Ferromagnetism and Electronic Transport. Ordinary magnetoresistance (OMR)

Transcription

1 Ferromagnetism and Electronic Transport There are a number of effects that couple magnetization to electrical resistance. These include: Ordinary magnetoresistance (OMR) Anisotropic magnetoresistance (AMR) Giant magnetoresistance (GMR) Tunneling magnetoresistance (TMR) Ballistic magnetoresistance (BMR) Colossal magnetoresistance (CMR) We ll look briefly at the physics taking place in each of these. Ordinary magnetoresistance (OMR) This is the same magnetoresistance that comes about in normal metals. (Total internal) magnetic induction affects orbits of electrons at Fermi surface. Result of tightening orbits at higher fields is a positive magnetoresistance. H J has a weaker effect than H perp. to J. In polycrystalline materials and those where boundary scattering is important, again can have interplay between magnetic induction and disorder. In clean metals, OMR typically ~ B, and can be ~ 10% at 10 Tesla 1

2 Anisotropic magnetoresistance (AMR) Pointed out experimentally as early as 1930s (though really solidly confirmed in 1960s) that FM material measured resistance depends on relative directions of M and J. Figure from HP Data from bulk permalloy (80% Ni, 0% Fe). Resistance lower if M perpendicular to J. Resistance higher if M parallel to J. Anisotropic magnetoresistance Physical origin: spin-orbit coupling leads to spin-dependent scattering of conduction electrons. Conduction in (for ex.) Ni due to 4s and 3d electrons. Crudely, the 3d orbitals are affected by M, and are mixed (slightly reoriented) so that they present a larger scattering cross-section to electrons moving parallel to M. More scattering = higher resistance. M M

3 Anisotropic magnetoresistance Typical size of effect: ~ 1%. Typical field scale: determined by physics of reorienting M. In bulk permalloy, 5-10 Oe. In permalloy wires with large aspect ratios, ~ 1 T. Weak temperature dependence (competition between this scattering and other scattering mechanisms) - effect gets slightly larger at lower T. Nickel nanowire Cobalt nanowire Figure from Fert et al., JMMM 00, 338 (1999). Giant Magnetoresistance Discovered in laboratory c Not a trait of pure FM materials! Requires nanostructured composites of FM and nonmagnetic metals. Superlattice of very thin alternating layers of FM and N metals. Original configuration = current in plane Figure from Baibich et al., PRL 61, 47 (1988). FM N 3

4 Giant Magnetoresistance Physical origin: spin-dependent transmission of carriers at interfaces between N and FM metals. There is some net spin polarization of the carriers. Density of states as a function of spin orientation is affected by M. Spins that are the majority species in one orientation of M are the minority species (lower DOS at Fermi level) in regions when M is oppositely directed. aligned = low resistance antialigned = high resistance Giant Magnetoresistance Typical size of effect: ~ 100%. Depends on percentage of spin polarized carriers at Fermi level, and zero-field coupling between neighboring FM layers. Note that relative directions of M and J not directly relevant. Typical field scale: determined by physics of reorienting relative directions of adjacent FM layer magnetizations. Usually arranged to be low. Interface quality is crucial. Coupling between adjacent FM layers oscillates with thickness of N layers: layer thicknesses and roughness must also be controlled. Effect gets larger at lower T and for cleaner metal layers as other scattering contributions are reduced. 4

5 Device geometries Original device was current-in-plane (CIP). If interfaces really dominate, GMR effects should be substantial in current-perpendicular-to-plane (CPP) devices, and they are. How are these systems actually used? Spin-valve geometry. Use antiferrromagnetic pinning layer to lock magnetization of one FM layer. Other FM is made to be magnetically soft - M easily realigns in small fields. Image from Leeds Univ. Tunneling magnetoresistance Spin-dependent density of states should have other consequences. Is possible to make FM-I- FM tunnel junctions. Tunneling conductance depends strongly on relative densities of states of spin species. In fact, tunneling magnetoresistance (TMR) is used to determine spin polarization at Fermi level. 5

