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We report on a study on a representative set of FeP-based MnFePSi samples by means of Mn NMR in both zero and applied magnetic field. The first-order nature of the magnetic transition is demonstrated by truncated order parameter curves with a large value of the local ordered moment at the Curie point, even at compositions where the transition appears second order from magnetic measurements. No weak ferromagnetic order could be detected at Si-poor compositions showing the kinetic arrest phenomenon, but rather the phase separation of fully ferromagnetic domains from volume fractions where Mn spins are fluctuating. The more pronounced decrease of the ordered moment at the $3f$ sites than at the $3g$ sites on approaching ${T}_{C}$, characteristic of the mixed magnetism of these materials, is shown to be driven by the drop of the $3f$ spin density instead of enhanced spin fluctuations. The temperature-driven $3f$ spin extinction is demonstrated to evolve into a truly nonmagnetic state of the $3f$ Mn ions well above ${T}_{C}$, in agreement with theoretical models and in contrast with previous experiments who detected just a partial moment quenching. Besides ``normal'' $3f$ Mn ions undergoing a magnetic to spinless state transition, NMR in Mn-rich compositions detects the disproportionation at $3f$ sites of a significant minority Mn fraction with negligible hyperfine couplings, which retains its diamagnetic character independent of temperature. Such a diamagnetic fraction qualitatively accounts for the reduced average $3f$ moment previously reported at large Mn concentrations.

55 Mn NMR was used to perform the atomic-scale study of the anisotropic properties of Mn 5 Ge 3 /Ge(111) epitaxial films with thicknesses between 9 and 300 nm. The NMR spectra have been recorded as a function of strong external magnetic field applied in the film plane and perpendicular to it. Two 55 Mn NMR resonances have been observed, corresponding to the two manganese sites 4d and 6g, in the hexagonal D8 8 structure; in zero field their frequency is centered around 207.5 and 428 MHz, respectively. The anisotropy of 55 Mn hyperfine fields between the hexagonal c direction and the c plane at both Mn sites was evidenced and attributed to the anisotropic term due to the unquenched Mn orbital momentum. The anisotropy of the orbital contribution to hyperfine fields was determined as 1.52 T in the 4d site and up to 2.77 T in the 6g site. The 4d site reveals a quadrupolar interaction due to the strong electric field gradient: V zz = 5.3 × 10 19 V/m 2 in this site, which is shown to be oriented along the hexagonal c axis.

We report the results of a muon-spin relaxation (μSR) investigation of La2−xSrxCoO4, an antiferromagnetic insulating series which has been shown to support charge ordered and magnetic stripe phases and an hourglass magnetic excitation spectrum. We present a revised magnetic phase diagram, which shows that the suppression of the magnetic ordering temperature is highly sensitive to small concentrations of holes. Distinct behavior within an intermediate x range (0.2≤x≲0.6) suggests that the putative stripe ordered phase extends to lower x than previously thought. Further charge doping (0.67≤x≤0.9) prevents magnetic ordering for T≳1.5K.

We investigate the magnetic properties and the phonon-induced relaxation dynamics of the first regular ${\mathrm{Cr}}_{9}$ antiferromagnetic (AF) ring, which represents a prototype frustrated AF ring. Geometrical frustration in ${\mathrm{Cr}}_{9}$ yields an energy spectrum with twofold degenerate low-lying levels and a low-spin ground state. The electronic relaxation dynamics is probed by ${}^{1}\mathrm{H}$-NMR through the temperature dependence of the spin-lattice relaxation rate $1/{T}_{1}$. We develop a microscopic model that reproduces $1/{T}_{1}(T)$ curves, taking also into account the wipeout effect. By interpreting these measurements we determine the spin-phonon coupling strength and we investigate the decay of the cluster magnetization due to the spin-phonon interaction. We find that at very low temperatures, the relaxation is characterized by a single dominating Arrhenius-type relaxation process, whereas several relevant processes emerge at higher temperatures. In addition, we calculate the temperature and magnetic field dependence of level lifetimes.

