Novel Ordering of an $S=1/2$ Quasi-1d Ising-Like Antiferromagnet in Magnetic Field

High-field specific heat measurements on ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$, which is a good realization of an $S=1/2$ quasi-one-dimensional (1D) Ising-like antifferomagnet, have been performed in magnetic fields up to 12 T along the chain and at temperature down to 200 mK. We have found a new magnetic ordered state in the field-induced phase above $H_c\simeq 3.9\mathrm{T}$. We suggest that a novel type of the incommensurate order, which is caused by the quantum effect inherent in the $S=1/2$ quasi-1D Ising-like antiferromagnet, appears in the field-induced phase.

Figure 1

$ϵ\mathrm{\text{−}}h$ phase diagram for 1D $S=1/2$ antiferromagnetic $XXZ$ model. Black and gray lines show the transition field and the saturation field, respectively. Dashed line shows the field at which the crossover of the TL components occurs.

Figure 2

Magnetization curve of ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ and the calculated TL exponents. Solid and gray dashed lines are the experimental and theoretical magnetization curves, respectively. The Van Vleck paramagnetic contribution is subtracted from the experimental magnetization curve. Dotted and dot-dashed lines shows the TL exponents ${\eta }_x$ and ${\eta }_z$, respectively.

Figure 3

Temperature dependence of the specific heat of ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ for $H\parallel c$. Each specific heat shifts up by $400\mathrm{mJ}/\mathrm{K}\mathrm{mol}$ with decreasing field from 12 T. Inset shows an extended figure of the specific heat observed at 9 T. Open circles are the experimental data and dashed line is a guide for the eye.

Figure 4

Phase diagram of ${\mathrm{BaCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ for $H\parallel c$. Closed circles show the transition temperatures determined from the heat capacity measurements. Diamonds and triangles show the transition fields determined from the magnetization curves and magnetocarolic effects, respectively. Solid curves are guides for the eye. Inset shows the results of the magnetocarolic effect measurements. Gray and black curves show the experimental data, observed in the field-ascending and descending processes, respectively.