Structural and magnetotransport properties of the colossal magnetoresistance material<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Tl</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Mn</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml

Shimakawa, Yuichi; Kubo, Yoshimi; Manako, T.; Sushko, Yu. V.; Argyriou, D. N.; Jorgensen, J. D.

doi:10.1103/physrevb.55.6399

Cited by 52 publications

(44 citation statements)

References 24 publications

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“…First, there is a considerable deviation from the linear Mn-O-Mn bond with a Mn-O-Mn bend angle of Ϸ133°. 12 This weakens the AF superexchange between the Mn core spins mediated via the O(2p) electrons as we shall see below. Second, the Mn sublattice being magnetically frustrated ͑see Fig.…”

Section: Double-exchange In Metallic Pyrochloresmentioning

confidence: 99%

“…The two types of oxygen atoms O͑1͒ and O͑2͒ form, respectively, the Tl 4 O͑1͒ tetrahedra and the MnO͑2͒ 6 octahedra. 7,12 It is interesting that the Mn e g orbitals responsible for the Zener carriers in the perovskites contribute very little in the case of the pyrochlores. In fact, from symmetry considerations ͑see Table I͒, the Mn e g orbitals cannot contribute to the ⌫ 1 state at the center of the Brillouin zone.…”

Section: Electronic Band Structure Of Tl 2 Mn 2 Omentioning

confidence: 99%

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Electronic structure and exchange interactions in the manganese-based pyrochlore oxides

Mishra

Satpathy

1998

Phys. Rev. B

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We describe the ferromagnetism in the manganese pyrochlores, such as Tl 2 Mn 2 O 7 and Sc 2 Mn 2 O 7 , in terms of the interplay between superexchange, Zener double exchange, and indirect exchange, the first being antiferromagnetic ͑AF͒ while the last two are ferromagnetic. The tendency towards the antiferroalignment of the localized Mn spins is significantly weakened due to ͑i͒ the presence of spin frustration in the Mn sublattice and ͑ii͒ the large bend in the Mn-O-Mn bond, which reduces the magnitude of the AF superexchange. This helps the ferromagnetic interaction terms, viz., the indirect exchange and the Zener double exchange, to dominate. The indirect exchange on the Mn-O-Mn bond, which is modeled by a coupling of the O 2p electrons to the conduction-band states, produces a weak ferromagnetism in the insulating pyrochlores. In the metallic pyrochlores, the indirect exchange is also present, but it is now supplemented by the Zener double exchange, making the ferromagnetism more robust, as indicated from a higher value of the Curie temperature T c . Density-functional calculations for the metallic Tl 2 Mn 2 O 7 pyrochlore, show the itinerant Zener carriers mediating double-exchange to be electrons in the ⌫ 1 minority spin band crossing the Fermi energy, with predominantly Mn 3d, Tl 6s, and O 2p characters. These Zener carriers move in a lattice of localized Mn t 2g spins, with the carrier spins aligned antiparallel to the localized spins. ͓S0163-1829͑98͒05335-1͔

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Section: Double-exchange In Metallic Pyrochloresmentioning

confidence: 99%

Section: Electronic Band Structure Of Tl 2 Mn 2 Omentioning

confidence: 99%

Electronic structure and exchange interactions in the manganese-based pyrochlore oxides

Mishra

Satpathy

1998

Phys. Rev. B

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“…There is some additional scattering near T C due to critical scattering, but for a powder sample this is quite a sharp transition indicative of the high quality and uniformity of the sample. The inset shows a fit of the data to a power law, which yields an estimate for the transition temperature of T C = 123.2(3) K. The critical exponent we obtain from the fit is β = 0.312 (12), which is a typical value for a three dimensional ferromagnet. The errors quoted are statistical only.…”

