• Dissanayake, S.E.; Chan, C.; Ji, S.; Lee, J.; Qiu, Y.; Rule, K.C.; Lake, B.; Green, M.; Hagihala, M.; Zheng, X.G.; Ng, T.K.; Lee, S.-H.: Magnetic-field-induced instability of the cooperative paramagnetic state in ZnxCo4-x(OD)6Cl2. Physical Review B 85 (2012), p. 174435/1-6

10.1103/PhysRevB.85.174435
Open Access Version (externer Anbieter)

Abstract:
I. INTRODUCTION Recently, frustrated magnets where pairwise interactions cannot be satisfied simultaneously have been actively studied in the quest for novel quantum spin states in insulating materials.1–4 Among them, ZnxCu4−x(OD)6Cl2 5 has generated particular interest because the magnetic Cu2+ ions with quantum spin (s = 1/2) form a two-dimensional kagome lattice even though it has 10% site disorder. The interest was heightened when it was found that the system does not exhibit any magnetic ordering, short or long range, down to 50 mK for x > 0.66.6–8 The exact nature of the quantum disordered state is however controversial. It has been shown recently that in ZnCu3(OD)6Cl2 there are broad gapless antiferromagnetic fluctuations that persist up to 20 meV.9 The wave vector (Q) dependence of the spin fluctuations was explained by a simple spin dimer model and interpreted as evidence that the ground state was an algebraic spin liquid state. On the other hand, when nonmagnetic Zn atoms, which predominantly lie in triangular layers between the kagome planes, are replaced by the magnetic Cu atoms, long-range magnetic ordering sets in at low temperatures, for instance below TN = 6.7 K for Cu2(OD)3Cl.8 In the long-range ordered (LRO) state, the magnetic excitation spectrum obtained from a powder sample showed a peak around the energy transfer of ¯hω = 7 meV whose Q dependence resembled that of spin dimers8 but may also be due to a Van Hove singularity from the top of the spin-wave excitation band.10 It remains to be seen whether the gapless spin fluctuations observed in ZnCu3(OD)6Cl2 are characteristic of the quantum spin liquid state of the quantum kagome system11 or share the same origin as the ¯hω = 7 meV mode of Cu2(OD)3Cl. To address this issue, it is desirable to investigate other related materials that exhibit similar low-temperature behaviors. A case in point is an isostructural compound, ZnxCo4−x(OD)6Cl2, with magnetic Co2+ (3d7; s = 3/2) ions. This system has an x-T phase diagram similar to that of ZnxCu4−x(OD)6Cl2. Co4(OD)6Cl2 develops long-range magnetic order belowTN = 10.5Kwhich is suppressed upon doping with nonmagnetic Zn while for ZnCo3(OD)6Cl2 no static ordering has been observed down to 1.5 K. We have performed both elastic and inelastic neutron scattering measurements on powder samples of ZnxCo4−x(OD)6Cl2 with x = 0 and x = 1 in zero and nonzero external magnetic fieldH.Our results are as follows. For x = 0 in the absence of an applied magnetic field the magnetic moments in the kagome plane order in a canted antiferromagnetic structurewhere their in-plane components form the q = 0 120◦ structure and these moments are canted out of the plane by 40◦. The magnetic moments in the triangular plane are aligned ferromagnetically along the c axis. Allmoments have the same frozen moment of M = 3.77(3) μB/Co2+ which is close to the expected value for the high-spin state of Co2+ of 3.87 μB.