Hyperfine Field Distribution Research Articles

Hyperfine-interaction parameters and magnetic phase antiferromagnet–ferromagnet transition in Ce(Fe1–x Si x )2

Mossbauer spectroscopy study of Ce(Fe1–x Si x )2 compounds with x = 0 and 0.07 was performed at different temperatures. Easy magnetization axis of the CeFe2 ferromagnet at 130 K was shown to be in the {110} plane and to deviate from the [001] axis by ∼10°. Upon cooling, the Ce(Fe0.93Si0.07)2 compound undergoes the ferromagnet–antiferromagnet phase transition in a temperature range of 120–125 K, which is accompanied by the reduction of the lattice symmetry. The Debye temperature of the Ce(Fe0.93Si0.07)2 compound was estimated using temperature dependences of the integral intensity of Mossbauer spectrum; it is T D ≈ 310 K. When analyzing the P(H) hyperfine field distributions P(H) derived from the Mossbauer spectra of Ce(Fe0.93Si0.07)2, it was found that in the cubic structure of this compound in the ferromagnetic state there occur local rhomboherdal distortions typical of the antiferromagnetic state.

Hyperfine interactions in nanocrystallized NANOPERM-type metallic glass containing Mo

NANOPERM-type alloy with chemical composition Fe76Mo8CuB15 was studied by combination of 57Fe Mossbauer spectroscopy and 57Fe(10B, 11B) nuclear magnetic resonance in order to determine distribution of hyperfine magnetic fields and evolution of relative concentration of Fe-containing crystalline phases within the surface layer and the volume of the nanocrystallized ribbons with annealing temperature. Differential scanning calorimetry revealed two crystallization stages at Tx1 ∼ 510 ∘C and Tx2 ∼ 640 ∘C, connected to precipitation of α-Fe and Fe(Mo,B) nanocrystals, respectively. The amorphous and partially crystalline state was obtained by annealing at several temperatures in the range 510-650 ∘C. The combination of conversion electron (CEMS) and transmission Mossbauer spectrometry (TMS) showed that annealing induces crystallization starting from both surfaces of the ribbons. For the as-quenched sample, scanning electron microscopy (SEM) and CEMS revealed significant differences in the “air” and “wheel” sides of the ribbons, crystallites were preferentially formed at the latter. While SEM micrographs of annealed samples showed various mean diameters of the crystals at opposite sides of the ribbons, the amounts of crystalline volume derived from the CEMS spectra approximately equaled. Mossbauer spectra of annealed samples contained narrow sextet ascribed to crystalline α-Fe phase, three sextets with distribution of hyperfine field assigned to the interface regions of the nanocrystals and the contribution of the amorphous phases. In-field TMS performed at 4.2 K with magnetic moments aligned by external magnetic field enabled to properly determine in particular the contribution of the amorphous phases in the samples. Resulting distributions of the hyperfine fields were compared with 57Fe(10B, 11B) nuclear magnetic resonance (NMR) spectra.

The role of silicon on the microstructure and magnetic behaviour of nanostructured (Fe0.7Co0.3)100−xSix powders

Single-phase(Fe0.7Co0.3)100−xSix nanostructured powders (x=0,5, 10, 15 and 20) have been elaborated by mechanical alloying in order to investigate the effect of silicon on the microstructure and magnetic properties of these alloys. A disordered Fe(Co, Si) solid solution with body centred cubic (bcc) crystal structure is formed after 72h of milling for all the compositions. The addition of Si gives rise to a progressive decrease of the lattice parameter, from about 2.865Å for the binary Fe70Co30 compound down to 2.841Å for the powder with x=20. The sample with the uppermost Si content exhibits the lowest value for the mean grain size (≈10nm) as well as the largest microstrain (above 1.1%). All the samples are ferromagnetic at room temperature, although the saturation magnetization value reduces almost linearly by adding Si to the composition. A similar trend is observed for the hyperfine magnetic field obtained from the analysis of the room temperature Mössbauer spectra. The hyperfine field distributions show a broad double-peak shape for x>0, which can be ascribed to multiple local environments for the Fe atoms inside a disordered solid solution.

