Magnetic Data Storage

5 nm Optimum Hard Disk Reading Head

Filtering mechanisms: Bulk: Spin-dependent Scattering Interface: Spin-dependent Reflection Parallel Spin Filters  Resistance Low Opposing Spin Filters  Resistance High Giant Magnetoresistance (GMR): Two Spin Filters 2007 Nobel Prize in Physics to Fert and Grünberg

TMR has taken over GMR in hard disk reading heads. Get larger effect with the current perpendicular to the layers, no shorting. GMR vs. TMR (Tunnel Magnetoresistance): Replace Metal Spacer by Insulating Spacer (TMR) (GMR)

50 nm 10 nm CoPt particles (  superparamagnetic limit) Magnetic Storage Media Need about a hundred particles per bit (particles not uniform). Magnetic Force Microscope (MFM) Image

Barrier  E Energy Superparamagnetic Size Limit for Magnetic Particles Flip Rate  Attempt exp[-  E/kT]  10 9 s -1  40kT ~ Volume A superparamagnetic particle has all its spins aligned internally, but thermal energy keeps flipping the magnetic orientation of the whole particle. The magnetization of an ensemble of such particles is zero, as for a paramagnetic arrangement of very large spins.

Shape anisotropy: The magnetization prefers to be parallel to the axis of a needle-shaped particle or in the plane of a thin film. Crystalline anisotropy: The magnetization prefers to align with a specific crystallographic direction (e.g. the hexagonal axis in cobalt) Surface anisotropy: The magnetization at a surface/interface is often perpendicular to the interface (opposite to the shape anisotropy) Hard magnet (large anisotropy): Permanent magnet (NdFeB), storage medium (Co). Soft magnet (small anisotropy): Transformer core (pure Fe), sensor (permalloy). Magnetic Anisotropy: The Energy to Rotate the Magnetization

Blocking Temperature When cooling a superparamagnetic particle, the flip rate drops rather suddenly. The blocking temperature defines the point where the magnetization of a superparamagnetic particle becomes “frozen”. Such behavior resembles the transition from paramagnetism to ferromagnetism at the Curie temperature, but there is a conceptual difference: The Curie temperature defines a sharp phase transition, while the blocking temperature depends slightly on the time scale of the experiment (a bit fuzzy). The magnetization of a particle will flip even below the blocking temperature if one waits a very long time. An example from magnetic data storage: For a reasonable lifetime of stored data one needs an energy barrier  E ≈ 40 k B T. A typical attempt frequency of 10 9 s -1 gives a flip rate of 10 9 e -40 s -1 or about one flip in 7 years. Reducing the diameter of a magnetic particle by a factor of 2, their volume decreases by a factor of 8 and likewise  E in the exponent. The resulting flip time is only 150 nanoseconds !

3 atomic layers of Ru for antiferromagnetic coupling (AFC) Antiferromagnetically Coupled (AFC) Storage Media Make bits smaller while keeping the volume: Need to go deeper

Want a Storage Medium like this: Deep, Regular, Flat Top

As the bit size shrinks, the shape anisotropy works against shorter in-plane bits and favors perpen- dicular magnetization. Adjacent perpendicular bits with opposite magnetization repel each other, like bar magnets. A soft underlayer connects the field lines, like an iron bar across a horse- shoe magnet.

Patterned Media: The next Step

Europhysics News 39, 31 (2008)

Reading the Spin of a Single Atom by Scanning Tunneling Spectroscopy (STS) Polarized atom, unpolarized STM tip: See transitions between different m S as energy loss (inelastic tunneling). Use polarized atom, polarized STM tip for readout (not shown): Asymmetry reveals spin orientation (TMR, same as in hard disk reading heads, Slide 4). IBM Almaden Group, Science 317, 1199 (2007)

Something really far out: A Magnetic Virus S.D. Bader, Rev. Mod. Phys. 78, 1 (2006); Liu et al., JMMM 302, 47 (206)