71Ga Nuclear Magnetic Resonance Research Articles

Spin dynamics and spin freezing behavior in the two-dimensional antiferromagnetNiGa2S4revealed by Ga-NMR, NQR andμSRmeasurements

We have performed $^{69,71}\mathrm{Ga}$ nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and muon spin rotation and resonance on the quasi-two-dimensional antiferromagnet $\mathrm{Ni}{\mathrm{Ga}}_{2}{\mathrm{S}}_{4}$, in order to investigate its spin dynamics and magnetic state at low temperatures. Although there exists only one crystallographic site for Ga in $\mathrm{Ni}{\mathrm{Ga}}_{2}{\mathrm{S}}_{4}$, we found two distinct Ga signals by NMR and NQR. The origin of the two Ga signals is not fully understood, but possibly due to stacking faults along the $c$ axis which induce additional broad Ga NMR and NQR signals with different local symmetries. We found the spin freezing occurring at ${T}_{\mathrm{f}}$, at which the specific heat shows a maximum, from a clear divergent behavior of the nuclear spin-lattice relaxation rate $1∕{T}_{1}$ and nuclear spin-spin relaxation rate $1∕{T}_{2}$ measured by Ga-NQR as well as the muon spin relaxation rate $\ensuremath{\lambda}$. The main sharp NQR peaks exhibit a stronger tendency of divergence, compared with the weak broader spectral peaks, indicating that the spin freezing is intrinsic in $\mathrm{Ni}{\mathrm{Ga}}_{2}{\mathrm{S}}_{4}$. The behavior of these relaxation rates strongly suggests that the Ni spin fluctuations slow down towards ${T}_{\mathrm{f}}$, and the temperature range of the divergence is anomalously wider than that in a conventional magnetic ordering. A broad structureless spectrum and multicomponent ${T}_{1}$ were observed below $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, indicating that a static magnetic state with incommensurate magnetic correlations or inhomogeneously distributed moments is realized at low temperatures. However, the wide temperature region between $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{f}}$, where the NQR signal was not observed, suggests that the Ni spins do not freeze immediately below ${T}_{\mathrm{f}}$, but keep fluctuating down to $2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with the MHz frequency range. Below $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, all components of $1∕{T}_{1}$ follow a ${T}^{3}$ behavior. We also found that $1∕{T}_{1}$ and $1∕{T}_{2}$ show the same temperature dependence above ${T}_{\mathrm{f}}$ but different temperature dependence below $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. These results suggest that the spin dynamics is isotropic above ${T}_{\mathrm{f}}$, which is characteristic of the Heisenberg spin system, and becomes anisotropic below $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Multinuclear MAS NMR Investigation of Sol-Gel and Ball-Milled Nanocrystalline Ga2O3

71Ga magic-angle spinning (MAS) nuclear magnetic resonance (NMR) has been used to characterize the structural evolution of nanocrystalline Ga2O3 samples prepared by sol-gel and ball-milling techniques. 29Si and 27Al MAS NMR have also been used to characterize silica and alumina Zener pinning phases. 71Ga NMR parameters are reported for the α- and β-Ga2O3 phases, and more tentatively for the δ-Ga2O3 phase. By simulating the octahedrally coordinated gallium NMR line of β-Ga2O3 using Gaussian distributions in χQ, the extent of disorder in the Ga2O3 crystallites has been quantified. The ball-milled samples contain much more inherent disorder than the sol-gel samples in the nano-phase, which was observed from simulations of the 71Ga MAS NMR spectra. The silica pinning phase produced highly crystalline and densely aggregated nanocrystalline Ga2O3, as well as the smallest nanocrystal sizes.

