In-plane Angular Dependence Research Articles

In-plane angular dependence of superconducting gaps in FeSe probed by high-resolution specific heat measurements

In-plane angular dependence of superconducting gaps in FeSe probed by high-resolution specific heat measurements

Characterizations of Anisotropic Superconductivity in (BiSe)1+δNbSe2

Anisotropy of superconductivity in misfit layered compounds (BiSe)1+δNbSe2 was studied. We synthesized single crystals of (BiSe)1+δNbSe2 using CsCl/KCl flux method. There existed two phases with different T c depending on the starting composition δ, although their structures were almost unaffected by changing δ. Anisotropy of the upper critical field was determined by magneto-resistance measurements with various out-of-plane and in-plane field angles. The out-of-plane anisotropy of H c2 showed a cusp-like behavior for fields close to ab-plane similar to that predicted by 2D Tinkham model. Furthermore, a clear two-fold symmetry was observed in the in-plane angular dependence of H c2.

Tunable non-Gilbert-type damping in Ni80Fe20 films sputtered on the rippled Al2O3 substrates

The ferromagnetic damping and relaxation mechanisms of Ni80Fe20 (NiFe) films sputtered on rippled Al2O3 substrates were investigated by in-plane angular dependence ferromagnetic resonance (FMR) measurements where these rippled Al2O3 substrates were prepared by thermal annealing. The linewidth of intrinsic contribution due to Gilbert damping mechanism and linewidth of extrinsic contribution related to two-magnon scattering (TMS) and inhomogeneous broadening of films were identified by filtering different magnetic relaxation mechanisms of films. The results show that the magnitude of ferromagnetic damping of films deposited on rippled Al2O3 substrates can be manipulated by modifying the ripple morphology of substrates and it is strongly dependent on the angle between the direction of the ripple and the external field. The non-Gilbert-type damping for these films is an adjustable, anisotropic contribution to the overall relaxation. The two-magnon scattering and inhomogeneous broadening act as non-Gilbert damping mechanism in magnetization relaxation and they dominate the variation of total linewidth. This work revealed the tunable magnetization relaxation in films deposited on rippled Al2O3 substrates with different surface morphology formed by annealing and it would be helpful for utilizing these rippled substrates to design magnetization relaxation characteristics of spintronic and magnonic devices.

Annealing temperature dependence of non-collinear magnetic anisotropy in NiFe/NiO bilayers

The unidirectional anisotropy and uniaxial anisotropy in exchange bias systems determine the effective field that governs magnetization precession, which is critical to magnetic applications. Research has found that a misalignment between unidirectional and uniaxial anisotropies may exist, i.e., non-collinear anisotropy, where the intrinsic mechanism and contributing factors are poorly reported. In this work, exchange-biased NiFe/NiO bilayers were fabricated by using magnetron sputtering. We investigated the effects of different annealing temperatures on the non-collinear anisotropy of the NiFe/NiO bilayers by measuring the in-plane angular dependence of the magnetic hysteresis loops and the ferromagnetic resonance. We found that the non-collinear angle β varied gradually from 0° to 45° with the variation in annealing temperature, which showed an asymmetric magnetization phenomenon. The competition between unidirectional and uniaxial anisotropy and the interfacial magnetic frustration play a major role in the asymmetric reversal phenomena in an antiferromagnet/ferromagnet system. Our results provide insight into the exchange-biased field and promote its application in spintronic devices.

Ferromagnetic Resonance in Permalloy Metasurfaces

Permalloy films with one-dimensional profile modulation of submicron periodicity are fabricated based on commercially available DVD-R discs and studied using ferromagnetic resonance method and micromagnetic numerical simulations. The main resonance position shows in-plane angular dependence which is strongly reminiscent of that in ferromagnetic films with uniaxial magnetic anisotropy. The main signal and additional low-field lines are attributed to multiple standing spin-wave resonances defined by the grating period. The results may present interest in magnetic metamaterials and magnonics applications.

Parity-controlled spin-wave excitations in synthetic antiferromagnets

We report in this study the current-induced-torque excitation of acoustic and optical modes in Ta/NiFe/Ru/NiFe/Ta synthetic antiferromagnet stacks grown on SiO2/Si substrates. The two Ta layers serve as spin torque sources with the opposite polarizations in both spin currents and Oersted fields acting on their adjacent NiFe layers. This can create the odd symmetry of spatial spin torque distribution across the growth direction, allowing us to observe different spin-wave excitation efficiency from synthetic antiferromagnets excited by homogeneous torques. We analyze the torque symmetry by the in-plane angular dependence of symmetric and anti-symmetric line shape amplitudes for their resonance and confirm the parallel (perpendicular) pumping nature for the acoustic (optical) modes in our devices, which is in stark contrast to the modes excited by spatially homogeneous torques. We also present our macrospin model for this particular spin-torque excitation geometry, which excellently supports our experimental observation. Our results offer capability of controlling spin-wave excitations by local spin-torque sources, and we can explore further spin-wave control schemes based on this concept.

