Fourfold Anisotropy Research Articles

The role of Rh spacer layer thickness on the noncollinear interlayer exchange coupling

Abstract The relationship between noncollinear interlayer exchange coupling (IEC) and magnetic anisotropic behavior in Fe/Rh/Fe trilayers is studied in detail by using magneto-optical Kerr effect (MOKE) and anisotropic magnetoresistance (AMR) techniques. It is found that the Rh spacer layer thickness strongly affects IEC and magnetic anisotropy in these trilayers. The role of Rh spacer layer thickness is shown in the oscillatory behavior in the magnitude of the magnetic anisotropy, the transition from uniaxial to four-fold anisotropy, the shift of easy axis for magnetic anisotropy, and the unusual increase in the sheet resistance. As an outcome of this study, we discuss the underlying mechanism based on the noncollinear IEC across the Fe/Rh/Fe interlayer. As a result, it has been shown that the noncollinear IEC can be controlled by the various Rh spacer thicknesses among nonmagnetic transition layers.

Magnetization dynamics and domain reversal in electrodeposited permalloy thin films: impact of thickness and annealing treatment

The domain reversal and magnetization dynamics of electrodeposited permalloy (Fe20Ni80) thin films on conducting ITO/glass substrate was investigated using Magneto-optic Kerr effect microscopy and ferromagnetic resonance. Permalloy thin films were electrodeposited with thickness ranging from 66 nm to 330 nm. Synchrotron XRD confirmed the deposited permalloy in FCC phase without any impurity. The squared hysteresis with very low coercivity (Hc ∼ 5 Oe) established soft magnetic nature of the films. Further, angular MOKE hysteresis measurements with simultaneous domain imaging revealed four-fold surface anisotropy in as-deposited film ensuing magnetization reversal via branched and ripple domains. The annealing treatment in Ar+H2 atmosphere removed surface anisotropy and renovated the magnetization reversal through 180° branched domains with rapid magnetization switching. Ferromagnetic resonance spectroscopy discloses reduction in the gyromagnetic ratio (γ) as well as in Gilbert damping parameter (α) as the film thickness increases. The lowest Gilbert damping for 330 nm film measured at 0.022, which further reduced to 0.018 after annealing. The combination of rapid magnetization switching and low Gilbert damping in the electrodeposited permalloy thin films render them promising for implementation in high-frequency microwave devices devices and magnetic sensors.

Microstructural contribution to the magnetic anisotropy in Fe 1−x Ga x thin films: correlation between crystallographic texture and magnetic response

In this study, we present a systematic analysis of the relation between the structural properties and magnetic anisotropies of as-grown and heat-treated polycrystalline Fe 1−x Ga x (0.11 <x< 0.19) thin films deposited on glass and Si(100) substrates. The results show that the crystallographic texture of the films has a significant impact on the evolution of the magnetic anisotropies. The samples grown on glass do not exhibit a preferred texture while, for samples grown on Si(100), the appearance of a weak in-plane fourfold magnetic anisotropy is reported. Such anisotropy is enhanced and better defined in the heat treated samples. Via a phenomenological model we show that this anisotropy has a microstructural origin. These findings demonstrate the possibility of tuning magnetic anisotropies by growing on different substrates and/or performing thermal treatments, which in turn reinforces the possibility of developing smart magnetic switching materials for electronic applications.

Extreme terahertz magnon multiplication induced by resonant magnetic pulse pairs

Nonlinear interactions of spin-waves and their quanta, magnons, have emerged as prominent candidates for interference-based technology, ranging from quantum transduction to antiferromagnetic spintronics. Yet magnon multiplication in the terahertz (THz) spectral region represents a major challenge. Intense, resonant magnetic fields from THz pulse-pairs with controllable phases and amplitudes enable high order THz magnon multiplication, distinct from non-resonant nonlinearities such as the high harmonic generation by below-band gap electric fields. Here, we demonstrate exceptionally high-order THz nonlinear magnonics. It manifests as 7th-order spin-wave-mixing and 6th harmonic magnon generation in an antiferromagnetic orthoferrite. We use THz two-dimensional coherent spectroscopy to achieve high-sensitivity detection of nonlinear magnon interactions up to six-magnon quanta in strongly-driven many-magnon correlated states. The high-order magnon multiplication, supported by classical and quantum spin simulations, elucidates the significance of four-fold magnetic anisotropy and Dzyaloshinskii-Moriya symmetry breaking. Moreover, our results shed light on the potential quantum fluctuation properties inherent in nonlinear magnons.

