Session W4: Electric Voltages Generated by Magnetization Dynamics

Detection of electromotive force induced by domain wall motion

A magnetic domain wall can be displaced by current via the transfer of spin angular momentum from conduction electrons to the local magnetization. The capacity of spin-transfer torque to drive domain wall motion is now well established experimentally and theoretically [1], and is a central topic in the growing field of spintronics. This talk will describe the first experimental evidence [2] that the coupling between spin and charge also works in reverse; namely, that a domain wall driven by a field through a stationary electron gas generates an experimentally-detectible voltage. This new spintronic effect [3] was measured by precisely controlling the motion of a single domain wall in a Permalloy nanowire and isolated from other sources using a field modulation scheme to differentiate between the small domain wall-induced voltage and conventional inductive voltages. The domain wall-induced voltage was found to scale in proportion to the driving field magnitude, and its sign depends only on the direction of domain wall motion. These results are consistent with theoretical predictions [2, 4, 5], and will be discussed in terms of a generalized two-dimensional topological framework [2] capable of treating vortex DWs. \\[4pt] [1] G.S.D. Beach, M. Tsoi, and J.L. Erskine, J. Magn. Magn. Mater. 320, 1272 (2008). \\[0pt] [2] S. Yang, G.S.D. Beach, C. Knutson, D. Xiao, Q. Niu, M. Tsoi, and J.L. Erskine, Phys. Rev. Lett. 102, 067201 (2009). \\[0pt] [3]. R. McMichael, and M. Stiles, Physics 2, 11 (2009). \\[0pt] [4] L. Berger, Phys. Rev. B 33, 1572 (1986) \\[0pt] [5] S. E. Barnes and S. Maekawa, Appl. Phys. 89, 122507 (2006).   

Conduction electrons and the Landau-Lifshitz-Gilbert equation

In conducting ferromagnets, spin-polarized transport properties are strongly correlated with static and dynamic magnetic properties. These correlations have led to a number of interesting phenomena such as giant magnetoresistance, spin transfer torques, spin pumping, non-uniform and non-local magnetization damping, and electric voltage induced by domain wall motion. In this talk, we discuss the roles of non-equilibrium conduction electrons on the magnetization dynamics by taking into account both spin-orbit interactions and exchange interactions between conduction electrons and magnetization vectors. For a non-uniform time-dependent magnetization vector, the exchange interaction generates spin-dependent electric and magnetic potentials which can be viewed as new forms of spin-orbit coupling. By explicitly calculating the non-equilibrium spin density and spin current, we relate the magnetization damping to the angular momentum carried away by the conduction electrons. We numerically evaluate the contribution of the conduction electrons to the magnetization damping for several different domain walls. Finally, we discuss a possible realization of the Aharonov-Bohm effect in magnetic ring structures with controlled domain walls. Work was done in collaboration with Steven S. Zhang.   

Electric detection ofmagnetization dynamics through inverse spin Hall effects

Spin currents, flows of spin angular momentum, are essential in spintronics. To explore the physics of spin currents, effective methods for detecting and generating spin currents should be established. Here we report the observation of the inverse/direct spin-Hall effects in metallic films. These effects enable electric generation and detection of spin currents. We have applied these effects to the observation of the spin-Seebeck effect. The inverse spin-Hall effect (ISHE) is the generation of a charge current from a spin current via the spin-orbit interaction. We have observed ISHE in metallic films at room temperature. The sample used in the present study is a bilayer film comprising a 10-nm-thick ferromagnetic NiFe layer and a 7- nm-thick nonmagnetic metallic (NM=Pt, Pd, Cu, Nb, and Au) layer. In our sample system, a pure spin current is injected from the NiFe layer into the NM layer using the spin-pumping effect operated by ferromagnetic resonance (FMR). ISHE in the NM layer converts the spin current into an electric current, which causes charge accumulation at the edges of the NM layer, or a difference of electric potential between the edges. By measuring this potential difference, this method allows us to detect ISHE in the films. We also demonstrated that the reverse effect of this spin- pumping induced ISHE allows the electric manipulation of magnetization relaxation even in a large-area film. This result can be argued in terms of the combination of the spin-torque effect and the direct spin-Hall effect. A model calculation reproduces the experimental data. This effect can be applied to a quantitative measurement of spin currents without assuming microscopic parameters. We have applied ISHE to the observation of the spin-Seebeck effect. By means of ISHE, we measured spin voltage generated from a temperature gradient in NiFe. This thermally induced spin voltage persists even at distances far from the sample ends and its sign is reversed between the ends of the sample along the temperature gradient. These behaviors are consistent with a phenomenological two-band model for the spin-Seebeck effect. The spin-Seebeck effect can be applied directly to constructing thermal spin generators for driving spintronics devices, thereby opening the door to thermo-spintronics.   