69Ga NMR in Pu 1− xGa x ( x < 0.01) alloy

69Ga nuclear magnetic resonance (NMR) experiments have been carried out on the Pu 0.995Ga 0.005 alloy at temperatures between 20 and 420 K at magnetic field of 9.4 T to study the local charge symmetry and magnetic behavior of the Pu atoms surrounding solute Ga. In accordance with optical metallography and X-ray diffraction studies the investigated plates of alloy (20 mm × 3 mm × 0.2 mm) represent at room temperature the single α-phase material without any detectable macroscopic segregation of other phases (the corresponding volume fraction <0.03). It was revealed that the magnetic shift of the 69Ga NMR central line and the electric quadrupole broadening of the Ga NMR spectrum trace temperature dependence of the NMR parameters is observed for the δ-phase plutonium alloy Pu 0.95Ga 0.05. On the basis of 69Ga NMR and X-ray data obtained, it is suggested that specific microscopic areas with a size less than 100 nm exist in the α-phase plutonium alloys Pu 0.995Ga 0.005. The local charge and magnetic environment of the solute Ga in these microscopic areas are similar to the observed in the stabilized δ-Pu alloy.

Magnetic state of f electrons in δ-phase of Pu–Ga alloys studied by Ga NMR

69Ga nuclear magnetic resonance (NMR) line shift ( 69 K) and nuclear spin–lattice relaxation rate ( 69 T 1 −1) are measured for Pu 0.95Ga 0.05 alloy, stabilized in δ-phase, in the temperature range 10 and 650 K at magnetic field of 9.4 T. The shift and 69 T 1 −1 are determined correspondingly by the static and fluctuating-in-time parts of the local magnetic fields arisen at Ga due to transferred hyperfine coupling with the nearest f electron environment of more magnetic Pu. At T > 200 K, the temperature dependent part of the shift 69 K( T) scales macroscopic magnetic susceptibility χ( T), following the Curie–Weiss law, and the product ( 69 T 1 T) increases with temperature proportionally ( T + 255) 1.5(1). Both of the NMR observations are typical of the incoherent spin fluctuation regime of f electrons in nonmagnetic 3D Kondo lattice. An estimate of the effective magnetic moment μ eff,5f( g e = 2) = 0.15(5) μ B per Pu atom points out a strong suppression of the spin magnetism in the alloy.

Depth-resolved X-ray determination of surface strain in free-standing films of HVPE-grown GaN and 71Ga NMR characterization

Free-standing GaN films grown by hydride vapor phase epitaxy (HVPE) on c-plane sapphire have been studied for in-plane anisotropic strain. Lattice parameters are obtained from high-resolution X-ray diffraction data and the film quality is determined by measuring the rocking curves and by 71Ga nuclear magnetic resonance (NMR). The in-plane strain was determined using grazing incidence X-ray diffraction and conventional X-ray measurements. It is found that the in-plane lattice parameter varies with depth and has estimated surface strain anisotropy of 4.0791×10-3 up to a thickness of 0.3 μm. The 71Ga NMR experiments reveal different degrees of inhomogeneity amongst the three samples. This is shown by the appearance of an additional broad central-transition peak shifted to higher frequency by a Knight shift from conduction electrons in sample regions having high carrier concentrations.

Elucidation of the correlation between atomic-level defects in GaN nanoparticles and photoluminescence properties by NMR, XRD and TEM

We present evidence from X-ray diffraction data that chemical shift changes observed in the 71Ga nuclear magnetic resonance spectrum of polycrystalline 15N-labeled GaN originate from crystal structure defects on the nearest neighbor atomic-level and are unlikely to be due to electronic effects from conduction band electrons (Knight shift). Nuclear magnetic resonance data ( 15N and 71Ga) suggests that these structural defects are caused by nitrogen-deficiency of the material. For Ga 15N prepared by the reaction of 15N-labeled ammonia with different gallium precursors we observe a strong correlation between band edge related photoluminescence and the presence of nitrogen-deficient material in the sample. Quenching of photoluminescence by structurally bound protons has been observed by 1H nuclear magnetic resonance spectroscopy.

