Ultrafast Dynamic Study of Spin and Magnetization Reversal

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 Ø Introduction of dilute semiconductor Ga. Mn. As Ø The magnetic anisotropy of Ga. Mn. As and four-state magnetization switching Ø spin relaxation dynamics Ga. Mn. As Ø Ultrafast optical manipulation of four-state magnetization reversal in (Ga, Mn)As and magnetic domain wall dynamics Ø Conclusion

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

Advantages of Semiconductor Spintronics Ø Integration of magnetic, semiconducting and optical properties Ø Compatibility with existing microelectronic technologies. Ø Promise of new functionalities and devices for IT. Ø Nonvolatility Spin - FET D. D. Awschalom, M. E. Flatte, Nat. Phys. 3, 153 (2007) Ø Increased data processing speed Ø Decreased electric power consumption Ø 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: u Light u Electric field u Electric current in trilayer structures u 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 Ga. Mn. As 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 Ga. Mn. As – ----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 ØThe compressively strained (Ga, Mn)As grown on (001)Ga. As 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? Ø Spin Dynamics and mechanisms? --- s-d exchange coupling? --- p-d exchange couplng? --- electron-hole exchange coupling? --- carrier/impurity scattering? --- spin & disorder fluctuation? Ø Magnetization precession and switching? --- Thermal or Non-thermal effect?

TR-MOKE and MSHG Experiments Delay stage Mode-locked Ti: Sapphire laser Polarizer sample probe Filter 1 pump o BS chopper T PM Lock-In Amplifier HG S M Filter B Fields MOKE Waveplate n Mo photo bridge

Fabrication of (Ga, Mn)As TR-MOKR/MSHG Mn, Ga, As ◆ Mod. Gen. II MBE: -III-V Low Dimensional structures ◆VG V 80 MARKII MBE System: --- III-V Diluted magnetic semiconducutors and ferromagnetic metals

(Ga, Mn)As Sample Ø As grown Ø Tc ~ 50 K Ø Mn concentration ~ 6% Ø The compressively strained (Ga, Mn)As grown on (001)Ga. As 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) Relaxation time ~ 524 ps Rising time ~ 120 ps: the formation time for spin alignment of magnetic ions by the photoexcited holes Pump intensity hole density Relaxation time Mn-Mn coupling

Spin relaxation and dephasing (2) 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 Major Loop Measured at 8 K Minor Loop B 12= - B 34 = 33 G B 23 = - B 41= 264 G B field is applied in-plane of the sample along about 5 o 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 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

Ultrafast optical manipulation of switching fields ◆ Hc 1 increases abruptly to 108 Measured at 8 K Time evolution of small switching field Hc 1 Gauss upon pumping and then recovers back to the value before pumping within about 500 ps. ～ 500 ps ◆ However it is found that Hc 2 is almost independent of delay time. 2～ 3 ps The different time evolution behavior of Hc 1 and Hc 2 implies that different magnetization reversal mechanisms have been involved Appl. Phys. Lett. 95, 052108 (2009)

Temperature Dependence 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 Small switching field Hc 1 . Pumping power: laser pulses with pump fluences of about 2μJ/cm 2 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: Ø 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. Ø The thermal effect induced by laser heating does not play key role here. ----- Complex magnetic domain dynamics: Ø 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.

Ø New aspect 1: Femtomagnetism: Femotosecond laser pulse Coherent interaction between photons, charges and spins Incoherent ultrafast demagnetization Associated with thermalization of charges and spins into phonon bath (lattice) Nature Physics, 5, 515 (2009); 5, 499 (2009)

Ø New aspect 2: Ultrafast Magnetic Recording: PRL, 103, 117201(2009) The fastest “read-write” event is demonstrated to be 30 ps 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 2007 CB 924904, and the Knowledge Innovation Project of Chinese Academy of Sciences (No. KJCX 2. YW. W 09).