Introduction to Nanotechnology

7.6. GIANT AND COLOSSAL MAGNETORESISTANCE 181
could be the source of conduction electrons, which might bring about itinerant
ferromagnetism. ltinerant ferromagnetism corresponds to ferromagnetism from a
spin that can move through the lattice. The result has not yet been confirmed by other
researchers.
7.6. 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 the average distance an
electron travels in a metal when 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 4 K a field of 10 T
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 (GMR) 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 magnetiza-
tion in the ferromagnetic layer is shown in Fig. 7.15a. The effect was first observed
in films 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 nm.
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 Mare
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 the 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 the resistance of the material.