Seminarium Fizyki Ciała Stałego

Geometrical frustration provides a fundamental route to magnetic states that cannot be understood in terms of conventional, independently interacting magnetic moments. Instead, competing interactions and local constraints can suppress conventional magnetic order and give rise to highly degenerate states, unconventional excitations, and emergent phenomena. Rare-earth pyrochlores offer a particularly rich platform for exploring these effects, owing to their intrinsically frustrated lattice geometry and the strong interplay between magnetic anisotropy, exchange interactions, and spin-orbit coupling.The lecture will introduce the basic concepts of frustrated magnetism and illustrate how local magnetic interactions and crystal-field-induced anisotropies determine collective magnetic behaviour. The discussion will then focus on rare-earth pyrochlores and, in particular, on the A2Ir2O7 family, where magnetic 4f rare-earth ions coexist with strongly spin-orbit-coupled 5d Ir ions. This combination gives rise to competing interactions between the two magnetic sublattices and a remarkably broad range of magnetic and electronic phenomena.Particular attention will be given to all-in-all-out order, spin-ice physics, the role of f-d exchange and anisotropy, and the emergence of unconventional magnetic excitations. Recent theoretical proposals for monopole-like excitations associated with spin-ice defects and antiferromagnetic domain walls will be discussed, including their possible electric dipole moments and implications for magnetoelectric phenomena.The lecture will draw on a comprehensive analysis of 227 rare-earth iridate pyrochlores and on experimental studies using magnetic, thermodynamic, neutron-scattering, synchrotron, and transport techniques. The aim is to illustrate how combining microscopic experimental probes with theoretical concepts can reveal general principles governing emergent magnetic states in frustrated quantum materials.