We determine the low-field ordered magnetic phases of the S = 1 dimerized antiferromagnet Ba3Mn2O8 using single-crystal neutron diffraction. We find that for magnetic fields between mu H-0 = 8.80 T and 10.56 T applied along the [1 (1) over bar0] direction the system exhibits spin density wave order with incommensurate wave vectors of type (eta,eta,epsilon). For mu H-0 > 10.56 T, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the [hh0] direction. For both field-induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. The nature of these two transitions is fundamentally different: the low-field transition is a second-order transition to a spin density wave ground state, while the one at higher field, toward the spiral phase, is of first order.

Ba3Mn2O8 is a hexagonally coordinated Mn5+ S = 1 spin dimer system with small uniaxial single- ion anisotropy. Ba-135,Ba-137 NMR spectroscopy is used to measure the longitudinal (M-l) magnetization in the vicinity of the critical field at H-c1 for the onset of magnetic order for H parallel to c and H perpendicular to c. M-l parallel to(T, H-c1), M-l perpendicular to(T, H-c1) are reproduced by solving a low- energy model for a dilute gas of interacting bosons.

Heat-capacity and susceptibility measurements have been performed on the diluted spin dimer compound Ba-3(Mn1-xVx)(2)O-8. The parent compound Ba3Mn2O8 is a spin dimer system based on pairs of antiferromagnetically coupled S = 1, 3d(2) Mn5+ ions such that the zero-field ground state is a product of singlets. Substitution of nonmagnetic S = 0, 3d(0) V5+ ions leads to an interacting network of unpaired Mn moments, the low-temperature properties of which are explored in the limit of small concentrations 0 <= x <= 0.05. The zero-field heat capacity of this diluted system reveals a progressive removal of magnetic entropy over an extended range of temperatures, with no evidence for a phase transition. The concentration dependence does not conform to expectations for a spin-glass state. Rather, the data suggest a low-temperature random singlet phase, reflecting the hierarchy of exchange energies found in this system.

We present pressure dependent neutron diffraction and inelastic neutron scattering measurements of the dimerized antiferromagnet Ba 3 Mn 2 O 8 . The room temperature diffraction measurements reveal a linear decrease in lattice constant as a function of applied pressure. No structural transitions are observed. The low-temperature neutron spectroscopy measurements indicate a small change in magnetic scattering intensity in the vicinity of the spin gap for pressures up to P =0.6 GPa.

We present inelastic neutron scattering and thermodynamic measurements characterizing the magnetic excitations in a disordered spin-liquid antiferromagnet with non-magnetic substitution. The parent compound Ba(3)Mn(2)O(8) is a dimerized, quasi-two-dimensional geometrically frustrated quantum disordered antiferromagnet. We substitute this compound with non-magnetic V(5+) for the S=1 Mn(5+) ions, Ba(3)(Mn(1-x)V (x))(2)O(8), and find that the singlet-triplet excitations which dominate the spectrum of the parent compound persist for the full range of substitution examined, up to x=0.3. We also observe additional low-energy magnetic fluctuations which are enhanced at the greatest substitution values.

Heat capacity and magnetic torque measurements are used to probe the anisotropic temperature-field phase diagram of the frustrated spin dimer compound Ba3Mn2O8 in the field range from 0T to 18T. For fields oriented along the c axis a single magnetically ordered phase is found in this field range, whereas for fields oriented along the a axis two distinct phases are observed. The present measurements reveal a surprising non-monotonic evolution of the phase diagram as the magnetic field is rotated in the [001]-[100] plane. The angle dependence of the critical field (Hc1) that marks the closing of the spin gap can be quantitatively accounted for using a minimal spin Hamiltonian comprising superexchange between nearest and next nearest Mn ions, the Zeeman energy and single ion anisotropy. This Hamiltonian also predicts a non-monotonic evolution of the transition between the two ordered states as the field is rotated in the a-c plane. However, the observed effect is found to be significantly larger in magnitude, implying that either this minimal spin Hamiltonian is incomplete or that the magnetically ordered states have a slightly different structure than previously proposed.

