Magnetization switching at the interface between ferromagnetic and paramagnetic metals, controlled by current-induced torques, could be exploited in magnetic memory technologies. Compelling questions arise regarding the role played in the switching by the spin Hall effect in the paramagnet and by the spin-orbit torque originating from the broken inversion symmetry at the interface. Of particular importance are the antidamping components of these current-induced torques acting against the equilibrium-restoring Gilbert damping of the magnetization dynamics. Here, we report the observation of an antidamping spin-orbit torque that stems from the Berry curvature, in analogy to the origin of the intrinsic spin Hall effect. We chose the ferromagnetic semiconductor (Ga,Mn)As as a material system because its crystal inversion asymmetry allows us to measure bare ferromagnetic films, rather than ferromagnetic-paramagnetic heterostructures, eliminating by design any spin Hall effect contribution. We provide an intuitive picture of the Berry curvature origin of this antidamping spin-orbit torque as well as its microscopic modelling. We expect the Berry curvature spin-orbit torque to be of comparable strength to the spin-Hall-effect-driven antidamping torque in ferromagnets interfaced with paramagnets with strong intrinsic spin Hall effect. In one interpretation discussed in the literature so far, currentinduced switching at ferromagnet/paramagnet interfaces 1,2 originates from an antidamping component of the spin-orbit torque (SOT) 1, at the broken space-inversion-symmetry interface, while in another 2,23,25 , the spin Hall effect (SHE) [26][27][28][29][30][31][32] in the paramagnet combines with the antidamping spin-transfer torque (STT) [33][34][35][36] in the ferromagnet. Because, so far, the theories have considered a scattering-related SOT with an antidamping component that is expected to be relatively weak compared with the field-like SOT component 18,19 , much attention has been focused on the SHE-STT interpretation, in which the large SHE originates from the Berry curvature in the band structure of a clean crystal 2,28,29,37 . The focus of the present work is on a large antidamping SOT that stems from a Berry curvature origin analogous to intrinsic SHE.In conventional semiclassical transport theory, the linear response of the carrier system to the applied electric field is described by the non-equilibrium distribution function of carrier eigenstates, which are considered to be unperturbed by the electric field. The form of the non-equilibrium distribution function is obtained by accounting for the combined effects of the carrier acceleration in the field and of scattering. For the SOT, the non-equilibrium distribution function can be used to evaluate the currentinduced carrier spin density, which then exerts the torque on the magnetization via carrier-magnetic moment exchange coupling. The field-like component of the SOT reported in previous theoretical and experimental studies in (Ga,Mn)As films 4,8,9,11,24 and pred...

Ferromagnetic resonance is the most widely used technique for characterizing ferromagnetic materials. However, its use is generally restricted to wafer-scale samples or specific micro-magnetic devices, such as spin valves, which have a spatially varying magnetization profile and where ferromagnetic resonance can be induced by an alternating current owing to angular momentum transfer. Here we introduce a form of ferromagnetic resonance in which an electric current oscillating at microwave frequencies is used to create an effective magnetic field in the magnetic material being probed, which makes it possible to characterize individual nanoscale samples with uniform magnetization profiles. The technique takes advantage of the microscopic non-collinearity of individual electron spins arising from spin-orbit coupling and bulk or structural inversion asymmetry in the band structure of the sample. We characterize lithographically patterned (Ga,Mn)As and (Ga,Mn)(As,P) nanoscale bars, including broadband measurements of resonant damping as a function of frequency, and measurements of anisotropy as a function of bar width and strain. In addition, vector magnetometry on the driving fields reveals contributions with the symmetry of both the Dresselhaus and Rashba spin-orbit interactions.

We report the fabrication and measurement of silicon quantum dots with tunable tunnel barriers in a narrow-channel field-effect transistor. Low-temperature transport spectroscopy is performed in both the many-electron ( approximately 100 electrons) regime and the few-electron ( approximately 10 electrons) regime. Excited states in the bias spectroscopy provide evidence of quantum confinement. These results demonstrate that depletion gates are an effective technique for defining quantum dots in silicon.

Quantum computation requires a qubit-specific measurement capability to readout the final state of individual qubits. Promising solid-state architectures use external readout electrometers but these can be replaced by a more compact readout element, an in situ gate sensor. Gate-sensing couples the qubit to a resonant circuit via a gate and probes the qubit's radiofrequency polarizability. Here we investigate the ultimate performance of such a resonant readout scheme and the noise sources that limit its operation. We find a charge sensitivity of 37 me Hz À 1/2 , the best value reported for this technique, using the example of a gate sensor strongly coupled to a double quantum dot at the corner states of a silicon nanowire transistor. We discuss the experimental factors limiting gate detection and highlight ways to optimize its sensitivity. In total, resonant gate-based readout has advantages over external electrometers both in terms of reduction of circuit elements as well as absolute charge sensitivity.

An enhancement in Brillouin light scattering of optical photons with magnons is demonstrated in magneto-optical whispering gallery mode resonators tuned to a triple-resonance point. This occurs when both the input and output optical modes are resonant with those of the whispering gallery resonator, with a separation given by the ferromagnetic resonance frequency. The identification and excitation of specific optical modes allows us to gain a clear understanding of the mode-matching conditions. A selection rule due to wave vector matching leads to an intrinsic single-sideband excitation. Strong suppression of one sideband is essential for one-to-one frequency mapping in coherent optical-to-microwave conversion.