Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B28: Focus Session: YIG based systems |
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Sponsoring Units: GMAG DMP FIAP Chair: Mathias Weiler, Walther Meissner Institute Room: 205 |
Monday, March 2, 2015 11:15AM - 11:27AM |
B28.00001: Investigation on spin-orbit torque induced perpendicular switching through voltage controlled magnetism Chong Bi, Meng Xu, Marcus Rosales, Ty Newhouse-Illige, Hamid Almasi, Weigang Wang Spin-orbit torques are shown to induce perpendicular magnetic switching in ultrathin ferromagnets (FMs) adjacent to heavy metals (HMs). Here, we demonstrated that the critical current density ($J_{c})$ for such perpendicular switching in HM/FM/oxide structures can be dramatically modulated by gate voltage induced reversible oxidation at FM/oxide interfaces [1]. Through controlling perpendicular anisotropy and saturation magnetization ($M_{s})$ of FM layer, respectively, we show $M_{s}$, rather than anisotropy field as suggested in macrospin model [2], dominates $J_{c}$. Moreover, the measured external field dependent $J_{c}$ results indicate that the spin-orbit torques have either a bulk or interface origin under different magnetization states. These results not only provide a promising means toward energy-efficient switching, but also offer further insights in understanding the reversal mechanism of the ferromagnetic layer. This work was supported in part by NSF (ECCS-1310338) and by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA.[1] C. Bi et al, submitted, [2] L. Liu et al. Phys. Rev. Lett. 109, 096602 (2012). [Preview Abstract] |
Monday, March 2, 2015 11:27AM - 11:39AM |
B28.00002: Magnon-phonon interactions and spin transport in insulators Stephen R. Boona, Hyungyu Jin, Joseph P. Heremans The spin Seebeck effect (SSE) is now a well established phenomenon whereby a spin current can be thermally pumped from a ferromagnetic (FM) material into a normal metal (NM), where the spin current is then converted into a transverse charge current through the inverse spin Hall effect. The most interesting feature of SSE is that it occurs even if the FM is an insulator, $e.g.$, yttrium iron garnet (YIG). Although the existence of the effect is well established, its microscopic origins are still not completely understood; the detailed nature of interactions between the elementary excitations (electrons, phonons, and magnons) that give rise to SSE are complex and strongly dependent on factors like materials selection and temperature. One particularly important issue that remains unclear is the role that magnon-phonon interactions play in generating spin currents, especially within the context of diffusive vs drag currents. This talk will address this question by discussing some of our recent experiments aimed at exploring the temperature and length dependence of thermal and spin transport phenomena in magnetically ordered insulators. [Preview Abstract] |
(Author Not Attending)
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B28.00003: Temperaturedependent spin Hall magnetoresistance in ferromagnetic insulator/normal metal hybrids Matthias Althammer, Sibylle Meyer, Stefan Gepr\"ags, Matthias Opel, Rudolf Gross, Sebastian T.B. Goennenwein Pure spin currents, i.e. the net flow of spin angular momentum without an accompanying charge current, represent a new paradigm for spin transport and spintronics. We have experimentally studied a new type of magnetoresistance effect, which arises from the interaction of charge and spin current flows in ferromagnetic insulator/normal metal hybrid structures. The resistance changes observed can be quantitatively traced back to the combined action of spin Hall and inverse spin Hall effect in the normal metal layer, and are thus termed spin Hall magnetoresistance (SMR) [1,2]. In more detail, we studied the temperature dependence of the SMR in yttrium iron garnet / platinum hybrid structures via magnetization orientation dependent magnetoresistance measurements. Our experiments show that the SMR amplitude decreases with decreasing temperature, which can be quantitatively modeled in terms of a spin Hall angle in platinum decreasing from 0.11 at 300K to 0.075 at 10K [3], while the spin diffusion length and the spin mixing conductance of the ferrimagnetic insulator / normal metal interface remain almost constant.