Andrew Berger

Off-resonant manipulation of spins in diamond via precessing magnetization of a proximal ferromagnet

We report the manipulation of nitrogen vacancy (NV) spins in diamond when nearby ferrimagnetic insulator, yttrium iron garnet, is driven into precession. The change in NV spin polarization, as measured by changes in photoluminescence, is comparable in magnitude to that from conventional optically detected magnetic resonance, but relies on a distinct mechanism as it occurs at a microwave frequency far removed from the magnetic resonance frequency of the NV spin. This observation presents a new approach to transferring ferromagnetic spin information into a paramagnet and then transducing the response into a robust optical signal. It also opens new avenues for studying ferromagnetism and spin transport at the nanoscale.

The effect of spin transport on spin lifetime in nanoscale systems

Spin transport electronics – spintronics – focuses on utilizing electron spin as a state variable for quantum and classical information processing and storage [1]. Some insulating materials, such as diamond, offer defect centers whose associated spins are well-isolated from their environment giving them long coherence times [2–4]; however, spin interactions are important for transport [5], entanglement [6], and read-out [7]. Here, we report direct measurement of pure spin transport – free of any charge motion – within a nanoscale quasi 1D ‘spin wire’, and find a spin diffusion length ∼ 700 nm. We exploit the statistical fluctuations of a small number of spins [8] ( √ N < 100 net spins) which are in thermal equilibrium and have no imposed polarization gradient. The spin transport proceeds by means of magnetic dipole interactions that induce flip-flop transitions [9], a mechanism that can enable highly efficient, even reversible [10], pure spin currents. To further study the dynamics within the spin wire, we implement a magnetic resonance protocol that improves spatial resolution and provides nanoscale spectroscopic information which confirms the observed spin transport. This spectroscopic tool opens a potential route for spatially encoding spin information in long-lived nuclear spin states. Our measurements probe intrinsic spin dynamics at the nanometre scale, providing detailed insight needed for practical devices which seek to control spin.Spintronics use the electron spin as a state variable for information processing and storage. This requires manipulation of spin ensembles for data encoding, and spin transport for information transfer. Because of the central importance of lifetime for understanding and controlling spins, mechanisms that determine this lifetime in bulk systems have been extensively studied. However, a clear understanding of few-spin systems remains challenging. Here, we report spatially resolved magnetic resonance studies of electron spin ensembles confined to a spin nanowire formed by nitrogen ion implantation in diamond. We measure the spin lifetime of the ensemble--that is, its spin autocorrelation time--by monitoring the statistical fluctuations of its net moment, which is in thermal equilibrium and has no imposed polarization gradient. We find that the lifetime of the ensemble is dominated by spin transport from the ensemble into the adjacent spin reservoir that is provided by the remainder of the nanowire. This is in striking contrast to conventional spin-lattice relaxation measurements of isolated spin ensembles. Electron spin resonance spectroscopy performed on nanoscale spin ensembles by means of a novel spin manipulation protocol corroborates spin transport in strong field gradients. Our experiments, supported by microscopic Monte Carlo modelling, provide a unique insight into the intrinsic dynamics of pure spin currents needed for nanoscale devices that seek to control spins.

Magnetization dynamics of cobalt grown on graphene

Ferromagnetic resonance (FMR) spin pumping is a rapidly growing field which has demonstrated promising results in a variety of material systems. This technique utilizes the resonant precession of magnetization in a ferromagnet to inject spin into an adjacent non-magnetic material. Spin pumping into graphene is attractive on account of its exceptional spin transport properties. This article reports on FMR characterization of cobalt grown on chemical vapor deposition graphene and examines the validity of linewidth broadening as an indicator of spin pumping. In comparison to cobalt samples without graphene, direct contact cobalt-on-graphene exhibits increased FMR linewidth—an often used signature of spin pumping. Similar results are obtained in Co/MgO/graphene structures, where a 1 nm MgO layer acts as a tunnel barrier. However, magnetometry, magnetic force microscopy, and Kerr microscopy measurements demonstrate increased magnetic disorder in cobalt grown on graphene, perhaps due to changes in the growth process and an increase in defects. This magnetic disorder may account for the observed linewidth enhancement due to effects such as two-magnon scattering or mosaicity. As such, it is not possible to conclude successful spin injection into graphene from FMR linewidth measurements alone.

Correlating spin transport and electrode magnetization in a graphene spin valve: Simultaneous magnetic microscopy and non-local measurements

Using simultaneous magnetic force microscopy (MFM) and transport measurements of a graphene spin valve, we correlate the non-local spin signal with the magnetization of the device electrodes. The imaged magnetization states corroborate the influence of each electrode within a one-dimensional spin transport model and provide evidence linking domain wall pinning to additional features in the transport signal.

A versatile LabVIEW and field-programmable gate array-based scanning probe microscope for in operando electronic device characterization

Understanding the complex properties of electronic and spintronic devices at the micro- and nano-scale is a topic of intense current interest as it becomes increasingly important for scientific progress and technological applications. In operando characterization of such devices by scanning probe techniques is particularly well-suited for the microscopic study of these properties. We have developed a scanning probe microscope (SPM) which is capable of both standard force imaging (atomic, magnetic, electrostatic) and simultaneous electrical transport measurements. We utilize flexible and inexpensive FPGA (field-programmable gate array) hardware and a custom software framework developed in National Instruments LabVIEW environment to perform the various aspects of microscope operation and device measurement. The FPGA-based approach enables sensitive, real-time cantilever frequency-shift detection. Using this system, we demonstrate electrostatic force microscopy of an electrically biased graphene field-effect transistor device. The combination of SPM and electrical transport also enables imaging of the transport response to a localized perturbation provided by the scanned cantilever tip. Facilitated by the broad presence of LabVIEW in the experimental sciences and the openness of our software solution, our system permits a wide variety of combined scanning and transport measurements by providing standardized interfaces and flexible access to all aspects of a measurement (input and output signals, and processed data). Our system also enables precise control of timing (synchronization of scanning and transport operations) and implementation of sophisticated feedback protocols, and thus should be broadly interesting and useful to practitioners in the field.

