STRUCTURE AND MAGNETIC PROPERTIES OF ULTRA-THIN MAGNETIC LAYERS Justin M. Shaw Optical Sciences Center, University of Arizona, Tucson Department of Physics and Astronomy, Arizona State University, Tempe Sungkyun Park Los Alamos National Laboratory Sukmock Lee Department of Physics, Inha University, Inchon, 402-751, Korea Charles M. Falco This work supported by ONR/DARPA N00014-02-01-0627

Outline of Talk Introduction and Motivation; Sample Growth Annealed Fe layers on GaAs(001); Description of Spin-waves and Brillouin Light Scattering (BLS); Results; Summary.

Introduction and Motivation Why Fe LAYERS ON GaAs? Spin injection has been demonstrated in Fe on GaAs; High quality BCC Fe layers can be easily grown on GaAs  aFe/aGaAs = 1/2 (1.4% mismatch); These properties make Fe on GaAs(001) a candidate for FM metal-semiconductor contacts in spintronic devices;

Spintronics: What and Why? We want to exploit the spin degree of freedom in electrons. Simple ‘Spin-Valve’ Lower power; Higher speeds; Non-Volatile Memory; Quantum Computing;  Spin is a quantum state. spin injection “valve” non-FM FM M Current Out CURRENT No

Molecular Beam Epitaxy (MBE) Growth Pbase= 4 x 10–11 Torr Analysis Pbase= 3 x 10–11 Torr

Sample Growth SUBSTRATE PREPARATION; 1 keV Ar+ at 600 °C; GaAs(001) 15 Å Fe 50Å Al Sample Structure We deposited all samples in a Perkin-Elmer 4-chamber MBE system with a base pressure of ~5 x 10–11 Torr SUBSTRATE PREPARATION; 1 keV Ar+ at 600 °C; Streaky 4×6 surface; Fe DEPOSITION (15 Å); Knudsen cell (0.088 Å/s); Tsubstrate = 30–35 °C; ANNEALED (150–350 °C for 10 min.); OVERLAYER (Al ); E-beam evaporation; Thickness = 20–90 Å. [110] [110] Al overlayer

Spin-waves (Magnons) Spins (magnetic moments) oscillate slightly about their equilibrium position; Electron spins interact via exchange (short range) and dipole-dipole (long range); Ferromagnetic crystals form quantized “spin-waves”. From Kittel “Intro to Solid State Physics”

Brillouin Light Scattering (BLS)

Annealing Temperature Dependence on the Saturation Field Saturation fields are given by the minima in the spinwave frequencies along the hard axis; Saturation field changes behavior at Tanneal~ 225 °C. H=0 H Hsat Mspontaneos M Location of minima

Anisotropy Dependence of Tanneal Magnetic Field = 3kOe At RT the magnetic anisotropy is in-plane uniaxial; Annealing up to 225°C results in: decreasing anisotropy; mixed uniaxial and cubic form. Annealing above 250 °C results in: gradually increasing anisotropy; becoming uniaxial again at 300 °C; the onset of a second mode.

Anisotropy Dependence of Tanneal Magnetic Field = 3kOe At RT the magnetic anisotropy is in-plane uniaxial; Annealing up to 225°C results in: decreasing anisotropy; mixed uniaxial and cubic form. Annealing above 250 °C results in: gradually increasing anisotropy; becoming uniaxial again at 300 °C; the onset of a second mode.

Anisotropy Dependence of Tanneal Magnetic Field = 3kOe At RT the magnetic anisotropy is in-plane uniaxial; Annealing up to 225°C results in: decreasing anisotropy; mixed uniaxial and cubic form. Annealing above 250 °C results in: gradually increasing anisotropy; becoming uniaxial again at 300 °C; the onset of a second mode.

Additional Spinwave Mode An additional spinwave mode develops for annealing temperatures at or above 250 °C; This new mode is present only near a <110> direction (easy and hard axes). 90° 100° 110° 120° 135° 150° 160° 170° 180° hard axis easy axis 15 Å Fe Annealed 10 min. 275 °C

Magneto-Optical Kerr Effect (MOKE) RT 150 °C 200 °C 225 °C 250 °C 275 °C 300 °C Coercivity increases rapidly for annealing temperatures above 225 °C.

Scanning Tunneling Microscopy (1 μm × 1 μm ) Rectangular pits faceted along <110> directions form for annealing temperatures between 200 °C and 250 °C; X-ray Reflectometry  16% increase in Fe thickness for 300 °C anneal. RT Fe RMS roughness = 0.890 ±0.02 nm [110] 250 °C Anneal 0.883 ±0.04 nm [110] 200 °C Anneal 0.940 ±0.05 nm [110] 300 °C Anneal 0.887 ±0.01 nm (terraces: = 0.484 ±0.07 nm) [110] 400×400 nm

Summary Ultra-thin ferromagnetic layers interfaced with semiconductors are critical to future Spintronic technology; Magnetic properties can be studied with high sensisitivity using BLS; The form and magnitude of the magnetic anisotropy is highly dependent on annealing temperature; Magnetic and structural changes occur at annealing temperatures at ~ 225 °C.