Spin-orbit torque generated from perpendicularly magnetized Co/Ni multilayers

We report the existence of a spin-orbit torque generated from charge to spin conversion in perpendicularly magnetized ferromagnetic material through spin-torque ferromagnetic resonance (ST-FMR) measurement. We fabricated a trilayer structure consisting of a Cu spacer layer sandwiched between an in-plane magnetized FeB layer and perpendicularly magnetized Co/Ni multilayers. From the ferromagnetic resonance under DC bias current application, we observed a significant linewidth modulation in the FeB layer attributed to the charge to spin conversion generated from Co/Ni multilayers. In-plane angular dependence and the magnetization polarity dependence of the linewidth modulation reveal that two types of charge to spin conversion with different symmetry coexist in the single Co/Ni multilayers.

Highly accurate evaluation of spin-torque efficiency by measuring in-plane angular dependence of spin-torque ferromagnetic resonance

Spin-torque ferromagnetic resonance (ST-FMR) is a powerful tool to evaluate spin-torque efficiency (ξFMR), the efficiency of current-induced torque in metallic bilayers consisting of ferromagnetic and nonmagnetic materials. In general, ξFMR can be evaluated from the amplitude ratio between symmetric and anti-symmetric components of the ST-FMR spectrum. However, the ratio is also affected by an unnecessary magnetic field owing to the asymmetric alternating current flow used to excite the ST-FMR. In this article, we demonstrate that ξFMR can be precisely evaluated from the full-angular dependence of the ST-FMR spectrum even though the distribution of alternating current flow is highly asymmetric.

Identification of superconducting pairing symmetry in twisted bilayer graphene using in-plane magnetic field and strain

We show how the pairing symmetry of superconducting states in twisted bilayer graphene can be experimentally identified by theoretically studying effects of externally applied in-plane magnetic field and strain. In the low field regime, superconducting critical temperature $T_c$ is suppressed by in-plane magnetic field $\boldsymbol{B}_{\parallel}$ in singlet channels, but is enhanced by weak $\boldsymbol{B}_{\parallel}$ in triplet channels, providing an important distinction. The in-plane angular dependence of the critical $\boldsymbol{B}_{\parallel, c}$ has a six-fold rotational symmetry, which is broken when strain is present. We show that anisotropy in $\boldsymbol{B}_{\parallel, c}$ generated by strain can be similar for $s$- and $d$-wave channels in moir\'e superlattices. The $d$-wave state is pinned to be nematic by strain and consequently gapless, which is distinguishable from the fully gapped $s$-wave state by scanning tunneling measurements.

Thickness dependence of dynamic magnetic properties of soft (FeCo)-Si alloy thin films

${\mathrm{Fe}}_{69}{\mathrm{Co}}_{26}{\mathrm{Si}}_{5}$ alloy thin films of various thicknesses were deposited on MgO(100) single crystal substrates by magnetron sputtering at $230{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ substrate temperature. The thickness dependence of the dynamic magnetic properties was investigated using broadband ferromagnetic resonance (FMR). The x-ray diffraction results indicate that all films are of the bcc structure with an in-plane epitaxial alignment of ${[100]}_{\mathrm{FeCoSi}}//{[110]}_{\mathrm{MgO}}$. The exchange constant was determined from the field shift between the uniform precession FMR mode and the first order perpendicular standing spin wave resonance mode in the FMR spectra. The effective damping parameter decreases dramatically with increasing film thickness up to 16 nm due to the decrease of the spin pumping contribution and then remains relatively constant as the film thickness increases. In-plane angle dependent FMR measurements reveal that the in-plane anisotropy of these films is dominated by a fourfold magnetic anisotropy, which increases sharply with increasing film thickness up to 16 nm and then shows a slightly decreasing trend as the film thickness increases. In-plane angular dependence of the FMR linewidth shows a strong two-magnon scattering contribution.

Thickness and angular dependent ferromagnetic resonance of ultra-low damping Co25Fe75 epitaxial films

Metallic ferromagnets with ultra-low damping are highly desirable for charge-based spintronic applications. Here, we systematically investigate the magnetic dynamics of Co25Fe75 epitaxial films with a Gilbert damping constant as low as 7.1 × 10−4. The in-plane angular dependence of ferromagnetic resonance (FMR) was measured on various thicknesses of Co25Fe75 films grown on MgO and MgAl2O4, from which the mechanisms for FMR linewidth broadening can be distinguished and quantified. The thickness dependencies of the magnetic anisotropy and inhomogeneous broadening of the linewidth are good indicators of crystal quality and magnetic uniformity. Additionally, it is shown that anisotropic two-magnon scattering is induced by defects at the surfaces.

