Preparation and properties of some new magnetic materials

Detailed techniques and suitable apparatus for producing single crystals of rare earth and yttrium gallium and iron garnets, according to the flux technique of Nielsen, are described. A similar method has been shown to be successful in producing single crystals of rare earth and yttrium aluminium garnets, and an improved melt composition has been discovered. A number of crystals of yttrium and lutecium gallium and aluminium garnets doped with various concentrations of other rare earth and paramagnetic ions have been prepared for resonance and spectroscopic work. In addition, the pure rare earth gallium garnets from gadolinium to ytterbium inclusive, as well as neodymium, and the aluminium garnets from gadolinium to ytterbium inclusive have been obtained as single crystals.
Methods for producing polycrystalline garnet samples are reviewed, and a satisfactory and convenient way of preparing pure and doped polycrystalline aluminium garnet samples developed. In particular, a number of small samples of dysprosium aluminium garnet (DAG) doped with various impurities was made.
Large polycrystalline blocks of yttrium, neodymium, erbium and ytterbium gallium. and yttrium, dysprosium, terbium holmium and erbium aluminium garnets were prepared for specific heat samples, but the conditions necessary for sintering to a highly compact form were not discovered in every case.
Samples of praseodymium gallium garnet and dysprosium aluminate (DyAlO3) were prepared in a similar way.
The possibility of using the Verneuil flame fusion technique for preparing single crystals of dysprosium aluminium garnet is discussed, and various attempts described.
The paramagnetic susceptibilities of all the pure rare earth garnets mentioned, and of some of the dilute samples, were measured in the liquid hydrogen and helium temperature ranges, and the experimental method is discussed in some detail. The results are considered with respect to the paramagnetic resonance measurements made on the 'doped' crystals, and with respect to the effects of crystal field splittings upon the rare earth ions.
A historical survey of the experiments performed upon DAG is given, together with the theories and motives inspiring them. The effects of impurities on the susceptibility of DAG, and the anomalous a.e. and d.c. susceptibility effects observed in DAG samples in different physical states are described and discussed, and tentatively regarded as 'super-paramagnetism'. The susceptibility of a sphere of single crystal DAG was very carefully measured, and shown to undergo an antiferromagnetic transition at 2.49°K.
Using crystallographic data on the garnets, end resonance results on Dy in aluminium garnets, the sublattice ordering below the transition is deduced.
Specific heat measurements on DAG are reported, and the results after correction confirm the sublattice ordering and transition temperature deduced from the susceptibility measurements.
The magnetic moment of an oriented spherical single crystal of DAG has been measured as a function of strength end angle of applied magnetic field, end these results are shown to accord with the deduced sublattice ordering arrangement. Depending upon the direction of the applied field, one, two or three of the antiferromagnetic sublattices are forced to become ferromagnetic in a sufficiently high field below the transition temperature. The observed magnetisations are compared with expected values deduced from crystallographic and resonance work. A method for working single crystals of garnet into a sphere is described, together with an orientation procedure using x-ray back reflection photographs, and the anisotropy of the magnetisation with angle.
The magnetic properties of various oxygen-europium phases have been investigated at liquid nitrogen, hydrogen and helium temperatures. The reported ferromagnetism of EuO has been confirmed, but the published Curie temperature, extrapolated from magnetic moment measurements. has been corrected from 77 to 69°K by direct observation. A phase previously thought to be Eu16O21, or Eu3O4 has been tentatively identified as Eu2SiO4, and shown to undergo a ferromagnetic transition between 4.2 and 14°K, conclusions very recently vindicated by magnetic moment and other measurements on Eu2SiO4. The compound Eu3O4 has a magnetic transition between 4.2 and 14°K but the susceptibility measurements do not show unambiguously whether this is to ferro- or antiferromagnetism. The magnetic properties at higher temperatures are very unusual, and are probably to be explained by the presence of 1% of EuO as impurity in the sample. Eu2O3, in accordance with expectation seems to be almost diamagnetic.
DyAlO3 has been shown to undergo an antiferromagnetic ordering transition at 3.4°K while Dy2O3 remains paramagnetic to the lowest temperatures measured.
The susceptibilities of GdI3 and TbF4 have been measured at liquid helium and hydrogen temperatures, and their chemical purities deduced from the results. The susceptibilities of GdOF, DyOF, NdOF and ErOF are reported, and the expected general similarities of their behaviour with the corresponding garnets discussed. GdOF and DyOF both appear to undergo antiferromagnetic ordering, though the results for GdOF are not unambiguous.
The susceptibilities parallel with, and perpendicular to the crystallographic c-axis of lanthanum nickel and lanthanum manganese nitrates have been carefully measured.