M. W. Shafer

Ferromagnetic Europium Compounds

Ferromagnetism has been found in several divalent europium compounds. Some of these materials are particularly simple in both crystal and magnetic structure and are ideal for experimental and theoretical study. The materials we have investigated can be divided into three groups, europium chalcogenides, europium halogens, and europium silicates. Magnetic information on these compounds are summarized and theoretical results reviewed.The europium chalcogenide series all have the rocksalt structure, the oxide, sulfide, and selenide being ferromagnetic, and it is on these materials that the most work has been done. This is especially true of EuS where detailed moment, specific heat, and nuclear magnetic resonance measurements are available. The oretical analysis indicates that there is positive exchange between nearest‐neighbor europium atoms which decreases with increasing atom spacings. There is also a weaker negative superexchange via the anions in these compounds which is responsible for antiferromagnetism...

Preparation and Crystal Chemistry of Divalent Europium Compounds

The discovery that the europium chalcogenides are a particularly simple and important group of magnetic materials has resulted in considerable work being done on the synthesis of these materials. Several methods of preparing pure EuO have been investigated and the results are reported. The method which resulted in the best EuO is when an excess of europium metal is reacted with Eu2O3 at 800°C and then distilled off at 1150° in a high vacuum. Chemical analyses, microstructures, and magnetization measurements were used to determine the stoichiometry of EuO as a function of temperature. The results indicated that the rock salt phase can accommodate about 4 mole% trivalent europium in solid solution at 1300°C. Very little (<1%) europium metal was found to go into the EuO phase, but by the addition of 5% to 7% the paramagnetic Curie temperature θ was increased from 73° to 79°K.A number of new divalent europium compounds have been prepared and their magnetic properties investigated. Ferromagnetism was found in ...

Magnetic Structure of EuTiO3

Europium titanate has the cubic perovskite structure containing divalent Eu (7 μB) and tetravalent Ti. From magnetic measurements we find that EuTiO3 is one of the few antiferromagnetic materials with a positive θ (TN=5.3°K, θ=3.8°K). At 1.3°K the magnetic moment (σ) increases linearly with field to 10 kOe; above 14 kOe the moment saturates and σ=156 emu/gm (6.93 μB) at 20 kOe. Powder neutron‐diffraction work indicates that EuTiO3 has the Type G magnetic structure in which a given Eu++ has six nearest‐neighbor europium ions antiparallel and 12 next‐nearest‐neighbor europium ions parallel. In a perovskite structure where only the 12‐coordinated ion is magnetic, i.e., Eu++, the molecular field relations for a two sublattice model yield J1/k=−0.021°K, where J1 is the effective intersublattice exchange interaction, and J2/k=0.040°K, where J2 is the effective intrasublattice exchange interaction. The signs of J1 and J2 are opposite to those found in the europium chalcogenide series. The chalcogenides, however,...

Studies of Curie‐Point Increases in EuO

The ferromagnetic transition temperature (TC) of EuO has been increased from 69° to 135°K by selectively doping to increase the electrical conductivity. Three different chemical systems have been studied: the EuO‐Eu, the EuO‐Eu‐RE2O3, and the EuO‐RES systems (RE is a rare earth other than europium). Compositions in the latter system were multiphase because the solubility of the rare‐earth sulfides in EuO is very small and it was necessary to add about 10% to cause significant Curie‐temperature increase. Single crystals have been grown from ternary compositions in the EuO‐Eu‐RE2O3 system with magnetizations at 77°K as high as 170 G cm3/g and ferromagnetic Curie temperatures (TC) close to the paramagnetic θ values, a condition not observed in the other doped europium chalcogenides. The effect of the Eu metal on the formation of these crystals and its role in “forcing” the trivalent rare‐earth oxide into the rock‐salt lattice is discussed. From Kerr measurements and the optical absorption data it is shown th...

The chemistry of and physics with porous sol‐gel glasses

Porous media with well‐defined pore diameters can be employed as hosts within which one may study a wealth of physical phenomena in well‐characterized confining geometries. In this paper we describe the procedures for forming variable pore size glasses via the sol‐gel process. Two general methods were used to form the gels from which the porous glasses were formed; the hydrolysis of the alkoxides and the gelling of colloidal silica, e.g., DuPont LUDOX. By controlling the pH of the gelling solution, the water/silica ratio and the firing temperature, glasses with pore diameters ranging from less than 2–12 nm were formed from the alkoxide hydrolysis. Larger pore diameters were obtained from colloidal silica gels, the largest being in the 45‐nm range in glasses made from leached alkali silicate gels. Extensive high resolution transmission electron microscopy analysis showed both ‘‘colloidal’’ and ‘‘polymeric’’ type glasses, in addition to an intraparticle structure in the 1–3 nm range. An example is given whe...

