182
NANOSTRUCTURED FERROMAGNETISM
NON MAGNETIC METAL
NON MAGNETIC METAL
0
NiFe 4
Figure 7.15. Three arrangements for producing colossal magnetoresistance: (a) layers of
nonmagnetic material alternating with oppositely magnetized (arrows) ferromagnetic layers;
(b) randomly oriented ferromagnetic cobalt nanoparticles (large circles) in a nonmagnetic copper
matrix (small circles); (c) hybrid system consisting of cobalt nanoparticles in a silver (Ag) matrix
sandwiched between nickeliron (NiFe) magnetic layers, with alternating magnetizations
indicated by arrows.
The magnetoresistance effect in these layered materials is a sensitive detector of
DC magnetic fields, and is the basis for the development of a new, more sensitive
reading head for magnetic disks. Prior to this, magnetic storage devices have used
induction coils to both induce an alignment of the magnetization in a small region of
the tape (write mode), and to sense the alignment of a recorded area (read mode).
The magnetoresistive reading head is considerably more sensitive than the inductive
coil method.
Materials made of single-domain ferromagnetic nanoparticles with randomly
oriented magnetizations embedded in a nonmagnetic matrix also display giant
magnetoresistance. Figure 7.15b shows a schematic of this system. The magnetoresistance in these materials, unlike the layered materials, is isotropic. The application of the DC magnetic field rotates the magnetization vector of the ferromagnetic