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

Ferromagnetism is rare……

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

Write Coil Write Pole2 Read Head = 8.5 nm +/- 2.5 nm 100 nm Disk Head Courtesy of Eric Fullerton

Recording Media = 8.5 nm +/- 2.5 nm 100 nm 1000 nm # grains/bit Courtesy of Eric Fullerton

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

Magnetostatics (Not as bad as it looks)

Magnetostatics: equilibrium condition Variational method to find the equilibrium condition, where Torque = 0 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/kOe

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

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

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….

Magnetic vortices 1) Lorentz Microscopy on 200 nm Co disk 2) MFM on 1  m Permalloy disk 3) SP-STM on 200 nm wide and 500 nm long Fe island  Observation of magnetic vortices 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) A.A. Thiele, Phys. Rev. Lett. 30, 230 (1973). See also B. Argyle et al., Phys. Rev. Lett. 53, 190 (1984).  Gyromagnetic force acting on a shifted vortex where = static force for an applied field H = gyrovector (antiparallel to the direction of vortex polarity P ) = magnetic energy dissipation dyadic Landau-Lifshitz-Gilbert Equation: Equivalent force equation When P changes sign, changes sign!

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

sec (chemical reaction dynamics) sec (semiconductors) sec (magnetism) sec Time Scales How do you make a movie on picosecond time scales?

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

Experimental Setup

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

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

22 Core reversal

Phase Diagram of Vortex Dynamics 23 Pinned & Depinned

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

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) FF Integration of ferromagnets with insulators, semiconductors, and normal metals Example: the spin valve Magnetic Heterostructures

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

Magnetic Tunnel Junctions FM 1 FM 2 Insulator

Spin transfer torque oscillators MgO-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 CoFeB on damping (with Data Storage Institute, Singapore) Modification of CoFeB/MgO interface anisotropy (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

Materials science of magnetic heterostructures Co 2 MnGe GaAs TEM Electronic Structure Calculations Growth and characterization Interfacial characterization Transport Simulations Spin dynamics

Epitaxial Fe/In x 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 In x 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