Relaziation of an ultrahigh magnetic field on a nanoscale

S. T. ChuiUniv. of Delaware

• chui@udel.edu

• 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

Many unexpected surprises:

Strong bias dependence of the magnetoresistance: Miyazaki et

al., Moodera et al.

A very large Hall resistivity in a F-I-P junction, Otani et al.

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

Negative bias dependence for a F-I-F-P structure by Moodera et

al.

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.

Additional Physics

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

Bias dependence

• X axis is the decrease of the normalized resistance with bias.

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)