Ultrafast Dynamic Study of Spin and Magnetization Reversal in (Ga,Mn)As
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Transcript of Ultrafast Dynamic Study of Spin and Magnetization Reversal in (Ga,Mn)As
Ultrafast Dynamic Study of Spin and Magnetization Reversal in (Ga,Mn)As
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)
Xinhui Zhang ( 张新惠)
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
T. Dietl, Science 287,1019,(2000)
III-Mn-V group: intrinsic DMS
GaMnAs, Ohno (Tohoko),APL’96
Mn% ~ 15%
Tc up 190K is now achieved
InMnAs, Ohno et al, (IBM,’92)
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
Increased data processing speed
Decreased electric power consumption
Increased integration densitiesD. D. Awschalom, M. E. Flatte, Nat. Phys. 3, 153 (2007)
Spin - FET
Carrier- mediated ferromagnetism in DMS
Ohno (Science,1998 Dietl (Science,2000)
Jungwirth PRB (1999)
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
Strong p-d coupling between Mn spin and holes
Carrier- mediated ferromagnetism in DMS: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
Manipulation of Spin
Hole density & Tc
Magnetic Anisotropy in (Ga,Mn)As
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.
The primary biaxial anisotropy originates from the hole-mediated ferromagnetism In combination with the strong spin-orbit coupling, based on the mean-field theory.
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
◆ 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)
◆ The magnetic properties related to theMagnetization reversal can be controlled by varing carrier density through electric field or optical excitation.
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!
◆ A switching of the magnetization between the four orientations of the magnetization can be significantly changed by ultrafast laser excitation
◆ 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.
G. V. Astakhov et al, APL, 86,152506 (2005)A.V.Kimel et al, PRL, 92,237203(2004)
Magnetization Switching in (Ga,Mn)As by subpicosecond optical excitation
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
B Fields
Delay stage
BS
probe
pump
Mode-locked Ti:Sapphire laser sample
Filter
MSHG
Polarizer
PMTMono
chopperWaveplate
Filter1
MO
KE
Lock-In Amplifier
photobridge
Fabrication of (Ga,Mn)As
◆ ModGenII MBE:-III-V Low Dimensional structures
◆VG V80 MARKII MBE System:--- III-V Diluted magnetic semiconducutors and ferromagnetic metals
Mn, Ga, As
TR-MOKR/MSHG
(Ga,Mn)As Sample
As grown
Tc ~ 50 K
Mn concentration ~ 6%
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
Spin relaxation and dephasing (1)
0 20 40 60 80 100460
480
500
520
540
560
Rel
axat
ion
Tim
e T 1
(ps
)Temperature (K )
6 9 12 15 18 21 24 27520
528
536
544
552
560
Pump Intensity (mW )
Pump intensity hole density
Mn-Mn coupling Relaxation time
0 200 400 600 800 1000 1200-30
-20
-10
0
10
20
30
Ker
r Rot
atio
n
deg
Delay time (ps)
linear polarization
B = 0 T, T = 8 K
Relaxation time ~ 524 ps
Rising time ~ 120 ps: the formation time for spin alignment of magnetic ions by the photoexcited holes
Spin relaxation and dephasing (2)
101520253035
A0 (
udeg
) (a)
20406080
100120 (b)
288312336360384
T* 2 (p
s)
(c)
288300312324336
(d)
0 20 40 60 80 10014.415.015.616.216.8
(G
Hz)
Temperature (K)
(e)
6 9 12 15 18 21 24 2714.014.414.815.215.6
Pump Intensity (mW)
(f)
0 200 400 600 800 1000 1200 1400
Ker
r Rot
atio
n (a
rb. u
nits
)
Delay time (ps)
100K90K80K70K60K50K40K30K20K8K
B = 1 T
CtTtAtK )cos()/exp()( *20
Appl. Phys. Lett. 94, 142109 (2009)
effB Bg g ~ 0.2 further proves the formation of hole-Mn complex
The static photo-induced four-state magnetization switching measurement
Major Loop
Minor Loop
B field is applied in-plane of the sample along about 5o off the [110] direction
B12= - B34 = 33 G B23 = - B41= 264 G
-600 -400 -200 0 200 400 600
-30
0
30
60
(2)(1)
B21 B12
Ker
r Rot
atio
n (
deg)
Magnetic Field (G)
(b)
-30
0
30
60B23B12B34
(3)
(4)
(1)
(2)
(a) B41
Measured at 8K
Ultrafast optical manipulation of four-state magnetization reversal in (Ga,Mn)As
Strong manipulation of the magnetic property and anisotropy fields by polarized holes injected by the circularly polarized pump light
◆ 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]
-400 -200 0 200 400
-200
-150
-100
-50
0
50
100
150
200
250
Ker
r Rot
atio
n (de
g)
Magnetic Field (G)
+5 ps
-30 ps
-60 ps
+67 ps
+134 ps
+267 ps
+550 ps
◆The magnetic reversal signals are dramatically suppressed at positive delay time and gradually recover back
within ~ 500 ps to that measured before arrival of pump pulse.
Ultrafast optical manipulation of switching fields
◆ Hc1 increases abruptly to 108 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.
The different time evolution behavior of Hc1 and Hc2 implies that different magnetization reversal mechanisms have been involved
Appl. Phys. Lett. 95, 052108 (2009)
0 100 200 300 400 500 600
40
60
80
100
Coe
rciv
e Fi
eld
(G)
Delay Time (ps)
Measured at 8KTime evolution of small switching field Hc1
~ 500ps
2 ~ 3ps
M-shaped major hysteresis loop could not be observed above 20 K, due to the vanished fourfold magnetic anisotropy in (Ga,Mn)As at
T ≈ 1/2 Tc .
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.
Pumping power:
Temperature Dependence
5 10 15 20 25 30 35 40 45
0
5
10
15
20
25
30
35
10
20
30
40
50
60
70
80
Coerc
ive Fi
eld (
G )
Am
plitu
de (
deg
)
Temperature (K)
Small switching field Hc1
Conclusion
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.
---- Non-thermal manipulation of magnetization:
Magnetic reversal is governed by domain nucleation/propagation at lower magnetic fields and magnetization rotation at higher magnetic fields.
----- Complex magnetic domain dynamics:
Manipulation of magnetization
and magnetic switching
Optical pumping
Magnetic field Electric field (or current)
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.
Challenge: is there any other mechanism for faster manipulation of magnetization?
New aspect 1: Femtomagnetism:
Nature Physics,5,515 (2009); 5, 499 (2009)
Femotosecond laser pulse
Coherent interaction between photons, charges and spins
Incoherent ultrafast demagnetizationAssociated with the thermalization of charges and spins into phonon bath (lattice)
New aspect 2: Ultrafast Magnetic Recording: PRL, 103,117201(2009)
The fastest “read-write” event is demonstrated to be 30ps for magnetic recording
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).
AcknowledgementAcknowledgement
Mrs. Yonggang Zhu ( 朱永刚) , Lin Chen (陈林) Prof. Jinhua Zhao (赵建华)