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...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.