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Invited Review Article: Microwave spectroscopy based on scanning thermal microscopy: Resolution in the nanometer range
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Image of FIG. 1.
FIG. 1.

Lateral and depth-dependent thermal diffusion length vs size of thermal source at a constant heating modulation frequency of .

Image of FIG. 2.
FIG. 2.

Schematic of a semi-infinite silicon substrate with a magnetic nanostructure consisting of a Py-stripe of limited dimensions (, , ), representing the modulated homogeneous heat source. The component of the Py film is not shown in the figure due to its small size.

Image of FIG. 3.
FIG. 3.

Lateral thermal wave distribution perpendicular to the stripe (as shown in the inserted sketch) calculated for a modulated heat source of finite dimensions, in ideal thermal contact with semi-infinite silicon substrate. The heating modulation frequencies are varied from (red line) to (blue line).

Image of FIG. 4.
FIG. 4.

Techniques of thermal microscopy (from left to right): schematic of thermoreflectance (TR), scanning thermal microscopy (SThM), and scanning thermoelastic microscopy (SThEM).

Image of FIG. 5.
FIG. 5.

SEM images of two commercially available SThM probes: (a) Wollaston wires with Pt tip from Veeco; (b) coated Si tip from PSIA.

Image of FIG. 6.
FIG. 6.

Scheme of ferromagnetic resonance measurement. The magnetization precession in the sample is illustrated with driving microwave field distribution in the surrounding cavity.

Image of FIG. 7.
FIG. 7.

Illustration of the principle of demagnetizing field. An externally applied field leads to a virtual polarization of the edges of the sample S,N. The white area represents a small area of the ferromagnetic material which sees a demagnetizing field resulting from this polarization.

Image of FIG. 8.
FIG. 8.

Precession of relaxing magnetization vector illustrating the Landau Lifshitz Gilbert [Eq. (4)].

Image of FIG. 9.
FIG. 9.

Scheme of the SThM-FMR setup.

Image of FIG. 10.
FIG. 10.

Cavity with waveguide part of setup.

Image of FIG. 11.
FIG. 11.

Design of the cavity top.

Image of FIG. 12.
FIG. 12.

Schema of the Explorer setup providing optimal scanning abilities.

Image of FIG. 13.
FIG. 13.

Pictures of the Explorer setup. left: top view of the FMR part with open cavity and removed AFM; right: side view of the explorer setup with AFM placed on the FMR part; bottom: thermal probe placed on a magnetic nanostructure in the cavity taken by the camera of the Explorer.

Image of FIG. 14.
FIG. 14.

Picture of the SThM-FMR setup capable of local FMR spectroscopy. The inset shows the mounted thermal nano probe and the larger cavity hole.

Image of FIG. 15.
FIG. 15.

Scheme of the SThEM-FMR.

Image of FIG. 16.
FIG. 16.

Picture of the STM scan head mounted on the Nanocube.

Image of FIG. 17.
FIG. 17.

Sketch of the PM-FMR setup.

Image of FIG. 18.
FIG. 18.

Typical PM-FMR signal.

Image of FIG. 19.
FIG. 19.

Top: Optical microscope image of structured Fe surface. Bottom: Gray scale image of the field independent PM-FMR-amplitude. The spots marked by a and b indicate the position for the PM-FMR spectra provided in Fig. 20.

Image of FIG. 20.
FIG. 20.

Amplitude of FMR response versus applied magnetic field. Spectra top to bottom: Integral conventional FMR, PM-FMR of Fe mesa on pure GaAs marked by a in Fig. 19, PM-FMR spectrum of Fe-mesa with HEMT below marked by b in Fig. 19.

Image of FIG. 21.
FIG. 21.

Left: Sketch of the Fe film sample prepared by molecular beam epitaxy, including the two different oxidation stages due to the removal of the two top layers. Right: Optical microscope image of the sample under investigations. The dark blue areas are regions where the protecting ZnS film is still in place. In the white shining areas only Ag is covering the Fe film. In the brown rough areas iron has been completely oxidized.

Image of FIG. 22.
FIG. 22.

FMR spectra of the Fe film. Upper three spectra: Derivative of the resonance absorption of the TM-FMR: (from top to bottom) uncovered Fe, Ag-covered area, and ZnS-protected area of the sample. Lower spectrum: conventional amplitude modulated FMR. The dashed lines indicate the pure Fe line position and the dash-dotted line the shifted resonance of the spectrum for the Ag-covered area.

Image of FIG. 23.
FIG. 23.

Illustrating model of the sample structure: Au is evaporated on mica and flame annealed. On top Ni is grown electrolytically forming differently shaped dots in a Ni film matrix. Each dot is a separate heat source for the thermoelastic expansion while in FMR.

