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GIANT MAGNETORESISTANCE (GMR)

GIANT MAGNETORESISTANCE (GMR). By R.S.Ragava Vignesh. Introduction. In general, magnetoresistance refers to the change in resistance of any material when placed in a magnetic field. With a non-magnetic material the change in resistivity is very small.

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GIANT MAGNETORESISTANCE (GMR)

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  1. GIANT MAGNETORESISTANCE (GMR) By R.S.Ragava Vignesh

  2. Introduction • In general, magnetoresistance refers to the change in resistance of any material when placed in a magnetic field. • With a non-magnetic material the change in resistivity is very small. • For a magnetic material, the change in resistivity depends on the direction of the current flow w.r.t the magnetic field. • The resistivity ρ(//) for current flow parallel to magnetic field decreases and ρ(l_), perpendicular to the field increases by the same amount. • This change in resistivity is anisotropic and is hence called anisotropic magnetoresistance.

  3. Variation of resistivities with Bo Resistivity ρ(//) ρ(l_) Magnetic field, Bo = µo H

  4. Physical Origin • The applied field tilts the angular momenta of the 3d electrons. The field rotates the 3d orbitals which changes the scattering of the conduction electrons w.r.t the direction of their travel.

  5. GMR • On the other hand, a very large magnetoresistance, called giant magnetoresistance(GMR) has been observed in certain special multilayer structures that exhibit substantial changes in resistance(upto 10%). • The multilayer structure in its simplest form has 2 ferromagnetic layers(Fe, Co or its alloys) separated by a non-magnetic transition metal layer(Cu etc.) called a SPACER. • The magnetic layers are thin(10 nm) and the spacer even thinner. The magnetization of the 2 layers depends on the thickness of the space because they both are coupled indirectly through the spacer.

  6. Configurations • In the absence of magnetic field, the magnetizations of the 2 layers are antiparalleland in opposite directions called an antiferromagneticallycoupled configuration denoted by FNA. • An external magnetic field is applied to one of the layers so that the 2 magnetizations are now parallel called ferromagnetically coupled layers denoted by FNF. • The 2 structures have a a giant difference in their resistances, hence the term giant magnetoresistance. • The resistance of the antiparallel structure FNA is much greater than the parallel FNF.

  7. Principle • Consider FNA, the magnetic moment up electron in the 1st magnetic layer is the favoured conduction electron and suffers very little scattering. • However when it arrives at the other layer in which the magnetization is reversed, it has a wrong spin and is an unfavoured electron subject to scattering. • The electron is also scattered at the interface between N and A. This has high resistance and is denoted by RAP. • When the magnetizations are parallel, the moment-up electron is favoured in both layer with little scattering and has al lower resistance denoted by RP.

  8. Principle F N F F N A RP RAP

  9. Principle • The differences in the resistivities is roughly 10% or less. • But in multilayered structures (FNANFANFA...), the change can be large exceeding 100% at low temperatures and 60-80% at room temperature. • GMR effect measured by the change in resistance w.r.t RP: (ΔR/RP)GMR = (RAP – RP)/RP • The GMR effect can be measured by passing a current in the plane of layers or perpendicular to the plane.

  10. Principle • The current-in-plane measurements are used mostly, but the biggest change is observed for currents perpendicular to the plane. • If the angle between the magnetization vectors M1 and M2of the 2 layers is θ, min. For θ=0 (FNA) and max. For θ=180o (FNA).The fractional change in resistance depends on θ as ΔR/RP=(ΔR/RP)MAX(1-cos θ)/2 • The change is max. For θ=180o

  11. Applications • One of the best applications of GMR is in a spin valve, in which the current flow is controlled by an external magnetic field. • The resistance of the valve is controlled by an applied field. The magnetization of the Co magnetic layer is fixed, by the adjacent antiferromagnetic layer, called the pinning layer. • A Cu spacer separates the Co and the next magnetic FeNi layer. The FeNi layer is called a free layer because its magnetization can be changed by an external magnetic field.

  12. Applications • The magnetization of the FeNi layer is antiparallel to Co layer and this has a high resistance RAP. • An applied external field Bo = µo H aligns the magnetization parallel to the Co layer and this has a low resistance RP. • The pinned layer must have high coercivity and the free layer must be soft...Why??(Question) • The spin valve exhibits hysteresis.

  13. Principle of a spin valve Resistance change vs applied magnetic field The four layers

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