RADIO-FREQUENCY MÖSSBAUER SPECTROSCOPYIN THE INVESTIGATIONOF NANOCRYSTALLINE ALLOYS

M. KOPCEWICZInstitute of Electronic Materials Technology Wólczy ska 133, 01-919 Warszawa, Poland

Corresponding author: M. Kopcewicz, e-mail: kopcew_m@sp.itme.edu.pl

Abstract: Conventional Mössbauer studies allow the identification of phases formed in the process of annealing the amorphous precursor and the evaluation of their relative content. However they not provide information regarding the anisotropy fields or magnetostriction. The unconventional rf-Mössbauer technique which employs the rf-collapse and sideband effects is discussed in detail. This method permits us to distinguish the magnetically soft nanocrystalline bcc-Fe phase from the magnetically harder microcrystalline -Fe. Qualitative information concern-ing the distribution of anisotropy fields in the nanocrystalline grains can be inferred from the dependence of the rf-collapsed spectra on the rf-field intensity. The rf-sidebands effect, directly related to magnetostriction, reveals strong reduction of magnetostriction due to the formation of the nanocrystalline phase. The principles of the rf-Mössbauer technique and examples of its application will be discussed for FeZrBCu, Fe-M-B(Cu) (M: Ti, Ta, Nb, Mo) and FeNiZrB nanocrystalline alloys.

1. PRINCIPLES OF THE rf-MÖSSBAUER TECHNIQUE

The Mössbauer effect allows not only the study of static magnetic fields but also time dependent relaxation effects. Every spectroscopy has a characteristic observation time of interaction of the radiation with the solid. The observation time of the Möss-bauer spectroscopy corresponds to the mean lifetime ( N) of the nuclear resonant level. For hyperfine interaction measurements, with the characteristic energy E, it cor-responds to the Larmor precession period L = / E. Times N and L are typically of the order of 10 7-10 8 s. In the presence of time dependent changes in the environment of the resonant nucleus, the experimental observation (the shape of the Mössbauer spec-trum and the hyperfine parameters) depends on the relative order of magnitude of the observation time ( N or L) as compared with the residence time of the fluctuating environment, R (relaxation time). When the observation time is much longer than

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the relaxation time ( N, L » R, fast relaxation) then a fully time-averaged situation occurs. If the opposite relation occurs ( N, L « R, slow relaxation) the quasi-static situation is observed. For intermediate relaxation rates we obtain complex spectra.

The time dependence can be introduced into the hyperfine interactions by using an external high frequency magnetic field. When the frequency of such a field is higher than the Larmor frequency, the magnetic hyperfine splitting collapses and the Möss-bauer spectrum consists of a single line or a quadrupole doublet instead of a Zeeman sextet. The shape of the spectra depends on the relation between the frequency of the field and the Larmor frequency.

The rf-Mössbauer technique is an unconventional specialized technique which combines the Mössbauer effect with the phenomena induced in ferromagnetic materials by an external radio-frequency (rf) magnetic field: (i) the rf-collapse and (ii) rf-side-bands.

(i) The rf-collapse effect occurs when the magnetic rf-field forces a fast reversal of sample magnetization. This results in the oscillations of the hyperfine field at the Mössbauer nuclei in response to the applied rf-field. The rf-collapse effect is interpreted in terms of the extreme ferromagnetic enhancement of the rf-field. Let us consider first the static situation when an external magnetic field is applied to a ferro-magnetic material. When this field is larger than the anisotropy field, Han, saturation magnetization is reached. The magnetic field at the 57Fe Mössbauer atom is then of the order of 104 Oe and directed parallel to the external field. This field causes the polarization of the iron core electrons and results in a hyperfine field of about 300 kOe at the sites of the Mössbauer nuclei directed antiparallel to the magnetization vector. If the material is magnetically soft, a small external field of several Oe is able to control the direction of a very large hyperfine field. If the direction of the external field is reversed, the hyperfine field is rotated to the same extent. When a radio-frequency magnetic field is used instead of the static field, the magnetic hyperfine field will be forced to oscillate in response to the external rf-field, and, as a result, the hyperfine field at the Mössbauer nuclei will be reduced. In the extreme case, when the frequency of the magnetization reversal, and consequently, the reversal of the hyperfine field, is higher than the Larmor precession frequency of the magnetic moment of an 57FeMössbauer nucleus about the local hyperfine field, the magnetic hyperfine field experi-enced by the Mössbauer nuclei will average to zero. As a result a collapse of the entire Zeeman sextet to a single line or a quadrupole doublet is observed in the Mössbauer spectrum in spite of the sample being in the ferromagnetic state. The apparent magnetic splitting will disappear because of the fast reversal of magnetization when all the spins are coupled and reversed together.

