Spin transfer torques in high anisotropy magnetic nanostructures S. Mangin 1, Y. enry 2, D. Ravelosona 3, J.A. Katine 4, and S. Moyerman 5, I. Tudosa 5, E. E. Fullerton 5 1) Laboratoire de Physique des Matériaux Vandoeuvre-les-Nancy 2) Institut de Physique et chimie des Matériaux de Strasbourg 3) Institut d'electronique Fondamentale - Orsay 4) itachi GST San Jose research center - San Jose 5) University of California, San Diego
Spin transfer torques in high anisotropy magnetic nanostructures Motivation Co/Ni multilayers 2 layer results ( - ) switching currents angular dependence (SW astroid) Conclusions
Spin transfer torques in heterostructures Γ p Angular momentum conservation spin transfer torques m Γ = m d e L dt see J. Magn. Magn. Mater. 32 (28) articles on spin torque edited by M. Stiles and D. Ralph I
Spin torque dynamics (LLG) dm dt = γ m Field torque (precession) + α m dm dt Damping torque (dissipation) eff m IPg i μ em t s B ( ( )) m x m x p Spin torque (negative friction ) J. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) L. Berger, Phys. Rev. B 54, 9353 (1996) I α C eff
Spin transfer torque nanotechnologies Noise in read heads STT MRAM Bit Line MTJ Source Gate Drain Oscillators Domain wall devices MTJ e- e- MTJ 1 Rippard et al., PRL 92, 2721 (24) Chappert, Fert, and Van Dau, Nature Mater. 6, 813 (27). Katine and Fullerton, J. Magn. Magn. Mater. 32, 1217 (28). Parkin et al., Science 32, 19 (28).
Perpendicular anisotropy devices Less sensitive to structure/lithography igher thermal stability More efficient reversal igher frequency oscillations Narrow domain walls New functionality e- Mangin et al., Nature Mater. 5, 21 (26) e- Burrowes et al., APL 93, 172513 (28). Mihai et al, Nature Phys. (in press)
Stability analysis of the LLG equations F1 F2 e- in-plane magnetization Demagnetization field suppresses out-of-plane precessions I C 2e h αm g S ( θ ) V p ( + + 2πM ) dip + K // : in-plane applied fied, dip : dipole field, k// in-plane anisotropy field S Stability U K = M S V K// / 2 Critical current must overcome 2πM S ~ 5-1 koe
Stability analysis of the LLG equations F1 e- out-of-plane magnetization ( K > 4πM S ) F2 K,eff I C 2e h αm g S ( θ ) V p k out of plane anisotropy field ( + + 4πM ) dip K S U = K ( M V )/ 2 S K,eff
Stability analysis of the LLG equations F1 e- out-of-plane magnetization ( K > 4πM S ) F2 I C 2e h g 2α θ U ( ) K p Zero applied field Critical current directly proportional to thermal stability More efficient reversal assuming low α and high p
Magnetic layers Films grown on 5 inch Si wafers by e-beam and sputtering (111) - Co(1Å)/Ni(6Å) Pd or Pt [Co/Ni]x4 Cu [Co/Ni]x2 [Co/Pd] or [Co/Pt] Pt K u 4x1 6 erg/cm 3 M s = 65 emu/cm 3 Normalized Kerr signal 1..5. -.5-1. (daalderop et al, Phys. Rev. Lett 68 (1992)) -4-2 2 4 (koe) C1 =.7 koe C2 = 2.7 koe
Co/Ni multilayers MBE grown (111) - Co(X)/Ni(3 ML) REED Intensity (a.u) 28 26 24 22 2 18 5 1 15 2 25 Time (s) Co / Ni(111) T cell =147 C K 1 (1 5 J/m 3 ) 1 5 Magnetometry) FMR 1 2 3 4 5 Co thickness (ML) S. Girod et al., Appl. Phys. Lett. 94, 26254 (29)
Nanopillars fabrication - Use of negative SQ resist as a high fidelity mask - 1 devices/5 inch wafer: circles and hexagons from 45nm to 15nm Au 1 nm Au I< Ta Cu Pt [Co/Ni]x4 Cu [Co/Ni]x2 [Co/Pt]x4 Pt > Ta (5 nm) Cu (15nm) Pt (3nm) [Co/Ni] (3 nm) Cu (4nm) [Co/Pt]/[Co/Ni] (4nm Pt (3nm) Cu Ta Cu (35nm) 5-1 nm 1-2 nm 45 nm Ta (5 nm)