Fourfold anisotropic magnetic damping induced by anisotropic spin absorption in amorphous CoFeB/epitaxial IrMn3 bilayers

Magnetic damping rooted in various relaxation processes of spin angular momentum plays a crucial role in determining the energy consumption and the operating speed of emerging spintronic devices. Here, we reported a fourfold anisotropic magnetic damping extrinsically caused by spin current absorption in amorphous ferromagnetic/antiferromagnetic bilayers. The angular dependent broadband ferromagnetic resonance measurements were employed to investigate the effect of interfacial exchange coupling on magnetization dynamics of the CoFeB/IrMn3 bilayers. In addition to the conventional exchange bias, the amorphous CoFeB layer exhibits an induced fourfold magnetic anisotropy along the IrMn3⟨100⟩ axes because of the exchange coupling with the uncompensated IrMn3 moments along IrMn3⟨100⟩ caused by the 3Q antiferromagnetic structure. The magnetic damping of the CoFeB/IrMn3 bilayer also exhibits an obvious fourfold symmetry, which is ascribed to the anisotropic spin absorption caused by the uncompensated IrMn3 moments. The ratio of the fourfold anisotropic magnetic damping decreases dramatically with reducing the interfacial exchange coupling. When the interfacial exchange coupling is isolated by a Cu spacer layer, both the induced magnetic anisotropy and the magnetic damping exhibit a uniaxial symmetry.

Giant four-fold magnetic anisotropy in nanotwinned NiMnGa epitaxial films

A giant four-fold magnetic anisotropy (with an anisotropy field up to 4 kOe) was observed in the twinned NiMnGa epitaxial film. Its appearance is explained in terms of moderate coupling between twin variants having strong uniaxial magnetocrystalline anisotropies directed orthogonally when the intertwin exchange field is comparable with the anisotropy field. This finding paves the way to increase the order of magnetic anisotropy in a many-component system while keeping the value of the anisotropy field by tuning the intercomponent exchange strength and can be extended to exchange-coupled multilayers and arrays of nanoelements.

Twisted Integration of Complex Oxide Magnetoelectric Heterostructures via Water-Etching and Transfer Process

HighlightsThe (001)-oriented ferromagnetic La0.67Sr0.33MnO3 films are stuck onto the (011)-oriented ferroelectric single-crystal 0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 substrate with 0° and 45° twist angle.By applying a 7.2 kV cm−1 electric field, the coexistence of uniaxial and fourfold in-plane magnetic anisotropy is observed in 45° Sample, while a typical uniaxial anisotropy is found in 0° Sample.Manipulating strain mode and degree that can be applied to epitaxial complex oxide thin films have been a cornerstone of strain engineering. In recent years, lift-off and transfer technology of the epitaxial oxide thin films have been developed that enabled the integration of heterostructures without the limitation of material types and crystal orientations. Moreover, twisted integration would provide a more interesting strategy in artificial magnetoelectric heterostructures. A specific twist angle between the ferroelectric and ferromagnetic oxide layers corresponds to the distinct strain regulation modes in the magnetoelectric coupling process, which could provide some insight in to the physical phenomena. In this work, the La0.67Sr0.33MnO3 (001)/0.7Pb(Mg1/3Nb2/3)O3–0.3PbTiO3 (011) (LSMO/PMN-PT) heterostructures with 45º and 0º twist angles were assembled via water-etching and transfer process. The transferred LSMO films exhibit a fourfold magnetic anisotropy with easy axis along LSMO < 110 >. A coexistence of uniaxial and fourfold magnetic anisotropy with LSMO [110] easy axis is observed for the 45° Sample by applying a 7.2 kV cm−1 electrical field, significantly different from a uniaxial anisotropy with LSMO [100] easy axis for the 0° Sample. The fitting of the ferromagnetic resonance field reveals that the strain coupling generated by the 45° twist angle causes different lattice distortion of LSMO, thereby enhancing both the fourfold and uniaxial anisotropy. This work confirms the twisting degrees of freedom for magnetoelectric coupling and opens opportunities for fabricating artificial magnetoelectric heterostructures.

