E. J. McNiff

Surface spin disorder in ferrite nanoparticles (invited)

Anomalous magnetic properties of organic coated NiFe2O4 nanoparticles have been reported previously (Berkowitz et al.).5 These properties included low magnetization with a large differential susceptibility at high fields and shifted hysteresis loops after field cooling, while Mossbauer spectra indicated that all of the material was magnetically ordered. In the present study, we find that the lack of saturation in high fields is accompanied by irreversibility (i.e., hysteresis loops are open) up to 160 kOe. In addition, the particles exhibit time dependent magnetization in 70 kOe applied field. The high field irreversibility and the loop shift both vanish above 50 K. We propose a model of the magnetization within these particles consisting of ferrimagnetically aligned core spins and a spin- glass-like surface layer. We find that qualitative features of this model are reproduced by a numerical calculation of the spin distribution. The implications of this model for possible macroscopic quantum tunneling in ...

600 kG superconductors

Abstract Upper critical fields, H C2 ( T ), for Sn-, SnAl-, and Pb- molybdenum sulfide superconducting materials were measured in dc fields to 215 kG and pulsed fields to 495 kG. For the highest T c (14.4 K) Pb compounds H c2 (4.2K) is approximately 510 kG and extrapolates to H C 2 (0)=600 kG. These are the highest critical fields reported for any superconductor.

Upper critical fields of high-temperature superconducting Nb1−y(Al1−xGex)y and Nb3Al: Measurements of Hc2 > 400 kG at 4.2°K

Abstract Pulsed field measurements of the upper critical fields at 4.2°K, H c2 (4.2°K), in Nb 1− y (Al 1− x Ge x ) y and Nb 3 Al are approximately 410 kG and 295 kG respectively. The data are consistent with complete suppression of Pauli paramagnetic limiting.

Upper critical fields of Nb3Ge thin film superconductors

The upper critical field Hc2(T) of the highest Tc(∼23K) Nb3Ge superconducting films has been found to be ≈370kG at 4.2K. Measurements on lower Tc films show very broad transitions reflecting nonuniformity. The Hc2(T) characteristics are consistent with other Nb3X type II superconductors.

Magnetic field induced properties of manganite perovskites with colossal magnetoresistance (invited)

We present a systematic study of the magnetotransport and magnetic properties of the half-doped La0.5Ca0.5MnO3+δ system. The solid is a metamagnet which undergoes a first-order antiferromagnet (AFM) to ferromagnet (FM) phase transition under a field or by changing temperature. Associated with the AFM–FM transition is an insulator to metal transition. A maximum 109-fold magnetoresistance ratio has been observed at 4.2 K between the least and the most conductive states. At low T (⩽50 K), we have also observed two additional metastable electronic states in the canted AFM state at certain fields. The resistivity of each state differs from one another by at least one order of magnitude. The existence of these multiple states may be related to the unique charge- and spin-ordered state of the half-doped manganite.

Anisotropy of Hc2 in single crystal Nb3Sn and V3Si at high magnetic fields: Limitations of linear chain model

Abstract The anisotropy of H c2 for single crystal Nb 3 Sn and V 3 Si in the low temperature tetragonal phase was measured in high magnetic fields. At ∼ 8K, δ = [ H c2 [100]- H c2 [110])/ H c2 ] ≈0.05 for Nb 3 Sn and δ ≈ 0.02 for V 3 Si. The data do not show evidence of a highly anisotropic Fermi surface for Nb 3 Sn or V 3 Si.

The Pure Pd Problem

Magnetic‐moment measurements vs field and temperature are presented for both Pd and Pt. An extreme sensitivity of χ for Pd at low T has been observed and the conditions for unambiguous measurements of χ at low T are discussed. Results for Pt show an analogous sensitivity to trace amounts of impurities and indicate a maximum in the susceptibility near 100°K. Brief comments concerning estimates of exchange enhancement by means of high‐field measurements are also made.

Rare Earth Metal Single Crystals. I. High‐Field Properties of Dy, Er, Ho, Tb, and Gd

Magnetic‐moment measurements have been made along principal axes of Dy, Er, Ho, Tb, and Gd single crystals at 4.2°K and dc fields to 150 kOe. Dy and Tb showed similar magnetic properties. Saturation along the easy axis was achieved at low fields, while along the hard (c) axis the moment was nearly linear with field and did not approach saturation. Anisotropy was observed in the basal plane at low fields. An irreversible change in Dy and Tb occurred below 150 kOe for fields along the hard axis, such that subsequently saturation could not be achieved along the easy axis. Extrapolation of the low‐field (unstrained) data along the hard axis indicated anisotropy energies of about 109 ergs/cm3. In Er significant modifications of the zero‐field conical moment state were observed and theoretical saturation was almost achieved along the c axis. Fields applied in basal‐plane directions collapsed the conical state and produced fan‐moment distributions approaching saturation near 150 kOe.

Upper critical fields of layered superconducting NbSe2 at low temperature

Abstract The upper critical field versus temperature Hc2 (T) is presented for several NbSe2 samples. Hc2 (T) is above the dirty or clean limit, and at low T is greater than the paramagnetic limiting field. No simple T-dependence is observed. The anisotropy is 3.23 ± 0.03 for 0 ⩽ Ho ⩽ 150 kG and for 1.2 K ⩽ T ⩽ 4.2 K.

High field measurements of giant intrinsic magnetic hardness in SmCo5−xNix and SmCo5−yCuy

The phenomenon of giant intrinsic magnetic hardness in SmCo5−x Ni x and SmCo5−y Cu y is investigated in applied fields up to 23.5 T. At 4.2 K, the average propagation field (approximate coercive field)H p is ≃13.0 T, 22.0 T and 18.2 T for x=3, 3.6 and 4.2 respectively, and H p is ≃12.0 and ≃23.0 T for y=2.5 and 3.6 respectively. At 77 K, values of H p ≃5.0 and ≃5.5 T for y=2.5. Some values of H p at 4.2 K are the largest observed to date for any material. The results are consistent with models for thermally activated domain wall motion. For several materials the irreversible portion of the magnetization is not saturated at 23 T and 4.2 K. In these cases the maximum magnetization is less than that observed at 77 K.