Mesoscopic magnetism and superconductivity

© 2015 Materials Research Society MRS BULLETIN • VOLUME 40 • NOVEMBER 2015 • www.mrs.org/bulletin Introduction Magnetism and superconductivity provide excellent foundations for the development of novel ideas in materials physics and serve as examples for new mesoscopic science. Magnetic and superconducting materials have a very interesting admixture of short and long length scales. The short, nanoscale (atomic, <1 nm) length scales, govern the ordering into specifi c crystalline structures and produce complex atomic-scale electronic interactions. It is probably safe to state that the appearance of superconductivity and magnetism in specifi c materials systems is often not well understood, and predicting specifi c, characteristic parameters such as the ordering temperatures, is even harder. Typical length scales that govern the appearance of superconductivity are given by the atomic arrangements of elements within a unit cell or the exchange length in a magnetic material. On the other hand, many superconducting and magnetic phenomena are governed by longer, mesoscopic (often 10 to 1000 times longer than interatomic distances) length scales. At these scales, there are well-defi ned predictions in agreement with experimental observations. Typical long length scales that appear in superconductivity are the coherence length and penetration depth, and in magnetic materials, the dipolar and Ruderman–Kittel–Kasuya– Yosida length scales, all of which are longer than interatomic distances and independent of detailed atomic arrangements. 1 Figure 1 shows a comparison of length scales and structural characterization techniques. 2 It is important to note that at the nanoscale (<1 nm), structural characterization techniques have diffi culties in providing high-accuracy quantitative structural or chemical information. Whereas at longer, mesoscopic (>1 nm) length scales, there are many structural and chemical tools that provide quantitative measurements. At the mesoscale, structural and chemical issues are much less pertinent to the understanding of the origin of unusual phenomena encountered. Long magnetic and superconducting length scales have been known to produce mesoscopic phenomena as described in this article, see for example, Table I . 3 – 17 The full complexity and current research opportunities of mesoscale phenomena in magnetism 18 can be illustrated by the behavior of spin waves (“magnons”) in magnetic fi lms. These fundamental excitations are due to the precession of the electron spin around an effective magnetic fi eld, which may include internal fi elds, such as anisotropy fi elds, as well as externally applied fi elds. Besides the interaction with the effective magnetic fi eld, the energy of the collective excitation is determined by the mutual interactions between spins, which includes short-scale exchange interactions, and mesoscopic-scale dipolar interactions. These dipolar interactions may stabilize inhomogeneous magnetic Mesoscopic magnetism and superconductivity