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Spin-transfer Torque Induced Magnetization Dynamics

A spin-polarized current entering into a ferromagnetic material exerts a torque on the magnetization by transferring spin angular momentum from the current to the ferromagnet, if the polarization direction differs from the quantization (magnetization) direction in the ferromagnet. This so-called spin-transfer torque gives rise to current-driven magnetization dynamics with unprecedented properties, such as the switching of the magnetization without applying an external field. This current-induced magnetization switching mechanism provides a smart alternative to magnetization switching by induction and is applied in magnetic random access memories (MRAM) for the writing process. Spin-transfer torques can also excite persistent large-angle precessions of the magnetization with frequencies in the GHz range, which are not accessible using magnetic field excitation alone. Persistent current-driven precessions are the basis for the so-called spin-transfer oscillators that are envisaged for applications in communication technology

Dr. Daniel E. Bürgler

Results


1. Fabrication process for nanopillars
2. Normal and inverse current-induced magnetization switching
3. Asymmetric spin-transfer torque
4. Spin-Transfer Induced Dynamic Modes in Single-Crystalline Fe/Ag/Fe Nanopillars
5. Magnetization dynamics in STOs: Vortex state versus uniform state
6. Injection locking of the gyrotropic vortex motion in a nanopillar
7. Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars
8. Quenched Slonczewski windmill in spin-torque vortex oscillators

Results

1. Fabrication Process for Nanopillars


STT_sample_jpg

We have developed a structuring process involving optical and e-beam lithography to fabricate nanopillar structures from epitaxially grown multilayers with pillar diameters down to 70 nm. The nanopillars are connected by relatively thick top and bottom electrodes in order to apply the high current densities, which are required for the observation of spin-transfer torque effects. The process has been successfully tested with epitaxial Fe/Ag/FeFe(001) and Fe/Ag/Fe/Cr/Fe(001) structures for which we measured current-induced hysteretic switching of the magnetic layers as well as microwave signals in the frequency range between 3 and 10 GHzH.

H. Dassow, R. Lehndoff, D. E. Bürgler, M. Buchmeier, P. A. Grünberg, C. M. Schneider, and A. van der Hart
Normal and inverse current-induced magnetization switching in a single nanopillar
Appl. Phys. Lett. 89, 222511, (2006).

2. Normal and Inverse Current-induced Magnetization Switching

We observed normal and inverse current-induced magnetization switching in single-crystalline nanopillars prepared from Fe(14 nm)/Cr(0.9 nm)/Fe(10 nm)/Ag(6 nm)/Fe(2 nm) multilayers. The nanopillars have a diameter of 150 nm. The central Fe layer is coupled to the thick one by interlayer exchange coupling over Cr, while the topmost Fe layer is decoupled. The opposite spin scattering asymmetries of the Fe/Cr and Fe/Ag interfaces lead to normal and inverse CIMS for the two subsystems, which are combined in a single device. At high magnetic fields, step-like resistance changes are measured at positive currents and are attributed to current-driven magnetic excitations.

Figure: At -20mT, we observe hysteretic magnetization switching at both current polarities, which is related to the opposite spin scattering asymmetries of the Fe/Cr and Fe/Ag interface, respectively. Fe/Ag exhibits normal switching behaviour, whereas for Fe/Cr the direction of the spin-transfer torques is reversed giving rise to inverse switching. At -1166 mT both switching event are suppressed.

H. Dassow, R. Lehndoff, D. E. Bürgler, M. Buchmeier, P. A. Grünberg, C. M. Schneider, and A. van der Hart
Normal and inverse current-induced magnetization switching in a single nanopillar
Appl. Phys. Lett. 89, 222511, (2006).


3. Asymmetric Spin-transfer Torque


STT_twostep_jpg


Figure: Two-step current-induced switching of the free layer magnetization at 5 K. Ic1 and Ic2 denote the critical currents for the switching from parallel to perpendicular and from perpendicular to antiparallel alignment, respectively. The perpendicular state is stabilized by the cubic anisotropy of bcc-Fe. Inset: Schematic sample structure.

