Spin-lattice relaxation of 25Al and 28P in Pt

The spin-lattice relaxation times T ₁ for short-lived β emitters ²⁵Al(I?=?5/2, T _1/2?= 7.2 s) and ²⁸P(I?=?3, T _1/2?= 270 ms) in Pt were measured by means of the β-NMR technique. As a result, T ₁[²⁵Al in Pt] = (1.1 $^{+\ 0.7}_{-\ 0.3})$ s and T ₁[²⁸P in Pt] >0.5 s were obtained at temperatures of 17 and 20 K, respectively. The Knight shifts were estimated from the Korringa relation, which were evaluated by comparing to the first principle calculations.     

Knight shifts for short-lived β emitters in Pt

The Knight shifts K for short-lived β emitters ¹²N and ²⁷Si implanted in Pt have been measured by means of β-NMR technique. The results were K(¹²N in Pt)= +(5.8 ± 2.1)× 10^-4 and K(²⁷Si in Pt)= +(1.4 ± 0.8)× 10^-3. The spin–lattice relaxation time T₁ was measured for ¹²N in Pt. The result was T₁(¹²N in Pt, T=300 K)= 66 ± 8 ms, thus, T₁T= 20 ± 2 Ks. The present Knight shifts are in good agreement with the KKR band structure calculation with local lattice relaxation determined theoretically. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Hyperfine interaction of13O and23Mg implanted in Pt

The spin relaxation timeT ₁ for short-lived beta emitters¹³O and²³Mg implanted in Pt have been measured for the first time;T ₁T¹³O) = 2.90 ±0.65 Ks andT ₁ T(²³Mg) = 1665 ±140 Ks. The Knight shift for¹³O in Pt was measured at 300 K to beK(¹³O) = +(4.23 ±0.14) × 10^–3. In the case of¹³O, the Knight shift is unusually large and the relaxation time is unusually fast compared with other interstitial impurities in Pt. A KKR band-structure calculation reproduces the present large Knight shift fairly well.     

NMR relaxation of 7Li in concentrated lithium-ammonia solutions

The Knight shift and the spin-lattice relaxation time of ⁷Li in lithium-ammonia solutions have been measured at -57°C over the concentration range X_Li = 0.01–0.20 (X_Li: mole fraction of Li). The Knight shift increases with increasing metal concentration, while the relaxation rate, 1/T₁, shows a broad minimum around X_Li = 0.07.     

Critical nuclear relaxation in cobalt metal

Nuclear magnetic resonance of cobalt metal was investigated in the paramagnetic and ferromagnetic states and in the critical region below T_c. The Knight shift and spin lattice relaxation times were measured in the paramagnetic phase in the solid and liquid states from 1578 K to 1825 K. The resonant frequency, spin-lattice and spin-spin relaxation times were measured in the ferromagnetic phase from room temperature to 1385 K. The main part of (T₁T)^-1 results from fluctuating orbital moments in both phases except near T_c where this process forms the background for critical spin relaxation. The critical exponents for T^-1₁ and for the magnetization in the ferromagnetic state were found to be n' = 0.96 ± 0.07 and β = 0.308 ± 0.012, respectively.     

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.     

Studies of impure cadmium oxide semiconductors using NMR,EPR and conductivity measurements

Cadmium oxide semiconductors were prepared by heating CdO which also contained hydroxide and carbonate species formed on storage. Reaction of the impurities on heating, and high temperature annealing, produced an unusually wide range of electrical conductivities, EPR spectra and NMR spectra. Changes in EPR spectra were correlated with the ¹¹³Cd shift, which was shown to be proportional to the square root of the relaxation rate, and also proportional to the line width. This allowed the Knight shift contribution to the resonance position to be separated from the paramagnetic shielding (due to covalency). The latter was found to be 234 ± 6 ppm relative to Cd(H₂O)²⁺₆. It could then be shown that the Korringa relationship (for degenerate electrons) was satisfied, with T₁TK² = 3.9 ± 0.2 × 10⁻⁶ sK. The variations of the asymmetric line shape with Knight shift were examined in the light of various models for impurity semiconductors.     

NMR and magnetic susceptibility study of rapidly quenched Ni100-XPX metallic glasses

In order to investigate the electronic structure of the rapidly quenched Ni_100-XP_X metallic glass system (18 ≦ x ≦ 22), NMR and magnetic susceptibility measurements have been carried out for temperatures 4.2 °K ≦ T ≦ 295 °K and magnetic fields H ≦ 20 kOe. The ³¹P Knight shift and relaxation rate behavior demonstrate a metalloid concentration dependence which is consistent with earlier work on the ternary NiPdP and NiPtP metallic glass systems. A consideration of the trends in the magnetic susceptibility indicates that, relative to the Fermi energy, the d-states associated with Ni are higher (the number of d-holes are greater) than those for Pd or Pt.     