Xenon-ion irradiation of Co/Si bilayers: Magnetic and structural properties

Evolution of the structure of cobalt-silicon films during Xe ions irradiation has been studied and the same is correlated with magnetic properties. The polycrystalline cobalt films were deposited by electron beam evaporation method to a thickness of 50 nm on crystalline silicon (c-Si) and silicon with pre-amorphized surface (a-Si). After deposition the layers were irradiated with 400 keV Xe ions to the fluences in the range of 2–30 × 1015 ions/cm2. Structural analysis was done by means of transmission electron microscopy, atomic force microscopy (AFM) and X-ray diffraction (XRD), while the magnetic properties were analyzed by using magneto-optical Kerr effect (MOKE) technique. For the both types of substrate the AFM and XRD results show that after Xe ions irradiation the layers become more rough and the grain size of the crystallites increases; the effects being more evidenced for all fluences for the layers deposited on pre-amorphized Si. The MOKE measurements provided the in-plane azimuthal angular dependence of the hysteresis loops and the change of magnetization with the structural parameters. Although the coercive field is influenced by the surface roughness, in the case of c-Si substrate we found it is much more determined by the size of the crystallites. Additionally, independently on the substrate used the magnetic anisotropy in the Co films disappeared as the Xe ion fluence increased, indicating that the changes of magnetization in both systems occur for similar reasons.

Lack of correlation between the spin-mixing conductance and the inverse spin Hall effect generated voltages in CoFeB/Pt and CoFeB/Ta bilayers

We investigate spin pumping phenomena in polycrystalline CoFeB/Pt and CoFeB/Ta bilayers and the correlation between the effective spin mixing conductance $g^{\uparrow\downarrow}_{\rm eff}$ and the obtained voltages generated by the spin-to-charge current conversion via the inverse spin Hall effect in the Pt and Ta layers. For this purpose we measure the in-plane angular dependence of the generated voltages on the external static magnetic field and we apply a model to separate the spin pumping signal from the one generated by the spin rectification effect in the magnetic layer. Our results reveal a dominating role of anomalous Hall effect for the spin rectification effect with CoFeB and a lack of correlation between $g^{\uparrow\downarrow}_{\rm eff}$ and inverse spin Hall voltages pointing to a strong role of the magnetic proximity effect in Pt in understanding the observed increased damping. This is additionally reflected on the presence of a linear dependency of the Gilbert damping parameter on the Pt thickness.

Defect-induced magnon scattering mechanisms in exchange-coupled bilayers

The influence of two-magnon scattering mechanisms, which may be activated by different sorts of defects, is theoretically studied in ferromagnetic/antiferromagnetic exchange-biased bilayers. The spin-wave based model considers the influence of geometrical defects in the ferromagnetic (FM) layer as well as small domains in the antiferromagnetic (AFM) sub-lattice of the FM/AFM interface in such a way that both kinds of defects are randomly distributed over their respective surfaces. The in-plane angular dependence of the ferromagnetic resonance (FMR) linewidth allows detection of the relevant influence of such defects in the relaxation mechanisms, where the role of the exchange-bias field is clearly identified. Typical experimental findings, such as quadratic dependence of the linewidth with the exchange-bias field and the in-plane angular dependence, are well explained within the proposed model. This lends confidence in the model's utility and leads to a better understanding of the role of the magnon-magnon scattering in the magnetization dynamics of exchange-coupled antiferromagnetic/ferromagnetic bilayers.

Enhancing the Spin-Orbit Coupling in Fe3O4 Epitaxial Thin Films by Interface Engineering.

By analyzing the in-plane angular dependence of ferromagnetic resonance linewidth, we show that the Gilbert damping constant in ultrathin Fe3O4 epitaxial films on GaAs substrate can be enhanced by thickness reduction and oxygen vacancies in the interface. At the same time, the uniaxial magnetic anisotropy due to the interface effect becomes significant. Using the element-specific technique of X-ray magnetic circular dichroism, we find that the orbital-to-spin moment ratio increases with decreasing film thickness, in full agreement with the increase in the Gilbert damping obtained for these ultrathin single-crystal films. Combined with the first-principle calculations, the results suggest that the bonding with Fe and Ga or As ions and the ionic distortion near the interface, as well as the FeO defects and oxygen vacancies, may increase the spin-orbit coupling in ultrathin Fe3O4 epitaxial films and in turn provide an enhanced damping.

Polarized Raman scattering study of kesterite type Cu2ZnSnS4 single crystals.

