For low-dimensional frustrated quantum magnets, the dependence of the staggered moment m_s on a magnetic field is nonmonotonic: For small and intermediate fields, quantum fluctuations are gradually suppressed, leading to an increase of ms(H). For large applied magnetic fields however, the classically expected monotonous decrease is recovered. For the same reasons, the Néel ordering temperature T_N of such compounds first increases and then exhibits a reentrant behavior as a function of the field strength. The quantitative analysis of this behavior is an excellent tool to determine the frustration parameter of a given compound. We have derived a general linear spin-wave (LSW) theory in the presence of a magnetic field. Based on our LSW'theory, including a small interlayer coupling, we use a self-consistent approach determining T_N by the condition of a vanishing total moment. We apply our findings to the recently measured field dependence of the magnetic ordering temperature T_n of Cu(pz)_2(ClO_4)_2 in the framework of the S = 1/2 two-dimensional J_1-J_2 Heisenberg model. The observed increase with increasing field strength can be understood naturally using an intermediate frustration ratio J_2/J_1 ≈0.2, which is in accordance with the field dependence of the staggered moment.
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