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Relaziation of an ultrahigh magnetic field on a nanoscale
S. T. ChuiUniv. of Delaware
• 302-831-8115
Collaborators:
• J. Cullen
• K. Esfarjani
• L. B. Hu
• Z. F. Lin
• Y. Kawazoe
• Jian-Tao Wang
• Y. Yu
Different physics of spin polarized transport in different
systems:• AMR (Anisotropic magnetoresistance)
• GMR (Giant magnetoresistance)
• Spin Polarized Tunnelling
• Giant impedence effect
• CMR (Collosal magnetoresistance)
• Spin polarized transistor
Tunnelling between ferromagnets• Miyazaki et al,
Moodera et al.• room temperature
magnetoresiatance is about 30 %
• strong bias dependence
• large resistance: 100 ohm for 10^(-4) cm^2, may save power
Conclusion:
• In tunnel junctions of which at least one side is a ferromagnet, very large magnetic polarization change ( 0.1 ) and splitting of the spin up and spin down Fermi energy (0.1 eV) can be created under steady state finite current conditions. This splitting can be created by a 1000 T field and is much higher than can be created by the highest magnetic field on earth.
B
Tunnelling: Simple picture
• conductance for spin s (s)= C Ns(L)Ns(R)
• Parallel: conductance =C[N(+)^2+N(-)^2]
• Antiparallel: conductance=2CN(+)N(-)
• magnetoresistance: difference of the above two terms
A ten-fold increase in the magnetoresistance from the on-
state to the off-state of a magnetic single electron tunnel
transistor: Ono Shimada and Ootuka
Magneto-capacitance effect
• A tunnel junction can be thought of as a capacitor
• The capacitance can change as the magnetizations change from a parallel to an antiparallel configuration.
Spin accumulation
• Discussed by Johnson and Silsbee• Due to a spin bottleneck effect, when a
current goes from a first ferromagnetic layer into a second one, a magnetization S is induced in the second layer. S=I(m)T(2)/V where I(m) is the magnetization current. T(2) is the spin relaxation time. V is the volume of the second layer.
Spin accumulation:
• Spin accumulation also produces a splitting between the spin up and the spin down chemical potential
• The ratio of the splitting to the current is of the order of the resistance of the metal.
• This is much smaller than our effect.
Effect of the electron-electron interaction: Steady State non-
equilibrium charge and magnetization dipole layer.
Very different length scales for charge and spin fluctuation leads
to new unexpected physics.
• For charge fluctuation, the length scale of change is the screening length, which is less than 5 A.
• For magnetization fluctuation, the length scale is the spin diffusion length, which is more than 1000 A.
Dipole layers: No magnetization dipole layer in equilibrium
• The external voltage leads to a charge and magnetization transfer across the interface.
• charge (single line) and magnetization (double line) shown.
• Magnetization density >> charge density.
An example:
• Hall conductivity in a F-I-Al junction
• Experiments by Y. Otani, T. Ishiyama, S. G. Kim and K. Fukamichi, Jour. Appl. Phys. 87, 6995 (2000).
Numerical details:• Band structure obtained with the self-
consistent FLAPW method under the GGA with spin-orbit coupling.
• The conductivity is calculated using the Kubo formula.
• The Brillouin zone sampling is performed using 4000 special k-points.
• The spin up and spin down Fermi energies move apart by an amount 2 proportional to the external voltage V.
Magneto-capacitance effect
• A tunnel junction can be thought of as a capacitor.• As the magnetizations are changed from a parallel
to an antiparallel configuration, the charge and magnetization dipole layers are changed.
• The capacitance changes as the magnetizations change from a parallel to an antiparallel configuration.
Bias dependence of the magnetoresistance • Spin up and spin down
potential barriers are different because of the splitting of the spin up and down chemical potentials.
• Tunnelling probability
• • This changes from a
parallel to an antiparallel configuration.
)exp( dxkss
MR ratio for a F-Semiconductor-F structure for different doping.
• V is bias• U is
barrier height.
• Previous MR very small.
Magnetic Reversal Induced by a Spin-Polarized Current
Large (~107-109 A/cm2) spin-polarized currents can controllably reverse the magnetization in small (< 200 nm) magnetic devices
Parallel
(P)
Antiparallel
(AP)
Ferromagnet 1 Ferromagnet 2
Nonmagnetic
Cornell THALES/Orsay NIST
Positive Current
Nanopillar Technique (Katine, Albert, Emley)
-Multilayer film deposited (thermal evaporation, sputtering) on insulating substrate
Au (10 nm)Co (3 nm)Cu (6 nm)
Co (40 nm)
Cu (80 nm)
-Current densities of 108 A/cm2 can be sent vertically through pillar
-Electron-beam lithography, ion milling form pillar structure (thicker Co layer left as extended film)
-Polyimide insulator deposited and Cu top lead connected to pillar
Polyimide insulator
Cu
Enhanced effect in tunnel junctions.
• In metallic multilayers, the current required is too high.
• In tunnel junctions, from a capacitor point of view, we get an additional physics in that there is a magnetization dipole layer controlled by the voltage, not the current.
• The current required for switching is much reduced.
References• S. T. Chui and J. Cullen, Phys. Rev. Lett. 74, 2118 (1995).• S. T. Chui, Phys. Rev. B 52, R3822 (1995).• S. T. Chui, Jour. App. Phys., 80, 1002 (1996). • S. T. Chui, Jour. Mag. Mag. Mat., 151, 374 (1995). • S. T. Chui, Phys. Rev. B55, 5600, (1997).• S. T. Chui, US Patent no. 5757056.• S. T. Chui, Jian-Tao Wang, Lei Zhou, K. Esfarjani and Y.
Kawazoe, J. Phys. Conds. Matt. 13, L49 (2001).• S. T. Chui and L. B. Hu, Appl. Phys. Lett. 80, 273 (2002)