The mapon technique and recent developments

Typical linewidths observed in NMRON on dilute impurities in ferromagnetic metals are of order 1 MHz, making difficult the observation of structure in the resonance with splitting Δv much less than this value. The technique of Modulated Adiabatic Passage on Oriented Nuclei (MAPON) was recently developed as a means of measuring the weak electric quadrupole splitting Δv_Q of the nuclear hyperfine interaction due to an electric field gradient V_zz. MAPON has successfully been applied to measure Δv_Q as low as 4 kHz, i.e. less than 0.5% of the inhomogeneously broadened NMRON CWFM resonance line. The isotopes⁵⁶Co,⁵⁷Co,⁵⁸Co and⁶⁰Co have been studied in iron single crystal hosts, yielding Δv_Q consistent with known and estimated quadrupole moments. In addition the results to date give striking confirmation of analyses based on the single impurity relaxation model. Following a brief summary of the theoretical development of MAPON a review of experimental data is given for the CoFe<100> system. The variation of Δv_Q with direction of magnetization, measured in⁵⁸CoFe and⁶⁰CoFe single crystal samples, is also described. Further MAPON measurements are described for a⁵⁶CoFe polycrystalline sample, for which the most probable value and width of the distribution of V_zz can be described simply in terms of the single crystal principal axis results. The application of the MAPON technique to the measurement of nuclear electric quadrupole moments in implanted and diffused samples is discussed.     

Studies of weak electric quadrupole interaction in57,60CoFe using modulated adiabatic passage on oriented nuclei

The electric quadrupole interactions at⁵⁷Co and⁶⁰Co in co-diffused^57,60CoFe single crystal have been measured using Modulated Adiabatic Passage on Oriented Nuclei (MAPON). The quadrupole splittings Δν_Q<100> are + 18(2) kHz for⁵⁷Co and +9(2) kHz for⁶⁰Co, corresponding to a principal electric field gradient (efg) tensorV _zz =2.0(5)×10¹⁹ Vm⁻² and 2.5(6)×10¹⁹ Vm⁻², respectively, in broad agreement with previous MAPON results for CoFe. The distributions of the efg’s are very similar for the two isotopes, verifying that previously reported differences in⁵⁸CoFe and⁶⁰CoFe could be attributed to different host preparations. The measurement of such weak efg’s which are not spectroscopically resolved, allows determination of new nuclear electric quadrupole moments, not accessible by other techniques. Applications to other systems are discussed.     

Recent experiments with mapon spectroscopy

Electric quadrupole interactions of impurity nuclei in single crystals of⁵⁴MnNi,¹²⁵SbNi and¹²⁵SbFe have been studied as a function of crystal field direction and applied magnetic field using MAPON. Distributions are in all cases broad compared with the mode values of the EQI's. For⁵⁴MnNi the mode efg is isotropic to better than 5% between the easy <111> axis and a hard <100> axis. The efg is +0.88 (15)x 10¹⁹ Vm⁻². The mode efg for¹²⁵SbFe along its easy <100> axis is one half of that along a hard <112> axis, and one third of that measured along the easy <111> axis of¹²⁵SbNi. The much larger efg mode and distribution seen in¹²⁵SbNi, for four to eight times greater dilution than for¹²⁵SbFe, suggests intrinsic contributions due to valence screening effects in the more itinerant nickel host.     

MAPON EFG Result for 54Mn in Cubic Cobalt

Modulated adiabatic passage on oriented nuclei (MAPON) spectroscopy has been used to measure the electric quadrupole interaction at dilute ⁵⁴Mn impurity probes in crystallographically cubic (fcc) cobalt. The measured value is P/h=+5.2(5) kHz and using Q=+0.33(3) b, leads to an electric field gradient of V _zz =+1.3(2)×10¹⁹ V m⁻². This result is consistent with established trends for Mn and Co probes in the 3d ferromagnetic hosts. This revised version was published online in September 2006 with corrections to the Cover Date.     

Transient NMRON of brute force oriented110mAgAg

A novel technique involving electrodeposition has been used to prepare an elemental silver specimen wherein radioactive^110mAg probe nuclei are confined to a surface layer of approximately 0.1 μm depth. This specimen was then brute force oriented in a field of 8T and the continuous wave (CW) nuclear magnetic resonance on oriented nuclei (NMRON) spectrum showed an integrated destruction of some 58%. Moreover the transient NMRON techniques of single and modulated adiabatic passage were successfully applied, resulting in a determination of the electric quadrupole interaction strength at the^110mAg site in polycrystalline silver of P/h=+0.74(5)kHz with a FWHM of ΔP/h=0.75(8)kHz. The resulting mode electric field gradient is V_zz=+1.87(23)×10¹⁸Vm⁻¹. Analysis of the single passage data provides an estimate for the nuclear spin lattice relaxation constant in silver at 8T of C_k=1.6(3)s.K.     