Magnetic Properties of Some Divalent Europium Compounds

The compounds of divalent europium form an attractive series for magnetic investigations because the magnetic ions are in S states and the crystal structures are generally simple. Moreover, true ferromagnetism, which is quite unusual in compounds, has been observed in EuO by Matthias, Bozorth, and Van Vleck.We have prepared samples of EuS, EuSe, EuTe, EuCO3, EuTiO3, EuZrO3, EuSO4, and EuCl2, and studied their magnetic properties in the temperature range 4–300°K. The sulfide, selenide, and telluride all have the same NaCl crystal structure as EuO; we find that the first two also become ferromagnetic at low temperature, while EuTe is paramagnetic, or possibly antiferromagnetic, at 4.2°K. The Curie temperatures for EuS and EuSe are about 18° and 7°K, respectively. The saturation moments of EuS and EuSe extrapolated to 0°K correspond to 6.8 and 6.7 μB per Eu++ ion, respectively, in good agreement with the expected value of 7 μB. None of the other compounds were ferromagnetic at liquid helium temperatures, and...

Annealing treatment effects on structure and superconductivity in Y1Ba2Cu3O9−x

We report the effects of heat treatment and ambient on the structure and superconducting properties of Y1Ba2Cu3O9−x. The structure undergoes an orthorhombic‐to‐tetragonal transition on heating at about 700 °C, caused by oxygen loss and disordering of oxygen vacancies on the copper plane between the barium layers. Heat treatments that promote maximum ordering of the oxygen vacancies result in superior superconducting properties.

Relationship Between Stoichiometry and Properties of EuO Films

The preparation of EuO films by co‐evaporation of Eu and Eu2O3 has been studied. The two most important parameters to control are the ratio r=Eu/Eu2O3 and the background pressure. The latter should be in the 10−6‐Torr range, and only by starting with high density (>95%) Eu2O3 is it possible to maintain this. As r increases from 0.8 to 1.4 we observe the following: (1) A large increase in Tc is observed from 51° to 117°K; (2) the absorption a increases from 0.85×105/cm to 1.6×105/cm and the peak shifts toward the red (from 5900 to 6000 A); (3) the longitudinal Faraday rotation 2φ (with 25° incident angle at 6328 A) increases from 0.9×105 deg/cm to a maximum of 3.7×105 deg/cm, followed by a slight decrease; and (4) Hc (10°K) decreases sharply from 140 Oe to a minimum of 43 Oe for r=0.94, followed by a slight increase. The best films which were single‐phase stoichiometric EuO, with bulk‐like properties, are obtained with r=0.94. These have well‐defined x‐ray diffraction patterns with lattice constants a0=5.14 A, Tc of 71°K, and the normal Faraday rotation (at 20°K with H=20 kOe) of 8.5×105 deg/cm. In films with r>0.9 the increase of Tc is attributed to the presence of Eu‐metal which contributes conduction electrons to enhance the Eu2+–Eu2+ interaction. For r 95%) Eu2O3 is it possible to maintain this. As r increases from 0.8 to 1.4 we observe the following: (1) A large increase in Tc is observed from 51° to 117°K; (2) the absorption a increases from 0.85×105/cm to 1.6×105/cm and the peak shifts toward the red (from 5900 to 6000 A); (3) the longitudinal Faraday rotation 2φ (with 25° incident angle at 6328 A) increases from 0.9×105 deg/cm to a maximum of 3.7×105 deg/cm, followed by a slight decrease; and (4) Hc (10°K) decreases sharply from 140 Oe to a minimum of 43 Oe for r=0.94, followed by a slight increase. The best films which were single‐phase stoichiometric EuO, with bulk‐like properties, are obtained with r=0.94. These have well‐defined x‐ray diffraction patterns with lattice constants a0=5.1...

MAGNETIC AND OPTICAL PROPERTIES OF TRANSPARENT RbNiF3

Magnetic and optical data on single‐crystal RbNiF3 are presented. We conclude that this material is ferrimagnetic, rather than antiferromagnetic as previously reported, with a transition temperature of 139°K and a saturation moment of 21 emu/g at 4.2°K. The basal plane is the preferred plane and the out‐of‐plane anisotropy is 106 ergs/cc. The Faraday rotation measured along the c axis at 77°K and 80% of magnetic saturation exhibits a peak value of 400° per cm at a wavelength of 4950 A and values of 250° to 50° per cm in the region 5000 to 6000 A where the optical absorption is lowest.

On the Magnetic Anisotropy in Manganese‐Iron Spinels

The magnetic anisotropy energy is measured as a function of temperature in eleven single crystal compositions corresponding to MnxFe3−xO4, where x is varied from 0.00 to 1.90. The interesting features observed are as follows.(1) Small manganese concentrations in magnetite (x=0) are shown to decrease the temperature of the “isotropy point” known to exist in magnetite at low temperatures.(2) An unexpected variation in the temperature dependence and magnitude of the anisotropy energy in the x=0.4 to 1.0 composition region is shown.(3) The anisotropy energy in the composition region between x=1.0 and 1.8 is semiquantitatively shown to be related to cubic crystalline field splitting of the energy levels of ferric ions.(4) A large planar anisotropy is measured in a crystal composition given by x = 1.90 and is discussed in terms of the expected tetragonal nature of trivalent manganese.(5) Finally, a minute magnetic annealing effect at a temperature of 370°K has been shown to exist in a crystal composition corres...