In-plane Uniaxial Magnetic Anisotropy Research Articles

Effects of swift heavy ion-irradiation on magnetic properties of Co-doped TiO2

Ferromagnetic Co-doped TiO2 thin films at room temperature have been synthesized by RF sputtering and ion-irradiated with 100 MeV Au ions. The surface roughness of the film increases with increasing Au-ion fluence. Kerr microscopy measurements at room temperature give isotropic hysteresis loop with negligible coercivity, which gets modified into in-plane uniaxial magnetic anisotropy after anion fluence of 5×1011 ions/cm2. Further ion-irradiation reduced the coercivity of the film. X-ray magnetic circular dichroism (XMCD) measurements have been performed in total electron yield mode, to investigate the effects of ion-irradiation on magnetic properties of the Co-doped TiO2 thin films. From the XMCD analysis, we observed that the total magnetic moment of Co atoms in case of as-deposited film is 1.771±0.016μB. After the ion-irradiation it became 0.3818±0.014μB at room temperature.

Polarization control of THz emission using spin-reorientation transition in spintronic heterostructure

Polarization of electromagnetic waves plays an extremely important role in interaction of radiation with matter. In particular, interaction of polarized waves with ordered matter strongly depends on orientation and symmetry of vibrations of chemical bonds in crystals. In quantum technologies, the polarization of photons is considered as a “degree of freedom”, which is one of the main parameters that ensure efficient quantum computing. However, even for visible light, polarization control is in most cases separated from light emission. In this paper, we report on a new type of polarization control, implemented directly in a spintronic terahertz emitter. The principle of control, realized by a weak magnetic field at room temperature, is based on a spin-reorientation transition (SRT) in an intermetallic heterostructure TbCo2/FeCo with uniaxial in-plane magnetic anisotropy. SRT is implemented under magnetic field of variable strength but of a fixed direction, orthogonal to the easy magnetization axis. Variation of the magnetic field strength in the angular (canted) phase of the SRT causes magnetization rotation without changing its magnitude. The charge current excited by the spin-to-charge conversion is orthogonal to the magnetization. As a result, THz polarization rotates synchronously with magnetization when magnetic field strength changes. Importantly, the radiation intensity does not change in this case. Control of polarization by SRT is applicable regardless of the spintronic mechanism of the THz emission, provided that the polarization direction is determined by the magnetic moment orientation. The results obtained open the prospect for the development of the SRT approach for THz emission control.

Effects of substrate annealing on uniaxial magnetic anisotropy and ferromagnetic resonance frequency of Ni80Fe20 films deposited on self-organized periodically rippled sapphire substrates

Ni80Fe20 (NiFe) films have been deposited on self-organized sapphire substrates with periodical surface ripples formed by 2–10 h thermal annealing at 1400°C. By extracting static and dynamic magnetic parameters from magneto-optical Kerr effect (MOKE) loops and ferromagnetic resonance (FMR) spectra, we quantified and analyzed the substrate annealing effects on the induced in-plane uniaxial magnetic anisotropy and ferromagnetic resonance frequency in these films. The study demonstrates that the uniaxial anisotropy of these NiFe films is strongly related to the annealing-time dependent substrate faceting and coarsening mechanisms. The uniaxial anisotropy field Hu changes from ~170 Oe to ~250Oe, and the maximum value of it is obtained in film on 8-h annealed substrate. The ferromagnetic resonance response and high frequency characteristics of films can be modulated by self-organized substrate is also confirmed. When external magnetic field is parallel to the direction of ripple, the ferromagnetic resonance frequency of films grown on the self-organized substrates is increased by ~0.3−1 GHz compared with that of reference film on plane substrate. This work would be helpful to design the high frequency properties of spintronic devices and micro-wave magnetic devices by utilizing self-organized substrates.