The effect of temperature and concentration on the electric field gradient in binary indium alloys

The concentration and temperature dependence of the quadrupole hyperfine interaction of¹¹¹Cd in InTl (hcp),InPb (fct),InTl (fct) andInCd (fct and fcc) alloys were studied using the perturbed angular correlation technique. The change in the observed quadrupole interaction frequency with concentration can be described by a linear dependence on the axial ratioc/a in all cases. In the alloys with identical crystal structures the strength of thec/a dependence is independent of the solute, in contrast to the strength of the concentration dependence. In all cases where no phase transition occurs, the change in the electric field gradient with temperature follows the empirical relationV _zz (T)=V _zz(0) · (1–B·T ^3/2), where the coefficientB depends on the lattice structure, on the solute-solvent combination and on the concentration. The phase transitions ofInCd alloys at 293 K could clearly be seen as discontinuities in the temperature curves. A similar series of discontinuities observed around 116 K suggests the existence of a cubic low temperature phase.     

NMR-ON measurements of187WFe,182,183,186ReNi,186ReFe and203PbFe

The magnetic hyperfine splitting frequencies of¹⁸⁷WFe,¹⁸²Re(j ^π=2⁺)Ni,¹⁸³ReNi,¹⁸⁶ReNi,¹⁸⁶ReFe and²⁰³PbFe in a zero external magnetic field have been determined by the NMR-ON method at about 7 mK as 225.56(6), 130.9(1), 98.17(4), 136.6(4), 1007.0(3) and 58.43(3) MHz, respectively. With the knowng-factors ofg(¹⁸⁶Re, 1⁻)=1.739(3) andg(²⁰³Pb, 5/2⁻)=0.27456(20), the following hyperfine fields were deduced:B _HF(¹⁸⁶ReNi)=−103.05(35) kG;B _HF(¹⁸⁶ReFe)=−759.7(13) kG;B _HF(²⁰³PbFe)=+279.18(25) kG. Taking hyperfine anomalies into account, theg-factor of¹⁸³Re was deduced as |g(¹⁸³Re, 5/2⁺)|=1.267(6). With the assumption of Knight shift factorK=0, theg-factors of¹⁸²Re and¹⁸⁷W and the hyperfine field of¹⁸⁷WFe were determined as |g(¹⁸²Re, 2⁺)|=1.63(5), |g(¹⁸⁷W, 3/2⁻)|=0.414(10) andB _HF(¹⁸⁷WFe) =−714(18) kG. The large hyperfine anomaly was deduced to be¹⁸³W Δ¹⁸⁷W =−0.124(22).     

Magnetic hyperfine field of arsenic in nickel by NMR/ON of71,72As

The magnetic hyperfine splitting frequencies of⁷¹AsNi and⁷²AsNi in a 0.11 Tesla external magnetic field have been determined by NMR/ON method as 66.00(6) MHz and 106.17(13) MHz respectively. Using the known magnetic moments of μ(⁷¹As)=1.6735(18) and μ(⁷²As)=−2.1566(3), the hyperfine fields were deduced asB _hf(⁷¹AsNi)=12.824(19) Tesla andB _hf(⁷²AsNi)=12.807(16) Tesla.     

Mapon and single passage NMRON spectroscopy on125SbFe and125SbNi single crystals

The signs, mode magnitudes and distributions of the electric quadrupole hyperfine interactions (EQI) in single crystal¹²⁵SbFe and¹²⁵SbNi have been examined along a principal hard axis for each system and are compared with earlier easy axis results. The surface preparation was essentially the same for both hosts but the results of the distribution width and anisotropy in the mode values of the measured EQI’s are remarkably different between the two hosts. For¹²⁵SbFe the mode value of the EQI is a factor of two larger along the hard axis <111> and the same sign, negative, as for the easy axis <100>. For¹²⁵SbNi the mode values of the EQI along the same two principal directions are comparable in magnitude but the efg distributions are much broader than in the¹²⁵SbFe case. Single passage results on¹²⁵SbNi provide a weak sweep asymmetry with indications of an apparent change in sign in EQI from negative along the <111> easy axis to positive along the hard <100> axis.     