The complete collapse of the magnetic hyperfine splitting occurs when two condi-tions are fulfilled. Firstly, the frequency of the external driving rf-field must be considerably larger than the Larmor frequency. Secondly, the switching time of magnetization reversal must be comparable to, or shorter than, the period of the rf-field applied. The switching time depends on the properties of the material and decreases with the increasing intensity of the driving rf-field. Hence, the shape of the rf-collapsed Mössbauer spectrum depends on both the frequency and intensity of the applied rf-field.

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Because the rf-collapse effect allows us to average the magnetic hyperfine field to zero in the ferromagnetic state, the magnetic hyperfine structure disappears in the Mössbauer spectrum, so one can observe directly the quadrupole interaction in the ferromagnetic state. Thus, the rf-collapse gives us a unique opportunity to separate the magnetic dipole and the electric quadrupole hyperfine interactions in the ferro-magnetic state.

Since the rf-collapse effect strongly depends on the switching time of magnetization reversal, which, in turn, is very sensitive to the relation between the driving field intensity and the local magnetic anisotropy field, information regarding the anisotropy fields can be derived from the dependence of the shape of the rf-collapsed spectra on the rf-field intensity.

The rf-collapse effect is very sensitive to even small changes of the magnetic anisotropy and occurs only in soft ferromagnets. Thus one can distinguish the very soft nanocrystalline phase from the magnetically harder microcrystalline ones.

The importance of these features of the rf-collapse effect for the application of the rf-Mössbauer technique for the study of the structure and magnetic properties of soft magnetic nanocrystalline alloys will be demonstrated below.

(ii) The rf-sidebands effect originates from the rf-induced vibrations of atoms viamagneto-acoustic coupling, which is magnetostriction. When the rf magnetic field is applied to the ferromagnetic material, additional lines appear in the Mössbauer spectrum. These new lines are separated from the originally observed lines by integral multiples of the frequency of the applied field. An infinite set of satellite lines (called sidebands) is formed in relation to each carrier line of the Zeeman sextet (or of the rf-collapsed pattern). The rf-sidebands effect can be described by a classical fre-quency modulation model which determines the positions of sideband lines and the intensities (modulation index) of each order of sidebands

The modulation index can be calculated in terms of a simple magnetostriction model under the assumption that the magnetostriction strain is given by a static model, the radio-frequency component of magnetization is much smaller than the static mag-netization, the static magnetostriction constant is appropriate at high frequencies, and, as the acoustic vibrations generated by the rf-field propagate in the sample, interaction with grain boundaries, defects, surfaces, dislocations, etc., they develop components in directions other than their original one. The modulation index resulting from this model is proportional to the magnetostriction constant. Therefore the intensities of rf-sidebands are directly related to the magnetostriction thus allowing us to follow the changes of magnetostriction in the nanocrystalline alloys related, e.g., to the formation and devel-opment of the nanocrystalline grains in the residual amorphous matrix.

The rf-collapse effect can be induced only in the soft ferromagnetic materials because the rf-field intensity should exceed the magnetic anisotropy. The rf-sidebands effect can occur only in the magnetostrictive ferromagnetic materials, i.e., below the Curie point. It disappears when magnetostriction vanishes.

