Magnetic Damping Effect Research Articles

Effects of spin torque within ferromagnetic infinite medium: The short-wave approximation and Painlevé analysis

In this paper, we investigate the effects of spin-transfer torques within the ferromagnetic infinite medium through the short-wave approximation method. As a result, we have derived the new (1+1) dimensional nonlinear evolution system, which describes the propagation of electromagnetic short waves within the ferromagnet in the presence of electric current density. Using the Painlevé analysis and Hirota’s bilinearization, we unearth the integrability properties of this new evolution system. In the wake of such an analysis, the typical class of excitations and its physical implications are presented. We remark that the current density acts on magnetization like an effective magnetic damping, which is important for the stabilization of magnetic information storage and data process elements.

Enhanced effective spin Hall efficiency contributed by the extrinsic spin Hall effect in Pt1- x Ta x /CoFeB structures

High effective spin Hall efficiency and low magnetic damping constant are essential to achieve efficient spin-charge conversion for energy-efficient spintronic devices. We report the measurements of effective spin Hall efficiency and magnetic damping constant of Pt1- x Ta x /CoFeB structures using spin-torque ferromagnetic resonance (ST-FMR) techniques. With the increase of Ta content, the spin Hall efficiency increases and peaks x = 0.15 with the value of θSHeff=0.35 , before decreases for further Ta content. This non-monotonic variation is attributed to the interaction between the contribution of the extrinsic skew scattering mechanism and the spin-orbit coupling (SOC). The weakening of the SOC in the Pt1-x Ta x layer leads to a decrease in the effective magnetic damping constant with increasing Ta concentration. High-temperature ST-FMR measurements confirm the influence of the Ta concentration on the spin Hall effect and magnetic damping mechanism. Additionally, the decrease in spin mixing conductance and the increase in spin diffusion length leads to a decrease in the interfacial spin transparency.

Stability analysis of wall‐attached Bénard–Marangoni convection in a vertical magnetic field

AbstractThe threshold for the onset of thermocapillary flow in a planar liquid layer heated from below is increased by a vertical magnetic field when the liquid is a good electric conductor. The magnetic damping effect is reduced when the induced eddy currents are blocked by insulating side walls. Neutral conditions for this specific Bénard–Marangoni stability problem with a vertical field and side walls are obtained numerically for three‐dimensional perturbations assumed periodic in one horizontal direction. The domain is bounded by a free‐slip wall at the bottom, a free surface at the top and two free‐slip lateral walls in the other horizontal direction. Buoyancy forces and surface deformations are neglected and a constant heat flux is imposed on the free surface. Upon increasing the magnetic induction, the least stable modes become localized near the side walls and the convective threshold increases at a lower rate than for the least stable bulk mode.

Searching for magnetic fields in pulsating A-type stars: the discovery of a strong field in the probable δ Sct star HD 340577 and a null result for the γ Dor star HR 8799

ABSTRACT Numerous δ Sct and γ Dor pulsators are identified in the region of the Hertzsprung–Russell diagram that is occupied by chemically peculiar magnetic Ap stars. The connection between δ Sct and γ Dor pulsations and the magnetic field in Ap stars is however not clear: theory suggests for magnetic Ap stars some critical field strengths for pulsation mode suppression by computing the magnetic damping effect for selected p and g modes. To test these theoretical considerations, we obtained PEPSI spectropolarimetric snapshots of the typical Ap star HD 340577, for which δ Sct-like pulsations were recently detected in Transiting Exoplanet Survey Satellite data, and the γ Dor pulsator HR 8799, which is a remarkable system with multiple planets and two debris discs. Our measurements reveal the presence of a magnetic field with a strength of several hundred Gauss in HD 340577. The measured mean longitudinal field would be the strongest field measured so far in a δ Sct star if the pulsational character of HD 340577 is confirmed spectroscopically. No magnetic field is detected in HR 8799.

Magnetic field modulated ultrafast spin dynamics at Ni80Fe20/Neodymium interface.

