Growth-induced Magnetic Anisotropy Research Articles

Characteristics of spin-thermoelectric generators enhanced by incorporating liquid phase epitaxial Bi-substituted YIG films as a ferrimagnetic insulator

Liquid phase epitaxial (LPE) Bi-substituted YIG films exhibit a large growth-induced magnetic anisotropy (GIMA) perpendicular to the film surface. The GIMA depends on the melt composition and supercooling in the LPE growth. Our film shows a large Ku value of 0.1 × 104 J/m3 even at a Bi content, x, of 0. We assume that GIMA enhances the spin-orbital coupling (SOC) in the local d-electron spins of Fe3+, and that the SOC-enhanced thermal energy transfer from the phonon system to the spin system increases spin currents in spin-thermoelectric (STE) generation. We discuss the characteristics of STE generators on the basis of data obtained from ferromagnetic resonance measurements.

Enhancement of the spin Seebeck effect owing to bismuth substitution in thermoelectric generators fabricated from LPE Bi-substituted YIG films

Raising the thermoelectric voltage in spin thermoelectric generators is an important subject. We investigated the substitution of bismuth for yttrium to increase spin pumping at the paramagnetic metal and ferrimagnetic insulator (PM/FMI) interface, and tested bismuth-substituted iron garnet films grown by liquid phase epitaxy. Epitaxial Bi-substituted iron garnet films exhibit large growth-induced magnetic anisotropy perpendicular to the film surface. This anisotropy increases the magnetic damping α in the FMI; α also increases with increasing Bi content. We report the rise in voltage observed in a spin thermoelectric generator incorporating Bi-substituted YIG films grown by liquid phase epitaxy, and explain the origin of the voltage rise based on the results of FMR measurements.

Anisotropic magnetoresistance and planar Hall effect in magnetoresistive NiFe/Pt thin film

The magnetoresistive NiFe/Pt sensor was fabricated by using standard microlithography and sputter deposition techniques and then the magnetic and magnetotransport properties were investigated. The magnetic properties of the sample show that the easy axis of magnetization prefers a predefined direction along the Hall bar. Moreover, the sample has uniaxial magnetic anisotropy due to the growth-induced magnetic anisotropy. On the other hand, the Anisotropic magnetoresistance (AMR) and the Planar Hall effect (PHE) of the sample were investigated in detail between 0° and 360°. The angle-dependent AMR and PHE signals at the in-plane magnetic field of constant magnitude intersect at certain angles. At these angles, the presence of unusual peaks and valleys was observed. This is proof that AMR and PHE curves had strongly affected each other at certain angles.

Magnetoelectric Interactions in Composites of Ferrite Films on Lattice-Matched Substrates and Ferroelectrics

The converse magnetoelectric effect (CME, $i.e.$ the magnetic response to a static electric field) in ferrite-ferroelectric composites is important for an innovative class of voltage-tunable, high-frequency magnetic devices. Technical challenges persist, though, due to weak CME coupling, large ferromagnetic-resonance (FMR) linewidth, and the need for a large bias magnetic field. This study aims to overcome those challenges by using single-crystal-like nickel ferrite films exhibiting high magnetostriction and narrow FMR linewidth. These films present a very high growth-induced magnetic anisotropy field that eliminates the need for a bias field, and facilitates device integration.

Thick permalloy films for the imaging of spin texture dynamics in perpendicularly magnetized systems

We demonstrated that thick Permalloy films exhibiting a weak growth-induced perpendicular magnetic anisotropy can be employed as an ideal test system for the investigation of gyration dynamics in topologically trivial and non-trivial magnetic states ranging from an isolated magnetic skyrmion to more complex n$\pi$ spin configurations.

