Fourfold Anisotropy Research Articles

High frequency dynamics and magnetic anisotropy of bcc Co films grown on Si (0 0 1) substrate

Dynamic magnetization and magnetic anisotropy of Cu/bcc-Co/CoSi2 /Si(001) system were investigated by Magneto-Optical Kerr Effect (MOKE) analysis and vector network analyzer ferromagnetic resonance (VNA-FMR). Typical fourfold magnetocrystalline anisotropy of bcc Co film and superposed uniaxial magnetic anisotropy were observed. The uniaxial magnetic anisotropy originated from shape anisotropy as confirmed by micromagnetic simulation. The large magnetocrystalline anisotropy results in high resonance frequency even in the zero applied field. The Gilbert damping parameter α obtained by analysis of the dynamic process is close to the theoretically estimated value of bcc phase Co. The Gilbert damping parameter of the bcc-Co film is larger than those of fcc-Co and hcp-Co films. This work indicates the Co damping parameter can be enhanced by using bcc structure, which will help create high-speed magnetoelectronic devices.

Growth and in-situ characterization of magnetic anisotropy of epitaxial Fe thin film on ion-sculpted Ag (001) substrate

Growth and magnetic characterization of Fe film on ion-sculpted Ag(001) substrate have been studied in-situ using reflection high energy electron diffraction and magneto-optical Kerr effect. Fe film is found to grow epitaxially on the Ag surface and exhibits epitaxial island formation up to ∼ 1 nm film thickness. Ion beam sculpting of the substrate surface, prior to the deposition of the Fe film, induces an uniaxial magnetic anisotropy in Fe film, which couples with the cubic four-fold anisotropy of bcc-Fe, lead to the multiple-step jump in magnetic hysteresis loops. Magnetic switching and its correlation with the domain dynamics of the film, as studied by the Kerr microscopy technique, revealed a striking difference in domain structure depending upon the direction of the applied magnetic field with respect to crystallographic direction. It is found that the magnetization reversal takes place via a “single-step jump” mechanism through sweeping of 180° domain walls, whereas the combination of coherent rotation and the sweeping of 90° domain walls is responsible for a “double step jump” magnetization reversal.

Fourfold Polarization‐Sensitive Photodetector Based on GaTe/MoS2 van der Waals Heterojunction

AbstractIntegrated polarization‐sensitive photodetectors fabricated by geometric anisotropic 2D materials have become attractive in recent years. In this work, the successful construction of self‐driven and polarization‐sensitive photodetectors based on GaTe/MoS2 p–n van der Waals (vdW) heterojunction is demonstrated by mechanical exfoliation and dry transfer methods. The fabricated GaTe/MoS2 vdW heterojunctions show ambipolar behavior, and the highest rectification ratio can reach 93.4. The highest responsivity under 532 nm illumination reaches 145 mA W−1 and the response time is less than 10 ms. Moreover, the photocurrent polarization of the fabricated GaTe/MoS2 photodetectors manifests in fourfold anisotropy with a high polarization ratio of 2.9, which can be ascribed to the highly anisotropic monoclinic structure of layered m‐GaTe. This finding thus offers more information and creates new opportunities about how to fabricate integrated polarization‐sensitive photodetectors.

Theoretical investigation of ferromagnetic resonance in a ferromagnetic thin film with external stress anisotropy

We use the ferromagnetic resonance (FMR) method to study the properties of ferromagnetic thin film, in which external stress anisotropy, fourfold anisotropy and uniaxial anisotropy are considered. The analytical expressions of FMR frequency, linewidth and the imaginary part of magnetic susceptibility are obtained. Our results reveal that the FMR frequency and the imaginary part of magnetic susceptibility are distinctly enhanced, and the frequency linewidth or field linewidth are broadened due to a strong external stress anisotropy field. The hard-axis and easy-axis components of magnetization can be tuned significantly by controlling the intensity and direction of stress and the in-plane uniaxial anisotropy field.

The Gilbert damping of thickness-dependent epitaxial single-crystal Heusler Co2FeAl films at various temperatures

Gilbert damping in epitaxial Heusler Co2FeAl films with thickness varying from 3 nm to 9 nm are investigated by broadband ferromagnetic resonance (FMR) with a temperature range of 5 K–300 K. Gilbert damping shows a continuous decrease with the increasing thickness of Co2FeAl films. Moreover, an enhanced peak of the Gilbert damping is observed with increasing temperature up to approximately 50 K for 3 nm, 6 nm and 9 nm thick Co2FeAl films, which may be attributed to the spin reorientation transition at Co2FeAl/MgO interface. Further, we analyzed the linewidth with a superposition of a uniaxial and a fourfold anisotropy for all samples, which suggests that FMR linewidth in epitaxial Co2FeAl films stems from two-magnon scattering. The change of Gilbert damping is confirmed to originate from various order degrees of the B2 phase with the growth of CFA film, impacting the control of magnetic damping in spin-based nanodevices.

