Stabilization of Néel order in frustrated magnets with increasing magnetic field

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 Neel 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.