Transcript Document

Ultrafast Dynamic Study of Spin and
Magnetization Reversal in (Ga,Mn)As
Xinhui Zhang (张新惠）
State Key Laboratory for Superlattices and Microstructures
Institute of Semiconductors
Chinese Academy of Sciences, Beijing, China
（中科院半导体研究所超晶格国家重点实验室）
Outline
 Introduction of dilute semiconductor GaMnAs
 The magnetic anisotropy of GaMnAs
and four-state magnetization switching
 spin relaxation dynamics GaMnAs
 Ultrafast optical manipulation of four-state
magnetization reversal in (Ga,Mn)As and
magnetic domain wall dynamics
 Conclusion
III-Mn-V group: intrinsic DMS
GaMnAs, Ohno (Tohoko),APL’96
InMnAs, Ohno et al, (IBM,’92)
Tc up 190K is now
achieved
1366
as grown
o
Ta=150 C t=16h
LT-GaAs 0.5nm
45
3
M (emu/cm )
60
T. Dietl, Science 287,1019,(2000)
Mn% ~ 15%
30
(Ga,Mn)As 70 nm
GaAs
15
0
0
50
100
Temperature (K)
150
200
Advantages of Semiconductor
Spintronics
 Integration of magnetic, semiconducting
and optical properties
 Compatibility with existing
microelectronic technologies.
 Promise of new functionalities and
devices for IT.
 Nonvolatility
Spin - FET
D. D. Awschalom, M. E.
Flatte, Nat. Phys. 3, 153
(2007)
 Increased data processing speed
 Decreased electric power consumption
 Increased integration densities
Carrier- mediated ferromagnetism in DMS
 p-d Zener model + kp theory describes
quantitatively or semi-quantitatively:
-----
Thermodynamics [Tc, M(T,H)]
Micromagnetic
Dc and ac charge and spin transport
Optical properties
Ohno (Science,1998
Dietl (Science,2000)
Jungwirth PRB (1999)
Strong p-d coupling between Mn spin and holes
Manipulation of Spin
Carrier- mediated ferromagnetism in DMS:
---- A base for magnetization manipulation through:
 Light
 Electric field
 Electric current in trilayer structures
 Domain-wall displacement induced by electric current
Hole density & Tc
Magnetic Anisotropy in (Ga,Mn)As
The primary biaxial
anisotropy originates from
the hole-mediated
ferromagnetism In
combination with the
strong spin-orbit coupling,
based on the mean-field
theory.
The magnetic anisotropy of GaMnAs is quite complex,
arising from the competition between cubic and uniaxial
contribution, which depends on temperature, strain, and
carrier density.
Magnetic Anisotropy in (Ga,Mn)As
Hamaya, PRB, 74,045201(2006)
 Shin, PRB, 74,035327(2007)
Spin memory device
◆ The most practical application of GaMnAs –
----spin memory device: the information can be stored via the
direction of magnetization
◆ The magnetic properties related to the
Magnetization reversal can be controlled
by varing carrier density through electric
field or optical excitation.
◆
Current-driven magnetization
switching could be performed by
using giant planar Hall Effect of
(Ga,Mn)As epilayers. The required
driven current density is 2-3
orders of magnitude lower than
ferromagnetic metals!
H.X.Tang ,90,107201(2003)
In-plane biaxial magnetocrystalline anisotropy
& four-state magnetic reversal
The compressively strained (Ga,Mn)As grown on (001)GaAs
substrate is known to be dominated by in-plane biaxial
magnetocrystalline anisotropy with easy axes along [100] and [010] at
low temperatures
--- Allowing magnetization switching between two pairs of states
--- Leading to doubling of the recording density!
