Magnetism on the Move Ferromagnetism Inhomogenous magnetization Magnetic

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

Ferromagnetism is rare……

…. but useful Inductive Write Element GMR Read Sensor “Compass” that responds to local magnetic field and varies the resistance W t B B = 25 nm (s<3 nm), W=150 nm, t = 14 nm data rate ~ GHz tion c e r Di of D ot isk M ion

Head Disk Write Pole 2 100 nm <D> = 8. 5 nm +/- 2. 5 nm Write Coil Read Head Courtesy of Eric Fullerton

Recording Media 100 nm <D> = 8. 5 nm +/- 2. 5 nm # grains/bit 1000 nm Courtesy of Eric Fullerton

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

Magnetostatics (Not as bad as it looks)

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

Micromagnetics Simulation

Excitations [Equilibrium State] [Excited State] [Dynamic motion] Landau-Lifshitz-Gilbert Equation = 17. 6 GHz/k. Oe

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

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

Four different configurations of the vortex state q 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….

Magnetic vortices q 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 1) J. Raabe et al. J. Appl. Phys. 88, 4437 (2000) 2) T. Shinjo et al. Science 289, 930 (2000) 3) A. Wachowiak et al. Science 298, 577 (2002) What about the dynamics?

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

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

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

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.

Experimental Setup

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

Large Amplitude: Core Switching Counterclockwise orbit Clockwise orbit 1 s 0. 5 ns B. Van Waeyenberge et al. , Nature 444, 461 (2006) 21

Core reversal 22

Phase Diagram of Vortex Dynamics Pinned & Depinned 23

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

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

Read Head Technology Pole 2 Gap Write Shield 2 Pole 1 MR Leads Read Shield 1 Scale: 50 nm Compound Pt. Mn Free Layer Cu Pinned Layer Leads

Magnetic Tunnel Junctions FM 1 Insulator FM 2

Spin transfer torque oscillators • • • Mg. O-based tunnel junction devices for maximizing signal and reducing threshold current Built-in hard-axis polarizer enhances output power and allows for zero-field operation Influence of Co. Fe. B on damping (with Data Storage Institute, Singapore) Modification of Co. Fe. B/Mg. O interface anisotropy (with DSI) Spin transfer torque FMR (with DSI) J. Appl. Phys. 109, 07 D 307 (2011) J. Appl. Phys. 109, 07 C 714 (2011) Appl. Phys. Lett. (accepted, 2012) Wang, Crowell

Materials science of magnetic heterostructures Electronic Structure Calculations Growth and characterization Co 2 Mn. Ge Ga. As TEM Interfacial characterization Simulations Transport Spin dynamics

Epitaxial Fe/Inx. Ga 1 -x. As heterostructures • Epitaxial structures: low temperature growth to minimize interfacial reactions • Transport and modeling techniques developed by the IRG • Increase spin-orbit coupling by shifting to Inx. Ga 1 -x. As Palmstrøm, Crowell

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 physics and new devices