Quantifying Spin Hall Effects from Spin Pumping

Recent activity in spin transport research has included a focus on spin Hall effects, which arise from spin-orbit interactions. Spin orbit coupling in normal metals (NM) results in a conversion of pure spin currents into charge currents, which are perpendicular to both the spin current direction and the spin polarization. This phenomenon is known as the inverse spin Hall effect and it generates a voltage across a spin-current-carrying sample. The strength of the inverse spin Hall effect is characterized by a single dimensionless parameter, the spin Hall angle, which is materials-specific. Here we present a new method to quantify spin Hall angles for many different materials. We studied the inverse spin Hall effect in Ni$_{80}$Fe$_{20}$/NM bilayer structures by generating pure spin currents inside the NM layer through spin pumping at the Ni$_{80}$Fe$_{20}$/NM interface. Integrating a patterned Ni$_{80}$Fe$_{20}$/NM bilayer into a coplanar waveguide transmission line enables us to excite large angle magnetization precession in Ni$_{80}$Fe$_{20}$ via {\em rf} excitation, which in turn generates a {\em dc} spin current in the adjacent NM. A strong {\em dc} signal across the Ni$_{80}$Fe$_{20}$/NM is observed at the FMR position, and its magnitude is dependent on the power of the {\em rf} excitation and the direction of the applied magnetic field. We identified two distinct contributions to the {\em dc} voltage: one symmetric with respect to the FMR resonance position, and the other antisymmetric. Our analysis shows that the antisymmetric contribution is due to anisotropic magnetoresistance (AMR) in the Ni$_{80}$Fe$_{20}$ layer and is present even in single-layer Ni$_{80}$Fe$_{20}$ films. The second, symmetric, contribution to the {\em dc} voltage is attributed to the inverse spin Hall effect. The main advantage of our approach is that this second contribution scales with the device dimension and thus even small spin Hall signals can be detected with large accuracy. Using this approach we determined the spin Hall angle for Pt, Au and Mo by fitting the experimental data to a self-consistent theory, which accounts for both AMR and inverse spin Hall effect contributions.\footnote{O.~Mosendz, J.~E.~Pearson, F.~Y.~Fradin, G.~E.~W.~Bauer, S.~D.~Bauer, and A.~Hoffmann, ArXiv:0911.2725.} Our technique allows to electrically detect the spin accumulation in the NM. Using this connection, we also showed that spin pumping is suppressed when MgO tunneling barrier is inserted at the Ni$_{80}$Fe$_{20}$/NM interface.\footnote{O.~Mosendz, J.~E.~Pearson, F.~Y.~Fradin, S.~D.~Bauer, and A.~Hoffmann, ArXiv:0911.3182}   

Quantitative measurement of spin transfer torque in magnetic tunnel junctions by spin-transfer-driven ferromagnetic resonance

MgO-based magnetic tunnel junctions (MTJs) whose magnetic dynamics are controlled by spin-transfer torque are prime candidates for practical memory devices. Yet the bias dependence of the spin torque in these MTJs remains controversial, especially at high bias. Here we present spin-torque-driven ferromagnetic resonance (ST-FMR) measurements of the magnitude and direction of the spin torque. We use two complementary methods to determine the amplitude of the magnetic response during ST-FMR: (a) measurement near zero frequency of a mixing signal generated by the oscillating resistance and the applied microwave current that provides accurate torque measurements for small biases $\vert V\vert $ less than 0.15-0.3 V and (b) a new type of direct time-domain detection at GHz frequencies that enables quantitative measurements of the spin torque at higher biases from $\vert $\textit{V$\vert $ }$\sim $ 0.15 V up to the breakdown voltage of the MTJ. We find that the two types of detection give consistent results at intermediate biases where they can both be employed. We measure a significant deviation from linear behavior for the bias dependence of the in-plane component of the spin torque (in the plane defined by the electrode magnetizations) with increasing bias, with a weaker torque for electron flow from the free magnetic layer to the fixed layer, compared to the opposite bias. The perpendicular component of the spin torque in symmetric CoFeB/MgO/CoFeB tunnel junctions is quadratic in bias at low bias, but is close to linear at high bias.