Samples of aligned MgB2 crystallites have been prepared, allowing for the first time the direct identification of an upper critical field anisotropy H ab c2 /H c c2 = ξ ab /ξc ≃ 1.7; with ξ o,ab ≃ 70Å, ξo,c ≃ 40Å, and a mass anisotropy ratio m ab /mc ≃ 0.3. A ferromagnetic background signal was identified, possibly related to the raw materials purity.PACS numbers: 74.25. Ha, 74.60.Ec, 74.60.Jg, 74.70.Ad The recent discovery of superconductivity at 39 K in Magnesium Diboride (MgB 2 ) 1 has brought new excitement to the area of basic and applied research on superconducting materials. The observation of an isotope effect 2 , a BCS-type energy gap measured by Scanning Tunneling Spectroscopy 3 , as well as band structure studies 4,5 , point to a phonon-mediated superconductivity in MgB 2 . Some reports 6,7 have suggested that MgB 2 has an isotropic (or 3D) behavior, based on measurements done in polycrystalline samples. However, other studies 8,9 have also discussed its possible anisotropic nature. The relatively high values reported for the critical current density 6,10 (J c ) are possibly indicating the absence of weak link problems, which are well known in the high-T c materials. While polycrystalline MgB 2 is very easy to grow and is a readily available reagent, good-sized single crystals of this material have not yet been reported, and their development promises to be a greater challenge. Here we present results from samples of aligned MgB 2 crystallites that establish the anisotropy of the upper critical field (H c2 ), thus implying an anisotropic character for other superconducting properties, e.g., the energy gap, coherence length (ξ), field penetration depth (λ), and J c .In this work, a weakly sintered sample of MgB 2 was prepared, starting with a stoichiometric mixture of 99.5 at% pure Boron and 99.8 at% pure Magnesium, both in chips form (Johnson Matthey Electronics). The loose mixture was sealed in a Ta tube under Ar atmosphere, which was then encapsulated in a quartz ampoule and put into the furnace. The compound formation was processed by initially holding the furnace temperature at 1200 • C for 1 hour, followed by a decrease to 700 • C (10 • C/h), then to 600 • C (2 • C/h), and finally to room temperature at a rate of 100 • C/h. The weakly sintered product was easily crushed and milled employing mortar and pestle. Using a stereomicroscope we could observe a very uniform powder consisting mainly of shiny crystallites, with aspect ratios ranging from 2 to 5. This is mainly due to the main surface size distribution ranging from 5 to 40 µm for the larger linear dimension, since the crystallites' thickness is very regular, around 2 µm. The powder was then sieved into a range of particle sizes between 5 -20 µm, which allows the crystallites fraction to be maximized to almost 100%. Small amounts of the powder were then patiently spread on both sides of a small piece of paper, producing an almost perfect alignment of the crystallites, as shown in the SEM picture in the upper part of Fig. 1. The lower part ...

The dynamics of the magnetic properties of polycrystalline RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10Ϫ␦ ͑Ru-1222͒ have been studied by ac susceptibility and dc magnetization measurements, including relaxation and ageing studies. Ru-1222 is a reported magnetosuperconductor with Ru spins magnetic ordering at temperatures near 100 K and superconductivity in Cu-O 2 planes below T c ϳ40 K. The exact nature of Ru spins magnetic ordering is still being debated, and no conclusion has been reached yet. In this work, a frequency-dependent cusp was observed in ac vs T measurements, which is interpreted as a spin glass transition. The change in the cusp position with frequency follows the Vogel-Fulcher law, which is commonly accepted to describe a spin-glass with magnetically interacting clusters. Such an interpretation is supported by thermoremanent magnetization ͑TRM͒ measurements at Tϭ60 K. TRM relaxations are well described by a stretched exponential relation, and present significant aging effects.

