MAGNETIZATION STEPS IN DILUTE MAGNETIC SEMICONDUCTORS

At low temperatures the magnetization curve of a dilute magnetic semiconductor (DMS) exhibits an apparent saturation which is followed, at higher fields, by magnetization steps (MSTs). These MSTs are caused by crossings of energy levels for pairs of nearest-neighbor (NN) magnetic ions. The fields where the MSTs occur yield the NN exchange constant JNN. Examples of data obtained in both dc and pulsed magnetic fields are presented, and values of JNN for several II-VI DMSs containing Mn++ are given. The apparent (or technical) saturation value Ms, and the magnitude δM of each MST, indicate that the Mn++ ions are randomly distributed over the cation sites. A dramatic narrowing of the MSTs, and a fine structure, are observed in pulsed-field experiments on some DMSs. The fine structure in Cd1-xMnxS is attributed to two types of NNs (with different JNN) in the wurtzite structure of this material. The theory of the MSTs, including the effects of distant neighbors and of the Dzyaloshinski-Moriya interaction, is reviewed. Although thus far the predicted effects have been observed only in DMSs, they should also occur in many other dilute magnetic materials with antiferromagnetic interactions.