9/2/2015 Spin Currents An overview Sources of Spin Currents Spin - PDF document

9/2/2015 Spin Currents An overview Sources of Spin Currents Spin current introduction Spin angular momentum current sources Sergio O. Valenzuela ICREA and Catalan Institute of Nanoscience and Nanotechnology (ICN2), Barcelona, Spain Ferromagnetic materials (electric and thermal driving) Optical orientation EXCELENCIA SEVERO Spin-orbit effects OCHOA Topological insulators Mechanical motion ….. Implementations Nature of spin currents http:// nanodevices.icn2.cat ESM Cluj, Romania, Sept 2, 2015 Cluj, September 2 nd 2015 Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela SEVERO SEVERO OCHOA OCHOA Spin currents vs. charge currents Spin currents vs. charge currents Basic concepts Basic concepts Charge conservation law Charge conservation law j c  closed surface  Charge q in position r change in total charge enclosed          j d q r j c q v c dt . ρ ρ: charge density electron j c : charge current density Gauss theorem Electron current j c Cluj, September 2 nd 2015 Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela SEVERO SEVERO OCHOA OCHOA Spin currents vs. charge currents Spin currents vs. charge currents Basic concepts Basic concepts Spin angular momentum current (second-rank tensor) Spin angular momentum conservation law j s  Spin  in position r closed surface  change in total spin enclosed     dt         s  d  j r j s v r   M : local magnetization magnetic moment density electron + spin . j s : spin current density M Spin angular momentum current Spin angular momentum is generally not conserved j s Costache and SOV Science 2010 T : non-conservation of angular momentum Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela SEVERO SEVERO OCHOA OCHOA 1

9/2/2015 Spin currents vs. charge currents Spin currents vs. charge currents Bouncing balls Bouncing balls Ball with no spin, velocity Ball spinning, angular momentum Interaction with the environment: nontrivial flow of angular momentum In few cases spin currents can be defined in terms of a conservation law z Basics of spintronics, by G. Tatara (2009) t Sign( v z ) Sign( w x ) t t http://www2.eng.cam.ac.uk/~hemh/movies.htm Cluj, September 2 nd 2015 Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela SEVERO SEVERO OCHOA OCHOA Spin currents vs. charge currents Spin current generation Bouncing balls Angular momentum “reservoir/source” Interaction with the environment: nontrivial flow of angular momentum In few cases spin currents can be defined in terms of a conservation law Mechanical Basics of spintronics, by G. Tatara (2009) motion Mechanical resonance Magnetic Non-uniform fluid flow Spin-orbit materials effects (Spin) angular momentum Metallic (carrier motion) Spin Hall and spin galvanic effects Insulator (tunneling filter, spin waves) (Dyakonov Perel 1971, Hirsch 1999, Spin currents (Tedrow and Meservey 1971, Aronov 1976, Murakami 2003, Sinova 2004, Ganichev Aronov and Pikus 1976) 2003) Magnetization Nuclear spins dynamics Optical Orientation Overhauser/Feher effect Ferromagnetic resonance (Clark and Feher 1963) Spin pumping (Kastler 1950, Lampel 1968, Meier Zakharchenya 1984) Cluj, September 2 nd 2015 Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela SEVERO SEVERO OCHOA OCHOA Magnetic Materials Spin current generation Spin generation and spin injection Magnetic materials • Two spin channel model Ni Mechanical – Metallic ferromagnets. Spin-up and spin- motion down are two independent families of carriers (Mott 1936) Mechanical resonance Magnetic Non-uniform fluid flow Spin-orbit materials effects Zumsteg and Parks 1970 (Spin) angular momentum Metallic (carrier motion) Spin Hall and spin galvanic effects • Unusual behaviour of the resistance of FM metals Insulator (tunneling filter, spin waves) (Dyakonov Perel 1971, Hirsch 1999, Spin currents Murakami 2003, Sinova 2004, Ganichev (Tedrow and Meservey 1971, Aronov 1976, 2003) Aronov and Pikus 1976) − Low T, magnon scattering becomes vanishingly small − Electrons of majority and minority spin (parallel or Magnetization Nuclear spins dynamics antiparallel to magnetization) do not mix in scattering processes Optical Orientation Overhauser/Feher effect Ferromagnetic resonance Nevill Francis Mott (Clark and Feher 1963) Spin pumping − The conductivity can be expressed as the sum of two independent parts (see also Campbell 1967, Fert and Campbell 1968, Valet and (Kastler 1950, Lampel 1968, Meier Zakharchenya 1984) Fert 1993) Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela SEVERO SEVERO OCHOA OCHOA 2

9/2/2015 Magnetic Materials Magnetic Materials Spin generation and spin injection Spin generation and spin injection • Exchange splitting (Stoner 1938) • Spin polarized current in a nonmagnetic metal – Different density of states at the Fermi level • Spin accumulation decays exponentially for spin up and down carriers • Characteristic length. Spin diffusion/relaxation length  sf – Different mobility for spin up and down carriers Different m *, v F , k F , g ( E F ), thus different conductivity   N N  M m P  N N M m Edmund Clifton Stoner -1≤ P ≤ 1 Minority Majority Johnson and Silsbee PRB 35 , 4959 (1987) van Son et al., PRL 58 , 2271 (1987) Cluj, September 2 nd 2015 Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela SEVERO SEVERO OCHOA OCHOA Spin current outside a ferromagnet Spin current outside a ferromagnet Meservey and Tedrow experiment Meservey and Tedrow experiment • Meservey-Tedrow technique. Superconductor with Zeeman Meservey and Tedrow, PRL 1971 Ni split density of states as a spin detector – P is obtained at high field, low temperatures and zero bias Partially polarized materials: Fe, Co, Ni ( P ~ 25-45 %) Fe Ni Co H=1-2T Meservey and Tedrow, PRL 1971, Review: Phys. Rep. 238 , 173 (1994) Meservey and Tedrow, PRB 1973 Cluj, September 2 nd 2015 Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela SEVERO SEVERO OCHOA OCHOA Spin current outside a ferromagnet Solid-state spin filter Meservey and Tedrow experiment Spin tunnelling through a ferromagnetic insulator The observation of internal field emission (Fowler-Nordheim tunneling} in magnetically ordered insulators is reported. A large magnetic field effect was observed and interpreted 18 as a decrease in the barrier height due to spin ordering (Esaki et al. PRL 1967) 15 P(%) 12 9 0 200 400 600 800 2 ) Junction Resistance (  m Au/EuS/Al 2.0 dI/dV (arb. unit) NiFe CoFe 1.5 1.0 0.5 0.0 -0.5 0.0 0.5 -0.5 0.0 0.5 V Al -V NiFe (mV) V Al -V CoFe (mV) H= 0; 2T Al/EuS/Al (Moodera et al. PRL 1988, PRB 1990) SOV and M. Tinkham, APL 85 , 5914 (2004) Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela Cluj, September 2 nd 2015 EXCELENCIA PEND nanodevices.icn2.cat Sergio O. Valenzuela SEVERO SEVERO OCHOA OCHOA 3