We report on a systematic study of the structural, magnetic, and transport properties of high-purity 1T-VS2 powder samples prepared under high pressure. The results differ notably from those previously obtained by deintercalating Li from LiVS2. First, no charge-density wave (CDW) is found by transmission electron microscopy down to 94 K, though ab initio phonon calculations unveil a latent CDW instability driven by an acoustic phonon softening at the wave vector qCDW≈(0.21,0.21,0) previously reported in deintercalated samples. A further indication of latent lattice instability is given by an anomalous expansion of the V-S bond distance at low temperature. Second, infrared optical absorption and electrical resistivity measurements give evidence of nonmetallic properties, consistent with the observation of no CDW phase. On the other hand, magnetic susceptibility and NMR data suggest the coexistence of localized moments with metallic carriers, in agreement with ab initio band structure calculations. This discrepancy is reconciled by a picture of electron localization induced by disorder or electronic correlations leading to a phase separation of metallic and nonmetallic domains in the nm scale. We conclude that 1T-VS2 is at the verge of a CDW transition and suggest that residual electronic doping in Li deintercalated samples stabilizes a uniform CDW phase with metallic properties.

The oxygen-order dependent emergence of superconductivity in YBa2Cu3O6+x is studied, for the first time in a comparative way, on pair samples having the same oxygen content and thermal history, but different Cu(1)Ox chain arrangements deriving from their intercalated and deintercalated nature. Structural and electronic non-equivalence of pairs samples is detected in the critical region and found to be related, on microscopic scale, to a different average chain length, which, on being experimentally determined by nuclear quadrupole resonance (NQR), sheds new light on the concept of critical chain length for hole doping efficiency.

The antiferromagnetic manganite ${\mathrm{Pr}}_{0.5}{\mathrm{Sr}}_{0.5}\ensuremath{-}{\mathrm{MnO}}_{3}$ was investigated at low temperature by means of magnetometry and ${}^{55}\mathrm{Mn}$ NMR. A field-induced transition to a ferromagnetic state is detected by magnetization measurements at a threshold field of a few tesla. NMR shows that the ferromagnetic phase develops from zero field by the nucleation of microscopic ferromagnetic domains, consisting of an inhomogeneous mixture of tilted and fully aligned parts. At the threshold the NMR spectrum changes discontinuously into that of a homogeneous, fully aligned, ferromagnetic state, suggesting a percolative origin for the ferromagnetic transition.

We present ${}^{139}\mathrm{La}$ NMR experiments on five powder samples of lanthanum manganites, with a ${\mathrm{Mn}}^{4+}$ concentration ranging from the antiferromagnetic-insulator (AFM) to the ferromagnetic-conducting (FM) region of the phase diagram. We measure a positive hyperfine coupling $\mathcal{C}=0.13 \mathrm{T}/{\ensuremath{\mu}}_{B}.$ A signal from nuclei in a FM environment is present at all compositions, as evidenced by a hyperfine frequency in zero-field experiments, by a positive hyperfine shift in NMR experiments below ${T}_{c},$ and by a paramagnetic frequency shift following Curie-Weiss law. A signal from nuclei in an AFM environment is identified by a similar negative intercept Curie-Weiss law. The NMR spectra reveal a large temperature dependent fraction of static spin defects below ${T}_{c}$ in the FM domains. Nuclear relaxation indicates that the FM regions are influenced by diffusing, AFM-correlated excitations, while the AFM regions probe spin fluctuations from diffusing, FM correlated excitations. These results are interpreted in terms of electronic inhomogeneities due to the presence of a magnetic small polaron.