Magnetization Switching in (Ga,Mn)As
by subpicosecond optical excitation
◆ A switching of the magnetization
between the four orientations of the
magnetization can be significantly changed
by ultrafast laser excitation
G. V. Astakhov et al, APL, 86,152506 (2005)
A.V.Kimel et al, PRL, 92,237203(2004)
◆ The giant magnetic linear
dichroism comes from the difference
of optical refractive index for the
projection of polarization plane of
incident light in two perpendicular
easy axes [100] and [010] of
(Ga,Mn)As plane.
A.V.Kimel et al, PRL, 94,227203(2004)
From: G. V. Astakhov et al,
APL, 86,152506 (2005)
Questions?
 Spin Dynamics and mechanisms?
--- s-d exchange coupling?
--- p-d exchange couplng?
--- electron-hole exchange coupling?
--- carrier/impurity scattering?
--- spin & disorder fluctuation?
 Magnetization precession and switching?
--- Thermal or Non-thermal effect?
TR-MOKE and MSHG Experiments
Delay
stage
Mode-locked
Ti:Sapphire laser
Polarizer
sample
probe
Filter1
pump
BS chopper
B Fields
MOKE
Waveplate
Filter
Lock-In
Amplifier
photo
bridge
Fabrication
of (Ga,Mn)As
TR-MOKR/MSHG
Mn, Ga, As
◆ ModGenII MBE:
-III-V Low Dimensional
structures
◆VG V80 MARKII MBE System:
--- III-V Diluted magnetic
semiconducutors and ferromagnetic
metals
(Ga,Mn)As Sample
3
Mr (emu/cm )
 As grown
93# B = 0 T
30
 Tc ~ 50 K
(Ga,Mn)As 200 nm
GaAs buffer 500 nm
15
 Mn concentration ~ 6%
S-I GaAs substrate
0
0
50
100
150
Teperature (K)
200
 The compressively strained
(Ga,Mn)As grown on (001)GaAs
substrate is known to be
dominated by in-plane biaxial
magnetocrystalline anisotropy
with easy axes along [100] and
[010] at low temperatures
Relaxation time ~ 524 ps
Kerr Rotation deg
30
B = 0 T, T = 8 K
20

10
linear polarization
0
-10

-20
Relaxation Time T1 (ps )
Spin relaxation and dephasing (1)
560
560
540
552
520
544
500
536
480
528
460
520
0
20
40
60
80
Temperature (K )
100
6
9
12
15
18
21
-30
0
200
400
600
800 1000 1200
Delay time (ps)
Rising time ~ 120 ps: the
formation time for spin
alignment of magnetic ions by
the photoexcited holes
24
Pump Intensity (mW )
Pump intensity
hole density
Relaxation time
Mn-Mn coupling
27
B=1T
0
200
400
600
800 1000 1200 1400
A0 (udeg)
2
Delay time (ps)
(a)
336
312
288
16.8
16.2
15.6
15.0
14.4
15.6
(e) 15.2 (f)
14.8
14.4
14.0
0
K (t )  A0 exp(t / T2* ) cos(t   )  C
g   B Beff
120
(b)
100
80
60
40
20
336
(c) 324 (d)
312
300
288
35
30
25
20
15
10
384
360
T* (ps)
100K
90K
80K
70K
60K
50K
40K
30K
20K
8K
(GHz)
Kerr Rotation (arb. units)
Spin relaxation and dephasing (2)
20
40
60
80 100
Temperature (K)
6
9
12 15 18 21 24 27
Pump Intensity (mW)
g ~ 0.2 further proves the
formation of hole-Mn complex
Appl. Phys. Lett. 94, 142109 (2009)
The static photo-induced four-state
magnetization switching measurement
Kerr Rotation (deg)
60
(a)
B34 B12
B41
30
(4)
0
B23
Major Loop
(2)
-30
(3)
(1)
60
(b)
Measured at 8K
B21 B12
30
(2)
0
Minor Loop
(1)
-30
-600
-400
-200
0
200
400
600
Magnetic Field (G)
B12= - B34 = 33 G
B23 = - B41= 264 G
B field is applied in-plane of the sample
along about 5o off the [110] direction
Ultrafast optical manipulation of four-state
magnetization reversal in (Ga,Mn)As
◆The magnetic reversal signals are
dramatically suppressed at positive
delay time and gradually recover back
◆ photo-induced magnetic anisotropy
change upon applying pump pulse:
hole density increase upon pumping
significantly reduces the cubic
magnetic anisotropy (Kc) along the
[100] direction, while enhances the
uniaxial magnetic anisotropy (Ku) along
[110]
-60 ps
200
Kerr Rotation (deg)
within ～500 ps to that measured
before arrival of pump pulse.
