Current research in current-driven magnetization dynamics

1. Current research in current-driven magnetization dynamics S. Zhang, University of Missouri-Columbia Beijing, Feb. 14, 2006

2. Outlines • Magentoelectronics started from discovery of giant magnetoresistive (GMR) effect • Spin-dependent transport in magnetic metal based nanostructures • Spin angular momemtum transfer: physics and potential technology • Perspectives and conclusions

3. What is giant magnetoresistance? R M.N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988).

4. Origin of GMR—two current model EF Up and down resistances Different numbers of up and down electrons A ferromagnet Low resistance High resistance e e e e

5. GMR Reading head Spin valve OR Conductor lead M NM M AF M Spin valve J “1” “0” Bit length Bit width

6. Concert efforts: theorists, experiments and technologists on GMR • Theorists: predict, explain, model and design GMR effects and devices • Experimentalists: design, fabricate, characterize, and measure GMR devices • Technologists: produce, evaluate, pattern, integrate, and deliver GMR devices It would be otherwise impossible to push the information storage so rapidly

7. History of magentic tapes and hard disks Now: 80Gbits/in2 5 years: 1 Terabits/in2 In 1988, giant Magnetoresistance (GMR) was discovered; in 1996, GMR reading heads were commercialized Since 2000: Virtually all writing heads are GMR heads

8. Magnetoelectronics: Magnetic Tunnel Junctions GND High tunneling probability Low resistance Low tunneling probability High resistance

9. Ta (5) Cu (20) Co-Fe-B(4) Al-O (0.8) Al-O barrier Co-Fe-B(4) IrMn IrMn (12) Py (5) Ta (5) Cu (30) Ta (5) Source: Dr. Xiufeng Han V

10. Brief History of TMJ • 1974, M. Julliere (a graduate student) published an experiment article which claimed 14% TMR in Fe/Ge/Fe trilayers. A simple model was proposed (the paper became a sleeping giant). • 1982, IBM reported 2% TMR on Ni/AlO/Ni. • 1995, Moodera (MIT) and Miyazaki (Japan) reported 10% TMR for Co/AlO/Co. • 1998, DARPA launched MRAM solicitation • 1999, Motorola’s 128kB MRAM demo • 2003, IBM, Motolora, 4Mb MRAM chip demo • More than 10 startup MRAM companies formed. • MRAM becomes internationally recognized future technology

12. FRAM PFRAM SiC Bipolar Molecular NanoX’tal Quantity PMC Polymer Perovskite 3DROM PCRAM Flow MRAM Spin Emerging non-volatile memory technologies

13. Current-driven spin torques • GMR/TMR: magnetization states control spin transport (low-high resistance). • Adverse effect: spin transport (spin current) affects magnetization states? • Every action will have reaction—spin transfer

14. spin angular momentum transfer? • Momentum transfer—electromigration • Angular momentum transfer—magnetization dynamics An impurity atom receives a force by absorbing a net momentum of electrons: electromigration is one of the major failure mechanisms in semiconductor devices. F The atom receives a torque by absorbing a net spin angular momentum of electrons: the spin torque can be used for spintronics T

15. Interaction between spin polarized current and magnetization (J. Slonczewski, IBM) Mp M Spin torque on the magnetic layer M

16. Current induced Domain wall motion Current  torque on DW Massless motion!! (Magnetic field  pressure on DW, ) From Sadamichi Maekawa

17. Magnetization dynamics: LLG equation (micromagnetics) LLG+spin torque Spin valve Domain wall Where

18. Novelty of spin transfer torques • Manipulation of magnetization states by currents • Self-sustained steady state magnetization dynamics • Unusual thermal effects • Interesting domain wall dynamics • Dynamic phases: synchronization, modification and chaos

19. First observation of current induced magnetic switching by Spin torques Co1=2.5nm Co2=6.0nm Katine et. al., PRL (2000).

20. Self-sustained steady-states precession Energy damping and pumping: The first term is always negative (damping), the second term could be positive or negative (it even changes sign at different times). Limit cycle: the energy change is zero in an orbit

21. Calculated limit cycles

22. Experimental identification of limit cycles Kiselev et al., Nature (2003)

23. Unusual Thermal effects Neel-Brown relaxation: Eb is algebraic dependent on T, E where Thermal activation Question: in the presence of the spin torque, how do we formulate the relaxation time? Difficulty: the spin torque is not conservative:

24. LLG equation at finite temperatures random field The magnetization receives following fields Precessional conservative field Non-conservative damping field Non-conservative spin torque field Diffusion field

25. Solution of Fokker-Planck equation is diffusion constant (dissipation-fluctuation relation) The probability energy density is: where

26. Experimental data interpretation R Telegraph noise H J J H + J H Current alone Field alone

27. Comparison with experiments Equal dwell times for P and AP states By simultaneously changing H and J, one can always have H-I phase boundary of equal dwell times.

28. Synchronization, modification and chaos 1. Another oscillator 2. AC external field 3. AC external current + Linear oscillator Limit cycle

29. Calculated limit cycles

30. Observation of synchronization by an AC current Rippard et al, PRL (2005)

31. Observation of mutual synchronization Kaka et al., Nature (2005); Mancoff et al, Nature (2005)

32. Observation of mutual synchronization

33. Narrower spectrum width at synchronization

34. Dynamic phases due to AC currents M M M M

35. Synchronization spectra x1

36. Modification spectra (beating) x2

37. Synchronization and modification agree well with experiments

38. x3 Chaos spectra

39. Theories of spin torques in ferromagnets e M • Berger, domain drag force, based an intuitive physics picture • Bazaliy, et al, • Waintal and Viret, a global pressure and a periodic torque • Tatara and Kohno, spin transfer torque and momentum transfer torque. • Zhang and Li, adiabatic torque and non-adiabatic torques • Barnas and Maekawa, non-adiabatic torque relates to the damping of the adiabatic torque within a ballistic transport model for half-metallic materials

40. Spin torques in a domain wall Interaction between conduction electrons and magnetization: Equation of motion for conduction electrons where

41. If the wall is in steady motion, the current driven wall velocity is independent wall structure and pinning potentials Steady state wall motion Steady state wall velocity is thus

42. Wall velocity for different materials in a perfect wire

43. Observed Domain wall motion in a nanowire Observed Wall velocity for Yamagushi et al., PRL (2004)

44. Vortex domain wall motion driven by current Wall transition: from vortex all to transverse wall

45. Goal: using a reasonable current to switch magnetization, ideally less than 106 A/cm2 Magnetic tunnel Junction 0 1

46. Application 2: local AC magnetic field oscillators (generators) Conductor lead J Oscillation of M (GHz)by a DC current Local AC field (1000 Oe) with spatial resolution 10nm!

47. Application IV: concerns of CPP reading heads Conductor lead The large current density in CPP reading heads may produce unwanted switching! Goal: eliminates current-induced switching for current density larger than 107A/cm2 M Spin valve J “1” “0” Bit length Bit width

48. Acknowledgement Students: Dr. Yu-nong Qi, Mr. Zhao-yang Yang, Mr. Jie-xuan He Postdoctoral: Dr. Z. Li (Postdoctoral) Collaborators: P. M. Levy (NYU) A. Fert (Orsay-Paris) Funded by: NSF-DMR, NSF-ECS, DARPA, NSFC