Distribution Of Electric Field Gradients Research Articles

A Mössbauer spectroscopy study of antiferromagnetic iron arachidate Langmuir-Blodgett films

Langmuir-Blodgett multilayers of ferric arachidate (abbreviated: FeA) on silicon wafers have been investigated by means of conversion electron Mossbauer spectroscopy (CEMS) and absorption Mossbauer spectroscopy at different temperatures between room temperature and 4.2 K, without and with external magnetic fields up to 5 T. The films show a quasicrystalline structure with ac-axis orientation perpendicular to the plane and a random distribution of orientations in the plane of the films. All Mossbauer spectra show Fe3+. At low temperatures, antiferromagnetic ordering has been observed. The lines in both the room temperature and the low-temperature spectra are significantly broadened, which is due to a distribution of electric field gradients and hyperfine fields. The reason for this is a slightly disordered iron environment. The orientation and the distribution function of the internal magnetic fields have been determined.

NMR and NQR study of the electronic and structural properties of Al-Cu-Fe and Al-Cu-Ru quasicrystals.

[sup 27]Al and [sup 63,65]Cu NMR is reported for powdered stable Al-Cu-Fe and Al-Cu-Ru icosahedral quasicrystals and crystalline approximants, and for an Al-Pd-Mn single-grain quasicrystal. [sup 27]Al NQR spectra at 4.2 K were observed in Al-Cu-Fe and Al-Cu-Ru samples. From quadrupole-perturbed NMR spectra at different magnetic fields, and from zero-field NQR spectra, a wide distribution of local electric-field gradient (EFG) tensor components and principal-axis-system orientations was found at the Al site. A model EFG calculation based on a 1/1 Al-Cu-Fe approximant successfully explained the observed NQR spectra. The average local gradient is largely determined by the [ital p]-electron wave function at the Al site, while the width of the distribution is due to EFG lattice contribution. Comparison of [sup 63]Cu and [sup 27]Al NMR shows the EFG distribution at the two sites is similar, but the electronic contribution to the EFG is considerably smaller at the Cu site, in agreement with a more [ital s]-type wave function of the conduction electrons. Overall spread of EFG values is well reproduced by calculation based on the approximant. However, the experimental spectra indicate a much larger number of nonequivalent sites when compared with the simulated NQR spectra based on the 1/1 approximant. Themore » short-range, local chemical order is well represented by the approximant, but differences in coordination must be included at intermediate range in the quasicrystal. Measured [sup 27]Al Knight shift, magnetic susceptibility, and nuclear spin-lattice relaxation time as a function of temperature indicate reduced density of states at the Fermi level by a factor of 7 or 8 from the value in Al metal, consistent with the notion of a pseudogap for these quasicrystals. No differences in measured parameters were detected as a function of composition of the quasicrystalline alloys.« less

Neutron Diffraction and Mössbauer Spectroscopic Studies of β-Mn(Sn) Alloys

About 90% of Sn atoms in β-Mn structure are found by means of neutron diffraction to occupy site II and neither antiferromagnetic long range order nor remarkable short range order was observable. By Mössbauer spectroscopy of 119m Sn, the magnitude of the electric field gradient (EFG) decreases with an increase in Sn concentration. The EFG distribution functions are expressed by the sum of two Lorentz distributions. From the stable distribution analysis of internal field distributions, it becomes clear that the magnetic moments randomly freeze in the noncollinear manner.

The129I hyperfine interaction in fatty acids studied by Mössbauer spectroscopy

Oleic acid substituted by iodine and saponified by Ca2+ cations has been studied by129I Mossbauer spectroscopy. The quadrupole coupling constants and isomer shifts, determined from the γ-resonance spectra recorded at 4.2 K, have been described by 5p and 5s orbital populations of iodine. It was also found that saponification of the fatty acid has no significant influence on the measured iodine bonds. However, the increased order of fatty acids in soap form is reflected by narrowing of the resonant linewidth due to the reduction of the electric field gradient distribution.

Microscopic observation of atomic disorder near the roughening transition at vicinal copper surfaces.

The appearance of a reversible order-disorder transition at Cu(115) and Cu(117) surfaces has been investigated using perturbed angular correlation spectroscopy. The local atomic arrangement at $^{111}\mathrm{In}$ probes is detected via nuclear quadrupole hyperfine interaction. A broadening of the electric field gradient distribution is measured for elevated temperatures due to structural changes around the probes. The onset of disorder on the atomic scale is found to be well above the roughening transition temperatures measured by various scattering techniques.

