Slowdown of atomic diffusion in liquid gallium–indium alloy under different nanoconfinements

NMR studies were carried out on three isotopes, ⁷¹Ga, ⁶⁹Ga, and ¹¹⁵In, in liquid gallium-indium (Ga–In) alloy embedded into porous glasses with 200 and 5 nm pore sizes at two magnetic fields, 9.4 and 17.6 T. Spin-lattice relaxation and the Knight shift were found to depend on pore size. For porous glass with 5 nm pores the relaxation rate was field-dependent which evidenced that the extreme narrowing limit was no longer valid. Magnetization recovery data were used to evaluate the correlation times of atomic mobility and the quadrupole constants under nanoconfinement.     

Diffusion slowdown in the nanostructured liquid Ga‐Sn alloy

The diffusion of gallium in liquid Ga‐Sn alloy embedded into different porous silica matrices was studied by NMR. Spin relaxation was measured for two gallium isotopes, ⁷¹Ga and ⁶⁹Ga, at two magnetic fields. Pronounced rise of quadrupole contribution to relaxation was observed for the nanostructured alloy which increased with decreasing the pore size. The correlation time of atomic mobility was evaluated and found to be much larger than in the relevant bulk melt which evidenced a pronounced diffusion slowdown in the Ga‐Sn alloy under nanoconfinement. It is shown that the diffusion was slower by a factor of 30 for the alloy within 7 nm pores. The spectral densities of electric field gradients at zero frequency were found to double for the finest pores. The Knight shift was found to decrease but slightly for the nanostructured alloy.     

Atomic mobility in a ternary liquid Ga–In–Sn alloy of the eutectic composition

The nuclear spin–lattice relaxation and Knight shift of ⁷¹Ga, ⁶⁹Ga, and ¹¹⁵In nuclei in a ternary liquid gallium–indium–tin alloy of the eutectic composition, which was introduced into pores of an opal matrix and porous glasses with pore sizes of 18 and 7 nm, have been investigated and compared with those for the bulk melt. It has been found that longitudinal relaxation is accelerated and the Knight shift is decreased, depending on the size of pores. The correlation time of the atomic motion has been calculated for the nanostructured melt in porous matrices. It has been shown that the atomic mobility in the melt decreases with decreasing size of pores in the glasses.

     

Low temperature nuclear orientation of a dilute RhCr alloy

A dilute^101mRhCr alloy has been investigated by means of low temperature nuclear orientation in the temperature range of 6–60 mK. The magnetic hyperfine field has been found proportional to the macroscopic chromium magnetization and follows even at these low temperatures an Overhauser distribution. The maximum hyperfine field value is B_o (T=6 mK)=7.0(5) T. A Knight shift of –16% affects the external magnetic field at the nuclear site.     

Ga and Pt NMR study of UPtGa5

^69,71Ga and ¹⁹⁵Pt NMR/NQR measurements have been carried out for the 5f-antiferromagnet, UPtGa₅. From a NMR study using a single crystal sample, Knight shift measurements are reported for two different Ga sites and for Pt. The principal axes of the electrical field gradient tensor at both Ga sites have been determined and the values of the splitting parameter ν_Q and the asymmetry parameter η have been evaluated. The hyperfine coupling constants with the external field along various directions are also reported for the two Ga sites and Pt.     

Nuclear resonance measurements of CePd2Ga3

CePd₂Ga₃, a Kondo lattice exhibiting ferromagnetic order below T _C = 6.3 K, has been studied using Ga NMR technique. Measurements of the magnetic susceptibility on an oriented sample proved the strong anisotropy of this quantity, whose major component is in the basal plane. From the analysis of NMR spectra of differently oriented samples, the quadrupole parameters and the temperature-dependent anisotropic Knight-shift have been determined. While the anisotropy of the susceptibility can sufficiently account for the axial anisotropy of the ⁷¹Ga Knight shift, the in-plane anisotropy of the shift points towards dipolar effects enhanced by hybridization of the Ce-4f and Ga-s electrons.     

Magnetic properties of CuRh alloys

By means of rapid quenching techniques single phased samples of Cu_xRh_1?x(0?x?1) were obtained. For these alloys the Knight shift of ⁶³Cu and ¹⁰³Rh has been determined employing pulsed NMR at low temperatures, furthermore the magnetic susceptibility was measured for temperatures between 4.2 and 300 K. While the Knight shift of ¹⁰³Rh is dominated by s-electron contributions in spite of a high density of d-states at the Fermi level, for the susceptibility, however, the d-electron contributions prevail. In addition the susceptibility shows a pronounced maximum at about 10 at.% Cu. Using the extrapolated Knight shift of copper (x→0) we estimate a net copper hyperfine field of — 15 T in close agreement with the corresponding values for CuPd and CuPt.     

