Hyperfine Field Distribution Research Articles

Structural, magnetic, and Mössbauer studies of magnetite and nickel-copper and nickel-zinc ferrites

This work aims to investigate the magnetite obtained by coprecipitation, as well as the mixed ferrites of nickel-copper and nickel-zinc. The as-prepared samples were then annealed at a temperature (Tan) of 1000 °C. The as-prepared magnetite shows a polycrystalline spinel structure having an average of crystallite size (DXRD) of 8 nm, while other as-prepared mixed ferrites are structurally amorphous. Annealing at Tan has resulted in an improvement in the crystallinity of both nickel-copper ferrites (NCF, DXRD = 92 nm) and nickel-zinc ferrites (NZF, DXRD = 83 nm), both of which exhibit a cubic spinel structure. The thermal treatment in air atmosphere has also led to the oxidation of magnetite into hematite. Morphological observations suggest that the magnetite nanoparticles are nearly spherical, whereas the NCF and NZF particles indicate irregular shapes, which tend to aggregate. Magnetic curves reveal that the NCF exhibits a blocking temperature (TB) close to room temperature, while the NZF sample displays the lowest TB at 45 K. Additionally, the NZF exhibits a higher Ms value of 80.4 emu/g and displays a narrow hysteresis loop, which makes it suitable for applications in hyperthermia. The 57Fe Mössbauer study on magnetite and NZF indicates broadening spectra, which is due to superparamagnetic relaxation, thus the fit their spectra was established with hyperfine field distribution component. Regarding the NCF Mössbauer spectrum was adjusted by superimposing two sextets reflecting the presence of Fe3+ at the A and B sites of spinel. The hyperfine parameters, including the isomer shift, and hyperfine field, have been systematically reported in the present study.

Insights into the Role of Defects in Fe2(MoO4)3 Catalysts

The catalytic activity and selectivity of Fe2(MoO4)3 obtained from solid-state synthesis protocols is investigated for the oxidative dehydrogenation (ODH) of ethanol to acetaldehyde. While Fe2(MoO4)3 annealed in a MoO3 atmosphere is found to be inactive, ball-milling of such solid-state synthesis precursors leads to an increase in activity, which cannot be attributed purely to the change in the specific surface area nor to a geometric effect of smaller particles. In a systematic study of the synthesis of Fe2O3-free Fe2(MoO4)3 samples, the correlation of ball-milling time, defect concentration, and catalytic activity is presented. In-depth X-ray diffraction studies, magnetic susceptibility, and 57Fe-Mössbauer spectroscopic measurements disclose characteristic signatures of the defect sites in the solid materials introduced by ball-milling and indicate: (i) MoO3-enriched amorphous layers of variable thickness (shell-like) and (ii) crystalline bulk defects corresponding to a reduction in the molar volume of the solid. The onset of recrystallization processes sets in around 300 °C and is accompanied by an enhanced MoO3 mobility in the particles, which adds to an increase in activity and a decreasing selectivity under catalytic conditions. Accordingly, the defects associated with the decreasing amorphous fraction are converted to crystalline bulk defects, which are monitored by the magnetic hyperfine field distribution of the respective Fe sites. Overall, this study shows the importance of the amorphous layer thickness with a sufficient defect concentration and of the bulk defects’ contribution to the chemical gradients, important for the MoO3 mobility and reduction of the MoO3 loss as a volatile species at the solid/gas phase boundary in ODH catalysis.

Cluster-layered [Fe/Cr]30 structure exhibited Kondo-like effect studied by GISAXS and Mössbauer spectroscopy

Structure and morphology of epitaxial [Fe/Cr]30 multilayers with ultrathin Fe layers (nominally 0.12 nm and 0.08 nm) have been investigated by X-ray reflectivity, synchrotron Mössbauer spectroscopy at low temperature and grazing-incidence small angle X-ray scattering (GISAXS). The films demonstrate Kondo-like behavior of electrical resistivity. The GISAXS patterns reveal their cluster-layered structure. The observed side maxima give the information about the sizes and distances between lateral inhomogeneities. Mössbauer reflectivity spectra measured below the critical angle of the total reflection support the existence of the cluster-layered structure of the samples. Magnetic hyperfine field distributions show that largest number of 57Fe atoms is situated in interfaces of iron clusters and their number increases in the thinnest films.

The origin of the large magnetoelectric coupling in the ceramic Ba0.1Bi0.9(Ti0.9Zr0.1)0.1Fe0.9O3

57Fe Mössbauer spectrometry and x-ray powder diffraction are conducted to examine the structure and the local magnetic order at the level of Fe sites of Ba0.1Bi0.9(Ti0.9Zr0.1)0.1Fe0.9O3 ceramic. The Mössbauer spectra in the temperature range of 77 K–623 K were analyzed using a discrete distribution of hyperfine field, indicating that the Néel temperature T N was about 603 K. The ceramic Ba0.1Bi0.9(Ti0.9Zr0.1)0.1Fe0.9O3 remains stable in the rhombohedral structure (R3c) in the temperature range of 300 K–800 K. The anomaly of volume below T N, as detected from the model Debye–Gruneissen, reveals the presence of magnetoelastic coupling in this compound. The total polarization (P) obtained from Rietveld refined atomic positions is found to depend on the magnetic order that leads to the decrease of the total polarization (P) through the anomaly volume. The reduction in the polarization by ΔP ∼ −2.4 μC cm−2 suggests negative magnetoelectric interaction. The total polarization (P) obtained from Rietveld refined atomic is coupled with the magnetic ordering mediated by magnetoelastic coupling. The hyperfine field (B hf) dependence of polarization (P) at a temperature range below T N exhibits a linear evolution, confirming the linear magnetoelectric coupling. At room temperature, the linear magnetoelectric coefficient is about α ME ≈ 1.84 × 10−9 s m−1.

Radiation Effects in Amorphous Metallic Alloys as Revealed by Mössbauer Spectrometry: Part I. Neutron Irradiation

Iron-based amorphous metallic alloys (AMAs) of several compositions were exposed to neutron irradiation with fluences of up to 1019 n/cm2. These materials exhibit excellent magnetic properties which predetermine them for use in electronic devices operated also in radiation-exposed environments. Response of the studied AMAs to neutron irradiation is followed by Mössbauer spectrometry which probes the local microstructure. Neutron irradiation leads to rearrangement of constituent atoms, their clustering, and formation of stress centers. The observed modifications of topological short-range order result in changes of spectral parameters including average hyperfine magnetic field, ⟨B⟩, standard deviation of the distribution of hyperfine fields, and position of the net magnetic moment. After irradiation, especially differences in ⟨B⟩-values develop in two opposite directions. This apparent controversy can be explained by formation of specific atomic pairs with different exchange interactions, which depend on the composition of the samples. Part II of this paper will be devoted to radiation effects caused in Fe-based AMAs by ion irradiation.

Spin Canting in Silica-Coated Nickel Ferrite (NiFe2O4@SiO2) Nanoparticles: a Mössbauer Spectroscopy Study