Self-organised stripe domains and elliptical skyrmion bubbles in ultra-thin epitaxial Au0.67Pt0.33/Co/W(110) films

We studied the symmetry of magnetic properties and the resulting magnetic textures in ultra-thin epitaxial Au0.67Pt0.33/Co/W(110), a model system exhibiting perpendicular magnetic anisotropy and interface Dzyaloshinskii–Moriya interaction (DMI). As a peculiar feature, the C2v crystal symmetry induced by the Co/W interface results in an additional uniaxial in-plane magnetic anisotropy in the cobalt layer. Photo-emission electron microscopy with magnetic sensitivity reveals the formation of self-organised magnetic stripe domains oriented parallel to the hard in-plane magnetisation axis. We attribute this behavior to the lower domain wall energy when oriented along this axis, where both the DMI and the in-plane magnetic anisotropy favor a Néel domain wall configuration. The anisotropic domain wall energy also leads to the formation of elliptical skyrmion bubbles under a weak out-of-plane magnetic field.

Quantification of residual stress governing the spin-reorientation transition (SRT) in amorphous magnetic thin films

Soft magnetic thin films with in-plane uniaxial magnetic anisotropy are of significant importance for a broad range of technological applications, including high-frequency power conversion. In-plane uniaxial anisotropy in amorphous films is of particular interest for ultra-low materials loss and MHz frequency operations. The present work is focused on one of the fundamental mechanisms, i.e., residual stress, that can negate the uniaxial anisotropy in amorphous films by engendering perpendicular magnetisation and hence, undermining the soft magnetic performance. It is quantified how the nature of residual stress, compressive or tensile, transforms the magnetisation from an in-plane to an out-of-plane configuration, also well-known as spin-reorientation transition (SRT). A correlation between engineered residual stress in multilayer stacks, induced by the uneven expansion of metallic/dielectric layers following a thermal-shock scheme, and SRT mechanism demonstrates tensile stress inside the films undermines the soft magnetic performance. We suggest the magnetic softness can be retained by eluding sources of tensile stress during fabrication or post-processing of the amorphous films.

Roles of cobalt and boron in FeCoBSi soft magnetic composite with in-plane uniaxial magnetic anisotropy

The roles of cobalt and boron in an FeCoBSi soft magnetic composite (SMC) containing magnetic flakes with in-plane uniaxial magnetic anisotropy were investigated. When cobalt was added to the magnetic flakes in the SMC, in-plane uniaxial magnetic anisotropy was effectively induced, and coercivity in the easy axis direction remarkably decreased after annealing under a magnetic field in comparison with the case without addition of cobalt. In particular, the addition of cobalt was essential to decrease coercivity in the SMC, in marked contrast to simple amorphous ribbons in which coercivity decreased even without addition of cobalt but in-plane uniaxial magnetic anisotropy was not induced. Boron prevented crystallization of the magnetic amorphous phase, with boron content of more than approximately 18 at% relative to FeCo necessary to produce an amorphous phase without crystallization, resulting in low coercivity and in-plane uniaxial magnetic anisotropy. High saturation magnetization of 1.12 T and low coercivity of 0.32 Oe with in-plane uniaxial magnetic anisotropy were obtained with a composition of Fe55.5Co23.7B14.2Si6.6 (Co/(Fe + Co) = 30 at% and B/(Fe + Co) = 18 at%) in the fabricated SMC.

Investigation on interfacial effect of CoFeB/GaAs heterostructure

Interfacial effect of amorphous cobalt-iron-boron (Co60Fe20B20) films deposited on GaAs(001) by DC magnetron-sputtering has been precisely studied. In-plane uniaxial magnetic anisotropy (UMA) as a significant magnetic property has been found in Co60Fe20B20/GaAs heterostructure and the UMA field (Hu) of (∼75 Oe) is achieved. Magnetic Schottky barrier contact has been achieved by current-voltage (I–V) characteristic through the interface with a barrier height of 0.72 eV, which is an appropriate for tunneling injection of spin current from ferromagnet to semiconductors in spin-injection devices.

