446
L. Shen et al.
16.3 Spin Torque Nano-oscillators Based
on Antiferromagnetic Skyrmions
The antiferromagnetic textures can be used not only as chaotic oscillators, but also
as microwave signal generators (or spin torque nano-oscillators). In this section, we
first review recent advances in skyrmion-based spin torque nano-oscillators, and
then focus on the discussion of the nano-oscillators based on antiferromagnetic
skyrmions. Spin torque nano-oscillators based on magnetic skyrmions have received
great attention, because they can excite microwave signals with small linewidth and
are expected to improve the output power [43–46]. In addition, the arrays of such
nano-oscillators can be used to perform the neuromorphic computing [47]. In 2015,
Zhang et al. [45] firstly demonstrated that applying a uniform current to the nanocontact oscillators, the ferromagnetic skyrmion can be induced to move in a circular
motion, and then an oscillating signal is obtained by detecting the skyrmion position and using the magnetoresistance effect. In 2016, Garcia-Sanchez et al. [44]
showed an alternative skyrmion-based nano-oscillator, in which a fixed layer with a
vortex magnetic configuration is used to generate the spin-polarized current with a
vortex-like polarization.
Generally, for the ferromagnetic skyrmion-based nano-oscillators, the oscillation
frequencies are low (about 1 gigahertz), because the fast-moving skyrmion results
in the presence of a large Magnus force and then the skyrmion will be destroyed
at the nanodisk edge. In order to overcome this obstacle, various methods have
been proposed. For example, in 2019, Feng et al. [48] showed that the oscillation frequency can be increased by 75%, where the nanodisk edge is enhanced
by applying high perpendicular magnetic anisotropy. In addition, modifying the
profile of Dzyaloshinskii-Moriya interaction can also lead to the increase in oscillation frequency [49]. Besides, in 2020, Jin et al. [50] showed that creating an annular
groove in the surface of the free layer, the frequency tunability of the nano-oscillators
reaches to 15.63 GHz. However, the above methods require sophisticated reprocessing and are not favorable from the point of view of device applications. Shen
et al. [51] proposed to use the circular motion of an antiferromagnetic skyrmion
to create the oscillation signal, where sophisticated reprocessing is not required. In
addition to the cancellation of the Magnus force, the antiferromagnetic skyrmion
obeys the inertial dynamics, as the motion equation of the antiferromagnetic order
parameter is of second order with respect to time (for ferromagnetic dynamic equation it is of first order). Thus, the motion behavior of the antiferromagnetic and
ferromagnetic skyrmions in a nanodisk is different, and the oscillation frequency of
the antiferromagnetic skyrmion (tens of gigahertz) is higher than that of the ferromagnetic skyrmion [51]. Figure 16.4 shows the comparison of ferromagnetic and
antiferromagnetic skyrmion-based nano-oscillators. We can see that the ferromagnetic skyrmion moves toward the nanodisk center when the positive current is applied,
while for the negative current, the skyrmion is destroyed at the nanodisk edge. For
the antiferromagnetic skyrmion, it moves steadily in the nanodisk under the action
of the same currents, and its motion is independent of the sign of the applied current.
L. Shen et al.
16.3 Spin Torque Nano-oscillators Based
on Antiferromagnetic Skyrmions
The antiferromagnetic textures can be used not only as chaotic oscillators, but also
as microwave signal generators (or spin torque nano-oscillators). In this section, we
first review recent advances in skyrmion-based spin torque nano-oscillators, and
then focus on the discussion of the nano-oscillators based on antiferromagnetic
skyrmions. Spin torque nano-oscillators based on magnetic skyrmions have received
great attention, because they can excite microwave signals with small linewidth and
are expected to improve the output power [43–46]. In addition, the arrays of such
nano-oscillators can be used to perform the neuromorphic computing [47]. In 2015,
Zhang et al. [45] firstly demonstrated that applying a uniform current to the nanocontact oscillators, the ferromagnetic skyrmion can be induced to move in a circular
motion, and then an oscillating signal is obtained by detecting the skyrmion position and using the magnetoresistance effect. In 2016, Garcia-Sanchez et al. [44]
showed an alternative skyrmion-based nano-oscillator, in which a fixed layer with a
vortex magnetic configuration is used to generate the spin-polarized current with a
vortex-like polarization.
Generally, for the ferromagnetic skyrmion-based nano-oscillators, the oscillation
frequencies are low (about 1 gigahertz), because the fast-moving skyrmion results
in the presence of a large Magnus force and then the skyrmion will be destroyed
at the nanodisk edge. In order to overcome this obstacle, various methods have
been proposed. For example, in 2019, Feng et al. [48] showed that the oscillation frequency can be increased by 75%, where the nanodisk edge is enhanced
by applying high perpendicular magnetic anisotropy. In addition, modifying the
profile of Dzyaloshinskii-Moriya interaction can also lead to the increase in oscillation frequency [49]. Besides, in 2020, Jin et al. [50] showed that creating an annular
groove in the surface of the free layer, the frequency tunability of the nano-oscillators
reaches to 15.63 GHz. However, the above methods require sophisticated reprocessing and are not favorable from the point of view of device applications. Shen
et al. [51] proposed to use the circular motion of an antiferromagnetic skyrmion
to create the oscillation signal, where sophisticated reprocessing is not required. In
addition to the cancellation of the Magnus force, the antiferromagnetic skyrmion
obeys the inertial dynamics, as the motion equation of the antiferromagnetic order
parameter is of second order with respect to time (for ferromagnetic dynamic equation it is of first order). Thus, the motion behavior of the antiferromagnetic and
ferromagnetic skyrmions in a nanodisk is different, and the oscillation frequency of
the antiferromagnetic skyrmion (tens of gigahertz) is higher than that of the ferromagnetic skyrmion [51]. Figure 16.4 shows the comparison of ferromagnetic and
antiferromagnetic skyrmion-based nano-oscillators. We can see that the ferromagnetic skyrmion moves toward the nanodisk center when the positive current is applied,
while for the negative current, the skyrmion is destroyed at the nanodisk edge. For
the antiferromagnetic skyrmion, it moves steadily in the nanodisk under the action
of the same currents, and its motion is independent of the sign of the applied current.