Spin Pumping Efficiency Research Articles

Unconventional exchange bias and enhanced spin pumping efficiency due to diluted magnetic oxide at the Co/ZnO interface

Exchange bias (EB) is normally created by the interfacial exchange coupling at a ferromagnetic/antiferromagnetic (FM/AFM) interface. FM/AFM interfaces have also been proved to perform enhanced spin angular momentum transfer efficiency in spin pumping (SP), compared with typical FM/nonmagnetic interfaces. Here, we report an unexpected EB and enhanced SP between a ferromagnet and semiconductor. Considerable EB has been observed in Co films grown on ZnO single crystal due to the interface antiferromagnetism of the Zn1−xCoxO (x depends on the Co solubility limit in ZnO) layer. Moreover, SP measurements demonstrate a giant spin pumping efficiency at the Co/ZnO interface with a bump (spin mixing conductance Geff↑↓= 28 nm−2) around the blocking temperature TB ∼ 75 K. The enhanced SP is further confirmed by inverse spin Hall effect measurements and the spin Hall angle θISHE of Zn1−xCoxO is estimated to be 0.011. The bound magnetic polarons with s–d exchange interaction between donor electrons and magnetic cation ions in Zn1−xCoxO play a key role in the formation of antiferromagnetism with giant Geff↑↓. Our work provides a new insight into spin physics at FM/semiconducting interfaces.

Thermal and Coherent Spin Pumping by Noncollinear Antiferromagnets.

Antiferromagnets attract much interest because of their potential for spintronic applications and open fundamental physics questions, but especially noncollinear antiferromagnets remain relatively unexplored. Here, we formulate the thermal and coherent pumping of spins in noncollinear antiferromagnets|normal metal bilayers. We find that the spin current polarization is a vector with components along both the Néel vector and net magnetic moment. The spin mixing conductance for the coherent spin pumping is a tensor with elements depending on the degree of noncollinearity and interface spin configuration. We explain the controversial sign problem of the antiferromagnetic spin Seebeck effect by interface effects and suggest that interface engineering may enhance the spin pumping efficiency.

Epitaxial Fe/Rh bilayers for efficient spin-to-charge conversion

To address the spin pumping in the conventional ferromagnetic/“normal” metal systems, we fabricated 6 nm Fe/1–15 nm Rh epitaxial bilayers and determined the g-factor, magnetic anisotropy, and magnetization damping by combining both 0–40 GHz CPW-based frequency-dependent and cavity-based 9.56 GHz in-plane angular-dependent ferromagnetic resonance measurements at room temperature. Auger electron spectroscopy and low-energy electron diffraction show that Rh grows epitaxially on Fe. The epitaxial bilayers exhibit a high spin mixing conductance gmix↑↓=(2.9±0.2)×1019 m−2 and a spin diffusion length in Rhodium λsd=9.0±1.3 nm. This makes Rh comparable to Pt and Pd in terms of spin pumping and spin transport efficiency at room temperature.

Chemical Approach Towards Broadband Spintronics on Nanoscale Pyrene Films

AbstractThe injection of pure spin current into the non‐magnetic layer plays a crucial role in transmitting, processing, and storing data information in the realm of spintronics. To understand broadband molecular spintronics, pyrene oligomer film (≈20 nm thickness) was prepared using an electrochemical method forming indium tin oxide (ITO) electrode/pyrene covalent interfaces. Permalloy (Ni80Fe20) films with different nanoscale thicknesses were used as top contact over ITO/pyrene layers to estimate the spin pumping efficiency across the interfaces using broadband ferromagnetic resonance spectra. The spintronic devices are composed of permalloy/pyrene/ITO orthogonal configuration, showing remarkable spin pumping from permalloy to pyrene film. The large spin pumping is evident from the linewidth broadening of 5.4 mT at 9 GHz, which is direct proof of spin angular momentum transfer across the interface. A striking observation is made with the high spin‐mixing conductance of ≈1.02×1018 m−2, a value comparable to the conventional heavy metals. Large spin angular moment transfer was observed at the permalloy‐pyrene interfaces, especially at the lower thickness of permalloy, indicating a strong spinterface effect. Pure spin current injection from ferromagnetic into electrochemically grown pyrene films ensures efficient broadband spin transport, which opens a new area in molecular broadband spintronics.

