Stabilization of Néel order in frustrated magnets with increasing magnetic field

Max-Planck-Institut für Chemische Physik fester Stoffe, Dresden, Germany

^a e-mail: bs@cpfs.mpg.de

Abstract

For low-dimensional frustrated quantum magnets, the dependence of the staggered moment ms on a magnetic field is nonmonotonic: For small and intermediate fields, quantum fluctuations are gradually suppressed, leading to an increase of m_s (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 (ClO₄)₂ in the framework of the S = 1/2 two-dimensional J₁-J₂ Heisenberg model. The observed increase with increasing field strength can be understood naturally using an intermediate frustration ratio J₂/J₁ ≈ 0.2, which is in accordance with the field dependence of the staggered moment.