For the past several years, the spinel vanadates, AV2O4, have been central to the study of orbital degeneracy and the complex coupling of spin, charge and lattice degrees-of-freedom in frustrated antiferromagnets. They are of interest not only to condensed matter physicists who study orbital order and frustration, but also applied physicists who seek to take of advantage of the useful multiferroic effects which often result. Systems with diamagnetic (e.g. Zn2+, Cd2+, Mg2+) and spin-only (e.g. Mn2+) cations on the A-site sublattice have been studied extensively, demonstrate multiple magnetic and structural phase transitions, and reveal ground state properties heavily influenced by V3+ orbital degrees-of-freedom. Here, I report on elastic and inelastic neutron scattering investigations of a relatively new member of the spinel family, FeV2O4. In addition to the orbital degeneracy of vanadium cations, this system has a orbital doublet degree-of-freedom on the iron sublattice. The resultant interactions lead to complex and interesting behavior, including reports of four separate structural phase transitions and the emergence of net magnetic and electric dipole moments at lowest temperatures. Our data confirm the existence of three of the four reported structural transitions, and associate the lowest two with the onset of collinear and canted ferrimagnetic structures. Through consideration of local crystal and spin symmetry and the magnetic excitation spectrum, we are able comment on the physics driving each of the reported transitions and the likely ordering of electron orbitals at all temperatures. I will discuss each of these observations in the context of the current literature on other spinel-vanadates, and lay out potentially interesting paths for future research.

Recent advances in nanolithography allow researchers to fabricate artificially tailored magnetic metamaterials. These artificial metamaterials have gathered considerable interest from high tech as well as from basic science community. Although substantial progress has been made in last 15 years or so in the fabrication and the study of different types of magnetic metamaterials, the focus has been limited to only periodic metamaterials. In this seminar, I will present our group’s recent results on novel artificial quasicrystal—Penrose P2 tiling—magnetic metamaterials. In particular, I will show how such complex artificial quasicrystals give reproducible knee anomalies in the DC magnetization—obtained using SQUID magnetometer and static micromagnetic simulations—and their possible dynamic signatures—obtained using ferromagnetic resonance experiment and simulation. Furthermore, I will also discuss occurrences of chiral loops in these metamaterials, and how artificial magnetic quasicrystals can be viewed as a candidate for artificial spin ice and our recent results on that.

Joe Straley (UK)