Synthesis and Characterizatin of Spin-Chain Compounds of the AM2V2O8 Type

Different representatives of the group of quasi-one-dimensional spin-chain compounds Ba(1-x)Sr(x)(Co(1-y)M(y))2V2O8 with M= Mn, Cu, Ni, Mg and (Ba,Sr)Mn2V2O8 were synthesized, characterized and their magnetic phase diagrams determined for H||c, H||a and H||[110]. The tetragonal structure of these compounds consists of screw chains of edge-sharing MO6 octahedra, where depending on the transition metal M different spin values are realized. The chains are separated by nonmagnetic Ba or Sr and V ions. Large single crystals were grown via the floating-zone method in a mirror furnace employing the twice scanning method. The single crystal studies of the S=5/2 Heisenberg spin chains BaMn2V2O8 (T_N = 37 K) and SrMn2V2O8 (T_N = 42 K) reveal a weak positive field dependence. BaMn2V2O8 shows a spin-flop transition at H^(perpendicular c} = 300 mT as is observed in the susceptibility and the magnetostriction. Deduced from this data a magnetic structure is suggested similar to the magnetic structure of SrMn2V2O8 which reveals ferromagnetic planes (spins along (+-)a) that are ordered antiferromagnetically to each other along the chain direction c [100]. BaCo2V2O8 is a quasi-one dimensional effective S=1/2 Ising chain where the antiferromagnetic Ising chains along c have a frustrated inter-chain arrangement leading to the presence of two magnetic domains. Below T_N = 5.4 K the system undergoes a structural distortion as confirmed by zero-field high-resolution thermal-expansion. Via magnetoelastic coupling the structural domains couple to the magnetic domains and can be manipulated by uniaxial pressure or magnetic fields along a. The expected distortion of the order of 10^(-5) could hardly be detected in high-resolution single crystal X-ray diffraction. For H||c, the Neel ordered state breaks down at H_c= 3.9 T where an incommensurate (IC) phase is entered in the low-T region [24,110]. A first order transition is confirmed by a hysteresis in the low-T magnetostriction. Thermal expansion yields very broad anomalies in the low-T region of the high-field curves and the IC-to-paramagnetic phase boundary was traced up to 17 T. For H perpendicular c, an in-plane anisotropy was found with a weak field dependence of T_N for H||[110], but a complete suppression of T_N at H^(||a} = 10 T. Kimura et al. developed a model where a magnetic field H||a is accompanied by a staggered transverse field component via a staggered g tensor [96]. The predicted breakdown of the magnetic order for H||a is evidenced in magnetostriction and the collapse of magnetic superstructure peaks in single-crystal neutron diffraction. The field-induced detwinning of the structural/magnetic domains is clearly observed in the different field hysterses of magnetic Bragg peaks being sensitive to different domains. Moreover, the magnetic field induces an increasing intensity for nuclear reflections due to a uniform ferromagnetic component parallel to to the external field. In addition, the presence of forbidden reflections and their field-dependent behavior are explained by the presence of the staggered moments giving a qualitative confirmation of the Kimura model. Different measures (thermal conductivity, heat capacity, neutron diffraction) show additional anomalies around 6 T whose origin remains unknown. The investigation of out-of-chain substitutions in the Ba(1-x)Sr(x)Co2V2O8 series and of in-chain substitutions in the BaCo1.9M0. 1V2O8 series (M= Mn, Ni, Cu, Mg) yields a preservation of the orthorhombic distortion (below T_N) and of both the Ising and in-plane anisotropy. Judging from these experimental results the magnetic structure is essentially preserved for all substitutions which was recently confirmed for SrCo2V2O8 by neutron diffraction [100]. Only indirect hints for a transition from the \neel ordered into an IC phase for H||c are given by hysteretic magnetostriction curves in the low-T regime. The Sr substitution yields a slight decline of the Ising anisotropy but an increase of the in-plane anisotropy as is reflected in the magnetic phase diagrams. These changes in the anisotropies can be understood by a systematic increase of the distortion of the CoO6 octahedra with increasing Sr content. Due to the decreasing inter-chain distance with increasing Sr content a change of J^(perp)_(NN) and J^(perp)_(NNN) is suggested in a way that the resulting frustration increases, which in turn results in a weakened effective inter-chain interaction and a weakly decreasing T_N with increasing Sr content. The substitution of, e.g., Cu and Mg yields a drastic reduction of the ordering temperature. Due to the strong suppression of T_N by Mg in contrast to, e.g., Ni the NN intra-chain interaction is suggested to be the dominant coupling mechanism of the finite chain segments.