Electric field gradient of iron in oxide glasses

57Fe Mössbauer spectroscopy has been carried out at room temperature on oxide glasses of the binary oxide systems, i.e., BaO-Fe 2O 3, SrO-Fe 2O 3, PbO-Fe 2O, and TeO 2-Fe 2O 3 in order to estimate the distribution of electric field gradient of Fe 3+ ion sites in these materials. The Mössbauer spectra obtained experimentally were reproduced by taking into account an asymmetric non-Gaussian distribution of electric field gradient around Fe 3+ ions. From the analyses, the average value of the electric field gradient was evaluated for each glass. The average electric field gradient tends to increase monotonically with an increase in concentration of Fe 3+ ions, irrespective of kinds of glass systems. It is thought that in the present oxide glasses with larger concentration of Fe 3+ ions, more distorted local structure is required for Fe 3+ ions to take a random state.

A cems and absorption Mössbauer spectroscopy study of iron arachidate Langmuir-Blodgett films

Abstract Langmuir-Blodgett multilayers of ferric arachidate on silicon wafers have been investigated by means of conversion electron Mossbauer spectroscopy (CEMS) and absorption Mossbauer spectroscopy at different temperatures between room temperature and 4.2 K without and with an external magnetic field of 5 T. The Mossbauer spectra show Fe 3+ . A change of the relative line intensities in the measured spectra dependent on the observation direction is visible. It was found that the lines in the spectra are significantly broadened, which is due to a distribution of electric field gradients and isomer shifts. The reason for this is a slightly disordered iron environment. The quadrupole splitting is linearly correlated to the isomer shift. At low temperatures antiferromagnetic ordering has been observed and the orientations of the internal magnetic field and the electric field gradient in the crystal system have been determined. Due to its chemical structure this substance is an example of a quasi two dimensional system.

High field MAS NMR, with simulations of the effects of disorder on lineshape, applied to thermal transformations of alumina hydrates

Samples of gibbsite, boehmite, pseudoboehmite and bayerite, calcined at temperatures up to 1000°C, were studied using27Al MAS NMR with spinning speeds up to 15 kHz in magnetic fields of 14.1, 11.8 and 7 T (156, 130 and 78 MHz27Al frequencies). The high magnetic fields reduce the effects of the second order quadrupolar interactions, giving much better resolution of the peaks than previous studies at 7 T and allowing quantitation of the different species. The samples generally show only octahedral and tetrahedral species, with the latter increasing with calcination temperature. The exceptions are ϱ-alumina and χ-alumina which show a peak at 33 ppm (in the 130 MHz spectra) with about 20% and 7% respectively of the total intensity. The peaks show large upfield tails and a dependence on magnetic field which differs from that expected for a single electric field gradient. Spectra were simulated assuming a random distribution of electric field gradients, and the dependence of the spectra on the field strength, quadrupolar parameters and the range of electric field gradients is illustrated and empirical equations derived. The method provides a good fit to the observed spectra at all 3 field strengths. These materials show considerable disorder which can be categorized by the parameter used to describe the range of electric field gradients.

High-field27Al mas NMR studies of the formation of metakaolinite by flash calcination of kaolinite

Flash calcination was used to dehydroxylate kaolinite and produced calcines kinetically frozen at various stages during metakaolinite formation. High speed magic-angle spinning27Al NMR, in magnetic fields of 14.1 and 11.8 T, enabled new features to be observed in the metakaolinite spectrum. The dependence on magnetic field of simulated and experimental spectra is discussed, and the effects of a distribution of electric field gradients is examined. The evolution of the metakaolinite peaks is followed during dehydroxylation.

NMR, magnetism and Mössbauer effect in icosahedral Al 63Cu 24.5Fe 12.5 — a thermodynamically stable quasi-periodic alloy

We report results of 27Al nuclear magnetic resonance (NMR), magnetism and iron-site Mössbauer experiments on the thermodynamically stable and “perfectly” quasicrystalline icosahedral alloy Al 63Cu 24.5Fe 12.5. NMR experiments were performed at 11.10, 17.8 and 45.7 MHz and at temperatures as low as 50 K. Magnetization was measured at 295, 100 and 5 K, while iron site Mössbauer was measured at 295 and 4.2 K. We find very small NMR Knight shifts and long relaxation times that we interpret as consistent with a pseudo-gap in the density of states near the Fermi level. NMR line shapes in AlCuFe do not show quadrupolar structure consistent with a single aluminum site. We report results of numerical simulations that effectively reproduce the features and trends in observed line shapes by means of a broad distribution of electric field gradients (EFGs) at aluminum sites. Our magnetization and Mössbauer effect experiments show that there is a very small fraction of the material that is magnetically ordered at low temperatures. This magnetic behavior was only observed well below the lowest temperature of NMR experiments and cannot be responsible for the broad NMR lines. Iron-site Mössbauer lines show significant broadening characteristic of a distribution of EFGs that is qualitatively similar to that indicated for the aluminum site.

