Large lattice distortions associated with the magnetic transition in La0.7Sr0.3MnO3

Abstract

Col ossal magnetoresistance (CMR) is associated with the phase transition from a metallic ferromagnetic to insulating paramag netic phase, which can be controlled by an applied magnetic fiel d. The insulating phase occurs due to trapping of the charge carriers by polaronic lattice distortions, which raise the resistivity. Theories based on local physics predict that the magnitude of the resistivity jump at T c is determined by how much, on average, the amplitude of these distorti ons increases at the phase transition. Using neutron scattering, we measured the average distortion amplitude in La 0.7 Sr 0.3 MnO 3 . Surprisingly, its increase from below to above T c is just as large as in other mang anites, which have a much l arger resistivity jump. This result suggests that the strength of CMR is determined not by the size of distortions, but by their cooperati ve nature s pecific to each compound. Existing theories need to be extended to include correlations between different unit cells to explain and predict the strength of CMR. Manganese perovskite o xides have a rich temperaturedoping phase diagram spanning a wide range of physical phenomena of both practical and fundamental interest such as metal-insulator transitions, colossal magnetoresistance, mu ltiferro icity, polaron format ion etc. Th is behavior is driven by strong coupling between magnetic, orbital, charge, and atomic lattice degrees of freedo m with ordering in one channel having a strong, often transformational effect on the other. Electron-phonon coupling through the Jahn-Teller effect plays a special role by tying electronic orb ital states to the atomic latt ice whose deformat ions trap charge carriers and thus raise the electrical resistivity. In half-doped manganites the Jahn-Teller effect leads to CE order characterized by very large resistivity, which then drops by several orders of magnitude when the CE order melts at elevated temperatures. At lower doping Jahn-Teller d istortions become frustrated: A distortion in one unit cell interferes with the distortion in the neighboring unit cells and long range CE order cannot form. This part of the phase diagram is famous for its outstandingly large (colossal) magnetoresistance (CM R) [6] , [7]. CM R occurs due to the ferro magnetic transition accompanied by a large resistivity change. The resistivity is metallic-like at temperatures, T, less than the ferro magnetic t ransition temperature, T C , i.e. it is relatively low and increases with T, whereas at T > T C it is very large and typically decreases with T. Due to h igh sensitivity of T C to the applied magnetic field, resistivity changes dramat ically as a function of field in the vicinity of T C . The CM R mechanism itself is straightforward, but what is unusual is the very existence of the "metal-insulator" transition at T C . It has been proposed that the resistivity increases because of competition between double-exchange and Jahn-Teller interactions [8,9]. The former favors ferro magnetism and an undistorted lattice and wins below T C . T...

Spin wave damping arising from phase coexistence below Tc in colossal magnetoresistive La0.7Ca0.3MnO3Helton1, Jones2, Parshall3 et al. 2017Phys. Rev. B111100View full textAdd to dashboardCite

“…However, experiments on manganite materials have not found evidence of significant coupling at the crossing between the spin wave and phonon branches [20][21][22]. The lattice and magnetic degrees of freedom are however coupled in the polaron regime above T C [23], and in a variety of manganite compounds the local distortions in the crystallographic position of oxygen atoms have been found to increase at T C [24,25] while acoustic phonons in La 0.7 Sr 0.3 MnO 3 soften and broaden when heated through T c [26]. A later work by Ye, et al [12] reported significantly damped excitations, with Γ (the full-width at half-maximum, FWHM) reaching 20 meV along the [110] direction and 60 meV along the [111] direction, but no abrupt change in broadening.…”

Polaronic metal phases inLa0.7Sr0.3MnO3uncovered by inelast

“…The polaronic metal character of La 0.7 Ca 0. become paramagnetic polaronic insulators with static polarons [8,14], whereas LSMO becomes a paramagnetic polaronic metal with mobile polarons. This explains why the absolute strength of magnetoresistance near 𝑇 𝐶 in LSMO and La 1.2 Sr 1.8 Mn 2 O 7 differs by more than an order of magnitude [20]. Phase separation between charge-ordered (CO) and ferromagnetic metallic regions is discussed in CMR manganites since more than 15 years [29] and, recently, CMR in undoped LaMnO 3 under an applied pressure of 𝑝 = 32 − 35 GPa was also explained in this scenario [30].…”

“…So the point of view emerged that the metallicity, high T C, and small CMR effect in LSMO, are well explained by DE physics without JT polarons [3]. However, in a recent experiment [20] some of us demonstrated that the lattice distortions in LSMO increase at 𝑇 𝐶 similarly to other manganites having a much larger magnetoresistance effect. Small polarons in LSMO were reported by pulsed neutron diffraction as well a [21].…”

Magnetodielectric behaviour in La0.53Ca0.47MnO3

“…Acting as extended defects though, both these interface-effects provide increased scattering of the charge-carriers, adding to the dc resistivity ρ 0T [37]. Consequently, field-dependent topological rearrangement of coexistent phases (FM tubules in PM matrix say, [14]) and of the associated local isostructural variations across their "domain-walls" reflect in the (ac) permittivity directly, and alter the (dc) resistance mainly via their across-the-sample percolative-reorganization; manifesting MR dcindependent (magneto-dielectric) ac effect at high-frequencies.…”