Ultrafast spin dynamics is crucial for the next-generation spintronic devices towards high-speed data processing. Here, we investigate the ultrafast spin dynamics of Neodymium/Ni80Fe20 (Nd/Py) bilayers by the time-resolved magneto-optical Kerr effect. The effective modulation of spin dynamics at Nd/Py interfaces is realized by an external magnetic field. The effective magnetic damping of Py increases with increasing Nd thickness, and a large spin mixing conductance (∼19.35×1015 cm-2) at Nd/Py interface is obtained, representing the robust spin pumping effect by Nd/Py interface. The tuning effects are suppressed at a high magnetic field due to the reduced antiparallel magnetic moments at Nd/Py interface. Our results contribute to understanding ultrafast spin dynamics and spin transport behavior in high-speed spintronic devices.

The influence of Ti ultrathin insertion layer on the effective magnetic damping and effective spin Hall angle

We report the influence of ultrathin Ti insertion layer on the effective magnetic damping and effective spin Hall angle in Co/[Pt/Ti]n/Pt structures via spin-torque ferromagnetic resonance measurements. The effective magnetic damping shows a non-monotonic variation as a function of insertion layers number n, reaching a minimum at n = 5. Our analysis shows that when n is less than 5, the damping is mainly related to the thickness of the bottom Pt layer, and when it is greater than 5, the attenuation of the spin currents leads to increased damping. The effective magnetic damping first decreases as the number of layers n increases, reaching a minimum at n=5, and then increases with further increases in n. The observation can be ascribed to a competition between the increased longitudinal resistivity, which is due to the strong interfacial scattering, and the reduced effective spin Hall conductivity that originates from the shortening of the carrier lifetime. Additionally, the extracted interfacial spin transparency is found to be improved with the effect of the insertion layer.

Effect of axial static magnetic field on microstructure evolution, performance, and melt pool signals of AlSi10Mg fabricated by laser powder bed fusion

Static magnetic field (SMF) assisted laser powder bed fusion (LPBF) has significant potential to improve the mechanical properties of AlSi10Mg parts. In this study, the tensile properties of AlSi10Mg are improved under the SMF, mostly due to the increased relative density and finer microstructure. Under the SMF, the epitaxial growth in the building direction is inhibited, indicating a weakening 〈001〉 texture. However, the mechanism of the magnetic field within the melt pool remains unknown. The melt pool area, temperature gradient, radiation emission, and spatter extracted by in-situ monitoring technology provide an excellent opportunity for increasing the understanding of the SMF-assisted LPBF process. The Marangoni convection in the melt pool is limited by the magnetic damping effect, which improves the melt pool and radiation stability and reduces spatters. Further, thermoelectric magnetic force at the cell surface in the mushy zone improves the local convection, increasing the thermal diffusion rate in the melt pool boundary during the solidification. This results in smaller melt pool areas, reduced radiation emission magnitude, and a steep temperature gradient. As a result, the directional solidification in the thermal diffusion direction is inhibited, and the microstructure is refined. Finally, the thermoelectric magnetic convection also facilitates the escape of the entrapped pores in the melt pool, increasing relative density.

Magnetocrystalline anisotropy of epitaxially grown FeRh/MgO(001) films

We quantitatively investigated the magnetocrystalline anisotropy (MCA) of metamagnetic FeRh films epitaxially grown on MgO(001) substrates. The in-plane fourfold magnetic anisotropy of epitaxial FeRh films is reoriented from the<100>to<110>directions and meanwhile the strength is obvious enhanced when FeRh transfers from the ferromagnetic (FM) to antiferromagnetic (AF) states. In the nominal AF state, the observed fourfold magnetic anisotropy originates from the exchange coupling between the residual FM moments caused by antisite defects and the AF matrix, which therefore indicates the orientation and strength of the MCA of AF FeRh. The steep increase of the FM resonance linewidth when crossing the FM-AF phase transition temperature reveals the enhanced effective magnetic damping and an increased fourfold anisotropic two-magnon scattering. The first-principles calculations suggest that the in-plane lattice compressive strain imposed by the MgO substrate plays an important role in determining the magnetic structures FeRh films.

