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Zusammenfassung:
We present results from our analysis of the finite-temperature properties of the spin View the MathML source12J1J1–J2J2 Heisenberg model on a square lattice. The analysis is based on the exact diagonalisation of small clusters with 16 and 20 sites utilising the finite-temperature Lanczos method. In particular, we focus on the temperature dependence of the third-order magnetic susceptibility as a method to resolve the ambiguity of exchange constants. We discuss the entire range of the frustration angle φ=tan-1(J2/J1)φ=tan-1(J2/J1) parameterising the different possible phases of the model, including the large region in the phase diagram with at least one ferromagnetic exchange constant.
