Forskjell mellom versjoner av «Giant magnetoresistance»
(→Multilayer stack configuration) |
(→How it works) |
||
| Linje 136: | Linje 136: | ||
=== How it works === |
=== 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. |
||
| + | ==== Hard disk drives (HDD) ==== |
||
| ⚫ | A GMR read-write head is located on |
||
| + | [[Bilde:harddrv.jpg |300px|thumb|right|A cutaway view of an HDD. Image courtesy of [http://www.cybercollege.com/frtv/www2.htm Cybercollege]]] |
||
| + | |||
| + | 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. It is extremely important that these platters are completely flat (less than a few nanometers in height-difference on the entire disk), because the read-write head is scanning the surface at a relative speed of 10-30 m/s at ~15 nm distance. This is also the reason why you should never move a HDD in use, because the read-write heads might come in contact with the disks and scratch the surface, which would leave the disk useless. The capacity of the drive depends on the number of platters, typically from 4 to 8 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. |
||
| + | When the disks spin the data is read and written by a read-write heads located on the "arms". The position of the read-write heads is controlled by an actuator operating the arm. |
||
| + | |||
| + | ==== Read-write head ==== |
||
| ⚫ | |||
| ⚫ | A GMR read-write head is located on the arm sweeping over the revolving platters typically separated from the platters by a mere 15 nm. The read head measures the resistance, and depending on the magnetization the resistance is measured as either high or low, corresponding to 1's and 0's which is provided back to the computer by the means of a control circuit. The write part of the read-write head consist of an electro-magnet which is able to shift the polarization of the mentioned bits and thereby writing the 1's and 0's. |
||
=== Problems === |
=== Problems === |
||
Revisjonen fra 17. mar. 2009 kl. 14:46
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.
Innhold
- 1 Theoretical background
- 2 GMR in hard-drives
- 3 References
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
Review article
Multilayer stack configuration
With increasing number of FM/NM bilayers within a multilayer the value of GMR monotonically increases until it reaches saturation. This is due to the diffuse scattering at the outer boundaries of the multilayer. If the longest mean free path is larger than the total thickness of the multilayer, then the diffuse outer-boundary scattering reduces the conductivity of the “good” spin channel and hence effects GMR negatively. The magnetoresistance ratio becomes independent of the number of bilayers when the total thickness of the multilayer is much larger than the longest mean free path.
Due to antiferromagnetic interlayer exchange coupling they are aligned antiparallel at zero magnetic field as is indicated by the dashed and solid arrows. At the saturation field the magnetic moments are aligned parallel. By choosing an appropriate thickness of the non-magnetic layer it is, therefore, possible to create an antiparallel configuration of the ferromagnetic layers and then reorient (align) the moments by an applied magnetic field.
Although the highest values of GMR were measured in antiferromagnetically-coupled magnetic multilayers, these multilayers are not the best materials for technological applications. This is due to the large magnetic fields, which are required to saturate the magnetization of the multilayers and to obtain a sizeable change in the resistance. For example, as is evident from Fig.6, the saturation fields in the Fe/Cr multilayers are of the order of 10-20 kG which is three orders of magnitude higher than the fields required for applications. The sensitivity, which is defined as ΔR/R per unit magnetic field, is much too small.
Spin valve configuration
Although the measured values of GMR are higher in magnetic multilayers, spin valves are more attractive from the point of view of applications, because only small magnetic fields need to be applied to change the resistance.
Most experiments on GMR are performed by measuring the electric current in the plane of the multilayer, i.e. within the current-in-the-plane (CIP) geometry. This geometry is currently used for the industrial applications of GMR. Measuring the current perpendicular to the multilayer plane, i.e. within the current-perpendicular-to-the-plane (CPP) geometry, is much more difficult. This is due to the very small thickness of the multilayer and consequently the very low CPP resistance, which is not easy to detect.
