Lodestone

Abstract: Magnetism has fascinated and served humanity for almost 3000 years. Since the discovery of the lodestone (FeO-Fe_2O_3), many different magnetic materials have been developed, almost all based on transition metals and/or rare-earth elements. Technological application of magnetism also has a long history, from the compass to today's sophisticated magnetic memory systems. In contrast, the theory of magnetism has progressed more slowly, despite the efforts of great minds throughout history. The reason early theoretical models were not very valuable is now clear. Any viable theory of magnetism must be based on two inherently quantum mechanical concepts: electron spin and the Pauli exclusion principle. As such, only the 20th century has produced a competent model for magnetism, and that model continues to evolve. Studies over the last 20 years have revealed a bewildering array of new magnetic phenomena that continue to challenge our understanding of solid-state physics.

Abstract: To the man-in-the-street, matter is often thought to be either magnetic or non-magnetic. An ordinary magnet attracts magnetic material, e.g. iron filings, pins, lodestone, whereas non-magnetic material, e.g. wood, chalk, is not attracted to the magnet. In fact, all materials show some reaction to a magnetic field though in the case of conventionally ‘non-magnetic’ materials the reaction will be very weak. A powerful electromagnet and sensitive measuring instrument are needed to demonstrate these weak reactions.

Abstract: The discovery of magnetism by the ancient Greeks was enabled by the natural occurrence of lodestone -- a magnetized version of the mineral magnetite. Nowadays, natural minerals continue to inspire the search for novel magnetic materials with quantum-critical behavior or exotic ground states such as spin liquids. The recent surge of interest in magnetic frustration and quantum magnetism was largely encouraged by crystalline structures of natural minerals realizing pyrochlore, kagome, or triangular arrangements of magnetic ions. As a result, names like azurite, jarosite, volborthite, and others, which were barely known beyond the mineralogical community a few decades ago, found their way into cutting-edge research in solid-state physics. In some cases, the structures of natural minerals are too complex to be synthesized artificially in a chemistry lab, especially in single-crystalline form, and there is a growing number of examples demonstrating the potential of natural specimens for experimental investigations in the field of quantum magnetism. On many other occasions, minerals may guide chemists in the synthesis of novel compounds with unusual magnetic properties. The present review attempts to embrace this quickly emerging interdisciplinary field that bridges mineralogy with low-temperature condensed-matter physics and quantum chemistry.