Dynamic configurational anisotropy in Ni80Fe20 antidot lattice with complex geometry

Efficient tunability of spin-wave dynamics has been demonstrated in Ni80Fe20 diamond-shaped antidot lattices (DADLs) arranged in square and hexagonal symmetry using all-optical time-resolved magneto-optical Kerr effect (TRMOKE) microscope. A stark variation in the spin-wave spectra is obtained with the strength and orientation of the in-plane bias magnetic field. The spin-wave modes in the square lattice exhibit four-fold rotational anisotropy, which is in contrast to a strong six-fold rotational anisotropy superposed with a weak four-fold anisotropy observed in the hexagonal lattice. Micromagnetic simulations qualitatively reproduce the experimentally observed spin-wave modes and the simulated mode profiles reveal the presence of diverse genres of extended, pseudo extended and quantized standing spin-wave modes in these lattices. Additionally, some lower frequency localized edge modes are obtained for some specific bias field orientations due to the presence of demagnetizing fields at the sharp corners of the diamond antidots. The strong modifications of the demagnetizing regions, as well as the internal field strengths around the diamond-shaped antidots, can explain the observed variation of the spin-wave dynamics. These findings are important for potential applications of the DADLs in future magnonic devices, leading towards the development of on-chip microwave logic and communication systems.

Electric control of magnetic properties in epitaxially grown FeRh/MgO/PMN-PT heterostructures

We epitaxially grew the metamagnetic FeRh films on ferroelectric PMN-PT(001) substrates and investigated the electric control of magnetic properties. An MgO buffer layer is inserted to reduce the considerably large lattice mismatch of 5.44% between FeRh and PMN-PT, leading to a nice epitaxial growth with a relationship of FeRh[110](001)||MgO[100](001)||PMN-PT[100](001). The as-grown FeRh film shows a typical antiferromagnetic-to-ferromagnetic phase transition by means of varying the temperature. Because a growth-induced uniaxial magnetic anisotropy is superimposed on the in-plane four-fold magnetocrystalline anisotropy, the epitaxial FeRh film displays different kinds of multi-step hysteresis loops at various field orientations. An electric field applied on the PMN-PT substrate can remarkably increase the coercivity of FeRh film, which presents a butterfly type shape, indicating a strain-mediated magnetoelectric coupling. Because of the inhibitory effect of the remnant compressive strain on the phase transition, the critical transition temperature and the magnetization are obviously enhanced and reduced, respectively, after removing the electric effect.

Magnonic crystals with complex geometry

Magnonic crystals offer a wide playground to study the emergent properties of spin waves, and ferromagnetic antidot lattices are leading candidates for magnonic devices due to the faster propagation of spin waves combined with wide-frequency tunability. Despite having a broad range of studies on periodic and quasiperiodic systems, a combination of quasiperiodic lattice with a complex basis is absent in the literature. The quasiperiodicity of octagonal lattice, along with a complex triangular antidot basis lacking reflection symmetry provides newer and richer spin-wave dynamics. Such complex magnonic crystal exhibits a strong eightfold anisotropy superposed with a weak threefold anisotropy. This is in contrast to a strong fourfold anisotropy superposed with a weak threefold anisotropy observed in a square lattice with triangular antidot basis. The spatial profiles of spin waves revealed the presence of resonant modes with both even and odd-mode quantization number, besides a mode conversion from extended to quantized mode with the systematic variation of the in-plane bias magnetic field orientation. These are in consonance with the strong anisotropic behavior of the spin-wave modes. The strong modifications of the asymmetric potential energy landscape in these magnonic crystals lead to the stark modulation of the rich spin-wave dynamics, thus opening avenues to reprogrammable magnonics with complex geometry.

