Magnetic Anisotropy In Thin Films Research Articles

Ultralow Electric Current-Assisted Magnetization Switching due to Thermally Engineered Magnetic Anisotropy.

Control of magnetic anisotropy in thin films with perpendicular magnetic anisotropy is of paramount importance for the development of spintronics with ultralow-energy consumption and high density. Traditional magnetoelectric heterostructures utilized the synergistic effect of piezoelectricity and magnetostriction to realize the electric field control of magnetic anisotropy, resulting in additional fabrication and modulation processes and a complicated device architecture. Here, we have systematically investigated the electric current tuning of the magnetic properties of the metallic NiCo2O4 film with intrinsic perpendicular magnetic anisotropy. Ferrimagnetic-to-paramagnetic phase transition has been induced through Joule heating, resulting in a rapid decrease of both magnetic coercivity and moment. An ultralow current density of 2.5 × 104 A/cm2, which is 2 to 3 orders magnitude lower than that of conventional spin transfer torque devices, has been verified to be effective for the control of the magnetic anisotropy of NiCo2O4. Successful triggering of magnetic switching has been realized through the application of a current pulse. These findings provide new perspectives toward the electric control of magnetic anisotropy and design of spintronics with an ultralow driving current density.

Effect of magnetic anisotropy relaxation on laser-induced magnetization precession in thin galfenol films

The rate and pathways of relaxation of a magnetic medium to its equilibrium following excitation with intense and short laser pulses are the key ingredients of ultrafast optical control of spins. Here we study experimentally the evolution of the magnetization and magnetic anisotropy of thin films of a ferromagnetic metal galfenol (${\mathrm{Fe}}_{0.81}{\mathrm{Ga}}_{0.19}$) resulting from excitation with a femtosecond laser pulse. From the temporal evolution of the hysteresis loops we deduce that the magnetization ${M}_{S}$ and magnetic anisotropy parameters $K$ recover within a nanosecond, and the ratio between $K$ and ${M}_{S}$ satisfies the thermal equilibrium's power law in the whole time range spanning from a few picoseconds to 3 nanoseconds. We further use the experimentally obtained relaxation times of ${M}_{S}$ and $K$ to analyze the laser-induced precession and demonstrate how they contribute to its frequency evolution at the nanosecond timescale.

Magnetic property modulation of Ni thin films transferred onto flexible substrates

The integration of highly crystallized thin films on flexible plastic substrates has so far been limited by the instability of the plastic substrates at high growth temperatures. In this paper, we report a robust and facile method to transfer magnetic Ni thin films onto flexible substrates, which enables the imposition of strain on the substrates to take advantage of the magnetoelastic properties of the films. Ni thin films were grown on water-soluble NaCl substrates by sputtering at high temperature and, thereafter, successfully transferred onto flexible polyethylene terephthalate substrates. Magnetic hysteresis loops with strong magnetic anisotropy were observed for the films grown at high temperatures on NaCl substrates; the anisotropy increased after the films were transferred onto the plastic substrate. The magnetic anisotropy of the transferred films was modulated under bending; this phenomenon of magnetic modulation is more apparent at a small radius of curvature. The modulation of magnetic anisotropy in magnetic thin films on flexible substrates using our innovative thin film transfer method presents significant potential for application in future flexible multifunctional devices.

Tuning magnetic anisotropy in thin films helps create magnon valves

Tuning of perpendicular magnetic anisotropy in iron garnets is used to create trilayer valve structures.

Study of roughness effect in Fe and Co thin films prepared by plasma magnetron sputtering

We report the experimental and theoretical study of a substrate roughness surface induced magnetic anisotropy in thin films of Fe and Co. The experimental results confirm previously reported data on NiFe thin films and indicate that rough substrates increase the magnetic coercive fields in magnetic thin films. This effect is most prominent in films of thickness comparable to the surface roughness values and materials of small volume magnetic anisotropy. We determined the coercive field of 15 nm Fe thin film sample deposited onto rough PVDF to be 256 Oe, which is more than doubled the value of the coercive field of 15 nm Fe films coated under identical conditions, onto smooth Si substrates. The effect is visible for Co films but weaken by its increased volume magneto-crystalline anisotropy. These results are important for applications based on magnetic thin film where the magnetic properties could be adjusted via substrate roughness engineering.

