T. J. Coburn

Charge Growth Equation for Belts of Insulating Materials Passing over Grounded Rollers

The magnitude of the electrostatic charge density on an insulating belt of photographic film increases as the belt passes repeatedly over a set of grounded metal rollers. The charge density q asymptotically approaches an equilibrium value qe according to the relation q = qe (1 − e−n/τ), where n is the number of roller passages and τ is an arbitrary constant. An equivalent circuit which describes this behavior is presented. The circuit elements are an electromotive force, a contact resistance, a leakage resistance, and a capacitance between the belt and the roller. The physical equivalents of these elements and the possibility of their measurement are discussed.

Hot pressed CoCr2S4: A potentially useful magneto-optic material

Transparent CoCr2S4 has been vacuum hot pressed at 850°C and 50,000 psi. This material is a ferrimagnetic semiconducting spinel with a Curie temperature of 221°K. In the spectral range from 1500 to 800 cm−1 (6.6–12.5μ) it has a refractive index of 3.56 and a residual absorption coefficient of about 7 cm−1. The Faraday rotation at 2000 cm−1 (5μ) and 944 cm−1 (10.6μ) at 80°K in a magnetic field of 7 kOe is 2100°/cm and 320°/cm, respectively. CoCr2S4 has been used to modulate a CO2 laser.

Reflectance−circular dichroism of the magnetic insulator CoxCd1−xCr2S4

Large infrared magneto−optic effects are observed in the series of magnetic compounds CoxCd1−xCr2S4 by means of reflectance−circular dichroism. These effects are associated with the crystal−field transitions of divalent cobalt. Comparison of surface and bulk magneto−optic effects indicate that the surface optical coefficients are representative of bulk phenomena. The magneto−optical effects associated with the 4A2(F) −to−4T1(F) transition are nearly linear with cobalt concentration x. The 4A2(F) −to−4T1(P) transition at about 1.0 μm exhibits a red shift of about 900 cm−1 as x increases from 0.05 to 1.00. Also, the magnitude of the magneto−optic coefficient is not linear in cobalt concentration and reaches a maximum value near x=0.25. The unusual magneto−optical effects associated with the 4T1(P) transition may be utilized in tailoring materials for particular applications.

Magnetooptical properties of ferrimagnetic CoCr 2 S 4 in the near infrared

CoCr 2 S 4 , which is a ferrimagnetic insulator, has been prepared by hot-pressing microcrystalline powder. The infrared magneto-reflectance effects have been measured at several temperatures below the transition temperature of 221K. Very large Kerr effects, which are centered at 1.0 and 1.7 μm, have been associated with the principal crystal field transitions of divalent cobalt. These samples exhibit a normal remanence of at least 95 percent, which is attributed to strain-induced anisotropy in the sample. The transmission and Faraday rotation of the material are deduced from Kramers-Kronig and other analysis of the reflectance data. The use of this material in a digital-storage application is discussed

Optical and Magneto‐Optic Properties of Polycrystalline CdCr2S4

The optical absorptionspectrum of hot‐pressed polycrystalline CdCr2S4 has been measured between 700 nm and 50 μ, with particular emphasis on the region of weak absorption between 700 and 1200 nm, in which Faraday rotationspectra were also obtained. In this region, two series of phonon‐assisted transitions have been observed, involving phonon energies of 0.041 and 0.046 eV. These energies are similar to the optical phonon energies obtained from infrared measurements on the same crystals. At 4.2°K, and with an applied magnetic field of 3000 Oe, the Faraday rotation reaches a value of about 1.6×104 deg per cm near 750 nm. At 80°K, the rotation is decreased by about a factor of two. These crystals have very low optical scatter or impurity absorption, resulting in high transparency in the near infrared. At 4.2°K, the figure of merit is about 60 deg dB−1 in this spectral region, and decreases to about 30 deg dB−1 at 80°K.

