“…. 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

The magnetic phase diagram of γ Fe-Mn alloys has been investigated in the whole range of composition by neutron diffraction and other techniques. The f.c.c. phase of Fe-Mn alloys with less than 40 at. % iron and more than 85 at. % iron was stabilized by addition of small amounts of copper and carbon respectively in the alloys. An antiferromagnetic long range order was detected in the whole range composition. The antiferromagnetism of this alloy system Fe x -Mn 1- x was found to be classified into three distinct groups according to the composition. They are 1) 0≤ x ≤0.3, 2) 0.4≤ x ≤0.8, and 3) 0.8≤ x ≤1. The magnetic phase of the second group is markedly different from other two groups. The results agree qualitatively with the prediction based on the band theory.

Antiferromagnetism of γ Iron Manganes Alloys

The thermodynamic properties and the phase diagram of the Fe-Mn system have been reassessed using a revised description of manganese and some new experimental information. Some improvements have been made compared to the previous assessment. The metastable hep phase is also included.

An assessment of the Fe-Mn system

Both natural and synthetic pyrope have been compressed to pressures above 300 kbar in a diamond-anvil press and heated to temperatures above 800°C by a continuous YAG laser. After quenching and release of pressure, X-ray diffraction shows that the natural pyrope transforms to a single phase with perovskite-like structure whereas the synthetic pyrope disproportinates into a mixture of MgSiO3 (perovskite) plus Al2O3 (corundum). The orthorhombic perovskite lattice parameters for the natural pyrope are ao = 4.816 ± 0.004 A, bo = 4.973 ± 0.004 A, and co = 6.997 ± 0.006 A with Z = 4. The orthorhombic cell dimensions for MgSiO3 (perovskite) are ao = 4.818 ± 0.005 A, bo = 4.869 ± 0.005 A, and co = 6.906 ± 0.007 A with Z=4. The calculated density of MgSiO3 perovskite is thus 4.12 g/cm³, or 3.7% denser than the isochemical mixed oxides. These experiments are the first demonstration of static high pressure phase transformations in silicate garnets and the first examples of a pure silicate existing in the perovskite structure.

Silicate perovskite from phase transformations of pyrope‐garnet at high pressure and temperature

Survey on tables and figures containing data on alloys and compounds between 3d-elements and Cu, Ag and Au, respectively.

Alloys and compounds of 3d elements with main group elements

The magnetic properties of single crystals of Cr alloys containing a small amount of Fe have been studied mainly by neutron diffraction. The alloys with less than 1.5 at.% Fe have been found to have the same magnetic structure as that of pure Cr. With increasing Fe concentration, which corresponds to an increase of the electron to atom ratio, the Neel temperature decreases, the wave vector of the sinusoidally modulated spin density wave increases, while the magnetic moment remains almost constant. The spin-flip temperature also decreases rapidly. The alloys with concentrations between 2 and 4 at% Fe have a simple antiferromagnetic structure at low temperatures, and the first order transition from the antiferromagnetic state to a sinusoidal spin state has been observed with increasing temperature. When the concentration of Fe exceeds 4 at%, the alloy is a simple antiferromagnet up to the Neel temperature. These results are discussed on the basis of the two band model.

Antiferromagnetism in Dilute Iron Chromium Alloys

The antiferromagnetism of disordered γ-FeMn alloys has been studied by neutron diffraction and Mossbauer effect. The average magnetic moment per atom at 0°K decreases with decreasing Fe concentration from 1.94µ B for 69.1 at% Fe to 1.08µ B for 47 at% Fe. The hyperfine field at 0°K acting on the Fe nucleus, however, remains almost constant at about 40 kOe±3 kOe for this concentration range. The temperature dependence of the sublattice moment for 69.1 at% Fe fits with the Brillouin function B J ( x ) with J =1. The decrease of the moment near the Neel temperature becomes steeper when the concentration of Fe is lower. In contrast with it, the temperature dependence of the hyperfine field is independent of concentration, and is markedly different from the temperature dependence of the sublattice moment for any concentration. The results indicate that the magnetic moments on Fe and Mn atoms behave with temperature in fairly different ways in this alloy system. The magnetic form factor of γ-FeMn alloys was also...

Antiferromagnetism of γ-FeMn Alloys. II. Neutron Diffraction and Mössbauer Effect Studies

A new high pressure phase of MnTiO3 and its magnetic property

High pressure synthesis of ilmenite and perovskite type MnVO3 and their magnetic properties

The magnetic properties of single crystals of Cr alloys containing a small amount of Co and Ni have been studied by neutron diffraction and thermal expansion. The alloy with less than 1 at%Co has the same magnetic structure as that of pure Cr. With increasing Co concentration, the Neel temperature decreases, and the wave vector of the S.D.W. increases, while the magnetic moment remains constant. For 2 at%Co-Cr, the commensurable antiferromagnetic structure becomes stable. The Neel temperature slightly increases and the magnetic moment still remains constant. With the further increase of Co concentration, the antiferromagnetic structure tends to be destroyed by Co-Co pairs. The alloys with Ni have been found to show the magnetic behavior as the alloys with impurities which decrease the electron to atom ratio. All of wave vector, Neel temperature and magnetic moment decrease with an increase of Ni concentration.

Yasuo Endoh

Papers

Antiferromagnetism in Dilute Iron Chromium Alloys

Antiferromagnetism of γ-FeMn Alloys. II. Neutron Diffraction and Mössbauer Effect Studies

A new high pressure phase of MnTiO3 and its magnetic property

High pressure synthesis of ilmenite and perovskite type MnVO3 and their magnetic properties

Antiferromagnetism of Dilute Cr Alloys with Co and Ni