We demonstrate a technique that enables ferromagnetic resonance measurements of the normal modes for magnetic excitations in individual nanoscale ferromagnets, smaller in volume by more than a factor of 50 compared to individual ferromagnetic samples measured by other resonance techniques. Studies of the resonance frequencies, amplitudes, linewidths, and line shapes as a function of microwave power, dc current, and magnetic field provide detailed new information about the exchange, damping, and spin-transfer torques that govern the dynamics in magnetic nanostructures.

The discovery of the quantum spin Hall (QSH) state, and topological insulators in general, has sparked strong experimental efforts. Transport studies of the quantum spin Hall state have confirmed the presence of edge states, showed ballistic edge transport in micron-sized samples, and demonstrated the spin polarization of the helical edge states. While these experiments have confirmed the broad theoretical model, the properties of the QSH edge states have not yet been investigated on a local scale. Using scanning gate microscopy to perturb the QSH edge states on a submicron scale, we identify well-localized scattering sites which likely limit the expected nondissipative transport in the helical edge channels. In the micron-sized regions between the scattering sites, the edge states appear to propagate unperturbed, as expected for an ideal QSH system, and are found to be robust against weak induced potential fluctuations.

Hexagonal boron nitride (h-BN) films have attracted considerable interest as substrates for graphene. ( Dean, C. R. et al. Nat. Nanotechnol. 2010 , 5 , 722 - 6 ; Wang, H. et al. Electron Device Lett. 2011 , 32 , 1209 - 1211 ; Sanchez-Yamagishi, J. et al. Phys. Rev. Lett. 2012 , 108 , 1 - 5 .) We study the presence of organic contaminants introduced by standard lithography and substrate transfer processing on h-BN films exfoliated on silicon oxide substrates. Exposure to photoresist processing adds a large broad luminescence peak to the Raman spectrum of the h-BN flake. This signal persists through typical furnace annealing recipes (Ar/H(2)). A recipe that successfully removes organic contaminants and results in clean h-BN flakes involves treatment in Ar/O(2) at 500 °C.

We measure the temperature, magnetic-field, and current dependence for the switching of nanomagnets by a spinpolarized current. Depending on current bias, switching can occur between either two static magnetic states or a static state and a current-driven precessional mode. In both cases, the switching is thermally activated and governed by the sample temperature, not a higher effective magnetic temperature. The activation barriers for switching between static states depend linearly on current, with a weaker dependence for dynamic to static switching.The interaction between the magnetic moment of a metallic ferromagnet and a spin-polarized electrical current results in the spin-transfer effect [1][2][3][4][5][6][7][8], whereby the current can apply a torque to the magnet via transfer of angular momentum. The manipulation of nanomagnets by spin-transfer torques is under investigation for use in the switching of nonvolatile memory elements [9] and for current-tunable microwave sources [10,11]. Previous measurements of magnetic switching driven by spin-polarized currents have suggested that the process is thermally activated [12][13][14], but there remains disagreement about the switching mechanism. One set of models describes the effect of a spin-polarized current in terms of a torque that rotates the local moment of the magnet uniformly, as described within the framework of the LandauLifshitz-Gilbert (LLG) equation [1,[15][16][17]]. An alternative model proposes that when the polarization of the current is opposite to the moment of the magnet, spin-flip scattering of electrons excites non-uniform magnons, effectively raising the magnetic temperature so as to accelerate switching [13,14]. In this Letter, we use measurements of the switching rates as a function of temperature, magnetic field, and current to distinguish between these mechanisms. We find that a single sample can undergo different switching processes between separate static and dynamic states, that were not all distinguished in previous studies. In all cases, switching is thermally activated and governed by the actual background sample temperature. We observe no magnetic-configuration-dependent heating. The data are described well by current-dependent activation barriers that agree with predictions of the LLG-based models.The samples we study are made from magnetic multilayers deposited by magnetron sputtering, with the structure Cu(100 nm)/ Py(X nm)/ Cu(6 nm)/ Py(2 nm)/ Cu(Y nm)/ Pt(30 nm), where X = 12 or 20 nm for the thicker permalloy (Py = Ni 80 Fe 20 ) layer and Y = 2 or 20 nm. Electron-beam lithography and ion milling are used to define an elliptical pillar structure with a size 130 nm × 60 nm and with both magnetic layers etched through [5,18]. Top contact is made with a deposited Cu electrode. We will analyze the switching properties of the 2-nm thick Py "free layer". Positive currents are defined so that electrons flow from the thinner to thicker Py layer. Although we focus below on one Py(20 nm)/Cu(6 nm)/Py(2 nm) device, similar results wer...

We have fabricated nanoscale magnetic tunnel junctions (MTJs) with an additional fixed magnetic layer added above the magnetic free layer of a standard MTJ structure. This acts as a second source of spin-polarized electrons that, depending on the relative alignment of the two fixed layers, either augments or diminishes the net spin-torque exerted on the free layer. The compound structure allows a quantitative comparison of spintorque from tunneling electrons and from electrons passing through metallic spacer layers, as well as analysis of Joule self-heating effects. This has significance for current-switched magnetic random access memory (MRAM), where spin torque is exploited, and for magnetic sensing, where spin torque is detrimental. Recent demonstrations1,2 of magnetic random access memory (MRAM) have shown that magnetic memory may compete with silicon based memory in some applications. In order to scale cell size and decrease write currents, future generations of MRAM may use spin transfer as the write mechanism 3,4 . A key to spin-transfer switched MRAM is the use of magnetic tunnel junctions 5,6 (MTJs) to obtain an impedance match to CMOS technology. Initial demonstrations of spin transfer in MTJs indicate critical current densities similar to those observed in current perpendicular to the plane (CPP) spin valves 3 , but with higher bias voltages (~0.5 V). This poses questions regarding self-heating effects and the possible degradation and breakdown of the ultra-thin insulating barrier layer. Therefore, it is beneficial to boost the magnitude of spin torque to enable switching at lower current and voltage levels. In contrast, for magnetic sensing, spin-transfer torques introduce noise and instability 7,8 . In this letter, we adapt a proposal due to Berger 9 to demonstrate that adding a copper spacer and a third magnetic fixed layer above the MTJ structure can, depending on the relative orientation of the two fixed layers, either increase by ~70% or nearly cancel the net spin-torque acting on the free magnetic layer. We analyze results by considering the effects of the spintorque and of the substantial Joule heating that occurs in tunnel junction devices at switching current levels. This investigation provides fundamental insight into the nature of spin torques in MTJs and spin valves by allowing a direct comparison, in the same device, of the magnitude of the spin-torque exerted by tunnel currents and by electrons that transport spin through the copper spacer layer.The multilayers for our samples were prepared on a thermally oxidized Si wafer in a DC magnetron sputtering chamber with a base pressure of 3×10 -8 torr. The 4 nm of Py above the AlO x serves as the magnetic free layer and the two CoFe layers are both magnetic fixed layers in our studies. Since the tunnel barrier limits the conductance, the resistance state reflects whether the bottom fixed layer (8 nm CoFe) and the free layer are aligned parallel (P, low resistance) or antiparallel (AP, high resistance). The role of the top fixed layer (5 nm Co...