226
I. Proskurin and R. L. Stamps
mechanism beyond this phenomenon can be based on the adiabatic spin transfer
torque from a pure spin current, or on the DMI between the same sublattices, which
breaks the inversion symmetry and lifts the degeneracy between the left and right
polarized magnon modes. In contrast to optical metamaterials, where the asymmetry
in light-matter interactions is related to structural chirality, the symmetry breaking
mechanism, which is based on the spin current, induces chirality of the material in
controllable way. For a spin current density j s ≈ 10
11 A/m
2 (in the electric units), we
obtain v s = μ B j s /(eM s ) ≈ 30 m/s for M s ≈ 3.5 × 10
5 A/m. This parameter should
be compared to the typical velocity of the spin waves in antiferromagnetic insulators
c s ≈ 10
−4 m/s, which gives v s /c s ≈ 10
−3 . The characteristic length of the magnon
circular dichroism, in this situation, CD ≈ 5 mm for the magnon frequencies about
1 THz and η ≈ 10
4 . Curiously, the effective strength of the DMI, D eff = v s /(k B a 0 )
is about 0.5 K (a 0 is the lattice spacing), which is comparable to a typical DMI
strength in magnetic materials.
9.4 Excitation of Magnon Spin Photocurrents with
Polarized Fields
Among the major goals of spintronics are generation of spin currents, their transmission over large distances, and conversion from one form to another because the
spin angular momentum can be carried by different types of carriers. Since magnons
are able to carry spin angular momentum, spin excitations in low damping magnetic
insulators are good candidates for being spin current mediators. The absence of the
net magnetization and the existence of two polarization states per magnon make
antiferromagnetic insulators particularly suitable for applications as spin current
conductors. It was demonstrated that an introduction of a thin layer of the antiferromagnetic insulator can enhance the spin current transmission in interface systems
[62, 63].
Magnon spin currents in antiferromagnetic insulators can be excited by several
methods. For example, it can be done by pumping a magnon spin current from a
neighboring ferromagnetic layer [62]. Thermal excitation of spin currents via the spin
versions of the Seebeck and Nernst effects also has attracted considerable attention
[64–68]. The latter is especially interesting in low-dimensional materials, where it
is provided by topological terms in magnon dynamics [69–71].
Optical control of spin states in antiferromagnetic insulators [72, 73] is a feature
in the emerging field of antiferromagnetic optospintronics [5]. In this respect, it is an
intriguing problem to investigate whether it is possible to find some sort of magnon
photo-effect [44]. Symmetry considerations suggest that this is indeed possible. As
we have already mentioned, spin currents satisfy the definition of true chirality [11],
which can be directly seen from the conservation law for the μth component of the
spin density
∂s
μ
(t, r)
∂t
+ ∇ · j
μ
(t, r) = 0.
(9.65)
I. Proskurin and R. L. Stamps
mechanism beyond this phenomenon can be based on the adiabatic spin transfer
torque from a pure spin current, or on the DMI between the same sublattices, which
breaks the inversion symmetry and lifts the degeneracy between the left and right
polarized magnon modes. In contrast to optical metamaterials, where the asymmetry
in light-matter interactions is related to structural chirality, the symmetry breaking
mechanism, which is based on the spin current, induces chirality of the material in
controllable way. For a spin current density j s ≈ 10
11 A/m
2 (in the electric units), we
obtain v s = μ B j s /(eM s ) ≈ 30 m/s for M s ≈ 3.5 × 10
5 A/m. This parameter should
be compared to the typical velocity of the spin waves in antiferromagnetic insulators
c s ≈ 10
−4 m/s, which gives v s /c s ≈ 10
−3 . The characteristic length of the magnon
circular dichroism, in this situation, CD ≈ 5 mm for the magnon frequencies about
1 THz and η ≈ 10
4 . Curiously, the effective strength of the DMI, D eff = v s /(k B a 0 )
is about 0.5 K (a 0 is the lattice spacing), which is comparable to a typical DMI
strength in magnetic materials.
9.4 Excitation of Magnon Spin Photocurrents with
Polarized Fields
Among the major goals of spintronics are generation of spin currents, their transmission over large distances, and conversion from one form to another because the
spin angular momentum can be carried by different types of carriers. Since magnons
are able to carry spin angular momentum, spin excitations in low damping magnetic
insulators are good candidates for being spin current mediators. The absence of the
net magnetization and the existence of two polarization states per magnon make
antiferromagnetic insulators particularly suitable for applications as spin current
conductors. It was demonstrated that an introduction of a thin layer of the antiferromagnetic insulator can enhance the spin current transmission in interface systems
[62, 63].
Magnon spin currents in antiferromagnetic insulators can be excited by several
methods. For example, it can be done by pumping a magnon spin current from a
neighboring ferromagnetic layer [62]. Thermal excitation of spin currents via the spin
versions of the Seebeck and Nernst effects also has attracted considerable attention
[64–68]. The latter is especially interesting in low-dimensional materials, where it
is provided by topological terms in magnon dynamics [69–71].
Optical control of spin states in antiferromagnetic insulators [72, 73] is a feature
in the emerging field of antiferromagnetic optospintronics [5]. In this respect, it is an
intriguing problem to investigate whether it is possible to find some sort of magnon
photo-effect [44]. Symmetry considerations suggest that this is indeed possible. As
we have already mentioned, spin currents satisfy the definition of true chirality [11],
which can be directly seen from the conservation law for the μth component of the
spin density
∂s
μ
(t, r)
∂t
+ ∇ · j
μ
(t, r) = 0.
(9.65)