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Hans T. Nembach

Publications and source records attributed to Hans T. Nembach.

34 records · Page 2Linked to original sources
arXiv2018

Determination of spin Hall effect and spin diffusion length of Pt from self-consistent fitting of damping enhancement and inverse spin-orbit torque measurements

Understanding the evolution of spin-orbit torque (SOT) with increasing heavy-metal thickness in ferromagnet/normal metal (FM/NM) bilayers is critical for the development of magnetic memory based on SOT. However, several experiments have revealed an apparent discrepancy between damping enhancement and damping-like SOT regarding their dependence on NM thickness. Here, using linewidth and phase-resolved amplitude analysis of vector network analyzer ferromagnetic resonance (VNA-FMR) measurements, we simultaneously extract damping enhancement and both field-like and damping-like inverse SOT in Ni$_{80}$Fe$_{20}$/Pt bilayers as a function of Pt thickness. By enforcing an interpretation of the data which satisfies Onsager reciprocity, we find that both the damping enhancement and damping-like inverse SOT can be described by a single spin diffusion length ($\approx$ 4 nm), and that we can separate the spin pumping and spin memory loss (SML) contributions to the total damping. This analysis indicates that less than 40% of the angular momentum pumped by FMR through the Ni$_{80}$Fe$_{20}$/Pt interface is transported as spin current into the Pt. On account of the SML and corresponding reduction in total spin current available for spin-charge transduction in the Pt, we determine the Pt spin Hall conductivity ($σ_\mathrm{SH} = (2.36 \pm 0.04)\times10^6 Ω^{-1} \mathrm{m}^{-1}$) and bulk spin Hall angle ($θ_\mathrm{SH}=0.387 \pm0.008$) to be larger than commonly-cited values. These results suggest that Pt can be an extremely useful source of SOT if the FM/NM interface can be engineered to minimize SML. Lastly, we find that self-consistent fitting of the damping and SOT data is best achieved by a model with Elliott-Yafet spin relaxation and extrinsic inverse spin Hall effect, such that both the spin diffusion length and spin Hall conductivity are proportional to the Pt charge conductivity.

cond-mat.mes-hall↗
arXiv2017

Inductive detection of field-like and damping-like AC inverse spin-orbit torques in ferromagnet/normal metal bilayers

Functional spintronic devices rely on spin-charge interconversion effects, such as the reciprocal processes of electric field-driven spin torque and magnetization dynamics-driven spin and charge flow. Both damping-like and field-like spin-orbit torques have been observed in the forward process of current-driven spin torque and damping-like inverse spin-orbit torque has been well-studied via spin pumping into heavy metal layers. Here we demonstrate that established microwave transmission spectroscopy of ferromagnet/normal metal bilayers under ferromagnetic resonance can be used to inductively detect the AC charge currents driven by the inverse spin-charge conversion processes. This technique relies on vector network analyzer ferromagnetic resonance (VNA-FMR) measurements. We show that in addition to the commonly-extracted spectroscopic information, VNA-FMR measurements can be used to quantify the magnitude and phase of all AC charge currents in the sample, including those due to spin pumping and spin-charge conversion. Our findings reveal that Ni$_{80}$Fe$_{20}$/Pt bilayers exhibit both damping-like and field-like inverse spin-orbit torques. While the magnitudes of both the damping-like and field-like inverse spin-orbit torque are of comparable scale to prior reported values for similar material systems, we observed a significant dependence of the damping-like magnitude on the order of deposition. This suggests interface quality plays an important role in the overall strength of the damping-like spin-to-charge conversion.

cond-mat.mtrl-sci↗
arXiv2017

Observation of Spin-Orbit Effects with Spin Rotation Symmetry

The spin-orbit interaction enables interconversion between a charge current and a spin current. It is usually believed that in a nonmagnetic metal (NM) or at a NM/ferromagnetic metal (FM) bilayer interface, the symmetry of spin-orbit effects (SOE) requires that the spin current, charge current and spin orientation are all orthogonal to each other. Here we show the observation of a SOE near the NM/FM interface that exhibits a very different symmetry from the conventional spin Hall effect, insofar as the spin polarization is further rotated about the magnetization. These results imply that a perpendicularly polarized spin current can be generated with an in-plane charge current simply by use of a FM/NM bilayer with magnetization collinear to the charge current. The ability to generate a spin current with arbitrary polarization using typical magnetic materials will greatly benefit the development of magnetic memories.