We have measured the nuclear spin-lattice relaxation of 59Co in the geometrically frustrated Ising-type spin-chain system Ca3Co2O6 as a function of temperature and applied magnetic field. In the nearly-saturated ferrimagnetic and ferromagnetic regimes considered here, the field-dependent energy scales governing the spin-lattice relaxation rates reflect the energy cost of a spin flip in the Co ions adjacent to the probed nuclei. This results in a thermally activated form for the nuclear relaxation rates, which can be exploited as a local probe of the exchange interactions. In particular, the measurements enable the values of intra- and interchain exchange constants J1, J2+J3 to be directly extracted from the data. By using a quasi-one-dimensional model, we then determine the value of J2 consistent with the spin-density-wave order observed in zero field.

We investigate the magnetic ordering and dynamics of the stripe phase of La 5/3 Sr 1/3 CoO 4 , a material shown to have an hourglass magnetic excitation spectrum. A combination of muon-spin relaxation, nuclear magnetic resonance, and magnetic susceptibility measurements strongly suggest that the physics is determined by a partially disordered configuration of charge and spin stripes whose frustrated magnetic degrees of freedom are dynamic at high temperature and which undergo an ordering transition around 35 K with coexisting dynamics that freeze out in a glassy manner as the temperature is further reduced.

{sup 55}Mn NMR experiments were carried out at 1.3{endash}4.2 K on six samples of lanthanum manganites, with a hole doping concentration ranging from the antiferromagnetic-insulator to the ferromagnetic-conductor region of the phase diagram. The dependence of the resonance frequencies on external fields is either as expected for ferromagnets (FM) or for antiferromagnets (AF). No indication of a canted phase could be detected. FM and AF resonances were found to coexist in all samples and are grouped in three broad bands, whose frequencies do not depend in first order on hole concentration. Strong evidence of interaction between AF and FM regions was provided by longitudinal relaxation experiments in several samples. An explanation of this behavior is offered in terms of intrinsic phase separation of the holes into FM microdomains. We determine the {sup 55}Mn hyperfine coupling to be isotropic in all cases, with A=6.2T/{mu}{sub B} for the on-site term in the AF domains. {copyright} {ital 1997} {ital The American Physical Society}

We report on a (59)Co NMR investigation of the trigonal cobaltate Ca(3)Co(2)O(6) carried out on a single crystal, providing precise determinations of the electric field gradient and chemical shift tensors, and of the internal magnetic fields at the nonmagnetic Co I sites, unavailable from former studies on powders. The magnetic-field-induced ferri-and ferromagnetic phases at intermediate temperature (e.g., 10 K) are identified by distinct internal fields, well accounted for by purely dipolar interactions. The vanishing transferred hyperfine field at the Co I site indicates that the Co(3+) (I) orbitals do not participate in the intrachain superexchange, in disagreement with a previous theoretical model. The strong Ising character of the system is confirmed experimentally by the field dependence of the resonance lines, indicating that local moments are saturated even at the phase boundaries. In the vicinity of the critical fields, nuclear spin-spin relaxations detect the spin-reversal dynamics of large magnetic assemblies, either Ising chain fragments or finite-size domains, which drive the metamagnetic transitions. Such collective excitations exhibit a glassy behavior, slowing down to subacoustic frequencies and freezing at low temperature. The relevance of such slow-fluctuation modes for the low-temperature multistep behavior reported in the magnetization is discussed.

We report on an investigation of the double-layer manganite series (La(1-z)Pr(z))1.2Sr1.8Mn2O7 (0 = 0.6), while more moderate Pr substitutions (0.2

