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Antiferromagnetic Spintronics

Antiferromagnetic Spintronics. School of Physics, Peking University Pingfan Gu 2018/ 12 / 15. Antiferromagnetc : Interesting and Useless. Robust against perturbation due to magnetic fields No stray fields Display ultrafast dynamics Capable of generating large magnetotransport effects.

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Antiferromagnetic Spintronics

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  1. AntiferromagneticSpintronics SchoolofPhysics,PekingUniversity PingfanGu 2018/12/15

  2. Antiferromagnetc:InterestingandUseless • Robust against perturbation due to magnetic fields • No stray fields • Display ultrafast dynamics • Capable of generating large magnetotransport effects 1970 Nobel lecture

  3. Contents • Antiferromagnetism • Electrical writing and reading by non-relativistic effects • Spintransfertorque • Electricalwritingandreadingbyrelativisticeffects • Anisotropicmagnetoresistance(AMR) • SpinHalleffect(SHE)andinversespinHalleffect • Spingalvaniceffect(SGE)andinversespingalvaniceffect(ISGE)

  4. Antiferromagnetism • Nearest-neighborHeisenbergHamiltonian: • :groundstateformsaferromagneticchain|or| • :groundstateformsaantiferromagneticchain|

  5. StoringInformationinAntiferromagneticMaterial • Controlled switching of antiferromagnets has required the help of nearby ferromagnetic domains, magnetoelectricity, or optical pulses. Science, 2012, 335(6065): 196-199.

  6. Reorient the Néel Order Parameter • Effects that depend on evenfunction of the spontaneous (sublattice) magnetization show the same variation in antiferromagnets as in ferromagnets.

  7. SpinTransferTorque • Torqueonacurrentloopcanbecalculatedby • Analogically,thespintransfertorquecanbeexpressedby

  8. SpinTransferTorque • Shortcarrierspinlifetime:,(LocalField-likeSTT) • ispolarizationoftheinjectionspincurrent • Longcarrierspinlifetime:,(Antidamping-likeSTT) • Landau–Lifshitz–Gilbert equation:)

  9. SpinTransferTorqueinAntiferromagnets

  10. AntiferromagneticSpinValve • The in-plane component is staggered, the out-of-plane component is spatially inhomogeneous • The order parameters of the right and left leads are rotated by π=2 around the current direction, meaning that the out-of- plane torque points toward the current direction. PHYSICAL REVIEW B 73, 214426 2006

  11. SpinTransferTorqueinAntiferromagnets • Realistic magnetic multilayers possess dislocations, defects, and grain boundaries as well as interfacial roughness, resistivity mismatch, randomly spread spin-glass-like phases at antiferromagnetic interfaces, peculiar spin structures, etc. • This is independent of the polarity of the vertical electrical current, so the antiferromagnet cannot be electrically switched back to the parallel configuration. • Quantum coherence is crucial to enable the transmission of staggered spin density from one part of the spin valve to the other. Physical Review B, 2014, 89(17): 174430.

  12. ElectricalReadingandWritingbyRelativisticEffects • Anisotropicmagnetoresistance • SHEandISHE • SGEandISGE

  13. AnisotropicMagnetoresistance • The scattering of itinerant electrons (and hence the conductivity) depends on the magnetization direction because of the anisotropy of the electronic structure induced by spin-orbit coupling. • Anisotropic magnetoresistance is even in magnetization ∼ , i.e., it is invariant upon magnetization reversal. Hence, such an effect also exists in antiferromagnets.

  14. ReadoutbyAntiferromagneticOhmicAMR • One state has antiferromagnetic moments aligned parallel and the other state perpendicular to the probing current direction • For writing, the sample is cooled in a field HFC from a temperature above the antiferromagnetic–ferromagnetic transition in FeRh. • AMR signals in ferromagnets and antiferromagnets are typically limited to a few per cent. Nature Mater. 13, 367–374 (2014).