cond-mat.mtrl-sci↗
arXiv2017

Magnetic properties in ultra-thin 3d transition metal alloys II: Experimental verification of quantitative theories of damping and spin-pumping

A systematic experimental study of Gilbert damping is performed via ferromagnetic resonance for the disordered crystalline binary 3d transition metal alloys Ni-Co, Ni-Fe and Co-Fe over the full range of alloy compositions. After accounting for inhomogeneous linewidth broadening, the damping shows clear evidence of both interfacial damping enhancement (by spin pumping) and radiative damping. We quantify these two extrinsic contributions and thereby determine the intrinsic damping. The comparison of the intrinsic damping to multiple theoretical calculations yields good qualitative and quantitative agreement in most cases. Furthermore, the values of the damping obtained in this study are in good agreement with a wide range of published experimental and theoretical values. Additionally, we find a compositional dependence of the spin mixing conductance.

cond-mat.mtrl-sci↗
arXiv2017

Magnetic properties of ultra-thin 3d transition-metal binary alloys I: spin and orbital moments, anisotropy, and confirmation of Slater-Pauling behavior

The structure and static magnetic properties - saturation magnetization, perpendicular anisotropy, spectroscopic g-factor, and orbital magnetization - of thin-film 3d transition metal alloys are determined over the full range of alloy compositions via X-ray diffraction, magnetometry, and ferromagnetic resonance measurements. We determine the interfacial perpendicular magnetic anisotropy by use of samples sets with varying thickness for specific alloy concentrations. The results agree with prior published data and theoretical predictions. They provide a comprehensive compilation of the magnetic properties of thin-film Ni-Co, Ni-Fe and Co-Fe alloys that goes well beyond the often-cited Slater-Pauling dependence of magnetic moment on alloy concentration.

cond-mat.mtrl-sci↗
arXiv2015

Ultra-low magnetic damping of a metallic ferromagnet

The phenomenology of magnetic damping is of critical importance for devices that seek to exploit the electronic spin degree of freedom since damping strongly affects the energy required and speed at which a device can operate. However, theory has struggled to quantitatively predict the damping, even in common ferromagnetic materials. This presents a challenge for a broad range of applications in spintronics and spin-orbitronics that depend on materials and structures with ultra-low damping. Such systems enable many experimental investigations that further our theoretical understanding of numerous magnetic phenomena such as damping and spin-transport mediated by chirality and the Rashba effect. Despite this requirement, it is believed that achieving ultra-low damping in metallic ferromagnets is limited due to the scattering of magnons by the conduction electrons. However, we report on a binary alloy of Co and Fe that overcomes this obstacle and exhibits a damping parameter approaching 0.0001, which is comparable to values reported only for ferrimagnetic insulators. We explain this phenomenon by a unique feature of the bandstructure in this system: The density of states exhibits a sharp minimum at the Fermi level at the same alloy concentration at which the minimum in the magnetic damping is found. This discovery provides both a significant fundamental understanding of damping mechanisms as well as a test of theoretical predictions.

cond-mat.mtrl-sci↗
arXiv2015

Radiative damping in wave guide based FMR measured via analysis of perpendicular standing spin waves in sputtered Permalloy films

The damping $α$ of the spinwave resonances in 75 nm, 120 nm, and 200nm -thick Permalloy films is measured via vector-network-analyzer ferromagnetic-resonance (VNA-FMR) in the out-of-plane geometry. Inductive coupling between the sample and the waveguide leads to an additional radiative damping term. The radiative contribution to the over-all damping is determined by measuring perpendicular standing spin waves (PSSWs) in the Permalloy films, and the results are compared to a simple analytical model. The damping of the PSSWs can be fully explained by three contributions to the damping: The intrinsic damping, the eddy-current damping, and the radiative damping. No other contributions were observed. Furthermore, a method to determine the radiative damping in FMR measurements with a single resonance is suggested.

cond-mat.mtrl-sci↗
arXiv2015

Localization of Fe d-states in Ni-Fe-Cu alloys and implications for ultrafast demagnetization

