Geometrically Frustrated Magnetism

This investigation was centred on exploring the question: Do lanthanide borates offer the prospect of an alternative to gadolinium gallium garnet in respect to the magneto caloric effect? Two different forms of lanthanide borates were tested for the probability of their success in order to achieve this: GdDyBO3 and GdTbBO3, which had similar properties, yet produced various results. Through the use of high temperature synthesis, the lanthanide borates are produced, and then, using an x-ray diffraction, analysed. Overall, GdDyBO3 had a much higher probability of being able to reproduce the magneto caloric effect due to it having much larger radii of Ln3+ ions relative to its cell volume, but further testing is required for the exact measurements of its magnetic properties.

We report the bulk magnetic properties of a yet unexplored vanadium-based multi-valenced
spinel system, Zn3V3O8. A Curie–Weiss fit of our dc magnetic susceptibility χ(T ) data
in the temperature region of 140–300K yields a Curie constant C = 0.75 cm3Kmole−1 V−1,
θCW = −370 K. We have observed a splitting between the zero field cooled (ZFC) and
field cooled (FC) susceptibility curves below a temperature Tirr of about 6.3K. The value of
the ‘frustration parameter’ ( |θCW|
TN
∼ 100) suggests that the system is strongly frustrated. From
the ac susceptibility measurements we find a logarithmic variation of freezing temperature (Tf )
with frequency ν attesting to the formation of a spin glass below Tf . However, the value of the
characteristic frequency obtained from the Vogel–Fulcher fit suggests that the ground state is
closer to a cluster glass rather than a conventional spin glass. We explored further consequences
of the spin glass behavior and observed aging phenomena and memory effect (both in ZFC and
FC).We found that a positive temperature cycle erases the memory, as predicted by the hierarchical
model. From the heat capacityCP data, a hump-like anomalywas observed inCP /T at about
3.75 K. Below this temperature the magnetic heat capacity shows a nearly linear dependence
with T which is consistent with the formation of a spin glass state below Tf in Zn3V3O8.

The search for quantum spin liquids (QSL) -- topological magnets with fractionalized excitations -- has been a central theme in condensed matter and materials physics. While theories are no longer in short supply, tracking down materials has turned out to be remarkably tricky, in large part because of the difficulty to diagnose experimentally a state with only topological, rather than conventional, forms of order. Pyrochlore systems have proven particularly promising, hosting a classical Coulomb phase in the spin ices Dy/Ho2Ti2O7, with subsequent proposals of candidate QSLs in other pyrochlores. Connecting experiment with detailed theory exhibiting a robust QSL has remained a central challenge. Here, focusing on the strongly spin-orbit coupled effective S=1/2 pyrochlore Ce2Zr2O7, we analyse recent thermodynamic and neutron scattering experiments, to identify a microscopic effective Hamiltonian through a combination of finite temperature Lanczos, Monte Carlo and analytical spin dynamics calculations. Its parameter values suggest a previously unobserved exotic phase, a π-flux U(1) QSL. Intriguingly, the octupolar nature of the moments makes them less prone to be affected by crystal imperfections or magnetic impurities, while also hiding some otherwise characteristic signatures from neutrons, making this QSL arguably more stable than its more conventional counterparts.

We present a magnetic phase diagram of rare-earth pyrochlore Yb2Ti2O7 in a ⟨111⟩ magnetic field. Using heat capacity, magnetization, and neutron scattering data, we show an unusual field-dependence of a first-order phase boundary, wherein a small applied field increases the ordering temperature. The zero-field ground state has ferromagnetic domains, while the spins polarize along ⟨111⟩ above 0.65T. A classical Monte Carlo analysis of published Hamiltonians does account for the critical field in the low T limit. However, this analysis fails to account for the lobed phase diagram, suggesting that either long-range interactions or quantum fluctuations govern low field properties.