Session V19: Magnetic Oxide Thin Films and Heterostructures: Spin Seebeck Effects

Paramagnetic and Antiferromagnetic Spin Seebeck Effect

We report on the observation of the longitudinal spin Seebeck effect in both antiferromagnetic and paramagnetic insulators. By using a microscale on-chip local heater, it is possible to generate a large thermal gradient confined to the chip surface without a large increase in the total sample temperature. This technique allows us to easily access low temperatures (200 mK) and high magnetic fields (14 T) through conventional dilution refrigeration and superconducting magnet setups. By exploring this regime, we detect the spin Seebeck effect through the spin-flop transition in antiferromagnetic MnF$_{2}$ when a large magnetic field (\textgreater 9 T) is applied along the easy axis direction. Using the same technique, we are also able to resolve a spin Seebeck effect from the paramagnetic phase of geometrically frustrated antiferromagnet Gd$_{3}$Ga$_{5}$O$_{12}$ (gadolinium gallium garnet) and antiferromagnetic DyScO$_{3}$ (DSO). Since these measurements occur above the ordering temperatures of these two materials, short-range magnetic order is implicated as the cause of the spin Seebeck effect in these systems. The discovery of the spin Seebeck effect in these two materials classes suggest that both antiferromagnetic spin waves and spin excitations from short range magnetic order may be used to generate spin current from insulators and that the spin wave spectra of individual materials are highly important to the specifics of the longitudinal spin Seebeck effect. Since insulating antiferromagnets and paramagnets are far more common than the typical insulating ferrimagnetic materials used in spin Seebeck experiments, this discovery opens up a large new class of materials for use in spin caloritronic devices. All authors acknowledge support of the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. The use of facilities at the Center for Nanoscale Materials, was supported by the U.S. DOE, BES under contract No. DE-AC02-06CH11357.   

Spin Seebeck measurements of current-induced switching in YIG

Quantifying spin torques generated at the interface between a normal metal (NM) and a ferromagnetic insulator (FI) is an important step in understanding the spin hall effect without charge transport. Measuring magnetization in NM/FI devices is challenging, however, because both magnetoresistive and magneto-optical signals are tiny in thin-film bilayers. We show that a promising alternative measurement approach is the use of picosecond thermal gradients to study spin torques in Pt/Yttrium Iron Garnet (YIG) bilayers. Recently, we demonstrated the application of heat to stroboscopically transduce a local magnetic moment into an electrical signal via the time resolved anomalous Nernst effect (TRANE) in ferromagnetic metals [1]. Using a similar geometry the spin Seebeck effect of YIG combined with the inverse spin Hall effect of Pt enables measurement of local magnetization [2]. Here we describe our study using this technique to study current-induced switching in Pt/YIG with sub-10 nm thick YIG films. [1] Bartell et al., Nat. Commun. 6, 8460 (2015). [2] Weiler et al., Phys. Rev. Lett. 108, 106602 (2012).   

Irreversible Thermodynamics of Uniform Ferromagnets with Spin Accumulation: Bulk and Interface Phenomena

We extend the irreversible thermodynamics of uniform ferromagnets to include the non-equilibrium phenomenon of spin accumulation, both for conductors and for insulators. The dynamics of the quantization axis $\hat{M}$ is governed by the Landau-Lifshitz equation. The spin accumulation, whose longitudinal and transverse parts we label $\delta M$ and $\vec{m}$, is due to a non-equilibrium distribution of magnetic excitations. Its dynamics is governed by a Bloch equation that includes spin diffusion. We also consider transport across surfaces, including boundary conditions for $\hat{M}$, $\delta M$, and $\vec{m}$, and apply the results to the nature of the reciprocity between spin transfer torque and spin pumping.   

\textbf{Temperature dependences of magnetic anisotropy and longitudinal spin Seebeck effect in Y}$_{\mathbf{3}}$\textbf{Fe}$_{\mathbf{5}}$\textbf{O}$_{\mathbf{12}}$

Spin caloritronics is an emerging, exciting research area in condensed matter owing to its potential use in advanced spintronics devices. Pure spin current without having charge current has been achieved though spin Seebeck effect (SSE). Over the last 7 years SSE has been observed in ferromagnetic metals, insulators, and semiconductors using longitudinal and transverse SSE measurement configurations. In this work, we have carried out an experimental study to understand the effect of magnetic anisotropy on the temperature evolution of longitudinal spin Seebeck effect (LSSE) in a single crystalline yttrium iron garnet (YIG). The effective anisotropy field (H$_{K})$ and inverse spin Hall (ISH) voltage (V$_{ISH})$ were measured using the radio-frequency transverse susceptibility (TS) and LSSE configuration, respectively. The V$_{ISH}$ of a 15 nm Pt strip on (6*2*1 mm) YIG slab with a temperature gradient of 3 K was measured in the temperature range of 120 to 300 K. The observed values of V$_{ISH}$ vary from 1 microV for 120 K to 0.5 microV for 300 K, These values fall into the previously reported theoretical and experimental results. The temperature evolution of H$_{K\, }$has been compared with that of V$_{ISH}$ to gain better fundamental understanding.   