6 Tunneling magnetoresistance Define spin polarization P as N P = N Model of Julliere (1975): N + N Here the Ns are proportional to the relevant densities of states at the Fermi level. Tunneling conductance will be proportional to tunneling rates, and tunneling rates will be proportional to number of carriers trying to tunnel (DOS of one FM) and number of available states (DOS of other FM). Assume that tunneling preserves spin (clean interfaces, etc.). G ~ ( N ) + ( N ) G ~ N N Tunneling magnetoresistance G ~ ( N ) + ( N ) G ~ N N Combining with definition of P, 1 G ~ (1 + P )( N + N ) 1 G ~ (1 P )( N + N ) Compute ratios, and unknown factors drop out: G G G R P = R 1+ P (1/ R ) (1/ R = (1/ R ) ) R P = = R 1 P Some measured numbers: (from Meservey et al., Phys. Rep. 38, 173 (1994) Ni: 3% Fe: 40% Co: 35% NiFe: 3% 6

7 Tunneling magnetoresistance Typical size of effect: ~ 100%. As in GMR. depends on percentage of spin polarized carriers at Fermi level. Note that relative directions of M and J not directly relevant. Typical field scale: determined by physics of reorienting relative directions of adjacent FM layer magnetizations. Usually arranged to be low. Interface quality is again crucial - growing good tunnel barriers is very tough without doing odd things to the magnetic properties at the interface. Should be ~ temperature independent. Ballistic magnetoresistance GMR (1988) > BMR (1988) >> Nano- or atomic-scale point contacts between FM electrodes Magnetization apparently changes direction on length scale shorter than (elastic?) scattering length. Result: very large magnetoresistive effects, postulated to be for similar physics reasons as TMR. 7

8 Ballistic magnetoresistance Garcia et al., PRL 8, 93 (1999). First measured using mechanical break junction technique. Effect can be large factor of 3 change in conductance! Unanswered questions: Truly ballistic? Magnetic dead layer? Magnetostriction? How large can effect be? Colossal magnetoresistance von Helmolt et al., PRL 71, 331 (1993) Lin et al., Science 64, 413 (1994) (Re)discovered in Takes place in specific family of compounds, perovskites of the form A 1- xb x MnO 3, where A = (La, Pr, Nd, Sm), B = (Ca, Sr, Ba). Physical mechanism is completely different than any described so far. 8

9 Colossal magnetoresistance Size of effect: ~ % (!) Extremely temperature and doping dependent - challenging to get useful, reproducible behavior at room temperature. Mechanism: phase transition between conductive FM ordering of Mn ions and insulating AFM ordering of Mn ions. Replacing rare earths with light metals changes some of the Mn from Mn 3+ to Mn 4+. Charge can hop from Mn to Mn via the oxygen anions. Strong FM exchange favors hopping of aligned spins (high conductance). Still not well understood! Of much interest because of large effects and very high spin polarization of carriers. Spin currents and magnetization We ve been talking about how M affects J, ability to transport charge, as manifested through magnetoresistive effects. One can also consider the converse: can a current J of carriers with a net spin polarization affect M? Yes! A current with a net spin polarization means a flow of angular momentum from one region to another. This results in a net torque on the spins in those regions, and for high enough torques, it can be energetically favorable for domains to rearrange themselves. 9

10 Spin currents and magnetization Myers et al., Science 85, 867 (1999) Here s an example, involving current flow into a GMR multilayer. Change in magnetization direction of one of the layers is detectable by GMR effect - change in electrical resistance. Note, should be asymmetric in current direction! Spin currents and magnetization Katine et al., PRL 84, 3149 (000) Clear demonstration of current-induced magnetization reversal. Ability to manipulate M without applying external fields is potentially very attractive technologically. One example of spintronics that we ll cover soon. 10

11 Summary Magnetization can affect electronic conduction through several mechanisms: Local B field (OMR) Band structure and scattering (AMR) Spin-dependent transmission and scattering (GMR, TMR) Coupling between charge and magnetic order (CMR) Current can also affect magnetization, as shown in currentinduced magnetization reversal experiments. Next time Demands of data storage industry, particularly magnetic. 11

12 Needs of the data storage industry Focus on magnetic media first, and, as in the lecture on the electronics industry, will try to keep an eye on areas where nano will be relevant. Historic trends State-of-the-art Media Read and write mechanisms Scaling concerns Nonvolatile approaches Where it all began 1878 Oberlin Smith invents magnetic recording - patterns of domains in steel wire Valdemar Poulsen invents reel-to-reel metal tape recording, and the telephone answering machine Sony introduces reel-to-reel recorder using coated tape RCA introduces stereo tape - cartridge needs special player. 196 Phillips introduces cassettes Motorola, RCA introduce 8-tracks Cassette outsell LP records. 1

13 Where it all began Core memory - use magnetization of ferrite cores as computer memory. $6000 per 1kb bit line 6.6 kb in 8 x8 x8 word line Image from Columbia Univ. ACIS Image from pcbiography.net Where it all began Image from pcbiography.net RAMAC (1956) First hard disk drive 50 4 disks Stored a total of 5 MB of information. Areal density = kb/in Data rate = 70 kb/sec.