Giant anisotropy of magnetic damping and significant in-plane uniaxial magnetic anisotropy in amorphous Co40Fe40B20 films on GaAs(001)**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300803), the National Natural Science Foundation of China (Grant Nos. 51971109, 51771053, and 51471085), and Scientific Research Foundation of Nanjing Institute of Technology (Grant Nos. ZKJ201708 and CKJB201708).

Tuning magnetic damping constant in dedicated spintronic devices has important scientific and technological implications. Here we report on anisotropic damping in various compositional amorphous CoFeB films grown on GaAs(001) substrates. Measured by a vector network analyzer-ferromagnetic resonance (VNA-FMR) equipment, a giant magnetic damping anisotropy of 385%, i.e., the damping constant increases by about four times, is observed in a 10-nm-thick Co40Fe40B20 film when its magnetization rotates from easy axis to hard axis, accompanied by a large and pure in-plane uniaxial magnetic anisotropy (UMA) with its anisotropic field of about 450 Oe. The distinct damping anisotropy is mainly resulted from anisotropic two-magnon-scattering induced by the interface between the ferromagnetic layer and the substrate, which also generates a significant UMA in the film plane.

Magnetic properties and thermal stability of nanocrystalline Fe films prepared by oblique sputtering deposition method

In this work, nanocrystalline Fe thin films on a Si (100) substrate were successfully prepared using the oblique-angle sputtering technique coupled with applying an external magnetic field of 200 Oe. The crystalline structure, the static magnetic properties, and the high-frequency magnetic characteristics of the as-deposited film were examined using the X-ray diffraction, the M–H loop tracer and the network analyzer, respectively. The results indicate that the nanocrystalline BCC phase of Fe with the average crystallite size of 9.4–11.3 nm and (110) preferred orientation is formed during the deposition process. The difference between M–H loops measured along easy and hard axes confirms the existence of a high in-plane uniaxial magnetic anisotropy which increases with increasing the incident angle. As a result, the ferromagnetic resonance frequency is shifted from 1.02 to 1.88 GHz with an increase of the oblique angle from 0 to 45°. In addition, the film deposited at the oblique angle of 45°, indicates good thermal stability with the maximum imaginary permeability of 513 at 420 K.

Large anisotropy of magnetic damping in amorphous CoFeB films on GaAs(001)

Amorphous CoFeB films grown on GaAs(001) substrates demonstrating significant in-plane uniaxial magnetic anisotropy were investigated by vector network analyzer ferromagnetic resonance. Distinct in-plane anisotropy of magnetic damping, with a largest maximum–minimum damping ratio of about 109%, was observed via analyzing the frequency dependence of linewidth in a linear manner. As the CoFeB film thickness increases from 3.5 nm to 30 nm, the amorphous structure for all the CoFeB films is maintained while the magnetic damping anisotropy decreases significantly. In order to reveal the inherent mechanism responsible for the anisotropic magnetic damping, studies on time-resolved magneto-optical Kerr effect and high resolution transmission electron microscopy were performed. Those results indicate that the in-plane angular dependent anisotropic damping mainly originates from two-magnon scattering, while the Gilbert damping keeps almost unchanged.

Observation of unexpected uniaxial magnetic anisotropy in La2/3Sr1/3MnO3 films by a BaTiO3 overlayer in an artificial multiferroic bilayer.

