EF + - EF - +
Ffix I Ffree
+ - EF + - EF
2 nm
Crystal electrodes
2 nm
Amorphous electrodes
Annealing • Enhances asymmetry of switching phase diagram
P
AP
P/AP
P
P/AP
AP
Switching phase diagram
ST-FMR measurement
Adjusted torkance
Results
• Annealing enhances
asymmetry in plane
torkance
• Annealing enhances
magnitude of the field
like torkance
~ 70θ o
As grown: TMR=12% P=24% Annealed: TMR=85% P=55%
Asymmetry of distribution of inelastic tunneling centers
Inelastic tunneling dominates
Weak effect of dependence of ρ- on energy
A peak above EF in minority DOS
Dependence of ρ- on energy
Elastic tunneling dominates
• T h e m i n o r i t y o f t h e interface states of Fe (100)
• Distinct feature in DOS shows up in the crystalline electrodes
Symmetric junction: FeCoB/MgO/FeCoB
V
A
B
0
C
D
Cross-‐channel differenCal conductance A: symmetric reference
B: asymmetry of elasCc tunneling
C: First order dependence of DOS on energy
D : A s y m m e t r y o f distribution of inelastic tunneling centers
J.C. Slonczewski, J.Z. Sun JMMM (2007) Where
Adjusted torkance
FeCoB
FeCoB MgO FeCoB
IntroducCon
Conclusions
High ResoluCon HSQ nanopillar liX-‐off process
School of Applied & Engineering Physics, Cornell University, Ithaca, NY Hitachi Global Storage Technology, San Jose, CA*
Band Structure Effect and Spin-‐Transfer Torque in MgO-‐Based Symmetric and Asymmetric MagneMc Tunnel JuncMons
Hsin-‐wei Tseng, Yun Li, Pinshane Huang J.A. KaMne*, John C. Read*, Patrick M. Braganca*, D.A. Muller, D.C. Ralph and Robert A. Buhrman
• FeB/MgO/FeCoB show opposite asymmetry in TMR bias dependence while simply exchange electrodes, which suggest spin-dependent transport is affected by electrodes or electrodes/interfaces. • Switching phase diagram in as-grown samples is symmetric and similar for both asymmetric MTJs. Switching phase diagram in annealed samples exhibit non-standard switching phase diagram, which indicates the spin-transfer torque is highly asymmetric under different current polarities.
Pulse-based Phase diagram in Symmetric MTJs
Individual Microwave Spectra Contour Plot
• Pulse-based microwave
measurement reveal strong
field-like effect under high
voltage bias
• We have developed using HSQ/PMMA/Omnicaot triplayer E-beam
lift-off process of quick turn-around nanopillar fabrication process for
spin-transfer torque characterization.
Highly asymmetric phase diagram in asymmtric MTJS
As grow Symmetric phase
diagram show
symmetric spin
transfer effect.
Annealed Highly asymmetric
phase diagram
suggest
unconventional spin-
transfer effect.
• Broadband microwave emission
• Spin-torque excited FMR microwave emission
• No microwave emission
• Coherent microwave oscillation
Spin-transfer torque enables direct ultrafast manipulation of nanomagnet and read-out through magnetoresistance (MR). However, while pursuing ultrafast switching which requiring high current density, spin-transfer switching has been more unreliable and complicated nanomagnet dynamics has been deviated from theoretical prediction under high votage. Here, we have found spin-transfer torque effect, which is higly depending on spin-dependent transport in mangetic tunnel junction (MTJs), can be stronger affected by the electronic structure in electrodes or electrode/interfaces. Spin-transfer effect in as-grow samples are highly symmetric in both symmetric and asymmetric MTJs. After annealed, symmetric MTJs show asymmetric in-plane torque, which can be explained by Slonczewski simple band structure model. In addition, in asymmetric MTJs (two electrodes are consisted of different ferromagnetic materials), spin-transfer effect exhibited complicated spin-transfer effect.
Symmetric FeCoB/MgO/FeCoB MTJs
Asymmetric FeB/MgO/FeCoB MTJs
Switching voltage is asymmetric in terms of bias direction
• Annealing enhances asymmetry of the in-plane STT.
• Band structure effect, a peak in minority DOS of the
electrodes may be the reason for the asymmetry of the In-
plane STT.
• High voltage spin-transfer torque
• Pulse-based contour plot
of microwave spectra
exhibit four different kind
of microwave emission
within coercive regime.
100nm
