Pressure Dependence of Ferromagnetic Curie Temperature and Colossal Magnetoresistance Effect in pyrochlore manganites.

Abstract

Negative pressure dependence of ferromagnetic Curie temperature Tc is established by the susceptibility measurememeasuremeesurement on a series of isostructural pyrochlores A2Mn2O7, (A=Tl,In,Y,Lu).At temperatures well below and above Tc the pronounced magnetothermohistory effects are revealed, indicating the presence of competing antiferromagnetic interaction. The results contradict not only the double exchange model, which dictates positive pressure shift of Tc, but also the simple superexchange scenario where the ferromagnetic character of the nearest-neighbor interaction allows no deviations from the standard ferromagnetic behaviour. We propose a model where a long-range ferromagnetic ordering originates from next-near-neighbor ferromagnetic interactions which compete with the antiferromagnetic nearest-neighbor superexchange, greatly reduced by the frustrated topology of pyrochlores. We also report a dramatic increase under pressure of the colossal magnetoresistance effect.

Carrier density change in the colossal-magnetoresistance pyrochloreTl2Mn2OImai1, Shimakawa2, Sushko3 et al. 2000Phys. Rev. BSelf Cite293300View full textAdd to dashboardCite

“…11,12 The negative pressure dependence of T C also conflicts with the double-exchange model. 13,14 Although a very small number of oxygen vacancies could yield such a small number of carriers, the electronic band structure calculation for ferromagnetic Tl 2 Mn 2 O 7 with a stoichiometric composition revealed that a small number of free-electron-like carriers were produced in one of the down-spin bands hybridized among the Tl 6s, Mn 3d, and O 2p orbitals. [15][16][17] The calculated number of carriers is ϳ0.005 electrons per formula unit, which is consistent with the results of Hall effect measurements.…”

Magnetic properties and magnetoresistance of the Ru-substitutedTl2Mn2−xSenis1, Martı́nez2, Fontcuberta3 et al. 2000Phys. Rev. B

“…In this sense, the possibility of having AF NN superexchange coupling has been discussed on the basis of some neutron scattering experiments 16 and pressure-dependent magnetization measurements, 17 in whose case the FM behavior experimentally observed has been explained in the framework of a more complex scenario, in which ferromagnetism originates from Mn 3d -Mn 3d long-range superexchange interactions via Tl 6s orbitals. 17 The observation of irreversibility in the ZFC-FC curves and a certain degree of magnetic frustration f ϭ/T c in the pure Tl 2 Mn 2 O 7 compound imply the existence of competing interactions and may be considered as proof of the existence of AF interactions even in the xϭ0 pure compound.…”

“…17 The observation of irreversibility in the ZFC-FC curves and a certain degree of magnetic frustration f ϭ/T c in the pure Tl 2 Mn 2 O 7 compound imply the existence of competing interactions and may be considered as proof of the existence of AF interactions even in the xϭ0 pure compound. Therefore, our results are in agreement with the predictions of Refs.…”

Pressure effects on superconducting and magnetic transitions in layered sodium cobalt oxides, NaxCoO2

“…The temperature-and field-dependencies of the magnetization were measured with a commercial MPMS-5 (Quantum Design™) SQUID magnetometer. For measurements under applied pressure we combined the MPMS magnetometer with a helium-gas high-pressure system in a set-up identical to that described in connection to the earlier pressure-dependent studies 17,18 of magnetic susceptibility of the CMR pyrochlore The temperature dependence of the magnetic moment along c-axis of Na 0.75 CoO 2 measured under initial value of pressure of 50 bar is presented in the left panel of Figure 1. The raw data, that include the sample signal plus the negligible small background from the pressure cell, are shown.…”

“…The temperature-and field-dependencies of the magnetization were measured with a commercial MPMS-5 (Quantum Design™) SQUID magnetometer. For measurements under applied pressure we combined the MPMS magnetometer with a helium-gas high-pressure system in a set-up identical to that described in connection to the earlier pressure-dependent studies 17,18 of magnetic susceptibility of the CMR pyrochlore Tl 2 Mn 2 O 7 and double-layer ruthenium oxide perovskite Sr 3 Ru 2 O 7 . The main advantage of using helium as a pressure transmitting medium is a truly hydrostatic, and easily controlled during the cooling/warming cycles, pressure.…”