The Heisenberg Hamiltonian represented in the reciprocal-lattice space (q-space) is introduced where spins are treated as classical vectors. According to the model, the q-vector which gives the lowest energy of the spin system determines the magnetic structure at the lowest temperature. Features of the ferro-,antiferro-,ferri- and helical magnetism are discussed. After magnetism at finite temperatures is discussed within the molecular-field approximation, spin waves at low temperatures, critical phenomena near the ordering temperature, and spin glasses are discussed.
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