When a ferromagnet is deposited on the surface of a topological insulator (TI) the topologically protected surface state develops a gap and becomes a 2-dimensional quantum Hall liquid. We demonstrate that the Hall current in such a liquid, induced by an external electric field, can have a large effect on the magnetization dynamics of the ferromagnet by changing the effective anisotropy field. This change is dissipationless and may be substantial even in weakly spin-orbit coupled ferromagnets. We study the possibility of dissipationless current-induced magnetization reversal in monolayer-thin, insulating ferromagnets with a soft perpendicular anisotropy and discuss possible applications of this effect.

Current-induced torques are commonly used to manipulate non-collinear magnetization configurations. In this article we discuss current-induced torques present in a certain class of collinear magnetic systems, relating them to current-induced changes in magnetic anisotropy energy. We present a quantitative estimate of their characteristics in uniform strained ferromagnetic (Ga,Mn)As.PACS numbers:

We demonstrate that the charge-spin conversion efficiency of topological insulators (TI) can be experimentally determined by injecting spin-polarized tunneling electrons into a TI. Through a comparative study between bismuth selenide and bismuth antimony telluride, we verified the topologicalsurface-state origin of the observed giant spin signals. By injecting energetic electrons into bismuth selenide, we further studied the energy dependence of the effective spin polarization at the TI surface. The experimentally verified large spin polarization, as well as our calculations, provides new insights into optimizing TI materials for near room-temperature spintronic applications. * cchen3@us.ibm.com. 2Topological insulators (TI) exhibit coupled spin and momentum orientations in their gapless surface states [1][2][3][4][5] . Recent experiments using TI/ferromagnet bilayer structures suggest that TI can exhibit ultra-high efficiencies in converting electrical charge current into spin accumulation (or vice versa) 6-9 , providing great potential for various spintronic applications. To further optimize this effect, it is highly desirable to quantitatively characterize the spin structures of TI, especially in the absence of direct contact between the TI film and ferromagnetic material, as the latter can break time reversal symmetry and change the intrinsic electronic structure of the TI 10 . So far, the mechanism of the observed phenomenawhether they derive mainly from the topological surface states (TSS) [6][7][8]11 or from the bulk states 9 -remains largely unclear in TI based spintronic studies. Moreover, large discrepancies both in the magnitude and the temperature dependence of the measured effects still exist in the two currently utilized experimental approaches -injecting a spin current with spin pumping and measuring the induced voltage 8,9 versus applying a charge current and detecting the spin generation 6,7,11,12 . The device geometry used in our experiments is illustrated in Fig. 1 the Fermi level of the sample is located within the bulk band gap. As is discussed below, this ratio of the total carrier density in the Bi 2 Se 3 sample to that in (Bi 0.5 Sb 0.5 ) 2 Te 3 is order-of-magnitude consistent with the spin signal (R H ) ratio after taking into account of the difference in the sample sheet resistance.To achieve optimal spin polarization in the tunnel junction, a 250 °C post-sputtering annealing was applied. The resistance-area (RA) product of the tunnel junctions studied in our experiment ranges from 500 kΩ·µm 2 to 2 MΩ·µm 2 . Fig experiments that used the inverse geometry (injecting current between leads 2 and 4 while measuring the voltage between leads 1 and 3) 11, 12 , the tunneling configuration has the advantage of allowing us to investigate the spin texture for states away from the Fermi level by superimposing a DC voltage across the barrier. Moreover, as described below, with our device structure, we can perform measurements in both of the two reversible configurations, providing a self-consiste...