Quantum matter in high magnetic fields

Abstract:

Oxidation of the ferromagnetic B-site diluted manganese(III) perovskite La2GaMnO6 is investigated by synthesis of the La2-xSrxGaMnO6 series. At the x = 0.3 composition, which corresponds to diamagnetic element substitution at the B-site of metallic LaO.7SrO.3MnO3, the MnIII and MnIV valences order in real space at low temperature with an unusual lamellar motif. Orbital ordering at the MnIII centers in this array maintains ferromagnetism and enhances the static coherent Jahn-Teller distortion over that found for the pure MnIII end-member, facilitating eg electron hopping in the insulating state. Further oxidation to x = 0.5 completely suppresses the charge and orbital ordering, leading to glassy rather than long-range ordered magnetism.

Abstract:

We present the results of muon-spin-relaxation experiments for two materials which show geometric frustration. ZnCr22O4 has a spinel structure with S = 3/2 spins on a lattice of corner-sharing tetrahedra. Our experiments show that a local magnetic field which is quasi-static on the muon timescale develops below Tc = 12.5 K, a transition which has been associated with a three-dimensional analogue of the spin-Peierls transition. In contrast, Gd3Ga5O12 has a garnet structure with S = 7/2 spins arranged on interpenetrating triangular sublattices. In this material the muon data exhibit a temperature-dependent spin-relaxation rate indicative of slow spin fluctuations. We discuss these differing behaviours and relate them to the underlying physics in the two materials.

Abstract:

We report a series of magnetic and magnetotransport studies performed on Ga-substituted manganite compounds La2-xSrxMnGaO6 (x≤0.7). We replace half of the Mn ions in LaMnO3 with diamagnetic ions Ga³⁺ and find that the system transforms into a ferromagnet as the result of local coherent and incoherent distortions. Following additional doping with Sr ions we find that the La2-xSrxMnGaO6 compounds remain ferromagnetic up to x∼0.3 but for higher concentrations the competition between the ferromagnetic and antiferromagnetic superexchange interactions, together with the randomness in distribution of the magnetic ions, lead to magnetic frustration, cluster-glass and spin-glass behavior. All compounds are insulating and the activation energy decreases almost linearly with x. μSR measurements suggest the existence of short-range magnetic correlations in the high-temperature region. We measure a significant increase in the muon-spin relaxation rate at low temperatures due to fluctuations of random internal magnetic fields. The observed magnetoresistance in the high-temperature region is mainly a consequence of the reduction of the spin disorder on a local scale, and is as high as 50% in 14 T and T>100 K for samples with x=0.3 and x=0.5.

Abstract:

The room-temperature crystal structure of the brownmillerite SrCaMnGaO5+δ (δ = 0.035) has been refined from neutron powder diffraction data; space group Ima2, a = 15.7817(6), b = 5.4925(2), c = 5.3196(2) Å. Mn and Ga occupy 99.0(2)% of the 6- and 4-coordinate sites, respectively. A combination of magnetometry, neutron diffraction and μSR spectroscopy has shown that the compound orders magnetically at 180 K, and that the low-temperature phase has a G-type antiferromagnetic structure, with an ordered magnetic moment of 3.30(2) μB per Mn at 2 K. Displaced hysteresis loops provide evidence that the atomic moment has an additional, glassy component. Magnetometry shows that significant short-range magnetic interactions persist above 180 K, and μSR that the spin fluctuations are thermally activated in this temperature region. The compound is an electrical insulator which at 159 K shows an unusually large magnetoresistance of 85% in 6T, increasing to 90% in 13 T. © 2002 Elsevier Science (USA).