Investigation of the timescale of the spin-Seebeck effect in yttrium iron garnet from pico to nanoseconds

We investigate the timescale of the spin-Seebeck effect (SSE) in yttrium iron garnet (YIG) by exciting transient thermal gradients with 150-fs laser heating pulses. The transient thermal gradient generates a spin current which is measured by a Pt top contact via the inverse spin Hall-effect (ISHE). A pulse selection system is used to lower the repetition rate of the laser to low frequencies (e.g. 10 kHz) such that the transient thermal gradient decays completely before the arrival of the next pulse. Lock-in detection, referenced at the laser repetition rate, is used to measure ISHE as a function of magnetic field, verifying that SSE is generated from the individual ultrafast laser pulses. Next, utilizing an optical delay line we vary the time delay between two equal fluence pulses. The correlated ISHE signal is measured with lock-in detection as a function of delay time with 0.1 ps resolution out to 1 ns to examine the characteristic decay times of the ultrafast laser pulse induced spin-Seebeck effect.   

Heat Transport between Antiferromagnetic Insulators and Normal Metals

Antiferromagnetic insulators can become active spintronics components by controlling and detecting their dynamics via spin currents in adjacent metals. This cross-talk occurs via spin-transfer and spin-pumping, phenomena that have been predicted to be as strong in antiferromagnets as in ferromagnets. In a recent article,\footnote{A. Brataas, H. Skarsv{\aa}g, E. G. Tveten and E. L. Fj{\ae}rbu, arXiv:1506.06705 (2015)} we demonstrate that a temperature gradient drives a significant heat flow from magnons in antiferromagnetic insulators to electrons in adjacent normal metals. The same coefficients as in the spin-transfer and spin-pumping processes also determine the thermal conductance. However, in contrast to ferromagnets, the heat is not transferred via a spin Seebeck effect which is absent in antiferromagnetic insulator-normal metal systems. Instead, the heat is proportional to a large staggered spin Seebeck effect.   

Spin Nernst and torque effects in Dzyaloshinskii-Moriya ferromagnets.

We predict that a temperature gradient can induce a magnon-mediated intrinsic torque and a transverse spin current in ferromagnets with non-trivial magnon Berry curvature. With the help of a microscopic linear response theory of nonequilibrium magnon-mediated torques and spin currents we identify the interband and intraband components that manifest in ferromagnets with Dzyaloshinskii--Moriya interactions and magnetic textures. In addition to the torque and spin current, we also identify the mechanical torque effect in accordance with the conservation of angular momentum. To illustrate and assess the importance of such effects, we apply our theory to the magnon-mediated spin Nernst and torque responses in a kagome lattice ferromagnet.   

Spin Seebeck Effect Signals from Antiferromagnets

The Longitudinal Spin Seebeck Effect (LSSE), in which a heat current stimulates spin propagation across an interface between a magnetic material and a normal metal, is well established and observed in ferromagnetic systems [1]. Data have been presented indicating that antiferromagnetic systems could also give rise to LSSE signals [2]. We report here on LSSE signal measured on the Pt/NiO/YIG structure, where NiO is an antiferromagnet. This system is reported to exhibit antiferromagnonic transport [3]. We explore the dependence of the signal on the thickness of the NiO and YIG layers. We also report its temperature dependence, which was not explored before [3]. The results are interpreted in terms of the temperature dependence of the magnon density of states. It appears that magnon modes with energies below about 40 K are most involved in the process, as was the case to the LSSE on YIG itself [4]. Preliminary results using other antiferromagnets and other inverse spin-Hall layers look promising and will also be reported. [1] S. R. Boona et al., Energy Environ. Sci. 7 885-910 (2014) [2] Y. Ohnuma et al. Phys. Rev. B 87 014423 (2012) [3] H. Wang, Phys. Rev. Lett. 113, 097202 (2014) [4] Y. Jin et al., Phys. Rev. B Phys. Rev. B 92, 054436 (2015)   

Effects of thermal magnetic fluctuations on spin transport in Pt.

