Confined Spin Waves Reveal an Assembly of Nanosize Domains in FerromagneticLa1−xCax

Abstract

We report a study of spin-waves in ferromagnetic La1−xCaxMnO3, at concentrations x=0.17 and x=0.2 very close to the metallic transition (x=0.225). Below TC , in the quasi-metallic state (T=150K), nearly q-independent energy levels are observed. They are characteristic of standing spin waves confined into finite-size ferromagnetic domains, defined only in (a, b) plane for x=0.17, and in all directions for x=0.2. They allow an estimation of the domain's size, a few lattice spacings, and of the magnetic coupling constants inside the domains. These constants, anisotropic, are typical of an orbital-ordered state, allowing to characterize the domains as "hole-poor". The precursor state of the CMR metallic phase appears, therefore, as an assembly of small orbital-ordered domains. In metallic La 1−x Ca x MnO 3 , for x≈0.3, the origin of the colossal magneto-resistance (CMR) which occurs at the metal-insulator transition is still a challenge for theories. The role played by an inhomogeneous ground state has been outlined by several theoretical[1] and experimental works [2,3]. In the semi-conducting parent LaMnO 3 , the magnetic coupling is of super-exchange (SE) type, ferromagnetic (F) in the (a, b) plane (J a,b > 0) and antiferromagnetic (AF) along c (J c <0), consistent with the orbital ordering which occurs at the Jahn-Teller transition T JT . 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). The ferromagnetic state which occurs in the range 0.125≤x<0.225 shows a quasi-metallic behavior below T C and an insulating one at lower temperature [6]. There, several works have also proposed an inhomogeneous picture using NMR [7], Mossbauer[8] or magnetization measurements [9].In the present paper, we report a determination of spin-waves in ferromagnets La 1−x Ca x MnO 3 at x=0.17 and 0.2 very close to the "true" metallic state (x≥0.225). Below T C , in the quasi-metallic state, q-independent energy levels of magnetic origin are observed in the large q-range. The q-dependence of their intensity, their evolution with temperature and magnetic applied field suggest that they can be ascribed to standing spin waves characteristic of a confinement into small domains. The shape and the size (a few lattice spacings) of the domains can be estimated as well as the magnetic coupling inside the domains. All the observations are consistent with a picture of hole-poor and orbital-ordered domains.Inelastic neutron experiments have been carried out at the reactor Orphée (Laboratoire Léon Brillouin) and at the Institut Laue-Langevin, on triple axis spectrometers, using fixed final wave vector of neutrons varying from 1.05 to 4.1Å −1 , and appropriat...

Ab initiostudy of the influence of nanoscale doping inhomogeneities in the phase separated state of La1−xCaxMnO3

“…In fact, different nanosize domains have been experimentally detected around this compositional range: FM clusters of 16 A for x = 0.17 whereas an approximate size of 8Å has been observed for x = 0.2. 21 This last result confirms the existence of FM clusters confined in about two perovskite subcells, in a similar scale than the chemical inhomogeneities we find in our calculations for x = 0.1875. The zones with a lower Ca 2+ concentration will have an AF ground state and the zones with higher values of x will be in their FM ground state, according to the phase diagram of the system.…”

Detailed Mapping of the LocalIr4+Dimers through the Metal-Insulator Transitions ofCuIr2S4Thiospinel by X-Ray Atomic P

“…However, the ubiquity of these fluctuations and the exact role that they play in the rich observed physical phenomena are not understood. Part of the reason is that such fluctuations are extremely difficult to study because they are not long-range ordered [6,7]. In cases where there is strong electron-phonon coupling a signature of the local electronic states is evident in the local atomic structure and the electronic or magnetic states may be studied using a local structural probe [8,9] such as the atomic pair distribution function (PDF) analysis of powder diffraction data [10,11].…”

Effect of disorder in an orbitally ordered Jahn-Teller insulator

“…Hole doping leads to progressive loss of orbital order (OO), a reduction of the transport gap, increasing isotropy in the effective magnetic exchange, and an insulator (I) to metal (M) transition [2,18,19]. Beyond transport and thermodynamic indicators, NMR and neutron scattering experiments have revealed an inhomogeneous -possibly phase separated -state in the low doping regime [20,21].…”