Chemical Approach Towards Broadband Spintronics on Nanoscale Pyrene Films.

The injection of pure spin current into the non-magnetic layer plays a crucial role in transmitting, processing, and storing data information in the realm of spintronics. To understand broadband molecular spintronics, pyrene oligomer film (~20 nm thickness) was prepared using an electrochemical method forming indium tin oxide (ITO) electrode/pyrene covalent interfaces. Permalloy (Ni80Fe20) films with different nanoscale thicknesses were used as top contact over ITO/pyrene layers to estimate the spin pumping efficiency across the interfaces using broadband ferromagnetic resonance spectra. The spintronic devices are composed of permalloy/pyrene/ITO orthogonal configuration, showing remarkable spin pumping from permalloy to pyrene film. The large spin pumping is evident from the linewidth broadening of 5.4 mT at 9 GHz, which is direct proof of spin angular momentum transfer across the interface. A striking observation is made with the high spin-mixing conductance of ~1.02 1018 m-2, a value comparable to the conventional heavy metals. Large spin angular moment transfer was observed at the permalloy-pyrene interfaces, especially at the lower thickness of permalloy, indicating a strong spinterface effect. Pure spin current injection from ferromagnetic into electrochemically grown pyrene films ensures efficient broadband spin transport, which opens a new area in molecular broadband spintronics.

Magnetic Damping and Dzyaloshinskii-Moriya Interactions in Pt/Co2FeAl/MgO Systems Grown on Si and MgO Substrates.

Spin-pumping-induced damping and interfacial Dzyaloshinskii-Moriya interaction (iDMI) have been studied in Pt/Co2FeAl/MgO systems grown on Si or MgO substrates as a function of Pt and Co2FeAl (CFA) thicknesses. For this, we combined vibrating sample magnetometry (VSM), microstrip ferromagnetic resonance (MS-FMR), and Brillouin light scattering (BLS). VSM measurements of the magnetic moment at saturation per unit area revealed the absence of a magnetic dead layer in both systems, with a higher magnetization at saturation obtained for CFA grown on MgO. The key parameters governing the spin-dependent transport through the Pt/CFA interface, including the spin mixing conductance and the spin diffusion length, have been determined from the CFA and the Pt thickness dependence of the damping. BLS has been used to measure the spin wave non-reciprocity via the frequency mismatch between the Stokes and anti-Stokes lines. iDMI has been separated from the contribution of the interface perpendicular anisotropy difference between Pt/CFA and CFA/MgO. Our investigation revealed that both iDMI strength and spin pumping efficiency are higher for CFA-based systems grown on MgO due to its epitaxial growth confirmed by MS-FMR measurements of the in-plane magnetic anisotropy. This suggests that CFA grown on MgO could be a promising material candidate as a spin injection source via spin pumping and for other spintronic applications.

Heavy metal deposition temperature tuned spin pumping efficiency control in permalloy/tantalum bilayers

Spin pumping is a key property for spintronic application that can be realized in heavy metal/ferromagnet bilayers. Here we demonstrate the possibility of improving spin pumping in permalloy (Py)/tantalum (Ta) bilayers through control of Ta heavy metal deposition temperature. Through a combination of structural and ferromagnetic resonance based magnetization dynamics study, we reveal the role of Ta deposition temperature in improving spin mixing conductance which is a key parameter for spin pumping across the Py/Ta interface. The results show that by depositing Ta above room temperature, a high spin mixing conductance of 7.7 ×1018 m−2 is obtained with α-Ta layer. The results present an understanding of the correlation between heavy metal deposition temperature and interface structure improvement and consequent control of spin pumping in Py/Ta bilayers.