We studied in detail the in-plane magnetic properties of heterostructures based on a ferroelectric BaTiO3 overlayer deposited on a ferromagnetic La2/3Sr1/3MnO3 film grown epitaxially on pseudocubic (001)-oriented SrTiO3, (LaAlO3)0.3(Sr2TaAlO6)0.7 and LaAlO3 substrates. In this configuration, the combination of both functional perovskites constitutes an artificial multiferroic system with potential applications in spintronic devices based on the magnetoelectric effect. La2/3Sr1/3MnO3 single layers and BaTiO3/La2/3Sr1/3MnO3 bilayers using the pulsed-laser deposition technique. We analyzed the films structurally through X-ray reciprocal space maps and high-angle annular dark field microscopy, and magnetically via thermal demagnetization curves and in-plane magnetization versus applied magnetic field loops at room temperature. Our results indicate that the BaTiO3 layer induces an additional strain in the La2/3Sr1/3MnO3 layers close to their common interface. The presence of BaTiO3 on the surface of tensile-strained La2/3Sr1/3MnO3 films transforms the in-plane biaxial magnetic anisotropy present in the single layer into an in-plane uniaxial magnetic anisotropy. Our experimental evidence suggests that this change in the magnetic anisotropy only occurs in tensile-strained La2/3Sr1/3MnO3 film and is favored by an additional strain on the La2/3Sr1/3MnO3 layer promoted by the BaTiO3 film. These findings reveal an additional mechanism that alters the magnetic behavior of the ferromagnetic layer, and consequently, deserves further in-depth research to determine how it can modify the magnetoelectric coupling of this hybrid multiferroic system.

Exploring the shear strain contribution to the uniaxial magnetic anisotropy of (Ga,Mn)As

We present an experimental and k⋅p theoretical study on the origin of the strong in-plane uniaxial magnetic anisotropy in (Ga,Mn)As layers, unexpected from the cubic crystalline structure. The symmetry lowering can be accounted for by structural or effective shear strains. We find theoretically out-of-plane and in-plane magnetic anisotropy constants being linear with the shear strain. Searching for a real shear strain arising from lattice relaxation, we perform two types of measurements: anomalous x-ray diffraction and strain-induced optical birefringence, at room temperature. Working on a strongly anisotropic (Ga,Mn)As layer, the estimated ϵxy=10−4 was not found although it lied an order of magnitude above the detection threshold. This ensemble of results indicates as unlikely a relaxation-driven uniaxial anisotropy. As previously suggested theoretically, the magnetic symmetry-lowering could instead originate from the anisotropic incorporation of Mn atoms during growth. This would yield a perfectly in-plane matched lattice, with an anisotropy that could nevertheless be modeled as an effective shear strain and modified by an external shear stress, in agreement with the existing experimental literature.

Tailoring of uniaxial magnetic anisotropy in Permalloy thin films using nanorippled Si substrates

In this work the investigation of in-plane uniaxial magnetic anisotropy induced by the morphology due to ion beam erosion of Si(1 0 0) has been done. Ion beam erosion at an oblique angle of incidence generates a well-ordered nanoripple structure on the Si surface and ripple propagates in a direction normal to ion beam erosion. Permalloy thin films grown on such periodic nanopatterns show a strong uniaxial magnetic anisotropy with easy axis of magnetization in a direction normal to the ripple wave vector. The strength of uniaxial magnetic anisotropy is found to be high for the low value of ripple wavelength; it is decreasing with increasing value of ripple wavelength. Similarly, the strength of uniaxial magnetic anisotropy decreases with increasing Permalloy film thickness. Grazing incidence small angle x-ray scattering data reveals an anisotropic growth of Permalloy thin films with preferential orientation of grains in the direction normal to the ripple wave vector. Permalloy thin film growth is highly conformal with the film surface replicating the substrate ripple morphology up to a film thickness of 50 nm has been observed. Correlation between observed uniaxial magnetic anisotropy to surface modification has been addressed.

Electric field-induced non-volatile 90° rotation of in-plane magnetic anisotropy in TbFeCo film grown on PMN-PT(0 1 1) substrate

A TbFeCo alloy film with in-plane uniaxial magnetic anisotropy was fabricated by pulsed laser deposition on a (0 1 1)-oriented Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT) substrate. By applying electric field along the surface normal, the magnetic easy axis was rotated 90° from the [0 1 ] to the [1 0 0] direction by strain-mediated magnetoelectric coupling. The modulation effect was further confirmed microscopically from variation of the magnetic domain configuration. Importantly, the magnetic easy axis remains along the [1 0 0] direction after removal of the electric field. We speculate that this non-volatile modulation was triggered by an irreversible rhombohedral to orthorhombic phase transition of PMN-PT.

