Other types of magnetic order

The spontaneous magnetization of a ferromagnet is the result of alignment of the magnetic moments of individual atoms. But parallel alignment is not the only – or even the most common – type of magnetic order. In an antiferromagnet, the atomic moments form two equivalent but oppositely oriented magnetic sublattices. Although M_s = 0, the material nonetheless exhibits a phase transition with a λ-shaped specific heat anomaly where the moments begin to order. The antiferromagnetic transition occurs at the Ne´ el temperature T_N. Occasionally it is possible to switch an antiferromagnet into a ferromagnet if a sufficiently large field is applied. This discontinuous change of magnetic order is known as a metamagnetic transition.
If the sublattices are inequivalent, with sublattice magnetizationsM_A and M_B where M_A ≠ −M_B, there is a net spontaneous magnetization. The material is a ferrimagnet. Most of the useful magnetic oxides, including magnetite, are ferrimagnetic.
The alignment of the atomic moments in the ordered state need not be collinear. Multiple noncollinear sublattices are found in manganese and some of its alloys. Other materials such as MnSi orMn₃Au have helical or spiral magnetic structures that are incommensurate with the underlying crystal lattice. In some disordered and amorphous materials the atomic moments freeze in more or less random directions. Such random, noncollinear magnets are known collectively as spin glasses. The original spin glassses were magnetically dilute crystalline alloys, but several different varieties of random spin freezing are encountered in noncrystalline (amorphous) solids. Finally, we remark on the behaviour of ferromagnetic fine particles whose volume V is so small that the product K_uV is less than or comparable to the thermal energy k_BT ; in that case the total sum moment, m, of all the coupled atoms fluctuates randomly like that of a large paramagnetic atom or macrospin. The susceptibility follows a Curie law with a huge value of C. The name superparamagnetism was coined by N´eel for this phenomenon, which is important for ferrofluids (magnetic liquids which are really colloidal suspensions of ferrimagnetic fine particles) and in rock magnetism.

Figure 1. portrays the magnetic family tree, summarizing the behaviour of the magnetization or susceptibility for the different types of magnetic order in crystalline and amorphous solids.