Relaziation of an ultrahigh magnetic field on a nanoscale S. T. Chui Univ. of Delaware

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.

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.

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 (~ A/cm 2 ) 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 10 8 A/cm 2 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 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)