Magnetic phases of the frustrated spin dimer compound Ba3Mn2O8 [electronic resource]
Keywords: Applied Physics

Spin dimer compounds are based on pairs of spins with antiferromagnetic exchange. At low fields the ground state is a product of singlets, with excited triplet states at higher energies. Application of a magnetic field closes the spin gap between the excited triplet state and singlet state. Interactions between dimers broaden the triplet bands, such that above a critical field where the minimum of the triplet band crosses the singlet, long range magnetic order (LRMO) can arise. The ordered states can take several novel forms, including a spin superlattice or a Bose-Einstein condensate of magnons, depending upon the spin Hamiltonian describing the system. Ba3Mn2O8 is a spin dimer system based on dimers of S = 1, 3d2, Mn5+ ions arranged on a triangular lattice. A pair of antiferromagnetically linked S = 1 ions has total spin 0, 1 or 2, leading to, in zero field, excited quintuplet states in addition to the excited triplet states above the singlet ground state. The triangular lattice is composed of vertical dimers on hexagonal layers which are stacked according to an 'ABC' structure. In this thesis, I describe the results of experiments which probed this system via different thermodynamic measurements of single crystals, revealing at least three novel ordered states. Measurements of heat capacity, magnetocaloric effect, torque magnetometry and magnetostriction revealed significant anisotropy in the singlet-triplet regime, with a single ordered state observed for fields along the easy c axis and two states observed for fields away from that direction. Analysis of the minimal spin Hamiltonian yields candidate phases for the canted antiferromagnetic order observed, including incommensurate order close to the archetypal 120° order for triangular systems as well as modulated order for fields away from the c axis. The triplet-quintuplet regime was probed via heat capacity, magnetocaloric effect and magnetization measurements, the first experiments to probe such ordered states of a spin dimer compound. A significant asymmetry in the quintuplet condensate was revealed in both the magnetization and the phase boundary. This asymmetry is understood as a consequence of zero point phase fluctuations, which are absent at the saturation field but present everywhere else. Finally, the effect of disorder in this spin dimer compound was studied by substitution of non-magnetic S = 0 3d0, V5+ ions for the S = 1, 3d2, Mn5+ ions in Ba3(Mn1−xVx)2O8. This work was motivated in part by theoretical predictions that substitution of non-magnetic species on a square lattice of dimers would result in a low-temperature ordered magnetic state, for which interactions between the unpaired magnetic moments is mediated by short range correlations of the background singlet ground state. We do not find any evidence for such a state down to 50 mK. Rather, the magnetic entropy is progressively removed over an extended range of temperature, from [approximately equal to]2 K down. The temperature and doping dependence of the heat capacity do not conform to expectations for a spin glass, leading us to suggest that Ba3(Mn1−xVx)2O8 manifests a random singlet state for the range of compositions and temperatures studied.