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The phenomenon of negative remanent magnetization (NRM) has been observed experimentally in a number of heterogeneous magnetic systems and has been considered anomalous. The existence of NRM in homogenous magnetic materials is still in debate, mainly due to the lack of compelling support from experimental data and a convincing theoretical explanation for its thermodynamic validation. Here we resolve the long-existing controversy by presenting experimental evidence and physical justification that NRM is real in a prototype homogeneous ferromagnetic nanoparticle, an europium sulfide nanoparticle. We provide novel insights into major and minor hysteresis behavior that illuminate the true nature of the observed inverted hysteresis and validate its thermodynamic permissibility and, for the first time, present counterintuitive magnetic aftereffect behavior that is consistent with the mechanism of magnetization reversal, possessing unique capability to identify NRM. The origin and conditions of NRM are explained quantitatively via a wasp-waist model, in combination of energy calculations.

The phase transformation between the orthorhombic and tetragonal structures of six high-T c superconductors, Ba2RCu3O7- δ , where R = Nd, Sm, Gd, Y, Ho, and Er, and δ = 0 to 1, has been investigated using techniques of x-ray diffraction, differential thermal analysis/thermogravimetric analysis (DTA/TGA) and electron diffraction. The transformation from the oxygen-rich orthorhombic phase to the oxygen-deficient tetragonal phase involves two orthorhombic phases. A superlattice cell caused by oxygen ordering, with a' = 2a, was observed for materials with smaller ionic radius (Y, Ho, and Er). For the larger lanthanide samples (Nd, Sm, and Gd), the a' = 2a type superlattice cell was not observed. The structural phase transition temperatures, oxygen stoichiometry and characteristics of the T c plateaus appear to correlate with the ionic radius, which varies based on the number of f electrons. Lanthanide elements with a smaller ionic radius stabilize the orthorhombic phase to higher temperatures and lower oxygen content. Also, the superconducting temperature is less sensitive to the oxygen content for materials with smaller ionic radius. The trend of dependence of the phase transformation temperature on ionic radius across the lanthanide series can be explained using a quasi-chemical approximation (QCA) whereby the strain effect plays an important role on the order-disorder transition due to the effect of oxygen content on the CuO chain sites.

We report a newly observed phenomenon, namely, the observation of a visible anomaly in the Bloch T(3/2) law for the temperature dependence of magnetization of nanostructured ferromagnets, for which we give a thermodynamic explanation. Our interpretation extends the Bloch law by introducing the system's chemical potential and assumes a finite Bose-Einstein condensation temperature T(BE) of the magnons. Our extension involves accounting for the possibility of a magnon or magnetic entropy term, leading to a magnon chemical potential (hitherto omitted in the traditional derivation) which varies with temperature. The result is a subtle upturn of the magnetization curves of ferromagnetic nanoparticles in the 10 to 50 K temperature range. Here we show experimental data for nanomaterials and an outline of the extended theory which includes T(BE).

Gadolinium nanoparticles have been produced at subambient temperature by alkalide reduction. The nanoparticles display maxima in the temperature dependence of their magnetization, cooled in the absence of an applied external field, at T(max) of 5.0 and 17.5 K for unheated samples and samples annealed at 1000 degrees C for 4 h, respectively. Field cooled behavior deviates at temperatures slightly above T(max), increasing at lower temperature. Curie-Weiss law fits of the high-temperature data yield magnetic moments in close agreement with those expected for noninteracting Gd(3+) ions, suggesting that the behavior seen is due to a magnetic transition rather than superparamagnetism. Magnetization is linearly dependent on field at temperatures higher than 7-8 times T(max) and shows remanence-free hysteresis at lower temperature, suggesting metamagnetism. Some annealed samples show evidence of additional ferromagnetic interactions below approximately 170 K. Magnetic entropy curves generated from magnetization data are consistent with that expected for a paramagnet.

Tungsten carbide has been shown to be an effective catalyst for a number of reactions that are readily catalyzed by platinum, but not at all by tungsten, and it was speculated that this behavior is due to changes in the electron distribution when carbon is added to tungsten. A test of this hypothesis, made by measuring the valence band x-ray photoelectron spectrum of tungsten carbide and comparing it with the spectra of tungsten and platinum, shows that, near the Fermi level, the electronic density of states of tungsten carbide more nearly resembles that of platinum than that of tungsten.

The purpose of this paper is to review in vivo NMR experiments [1, 2] on a transplantable tumor in mice and to discuss the feasibility of using noninvasive NMR for cancer detection in humans.

A brief overview of recent superconductivity research at NIST is presented. Emphasis is placed on the new high-temperature oxide superconductors, though mention is made of important work on low-temperature superconductors, and a few historical notes are included. NIST research covers a wide range of interests. For the new high-temperature superconductors, research activities include determination of physical properties such as elastic constants and electronic structure, development of new techniques such as magnetic-field modulated microwave-absorption and determination of phase diagrams and crystal structure. For the low-temperature superconductors, research spans studying the effect of stress on current density to the fabrication of a new Josephson junction voltage standard.

