The internal field distribution in the vortex state of YBa 2 Cu 3 O 6.60 is shown to be a sensitive measure of both the magnetic penetration depth l ab and the vortex-core radius r 0 . The temperature dependence of r 0 is found to be weaker than in the conventional superconductor NbSe 2 and much weaker than theoretical predictions for an isolated vortex. The effective vortex-core radius decreases sharply with increasing H, whereas l ab ͑H͒ is found to be much stronger than in NbSe 2 . [S0031-9007 (97)04251-8] PACS numbers: 74.60.Ec, 74.72.Bk, 76.75. + iThe magnetic penetration depth l and the coherence length j are the two fundamental length scales in a superconductor. l is directly related to the superfluid density n s , whereas j is the length scale for spatial variations in the superconducting order parameter. In recent years, measurements of l have provided strong evidence for unconventional pairing in the high-T c materials in that there are line nodes in the superconducting energy gap. In particular, in both the Meissner and vortex states, l has been observed to increase linearly with temperature T [1,2] and magnetic field H [3,4] at low T in single crystals of YBa 2 Cu 3 O 6.95 .Much less is known about the behavior of j. In fact, until now there has been no measurement of j deep in the superconducting state of a high-T c superconductor. The magnitude of j near the phase boundary has been estimated from measurements of the upper critical field H c2 using Ginzburg-Landau (GL) theory. For a type-II superconductor for H near H c2 , the coherence length is related to H c2 ͑T ͒ in GL theory byReliable estimates of j in YBa 2 Cu 3 O 72d made in this way are extremely difficult because the value of H c2 at T 0 is so large (i.e., ϳ100 T). Consequently, H c2 measurements are limited to high temperatures above T ͞T c ϳ 0.85, so that the low T behavior of j͑T͒ can be determined only by extrapolation. The situation is further complicated in the high-T c compounds by strong thermal fluctuations over a sizable region near T c which result in broad transitions and poor estimates of H c2 ͑T ͒. Furthermore, Eq. (1) may not be valid for an unconventional superconductor.It is desirable to have direct measurements of the coherence length, which in the vortex state is related to the size of the vortex cores. In particular, for a conventional superconductor j ϳ r 0 , where r 0 is the vortex-core radius [5]. In principle, both scanning tunneling microscopy (STM) and muon spin rotation (mSR) can be used to characterize the size of vortex cores and thereby determine j directly deep in the superconducting state. Recent STM [6] and mSR [7] measurements on NbSe 2 show that r 0 decreases with increasing H, as a result of the increased interaction between vortices. Similar studies on high-T c materials have not yet been performed with STM. Pinning of the vortices due to surface roughness and oxygen vacancies eliminates the long-range order in the vortex lattice and results in variations in the electronic structure of the cores. Further...

Nickel ferrites have been produced by the rf plasma deposition technique for the first time. This technique has promise for large scale fine particle production of ferrites. Powder x-ray diffraction linewidth measurements show an average particle size of about 55 nm. Mossbauer measurements show the presence of both the nickel ferrite and the nickel zinc ferrite with Fe3O4 created in each ferrite production process as an impurity. Magnetization and Mössbauer measurements show evidence that the aerosol prepared samples have small particle characteristics as compared to solid state reacted bulk materials.

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