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）