Ni$_{80}$Fe$_{20}$ (Py) and Py-Cu exhibit intriguing ultrafast demagnetization behavior, where the Ni magnetic moment shows a delayed response relative to the Fe [S. Mathias et al., PNAS {\bf 109}, 4792 (2012)]. To unravel the mechanism responsible for this behavior, we have studied Py-Cu alloys for a wide range of Cu concentrations using X-ray magnetic circular dichroism (XMCD). The magnetic moments of Fe and Ni are found to respond very differently to Cu alloying: Fe becomes a strong ferromagnet in Py, with the magnetic moment largely unaffected by Cu alloying. In contrast, the Ni magnetic moment decreases continuously with increasing Cu concentration. Our results are corroborated by ab-initio calculations of the electronic structure, which we discuss in the framework of virtual bound states (VBSs). For high Cu concentrations, Ni exhibits VBSs below the Fermi level, which are likely responsible for an increased orbital/spin magnetic ratio at high Cu concentrations. Fe exhibits VBSs in the minority band, approximately 1 eV above the Fermi level in pure Py, that move closer to the Fermi level upon Cu alloying. A strong interaction between the VBSs and excited electrons above the Fermi level enhances the formation of localized magnons at Fe sites, which explains the different behavior between Fe and Ni during ultrafast demagnetization.

cond-mat.mtrl-sci↗
arXiv2015

Suppression of Spin Pumping Between Ni$_{80}$Fe$_{20}$ and Cu by a Graphene Interlayer

We compare ferromagnetic resonance measurements of Permalloy Ni$_{80}$Fe$_{20}$ (Py) films sputtered onto Cu(111) films with and without a graphene (Gr) interlayer grown by chemical vapor deposition before Py deposition. A two-angle sputtering method ensured that neither Gr nor Py was degraded by the sample preparation process. We find the expected damping enhancement from spin pumping for the Py/Cu case and no detectable enhancement for the Py/Gr/Cu case. Since damping is sensitive to effects other than spin pumping, we used magnetometry to verify that differences in Py magnetostatic properties are not responsible for the difference in damping. We attribute the suppression of spin pumping in Py/Gr/Cu to the large contact resistance of the Gr/Cu interface.

cond-mat.mes-hall↗
arXiv2014

Spectroscopic confirmation of linear relation between Heisenberg- and interfacial Dzyaloshinskii-Moriya-exchange in polycrystalline metal films

Proposals for novel spin-orbitronic logic1 and memory devices2 are often predicated on assumptions as to how materials with large spin-orbit coupling interact with ferromagnets when in contact. Such interactions give rise to a host of novel phenomena, such as spin-orbit torques3,4 , chiral spin-structures5,6 and chiral spin-torques. These chiral properties are related to the anti-symmetric exchange, also referred to as the interfacial Dzyaloshinskii-Moriya interaction (DMI)9,10. For numerous phenomena, the relative strengths of the symmetric Heisenberg exchange and the DMI is of great importance. Here, we use optical spin-wave spectroscopy (Brillouin light scattering) to directly determine the DMI vector for a series of Ni80Fe20/Pt samples, and then compare the nearest-neighbor DMI coupling energy with the independently measured Heisenberg exchange integral. We find that the Ni80Fe20-thickness-dependencies of both the microscopic symmetric- and antisymmetric-exchange are identical, consistent with the notion that the basic mechanisms of the DMI and Heisenberg exchange essentially share the same underlying physics, as was originally proposed by Moriya11. While of significant fundamental importance, this result also leads us to a deeper understanding of DMI and how it could be optimized for spin-orbitronic applications.

cond-mat.mes-hall↗
arXiv2014

Detection of the dc inverse spin Hall effect due to spin pumping in a novel meander-stripline geometry

The dc voltage obtained from the inverse spin Hall effect (iSHE) due to spin pumping in ferromagnet/normal-metal (NM) bilayers can be unintentionally superimposed with magnetoresistive rectification of ac charge currents in the ferromagnetic layer. We introduce a geometry in which these spurious rectification voltages vanish while the iSHE voltage is maximized. In this geometry, a quantitative study of the dc iSHE is performed in a broad frequency range for Permalloy/NM multilayers with NM={Pt, Ta, Cu/Au, Cu/Pt}. The experimentally recorded voltages can be fully ascribed to the iSHE due to spin pumping. Furthermore we measure a small iSHE voltage in single CoFe thin films.