\\[0pt] [1] Nakayama et al., PRL, \textbf{110}, 206601 (2013)\\[0pt] [2] Althammer et al., PRB, \textbf{87}, 224401 (2013)\\[0pt] [3] Meyer et al., APL, \textbf{104}, 242411 (2014) [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B28.00004: Spin Seebeck effect in YIG-based systems Gene Siegel, Megan Prestgard, Shiang Teng, Ashutosh Tiwari Recently, the use of magnetic insulators (yttrium iron garnet, YIG) in conjunction with platinum has sparked interest in spintronics research. This is due to the existence of the spin Seebeck effect which could potentially be a source of pure spin current for spintronic devices. Furthermore, these coatings could potentially show the versatility of spintronics by acting as a spin-based thermoelectric generator, thereby providing a new method of transforming heat into power. However, there remain questions regarding the origins and legitimacy of the spin Seebeck effect. Moreover, recent publications claim that the observed effects are a manifestation of magnetic proximity effects in platinum and not a true SSE signal. Because of these concerns, we are providing supporting evidence that the voltages observed in the YIG/Pt films are truly SSE voltages. We are reaffirming claims that magnon transport theory provides an accurate basis for explaining SSE behavior. Finally, we illustrate the advantages of pulsed laser deposition, as these YIG films possess a large SSE voltage compared to those films grown using liquid phase deposition techniques. [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B28.00005: Subthermal-magnon-driven longitudinal spin Seebeck effect in yttrium iron garnets (YIG) Hyungyu Jin, Stephen Boona, Zihao Yang, Roberto Myers, Joseph Heremans Since its discovery in 2008, the spin Seebeck effect (SSE) has intrigued many interesting research all around the world, which has led to the birth of a new field of research, called ``spin-caloritronics''. Of the two different experimental configurations used for detecting SSE, the longitudinal geometry (LSSE) seems to be generally accepted [1]. The yttrium iron garnet (YIG) / Pt bilayer structure has been most commonly used for LSSE experiments because absence of electrons in YIG excludes contaminations from other thermomagnetic effects. The dependence of the LSSE on YIG film thickness [2] and on temperature [3] have been reported, but not yet both together. Here we present experimental data on the temperature dependence of LSSE in Pt/YIG below room temperature in systems in which the thickness of YIG varies. Detailed discussion is given on the experimental results, with emphasis on the role of subthermal-magnons in the temperature dependence of LSSE in the YIG/Pt system. \\[4pt] [1] S. R. Boona et al., Energy Environ. Sci. 7, 885-910 (2014).\\[0pt] [2] A. Kehlberger et al., arXiv 1306.0784 (2013).\\[0pt] [3] K. Uchida et al., Phys. Rev. X 4, 041023 (2014). [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B28.00006: Spin Seebeck Effect in a Compensated Ferrimagnet Sebastian T.B. Goennenwein, S. Gepr\"{a}gs, A. Kehlberger, T. Schulz, C. Mix, F. della Coletta, S. Meyer, A. Kamra, G. Jakob, M. Althammer, H. Huebl, R. Gross, M. Kl\"{a}ui Thermal gradients allow for driving pure spin currents in electrically insulating magnetic materials. In magnetic insulator/normal metal heterostructures, such thermally driven spin currents can be electrically detected via the inverse spin Hall effect in the normal metal, in so-called spin Seebeck effect (SSE) experiments. We have fabricated Gadolinium Iron Garnet/Platinum (GdIG/Pt) thin film heterostructures, and measured the spin Seebeck effect in these samples as a function of temperature. We observe two sign changes as a function of T in the SSE signal. The first sign change occurs around the GdIG magnetic compensation temperature, and can be straightforwardly understood in terms of the reorientation of the iron sublattice magnetizations at this temperature. The second, more gradual SSE sign change takes place around the ordering temperature of the Gd magnetic sublattice, suggesting that the thermally driven spin current is mainly determined by the Gd sublattice at low T. Our results thus show that the SSE spin currents do not simply replicate the effective magnetization of the magnetic insulator, but rather reflect a complex interplay of magnetic sublattice properties. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 12:39PM |
B28.00007: Transverse Spin Seebeck Effect on YIG/Pt Arati Prakash, Stephen Boona, Hyungyu Jin, Joseph Heremans The existence of the longitudinal spin-Seebeck effect (LSSE) is well established and supported by theory. Much more controversial is the nature of the signals observed in the transverse spin-Seebeck (TSSE) geometry, where the heat current (x) is orthogonal to the direction of spin current propagation (y). TSSE has been described as simply non-local thermal spin-injection [1], but questions remain about the fact that the effect is observed at macroscopic length scales. To explore possible explanations for the observed TSSE signals, we report data from new TSSE measurements on the YIG/Pt system. The system studied has multiple Pt strips deposited in series upon bulk single crystals of YIG. We investigate the TSSE coefficient as a function of four variables: (1) sample temperature; (2) magnitude of the temperature gradient; (3) position of Pt strips along x; and (4) width of Pt strips along x. We consider nonlinear effects and the role of magnon density in the interpretation of our results. [1] Boona et al., Energy and Environ. Sci. 7 885 (2014) [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B28.00008: DC Rectification of Microwaves in YIG/Pt/Py Trilayers Joseph Sklenar, John Ketterson, Matthias Jungfleisch, Wanjun Jiang, Wei Zhang, John Pearson, Axel Hoffmann, Qinghui Yang, Qiye Wen, Huaiwu Zhang The DC voltage arising from the rectification of microwaves passing through a ferromagnet/spin Hall metal bilayer structure at ferromagnetic resonance is a powerful tool in understanding spin-orbit torques from spin Hall effects. Rectification mechanisms such as anisotropic magnetoresistance of the ferromagnetic or spin Hall magnetoresistance of the spin Hall metal can contribute depending on whether the ferromagnet is conductive or insulating. For both types of ferromagnets, spin pumping acting in concert with the inverse spin Hall effect can also generate additional DC voltages. We have studied rectification in a trilayer system of YIG/Pt/Py under conditions where both ferromagnets are simultaneously excited. By tipping the DC magnetic field out of the sample plane we can make the resonances of both ferromagnet materials degenerate. In this simultaneous resonance regime we observe an enhancement in the voltage of the YIG lineshape coming at the expense of the Py signal. Furthermore, at arbitrarily tipped out-of-plane tipping angles we observe asymmetries of the Py signal under field reversal. We compare this observation with the behavior of Py/Pt bilayer samples. [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B28.00009: Surface sensitivity of the spin Seebeck effect in the Pt/YIG system Aisha Aqeel, Ivan J. Vera-Marun, Bart J. van Wees, Thomas T.M. Palstra It is well-known that the surface plays an important role in the spin Seebeck effect (SSE) [1]. However the effect of mechanical treatment on the SSE has not been systematically studied yet. Here, we have investigated the influence of the interface quality on the SSE in a bilayer system of platinum and yttrium iron garnet (Pt/YIG). The surfaces of the YIG crystals are modified by different types of mechanical polishing before Pt deposition for different samples. We observed that the magnitude and magnetic field dependence of the SSE is strongly influenced by mechanical treatment of the YIG surface. No definite relation has been found between the SSE response and the sample roughness. However, we observe a direct correlation between the saturation magnetic field (H$_{sat})$ of the SSE and the roughness of sample, as the former increases by moving from soft toward coarse particle polishing. The change in the magnitude of H$_{sat}$ can be attributed to the presence of a perpendicular magnetic anisotropy due to the treatment induced surface strain or shape anisotropy in the Pt/YIG system [2]. \\[4pt] [1] G. E. W. Bauer, \textit{et al}., Nature Mater. \textbf{11}, 391, (2012).\\[0pt] [2] A. Aqeel \textit{et al}., J. Appl. Phys. \textbf{116}, 153705, (2014). [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B28.00010: Magnons and Phonons Driven out of Equilibrium in a Magnetic Insulator Kyongmo An, Kevin Olsson, Nikita Klimovich, Sean Sullivan, Annie Weathers, Luke Marshall, Xi Chen, Jianshi Zhou, Li Shi, Xiaoqin Li We investigate magnons and phonons in a bulk Y$_{3}$Fe$_{5}$O$_{12~}$(YIG) under a large temperature gradient created by laser radiation. YIG is a good playground to study the interaction between phonons and magnons. Because of its absence of itinerant electrons, energy transport is only carried by magnons and phonons. Understanding the coupling between them is a key to the thermally driven spin transport such as Spin Seebeck Effect. We use Brillouin light scattering technique to measure phonon and magnon temperature. We found that they can be driven out of equilibrium under a large temperature gradient. We numerically simulate the phonon and magnon temperatures using two-temperature model. Our results suggest a lower bound of magnon phonon relaxation time in YIG. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B28.00011: Engineering damping in insulating magnet-metal bilayers using ultrathin spacer layers Sriharsha V. Aradhya, Colin L. Jermain, Hanjong Paik, John T. Heron, Darrell G. Schlom, Daniel C. Ralph, Robert A. Buhrman Insulating magnetic materials, particularly yttrium iron garnet (YIG), are of significant interest for fundamental research as well as technological applications. Thus far copper spacer layers of $\sim$10 nm - 1 $\mu$m thickness sandwiched between YIG and heavy metal films have been shown to modulate the damping of the magnetic layer either higher or lower. We report on the effect of ultrathin nonmagnetic spacer layers on the damping of YIG with different heavy metal overlayers. We start with YIG films grown by oxide molecular beam epitaxy with thicknesses below 20 nm and Gilbert damping as low as 0.0005. We observe that a spacer layer can increase the damping by 50\% in YIG/spacer/Ta samples compared to YIG/Ta, and the increase can be as large 500\% for YIG/spacer/Pt compared to YIG/Pt. These observations suggest a significant increase in the effective spin mixing conductance at the YIG-heavy metal interface that might be used to improve the efficiency of the spin torque produced by the spin Hall effect. [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B28.00012: Low-temperature magnetization dynamic properties of yttrium iron garnet films Yue Zhao, Qi Song, Wei Yuan, Tang Su, Yong Wu, Yong Jiang, Wei Han, Jing Shi The long lifetime of collective spin excitations in yttrium iron garnet (YIG) has spurred intense interest in spintronics. Ferromagnetic resonance (FMR) is a sensitive tool for investigating the magnetization dynamics in YIG films as well as YIG-based heterostructure devices. Previous studies have been mainly focused on the room temperature magnetization dynamics of YIG thin films. In this talk, I will first describe a low-temperature broad-band FMR setup with a wide frequency range from 1 to 18 GHz incorporated in Quantum Design's Physical Properties Measurement System. Using this setup, we study the magnetization dynamic properties of the YIG thin films. In YIG films we observe an intriguing non-monotonic behavior in the temperature dependence of the FMR linewidth from which the Gilbert damping parameter can be extracted. Furthermore, various thicknesses of YIG films are studied to probe the underlying mechanisms that dominate the low-temperature magnetization dynamic behavior. Our temperature-dependence FMR results are very important for the understanding of the relaxation processes of YIG thin films. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B28.00013: Growth of high-quality nanometre-thick yittrium iron garnet by sputtering and their magnetic properties Arpita Mitra, Oscar Cespedes, Mannan Ali, B.J. Hickey Observation of Spin Seebeck effect(SSE) in magnetic insulators has led to dramatic advances in spin currents research and its applications for thermo-spintronics devices. Here we report deposition of high quality nm-thick yittrium iron garnet(YIG) film on gadolinium gallium garnet(GGG) by RF magnetron sputtering. The morphology and magnetic properties of the films were studied by using AFM and SQUID VSM respectively. 10-60 nm thick films have surface roughness of 1-3{\AA},and (111) orientation. Our results show that magnetic properties of YIG depend strongly on thickness: magnetic moment has linear dependence at room temperature. The saturation magnetization and coercive field observed in thick films are 136 emu/cc and 0.50 Oe, respectively. Temperature dependence of magnetization of nm-thick YIG films has revealed an interesting result,which can be attributed to an additional magnetic phase at the YIG/GGG interface. The reduction in magnetization at low temperatures up to now has not been reported, but has significant relevance to the spin hall magnetoresistance(SMR) at low temperature.Our results on the temperature dependence of Gilbert damping factor of YIG and YIG/Pt films will lead to new physics, to understand its effect on spin mixing conductance and SMR in magnetic insulators [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B28.00014: Modeling the Interface of a Heavy Metal/Ferromagnetic Bilayer Using the Boltzmann and Drift-Diffusion Equations Vivek Amin, Kyoung-Whan Kim, Kyung-Jin Lee, Hyun-Woo Lee, M.D. Stiles Accurate models for spin torques due to electrical currents in heavy metal/ferromagnetic bilayers allow for the meaningful extraction of parameters from measurements and optimization of devices.~Descriptions of coherent spin transport across magnetic interfaces used to date typically take the form of resistor-like components that neglect the spin-orbit interaction. Using the Boltzmann equation, we develop new boundary conditions for the drift-diffusion equations to include both interfacial spin-orbit coupling and the exchange interaction due to the magnetization.~These boundary conditions capture interfacial spin-flip scattering, coupling between longitudinal and transverse spin components, and significant modifications to the interfacial torques on the magnetization. They address the spin-orbit interaction at the interfaces of heavy metal/ferromagnetic bilayers, and allow for the calculation of interfacial spin-orbit torques within the drift-diffusion approach. [Preview Abstract] |
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