Rotational symmetry breaking in superconducting nickelate Nd0.8Sr0.2NiO2 films

The infinite-layer nickelates, isostructural to the high-Tc cuprate superconductors, have emerged as a promising platform to host unconventional superconductivity and stimulated growing interest in the condensed matter community. Despite considerable attention, the superconducting pairing symmetry of the nickelate superconductors, the fundamental characteristic of a superconducting state, is still under debate. Moreover, the strong electronic correlation in the nickelates may give rise to a rich phase diagram, where the underlying interplay between the superconductivity and other emerging quantum states with broken symmetry is awaiting exploration. Here, we study the angular dependence of the transport properties of the infinite-layer nickelate Nd0.8Sr0.2NiO2 superconducting films with Corbino-disk configuration. The azimuthal angular dependence of the magnetoresistance (R(φ)) manifests the rotational symmetry breaking from isotropy to four-fold (C4) anisotropy with increasing magnetic field, revealing a symmetry-breaking phase transition. Approaching the low-temperature and large-magnetic-field regime, an additional two-fold (C2) symmetric component in the R(φ) curves and an anomalous upturn of the temperature-dependent critical field are observed simultaneously, suggesting the emergence of an exotic electronic phase. Our work uncovers the evolution of the quantum states with different rotational symmetries in nickelate superconductors and provides deep insight into their global phase diagram.

Fourfold magnetic anisotropy varied by interfacial exchange coupling in epitaxial Fe/IrMn bilayers

We report variable in-plane fourfold magnetic anisotropy in epitaxial exchange-biased Fe/IrMn bilayers with different layer thicknesses. The fourfold magnetic anisotropy of the bilayers continuously decreases and even appears negative as the thickness of the Fe layer decreases, indicating a change in the easy axes from Fe 100 to Fe 110, which can be interpreted by the competition between the intrinsic magnetocrystalline anisotropy along Fe 100 and the extrinsic fourfold magnetic anisotropy along Fe 110 induced by the interfacial exchange coupling to IrMn. As the IrMn thickness increases, the fourfold magnetic anisotropy of the bilayers decreases rapidly from a value close to the intrinsic magnetocrystalline anisotropy of Fe and then gradually reaches saturation. The extrinsic fourfold magnetic anisotropy depends on the amount of the induced uncompensated IrMn spins near the edge of antiferromagnetic domains. In contrast, the exchange bias is determined by the net difference of the opposite uncompensated IrMn spins.

Electric field control of the ferromagnetic resonance linewidth in synthetic antiferromagnetic heterostructure

In this study, we present a novel approach to control the microwave properties of a Co/Ru/FeNi film, characterized by strong antiferromagnetic coupling and in plane uniaxial anisotropy. The film is fabricated on a Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) ferroelectric substrate using the magnetron oblique sputtering. Our experimental results demonstrate a remarkable non-volatile mediation of the ferromagnetic resonance (FMR) field and linewidth by applying the perpendicular-bias electric field. The observed asymmetric butterfly curve indicates the existence of electric mediation dominated by strain effect. Moreover, we investigate the angular dependence of FMR under different electric field, which reveals changes in magnetic anisotropy including FMR field. Notably, the FMR linewidth exhibits a distinctive four-fold anisotropy. Through microstructure analysis of the film surface, we identify that the origin of this FMR linewidth anisotropy is attributed to the two-magnon scattering induced by the oblique sputtering.