Controlling stability and emergent rotation of the skyrmion crystal in thin films of helimagnets via tilted magnetic field

The interaction of a tilted magnetic field and the intrinsic anisotropy of magnetic thin films leads to profound effects on the deformation, orientation, and stability of the skyrmion crystal (SkX) phase, which has yet to be explored comprehensively. Here, we study the combined effects of a tilted magnetic field and the exchange anisotropy on SkX in thin films of helimagnets. By applying an analytical framework suitable for deformed or rotated SkX, not only the strength of the magnetic field but also the polar angle and the azimuthal angle, which describe the direction of the magnetic field, are found to affect the stability of SkX. Moreover, they reorientate SkX at certain conditions. Our findings pave a way for the manipulation of SkX.

Magnetic Anisotropy in Thin Films of FePt Detected by the Ferromagnetic Resonance Method

FePt thin films are made via the heat treatment of structures containing Pt (001) and Fe (001) epitaxial layers. X-ray spectral analysis and field-emission scanning electron microscopy show that the samples contain the cubic L12–Fe3Pt phase and the tetragonal L10–FePt phase. The magnetic anisotropy of the thin films is studied by ferromagnetic resonance and magnetometric methods. The contributions from the cubic and tetragonal phases to the ferromagnetic-resonance spectrum of the sample are distinguished. The key characteristics of thin films which determine their practical applicability are determined: the Gilbert damping parameter G ~ 0.4, the first and second crystal anisotropy constants K1⊥ = –4.08 × 106 erg/cm3, K2⊥ = 1.34 × 106 erg/cm3, K2|| = –9.76 × 104 erg/cm3.

Observation of surface step bunch induced perpendicular magnetic anisotropy using spin-polarized low energy electron microscopy

Using spin-polarized low energy electron microscopy (SPLEEM), we observed surface step bunch induced perpendicular magnetic anisotropy in Fe/Ni bilayers grown on Cu(001) single crystal as well as in Ni/Co/Pd trilayers grown on W(110) crystal. On Cu(100) the formation of step bunches can be stimulated or suppressed by high- or low-temperature annealing cycles, respectively. SPLEEM images resolving the three dimensional magnetization vector in the Fe/Ni films grown on step bunched Cu(100) reveal an additional perpendicular magnetic anisotropy in regions near step bunches. In contrast, no extra perpendicular magnetic anisotropy is observed on low-temperature annealed Cu(100) featuring single-atom height step arrays. Additional investigation of Ni/Co/Pd trilayers on W(110) reveals the influence of step bunch orientation on magnetic anisotropy. Our observations may lead to opportunities for tailoring or patterning anisotropy in magnetic thin-films by controlling film morphology.

Creation and Destruction of Skyrmions via Electrical Modulation of Local Magnetic Anisotropy in Magnetic Thin Films

Magnetic skyrmions are topologically protected spin textures that are promising as carriers of information for computing and storage beyond CMOS-based systems. An electrostatic approach to creating a skyrmion can be more energy efficient than those based on a driving current. In this work, formation and dissolution of N\'eel skyrmions are examined theoretically via electrical modulation of magnetic anisotropy, in both ferromagnetic and antiferromagnetic structures. Simulations clearly illustrate the feasibility of the mechanism, enabling local control for versatility in applications. Also noteworthy is that the dynamical processes involved are much faster in antiferromagnetic materials.

