Recent EPR and susceptibility measurements in La 1Ϫx Ca x MnO 3 support the existence of a bottleneck EPR regime up to 1000 K quantitatively. The EPR linewidth and electrical conductivity follow the same temperature dependence in the range of 250 to 650 K predicted by the small polaron hopping model. This indicates that spin-lattice relaxation in manganates is due to the relaxation of spins of e g Jahn-Teller polarons to the lattice.

A detailed electron-spin-resonance ͑ESR͒ investigation is performed in the one-dimensional magnet CuSb 2 O 6 on both single crystals and polycrystals. Angular-dependent ESR data on the single crystal are interpreted in terms of the anisotropic Zeeman interaction due to the Jahn-Teller distortion of the two inequivalent CuO 6 octahedra. The very strong increase of the ESR linewidth with increasing temperature on approaching the monoclinic-to-tetragonal phase transition at about 400 K indicates the thermal activation of a dynamic Jahn-Teller process. Utilizing temperature-dependent x-ray diffraction experiments we propose the monoclinic angle ␤ as the order parameter of this phase transition.

Abstract. We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancellation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh2Si2 in terms of their dependence on temperature and magnetic field.

Electron-paramagnetic resonance (EPR) measurements on La 22x Sr x CuO 4 provide experimental evidence of a three-spin polaron, consisting of two Cu 21 ions and one p hole. The symmetry properties and the peculiar temperature dependence of the g values of the EPR line indicate the presence of dynamical Jahn-Teller distortions and formation of a collective mode of polarons and surrounding strongly correlated Cu ions (bottlenecked regime). [S0031-9007(97)04581-X]

We consider the local properties of the Yb 3+ ion in the crystal electric field in the Kondo lattice compounds YbRh 2 Si 2 and YbIr 2 Si 2 . On this basis we have calculated the magnetic susceptibility taking into account the Kondo interaction in the simplest molecular field approximation. The resulting Curie-Weiss law and Van Vleck susceptibilities could be excellently fitted to experimental results in a wide temperature interval where thermodynamic and transport properties show non-Fermiliquid behaviour for these materials.

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 ...

In the low doping range of x from 0.01 to 0.06 in La2−xSrxCuO4, a narrow electron paramagnetic resonance (EPR) line has been investigated. This line is distinct from the known broad line, both due to probing Mn 2+ ions. The narrow line is ascribed to metallic regions in the material, and its intensity increases exponentially upon cooling below ∼ 150 K. The activation energy deduced ∆ = 460(50) K is nearly the same as that found in the doped superconducting regime by Raman and neutron scattering. The intensity of the narrow EPR line follows the same temperature dependence as the resistivity anisotropy in lightly doped La2−xSrxCuO4 single crystals.