1. 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）

2. 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

3. III-Mn-V group: intrinsic DMS
T. Dietl, Science 287,1019,(2000)
GaMnAs, Ohno (Tohoko),APL’96
InMnAs, Ohno et al, (IBM,’92)
Mn% ~ 15%
Tc up 190K is now
achieved

4. Advantages of Semiconductor Spintronics
D. D. Awschalom, M. E. Flatte, Nat. Phys. 3, 153 (2007)
Spin - FET
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 densities

5. 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

6. Carrier- mediated ferromagnetism in DMS:
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

7. Hole density & Tc

8. 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.

9. Magnetic Anisotropy in (Ga,Mn)As
Hamaya, PRB, 74,045201(2006)
Shin, PRB, 74,035327(2007)

10. 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)

11. 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!

12. Magnetization Switching in (Ga,Mn)As
by subpicosecond optical excitation
From: G. V. Astakhov et al,
APL, 86, (2005)
◆ 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, (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)

13. 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?

14. TR-MOKE and MSHG Experiments
B Fields
Delay stage BS
probe
pump
Mode-locked Ti:Sapphire laser
sample
Filter
MSHG
Polarizer
PMT
Mono
chopper
Waveplate
Filter1
MOKE
Lock-In Amplifier
photo
bridge

15. Fabrication of (Ga,Mn)As
TR-MOKR/MSHG
Fabrication of (Ga,Mn)As
Mn, Ga, As
◆ ModGenII MBE:
-III-V Low Dimensional structures
◆VG V80 MARKII MBE System:
--- III-V Diluted magnetic semiconducutors and ferromagnetic metals

16. (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

17. Spin relaxation and dephasing (1)
Pump intensity
hole density
Mn-Mn coupling
Relaxation time
Relaxation time ~ 524 ps
Rising time ~ 120 ps: the formation time for spin alignment of magnetic ions by the photoexcited holes

18. Spin relaxation and dephasing (2)
Appl. Phys. Lett. 94, (2009)
g ~ 0.2 further proves the formation of hole-Mn complex

19. The static photo-induced four-state magnetization switching measurement
B12= - B34 = 33 G
B23 = - B41= 264 G
Measured at 8K
Major Loop
Minor Loop
B field is applied in-plane of the sample
along about 5o off the [110] direction

20. 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 within ～500 ps to that measured before arrival of pump pulse.
◆ 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]
Strong manipulation of the magnetic property and anisotropy fields by polarized holes injected by the circularly polarized pump light

21. Ultrafast optical manipulation of switching fields
Measured at 8K
Time evolution of small
switching field Hc1
～500ps
2～3ps
◆ 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, (2009)

22. Temperature Dependence
Small switching field Hc1
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 .
Pumping power:
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.

23. 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.
---- 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:

24. 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.

25. New aspect 1: Femtomagnetism:
Nature Physics,5,515 (2009); 5, 499 (2009)
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)

26. New aspect 2: Ultrafast Magnetic Recording: PRL, 103,117201(2009)
The fastest “read-write” event is demonstrated to be 30ps for magnetic recording

27. 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 , ), 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).

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