NMR probe of bulk electronic structures in H+ beam irradiated Bi2Te3 topological insulator

We report a nuclear magnetic resonance (NMR) study on H⁺ beam irradiated Bi₂Te₃ powdered single crystals. In this work, we demonstrate that the beam creates defects within its penetration range giving rise to delocalized charge carriers, thereby making further ¹²⁵Te NMR Knight shift and line broadening. Upon increasing temperature, the NMR line narrowing manifests the activated motions of thermally excited charge carriers in the irradiated sample. In contrast, it reveals that in the unirradiated sample the free-charge carriers at the Fermi level dominantly contribute to the Knight shift. Our results show that the orbital contribution to the Knight shift in the bulk state of Bi₂Te₃ becomes predominant in the system with the higher density of defects, as evidenced by modified electronic structures induced by the beam irradiation.     

A comparative study of the magnetic heavy-electron materials U2Zn17 and UCu5 by μ+SR

A systematic μ⁺SR study of the magnetic heavy-electron systems U₂Zn₁₇ and UCu₅ in the paramagnetic and in the magnetically ordered state is presented. In both systems the antiferromagnetic nature of the low-temperature phase could be at least partially confirmed, but the muon reveals significant differences with regard to the phase transition itself. UCu₅ behaves like a model-antiferromagnet showing a drastic increase of the relaxation rate both below and above T_N, two spontaneous frequencies in the ordered phase, and a Knight shift above T_N which scales with the bulk susceptibility. In contrast U₂Zn₁₇ shows a loss of μ⁺ asymmetry by 20% below T_N, which is independent of the external field but can be quenched in sufficiently strong longitudinal fields. No scaling of the Knight shift and the susceptibility was observed and no critical increase of the relaxation rate λ. Most astonishing is the strong and nonlinear field dependence of λ above and below T_N in both compounds. The absence of longitudinal relaxation demonstrates the static origin of λ.     

NMR Knight shift studies in SmSn3 — Extension to low temperatures

The ¹¹⁹Sn Knight shift in SmSn3 has been measured in the temperature range 15—300 K, and it is found that the 4f-contribution to the Knight shift does not show a sign reversal in this temperature interval. The range of crystal field parameters for which simultaneous fit to the magnetic susceptibility and the extended Knight shift results in SmSn3 is obtained can be substantially narrowed from that reported earlier.     

Controversy about the reference compound for correct chemical shift determination of 195Pt nuclei

Several groups exploring the ¹⁹⁵Pt NMR in solids, including metallic and magnetic materials, use different standards for chemical shift (Knight shift) determination. Commonly applied H₂PtCl₆ and Na₂PtCl₆ (IUPAC δ scale) lead to considerable underestimation of the shifts since H₂PtCl₆ has considerable own ¹⁹⁵Pt NMR shift due to its Van Vleck paramagnetism. In this Letter new results on ¹⁹⁵Pt NMR in heavy fermion system CeInPt₄ are presented and rationalized scale for the Knight shift determination is discussed.     

Muon Knight shift in palladium

The Knight shift at positive muons implanted in pure palladium has been measured as a function of temperature from 19.8 to 883 K. The Knight shift variation is strictly proportional to the Pd magnetic susceptibility with ΔK^μ/Δ_x=-(0.43±0.02) mole/emu=-(2.39±0.11)kG/μ_B. A temperature independent term K^μ(x=0)=+45±10 ppm is found. The results are discussed in terms of the electronic structure of H in Pd.     

NMR of 69Ga, 71Ga,and 51V in V3−xFex Ga alloys

We have studied at room temperature the intensities and the Knight shifts of the nuclear magnetic resonances of ⁶⁹Ga, ⁷¹Ga, and ⁵¹V in V_3?xFe_xGa for Fe concentrations ranging from x = 0.04 to x = 0.6. The results interpreted in terms of a redistribution of the conduction electrons and a paramagnetism due to the Fe impurities.     