Selective catalytic reduction of NO x with ammonia on gallium-exchanged ferrierites

Selective catalytic reduction of NO x on Ga 3+ exchanged ferrierite zeolites (FER) using ammonia as reductant shows high rates of NO x conversion, although neither the catalytic reaction mechanism nor the role of the gallium is understood. We investigate the features of the structure of the catalyst and the mechanism of the catalytic reduction of NO x to N 2 using nuclear magnetic resonance (NMR) spectroscopy, complemented by temperature programmed desorption (TPD) and catalytic conversion measurements. The NMR studies are enabled by the diamagnetic nature of the gallium-exchanged zeolites in contrast to many of the transition metal exchanged zeolites which are often paramagnetic. Two different ferrierite samples were chosen for comparison, one containing significant quantities of K + and Na +, the other being largely alkali-free. The gallium-exchanged zeolites and their parent materials were characterized using 29Si, 27Al, 71Ga, and 129Xe NMR. Reactions run under batch-mode conditions using ammonia as the reducing agent clearly show the formation of nitrogen in the 15N NMR spectra. Our results show that the acidity of the zeolite is more important for catalytic reduction than the presence of gallium when NO x is supplied to the catalyst as a 1:1 mixture of NO and NO 2.

The Knight shift in liquid gallium confined within porous glasses andopals

71Ganuclear magnetic resonance studies were carried out for liquid gallium embeddedinto porous glasses with different pore sizes and into artificial opals within thetemperature range from about 320 K to complete confined gallium freezing. Ageneral decrease in the Knight shift compared to the bulk melt depending on poresizes was observed in contrast to theoretical predictions. Correlations betweenalterations in the Knight shift and pore sizes were established for particularpore geometry. It was also observed that confined geometry affects thetemperature dependence of the Knight shift in liquid gallium.

Ga nuclear magnetic resonance study of UTGa5(T = Ni,Pt)

Ga nuclear magnetic resonance measurements havebeen carried out for the 5f antiferromagnets UNiGa5 andUPtGa5.The transferred field at the Ga nuclei has been evaluated. The magnetic structurein the antiferromagnetic region has been confirmed from the microscopic pointof view. The mechanism of the hyperfine interaction is discussed.

69,71GaNMR spectra and relaxation in wurtzite GaN

Magnetic properties of wurtzite GaN are studied by Ga nuclear magnetic resonance (NMR) in a GaN bulk crystal containing a high carrier concentration, as well as in highly resistive Mg-doped crystals. The quadrupole coupling constant is derived from satellite lines and from the shift of the central line in the quadrupolar perturbed 6 9 , 7 1 Ga NMR spectra. The electric-field gradient is in good agreement with the value calculated by the ah initio full-potential linear-muffin-tin-orbital method, using the local-density approximation to describe exchange and correlation effects. The 6 9 , 7 1 Ga spin-lattice relaxations in the Mg-compensated sample in the range 80-400 K are due to the quadrupolar interaction mechanism, similar to what is observed in GaAs, and the proper scaling factor, given by the square of the ratio of the nuclear quadrupole moments, ( 6 9 Q/ 7 1 Q) 2 , is observed for the two isotopes. In contrast, in the conductive sample, the relaxation mechanism is caused by magnetic interaction with the conduction electrons and one finds a relaxation rate, WT. The isotope ratio 6 9 W/ 7 1 W is close to ( 6 9 γ/ 7 1 γ) 2 , where y is the gyromagnetic factor. Only above 200 K, there is evidence of a relaxation process due to interactions with phonons also in the degenerate sample, with a temperature dependence approximately of the form WT 3 , consistent with a Debye temperature around 600 K.

Local structural orders in nanostructured fluoride powders

Nanostructured fluoride powders were prepared by high-energy ball milling under different milling conditions (time and intensity). To investigate the local structural orders, we combined suitable local probe techniques such as 57Fe Mössbauer spectrometry (MS) and 19F, 69Ga and 71Ga nuclear magnetic resonance (NMR) applied to FeF 3 and GaF 3 milled samples, respectively. Both isomer shifts and chemical shifts are consistent with the presence of only corner-sharing octahedral units in these systems. After analysis of the different hyperfine magnetic and quadrupolar parameters, the corresponding Mössbauer and nuclear magnetic resonance data were compared to those of crystalline and amorphous phases and then mutually compared. Two types of local structural orders are unambiguously detected and attributed to the presence of crystalline grains and grain boundaries (GB).