Spin Hall angle of rhodium and its effects on magnetic damping of Ni80Fe20 in Rh/Ni80Fe20 bilayer

We report the investigation of the spin Hall angle (SHA) of Rhodium (Rh) and the effective magnetic damping constant of Ni80Fe20 in Rh (10 nm)/Ni80Fe20 (10 nm) bilayer system using Spin Torque Ferromagnetic Resonance (STFMR) spectroscopy and density functional theory based analysis. The filtered DC output voltages from Rh (10 nm)/Ni80Fe20 (10 nm) bilayer system at different input signal frequencies and powers for a complete 360° angle were measured. The average magnetic damping constant of Ni80Fe20 is found to be 0.017 and the angle-dependent study shows a 2-fold symmetry with the in-plane anisotropy field of 120 Oe. The SHA of Rh is found to be 0.2 %. The results agrees well with the calculations based on density functional theory where we found spin Hall conductivity (σxyspinz) at the charge-neutral Fermi level (EF) is positive and it’s magnitude is 423ℏe(S/cm).

Unconventional Spin Pumping and Magnetic Damping in an Insulating Compensated Ferrimagnet.

Recently, the interest in spin pumping (SP) has escalated from ferromagnets into antiferromagnetic systems, potentially enabling fundamental physics and magnonic applications. Compensated ferrimagnets are considered alternative platforms for bridging ferro- and antiferromagnets, but their SP and the associated magnetic damping have been largely overlooked so far despite their seminal importance for magnonics. Herein, an unconventional SP together with magnetic damping in an insulating compensated ferrimagnet Gd3 Fe5 O12 (GdIG) is reported. Remarkably, the divergence of the nonlocal effective magnetic damping induced by SP close to the compensation temperature in GdIG/Cu/Pt heterostructures is identified unambiguously. Furthermore, the coherent and incoherent spin currents, generated by SP and the spin Seebeck effect, respectively, undergo a distinct direction change with the variation of temperature. The physical mechanisms underlying these observations are self-consistently clarified by the ferrimagnetic counterpart of SP and the handedness-related spin-wave spectra. The findings broaden the conventional paradigm of the ferromagnetic SP model and open new opportunities for exploring the ferrimagnetic magnonic devices.

Magnetization Precession at Sub-Terahertz Frequencies in Polycrystalline Cu2 Sb-Type (Mn-Cr)AlGe Ultrathin Films.

A ferromagnetic metal nanolayer with a large perpendicular magnetic anisotropy, small saturation magnetization, and small magnetic damping constant is a crucial requirement for high-speed spintronic devices. Fabrication of these devices on Si/SiO2 amorphous substrates with polycrystalline structure is also strongly desired for the mass production industry. This study involves the investigation of sub-terahertz (THz) magnetization precessional motion in a newly developed material system consisting of Cu2 Sb-type MnAlGe and (Mn-Cr)AlGe films by means of an all-optical pump-probe method. These materials exhibit large perpendicular magnetic anisotropy in regions of a few nanometers in size. The pseudo-2D crystal structures are clearly observed in the high-resolution transmission electron microscopy (TEM) images for the film samples grown on thermally oxidized silicon substrates. The TEM images also show a partial substitution of Cr atoms for the Mn sites in MnAlGe. A magnetization precession frequency of 0.164 THz with a relatively small effective magnetic damping constant of 0.012 is obtained for (Mn-Cr)AlGe. Theoretical calculation infers that the modification of the total density of states by Cr substitution decreases the intrinsic magnetic damping constant of (Mn-Cr)AlGe.

