Spin Dynamics of the Magnetoresistive PyrochloreTl2Mn2O7

Abstract

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 ...

The Origin of Bulk Magnetoresistivity in Manganites

“…The local spin density approximation (48) and the tight bonding band calculations (49) show that the Fermi surface is derived from strongly mixed combinations of Tl, Mn, and O orbitals. It is interesting to note that Lynn et al (50) did not see the central di!usive component to the neutron scattering #uctuation spectrum in sharp contrast to the La \V (Ca, BaSr) V MnO system. This central di!usive component was interpreted as a signature of magnetic clusters/polarons.…”

Structure and spin dynamics ofLa0.85Sr0.15MnO3Vasiliu-Doloc1, Lynn2, Moudden3 et al. 1998Phys. Rev. BSelf Cite785650View full textAdd to dashboardCite

“…We have observed a similar anomalous behavior of the central peak in the strongly-doped system La 0.70 Sr 0.30 MnO 3 , 23 but not in the pyrochlore Tl 2 Mn 2 O 7 . 24 It thus appears that the coexistence of spin-wave excitations and spin diffusion is a common characteristic for many perovskite manganites, and it raises the question about its relevance for the colossal magnetoresistance property of the perovskite manganites.…”

Neutron investigation of the magnetic scattering in an iron-based ferromagnetic superconductor

“…An alternative possibility is to note that the Q and T dependence of this scattering is quite similar to the expected scattering from ferromagnetic domains/domain walls, such as has been observed in the polycrystalline Tl 2 Mn 2 O 7 ferromagnet. [39] To investigate the field dependence of this small angle magnetic scattering as well as search for scattering from the field-induced vortex lattice, we carried out additional SANS measurements under identical instrumental conditions in a horizontal field (parallel to the incident neutrons) superconducting SANS magnet. For the field-dependent measurements the sample was pressed into a pellet to prevent the grains from possibly changing orientation with field.…”