J. P. Remeika

Effect of Impurities on the Optical Properties of Yttrium Iron Garnet

The optical absorption spectrum of single‐crystal YIG is dominated at the short wavelengths by electronic transitions due to iron ions. For the long wavelengths, it is dominated by lattice vibrational transitions. Between about 1 μ and 6 μ there is a region of great transparency which can be modified in several ways. The valence of the iron ions can be altered by heat‐treatment of crystals in oxidizing or reducing atmospheres, or by doping with divalent or tetravalent ions. The altered valence leads to increased optical absorption, as well as increased electrical conductivity.Foreign ions such as cobalt or the rare earths may have characteristic absorption lines in the region of transparency, and when yttrium is completely replaced by a rare earth, the short‐wavelength edge absorption may be shifted appreciably due to lattice expansion or contraction.Another anomaly in the transparent region of YIG is found in hydrothermally‐grown crystals, where OH‐group stretching vibrations occur.

Application of orthoferrites to domain-wall devices

Orthoferrites are of general formula RFeO 3 where R is any rare earth or yttrium. They are usually flux grown as large single crystals and then processed to provide platelets several mils in thickness. A high uniaxial anisotropy gives rise to a single unique easy magnetization direction parallel to the c axis above room temperature in all orthoferrites except SmFeO 3 . A low saturation magnetization results from the canting of a pair of anti-parallel spin systems. Platelets prepared so that the easy axis of magnetization is normal to the planar surface display a serpentine domain pattern made visible by the Faraday effect. Under specific conditions cylindrical domains are observed. These domains, which in Sm 0.55 Tb 0.45 FeO 3 are as small as 0.8 mil in diameter, can be manipulated to perform memory and logic. Three techniques can be used to propagate cylindrical domains. The first uses a sequence of current pulses applied to a conductor array. The second requires an in-plane rotating field acting on a structured Permalloy pattern to generate traveling positive and negative poles. These poles selectively attract and repel a cylindrical domain and thereby control its motion. The movement of an inchworm most closely approximates the propagation mechanism of the third technique. Interacting a pulsating domain with a wedge-like Permalloy pattern results in a unidirectional movement.

The growth and ferroelectric properties of high resistivity single crystals of lead titanate

Abstract Flux growth of ferroelectric PbTiO3 containing less than 0.1 weight % of uranium is found to produce single crystals of high resistivity (> 1010 ohm cm). The Curie temperature of these crystals, Tc = 492 ± 5°C, is the same as that in undoped crystals, and it is possible to reverse the polarization of the crystals at room temperature. Measurements of the dielectric constant, conductivity, specific heat, polarization and pyroelectric coefficients as a function of temperature are reported.

Superconductivity of ternary sulfides and the structure of PbMo6S8

The compound Pb0.92Mo6S7.5, a derivation from the ideal stoichiometric compound PbMo6S8, is a member of the ternary sulfide series MXMo3S4 where M = Ag, Sn, Ca, Sr, Pb, Ba, Cd, Zn, Mg, Cu, Mn, Cr, Fe, Co, Ni, Li, Na, Sc, and Y. Members of the series with M = Pb, Sn, Ag, Cu, Zn, Mg, Cd, Sc and Y have been found to be superconducting at TC = 15.2, 14.2, 9.1, 10.9, 3.6, 3.5, 3.5, 3.6 and 3.0oK respectively. The lead compound has rhombohedral symmetry R3, lattice parameters a = 6.551A and α = 89.33°, and one formula per unit cell. The structure consists of a distorted primitive-cubic network of sulfur atoms with every fourth sulfur cube occupied by either a Pb atom or a Mo6 octahedron. The vertices of the Mo6 octahedron lie in the face centers of the sulfur cube which is tilted approximately 15o with respect to the rhombohedral axes. This arrangement yields open channels running through the structure parallel to the unit cell edges.

Perovskite-like La1−xKxMnO3 and related compounds: Solid state chemistry and the catalysis of the reduction of NO by CO and H2

The new compounds La1−xMxMnO3 (0.05 ⩽ x ⩽ 0.4 for M = K; x = 0.2 for M = Na, Rb) have been prepared. La1−xKxMnO3 (0.05 ⩽ x ⩽ 0.4), LaMnO3.01, LaMnO3.15, La0.8Na0.2MnO3, and La0.8Rb0.2MnO3 have been used as catalysts in the reduction of NO. La0.8K0.2MnO3 has also been used in the catalytic decomposition of NO. The activity of these catalysts is related to the presence of a Mn3+/Mn4+ mixed valence and to the relative ease of forming oxygen vacancies in the solid. The presence of cation vacancies in LaMnO3.15 and the substitution of La3+ by alkali ions in LaMnO3 increases the catalytic activity. The reduction of NO involves both molecular and dissociative adsorption of NO.

