Forskjell mellom versjoner av «Giant magnetoresistance»

Revisjonen fra 17. mar. 2009 kl. 13:14

In production by Einar, Fabio og Fabio, please do not edit unless you are one of us.

When magnetic fields are appplied to metals, most show an increase in resistance, known as magnetoresisitance (MR), but GMR exploits the fact that when these junctions is under a critical length the magnitude of this magnetoresistance is greatly amplified. This has been used as a way of packing data extremely dense, and most modern hard-drives (HDD) uses this technology.

Theoretical background

History

Gigant Magnetoresistance (GMR) was discovered in 1988 simoltanously by Peter Grünberg at the Jülich Research Centre (DE) and Albert Fert at the University of Paris-Sud (FR) who shares the 2007 Nobel prize in physics for this discovery.

Magnetoresistance

Giant magnetoresistance

Materials and synthesis methods

The increase of GMR in multi as compared to double layers apparent from the figures in Table 3, can be explained on the basis of an increased spin dependent scattering probability when the electron has to pass many interfaces instead of only two. Source Need superstructure with layers of ferro/antiferromagnetic materials. Ferromagnetism is direction dependent. The spins for a crystal material will point in a certain direction. Need to consider this when building the superlattice! Materials of many different crystal structures display ferromagnetic behaviour. Not always fully understood. Need unbonding electrons.

Antiferromagnetic coupling: External IBM link

Wikipedia

Antiferromagnets can couple to ferromagnets, for instance, through a mechanism known as exchange bias, in which the ferromagnetic film is either grown upon the antiferromagnet or annealed in an aligning magnetic field, causing the surface atoms of the ferromagnet to align with the surface atoms of the antiferromagnet. This provides the ability to "pin" the orientation of a ferromagnetic film, which provides one of the main uses in so-called spin valves, which are the basis of magnetic sensors including modern hard drive read heads. The temperature at or above which an antiferromagnetic layer loses its ability to "pin" the magnetization direction of an adjacent ferromagnetic layer is called the blocking temperature of that layer and is usually lower than the Néel temperature.

Layers in a GMR readhead are made of two materials with different magnetic coercivity, which can be seen in the layers' hysteresis curves. Due to the different coercivities one layer ("soft" layer) changes polarity at small magnetic fields while the other ("hard" layer) changes polarity at a higher magnetic field. As the magnetic field across the sample is swept two distinct states can exist, one with the magnetisations of the layers parallel, and one with the magnetisations of the layers antiparallel. Spin valves work because of a quantum property of electrons (and other particles) called spin. When a magnetic layer is polarized, the unpaired carrier electrons align their spins to the external magnetic field. When a potential exists across a spin valve, the spin-polarized electrons keep their spin alignment as they move through the device. If these electrons encounter a material with a magnetic field pointing in the opposite direction, they have to flip spins to find an empty energy state in the new material. This flip requires extra energy which causes the device to have a higher resistance than when the magnetic materials are polarized in the same direction.

Nobel article

To obtain well-behaved metallic multilayers it is important that the lattice parameters for the different metallic layers match each other and it is also an advantage if the two metals forming the multilayer have the same crystal structure. This is the case for chromium and iron, where both metals adapt the bcc (body-centred cubic) crystal structure and where in addition they have very similar lattice spacings. This was important for the studies for which this Nobel Prize is awarded under-taken by the groups of Fert and Grünberg. In addition it was also extremely important that it was now possible to grow multilayers where the spatial separation between the magnetic layers is of the order of nanometres. In order to exhibit the GMR effect the mean free path length for the conduction electrons has to greatly exceed the interlayer separations so that the electrons can travel through magnetic layers and pick up the GMR effect.

The magnetic moment of a second impurity placed relatively close to the first one, will become aligned parallel or antiparallel to the magnetic moment of the first moment de-pending on the sign of the induced polarization at that particular distance. The second Gd layer happens to be at a distance where an antiferro-magnetic alignment is preferred. The important role of the electrons of the non-magnetic layer(s) is that they provide the coupling mechanism between the magnetic layers. Can be paramagnetic.

My questions

• How does the alignment change from parallel to unparallel when we apply a magnetic field?? It only says it depents on the distance... "In the absence of a magnetic field (at the top) the two FM layers are separated from each other in such a way that they have opposite magnetization directions" Does that mean that changing the magnetic field changes the density of the nonmagnetic spacer material..?
• In the Fe/Cr/Fe system, Cr is non-magnetic (according to Nobel article). So it doesn't use a spin valve..? All superlattices use ferro/non/ferro-structures..?
• Does current pass perpendicular or along with the superlattice structure?

Answer:The GMR effect has been investigated in two different geometries, namely the current in plane (CIP) and the current perpendicular plane (CPP) geometry. The relative effect is stronger in the CPP geometry but without special structuring, due to the extremely unfavorable geometrical conditions (lateral dimensions some orders of magnitude larger than film thickness) the voltage drop perpendicular to the layers, in the CPP geometry, is very difficult to detect. On the other hand by structuring, GMR in the CPP geometry can become sufficiently strong to be of interest even for applications Source

• Free layer vs pinned layer

Requirements for the magnetic materials in the hard drive itself:

• Soft magnet. Low coercivity, meaning that you don't need a strong magnetic field to magnetize or demagnetize (1 or 0), and the process doesn't require a lot of energy.
• Hysterisis loop must be narrow (--> low energy losses for (de)magnetization) and square shaped (--> swift change from 1 to 0

GMR in hard-drives

For a nice graphical presentation, click here (IBM pages).

How it works

In GMR Hard disk drives (HDD) data is stored on magnetically coated platters made of metal or glass that revolve at several thousand rotations per minute (RPM), driven by an electric motor. The capacity of the drive depends on the number of platters (which may be as many as eight) and the type of magnetic coating. The magnetic coating is divided into small areas called bits, where one bit represent either a 1 or a 0, depending on the direction of the magnetization.

A GMR read-write head is located on an arm sweeping over the revolving platters typically separated from the platters by a mere 15 nm, kept in the right place by an actuator. This head can read the polarization of the bits and by the means of a control circuit it can provide the computer with the information stored on the platter. In write mode another

Problems

Superparamagnetic effect (SPE)

Simply described, SPE is a physical phenomenon that occurs in data storage when the energy that holds the magnetic spin in the atoms making up a bit either a 0 or 1) becomes comparable to the ambient thermal energy. When that happens, bits become subject to random flipping between 0’s and 1’s, corrupting the information they represent.

Future

Vertical bits

Investigation has begun into what is known as vertical bits, which instead of reducing the total size of a single bit, tries to reduce the average surface of one bit by increasing the depth of the magnetic layer and thereby increasing the density of the bits without reducing the volume of one bit (and making it susceptible to SPE).

Tunneling magnetoresistance (TMR)

Antoher way of increasing the signal is by the means of TMR.

Current-perpendicular-to-plane GMR (CPP-GMR)

References

• The Discovery of Gigant Magnetoresistance, Scientific background for the 2007 Nobel prize in physics.