Using single crystal inelastic neutron scattering with and without the application of an external magnetic field and powder neutron diffraction, we have characterized magnetic interactions in Ba3Cr2O8. Even without a field, we found that there exist three singlet-to-triplet excitation modes in the (h, h, l) scattering plane. Our complete analysis shows that the three modes are due to spatially anisotropic interdimer interactions that are induced by lattice distortions of the tetrahedron of oxygens surrounding the Jahn-Teller active Cr5+(3d1). The strong intradimer coupling of J0=2.38(2) meV and weak interdimer interactions (|Jinter|< or =0.52(2) meV) makes Ba3Cr2O8 a good model system for weakly coupled s=1/2 quantum spin dimers.

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Resonant x-ray scattering experiments at the Gd L3 edge show interference between magnetic and anisotropic tensor susceptibility (ATS) reflections in GdB4. Energy profiles obtained from the magnetic and ATS resonances exhibited approximately 10 eV separation between the maximum resonance energies. The findings show that the Gd 5d band experienced hybridization giving rise to a significant split into isotropic lower energy band and distorted upper band states that account for the magnetic and ATS scattering, respectively.

Using synchrotron X-rays and neutron diffraction we disentangle spin-lattice order in highly frustrated ZnCr2O4 where magnetic chromium ions occupy the vertices of regular tetrahedra. Upon cooling below 12.5 K the quandary of anti-aligning spins surrounding the triangular faces of tetrahedra is resolved by establishing weak interactions on each triangle through an intricate lattice distortion. The resulting spin order is however, not simply a Néel state on strong bonds. A complex co-planar spin structure indicates that antisymmetric and/or further neighbor exchange interactions also play a role as ZnCr2O4 resolves conflicting magnetic interactions. PACS numbers:While tetrahedral atomic clusters are a natural consequence of close packing, they are particularly inconvenient for antiferromagnetically interacting spins. This is because no spin configuration can simultaneously satisfy all six antiferromagnetic interactions amongst spins on the vertices of a tetrahedron [1,2,3,4,5]. The consequence of such "geometrical frustration" is deep suppression of magnetic order and a range of temperatures where spins remain fluctuating despite interactions that far exceed thermal energies [6,7]. Indeed for spins on a lattice of corner-sharing tetrahedra, it appears there is no conventional order in the quantum limit (S = 1/2, T = 0) [5]. Because they entail higher energy spin configurations, geometrically frustrating lattices however typically do not survive in the low temperature limit. Instead a compromise between spin and lattice energy is reached through a first order phase transition that freezes the spin liquid and distorts the lattice [8,9,10,11,12,13]. Such phase transitions challenge conventional theories of magnetism because they involve strongly correlated spins and the collapse of the rigid lattice approximation [14,15,16].A case in point is ZnCr 2 O 4 . At room temperature, it has a cubic F d3m crystal structure where Cr 3+ (S = 3/2) ions form a network of corner-sharing tetrahedra [9]. The Curie-Weiss temperature is -390 K indicating strong antiferromagnetic frustration, yet chromium spins remain in a cooperative paramagnetic phase down to T C = 12.5 K [6,9]. There, a first order phase transition from a cubic paramagnet to a tetragonal antiferromagnet signals the end of distinct spin and lattice degrees of freedom. Tetragonal strain energy alone does not account for the difference between magnetic energy gain and overall latent heat and this was a first indication of a more comprehensive rearrangement of the lattice [9]. Subsequently X-ray superlattice peaks were detected at ( 2 ) c type reflections (see Fig. 1 (a)) [17]. This indicates that below T N the tetragonal lattice has I4m2 symmetry and a √ 2 × √ 2 × 2 chemical unit cell [18]. Theoretical efforts to understand the nature of the phase transition have focused on magneto-elastic couplings that involve symmetric isotropic nearest neighbor (NN) exchange interactions [14,15,16].Here we report a combined synchrotron X-ray and magnetic neutron diffraction study ...