Ultrafast laser-induced changes of the magnetic anisotropy in a low-symmetry iron garnet film

We explore a thermal mechanism of changing the anisotropy by femtosecond laser pulses in dielectric ferrimagnetic garnets by taking a low symmetry (YBiPrLu)3(FeGa)5O12 film grown on the (210)-oriented Gd3Ga5O12 substrate as a model media. We demonstrate by means of spectral magneto-optical pump-probe technique and phenomenological analysis, that the magnetization precession in such a film is triggered by laser-induced changes of the growth-induced magnetic anisotropy along with the well-known ultrafast inverse Faraday effect. The change of magnetic anisotropy is mediated by the lattice heating induced by laser pulses of arbitrary polarization on a picosecond time scale. We show that the orientation of the external magnetic field with respect to the magnetization easy plane noticeably affects the precession excited via the anisotropy change. Importantly, the relative contributions from the ultrafast inverse Faraday effect and the change of different growth-induced anisotropy parameters can be controlled by varying the applied magnetic field strength and direction. As a result, the amplitude and the initial phase of the excited magnetization precession can be gradually tuned.

Magnetic properties and thermal stability of Co/HfN multilayer films for high-frequency application

Sputtered metal–nonmetal granular films tend to exhibit growth-induced perpendicular magnetic anisotropy. In this work, Co/HfN multilayers were synthesized to suppress the development of columnar clusters along the deposition direction. The results reveal that a HfN interlayer thickness of less than 0.4 nm is insufficient to separate the columnar clusters; however, increasing the interlayer thickness to 0.8 nm with increasing the sputtering duration successfully led to typical in-plane magnetic anisotropy with a coercivity as low as 3 Oe. The Co(4 nm)/HfN(1.5 nm) multilayers exhibited high permeability of approximately 260 up to 1.6 GHz. The resonance frequency increased from 1.8 to 2.3 GHz with increasing annealing temperature, which is attributed to the increased magnetic anisotropy. Our results suggest that the multilayers still show high-frequency performance even after annealing at 450 °C, which would be a big advantage for complementary metal–oxide–semiconductor (CMOS) fabrication technology.

Domain wall dynamics of periodic magnetic domain patterns in Co2MnGe-Heusler microstripes

Highly symmetric periodic domain patterns were obtained in Co2MnGe-Heusler microstripes as a result of the competition between growth-induced in-plane magnetic anisotropy and shape anisotropy. Zero field magnetic configurations and magnetic field-induced domain wall (DW) motion were studied by magnetic force microscopy-image technique for two different cases: dominant uniaxial- and dominant cubic in-plane anisotropy. We implemented a magneto-optical Kerr effect susceptometer to investigate the DW dynamics of periodic domain structures by measuring the in-phase and out-of-phase components of the Kerr signal as a function of magnetic field frequency and amplitude. The DW dynamics for fields applied transversally to the long stripe axis was found to be dominated by viscous slide motion. We used the inherent symmetry/periodicity properties of the magnetic domain structure to fit the experimental results with a theoretical model allowing to extract the DW mobility for the case of transverse DWs (μTDW = 1.1 m s−1 Oe−1) as well as for vortex-like DWs (μVDW = 8.7 m s−1 Oe−1). Internal spin structure transformations may cause a reduction of DW mobility in TDWs as observed by OMMFF simulations.

Growth-induced non-planar magnetic anisotropy in FeCoZr-CaF2 nanogranular films: Structural and magnetic characterization

The relation between nanoscale structure, local atomic order and magnetic properties of (FeCoZr)x(CaF2)100−x (29 ≤ x ≤ 73 at. %) granular films is studied as a function of metal/insulator fraction ratio. The films of a thickness of 1–6 μm were deposited on Al-foils and glass-ceramic substrates, by ion sputtering of targets of different metal/insulator contents. Structural characterization with X-ray and electron diffraction as well as transmission electron microscopy revealed that the films are composed of isolated nanocrystalline bcc α-FeCo(Zr) alloy and insulating fcc CaF2 matrix. They grow in a columnar structure, where elongated metallic nanograins are arranged on top of each other within the columns almost normal to the substrate surface. Mössbauer spectroscopy and magnetometry results indicate that their easy magnetization axes are oriented at an angle of 65°–74° to the surface in films with x between 46 and 74, above the electrical percolation threshold, which is attributed to the growth-induced shape anisotropy. Interatomic distances characteristic for metallic state of α-FeCo(Zr) nanograins were revealed by X-ray Absorption Spectroscopy. The results show a lack of surface oxidation of the alloy nanograins, so the growth-induced orientation of nanograins in the films cannot be attributed to this effect. The study is among the first to report a growth-induced non-planar magnetic anisotropy in metal/insulator granular films above the percolation threshold and to reveal the origin of it.