We present a comparative study on superparamagnetic uncoated and silica-coated NiFe2O4@SiO2 nanoparticles with identical NiFe2O4 cores and two different silica coating thicknesses, focusing on the surface spin arrangement on NiFe2O4. For the investigation of surface spin dynamics, in addition to the SQUID magnetometry experiments which probe the macroscopic behaviour of particle magnetization, Mossbauer spectroscopy was utilized as being a local technique with superior characteristic measurement time. It was revealed that two common aspects observed in nanoscaled magnetic particles, namely the surface spin canting and interparticle dipolar interactions, are intertwined with respect to their contributions to the total magnetic anisotropy, where the latter is significantly decreased with silica coating thickness and allowed surface effects to manifest itself in Mossbauer spectra. The reduced saturation magnetization of uncoated NiFe2O4 nanoparticles with respect to their bulk counterparts indicated the existence of a “magnetically dead” layer on NiFe2O4 surface. In opposite to some assumptions in the literature, the Mossbauer spectra confirmed that such reduced magnetization arises from spin disorder, rather than a change in coordination from inverse spinel to the mixed spinel structure. On the other side, emergence of broad sextet peaks at room temperature Mossbauer spectra showed that for NiFe2O4@SiO2 samples, silica coating further enhanced the spin canting on NiFe2O4 surface. Similarly, for NiFe2O4@SiO2 sample, an unusual asymmetric hysteresis loop at spin freezing temperature was observed as a signature of the existence of disordered spins in the SiO2/NiFe2O4 interface. It was shown that the sextet components in Mossbauer spectra can be reproduced with a continuous hyperfine field distribution due to the frustrated local spin arrangement at the surface. The thickness of the “magnetically dead” disordered surface spin layer was estimated for uncoated NiFe2O4, and average spin canting angle was calculated for NiFe2O4@SiO2 nanoparticles.

Spin-Dependent Charge-Carrier Recombination Processes in Tris (8-Hydroxyquinolinato) Aluminum

We have studied the nature and dynamics of spin-dependent charge carrier recombination in Tris(8-hydroxyquinolinato) aluminum (Alq$_3$) films in light emitting diodes at room temperature using continuous wave and pulsed electrically detected magnetic resonance (EDMR) spectroscopy. We found that the EDMR signal is dominated by an electron-hole recombination process, and another, weaker EDMR signal whose fundamental nature was investigated. From the pulsed EDMR measurements we obtained a carrier spin relaxation time, $T_2 = 45\pm 25$ ns which is much shorter than $T_2$ in conjugated polymers, but relatively long for a molecule containing elements with high atomic number. Using multi-frequency continuous wave EDMR spectroscopy, we obtained the local hyperfine field distributions for electrons and holes, as well as their respective spin-orbit coupling induced g-factor and g-strain values.

59Co Internal field NMR analysis of Co35Fe35Ni30 alloy synthesized via novel low cost chemical reduction technique

We have synthesized ternary cobalt alloy using a chemical reduction process using hydrazine as a reducing agent . X-Ray Diffraction (XRD) measurements shows that the synthesized cobalt ternary alloy exists in the pure face centred cubic (fcc) phase with lattice parameter of 3.5562 ± 5 Å. The SEM micrograph shows that the particles exist in the spherical shape which suggests that the particles are crystallized in fcc phase. The magnetic properties are determined using Vibrating Sample Magnetometry (VSM) and 59Co IFNMR technique. The saturation magnetization (MS) of the synthesized sample is ~80 emu/g, a slightly lower value compared to the pure cobalt metal. 59Co IFNMR technique shows that there is a broad distribution of hyperfine field (~6 T) compared to the pure fcc cobalt metal. Further, from the deconvoluted NMR peaks suggests that the variation in the nearest neighbours to the central cobalt atom leads to the different hyperfine field.

Substitutional effect of Mg2+ on structural and magnetic properties of cobalt nanoferrite