14 Historical trends Image from IBM presentation Historical trends Image from IBM presentation 3

15 Historical trends Image from IBM presentation Magnetic storage: state-of-the art Image from IBM website Disk medium:.5 diameter, 34 Gb/in (typical size ~ 140 nm) CoPtCr alloy (M sat = 4 x 10 5 A/m, H c =.7 x 10 5 A/m, K = 1.5 x 10 5 J/m 3 ) Layered structure, including special AFM layer: 4

16 Magnetic media Why the AFM layer? Consider superparamagnetism for this disk material. Individual grain in a bit ~ 10 nm on a side. Energy barrier = KV ~ 1.5 x J. Attempt frequency ~ 10 9 leads to effective rate of τ 1 ~ Ω exp( KV / k T ) ~ Means typical timescale for a particular grain to thermally reverse itself would be months. B 7 s 1 Need some way of reducing superparamagnetic effects! Magnetic media Image from IBM website Antiferromagnetic pixie dust layer stiffens disk medium without strongly altering its coercivity (write-ability). 5

17 Hard drives - read heads Permalloy yoke with integrated, microfabricated Cu coils. Shield of high permeability material between yoke and GMR sensor - guides B to prevent record head from affecting GMR FM layers. Minimum size of transverse bit set by write-gap width. Head attached to end of piezoactuated arm. Aerodynamics keeps head suspended above surface at flight height. Images from IBM website Hard drives - read heads Image from IBM website Basic spin valve design. Exchange layer pins one of the GMR FM layers. Note that this is a standard current-in-plane geometry. 6

18 Where are we headed? Image from IBM presentation Advances coming: Different media Thermally assisted recording Vertical recording Patterned media Different read heads CPP GMR heads CPP magnetic tunnel junction heads CMR? EMR? Different technology altogether MRAM Millipede Holographic storage 7

19 Thermally assisted recording Image from IBM website Simple idea: use media with significantly higher anisotropies. Benefit: added thermal stability against superparamagnetism. Downside: harder to write bits. Solution: locally heat medium (optically) to drop coercive field. Tricky - need short thermal relaxation times. Vertical media 8

20 Vertical media Potential advantages: Can get better thermal stability by larger bit volumes without sacrificing bit area density. Better signal to noise under some circumstances because fringing fields over larger area tend to influence read head. Disadvantages: Textured growth of medium on substrate can be quite tricky. Patterned media image from Nat. Univ. Singapore Don t rely just on film growth: make nanostructured media with prearranged bit locations - single domain particles. Several different approaches: E-beam lithography (far too slow) Ion beam patterning through mask Nanoimprint lithography Electrochemistry through porous mask Self-organization / self-assembly Real possibility of > 1TB/in! 9

21 CPP read heads Competing storage technologies Other approaches (nonmagnetic): AFM-based storage ( Millipede - IBM) Electron-based storage (HP) STM-based storage (IBM) Holographic storage 10

22 Millipede AFM storage Image from IBM website Millipede AFM storage Array of 104 piezoresistively sensed AFM cantilevers. Physical deformation of polymer medium to write, using integrated resistor to heat individual tip by ~ 100 degrees. Healed by local heating to erase. Demonstrated density of 00 Gb/in, with an eye toward 1Tb by summer, 003. Can this ever be cheap and reliable? IBM really seems to think so. Think 300 DVDs in a space the size of a credit card. 11

23 Electron-based storage image from Scientific American, May 000 HP plan: nanofabricated field emission tips. In principle, can get very high resolution, approaching atomic scale. Holographic storage image from Bell Labs Use phase-sensitive interference patterns from lasers to write data pages into an optically changeable medium. Can be read pages at a time. Different pages can be stored at different depths in medium. Potential storage densities and speeds are enormous! Problems: Needs serious lasers and optics. Materials problems with media. 1

24 Conclusions Magnetic data storage still has a lot of life left to it, but progress rates are so fast they make Moore s law look relaxed. Physics (superparamagnetism) demands changes in media soon. New read head technologies also likely to be relevant. Competing technologies have incredible potential, and will eventually supplant magnetic storage for certain applications. 13