Study of the Induced Anisotropy in Field Annealed Hitperm Alloys by Mössbauer Spectroscopy and Kerr Microscopy

Samples of Fe39Co39Nb6B15Cu1 alloy were nanocrystallized under zero field annealing (ZF) and transverse field annealing (TF) conditions. A reduction in coercivity for TF samples with respect to ZF sample (16 and 45 A/m, respectively) is observed. Kerr microscopy images show a well-defined parallel domain structure, transversally oriented to the ribbon axis for the TF sample unlike for the ZF sample, for which a complex pattern is observed with large and small domains at the surface of the ribbon. Although Mossbauer spectra are clearly different for the two studied samples, Mossbauer studies confirm that there is no significant difference between the hyperfine field distributions of TF and ZF samples but only the relative intensity of the 2nd and 3rd lines A23 (related to the angle between the gamma radiation and the magnetic moments, α). However, for TF annealed samples α = 90 deg (A23 = 4), indicating that the magnetic moments lay on the plane of the ribbon in agreement with the well-defined domain structure observed by Kerr microscopy, ZF annealed samples show A23 = 1.8. This value is close to that of a random orientation (A23 = 2) but smaller, indicating a slight preference for out of plane orientations. Moreover, it is clearly smaller than that of the as-cast amorphous samples A23 = 2.8, with a preference to in-plane orientations. The application of the law of approach to saturation yields a larger effect of the inhomogeneities in ZF sample with respect to TF one.

Modification of structural and magnetic properties of soft magnetic multi-component metallic glass by 80 MeV 16O6+ ion irradiation

Effect of 80MeV 16O6+ ion irradiation in amorphous Fe77P8Si3C5Al2Ga1B4 alloy is reported. Electronic energy loss induced modifications in the structural and, magnetic properties were monitored by synchrotron X-ray diffraction (SXRD), Mössbauer and, magnetic measurements. Broad amorphous hump seen in SXRD patterns reveals the amorphous nature of the studied specimens. Mössbauer measurements suggest that: (a) alignment of atomic spins within ribbon plane, (b) changes in average hyperfine field suggests radiation-induced decrease in the inter atomic distance around Mössbauer (Fe) atom, (c) hyperfine field distribution confirms the presence of non-magnetic elements (e.g. – B, P, C) in the first near-neighbor shell of the Fe atom, thus reducing its magnetic moment, and (d) changes in isomer shift suggests variation in average number of the metalloid near neighbors and their distances. Minor changes in soft magnetic behavior – watt loss and, coercivity after an irradiation dose of 2×1013ions/cm2 suggests prospective application of Fe77P8Si3C5Al2Ga1B4 alloy as core material in accelerators (radio frequency cavities).

Sensitivity of 57Fe emission Mössbauer spectroscopy to Ar and C induced defects in ZnO

Emission Mossbauer Spectroscopy (eMS) measurements, following low fluence (<1012 cm−2) implantation of 57Mn (t1/2 = 1.5 min.) into ZnO single crystals pre-implanted with Ar and C ions, has been utilized to test the sensitivity of the 57Fe eMS technique to the different types of defects generated by the different ion species. The dominant feature of the Mossbauer spectrum of the Ar implanted ZnO sample was a magnetic hyperfine field distribution component, attributed to paramagnetic Fe3+, while that of the C implanted sample was a doublet attributed to substitutional Fe2+ forming a complex with the C dopant ions in the 2− state at O vacancies. Magnetization measurements on the two samples, on the other hand, yield practically identical m(H) curves. The distinctly different eMS spectra of the two samples display the sensitivity of the probe nucleus to the defects produced by the different ion species.

Cs6 Cl][Fe24 Se26 ]: A Host-Guest Compound with Unique Fe-Se Topology.

The novel host-guest compound [Cs6Cl][Fe24Se26] (I4/mmm; a=11.0991(9), c=22.143(2) Å) was obtained by reacting Cs2Se,CsCl, Fe, and Se in closed ampoules. This is the first member of a family of compounds with unique Fe-Se topology, which consists of edge-sharing, extended fused cubane [Fe8Se6Se8/3] blocks that host a guest complex ion, [Cs6Cl](5+). Thus Fe is tetrahedrally coordinated and divalent with strong exchange couplings, which results in an ordered antiferromagnetic state below TN =221 K. At low temperatures, a distribution of hyperfine fields in the Mössbauer spectra suggests a structural distortion or a complex spin structure. With its strong Fe-Se covalency, the compound is close to electronic itinerancy and is, therefore, prone to exhibit tunable properties.