If the material exposed to the rf-field is a soft magnetostrictive ferromagnet then both rf-induced effect coexist. They may be separated either by suppressing the rf-collapse (e.g., by inducing external anisotropy or by a static magnetic field su-perimposed on the rf-field or by suppressing the rf-sidebands (e.g., by covering

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the sample surface with a damping medium). Therefore, although both effects are induced by the same rf-field their origin is different: the rf-collapse is related to the fast relaxation of the hyperfine field resulting from the fast magnetization reversal, and the rf-sidebands originate from the frequency modulation of the Mössbauer gamma radiation by acoustic vibrations induced by the rf-field via magnetostriction. The rf-collapse requires the rf-field frequency being several times larger than the Lar-mor frequency, while the rf-sidebands effect occurs at all frequencies. However, to observe the well-resolved sidebands in the Mössbauer spectrum, the rf-field frequency should be sufficiently large so that the sidebands separation from the carrier lines could exceed the linewidth of the carrier line. The sideband intensity depends on frequency and strongly decreases for higher frequencies.

For a detailed discussion of the rf-collapse and rf-sideband effects, see, e.g. Refs. [1-5].

2. TYPICAL rf-MÖSSBAUER EXPERIMENT

A typical rf-Mössbauer experiment is performed in transmission geometry. The sample, playing a role of an absorber, is placed inside the coil to which the sinusoidal signal of a given frequency is delivered from the rf-power generator. The rf-magnetic field is applied to the plane of the sample perpendicularly to the direc-tion of propagation of the Mössbauer gamma radiation. Usually, because of the rf-heating (eddy currents and hysteresis heating), the sample must be cooled to keep its temperature well below the Curie point. In the experiments described in this chapter, the sample holder was water-cooled. In order to observe a complete rf-collapse of the magnetic hyperfine structure in the Mössbauer spectrum of ferromagnetic amor-phous or nanocrystalline alloys it was sufficient to apply the rf-field frequency of about 60 MHz (about 3-4 times larger than Larmor frequency). Typical rf-field intensity was varied between 0 and 20 Oe. Simple rf-power generator of about 100 W was used. In most experiments discussed here, the frequency was kept constant and the intensity of the rf-field was changed by changing the anode voltage, which controlled the rf-current in the coil. The pick-up coil was used to monitor the rf-field intensity. The rf-coil with the sample inside has to be well screened from the rest of the Mössbauer spectrometer to prevent the rf-signal from affecting the Mössbauer electronics. The rest of the spec-trometer is conventional.

The Mössbauer studies of the rf-induced effects evolved from the investigations of the origin and basic properties of these effects [1, 2] to the specialized rf-Mössbauertechnique [3, 4] which may provide unique information regarding the structural and magnetic properties of various ferromagnetic materials, including amorphous and nanocrystalline alloys.

3. THE rf-MÖSSBAUER STUDY OF Fe-BASED NANOCRYSTALLINE ALLOYS

The rf-Mössbauer technique was applied for the first time for the study of nanocrystalline FeCuNbSiB alloys [6]. The rf-Mössbauer experiment allowed us to

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distinguish the nanocrystalline Fe3Si phase from the microcrystalline phases formed in the course of annealing.

A detailed study of the magnetic properties have been performed for Fe93 x yZr7BxCuy

(x = 4, 6, 8, 12; y = 0, 2) nanocrystalline alloys using the conventional and rf-Mössbauer techniques [7]. The most important results can be summarized as follows: (i) in all FeZrB(Cu) alloys the bcc-Fe phase is formed due to annealing (Figs. 1b-1f

and 1a-1f). The increase of boron content in the alloys increases the crystallization temperature resulting in the enhanced thermal stability of the amorphous phase (Figs.1a, 2a). The relative abundance of the bcc-Fe phase increases with decreasing boron content. The presence of Cu in alloys with the same boron content decreases the crystallization temperature and dramatically promotes the precipitation of the bcc-Fe phase (e.g. Figs. 1b, 1d, 1f);

(ii) a complete rf-collapse effect is observed only in fully amorphous alloys (e.g.Fig. 1a’). The rf-Mössbauer spectra consist of the fully collapsed component (QS doublet) accompanied by the rf-sidebands; the rf-sidebands, clearly seen for the as-quenched amorphous alloys, nearly disappear due to the formation of the nanocrystalline phase. This is related to the reduction of magnetostriction;