This work focuses on the study of structural and magnetic properties of Mg2+-substituted cobalt nanoferrite (Co1-xMgxFe2O4, 0.0 ≤ x ≤ 1.0). The samples were synthesized by a coprecipitation route and then annealed at 850ؘ °C for 2 h under atmospheric air. The X-ray diffraction patterns show that the samples crystallize in a cubic spinel structure with space group Fd3‾m, and 3% secondary phase was identified as hematite (R3‾c) for x = 1.0. The average crystallite size (D) and the lattice parameter were greatly influenced by Mg content (D decreased from 38 to 20 nm). Transmission electron microscopy investigations revealed that the grains have an almost spherical shape and furthermore that the samples are chemically homogeneous. Magnetic measurements at 5 K revealed a wasp-waist magnetic hysteresis behavior of Co–Mg ferrite. The coercivity of Co0.4Mg0.6Fe2O4 with D = 25 nm was enhanced, with a maximum value of 4000 Oe. The room temperature Mössbauer spectra were fitted with two Zeeman sextets, which reflects the incorporation of Fe3+ in the A and B sublattices of the spinel structure. The hyperfine field of these sites decreases similarly to the saturation magnetization with increasing Mg content. The spectrum corresponding to x = 1.0 was calculated with the help of the hyperfine field distribution. Moreover, Mössbauer spectra, fore x = 0.0 and 1.0 were recorded at 6 and 80 K and were adjusted by the superposition of three sextets: one sextet for the A site and two sextets for the B sites (B1 and B2). The hyperfine parameters such as the hyperfine field and the isomer shift were improved at 6 and 80 K.

57Fe Mossbauer spectroscopy investigation of NiFe2O4 and MnFe2O4 ferrite nanoparticles prepared by thermal treatment method

In this investigation, we have used the thermal treatment method for the preparation of nickel and manganese ferrite nanoparticles. All the samples were calcined at different temperatures from 420 to 570 °C. The results of XRD demonstrated the variation of average nanocrystallite size as a function of calcination temperature. Mossbauer spectra of calcined NiFe2O4 nanoparticles at 570 °C exhibit two sextets, indicating the samples are ferromagnetic, while calcined MnFe2O4 nanoparticles at 420 °C reveal simultaneous presence of two paramagnetic doublets with a central magnetic sextet. The Mossbauer spectroscopy is also applied here for investigating the magnetic hyperfine characteristics and cation distribution of the above-mentioned ferrite nanoparticles. Mossbauer results demonstrate that higher calcination temperature assistances M2+ ions enter into the octahedral sites, while simultaneously Fe3+ ions transfer from the octahedral (B) sites to the tetrahedral (A) sites. As the results indicate, magnetic properties of the samples can be attributed to the hyperfine parameters such as hyperfine field and cation distribution. The results of Mossbauer showed the change in hyperfine parameters along with changing temperature and nanocrystallite size.

Enhancement in magnetoelectric properties of lead-free multiferroic composite through high-energy mechanical milling

Ferrite-Ferroelectric composites with the generic formula y(Mn0.5Cu0.5Fe2O4)1 − y[Ca0.1Ba0.9Zr0.1Ti0.9] (y = 25%, 50% and 75%) are prepared by solid-state reaction. One of the prepared samples of the composites 25%Mn0.5Cu0.5Fe2O4–75%Ca0.1Ba0.9Zr0.1Ti0.9O3 (MCF-CBZT) is subjected to high-energy mechanical milling for different durations (12 h, 18 h and 30 h). The compositional stoichiometry of all the samples is checked by Energy Dispersive Spectroscopy. All the un-milled and milled samples are characterized by X-ray diffraction, scanning electron microscopy, FTIR, magnetometry, and dielectric and magnetoelectric coefficient measurements. Magnetoelectric composite possesses biphasic surrounding to exhibit complex behavior of ME effect. The present study reveals influence of mechanical milling on MCF-CBZT magnetoelectric composite. The XRD confirms the presence of ferrite and ferroelectric phases for all the samples and microstructural changes appear in SEM images of milled specimens. FTIR spectra show four characteristic bands in 400–800 cm−1 range. Saturation magnetization and Curie temperature decrease as milling duration increases. Relaxed broad doublet with the distribution of nuclear hyperfine fields is found in Mossbauer spectra indicating good coupling between ferrite-ferroelectric phases caused by mechanical milling for the multiferroic composite. The frequency response of dielectric constant and loss tangent is recorded in the frequency range from 100 Hz to 1 MHz. The static value of magnetoelectric factor has been studied as a function of magnetic field for un-milled and milled MCF-CBZT samples. The maximum value 382 μV/cm·Oe of (ME)H is observed for the 18 h milled MCF-CBZT sample.