Spin-pumping investigations in as-deposited and 400 °C annealed Co60Fe20B20/Mo heterostructures

The spin-pumping behavior in the as-deposited and post-deposition annealed (at 400 °C) Co60Fe20B20/Molybdenum (CoFeB/Mo) heterostructures is investigated. It is found that while in both the as-deposited and annealed CoFeB/Mo heterostructures, Mo crystallizes in the form of mixed phases, i.e., body centered cubic (bcc) and face centered cubic (fcc), the dominance, however, changes from fcc to bcc on annealing. The ferromagnetic resonance measurements reveal that despite relatively low spin orbit coupling of Mo, the spin-pumping efficiency and magnetic properties in both the as-deposited as well as in the annealed heterostructures was comparable with those reported in similar CoFeB based heterostructures comprising of other nonmagnetic metals such as W and Ta. The spin efficiency parameters are, however, slightly better when the Mo film is predominantly in the fcc phase as compared to the case when the Mo film dominantly comprises of the bcc phase. The different spin-pumping efficiencies in the two different combinations of phases of Mo are attributed to the different band structures and the density of states in different phases of Mo, as confirmed by density functional theory calculations. The slight changes observed in the spin-pumping response are possibly attributed to the structural manifestations that result at the interface that the two phases of Mo share with CoFeB. Importantly, despite the weak spin–orbit interaction, the 4d transition metal Mo could be a suitable choice of non-magnetic material for spin pumping when the thermal stability of different layers in the CoFeB based spintronic devices is of paramount concern.

Spin pumping at interfaces with ferro- and paramagnetic Fe60Al40 films acting as spin source and spin sink

We present a study of spin pumping efficiency and determine the spin mixing conductance and spin diffusion length in thin bilayer films based on 3d transition metal alloy Fe60Al40. Due to its magnetostructural phase transition, Fe60Al40 can be utilized as a ferromagnetic (FM) or paramagnetic (PM) material at the same temperature depending on its structural order; thus a thin Fe60Al40 film can act as a spin source or a spin sink when interfaced with a paramagnet or a ferromagnet, respectively. Ferromagnetic resonance measurements were performed in a frequency range of 5–35 GHz on bilayer films composed of FM–Fe60Al40/Pd and PM–Fe60Al40/Ni80Fe20 (permalloy). The increase in damping with the thickness of the paramagnetic layer was interpreted as a result of spin pumping into the paramagnet. We determine the spin mixing conductance gPd↑↓=(3.8±0.5)×1018m−2 at the FM–Fe60Al40/Pd interface and the spin diffusion length λPd=9.1±2.0nm in Pd. For the PM–Fe60Al40/permalloy interface, we find a spin mixing conductance gFeAl↑↓=(2.1±0.2)×1018m−2 and a spin diffusion length λFeAl=11.9±0.2nm for PM–Fe60Al40. The demonstrated bi-functionality of the Fe60Al40 alloy in spin pumping structures may be promising for spintronic applications.

Crystallographic direction related spin current transmission in MgO(001)/Fe0.79Si0.21(001)/Pt(111) epitaxial bilayers

Spin-pumping and spin-orbit torque efficiencies in ferromagnetic/heavy metal bilayers, which usually consist of the polycrystalline grains, are related to the spin current transmission from ferromagnets to heavy metals and vice versa, respectively. In this work, epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}(001)/\mathrm{Pt}(111)$ films were fabricated to investigate the spin-pumping efficiency and spin-orbit torque efficiency by ferromagnetic resonance and harmonic Hall resistance measurement techniques, respectively. Ferromagnetic resonance results show that the incremental magnetic damping constants in epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}(001)/\mathrm{Pt}(111)$, compared with those of the epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}(001)/\mathrm{Al}{\mathrm{O}}_{x}$ reference sample, depend on whether the external magnetic field is applied along the in-plane easy or hard axes of ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}$. The Pt thickness-dependent anisotropic damping constant was ascribed to the anisotropic spin current absorption in epitaxial Pt(111) layer. When electric currents were applied along the easy and hard axes of ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}$ in epitaxial ${\mathrm{Fe}}_{0.79}{\mathrm{Si}}_{0.21}(001)/\mathrm{Pt}(111)$ bilayers, a large difference between spin-orbit torques generated from Pt(111) was observed by a harmonic Hall resistance measurement method. Both of the results suggest that spin current transmission efficiency is related to the anisotropic spin Hall effect of epitaxial Pt(111) layer due to the different spin-orbit interaction energies along the different crystallographic directions of Pt.

