N. Yazidjoglou

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 EQIs. 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.

Discrepancies in NMRON Knight Shifts of impurity nuclei in ferromagnets

Precision NMRON field shift studies have been carried out up to 8T on a54MnNi single crystal along a hard [100] direction and on a125SbFe single crystal along a hard [110] direction. For both systems, high field (Bapp>-1T) and low field (0.3T<Bapp<0.8T) data sets are obtained. The analysis reveals a significant discrepancy between the apparent Knight Shift dependent on whether the low field or high field data set is utilised. For both systems, consideration of the high field data sets yield a zero Knight Shift K(54MnNi)=+0.0(0.2)% and K(125SbFe)=+0.2(1.4)%, whereas the low field data sets yield K(54MnNi)=+7.5(3.9)% and K(125SbFe)=−5.4(3.3)% respectively. The field range dependence of K suggests that only Knight shifts measured in large fields (>-1T) are meaningful for establishing systematics. This casts some doubt on the greater bulk of the literature’s NMRON Knight shift studies, where predominantly low fields have been used to determine K.

Nuclear magnetic resonance of brute force oriented110mAgAg

NMR on brute force oriented nuclei (NMRON/BF) has been observed for110mAg in elemental silver. Resonances have been measured at approximately 7 T and 8 T with the resulting gradient of 4.583(5) MHzT−1 which produces a value for the110mAg moment of μ=3.589(4) nm (uncorrected). Factors pertinent to successful NMRON/BF in systems such as110mAgAg, with a complex decay scheme and modestg-factor, are also discussed.

MAPON spectroscopy and its application to transition metal ferromagnets

Modulated adiabatic passage on oriented nuclei (MAPON) is a technique developed specifically to measure very small electric quadrupole interactions (EQIs) at radioactive probe nuclei even when such interactions are masked in the frequency domain by inhomogeneous magnetic broadening associated with a dominant magnetic hyperfine interaction. In this paper an overview of the technique is presented with particular reference to 60CoCo EQI measurements which were an important step in establishing MAPON methodology. MAPON results for dilute nd impurities in ferromagnetic Fe and Ni metal hosts are reviewed including presentation of new data for 58CoNi and 60CoNi. Priorities for future studies are provided on the basis of this review.

NMRON studies of ferromagnetic hcp and fcc Co

The high resolution hyperfine spectroscopy, modulated adiabatic passage of oriented nuclei (MAPON), has been applied for the first time to high purity, elemental systems. Detailed comparisons between the electric quadrupole hyperfine interactions (EQI’s) and, in particular, their distributions, are obtained for60CoCo where the hosts are a single crystal of hcp cobalt and a polycrystalline cobalt foil of predominantly fcc character. For hcp Co, with the electronic magnetization, M, parallel to the c-axis, the mode value P/h=3e2qQ/4I(2I−1)h=−48.5(5) kHz. This fractional distribution implies the sharpest electric field gradient (efg) measured in a metal to date, using MAPON spectroscopy, in excess of two times sharper than that of the most dilute impurity efg in a crystallographically cubic ferromagnetic host. The mode efg is Vzz=−27.3(32)×1019 Vm−2. For the polycrystalline, predominantly fcc foil, prepared by quenching, the EQI mode value is P/h=−6.2(4) kHz with a FWHM of 12.0(7) kHz yielding a mode efg of Vzz=−3.5(5)×1019 Vm−2.

Transient NMRON of brute force oriented110mAgAg

A novel technique involving electrodeposition has been used to prepare an elemental silver specimen wherein radioactive110mAg 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 the110mAg 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 Vzz=+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 Ck=1.6(3)s.K.

Transient NMRON studies of dilute 54Mn in single-crystal Fe

The sign, mode magnitude, and distribution of the electric quadrupole interaction (EQI) in single-crystal 54MnFe have been examined along an easy (100) direction for the first time using transient NMR on oriented nuclei (NMRON) techniques. Single-passage experiments yielded the sign of the EQI as being negative, and the modulated adiabatic passage on oriented nuclei (MAPON) technique yielded a mode magnitude and distribution for the EQI of P/h=-4.4(4) kHz and Delta P/h=4.0(5) kHz respectively. The resulting mode electric field gradient is Vzz=-1.11(10)*1019 V m-2.

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.

Pulsed fm NMR/on studies of single crystal54MnNi and125SbFe

Precision field shift studies of Bappl-8T using pulsed FM NMR/ON along hard directions (eg <100>54MnNi, <112>125SbFe and the easy axis (<111>54MnNi, <100>125SbFe) yield no measurable Knight shifts within the uncertainty allowed by current accuracy of the nuclear moments. This is in striking contrast to earlier measurements with the same apparatus that yielded K=+1.5 (4)% for60CoFe.The modulation frequency dependences of the pulsed FMNMR/ON signals are investigated for a variety of rf parameters and compared with model predictions.

Magnetic ordering in two dimensional manganese stearate films : A nuclear orientation study

Abstract Two dimensional manganese stearate films incorporating radioactive 54 Mn have been produced using the Langmuir–Blodgett technique. Low temperature nuclear orientation measurements on these films show a degree of magnetic order even in zero applied magnetic field; a result which is discussed in relation to the Mermin–Wagner theorem which rules out ordering of Heisenberg spins in low dimensional systems with continuously symmetric interactions.