Ordered and disordered variants of the triangular lattice antiferromagnet Ca3NiNb2O9 : Crystal growth and magnetic properties

Abstract

Single crystals of the triangular lattice antiferromagnet (TLAF) ${\mathrm{Ca}}_{3}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{9}$ and its nonmagnetic analog ${\mathrm{Ca}}_{3}{\mathrm{MgNb}}_{2}{\mathrm{O}}_{9}$ are grown using the four-mirror optical floating-zone furnace. By varying the growth conditions, two types of mm-sized crystals are obtained: monoclinic crystals of the form ${\mathrm{Ca}}_{3}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{9}$ where ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Nb}}^{5+}$ exhibits 1:2 ordering and Ni ions (spin 1) are arranged on a triangular lattice, and orthorhombic crystals of the form ${\mathrm{CaBO}}_{3}$ where B site is occupied randomly by 1/3 Ni and 2/3 Nb. Low-temperature specific heat and low-field magnetization on these crystal modifications revealed contrasting ground states. In B-site ordered ${\mathrm{Ca}}_{3}{\mathrm{NiNb}}_{2}{\mathrm{O}}_{9}$, the Ni spins undergo two successive phase transitions at ${\mathrm{T}}_{{\mathrm{N}}_{1}}=4.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${\mathrm{T}}_{{\mathrm{N}}_{2}}=4.2\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The magnetic propagation vector below ${\mathrm{T}}_{{\mathrm{N}}_{2}}$ is found to be $\stackrel{P\vec}{k}=(0,1/3,0)$ confirming the ${120}^{\ensuremath{\circ}}$ spin structure. The high-field magnetization reveals the presence of plateaus characteristic of TLAFs. In contrast, the orthorhombic ${\mathrm{CaBO}}_{3}$ crystals exhibit a spin-glass-like ground state.