In this paper we report on the magnetic properties of pure bulk ferromagnetic graphite, obtained by a chemical route previously described. This magnetic graphite has been obtained by a vapor reaction consisting of a controlled etching on the graphite structure. By magnetic force microscopy we have verified that its magnetic properties are related to the topographic defects introduced in the pristine material. Also, the magnetic properties have been verified through magnetization measurements as a function of temperature and applied magnetic field. At low temperatures ͑2 K͒ the saturation magnetization reaches a value of 0.58 emu/ g, leading to a defect concentration of 1250 ppm. The system is highly irreversible due to the inhomogeneity of the distribution of defects in the material. Two transition temperatures are detected, T c1 = 115͑5͒ K and T c2 = 315͑5͒ K. These transitions could be associated to the weak coupling between ferromagnetic regions related to defects and to the ferromagnetism inside the defect regions.

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The magnetic and superconducting properties of RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10−␦ polycrystalline samples with different oxygen-doping levels are presented. A strong suppression of the superconducting temperature ͑T c ͒, as well as a reduction in the superconducting fraction, occurs as the oxygen content is reduced by annealing the samples in oxygen-deprived atmospheres. Drastic changes in the electrical resistivity are observed above T c , possibly associated with oxygen removal, mainly from grain boundaries. However, the magnetic ordering is relatively less affected by the changes in oxygen content of the samples. The spin-glass transition is enhanced and shifted to higher temperatures with the reduction in oxygen content. This could be correlated with an increase in the spin disorder and frustration for the oxygen-depleted samples. Also, the same oxygen-vacancyinduced disorder could explain the reduction in the fraction of the sample showing antiferromagnetic order. We also report significant changes in the measured properties of the samples as a function of time.

Articles you may be interested inEnhanced metal-insulator transition and magnetoresistance in melt-processed La 0.67 Ca 0.33 MnO 3 and Hodoped manganites Structural, magnetic, and thermal measurements are carried out on the La 0.67 Ca 0.33 Mn 1Ϫx Zn x O 3 system with xϭ0.0-0.50. The structural characterization of the samples is done by Rietveld analysis of their neutron diffraction patterns. Zn substitutes at Mn site isostructurally until x ϭ0.30. Oxygen content remains nearly invariant with x. Magnetic and thermal measurements as well as the electrical resistance show a para-to-ferromagnetic transition at T p , which decreases with an increase of x. For low Zn concentration ͑until xϭ0.075͒ the decrease dT p /dx is smaller than for the larger concentrations of Zn. Relative decrease dT p /dx at higher concentrations (xϾ0.10) is similar to that observed earlier for the La 0.67 Ca 0.33 Mn 1Ϫx Fe x O 3 system. For the transition at T p , the related change of magnetic entropy (⌬S trs ) are calculated from the heat capacity data and indicate that for xϭ0 the expected value ⌬S trs ϭ12.8 J/mol K is recovered.

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The angular dependence of the bulk nucleation field of a sample made of aligned MgB 2 crystallites was obtained using dc magnetization and ac susceptibility measurements. A good fitting of the data by the three-dimensional anisotropic Ginzburg-Landau theory attests the bulk nature of the critical field H c2 . We found a mass anisotropy ratio ε 2 ≈ 0.39 that implies an anisotropy of the Fermi velocity, with a ratio of 1.6 between the in-plane and perpendicular directions, if an isotropic gap energy is assumed. For an swave anisotropic gap this ratio could increase to 2.5. Besides the fundamental implications of this result, it also implies the use of texturization techniques to optimize the critical current in wires and other polycrystalline forms of MgB 2 . 74.25.Ha, 74.60.Ec, 74.60.Ge, 74.70.AdTypeset using REVT E X Recent studies on the new MgB 2 superconductor 1 , with a critical temperature T c = 39 K, have evidenced its potential for applications 2,3 , although intense magnetic relaxation effects limit the critical current density, J c , at high magnetic fields 4 . This means that effective pinning centers must be added 5 into the material microstructure, in order to halt dissipative flux movements. Concerning the basic microscopic mechanism to explain the superconductivity in MgB 2 , several experimental 6-12 and theoretical [13][14][15] works have pointed to the relevance of a phonon-mediated interaction, in the framework of the BCS theory.Questions have been raised about the relevant phonon modes, and the gap and Fermi surface anisotropies, in an effort to interpret spectroscopic and thermal data that give values between 2.4 and 4.5 for the ratio 2∆ 0 /kT c , where ∆ 0 is the gap energy and k is the Boltz-