We have performed Cr-53 NMR measurements on a high-purity polycrystalline sample to investigate the static and dynamic properties of a half-metallic ferromagnet CrO2. Two Cr-53 NMR lines, corresponding to magnetically nonequivalent Cr nuclei, were observed in the ferromagnetic phase of CrO2 despite all of the Cr ions being situated on the crystallographic equivalent sites. We measured the temperature dependences of the Cr-53 spin-lattice relaxation rate (T-1)(-1) in the ferromagnetic phase for the temperature range T = 4.2-360 K. It was found that in the range of low temperatures (T <= 60 K) the relaxation of nuclear magnetic moments is determined mainly by the orbital contribution proportional to the temperature, conditioned by the fluctuation of the orbital currents of d-band electrons. At temperatures T > 60 K, the main mechanism leading to the nuclear spin-lattice relaxation is a three-magnon process of scattering at which the relaxation of the nuclear spin is accompanied by the absorption of a magnon and the creation of two magnons. Based on the analysis of temperature dependences of (T-1)(-1) for two nonequivalent Cr ions, we found that their valence state is the same and corresponds to valence Cr4+, whereas the difference of resonance frequencies for these ion nuclei is conditioned by the different magnetic local fields in their location.

In the paramagnetic phase of cubic antiferromagnet SrMnO2.997 (T-N = 236 K), the spin susceptibility of the localized Mn(t(2g)) electrons exhibits a gapped behavior with d chi(s) (T) >= 0, suggesting the existence of a low-dimensional short-range magnetic order above T-N. The low-frequency fluctuations of the spin correlations of neighboring Mn4+ ions were probed by measuring the spin-lattice relaxation rate T-1(-1) and the echo-decay rate T-2(-1) of O-17 and Sr-87 nuclei up to 420 K. O-17 being involved in an Mn-O-Mn bond, the echo-decay rate T-17(2)-1 probes the fluctuations of the two neighboring S(t(2g)) spins at low frequency, omega <= 10(3) s(-1). It is shown that there exist local changes of the double-exchange interaction, which favor FM correlated pairs of neighboring S(t(2g)) spins in the Mn-O-17-Mn bond. The unusual thermal behavior of T-17(2)-1 (T) indicates low-frequency fluctuations of the short-range magnetic order which may include the change of AF <-> FM spin alignment of neighboring magnetic ions. For Sr-87 nuclei, which probe the spin configuration of eight neighboring Mn ions in the cubic unit cell, T-87(2)-1 has no such anomaly, implying that FM order should be excluded within the cubic unit cell. With both NMR probes, it is deduced that the only magnetic orders which may exist in the cubic unit cells are the following: AF-G [q = pi/a(1, 1, 1)], AF-C [q = pi/a(1, 1, 0)], and AF-A [q = pi/a(0, 0, 1)] so that the slow fluctuating short-range magnetic order is built from these three AF ordered unit cells. Furthermore, we deduce from T-17(1)-1 results that the fluctuating short-range magnetic order, i.e., corresponding to large q, has a high thermal stability in the PM phase of SrMnO2.997.

Field-swept Cu-63,Cu-65 NMR spectra under magnetic fields up to 8.3 T at a constant NMR frequency and temperatures T < 12 K on a single crystalline sample of multiferroic CuFeO2 were measured and analyzed. When the magnetic field is applied along the c axis, a nearly zero internal magnetic field at the Cu site in magnetic ordered state was observed. This is explained by the perfect cancellation of the internal fields produced by the 6 nearest neighbor Fe3+ (S = 5/2) ions, revealing the magnetic structure to be a collinear four-sublattice structure. On the other hand, when the magnetic field is applied along the ab plane, we observed a finite internal field at the Cu sites, which is due to the canting of the Fe moments. Strong change in the NMR signal intensity is observed around 7-8 T, corresponding to the magnetic phase transition from the collinear magnetic to ferroelectric incommensurate states. The ratio of the two magnetic phases significantly depends on the history of the change in the external magnetic field and the temperature of the sample. The details of history dependence of the ratio were discussed.

