Field-Induced Magnetic Order and Simultaneous Lattice Deformation in ${\mathrm{TlCuCl}}_3$

We report the results of Cu and Cl nuclear magnetic resonance experiments and thermal expansion measurements in magnetic fields in the coupled dimer spin system ${\mathrm{TlCuCl}}_3$. 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.

Figure 1

The crystal structure of ${\mathrm{TlCuCl}}_3$ viewed along the $a$ axis.

Figure 2

Temperature dependencies of NMR frequency shifts at $H=8.56\mathrm{T}$. (a) Central transition of ${}^{63}\mathrm{Cu}$. (b) High-frequency satellite of ${}^{65}\mathrm{Cl}$ (closed squares, right axis) and low-frequency satellite of ${}^{37}\mathrm{Cl}$ (open squares, left axis).

Figure 3

(a) Uniform frequency shift for the ${}^{63}\mathrm{Cu}$ central transition. The line is a guide to the eye. (b) The staggered hyperfine field obtained from the splitting of the ${}^{63}\mathrm{Cu}$ central line. At 5.5 K, the spectrum consists of unsplit (${}^{63}h_{\mathrm{s}}=0$) and split (finite ${}^{63}h_{\mathrm{s}}$) lines. (c) Evolution of the high-frequency satellite ${}^{35}\mathrm{Cl}$ NMR spectrum across the phase boundary. All data were taken at $H=8.56\mathrm{T}$.

Figure 4

(a) First-order quadrupole shift for ${}^{35}\mathrm{Cl}$ extracted with Eq. (3) from the data in Fig. 2b. (b) $b$-axis thermal expansion in different magnetic fields. The inset shows the $b$-axis strain vs magnetic field at 2 K.

Figure 5

Temperature dependence of ${}^{63}\mathrm{Cu}$ spin-lattice relaxation rate at $H=5.2\mathrm{T}$ (squares), 4.8 T (crosses), and 8.5 T (circles). Inset: field dependence of the relaxation rate at 1.5 K.