Department of Physics, University of California at Berkeley and

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Palo Alto Colloquium ATC Feb 19, 2009
Manipulation of Long-Range Order in Ferromagnetic
Semiconductors by Ultrafast Laser Pulses
Ingrid Cotoros
Department of Physics, University of California
at Berkeley and Materials Sciences Division,
Lawrence Berkeley National Laboratory Work
supported by the Office of Basic Energy Sciences
of the DOE
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Palo Alto Colloquia, ATC Feb 19, 2009
Collaborators
Berkeley Lab UC Berkeley Jigang Wang Daniel S.
Chemla
Department of Physics, University of California
at Berkeley and Materials Sciences Division,
Lawrence Berkeley National Laboratory Work
supported by the Office of Basic Energy Sciences
of the DOE
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Outline

• 1. Introduction
• - Spin Manipulation Ultrasmall Ultrafast
• - Carrier-mediated Ferromagnetism
• 2. Time-resolved MOKE spectroscopy
• 3. Ultrafast Ferromagnetic Phase Enhancement
• 4. Memory Effects in Femtosecond Magnetization
  Rotation

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Spin the Magical Property
Compass - ancient China before 1044 -
Earths magnetic field - observed for centuries -
Electron spin - Uhlenbeck and Goudsmit-
L
S
Spin manipulation a fundamental challenge
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Spin Manipulation Ultrasmall
nanometer
MRAM
GMR head
Peter Grünberg
Albert Fert
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Spin Manipulation Ultrafast
Fundamental time scales (sec)
Domain wall motion
Spin-lattice heating
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Ultrafast Manipulation of Spins?
External Stimuli
Spin Current
Magnetic Field
Laser pulse
1 ms
1 ms
1 ns
1 ps
1 fs
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Outline

• 1. Introduction
• - Spin Manipulation Ultrasmall Ultrafast
• - Carrier-mediated Ferromagnetism
• 2. Time-resolved MOKE spectroscopy
• 3. Ultrafast Ferromagnetic Phase Enhancement
• 4. Memory Effects in Femtosecond Magnetization
  Rotation

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Ultrafast Demagnetization
Ultrafast demagnetization ( hundred fs)
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Ultrafast, Non-thermal Cooperative

• Ultrafast enhancement of ferromagnetic order

Magnetic Memory
M

• Ultrafast creation of a
• ferromagnetic phase

No Pump
T

• Femtosecond detection of
• magnetic memory states

Curie Temp Tc
Ferromagnetism as an Example
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Interest in Spintronics

• Integration of coupled and inter-tunable
  electrical and magnetic properties
• Ferromagnetic semiconductors at room temperature
  and above are needed
• III-V derived magnetic
• semiconductors show
• Curie temperatures above
• 100K.

• Low-temperature MBE grown III1-xMnxV
• InMnAs, GaMnAs, InGaMnAs, GaMnSb, InMnSb, GaMnN,
  etc.

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Carrier-mediated Ferromagnetic Order

• Conventional itinerant ferromagnets particles
  interact directly to form FM phase.
• Ferromagnetism (FM) in III-Mn-V semiconductors is
  carrier mediated Mn ions interact through
  holes.

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III-Mn-V Ferromagnetic Semiconductor

• AFM correlation between holes and Mn ions
  stabilize Mn ions (local magnetic moments 3d5, S
  5/2) into FM phase

Mn-Mn interaction ? hole mediated Magnetism ?
sensitive to hole distribution and polarization
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III-Mn-V Ferromagnetic Semiconductor
Manganese doped III-V Semiconductor (e.g.
GaMnAs, InMnAs)
Mn ions (Mn2, 3d5) local magnetic moments
acceptors
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Outline

• 1. Introduction
• - Spin Manipulation Ultrasmall Ultrafast
• - Carrier-mediated Ferromagnetism
• 2. Time-resolved MOKE spectroscopy
• 3. Ultrafast Ferromagnetic Phase Enhancement
• 4. Memory Effects in Femtosecond Magnetization
  Rotation

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Magneto-optical Kerr Effect (MOKE)
MOKE at 1.55 eV ? Macroscopic M from Mn ions
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Optical Pump-Probe MOKE
Excitation
3.1 eV, 95 fs
Carriers
Polar MOKE
Dqk DMz
Magnetic Ions
Tunable pump-probe from MIR, NIR to visible
J. Wang, et al., J. Phys. Condens. Matter 18,
R501 (2006)
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Polar MOKE Spectroscopy