Thickness dependent domain wall dynamics in Fe2CoSi thin films

We report on the effect of thickness on the magnetic properties and domain wall motion in as deposited Fe2CoSi thin films. Reflections present in selected area electron diffraction (SAED) micrograph infer that they are allowed reflections and are related to the Fe2CoSi. The coercivity is found to increase with the film thickness and the same is explained on the basis of Néel domain wall model. Longitudinal magneto-optical Kerr effect (L – MOKE) infers that the formation of ripple kind of domains that depend on the orientation of the field. L – MOKE data also infers nearly perfect 4-fold anisotropy for the film with thickness 5 nm and a weaker 4-fold anisotropy at higher thicknesses. Up on varying the angle between magnetic field and the film edge to 0° or 90°, indeed there is a transformation of the film magnetization from easy direction to hard direction. Fast Fourier transform of magnetic force microscopy (MFM) and polar plots infer a uniaxial anisotropy and a four-fold anisotropy in the films. Micromagnetic simulations infer that the coercivity value depends on the void size and enhances up to certain radius, above which it decreases. Present results would indeed be helpful for the future spintronic applications.

Epitaxial thin-film Pd1−xFex alloy: a tunable ferromagnet for superconducting spintronics

In the paper we present the results of extensive studies of palladium-rich Pd1-xFex alloy films epitaxially grown on MgO single-crystal substrate. In a composition range of x = 0.01-0.07 these materials are soft ferromagnets, the saturation magnetization and magnetic anisotropy of which can be tuned by its composition. Vibrating sample magnetometry was used to study temperature dependences of spontaneous magnetic moment and to establish the temperature of magnetic ordering (Curie temperature). Ferromagnetic resonance (FMR) measurements at low temperatures in the in-plane and out-of-plane geometries revealed the four-fold in-plane magnetic anisotropy with the easy directions along the <110> axes of the substrate. The modelling of the angular dependence of the field for resonance allowed to extract the cubic and tetragonal contributions to the magnetic anisotropy of the films and establish their dependence on the concentration of iron in the alloy. Experimental data are discussed in the framework of existing theories of dilute magnetic alloys. Using the anisotropy constants established from FMR, the magnetic hysteresis loops are reproduced utilizing the Stoner-Wohlfarth model thus indicating the predominant coherent magnetic moment rotation at low temperatures. The obtained results compile a database of magnetic properties of a palladium-iron alloy considered as a material for superconducting spintronics.

Magnetocrystalline anisotropy imprinting of an antiferromagnet on an amorphous ferromagnet in FeRh/CoFeB heterostructures

Magnetic anisotropy is a fundamental key parameter of magnetic materials that determines their applications. For ferromagnetic materials, the magnetic anisotropy can be easily detected by using conventional magnetic characterization techniques. However, due to the magnetic compensated structure in antiferromagnetic materials, synchrotron measurements, such as X-ray magnetic linear dichroism, are often needed to probe their magnetic properties. In this work, we observed an imprinted fourfold magnetic anisotropy in the amorphous ferromagnetic layer of FeRh/CoFeB heterostructures. The MOKE and ferromagnetic resonance measurements show that the easy magnetization axes of the CoFeB layer are along the FeRh〈110〉 and FeRh〈100〉 directions for the epitaxially grown FeRh layer in the antiferromagnetic and ferromagnetic states, respectively. The combined Monte Carlo simulation and first-principles calculation indicate that the fourfold magnetic anisotropy of the amorphous CoFeB layer is imprinted due to the interfacial exchange coupling between the CoFeB and FeRh moments from the magnetocrystalline anisotropy of the epitaxial FeRh layer. This observation of imprinting the magnetocrystalline anisotropy of antiferromagnetic materials on easily detected ferromagnetic materials may be applied to probe the magnetic structures of antiferromagnetic materials without using synchrotron methods.

Uniaxial and fourfold basal anisotropy in GdRh2Si2

The magnetocrystalline anisotropy of GdRh2Si2 is examined in detail via the electron spin resonance (ESR) of its well-localised Gd3+ moments. Below TN = 107 K, long range magnetic order sets in with ferromagnetic layers in the (aa)-plane stacked antiferromagnetically along the c-axis of the tetragonal structure. Interestingly, the easy-plane anisotropy allows for the observation of antiferromagnetic resonance at X- and Q-band microwave frequencies. In addition to the easy-plane anisotropy we have also quantified the weaker fourfold anisotropy within the easy plane. The obtained resonance fields are modelled in terms of eigenoscillations of the two antiferromagnetically coupled sublattices. Conversely, this model provides plots of the eigenfrequencies as a function of field and the specific anisotropy constants. Such calculations have rarely been done. Therefore our analysis is prototypical for other systems with fourfold in-plane anisotropy. It is demonstrated that the experimental in-plane ESR data may be crucial for a precise knowledge of the out-of-plane anisotropy.

