Quantum matter in high magnetic fields

Abstract:

The non-metallic, magnetically-dilute, cation-disordered perovskite La1.5Sr0.5RhMnO6 is shown to be a soft ferromagnet with T(c) = 105 K with an ordered magnetic moment of 3.56(8) μ(B) per Mn.

Abstract:

The room-temperature crystal structures of the n = 3 Ruddlesden-Popper phases Ca4Mn2FeO9.75 and Sr4Mn2FeO9.80 have been refined from neutron and X-ray powder diffraction data. Both adopt space group 14/mmm with (a,c) = (Ca, 3.73683(1), 27.0860(1) ̊), (Sr, 3.83393(1), 27.8148(1) ̊). In both compounds the cation site at the center of the perovskite blocks is preferentially occupied by Fe (Ca, Mn:Fe = 0.424:0.576(4)), and the anion vacancies are found around this site. The occupied anion sites show static disorder in Ca4Mn2FeO9.75 but not in Sr4Mn2FeO9.80. Both compounds are electrical insulators which order antiferromagnetically at TN = 75 K (Ca) or 90 K (Sr). Susceptibility and M(H) data suggest that not all the Mn and Fe cations take part in the long-range magnetic ordering, and there is evidence of a spin glass transition in both compounds at ̃11 K. The magnetic structure of Ca4Mn2FeO9.75 at 5 K has been determined by neutron diffraction. No ordered moment was detected on the Mn/Fe site at the center of the perovskite blocks; 0.74(1) μB per transition metal cation was measured at the sites on the block edges. Possible causes of magnetic frustration in this crystal structure are considered. Ca4Mn2FeO9.75 has a magnetotresistance of -4% at 137 K in a 14 T field. © 1999 American Chemical Society.

Abstract:

We present the results of a combined magnetization, muon-spin rotation, transport and magnetotransport study of the n = 3 Ruddlesden-Popper (RP) compound Ca4Mn3O10. This compound adopts a layered structure in which groups of three perovskite layers alternate with single rock-salt layers. The muon-spin rotation data show that there is a sharp magnetic phase transition at 115 K. The resistance and magnetoresistance of the sample show no particular features at this temperature, but the transition affects the energy barriers associated with hopping transport. The magnetoresistance is proportional to the square of the magnetization, and is largest at low temperatures; a 40% drop in resistivity is observed in a magnetic field of 14 T at 61 K, much smaller than that measured in the related n = ∞ RP (perovskite) manganites which exhibit colossal magnetoresistance (CMR).

Abstract:

Experiments on three double-layer (n=2) Ruddlesden-Popper (RP) systems are reported. Doping Sr1.8La1.2Mn2O7 (T-c = 126 K) with Nd to form Sr1.8La1.2-xNdxMn2O7 leads to a reduction in Curie temperature for low doping levels (x = 0.2), and to behavior reminiscent of Sr1.8Nd1.2Mn2O7 for x greater than or equal to 0.7. This suggests that it may be possible to control the temperature of maximum magnetoresistance chemically in these phases. The application of pressure (0 < P/GPa less than or equal to 1.8) is shown to modify the magnetotransport properties of Sr2NdMn2O7 to resemble those of Sr1.9Nd1.1Mn2O7. The changes can be explained by considering the relative strength of ferromagnetic and antiferromagnetic interactions within the material. Finally, the need for careful phase analysis of n = 2 RP materials is demonstrated by the misleading magnetization data recorded for a sample of Sr1.8Sm1.2Mn2O7 containing similar to 2.8% of an n = infinity perovskite phase. (C) 1998 American Institute of Physics. [S0021-8979(98)20911-X].