NMR study on equilateral triangular lattice antiferromagnet Ba2La2CoTe2O12

Abstract

We report a 139La-NMR study of Ba2La2CoTe2O12, S = 1/2 equilateral triangular-lattice antiferromagnet with easy-plane anisotropy at low temperatures. This compound undergoes a magnetic phase transition at TN = 3.26 K into an ordered state with the 120 degree spin structure. Under magnetic fields above 3T, TN splits into TN1 and TN2, which correspond to the transitions from the paramagnetic phase to the up-up-down (uud) phase and from the uud phase to the triangular coplanar phase, respectively. The NMR spin-lattice relaxation rate 1/T1 exhibits a critical divergence at TN1, indicating the onset of long-range magnetic order. At TN2, the NMR-linewidth measured at 5.4 T exhibits an anomalous decrease, which we attribute to a change in the spin structure from the uud to the triangular coplanar phase.

Figures (2)

• Figure 1: (Left) NMR shift and linewidth versus magnetic susceptibility in the paramagnetic region 10 - 50 K. The dashed lines show the isotropic and anisotropic part of hyperfine coupling constans $A_{\rm iso} = -660$ and $3A_{\rm an} = +190$ (Oe/$\mu_{\rm B}$), respectively. Insets show typical spectra at low temperatures. The $eqq$ interaction parameter was obtained from the distances between the singular points, to be $^{139}\nu_{\rm Q} =$ 1.43 MHz. (Top Right) Schematic of the local structure around the NMR site La. (Bottom Right) Spin-echo amplitude versus time span between the saturating pulse train and the echo-measurement pulses. Data for each temperature is shifted vertically. Solid curves show the theoretical recovery function for $I = 7/2$ nuclei shown in the text.
• Figure 2: (left) B-T phase diagram for the magnetic field parallel with the triangular plane, expected from specific heat and magnetization measurements. The error bars of specific heat come from the fact that observed peaks at high fields are broad BLCTO_Kojima. The position of the one-third plateau experimentally determined is shown by the vertical bar BLCTO_Kojima. Gray solid curves show the expected phase boundaries between the paramagnetic phase and uud ($T_{\rm N1}$) and between the uud and the triangular coplanar phase $T_{\rm N2}$. Thick solid and dashed arrows show the schematic spin structures and the dirrections of applied fields for each phases. Horizontal dotted arrows show the experimental conditions of temperature range and of magnetic fields. (right) Temperature dependence of $^{139}$La-NMR $1/T_1$ and linewidth measured under the magnetic fields at around 5.4 and 7.5 T. Vertical arrows show maxima of each quantities.