Electronic, magneto-optical and magnetic anisotropy properties of tetragonal BiFeO3

The full potential linear augmented plane wave method including Hubbard potential and spin-orbit coupling are performed to study the structural, electronic, magneto-optical and magnetic anisotropy properties of tetragonal BiFeO3. Using the exchange correlations potentials generalized gradient plus Hubbard parameter (GGA + U) approximations are used for the description of electron-electron interactions. We studied first the structural properties which present a tetragonal distortion results from the stereochemical 6s2 lone pair of Bi+2 and the Jahn-Teller (JT) distortion effect of Fe+3 and the value of c∕a = 1.28. The calculated gap is 2.0 eV at Ueff = 4 eV. The magnetic moment of Fe in phase is 3.65 μB. Kerr and ellipticity are calculated by using a spin-orbit coupling and Hubbard potential which present a high angles values −1.0° and 1.5° respectivly. In plane uniaxial and fourfold anisotropy constants are determined from the fit curves of DFT calculation. We observed a predominance of uniaxial anisotopy on the fourdfold anisotropy.

Spin Pumping and Magnetic Anisotropy in La2/3Sr1/3MnO3/Pt Systems

La2/3Sr1/3MnO3 (LSMO) thin films of various thicknesses (6, 8, 10, 20, and 30 nm), capped by 7 nm‐thick Pt layer, are grown by pulsed laser deposition on SrTiO3 (001) substrates. X‐ray diffraction revealed that LSMO films are (001) oriented. Vibrating sample magnetometer is used to determine the magnetization at saturation and the magnetic dead layer thickness. This latter is around 3.4 nm, significantly thicker compared with the one induced at interfaces of Pt with ferromagnetic transition metals. Microstrip line ferromagnetic resonance (MS‐FMR) is used to extract the gyromagnetic ratio, which is found to increase with LSMO thickness. MS‐FMR revealed that the in‐plane magnetic anisotropy is dominated by a uniaxial contribution for the Pt capped film, whereas the noncapped 10 nm‐thick LSMO layer shows a fourfold anisotropy. Furthermore, the thickness dependence of the effective magnetization reveals the existence of a second‐order perpendicular anisotropy term, which is thickness‐dependent, and of a weak uniaxial interface anisotropy. The Gilbert damping coefficient is found to vary linearly with the inverse of the effective LSMO thickness due to spin pumping leading to relatively low spin mixing conductance of LSMO/Pt interface.

Magnetocrystalline anisotropy of the easy-plane metallic antiferromagnet Fe2As

Magnetocrystalline anisotropy is a fundamental property of magnetic materials that determines the dynamics of magnetic precession, the frequency of spin waves, the thermal stability of magnetic domains, and the efficiency of spintronic devices. We combine torque magnetometry and density functional theory calculations to determine the magnetocrystalline anisotropy of the metallic antiferromagnet Fe$_2$As. Fe$_2$As has a tetragonal crystal structure with the N\'eel vector lying in the (001) plane. We report that the four-fold magnetocrystalline anisotropy in the (001)-plane of Fe$_2$As is extremely small, ${K_{22}} = - 150~{\rm{ J/}}{{\rm{m}}^{\rm{3}}}$ at T = 4 K, much smaller than perpendicular magnetic anisotropy of ferromagnetic structure widely used in spintronics device. ${K_{22}}$ is strongly temperature dependent and close to zero at T > 150 K. The anisotropy ${K_1}$ in the (010) plane is too large to be measured by torque magnetometry and we determine ${K_1} = -830~{\rm{ kJ/}}{{\rm{m}}^{\rm{3}}}$ using first-principles density functional theory. Our simulations show that the contribution to the anisotropy from classical magnetic dipole-dipole interactions is comparable to the contribution from spin-orbit coupling. The calculated four-fold anisotropy in the (001) plane ${K_{22}}$ ranges from $- 292~{\rm{ J/}}{{\rm{m}}^{\rm{3}}}$ to $280~{\rm{ J/}}{{\rm{m}}^{\rm{3}}}$, the same order of magnitude as the measured value. We use ${K_1}$ from theory to predict the frequency and polarization of the lowest frequency antiferromagnetic resonance mode and find that the mode is linearly polarized in the (001)-plane with $f = $ 670 GHz.

Efficient Spin Torques in Antiferromagnetic CoO/Pt Quantified by Comparing Field- and Current-Induced Switching.