GaFeO3: A Ferromagnetic-Piezoelectric Compound

Piezoelectricity and ferromagnetism have been found to occur simultaneously in the compound Ga(2−x)FexO3. Single crystals of approximately equimolar gallium to iron ratio have been grown from a flux consisting of Bi2O3 and B2O3. X-ray diffraction data on ceramic compositions have shown that the gallium to iron ratio can be varied between x = 0.7 to x = 1.4. Magnetic data showing the shift in magnetic Curie point vs Fe concentration and magnetization vs Fe concentration will be given.

Spin Resonance of Transition Metal Ions in Corundum

The results of electron spin resonance experiments on a series of transition metal ions in single crystals of corundum are reviewed and interpreted in terms of crystal field theory emphasizing three topics: (1) The strength of the cubic crystal field, (2) the axial field as an indicator of position of the impurity ion, and (3) Jahn‐Teller effects in the ground state. The 4d and 5d ions which were examined, i.e., Ru3+, Rh3+, and Ir3+ are found to be strong crystal field cases. In addition, the increased charge of Ni3+(3d7) is shown to be sufficient to drive it to a strong crystal field case (t26e) with S = ½ compared to Co2+ (3d7) which is t25e3 with S = 32. The parameter v which measures the trigonal field potential, was found to be +850 cm−1 in Ru3+ in good agreement with other ions such as Co2+ and V3+ whose cubic field ground states are also orbitally degenerate. In contrast, the determination of v from ESR data on ions with singlet ground states such as Cu3+ and Ni2+ is regarded as less reliable. The ...

Domain Behavior in Some Transparent Magnetic Oxides

Magnetic domains were observed by means of the Faraday effect, and by the Bitter technique in a number of compounds having the spinel, magnetoplumbite, and perovskite‐like structures. The latter compounds are orthorhombic and have the general formula MFeO3 where M is Y, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu. Domain boundaries in these compounds move for fields less than one oersted, and after saturation, crystals as large as (∼0.2×0.2×0.1 in.) remain single domains. Fields of 2500 oersteds are required to reverse the magnetization in the single domain crystals. Reversal occurs by nucleation and growth of a reverse domain. This is evident from the unusual torque curves. The fact that domain walls occur and move in such weak fields in these compounds indicate that the slight or parasitic magnetization is a uniform body property of the crystals. The parasitic magnetization of these compounds was determined from torque measurements. These crystals are well suited for light modulators because a uniaxial anisotropy holds the magnetization vectors normal to the large surfaces of thin flat crystals and weak fields produce changes in magnetization. Absorption coefficients were measured for some compounds for radiation having wavelengths from 0.6 μ to 13 μ. Thin sections were found to be transparent to light at the red end of the visible spectrum. The Faraday rotation was measured for Li0.5Fe2.5O4, MgFe2O4, YFeO3, and PbFe11Al1O19.

Optical Transparency of Rare‐Earth Iron Garnets

Optical absorption and electrical resistivity have been measured for single‐crystal flux grown rare‐earth iron garnets doped with Si4+, Sn4+, Ge4+, and Ca2+. Strong optical absorption near 9000 cm−1 is observed due to relaxation mechanisms involving transfer of an electron from Fe2+ ions in the case of Si, Ge, Sn doping, or a hole from Fe4+ ions in the case of Ca doping to Fe3+ ions in the vicinity. This is accompanied by n‐type electrical conductivity for crystals containing Fe2+ introduced by Si, Sn, Ge, and by p‐type conductivity for those containing Fe4+ produced by Ca doping. We find that pure rare‐earth iron garnets contain small amounts (0.004 atom/formula) of Fe4+ producing undesirable optical absorption, and this can be eliminated by small additions of tetravalent Si, Sn, or Ge.

Structural aspects of the metal-insulator transitions in Ti4O7

Abstract Ti4O7 has two transitions with decreasing temperature and the structure has been refined in each phase. The triclinic structure (A l ) consists of rutile-like layers of TiO6 octahedra extending indefinitely in the a-b plane and four octahedra thick along the c axis. The average TiO distances for the four independent Ti atoms are 2.006(2), 2.006(2), 2.004(2), and 2.018(2) A at 298K; 2.011(4), 2.000(4), 2.015(4), and 2.012(4) A at 140K; 2.043(4), 1.973(4), 2.044(4), and 1.996(4) A at 120K. At 120K there is a clear separation into strings of Ti3+ or Ti4+ ions running parallel to the pseudorutile c axis. In addition the 3+ sites are paired to form short TiTi bonds, whereas the Ti4+ atoms are displaced toward one oxygen giving short TiO distances of 1.776(4) and 1.787(4) A and thus forming a zig-zag chain. In the intermediate phase the interatomic distances are only slightly different from those at 298K but the thermal factors are anomalously large.