Influence of nanoholes array geometrical parameters on magnetic properties of Dy–Fe antidot thin films

Nanoscale artificially engineered spintronic materials could be used to enlarge the storage density of magnetic recording media. For this purpose, magnetic nanostructures such as antidot arrays exhibiting high uniaxial magnetic anisotropy are new contestants in the field of ultrahigh density magnetic data storage devices. In this context, we focus on the synthesis of nanostructured magnetic materials consisting of Dy–Fe alloyed antidot thin films, deposited onto the surface of nanoporous alumina membranes served as patterned templates. Noticeable variations of in the in-plane magnetic anisotropy have been observed by modifying the layer thickness at both microscopic and macroscopic scales. The microscopic magnetic properties have been locally studied by Nano-MOKE magnetometry. For thinner antidot samples with 15, 20 and 25 nm in thickness, a tri-axial in-plane magnetic anisotropy has been detected. Meanwhile, for thicker antidot samples (40–60 nm of layer thickness), an in-plane uniaxial magnetic anisotropy has been noted. We attribute these changes in the magnetic anisotropy to the strong correlation between the edge-to-edge distance among adjacent nanoholes, W, and the local magnetic anisotropy of antidot samples. The effective magnetic anisotropy exhibits an unexpected crossover from the in-plane to out-of-plane direction due to the increasing of the effective perpendicular magnetic anisotropy with varying the layer thickness of antidot thin films. Therefore, we detected a critical layer thickness, t = 25 nm for the Dy–Fe alloy antidot arrays, at which the appearance of the perpendicular magnetization is observed. Furthermore, an enhancement in the Curie temperature of the antidot arrays compared to the continuous thin films has been obtained. We attribute these effects to the complex magnetization reversal processes and the high thermal stability of the hexagonal structure of antidot arrays. These findings can be of high interest for the development of novel magnetic sensors and for thermo-magnetic recording patterned media based on template-assisted deposition techniques.

Thickness-dependent magnetic anisotropy in obliquely deposited Fe(001)/Pd thin film bilayers probed by VNA-FMR**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921403 and 2016YFA0300701), the National Natural Science Foundation of China (Grant Nos. 51427801, 11374350, and 51671212), and the Chinese Government Scholarship (Grant No. 2015GXYG37).

The thickness-dependent magnetic anisotropy of obliquely deposited Fe(001)/Pd thin films on Mg(001) is investigated by fitting the field-dependent resonant field curve using the Kittel equation. In this study, three Fe film samples with thicknesses of 50 monolayers (ML), 45 ML, and 32 ML deposited at 0°, 45°, and 55°, respectively, are used. The magnetic anisotropy constant obtained from ferromagnetic resonance (FMR) spectra exhibits a dominant fourfold magnetocrystalline anisotropy (MCA) at the normal deposition angle with larger Fe thickness. However, the in-plane uniaxial magnetic anisotropy (UMA) is induced by a higher oblique deposition angle and a smaller thickness. Its hard axis lies between the [100] and [010] directions. The FMR data-fitting analysis yields a precise measurement of smaller contributions to the magnetic anisotropy, such as in-plane UMA. Due to MCA, when the magnetic field is weaker than the saturated field, the magnetization direction does not always align with the external field. The squared frequency-dependent resonant field measurement gives an isotropic Landé g-factor of 2.07. Our results are consistent with previous experiments conducted on the magneto-optical Kerr effect (MOKE) and anisotropic magnetoresistance (AMR) systems. Thus, a vector network analyzer ferromagnetic resonance (VNA-FMR) test-method for finding UMA in obliquely deposited Fe(001)/Pd bilayer ferromagnetic thin films, and determining the magnetic anisotropy constants with respect to the film normal deposition, is proposed.