Exchange-coupling-induced fourfold magnetic anisotropy in CoFeB/FeRh bilayer grown on SrTiO3(001)

Exchange coupling across the interface between a ferromagnetic (FM) layer and an antiferromagnetic (AFM) or another FM layer may induce a unidirectional magnetic anisotropy and/or a uniaxial magnetic anisotropy, which has been extensively studied due to the important application in magnetic materials and devices. In this work, we observed a fourfold magnetic anisotropy in amorphous CoFeB layer when exchange coupling to an adjacent FeRh layer which is epitaxially grown on an SrTiO3(001) substrate. As the temperature rises from 300 K to 400 K, FeRh film undergoes a phase transition from AFM to FM phase, the induced fourfold magnetic anisotropy in the CoFeB layer switches the orientation from the FeRh〈 110〉 to FeRh〈 100 〉 directions and the strength is obviously reduced. In addition, the effective magnetic damping as well as the two-magnon scattering of the CoFeB/FeRh bilayer also remarkably increase with the occurrence of magnetic phase transition of FeRh. No exchange bias is observed in the bilayer even when FeRh is in the nominal AFM state, which is probably because the residual FM FeRh moments located at the interface can well separate the exchange coupling between the below pinned FeRh moments and the CoFeB moments.

Optimized growth of compensated ferrimagnetic insulator Gd3Fe5O12 with a perpendicular magnetic anisotropy* *Project supported by the National Key R&D Program of China (Grant Nos. 2017YFA0206200 and 2016YFA0302300), the Basic Science Center Project of the National Natural Science Foundation of China (Grant No. 51788104), the National Natural Science Foundation of China (Grant Nos. 11774194, 11804182, 51831005, and 11811082), Beijing Natural Science

Compensated ferrimagnetic insulators are particularly interesting for enabling functional spintronic, optical, and microwave devices. Among many different garnets, Gd3Fe5O12 (GdIG) is a representative compensated ferrimagnetic insulator. In this paper, we will study the evolution of the surface morphology, the magnetic properties, and the magnetization compensation through changing the following parameters: the annealing temperature, the growth temperature, the annealing duration, and the choice of different single crystalline garnet substrates. Our objective is to find the optimized growth condition of the GdIG films, for the purpose of achieving a strong perpendicular magnetic anisotropy (PMA) and a flat surface, together with a small effective damping parameter. Through our experiments, we have found that the surface roughness approaching 0.15 nm can be obtained by choosing the growth temperature around 700 °C, together with an enhanced PMA. We have also found the modulation of magnetic anisotropy by choosing different single crystalline garnet substrates which change the tensile strain to the compressive strain. A measure of the effective magnetic damping parameter (α eff = 0.04±0.01) through a spin pumping experiment in a GdIG/Pt bilayer is also made. Through optimizing the growth dynamics of GdIG films, our results could be useful for synthesizing garnet films with a PMA, which could be beneficial for the future development of ferrimagnetic spintronics.

Magnetoelastic interactions and magnetic damping in Co2Fe0.4Mn0.6Si and Co2FeGa0.5Ge0.5 Heusler alloys thin films for spintronic applications

Co2Fe0.4Mn0.6Si (CFMS) and Co2FeGa0.5Ge0.5 (CFGG) Heusler alloys are among the most promising thin film materials for spintronic devices due to a high spin polarization, low magnetic damping and giant/tunneling magnetoresistance ratios. Despite numerous investigations of Heusler alloys magnetic properties performed up to now, magnetoelastic effects in these materials remain not fully understood; due to quite rare studies of correlations between magnetoelastic and other magnetic properties, such as magnetic dissipation or magnetic anisotropy. In this research we have investigated epitaxial CFMS and CFGG Heusler alloys thin films of thickness in the range of 15–50 nm. We have determined the magnetoelastic tensor components and magnetic damping parameters as a function of the magnetic layer thickness. Magnetic damping measurements revealed the existence of non-Gilbert dissipation related contributions, including two-magnon scattering and spin pumping phenomena. Magnetoelastic constant B11 values and the effective magnetic damping parameter αeff values were found to be in the range of − 6 to 30 × 106 erg/cm3 and between 1 and 12 × 10–3, respectively. The values of saturation magnetostriction λS for CFMS Heusler alloy thin films were also obtained using the strain modulated ferromagnetic resonance technique. The correlation between αeff and B11, depending on magnetic layer thickness was determined based on the performed investigations of the above mentioned magnetic properties.