Ultra-soft magnetic properties and correlated phase analysis by 57Fe Mössbauer spectroscopy of Fe74Cu0.8Nb2.7Si15.5B7 alloy

A detailed study of magnetic softness has been performed on FINEMENT type of ribbons by investigating the BH loop with maximum applied field of 960 A/m. The ribbon with the composition of Fe74Cu0.8Nb2.7Si15.5B7 was synthesized by rapid solidification technique and the compositions volume fraction was controlled by changing the annealing condition. Detail phase analysis was performed through X-ray diffraction (XRD), Differential scanning calorimetry (DSC), Vibrating sample magnetometer (VSM) and Mössbauer spectroscopy in order to correlate the ultrasoft magnetic properties with the volume fraction of amorphous and α-Fe(Si) soft nano composites. Bright (BF) and dark field (DF) image with selective area diffraction (SAD) patterns by the transmission electron microscopy (TEM) of the sample annealed for the optimized annealed condition at 853 K for 3 min reveals nanocrystals with an average size between 10-15 nm possessing the bcc structure which matches with the grain size revealed by the X-ray diffraction. Kinetics of crystallization of α-Fe(Si) phases has been determined by DSC curves. Extremely small coercivity of 30.9 A/m and core loss of 2.5 W/Kg for the sample annealed at 853 K for 3 min was found. Similar values for other crystalline conditions were determined by using BH loop tracer with a maximum applied field of around 960 A/m. Mössbauer spectroscopy was used to determine chemical shift, hyperfine field distribution (HFD), and peak width of different phases. The volume fractions of the relative amount of amorphous and crystalline phases are also determined by Mössbauer spectroscopy. High saturation magnetization along with ultrasoft magnetic properties exhibits very high potentials technological applications.

Carbon-Substituted Hematite and Magnetite Nanoparticles

ABSTRACTGraphite-doped hematite and magnetite nanoparticles systems (∼50 nm) were prepared by mechanochemical activation for milling times ranging from 2 to 12 hours. Their structural and magnetic properties were studied by 57Fe Mössbauer spectroscopy. The spectra corresponding to the hematite milled samples were analyzed by considering two sextets, corresponding to the incorporation of carbon atoms into the iron oxide structure. For ball milling time of 12 hours a quadrupole split doublet has been added, representing the contribution of ultrafine particles. The Mössbauer spectra of graphite-doped magnetite were resolved considering a sextet and a magnetic hyperfine field distribution, corresponding to the tetrahedral and octahedral sublattices of magnetite, respectively. A quadrupole split doublet was incorporated in the fitting of the 12-hour milled sample. The recoilless fraction for all samples was determined using our previously developed dual absorber method. It was found that the recoilless fraction of the graphite-doped hematite nanoparticles decreases as function of ball milling time. The f factor of graphite-containing magnetite nanoparticles for the tetrahedral sites stays constant, while that of the octahedral sublattice decreases as function of ball milling time. These findings reinforce the idea that carbon atoms exhibit preference for the octahedral sites of magnetite.

Mössbauer Study of Graphite-Containing Iron Oxide Nanoparticles

Graphite-doped hematite and magnetite nanoparticles systems (~50 nm) were prepared by mechanochemical activation for milling times ranging from 2 to 12 hours. Their structural and magnetic properties were studied by 57Fe Mossbauer spectroscopy. The spectra corresponding to the hematite milled samples were analyzed by considering two sextets, corresponding to the incorporation of carbon atoms into the iron oxide structure. For ball-milling time of 12 hours a quadrupole split doublet has been added, representing the contribution of ultrafine particles. The Mossbauer spectra of graphite-doped magnetite were resolved considering a sextet and a magnetic hyperfine field distribution, corresponding to the tetrahedral and octahedral sublattices of magnetite, respectively. A quadrupole split doublet was incorporated in the fitting of the 12-hour milled sample. The recoilless fraction for all samples was determined using our previously developed dual absorber method. It was found that the recoilless fraction of the graphite-doped hematite nanoparticles decreases as function of ball-milling time. The f factor of graphite-containing magnetite nanoparticles for the tetrahedral sites stays constant, while that of the octahedral sublattice decreases as function of ball-milling time. These findings reinforce the idea that carbon atoms exhibit preference for the octahedral sites of magnetite.

