32
P. K. Muduli et al.
9.1 Approaches to Enhance the Sensitivity of Microwave
Detection
9.1.1 Enhancement of Sensitivity Using Parametric Synchronization
Parametric synchronization, as discussed in Sect. 7, can be used to efficiently enhance
the signal sensitivity of an MTJ-based radio frequency STNO [133]. The synchronization increases the signal sensitivity at 2f 0 , where f 0 is the eigen frequency of the
free-running STNO. This happens when the frequency of the external microwave
signal (f e ) is close to twice the eigen frequency f 0 of the STNO [133]. Figure 19a
shows the spin-torque diode voltage signal obtained at an injected frequency of f e
∼ 9 GHz, which is twice the resonance frequency of the STNO, f 0 ∼ 4.5 GHz. The
signal at ∼ 9 GHz increases with dc current I dc , and exceeds the amplitude of the signal at ∼4.5 GHz for a current of 7 mA. The threshold current I th for auto-oscillation
is around 6.4 mA [40, 72, 96]. Hence, the sensitivity at 2f 0 becomes higher than at f 0
only above the threshold of the auto-oscillations. Figure 19b, c shows the macrospin
simulated behavior of the STNO below and above the threshold, respectively. Parametric synchronization happens above the threshold, as shown in Fig. 19c, and this
enhances the microwave sensitivity. The advantage of this approach is that the frequency of detection is twice that of the eigen frequency of the STNO, and hence the
technique also offers higher operation frequencies.
Fig. 19 Enhancement of sensitivity using parametric synchronization. a FM-STFMR spectra measured at varying dc biases at H ext = 400 Oe and an injected RF power of P e = −10 dBm. b and
c Macrospin simulated spectra of the STNO as a function of f rf at H ext = 400 Oe for I dc = 2 mA
and 7 mA, respectively
P. K. Muduli et al.
9.1 Approaches to Enhance the Sensitivity of Microwave
Detection
9.1.1 Enhancement of Sensitivity Using Parametric Synchronization
Parametric synchronization, as discussed in Sect. 7, can be used to efficiently enhance
the signal sensitivity of an MTJ-based radio frequency STNO [133]. The synchronization increases the signal sensitivity at 2f 0 , where f 0 is the eigen frequency of the
free-running STNO. This happens when the frequency of the external microwave
signal (f e ) is close to twice the eigen frequency f 0 of the STNO [133]. Figure 19a
shows the spin-torque diode voltage signal obtained at an injected frequency of f e
∼ 9 GHz, which is twice the resonance frequency of the STNO, f 0 ∼ 4.5 GHz. The
signal at ∼ 9 GHz increases with dc current I dc , and exceeds the amplitude of the signal at ∼4.5 GHz for a current of 7 mA. The threshold current I th for auto-oscillation
is around 6.4 mA [40, 72, 96]. Hence, the sensitivity at 2f 0 becomes higher than at f 0
only above the threshold of the auto-oscillations. Figure 19b, c shows the macrospin
simulated behavior of the STNO below and above the threshold, respectively. Parametric synchronization happens above the threshold, as shown in Fig. 19c, and this
enhances the microwave sensitivity. The advantage of this approach is that the frequency of detection is twice that of the eigen frequency of the STNO, and hence the
technique also offers higher operation frequencies.
Fig. 19 Enhancement of sensitivity using parametric synchronization. a FM-STFMR spectra measured at varying dc biases at H ext = 400 Oe and an injected RF power of P e = −10 dBm. b and
c Macrospin simulated spectra of the STNO as a function of f rf at H ext = 400 Oe for I dc = 2 mA
and 7 mA, respectively