The inhomogeneous magnetic state arising from isothermal aging of the rapidly quenched U–6Nb alloy (6.3 wt % or 14 at % niobium) was studied for the first time using nuclear magnetic resonance of the 93Nb nucleus. In the process of phase transformation during isothermal annealing at Тan = 500°C, the fraction of niobium atoms in the bulk of the alloy increases in regions with a magnetic susceptibility corresponding to the alloys U1–хNbх (х > 0.14). It is shown that the process of isothermal transformation, accompanied by the formation of niobium-enriched structural precipitates, is fully completed after 60 hours.

The single crystalline and powder samples of multiferroic CuCrO2 have been investigated by O-17 NMR techniques in the paramagnetic and magnetically ordered states. The unusual negative hyperfine field on oxygen nuclei, H-hf = -5.2 kOe/mu B, has been determined in paramagnetic state. The O-17 NMR spectrum in ordered state has a shape typical for incommensurate magnetic structure. It can be well described taking into account only dipole fields which are produced by Cr3+- ions with magnetic moment mu = 2.2 mu(B). The parameters of the EFG (electric field gradient) tensor - the quadrupole frequency (upsilon(Q)) and the asymmetry parameter, have been determined. The value of upsilon(Q) increases monotonously with decreasing temperature in the range 90 K > T > 1.5 K. The estimates of the population of the ligands p - orbitals have been obtained. (C) 2018 Elsevier B.V. All rights reserved.

The spin density of doped electrons was investigated by Sr-87 NMR in the paramagnetic (PM) and antiferromagnetic (AF) G-type phases of electron-doped Sr1-xLaxMnO3 (x = 0.00, 0.02, 0.04; T-N = 236-200 K) ceramics with the cubic structure. It is shown that the Sr-87 NMR shift is proportional to the local density of the itinerant doped electrons surrounding the Sr sites; these electrons have mainly the e(g) character. In the PM phase, all the doped electrons are itinerant; however, they are inhomogeneously distributed in the La-containing oxides, creating electron-doped regions (EDRs) with a number of eg electrons perMn larger than in the rest of the oxide. At room temperature, the network of the overlapping EDRs does not cover all Sr sites. Nevertheless, the number of the Sr sites inside an EDR exceeds the site percolation threshold even for x = 0.02, so that the eg electrons can move on large distances. In the AF phase, below 80 K the EDRs cover the entire crystal. In this T range the doped electrons separate into two species: some of them slow down their motion and form below 50 K static FM domains, which are considered as bound magnetic polarons (MPs) of small size with the effective moment p(eff) = 23(10) mu(B) and a MP formation energy similar to 40 meV. The second species concerns the electrons which remain itinerant at low temperature participating in the fast hopping in the AF G-type ordered lattice of the Mn4+ ions. Nevertheless, their motion is slower than what is expected in an AF metal phase without cation disorder; this is probably due to the imperfect shielding of the (La3+/Sr2+) charge disorder.

The spin susceptibility of the localized Mn(t(2g)) electrons, chi(s), and the spatially distributed spin density of the doped electrons were investigated by O-17 nuclear magnetic resonance (NMR) in the paramagnetic (PM) and antiferromagnetic (AF) phases of electron-doped SrMnO3-x ceramics with the cubic structure. Three lightly doped samples (2x < 0.015) were studied with T-N = 220 K-240 K. In the PM state chi(s) increases gradually from T-N and reaches a broad maximum above similar to 1.5T(N). The gapped behavior of chi(s) indicates a low-dimensional short-range spin order persisting above T-N. These short-range one-dimensional correlations are consistent with O-17 NMR results obtained at room temperature, which show that Mn magnetic moments are aligned along the edges of the cubic unit cell. Above 350 K all doped electrons are fast-moving e(g) electrons. They provide the uniform polarization of the localized spins which increases chi(s) and the increasing doping shifts the oxygen-deficient SrMnO3-x oxide towards a ferromagnetic (FM) metallic state. At lower T the doped electrons are heterogeneously distributed in the oxide: The fraction of the fast-moving electrons diminishes and vanishes below 100 K, while the remaining doped electrons slow down their hopping and each of them creates a FM domain. These FM domains which are detected below 10 K by Mn-55 NMR can be considered as small-size magnetic polarons. Their T-activated hopping in the G-type AF lattice was probed by O-17 spin-echo experiments. The energy barrier of hopping shows a trend to grow with increasing doping, indicating that the de Gennes metallic ground state cannot be achieved in oxygen-deficient SrMnO3-x oxides, probably due to detrimental oxygen vacancy defects.