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confidence: 99%

Spin Dynamics of the Magnetoresistive PyrochloreTl2Mn2O7

1998

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Neutron scattering has been used to study the magnetic order and spin dynamics of the colossal magnetoresistive pyrochlore Tl2Mn2O7. On cooling from the paramagnetic state, magnetic correlations develop and appear to diverge at TC (123K). In the ferromagnetic phase well defined spin waves are observed, with a gapless (∆ < 0.04 meV) dispersion relation E = Dq 2 as expected for an ideal isotropic ferromagnet. As T → TC from low T , the spin waves renormalize, but no significant central diffusive component to the fluctuation spectrum is observed in stark contrast to the La1−x(Ca,Ba,Sr)xMnO3 system. These results argue strongly that the mechanism responsible for the magnetoresistive effect has a different origin in these two classes of materials.PACS numbers: 75.40. Gb, 75.70Pa, 75.30.Kz, 75.25.+z Pure LaMnO 3 is an antiferromagnetic insulator in which the Mn 3+ O 3 octahedra exhibit a Jahn-Teller distortion that strongly couples the magnetic and lattice system [1,2]. Doping with divalent ions such as Ca, Sr, or Ba introduces Mn 4+ , and with sufficient doping (x > ∼ 0.1) the holes become mobile and the system transforms into a metal. In this metallic regime the double-exchange mechanism allows holes to move only if adjacent spins are parallel, which results in a dramatic increase in the conductivity when the spins order ferromagnetically, either by lowering the temperature or applying a magnetic field. The carrier mobility is thus intimately tied to both the lattice and magnetism, and considerable effort has been devoted to identifying the basic interactions that dominate the energetics and control the magnetoresistive properties. One avenue to unraveling these interactions is by measuring the spin dynamics, and a number of anomalous features have been identified including very strong damping of the spin waves in the ground state [3] and as a function of temperature [4], anomalous spin wave dispersion [5], and the development of a strong spin-diffusion component to the fluctuation spectrum well below T C [6][7][8]. Recently a new "colossal" magnetoresistive (CMR) compound has been discovered, namely the pyrochlore Tl 2 Mn 2 O 7 [9,10], and an important question concerns whether this new class of CMR materials contains the same underlying physics, or represents a completely new and different CMR mechanism. We have investigated the magnetic correlations, phase transition, and long wavelength spin dynamics using neutron scattering techniques, and find no evidence of the anomalous spin-diffusion component of the magnetic fluctuation spectrum that dominated the phase transition in the optimally-doped La 1−x Ca x MnO 3 (LCMO) manganites and appears to be associated with the spin component of the polaron in these materials. These results, coupled with the absence of any Jahn-Teller effects due to Mn 3+ in the system [11][12][13], argues strongly that the Mn pyrochlore belongs to a new class of CMR systems with a different underlying magnetoresistive mechanism.The neutron scattering measurements were carried out at the ...

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“…At a glance, DE model could be a plausible universal scenario since, exactly as in perovskites, the negative CMR in pyrochlore stems from the magnetic field dependent ferromagnetic (FM) ordering coupled with a sharp metal-insulator transition. However, such a prospect has received a strong criticism because of the small carrier density in Tl2Mn2O7 [6], and also due to the absence [7,8] Thus, the observed pressure effect in pyrochlores appears to contradict not only the DE model, but also the main postulate of the simple superexchange model that requires a FM character of the nearest-neighbor superexchange Simultaneous reduction under pressure of both the Tc and a can be understood, however, assuming that ferromagnetism in pyrochlore manganites originates from the longer-range (i.e. second-, third-nearest neighbor) interactions, while the nearest-neighbor coupling is an AF one.…”

Section: Introductionmentioning

confidence: 99%

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

Sushko

Kubo

Shimakawa

et al. 1998

THE REVIEW OF HIGH PRESSURE SCIENCE AND TECHNOLOGY

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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.

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Structural and magnetotransport properties of the colossal magnetoresistance materialTl2Mn2

Cited by 52 publications

References 24 publications

Electronic structure and exchange interactions in the manganese-based pyrochlore oxides

Electronic structure and exchange interactions in the manganese-based pyrochlore oxides

Spin Dynamics of the Magnetoresistive PyrochloreTl2Mn2O7

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

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