Presented by:
GEO G KAPPEN
Jison T JAMES

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GMR TECHNOLOGY
INTRODUCTION
 MR is the change of resistance of a conductor when it is placed in an external magnetic field.
 GMR was independently discovered in 1988 in Fe/Cr/Fe trilayers by a research team led by Peter Grünberg of the Jülich Research Centre and in Fe/Cr multilayers by the group of Albert Fert of the University of Paris-Sud.
 The discovery of GMR is considered as the birth of spintronics. Grünberg and Fert have received a number of prestigious prizes and awards for their discovery and contributions to the field of spintronics, including the Nobel Prize in Physics in 2007
 GMR effect is shown by ferromagnetic materials like iron,cobalt and nickel.
 The change of resistance is in accordance to the direction of external magnetic field applied.
 These experiments were performed at low temperatures and in the presence of a very high magnetic field.
 Initially the resistance change was in between 6 percent and 50 percent.
 Materials showing GMR is known as magnetic multilayers,where layers of ferromagnetic and non-magnetic metals are stacked on each other.The width of individual layers are of nanometre size.
 A trilayer system of Fe/Cr/Fe is used for the discovery of GMR, led by Peter Grunberg.
 Fert used multilayers of (Fe/Cr)n,where n is as high as 60.
 The single layer experiment is performed at room temperature and the multilayer experiment is performed at 4.2K.
 The multilayer experiment shows a resistance change about 50%.
 The resistance change for a system for a system with three Fe layer and two Cr layer is about 10%.
BACKGROUND
Resistance and magnetization
 The current is conducted because of the movement of electrons in a specific direction, the straighter the path of the electrons, the greater the conductance of the material.
 Electric resistance is due to electrons diverging from their straight path when they scatter on irregularities and impurities in the material. The more the electrons scatter, the higher the resistance.
 In a magnetic material the scattering of electrons is influenced by the direction of magnetization.
 Giant magnetoresistance arises because of the intrinsic rotation of the electron that induces a magnetic moment – the quantum mechanical property called spin – which is directed in either one of two opposite directions.
 In a magnetic material, most of the spins point in the same direction (in parallel). A smaller number of spins, however, always point in the opposite direction, anti-parallel to the general magnetization.
In a magnetic conductor the direction of spin of most electrons is parallel with the magnetization (red). A minority of electrons have spin in the opposite direction (white). In this example electrons with antiparallel spin are scattered more.
ANISOTROPIC MAGNETORESISTANCE
AMR is the property of a material in which a dependence of electrical resistance on the angle between the direction of electrical current and orientation of magnetic field is observed.
 The net effect is that the electrical resistance has maximum value when the direction of current is parallel to the applied magnetic field.
 AMR up to 50% has been observed in some ferromagnetic uranium compounds .
SPIN VALVE GMR
 Two ferromagnetic layers are separated by a thin (about 3 nm) non- ferromagnetic spacer.
 When the two layers have parallel spin polariza-tions (magnetizations), i.e. in the presence of an external magnetic field,
then total resistance RH = 2R↑R↓/(R↑ + R↓).
 In the case of no external field(H=0), the configuration between the two magnetic layers is antiparallel,then toatal resistance affected by the ē,
R0 = (1/2)(R↑+R↓).
Thus the difference in resistance between the two cases (magnetic field or not) becomes:
ΔR = RH – R0 = – (1/2)(R↑ – R↓)2 /(R↑ + R↓).
 The larger the difference between R↑ and R↓ the larger the negative magnetoresistance. This expression clearly shows that the magnetoresistance effect.
GRANULAR GMR
 Granular GMR is an effect that occurs in solid precipitates of a magnetic material in a non-magnetic matrix.
 Granular GMR has only been observed in matrices of copper containing cobalt granules. The reason for this is that copper and cobalt are immiscible, and so it is possible to create the solid precipitate by rapidly cooling a molten mixture of copper and cobalt.
 Granule sizes vary depending on the cooling rate and amount of subsequent annealing. Granular GMR materials have not been able to produce the high GMR ratios found in the multilayer counterparts.