Orbital order parameter inLa0.95Sr0.05MnO3

Abstract

The temperature dependence of the electron-spin resonance linewidth in La0.95Sr0.05MnO3 has been determined and analyzed in the paramagnetic regime across the orbital ordering transition. From the temperature dependence and the anisotropy of linewidth and g-value the orbital order can be unambiguously determined via the mixing angle of the wave functions of the eg-doublet. The linewidth shows a similar evolution with temperature as resonant x-ray scattering results.PACS numbers: 77.22. Gm, 64.70.Pf In transition-metal oxides the orbital degrees of freedom play an important role for the electric and magnetic properties. Their coupling to spin, charge and lattice is responsible for the occurrence of a variety of complex electronic ground states. Orbital order (OO) can be derived via the Jahn-Teller (JT) effect or via superexchange (SE) between degenerate orbitals under the control of strong Hund's-rule coupling [1]. Strong correlations exist between spin and orbital order and between OO and lattice distortions, but of course a one-to-one correspondence cannot be expected. While spin and lattice order can easily be detected experimentally, this is not true for OO and so far the OO parameter remains hidden. In recent years resonant x-ray scattering (RXS) has been used to derive information on the OO parameter [2], but there is an ongoing dispute, whether RXS probes the JT distortion or the orbital charge distribution [3,4]. Indirectly, OO can also be derived from diffraction experiments via lattice distortions and bond lengths [5]. In this Letter we demonstrate that electron-spin resonance (ESR) can be used to detect OO and to monitor the evolution of the OO parameter. Probing the spin of the partially filled d-shell of Mn 3+ ions by ESR, the anisotropy and T -dependence of g-value and linewidth ∆H provide clear information on OO via spin-orbit (SO) coupling.The power of ESR to gain insight into OO will be demonstrated on A-type antiferromagnetic (AFM) LaMnO 3 (T N =140 K), the parent compound of the magneto-resistance manganites and a paradigm for a cooperative JT effect that suggests a d 3x 2 −r 2 /d 3y 2 −r 2 -type OO below T N =750 K [6]. However, it has been shown that SE interactions play an important role, too [7]. Several recent studies exhibit clear anomalies of the ESR parameters at the JT transition in both doped and pure LaMnO 3 [8,9,10,11]. The orbitally ordered O ′ -phase is characterized by an anisotropy of ∆H [11,12], which for polycrystalline samples reduces to a broad maximum in ∆H(T ) [9,10]. Previously, the angular dependencies of ∆H and the resonance field H res had been analyzed for 200 K and 300 K in high-temperature approximation, allowing to estimate the Dzyaloshinsky-Moriya (DM) interaction and the strength of the zero-field splitting (ZFS) parameters [13]. At X-Band frequencies (9 GHz) ∆H was of the same order of magnitude as H res and due to the overlap with the resonance at −H res and their mutual coupling via the nondiagonal elements of the dynamic susceptibility [14] the values for ...

ESR evidence for partial melting of the orbital order inLaMnO3below the Jahn-Teller transition

“…16,29 The present results reveal, however, more details on the character of the transition from the O ′ into the O phase, because it is not artificially broadened by inhomogeneities due to doping. For further discussion we refer to the measurements on LaMnO 3 powder samples which are also shown in Fig.…”

“…Like for the g tensor, the anisotropy for T < T JT mainly results from the zero-field splitting of the S = 2 spin state of Mn 3+ due to spin-orbit coupling and crystal-electric field of the ligands. 16,28,29 The relative anisotropy ratio ∆H a /∆H b of the linewidth with the magnetic field parallel to the ferromagnetically coupled spins along the a-axis to the one along the antiferromagnetically coupled spins along the b-axis (see Fig. 3 b)) is rather constant at a value of 1.4 below the JT-transition, but approaches 1 as the system enters the paradistortive phase with only local distortions.…”

“…This finding was supported by results obtained from different experimental techniques on La 0.95 Sr 0.05 MnO 3 single crystals, a series of Sr-doped powder samples and LaMnO 3 thin films. [16][17][18] Besides this general assessment concerning the character of the JT transition, the dynamical properties observed in a rather broad temperature range around T JT deserve a closer inspection. Recent Nuclear Magnetic Resonance (NMR) experiments 19 show changes in the spin-relaxation and the Knight-shift of the 17 O ligand ions already far below T JT , which are interpreted as a melting of the orbital order.…”

Neutron diffraction, magnetization, and ESR studies of pseudocubicNd0.75Ba0.25MnO3and its critical behavior aboveTCRyzhov1, Lazuta2, Smirnov3 et al. 2005Phys. Rev. B190130View full textAdd to dashboardCite

“…Our M(τ ) data in this region are related mainly to an intermediate H-regime between the weak (T > 143 K) and strong (T = T C ) magnetic fields. As can be easily verified, a modified Arrot plot scheme frequently employed for analysis of the critical M(τ, H) dependences [22,23] has the incorrect analytic properties in the weak fields. Therefore we use another approximate equation of state [24] with a correct behavior in the weak and strong fields.…”

“…Here Γ c is controlled by the dipolar forces, a single ion anisotropy (∝ S α S β ) and anisotropy of the exchange interaction (the two latter appear in the P bnm symmetry) [22,28]. The term with Γ unc describes the uncritical contribution including a spin-lattice relaxation rate.…”