Field switching in 5x1nm 2 nanopillars dv/di (Ohms) 12.95 12.9 12.85 AP a) P c free = 2.65 koe c ref = 1 koe d = 65 Oe -1-5 5 1 (koe)
Current induced switching in 5x1nm 2 nanopillars dv/di (Ohms) dv/di (Ohms) 12.95 12.9 12.85 13.1 13. 12.9 AP -1-5 5 1 (koe) a) P b) AP P c free = 2.65 koe c ref = 1 koe d = 65 Oe I C AP-P = -2.6x1 7 A/cm 2 I C P-AP = 7x1 7 A/cm 2 I c (Co/Pt) 4xI c (Co/Ni) 12.8-3 -2-1 1 2 3 I B (ma) Mangin et al., Nature Materials 5, 21 (26) Ravelosona et al., Phys. Rev. Lett. 96, 18664 (26)
Lower anisotropy free layer 31.2 45 nm circle dv/di (ohms) 31.1 31 31.2 31.1 Pt [Co/Ni]x4 Cu [Co/Ni]x2 [Co/Pt]x4 Pt 3.9-1 -5 5 1 (Oe) dv/di (ohms) 31.1 31 3.9 31 c ~ 4 Oe d ~ 8 Oe I C ~11 μa 6x1 6 A/cm 2 3.8 3.9-2 -.6 -.4-1 -.2.2 1.4 2.6 I (ma) (Oe)
Critical currents I C 2e h g 2α θ U ( ) K p sample 1 sample 2 ratio I C (ma) 145 11 13 V (1-18 cm 3 ) 11.25 5.8 2 C (Oe) 265 42 6.5 Mangin et al., Appl. Phys. Lett. 94, 1252 (29)
AF-coupled pinned layer 7.4 Resistance R ac (Ohms) 7.39 7.38 7.37 CoNix2/CoPd Cu CoPdx2/CoNix2 Ru CoPdx4-4.k -2.k. 2.k 4.k Magnetic Field (Oe)
AF-coupled pinned layer 7.4 Resistance R ac (Ohms) 7.39 7.38 7.37 major loop AF layer free layer.5 koe CoNix2/CoPd Cu CoPdx2/CoNix2 Ru CoPdx4-4.k -2.k. 2.k 4.k Magnetic Field (Oe) 2.5 koe
Stoner-Wohlfarth astroid M 1 M θ SW K = ( 2/3 2/3 sin θ + cos θ ) K 1.8.6.4.2 45 3 7 3/ 2 easy axis 33 3 hard axis 27 9 7 45 1 3-1 -1.5-1 -.5.5 1 1.5 12 15 18 21 24 K
Angle-dependent field switching 1x2nm 2 I θ 2.86 +87 +86 2.67-2 P dv/di (Ω) +85 +8 +6 +4-4 -6-8 -85 27 9 +2-86 AP 18-87 2.64 2.46-1 1-1 1 (T) (T) enry et al., Phys. Rev. B, 79, 214422 (29).
4 Comparison theory-experiment experiment I=+3mA I=+4.5mA I=+6mA I=+9mA z (mt) -4-5 5-5 5-5 5-5 5 y (mt) Field torque Damping torque Spin torque dm dt = γ m eff +α ( m m ) eff ( m m ) + β I u z
4 Comparison theory-experiment experiment I=+3mA I=+4.5mA I=+6mA I=+9mA z (mt) z (mt) -4 3-5 5-5 5-5 5-5 5 I= y (mt) -3-3 3-3 3-3 3-3 3 J.Z. Sun, PRB 62, 56 (2). y (mt)
Angular dependence of the spin torque Current has large effect // K +2 x 1 7 A/cm 2 K Threshold current for K I onset α eff on the SW astroid eff = K sin 2 (θ M ) 18 Y. enry et al., Phys. Rev. B, 79, 214422 (29).
r m t where r r r * r m = γ ( m eff ) + α m t r * eff Analytic solution r β = eff + I γ m r * r Equilibrium conditions: magnetization parallel to r r ( m z) eff x z θ m ϕ e r θ y e r ϕ Stability condition: total damping positive Linear stability analysis in the small current limit (2D problem) θ r [ αγ ( e ) β I sin θ] eff r θ θ=θ N. Smith et al, IEEE Trans Mag. 41,2935 (25) Y. enry et al., Phys. Rev. B, 79, 214422 (29).
Analytic expression for the astroid h h y z = sinθ [sin = cosθ [cos 2 2 θ θ C( θ )] + C( θ )] with C( θ ) = 1 αγ K ( β I sin θ ) θ θ=θ 1 Analytical Numerical Z / K - -1-1 -1-1 y / K in the small current limit
Conclusions Demonstration of efficient spin transfer in Nano-pillars with perpendicular anisotropy I c scales with thermal stability (Co/Ni multilayers: : higher p and lower α compared to Co/Pt) Role of current on the SW astroid. Mangin et al., Nature Materials 5, 21 (26) Ravelosona et al., Phys. Rev. Lett. 96, 18664 (26) Mangin et al., Appl. Phys. Lett. 94, 1252 (29) Cucchiara et al., Appl. Phys. Lett. 94, 1253 (29) enry et al., Phys. Rev. B, 79, 214422 (29). S. Girod, et al, Appl. Phys. Lett. 94, 26254 (29).