NMR studies of magnetic properties in Heusler-type Mn 3 Si

In order to microscopically investigate the magnetic properties of both paramagnetic and antiferromagnetic phases in Mn₃Si (T _N?=?23 K), the ⁵⁵Mn NMR has been carried out at temperatures between 2.2 K and 300 K. The temperature dependences of the spectrum, Knight shift (or resonance frequency shift) and spin-lattice relaxation time T ₁ of ⁵⁵Mn NMR have been measured. In the paramagnetic phase, only one resonance spectrum can be obtained. The observed spectrum is identified to be a signal corresponding to the Mn(II) site. In the antiferromagnetic phase, two different spectra corresponding to the Mn(I) and Mn(II) sites are found at the resonance frequencies of 145 and 6 MHz, respectively, by the zero field NMR at 4.2 K. From these results, the internal magnetic fields on the ⁵⁵Mn(I) and ⁵⁵Mn(II) nuclei are found to be 13.6 and 0.6 T, respectively. According to the NMR results, the helical structure in incommensurate Mn spin states is better explained compared with the transverse sinusoidal structure.     

Biogenic Pt uptake and nanoparticle formation in Medicago sativa and Brassica juncea

The ability of the facultative metallophyte plants, Medicago sativa (M. sativa) and Brassica juncea (B. juncea) to accumulate and translocate platinum (Pt) from aqueous substrates is reported. The influence of Pt concentration in the substrate (5, 10, 20, 40 and 80 ppm), exposure time (24, 48 and 72 h) and substrate pH (2, 3, 5, 7 and 9) was determined. In both plants the concentration of Pt increased with substrate concentration and exposure time. Greater accumulation was detected in the roots of M. sativa than B. juncea, up to a maximum of 94.19 mg Pt g^?1 (dry biomass) compared with 38.5 mg Pt g^?1 (dry biomass) following exposure to 80 ppm Pt after 72 h exposure, respectively. However, at lower substrate concentrations (5 and 20 ppm) greater quantities of Pt were detected in the shoots of B. juncea, ranging between 0.02 and 0.32 mg Pt g^?1 (dry biomass) at 5 ppm across the different time intervals studied, compared with 0.02?0.14 mg Pt g^?1 (dry biomass) for M. sativa, suggesting B. juncea to be a better translocator of Pt under idealised conditions at low concentrations. Higher Pt uptake was also observed in acidic media, with a maximum at pH 2 for M. sativa and pH 3 for B. juncea, indicating the role of net surface charge on the bioaccumulation of Pt. Once sequestered Pt(II) was reduced to Pt(0) due to the action of local metabolites. TEM images of M. sativa root samples showed the in vivo formation of Pt nanoparticles between 3 and 100 nm in size and of varying morphologies in the epidermal root cells. In vivo Pt distribution profiles were assessed using proton induced X-ray emission (μ-PIXE) spectroscopy, which showed even distribution across all tissue systems (epidermal, cortical and vascular) within the roots of both M. sativa and B. juncea.     

Precise measurement of magnetic moment of short-lived β-emitting nuclei12B ( I π=1+, T 1/2=20.18 ms)

The spin polarized β-emitting nuclei¹²B (I ^π=1⁺,T _1/2=20.18 ms) were produced by the nuclear reaction¹¹B(d, p)¹²B and by the selection technique of the incident deuteron energy and the¹²B recoil angle following the nuclear reaction. The nuclear magnetic moment of the short-lived nuclei¹²B was measured by β-NMR with the β-NMR and β-NQR setup established for the first time in China. The nuclear magnetic moment of¹²B was determined to be μ=0.99993±0.00048 nm org=0.99993±0.00048 after the precise correction of the Knight shift.     

Angular dependence of the Knight shift, electric field gradient, and spin-lattice relaxation time of 9Be NMR in beryllium metal

Angular dependences of ⁹Be NMR (±1/2) and (±1/2 ? ±3/20 transition frequencies are measured in a single-crystal beryllium metal plate in a field of 7.04T. The isotropic K _iso and anisotropic K _aniso components of the Knight shift are determined. The measured values of K _iso and T ₁ are considered in terms of the contact, polarization, and orbital contributions.     