250
150
-30 ps
100
+5 ps
50
+67 ps
0
+134 ps
-50
-100
+267 ps
-150
+550 ps
-200
Strong manipulation of the magnetic property
and anisotropy fields by polarized holes
injected by the circularly polarized pump light
-400 -200
0
200
400
Magnetic Field (G)
Ultrafast optical manipulation of
switching fields
Gauss upon pumping and then
recovers back to the value
before pumping within about
500 ps.
◆ However it is found that Hc2
is almost independent of delay
time.
Coercive Field (G)
◆ Hc1 increases abruptly to 108
Measured at 8K
Time evolution of small
switching field Hc1
100
80
～500ps
60
40
2～3ps
0
The different time evolution behavior of Hc1
and Hc2 implies that different magnetization
reversal mechanisms have been involved
100 200 300 400 500 600
Delay Time (ps)
Appl. Phys. Lett. 95, 052108 (2009)
Temperature Dependence
T ≈ 1/2 Tc
.
70
30
60
25
20
50
15
40
10
30
5
20
0
10
5
Pumping power:
35
Small switching field Hc1
Amplitude (deg)
M-shaped major
hysteresis loop could not
be observed above 20 K,
due to the vanished
fourfold magnetic
anisotropy in (Ga,Mn)As at
Coercive Field (G )
80
10
15
20
25
30
35
40
45
Temperature (K)
laser pulses with pump fluences of about 2μJ/cm2 can effectively manipulate
the magnetization reversal and switching field, which is about five orders of
magnitude lower than that achieved by Astakhov et al, which is favorable for
magneto-optical recording in (Ga,Mn)As.
Conclusion
---- Non-thermal manipulation of magnetization:
 The similar time evolution of spin relaxation and magnetic
reversal switching within the SAME sample suggests that
the polarized holes injected by optical pumping account for
the observed phenomena.
 The thermal effect induced by laser heating does not play
key role here.
----- Complex magnetic domain dynamics:
 Magnetic reversal is governed by domain
nucleation/propagation at lower magnetic fields and
magnetization rotation at higher magnetic fields.
Challenge: is there any other mechanism for
faster manipulation of magnetization?
Magnetic field
Electric field (or current)
Manipulation of
magnetization
and
magnetic switching
Optical pumping
Manipulation of magnetization in the ultrafast fashion:
---- A torque can be induced optically through the non-thermal pass,
and results in the non-equilibrium state of magnetization. The
state is controllable by optical pulses.
 New aspect 1: Femtomagnetism:
Femotosecond laser pulse
Coherent interaction between
photons, charges and spins
Incoherent ultrafast
demagnetization
Associated with the thermalization
of charges and spins into phonon
bath (lattice)
Nature Physics,5,515 (2009); 5, 499 (2009)
 New aspect 2: Ultrafast Magnetic
Recording:
PRL, 103,117201(2009)
The fastest “read-write” event is demonstrated to be 30ps for magnetic recording
Acknowledgement
Mrs. Yonggang Zhu (朱永刚）, Lin Chen（陈林）
Prof. Jinhua Zhao （赵建华）
This work is supported by the National Natural
Science Foundation of China (No. 1067 4131, 60836002),
the National Key Projects for Basic Research of China
under Grant No 2007CB924904, and the Knowledge
Innovation Project of Chinese Academy of Sciences (No.
KJCX2. YW. W09).