Magnetic and 57Fe hyperfine structural features of nitrided austenitic stainless steel

Samples of an austenitic stainless steel were plasma-nitrided at 673 K, for 4 h, in 80% H2−20% N2 gas mixture, at different pressures: 4; 6 and 10 Torr. The samples were characterized by four different techniques: X-ray diffraction, Mössbauer spectroscopy, magnetic force microscopy and energy dispersive X-ray. The nitrogen concentration and thickness of the nitrided layer was found to be 7 at.% and 2 μm for the 4 Torr-sample; for those at 6 and 10 Torr, the concentration was 16 and 24 at.%, with nearly the same thickness, namely 7 μm. These values are well correlated with the framework of an expanded austenite, as pointed by the X-ray diffraction data. The magnetic force microscopy data are well correlated with the observed nitrogen concentration, evidencing the absence of any magnetic domain for the 4 Torr-sample; the magnetic patterns for the 6 Torr and 10 Torr-samples are quite similar, despite of the great difference in their nitrogen concentrations. The 57Fe hyperfine parameters at two different sample depths were assessed by Mössbauer spectroscopy, by collecting data with two distinct backscattering setup geometries. Fitting the corresponding spectra with model-independent hyperfine field distributions, starting from very low hyperfine field values, allowed correlating the magnetic force microscopy data to find out that the transition from paramagnetic to magnetic for the expanded austenite occurs at smaller hyperfine magnetic fields than it is usually reported in the scientific literature. Such correlations point to the formation of a submicrometer layer occurring on the outermost surface of the sample. Despite the complexity and difficulties in differentiating the nitrides and the martensitic structure on the surface of the samples obtained at 6 Torr and 10 Torr pressures, the results obtained in this work strongly indicate the existence of them in the surface layer.

Nickel-doped nanostructured Fe70Al30 alloys: The role of Ni on the microstructure and the evolution of hyperfine and magnetic properties

We have investigated the effect of the Nickel concentration on the crystal structure and microstructure, as well as on the hyperfine and magnetic properties of nanostructured (Fe0.7Al0.3)100-xNix powders (x = 0, 5, 10 and 15 at.%) fabricated by mechanical alloying. After 72 h of milling, all the Fe(Al, Ni) samples are single-phase disordered solid solutions with body centred cubic crystal structure. A progressive reduction of the lattice parameter value from 2.9084 Å for the Ni-free FeAl binary alloy down to 2.8897 Å for the x = 15 ternary compound is observed. The mean grain size of the nanocrystals also diminishes down to c.a. 20 nm as the Ni concentration is increased, whereas the mechanically-induced microstrain remains almost constant at around 0.5%. The magnetic measurements show a ferromagnetic character of all the samples at room temperature with a slight decrease of both saturation magnetization and coercivity as the Ni concentration rises. The fit of Mössbauer spectra collected at room temperature gives an almost constant value (around 26 T) for the mean hyperfine magnetic field, <Bhf>(T). Hence, the addition of Ni to Fe (Ni,Al) solid solution has not a clear influence on the value of <Bhf>, although the hyperfine field distributions suggest somewhat different local environments of Fe atoms.

Effect of etching on spin canting in hydrothermally synthesized Co-Ni ferrite particles

Spinel cobalt-nickel ferrite nanoparticles, having a particle size of around 40 nm, were produced by undergoing a succession of synthesis routes comprised of chemical co-precipitation, hydrothermal treatment (HT), and etching in hydrochloric acid (ET) of concentration 2.0 mol/L. The room temperature high-field Mossbauer spectra were analyzed in detail and significant spin canting associated with Fe3+ ions at both A- and B-sites were observed for both the HT and ET samples in the presence of external magnetic field of 5 T. The effect of etching on various Mossbauer parameters such as the hyperfine field distribution, isomer shift, quadrupole splitting and the linewidth was noted. The A site and B site canting angles of the HT sample were respectively 17° and 22°, while canting angle of approximately 13° was observed for both A and B sites in the ET sample.

