Evidence of anisotropic magnetic polarons inLa0.94Sr0.06MnO3

Abstract

Elastic and inelastic neutron scattering experiments have been performed in a La0.94Sr0.06MnO3 untwinned crystal, which exhibits an antiferromagnetic canted magnetic structure with ferromagnetic layers. The elastic small q scattering exhibits a modulation with an anisotropic q-dependence. It can be pictured by ferromagnetic inhomogeneities or large magnetic polarons with a platelike shape, the largest size (≈17Å) and largest inter-polaron distance (≈ 38Å) being within the ferromagnetic layers. Comparison with observations performed on Ca-doped samples, which show the growth of the magnetic polarons with doping, suggests that this growth is faster for the Sr than for the Ca substitution. Below the gap of the spin wave branch typical of the AF layered magnetic structure, an additional spin wave branch reveals a ferromagnetic and isotropic coupling, already found in Ca-doped samples. Its q-dependent intensity, very anisotropic, closely reflects the ferromagnetic correlations found for the static clusters. All these results agree with a two-phase electronic segregation occurring on a very small scale, although some characteristics of a canted state are also observed suggesting a weakly inhomogeneous state.

Approach to the metal-insulator transition inLa1−xCaxMnO<

“…Such a model is based upon two assumptions : (i) The diffuse scattering intensity arises from the < S z i S z j > spin correlations, with Oz//c. This assumption was actually checked in a previous study on a x = 0.06 Sr-doped sample, thanks to the absence of twinnning 19 . The "magnetic" contrast |∆m z | is defined as the difference between the average magnetizations m z inside and outside the cluster.…”

Confined Spin Waves Reveal an Assembly of Nanosize Domains in FerromagneticLa1−xCaxHennion1, Moussa2, Lehouelleur3 et al. 2005Phys. Rev. Lett.Self Cite467210View full textAdd to dashboardCite

“…The small-q dispersion (0), which determines T C , could be attributed to a coupling induced by mobile-holes through the clusters. These clusters differ from those found in the CAF state where the "holepoor" region corresponds to the matrix and the clusters, described as "hole-rich" platelets [4]. The general evolution with x in direct space is schematically shown in Fig.…”

“…Doping with holes induces a canted AF state (CAF). There, neutron diffuse scattering indicates the existence of magnetic inhomogeneities attributed to hole-rich clusters embedded in a hole-poor matrix [4], and a new spin-wave branch appears at smallq around F Bragg peaks. This new branch coexists with the spin-wave branch close to that of LaMnO 3 (SE type of coupling), attributed to the hole-poor matrix [5], which disappears at x=0.125 (F state).…”

Inhomogeneous magnetism in La-dopedCaMnO3. II. Nanometric-scale spin clusters and long-range spin canting

“…While the classic de Gennes theory for lightly-doped manganites describes the weak ferromagnetism in terms of spin-canted ground states 7 , a number of more recent theoretical studies indicate that homogeneous canted magnetic structures may not be energetically stable, suggesting a tendency towards magnetic and electronic phase segregation for both hole-doped 8,9,10,11,12,13,14,15,16,17 and electrondoped 14,15,16,17,18 manganites. In fact, for moderately hole-doped LaMnO 3 (5 − 8 % Ca-or Sr-doping), single crystal neutron-scattering studies revealed the existence of nanometric-scale magnetic inhomogeneities at low T 19,20,21 . Whether electron-doped manganites actually mirror this effect is an open experimental problem and a fundamental issue, since the phase diagram of electron-doped manganites is in general asymmetrical with respect to their hole-doped counterparts.…”