A non-destructive Raman spectroscopy has been widely used as a complimentary method to X-ray diffraction characterization of Cu2ZnSnS4 (CZTS) thin films, yet our knowledge of the Raman active fundamental modes in this material is far from complete. Focusing on polarized Raman spectroscopy provides important information about the relationship between Raman modes and CZTS crystal structure. In this framework the zone–center optical phonons of CZTS, which is most usually examined in active layers of the CZTS based solar cells, are studied by polarized resonant and non-resonant Raman spectroscopy in the range from 60 to 500 cm−1 on an oriented single crystal. The phonon mode symmetry of 20 modes from the 27 possible vibrational modes of the kesterite structure is experimentally determined. From in-plane angular dependences of the phonon modes intensities Raman tensor elements are also derived. Whereas a strong intensity enhancement of the polar E and B symmetry modes is induced under resonance conditions, no mode intensity dependence on the incident and scattered light polarization configurations was found in these conditions. Finally, Lyddane-Sachs-Teller relations are applied to estimate the ratios of the static to high-frequency optic dielectric constants parallel and perpendicular to c-optical axis.

Interface Magnetic Properties in Piezoelectric/Ferromagnetic AlN/FeNi Heterostructures

The spin wave resonance technique was utilized to investigate the interface magnetic properties in AlN/FeNi heterostructures. Obtained surface spin wave (SSW) mode becomes the uniform mode at a critical angle as applied field rotating to the normal direction. Beyond this angle SSW transforms into the uniform mode and the higher-order spin wave modes are also shown. The surface anisotropy caused by the interface pinning between AlN and FeNi resulted in an additional in-plane biaxial anisotropy, which was confirmed by the in-plane angular dependence of remanence measurement. The observed four-fold anisotropy may have potential application in magnetoelectric coupling based information devices.

In-Plane Anisotropy of Upper Critical Field and Flux-Flow Resistivity in Layered Organic Superconductor β′′-(ET)2SF5CH2CF2SO3

We report the in-plane anisotropy of the upper critical field (Hc2) and the flux-flow resistivity (FFR) for a layered organic superconductor β′′-(ET)2SF5CH2CF2SO3 with the incoherent nature of the interlayer transport. The in-plane angular dependence of Hc2 showed a fourfold oscillation with maxima in the directions around H || b and a axes. This result is compatible with a \(d_{x^{2} - y^{2}}\) order parameter. The determined nodal structure is consistent with theoretical predictions of superconductivity mediated by charge fluctuations. Moreover, the dependence of the FFR on in-plane field-orientation showed a fourfold symmetry with cusp-like minima in the directions around H || b and a axes, which is very similar to the FFR for κ-(ET)2Cu(NCS)2 with d-wave pairing symmetry. From these results, we claim that the vortex dynamics is strongly affected by the superconducting gap structure for highly two-dimensional superconductors with d-wave pairing symmetry.

In-plane angular dependence of the spin-wave nonreciprocity of an ultrathin film with Dzyaloshinskii-Moriya interaction

The nonreciprocal propagation of spin waves in an ultrathin Pt/Co/Ni film has been measured by Brillouin light scattering. The frequency nonreciprocity, due to the interfacial Dzyaloshinskii-Moriya interaction (DMI), has a sinusoidal dependence on the in-plane angle between the magnon wavevector and the applied magnetic field. The results, which are in good agreement with analytical predictions reported earlier, yield a value of the DMI constant which is the same as that obtained previously from a study of the magnon dispersion relations. We have demonstrated that our magnon-dynamics based method can experimentally ascertain the DMI constant of multilayer thin films.

Study of angular dependence of exchange bias and misalignment in uniaxial and unidirectional anisotropy in NiFe(111)/FeMn(111)/CoFeB(amorphous) stack

We report the investigation of the in-plane azimuthal angular dependence of the magnetization reversal in the ion beam sputtered exchanged biased NiFe(111)/FeMn(111)/CoFeB(amorphous) stack. Compared to the as-deposited case, the magnetic annealing resulted in 3 fold enhancement in exchange bias but decrease in coercivity. The observed cosine dependence of exchange biased CoFeB layer on the in-plane azimuthal angle of applied field is corroborated with Meiklejohn and Bean model. The training effect associated with the exchange bias showed unconventional increase in coercivity after first cycle of hysteresis loop, while the exchange bias decreases sharply, and for subsequent cycles the exchange bias follows the empirical relation based on the energy dissipation in the AF layer. The ferromagnetic resonance (FMR) measurements also exhibited the in-plane azimuthal angle dependence of the magnetic resonance field indicating that the uniaxial and unidirectional anisotropies are not collinear, although they lie in the same plane. However, no misalignment between the unidirectional anisotropy and the exchange bias direction is observed. The misalignment angle between the uniaxial and unidirectional anisotropy, as measured by FMR, is found to be 10° and 14° for CoFeB and NiFe, respectively. This misalignment is attributed to the interface roughness as revealed by x-ray reflectance measurements.