Despite extensive studies and applications of Pt as a spin Hall material in spintronic devices, its spin-dependent transport properties are still debated. We present a comprehensive experimental study of spin transport in Pt, utilizing measurements of giant magnetoresistance (GMR) in nanoscale Permalloy (Py)-based spin valves with Pt inserted in the nonmagnetic spacer. The spin diffusion length and the interfacial spin flipping coefficients are extracted from the dependence of MR on the Pt thickness. For samples with Pt separated from Py by Cu spacers, the spin diffusion length is 6 nm at 7K, and decreases to 3 nm at room temperature. The interfacial spin flipping decreases with increasing temperature, resulting in nonmonotonic temperature dependence of MR in samples with thin Pt. In contrast, in samples with Pt in direct contact with Py, we do not observe such a nonmonotonic dependence, and the spin diffusion length is significantly larger than in samples with Pt surrounded by Cu spacers. Our results indicate a large effect of the giant paramagnetic fluctuations in the nearly ferromagnetic Pt. These fluctuations are suppressed due to the proximity magnetism when Pt is in contact with Py, resulting in enhanced spin diffusion length and reduced spin flipping at the Pt interfaces. These observations indicate the need for a critical revision of spin transport and spin Hall-related properties of Pt-based structures.   

Magnon-drag contribution to the Nernst effect of single-crystal iron

The thermopower of single-crystal iron has recently been proven to be dominated by magnon-drag [1]. Experimental results align with hydrodynamic and microscopic theories [2] that underline the similarity between the magnon-drag charge Seebeck effect and the spin-Seebeck effect. Here, the results are expanded to the Nernst effect. The Nernst coefficient of iron is shown to be quite large and is expected to contain a contribution similar to the spin-Seebeck effect. In this case, it is present in the absence of a ferromagnet-normal metal interface or spin-orbit interactions. This talk will present a new model based on ambipolar transport. Spin-up and spin-down electrons are considered as charge carriers with separate magnon-drag Seebeck coefficients. The difference between these partial Seebeck coefficients leads to a large magnon-drag Nernst coefficient in the absence of a skew force. Furthermore, methods to increase the thermopower of iron while maintaining its magnon-drag effects will be presented with preliminary results. 1. S. J. Watzman et al., San Antonio APS March Meeting talk (2015) 2. M. E. Lucassen et al., Appl. Phys. Lett.\textbf{ 99} 262506 (2011)   

Time-domain measurement of spin-Seebeck effect as a function of temperature: interface magnon effect

Time-resolved longitudinal spin Seebeck effect (LSSE) measurements allow a means to separate the influence of thermally excited electrons, phonons and magnons on the detected spin current. In this study, we measured the time dependence of the LSSE signal in Pt/YIG structures using a high bandwidth oscilloscope and a modulated CW laser from 20 K to 300 K. The rise of the LSSE signal is sharp and not truncated indicating that the measurement is not limited by the bandwidth of the setup. The temporal profile of the LSSE signal consists of two distinct components, a fast rise (200 ns) and a slow rise. The fast component is temperature independent and roughly on par with the rise time of the modulated laser intensity, while the slow component does not saturate upto 50 $\mu $s. We model the temporal evolution of the LSSE signal by carrying out three-temperature 3D time domain heat diffusion finite element modeling of the magnon temperature gradient profile in YIG to determine the electron, magnon, and phonon temperature profile versus time. It is found that the magnon temperature gradient near the YIG interface exhibits the same fast rise time that is measured in the LSSE signal. We discuss implications for this measurement on the existing models of LSSE.   

Non-local thermal spin injection to study spin diffusion in yttrium iron garnet

Understanding the generation, detection, and manipulation of spin current is critical for the development of devices that depend on spin transport for information processing and storage. Recent studies have shown that spin transport over long distances is possible in the magnetic insulator yttrium iron garnet (YIG) through the diffusion of non-equilibrium magnons. Electrically excited magnons have been shown to diffuse up to 40um at room temperature [1], while thermally injected magnons were detected at ranges greater than 125um at 23K [2]. However, much work is still required to fully understand the processes responsible for magnon diffusion. Here, we present an in-depth study of the diffusion of magnons in YIG. By using the non-local thermal spin detection method [2], we analyze spin transport as a function of temperature. Spin diffusion maps, which can be used to experimentally determine the spin diffusion length in YIG as a function of temperature, are presented. [1] L. J. Cornelissen, \textit{et al.} Nat Phys (2015). [2] B. L. Giles, \textit{et al.} arXiv:1504.02808 [cond-Mat] (2015).