The spin-1/2 coupled tetramer system Ba(TiO)Cu${}_4$(PO${}_4$)${}_4$ probed by magnetization, specific heat, and ${}^{31}$P-NMR

Abstract

We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline Ba(TiO)Cu${}_4$(PO${}_4$)${}_4$ (BTCPO) via x-ray diffraction, magnetic susceptibility, heat capacity, and ${}^{31}$P Nuclear Magnetic Resonance (NMR) measurements. BTCPO has a 2D layered structure with interlinked Cu${}_4$O${}_{12}$ tetramer units. A broad maximum is observed around 16.5 K in our magnetization data accompanied by a sharp anomaly around $T$ = 9.5 K in the heat capacity. An anomaly at $T$ = 9.5 K is also found in the temperature dependence of the ${}^{31}$P NMR spin-lattice relaxation rate $1/T_1$. A power law behavior for the heat capacity as well as for the ${}^{31}$P $1/T_1$ below the ordering temperature could be obtained. The ${}^{31}$P NMR lineshape is asymmetric and the NMR shift tracks the bulk spin-susceptibility. We estimated the isotropic and axial components of the hyperfine coupling tensor to be as the $A_{hf}^{iso}\backsimeq 6794$ $Oe/{\mu }_{\mathrm{B}}$ and $A_{hf}^{ax}\backsimeq 818$ $Oe/{\mu }_{\mathrm{B}}$, respectively.

8 pages, 3 tables, 13 figures