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Register a new userRole of anisotropy in the spin-dimer compound BaCuSi2O6
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We present results of magnetisation and electron paramagnetic resonance experiments on the spin-dimer system BaCuSi2O6. Evidence indicates that the origin of anisotropic terms in the spin Hamiltonian is from magnetic dipolar interactions. Axial symmetry-breaking is on a very small energy scale of ~11 mK, confirming Bose Einstein condensation critical scaling over an extended temperature range in the vicinity of the quantum critical point.
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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.
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.
Ba3Mn2O8 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. Here we present the first results of thermodynamic measurements for single crystals probing the high-field ordered states of this material associated with closing the spin gap to the excited triplet states. Specific heat, magnetocaloric effect, and torque magnetometry measurements were performed in magnetic fields up to 32 T and temperatures down to 20 mK. For fields above H_{c1} ~ 8.7 T, these measurements reveal a single magnetic phase for H parallel to c, but two distinct phases (approximately symmetric about the center of the phase diagram) for H perpendicular to c. Analysis of the simplest possible spin Hamiltonian describing this system yields candidates for these ordered states corresponding to a simple spiral structure for H parallel to c, and to two distinct modulated phases for H perpendicular to c. Both single-ion anisotropy and geometric frustration play crucial roles in defining the phase diagram.
We report the study on the low-temperature heat transport of Ba_3Mn_2O_8 single crystal, a layered spin-dimer compound exhibiting the magnetic-field-induced magnetic order or the magnon Bose-Einstein condensation. The thermal conductivities (kappa) along both the ab plane and the c axis show nearly isotropic dependence on magnetic field, that is, kappa is strongly suppressed with increasing field, particularly at the critical fields of magnetic phase transitions. These results indicate that the magnetic excitations play a role of scattering phonons and the scattering effect is enhanced when the magnetic field closes the gap in the spin spectrum. In addition, the magnons in the BEC state of this materials do not show notable ability of carrying heat.
A series of compounds M$_{0.1}$Sr$_{0.9}$Cu$_2$(BO$_3$)$_2$ with Sr substituted by M=Al, La, Na and Y were prepared by solid state reaction. XRD analysis showed that these doping compounds are isostructural to SrCu$_2$(BO$_3$)$_2$. The magnetic susceptibility from 1.9K to 300K in an applied magnetic field of 1.0T and the specific heat from 1.9K to 25K in applied fields up to 14T were measured. The spin gap is deduced from the low temperature susceptibility as well as the specific heat. It is found that the spin gap is strongly suppressed by magnetic fields. No superconductivity is observed in all four samples.
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