Classification of magnetic materials

Magnetic materials are those materials that can be attracted or repelled b)f a magnet and be magnetized themselves. The magnetic properties of materials are of microscopic origin. 

There exist five classes of magnetic materials  

In the interests of simplicity, attention is confined to a crystal consisting of N atoms per unit volume for which the magnetic moment per atom arises from a 

If n denotes the number of atoms per unit volume with moments in the direction of B and n the number in the antiparallel sense, the net induced magnetization is M = (wf —It follows that 

Under most practical conditions /ibB C kT, and then (using the approximation e 1 + v for x 1) it follows that 

In 1907 Weiss suggested that the magnetic field //, seen by an individual dipole in a solid was the macroscopic internal field //modified by the presence of dipoles in the neighbourhood of the individual. This idea was expressed by H = H+wM, (c.f. Eq. (2.84)) where w is known as the molecular field constant . Under this assumption Eq. (9.22) is modified to 

Spontaneous magnetization implies a value for M when H = 0, and the practical feasibility of this can be explored by putting H = 0 in Eq. (9.24) and then examining whether the equation can be satisfied by non-trivial values for M. Therefore, putting N/ib = Ms, where Ms is the saturation magnetization, we can write Eq. (9.24) as 

In materials exhibiting paramagnetism, some of the atoms or ions in the material have a net magnetic moment due to unpaired electrons in partially filled orbitals. 

In the Pauli model of paramagnetism, the conduction electrons are considered essentially to be free and under an applied field an imbalance between electrons with opposite spin is set up leading to a low magnetization in the same direction as the applied field. The susceptibility is independent of temperature, although the electronic band structure may be affected, which will then have an effect on the magnitude of the susceptibility. 

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Figure 2. Neel s classification of magnetization curves for ferrimagnetic materials.

In previous chapters we focused on physical properties for which electrons are only important in the sense that we must know the ground state of the electrons to understand the material s energy. There is, of course, a long list of physical properties where the details of the electronic structure in the material are of great importance. Two examples of these properties are the classification of a bulk material as a metal, a semiconductor, or an insulator and the existence and characteristics of magnetic properties. In this chapter we examine how information related to these questions can be obtained from DFT calculations. 

In (2.5) and (2.6), e, /x, a, and a are the constitutive parameters that characterize the electric and magnetic properties of the material. Hence, s is the electric permittivity, /i the magnetic permeability, a the electric conductivity, and a the equivalent magnetic resistivity. The variation of the constitutive parameters as a function of diverse field characteristics (such as intensity, position, direction, and frequency) leads to their classification according to structure and behavior. 

The determination of the y-values is important for the classification of YbTX intermetallics as heavy-fermion systems which exhibit significantly enlarged y-values. The source of the large y-values arises from the compensation of the magnetic moment associated with a nearly localized electron state, generally of f character, by the spins of the conduction electrons in the metallic material. 

This review is divided into four major sections (i) a general survey of n molecular conductors, including the chemistry and the structural classification of radical-ion salts (ii) a summary of the physics of these electronic and magnetic low-dimensional systems (iii) a discussion of the present "state-of-the-art" in these materials, with an emphasis on their mix -valence character, and the impetus to create new compounds with specific low-temperature physical properties (iv) a description of the current trends, which show an explosion in different directions, l cause one can get new materials with specific properties, by playing with the molecular organization in stable or metastable phases. 

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