Maynooth Maths.Physics seminars

Over the past few years research in magnetism has seen significant progress in the understanding of low-dimensional systems and in the production of artificial nanostructures. The description of such systems refutes many of the traditional textbook concepts which are based on linear approaches such as spin wave theory. Instead, the nonlinearity of the magnetization field plays an important role for various nanoscale phenomena. Moreover, magnetism at the nanoscale forces us to understand the delicate transition between classical and quantum descriptions.

I will discuss how solitons govern the onset of superparamagnetism in nanostructures and are responsible for the extremely low coercivity of nanostructured magnetic materials, results that are important for applications such as data storage. Then I will show how geometric quantization using Berry's phase offers a straightforward description of systems such as magnetic molecules, excitations of dipolar vortex lattices and the semiclassical interpretation of spinon-type excitations in quasi 1D magnets. In particular, I discuss our recent neutron experiments that demonstrate that solitons carry a spin-current in quantum antiferromagnets of half-integer spin [1]. These experiments provide the first example of the spontaneous emergence of spin currents and hidden chiral order that accompany the disappearance of antiferromagnetic order, a scheme believed to lie at the heart of the enigmatic normal state of cuprate superconductors.

[1] H.B. Braun, J. Kulda, P. Böni, B. Roessli, D. Visser, K. Krämer & H.U. Güdel, ``Emergence of soliton chirality in a quantum antiferromagnet", Nature Physics 1, 159 (2005).