On the Structural Stability of Melt Spun Ribbons of Fe95−x Zr x B4Cu1 (x = 7 and 9) Alloys and Correlation with Their Magnetic Properties

Melt spun ribbons of Fe95−x Zr x B4Cu1 with x = 7 (Z7B4) and 9 (Z9B4) alloys have been prepared, and their structure and magnetic properties have been evaluated using XRD, DSC, TEM, VSM, and Mossbauer spectroscopy. The glass forming ability (GFA) of both alloys has been calculated theoretically using thermodynamical parameters, and Z9B4 alloy is found to possess higher GFA than that of Z7B4 alloy which is validated by XRD results. On annealing, the amorphous Z7B4 ribbon crystallizes into nanocrystalline α-Fe, whereas amorphous Z9B4 ribbon shows two-stage crystallization process, first partially to bcc solid solution which is then transformed to nanocrystalline α-Fe and Fe2Zr phases exhibiting bimodal distribution. A detailed phase analysis using Mossbauer spectroscopy through hyperfine field distribution of phases has been carried out to understand the crystallization behavior of Z7B4 and Z9B4 alloy ribbons. In order to understand the phase transformation behavior of Z7B4 and Z9B4 ribbons, molar Gibbs free energies of amorphous, α-Fe, and Fe2Zr phases have been evaluated. It is found that in case of Z7B4, α-Fe is always a stable phase, whereas Fe2Zr is stable at higher temperature for Z9B4.

Interface influence on the properties of Co90Fe10 films on soft magnetic underlayers – Magnetostrictive and Mössbauer spectrometry studies

The main aim of the work was to show the correlation between magnetostrictive properties and microstructure of 25nm thick Co90Fe10 films deposited on soft magnetic underlayers. A special attention was paid to the role of the interface region. In the case of Co90Fe10 on 25nm and 35nm thick METGLAS underlayers one can resolve in conversion electron Mössbauer spectra two hyperfine field distributions (high-field and medium-field ones) corresponding to both constituents of bilayers. Analogical distributions describe the spectra of Co90Fe10 on 25nm and 35nm thick Ni81Fe19 underlayers, however an additional low-field, smeared component has been observed. It has been attributed to the interface layer (of partially disordered structure) between magnetostrictive layer and soft magnetic layer. Such interpretation is backed up by the obtained strong correlation between mean hyperfine field value and magnetostriction constant of the films. The investigated bilayers are good candidates for MRAM devices.

Mössbauer and magnetization studies of mechanically milled nanocrystalline Fe1–xAlx alloys

Changes in the magnetic behavior of Fe1–xAlx (x = 0.3, 0.4, 0.5, 0.6) powders during mechanical alloying have been studied. The ball milling process leads to formation of solid state reaction assisted by severe plastic deformation because of which crystallite size is reduced and as a result of which interesting magnetic properties are developed. The evolution of magnetic order in high-energy ball-milled Fe–Al solid solution is investigated using 57Fe Mössbauer spectroscopy and vibrating sample magnetometer. Mössbauer spectra and the hyperfine field distributions of all the samples show the presence of both magnetic and paramagnetic components in the samples. The corresponding bulk magnetization studies also show that the Al rich samples are also ferromagnetic, which can be attributed to the presence of disordered Fe-rich phases due to the non-equilibrium process of alloying. In Fe-rich samples, the formation of an off stoichiometric Fe3Al phase is favored while in the case of Al-rich samples both Al-rich phases and clustering of Fe and Al atoms are present. The systematic variation in the magnetic properties has been qualitatively correlated with the evolution of microstructure, reduction in grain size (obtained using transmission electron microscopy) and enhanced intergranular exchange coupling.

Occurrence of two amorphous phases in an Fe40Co40B20 alloy

A process for the preparation of amorphous alloys is introduced to realize physical properties, which could not previously be achieved when using known methods. The method of preparation is based on the sputtering of metal–metalloid alloys on a substrate. Unlike presently known steady sputtering techniques, in this method of preparation, the shutter that is located between substrate and target frequently interrupts the plasma between the target and the substrate. Thereby, it possible to create interfaces, different phases, and defects between the layers, which leads to drastic changes in the physical characteristics of the sample. In order to demonstrate the effect of interfaces on the magnetic parameters, an amorphous Fe40Co40B20 alloy with a thickness of 200nm was prepared by repeated interruption of plasma between the target and the substrate. The interruption of plasma took place after the growth of each 2nm amorphous film (8 atomic layers). It means that for the preparation of an amorphous Fe40Co40B20 alloy with a thickness of 200nm, the plasma has to be interrupted 100 times. Each interruption duration was 30s. The X-Ray MicroDiffraction measurements confirm the formation of amorphous structures. The analyses of Conversion Electron Mössbauer Spectroscopy, Transmission Electron Microscopy (TEM), and magnetization measurements show that the sample is amorphous. As the result of the interruption, the structure of the amorphous alloy is affected. An interesting result of this kind of preparation is the occurrence of two amorphous states or atomic arrangements of amorphous Fe40Co40B20, which leads to two different magnetic hyperfine field distributions. The TEM image shows that the formation of two amorphous states is coherent. It means that the two states are connected together without the formation of grain boundaries or interfaces.

