Spintronics Tomas Jungwirth University of Nottingham Institute of Physics ASCR, Prague

1. Current spintronics in HDD read-heads and MRAMs 2. Basic physical principles of the operation of current spintronic devices devices 3. Spintronics research 4. Summary

Hard disk drive First hard disc (1956) - classical electromagnet for read-out From PC hard drives ('90) to micro-discs - spintronic read-heads MB’s 10’s-100’s GB’s 1 bit: 1mm x 1mm 1 bit: mm x mm

Dawn of spintronics Anisotropic magnetoresistance (AMR) – 1850’s  1990’s Giant magnetoresistance (GMR) – 1988  1997 Inductive read/write element Magnetoresistive read element

MRAM – universal memory fast, small, low-power, durable, and non-volatile First commercial 4Mb MRAM

RAM chip that actually won't forget  instant on-and-off computers Based on Tunneling Magneto-Resistance (similar to GMR but insulating spacer)

1. Current spintronics in HDD read-heads and MRAMs 2. Basic physical principles of the operation of current spintronic devices devices 3. Spintronics research 4. Summary

Spin-orbit coupling from classical E&M and postulated electron spin nucleus rest frame electron rest frame Lorentz transformation  Thomas precession 22 e-e-e-e- … it’s all about spin and charge of electron communicating

quantum mechanics & special relativity  Dirac equation E=p 2 /2m E  ih d/dt p  -ih d/dr E 2 /c 2 =p 2 +m 2 c 2 (E=mc 2 for p=0) & H SO (2 nd order in v/c around the non-relativistic limit) Spin Anisotropic Magneto-Resistance Current sensitive to magnetization direction ~ 1% MR effect SO coupling from relativistic QM

Ferromagnetism = Pauli exclusion principle & Coulomb repulsion total wf antisymmetric = orbital wf antisymmetric * spin wf symmetric (aligned) Robust (can be as strong as bonding in solids) Robust (can be as strong as bonding in solids) Strong coupling to magnetic field Strong coupling to magnetic field (weak fields = anisotropy fields needed (weak fields = anisotropy fields needed only to reorient macroscopic moment) only to reorient macroscopic moment) DOS e-e-e-e- e-e-e-e- e-e-e-e-

Giant Magneto-Resistance      ~ 10% MR effect DOS  AP PP >

Tunneling Magneto-Resistance ~ 100% MR effect DOS   DOS 

1. Current spintronics in HDD read-heads and MRAMs 2. Basic physical principles of the operation of current spintronic devices devices 3. Spintronics research 4. Summary

TMR Tunneling Anisotropic Magneto-Resistance Magneto-Resistive transistors Discovered in ferromagnetic semiconductors (T c < room T) First successful attempts in metals Mn Ga As Mn Au

Spin Transfer Torque writing

Magnetic domain “race-track” memory

Datta-Das transistor Spintronics in nominally non-magnetic materials

intrinsic skew scatteringside jump I _ F SO _ _ _ Spin Hall effect spin-dependent deflection  transverse edge spin polarization

n n p SHE mikročip, 100  A supercondicting magnet, 100 A Spin Hall effect detected optically in GaAs-based structures Same magnetization achieved by external field generated by a superconducting magnet with 10 6 x larger dimensions & 10 6 x larger currents Cu SHE detected elecrically in metals SHE edge spin accumulation can be extracted and moved further into the circuit

Information reading  Ferro Magnetization  Current Information reading & storage Tunneling magneto-resistance sensor and memory bit Information reading & storage & writing Current induced magnetization switching Information reading & storage & writing & processing : Spintronic transistor: magnetoresistance controlled by gate voltage New materials Ferromagnetic semiconductors Non-magnetic SO-coupled systems Mn Ga As Mn Spintronics explores new avenues for: