“…. For the truth of the conclusions of physical science, observation is the supreme Court of Appeal….” (Sir Arthur Eddington, The Philosophy of Physical Science.) In this paper we shall review the theoretical and experimental results obtained on simple magnetic model systems. We shall consider the Heisenberg, XY and Ising type of interaction (ferro and antiferromagnetic), on magnetic lattices of dimensionality 1, 2 and 3. Particular attention will be paid to the approximation of these model systems in real crystals, viz. how they can be realized or be expected to exist in nature. A large number of magnetic compounds which, according to the available experimental information, meet the requirements set by one or the other of the various models are considered and their properties discussed. Many examples will be given that demonstrate to what extent experiments on simple magnetic systems support theoretical descriptions of magnetic ordering phenomena and contribute to their understanding. It will a...

Experiments on simple magnetic model systems

It is shown by a general theoretical argument that the variation of the magnetic specific heat of a ‘simple’ antiferromagnet, in particular the singular behaviour in the region of the transition, should be closely similar to the behaviour of the function ∂(χT)∂T, where χ is the zero-field susceptibility. This conclusion is compared with the experimental results for MnO and MnF2 and its general implications for the shape of the susceptibility curve near the transition are discussed.

Relation between the specific heat and susceptibility of an antiferromagnet

The Magnetic Properties of Transition Metal Complexes

The paper presents the review of AC susceptibility measurements in study of magnetic phase transitions and spin dynamics. Examples of several types of phase transitions in conventional and molecular magnets are shown. The phenomenon of magnetic relaxation is brie y ascribed and illustrated with results on relaxation in low-dimensional molecular magnets, such as single molecule magnets, single chain magnets and quasi 1-dimensional systems showing magnetic phase transition to the long-range ordered phase. The following experimental possibilities of the AC technique, which allow to get insight into properties of novel magnetic materials are mentioned: (i) change of the AC eld amplitude (in order to distinguish ferro-(ferri) and antiferromagnets); (ii) use of a static eld HDC (to conclude on the type of magnetic order and to study the phase diagram); (iii) measurement of nonlinear susceptibilities (to conclude on the type of phase transition); (iv) probing the dependence of both components of susceptibility on frequency of the AC eld (to study magnetic relaxation); (v) tests of the DC eld e ect on the frequency dependence of χAC (to analyze the possibility of blocking or reentrant spin-glass transition).

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C.J. Gorter

Papers

The magnetic susceptibility and relaxation of a MnCl2.4H2O single crystal in the paramagnetic and antiferromagnetic states

Determination of very low thermodynamic temperatures in chromium potassium alum

On paramagnetic relaxation in the temperature region of liquid helium

On the susceptibility of CuCl2.2H2O powder in the antiferromagnetic and paramagnetic states

Paramagnetic relaxation in crystals of copper potassium chloride and chromium potassium alum at liquid helium temperatures