N. Menyuk

NaYF4 : Yb,Er—an efficient upconversion phosphor

Studies of the infrared‐pumped visible luminescence of NaYF4 : Yb, Er show it to have at least twice the visible output of YF3 : Yb, Er when pumped with a narrow‐band excitation source at the optimum frequency. It is also free from saturation effects until very intense excitation levels are reached. The mechanism for red upconversion appears to change with increasing excitation from a two‐step process to one involving a significant amount of three‐step upconversion, which change is accompanied by an increase in the response time for visible emissions.

Magnetic Materials for Digital‐Computer Components. I. A Theory of Flux Reversal in Polycrystalline Ferromagnetics

It is proposed that magnetization reversal in polycrystalline ferro‐ and ferrimagnetic materials is primarily due to the nucleation and growth of 180° Bloch walls. The origin of domains of reverse magnetization is discussed. The rate of growth of these domains is determined by a study of the elastic and frictional forces which retard the motion of their 180°‐Bloch‐wall boundaries. This theoretical model successfully explains the output‐voltage wave forms of polycrystalline materials. A figure of merit for the magnetization reversal of magnetic cores is defined as the switching coefficient Sw=(Hm−H0)τ, where τ is the time required to reverse the magnetization, Hm is the applied magnetic field, and H0 is the threshold field value at which the average domain‐wall velocity is zero. Sw is composed of an eddy‐current contribution Swe and a spin‐relaxation contribution Swr. The value of Sw is derived in terms of various fundamental parameters of the material. It is shown that in ferrites and ultra‐thin metal tap...

Ferromagnetism in CdCr2Se4 and CdCr2S4

A study has been made of the magnetic properties of the spinels ACr2X4, where A is diamagnetic zinc or cadmium, and X is oxygen, sulfur, or selenium. CdCr2Se4 and CdCr2S4 are apparently ferromagnetic with moments somewhat below the predicted value of 6.0 μB/molecule. By contrast, ZnCr2Se4 and ZnCr2S4 are antiferromagnetic at low temperature despite having positive asymptotic Curie points (TA). We found very little thermal variation of cell size in these materials, which further corroborates Lotgerings conclusion that the antiferromagnetism is due to next‐nearest‐neighbor interactions.Disagreement between experimental values of TA/TC and the theoretical value calculated assuming nearest‐neighbor interactions indicate that next‐nearest‐neighbor interactions are playing a role in the ferromagnetic materials as well.Comparisons of TA and TN in these materials indicate that the magnetic interactions are strongly dependent upon the diamagnetic A‐site ion. However, the data cannot uniquely attribute this effect...

Laser Remote Sensing of the Atmosphere

Laser beams can be used as long-range spectroscopic probes of the chemical composition and physical state of the atmosphere. The spectroscopic, optical, and laser requirements for atmospheric laser remote sensing are reviewed, and the sensitivity and limitations of the technique are described. A sampling of recent measurements includes the detection of urban air pollution and toxic chemicals in the atmosphere, the measurement of global circulation of volcanic ash in the upper atmosphere, and the observation of wind shear near airports.

Limitations of signal averaging due to temporal correlation in laser remote-sensing measurements.

Laser remote sensing involves the measurement of laser-beam transmission through the atmosphere and is subject to uncertainties caused by strong fluctuations due primarily to speckle, glint, and atmospheric-turbulence effects. These uncertainties are generally reduced by taking average values of increasing numbers of measurements. An experiment was carried out to directly measure the effect of signal averaging on back-scattered laser return signals from a diffusely reflecting target using a direct-detection differential-absorption lidar (DIAL) system. The improvement in accuracy obtained by averaging over increasing numbers of data points was found to be smaller than that predicted for independent measurements. The experimental results are shown to be in excellent agreement with a theoretical analysis which considers the effect of temporal correlation. The analysis indicates that small but long-term temporal correlation severely limits the improvement available through signal averaging.

Research on nonlinear optical materials: an assessment

The seven papers making up this assessment are based on the Workshop on Nonlinear Optical Materials held in April 1986.

Laser remote sensing of hydrazine, MMH, and UDMH using a differential-absorption CO 2 lidar

A dual mini-TEA CO(2) laser differential-absorption lidar system has been used to test the remote sensing of hydrazine, unsymmetrical dimethylhydrazine (UDMH), and monomethylhydrazine (MMH) in atmospheric conditions. Average concentrations of these compounds were measured using backscattered laser radiation from a target located at a range of 2.7 km. The experimental results indicate that average atmospheric concentration levels of the hydrazine compounds of the order of 40-100 ppb can be detected over ranges between 0.5 and 5 km. The level of concentration sensitivity over this interval was found to be limited primarily by atmospheric fluctuations. An investigation of the effect of these fluctuations on measurement uncertainties indicated that the fluctuations reduce the benefits of signal averaging over N pulses significantly below the expected square root of N improvement. It is also shown that uncertainties due to long-term atmospheric drifts can be effectively reduced through use of dual-laser lidar return ratios.

Classical Theory of the Ground Spin State in Spinels

An investigation of the ground state of the classical Heisenberg exchange energy in spinels has led to several new results. By means of a recent generalization of the method of Luttinger and Tisza, we have rigorously obtained the ground state in part of the exchange parameter space defined by nearest neighbor AB and BB interactions for tetragonally distorted spinels. The regions in this space where the Neel and Yafet-Kittel configurations minimize the energy are much smaller than predicted by the Yafet-Kittel six-sublattice theory. Outside of these regions, there exists a domain where the ground state is an antiferromagnetic spiral similar to those recently introduced in the literature. In still other regions, the ground state is a new type of spiral which is ferrimagnetic. An investigation of a parameter region including cubic spinels shows that such a ferrimagnetic spiral yields an appreciably lower energy than that given by the Yafet-Kittel model and is locally stable up to a specified ratio of BB to A...

Remote sensing of NO using a differential absorption lidar

Single-ended remote sensing measurements of atmospheric NO have been made using differential absorption of frequency-doubled pulsed CO(2) laser radiation backscattered from topographic targets. Returns were obtained from targets at ranges out to 1.4 km, and significant NO concentrations above ambient were observed over a path which crossed a traffic roadway at a range of 0.5 km. In view of the severe atmospheric water vapor absorption in the spectral region containing the NO absorption band, the range dependence of the lidar returns was also measured in order to determine the differential absorption of the ambient atmosphere.The results differed significantly from those computed from atmospheric transmission data tapes.

Magnetic Properties of MnCr2S4

The neutron diffraction pattern of MnCr2S4 at room temperature shows it to be a normal cubic spinel. The magnetization of this material was measured from 4.2° K through the Curie temperature. It was then determined, on the basis of a molecular field calculation, that the shape of the magnetization curve requires a large and ferromagnetic B‐B interaction such that JBB/JAB∼−4. This is in general accord with Lotgerings conclusion, which was based on paramagnetic susceptibility data, and indicates that the material should have a simple Neel (collinear) spin configuration. This expectation was confirmed by a 4.2°K neutron diffraction investigation which also established the cation moments to be 3μB per chromium ion and 4.7μB per manganese ion, in agreement with the measured net magnetization of 1.3μB. Furthermore, the 4.2°K diffraction pattern clearly indicates that the reduction of the manganese moment below its spin‐only value cannot be attributed to A‐site canting. On the basis of the above results, we are...