Pseudogap inFe2VGa:NMR evidence

We report the results of a V and Ga nuclear magnetic resonance ~NMR! study of Fe2VGa at temperatures between 4 and 450 K. The presence of magnetic antisite defects is deduced from the NMR linewidth, which displays a Curie-law temperature dependence. The absence of associated NMR shifts indicates the material to be intrinsically nonmagnetic. At low temperatures the NMR spin-lattice relaxation rate exhibits Korringa behavior, indicating a small carrier density at the Fermi level. At elevated temperatures, the Knight shifts and NMR relaxation rates go over to a thermally activated response, a semiconductor-like behavior. These results are consistent with pseudogap features identified by recent band structure calculations. The Fermi level density of states deduced from NMR is considerably smaller than given by the specific heat coefficient, g . The electronic properties are compared to the isostructural semimetal Fe2VAl.

Characterization of Ordered Mesoporous Gallium MCM-41 Synthesized at Room Temperature

A series of highly ordered mesoporous gallium-containing MCM-41 molecular sieves with variable Si/Ga (100−10) ratios have been synthesized at room temperature. The synthesized Ga−MCM-41 was investigated by powder X-ray diffraction, transmission electron microscopy, thermal analysis, N2 adsorption measurement, and 29Si and 71Ga nuclear magnetic resonance (NMR); optimal synthesis conditions have thus been established. We were successful in synthesizing highly ordered material at room temperature using gallium nitrate as the source of gallium, and the surfactant was the cetyltrimethylammonium cation [(C16H33N(CH3)3]+. 71Ga NMR of calcined and uncalcined Ga−MCM-41 demonstrated that gallium was substituted for Si with tetrahedral coordination in the Ga−MCM-41 structure. N2 adsorption measurements indicate that high mesoporosity exists over the entire range of Ga−MCM-41 with an average pore diameter of 38.3 Å.

Characterization of an Al-, Ga-based catalyst by Ga NMR and XAS

Both Nuclear Magnetic Resonance (NMR) and X-Ray absorption spectroscopies characterize local order around the observed nuclei. With the recent progresses of 71Ga solid state NMR, it has become possible to take advantage of the complementary information that can be obtained using these two methods. This opens the possibility of a more thorough description of the first coordination shells of Ga in oxide. We present and discuss the example of an Al-, Ga-based catalyst.

71Ga nuclear magnetic resonance investigation of aqueous gallium(III) and its hydrolysis

AbstractThe 71Ga resonance line width of the aquagallium cation [Ga(H2O)6]3+ is shown to vary with pH, and to have a minimum value of 53 Hz in the presence of excess acid and provided that the gallium concentration is not greater than 22 mM. It is shown that the dependence of line width on pH is due to self‐hydrolysis of the cation and is consistent with the known value of the 1,1 hydrolysis constant of pK1,1 = 2.6.

Local order and defects in MBE-grown a-GaAs

Electron spin resonance (ESR), and 71Ga and 75As nuclear magnetic resonance (NMR) measurements have been performed on a 20μ thick film of a-GaAs deposited on a SiO 2 substrate by molecular beam epitaxy. The ESR spectrum exhibits the four-line S= 1 2 , I= 3 2 hyperfine pattern characteristic of the singly ionized arsenic antisite defect (As Ga) commonly observed in crystalline GaAs. The concentration of As Ga centers is estimated to be 10 17−10 18 cm −3 which is 2–3 orders of magnitude higher than those typically found in as-grown (unirradiated) crystalline samples. 71Ga and 75As NMR linewidths suggest substantial quadrupole broadening due to slight deviations from crystalline bonding parameters, but with overall preservation of short-range tetrahedral order.