Magnetic transitions at low temperatures

Abstract

INTRODUCTION In many crystalline substances there exists a pattern of electronic energy levels such that in experimentally accessible range of temperature it is possible to pass from a low-temperature, completely ordered, form of essentially zero entropy to a higher temperature form where there is a nearly random distribution among several electronic states per molecular unit. There is, therefore, in higher temperature form a substantial electronic contribution to entropy of crystal, of magnitude R In n per mole, where n is number of electronic states per molecular unit and R is gas constant. Such a pattern of electronic energy levels is found in substances which chemist would describe as having unpaired electrons and, in particular, in ionic crystals containing elements with partially filled d or f electronic shells. Because of magnetic moment accompanying spin and orbital angular momenta of electrons, these substances are, in higher te:mperature region, paramagnetic. The changes from completely ordered low temperature form to a high temperature form where there is a random distribution among several electronic states may be called a magnetic transition. Such transitions always occur over an extended range of temperature and increase in entropy of electronic system (we shall for short call this the magnetic entropy) is never found to occur discontinuously at a single temperature as happens in more familiar first order transitions associated with melting, vaporization or change from one crystal form to another. In this paper we shall choose for more detailed discussion some examples of magnetic transitions in salts of divalent ions of manganese, iron, cobalt and nickel. No attempt will be made at a comprehensive survey of considerable amount of experimental and theoretical information concerning magnetic transitions in ionic crystals which has become available in recent years, nor will more complicated phenomena of ferromagnetism and antiferromagnetism in metals be discussed. There are several excellent recent review articles on antiferromagnetism 1-4 and ferromagnetism, The examples, chosen from among substances in which writer and his collaborators have an experimental interest, will be typical of many magnetic salts at low temperatures. The main emphasis in this paper will be on thermal properties rather than many interesting phenomena disclosed by measurements of magnetic susceptibility, of magnetic structure by neutron diffraction, and electronic and nuclear resonance measurements.