cond-mat.mes-hall↗
arXiv2014

Phase-sensitive detection of spin pumping via the ac inverse spin Hall effect

An intriguing feature of spintronics is the use of pure spin-currents to manipulate magnetization, e.g., spin-currents can switch magnetization in spin-torque MRAM, a next-generation DRAM alternative. Giant spin-currents via the spin Hall effect greatly expand the technological opportunities. Conversely, a ferromagnet/normal metal junction emits spin-currents under microwave excitation, i.e. spin-pumping. While such spin-currents are modulated at the excitation frequency, there is also a non-linear, rectified component that is commonly detected using the corresponding inverse spin Hall effect (iSHE) dc voltage. However, the ac component should be more conducive for quantitative analysis, as it is up to two orders of magnitude larger and linear. But any device that uses the ac iSHE is also sensitive to inductive signals via Faraday's Law and discrimination of the ac iSHE signal must rely on phase-sensitive measurements. We use the inductive signal as a reference for a quantitative measurement of the magnitude and phase of the ac iSHE.

cond-mat.mes-hall↗
arXiv2013

Precise determination of the spectroscopic g-factor using broadband ferromagnetic resonance spectroscopy

We demonstrate that the spectroscopic g-factor can be determined with high precision and accuracy by broadband ferromagnetic resonance measurements and applying an asymptotic analysis to the data. Spectroscopic data used to determine the g-factor is always obtained over a finite range of frequencies, which can result in significant errors in the fitted values of the spectroscopic g-factor. We show that by applying an asymptotic analysis to broadband datasets, precise values of the intrinsic g-factor can be determined with errors well below 1 %, even when the exact form of the Kittel equation (which describes the relationship between the frequency and resonance field) is unknown. We demonstrate this methodology with measured data obtained for sputtered Ni80Fe20 ("Permalloy") thin films of varied thicknesses, where we determine the bulk g-factor value to be 2.109 +/- 0.003. Such an approach is further validated by application to simulated data that includes both noise and an anisotropy that is not included in the Kittel equation that was used in the analysis. Finally, we show a correlation of thickness and interface structure to the magnitude of the asymptotic behavior, which provide insight into additional mechanisms that may induce deviations from the Kittel equation.

cond-mat.mes-hall↗
arXiv2013

Mode- and size-dependent Landau-Lifshitz damping in magnetic nanostructures: Evidence for non-local damping

We demonstrate a strong dependence of the effective damping on the nanomagnet size and the particular spin-wave mode that can be explained by the theory of intralayer transverse-spin-pumping. The effective Landau-Lifshitz damping is measured optically in individual, isolated nanomagnets as small as 100 nm. The measurements are accomplished by use of a novel heterodyne magneto-optical microwave microscope with unprecedented sensitivity. Experimental data reveal multiple standing spin-wave modes that we identify by use of micromagnetic modeling as having either localized or delocalized character, described generically as end- and center-modes. The damping parameter of the two modes depends on both the size of the nanomagnet as well as the particular spin-wave mode that is excited, with values that are enhanced by as much as 40% relative to that measured for an extended film. Contrary to expectations based on the ad hoc consideration of lithography-induced edge damage, the damping for the end-mode decreases as the size of the nanomagnet decreases. The data agree with the theory for damping caused by the flow of intralayer transverse spin-currents driven by the magnetization curvature. These results have serious implications for the performance of nanoscale spintronic devices such as spin-torque-transfer magnetic random access memory.

cond-mat.mes-hall↗
arXiv2013

Measurement of orbital asymmetry and strain in Co90Fe10/Ni multilayers and alloys: Origins of perpendicular anisotropy

We use broadband ferromagnetic resonance spectroscopy and x-ray diffraction to investigate the fundamental origin of perpendicular anisotropy in Co90Fe10/Ni multilayers. By careful evaluation of the spectroscopic g-factor, we determine the orbital moment along the out-of-plane and in-plane directions. For the multilayers, we find a direct relationship between the orbital moment asymmetry and the perpendicular anisotropy, consistent with the theory of Bruno [P.Bruno, Phys. Rev. B, 39, 865 (1989)]. A systematic x-ray diffraction study revealed the presence of a trigonal strain as high as 0.7 % in some samples. However, we found no direct correlation between the strain and the anisotropy, indicating that the anisotropy is not dominated by magnetoelastic effects. In order to further study the interface structure on the anisotropy, we prepared a set of equivalent alloy samples. The strain in the alloy samples was comparable to that of the multilayer samples; however the orbital moment asymmetry in the alloy samples showed a very different trend allowing us to isolate the effect of the interfaces in the multilayers.

cond-mat.mtrl-sci↗