Tunnel Barrier Enhanced Voltage Signal Generated by Magnetization Precession of a Single Ferromagnetic Layer

We report the electrical detection of magnetization dynamics in an $\mathrm{Al}/{\mathrm{AlO}}_x/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Cu}$ tunnel junction, where a ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ ferromagnetic layer is brought into precession under ferromagnetic resonance conditions. The dc voltage generated across the junction by the precessing ferromagnet is enhanced about an order of magnitude compared to the voltage signal observed when the contacts in this type of multilayered structure are Ohmic. We discuss the relation of this phenomenon to magnetic spin pumping and speculate on other possible underlying mechanisms responsible for the enhanced electrical signal.

Figure 1

Schematic diagram of the sample structure (a) and the measurement geometry (b). The arrow in the ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ layer indicates the magnetization direction. An external dc field $H_{\mathrm{ex}}$ is applied in the $y$ direction and the rf magnetic field $H_{\mathrm{rf}}$ is applied along the $x$ direction. A coplanar microwave probe feeds microwave signals through the coplanar waveguide. dc voltage across the junction (TJ) is measured between the top of the junction and signal line of the CPW.

Figure 2

The dc voltage $\Delta V$ generated across the $\mathrm{Al}/{\mathrm{AlO}}_x/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Cu}$ tunnel junction as a function of the externally applied static magnetic field. The frequency of the applied rf field ranges from 1.8 to 2.8 GHz. The background voltage is subtracted for comparison purpose.

Figure 3

The frequency dependence of static magnetic field at which the dc voltage peak (circles) appears in Fig. 2. The crosses label the frequency dependence of the resonance field obtained from FMR measurements on flip-chip structures in the CPW line. The curve is a fit to the Kittel formula [12].

Figure 4

The amplitude of the dc voltage $\Delta V$ measured across $\mathrm{Al}/{\mathrm{AlO}}_x/{\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}/\mathrm{Cu}$ device as function of (a) microwave power; (b) precession cone angle; and (c) microwave frequency at 10 dBm. Solid lines in (b) and (c) are the fit to Eq. (3) as described in the text.