Magnetization Plateau in the FCC Antiferromagnet

The diploma work is devoted to investigation of the magnetization process of the J1-J2 Heisenberg antiferromagnets on a face centered cubic lattice (fcc). Our main goal is to demonstrate theoretically the existence of two stable plateaus and other exotic phases in these materials. The relevance and motivation for studying fcc antiferromagnets is presented in Chapter 1. The J1-J2 Heisenberg model is considered with J1 antiferromagnetic exchange between nearest neighbors spins and J2 exchange between second neighbors. For the special ratio J2/J1 = 0.5, the degeneracy is enhanced, and the spin model can be represented as a sum of edge-sharing octahedra. The contribution of quantum fluctuations is modeled by a negative biquadratic exchange (Chapter 2). The magnetization curve is constructed for different values of the biquadratic exchange constant when considering one octahedron and for different temperatures when considering the whole cluster of the system. For this, Monte Carlo algorithms were used (Chapter 3). A phase diagram is constructed showing spin configurations that depend on the biquadratic exchange constant. As a result, the existence of two stable magnetization plateaus was shown. Chapter 4 summarizes the relevant conclusions and presents the further development of this topic.