The 1H nuclear magnetic resonance measurements were carried out to study proton dynamics features in the hydrated scandium-doped barium zirconate, BaZr1 − x Sc x O3 − x/2(OH) y , with x = 0.2 and 0.4 in the temperature range 300–600 K. The obtained data evidence a fast proton motion in both samples, which is characterized by the jump frequency of about 108 s−1 at 450 K. However, the microscopic nature of hydrogen diffusion in these two samples is quite different. For the sample with x = 0.2, hydrogen motion mechanisms are defined by a rapid chemical exchange between Sc–OH–Zr and Zr–OH–Zr positions. The estimated value of activation energy for hydrogen diffusion of about 0.5 eV is determined by the energy barrier produced by Sc3+ ion. The increase of Sc concentration to x = 0.4 leads to the drastic changes of sample properties. Experimental results allow to assume the formation of nanoscaled Sc-rich domains and the decomposition of hydrogen sublattice with formation of two proton subsystems: the protons in Zr–OH–Zr coordinations and those concentrated in Sc–OH–Sc environments. The proton motion in both these subsystems is rather fast, but the chemical exchange between them is highly suppressed. Hydrogen motion inside Sc-rich environments has most likely localized nature. Our estimates yield the energy values of about 0.25 and 0.55 eV for hydrogen motion in “free lattice” and in Sc-rich clusters, respectively. The Sc-rich domains can retain hydrogen up to 600 K.

Despite the fact that cobalt based catalysts are used at the industrial scale for Fischer-Tropsch synthesis, it is not yet clear which cobalt metallic phase is actually at work under operando conditions and what is its state of dispersion. As it turns out, the different phases of metallic cobalt, fcc and hcp, give rise to distinct ferromagnetic nuclear magnetic resonance. Furthermore, within one Co metal particle, the occurrence of several ferromagnetic domains of limited sizes can be evidenced by the specific resonance of Co in multi-domain particles. Consequently, by ferromagnetic NMR, one can follow quantitatively the sintering and phase transitions of dispersed Co metal particles in supported catalysts under near operando conditions. The minimal size probed by ferromagnetic Co NMR is not precisely known but is considered to be in the order of 10 nm for supported Co particles at room temperature and increases to about 35 nm at 850 K. Here, in Co metal Fischer-Tropsch synthesis catalysts supported on β-SiC, the resonances of the fcc multi-domain, fcc single-domain and hcp Co were clearly distinguished. A careful rationalization of their frequency and width dependence on temperature allowed a quantitative analysis of the spectra in the temperature range of interest, thus reflecting the state of the catalysts under near operando conditions that is without the uncertainty associated with prior quenching. The allotropic transition temperature was found to start at 600-650 K, which is about 50 K below the bulk transition temperature. The phase transition was fully reversible and a significant part of the hcp phase was found to be stable up to 850 K. This anomalous behavior that was observed without quenching might prove to be crucial to understand and model active species not only in catalysts but also in battery materials.

The Mn spin correlations were studied near the O'-O phase transition at T-JT = 750 K up to 950 K with O-17 and La-139 NMR in a stoichiometric LaMnO3 crystalline sample. The measured local hyperfine fields originate from the electron density transferred from the e(g) and t(2g) 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, phi(NMR) = 109 +/- 1.5 degrees, of the e(g) 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 nonpolarized e(g) orbitals since both e(g) orbitals are equally occupied. DOI: 10.1103/PhysRevB.87.125142

The 6,7Li NMR spectra and the 7Li spin–lattice relaxation rate were measured on polycrystalline samples of Li2ZrO3, synthesized at 1050K and 1300K. The 7Li NMR lines were attributed to corresponding structural positions of lithium Li1 and Li2 by comparing the EFG components with those obtained in the first-principles calculations of the charge density in Li2ZrO3. For both samples the line width of the central 7Li transition and the spin–lattice relaxation time decrease abruptly at the temperature increasing above ~500K, whereas the EFG parameters are averaged (〈νQ〉=42 (5)kHz) owing to thermally activated diffusion of lithium ions.