(iii) the onset of the bcc-Fe phase affects the shape of the rf-collapsed spectra; in addition to the QS doublet, a single line appears. The noncollapsed six line spectral component (Hhf about 33 T) appears in the rf-spectra of alloys without Cu (Figs. 1c', 1e', 2a') implying that a bimodal distribution of the bcc-Fe grain size appears at the early stage of crystallization;

(iv) an addition of Cu leads to a more homogeneous size distribution of bcc-Fe grains. However, the anisotropy of the nanocrystals is higher than that of the amorphous phase. The rf-collapse is not complete so a partly narrowed magnetic hyperfine split pattern appears in the spectra recorded during rf-exposure (Figs. 1d', 1f ', 2b'-2f ');

(v) the spectral contribution of the partly narrowed hyperfine structure of the nano-crystalline bcc-Fe phase increases, at a given annealing temperature, with decreas-ing boron content in Cu-containing alloys (Figs. 1d', 1f ', 2b', 2d', 2f ');

(vi) the nanoscale bcc-Fe grains are smaller (lower anisotropy) in the Cu-containing alloys with the same boron content, e.g., Figs. 2c' with 2d' and Figs. 2e' with 2f ' which show a larger spectral contribution of the rf-collapsed component (despite the fact that the abundance of the retained amorphous phase is lower) and stronger narrowing of the noncollapsed hyperfine split component. The rf-Mössbauer technique was used recently for studying the magnetic properties

of Fe73.5Nb4Cr5Cu1B16 [8] and Fe80.5Nb7B12.5 [9] alloys in the as-quenched state and after thermal treatment as a result of which the soft magnetic nanostructure was formed. Similarly to the FeZrBCu alloys, also in these alloys the rf-Mössbauer measurements permitted evaluation of the magnetic anisotropy fields. The magnetic anisotropy of the nanocrystalline bcc-Fe phase was found to be considerably larger than that corresponding to the amorphous phase. The rf-sidebands appear only in the fully amorphous as-quenched alloy. The sidebands disappear in the annealed alloys. This is related to the reduction of magnetostriction due to the formation of the nanocrystalline phase. The bcc-Fe sextet is only marginally narrowed by the rf-field indicating that the anisotropy in the bcc-Fe grains is considerably larger than that in the retained

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amorphous phase. The magnetic anisotropy in the nanograins is, however, considerably smaller than in the microcrystalline -Fe. These conclusions were confirmed by detailed rf-Mössbauer measurements as a function of the rf-field intensity.

Figure 1. The Mössbauer spectra recorded for the Fe93-x-yZr7BxCuy (x = 12, 8, 6; y = 0, 2) alloys annealed at 500 C in the absence (a-f) and during rf-exposure (a'-f ') to the rf-field of 20 Oe at 60.8 MHz (Ref. 7)

Figure 2. The Mössbauer spectra recorded for the Fe93-x-yZr7BxCuy (x = 12, 8, 6; y = 0, 2) alloys annealed at 550 C in the absence (a-f) and during rf-exposure (a'-f ') to the rf-field of 20 Oe at 60.8 MHz (Ref. 7)

The influence of the alloy composition on the magnetic properties of nanocrystalline Fe80M7B12Cu1 (M: Ti, Ta, Nb, Mo) alloys has been studied using the rf-Mössbauer technique [10, 11]. In the Ti-, Ta- and Nb-containing alloys the rf-induced effects occur in a similar way and show a similar behaviour of the rf-Mössbauer spectra. As an exam-ple the case of Fe80Ti7B12Cu1 alloy is discussed. Typical rf-Mössbauer spectra measured during the exposure to the rf-field of 12 Oe at 61.2 MHz are shown in Fig. 3. The hyperfine split spectrum of the alloy in the amorphous state (Fig. 3a) collapses completely to the quadrupole split doublet (QS) accompanied by the rf-sidebands thus revealing that the applied 12 Oe rf-field is sufficiently strong to overcome the magnetic anisotropy in the amorphous state. The spectrum recorded for the two-phase nanocrystalline alloy (bcc-Fe embedded in the retained amorphous matrix) consists, similarly to the case of the FeZrBCu alloys, of two components (Fig. 3b'-3e'): (i) a fully collapsed central QS doublet originating from the retained amorphous phase and (ii) a partially collapsed six-line component related to the nanocrystalline bcc-Fe phase. With increasing annealing temperature the partially collapsed component becomes better