Large spin Hall angle enhanced by nitrogen incorporation in Pt films

We report on the enhancement of spin Hall angle from the CoFeB/Pt interface by introducing nitrogen into the Pt thin film. Spin-torque ferromagnetic resonance measurements on the effective spin Hall angle (θSH) reveal a non-monotonic variation as a function of the amount of nitrogen gas introduced, Q in the film deposition, which peaks at θSH = 0.16 when Q is 8%. Our analysis shows that the θSH enhancement is mainly attributed to the increase in spin-dependent scattering at the interface. The effective magnetic damping decreases with increasing Q due to the reduced spin–orbit coupling. The interfacial spin transparency is also observed to show improvement after the introduction of nitrogen. Moreover, the additional damping-like torque from the interface may also lead to the enhancement of the linewidth modulation.

Electric-field control of spin dynamics during magnetic phase transitions.

Controlling magnetization dynamics is imperative for developing ultrafast spintronics and tunable microwave devices. However, the previous research has demonstrated limited electric-field modulation of the effective magnetic damping, a parameter that governs the magnetization dynamics. Here, we propose an approach to manipulate the damping by using the large damping enhancement induced by the two-magnon scattering and a nonlocal spin relaxation process in which spin currents are resonantly transported from antiferromagnetic domains to ferromagnetic matrix in a mixed-phased metallic alloy FeRh. This damping enhancement in FeRh is sensitive to its fraction of antiferromagnetic and ferromagnetic phases, which can be dynamically tuned by electric fields through a strain-mediated magnetoelectric coupling. In a heterostructure of FeRh and piezoelectric PMN-PT, we demonstrated a more than 120% modulation of the effective damping by electric fields during the antiferromagnetic-to-ferromagnetic phase transition. Our results demonstrate an efficient approach to controlling the magnetization dynamics, thus enabling low-power tunable electronics.

On the first δ Sct–roAp hybrid pulsator and the stability of p and g modes in chemically peculiar A/F stars

ABSTRACT Strong magnetic fields in chemically peculiar A-type (Ap) stars typically suppress low-overtone pressure modes (p modes) but allow high-overtone p modes to be driven. KIC 11296437 is the first star to show both. We obtained and analysed a Subaru spectrum, from which we show that KIC 11296437 has abundances similar to other magnetic Ap stars, and we estimate a mean magnetic field modulus of 2.8 ± 0.5 kG. The same spectrum rules out a double-lined spectroscopic binary, and we use other techniques to rule out binarity over a wide parameter space, so the two pulsation types originate in one δ Sct–roAp hybrid pulsator. We construct stellar models depleted in helium and demonstrate that helium settling is second to magnetic damping in suppressing low-overtone p modes in Ap stars. We compute the magnetic damping effect for selected p and g modes, and find that modes with frequencies similar to the fundamental mode are driven for polar field strengths ≲4 kG, while other low-overtone p modes are driven for polar field strengths up to ∼1.5 kG. We find that the high-order g modes commonly observed in γ Dor stars are heavily damped by polar fields stronger than 1–4 kG, with the damping being stronger for higher radial orders. We therefore explain the observation that no magnetic Ap stars have been observed as γ Dor stars. We use our helium-depleted models to calculate the δ Sct instability strip for metallic-lined A (Am) stars, and find that driving from a Rosseland mean opacity bump at ∼5 × 104 K caused by the discontinuous H-ionization edge in bound-free opacity explains the observation of δ Sct pulsations in Am stars.

Magnetic-Field-Induced Liquid–Solid Interface Transformation and Its Effect on Microsegregation in Directionally Solidified Ni-Cr Alloy

