Effect of the parameters of twin microstructure on the magnetic domain structure of CoPt-type alloys: A review

Twinned microstructure in CoPt and FePt alloys is formed in the process of atomic ordering at temperatures below T-K-temperature of the Al --> Ll(0) p hase transformation. Polytwinned CoPt and FePt crystals consist of regular systems (blocks, plates, packets) of lamellar tetragonal crystallites of tw in orientation called C domains. In our work, we determined the specific ro le of dispersity and the character of contact between the above structural elements in the formation of the magnetic domain structure (DS) and hystere sis properties of CoPt and FePt alloys. We showed that the magnetic DS of p olytwinned crystals has a cooperative nature and consists of magnetic micro and macro domains. We constructed models of a cooperative DS and introduce d the concept of a critical size of magnetic single-macrodomain structure o f a polytwinned crystal (particle). We theoretically studied the quantitati ve relationship between the main parameters of the cooperative DS (width of macrodomains and the critical single-macrodomain size) and the thickness o f C domains and the dimensions of plates and packets. The effect of the exc hange and magnetostatic interactions (that arise at the twin boundaries of C domains) on the parameters of the DS are discussed and the results of a n umerical calculation of hysteresis properties with allowance for these inte ractions are given. A conclusion is made that the exchange and magnetostati c interactions create additional metastable magnetic states in the twin sys tem of lamellar crystallites with high uniaxial anisotropy. This leads to e ffective multiaxiality of the polytwinned crystals.