Session L15: Focus Session: Spins in Metals - Resonance Phenomena I, Spin Wave Excitation and Spin Torque Oscillators

Spin-torque excited spin waves revealed by micro-focused Brillouin light scattering

Since the discovery of the spin transfer torque (STT) effect [1,2] a great effort has been devoted to the realization and study of spin torque oscillators (STOs) because of their potential applications as spin waves injectors in magnonic devices or current-tunable broad-band microwave sources. More recently the possibility to synchronize multiple STOs [3,4] via the emitted spin waves, propagating in the magnetic ``free'' layer, envisioned a way to overcome their main limitation in the output power. For these reasons it's now crucial to obtain a detailed knowledge and understanding of the emitted spin waves properties like: their spatial distribution, their propagating or localized character, their decay length, wavelength and group velocity. In the last two years micro-focused Brillouin light scattering ($\mu $-BLS) revealed to be a powerful tool in order to investigate several of this properties [5,6]. In this presentation we discuss the potentialities of $\mu $-BLS to the study of emitted spin waves in STOs systems with particular focus on the results of our latest work [6]. Here we took advantage of our $\mu $-BLS setup in order to study spin waves emitted by an out-of-plane magnetized nano-contact STO. Performing a ``wave-vector resolved'' $\mu $-BLS experiment we provided the first direct experimental evidence of the \textit{propagating} nature of SWs emitted from an out-of-plane magnetized STO. The decay of the propagating SW intensity up to several microns away from the nano-contact position showed great potential for STT based magnonic devices. We also investigated the STO tunability measuring the emitted SW frequency as a function of both the applied direct current and external field intensities. Micromagnetic simulations provided the theoretical support to quantitatively reproduce the results. \\[4pt] [1] Slonczewski, J. C. J. Magn. Magn. Mater. 159, L1 (1996).\\[0pt] [2] Berger, L. Phys. Rev. B 54, 9353 (1996).\\[0pt] [3] Kaka, S. et al. Nature 437, 389 (2005).\\[0pt] [4] Mancoff, F. B., Rizzo, N. D., Engel, B. N., Tehrani, S. Nature 437, 393 (2005).\\[0pt] [5] Demidov, V. E., Urazhdin, S., and Demokritov, S. O. Nature materials, 9(11), (2010).\\[0pt] [6] Madami, M., Bonetti, S. et al. Nature Nanotechnology, 6, 635 (2011).   

Probing spin wave excitations using magnetic tunnel junction structures

We propose a quantitative method based on the use of high tunneling magnetoresistance and small lateral dimension of magnetic tunnel junction (MTJ) to detect spin excitations in magnetic films with promising high spatial resolution and sensitivity. One ferromagnetic (FM) layer of the MTJ is pinned by an antiferromagnetic layer and the other one is free to rotate in response to an external magnetic field. In the presence of microwave magnetic fields, the free layer will precess, leading to the average resistance of MTJ change. By applying a constant dc current bias to the MTJ, a time dependent voltage can be introduced and measured which is related is the time dependent magnetization along the external static field direction. We have demonstrated the usefulness of this method by studying the spin wave excitations in a single elliptical Permalloy thin film (50 $\mu $m $\times $30 $\mu $m $\times $40 nm). At low microwave power, a uniform linear ferromagnetic resonance behavior has been observed. Surprisingly, above the spin wave instability threshold, the experimental results show a linear response of to the microwave field over a large range, which is followed by a phase limiting behavior. The linear behavior can be described by the theoretical model describing subsidiary resonance.   

Generating Damon-Eshbach Spin Waves in Py using a Conducting Diffraction Grating

We have patterned silver hole arrays directly on top of uniform permalloy (Py) films. Typical Py and Ag film thicknesses are 25nm and 40 nm respectively; the holes in the Ag have a 500nm diameter and are patterned on a 1 micron lattice constant. We have measured resonant modes arising from a quasi-uniform microwave excitation field, applied in the plane of the sample, as a function of the in-plane external field and the in-plane field orientation relative to the principal axes of the array. Measurements were done using our broadband meanderline-based ferromagnetic resonance (FMR) spectrometer.\footnote{C. C. Tsai, J. Choi, S. Cho, B. K. Sarma, C. Thompson, O. Chernyashevskyy, I. Nevirkovets, and J. B Ketterson, Rev. of Sci. Instr. \textbf{80}, 023904 (2009).} In addition to a uniform FMR mode we observe satellite modes that correspond to the Damon-Eshbach spin waves\footnote{R. W. Damon and J. R. Eshbach J. Phys. Chem. Solids \textbf{19}, 308 (1961).} with wave vectors having Fourier components of the reciprocal lattice of the silver array. Hence, in an otherwise uniform magnetic film the silver array acts as a \textit{diffraction grating} which excites spin waves with k $\ne $ 0 from the dynamic k $\approx $ 0 microwave magnetic field. The observed spin wave angular dispersion is in excellent agreement with a magnon dispersion relation for spin waves in a uniform film given by Kriesel et al.\footnote{A. Kreisel, F. Sauli, L. Bartosch, and P. Kopietz, Eur. Phys. J. B \textbf{71}, 59 (2009).}   

