Session Y9: Invited Session: Spin Mechanics

Autonomous and forced dynamics in a spin-transfer nano-oscillator: Quantitative magnetic-resonance force microscopy

In this talk, we will discuss how magnetic-resonance force microscopy, can provide quantitative measurement of the power emitted by a spin-transfer nano-oscillator, consisting of a normally magnetized Py|Cu|Py circular nanopillar, excited both in the autonomous and forced regimes.\footnote{A. Hamadeh, et al. PHYSICAL REVIEW B 85, 140408(R) (2012)} From the power behavior in the subcritical region of the autonomous dynamics, one obtains a quantitative measurement of the threshold current and of the noise level. Their field dependence directly yields both the spin torque efficiency acting on the thin layer and the nature of the mode which first auto-oscillates: the lowest energy, spatially most uniform spin-wave mode. We will then demonstrate that the observed spin-wave spectrum in the forced regime critically depends on the method of excitation. While the spatially uniform radio-frequency (RF) magnetic field excites only the axially symmetric modes having azimuthal index $\ell=0$, the RF current flowing through the nano-pillar, creating a circular RF Oersted field, excites only the modes having azimuthal index $\ell=+1$.\footnote{V.V. Naletov et al. PHYSICAL REVIEW B 84, 224423 (2011)} It is then demonstrated that in order to phase lock this auto-oscillating mode, the external source must have the same spatial symmetry as the mode profile, i.e., a uniform microwave field must be used rather than a microwave current flowing through the nanopillar.   

Magneto-mechanical detection and control of the nanoscale Barkhausen effect

Developments in nano- and spin mechanics are driving a resurgence of interest in mechanical approaches to magnetometry. Torque methods for measurement of quasi-static magnetization or detection of spin dynamics can very fruitfully be miniaturized for application to individual magnetic nanostructures, and are complementary to magnetic force microscopy and related techniques [1]. We report a complete study of the Barkhausen effect in torsional magnetometry measurements of a micromagnetic disk. The discovery of Barkhausen noise in 1919 [2] provided the first experimental evidence of ferromagnetic domains. Within three decades elegant experiments had been performed on individual domain walls and a firm qualitative understanding had emerged [3]. Quantitative treatments of the effect have relied on statistical analysis [4], due to the collective nature of domain wall pinning by many sites. However, a vortex core effectively localizes the domain wall to the scale of an individual pining site, thereby converting the Barkhausen effect into a quantitative 2D nanoscale probe of local energetics in thin magnetic films [5]. In addition to characterization of the intrinsic disorder in a polycrystalline film, point-like tailoring of the energy landscape through low dose focussed ion beam implantation is demonstrated, and can be exploited to tune the properties of integrated magneto-mechanical devices.\\[4pt] [1] J.P. Davis et al., Appl. Phys. Lett. {\bf 96}, 072513 (2010) and New J. Phys. {\bf 12}, 093033 (2010). \newline [2] H. Barkhausen, Phys Z. {\bf 20}, 401 (1919). \newline [3] C. Kittel, Rev. Mod. Phys. {\bf 21}, 541 (1949). \newline [4] B. Alessandro et al., J. Appl. Phys. {\bf 68}, 2901 (1990). \newline [5] J.A.J. Burgess et al., arxiv.org/abs/1208.3797, and to be published.   

Coherent mechanical control of a single electronic spin

Quantum control of spins via electrical, magnetic, and optical means has generated numerous applications in metrology and quantum information technology. In this talk we present an alternative control scheme that uses the mechanical motion of a resonator to coherently control spins. Specifically, by coupling the motion of a magnetically coated mechanical oscillator to a single nitrogen-vacancy (NV) defect in diamond, we demonstrate manipulations of both the amplitude and phase of the NV's electronic spin. Coherent control is achieved by synchronizing NV-addressing optical and microwave manipulations to the driven motion of the coupled mechanical oscillator, which additionally allows for a stroboscopic readout of the resonator's motion. We demonstrate applications of this mechanical spin control to sensitive nanoscale scanning magnetometry and discuss the potential for sensitive motion sensing of nanomechanical resonators.   

Laser-induced magnetization switching in ferrimagnetic alloys

This talk will discuss the recent studies of ultrafast switching of magnetization and the role of angular momentum in this process in ferrimagnetic rare-earth - transition metal alloys, e.g. GdFeCo, where both magnetization and angular momenta are temperature dependent. It has been experimentally demonstrated that the magnetization can be manipulated and even reversed by a single 40 fs laser pulse, without any applied magnetic field [1]. This switching is found to follow a novel reversal pathway [2], that is shown to depend crucially on the net angular momentum, reflecting the balance of the two opposite sublattices [3,4]. In particular, optical excitation of ferrimagnetic GdFeCo on a time-scale pertinent to the characteristic time of the exchange interaction between the rare earth (RE) and transition metal (TM) spins, i.e. on the time scale of tens of femtoseconds, pushes the spin dynamics into a yet unexplored regime, where the two exchange coupled magnetic sublattices demonstrate substantially different dynamics [3]. As a result, the reversal of spins appears to proceed via a novel transient state characterized by a ferromagnetic alignment of the Gd and Fe magnetic moments, despite their ground-state antiferromagnetic coupling [4]. This process is fully modeled by a system of coupled equations for the longitudinal relaxation of the sublattices [5]. The role of light helicity in this process, being a controversial issue for many years, is clarified as well [6].\\[4pt] [1] C.D. Stanciu et al., Phys. Rev. Lett. 99, 047601 (2007).\\[0pt] [2] K. Vahaplar et al., Phys. Rev. Lett. 103, 117201 (2009).\\[0pt] [3] I. Radu et al., Nature 472, 205 (2011)\\[0pt] [4] T.A. Ostler et al., Nature Comm. 3, 666 (2012).\\[0pt] [5] J.H. Mentink et al., Phys. Rev. Lett. 108, 057202 (2012).\\[0pt] [6] A.R. Khorsand et al., Phys. Rev. Lett. 108, 127205 (2012).   

Spin-current generation arising from mechanical motions

Spin current, the flow of spins, is a key concept in the field of spintronics.\footnote{S. Maekawa, S. O. Valenzuela, E. Saitoh, and T. Kimura ed. ``Spin Current,'' Oxford University Press (2012).} To create and control spin currents, magnetic dynamics, electromagnetic fields, and thermal gradient have been used. Recently, the acoustically generated spin current was observed in an insulating ferromagnet.\footnote{K. Uchida et al., Nat. Mater. 10, 737 (2011).} However, the conversion between mechanical motions and the spin current in non-magnetic materials has not been studied so far. In this talk, we will present our recent results on spin-current generation from mechanical motions, including rigid and elastic motions in non-magnetic metals and semiconductors. In a rigidly accelerating body, the spin-orbit interaction (SOI) is modulated by the mechanical motion.\footnote{M. Matsuo, J. Ieda, E. Saitoh, and S. Maekawa, Phys. Rev. Lett 106, 076601 (2011); Appl. Phys. Lett. 98, 242501 (2011); Phys. Rev. B 84, 104410 (2011).} The augmented SOI leads to the spin-current generation from both mechanical rotation and vibration. On the other hand, in the presence of the surface acoustic wave (SAW), the elastically driven rotational motion of the lattice couples to electron spins and the spin current is generated in the direction of depth. Dependence of amplitude and frequency of the SAW, the spin diffusion length, and elastic parameters on the spin current will be shown. We will also discuss the enhancement of the SOI and the spin-rotation coupling caused by an interband mixing, using an extended k.p perturbation with the gauge potential due to mechanical rotation.\footnote{M. Matsuo, J. Ieda, and S. Maekawa, arXiv:1211.0127.}