Determination of spin Hall effect and spin diffusion length of Pt from self-consistent fitting of damping enhancement and inverse spin-orbit torque measurements (Conference Presentation)

Functional spintronic devices rely on spin-charge interconversion effects, such as the reciprocal processes of electric field-driven spin torque and magnetization dynamics-driven spin and charge flow. Both damping-like and field-like spin-orbit torques have been observed in the forward process of current-driven spin torque and damping-like inverse spin-orbit torque has been well-studied via spin pumping into heavy metal layers. Here we demonstrate that established microwave transmission spectroscopy of ferromagnet/normal metal bilayers under ferromagnetic resonance can be used to inductively detect the AC charge currents driven by the inverse spin-charge conversion processes. This technique relies on vector network analyzer ferromagnetic resonance (VNA-FMR) measurements. We show that in addition to the commonly-extracted spectroscopic information, VNA-FMR measurements can be used to quantify the magnitude and phase of all AC charge currents in the sample, including those due to spin pumping and spin-charge conversion. Our findings reveal that Permalloy/Pt bilayers exhibit both damping-like and field-like inverse spin-orbit torques. While the magnitudes of both the damping-like and field-like inverse spin-orbit torque are of comparable scale to prior reported values for similar material systems, we observed a significant dependence of the damping-like magnitude on the order of deposition. This suggests interface quality plays an important role in the overall strength of the damping-like spin-to-charge conversion. Spin memory loss (SML) [1] and proximity-induced magnetic moments at the FM/NM interface [2] have been invoked to explain the large damping enhancement caused by thin NM films even when the NM thickness is less than its spin diffusion length. In this model, spin loss at the FM/NM interface acts as an additional parallel spin relaxation pathway to that of spin pumping and diffusion into the Pt bulk. From damping measurements alone, the relative contributions of these mechanisms are not resolvable. In this work, we show that a self-consistent fit of Gilbert damping and damping-like iSOT versus Pt thickness—where both sets of data are described by the same spin diffusion length—makes it possible to separate these sources of damping. Furthermore, this data analysis methodology allows for unambiguous determination of the spin-mixing conductance at the FM/NM interface. We therefore can determine the spin Hall conductivity (or spin Hall angle) without having to refer to spin transport parameters, e.g. the spin-mixing conductance and spin diffusion length, as determined from measurements performed on dissimilar samples or theoretical idealized values. For our samples of Pt deposited on Permalloy, only 37 ± 6% of the total damping enhancement from the Pt film is attributable to spin pumping into the Pt layer when the Pt thickness is much greater than the spin diffusion length. The self-consistent fit also results in a spin diffusion of length of (4.2 ± 0.1) nm, and a spin mixing conductance of (130,000 ± 20,000) 1/(μΩ cm^2), which is in good agreement with the maximum theoretical value for Pt of 107,000 1/(μΩ cm^2) [3], given the estimated error, and σ_SH = (2.36 ± 0.04) 1/(μΩ m). This corresponds to a spin Hall angle of 0.387 ± 0.008. While this θ_SH is among the largest reported for Pt [4, 5], it is a necessary logical conclusion that with less spin current driven into the NM (on account of SML), a larger spin-to-charge conversion efficiency is required to fit the data than would be otherwise obtained if the SML were negligible. We furthermore stress that the phenomenological value for the damping-like spin orbit torque is comparable to that measured with other techniques [5-7]. This indicates that the Pt layer in our samples behaves conventionally, and stresses the importance of characterizing spin loss mechanisms to optimize SOT for magnetic switching. nnREFERENCESnn[1] J.-C. Rojas-Sanchez, N. Reyren, P. Laczkowski, et al., Spin Pumping and Inverse Spin Hall Effect in Platinum: The Essential Role of Spin-Memory Loss at Metallic Interfaces, Phys. Rev. Lett., vol. 112, p. 106602 (2014).n[2] M. Caminale, A. Ghosh, S. Auffret, et al., Spin pumping damping and magnetic proximity effect in Pd and Pt spin-sink layers, Physical Review B, vol. 94, p. 014414 (2016).n[3] Y. Liu, Z. Yuan, R. J. H. Wesselink, et al., Interface Enhancement of Gilbert Damping from First Principles, Physical Review Letters, vol. 113, p. 207202 (2014).n[4] W. Zhang, W. Han, X. Jiang, et al., Role of transparency of platinum-ferromagnet interfaces in determining the intrinsic magnitude of the spin Hall effect, Nat Phys, Article vol. 11, pp. 496-502 (2015).n[5] C.-F. Pai, Y. Ou, L. H. Vilela-Leao, et al., Dependence of the efficiency of spin Hall torque on the transparency of Pt/ferromagnetic layer interfaces, Phys. Rev. B, vol. 92, p. 064426 (2015).n[6] K. Garello, I. M. Miron, C. O. Avci, et al., Symmetry and magnitude of spin-orbit torques in ferromagnetic heterostructures, Nat. Nano., vol. 8, pp. 587-593 (2013).n[7] M.-H. Nguyen, D. C. Ralph, and R. A. Buhrman, Spin Torque Study of the Spin Hall Conductivity and Spin Diffusion Length in Platinum Thin Films with Varying Resistivity, Physical Review Letters, vol. 116, p. 126601 (2016).