Spintronics Tomas Jungwirth Institute of Physics ASCR Prague

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

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

Hard disk drive First hard disc (1956) - classical electromagnet for read-out 1 bit: 1 mm x 1 mm MB’s From PC hard drives ('90) to micro-discs - spintronic read-heads 1 bit: 10 -3 mm x 10 -3 mm 10’s-100’s GB’s

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

MRAM – universal memory fast, small, low-power, durable, and non-volatile 2006 - First commercial 4 Mb MRAM

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

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

Spin-orbit coupling from classical E&M and postulated electron spin nucleus rest frame electron rest frame 2 2 Lorentz transformation Thomas precession e… it’s all about spin and charge of electron communicating

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

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

DOS Giant Magneto-Resistance > P AP ~ 10% MR effect

Tunneling Magneto-Resistance DOS ~ 100% MR effect

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

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

Spin Transfer Torque writing

Magnetic domain “race-track” memory

Spintronics in nominally non-magnetic materials Datta-Das transistor

Spin Hall effect spin-dependent deflection transverse edge spin polarization skew scattering intrinsic _ __ side jump FSO I

Spin Hall effect detected optically in Ga. As-based structures Same magnetization achieved by external field generated by a superconducting magnet with 106 x larger dimensions & 106 x larger currents p n n SHE mikročip, 100 A SHE detected elecrically in metals Cu supercondicting magnet, 100 A SHE edge spin accumulation can be extracted and moved further into the circuit

Magnetization Spintronics explores new avenues for: • Information reading Ferr o • 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 Ga As Mn Mn Current