The chemical state of iron ions implanted into silicon carbide

Conversion electron Mössbauer spectroscopy (CEMS) was used to determine the local environment of 57Fe implanted into single crystals of α-SiC at room temperature. Rutherford backscattering ion channeling measurements showed the specimens to be amorphous (random) for all fluences studied (1, 3, and 6 × 10 16ions/cm 2, 160 keV). The CEMS spectra were fit with a single component having a distribution of quadrupole split values, i.e., a distribution of electric field gradients. The results indicate a distribution of the implanted iron ions among several sites with slightly differing local environments.

Defects in Fe1-xO and the Fe1-xO to Fe3O4 phase transition studied by the perturbed angular correlation method

The time differential perturbed angular correlation (PAC) method has been applied to study defects in iron monoxides, using iron-implanted 111In probes. Besides a broad distribution of electric field gradients two well-defined quadrupole coupling constants have been detected nu Q=97(4) MHz, eta =0 and nu Q=71(2) MHz, eta =0.32(2). The first one was associated with an indium-iron vacancy complex, while the second one, visible in a high-temperature measurement, was tentatively assigned to indium having trapped two iron vacancies and one interstitial iron atom (the so-called Roth-type cluster). Structural transformations between different complexes and the Fe1-xO to Fe3O4 phase transition are discussed. From the analysis of the vacancy/interstitial ratio, a sizeable fraction of 4:1 defect clusters at low temperatures can be excluded. The proposed defects are compared with those recently observed in the chemically and structurally similar cobalt monoxide CoO.

Perturbed angular correlation measurements of 111Cd in cobalt (II, III) oxide

The time differential perturbed angular correlation of 111Cd has been measured after implantation of the radioactive mother isotope 111In into Co3O4. Besides a broad distribution of electric field gradients ((vQC)=105(5) MHz, eta C=0.8(1)) two fractions with vQA=0 and vQB=145 MHz, eta B<0.15 are found which have been associated with 111In on substitutional A and B sites of the spinel lattice. During isochronal annealings at various oxygen partial pressures, the distribution widths delta A and delta B and the asymmetry parameter eta B approach zero; simultaneously, vQB slightly decreases. The changes are correlated and quantitatively explained in the point charge model.

Order-disorder structural phase transition in La2-xSrxCuO4+ delta at 150 K.

Using perturbed {gamma}{gamma} angular correlations of {sup 111}In in the La site, we report a structurally disordered phase in oxygen stoichiometric La{sub 2{minus}{ital x}}Sr{sub {ital x}}CuO{sub 4+{delta}}. So far this phase has been defined for 0{le}{ital x}{le}0.07, involves a reversible order-disorder transition at {ital T}{sub {ital d}}=150(20) K, and is observed by changes in the electric-field gradient (EFG). These changes involve a continuous {ital broadening} of the EFG distribution and {ital decrease} of the average EFG asymmetry parameter as {ital T}{r arrow}0. The new disordered phase is associated with a loss of three dimensional magnetic order in the range 0.015{lt}{ital x}{lt}0.018, and may be due to short-range disorder in the tilting of the CuO{sub 6} octahedra as proposed by Egami {ital et} {ital al}.

Distribution of electric field gradient at the nucleus for a cluster of amorphous Zr

In this work we apply the recently developed real space scheme based on the recursion method and the LMTO-ASA formatism to investigate the dominant p valence contribution to the distribution of electric field gradient at the nucleus (EFG) in amorphons metals. The Zr eluster was obtained by random packing of spheres with periodic boundary conditions. We find that the distribution of EFG is over positive and negative values and has a gap around zero.

Solid-state ‘‘magic-angle’’ sample-spinning nuclear magnetic resonance spectroscopic study of group III–V (13–15) semiconductors