Depth-resolved X-ray determination of surface strain in free-standing films of HVPE-grown GaN and <Superscript>71</Superscript>Ga 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. PACS 72.80.Ey; 61.10.-i; 61.72.Hh     

Temperature dependence of the Knight shift for cadmium in palladium

The Knight shift and its temperature dependence for a Cd impurity in palladium metal were measured by means of DPAD- and DPAC-methods utilizing the well known 5/2⁺, 247-keV state in¹¹¹Cd. The shift at 80 K was found to be KS (CdPd, 80 K)=?0.8(2)%. The observed variation of the KS in the temperature range from 80 K up to 1400 K is 0.5%. For calibration purposes an accurate remeasurement of the magnetic moment of the 5/2⁺ state in¹¹¹Cd was necessary and yieldedμ(¹¹¹Cd, 5/2⁺, 247 keV)=?0.7697(20) n.m.     

Temperature dependence of the nuclear quadrupole interaction and the Knight shift in rhenium metal

The temperature dependences of the static nuclear quadrupole interaction and the Knight shift in rhenium metal were investigated for 2K ≦ T ≦150 K by the method of Nuclear Acoustic Resonance (NAR). For the quadrupole interaction ¦e²qQ/h¦ an increase occurs from 255.2(5) MHz at 2 K to 257.2(8) MHz at 150 K in¹⁸⁷Re [269.5(7) to 271.5(10) in¹⁸⁵Re] whereas for the Knight shift a slight decrease from 1.14(4)% at 2K to 1.10(8)% at 150 K [1.16(6) to 1.12(10)] is found.     

NMR parameters in gapped graphene systems

We calculate the nuclear spin-lattice relaxation time and the Knight shift for the case of gapped graphene systems. Our calculations consider both the massive and massless gap scenarios. Both the spin-lattice relaxation time and the Knight shift depend on temperature, chemical potential, and the value of the electronic energy gap. In particular, at the Dirac point, the electronic energy gap has stronger effects on the system nuclear magnetic resonance parameters in the case of the massless gap scenario. Differently, at large values of the chemical potential, both gap scenarios behave in a similar way and the gapped graphene system approaches a Fermi gas from the nuclear magnetic resonance parameters point of view. Our results are important for nuclear magnetic resonance measurements that target the ¹³C active nuclei in graphene samples.     

On the magnetic properties of Pt1−xAgx

The observed rapid continuous isothermal change from enhanced paramagnetism to weak diamagnetism with increasing silver mole fraction x of the Pt_1?xAg_x alloy system is interpreted by means of a semiphenomenological magnetic alloy susceptibility function X(x). The x-dependent Knight shift K(x) of both ¹⁰⁹Ag and ¹⁹⁵Pt nuclei in Pt_1?xAg_x is closely related to the alloy susceptibility and therefore can be used in a combined K(x)?X(x)-analysis.     

Crystal electric field next to a hydrogen-like interstitial— µ+ in PrNi5

Muon spin rotation measurements of the temperature dependence and the anisotropy of the μ⁺ Knight shift in single crystals of the crystal electric field singlet ground state system PrNi₅ reveal pronounced deviations from a linear scaling of the Knight shift with the bulk magnetic susceptibility atT≤50 K. They are explained by a μ⁺ induced modification of the atomic susceptibility of neighboring Pr³⁺ ions. From the Knight shift anisotropy atT> 50 K it is determined that the implanted μ⁺ occupy a single intersitial site, namely the 6i site (0.5, 0, 0.21±0.02). Using this site information, good model fits to the measured data are obtained assuming a μ⁺ induced perturbation of the crystal electric field at the Pr³⁺ ions next to the μ⁺. Apparently, the presence of the μ⁺ leads to a lowering of the local symmetry, causing a lifting of the degeneracy and a pronounced rearrangement of the low lying crystal electric field levels for these ions.     

Muon Knight shift and μ+ site in the monopnictide CeAs

The transverse field μSR-signal from μ⁺ implanted in a CeAs single crystal shows a splitting into two components, displaying a positive and a negative Knight shift, respectively which both scale reasonably well with the magnetic bulk susceptibility above 10 K. The orientation dependence of the frequency shifts, measured at 15 K, is fully accounted for by the demagnetization field, implying an isotropic Knight shift, consistent with the μ⁺ residing at the only interstitial site of high (cubic) symmetry, i.e., at the position (1/4,1/4,1/4). The occurrence of a split signal remains unexplained. This revised version was published online in July 2006 with corrections to the Cover Date.