Impact of interfacial chemical state on spin pumping and inverse spin Hall effect in YIG/Pt hybrids

The interface between ferromagnetic (FM) and nonmagnetic (NM) metal plays a fundamental role in the generation, transmission, detection of spin current, and spin-based devices. The impact of physical properties on the interface such as roughness, crystallization conditions, and even impedance matching has been widely investigated. In this work, the effects of interfacial chemical state at the interface of ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ and Pt (YIG/Pt) are systematically investigated through spin injection. Prior to Pt deposition, the spin mixing conductance and inverse spin Hall voltage of YIG/Pt hybrids increased up to 300%, using surface chemical processing on YIG. The interfacial chemical state is identified from x-ray photoelectron spectroscopy spectra and peak decomposition. Both ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ cations appear at the interface and higher content of ${\mathrm{Fe}}^{3+}$ enhances the interface magnetic moment which consequently plays a crucial role in spin injection efficiency and inverse spin Hall voltage. In order to prevent the influence of magnetic proximity effect of Pt layers close to the FM/NM interface, a Cu spacer is inserted. It is found that the improved spin injection efficiency due to iron cations still exists in the YIG/Cu/Pt structure, which confirms the universal influence of the interfacial chemical state on spin pumping efficiency. This discovery has potential implications for spin generation, detection, and even optimization of spintronic devices.

Nonsymmetric spin pumping in a multiferroic heterostructure

We present spin pumping experiments in Co/Pt bilayers grown on Al2O3(0001) and on ferroelectric Y-cut LiNbO3 substrates. Spin pumping is triggered by resonant ferromagnetic resonance induced by surface acoustic waves. We observe that spin pumping efficiency varies when both the magnetization vector and the acoustic wave vector are reversed in Pt/Co/LiNbO3. This phenomenon is not observed in Pt/Co/Al2O3. We propose that the in-plane electric polarization of LiNbO3 is at the origin of the observed phenomenon. These observations open up the perspective of an electric field control of spin pumping efficiency.

Spin Wave Excitation, Detection, and Utilization in the Organic-Based Magnet, V(TCNE)x (TCNE = Tetracyanoethylene).

Spin waves, quantized as magnons, have low energy loss and magnetic damping, which are critical for devices based on spin-wave propagation needed for information processing devices. The organic-based magnet [V(TCNE)x ; TCNE = tetracyanoethylene; x ≈ 2] has shown an extremely low magnetic damping comparable to, for example, yttrium iron garnet (YIG). The excitation, detection, and utilization of coherent and non-coherent spin waves on various modes in V(TCNE)x is demonstrated and show that the angular momentum carried by microwave-excited coherent spin waves in a V(TCNE)x film can be transferred into an adjacent Pt layer via spin pumping and detected using the inverse spin Hall effect. The spin pumping efficiency can be tuned by choosing different excited spin wave modes in the V(TCNE)x film. In addition, it is shown that non-coherent spin waves in a V(TCNE)x film, excited thermally via the spin Seebeck effect, can also be used as spin pumping source that generates an electrical signal in Pt with a sign change in accordance with the magnetization switching of the V(TCNE)x . Combining coherent and non-coherent spin wave detection, the spin pumping efficiency can be thermally controlled, and new insight is gained for the spintronic applications of spin wave modes in organic-based magnets.

