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Abstract

Low-dimensional spin systems constitute an important class of quantum materials in which
reduced dimensionality and lattice geometry strongly influence magnetic interactions.
From a materials science perspective, they provide model compounds for linking crystal
structure, chemical composition, and emergent magnetic behavior driven by quantum
effects in systems with reduced dimensionality. Despite the existence of several theoretical
models, experimentally realized materials that closely match them remain scarce. To bridge
the gap between theoretical predictions and their experimental implementation in twodimensional
systems, this dissertation investigates chemical doping of prototypical Shastry-
Sutherland compound, SrCu2(BO3)2, as well as single-crystal growth of rare-earth
heptatantalates, RETa7O19, with a perfect triangular lattice. This work focuses on the
synthesis, structural characterization, and magnetic behavior of these systems.
Chemical doping of SrCu2(BO3)2 has been predicted to drive the system towards exotic
magnetic states, such as superconductivity and quantum spin-liquid states. The first part
of the dissertation experimentally investigates the effects of chemical substitution on the
structure and magnetic properties of this system. A-site charge doping with M = La or Na
is studied up to nominal x = 0.15 in Sr1−xMxCu2(BO3)2, while B-site nonmagnetic doping,
i. e., magnetic dilution, with Mg explores nominal concentrations up to x = 0.10 in
SrCu2−xMgx(BO3)2. The incorporation of dopants into the parent structure is confirmed for
La- and Mg-doping, although at lower levels compared to nominal concentrations, whereas
Na likely incorporates only in trace amounts. All dopants induce spin-dimer breaking, with
the strongest effect observed for Mg-induced magnetic dilution. Both La and Mg
substitution significantly reduce the effective spin gap, with the reduction being more
pronounced for Mg. This reduction is accompanied by the development of
antiferromagnetic correlations between dimer-free Cu2+ spins. In particular, Mg-doping
gives rise to a distinct superstructure, evidenced by emergence of a high-field magnetic
anomaly and the suppression of the plateau at 1/8 of the saturation magnetization.
The second part of the dissertation focuses on optimization of a flux method to grow
high quality single crystals of NdTa7O19, the first Ising quantum spin liquid candidate on a
perfect triangular lattice. With lateral sizes up to 3.5 mm and thicknesses up to 2 mm, the
resulting crystals are the largest reported to date, allowing a precise characterization of the
magnetic ground state. The method is further refined to grow three additional
representatives of the rare-earth heptatantalate series. Two of them, ErTa7O19 and
DyTa7O19, exhibit similar magnetic anisotropy to that of NdTa7O19, potentially hosting a
disordered ground state, while GdTa7O19 is paramagnetic. The method could be applied to
further representatives of the series.
Overall, the dissertation advances the field of two-dimensional magnetism in two
complementary directions. First, the systematic study of SrCu2(BO3)2 doping elucidates
the intrinsic limitations and effects of chemical substitution on the parent lattice at both
atomic and macroscopic scales. Second, the flux growth of rare-earth heptatantalates
provides a new platform for exploring exotic magnetic phenomena in ideal triangular-lattice
materials, including potential quantum spin-liquid states.