Magnetic Defect Concentration Research Articles

Anion vacancy-driven magnetism in incipient ferroelectric SrTiO3 and KTaO3 nanoparticles

We predict that undoped nanoparticles (size ≤ 10–100 nm) of incipient ferroelectrics without any magnetic ions can become ferromagnetic even at room temperatures due to the inherent presence of a new type of magnetic defects with spin S = 1, namely oxygen vacancies, where the magnetic triplet state is the ground state in the vicinity of the surface (magnetic shell), while the nonmagnetic singlet is the ground state in the bulk material (nonmagnetic core). In particular, for a particle radii R less than the critical radius Rc(T) ferromagnetic long-range order appears in a shell region of thickness 5–50 nm once the concentration of magnetic defects N exceeds the magnetic percolation threshold Np. At vacancy concentrations N < Np and radii R < Rc(T), short-range ferromagnetic order and consequently a spin glass state may appear. For particle radii R > Rc(T), only the paramagnetic phase is possible.

Absence of structural correlations of magnetic defects in the heavy-fermion compoundLiV2O4

Magnetic defects arising from structural imperfections have pronounced effects on the magnetic properties of the face-centered cubic normal-spinel structure compound ${\text{LiV}}_{2}{\text{O}}_{4}$. High-energy x-ray diffraction studies were performed on ${\text{LiV}}_{2}{\text{O}}_{4}$ single crystals to search for superstructure peaks or other evidence of spatial correlations in the arrangement of the crystal defects present in the lattice. Entire reciprocal lattice planes were mapped out with the help of synchrotron radiation. No noticeable differences in the x-ray diffraction data between a crystal with high magnetic defect concentration and a crystal with low magnetic defect concentration were found. This indicates the absence of any long-range periodicity or short-range correlations in the arrangements of the crystal/magnetic defects.

Li7NMR study of heavy-fermionLiV2O4containing magnetic defects

We present a systematic study of the variations of the ^{7}Li NMR properties versus magnetic defect concentration up to 0.83 mol% within the spinel structure of polycrystalline powder samples and a collection of small single crystals of LiV2O4 in the temperature range from 0.5 to 4.2 K. We also report static magnetization measurements and ac magnetic susceptibility measurements at 14 MHz on the samples at low temperatures. Both the NMR spectrum and nuclear spin-lattice relaxation rate are inhomogeneous in the presence of the magnetic defects. The NMR data for the powders are well explained by assuming that (i) there is a random distribution of magnetic point defects, (ii) the same heavy Fermi liquid is present in the samples containing the magnetic defects as in magnetically pure LiV2O4, and (iii) the influences of the magnetic defects and of the Fermi liquid on the magnetization and NMR properties are separable. In the single crystals, somewhat different behaviors are observed. Remarkably, the magnetic defects in the powder samples show evidence of spin freezing below T ~ 1.0 K, whereas in the single crystals with similar magnetic defect concentration no spin freezing was found down to 0.5 K. Thus different types of magnetic defects and/or interactions between them appear to arise in the powders versus the crystals, possibly due to the substantially different synthesis conditions of the powders and crystals.

Crystallography, magnetic susceptibility, heat capacity, and electrical resistivity of heavy-fermionLiV2O4single crystals grown using a self-flux technique

Magnetically pure spinel compound ${\rm LiV_2O_4}$ is a rare $d$-electron heavy fermion. Measurements on single crystals are needed to clarify the mechanism for the heavy fermion behavior in the pure material. In addition, it is known that small concentrations ($< 1$ mol%) of magnetic defects in the structure strongly affect the properties, and measurements on single crystals containing magnetic defects would help to understand the latter behaviors. Herein, we report flux growth of ${\rm LiV_2O_4}$ and preliminary measurements to help resolve these questions. The magnetic susceptibility of some as-grown crystals show a Curie-like upturn at low temperatures, showing the presence of magnetic defects within the spinel structure. The magnetic defects could be removed in some of the crystals by annealing them at 700 $^\circ$C\@. A very high specific heat coefficient $\gamma$ = 450 mJ/(mol K${^2}$\@) was obtained at a temperature of 1.8 K for a crystal containing a magnetic defect concentration $n$${\rm_{defect}}$ = 0.5 mol%. A crystal with $n$${\rm _{defect}}$ = 0.01 mol% showed a residual resistivity ratio of 50.

Dynamics of Magnetic Defects in Heavy FermionLiV2O4from Stretched ExponentialLi7NMR Relaxation

7Li NMR measurements on LiV2O4 from 0.5 to 4.2 K are reported. A small concentration of magnetic defects within the structure drastically changes the nuclear magnetization relaxation versus time from a pure exponential as in pure LiV2O4 to a stretched exponential, indicating glassy behavior of the magnetic defects. The stretched exponential function is described as arising from a distribution of 7Li nuclear spin-lattice relaxation rates and we present a model for the distribution in terms of the dynamics of the magnetic defects. Our results explain the origin of recent puzzling 7Li NMR literature data on LiV2O4 and our model is likely applicable to other glassy systems.

Isomerization and n-type doping of polyacetylene: Pure t r a n s-(CH)x and t r a n s-(CD)x

A systematic study of thermal isomerization leads to a definition of standard conditions (5 min at 180 °C) to obtain trans-polyacetylene (CH)x and (CD)x. Light doping with Na derivatives of benzophenone enables the preparation of pure trans-(CH)x and (CD)x. The characteristics of these systems have been determined and, in particular, a peak-to-peak ESR linewidth as small as 0.28 G has been found for both systems. ESR and IR spectroscopic evidences of the inhomogeneity of the doping process have been obtained. The dopant is concentrated at the surface before penetrating inside the fibrils. The doping process is linked to the appearance of additional bands in the IR spectra, the position of which is sensitive to the nature of the polymer [(CH)x or (CD)x] and of the doping agent (Na or I2). Upon ’’compensation’’ by a subsequent doping, the properties of the insulating trans-(CH)x are recovered, but any doping inside the fibers is proved to create a high concentration of magnetic defects. After such compensation, a further doping allows recovery of the metallic state.