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A. Trokiner

Publications and source records attributed to A. Trokiner.

6 recordsLinked to original sources
arXiv2013

Melting of the orbital order in LaMnO3 probed by NMR

The Mn spin correlations were studied near the O'-O phase transition at T{JT} = 750 K, up to 950 K with 17O and 139La NMR in a stoichiometric LaMnO3 crystalline sample. The measured local hyperfine fields originate from the electron density transfered from the eg- and t2g-orbitals to the 2s(O) and 6s(La) orbits, respectively. By probing the oxygen nuclei, we show that the correlations of the Mn spins are ferromagnetic in the ab-plane and robust up to T{JT} whereas along the c-axis, they are antiferromagnetic and start to melt below T{JT}, at about 550 K. Above T{JT} the ferromagnetic Mn-Mn exchange interaction is found isotropic. The room temperature orbital mixing angle, ϕnmr = 109+-1.5°, of the eg ground state is close to the reported value which was deduced from structural data on Jahn-Teller distorted MnO6 octahedra. For T > T{JT} LaMnO3 can be described in terms of non-polarized eg-orbitals since both eg-orbitals are equally occupied.

cond-mat.str-el↗
arXiv2011

First 17O NMR study of a 17O enriched LaMnO3 stoichiometric crystal

We present the synthesis and the NMR characterization of a 17O enriched LaMnO3 crystalline sample. We checked that it is single phase and, more important, stochiometric in oxygen. Its 17O enrichment estimated by NMR is about 5.5%. These first 17O NMR results obtained at T=415K in a undoped parent LaMnO3 manganite demonstrate that the two oxygen sites of the structure probe very different Mn spin correlations in the paramagnetic orbital ordered phase. This work opens the way to study experimentally the interactions responsible for the orbital order.

cond-mat.mtrl-sci↗
arXiv2005

The charge ordered state in half-doped Bi-based manganites studied by $^{17}$O and $^{209}$Bi NMR

We present a $^{209}$Bi and $^{17}$O NMR study of the Mn electron spin correlations developed in the charge ordered state of Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ and Bi$_{0.5}$Ca$_{0.5}$MnO$_{3}$. The unusually large local magnetic field $^{209}H_{loc}$ indicates the dominant $6s^{2}$ character of the lone electron pair of Bi$^{3+}$-ions in both compounds. The mechanism connecting the $s$ character of the lone pairs to the high temperature of charge ordering $T_{CO}$ is still not clarified. The observed difference in $^{209}H_{loc}$ for Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ to Bi$_{0.5}$Ca$_{0.5}$MnO$_{3}$ is probably due to a decrease in the canting of the staggered magnetic moments of Mn$^{3+}$-ions from. The modification of the $^{17}$O spectra below $T_{CO}$ demonstrates that the line due to the apical oxygens is a unique local tool to study the development of the Mn spin correlations. In the AF state the analysis of the $^{17}$O spectrum of Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$ and Bi$_{0.5}$Sr$_{0.5}$MnO$_{3}$ prompts us to try two different theoretical descriptions of the charge-ordered state, a site-centered model for the first manganite and a bond-centered model for the second one.

cond-mat.str-el↗
arXiv2003

Isotopic disorder in Ge single crystals probed with 73Ge NMR

NMR spectra of 73 Ge (nuclear spin I = 9/2) in germanium single crystals with different isotopic compositions have been measured at the frequency of 17.4 MHz at room temperature. Due to the small concentration (~0.1%) of the magnetic (73 Ge) isotope, the magnetic dipole-dipole interaction is negligible in the samples studied, and the observed specific features of the resonance lineshapes (a narrow central peak and a wide plateau) are determined mainly by the quadrupole interaction of magnetic nuclei with the random electric field gradient (EFG) induced by the isotopic disorder. The second and fourth moments of the distribution function of the EFG are calculated taking into account local lattice deformations due to mass defects in the close neighborhood of the magnetic nuclei, as well as charge density redistributions and lattice strains induced by distant impurity isotopes. The simulated lineshapes, represented by a superposition of Gaussians corresponding to individual transitions between nuclear Zeeman sublevels, agree reasonably well with the measured spectra.

cond-mat↗
arXiv2002

Charge and Orbital Ordering in Pr_{0.5} Ca_{0.5} MnO_3 Studied by ^{17}O NMR

The charge and orbital ordering in Pr_{0.5} Ca_{0.5} MnO_3 is studied for the first time by ^{17}O NMR. This local probe is sensitive to spin, charge and orbital correlations. Two transitions exist in this system: the charge and orbital ordering at T_{CO} = 225 K and the antiferromagnetic (AF) transition at T_N = 170 K. Both are clearly seen in the NMR spectra measured in a magnetic field of 7T. Above T_{CO} there exists only one NMR line with a large isotropic shift, whose temperature dependence is in accordance with the presence of ferromagnetic (FM) correlations. This line splits into two parts below T_{CO}, which are attributed to different types of oxygen in the charge/orbital ordered state. The interplay of FM and AF spin correlations of Mn ions in the charge ordered state of Pr_{0.5} Ca_{0.5} MnO_3 is considered in terms of the hole hopping motion that is slowed down with decreasing temperature. The developing fine structure of the spectra evidences, that there still exist charge-disordered regions at T_{CO} > T > T_N and that the static (t > 10^{-6}s) orbital order is established only on approaching T_N. The CE-type magnetic correlations develop gradually below T_{CO}, so that at first the AF correlations between checkerboard ab-layers appear, and only at lower temperature - CE correlations within the ab-planes.

cond-mat.str-el↗