The expressions for !1, !2, and !4 in the original manuscript are correct. The sign of the quantities in the Fourier sum is corrected in the equation above. The resulting exchange constants are therefore J0 1⁄4 1:642ð3Þ, J1 1⁄4 0:118ð2Þ, ðJ2 J3Þ 1⁄4 0:1136ð7Þ, and J4 1⁄4 0:037ð2Þ meV. J1 and J4 were previously reported as being negative. The resulting dispersion, @!ðQÞ, of the magnetic excitations does not change due to this error. The only consequence of this change is that the interlayer exchange interactions are in fact antiferromagnetic for Ba3Mn2O8. Our other conclusions and results remain unchanged. The research at Oak Ridge National Laboratory was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This work utilized facilities supported in part by the National Science Foundation under Agreement No. DMR-0454672. Work at Stanford was supported by the National Science Foundation, under Grant No. DMR 0705087.

Ba3Mn2O8 is a spin-dimer compound based on pairs of S = 1, 3d2, Mn5+ ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero-field, with excited triplet and quintuplet states at higher energy. High field thermodynamic measurements are used to establish the phase diagram, revealing a substantial asymmetry of the quintuplet condensate. This striking effect, all but absent for the triplet condensate, is due to a fundamental asymmetry in quantum fluctuations of the paramagnetic phases near the various critical fields.

Ba3Mn2O8 is a hexagonally coordinated Mn5+ S=1 spin dimer system with small uniaxial single-ion anisotropy. 135,137Ba NMR spectroscopy is used to establish the lower critical field Hc1 of distinct field-induced phases for H parallel to c,H perpendicular to c, and measure the longitudinal (Ml) and transverse (Mt) magnetizations in the vicinity of the quantum critical point (QCP). Ml_parallel (T, Hc1), Ml_perpendicular (T, Hc1) are reproduced by solving a low-energy model for a dilute gas of interacting bosons. Ml_parallel(T goes to 0, H = Hc1) (Ml_perpendicular(T goes to 0, H = Hc1)) follows the expectation for a BEC (Ising-like) QCP.

Ba_{3}Mn_{2}O_{8} is a spin-dimer compound based on pairs of S = 1, 3d;{2}, Mn;{5+} ions arranged on a triangular lattice. Antiferromagnetic intradimer exchange leads to a singlet ground state in zero field, with excited triplet and quintuplet states at higher energy. High field thermodynamic measurements are used to establish the phase diagram, revealing a substantial asymmetry of the quintuplet condensate. This striking effect, all but absent for the triplet condensate, is due to a fundamental asymmetry in quantum fluctuations of the paramagnetic phases near the various critical fields.

We present single crystal inelastic neutron scattering measurements of the S=1 dimerized quasi-two-dimensional antiferromagnet Ba(3)Mn(2)O(8). The singlet-triplet dispersion reveals nearest-neighbor and next-nearest-neighbor ferromagnetic interactions between adjacent bilayers that compete against each other. Although the interbilayer exchange is comparable to the intrabilayer exchange, this additional frustration reduces the effective coupling along the c axis and leads to a quasi-two-dimensional behavior. In addition, the obtained exchange values are able to reproduce the four critical fields in the phase diagram.

We present powder inelastic neutron scattering measurements of the S=1 dimerized antiferromagnet Ba(3)Mn(2)O(8). The T=1.4 K magnetic spectrum exhibits a spin gap of Delta approximate to 1.0 meV and a dispersive spectrum with a bandwidth of approximately 1.5 meV. A comparison to coupled dimer models accurately describes the dispersion and scattering intensity and determines the exchange constants in Ba(3)Mn(2)O(8). The wave vector dependent scattering intensity confirms the proposed S=1 dimer bond. Temperature dependent measurements of the magnetic excitations indicate the presence of both singlet-triplet and thermally activated triplet-quintet excitations.