Materials and synthesis methods
Plan
Crucial material parameters:
- ferromagnetic layer must have spin decoupling
- lattice- and band matching between FM/NM layers
- purity (to avoid random scattering)
Synthesis:
- CVD/sputtering etc. Emphasis on thickness and purity control
Raw material
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
Review article
Explanation of ferromagnetism
In ferromagnetic 3d metals the d band is exchange-split. Due to the localized nature of the d electrons, two d electrons experience a strong Coulomb repulsion provided that they have antiparallel spins and occupy the same orbital. To reduce the energy it is advantageous for the d electrons to have parallel-oriented spins, because the Pauli exclusion principle does not permit two electrons with the same spin to approach each other closely (i.e. occupy the same orbital) and hence the Coulomb interaction is reduced. Therefore, the Coulomb repulsion in conjunction with the Pauli principle leads to the ferromagnetic exchange interaction and favors the formation of a spontaneous magnetic moment.
Synthesis problems
the growth of these multilayers represents a real problem. For example the Ni80Fe20/Ag multilayer has to be deposited at liquid-nitrogen temperatures in order to attain the required integrity of the layers.
A number of attempts have been made to use half-metallic materials in GMR structures.59-61 Halfmetallic compounds are characterized by the coexistence of metallic behaviour for one electron spin and insulating behaviour for the other spin. The electronic density of states is, therefore, 100% spinpolarized at the Fermi level, and the conductivity is dominated by single-spin charge carriers. The highest value of GMR of about 7% is found in CPP measurements on NiMnSb/Cu/NiMnSb trilayers at liquid helium temperature.60 This result is far short of the infinite value of CPP GMR expected from a half-metallic-based structure. A possible reason for this is the poor quality of the NiMnSb films, in particular at the NiMnSb/spacer interfaces, resulting in a reduced spin polarization and/or a significant spin-flip scattering due to misaligned magnetic moments.
NM Film thickness
However, at small spacer thicknesses the magnetic layers may become strongly coupled ferromagnetically due to the presence of pinholes in the nonmagnetic film, leading to a decreased GMR ratio.
FM film thickness
The decrease in GMR at large magnetic layer thickness is due to the increasing shunting of the current in the inner part of the ferromagnetic layers. The decrease in GMR at low thickness is due to the scattering at the outer boundaries (substrate, buffer layer or capping layer).
Interface roughness
It is expected that interface roughness will enhance the magnetoresistance due to an increase in spin-dependent scattering. Changing the sputtering gas pressure and varying the sputtering power. Spin-dependent scattering is very sensitive to the details of the microstructure of the interfaces. On the other hand, interface roughness associated with interdiffused regions and a high density of defects, such as grain boundaries, is likely to reduce GMR
Impurities
modifying the spin-dependent scattering by introducing appropriate impurities either at the interfaces or in the bulk of the ferromagnetic layers would enhance GMR.
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
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
For a nice graphical presentation, click here (IBM pages).
How it works
Hard disk drives (HDD)
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. It is extremely important that these platters are completely flat (less than a few nanometers in height-difference on the entire disk), because the read-write head is scanning the surface at a relative speed of 10-30 m/s at ~15 nm distance. This is also the reason why you should never move a HDD in use, because the read-write heads might come in contact with the disks and scratch the surface, which would leave the disk useless. The capacity of the drive depends on the number of platters, typically from 4 to 8 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. When the disks spin the data is read and written by a read-write heads located on the "arms". The position of the read-write heads is controlled by an actuator operating the arm.
Read-write head
A GMR read-write head is located on the arm sweeping over the revolving platters typically separated from the platters by a mere 15 nm. The read head measures the resistance, and depending on the magnetization the resistance is measured as either high or low, corresponding to 1's and 0's which is provided back to the computer by the means of a control circuit. The write part of the read-write head consist of an electro-magnet which is able to shift the polarization of the mentioned bits and thereby writing the 1's and 0's.
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)
TMR occurs when current flows through two ferromagnets separated by a thin (~1nm) insulator. The resistance is dramatically changed from the parallel to the anti-parallel alignment of the two ferromagnets, which gives higher signal-to-noise ratio for the TMR than GMR and possibility of higher density. The increase in resistance for TMR is in the orders of 200% and often much more.
