The anisotropy gamma of the superconducting state of high quality single crystals of MgB2 was determined, using torque magnetometry with two different methods. The anisotropy of the upper critical field was found to be temperature dependent, decreasing from gamma approximately 6 at 15 K to 2.8 at 35 K. Reversible torque data near T(c) reveal a field dependent anisotropy, increasing nearly linearly from gamma approximately equal to 2 in zero field to 3.7 in 10 kOe. The unusual temperature dependence is a true bulk property and can be explained by nonlocal effects of anisotropic pairing and/or the k--> dependence of the effective mass tensor.

We present single crystal neutron diffraction measurements on multiferroic LuFe2O4. Magnetic reflections are observed below transitions at 240 and 175 K indicating that the magnetic interactions in LuFe2O4 are 3-dimensional (3D) in character. The magnetic structure is refined as a ferrimagnetic spin configuration below the 240 K transition. Below 175 K a significant broadening of the magnetic peaks is observed along with the build up of a diffuse component to the magnetic scattering. 75.30.Kz, 28.20.Cz, 25.40.Dn Materials that offer the possibility of simultaneously controlling magnetic and electric degrees of freedom are the subject of intense interest [1]. Recently, multiferroic materials have been identified that show large coupling between electric and magnetic degrees of freedom. Ferroelectricity driven by either magnetic or charge ordering appears to be the origin of the large coupling and, hence, understanding the underlying electronic interactions is crucial for further insight into multiferroicity [1].LuFe 2 O 4 has attracted attention as a novel ferroelectric material where ferroelectricity is driven by the electronic process of charge ordering of Fe 2+ and Fe 3+ ions and for indications of coupling between electronic and magnetic degrees of freedom [2,3,4,5,6]. LuFe 2 O 4 is a member of the RFe 2 O 4 (R = rare earth element) family, the physical properties of which depend strongly on oxygen stoichiometry. For example, nearly stoichiometric YFe 2 O 4 exhibits three-dimensional (3D) magnetic order while oxygen deficient YFe 2 O 4 exhibits two-dimensional (2D) magnetic order [7]. LuFe 2 O 4 exhibits multiple phase transitions. 2D charge correlations are observed below 500 K, while below 320 K 3D charge order is established, roughly coinciding with the onset of ferroelectricity [2,8]. Magnetic order appears below 240 K and 2D ferrimagnetic order has been suggested by neutron scattering studies [9]. However, strong sample dependent behavior observed in other members of RFe 2 O 4 [7] suggests that unraveling the interesting behavior of LuFe 2 O 4 requires paying due attention to sample quality.In this letter we present extensive neutron diffraction measurements from 20 to 300 K on high quality single crystals of LuFe 2 O 4 . We report several new findings that provide information about the underlying magnetic interactions. First, our measurements indicate that below 240 K 3D magnetic correlations exist with magnetic intensity appearing at (1/3 1/3 L) where L may take on integer and 1/2-integer values. The magnetic structure is refined with a ferrimagnetic spin configuration with a propagation vector of (1/3 1/3 0). The magnetic intensity appearing on peaks where L is a 1/2-integer is a consequence of the charge ordering at ∼320 K. In addition, evidence is presented for a second transition at 175 K with significant changes in magnetic peak intensities and broadening of many reflections.Single crystals of LuFe 2 O 4 were grown by floatingzone-melting, using an oxygen partial pressure tuned by a CO/CO 2 mi...

We present the refinement of the crystal structure of charge-ordered LuFe2O4, based on single-crystal x-ray diffraction data. The arrangement of the different Fe-valence states, determined with bond-valence-sum analysis, corresponds to a stacking of charged Fe bilayers, in contrast with the polar bilayers previously suggested. This arrangement is supported by an analysis of x-ray magnetic circular dichroism spectra, which also evidences a strong charge-spin coupling. The nonpolar bilayers are inconsistent with charge order based ferroelectricity.

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