Ferrimagnetism


Ferrimagnets behave similarly to ferromagnets, in that they exhibit a spontaneous magnetization below some critical temperature, Tc, even in the absence of an applied field. However, as we see in Fig. 1.1, the form of a typical ferrimagnetic magnetization curve is distinctly different from the ferromagnetic curve.


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Figure 1.1 Comparison of magnetization and inverse susceptibility in typical ferriand ferromagnets.

external image placeholder?w=291&h=241In fact ferrimagnets are also related to antiferromagnets, in that the exchange coupling between adjacent magnetic ions leads to antiparallel alignment of the localized moments. The overall magnetization occurs because the magnetization of one sublattice is greater than that of the oppositely oriented sublattice. A schematic of the ordering of magnetic moments in a ferrimagnet is shown in Fig. 1.2.
Figure 1.2 Ordering of magnetic ions in a ferrimagnetic lattice.
external image placeholder?w=221&h=313In ionic compounds, such as oxides, more complex forms of magnetic ordering can occur as a result of the crystal structure. One type of magnetic ordering is call ferrimagnetism. A simple representation of the magnetic spins in a ferrimagnetic oxide is shown here. The magnetic structure is composed of two magnetic sublattices (called A and B) separated by oxygens. The exchange interactions are mediated by the oxygen anions. When this happens, the interactions are called indirect or superexchange interactions. The strongest superexchange interactions result in an antiparallel alignment of spins between the A and B sublattice. In ferrimagnets, the magnetic moments of the A and B sublattices are not equal and result in a net magnetic moment.






Fig. 1.3
Ferrimagnetism is therefore similar to ferromagnetism. It exhibits all the hallmarks of ferromagnetic behavior- spontaneous magnetization, Curie temperatures, hysteresis, and remanence. However, ferro- and ferrimagnets have very different magnetic ordering. Magnetite is a well known ferrimagnetic material. Indeed, magnetite was considered a ferromagnet until NĂ©el in the 1940's, provided the theoretical framework for understanding ferrimagnetism.



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