The oxygen vacancies distribution in the rigid lattice and the thermally activated motion of oxygen atoms are studied in La1−xSrxGa1−xMgxO3−x (x=0.00; 0.05; 0.10; 0.15 and 0.20) compounds. For that 71Ga, 25Mg and 17O NMR was performed from 100 K up to 670 K, and ion conductivity measurements were carried out up to 1273 K. The comparison of the electric field gradients at the Ga- and Mg-sites evidences that oxygen vacancies appear exclusively near gallium cations as a species trapped below room temperature in local clusters, GaO5/2-□-GaO5/2. These clusters decay at higher temperature into mobile constituents of the structural octahedra Ga(O5/6□1/6)6/2. At the same time, the nearest octahedral oxygen environment of magnesium cations persists at different doping levels. The case of two adjacent vacant anion sites is found highly unlikely within the studied doping range. The thermally activated oxygen motion starts to develop above room temperature as is observed from both the motional narrowing of 17O NMR spectra and the 17O nuclear spin-lattice relaxation rate. The obtained results show that two types of motion exist, a slow motion and a fast one. The former is a long-range diffusion whereas the latter is a local back and forth oxygen jumps between two adjacent anion sites. These sites are strongly differentiated by the probability of the vacancy formation, like the vacant apical site and the occupied equatorial site in the orthorhombic compositions x <0.15.

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.

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, stoichiometric in oxygen. Its 17O enrichment estimated by NMR is about 5.5%. These first 17O NMR results obtained at T=415K in an 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.

We used V-51 NMR to study magnetic ordering in the Ni3V2O8 single crystal with a Kagome staircase structure of Ni atoms. The NMR spectra were measured in the temperature range T= (3-300) K and magnetic fields H= (2-9.4) T directed along the main a, b, c axes of the orthorhombic (Cmca) crystal. The local magnetic field at the V-51 NMR probe determines position and the shape of the corresponding NMR line. These parameters yield an unique information, respectively, on the uniform and the staggered spin components of the ordered Ni. The NMR data collected at H >= 2 T are considered in line with predictions of the representation theory [A. Harris, Phys. Rev. B 76, 054447 (2007)] with a result that incommensurate amplitude-modulated structure of the spine Ni-s spins acquires in the high-temperature incommensurate (HTI) phase two prominent nearly equal spin components S-a approximate to S-c >> S-b instead of the longitudinal incommensurate spin-density wave (SDW) order with S-a >> S-c, S-b as it was deduced from neutron-diffraction data [M. Kenzelmann et al., Phys. Rev. B 74, 014429 (2006)]. No noticeable variation of SDW polarization in the ab plane was detected below the HTI-low-temperature incommensurate (LTI) transition. In both the HTI and LTI phases two almost equal spin components of the Ni-s spins S-a approximate to S-c >> S-b exist at H < 4.7 T. Their phasing is still not determined. The bulk magnetization in these phases is explained by contribution of the cross-tie Ni-c spins which antiferromagnetic structure in the LTI phase is canted along H.

An O-17 NMR study of the distribution of the spin and charge densities of a lightly electron-doped CaMnO3-x (x<0.01, T-N=123 K) ceramic in the paramagnetic phase up to T=670 K is reported. The isotropic and anisotropic components of the NMR line shift probe selectively the local spin susceptibility of the itinerant (e(g)) and the localized (t(2g)) electrons of the Mn neighbors whereas the nuclear quadrupole parameters look at the distribution of the charge density along the Mn-O bond. When approaching T-N, the spin density of the doped electrons becomes inhomogeneously distributed: a separation into slow carriers, forming magnetic polarons, and fast carriers develops few tens of degrees above T-N. The energy barrier corresponding to the change in carriers mobility was estimated to similar to 1100 K. The spin and valence charge densities at the oxygen ions were also measured. From the comparison of these data, the ground state of CaMnO3 appears as a charge-transfer insulator state.