Field-free spin-orbit-torque switching of a single ferromagnetic layer with fourfold in-plane magnetic anisotropy

We present the observation of field-free spin-orbit-torque (SOT) switching of magnetization in a ferromagnetic semiconductor (Ga,Mn)As film with four-fold in-plane magnetic anisotropy. Magnetization switching is demonstrated between two orthogonal in-plane easy axes through planar Hall resistance measurements as a current is scanned in the absence of a magnetic field. The chirality of the switching of the current hysteresis loop is consistent with SOT arising from spin polarization caused by Dresselhaus- and Rashba-type spin-orbit-induced effective magnetic fields in the (Ga,Mn)As film. The chirality of magnetization switching can be reversed either by changing the direction of the initial magnetization or by applying a constant external magnetic field bias. The SOT magnetization switching between two states was consistently repeated by applying sequential current pluses with alternating polarities, indicating the potential for developing a field-free SOT device.

Absence of Spin-Orbit Torque and Discovery of Anisotropic Planar Nernst Effect in CoFe Single Crystal.

Exploration of exotic spin polarizations in single crystals is of increasing interest. A current of longitudinal spins, the so-called "Dresselhaus-like" spin current, which is forbidden in materials lacking certain inversion asymmetries, is implied to be generated by a charge current at the interface of single-crystal CoFe. This workreports unambiguous evidence that there is no indication of spin current of any spin polarizations from the interface or bulk of single-crystalline CoFe and that the sin2φ second harmonic Hall voltage, which ispreviously assumed to signify Dresselhaus-like spin current, is not related to any spin currents but rather a planar Nernst voltage induced by a longitudinal temperature gradient within the sample. Such sin2φ signal is independent of large applied magnetic fields and interfacial spin-orbit coupling, inversely correlated to the heat capacity of the substrates and overlayers, quadratic in charge current, and appears also in polycrystalline ferromagnets. Strikingly, the planar Nernst effect (PNE) in the CoFe single crystal has a strong fourfold anisotropy and varies with the crystalline orientation. Such strong, anisotropic PNE has widespread impacts on the analyses of a variety of spintronic experiments and opens a new avenue for the development of PNE-based thermoelectric battery and sensor applications.

Antiferromagnetic fourfold anisotropy induced exchange bias and magnetization reversal behaviors in CoFeB/IrMn bilayers

Bilayers of an amorphous CoFeB ferromagnet coupled to an epitaxially grown IrMn antiferromagnet, whose anisotropy is fourfold symmetric, are modeled to study the angular (φ) dependence of magnetization reversal mechanism and exchange bias behavior, based on a modified Monte Carlo method. It is found that positive and negative exchange-bias field (HE) maximum values appear in the antiferromagnetic easy-axis directions, and the HE zero-value transition/return points exist in the antiferromagnetic hard-axis directions. Furthermore, the value of HE(φ) is well fitted by considering cos3φ/sin3φ terms with larger or comparable coefficients as compared to the first-order terms in the Fourier expansion. Finally, the microscopic spin configuration results evidence these phenomena originating from the tailoring of antiferromagnetic spins by simply rotating the field-cooling direction between antiferromagnetic easy- and hard-axis directions. We open an exchange-bias mode governed by antiferromagnetic fourfold anisotropy and propose a method to precisely detect antiferromagnetic higher-order anisotropies on an exchange-bias support.

