Nanoworld of terahertz magnetism opening new frontiers.

The quest to understand magnetic phenomena at ever-smaller length scales and faster time scales has long driven magnetism research, yielding numerous breakthroughs from the Nobel Prize-winning discovery of giant magnetoresistance to advanced applications. Understanding (sub)picosecond (THz) spin dynamics at the nanoscale has further motivated the development of unique instrumentation for nanofabrication and scanning probe microscopy, as well as multi-billion-dollar multinational projects such as X-ray free-electron lasers. In this perspective, we describe fundamental principles for exciting and detecting coherent spin-wave excitations-magnons-at THz frequencies and nanometer wavelengths without relying on X-rays or nanopatterning techniques. Our idea is to show that there is a practical approach to open up new avenues for exploring ultrafast magnetism at the nanoscale using a conventional femtosecond laser source, accessible in virtually any laboratory.