Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F40: Invited Session: New Magnetoresistance in Metal/Magnetic Insulator Heterostructures |
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Sponsoring Units: GMAG Chair: Sufeng Zhang, Arizona University Room: Mile High Ballroom 2B-3B |
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F40.00001: Theory of spin Hall magnetoresistance (SMR) and related phenomena Invited Speaker: Gerrit Bauer A new anisotropic magnetoresistance effect has recently been found for a Pt film on top of the insulating ferrimagnet Yttrium-Iron-Garnet (YIG) [1-6]. We interpret this effect by the simultaneous action of spin Hall and inverse spin Hall effects as a non-equilibrium proximity phenomenon dubbed spin Hall magnetoresistance (SMR). This mechanism does not require the equilibrium proximity magnetization in Pt, which was assumed in [5]. We computed the SMR in F\textbar N and F\textbar N\textbar F layered systems, where F is a magnetic insulator, treating the normal metal N by spin-diffusion theory with quantum mechanical boundary conditions at the interfaces in terms of the spin-mixing conductance [7]. Our results explain the experimentally observed spin Hall magnetoresistance in F\textbar N bilayers. An analysis of the Hall effect when magnetization is normal to the plane allowed the experimental observation of the imaginary part of the mixing conductance [4]. For F\textbar N\textbar F spin valves we predict enhanced SMR amplitudes when magnetizations are collinear. In this talk I review the state of the art and discuss recent extensions of the SMR theory. \\[4pt] [1] H. Nakayama et al., Phys. Rev. Lett. 110, 206601 (2013).\\[0pt] [2] C. Hahn, Phys. Rev. B 87, 174417 (2013).\\[0pt] [3] M. Althammer et al., Phys. Rev. B 87, 224401 (2013).\\[0pt] [4] N. Vlietstra, et al., Appl. Phys. Lett. 103, 032401 (2013).\\[0pt] [5] Y. M. Lu et al., Phys. Rev. B 87, 220409 (2013).\\[0pt] [6] M. Isasa et al., arXiv:1307.1267\\[0pt] [7] Y. Chen et al., Phys. Rev. B 87, 144411 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F40.00002: Spin Hall magnetoresistance in ferromagnetic insulator/normal metal hybrids Invited Speaker: Matthias Althammer 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. In more detail, we measured the resistance of yttrium iron garnet(YIG)/Pt, YIG/nonferromagnet/Pt, nickel ferrite/Pt, and magnetite/Pt hybrid structures as a function of the magnitude and the orientation of an external magnetic field. The resistance changes observed can be quantitatively traced back to the combined action of spin Hall and inverse spin Hall effect in the Pt metal layer, and are thus termed spin Hall magnetoresistance (SMR) $[1,2]$. We show that the SMR is qualitatively different from the conventional anisotropic magnetoresistance effect arising in magnetic metals. From the magnetoresistance measurements in YIG/Au/Pt and YIG/Cu/Pt structures and from x-ray magnetic circular dichroism measurements on YIG/Pt heterostructures we exclude a static proximity magnetization in Pt as the origin of the magnetoresistance, in contrast to the mechanism proposed by Huang et al.~$[3]$. Furthermore, the SMR enables us to quantify the spin Hall angle as a function of temperature in our Pt layers. In addition, we analyze the anomalous Hall type contribution of the SMR to quantify the imaginary part of the spin mixing conductance. Financial support by the DFG via SPP 1538 (project no. GO 944/4) and the Nanoinitiative Munich (NIM) is gratefully acknowledged.\\[4pt] [1] Nakayama et al., PRL, \textbf{110}, 206601 (2013)\\[0pt] [2] Althammer et al., PRB, \textbf{87}, 224401 (2013)\\[0pt] [3] Huang et al., PRL, \textbf{109}, 107204 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F40.00003: Hybrid magnetoresistance in the proximity of a ferromagnet Invited Speaker: Chia-Ling Chien A new type of magnetoresistance (MR) effect has recently been observed in nominally nonmagnetic metal (Pt) thin films in contact with either a ferromagnetic (FM) insulator such as yttrium iron garnet (YIG),\footnote{H. Nakayama \textit{et al., }Phys. Rev. Lett. \textbf{110}, 206601(2013).} or a FM metal,\footnote{Y. M. Lu \textit{et al,,} Phys. Rev. B \textbf{87}, 220409(R) (2013).} such as permalloy (Py). The resistivities with in-plane magnetic fields parallel ($\rho _{\mathrm{\parallel }})$ and transverse ($\rho_{\mathrm{T}})$ to a current and a perpendicular field ($\rho_{\mathrm{\bot }})$ at room temperature show the behavior of $\rho_{\mathrm{\bot }}\approx \rho _{\mathrm{\parallel }}$\textgreater $\rho_{\mathrm{T}}$, distinctively different from all other known MR effects, including the well-known anisotropic MR in FMs of $\rho_{\mathrm{\parallel }}$\textgreater $\rho _{\mathrm{T}}\approx \rho_{\mathrm{\bot }}$. The key question is whether the new MR is the proposed spin Hall MR (SMR) based on spin current conversion in Pt, or due to magnetic proximity effects (MPE), for which Pt is highly susceptible when in contact with a FM. Recent experiments show that the characteristics of $\rho_{\mathrm{\bot }}\approx \rho _{\mathrm{\parallel }}$\textgreater $\rho_{\mathrm{T}}$, for which the SMR theory accounts, do not hold at low temperatures nor at different magnetic fields. Furthermore, the new MR persists even after altering the Pt/YIG interface thereby blocking the spin current.\footnote{B. F. Miao \textit{et al.,} Phys. Rev. Lett. \textbf{111}, 066602 (2013).} The feature of new MR can also be reproduced when Pt is in contact with a non-magnetic insulator doped with a few percent of Fe impurities. These results show that the new MR is probably due to both spin current and MPE. Through tuning the YIG surface and the insertion of other layers between Pt and YIG, we are able to separate the two contributions of spin current and MPE of the new hybrid MR. This work, in collaboration with S. Y. Huang, D. Qu (JHU) B. F. Miao (JHU and Nanjing University), Y. M. Lu and J. W. Cai (Institute of Physics, Chinese Academy of Sciences), has been supported in part by NSF DMR1262253. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F40.00004: Distinguishing spin Hall MR from anisotropic MR by temperature dependence Invited Speaker: Jing Shi In bilayers consisting of a strong spin-orbit coupling metal such as Pt or Pd and a magnetic insulator such as yttrium iron garnet, magnetoresistance is found in the magnetic field range where the magnetization of YIG reverses. This magnetoresistance resembles the conventional anisotropic magnetoresistance (AMR) in ferromagnetic conductors, which arises from the difference in resistance between the current parallel and perpendicular to the magnetization. At room temperature, however, no difference is observed when the magnetization is rotated between these two orientations; therefore, this phenomenon is clearly not the conventional AMR. An alternative explanation is based on the magnetization-dependent spin current reflection effect, called the spin Hall magnetoresistance (SMR). In our bilayer systems, we find that a finite AMR appears and increases monotonically as the temperature is decreased. In the meantime, SMR increases first, reaches a peak at an intermediate temperature, and then decreases at low temperatures. We will show that this characteristic temperature dependence is consistent with the SMR model. The SMR peak occurs when the spin diffusion length is approximately equal to the metallic layer thickness. These two magnetoresistance effects coexist but can be distinguished from each other from their distinct temperature dependences. [Preview Abstract] |
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