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resolved. Even though the hyperfine splitting of this component for the sample annealed at 620oC is the largest in the set of the spectra in Figs. 3b'-3e' the average hyperfine field is markedly reduced as compared to bulk -Fe as well as to the bcc-Fe component in the spectrum recorded for this sample in the absence of the rf-field (Fig. 3e). The reduction of the hyperfine field due to the rf-field strongly suggests that the mag-netic anisotropy in the bcc-Fe nanograins is considerably smaller than in the bulk -Febut is sufficiently large to limit the fast magnetization reversal and is considerably larger than that in the retained amorphous phase for which a complete rf-collapse is observed. The spectral fraction of the partially collapsed six-line component increases with increasing annealing temperature (Figs. 3b'-3e') which is related to the increase of the amount of the bcc-Fe nanograins at the expense of the amorphous phase. The observed changes in Figs. 3a'-3e' in the rf-Mössbauer spectra are characteristic of all nanocrystalline alloys studied.

The rf-Mössbauer measurements performed as a function of the rf-field intensity resemble closely those obtained for the FeZrB(Cu) alloys and provide information on the anisotropy fields in each phase present in the nanocrystalline alloy.

The rf-Mössbauer spectra recorded for alloys with different M substitutions were compared. The spectra of samples annealed at similar temperatures and recorded during the exposure to the rf-field of the same intensity (16 Oe) are shown in Fig. 4. The spectra recorded in the absence of the rf-field (Figs. 4a-4e) reveal similar relative fractions of the bcc Fe phase in all alloys. However, the rf-Mössbauer spectra differ considerably. The most effective collapse of the magnetic hyperfine structure was

Figure 3. Mössbauer spectra of the as-quenched and annealed Fe80Ti7B12Cu1 alloys recorded without (a-e) with rf-field of 12 Oe at 61 MHz (a'-e') (Ref. 11)

Figure 4. Mössbauer spectra of the an-nealed Fe80M7B12 Cu1 alloys recorded without (a-e) and with rf-field of 16 Oe at 61 MHz (a'-e') (Ref. 11)

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observed in Figs. 4b’ and 4c’ for Nb- and Mo-containing alloys, respectively, and in Fig. 4a’ for FeZrBCu alloy. The fully collapsed central part dominates in these spectra and the partially collapsed six-line component is the most narrowed. This suggests that the alloys containing Nb, Mo or Zr are magnetically the softest, both as regards the retained amorphous phase and the nanocrystalline bcc Fe grains. The spectra of the Ti- and Ta-containing alloys (Figs. 4d’, 4e’) are much less narrowed. The spectral contribution of the central collapsed part is much smaller than in the case of Nb- and Mo-containing alloys. This suggests that the magnetic anisotropy is considerably larger in Ti- and Ta-containing alloys due to larger bcc Fe grains as compared with Nb-, Mo- and Zr-containing alloys. Similar characteristic features were observed for the samples annealed at lower temperatures.

Very interesting results have been obtained recently for amorphous and nano-crystalline Fe81 xNixZr7B12 (x = 10-40) alloys [12, 13]. The exposure of FeNiZrB alloys to the rf-field induces both effects: the rf-collapse and rf-sidebands, which are clearly seen in the Mössbauer spectra. Their behaviour vs. rf-field depends on Ni content and on the degree of the crystallization of the alloy. For low Ni-content (x = 10 and 20) the rf-Mössbauer spectra resemble those observed earlier for various NANOPERM alloys [7-11]. A typical example is shown in Fig. 5 for the Fe61Ni20Zr7B12 alloy. For an amorphous alloy (as-quenched and annealed at 470oC, Figs. 5a and 5b) the spectra recorded during sample exposed to the rf-field of 61 MHz at 20 Oe consist of the fully collapsed central doublet accompanied by intense rf-sidebands, whose shape repeats that of the central doublet (Figs. 5f and 5g). The applied rf-field is sufficiently strong to overcome the effective magnetic anisotropy in the amorphous phase and to induce a fast magnetization reversal, and as a result the magnetic hyperfine field acting on the Mössbauer nuclei is averaged to zero. The rf-sidebands appear because the sample is magnetostrictive.