We investigated current-perpendicular-plane giant magnetoresistance (CPP-GMR) and current-induced magnetization switching in single-crystalline Fe/Ag/Fe nanopillars of 70 nm diameter. The interplay between the in-plane, fourfold magnetocrystalline anisotropy of the Fe(001) layers and the spin-transfer torque (STT) gives rise to a two-step switching behaviour, which allows an investigation of the angular dependences of CPP-GMR and spin-transfer torque. Both behave asymmetrically with respect to the perpendicular alignment of the two Fe layer magnetizations as theoretically predicted, due to strong spin accumulation at the Fe/Ag(001) interfaces. The asymmetry parameter determined from the STT data quantitatively agrees with calculated spin-dependent interface resistances, whereas CPP-GMR yields a smaller degree of asymmetry.

R. Lehndoff, D. E. Bürgler, A. Kákay, R. Hertel, and C. M. Schneider
Asymmetric spin-transfer torque in single-crystalline Fe/Ag/Fe nanopillars
Phys. Rev. B 76, 214420 (2007)

4. Spin-Transfer Induced Dynamic Modes in Single-Crystalline Fe/Ag/Fe Nanopillars

STT_fish.jpg

Figure: Simulated STT-induced switching of a macrospin in the presence of cubic magnetocrystalline anisotropy and demagnetizing field. The free layer Mfree switches under the influence of a persistent DC current first from parallel (+x-direction) to a 90°-orientation (+y-direction) with respect to the fixed layer Mfixed and then from the 90°-orientation to the antiparallel alignment (−x-direction).  (a,b) Trajectories of the two switching events. (c,d) Representation of the STT (blue arrows) and damping torque (red arrows) viewed along (b) the initial, parallel and (c) the 90°-orientation of the macrospin. Only a fraction of the trajectory in the immediate vicinity of the switching event (a) is shown in (c) and (d).

We performed measurements and simulations of spin-transfer torque (STT)-induced magnetization dynamics in nanopillars containing a thin, circular, single-crystalline Fe nanomagnet with four-fold in-plane magnetocrystalline anisotropy as a free layer. The magnetocrystalline anisotropy inherent to bcc-Fe allows for a consecutive switching of the magnetization by 90° between parallel and antiparallel alignment to the fixed layer magnetization. Additionally, the anisotropy gives rise to steady-state precession of the magnetization at low or even zero applied magnetic fields as well as at large fields exceeding the coercive field. While the low-field mode is governed by the interplay between the STT and the anisotropy, the high-field dynamics result from the STT acting against the externally applied magnetic field.

R. Lehndoff, D. E. Bürgler, A. Kákay, R. Hertel, and C. M. Schneider
Spin-Transfer Induced Dynamic Modes in Single-Crystalline Fe-Ag-Fe Nanopillars
IEEE Trans. Magn. 44, 1951 (2008).


5. Magnetization Dynamics in STOs: Vortex State versus Uniform State

STT_compare.jpg

Figure: STT induced excitation of qualitatively different oscillatory modes in a nanodisk. (a) After preparation of a uniform state, a standing-wave mode with a transition from blue-to-red shift is excited. (b) The gyrotropic mode is excited after preparation of the vortex state. Note that the microwave output power generated by the gyrating vortex for a given DC current in (b) is much higher than for the standing-wave mode in (a).

We undertook experimental studies of current-driven high-frequency (HF) excitations of STOs for two fundamental magnetization states of the free layer, namely, the vortex state and uniform in-plane magnetization. Our ability to switch between the two states in a given STO enables a direct comparison to be made of the critical currents, agility, power, and linewidth of the HF output signals. We found that the vortex state has some superior properties; in particular, it maximizes the emitted HF power and shows a wider frequency tuning range at a fixed magnetic field.

R. Lehndorff, D. E. Bürgler, S. Gliga, R. Hertel, P. Grünberg, and C. M. Schneider
Magnetization dynamics in spin torque nano-oscillators: Vortex state versus uniform state
Phys. Rev. B 80, 054412 (2009).