Co2Fe(Ti0.5Al0.5) epitaxial thin films: Structural and magnetic properties of a Heusler alloy with Z-site transition metal substitution

We investigate the structural, static, and dynamic magnetic properties of epitaxial Heusler Co2Fe(Ti0.5Al0.5) (CFTA) alloy thin films with thickness varying from 6 nm to 80 nm grown by sputter beam epitaxy on cubic MgO(001), MgAl2O4(001), and hexagonal Al2O3(112̄0) substrates. X-ray diffraction measurements indicate epitaxial growth of CFTA thin films with B2 chemical ordering, with cubic [001] and [220] CFTA axes normal to the cubic and hexagonal substrates, respectively. Microstructure analysis of films grown on MgO substrates reveals a uniformly oriented epitaxial crystal with small variations consistent with strain distortions, providing an explanation for the relatively large X-ray rocking curve values found. Meanwhile, films on Al2O3(112̄0) substrates reveal columnar growth with frequent in-plane grain rotations. A pronounced four-fold magnetocrystalline anisotropy is observed in epitaxial thin films grown on cubic substrates. A pronounced uniaxial anisotropy for films grown on Al2O3(112̄0) substrates is observed. A saturation magnetization of ∼5.0μB/f.u. (where f.u. represents formula unit) is obtained at room temperature, slightly smaller compared to the expected value based on the Slater-Pauling rule. Ferromagnetic resonance spectroscopy finds an effective damping parameter and inhomogeneous linewidth broadening comparable to those found in parent compound Co2FeAl, which suggests that Ti substitution can be achieved without negatively affecting the magnetic properties of the system.

Three-dimensional experimental-scale phase-field modeling of dendrite formation in rechargeable lithium-metal batteries

This paper presents a phase-field based numerical study on the 3D formation of dendrites due to electrodeposition in an experimental-scale lithium metal battery. Small-scale 3D simulations were firstly conducted to elucidate the characteristics and resolution requirements of the numerical framework. Using a four-fold anisotropy model to simulate the growth of lithium deposition, the dependency of dendrite morphology on charging conditions (ϕb=−0.7V and ϕb=−1.4V) on a (larger) experimental-scale metal anode was demonstrated. The dendrite shape was found to shift from a smoother, tree-like formation at the lower applied voltage, to a more spike-like, highly branched structure at the higher voltage. The resulting morphological parameters, such as dendrite propagation rates, volume-specific area, and side branching rates, were compared against published experimental data and found to be comparable to the reported ranges for the electrodeposition of spike- or tree-like metal dendrites. This finding supports our previous observation that dendrite formation is connected to the competition between the lithium cation diffusion and electric migration, generating an uneven distribution of Li+ on the electrode surface. This observation also gives insight into dendrite inhibition strategies focusing on enhancing the diffusion of lithium ions to achieve a more uniform concentration field on the anode surface.

Anisotropy of magnetic damping in Ta/CoFeB/MgO heterostructures

Magnetic damping controls the performance and operational speed of many spintronics devices. Being a tensor quantity, the damping in magnetic thin films often shows anisotropic behavior with the magnetization orientation. Here, we have studied the anisotropy of damping in Ta/CoFeB/MgO heterostructures, deposited on thermally oxidized Si substrates, as a function of the orientation of magnetization. By performing ferromagnetic resonance (FMR) measurements based on spin pumping and inverse spin Hall effect (ISHE), we extract the damping parameter in those films and find that the anisotropy of damping contains four-fold and two-fold anisotropy terms. We infer that four-fold anisotropy originates from two-magnon scattering (TMS). By studying reference Ta/CoFeB/MgO films, deposited on LiNbO3 substrates, we find that the two-fold anisotropy is correlated with in-plane magnetic anisotropy (IMA) of the films, suggesting its origin as the anisotropy in bulk spin–orbit coupling (SOC) of CoFeB film. We conclude that when IMA is very small, it’s correlation with two-fold anisotropy cannot be experimentally identified. However, as IMA increases, it starts to show a correlation with two-fold anisotropy in damping. These results will be beneficial for designing future spintronics devices.

