Microwave Assisted Switching of Single Domain ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ Elements

We study the switching behavior of thin single domain magnetic elements in the presence of microwave excitation. The application of a microwave field strongly reduces the coercivity of the elements. We show that this effect is most profound at the ferromagnetic resonance frequency of the elements. Observations using time-resolved magneto-optic Kerr microscopy in combination with pulsed microwave excitation further support that the microwave assisted switching process is indeed based on the coherent motion of the magnetization.

Figure 1

Experimental configuration. A coplanar waveguide is used to excite the magnetic microstructures. The magnetic easy axis and the applied magnetic field are parallel to the coplanar waveguide.

Figure 2

The main graph shows the Kittel plot for the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ film. The resonance frequencies were determined by measuring the amplitude of the precession as a function of frequency at a low microwave power of 0 dBm. The inset on the top shows an optical microscope image of the microstructures. In the inset on the bottom resonance scans are shown. The red curve corresponds to the unpatterned ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ film. Blue and olive curves correspond to $1.5\times 3.0\mu {\mathrm{m}}^2$ and $0.7\times 1.4\mu {\mathrm{m}}^2$ hexagons, respectively. The microstructures have a built-in shape anisotropy of 0.5 mT and 1 mT leading to zero-field frequency offsets 250 and 700 MHz compared to the continuous film, respectively. The black dashed curve shows a resonance scan for the $1.5\times 3\mu {\mathrm{m}}^2$ hexagon measured in a field of 5 mT.

Figure 3

Series of hysteresis loops measured for the $1.5\times 3\mu {\mathrm{m}}^2$ hexagon as a function of microwave power at a fixed frequency of 2 GHz. The microwave frequency is 2 GHz with a power of (a) 8 dBm, (b) 15 dBm, (c) 16 dBm, and (d) 18 dBm. As the magnetization reverses, the sign of the magnetic response is inverted; see image scans in (a). The slight increase of the signal with increasing field is due to the fact that the amplitude of the magnetic precession increases with field since the resonance of the element at 2 GHz occurs at about 5 mT.

Figure 4

Coercive field of the of the $0.7\times 1.4\mu {\mathrm{m}}^2$ (diamonds) and $1.5\times 3.0\mu {\mathrm{m}}^2$ (circles) hexagons as a function of microwave power at a frequency of 2 GHz.

Figure 5

Coercive fields as a function of microwave power and frequency measured for the $1.5\times 3.0\mu {\mathrm{m}}^2$ hexagon. The zero-field resonance occurs at about 500 MHz (see lower inset in Fig. 2). The coercivity is color-coded and ranges between 0 and 0.4 mT. In addition to the power scale also a scale with the corresponding rf magnetic field is shown.

Figure 6

Pulse length dependence of the coercive field measured for the $0.7\times 1.4\mu {\mathrm{m}}^2$ hexagon. The pulse length is varied in 30 ps steps between 0.4 and 2.5 ns. The microwave carrier frequency is 2 GHz. The inset shows a 500 ps microwave burst acquired with a fast oscilloscope and the corresponding magnetic response.