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

Single crystals of the triangular lattice antiferromagnet (TLAF) Ca3NiNb2O9 and its nonmagnetic analog Ca3MgNb2O9 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 Ca3NiNb2O9 where Ni2+ and Nb5+ exhibits 1:2 ordering and Ni ions (spin 1) are arranged on a triangular lattice, and orthorhombic crystals of the form CaBO3 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 Ca3NiNb2O9, the Ni spins undergo two successive phase transitions at T-N1 = 4.6 K and T-N2 = 4.2 K. The magnetic propagation vector below T-N2 is found to be (k) over right arrow= (0, 1/3, 0) confirming the 120 degrees spin structure. The high-field magnetization reveals the presence of plateaus characteristic of TLAFs. In contrast, the orthorhombic CaBO3 crystals exhibit a spin-glass-like ground state.