Site-resolved hyperfine and lattice dynamical properties of a single-crystal Fe3Si

A single-crystal sample of a DO3-ordered Fe3Si compound was studied by means of the 57Fe Mössbauer spectroscopy. Spectra have been recorded in a transmission geometry in the temperature range between 5 and 850 K. They have been analyzed either in terms of five sextets or five hyperfine field distribution curves. Concerning the former three sub spectra have been attributed to Fe atoms occupying the B sites and having 8Fe, 7Fe1Si and 6Fe2Si atoms in the first coordination shell (1NN), respectively, and two have been attributed to Fe atoms residing on the A and C sites and having 4Fe4Si and 3Fe5Si atoms in the 1NN, respectively. Based on the temperature dependence of the center shifts values the Debye temperature, TD, have been determined. TD was found to be equal to 491 (11) K for the sites A and C, and to 403 (3) K the site B. The smaller value of TD coincides with the smaller values of the hyperfine field, H. Changes in H caused by one Si atom present in the 1NN have been determined as ΔH≈13 kOe for Fe atoms occupying the A and C sites and ΔH≈54 kOe for Fe atoms present on the B sites. These changes have been also expressed in terms of the underlying number of polarized s-like electrons, ΔNs. For the Fe atoms situated on the A and C sites ΔNs = 0.03 was found while for those occupying the B sites ΔNs = 0.17.

Mössbauer study of Andean páramo soil from Ventaquemada, Boyacá, Colombia

Samples of paramo soil for Mossbauer studies down to a temperature of 4.2 K were collected from a sampling site in the municipality of Ventaquemada, district of Montoya, sector of Matanegra. This location is close to Tunja, capital of Boyaca Province, Colombia. Paramo soils often result from weathering of volcanic ash and are rich in allophane and organic matter. The Mossbauer spectra show the iron in all samples to be present mainly as a ferric component that splits magnetically at temperatures below about 20 K into a sextet with a mean hyperfine field of about 44 T and broad resonance lines that indicate a distribution of hyperfine fields. Additionally, weak contributions of hematite, magnetite, goethite, and ferric and ferrous iron in clays, are present as well as a structure-less pattern (collapsed sextet) that appears at low temperatures. The broad magnetically split component represents up to 70% of the iron in the Paramo soils. It is quite stable towards heating the soils, converting to hematite mainly between 600 and 800 °C. This rules out that the component is ferrihydrite, which decomposes already below 400 °C. Presumably, it is attributable to the allophane component in the Paramo soils. Leaching by the DCB method removes this component as well as most of the goethite, but only part of the hematite, whereas oxalate leaching removes mainly the broad magnetically split component attributed to allophane, but leaves hematite, magnetite and goethite largely unchanged. Leaching with H2O2, which oxidises and removes the organic matter, has no effect on the Mossbauer spectra, showing that iron bound to organic substances is practically absent.

Is “expanded austenite” really a solid solution? Mössbauer observation of an annealed AISI 316L nitrided sample