La0.7Sr0.3MnO3 thin films having different thicknesses were grown by pulsed laser deposition with in situ reflection high energy electron diffraction diagnostics on LaAlO3 substrates. The mismatch between film and substrate gives rise to an in-plane compressive biaxial strain, which partially relaxes in films thicker than 30 unit cells. Accordingly, the ratio between the out-of-plane and the in-plane lattice parameter (c/a) varies between 1.06 (fully strained) and 1.03 (partially relaxed). In-plane compressive strain favors the stabilization of the 3z(2)-r(2) orbitals (chain-type antiferromagnetic phase), thus giving rise to a sizeable x-ray absorption linear dichroism signal. The shape of the linear dichroism depends weakly on the c/a ratio, while its intensity strongly increases with c/a. At the same time, the metal-insulator transition temperature shifts from about 360 K towards lower temperatures with decreasing thickness, eventually reaching an insulating state for the 30 unit cells film. Low-temperature nuclear magnetic resonance spectra show a decrease of the Mn-DE double-exchange metallic contribution with decreasing the thickness, which becomes negligible in the 30 unit cells thick film. The experimental results demonstrate a strain driven competition between two stable phases: the orbital ordered chain-type insulating antiferromagnetic and the orbital disordered metallic ferromagnetic. For intermediate values of the epitaxial strain the local minimum state of the system lies in a gap region between the two stable phases. Such a region has glassy characteristics with coexisting clusters of the two phases. The strain is used as a driving force to span the glassy region.

The magnetic properties of the substituted spinel ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{4}$, with $0\ensuremath{\leqslant}x\ensuremath{\leqslant}1$, were investigated by means of superconducting quantum interference device (SQUID) magnetometry and $\ensuremath{\mu}\mathrm{SR}$ spectroscopy. A zero-field muon spin precession is detected in the two end members and the appearance of static internal fields is evidenced in all samples below a weakly $x$-dependent ordering temperature ${T}_{N}$, different from that detected by macroscopic magnetization, ${T}_{M}$. For $x$ close to 1 a noncollinear spin structure as in ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ is compatible with our low temperature results. At the opposite end ($x$ close to zero) SQUID and $\ensuremath{\mu}\mathrm{SR}$ data suggest an antiferromagnetic arrangement of the Mn ions on the $B$ spinel site. A nontrivial finding is that the uniform $(q=0)$ component of the magnetic moment scales linearly with $x$. This evidence is discussed on the basis of the known magnetic structure of ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$.

$^{55}\mathrm{Mn}$ nuclear magnetic resonance and magnetization studies of the series of ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ thin films have been performed at low temperature. Two distinct lines were observed, at $322\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$ and $380\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$, corresponding to two different phases, the former located at the interface, with localized charges, and the latter corresponding to the film bulk, with itinerant carriers (as it was also found in Ca manganite films). The spin-echo amplitude was measured as a function of a dc magnetic field applied either in the film plane or perpendicular to it. The field dependence of the NMR signal intensity above the effective anisotropy field agrees well with a model of magnetically homogeneous sample. The enhanced magnetic anisotropy estimated for the NMR signal from the interface is consistent with its very small thickness, indicating similar magnetic properties in charge localized and itinerant regions.

We measured the 2 7 Al and 1 1 B NMR spin lattice relaxation rates and the isotropic Knight shifts in powder samples of Mg 1 - x Al x B 2 . The comparison with band structure calculations allows the experimental determination of the partial DOS and its dependence upon the Al concentration x. Agreement is obtained for its absolute values as well as for its behavior upon doping. The satisfactory understanding of the electronic wave function around the probe nuclei validates our determination of a surprisingly large Korringa ratio, from the comparison between 2 7 Al relaxations and shifts, which can only partially be attributed to Al disorder.

We present a study of a microtwinned single crystal of ${\mathrm{LaMnO}}_{3}$ by means of implanted muons. Two muon stopping sites are identified from the symmetry of the internal field in the ordered phase. The temperature dependence of these fields yields the behavior of the staggered magnetization, from which a static critical exponent $[\ensuremath{\beta}=0.36(2)]$ is extracted and discussed. The muon spin-spin relaxation rate shows a critical slowing down (contrary to preliminary findings), with a critical exponent $n=0.7(1),$ demonstrating the Ising nature of the dynamic fluctuations. The muon precession frequencies vs the applied magnetic field reveal the saturation of the weak ferromagnetic domain structure originated by the Dzialoshinski-Moriya antisymmetric exchange.