Mössbauer study of amorphous alloys Fe85–x Cr x B15 (x = 0–20)

Mossbauer spectra of amorphous alloys Fe85–x Cr x B15 (x = 0–20) are measured at 91 K. Distributions of 57Fe hyperfine fields are recovered from the experimental spectra. The data are analyzed using a nanocrystal model. It is found that atoms of chrome replace iron atoms in nanocrystals of type α-Fe with orthorhombic o-Fe3B and tetragonal Fe3B phases. The local magnetic moments of iron atoms in the nanocrystal phases are estimated. Replacing iron atoms with atoms of chrome sharply reduces the local magnetic moments of iron atoms in nanocrystals of amorphous alloys.

Studying mechanosynthesized Hägg carbide (χ-Fe5C2)

Methods of thermomagnetic analysis and Mossbauer experiments (57Fe) were used to investigate the formation of Hagg carbide (χ-Fe5C2) under the conditions of mechanical milling of α-Fe in a medium of liquid hydrocarbons. It has been established that, with the employed parameters of milling, the synthesis of χ carbide begins after the completion of the stage of the formation of cementite (θ phase). The borderline of temperature stability of the monophase state of the χ carbide has been determined to be no more than 800 K. At T > 800 K, χ carbide decomposes into cementite and free carbon. The optimum temperature of heating of the synthesized Hagg carbide at which the population of the crystallographically nonequivalent positions of the Fe atoms is close to the ideal (0.2: 0.4: 0.4) is 775 K; the Curie temperature is T C = 520 K. The analysis of the Mossbauer data and of the results of a geometrical simulation of the configurations of Fe atoms in the the χ carbide unit cell made it possible to establish that the above relationship between the populations of positions is satisfied with the allowance for the anisotropic component h an of the field of hyperfine interaction. Under the effect of h an, the crystallographically equivalent atoms Fe(4e) become nonequivalent (Fe(e 1) and Fe(e 2)) in the magnetic sense. This specific feature manifests in the appearance in the presence of the distribution of hyperfine fields P(H) of two Mossbauer contributions, i.e., p(e 1) and p(e 2) with equal fractions of iron atoms in each of the contributions f Fe(e 1) = = f Fe(e 2) = 0.1 with the magnitudes of the fields H ≈ 11 and 16 T, respectively.

The Iron Mineralogy of Eocene Fossil Wood—a Mössbauer Study of Samples From the Petrified Forest, New Cairo, Egypt

Abstract As part of an investigation of iron species in fossil wood at the New Cairo Petrified Forest, representative samples, collected from the area, were analyzed by inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffraction (XRD), and Mossbauer spectroscopy at room temperature. The samples are composed of silica, with ~1.2 to ~17 wt.% Fe 2 O 3 , 0.21−0.92 wt.% Al 2 O 3 , and 0.001−0.71 wt.% MnO. The other oxides—including TiO 2 , MgO, CaO, Na 2 O, K 2 O, and P 2 O 5 —constituted ~0.24 to ~1.4 wt.% of the fossil wood. Mossbauer spectra of the samples, fitted using the hyperfine distribution method, showed a major sextet of Fe 3+ in goethite (64.75–100%), another sextet of Fe 3+ in hematite (27.62–31.94%), and a doublet (2.01–5.72%) that is attributable to octahedral Fe 3+ in superparamagnetic phases. The presence of the iron in the trivalent state in all samples is consistent with high oxygen fugacity environments post-fossilization. Equivalent weights of goethite and hematite in these samples calculated using the relative spectral areas determined by Mossbauer spectroscopy and the iron contents by ICP-MS were 0.84–18.52 and 0.13–0.45 wt.%, respectively. Changes in goethite or hematite abundances in the samples had no effects on their hyperfine parameters. The observed changes in Mossbauer spectra of goethite in the samples studied have been attributed to poor crystallinity, rather than element substitutions. This resulted in a reduction of the average magnetic hyperfine field (to ~27 T) and an increase in the hyperfine field distribution widths. The effects of poor crystallinity were much less pronounced in the hematite spectrum, with the samples having average room-temperature magnetic hyperfine fields of ~50.2 T, as expected for bulk hematite. Silica substitutions for iron in the fossil wood samples cannot be ruled out, but the effects of such processes on hyperfine field distributions of the iron are unknown.

