Elucidation of quantum magnetic dynamics in semi-classical honeycomb lattice
Authors:   , Guo Jiasen 
Published: March 2024

[EMBARGOED UNTIL 12/1/2024] The artificial spin ice systems are essentially arrays of single domain nanomagnets with specifically designed patterns, e.g. squares, honeycombs, etc. As a natural extension of the classical spin ice system found in rare earth pyrochlore compounds, artificial spin ice systems are initially designed to mimic the tetrahedral geometry in the pyrochlore lattice. The geometrically frustrated spin configuration in classical spin ice often leads to novel electric and magnetic properties, in particular, the intriguing magnetic charge physics under the dumbbell formalism, so does the artificial spin ice. In fact, artificial spin ice has been proved a more versatile platform for such studies, both of scientific and technological importance. This is largely benefited from its easily tunable design parameters, including those related to the periodic lattice as well as those related to the individual building element, the nanomagnet. Hence a plethora of artificial spin ice has been designed and explored. Conventionally, artificial spin ice systems are often prepared via electric beam lithography which yields large element size typically in the range from hundreds of nanometers to micrometers. The large element size unavoidably leads to strong dipolar inter-elemental interaction energy in the order of 104 K, thus prohibiting the redistribution of magnetic charges via moment flipping without appealing to external stimulus. However, it is the aim of this thesis to explore a newly realized artificial magnetic honeycomb lattice through a hierarchical nanofabrication process. The as-mentioned magnetic honeycomb lattice features ultra small connected elements that have typically 11 nm in length and 4 nm in width and a variable thickness. The resulting small inter-elemental interaction energy ? 40 K thus enables the rearrangement of magnetic charges in a large temperature range, hence temperature dependent magnetic phases. In this thesis, we have extensively studied the as-mentioned artificial magnetic honeycomb lattice using a combination of neutron scattering techniques, electric mea- surements as well as theoretical calculations. Specifically, we have focused on the exploration of the dynamic properties of magnetic charge defects in the as-mentioned artificial magnetic honeycomb lattice of permalloy as well as the topological con- sequence imparted onto its transport properties by the ordering of these magnetic charges. We found that the magnetic charge defects resemble quasi-particles of quan- tum mechanical nature, and persistently move around the honeycomb lattice effort- lessly, exhibiting a temperature independent relaxation rate. On the other hand, an usual quasi-oscillatory Hall anomaly was detected in the Hall probe measurements, which can be attributed to the Berry phase effect invoked by the gauge potential due to the vortex magnetism. In a separate venue, Neodymium based honeycomb systems have also been studied extensively. In a remarkable observation, we have observed planar hall effect induced by the local spin ice order in a thin Nd-Sn hon- eycomb. More recently, neutron spin echo measurements reveals dynamic behavior of magnetic origin in the Nd-based honeycomb, that is, in many ways, similar to the dynamics of the magnetic charges defects in the Py-based honeycomb, despite the rather different magnetic properties of these two systems. Further experimental and theoretical investigation in this dynamic behavior is currently going on. In addition to the studies on artificial magnetic honeycomb systems, we also high- light two complementary studies on bulk magnetic materials, namely, Cr-doped ZnTe and NiSi. In both cases, we have utilized single crystal neutron scattering methods to elucidate the magnetic structure as well as its order parameter. Specifically, The study on Cr-doped ZnTe is supplemented by detailed theoretical calculation based on the density functional theory.