Hailemichael M. Seyoum

Adiabatic magnetocaloric temperature change in polycrystalline gadolinium - A new approach highlighting reversibility

The adiabatic temperature change (ΔT) during the magnetization and demagnetization processes of bulk gadolinium is directly measured for several applied magnetic fields in the temperature range 285 K to 305 K. During the magnetization process, ΔT measurements display the same maximum for each applied field when plotted against the initial temperature (Ti). However, during the demagnetization process, the maximum ΔT varies for each applied field. This discrepancy between the magnetization and demagnetization measurements appears inconsistent with the reversibility of the magnetocaloric effect. A new approach is undertaken to highlight the reversibility of the magnetocaloric effect by plotting ΔT against the average temperature change (Tavg) instead of Ti. The value of Tavg which corresponds to the maximum ΔT is found to increase linearly with the applied magnetic field, consistently for both the magnetization and demagnetization measurements. Solving the linear-fitting equations of these measurements gives...

Total photoabsorption cross section of molecular nitrogen near 83.4 nm

The photoabsorption cross section of molecular nitrogen near 83.4 nm has been measured at room temperature. The cross sections at the wavelengths of the 2s2p4 4P5/2, 4P3/2, and 4P1/2 (all - 2s²2p³ 4So3/2) O+ emission lines are measured to be 10.1 ± 1.6 Mb, 0.29 ± 0.02 Mb, and 0.049 ± 0.008 Mb respectively. These measurements were made with an O+ emission discharge source, and repeated using a synchrotron radiation source. Both measurements were made with the 0.008-nm resolution 6.65-m spectrometer at the SURF II storage ring of the National Institute for Standards and Technology.

Metastability in the Magnetic Structure of Ni 51 Mn 33.4 In 15.6 Heusler Alloy

The metastability of the nonstoichiometric Ni₅₁Mn_33.4In_15.6 Heusler alloy in the first-order transition region is studied using direct adiabatic temperature change (ΔT_ad) measurements for heating and cooling magnetization processes. During the heating process, ΔT_ad shows an unusual pronounced peak at 294 K in low applied magnetic fields. The height of this peak diminishes and eventually vanishes at higher fields. Kinks are as well observed in the 290-295 K temperature interval for both processes. The existence of the peak and kinks is indicative of the complex magnetic dynamics in Ni₅₁Mn_33.4In_15.6 and is attributed to a spin-reorientation transition near the first-order phase transition temperature.

The very high resolution spectrometer at the National Institute of Standards and Technology

Abstract The Very High Resolution Spectrometer at the Synchrotron Ultraviolet Radiation Facility of the National Institute of Standards and Technology has been upgraded with improved beamline throughput, instrument control and data acquisition. Since this upgrade, a number of high resolution measurements have been made in the vacuum ultraviolet region of the electromagnetic spectrum between 40 nm and 100 nm. Utilizing the features of this instrument, on-line photoelectric recording at intervals of 0.1 pm in first order from a synchrotron light source can be made. A resolution of 0.9 pm has been obtained at 80 nm.

Nanomaterials in Nanomedicine

Nanomedicine refers to the applications of nanotechnology to the field of medicine. Nanomaterials have led to the development of novel devices for the early detection of malignant tumors, as well as significant enhancements in efficient drug, gene and protein delivery mechanisms to targeted sites in the human body. As nanoparticles become increasingly smaller in size, they also present the potential for harming certain organs of the body. Safety issues involving nanoparticles need to be solved using in vivo techniques. Research in nanomedicine has improved biological therapies, such as vaccination, cell therapy and gene therapy. A particular kind of colloidal nanoparticle, called the liposome, which has properties similar to a red blood cell, has viscoelastic properties that make it extremely useful for a variety of applications in the pharmaceutical and consumer product sectors of the global market. Liposomes have been clinically established as efficient nanosystems for targeted drug delivery. Their efficacy has been demonstrated in reducing systemic effects and toxicity, as well as in attenuating drug clearance. The Maxwell Spring-Dashpot model has been reviewed for liposomes and the viscoelastic exponential equation shown to fit the liposome data. The relevance of this study is to the increasing use of viscoelastic characteristics of liposomes for efficient drug delivery and targeted destruction of malignant tumors. Nanobiotechnology has the potential to facilitate the integration of diagnostics with therapeutics, and in turn lead to personalized medicine tailored for a specific individual.

Evidence of metastability near the Curie temperature of polycrystalline gadolinium

The adiabatic temperature change (ΔT) during the magnetization and demagnetization processes of bulk gadolinium is directly measured for several applied magnetic fields in the temperature range 285 K to 305 K. In low magnetic fields, the measurements of ΔT display two maxima at 293.8 K and 291.5 K with an inflection point between 289 K and 293 K. The height of the second peak is more pronounced at low applied magnetic fields and diminishes in high fields. The occurrence of a second ΔT maximum is indicative of the complex spin dynamics in Gd and is related to the presence of metastable states near the Curie temperature.

Magnetic Properties of Zr

We report on a very interesting magnetic behavior in the intermetallic compound Zr9N11, namely, (i) spin-glass behavior and (ii) fluctuations in the magnetization versus temperature curves. The magnetic properties are consistent with structural results obtained by neutron diffraction and by scanning and transmission electron microscopies. The fluctuations are believed to arise from the Dzyaloshinskii-Moriya interactions occurring in the Zr9N11 compound. Between 7.86 kA/m and 31.85 kA/m, the temperature (Tp) of the zero-field cooled (Mzfc ) peak shifted to lower temperatures with increasing field. Tp is found to be proportional to Hb, where H is the applied field and the exponent b < 1. The value of b is found to be 0.68, which is close to the de Almeida-Thouless 2/3 power law that applies to spin glasses. Published neutron diffraction data showed that a Zr9N11 compound possessed Zr9Pt11-type tetragonal structure with columnar atomic chains parallel to the c axis and consisting of alternating Zr and Ni atoms. Small fluctuations present in the Mfc versus T plot at 31.85 kA/m are believed to be the result of atomic displacements along these chains over short distances. Another interesting featured observed in the Zr9N11 compound is the fact that at 3.98 kA/m the Mzfc values reside mostly above the corresponding Mfc values.

_{9}

Evidence suggesting reentrant behavior in the resistivity of a high-Tc superconductor is reported. As the temperature is lowered in a multiphase sample with nominal composition Tl1Ca1.7Ba0.8Cu3Ox, a partial (30%) drop in the resistivity and then reentranc e to the normal state are observed before the sample becomes a bulk superconductor. This quasi-reentrant behavior was recorded in the interval from 105 to 115 K both on cooling and warming the material. The magnetic susceptibility attains a minimum at low temperatures. This minimum shifts with applied external field.

Ni

The decay of magnetization with time is recorded for a quench‐melt‐growth processed YBa2Cu3O7−x sample. The vortex‐glass and collective pinning theories fit the data quite well over all time regimes with an exponent, μ, value of 0.78. The normalized logarithmic magnetic relaxation rate, S, attains a plateau value of 0.05, considerably higher than a variety of samples which fall within a universal plateau curve.

_{11}

BaCuO2 has a pronounced effect on the magnetization data of high-Tc superconductors. By subtracting the estimated magnetic contribution of BaCuO2 present in the superconducting samples, the susceptibility is seen to be purely diamagnetic.