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Versatile magneto-optic Kerr effect polarimeter for studies of domain-wall dynamics in magnetic nanostructures
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View: Figures


Image of FIG. 1.
FIG. 1.

MOKE instrument schematic and block diagram. Normal-incidence long-focal-length Kerr microscope ( resolution) facilitates microstructure alignment at laser focus. Polarimeter is based on a cw-laser source incident at 45° from normal. Beam expander and microscope objective produce beam focus at sample. Magnet assembly is mounted on stage ( accuracy with closed-loop optical encoders). Photomultiplier tubes with shortened dynode chains and amplifiers yield response.

Image of FIG. 2.
FIG. 2.

(a) Schematic of knife-edge measurement of beam profile. (b) Reflected beam intensity profile as the optimally focused laser spot is scanned over the cleaved edge of a Si substrate (solid symbols), along with numerically differentiated intensity profile (open symbols). The solid lines are fits to an error function profile and a Gaussian profile. (c) Plan-view scanning electron micrograph of a Permalloy nanowire, milled from a continuous film of Permalloy. Regions of light contrast correspond to unmilled regions of the metal layer. (d) Kerr susceptibility map of the same nanowire, presented with the same spatial scale as in (c). Lighter contrast corresponds to higher magnetic signal.

Image of FIG. 3.
FIG. 3.

Broadband magnet schematic and frequency characteristics. (a) Circuit diagram of the magnet design. The load resistors in the secondary circuit are absent in the high-frequency version. (b) Schematic of substrate orientation near the magnet wire (substrate not drawn to scale). [(c) and (d)] Calculated field amplitudes for the magnet wire vs position, including skin effects for several frequencies. Axes are defined by the schematic in (b).

Image of FIG. 4.
FIG. 4.

Measured current in the broadband magnet wire (solid line) and Kerr rotation measured in the phase calibration line (symbols) after the application of a current step wave form. The field wave form, proportional to the Kerr signal, closely follows the current wave form, indicating that the measured current wave form accurately characterizes the field wave form subjected to the sample. The temporal shift between the two is used to characterize the phase shift between the two measurement channels.

Image of FIG. 5.
FIG. 5.

(a) Measured magnet current (high-frequency version) as a function of frequency for sinusoidal input drives of two fixed amplitudes. The current point occurs at approximately . (b)–(d) show various examples of current/field wave forms produced in the magnet wire. (b) sinusoid. (c) Magnet current in response to a square-wave input (step) and a compensated step wave form (see text). (d) Magnet current pulse with a FWHM of .

Image of FIG. 6.
FIG. 6.

(a) Plan-view scanning electron micrograph of device structure designed for driving ferromagnetic nanowire with microwave currents. Regions of light contrast correspond to the unmilled metal regions. (b) Device impedance measured from .

Image of FIG. 7.
FIG. 7.

Dynamic coercive force measured for two thin Permalloy films over in drive frequency, showing frequency-dependent scaling behavior.

Image of FIG. 8.
FIG. 8.

Illustration of time-of-flight domain-wall velocity measurements. (a) Drive-field wave forms applied to drive a domain wall into and along a nanowire. Negative-going saturation pulse (not shown) preceded the positive-going field. (b) Time-resolved MOKE signal (symbols) and fits to the error function (solid lines), in response to the field wave form of (a). (c) The MOKE transients were measured at the nanowire locations indicated in the scanning electron micrograph. Ellipses approximate the width of the laser spot (incident at 45°). (d) Mean domain-wall arrival time vs position along the nanowire for different field-step amplitudes. The inverse slope of these wall trajectories yields the average velocity.

Image of FIG. 9.
FIG. 9.

Mobility curve (velocity vs field) for the nanowire shown in Fig. 8. The curve reveals a transition (Walker breakdown) from uniform domain-wall propagation (below velocity peak) to oscillatory propagation (above velocity peak).

Image of FIG. 10.
FIG. 10.

Schematic illustration of the relation of the reversal transient width to the domain-wall velocity for (a) a uniformly propagating domain wall and (b) a domain wall moving with an oscillatory trajectory. (c) Reversal transients measured at a fixed location showing variation of width with field. The solid lines are fits to the error function. The transients for 6.8 and , just above and just below the critical field, respectively, have been shifted horizontally for clarity. (d) Ratio of the measured reversal transient widths to those expected for a domain wall moving at velocity v across a laser spot of width , where is the average velocity shown in Fig. 9. Data are included for several measuring positions. The solid lines are visual guides.

Image of FIG. 11.
FIG. 11.

(a) Scanning electron micrograph of device structure used for current-driven domain-wall motion studies: nanowire and contiguous large-area film regions (light contrast) and milled regions (dark contrast). The large-area regions at either end of the wire form electrically isolated contact pads for current injection. Directions of domain-wall propagation and positive current (opposite to direction of flow) are indicated. (b) Average domain-wall trajectories during magnetic reversal under the application of a field step in the absence of current, and with injected current densities of .

Image of FIG. 12.
FIG. 12.

(a) Wall velocity vs dc for leftward- and rightward-moving walls at an applied field . Existing spin-torque models predict a linear dependence between and the current-density . The measured response can be decomposed into its even and odd components about , as shown in (b) and (c). The odd (linear) component agrees with the static “pressure” term of theoretical models, which would accelerate a wall traveling with the electron flow and decelerate a wall traveling against it. The even component, which tends to accelerate both walls regardless of the current direction, represents a new contribution of the spin torque to the wall-propagation dynamics related to the dynamic internal wall spin structure.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Versatile magneto-optic Kerr effect polarimeter for studies of domain-wall dynamics in magnetic nanostructures