PowerPoint Presentation Physics (PowerPoint)

Document Sample
PowerPoint Presentation Physics (PowerPoint) Powered By Docstoc
					Magnetic Tunnel Junctions


                                       2
                            16k1 k2 kb ex p(2kb d )
       G  (e 2 / ) 
                    k|| ,   ( k1 2  k b )( k2 2  kb )
                                         2           2



                 2m( F  V1, 2 ) /
                                                    2
       k1,2                                             k| |2
       kb      2 m(U   F ) /             2
                                                    k|2
                                                       |




                     GWKB  (e2 /   2
                                        )  | t | 2 k 2 / k1
                                         k|| , 


                          k1     d           
                     t      exp  kb (x)dx 
                          k2     0           
                                         d        
                     G  (e / )  exp2  kb (x)dx
                           2

                                k|| ,   0        
                                   Transfer Hamiltonian



                                       
                               
HT               Tk qck c q  h.c.
                       L    R

         k q




                C2
T 2
      
 kq
         L ( Ek ) R ( Eq )


I  e   d | Tk q |2 AR (q ,  )AL (k  ,   eV )nF ( )  nF (  eV )
       k q




Ap  mGss  2 Z k  (  E k )
                       p            p        p




          L   (Ek ) R (Eq  )dE k dEq 
kq




I     2 e C
      
                   2       2
                               Z Z V
                                L  R
Tunneling Magnetoresistance
Tunneling: see Phil. Mag. 83, 1255 (2003)
k|| resolved (partial) DOS in
units of states/atom. eV of
bcc(100) Fe plotted in the
first 2DBZ. Left column is
for the majority spin channel
and right column is for the
minority spin channel.
                                QuickTime™ and a TIFF (Uncompressed ) d ecompressor are needed to see this picture.




From top to bottom, DOS
for a bulk Fe layer, a free
surface Fe layer and the
isolated electrode surface
Fe layer.
k|| resolved (partial) DOS in
units of states/atom. eV of
fcc(100) Co plotted in the
first 2DBZ. Left column is
for the majority spin channel
and right column is for the
minority spin channel.
                                QuickTime™ and a TIFF (Uncompressed ) d ecompressor are needed to see this picture.

From top to bottom, DOS
for a bulk Co layer, a free
surface Co layer and the
isolated electrode surface
Co layer.
Also see for resonant states: O.Wunnicke et al. PRB 65, 064425 (2002).
k|| resolved (partial) tunneling
conductance of bcc (100)
Fe/vacuum(10)/Fe tunnel
junction at an energy 0.05eV
below the Fermi level (a) only
for the barrier region, and (b)
for the entire junction.
                              QuickTime™ and a TIFF (Uncompressed ) d ecompressor are needed to see this picture.
zoomed-in view.




                  QuickTime™ and a TIFF (Uncompressed ) d ecompressor are needed to see this picture.
k|| resolved (partial) tunneling
conductance of the barrier region of a
Co junction in the first 2DBZ when the
magnetizations of the two electrodes are
aligned in parallel(a) majority spin channel,
(b) minority spin channel, and
(c) antiparallelly aligned. The vacuum
barrier is 6 atomic layers thick.      QuickTime™ and a TIFF (Uncompressed ) d ecompressor are needed to see
       The need to diagonalize basis when more than one state
          transforms under same irreducible representation




C. Uiberacker and P.M. Levy, PRB 64, 193404 (2001);erratum
PRB 65, 169904 (2002).
Fe/ZnSe/Fe
Bias dependence of TMR
           Bias dependence of TMR-trapezoidal barrier

JMR ratio vs. bias for free electron trapezoidal barrier model tunnel
junction with a Fermi sea depth of 16eV for majority and 3eV for
minority spins; square barrier height at zero bias is 1eV measured
from Fermi level
Oscillating TMR S.Yuasa et al. Science 297, 234 (2002)
Ab-initio calculation of JMR for Co/Vac(6)/Cu(p)Co junction;
K. Wang private communication.
   Tunneling with semiconducting electrodes

  For metallic electrodes it is sufficient to have 2-3ML of the
  magnetic electrode for spin dependent tunneling (SDT) ;
  therefore it is not the spin polarization of the current that
  produces SDT. What happens for semiconducting electrodes?


As there is little screening of electrons in semiconductors I
surmise that the SDT with 2-3ML of magnetic semiconducting
electrodes is very different from 20-30 ML.

Also, there will be hole as well as electron conduction; particularly
for holes spin-orbit coupling plays an important role. The spin-orbit
coupling can be detrimental to the polarization of spin currents.
One can conceptualize spin dependent tunneling in two steps:

• Presenting spin polarized electrons at the interface of between
  the electrode and barrier. This is accounted for by the DOS.

• The decay of wavefunction inside the barrier. There are
  evanescent states inside the barrier whose decay is given by
  an imaginary k vector.
     So what’s new?


     The spin Hall effect: Hirsch (1999), Zhang (2000)*

In general the Hall effect is the current or voltage transverse
to the applied electric field. This usually appears when one
applies a magnetic field perpendicular to the electric field;
however in systems that contain spin-orbit coupling one can
have preferential scattering of spins to the left or right of the
current, so that one can produce a Hall voltage in the absence
of a magnetic field. This principle was first enunciated by Mott
and has the formed the basis of Mott spin scattering detectors.


 *
Then we were told there’s an intrinsic SHE




But can one detect the ISHE?     It seems not!
What do the experimentalists say about the ISHE?
Molecular spintronics
Published online March 6, 2005 in Nature Materials
Conclusion

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:6
posted:5/1/2012
language:Latin
pages:71