Selected Works of Louis Neel

Nicholas Kurti

• Publisher: CRC Press
• Year: 2019
• ISBN:
  9781000721973
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

One of the world's foremost authorities on magnetism, Professor Louis Neel was the recipient of the 1970 Nobel Prize in Physics. With all but ten of Neel's 150 original papers being written in French, the aim of this English edition is to bring this important work to a wider readership.

Chapter I

GENERAL THEORY OF MAGNETISM
A7, A17, A40, A22, A35, C5, C29

A7 (1932)

INFLUENCE OF FLUCTUATIONS IN THE MOLECULAR FIELD ON THE PROPERTIES OF MAGNETIC BODIES

Summary (1977)—Pages 1 to 50 and 55 to 70 of the original memoir A-7 (Thesis by L. Néel) are reproduced here.

The magnetic coupling between the atoms of a ferromagnetic body are only effective over a short distance and can therefore not be represented by a uniform Weiss molecular field. It is necessary to take into account fluctuations of that field. To determine the effects of this a study is made of the magnetic properties of a simple model consisting of an infinite chain of atoms satisfying Lenz’s law. Each atom is coupled to the p atoms which precede it in the chain and to the p atoms which follow it. One thus finds the origin of some anomalies which are not explained by the Weiss theory, such as the difference between the ferromagnetic Curie point θ_f, the temperature at which the spontaneous magnetization vanishes, and the paramagnetic Curie point θ_P deduced, in the paramagnetic region, by extrapolation of the Curie–Weiss line. In the same way, one can explain the persistence of an excess specific heat above θ_f.

It is also shown that, in solid solutions, it is necessary to use different molecular fields, depending upon the surroundings of the site at which they act. The temperature variation of the inverse susceptibility is no longer a straight line but rather a hyperbola.

Finally, a study is made of the case of negative interactions (§ 25, §§ 46–50) which tend to align antiparallel the magnetic moments of two neighbouring atoms. Using a very simple schematic model, it is shown that at low temperatures the body is paramagnetic with a susceptibility independent of temperature (constant paramagnetism). In this temperature range, and if the body has a body centred cubic lattice, it forms (§ 48) two simple cubic sub-lattices magnetized in opposite directions.

INTRODUCTION

§ 1. Theory of paramagnetism, — The theory of paramagnetism is based on the following basic hypothesis: inside a paramagnetic material there are carriers of magnetic moment. Their magnetic moment μ is fixed. One can, for example, identify the carriers as atoms.

Langevin [1] assumed that these carriers had complete freedom to rotate and that Boltzmann statistics could be applied to them. Under these conditions, the magnetization per carrier, in a magnetic field H, is given by the equation:

$\begin{array}{cc}\overline{\mu }=\mu \left(\coth a-\frac{1}{a}\right)& a=\frac{\mu H}{\text{KT}}.\end{array}$

(1)

The carriers are then independent of each other; this is certainly not true in a dense material. Weiss [2] suggested that the interactions between carriers are equivalent to a magnetic field: the molecular field proportional to the magnetization:

$h_m=n\overline{\mu }.$

(2)

In weak fields the law of magnetization is of the form:

$\begin{array}{cc}\mu =\frac{{\mu }^2}{3\text{K}}\frac{\text{H}}{\text{T}-\theta }& \theta =\frac{n{\mu }^2}{3\text{K}}\end{array}$

For T < θ the material remains spontaneously magnetized (ferromagnetism) in zer...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface to the English Edition
• Preface to the French Edition
• Louis Néel - Chronology
• I General theory of magnetism A7, A17, A40, A22, A35, C5, C29
• II Ferrimagnetism A6 (Translator P Johnston A70 A72 A77 A94 C4
• III Antiferromagnetism A29, A112, A113, A114, A115
• IV Magnetic interactions and their variations A18, A23, A33, A30
• V Approach to saturation in ferromagnets A62, A63, A93
• VI Elementary domains; phases and modes A52, C29, A58, A127, A85
• VII Bloch and Néel walls A50, A97, A118
• VIII Weak fields and anhysteretic magnetization A45, A44, A47, A48, A144
• IX Coercive force; magnets B1, A51, A57, A59, A60, A91
• X Random fields; reptation and tilting A101, A102, A106, A107, A109, A108
• XI Magnetic after-effects A73, A78, A84
• XII Directional order; irradiation A92, A119
• XIII Rocks and baked clays A6, C2 (originally published in English)
• XIV Surface problems A121, A122
• XV Antiferromagnetic hysteresis A142
• XVI Apparatus and Techniques A11, A31, A39
• XVII Experimental work A7, A9, A20, A36
• Index