Imaging small-amplitude magnetization dynamics in a longitudinally magnetized microwire

We have used time-resolved scanning Kerr microscopy to study spin waves in a magnetic microwire subjected to a bias magnetic field applied parallel to its long axis. The spin-wave spectra obtained from different points near one end of the wire reveal several normal modes. We found that modes of a higher frequency occupied regions located further from the end of the wire. This was interpreted in terms of the confinement of the spin-wave modes by a nonuniform demagnetizing field. Furthermore, at a particular distance from the end of the wire, two or more modes occupying different regions along the width of the wire were observed. This was interpreted in terms of the confinement of the spin-wave modes due to an asymmetric variation in the local angle between the static magnetization and the effective direction of the wave vector of the confined modes. Images of the dynamic magnetization that are acquired at fixed pump-probe time delays revealed stripes lying perpendicular to the long axis of the wire and, hence, to the applied magnetic field. We interpret the stripe pattern in terms of a collective mode of the quasiperiodic system of ripple domains existing within the polycrystalline sample. Our results give an additional insight into the connection between the nonuniform static magnetic state in small magnetic elements and their precessional dynamics, which is fundamentally important for the design of future high-speed switching and spin-wave logic devices of magnonics.

Figure 1

(a) The geometry of the sample is shown. The white circles show the points of the wire from which the time-resolved signals were measured. The dashed rectangle shows the part of the wire from which the images were acquired. (b) The FFT power spectra calculated from the time-resolved signals acquired from the different points shown in (a) are shown for the bias field values of 5.5 and 15.2 mT. The background due to the pulsed field profile was subtracted from the time-resolved signals prior to FFT.

Figure 2

(Color online) (a) The ground magnetic configuration assumed in the discussion is schematically shown. The arrows and the grayscale represent the direction of the static magnetization and the value of its in-plane component that is perpendicular to the direction of the applied field, respectively. The white circles show the points of the wire from which the time-resolved signals were measured. The dashed rectangle shows the part of the wire from which the images were acquired. (b) The FFT power spectra calculated from the time-resolved signals acquired from the different points shown in (a) are shown for the bias field values of 5.5 and 15.2 mT. The background due to the pulsed field profile was subtracted from the time-resolved signals prior to FFT. The peaks at frequencies smaller than 1 GHz are due to an imperfect subtraction of the background and so are irrelevant to the discussion in this Brief Report.

Figure 3

(a) The time-resolved signal acquired from point 7 in Fig. 1a is shown for the bias field of 5.5 mT. The dashed rectangle indicates the time interval in which the images of the dynamic magnetization were acquired. (b) Grayscale images of the dynamic magnetization acquired from the $6\times 6.5\mu {\text{m}}^2$ region near the end of the wire shown in Fig. 1a are presented for the indicated pump-probe time delays. The images correspond to the same scale, which was arbitrarily chosen to give the best contrast on average. See also Ref 21.

Figure 4

[(a)–(c)] The vector MOKE hysteresis loops are shown for different orientations of the bias magnetic field relative to the wire and the direction of the sensed component of magnetization. (d) A wide-field Kerr microscopy image of the magnetic configuration in the wire is shown for the field value of $-1.9\text{mT}$ approached from saturation in the opposite (positive) direction. The magnetization component parallel to the short edge of the wire is shown. (e) A qualitative sketch of the domain structure imaged in (d) is presented.