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AP Ramirez

Publications and source records attributed to AP Ramirez.

At least 19 recordsLinked to original sources
PubMed2000

Entropy balance and evidence for local spin singlets in a Kagome-like magnet

We have measured the specific heat C(T) of the S = 3 / 2 Kagome-lattice-containing compound SrCr (9p)Ga (12-9p)O (19). We find little field dependence of the low-temperature C(T), consistent with a low-energy spectrum dominated by many-body singlet excitations. At high temperatures, we recover only approximately 50% of the total R ln4 entropy.

Journal Article↗
PubMed2000

Enhanced critical currents of superconducting ErNi2B2C in the ferromagnetically ordered state

We report on transport and magnetization studies of the critical current in single crystal ErNi2B2C for applied fields below 4 kG. Below T approximately 2.5 K superconductivity coexists with weak ferromagnetism. We find that the critical currents are strongly enhanced for all field orientations in this ferromagnetic regime, corresponding to a threefold increase of the pinning force of the flux line lattice. We speculate that this increase is due to strong pair breaking by the ferromagnetism.

Journal Article↗
PubMed1996

Colossal Magnetoresistance Without Mn3+/Mn4+ Double Exchange in the Stoichiometric Pyrochlore Tl2Mn2O7

Structural analysis from powder neutron and single-crystal x-ray diffraction data for a sample of the Tl2Mn2O7 pyrochlore, which exhibits colossal magnetoresistance (CMR), shows no deviations from ideal stoichiometry. This analysis gives an Mn-O distance of 1.90 angstroms, which is significantly shorter than the Mn-O distances (1.94 to 2.00 angstroms) observed in phases based on LaMnO3 perovskites that exhibit CMR. Both results in Tl2Mn2O7 indicate oxidation states very close to Tl23+Mn24+O7. Thus, Tl2Mn2O7 has neither mixed valence for a double-exchange magnetic interaction nor a Jahn-Teller cation such as Mn3+, both of which were thought to have an important function in CMR materials. An alternate mechanism for CMR in Tl2Mn2O7 based on magnetic ordering driven by superexchange and strong spin-fluctuation scattering above the Curie temperature is proposed here.

Journal Article↗