Investigation of the magnetic properties of insulating thin films using the longitudinal spin Seebeck effect

The longitudinal spin Seebeck effect is used as a detector for the magnetic properties and switching characteristics of magnetic thin insulating films. We use a 300 nm and a 20 nm thick Yttrium Iron Garnet (YIG, Y3Fe5O12) film prepared by pulsed laser deposition and afterwards coated by platinum for the detection of the thermally excited magnons by the inverse spin Hall effect. The inverse spin Hall signals reveal a magnetic uniaxial anisotropy along the direction of the platinum stripe in the thicker film. For the thin film we find a more isotropic behavior, which is complementarily observed using the magnetoresistance occurring at the platinum/YIG interface. We explain our results on the basis of x-ray diffraction data, which reveal a miscut of the substrate and film surface and an expansion of the YIG lattice. Both findings favor a growth-induced magnetic anisotropy that we observe.

Growth-induced magnetic anisotropy behavior in thin garnet films fabricated by pulsed laser deposition

The effect of target rotation rate on the magneto-optical properties and induced magnetic anisotropy of pulse laser deposition grown Bi:YIG garnets has been investigated. The rotation rate dependence of magnetic properties of thin films has been studied using magneto-optical Faraday rotation combined with magneto-optical Kerr measurement setup. Our results show that decreasing the target rotation rate can also increase the polar coercive field and hence magnetic anisotropy. Also the spectral answer of samples show the capability of use of them in one off optical ranges such as visible, near infrared and far infrared regions.

Tunable magnetic properties in ultrathin Co/garnet heterostructures

We demonstrate how the magnetic properties of metal/dielectric Co/yttrium iron garnet heterostructures can be engineered by both changing the garnet thickness and adding an ultrathin Co cover layer. The observed magnetization reversal process in the heterostructures is explained by both cubic and perpendicular growth-induced magnetic anisotropy of the garnet films. In particular, the perpendicular magnetic anisotropy can be strongly increased for reduced thickness. A strong influence of a 2 nm Co layer on the domain structure geometry and magnetization processes has been found for 1.8 µm garnet films.

Miscut-angle dependence of perpendicular magnetic anisotropy in thin epitaxial CoPt3 films grown on vicinal MgO

The effect of vicinal substrates on the growth-induced perpendicular magnetic anisotropy of epitaxial CoPt3 films has been studied. A small (2°, 4°, or 10°) miscut angle of the vicinal substrate causes the crystallographic axes of the sample to be tilted along the miscut direction. The magnitude of the perpendicular anisotropy is unaffected by the presence of substrate steps produced by the miscut angle, while an additional, in-plane anisotropy develops with a larger miscut angle. Effects of the steps are seen in magnetic force microscopic images of domain wall pinning.

Growth-induced perpendicular magnetic anisotropy and clustering in Ni xPt 1− x alloys

Polycrystalline and epitaxial (1 0 0), (1 1 0), and (1 1 1)-oriented Ni 3Pt, NiPt, and NiPt 3 films were deposited over a range of growth temperatures from 80°C to 700°C. Films grown at moderate temperatures (200–400°C) exhibit growth-induced properties similar to Co–Pt alloys: enhanced and broadened Curie temperature, perpendicular magnetic anisotropy and large coercivity. As in Co–Pt, the magnetic properties suggest a clustering of Ni into platelets on the growth surface, as the films are being grown. Unlike Co–Pt, however, NiPt films exhibit a strong orientational dependence of anisotropy and enhanced Curie temperature, possibly resulting from different types of surface reconstructions which affect the growth surface.