The transformation of liquid–solid interface induced by the steady magnetic field (SMF) in the directionally solidified Ni-10 wt pct Cr alloy was studied experimentally. At the moderate pulling rate (50 μm s−1), it could be observed that the interface morphology gradually transformed from planar to cellular shape with increasing the SMF intensity (0 T, 3 T, 6 T). However, the cellular interface at the high pulling rate (100 μm s−1) was not influenced by the SMF. 3D numerical simulations suggested that the transformation of interface morphology originated from the thermoelectric magnetic convection near the wavelike interface at the early stage of solidification. From the composition measurement, it was found that the formation of microsegregation at the moderate pulling rate was associated with the interface morphology. Under the 3 T SMF, the liquid–solid interface remained planar and the microsegregation level increased in comparison with that without the SMF. Under the 6 T SMF, the liquid–solid interface became cellular and the microsegregation level was reduced. The factors affecting microsegregation were evaluated. The effective partition coefficient was estimated based on composition data. It was revealed that the effective partition coefficient increased with the 6 T SMF due to the thermoelectric magnetic and magnetic damping effects within the cellular structure. Additionally, the solid diffusivity was measured using the diffusion couple technique. It was found that the interdiffusion coefficient of Cr decreased with increasing the SMF intensity. The modified Brody model was used to predict the microsegregation behavior in the SMF. The predicted results were in agreement with experimental observation. It could be concluded that the decrease in solid diffusivity enhanced the formation of microsegregation for the planar interface, whereas the increase in effective partition coefficient in the SMF was beneficial for alleviating the extent of microsegregation for the cellular interface.

Instability analysis of bi-axial micro-scanner under electromagnetic actuation including small scale and damping effects

The objective of this work is to create an analytical framework to study the problem of instability and squeezed film damping in bi-axial micro-scanners under electromagnetic actuation considering Casimir effects and size dependence, simultaneously. Also, the concept of Eulerian angles has also been used to achieve this idea. The novelty of this work is analytical solution of squeeze film damping considering changes of pressure distribution for micro-mirror in the horizontal and vertical rotations. Moreover, the modified couple stress theory is employed to assess the effect of the small-scale on the dynamic instability of a biaxial micro-scanner, and the governing equations are derived based on the concept of Eulerian angles. This study addresses the nonlinear effect of air squeeze film damping on the stability of the micro-scanner according to its geometry. Then, the influence of magnetic field, Casimir force, and size are examined, followed by the generation of phase portrait and plot of parametric analysis. Based on the obtained results, the Casimir force accounts for the instability threshold of the system. Moreover, the phase portrait diagrams indicates that small size and air squeeze film damping improves the rotational stability of the micro-scanner. AC voltage is applied to induce electromagnetic actuation, and the graphs of nonlinear frequency response are plotted versus the micromirror rotation amplitudes considering various effects of magnetic field actuation and air squeeze film damping. Furthermore, increasing the magnetic field diminishes the instability threshold, and augmentation of the air squeeze film damping enhances the system stability and decreases the maximum rotation amplitude of the micro-scanner. The obtained results are validated against the empirical/theoretical results in the literature.

A Simplified Analytical Model for the Analysis of Magnetomechanical Dynamic Response of a Test Module in J-TEXT Tokamak

Test blanket modules (TBMs) are key components of the ITER for the demonstration of tritium breeding. During plasma disruption, the TBMs and other in-vessel components suffer large electromagnetic forces and vibrations accompanied by a significant magnetostructural coupling effect. Aiming to evaluate the coupled vibration of a test module in J-TEXT Tokamak, a simplified analytical model was developed in this paper. The model has one degree of torsional freedom and was established by the idealization of the geometry and the confining magnetic field. The analytical solution of the model indicates that the magnetic stiffness effect is not significant for this problem, and the model gives a conservation estimation of the magnetic damping effect. To demonstrate the feasibility of the analytical model, the dynamic response of the test module in J-TEXT during a typical plasma disruption event was simulated by using an FEM numerical code of the Lagrangian sequential coupling method and was found to be in good agreement with the analytical solution. To prove the validity of the numerical model and code, the coupled vibration of a cubic block in a large constant magnetic field and a pulsed magnetic field of an electromagnet was measured by fabricating an experimental setup in a large magnet and was also calculated by using the FEM code under the experimental conditions. The measured results agree well with the numerical ones, which proved the validity of the analytical model indirectly. Due to the explicit form of the coupling term, the analytical solution gives a good reference when considering the magnetostructural coupling effect of a typical in-vessel structure in the tokamak device.