Controlling the magnetic anisotropy in epitaxial Y3Fe5O12 films by manganese doping

Controlling the magnetic anisotropy in epitaxial ${\mathrm{Y}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$ (YIG) thin films is critical for magnonic and photonic device applications. In this paper, we report the crystal structure, magnetic properties, and magnetic anisotropy of epitaxial ${\mathrm{Y}}_{3}(\mathrm{F}{\mathrm{e}}_{5\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x}){\mathrm{O}}_{12}$ (Mn:YIG) thin films grown on $\mathrm{G}{\mathrm{d}}_{3}\mathrm{G}{\mathrm{a}}_{5}{\mathrm{O}}_{12}$ (111) (GGG) substrates by pulsed-laser deposition. Mn doping is observed to strongly enhance the magnetoelastic coefficient of YIG thin films, which leads to large tunability of the thin film magnetic anisotropy by lattice strain. With increasing Mn concentration from $x=0$ to $x=1.25$, a continuous increase of out-of-plane magnetic anisotropy ranging from \ensuremath{-}644.4 Oe to 1337.5 Oe is observed. In particular, a perpendicular magnetic anisotropy (PMA) is achieved in Mn:YIG thin films with a high Mn concentration of $x=1.12$. Ferromagnetic resonance (FMR) measurements show low FMR linewidths of 3.4 Oe to 129 Oe at 9.5 GHz in Mn:YIG thin films. Our paper demonstrates manganese doping as an effective way to enhance the magnetoelastic anisotropy of YIG thin films by strain, which is useful for magnonic and magneto-optical device applications.

Tunable anomalous hall effect induced by interfacial catalyst in perpendicular multilayers

The interfacial structures, playing a critical role on the transport properties and the perpendicular magnetic anisotropy in thin films and multilayers, can be modified by inserting an ultrathin functional layer at the various interfaces. The anomalous Hall effect (AHE) in the multilayers with core structure of Ta/CoFeB/X/MgO/Ta (X: Hf or Pt) is tuned by interfacial catalytic engineering. The saturation anomalous Hall resistance (RAH) is increased by 16.5% with 0.1 nm Hf insertion compared with the reference sample without insertion. However, the RAH value is decreased by 9.0% with 0.1 nm Pt insertion. The interfacial states were characterized by the X-ray photoelectron spectroscopy (XPS). The XPS results indicate that a strong bonding between Hf and O for Hf insertion, but no bonding between Pt and O for Pt insertion. The bonding between metal and oxygen leads to various oxygen migration behavior at the interfaces. Therefore, the opposite behavior about the RAH originates from the different oxygen behavior due to various interfacial insertion. This work provides a new approach to manipulate spin transport property for the potential applications.

Angular dependence of magnetization reversal in epitaxial chromium telluride thin films with perpendicular magnetic anisotropy

We investigate magnetic anisotropy and magnetization reversal mechanism in chromium telluride thin films grown by molecular beam epitaxy. We report existence of strong perpendicular magnetic anisotropy in these thin films, along with a relatively strong second order anisotropy contribution. The angular variation of the switching field observed from the magnetoresistance measurement is explained quantitatively using a one-dimensional defect model. The model reveals the relative roles of nucleation and pinning in the magnetization reversal, depending on the applied field orientation. Micromagnetic simulations are performed to visualize the domain structure and switching process.

Temperature dependence of perpendicular magnetic anisotropy in CoFeB thin films

We study perpendicular magnetic anisotropy in thin films of Ta/Co20Fe60B20/MgO by ferromagnetic resonance and find a linear temperature dependence for the first and second order uniaxial terms from 5 to 300 K. Our data suggest the possible hybridization of Fe-O orbitals at the CoFeB/MgO interface for the origin of the first order anisotropy. However, we also find that non-interfacial contributions to the anisotropy are present. An easy-cone anisotropy is found for the entire temperature range in the narrow region of film thicknesses around the spin reorientation transition 1.2–1.35 nm.

