Physics:Spin pumping

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Short description: Generation of spin current

Spin pumping is the dynamical generation of pure spin current by the coherent precession of magnetic moments, which can efficiently inject spin from a magnetic material into an adjacent non-magnetic material. The non-magnetic material usually hosts the spin Hall effect that can convert the injected spin current into a charge voltage easy to detect. A spin pumping experiment typically requires electromagnetic irradiation to induce magnetic resonance, which converts energy and angular momenta from electromagnetic waves (usually microwaves) to magnetic dynamics and then to electrons, enabling the electronic detection of electromagnetic waves. The device operation of spin pumping can be regarded as the spintronic analog of a battery.[1]

Spin pumping involves an AC effect and a DC effect:

  • The AC effect generates a spin current that oscillates at the same frequency with the microwave source.
  • The DC effect requires that the magnetic dynamic is circularly polarized or elliptically polarized, whereas a linear oscillation can only generate an AC component.
  • Both effects result in a net enhancement of the effective magnetic damping.[2]

Spin pumping in ferromagnets

The spin current pumped into an adjacent layer by a precessing magnetic moment is given by[2]

[math]\displaystyle{ \vec j_s = {\hbar \over {4 \pi}} g^{\uparrow \downarrow} {1 \over M_S^2} \langle \vec M(t) \times {{d \vec M(t))} \over {dt}} \rangle }[/math]

where [math]\displaystyle{ \vec j_s }[/math] is the spin current (the vector indicates the orientation of the spin, not the direction of the current), [math]\displaystyle{ g^{\uparrow \downarrow} }[/math] is the spin-mixing conductance characterizing the spin transparency of the interface, [math]\displaystyle{ M_S }[/math] is the saturation magnetization, and [math]\displaystyle{ \vec M(t) }[/math] is the time-dependent orientation of the moment.

Optical, microwave and electrical methods are also being explored.[3] These devices could be used for low-power data transmission in spintronic devices[4] or to transmit electrical signals through insulators.[5]

Spin pumping in antiferromagnets

Spin pumping in antiferromagnetic materials does not vanish because the antiparallel magnetic moments contribute constructively rather than destructively to spin current, which was theoretically predicted in 2014.[6] Since the frequency of antiferromagnetic resonance[7] is much higher than that of ferromagnetic resonance, spin pumping in antiferromagnets can be utilized to study electromagnetic signals in the sub-terahertz and terahertz regime, which had been demonstrated by two independent experiments in 2020.[8][9]

Besides higher frequency, spin pumping in antiferromagnets features the chirality degree of freedom of magnetic dynamics that does not exist in ferromagnets. For example, the spin currents pumped by the left-handed and the right-handed resonance modes are opposite in direction.

References

  1. ↑ Brataas, Arne; Tserkovnyak, Yaroslav; Bauer, Gerrit E. W.; Halperin, Bertrand I. (2002-08-29). "Spin battery operated by ferromagnetic resonance" (in en). Physical Review B 66 (6): 060404. doi:10.1103/PhysRevB.66.060404. ISSN 0163-1829. https://link.aps.org/doi/10.1103/PhysRevB.66.060404. 
  2. ↑ 2.0 2.1 Y Tserkovnyak (2002). "Enhanced Gilbert Damping in Thin Ferromagnetic Films". Physical Review Letters 88 (11): 117601. doi:10.1103/PhysRevLett.88.117601. PMID 11909427. Bibcode: 2002PhRvL..88k7601T. 
  3. ↑ C Sandweg (2011). "Spin Pumping by Parametrically Excited Exchange Magnons". Physical Review Letters 106 (21): 216601. doi:10.1103/PhysRevLett.106.216601. PMID 21699324. Bibcode: 2011PhRvL.106u6601S. 
  4. ↑ G E Bauer and Y Tserkovnyak (2011). "Spin-magnon transmutation". Physics 4: 40. doi:10.1103/Physics.4.40. Bibcode: 2011PhyOJ...4...40B. http://resolver.tudelft.nl/uuid:131da22c-f204-4682-a163-f1f0baf18489. 
  5. ↑ Y Kajiwara (2010). "Transmission of electrical signals by spin-wave interconversion in a magnetic insulator". Nature 464 (7286): 262–266. doi:10.1038/nature08876. PMID 20220845. Bibcode: 2010Natur.464..262K. 
  6. ↑ Cheng, Ran; Xiao, Jiang; Niu, Qian; Brataas, Arne (2014-07-29). "Spin Pumping and Spin-Transfer Torques in Antiferromagnets". Physical Review Letters 113 (5): 057601. doi:10.1103/PhysRevLett.113.057601. PMID 25126936. https://link.aps.org/doi/10.1103/PhysRevLett.113.057601. 
  7. ↑ Keffer, F.; Kittel, C. (1952-01-15). "Theory of Antiferromagnetic Resonance" (in en). Physical Review 85 (2): 329–337. doi:10.1103/PhysRev.85.329. ISSN 0031-899X. https://link.aps.org/doi/10.1103/PhysRev.85.329. 
  8. ↑ Li, Junxue; Wilson, C. Blake; Cheng, Ran; Lohmann, Mark; Kavand, Marzieh; Yuan, Wei; Aldosary, Mohammed; Agladze, Nikolay et al. (2020-02-06). "Spin current from sub-terahertz-generated antiferromagnetic magnons" (in en). Nature 578 (7793): 70–74. doi:10.1038/s41586-020-1950-4. ISSN 0028-0836. PMID 31988510. https://www.nature.com/articles/s41586-020-1950-4. 
  9. ↑ Vaidya, Priyanka; Morley, Sophie A.; van Tol, Johan; Liu, Yan; Cheng, Ran; Brataas, Arne; Lederman, David; del Barco, Enrique (2020-04-10). "Subterahertz spin pumping from an insulating antiferromagnet" (in en). Science 368 (6487): 160–165. doi:10.1126/science.aaz4247. ISSN 0036-8075. PMID 32273462. https://www.science.org/doi/10.1126/science.aaz4247. 

See also




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