Infrared absorption spectrum and faraday rotation of hot- pressed CdCr 2 S 4

Hot-pressed CdCr 2 S 4 has been prepared which has high optical quality in the infrared. The absorption coefficient and Faraday rotation have been measured in the wavelength range between 3 and 15 μm. The absorption coefficient was measured both spectrometrically, which includes absorption and scattering, and calorimetrically at 10.6 μm with a CO 2 laser, which measures only the absorption. At 10.6 μm both methods give a value of about 3.5 cm-1, indicating that scattering is not important. The Faraday rotation measurements agree essentially with those found for single-crystal CdCr 2 S 4 by Wittekoek and Bongers. At 10.6 μm and 80°K the rotation is 120 deg/cm at 6.5 kOe, giving a figure of merit of about 7.5-deg rotation per dB absorption. Therefore, application of this material to magnetooptical devices may be possible using liquid nitrogen as a coolant.

Low dark‐current MOS photodiodes using GaAs0.6P0.4

Anodic oxides have been grown on n‐type GaAs0.6P0.4 and MOS capacitors have been fabricated. The thermal generation rate of minority carriers, and therefore the dark current, is very small in this material. Consequently, MOS photosensors typically have storage times of 500 sec but can be read out in submillisecond times by pulsing into accumulation. The intrinsic quantum efficiency of these structures is about unity and applications to sample‐and‐hold and low‐light‐level devices are suggested.

Electronic properties of anodic oxides grown on GaAs0.6P0.4

Native oxides were grown on GaAsP by an anodic reaction and the properties of the resultant MOS structure were studied. The C-V measurements showed a hysteresis effect upon sweeping from depletion to accumulation, corresponding to an accumulation of negative charge in traps at the interface region. The magnitude of these effects depended strongly upon the heat treatment of the oxides. Oxides annealed in argon showed the lowest trap densities, which were about 1012 cm-2. Exposure at large negative gate voltages led to large negative flat-band shifts related to photo-holes trapped in the interface states. The emission times for these trapped holes could be very long.

The Preparation and Magneto‐Optical Properties of Hot‐Pressings of the Magnetic Semiconductors CdxCo1−xCr2S4

The preparation and hot‐pressing of microcrystalline spinel powders of the composition CdxCo1−xCr2S4, where x varies from 1 to 0, are described. The Curie temperatures increase and the magnetic moments decrease as x goes from 1 to 0. The optical absorption and Faraday rotation of these pressings have been measured in the wavelength range from 1 to 25 μm. Compositions with x slightly less than 1 (low Co doping) reveal optical absorption bands at 1.61, 1.72, and 1.96 μm, which are assigned to the 4A2 to. 4T1(F) crystal field transitions of the Co++ in a tetrahedral field. Between 5 and 12μm, as x goes from 1 to 0, the magnitude of the figure of merit decreases to zero and then increases to the same order at x=0 as at x=1. The increase of the Curie temperature (Tc) from 84.5°K at x=1 to 221°K at x=0 reduces the cooling requirements and CoCr2S4, will probably be a more useful magneto‐optical material than CdCr2S4.

Optical Storage in Hot‐Pressed Ferrimagnetic Spinels

Unusual optical storage effects in hot‐pressed CoCr2S4 have been recently reported.1 Even though thesample is cubic and polycrystalline, remanence of the polar Kerr effect is observed at normal incidence. Similar effects have been observed recently in hot‐pressed FeCr2S4 and CoFe2O4, which have transition temperatures of about 190°K and 790°K, respectively. The origin of the polar remanence is related to the residual hydrostatic strain in a hot‐pressed polycrystal. The magnetostriction of the material produces a strain anisotropy term in the magnetic energy. Assuming a single‐domain particle model for the individual crystallites in the hot‐pressed material, the magnetic energy of the ensemble has been calculated. The magnetization will be stable in the normal direction provided certain criteria are met. The magnetostriction coefficient λ must be negative and the product λS must be greater than 3/2 DM2, where S is the residual stress and D is the average inplane demagnetizing factor. The normal remanence, ...