Electric field gradient at 12 N implanted into ZnO

The nuclear quadrupole interaction of short-lived β-emitter ¹²N(I?=?1, T _1/2?=?11 ms) implanted into a ZnO single crystal has been studied by means of the β-NMR method. We have observed a quadrupole splitting of ¹²N in ZnO at room temperature, from which the electric field gradient (EFG) q?=?+?(8.6 ±1.1) ×10¹⁹ V/m² was deduced assuming axially symmetric EFG with respect to the crystalline c axis. A first principle calculation does not reproduce the data under assumption of the lattice location of ¹²N at the oxygen substitutional site.     

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.     

Perpendicular magnetic anisotropy and thermal stability in Co2FeAl0.5Si0.5/Pt multilayers

Perpendicular magnetic anisotropy (PMA) and thermal stability of Co₂FeAl_0.5Si_0.5 (CFAS)/Pt multilayer structure prepared by dc magnetron sputtering have been studied. It is found that the strength of PMA depends on the thickness of CFAS layer, and the maximum effective thickness of CFAS with PMA is demonstrated to be about 1.5 nm. The interfacial anisotropy energy K _s is estimated to be 0.45 erg/cm², which is larger than those of Co/Pd and Co/Ni multilayers and responsible for the strong PMA of the CFAS/Pt multilayer. The annealing treatments will give rise to the loss of PMA of CFAS/Pt multilayer and has the relatively less influence with large period number N. With the increasing of the period number N, the CFAS/Pt multilayers tend to have wasp-waist-shaped easy-axis hysteresis loops and multidomain structures.     

127I NQR and 1H NMR studies of 4-aminopyridinium tetraiodoantimonate(III); molecular motion and phase transition

The crystals of 4-NH₂PyHSbI₄ (Py = C₅H₄N) have been investigated by means of ¹²⁷I NQR, ¹H NMR T ₁ and DTA. The crystals can exist in two modifications of β and α(I) at room temperatures. The α(I)-phase is a metastable state which is obtained when the stable β form is heated. The α(I)-phase undergoes a first-order type phase transition of α(I) $\leftrightarrow \alpha $ (II) at 272 K (on heating), while the β-phase is stable down to 77 K. Four and two ¹²⁷I (m = ±1/2 $\leftrightarrow \pm $ 3/2) NQR lines have been found for the β- and α(II)-phases, respectively. One half of them is assignable to the terminal I atom(s) and the other to the bridging I atom(s) in each phase. All the resonance lines of the α(II)-phase underwent a disappearance above ca. 240 K and no resonance line was observed in the α(I)-phase. The second moment M ₂ value of ¹H NMR spectra with 8 G² at 290 K shows that the 4-NH₂PyH^?+? cations reside in the rigid lattice in the β-phase. In contrast, in the α(I)-phase the cation rotates about an axis more symmetric than pseudo threefold axis. The activation energy of 21 kJ mol^???1 was estimated for the reorientational motion in the α(I)-phase from the ¹H NMR T ₁ measurements. The nature of phase transitions in the 4-NH₂PyHSbI₄ is discussed in comparison with that in 4-NH₂PyHSbBr₄.     

Shifts of g Values in the Excited Triplet States of Metal Complexes Studied by Time-Resolved W-band EPR

A highly time-resolved high-frequency/high-field W-band electron paramagnetic resonance (EPR) (ν ~ 94 GHz) is a powerful technique to determine small g anisotropies of transient paramagnetic species. We applied this method to studies of the lowest excited triplet (T₁)³ ππ* states in metal complexes such as a platinum (Pt) diimine complex and metal (Zn and Mg) porphines in rigid glasses. From the analyses of time-resolved EPR spectra, g anisotropies were obtained as g _z  = 2.0048, g _x  = g _y  = 2.0035 for Pt(b-iq)(CN)₂ (b-iq = 3,3′bi-isoquinoline) and g _z  = 1.9968, g _x  = g _y  = 2.0022 for zinc tetraphenylporphine (ZnTPP). No measurable anisotropies were found for magnesium (Mg) TPP. The g values of the Pt complex are larger than g _e (=2.0023, g value of free electron) and that g _z of ZnTPP is smaller than g _e. These results were interpreted in terms of the nature of the perturbed states: the higher triplet ππ′* state mixes with T₁(ππ*) via spin–orbit coupling in ZnTPP. In contrast, the higher triplet dπ* state is involved in this coupling for the Pt complex. Thus, the nature of the perturbed state can be distinguished from the anisotropic g values of the T₁(ππ*) state.     

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.