Funding review_2011_ht5 copy

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