• Measure Mz dynamics reflecting
• amplitude changes
• reorientation

• Selectively pump in the
• Mn layer
• Probe the Mn level

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Experimental Setup
fs- vectorial resolved magnetization
dynamics at H lt 7T and T gt 1.5K
Ga1-xMnxAs
NIR PROBE
polarizer
fs TiSapphire Oscillator
B
BBO Xtal
UV PUMP
probe delay stage
7T magnet
131fs 800nm 0.6W
balanced detector
Transverse MOKE, DR/R
Lock-In Amplifier
Computer
balanced detector
Polar MOKE, MCD
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Sample Characterization
73nm
10nm
200nm
Dp 3x1020
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Outline

• 1. Introduction
• - Spin Manipulation Ultrasmall Ultrafast
• - Carrier-mediated Ferromagnetism
• 2. Time-resolved MOKE spectroscopy
• 3. Ultrafast Ferromagnetic Phase Enhancement
• 4. Memory Effects in Femtosecond Magnetization
  Rotation

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Photoinduced Ferromagnetism in (III,Mn)V
Light-induced ferromagnetism
Electrical-tuned Ferromagnetism
H. Ohno, et al., Nature 408, 944 (2000)
S. Koshihara et al., PRL 78, 4617 (1997)
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Ultrafast Photo-induced M Dynamics

• Study magnetization amplitude dynamics
• Typical data shows clear positive magnetization
  component (Dqk gt 0) on 100s of ps time scale!!!

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Magnetization Amplitude T-dependence

• Ultrafast enhancement of ferromagnetism!!
• Magnetization enhancement dominates
  demagnetization when approaching TC

TC
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The Dynamic Demagnetization

• (1) Disappears Tc
• (2) Resembles the static magnetization

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The Dynamic Magnetization Enhancement
Low-T Maximum
Long Tail

• Clear transient ferromagnetic enhancement
• The maximum below Tc long tail beyond Tc

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Ultrafast Photo-enhanced Ferromagnetism
- The time sequence -
Dt0 e-h pairs
Dtlt1ps e trap, demagnetization
Dt100ps M enhancement
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The LT Peak from Hpd Correlation
J. Wang, I. Cotoros et al., PRL 98, 217401 (2007)

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Outline

• 1. Introduction
• - Spin Manipulation Ultrasmall Ultrafast
• - Carrier-mediated Ferromagnetism
• 2. Time-resolved MOKE spectroscopy
• 3. Ultrafast Ferromagnetic Phase Enhancement
• 4. Memory Effects in Femtosecond Magnetization
  Rotation

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- GaMnAs-based four-state magnetic memory -
Ultrasensitive Spin Memory Detection
H. X. Tang et al., PRL 90, 107201 (2003)
A. V. Kimel et al., PRL 94, 227203 (2005)
Giant Planar Hall Effect
Giant Magnetic Linear Dichroism
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Writing Magnetic Memory
Write the magnetization in Y or X state at B0.
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Femtosecond Spin Rotation for Memory States
Z
M

• Unambiguous fs responses in pure spin rotation
• Opposite, state-dependent fs spin reorientations
• An cooperative spin rotation regime
• during non-equilibrium, non-thermal timescale

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Femtosecond Detection of Magnetic Memory
J. Wang, I. Cotoros et al., APL 94, 021101 (2009)
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Carrier Mediated Magnetic Field Pulse
Intuitive Picture
Experiment
Simulation (I. Perakis)
Y
1-10
Ku
Kc
X-
Kc
Thermal effects tens of ps Transient carrier
effects pulse width
J. Wang, I. Cotoros et al., APL 94, 021101 (2009)
Carrier-mediated, femtosecond response in
collective spin rotation
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Summary
Spin Manipulation Ultrafast, Non-thermal
Cooperative

• Ultrafast photoenhanced ferromagnetism on 100s of
  ps time scale
• Femtosecond spin reorientation via
  carrier-mediated magnetic field pulse
• Femtosecond detection of four-state magnetic
  memory

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Summary
Spin Manipulation Ultrafast, Non-thermal
Cooperative

• Thank you for your attention!
• Questions?

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Femtosecond Mn Spin Rotation
Step size 67 fs
J. Wang et al., in preparation
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UV pump/NIR probe Vectorial MOKE
fs- vectorially resolved Magnetization dynamics
at H lt 7 T and T gt 1.5K
GaMnAs
120 fs, Dq gt 0.2 mrad, DR/R gt 10-6
J. Wang, I. Cotoros, D. S. Chemla, in preparation
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Ultrafast Magneto-optics

• Ultrafast manipulation of magnetism with fs
  laser pulses
• Photoinduced demagnetization dynamics

M
No Pump

• Ultrafast enhancement of ferromagnetic order
• Ultrafast formation of a collective phase from an
  uncorrelated ground state?

T
Curie Temp Tc
The challenge understand and control spin
correlation at ultrafast, non-equilibrium
timescale