S. J. Pickart

Magnetic Structure and Exchange Interactions in Cubic Gadolinium Compounds

The two groups of metallic compounds Gd3+X3− (X=N, P, As, Sb, Bi) and Gd3+Z2− (Z=S, Se, Te) have the rocksalt structure and except for ferromagnetic GdN they are antiferromagnetic. Powder neutron diffraction measurements on GdS, GdSe, GdSb, and GdBi show them to have order of the second kind. The exchange interactions are discussed on the basis of the magnetic structure and susceptibility measurements.

Site preference of dilute transition metal solutes in Fe3Si

Abstract Dilute alloys of the type Fe 3− x T x Si (0⩽ x ⩽0.2), with T = V, Cr, Mn, Co, Ta, and W, were studied by means of neutron diffraction. The results indicate that V, Cr and Mn have a clear preference for the Fe site with 8 Fe first neighbors, Co for the Fe site with 4 Fe and 4 Si first neighbors, while only tentative conclusions can be drawn for Ta and W.

Applied field response of the spin glass state in amorphous NdFe2

The magnetic correlations and the response of the spin system to an applied field have been studied in sputtered amorphous NdFe2 using high‐resolution small angle neutron scattering (SANS) techniques. In zero field the line shape of the scattering is Lorentzian down to a spin freezing temperature of approximately 330 K. Below this temperature the line shape becomes increasingly non‐Lorentzian and has been analyzed using a Lorentzian plus Lorentzian‐squared cross section appropriate for a random anisotropy field system. The correlation length shows a broad maximum of 170 A near 285 K (below Tc) and then decreases to a constant value of ≊80 A for T< 100 K. SANS data taken in applied fields up to 18 kOe at 83 K produced constant intensity contours which are elliptically distorted with the Q∥H direction being the major axis. The overall intensity and the calculated spin correlation length both drop abruptly with increased field. This behavior is consistent with the field driving the dominant part of the spin ...

Flux lattice behavior in high‐Tc materials studied by neutron depolarization

The depolarization of a neutron beam passing through a sample of the high‐Tc superconductor YBa2Cu3O7−δ has been measured as a function of temperature and applied field. The difference in behavior between field‐cooled and zero‐field‐cooled states, the observation of hysteresis correlated with Hc1, and the disappearance of the effect near 55 K (below Tc) suggest an explanation in terms of vortex line lattice formation with possible connection to recently proposed flux line entanglement and melting.

Small angle magnetic scattering from amorphous DyCu

The unusual magnetic transition previously reported for the random anisotropy amorphous alloy DyCu was investigated by small−angle neutron scattering methods. The measurements were made on two 150‐μm sputtered films at the NBSR SANS spectrometer with incident 6.0‐A wavelength, at temperatures between 5 and 77 K, and for wavevectors 0.016≤q≤0.165 A−1. The small−angle magnetic intensity at fixed q shows a pronounced break at T≂17 K, corresponding to the weak‐field susceptibility cusp as measured on the same sample. The lineshapes at all temperatures were observed to be Lorentzian, with a correlation length slowly increasing as T decreases to a maximum of ∼13 A at 17 K and remaining constant thereafter. These results are compared with susceptibility and specific heat measurements for insight into the proposed mechanism for the transition.

Neutron depolarization effects in a high‐Tc superconductor (abstract)

Using the polarized beam small‐angle neutron scattering spectrometer at the Rhode Island Nuclear Science Center Reactor, we have observed significant depolarization of a neutron beam by passage through polycrystalline high‐Tc superconductors, specifically 123 Y‐Ba‐Cu‐O prepared and characterized at the IBM Watson Research Center. We believe that this technique will prove useful in studying aspects of these materials, such as the penetration depth of shielding currents, the presence and structure of trapped flux vortices, and grain size effects on the supercurrent distribution in polycrystalline samples. The two samples showed sharp transitions at 87 and 89 K, and have been studied at temperatures of 77 K; the second sample has also been studied at 4 K. The transition to the superconducting state was monitored by the shift in resonant frequency of a coil surrounding the sample. No measurable depolarization was observed in either sample at 77 K in both the field‐cooled and zero‐field‐cooled states, using ap...

Neutron depolarization by amorphous magnets in the critical region

Abstract The depolarization of neutrons by transmission through amorphous samples of NdFe 2 and HoFe 2 has been measured at temperatures in the neighborhood of their magnetic transitions. The results, when compared to other determinations of the Curie temperature ( T c ) suggest that T c may not be a well-defined quantity in these materials.

Small-angle magnetic scattering from a dilute amorphous Fe(Tb) alloy

Abstract Low temperature neutron scattering measurements on an amorphous alloy of composition Tb0.018 Fe0.982 reveal anomalous small-angle magnetic scattering similar to that observed in the more rare-earth-rich compounds TbFe2 and HoFe2. Inferences are drawn from the angular dependence of the scattering concerning cluster size and magnetic structure.

Contrasting magnetic and structural properties of two La manganites with the same doping levels

Powder and sintered samples of La0.9MnO3 (LMO) and La0.62Y0.07Ca0.31MnO3 (LYCMO) are used in this study. The two initial calcined in air powders have almost identical ferromagnetic properties with Tc≈250 K, and saturation magnetization of σ≈90 emu/gm corresponding to Mn4+(0.3)/Mn3+(0.7). Heating the calcined powders in argon to high temperatures lowers Tc to ≈120 K; however, subsequent oxygen annealing brings back the high Tc form. The LYCMO remains ferromagnetic with σ≈90 emu/gm in both heat treatments but only has a colossal magnetoresistance (MR) for Tc≈120 K. In contrast the LMO with Tc≈120 K has σ reduced to one-half its ferromagnetic value and the MR is almost zero. Neutron diffraction shows only the ferromagnetic structure below Tc for LYCMO but the low magnetization of LMO is best interpreted as a mixed ferro–antiferromagnetic structure rather than a canted one.

Magnetic field dependence of the small angle scattering in amorphous DyCu

The previously reported small angle neutron scattering (SANS) from the spin glass amorphous DyCu (TF =17 K) was examined in applied fields up to 15 kOe. Application of the field caused a decrease of the total SANS intensity and a decrease of the correlation length derived from the Lorentzian line shapes to ∼6 A from the zero field value of 13.5 A. Field‐cooling and relaxation effects were also observed in the SANS intensity and are compared with similar phenomena in magnetization measurements.