We achieve current-induced switching in collinear insulating antiferromagnetic CoO/Pt, with fourfold in-plane magnetic anisotropy. This is measured electrically by spin Hall magnetoresistance and confirmed by the magnetic field-induced spin-flop transition of the CoO layer. By applying current pulses and magnetic fields, we quantify the efficiency of the acting current-induced torques and estimate a current-field equivalence ratio of 4×10^{-11} T A^{-1} m^{2}. The Néel vector final state (n⊥j) is in line with a thermomagnetoelastic switching mechanism for a negative magnetoelastic constant of the CoO.

Exchange bias effect in Fe/LaAlO3: An interface induced effect

The present work reports the appearance of the exchange bias (EB) effect at low temperatures in epitaxial Fe thin film deposited on single crystalline LaAlO3(001) substrate. The observed EB effect vanishes around 30 K indicating that it could be due to the formation of antiferromagnetic FeO layer at the interface between Fe and LaAlO3 substrate. The EB in Fe/LaAlO3(001) is further substantiated by comparing the observations with polycrystalline Fe film deposited on silicon substrate, kept adjacent to LaAlO3 substrate during the deposition. Magnetic microstructure measured by Kerr microscopy corroborates the structural data of Fe film i.e., cubic four-fold anisotropy and isotropic nature for Fe film deposited on LaAlO3 and silicon substrates, respectively. Further, low temperature Kerr microscopy results reveal that the magnetization reversal process significantly gets modified due to the induced unidirectional anisotropy as a consequence of FeO layer formation at Fe/LaAlO3 interface.

The influence of interface strain on the magnetization switching process in FeSi/(011)PMN-0.3PT heterostructures

The magnetization switching process of Fe80Si20/(PbMg1/3Nb2/3O3)0.7–(PbTiO3)0.3 (FeSi/PMN-0.3PT) heterostructures under electric-field-induced interface strain effects was investigated using the magneto-optical Kerr effect. In the as-deposited FeSi films with low four-fold magnetic anisotropy, the one-jump and two-jump loops corresponding to the 180° and 90° domain walls were observed and could be regularly tuned by applying saturated and converse electric fields in these crystalline regions. After 300 °C annealing treatment, the heterostructures presented enhanced crystallinity and interface strain effect. By applying an impulse electric field, we achieved a reliable and non-volatile two-jump and three-jump magnetization switching transition. These regulation results further confirm the magnetic anisotropy transition and magnetic energy superposition relationship in FeSi/PMN-0.3PT. The controllable magnetization switching process is significant for understanding the mechanism of magnetic anisotropy competition and for designing the relevant magnetoelectric devices.

Magnetization states driven by spin-transfer torque in spin-valve nanopillars in presence of four-fold magnetocrystalline anisotropy

In this work, the magnetization states under the spin-transfer switching effect are investigated in a nanopillar device composed of a half-metallic Heusler Co1.5Fe1.5Ge alloy with a four-fold in-plane magnetocrystalline anisotropy. A comparative study with a half-metallic Heusler alloy of Co2FeAl0.5Si0.5 and a typical material of Fe is realized by numerically solving the Landau-Lifshitz-Gilbert (LLG) equation with the Spin-Transfer Torque (STT) contribution, using micromagnetic simulations. Our ultimate goal is to elucidate the origins of the intermediate state (IS) that occurs during magnetization switching by deeply examining the effects of magnetocrystalline anisotropy and shape anisotropy on the magnetization states through analyzing and comparing various switching processes. Our simulation results showed that the magnetization switching via a single-step under low current densities is possible at certain critical cross-sectional dimensions of the ellipse that are adjusted to increase the demagnetization field against the four-fold in-plane magnetocrystalline anisotropy. As well, we provided in this paper a set of appropriate geometric dimensions which allow the magnetization to reverse via a single-step by overcoming IS with minimal spin-polarized current densities.

Four-Fold Anisotropy of the Parallel Upper Critical Magnetic Field in a Pure Layered d-Wave Superconductor at T = 0

It is well known that a four-fold symmetry of the parallel upper critical magnetic field disappears in the Ginzburg-Landau (GL) region in quasi-two-dimensional (Q2D) $d$-wave superconductors. Therefore, it has been accurately calculated so far as a correction to the GL results, which is valid close to superconducting transition temperature and is expected to be stronger at low temperatures. As to the case $T=0$, some approximated methods have been used, which are good only for closed electron orbits and unappropriate for the open orbits which exist in a parallel magnetic field in Q2D superconductors. For the first time, we accurately calculate the four-fold anisotropy of the parallel upper critical magnetic field in a pure Q2D $d$-wave superconductor at $T=0$, where it has the highest possible value. Our results are applicable to Q2D $d$-wave high-Tc and organic superconductors.