Time-dependent spin-wave theory

We generalize the spin-wave expansion in powers of the inverse spin to time-dependent quantum spin models describing rotating magnets or magnets in time-dependent external fields. We show that in these cases the spin operators should be projected onto properly defined rotating reference frames before the spin components are bosonized using the Holstein-Primakoff transformation. As a first application of our approach, we calculate the re-organization of the magnetic state due to Bose-Einstein condensation of magnons in the magnetic insulator yttrium-iron garnet; we predict a characteristic dip in the magnetization which should be measurable in experiments.   

Autonomous and driven dynamics of spin torque nano-oscillators

Understanding the dynamical properties of autonomous spin torque nano-oscillators (STNO) and their response to external perturbations is important for their applications as nanoscale microwave sources. We used spectroscopic measurements to study the dynamical characteristics of nanopillar- and point contact-based STNOs incorporating a microstrip in close proximity to the active magnetic layer. By applying microwave current at frequency $f_{ext}$ to the microstrip, we were able to generate large microwave fields of more than 30 Oe rms at the location of STNO. We demonstrate that for a wide range of $f_{ext}$, STNO exhibits multiple synchronization regimes with integer and non-integer rational ratios between $f_{ext}$ and the oscillation frequency $f$. We show that the synchronization ranges are determined by the symmetry of the oscillation orbit and the orientation of the driving field relative to the symmetry axis of the orbit. We observe synchronization hysteresis, i.e. a dependence of the synchronization limits on the dynamical history caused by the nonlinearity of STNO. We also show that the oscillation can be parametrically excited in the subcritical regime of STNO by a microwave field at twice the frequency of the oscillation. By measuring the threshold and the frequency range of parametric excitation, we determine damping, spin-polarization efficiency, and coupling to the microwave signal. In addition, by measuring the frequency range of parametric synchronization in the auto-oscillation regime, we determine the dynamic nonlinearity of the nanomagnet. Thus, analysis of the driven oscillations provides complete information about the dynamical characteristics of STNO. Finally, we discuss several unusual dynamical behaviors of STNO caused by their strong nonlinearity.   

Improved coherence of a quasi-linear spin-torque nano-oscillator

We have fabricated tapered nanopillar spin-valve devices, $\sim $ 50 x 145 nm2, from a Py(5)/Cu(12)/Py(20) multilayer (thickness in nm) for spin torque nano-oscillator (STNO) studies. When biased with electron flow from the thick layer to the thin layer multiple, high power, but broad ($\Delta $f $>$50 MHz), spin torque excitation modes are obtained at hard-axis fields Hy $<$ 500 Oe. For Hy $\sim $ 700 Oe we obtain much more coherent ST oscillations, $\Delta $f $<$ 10 MHz, close to that predicted for a linear STNO at 300 K. A macrospin model successfully explains the optimum field bias as being where the amplitude-dependent red shift effect due to the demagnetization field is closely balanced by the blue shift effect due to the in-plane anisotropy field. We have also modeled the internal field within the free layer as a function of Hy and conclude that the multiple modes at lower fields originate from the relatively broad spatial distribution of the internal field, or equivalently from the broad natural frequency distribution of the individual magnetic elements. Our results suggest pathways for further enhancements in STNO performance.   

Time-Domain Measurements of Real-Space Magnetization Trajectories in Spin Torque Oscillators

We make time-domain measurements of the microwave signal emitted by spin torque nano-oscillators (STNOs) based on magnetic tunnel junctions with Fe-rich free layers. The perpendicular magnetic anisotropy of the free layer nearly cancels its easy-plane shape anisotropy, allowing the magnetization to undergo large amplitude precession. The microwave power emitted by such STNOs reaches values approaching 0.4 $\mu$W. We employ a high-gain low-noise amplifier to further amplify the emitted signal, thereby bringing it to a level ($\sim$ 0.5 V rms) far exceeding the noise floor (5mV rms) of a 12 GHz, 40 Gs/s storage oscilloscope used for time-domain measurements. Relying on the assumption that extrema of the measured voltage versus time trace correspond to the magnetization crossing the sample plane, we use these time-domain traces to reconstruct the statistical distributions of the azimuthal angles at which the magnetization vector of the free layer crosses the plane of the sample. We measure the evolution of these crossing angle distributions as a function of current density and compare to theoretical predictions.   

