Field-Induced Magnetic Order and Simultaneous Lattice Deformation in TlCuCl3

We report the results of Cu and Cl nuclear magnetic resonance experiments (NMR) and thermal expansion measurements in magnetic fields in the coupled dimer spin system TlCuCl3. We found that the field-induced antiferromagnetic transition as confirmed by the splitting of NMR lines is slightly discontinuous. The abrupt change of the electric field gradient at the Cl sites, as well as the sizable change of the lattice constants, across the phase boundary indicate that the magnetic order is accompanied by simultaneous lattice deformation.Comment: 4 pages, 5 figure

Microscopic model for the magnetization plateaus in NH4CuCl3

A simple model consisting of three distinct dimer sublattices is proposed to describe the magnetism of NH4CuCl3. It explains the occurrence of magnetization plateaus only at 1/4 and 3/4 of the saturation magnetization. The field dependence of the excitation modes observed by ESR measurements is also explained by the model. The model predicts that the magnetization plateaus should disappear under high pressure.Comment: 4 pages, 5 figures, REVTeX

Magnon Dispersion in the Field-Induced Magnetically Ordered Phase of TlCuCl3

The magnetic properties of the interacting dimer system TlCuCl3 are investigated within a bond-operator formulation. The observed field-induced staggered magnetic order perpendicular to the field is described as a Bose condensation of magnons which are linear combinations of dimer singlet and triplet modes. This technique accounts for the magnetization curve and for the field dependence of the magnon dispersion curves observed by high-field neutron scattering measurements.Comment: 4 pages, 4 figures, REVTeX

Observation of Field-Induced Transverse N\'{e}el Ordering in the Spin Gap System TlCuCl3_3

Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl$_3$ in magnetic fields parallel to the $b$-axis. The magnetic Bragg peaks which indicate the field-induced N\'{e}el ordering were observed for magnetic field higher than the gap field $H_{\rm g}\approx 5.5$ T at $Q=(h, 0, l)$ with odd $l$ in the $a^*-c^*$ plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced N\'{e}el ordering as a Bose-Einstein condensation of magnons.Comment: 4 pages, 5 eps figures, jpsj styl

Doping Induced Magnetization Plateaus

The low temperature magnetization process of antiferromagnetic spin-S chains doped with mobile spin-(S-1/2) carriers is studied in an exactly solvable model. For sufficiently high magnetic fields the system is in a metallic phase with a finite gap for magnetic excitations. In this phase which exists for a large range of carrier concentrations x the zero temperature magnetization is determined by x alone. This leads to plateaus in the magnetization curve at a tunable fraction of the saturation magnetization. The critical behaviour at the edges of these plateaus is studied in detail.Comment: RevTeX, 4 pp. incl. 3 figure

Theoretical analysis of the experiments on the double-spin-chain compound -- KCuCl3_3

We have analyzed the experimental susceptibility data of KCuCl$_3$ and found that the data are well-explained by the double-spin-chain models with strong antiferromagnetic dimerization. Large quantum Monte Carlo calculations were performed for the first time in the spin systems with frustration. This was made possible by removing the negative-sign problem with the use of the dimer basis that has the spin-reversal symmetry. The numerical data agree with the experimental data within 1% relative errors in the whole temperature region. We also present a theoretical estimate for the dispersion relation and compare it with the recent neutron-scattering experiment. Finally, the magnitude of each interaction bond is predicted.Comment: 4 pages, REVTeX, 5 figures in eps-file

Magnetization plateaus in antiferromagnetic-(ferromagnetic)_{n} polymerized S=1/2 XXZ chains

The plateau-non-plateau transition in the antiferromagnetic-(ferromagnetic)$_{n}$ polymerized $S=1/2$ XXZ chains under the magnetic field is investigated. The universality class of this transition belongs to the Brezinskii-Kosterlitz-Thouless (BKT) type. The critical points are determined by level spectroscopy analysis of the numerical diagonalization data for $4 \leq p \leq 13$ where $p(\equiv n+1)$ is the size of a unit cell. It is found that the critical strength of ferromagnetic coupling decreases with $p$ for small $p$ but increases for larger enough $p$. It is also found that the plateau for large $p$ is wide enough for moderate values of exchange coupling so that it should be easily observed experimentally. This is in contrast to the plateaus for $p = 3$ chains which are narrow for a wide range of exchange coupling even away from the critical point

The Origin of Magnetic Interactions in Ca3Co2O6

We investigate the microscopic origin of the ferromagnetic and antiferromagnetic spin exchange couplings in the quasi one-dimensional cobalt compound Ca3Co2O6. In particular, we establish a local model which stabilizes a ferromagnetic alignment of the S=2 spins on the cobalt sites with trigonal prismatic symmetry, for a sufficiently strong Hund's rule coupling on the cobalt ions. The exchange is mediated through a S=0 cobalt ion at the octahedral sites of the chain structure. We present a strong coupling evaluation of the Heisenberg coupling between the S=2 Co spins on a separate chain. The chains are coupled antiferromagnetically through super-superexchange via short O-O bonds.Comment: 5 Pages, 3 Figures; added anisotropy term in eq. 9; extended discussion of phase transitio

Pressure-Induced Magnetic Quantum Phase Transitions from Gapped Ground State in TlCuCl3

Magnetization maesurements under hydrostatic pressure were performed on an S=1/2 coupled spin system TlCuCl3 with a gapped ground state under magnetic field H parallel to the [2,0,1] direction. With increasing applied pressure P, the gap decreases and closes completely at Pc=0.42 kbar. For P>Pc, TlCuCl3 undergoes antiferromagnetic ordering. A spin-flop transition was observed at Hsf=0.7T. The spin-flop field is approximately independent of pressure, although the sublattice magnetization increases with pressure. The gap and Neel temperature are presented as function is attributed to to the relative enhancement of the interdimer exchange interactions compared with the intradimer exchange interaction.Comment: 4pages,3figures To be published in J. Phys. Soc. Jpn. Vol.73 No.1

Universal scaling at field-induced magnetic phase transitions

We study field-induced magnetic order in cubic lattices of dimers with antiferromagnetic Heisenberg interactions. The thermal critical exponents at the quantum phase transition from a spin liquid to a magnetically ordered phase are determined from Stochastic Series Expansion Quantum Monte Carlo simulations. These exponents are independent of the interdimer coupling ratios, and converge to the value obtained by considering the transition as a Bose-Einstein condensation of magnons, alpha_(BEC) = 1.5. The scaling results are of direct relevance to the spin-dimer systems TlCuCl_3 and KCuCl_3, and explain the broad range of exponents reported for field-induced ordering transitions.Comment: 4 pages, 4 eps-figure