Suppression of growth-induced perpendicular magnetic anisotropy in Co–Pt alloys by trace amounts of Si

( Co x Pt 1−x ) 1−y Si y alloys with Si content from 1 to 20 at. % have been grown over a range of growth conditions. Co-deposition of even trace amounts of Si with Co–Pt alloys causes the growth-induced magnetic anisotropy and chemical clustering found in these vapor-deposited alloy films to decrease or vanish. It also causes significant reduction in grain size. Addition of 5 at. % Si eliminates anisotropy completely. Addition of 1 at. % Si produces a film with magnetic properties identical to pure Co–Pt alloys grown at lower deposition temperatures. We suggest this suppression of anisotropy and related effects are due to a decrease in surface mobility during growth in the presence of even trace amounts of Si.

Growth-induced magnetic anisotropy and clustering in vapor-deposited Co-Pt alloy films

Polycrystalline and epitaxial (100)-, (110)-, and (111)-oriented ${\mathrm{CoPt}}_{3}$ and ${\mathrm{Co}}_{0.35}{\mathrm{Pt}}_{0.65}$ films were deposited at various growth rates and over a range of growth temperatures from -50 to 800 \ifmmode^\circ\else\textdegree\fi{}C. Films grown at moderate temperatures (200--400 \ifmmode^\circ\else\textdegree\fi{}C) exhibit remarkable growth-induced properties: perpendicular magnetic anisotropy and large coercivity, as well as enhanced Curie temperature and low-temperature saturation magnetization. Magnetic measurements indicate significant Co clustering in these epitaxial fcc films. These properties are independent of crystallographic orientation, increase with increasing growth temperature, and vanish with annealing. We propose that the correlation between magnetic inhomogeneity, magnetic anisotropy, and enhanced moment is explained by clustering of Co into thin platelets in a Pt-rich lattice. This clustering occurs at the growth surface and is trapped into the growing film by low bulk atomic mobility.

Growth-induced magnetic anisotropy behavior in Ni thin films

The magnetic anisotropy of Ni thin films fabricated under different growth conditions have been studied by the ferromagnetic resonance (FMR) method. The FMR data have been fitted theoretically. The results show that the anisotropy is an orthorhombic anisotropy, and there exists an optimal rotary speed of the sample for a fixed deposition rate. Near the optimal speed, the anisotropy field along the in-plane easy axis exhibits a maximum, while the anisotropy field perpendicular to the film plane exhibits a minimum. The origin of orthorhombic anisotropy has been discussed.

Characterization of Laser-Annealed Magnetic Garnet Films by Scanning Probe Microscopy

Surface damage and magnetic structures in laser-annealed magnetic garnet films investigated by the use of scanning probe microscopes, were i.e., Atomic Force Microscope (AFM), Magnetic Force Microscope (MFM), and Near-field Scanning Optical Microscope (NSOM). AFM images showed the atomically flat surface of the laser-annealed region in which the growth-induced magnetic anisotropy was strongly reduced. MFM and NSOM images showed sharp changes in the direction of magnetization at the boundary between the laser-annealed region and laser-annealed region. These results show that the laser-annealing technique is very useful for fabricating waveguide optical isolators.

Growth-induced uniaxial in-plane magnetic anisotropy for ultrathin Fe deposited on MgO(001) by oblique-incidence molecular beam epitaxy

In-plane uniaxial magnetic anisotropy is observed in the magnetic hysteresis loops of ultrathin Fe films grown on MgO(001) by means of oblique-incidence molecular beam epitaxy. The films are deposited from an angle of 45° from the surface normal in the (100) azimuth toward the [100]. The easy axis of magnetization is perpendicular to the incident Fe-beam direction. A strong structural asymmetry is observed in x-ray photoelectron diffraction and attributed to an oriented morphology that drives the growth-induced magnetic anisotropy.

Surface-induced ordering: A model for vapor-deposition growth of amorphous materials

During vapor deposition growth of amorphous materials, we propose that adatom clusters align such as to minimize surface energy. This alignment, a thermally activated process, causes the as-grown structure of the amorphous thin film to be anisotropic and stabilizes the structure against subsequent relaxation, presumably by reducing free volume. A two-level-systems analysis describes the effect of deposition temperature, deposition rate, and subsequent annealing. The model is used to describe the growth-induced magnetic anisotropy of amorphous Tb-Fe.