Macroscopic ferrimagnets as magneto-optic media (abstract)

Abstract

We have discovered a class of phase separated materials in which the two phases couple antiferromagnetically across the phase boundary. A prototypical example is cobalt containing ≂100-Å-diam precipitate particles of EuS with the NaCl structure. These materials show many of the properties of ferrimagnets such as compensation points so we have adopted the term ‘‘macroscopic ferrimagnet’’ to distinguish them from ferrimagnets in which the antiferromagnetic exchange couples individual atoms. As a result of the ferrimagnetic exchange between the Co and the EuS the net magnetization of the composite is low in the vicinity of compensation so the anisotropy field is high. Electron beam evaporated thin films have a perpendicular easy axis as a result of growth induced anisotropy. Another effect of the exchange between the Co and EuS is to order the EuS well above its normal Curie point of 16 K. A film with 80 mole % Co and 20 mole % EuS has a compensation point of 55 K showing that the Eu2+ has essentially its full moment of 7μB at this temperature in order to compensate the 1.65μB of Co. The EuS even shows evidence of magnetic order at room temperature. The wavelength dependence of the sign of the magneto-optic Kerr effect can only be explained if the EuS has sufficient rotation at high photon energies to be larger than the rotation of the Co. We have prepared more complex alloys that show square loop properties at room temperature, for example, Tb-Co-EuS and Tb-FeCo-EuO. X-ray diffraction analysis shows that these films consist of an amorphous matrix (Tb-Co or Tb-FeCo) with precipitates of a NaCl structure phase (e.g., defect solid solutions like Eu1−xTbxS1−x⧠x). Both atomic and macroscopic ferrimagnetism are operating in these compositions. Films with high remanence and coercivities over 4 kOe at room temperature have been obtained. Some of these compositions may be useful as magneto-optic storage media.