Magnetism on the Move

1. Magnetism on the Move • Ferromagnetism • Inhomogenous magnetization • Magnetic vortices • Dynamics • Spin transport US-Spain Workshop on Nanomaterials

2. Ferromagnetism is rare……

3. …. but useful Inductive Write Element “Compass” that responds to local magnetic field and varies the resistance GMR Read Sensor W t B Direction of Disk Motion B = 25 nm (s<3 nm), W=150 nm, t = 14 nm data rate ~ GHz

4. Write Coil Write Pole2 Read Head Head Disk <D> = 8.5 nm +/- 2.5 nm 100 nm Courtesy of Eric Fullerton

5. Recording Media <D> = 8.5 nm +/- 2.5 nm 100 nm 1000 nm # grains/bit Courtesy of Eric Fullerton

6. Why is ferromagnetism neither common nor “perfect”? Macroscopic Microscopic R. Schaefer, Dresden

7. Magnetostatics (Not as bad as it looks)

8. , where Torque Magnetostatics: equilibrium condition Variational method to find the equilibrium condition = 0 W. F. Brown, Jr., Micromagnetics (Interscience Publishers, New York, 1963)

9. Micromagnetics Simulation

10. Excitations [Equilibrium State] [Excited State] [Dynamic motion] Landau-Lifshitz-Gilbert Equation = 17.6 GHz/kOe

11. Spin waves • Uniform precession (q = 0 spin wave) • Spin Waves

12. The simple case (no magnetocrystalline anisotropy) LE = exchange length = Magnetic vortex K.L. Metlov et. al., J. Magn. Magn. Mater. 242-245 (2002) 1015 Of course there are intermediate cases - such as the S-state

13. Four different configurations of the vortex state • Schematic illustration of four different vortex states P= Polarity (the magnetization direction of the vortex core) C= Chirality (the winding direction of in-plane magnetization) The magnetostatic energies are obviously identical….

14. Magnetic vortices • Observation of magnetic vortices 1) Lorentz Microscopy on 200 nm Co disk 2) MFM on 1 mm Permalloy disk 3) SP-STM on 200 nm wide and 500 nm long Fe island • J. Raabe et al. J. Appl. Phys. 88, 4437 (2000) • T.Shinjo et al. Science289, 930 (2000) • A. Wachowiak et al. Science 298, 577 (2002) What about the dynamics?

15. Vortex-core dynamics (gyrotropic motion) where = static force for an applied field H =gyrovector (antiparallel to the direction of vortex polarity P) = magnetic energy dissipation dyadic • Gyromagnetic force acting on a shifted vortex Landau-Lifshitz-Gilbert Equation: Equivalent force equation When P changes sign, changes sign! A.A. Thiele, Phys. Rev. Lett. 30, 230 (1973). See also B. Argyle et al., Phys. Rev. Lett. 53, 190 (1984).

16. Gyrotropic Mode The lowest frequency excitation: Gyrotropic mode [Will be replaced with a movie: Gyrotropic motion in simulation] 1 s  1 ns

17. Time Scales 10-12 sec (semiconductors) 10-14 sec (chemical reaction dynamics) 10-9 sec (magnetism) 10-7 sec How do you make a movie on picosecond time scales?

18. Time-resolved Kerr microscopy (stroboscopic) What we measure: Polar Kerr Rotation Mz as a function of time delay, probe-beam position, and applied field [Freeman et al. J. Appl. Phys. 79, 5898 (1996)] Also Back, Hicken, and others.....This is a stroboscopic technique.

19. Experimental Setup

20. Different equilibrium positions Excitation off Not at a pinning site At a pinning site Pinning potential

21. Large Amplitude: Core Switching Counterclockwise orbit Clockwise orbit 1 s  0.5 ns B. Van Waeyenberge et al., Nature 444, 461 (2006)

22. Core reversal

23. Phase Diagram of Vortex Dynamics • Pinned & Depinned

24. Magnetic Heterostructures Magnetic Random AccessMemory Disk drives • New Technologies: • Magnetic Random Access Memory • Magnetic tunnel junction sensors • Patterned media • Semiconductor spintronics • Highly polarizable materials Field sensing(medical devices,security)

25. Magnetic Heterostructures Example: the spin valve • The electrical response of the device depends on the magnetic state of two or moreelectrodes (field sensors, read heads) • The magnetic state of the device can be changedby an electrical current (memory, oscillators) F F Integration of ferromagnets with insulators, semiconductors, and normal metals

26. Free Layer Pinned Layer Scale: 50 nm Read Head Technology Pole2 Gap Write Shield2 Pole1 MR Leads Leads Read Shield1 Compound PtMn Cu

27. Magnetic Tunnel Junctions FM 1 Insulator FM 2

29. Spin transfer torque oscillators • MgO-based tunnel junction devices for maximizing signal and reducing threshold current • Built-in hard-axis polarizer enhancesoutput power and allows for zero-fieldoperation • Influence of CoFeB on damping (withData Storage Institute, Singapore) • Modification of CoFeB/MgO interfaceanisotropy (with DSI) • Spin transfer torque FMR (with DSI) J. Appl. Phys. 109, 07D307 (2011) J. Appl. Phys. 109, 07C714 (2011) Appl. Phys. Lett. (accepted, 2012) Wang, Crowell

30. Materials science of magnetic heterostructures TEM Co2MnGe GaAs Growth and characterization Electronic Structure Calculations Interfacial characterization Simulations Transport Spin dynamics

31. Epitaxial Fe/InxGa1-xAs heterostructures • Epitaxial structures: low temperaturegrowth to minimize interfacial reactions • Transport and modeling techniques developed by the IRG • Increase spin-orbit coupling by shiftingto InxGa1-xAs Palmstrøm, Crowell

32. Summary • Magnetism is ubiquitous, although ferromagnetism is relatively rare • Ferromagnetism is useful if not always easy to understand • Imperfect magnets are more interesting than perfect ones • Dynamics are accessible by new tools • Integration of ferromagnets with other materials yields new physicsand new devices