Electrical detection of magnon-photon interactions via an auxiliary spin-wave mode

We report on the electrical detection of a hybrid magnon-photon system, which is comprised of a magnetic sample coupled to a planar cavity. While the uniform Kittel mode has the largest coupling strength among all the magnon modes, it only generates a modest voltage signal by means of inverse spin-Hall effect. We have found that the generated voltage can be significantly enhanced by introducing a higher order magnon mode, which possesses a much higher spin pumping efficiency and furthermore, it is nearly degenerated with the Kittel mode. The experimental results can be explained by our theoretical model, and suggest that the use of an auxiliary magnon mode can realize the configuration of a magnon-photon system with both strong coupling and large spin current.

The effect of graphene interlayer at Pt/YIG interface on spin pumping

Spin pumping and the spin transfer torque in the normal metal (NM)/ferromagnetic materials (FM) heterostructure strongly depend on the interfacial spin memory loss. Therefore, the interface engineering of the NM/FM bilayer is important for enhancing spin pumping and spin-orbit torque efficiency for spin-orbitronic devices.In this study, we investigate the influence of the graphene interlayer between Pt and yttrium iron garnet (YIG) on spin pumping. Measurements of both the coplanar wave guide ferromagnetic resonance and inverse spin Hall effect with ferromagnetic resonance are performed to study the role of graphene in the transmission of the spin current at the interface. Results indicate a decreased Gilbert damping constant with the insertion of graphene interlayer, which causes the spin mixing conductance and inverse spin Hall voltage to be significantly reduced. The observed decrease in damping constant and spin injection is attributed to the presence of magnetic proximity effect and interfacial spin-orbit scattering, which causes graphene to serve as a spin-sink layer.

All-optical investigation of anisotropic spin pumping in W/CoFeB/W heterostructure

Pure spin current generation and transport in magnetic heterostructures comprising of normal metal (NM)/ferromagnet (FM) interfaces are imperative for future energy efficient spintronic devices. The above properties are considered to be equivalent in terms of spin-mixing conductance and spin pumping efficiency. Correspondingly, the enhancement in Gilbert damping should be equal for every interface with same NM and FM layers. However, this equivalency may break down under certain circumstances, giving rise to anisotropy. Here, we have shown by time-resolved magneto-optical Kerr effect measurements that for W/CoFeB interface, the enhancement in Gilbert damping is dissimilar when the position of W and CoFeB are interchanged. The damping coefficient (α) is 0.010 for CoFeB/W, whereas it is 0.0135 for W/CoFeB. Our in-depth investigation reveals that the precessional trajectory for W/CoFeB system is strongly affected by the induced magnetic anisotropy which is not present in CoFeB/W, leading towards the observed directional spin pumping. This is clearly demonstrated in W/CoFeB/W trilayer where the spin current generated by spin pumping diffuses more towards W underlayer in comparison to W overlayer. Our findings add new insights in the role of ferromagnet/nonmagnet interfaces for controlled generation and utilization of pure spin current for application in advanced spintronic devices.

Parametric spin pumping into an antiferromagnetic insulator

We report the observation of spin transport through antiferromagnetic insulators using spin pumping driven by short-wavelength magnons. The short-wavelength magnons are excited by parametric pumping in a $\mathrm{Pt}/\mathrm{NiO}/{\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ trilayer. The parametrically excited magnons in the ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ layer inject a spin current into the NiO layer, which is detected electrically using the inverse spin Hall effect in the Pt layer. We found that the spin-pumping efficiency of the parametrically excited magnons in the $\mathrm{Pt}/\mathrm{NiO}/{\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ film is nearly an order of magnitude smaller than that in the $\mathrm{Pt}/{\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ film. The suppression of the parametric spin-pumping efficiency induced by inserting the NiO layer is comparable to that of the FMR spin-pumping efficiency. We further found that the suppression of the spin-pumping efficiency due to the NiO insertion for the dipole-exchange magnons with the wave number of $k\ensuremath{\sim}{10}^{3}\ensuremath{-}{10}^{4}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ is more significant than that for the exchange magnons with $k\ensuremath{\sim}{10}^{5}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. This difference is attributed to two-magnon scattering and spin pinning due to antiferromagnetic grains in the NiO layer. These results provide insight into the understanding of the spin pumping into antiferromagnetic insulators.