Single-Crystal EPR Identification of a Low Hyperfine Value and Interstitial Position of Copper Impurity in Diaquabis[malonato(1-)-κ2O,O′] Zinc(II)

Electron paramagnetic resonance studies have been carried out at 300 K on the Cu(II)-doped [Zn(C₃H₂O₄)₂(H₂O)₂] system in single-crystal and powder forms in order to rationalize the low parallel ⁶³Cu hyperfine value. Angular variation of the hyperfine resonances in the three orthogonal planes shows the presence of only one magnetic site with g and A values equal to g _zz = 2.455, g _yy = 2.121, g _xx = 2.105 and A _zz = 160.9 · 10⁻⁴ cm⁻¹, A _yy = 12.5 · 10⁻⁴ cm⁻¹, A _xx = 7.35 · 10⁻⁴ cm⁻¹. The crystal structure of the host lattice is isostructural with the corresponding cobalt complex and contains two molecules per unit cell. The low magnitude of A _zz value for the complex is rationalized in terms of an admixture of the ground state with the excited state and delocalization of the unpaired spin density onto the ligands. In addition, the highest hyperfine value obtained from the single-crystal data (160.9 · 10⁻⁴ cm⁻¹) is considerably larger than that obtained from the powder spectrum (138 · 10⁻⁴ cm⁻¹). Authors' address: P. Sambasiva Rao, Department of Chemistry, Pondicherry University, Pondicherry 605014, India     

Hyperfine Interaction Studies on Y, Zr, Nb, Mo, Rh, In and Xe in Co

Nuclear magnetic resonance on oriented nuclei and modulated adiabatic fast passage on oriented nuclei measurements were performed on several 4d and 5sp impurities in polycrystalline Co(fcc) foils and Co(hcp) single crystals. The hyperfine fields of Y and Zr in Co(fcc), the hyperfine fields of Y, Zr, Nb, Mo, Rh, In and Xe in Co(hcp), the electric field gradients of Zr, Nb and In in Co(hcp), and the nuclear spin-lattice relaxations of Zr, Nb, Rh and In in Co(hcp) were determined. The dependence of the hyperfine fields and electric field gradients in Co(hcp) on the angle between the magnetization and the c axis was investigated in most cases. The magnetic-field dependence of the spin-lattice relaxation was studied for Nb, Rh and In in Co(hcp), applying the magnetic field perpendicular to the c axis. The known hyperfine interaction parameters of the 4d and 5sp impurities in Co(fcc) and Co(hcp) are summarized. The new results provide a more detailed picture of the hyperfine interaction in Co.     

The electric field gradient of111Cd in an α-Al2O3 single crystal

Radioactive¹¹¹In⁺ ions were implanted into an α-Al₂O₃ single crystal. The hyperfine parameters of¹¹¹Cd at substitutional Al lattice sites were identified by measuring the perturbed angular correlation for different sample orientations. The electric field gradientV _zz =1.04(17)·10²²V/m² was obtained from the quadrupole coupling constant. This result is compared with the efg values of²⁷Al in α-alumina and¹¹¹Cd in α-Fe₂O₃, which also has the corundum structure. Two additional fractions with broad frequency distributions were observed, one of which is attributed to¹¹¹Cd atoms in a strongly distorted Al₂O₃-lattice.     

Hyperfine interaction of ZrO2 — Tetragonal phase

The quadrupole hyperfine interaction of the tetragonal phase of the zirconium oxide has been measured at 1523 K using the time-differential perturbed angular correlation technique. The electric field gradient at zirconium sites was determined to beV _zz =(17.5±0.4)·10¹⁷ Vcm⁻² and axially symmetric.     

Deformation in the light Br isotopes

The nuclear ground state spins of the odd-A Br nuclides^{75, 77, 79}Br with Z=35 are all 3/2⁻. Nilsson orbital calculations show that the 35^th proton occupies the f_5/2 [301]3/2⁻ orbital for ε<0.20 and the p_3/2 [312]3/2⁻ orbital at larger deformations. The magnetic moments of these two states differ by a factor of two, giving clear evidence for the magnitude of the ground state deformation. Low Temperature Nuclear Orientation measurements made in Oxford and Bonn on^{76, 77g}BrFe prepared at ISOLDE, and on line at the SERC Daresbury Laboratory on^{72, 74m, 75}BrFe, have yielded the magnetic moments of these isotopes, using a two non-zero field model with magnetic hyperfine fields of +81.38(6) T and 26(2) T. The spin of the⁷³Br ground state is also deduced. An interpretation of the ground state configurations of these isotopes is given.     

Fabrication and magnetic characterization of Co<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Pt<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> nanowire arrays

Co _x Pt_1−x (x≥0.7) alloy nanowires are grown into self-synthesized anodic alumina templates by electrodeposition. Magnetic and magnetization properties of Co _x Pt_1−x alloy nanowires are measured as functions of wire length, temperature, and field orientation. X-ray diffraction shows that as-prepared CoPt nanowires are of fcc polycrystalline structure. A crossover of easy axis of magnetization is observed from parallel to perpendicular of the nanowire axis as a function of length. The coercivity (H _c) and remanent squareness (SQ) of Co _x Pt_1−x nanowire arrays are derived from hysteresis loops measured at various angles (θ) between the field and wire axis. H _c(θ) and SQ(θ) curves show bell-shaped or otherwise bell-shaped behavior corresponding to the easy axis of their magnetization.     