Image of FIG. 24.
FIG. 24.

Topography of the Ni film grown on a (111)-textured flash annealed Au film grown on a mica substrate. The blue and the red markers give the position for the SThEM-FMR measurements shown in Fig. 25

Image of FIG. 25.
FIG. 25.

Red and blue lines represent SThEM-FMR spectra taken at the red and blue marked spots in Fig. 24. The black line is the conventional FMR spectrum of the whole sample measured by the microwave diode given in arbitrary units.

Image of FIG. 26.
FIG. 26.

Frequency dependence of the FMR line width: black dots represent the linewidth measured by conventional FMR at three different frequencies. The upper black line represents the fit to the line width due to the equation. The lower black line represents the slope of the fit with subtracted representing the frequency dependence of the intrinsic part of damping. The dotted red line give the intrinsic part of the line width at a microwave frequency of .

Image of FIG. 27.
FIG. 27.

Sketch of the combined MR and SThM-FMR setup.

Image of FIG. 28.
FIG. 28.

FMR investigations of Ni wires: (a) conventional angle dependent FMR on a Ni wire. (c) Locally resolved SThM-FMR spectra of the thick end of the Ni-wire . (d) of the thin part of the constricted Ni-wire . The spectra correspond to positions 1 and 4 in the scheme marked by (b). The microscope pictures show the thermal nanoprobe on the wire during SThM-FMR measurement.

Image of FIG. 29.
FIG. 29.

Relative magnetoresistance curves for positive increasing field only. The different curves are measured at the AFM probe positions on the Ni wire marked by numbers related to the scheme above.

Image of FIG. 30.
FIG. 30.

Scanning electron microscope image of the Py-stripe lattice.

Image of FIG. 31.
FIG. 31.

Amplitude modulated conventional FMR measured in the SThM-FMR setup providing the resonance fields marked by the dashed lines for the SThM-FMR images of Fig. 32.

Image of FIG. 32.
FIG. 32.

AFM topography (left) and thermal images of SThM-FMR (right) for different applied resonance fields of Fig. 31.

Image of FIG. 33.
FIG. 33.

Sketch of the Co stripes on Si. The dimension of a Co stripe is , the distance between the stripes is .

Image of FIG. 34.
FIG. 34.

Image of the angle dependent conventional FMR investigation of the Co-stripe lattice. The FMR absorption amplitude is plotted in rainbow colors against the in-plane angle on the axis and the external field on the axis. The arrows mark the four resonance line positions for the external field applied perpendicular to the stripes (compare Fig. 33).

Image of FIG. 35.
FIG. 35.

Conventional FMR absorption spectrum taken in the SThM-FMR setup for external field perpendicular to the Co stripes. The four observed resonances are marked according to Fig. 34.

Image of FIG. 36.
FIG. 36.

Calculated FMR frequency spectrum for applied external field perpendicular to the Co stripes deduced by Fourier transformation from the oscillating magnetization component in the direction of the microwave magnetic field displayed in the inset. I–III mark the resonances according to Figs. 34 and 35.

Image of FIG. 37.
FIG. 37.

SThM-FMR image of the uniform excitation marked by III in the Figs. 34–36. The size of the image is .

Image of FIG. 38.
FIG. 38.

SThM-FMR image of the rim-excitation marked by I in the Figs. 34–36. The size of the image is .

Image of FIG. 39.
FIG. 39.

Illustrations of the estimation for the temperature distribution perpendicular to the Co stripe due to the rim excitation. Upper part calculated magnetization distribution for the applied resonance field of . Lower part calculated temperature distribution taking into account the magnetization distribution above (Ref. 63).

Image of FIG. 40.
FIG. 40.

SThM-FMR image of a Co stripe taken at the resonance field of the backward volume mode marked by II in the Figs. 34–36. The size of the image is . Thermal amplitude and phase images are taken simultaneously not at an edge of the stripe to better prove the exponential decay of the amplitude towards the middle of the stripe and the homogeneity of the phase across the stripe.

Image of FIG. 41.
FIG. 41.

SThM-FMR image of the edge of a Co stripe taken at the resonance field of the not-aligned mode as marked in the conventional FMR spectra of Figs. 34 and 35. The size of the image is . The simultaneously recorded AFM image is displayed on the right, the thermal amplitude is displayed on the left.


Generic image for table
Table I.

Characteristics of locally resolved measurement techniques for the investigation of magnetic dynamics.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Invited Review Article: Microwave spectroscopy based on scanning thermal microscopy: Resolution in the nanometer range