A physical explanation is given for the finding that the spin-lattice relaxation time T1, obtained by nuclear magnetic resonance from equivalent malignant and non-malignant samples, is greater for the phosphorus isotype 31P than for the proton 1H. The nuclear relaxation of the phosphorus proceeds by way of its interaction with associated cell water. Within limits determined by other variables, T1 is expected to vary inversely as the square of the gyromagnetic ratio.

The current theoretical and experimental situation with respect to Knight shifts and bulk susceptibility of Cd in pure Cd and in Cd-Mg and Cd-Hg alloys is reviewed. New experimental isotropic and anisotropic Knight shift data on 113 Cd in Cd-Mg and Cd-Hg alloys and on 199 Hg in Cd-Hg alloys are presented. The behavior of the 199 Hg sites is found to be remarkably similar to that of the 113 Cd site in the Cd-Hg alloys. However the ratio of isotropic Knight Shift of 199 Hg to 113 Cd is somewhat greater than the ratio of 199 Hg to 113 Cd atomic hyperfine fields suggesting that there is more local electronic " s " character at the Fermi surface in the alloy than in the pure metals. This is consistent with the theoretical picture of Cd in which phonon scattering induced by increasing temperature or alloying smears and weakens the lattice potential which in turn leads to a more " s " like Fermi surface. Further, the 113 Cd Knight shift is found to be a useful tool for monitoring the phase segregation that occurs in Cd-Mg due to internal oxidation and for determining the concentrations of solutes such as Hg and Mg in Cd-Hg and Cd-Mg alloys.

Computerized listings of 10,000 research papers, indexed at the Alloy Data Center, are now being made available in two forms. In the first, the Permuted Materials Index, all records are arranged alphabetically by chemical symbol. Records referring to alloys or compounds are listed under each of the constituent elements. In the second, the Author Index, all papers are listed alphabetically by first author in three groupings: the first, the NMR papers; the second, the soft x-ray papers; and the third, a broad group of papers that have been of general interest to the Alloy Physics Section. The magnetic tape on which the indexed file is stored will be made available at a future date.

The splitting of the 57 Fe Mössbauer spectrum in TiFe has been carefully measured in a magnetic field and compared with the zero-field quadrupolar splitting of sodium nitroprusside. The comparison gives a value of 1.699 mm/s at 295 K for the sodium nitroprusside splitting. This determination makes use of the known Knight shift of Fe in TiFe and is independent of any velocity measurement. Comparison is made with the splitting of the inner two lines of a pure Fe spectrum.

Pulsed nuclear magnetic resonance has been used to differentiate in vivo between normal mouse tail tissue and a malignant transplanted melanoma, S91, located on the tail. The tumor displayed a nuclear (proton) spin-lattice relaxation time of approximately 0.7 second contrasted with the simultaneously measured normal tail tissue relaxation time of approximately 0.3 second.

The Knight shift, K , measures the magnetic hyperfine field at the nucleus produced by the conduction electrons which are polarized in a magnetic field. Knight shifts are often dominated by the Pauli term and, in its most simple form, can be written as K = 〈 a 〉 χ p . Here χ p is the conduction electron Pauli spin susceptibility which depends on the density of states at the Fermi level, N(E f ), and 〈 a 〉 is an average magnetic hyperfine coupling constant associated with the wave function character at the nucleus, ∣ ψ F (0)∣ 2 , for conduction electrons at the Fermi surface. The Knight shift therefore provides, through 〈 a 〉, insight into the wave-function character associated with N ( E F ). Calculations of 〈 a 〉 involving an averaging over k -space have been attempted for a few simple metals up to the present time. For alloys and intermetallic compounds, rather different 〈 a 〉's are experimentally observed for different local environments, indicating that K samples the variation in local wave-function character, or a variation in local density of states. There is no unique way of separating the local variation of N(E F ) from ∣ ψ F ∣(0)∣ 2 . In this article the methods developed for relating K to the electronic properties for most of the types of cases encountered in the literature are reviewed. We discuss "simple" metals including problems of orbital magnetism and changes in K caused by electronic transitions such as melting. Knight shifts and their temperature dependence in metals and intermetallic compounds involving unfilled d shells, are discussed. We give estimates of atomic hyperfine fields due to single electrons, appropriate to those cases where problems due to electronic configurations do not make deductions from experiment too ambiguous. A density of states curve calculated for Cu is given, showing the relative importance of s - p , and d character for that metal. In a qualitative sense this Cu curve implies such information for other transition metals. We discuss alloy solid solutions for the cases where a "rigid" band model might be used to explain the results, and for cases where local effects have to be taken into account. The charge oscillation and RKKY approaches and their limitations are reviewed for cases of dilute nonmagnetic and d - or f -type impurities.