Network Analyzer Measurements of Spin-Torque Dynamics

A microwave current flowing through a magnetic tunnel junction (MTJ) produces an oscillating spin torque. This oscillating spin torque is able to excite resonant magnetic dynamics and produce an oscillating resistance. The oscillating resistance combined with an applied DC current can generate a microwave voltage signal at the same frequency as the input microwave signal. We show that a network analyzer measurement of the amplitude and phase of this signal provides a simple way to make a quantitative measurement of the strength and direction of the spin transfer torque vector in MTJs at non-zero biases, the regime of primary interest for applications. Compared with a previous time-domain technique for measuring the spin torque vector, this technique requires no specialized equipment and provides roughly similar sensitivity. Compared to dc-detected spin-torque ferromagnetic resonance, the network-analyzer method is free of artifacts at high bias.   

Spin-transfer-driven parametric resonance in magnetic nanodomains

We study experimentally the parametric excitation of a magnetic nanodomain by spin-transfer-torque (STT). In our experiments, we use a nanoscale point contact to inject high-density ac (microwave frequency) and dc currents into an exchange-biased IrMn/NiFe/Cu/NiFe spin valve (EBSV) and to produce STT [1] on NiFe moments in a small contact region. Here a time-dependent STT associated with the microwave current produces a time-dependent modulation of the effective damping parameter which, in turn, drives the magnetic moments into parametric resonance [2]. The resonance was detected electrically by measuring a small rectified dc voltage which appears across the contact at resonance [3]. We study this resonance signal as a function of frequency and power of the applied microwaves. As expected for parametric excitation, this resonance has an ac threshold and occurs at double the natural frequency of magnetic precession (FMR frequency). We found that both the excitation threshold and the width of the resonance depend on the applied dc bias. Detailed dc bias dependent measurements of the resonance signal provide a means to characterize instability regions in parameter space known as Arnold tongues. The parametric excitation can be potentially used in magnetic memory technology for reducing power and increasing speed of logic and memory devices. [1] J. C. Slonczewski (1996); L. Berger (1996); M. Tsoi et al. (1998). [2] M. Faraday (1831). [3] T. Staudacher and M. Tsoi (2011).   

Spin-orbit coupling and spin excitations in nanoscopic strucutres

We have developed a formalism to calculate the spectra of spin excitations of structures of nanoscopic dimensions that takes into account spin-orbit coupling. We study structures composed by magnetic units (adatoms, clusters, ultrathin films) deposited on metallic substrates. The reduced symmetry of the magnetic units enhance the effects of spin-orbit coupling and activate mechanisms such as the Dzyaloshinskii-Moriya anti-symmetric exchange coupling. We are also able to predict anysotropic g-factors. In the case of ultrathin films, our formalism can span the entire Brillouin zone, being able to describe within the same framework FMR results and spectra obtained with SPEELS. We can also calculate the spectra that would be obtained by local probes such as inelastic scanning tunneling spectroscopy. We will present results for Fe ultrathin films on W(110) and for several transition metal adatoms on metallic substrates.   

Spin orbit driven ferromagnetic resonance and torques in single ferromagnetic layers

The coupling of spin and charge may convert electrical currents into spin currents in non-magnetic metals. In non-magnetic metals with strong spin orbit (SO) interaction in combination with magnetic metals one can also us the effect to excite magnetization dynamics; electrical currents in the non-magnetic metal transform to spin currents and the spin currents diffuse to the magnetic metal interacting with the magnetic moments. The combination of non-magnetic metals and magnetic metals has been recently used to determine spin hall angles. Here we demonstrate that spin currents in a ferromagnetic layer associated with SO interactions can excite ferromagnetic precession in the same layer. We have studied Co|Ni multilayers with both in-plane anisotropy and weak out-of-plane anisotropy. Results show that the samples have strong SO interactions. We have injected microwaves into patterned samples with several geometries and measured the mixed voltage in the same leads. Oscillatory currents drive FMR in the thin-film layer. We show that SO torques are primarily responsible for the magnetic excitations in samples with strong SO interactions, whereas samples with a weaker SO barely respond to the injected microwaves and show asymmetric components from charge current induced Oersted fields.