6. Injection Locking of the Gyrotropic Vortex Motion in a Nanopillar


STT_locking_jpg

Figure: Injection locking of the gyrotropic vortex motion. Power spectra (fdet, vertical axis) of the current-induced gyrotropic mode as a function of HF excitation frequency (fext, horizontal axis) measured for different excitation amplitudes as indicated. The intensity at fext exceeds the colour scale and appears as diagonal white lines.

Spin-torque oscillators (STOs) are a promising application for the spin-transfer torque effect. The major challenge lies in pushing the STO’s microwave output power to useful levels, e.g. by operating an array of STOs in a synchronized, phase-locked mode. Our experiment on metallic, giant magnetoresistance-type nanopillars focuses on the influence of external high-frequency signals on current-driven vortex dynamics and demonstrates the injection locking of the gyrotropic mode. We detected a gap of approximatelyt three orders of magnitude between the high-frequency power emitted by one oscillator and the power needed for phase-locking.

R. Lehndorff, D. E. Bürgler, C. M. Schneider, and Z. Celinski
Injection locking of the gyrotropic vortex motion in a nanopillar
Appl. Phys. Lett. 97, 142503 (2010).


7. Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars

STT_2vortex_jpg

Figure: Single and double vortex states in a nanopillar with a diameter of 150 nm. Experimental (left) and simulated (right) resistance vs. field dependence of a nanopillar under the influence of a DC current. Stars in the left part indicate high frequency excitations, and the symbols in the right part display the magnetization states in different field ranges.

We performed experimental and analytical work on spin-transfer torque-induced vortex dynamics in metallic nanopillars with in-plane magnetized layers. Our studies involved nanopillars with a diameter of 150 nm, containing two Fe layers with a thickness of 15nm and 30 nm respectively, separated by a 6nm Ag spacer. The sample geometry is such that it allows for the formation of magnetic vortices in the Fe disks. As confirmed by micromagnetic simulations, we are able to prepare states where one magnetic layer is homogeneously magnetized while the other contains a vortex. We  show experimentally for this configuration that spin-transfer torque can excite vortex dynamics and analyze their dependence on a magnetic field applied in the sample plane. The centre of gyration is continuously dislocated from the disk centre, and the potential changes its shape according to field strength. The latter is reflected in the field dependence of the excitation frequency.

In a second step, we propose a novel mechanism for the excitation of the gyrotropic mode in nanopillars with a perfectly homogeneously magnetized in-plane polarizing layer. We show analytically that in this configuration the vortex can absorb energy from the spin-polarized electric current if the angular spin-transfer efficiency function is asymmetric. This effect is supported by micromagnetic simulations.

V. Sluka, A. Kákay, A. M. Deac, D. E. Bürgler, R. Hertel, and C. M. Schneider
Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars
J. Phys. D: Appl. Phys. 44, 384002 (2011)

8. Quenched Slonczewski windmill in spin-torque vortex oscillators

We present a combined analytical and numerical study on double-vortex spin-torque nano-oscillators and describe a mechanism that suppresses the windmill modes. The magnetization dynamics is dominated by the gyrotropic precession of the vortex in one of the ferromagnetic layers. In the other layer, the vortex gyration is strongly damped. The dominating layer for the magnetization dynamics is determined by the sign of the product between sample current and the chiralities. Measurements on Fe/Ag/Fe nanopillars support these findings. The results open up a new perspective for building high quality-factor spin-torque oscillators operating at selectable, well-separated frequency bands.

NanopillarResistance

Figure: Experimental resistance vs. field measurements of a Fe(25 nm)/Ag(6 nm)/Fe(15 nm) nanopillar with a diameter of 230 nm for (a) increasing field at +21 mA and (b) decreasing field at -21 mA. The low-resistance region around B=0 indicates the existence of the double-vortex state. Simultaneously recorded high-frequency spectra yields the excitation frequencies shown in (c) and (d). The frequency ranges are clearly different and well separated for the two current polarities.

V. Sluka, A. Kákay, A. M. Deac, D. E. Bürgler, R. Hertel, and C. M. Schneider
Quenched Slonczewski windmill in spin-torque vortex oscillators
Phys. Rev. B 86, 214422 (2012)


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