Magnetically nanostructured state in a Ni-Mn-Sn shape-memory alloy

For certain compositions Ni-Mn-Sn and related magnetic shape-memory alloys undergo a martensitic transition at temperatures in the range 300--400 K, with the emergence of novel magnetic properties below the transition. While $\mathrm{N}{\mathrm{i}}_{50}\mathrm{M}{\mathrm{n}}_{50}$ is an antiferromagnet, substitution of Sn on some fraction of the Mn sites in $\mathrm{N}{\mathrm{i}}_{50}\mathrm{M}{\mathrm{n}}_{50\ensuremath{-}x}\mathrm{S}{\mathrm{n}}_{x}$ leads to competing ferromagnetic (F) and antiferromagnetic (AF) phases at low temperatures. Details of this magnetic phase coexistence are, however, significantly lacking, particularly with respect to the AF phase. The present investigations use zero applied magnetic field $^{55}\mathrm{Mn}$ NMR as a local probe of the magnetic properties of the alloy $\mathrm{N}{\mathrm{i}}_{50}\mathrm{M}{\mathrm{n}}_{50\ensuremath{-}x}\mathrm{S}{\mathrm{n}}_{x}$ with $x=10$. Rich multipeak spectra are observed, and the various components are definitively assigned to nanoscale F or AF regions. Measurements of the static nuclear hyperfine field distributions as a function of temperature, and in small applied fields, together with nuclear relaxation rates provide detailed information on the size distributions, relative concentrations, and physical natures of these F and AF regions. The results show that the nanoscale magnetic features of the $x=10$ system are substantially more complex than previous studies have suggested. We argue that the general approach used in these experiments is applicable to other such complex metal alloys, and could yield many additional insights.

Mössbauer and Electrochemical Investigations of Carbon-Rich Fe1-xCx Films

A thin film binary library of carbon-rich Fe1-xCx (0.47≤x≤0.97) alloys was prepared by combinatorial sputtering of carbon and iron. The sputtered library was characterized by X-ray diffraction and room temperature 57Fe Mössbauer effect spectroscopy to determine its microstructure. X-ray diffraction results show that the Fe1-xCx film is amorphous in the whole composition range of the library. For 0.52≤x≤0.59, a hyperfine field distribution and a quadrupole splitting distribution as obtained from Mössbauer spectra indicate the presence of a ferromagnetic phase and a paramagnetic phase in this regime. With increasing of carbon content, for 0.61≤x≤ 0.97, the sextet disappears and two paramagnetic doublets splitting appear suggesting two different Fe sites. The electrochemical performance of the Fe1-xCx film was investigated in lithium cells and the presence of Fe was found to increase the reversible capacity per mass of carbon over that of a pure carbon electrode.

Evolution of inhomogeneously broadened spin-noise spectrum with ac drive

In the presence of random hyperfine fields, the noise spectrum, \delta s_{\omega}^2, of a spin ensemble represents a narrow peak centered at \omega =0 and a broad "wing" reflecting the distribution of the hyperfine fields. In the presence of an ac drive, the dynamics of a single spin acquires additional harmonics at frequencies determined by both, the drive frequency and the local field. These harmonics are reflected as additional peaks in the noise spectrum. We study how the ensemble-averaged \delta s_{\omega}^2 evolves with the drive amplitude, \omega_dr (in the frequency units). Our main finding is that additional peaks in the spectrum, caused by the drive, remain sharp even when \omega_dr is much smaller than the typical hyperfine field. The reason is that the drive affects only the spins for which the local Larmour frequency is close to the drive frequency. The shape of the low-frequency "Rabi"-peak in \delta s_{\omega}^2 is universal with both, the position and the width, being of the order of \omega_dr. When the drive amplitude exceeds the width of the hyperfine field distribution, the noise spectrum transforms into a set of sharp peaks centered at harmonics of the drive frequency.