Magnetic hyperfine field of Se in nickel measured by means of the DPAC method

The magnetic hyperfine field of Se in nickel was measured by means of the time-differential perturbed angular correlation (DPAC) technique, using the 755–250 keV γ-ray cascade fed in the decay of⁷⁷Br. A value ofB _hf(NiSe)=+15.11(35) T was obtained at room temperature. The half-life of the 250 keV state and the anisotropy of the 755–250 keV cascade were found to beT _1/2=9.68(6) ns andA ₂₂=−0.454(9), respectively.     

Structure and impedance spectroscopy of Pr1? x Sr x Fe0.8Co0.2O3? δ ( x =0.1, 0.2, 0.3) thin films grown by laser ablation

Polycrystalline samples of Pr_1−x Sr _x Fe_0.8Co_0.2 O_3−δ (x=0.1, 0.2, 0.3) (PSFC) were prepared by the combustion synthesis route at 1200°C. The structure of the polycrystalline powders was analysed with X-ray powder diffraction data. The X-ray diffraction (XRD) patterns were indexed as the orthoferrite similar to that of PrFeO₃ having a single-phase orthorhombic perovskite structure (Pbnm). Pr_1−x Sr _x Fe_0.8Co_0.2O_3−δ (x=0.1, 0.2, 0.3) films have been deposited on yttria-stabilized zirconia (YSZ) single-crystal substrates at 700°C by pulsed laser deposition (PLD) for application to thin film solid oxide fuel cell cathodes. The structure of the films was analysed by XRD, scanning electron microscopy (SEM) and atomic force microscopy (AFM). All films are polycrystalline with a marked texture and present pyramidal grains in the surface with different size distributions. Electrochemical impedance spectroscopy (EIS) measurements of PSFC/YSZ single crystal/PSFC test cells were conducted. The Pr_0.7Sr_0.3Fe_0.8Co_0.2O_3−δ film at 850°C presents a lower area specific resistance (ASR) value, 1.65 Ω cm², followed by the Pr_0.8Sr_0.2Fe_0.8Co_0.2O_3−δ (2.29 Ω cm² at 850°C) and the Pr_0.9Sr_0.1Fe_0.8Co_0.2O_3−δ films (5.45 Ω cm² at 850°C).     

Hyperfine magnetic field of Zn in iron

Precise hyperfine field value of zinc in iron has been determined by nuclear magnetic resonance on oriented nuclei (NMR/ON): B_hf (ZnFe)=−18.785 (35) T at 7 mK. The relaxation constant of Zn in iron is established C_K=14(3) Ks. The new hyperfine field value of zinc in iron allows a more precise reevaluation of the magnetic moments of^69mZn and^71mZn measured with NMR/ON. and the NICOLE Collaboration, CERN     

Quadrupole effects in the NMR spectra of 27Al in disordered mixed compounds CaREAlO4 (RE=La, Pr, Eu, Y)

The NMR spectra of ²⁷Al are measured in several disordered rare-earth aluminates CaREAlO₄. A change in the shape of the resonance line of the transition 1/2↔−1/2 in the transverse orientation in the rare-earth series RE=La, Pr, Eu, Y is observed, from a narrow (∼16 kHz), structureless line in the La compound to a wide (∼130 kHz), three-component line in the Y compound. This is attributed to a nonuniform distribution of the electric field gradient tensor at the ²⁷Al site. The electric field gradient tensor at the ²⁷Al site in a cluster (AlO₆)⁻⁹ is calculated by a nonempirical method (MO LCAO SCF) in the “deformable oxygen octahedron” model. The results are found to be in satisfactory agreement with the experimental values of 〈V _zz〉 and can be used to obtain the experimental splitting of the resonance line in the Y compounds for reasonable values of the parameters of the local deformations of the AlO₆ octahedron. The nature of the strong axial correlations in the distribution of the Ca and Y atoms in CaYAlO₄ is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 618–623 (April 1997)     

Negative muon spin precession in ferromagnetic iron and the hyperfine anomaly

The hyperfine field (B _μ ^hf ) at the negative muon μ⁻ in ferromagnetic iron was investigated by means of the zero-field μ⁻ spin precession technique. In the temperature range 320–690 K,B _μ ^hf for μ⁻ Fe departs from the magnetization curve of pure iron in the same way as the hyperfine field seen by a⁵⁵Mn impurity in dilute MnFe measured by NMR. The hyperfine anomaly for μ⁻ Fe relative to dilute (1.5 at.%)⁵⁵Mn in iron is found to be −0.9(3)% and temperature independent over the temperature range investigated.