= Quantum magnetism
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<Quantum magnetism> studies collective magnetic behavior in which quantum <spin> states, noncommuting observables and <quantum fluctuations> matter. A lattice of local <spins> can be modeled by a <Heisenberg model>, whose exchange interaction favors particular relations between neighbours. Its states need not resemble a set of classical arrows pointing in definite directions.
A two-spin <spin singlet state> illustrates the difference: it has zero total <spin> while retaining nontrivial spin correlations, rather than two independently known opposite arrows. Large ordered magnets can support <spin waves>, whereas strong <quantum fluctuations> can invalidate an expansion around a chosen ordered state. Measurements of <magnetic susceptibility>, <heat capacity> and excitation spectra probe different aspects of the same interacting system. The subject is therefore about both collective order and correlated states that resist a simple ordered reference picture.
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