Giant And Colossal Magnetoresistance

Magnetoresistance is a phenomenon where the application of a DC magnetic field changes the resistance of a material. The phenomenon has been known for many years in ordinary metals, and is due to the conduction electrons being forced to move in helical trajectories about an applied magnetic field. The effect becomes evident only when the magnetic field is strong enough to curve the electron trajectory within a length equal to its mean free path. The mean free path is die average distance an electron travels in a metal what an electric field is applied before it undergoes a collision with atoms, defects, or impurity atoms. The resistance of a material is the result of the scattering of electrons out of the direction of current flow by these collisions. The magnetoresistance effect occurs in metals only at very high magnetic fields and low temperatures. For example, in pure copper at 4K a field of 10T produces a factor of 10 change in the resistance.

Because of the large fields and low temperatures, magnetoresistance in metals originally had few potential application possibilities. However, that changed in 1988 with the discovery of what is now called giant magnetoresistance (OMR) in materials synthetically fabricated by depositing on a substrate alternate layers of nanometer thickness of a ferromagnetic material and a nonferromagnetic metal. A schematic of the layered structure and the alternating orientation of the magnetization in die ferromagnetic layer is shown in Fig. 7.15a. The effect was first observed in Aims made of alternating layers of iron and chromium, but since then other layered materials composed of alternating layers of cobalt and copper have been made that display much higher magnetoresistive effects. Figure 7.16 shows the effect of a DC magnetic field on the resistance of the iron-chromium multilayered system. The magnitude of the change in the resistance depends on the thickness of the iron layer, as shown in Figure 7.17, and it reaches a maximum at a thickness of 7 mm.

The effect occurs because of the dependence of electron scattering on the orientation of the electron spin with respect to the direction of magnetization. Electrons whose spins are not aligned along the direction of the magnetization M are scattered more strongly than those with their spins aligned along M. The application of a DC magnetic field parallel to the layers forces the magnetization of all the magnetic layers to be in the same direction. This causes the magnetizations pointing opposite to die direction of the applied magnetic field to become flipped. The conduction electrons with spins aligned opposite to the magnetization are more strongly scattered at the metal-ferromagnet interface, and those aligned along the field direction are less strongly scattered. Because the two spin channels are in parallel, the lower-resistance channel determines die resistance of die material.

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