Control of magnetization states in full-Heusler [formula omitted] alloy-based MTJ with four-fold anisotropy using VCMA-assisted STT

The magnetization switching process in four-fold anisotropy materials is characterized by an intermediate state (IS) due to the interplay between magnetic anisotropy and spin-transfer torque (STT). Practically, the presence of an IS can impede the switching process in a single-step, causing switching errors during data storage. Here, we investigate the STT-induced magnetization switching in the MTJ device based on half-metallic full-Heusler Co2FeAl0.5Si0.5 (CFAS) alloy with four-fold anisotropy in the presence of voltage-controlled magnetic anisotropy (VCMA) using micromagnetic simulation. Our results demonstrate that VCMA-assisted STT can effectively tune the magnetic states during the switching by adjusting the VCMA effect at appropriate intervals and strengths. As a result, the magnetization can overcome the limitations posed by the IS in the CFAS alloy, leading to single-step switching modes for low spin current density. Additionally, we revealed that pulsed voltage-assisted STT results in shorter switching times compared to conventional switching in four-fold anisotropy materials.

Investigation of orthogonal spin–orbit fields in crystalline ferromagnetic films with four-fold in-plane magnetic anisotropy

We report an investigation of current-induced spin–orbit fields (SOFs) in a crystalline (Ga,Mn)As ferromagnetic film with four-fold in-plane anisotropy. By designing a Hall device with current channels along crystallographic 〈100〉 directions, we observe the effects of SOFs in all four magnetization transitions over the four 〈110〉 hard axes of the film. The SOF effects are observed in the form of opposite shifts of angles at which the magnetization of the film switches sign for opposite current polarities in anisotropic magnetoresistance (AMR) measurements carried out as the applied magnetic field was rotated. Note that the Dresselhaus- and Rashba-type SOFs generated by currents flowing along the 〈100〉 directions—as in the device designed for the present experiments—are orthogonal to one another and can thus be separated by appropriate analysis of AMR. The analysis of experimental results based on magnetic free energy, in which the effects of SOFs are included, reveals that the Dresselhaus-type SOF is much stronger than the Rashba-type SOF. The values of the two SOFs obtained independently with currents flowing either along the [100] or [010] channels are consistent with each other, showing the reliability of the above-mentioned approach for investigating SOFs in ferromagnetic films.

Modulating the magnetization dynamic properties of zigzag magnetic nanowires by the four-fold magnetic anisotropy

The magnetization dynamic properties of zigzag magnetic nanowires with varied fourfold magnetic anisotropy, which modulated novel microwave responses without any external magnetic field, were investigated. The microwave properties associated with two different remanent magnetic states were found to be highly sensitive to fourfold magnetic anisotropy. A large frequency shift between the two remanent magnetic states was observed when fourfold magnetic anisotropy was added at a particular direction. The results are valuable for the realization of bias field-free microwave applications. In addition, the microwave properties demonstrated by two different remanent magnetic states could be transformed using a nanosecond pulse magnetic field, and fast switching in the system could be achieved with the fourfold magnetic anisotropy.

Influence of substrate type and magnetic anisotropy on the spin Seebeck effect in ZnFe2O4 thin films

The longitudinal spin Seebeck effect (LSSE) has been investigated in ZnFe2O4 (ZFO) thin films on different substrates, such as Si (111), MgO (100), and SrTiO3 (100). The LSSE voltage signal exhibited a linear dependence on the temperature difference between both sides of the samples. The spin Seebeck coefficients were highly sensitive to the thermal conductivities of the magnetic layer and substrate, with values from 3 nV/k to 110 nV/K. Charge currents ({J}_{mathrm{c}}) and spin currents ({J}_{mathrm{s}}) densities were estimated. {J}_{mathrm{s}} values are similar for the samples deposited on MgO and STO. The saturation magnetic field for the LSSE signal was reached with a magnetic field lower than its bulk counterpart. These results were related to the lattice mismatch between the ZFO films and the substrate, the magnetic response, and the anisotropies present in the samples since a twofold in-plane anisotropy was observed for the sample deposited on Si, while the samples deposited on MgO and SrTiO3 showed a fourfold in-plane anisotropy. Moreover, the presence of a notable perpendicular magnetic anisotropy in the thin films may be a consequence of the lattice mismatch and at the same time, it can cause the saturation in the LSSE voltage hysteresis loop measured as a function of the external magnetic be reached with a low magnetic field.