Abrikosov vortex nucleation and its detrimental effect on superconducting spin pumping in Pt/Nb/Ni80Fe20/Nb/Pt proximity structures

We report Abrikosov vortex nucleation in Pt/Nb/Ni80Fe20/Nb/Pt proximity-coupled structures under oblique ferromagnetic resonance (FMR) that turns out to be detrimental to superconducting spin pumping. By measuring an out-of-plane field-angle {\theta}H dependence and comparison with Pt-absent control samples, we show that as {\theta}H increases, the degree of enhancement (suppression) of spin pumping efficiency in the superconducting state for the Pt-present (Pt-absent) sample diminishes and it reverts to the normal state value at {\theta}H = 90{\deg}. This can be explained in terms of a substantial out-of-plane component of the resonance field for the Ni80Fe20 layer (with in-plane magnetization anisotropy and high aspect ratio) that approaches the upper critical field of the Nb, turning a large fraction of the singlet superconductor volume into the normal state.

Dependence of spin pumping in W/CoFeB heterostructures on the structural phase of tungsten

We report the systematic dependence of spin pumping in tungsten (W)/CoFeB heterostructures on the structural phase of W, which is intricately related to argon gas pressure (${p}_{\mathrm{Ar}}$) maintained during the sputter deposition. We found that with increasing ${p}_{\mathrm{Ar}}$ the structural phase of W changes from mixed ($\ensuremath{\alpha}+\ensuremath{\beta}$) phase to pure $\ensuremath{\beta}$ phase. The $\ensuremath{\beta}$-W is stabilized in films for the high thickness of 40 nm which is desirable for spin devices. Using ferromagnetic resonance measurement of W(${p}_{\mathrm{Ar}}$)/CoFeB heterostructures, it is shown that enhancement of magnetic damping (${\ensuremath{\alpha}}_{\mathrm{eff}}$) from spin pumping is more in $\ensuremath{\beta}$-W compared to ($\ensuremath{\alpha}+\ensuremath{\beta}$)-W. The effective spin mixing conductance (${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$) is estimated for different phases of W from the linear evolution of ${\ensuremath{\alpha}}_{\mathrm{eff}}$ with the inverse thickness of the CoFeB layer. For $\ensuremath{\beta}$-W, the ${g}_{\mathrm{eff}}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$ is found to be larger than that of ($\ensuremath{\alpha}+\ensuremath{\beta}$)-W and it is attributed to different interface structure. Thus, effective tuning of spin pumping efficiency can be achieved using different W crystal phases. We also studied the dependence of ${\ensuremath{\alpha}}_{\mathrm{eff}}$ on $\ensuremath{\beta}$-W film thickness to calculate the value of spin-diffusion length (${\ensuremath{\lambda}}_{\mathrm{SD}}$) and intrinsic spin mixing conductance (${g}_{\ensuremath{\beta}\text{\ensuremath{-}}W}^{\ensuremath{\uparrow}\ensuremath{\downarrow}}$) using both the ballistic and diffusive spin transport models.

Exchange-field enhancement of superconducting spin pumping

A recent ferromagnetic resonance study [Jeon et al., Nat. Mater. 17, 499 (2018)] has reported that spin pumping into a singlet superconductor (Nb) can be greatly enhanced over the normal state when the Nb is coupled to a large spin-orbit-coupling (SOC) spin sink such as Pt. This behavior has been explained in terms of the generation of spin-polarized triplet supercurrents via SOC at the Nb/Pt interface, acting in conjunction with a nonlocally induced magnetic exchange field. Here we report the effect of adding a ferromagnet (Fe) to act as an internal source of an additional exchange field to the adjacent Pt spin sink. This dramatically enhances the spin pumping efficiency in the superconducting state compared with either Pt and Fe separately, demonstrating the critical role of the exchange field in generating superconducting spin currents in the Nb.