Robert M. White

Microstructure and magnetic properties of electroless Co‐P thin films grown on an aluminum base disk substrate

The microstructure and magnetic properties of electroless Co‐P films (∼100 to 1000 A) deposited on an aluminum base substrate are examined. The squareness and coercivity characterizing the magnetic hysteresis are found to correlate with the size and nature of the microcrystallites making up the film. It is argued that this correlation is due to coupling between the microcrystallites.

Exchangestriction in CdCr2S4 and CdCr2Se4

The ferromagnetic semiconductors CdCr2S4 and CdCr2Se4 both show large (∼0.2 eV) shifts in their absorption edges as the temperature decreases below their Curie temperatures. In CdCr2S4, the absorption edge increases with decreasing temperature; while in CdCrSe4, it decreases with decreasing temperature. It has been proposed that these shifts arise from a lattice expansion or contraction which is, in turn, produced by exchangestriction. Using x‐ray diffraction, we have measured the lattice parameters of these two materials from room‐temperature down to 4.2°K. CdCr2S4 is found to expand with decreasing temperature below its Curie point. The total fractional change is of the order of 10−3. CdCrSe4 contracts, but its fractional change is even smaller than that of the sulfide. Therefore, if this small expansivity is to explain the absorption‐edge shift, it requires a deformation potential of the order of 100 eV, which is at least an order of magnitude larger than what one would reasonably expect. Therefore, it...

Transit‐time measurements of domain‐wall mobilities in YFeO3

Using a transit‐time technique, we have measured domain‐wall velocities versus applied field H for Bloch, Neel, and the head‐to‐head walls in bulk single‐crystal bars of yttrium orthoferrite from 150 to 600 °K. Fields as high as 100 Oe were applied and wall velocities exceeding 106 cm/sec were recorded. After correcting for magnetostatic effects, all three walls display a well‐defined linear v‐H region, with a common mobility μ of 1.2×104 cm/sec Oe at 300 °K. As a function of temperature T the wall mobility varies as 1/T2. From these results it is argued that the mobility arises from thermal spin‐wave scattering. Velocities for the three walls become field independent abruptly at vs=4.2×105 cm/sec and maintain this velocity until H exceeds ∼60 Oe, whereupon the velocity again increases. We demonstrate that this ’’saturation’’ at vs is due to the interaction between the wall and the bulk acoustic phonons.

The lattice polarizability of PbI2

Abstract We have measured the far-i.r. reflectivity spectrum of the layer crystal PbI2 for the in-plane polarization. A strong reststrahlen band is observed, with TO and LO phonon frequencies located at 52 and 108 cm−1. The large lattice contribution accounts for three-quarters of the low-frequency dielectric constant of 26. The deduced infrared effective charge and electronic polarizability of PbI2 indicates that the intralayer bonding is closely related to the chemical bonding in the IV–VI rocksalt-structure crystals.

Reflectivity studies of Ti- and Ta-dichalcogenides: Phonons

Abstract The i.r.-spectra of the 1T phases of TiS2, TiSe2, TaS2 and TaSe2 reveal features attributed to large free carrier densities and to i.r.-active longwavelength phonons. We find a single phonon in TiS2, TiSe2 and TaS2, but six phonons in TaSe2, The oscillator strengths are significantly higher in the Ti-compounds yielding large values of the infrared effective charge, ∼6e, and electron-phonon coupling constant, 0.4–1.0. The observation of many modes in TaSe2 is attributed to a superlattice distortion driven by a charge density wave.

Chemical bonding and structure in layered transition metal dichalcogenides

Abstract Recent infrared reflectivity measurements on the transition metal dichalcogenides show striking differences among these compounds. The consequences of these differences on their band structures is discussed.

Theory of the Susceptibility of CeB6

Cerium hexaboride is an antiferromagnetic metal with a Neel temperature of 3°K. The high‐temperature inverse susceptibility of this material is observed to decrease with decreasing temperature more rapidly than would be expected from a Curie‐Weiss law. Since the temperature at which this deviation appears is much higher than the eventual ordering temperature, short‐range ordering effects should be negligible. Also, it can be shown that crystal‐field effects alone cannot produce such a decrease in χ−1. In this paper we offer a possible mechanism which explains this behavior. Basically, we assume that the Ce–Ce exchange arises through an indirect exchange involving the conduction electrons. We visualize the conduction electrons as moving in the spaces between the boron complexes so that the magnitude of the s‐f exchange will be different for the different crystal‐field states of cerium. In a cubic field the 2F5/2 state of Ce3+ splits into a Γ7 and a Γ8. Low‐temperature measurements of the saturation moment ...

Magnetic aftereffect in CoCr films

We have studied the relaxation or decay of the perpendicular remanent magnetization of CoCr films after the removal of a saturating magnetic field. The films have a perpendicular anisotropy and are similar to those being studied for application as perpendicular recording media. The time dependence of the decay of the magnetization was determined by measuring the magnitude of the magnetization continuously for time periods up to a thousand minutes using a SQUID magnetometer. This time dependence was studied over a range of temperatures for each of the samples (10–300 K). The decays are found to be quasilogarithmic over at least three decades of time and have been compared to two phenomological models. A fit to one model’s decay rate indicates the temperature dependence of the decay rate is nonmonotonic, i.e., the decay rate is a maximum in an intermediate temperature region for the films we have studied.

Nanosensor dosimetry of mouse blood proteins after exposure to ionizing radiation

Giant magnetoresistive (GMR) nanosensors provide a novel approach for measuring protein concentrations in blood for medical diagnosis. Using an in vivo mouse radiation model, we developed protocols for measuring Flt3 ligand (Flt3lg) and serum amyloid A1 (Saa1) in small amounts of blood collected during the first week after X-ray exposures of sham, 0.1, 1, 2, 3, or 6 Gy. Flt3lg concentrations showed excellent dose discrimination at ≥ 1 Gy in the time window of 1 to 7 days after exposure except 1 Gy at day 7. Saa1 dose response was limited to the first two days after exposure. A multiplex assay with both proteins showed improved dose classification accuracy. Our magneto-nanosensor assay demonstrates the dose and time responses, low-dose sensitivity, small volume requirements, and rapid speed that have important advantages in radiation triage biodosimetry.

Magnetic structure for a microundulator

A scheme is proposed for the fabrication of an undulator having a magnetic period in the millimeter range.