickness dependent structural and magnetic properties of ultra-thin Fe/Al structures

The structural and magnetic properties of electron beam evaporated ultra-thin Fe/Al structures are studied as a function of Fe layer thickness, while keeping the Al layer thickness constant. The grazing incidence X-ray reflectivity measurements carried out on the structures having Fe layer thickness ≤20 A show substantial intermixing between the layers during deposition, indicated by a loss of periodicity. These structures resemble a composite single layer film consisting of Fe and Al clusters. However, for thicker Fe layers (≥30 A), the appearance of a first order Bragg peak in the reflectivity patterns indicates the formation of a better-multilayered structure. These results are also supported by AFM and resistivity measurements. The X-ray diffraction measurements show that in all the multilayer films, deposited Fe layers are textured mainly along (110) direction. The corresponding magnetic measurements show a soft magnetic behaviour of the films with an in-plane easy direction of the magnetization. The observed soft magnetic behaviour in these samples is explained in terms of (i) weak crystalline magnetic anisotropy due to small crystal grains and magnetostriction and (ii) the morphological and structural changes occurring due to the variation in the Fe layer thickness below the critical value in the deposited structures.

Brillouin light scattering from spin waves in epitaxial hcp Co films

The first and second magnetic anisotropy constants ${K}_{1}$ and ${K}_{2}$ and the spin wave (SW) stiffness constant ${D}_{\mathrm{BLS}}$ of epitaxial $(101\ifmmode\bar\else\textasciimacron\fi{}0)$ Co films of $450\ifmmode\pm\else\textpm\fi{}10\mathrm{\AA{}}$ in thickness have been determined by Brillouin light scattering (BLS) at room temperature in magnetic fields of up to 5.0 kOe. We employed three scattering geometries: (A) magnetic fields applied along the in-plane easy direction, (B) magnetic fields applied along the in-plane hard direction, and (C) a constant magnetic field rotated from the in-plane easy direction to the in-plane hard direction. We always examined SW's propagating perpendicular to the magnetic field. Numerical analyses of the SW frequencies by a continuum dipole-exchange model with the magnetic anisotropy allowed us to obtain values of ${D}_{\mathrm{BLS}}=(3.46\ifmmode\pm\else\textpm\fi{}0.35)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\mathrm{Oe}{\mathrm{cm}}^{2},$ ${K}_{1}=(3.45\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{6}{\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$ and ${K}_{2}=(0.90\ifmmode\pm\else\textpm\fi{}0.1)\ifmmode\times\else\texttimes\fi{}{10}^{6}{\mathrm{e}\mathrm{r}\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}.$ These values are in good agreement with the BLS values reported for the $(101\ifmmode\bar\else\textasciimacron\fi{}0)$ epitaxial Co films by M. Grimsditch et al. [Phys. Rev. B 56, 2617 (1997)]. We also measured temperature development of film magnetization using a superconducting quantum inerference device magnetometer in a temperature range between 4.2 and 100 K. The temperature development of magnetization obeys the ${T}^{3/2}$ law and can be well reproduced by SW theory with BLS values of the magnetic constants.

Magnetic circular X-ray dichroism of metastable epitaxial Fe on Cu(1 0 0)

We report L-edge Magnetic Circular X-Ray Dichroism measurements at 30 K of Fe deposited at 100 K on Cu (1 0 0) in the form of ultrathin films and a wedge. The orbital and spin moments of the wedge differ from those obtained for uniform thickness films. For the FCT phase of Fe we stabilised both perpendicular and in-plane easy directions.

Magnetic bubbles, walls, and fine structure in ion-implanted garnets

The description and interpretation of bubbles with associated domains and charged walls in Sm YIG garnets implanted with H+ ions are presented. The nature of the contrast obtained with Bitter solution is shown to result from the polarization of the grains by the external bias field. The use of this contrast allows the determination of the direction of the magnetization in the plane of the film and perpendicular to it. The domain arrangements around the bubbles are found to be strongly dependent on the presence of an in-plane anisotropy. In the specimen studied here, the magnetocrystalline threefold symmetry and in-plane easy direction coming from a crystallographic tilt are predominent. The shape of the domains associated with bubbles changes significantly with the magnitude and the direction of the applied in-plane field. The structure can be interpreted in terms of the flux closure and bubble-wall configurations for the S=+1 states investigated here. The fine structure inside the bubbles is revealed and shown to be linked with external walls. The charged walls are attached to unimplanted disks. Their position, polarity, and shape are found to be highly dependent on the symmetry properties of the film as well as the characteristics of the applied in-plane field. Bubbles with charged walls of opposite polarity are strongly attracted to each other; however, a simple model based on magnetostatic interaction is found to be insufficient as the charged walls and the bubbles do not always rotate with exactly the same phase.