Microwave Oscillators and Detectors Based …
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high linewidth are the main limitations for commercialization of STNOs. MTJ-based
STNOs offer significantly greater output power than metallic GMR-based STNOs,
and have recently reached 10 µW [28], in vortex based MTJ oscillators, which meets
the requirements of commercial applications. However, the frequency of operation
of such vortex STNOs is around 300 MHz, and hence they do not take full advantage of the wide-frequency tunability of STNOs, which can usually be tuned in the
1–65 GHz range [17]. STNOs have the capability to go beyond 200 GHz [29] with
conventional materials in high fields, and can even reach the terahertz range [30]
with the use of antiferromagnets.
MTJ nanopillars can also be used to rectify radio frequency (RF) signals through
spin-transfer torque [31–36]. With an RF current injected into an MTJ nanopillar, a dc voltage develops over the device when the frequency of the RF current
is close to the eigenfrequency of the nanomagnet. Thus, an MTJ nanopillar can
act as a microwave detector, also known as spin-torque diode (STD). For applications, a high sensitivity—defined as the ratio of the voltage detected to the input RF
power—is required. Preliminary calculations predicted that optimized MTJs should
reach sensitivities exceeding 10,000 mV/mW [37]. With advances in the design
of the MTJs in terms of tunneling magnetoresistance (TMR) effect and voltagecontrolled magnetocrystalline anisotropy (VCMA) and with improvements in the
understanding of strong nonlinear effects such as stochastic resonance [38] and nonlinear resonance [35, 36], the sensitivity performance of STD has reached values as
high as 10
5 mV/mW, exceeding that of semiconductor Schottky diode detectors at
room temperature. These results open a path for the design of a new generation of
high-sensitivity microwave detectors for wireless power transfer, which can rectify
microwave power from different energy sources, such as satellite, sound, television,
and Wi-Fi signals. The resulting dc voltage could be used to operate low-powered
nanodevices [39].
2 Linear Spin Wave Modes in MTJ-Based Nanopillars
Spin waves can be excited by thermal excitations in MTJ nanopillars in the presence
of an external field. A frequency domain spectrum of the time-varying magnetization
in the MTJ will reveal that magnetic moments can oscillate at several different frequencies. Each of these coexisting frequencies is called a frequency eigenmode. The
study of these modes gives an insight into the coherence of spin motion inside the
ferromagnet. Because the excited spin waves are contained mostly in the ferromagnetic material and die out fast outside it, different spin wave modes are excited only
from the magnetic part of the nanopillars—i.e., the free layer and the fixed layer.
Moreover, as the fixed layer is pinned strongly in one particular direction due to
exchange bias, a very high field is usually required to excite spin waves in it. Figure 2
shows the field variation of the excited spin wave modes in an MTJ nanopillar as
reported in Ref. [40]. The MTJ investigated in [40] had a circular cross-section with
a diameter 240 nm consisting of multilayers of IrMn (5)/CoFe (2.1)/Ru (0.81)/CoFe
5
high linewidth are the main limitations for commercialization of STNOs. MTJ-based
STNOs offer significantly greater output power than metallic GMR-based STNOs,
and have recently reached 10 µW [28], in vortex based MTJ oscillators, which meets
the requirements of commercial applications. However, the frequency of operation
of such vortex STNOs is around 300 MHz, and hence they do not take full advantage of the wide-frequency tunability of STNOs, which can usually be tuned in the
1–65 GHz range [17]. STNOs have the capability to go beyond 200 GHz [29] with
conventional materials in high fields, and can even reach the terahertz range [30]
with the use of antiferromagnets.
MTJ nanopillars can also be used to rectify radio frequency (RF) signals through
spin-transfer torque [31–36]. With an RF current injected into an MTJ nanopillar, a dc voltage develops over the device when the frequency of the RF current
is close to the eigenfrequency of the nanomagnet. Thus, an MTJ nanopillar can
act as a microwave detector, also known as spin-torque diode (STD). For applications, a high sensitivity—defined as the ratio of the voltage detected to the input RF
power—is required. Preliminary calculations predicted that optimized MTJs should
reach sensitivities exceeding 10,000 mV/mW [37]. With advances in the design
of the MTJs in terms of tunneling magnetoresistance (TMR) effect and voltagecontrolled magnetocrystalline anisotropy (VCMA) and with improvements in the
understanding of strong nonlinear effects such as stochastic resonance [38] and nonlinear resonance [35, 36], the sensitivity performance of STD has reached values as
high as 10
5 mV/mW, exceeding that of semiconductor Schottky diode detectors at
room temperature. These results open a path for the design of a new generation of
high-sensitivity microwave detectors for wireless power transfer, which can rectify
microwave power from different energy sources, such as satellite, sound, television,
and Wi-Fi signals. The resulting dc voltage could be used to operate low-powered
nanodevices [39].
2 Linear Spin Wave Modes in MTJ-Based Nanopillars
Spin waves can be excited by thermal excitations in MTJ nanopillars in the presence
of an external field. A frequency domain spectrum of the time-varying magnetization
in the MTJ will reveal that magnetic moments can oscillate at several different frequencies. Each of these coexisting frequencies is called a frequency eigenmode. The
study of these modes gives an insight into the coherence of spin motion inside the
ferromagnet. Because the excited spin waves are contained mostly in the ferromagnetic material and die out fast outside it, different spin wave modes are excited only
from the magnetic part of the nanopillars—i.e., the free layer and the fixed layer.
Moreover, as the fixed layer is pinned strongly in one particular direction due to
exchange bias, a very high field is usually required to excite spin waves in it. Figure 2
shows the field variation of the excited spin wave modes in an MTJ nanopillar as
reported in Ref. [40]. The MTJ investigated in [40] had a circular cross-section with
a diameter 240 nm consisting of multilayers of IrMn (5)/CoFe (2.1)/Ru (0.81)/CoFe