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Experimental efforts to characterize and develop an understanding of non Fermi liquid (NFL) behavior at low temperature in f-electron materials are reviewed for

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We present the results of field-and temperature-dependent resistivity and magnetization M measurements which show that colossal magnetoresistance in semimetallic EuB 6 is not associated with its bulk Curie temperature T C ϭ12.6 K, determined by a scaling analysis of M (H,T), but with a second phase transition at T M ϭ15.5 K. We propose that metallization occurs at T M via the overlap of magnetic polarons. The separation of the charge delocalization and bulk magnetic ordering transitions implies electronic and magnetic phase separation.The discovery of colossal magnetoresistance ͑CMR͒ in rare-earth manganites and manganese pyrochlores has renewed interest in the properties of magnetic polarons. 1 As has often been noted, CMR was also observed in a very different class of rare-earth magnets ͓EuO, 2 EuB 6 , 3-5 Gd 1Ϫx Eu x Se ͑Ref. 6͔͒ almost 30 years ago. It is generally believed that magnetic polarons are present in all of these materials, and that the sensitivity of polaronic transport to field is responsible for CMR. Although EuB 6 has historically been grouped with charge-doped rare-earth chalcogenides and oxides, in which the polaronic carriers are extrinsic, it has recently been shown that this intriguing material is a rare example of a low carrier density semimetal which also orders ferromagnetically. 7-9 We present here a detailed study of the field-and temperature-dependent resistivity and magnetization M of EuB 6 which suggests the presence of magnetic polarons in this system, and demonstrates that bulk magnetic order and metallization occur at different temperatures. This is a phenomenon unique to EuB 6 , and we will argue that it implies electronic and magnetic phase separation.EuB 6 crystallizes in the simple cubic CaB 6 structure. The electronic structure is derived primarily from the hybridization of the boron p bands, filled by electrons donated by the Eu ions, which are found to be divalent by both photoemission 10 and Mössbauer 11 measurements. Electronic structure calculations 7 indicate that small dilations of the boron octohedra cause overlap of the conduction and valence bands at the X points, rendering EuB 6 semimetallic. Shubnikov-de Haas and de Haas-van Alphen measurements 8,9 as well as angle-resolved photoemission ͑ARPES͒ measurements 10 confirm that EuB 6 is a semimetal. Quantum oscillation studies found that the sample studied here has an intrinsic carrier density of 1.2ϫ10 20 cm Ϫ3 . 9 Neutron-diffraction measurements on EuB 6 ͑Ref. 12͒ find that the spontaneous moment first appears near ϳ16 K, increases slowly as the temperature is lowered, and then displays quasi-mean-field behavior below ϳ12-13 K. The saturation moment of 6.9Ϯ0.2 B , measured at 1.5 K, is somewhat reduced from the nominal Eu 2ϩ value of 7.94 B , but is in good agreement with the 7.02 B predicted by bandstructure calculations. 7 Signatures of a second phase transition at 15.5 K have been found in the resistivity, the lowfield magnetization, and the zero-field specific heat. 13,14 Currently, there is no explanation fo...

We have made time-of-flight neutron diffraction measurements on α-manganese metal. Powder diffraction measurements were made at 14 temperatures between 15 and 305 K, and single crystal measurements were made at 15 and 300 K. We found that the crystal structure of α-Mn is tetragonal below its Néel point of 100 K, with crystal symmetry I4̄2m and magnetic (Shubnikov) symmetry PI4̄21c. In agreement with the earlier results of Yamada et al., there are six independent magnetic atoms, and we found that their moments are weakly temperature dependent. The onset of magnetic order causes slight changes in the atomic positions and in the average atomic elastic constant.