Spin Electronics Peng Xiong Department of Physics and MARTECH Florida State University QuarkNet, June 28, 2002

Moore’s Law… is the end in sight? Speed: 100 Hz Size: 10-2 m Cost: $106/transistor Speed: 109 Hz Size: 10-7 m Cost: $10-5/transistor SOURCE GATE DRAIN MOSFET

Magnetic Information Storage: superparamagnetic limit Density: 20 Gb/in2 Speed: 200 Mb/s Size: f2.5” x 2 Capacity: 50 Gb Density: 2 kb/in2 Speed: 70 kb/s Size: f24” x 50 Capacity: 5 Mb

Superparamagnetic Limit: thermal stability of magnetic media

Semiconductor Random Access Memory: alternatives? High speed Low density High power consumption Volatile

Metal-based Spintronics: Spin valve and magnetic tunnel junction EF E H EF N(E) E H R H M Applications: magnetic sensors, MRAM, NV-logic

Spintronics in Semiconductor: spin transistor Dreams High performance opto-electronics Single-chip computer (instant on; low power) Quantum computation Datta and Das, APL, 1990 GATE H SOURCE DRAIN GaAs Issues Spin polarized material Spin injection Spin coherence Spin detection H

Spin Injection: the conductivity mismatch Schmidt et.al., PRB, 2000 I I­ RF­ RN­ I¯ SC mF­ RF¯ RN¯ mF¯ mN­ Solutions: Use injector with 100% spin polarization Non-diffusive injection Conductivity matching FM mN¯

Measurement of spin polarization: using a superconductor half-metallic ferromagnet E Uex normal metal E CrO2: a half metal Tc = 400 K m = 2mB/Cr p = 100% Schwarz, J. Phys. F, 1986 metallic ferromagnet E 4s 3d Measurement of spin polarization: using a superconductor

Andreev reflection: normal metal/superconductor Question: What could happen to an electron with energy eV < D when it hits S from N? bounce back; go into S as an electron; C. go into S in a Cooper pair. A and B B and C C and A A and B and C D eV EF -D N(E) N S

clean metallic contact Andreev reflection: normal metal/superconductor p = 0 Z = 0 clean metallic contact Z ~ 1 in-between Z >> 1 tunnel junction Blonder, Tinkham, and Klapwijk, PRB, 1982

Andreev reflection: ferromagnet/superconductor Z = 0 metallic contact D eV EF -D Z ~ 1 in-between DOS Z >> 1 tunnel junction V

Comparison: normal metal and ferromagnet Z = 0 metallic contact Z = 0 metallic contact Z ~ 1 in-between Z ~ 1 in-between Z >> 1 tunnel junction Z >> 1 tunnel junction V V

Spin Polarization of CrO2: our approach Planar junction  real device structure Artificial barrier  controlled interface Preservation of spin polarization at and across barrier Key step: controlled surface modification of CrO2 via Br etch

CrO2 Film Growth: Chemical Vapor Deposition Furnace, T=280° C O2 flow Heater block, T=400°C substrate Cr8O21 precursor Ivanov, Watts, and Lind, JAP, 2001

I ~ CrO2 TiO2 Junction Fabrication and Measurement Pb or Al Pb or Al Grow CrO2 film Pattern CrO2 stripe Surface modification: Br etch Deposit S cross stripes V ~ Lock-in Pb or Al Pb or Al I CrO2 CrO2 TiO2 dV/dI vs V in He4 (1K) or He3 (0.3K) cryostats

w/o inelastic scattering Results: CrO2/(I)/Pb junctions Metallic contact Z = 0 p = 97% T = 1.2 K = 1.44 meV Tunnel junction T = 400 mK High quality barrier w/o inelastic scattering

Measurement of spin polarization in high-Z junctions: using Zeeman splitting E mH D eV EF -D eV/D N(E) H Meservey and Tedrow, Phys. Rep., 1994 F S

Zeeman splitting in an F/I/S junction CrO2 H In order to get high Hc: Ultrathin S film Parallel field Negligible s-o interaction Al CrO2 Al

Results: Zeeman splitting T =400 mK

Summary (CrO2) Verified half-metallicity of CrO2 Engineered an artificial barrier on CrO2 surface Preserved complete spin polarization at interface Achieved full spin injection from a half metal Future Apply the technique to other systems Magnetic tunnel junction

CrO2/I/Co magnetic “tunnel” junction H CrO2 Co AlOx

The People Jeff Parker Jazcek Braden Steve Watts Pavel Ivanov Stephan von Molnár Pedro Schlottmann David Lind

Let’s build “computers with wires no wider than 100 atoms, a microscope that could view individual atoms, machines that could manipulate atoms 1 by 1, and circuits involving quantized energy levels or the interactions of quantized spins.” Richard Feynman – “There’s Plenty of Room at the Bottom” 1959 APS Meeting