Correlation Between Magnetism, Structure and Growth for Ultrathin Fe Films on Cu(100)

Abstract

To obtain a comprehensive understanding of the correlation between the magnetism and structure of materials has been a research goal of scientists for a long time. Early in the 1930s, Bethe and Slater found a phenomenological relationship between the direct exchange interaction and the atomic separation [1, 2] (Fig. 6.1). This is the well known Bethe-Slater curve. Recently, the correlation between the structure and the magnetism for 3d metals has been investigated with advanced self-consistent energy-band calculations [4, 5, 6]. These calculations show that the magnetic properties of a 3d metal are closely related to the atomic volume. Normally magnetic transition metals will lose their magnetic moment at a compressed volume and, on the other hand, nonmagnetic transition metals will become ferromagnetic at an expanded volume. Thus a transition from the nonmagnetic to a magnetic state is expected for all 3d transition metals when the atomic volume is increased. In the limit of large volume, the magnetic moment approaches the value determined by Hund’s rule for the free-atom configuration. Three types of transition behavior, classified as type I, type II and type III depending on the number (one, two and three, respectively) of critical points, have been predicted (see Fig. 6.2). Bcc Sc, Ti, Fe, Co and Ni have been predicted to show a type I transition in which the termination of the nonmagnetic behavior is followed by a ferromagnetic phase. This is a second-order transition, since the magnetic moment changes continuously. If the calculations take only the nonmagnetic and ferromagnetic states into account, bcc Cr is found to undergo a first-order type II transition, which is characterized by a discontinuous jump in magnetic moment at the transition from the nonmagnetic to the ferromagnetic state. Bcc V and Mn, which have a type III transition, exhibit a second-order transition from the nonmagnetic state to a low-spin state, followed by a first-order transition from a low-spin to a high-spin state at larger atomic volume. The type of transition is determined by the details of the nonmagnetic density of states and the location of the Fermi level. In general, if the Fermi level falls in a deep minimum of the nonmagnetic DOS, the system will undergo a first-order transition. On the other hand, if the Fermi level is located at a peak in the nonmagnetic DOS, the system will undergo a second-order transition. Considering the diversity in the structure of the DOS, the transition could be more complicated than that shown in Fig. 6.2. A typical example is provided by fcc iron.