1. 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, , Korea
Charles M. Falco
This work supported by ONR/DARPA N

2. 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.

3. 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;

4. 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

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

6. 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

7. 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”

8. Brillouin Light Scattering (BLS)

9. 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

10. 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 °C;
the onset of a second mode.

11. 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 °C;
the onset of a second mode.

12. 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 °C;
the onset of a second mode.

13. 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

14. 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 °C.

15. 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

16. 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.