We have obtained 27 Al, 69 Ga, 71 Ga, 113 In, and 115 In static and ‘‘magic-angle’’ sample-spinning (MASS) solid-state nuclear magnetic resonance (NMR) spectra of a series of polycrystalline III–V semiconductors (AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, and InSb) at magnetic field strengths of 8.45 and 11.7 T. Line-broadening mechanisms have been identified by comparing static and MASS NMR results. By applying the MASS technique, dipolar, first-order quadrupolar, and pseudodipolar interactions are removed. Thus, the MASS spectral linewidth (of the central, 1/2↔−1/2, transition) is dominated by exchange and/or second-order quadrupolar interactions. For compounds having the cubic zinc blende structure, the exchange interaction dominates, and exchange interaction constants can be determined. For AlP, AlAs, AlSb, GaP, GaAs, InP, and InAs, first-order quadrupole effects are evident as spinning sidebands (SSBs), due to the satellite transitions. These effects are due to a small distribution of electric field gradients caused by lattice defects, and result in an overall Lorentzian profile for the SSB envelope. For compounds having the hexagonal wurtzite structure (GaN and AlN), the second-order quadrupolar interaction is the main line-broadening mechanism, and we find isotropic chemical shifts δi of 333 ppm [from 1 M Ga(NO3 )3 ] for GaN, and of 115 ppm [from 1M Al(H2 O)6 Cl3 ] for AlN, and nuclear quadrupole coupling constant e2 qQ/h of 2.8 MHz for 69 GaN, 1.7 MHz for 71 GaN, and of 2.2 MHz for AlN.

Molecular relaxation versus orientational ordering ofH2ina-Si:H

Recently reported experimental results on a-Si:H have led to suggestions that ${\mathrm{H}}_{2}$ in the microvoids of a-Si:H undergoes a phase transition to an orientational ordered state at temperatures ranging from 3 to 20 K. The interpretation is based upon the onset of a Pake doublet in NMR experiments that is suggestive of an order parameter and also by peaks in the heat capacity of ${\mathrm{H}}_{2}$ in a-Si:H. We show that orientational ordering of microvoid ${\mathrm{H}}_{2}$ is extremely unlikely at any temperature and certainly not above the critical temperature of bulk single-crystal ${\mathrm{H}}_{2}$. Instead we shall show that the observed Pake doublet is a manifestation of the slow molecular relaxation regime and that the heat capacity can be explained entirely by a distribution of electric field gradients. Neither phenomenon requires a phase transition driven by the electric quadrupole-quadrupole molecular interactions. However, the sharp onset of the Pake doublet reflects the size and surface conditions of the microvoids.

Hyperfine and exchange field distributions in amorphous magnetic insulators (abstract)

The 57Fe Mössbauer Zeeman spectra of amorphous magnetic ferric insulators, a-Y3Fe5O12 (YIG), a-FeF3, and a-NaFeF4, consist of well-resolved broad experimental lines, which indicate the existence of a distribution of electric field gradients, hyperfine fields, and isomer shifts at the iron sites.1,2 The hyperfine-field distribution in amorphous magnetic insulator is shown to result dominantly from local spin disorder via its supertransferred component HST. Since the same spin disorder determines local exchange fields Hex, the hyperfine-field distribution indirectly contains much information concerning the distribution of exchange, in the spin-glass environment. This work probes the nature of the nearest-neighbor spin correlations in such systems, by analysis of hyperfine field distribution, as measured by 4.2 K Mössbauer spectroscopy. Low-field tails are found in the hyperfine field distribution spectra.3 They indicate the presence of low-energy exchange-field states caused by the cancellation of the nearest-neighbor contributions within the frustrated spin network. The distribution of exchange fields for a-YIG, a-FeF3, and a-NaFeF4 has been determined and, by comparing the results to those expected for a set of randomly oriented spins, the nature of nearest-neighbor spin frustration has been monitored in these amorphous magnetic insulators. The implied spin correlation is one of a quasiconstraint of the nearest neighbors of any given spin which restricts their vector sum to small values.

Grain size dependence of the electric field gradient

Precise measurements of the electric field gradient (EFG) with the perturbed angular correlation method (PAC) in Cd and Zn show significant differences between single-crystalline and poly-crystalline samples. These differences (up to four percent at low temperatures) decrease with increasing temperature and vanish at about 2 60 K. In all samples the lowest EFG values are obtained for single crystals. In poly-crystalline samples the EFG as well as the width of the EFG distribution increase with decreasing grain size. This effect is also observed in semiconducting ZnO.

Quadrupole interaction in In0.95 Ag0.045 Ga0.005 alloy

The temperature dependence of the quadrupole interaction of111Cd in the In0.95 Ag0.045 Ga0.005 alloy is determined by the time differential perturbed angular correlation method from 77 to 422 K. The electric field gradients produced by the lattice are derived. The temperature dependence of the electric field gradient is found to follow theT3/2 law observed in In between 77 K and room temperature and to deviate from it at higher temperatures. The widths of the electric field gradient distribution are 0.1147(259), 0.0924(340), 0.4954(601) and 1.344(175) at 77, 298, 373 and 422 K, respectively. This illustrates that the alloy is well ordered at 77 and 298 K and that it becomes less ordered at 373 and 422 K.