Tuning magnetic anisotropy of amorphous CoFeB film by depositing on convex flexible substrates

We have investigated the magnetic properties of amorphous Co40Fe40B20 (CoFeB) thin films grown on flexible polyimide (PI) substrates, which were fixed on convex molds with different curvatures during the magnetron sputtering deposition. When the flexible substrates were changed from convex to flat state after fabrication, a uniaxial magnetic anisotropy was induced in the CoFeB film due to magnetostrictive effect. Furthermore, the anisotropy also depends on the thicknesses of the film and substrate. Our results demonstrate a convenient method to tune the anisotropy of magnetic thin films grown on flexible substrates.

Measuring magnetic anisotropy with a rotatable ac electromagnet

Magnetic anisotropy of magnetic thin films was determined from a series of hysteresis loops measured via a rotatable alternate current (ac) electromagnet, the rotation angle of which could be continuously tuned at 360° with a high resolution. Co2FeAl Heusler alloy films grown by molecular beam epitaxy were used to show the benefits of the method for measuring magnetic anisotropy.

Field-dependent perpendicular magnetic anisotropy in CoFeB thin films

We report ferromagnetic resonance measurements of perpendicular magnetic anisotropy in thin films of Ta/Co20Fe60B20/MgO as a function of the Co20Fe60B20 layer thickness. The first and second order anisotropy terms show unexpectedly strong dependence on the external magnetic field applied to the system during the measurements. We propose strong interfacial spin pinning as a possible origin of the field-dependent anisotropy. Our results imply that high-field anisotropy measurements cannot be directly used for quantitative evaluation of zero-field performance parameters of CoFeB-based devices such as spin torque memory.

Peculiarities in the measurements of magnetic anisotropy in thin films and nanostructures by the magnetooptical Kerr effect

Cobalt polycrystalline thin films, sputtered by a magnetron system, and cobalt nanostructures, patterned by electron-beam lithography, were studied. We propose an improved magnetooptical method for measuring the magnetic anisotropy field in thin films and arrays of nanostructures. We demonstrate that our method is applicable for a precise investigation of the configurational anisotropy of magnetization reversal in nanostructure arrays.

The Improved Magneto-Optical Kerr Effect Method of Magnetic Anisotropy Measurements in Thin Films and Nanostructures

Magnetic anisotropy of thin films and nanodisk arrays made of cobalt has been studied by the magneto-optical anisometry with modulated field. The improved method of magnetic anisotropy field measurements in thin films has been proposed. This method has been used for cobalt nanodisk arrays in order to investigate anisotropy of magnetization reversal processes due to dipole-dipole interaction between nanodisks.

Determination of the distribution of transverse magnetic anisotropy in thin films from the second harmonic of Kerr signal

We describe a method to determine the magnetic anisotropy distribution in thin films based on Kerr magnetometry, well adapted for single micro- and nanostructures. When the sample is excited by an ac field of small amplitude, for each value of a longitudinal dc field H, the second harmonic of the Kerr signal gives the contribution of the corresponding transverse anisotropy field Hk=H to the anisotropy distribution. The method is tested on a Permalloy-based multilayer microstructure, revealing two anisotropy contributions, one of them deviated from the perfect transverse direction. This confirms and extends a previous characterization performed by far more sophisticated methods.

Control of magnetic anisotropy in (Ga,Mn)As with etching depth of specimen boundaries

We have found large effect of mechanical modifications on the magnetic anisotropy in thin films of (Ga,Mn)As on GaAs substrates and on (In,Ga)As buffer layers. Such modifications are simply brought about with etching two-dimensional films into thin-wires, which process partly releases compressive or tensile strains in the thin films. The effect of such mechanical processing depends on the etching depth, through which we can control the magnetic anisotropy. Uniaxial in-plane magnetic anisotropy has disappeared with releasing the strain, which result suggests that the uniaxial anisotropy is due to some extrinsic effect in one hand, that the dimer formation model is not the case on the other hand.