Local and Medium Range Order Influence on the Magnetic Behavior of Sputtered Ga-Rich FeGa Thin Films

We have investigated the influence of the growth power on the structural properties of Fe100–xGax (x ca. 29) films sputtered in the ballistic regime in the oblique incidence. By means of different structural characterizations, mainly X-ray diffraction and X-ray absorption spectroscopy, we have reached a deeper understanding about the influence of the local and medium range order on the magnetic behavior of Ga-rich FeGa thin films. On the one hand, the increase of the growth power reduces the crystallite size (medium order) that promotes the decrease of the coercive field of the layers. On the other hand, the growth power also determines the local order as it controls the formation of the A2, B2, and D03 structural phases. The increase of the uniaxial in-plane magnetic anisotropy with growth power has been correlated with the enhancement of both Ga pairs and a tetragonal distortion. The results presented in this work give more evidence about the magnetic anisotropy sources in Ga-rich FeGa alloys, and therefore, it helps to understand how to achieve a better control of the magnetic properties in this family of alloys.

On the origin of magnetic anisotropy of FeCo(Nb)B alloy thin films: A thermal annealing study

Abstract Influence of thermal annealing on the ion beam sputtered FeCo(Nb)B quaternary alloy thin films having thickness ∼20 nm and 80 nm was investigated. Regardless of amorphous microstructure, the as-deposited films possess uniaxial magnetic anisotropy (UMA) in the film plane. Annealing at temperature ∼503 K results in amorphous to nanocrystalline phase transformation with precipitation of bcc-FeCo phase. With increasing annealing temperature, magnetic anisotropy gradually decreases and thermal annealing at 773 K results in complete disappearance of UMA. In-plane strain distribution in such nanocrystalline FeCo(Nb)B thin films was obtained by using laboratory and synchrotron X-ray diffraction (XRD) measurements. In-plane and out-of-plane X-ray diffraction measurements suggest, growth induced long range non-uniform tensile stress, as the possible origin of in-plane uniaxial magnetic anisotropy in such films. It is demonstrated that the stress distribution becomes uniform after annealing at 773 K and results in complete disappearance of magnetic anisotropy. The study further suggests that the presence of Nb in FeCoB film improves the thermal stability, inhibits grain growth and stabilizes bcc-FeCo phase up to elevated temperatures. This information is useful for magnetic tunnel junction application, where annealing of FeCoB magnetic electrode is essential.

Evidence of substrate roughness surface induced magnetic anisotropy in Ni80Fe20 flexible thin films

Experimental and computational evidence of a surface roughness induced magnetic anisotropy in NiFe thin films coated onto substrates of various surface roughnesses is reported. Magnetic coercive fields of 15 nm NiFe thin films coated on substrates with approximately 7 nm average roughness were remarkably 233% larger than identical thin films coated onto smooth substrates with < 1 nm average roughness. The NiFe films coated onto rough substrates developed hard and easy axes, normally non-existent in NiFe Permalloy. A linear correlation of the incline angles of the hard axis hysteresis loops to the average roughness values of the individual substrates was observed, with 99% correlation level. Using a modified micromagnetics theory that incorporates the effects of surface roughness, it is shown the observed magnetic anisotropy arises due to the spatial anisotropy of the surface roughness, resulting in an effective in-plane uniaxial magnetic anisotropy with energy density up to 15 kJ/m3.

Возможности магнитного псевдорезонанса в изучении тонких ферромагнитных пленок с одноосной магнитной анизотропией

AbstractPossibilities of magnetic pseudoresonance (a non-resonance peak of magnetic susceptibility) were studied and compared with the ferromagnetic resonance (FMR) in measuring the parameters of thin ferromagnetic films with in-plane uniaxial magnetic anisotropy. The measurements were conducted with two characteristic samples of ferromagnetic films showing this effect. A Q -meter operating at a frequency near 300 MHz (for pseudoresonance) and a standard X -band magnetic resonance spectrometer (for FMR) were used. The Q -meter working at 300 MHz was shown to detect reliably the magnetic pseudoresonance in both epitaxial and polycrystalline films. It was found that the accuracy of determination of the magnetic anisotropy field and orientation of the easy magnetization axis provided by the pseudoresonance method is as good as with FMR, and in some cases the pseudoresonance method gives additional information.