  15. Readout by antiferromagnetic TAMR • Larger magnetoresistance effects in tunneling devices than ohmic resistors. • Antiferromagnet on one side and a non-magnetic metal on the other side of the tunnel barrier. • The positive/negative field-cooled magnetization loops provide clear evidence of the exchange coupling between the NiFe ferromagnet and the IrMn antiferromagnet. Nature Mater. 10, 347–351 (2011).

  16. SpinHallEffectandInverseSpinHallEffect • Extrinsic mechanism: carriers with opposite spin diffuse in opposite directions when colliding with impurities in the material. • Intrinsic mechanism: carrier's trajectories are distorted due to spin-orbit interaction as a consequence of the asymmetries in the material.

  17. FirstObservationofISHEinAntiferromagneticMetalIr20Mn80 • Large spin-orbit couplingmetal:Pt • Antiferromagnetic metal:Ir20Mn80 • Spinpumping:The magnetization in the FMI layer is driven by a rf magnetic field with microwave frequency perpendicular to the static field H. • SpinSeebeckEffect:A thermal gradient applied across the thickness of the bilayer produces a bulk magnon spin current that results in a spin-pumped spin current across the FMI/ML interface proportional to the temperature gradient and to the spin-mixing conductance. PHYSICAL REVIEW B 89, 140406(R) (2014)

  18. FirstObservationofISHEinAntiferromagneticMetalIr20Mn80 • SincethechargecurrentintheMLisproportionaltotheandthe spin polarization is parallel to the static field, sothe spin pumping voltage VSP vanishes when the field is along the film strip (φ = 0), is finite and positive for φ = 90◦ and changes sign when the field is reversed. PHYSICAL REVIEW B 89, 140406(R) (2014)

  19. SHE devices require no auxiliary ferromagnet and allow for reversible electrical switching. • The torques are sensitive to the quality of the non-magnetic/antiferromag-netic interface and are efficient only for antiferromagnetic film thicknesses of the order of the spin diffusion length,

  20. RashbaEffectandSpinGalvanic Effect • The spin-galvanic effect is caused by the asymmetric spin-flip relaxation of spin-polarized electrons in systems with k-linear contributions to the effective Hamiltonian. Nature, 2002, 417(6885): 153. Nature, 2016, 539(7630): 509.

  21. InverseSpinGalvanic Effect • Nonequilibrium redistribution of eigenstates in an applied electric field resulting in a nonzero spin density due to broken inversion symmetry of the spin texture. The opposite chirality spin texture with lower Fermi wave vector is not drawn for clarity. This reversed chirality will give and opposite but lower contribution to the one shown, hence not changing the basic physics illustrated here. PRL 113, 157201 (2014)

  22. InverseSpinGalvanic Effect • The ISGE responsible for these polarization effects requires the spin–orbit coupling to be combined with inversion asymmetries in the crystal structure. • The ISGE can generate local non-equilibrium spin polarizations with opposite sign and equal magnitude on the two inversion-partner atoms, while the global polarization integrated over the whole unit cell vanishes. Here a uniform electrical current induces a non-equilibrium anti- ferromagnetic spin polarization in the bulk crystal. Nature physics, 2016, 12(9): 855.

  23. Relativistic Néel-OrderSpin-OrbitTorque(NSOT) Fields Induced by Electrical Current in Antiferromagnets PRL 113, 157201 (2014)

  24. Electrical Switching of an Antiferromagnet • Mn atoms form two sublattices whose local environment has broken inversion symmetry, and the two Mn sublattices form inversion partners. • AFM moments will tend to align perpendicular to the applied current Science, 2016, 351(6273): 587-590.

  25. Electrical Switching of an Antiferromagnet • Successive Jwrite pulses in one direction increase the amplitude of the read- out R⊥ signal of one sign and pulsing in the orthogonal direction increases the amplitude of R⊥ of the opposite sign Science, 2016, 351(6273): 587-590.

  26. Electrical Switching of an Antiferromagnet Science, 2016, 351(6273): 587-590.

  27. OtherAntiferromagneticSpintronics • Spin-currenttransimitters • Spin-currentgenerators • Spininjectors • AnomalousHallEffect • …… ThanksforYourAttention!

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