Expanded austenite, also known as S phase, m phase or γN phase forms during low temperature (<420 °C) nitriding of austenitic materials containing chromium. In this communication, annealing of a nitrided sample is used as a tool to understand the nature of expanded austenite. Starting from an AISI 316LN sample plasma nitrided for 4 h at 410 °C, the decomposition of expanded austenite is studied by conversion electron Mössbauer spectroscopy (CEMS) and X ray diffraction (XRD) for different annealing times at 400 °C. For the as nitrided sample, the CEMS spectra are analysed using two hyperfine field distributions (HFD). HFD A is attributed to a nitrogen enriched expanded austenite, HFD B is attributed to a bcc environment without or with very low nitrogen content. Both the local deformation of the bcc lattice and the presence of other elements as iron neighbouring atoms explain the broad range of hyperfine field in HFD B. The vanishing of HFD A, after 2 h of annealing, is explained by the formation of short-range order (SRO) between chromium and nitrogen and by the redistribution of nitrogen in excess. For the 3 h of annealing, a bcc phase is observed by XRD. We propose to describe this phase as martensite. An interplay between local internal stress and local nitrogen content is proposed to explain the unexpected formation of such a martensitic phase in a strongly gamma stabilizing environment. From the microscopic point of view expanded austenite can be considered to be constituted of at least two different environments: a gamma environment supersaturated in nitrogen and a martensitic environment without nitrogen.

Debye temperature of single-crystal Cr with incommensurate and commensurate magnetic structure

Single crystals of Cr and Cr3%Mn doped with ∼0.2 at.% 119Sn were studied by means of 119Sn-site Mössbauer spectroscopy. Mössbauer spectra recorded in the temperature range of 78–300 K were analyzed using a hyperfine field distribution method. Additionally, the spectra recorded on the Cr3%Mn sample were analyzed in terms of three sextets. Derived therefrom values of a center shift, CS, were fitted to the Debye model yielded values of the Debye temperature, for Cr and for Cr3%Mn. The difference in the -value can be regarded as evidence that a spin-phonon coupling is different for the incommensurate and commensurate antiferromagnetic magnetic structure of chromium.

Mössbauer analysis of compositional tuning of magnetic exchange interactions in high entropy alloys

We measured the change in the average hyperfine field strength of several high entropy alloys in relation to small compositional deviations from the equiatomic alloy, FeCoNiCuMn. Mössbauer spectra of four psuedo-binary systems, in which Mn content is increased and another element was decreased in equal measure, reveal several discrete peaks in the hyperfine field distribution that show evidence of the discrete exchange interactions between magnetic elements in the alloy. A simple linear regression modelling the perturbation of the average hyperfine field when the composition is altered calculates the contribution of each atom to the overall average. The average hyperfine field is linear with Tc, so these values allow us to estimate Tc for alloys with more complex compositional variation within the window of linearity (&amp;lt;24% Mn based on other alloys). The results were confirmed experimentally by calculating Tc of two new alloys, Fe19Co20Ni19Cu19Mn23 and Fe19Co20Ni19Cu20Mn22.

Epitaxial Co1-xMox thin film alloys studied by 59Co NMR

59Co NMR (Nuclear Magnetic Resonance) has been used to investigate the distribution of 59Co hyperfine fields in the Co1-xMox thin film alloys (0.01 < x < 0.1) co-deposited on V buffer. At x = 0 the films consisted of a mixture of fcc and hcp Co, and Mo was found to enter randomly the two Co fractions in the investigated concentration range. However, alloying with Mo was found to increase the amount of stacking faults, in agreement with the previously published results of theoretical calculations for binary alloys. A certain deviation from a random distribution of Mo, most probably due to the presence of magnetically induced miscibility gap for Co:Mo, has been identified for the alloys with Mo content larger than 1%. It was found that in the limit of a very small Mo concentration x ≈ 0, each Mo nearest neighbor lowers the 59Co NMR resonance frequency by 43.8 MHz, i.e. the corresponding hyperfine field is reduced by 4.34 T, implying that Co atoms would become nonmagnetic when 5 Mo atoms enter as the nearest neighbors. However, the effect increases with concentration, due to the contribution from Mo in the more distant shells. The extrapolation to Mo concentration higher than the one used in the experiment predicts that Co becomes nonmagnetic when surrounded by 3 Mo nearest neighbors for the concentration as low as x = 0.2–0.25.