The formation of the nanocrystalline bcc-Fe phase strongly affects the shape of the collapsed spectrum. The central fully collapsed part corresponds to the retained amorphous phase in the nanocrystalline alloy and the partly narrowed magnetically split spectral component is related to the nanocrystalline phase (Figs. 5h, 5i and 5j). The rf-Mössbauer spectrum of the sample annealed at 520oC, which still contains a significant fraction of the amorphous phase, is dominated by the rf-collapsed central part, accompanied by the rf-sidebands (Fig. 5h). In addition, the magnetically split component could be distinguished (the second and fifth lines of the magnetic sextet appear between the sideband lines and the central collapsed quadrupole doublet while the first and the sixth lines of the sextet overlap the sidebands). At higher annealing temperatures, the samples contain less of the soft magnetic amorphous fraction and therefore the spectral contribution of the collapsed part decreases (Figs. 5i and 5j). The magnetically split sextet strongly dominates in the spectra. However, the hyperfine splitting is considerably reduced in comparison with the one recorded for the sample in the absence of the rf-field (Figs. 5d and 5e). The spectral contribution of the rf-collapsed doublet markedly increases for the sample annealed at TA = 620oC (Fig. 5j) as compared with that for TA = 570oC (Fig. 5i) which suggests that the magnetically soft nanocrystalline component increases at TA = 620oC. Since the annealing at temperatures

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exceeding 700oC causes the crystallization of the nonmagnetic phase, the spectrum is not affected by the rf-field.

Quite similar effects were observed for the Fe51Ni30Zr7B12 alloy. The magnetic hyperfine splitting (mhfs) fully collapses for the as-quenched amorphous alloy and that annealed at TA = 470oC. For the annealed alloys, similarly to the case of x = 20 sample, a resolved, but narrowed by the rf-field, mhfs spectral component is observed. Annealing at 520oC TA 570oC induces the formation of the bcc-Fe phase which prevails in the spectra recorded in the absence of the rf-field. The rf-collapsed spectra are similar to those shown in Figs. 5i and 5j for x = 20 sample. The rf-Mössbauer spectra contain two components: the rf-collapsed central quadrupole doublet related to the retained amorphous phase, and the partly narrowed magnetic sextet due to the nanograins. The latter reveals magnetic anisotropy larger than that in the amorphous phase, and large enough to limit the magnetization reversal. Therefore the hyperfine field is reduced but not averaged to zero.

Figure 5. The Mössbauer spectra recorded for as-as-quenched and annealed Fe61Ni20Zr7B12

alloy in the absence (a-e) and during rf-exposure (f-j) for the rf-field of 20 Oe at 61 MHz (Ref. 13)

Figure 6. The Mössbauer spectra re-corded for the as-quenched and annealed Fe41Ni40Zr7B12 alloy in the absence (a-e) and during rf-exposure (f-j) for the rf-field of 20 Oe at 61 MHz (Ref. 13)

The behaviour of the Fe41Ni40Zr7B12 alloy in the rf-field is completely different from that observed for x = 10, 20 and 30 alloys. The as-quenched and annealed at TA 595oCalloys reveal excellent soft magnetic properties. The amorphous alloy as well as the nanocrystalline alloys show complete rf-collapse of the mhfs (Figs. 6f-6i). The shape

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of the central collapsed component changes from the quadrupole doublet, for the samples with dominating amorphous phase (Figs. 6f and 6g), to the narrow single line resulting from the cubic nanocrystalline structures (Figs. 6h and 6i). The magnetostriction, being quite large in the amorphous alloy (Fig. 6f) and in the sample with prevailing amorphous phase (Fig. 6g), strongly decreases when the nanocrystalline phase is formed, as shown by the decreasing intensity of rf-sidebands (Figs. 6g and 6h). It almost vanishes in the alloy annealed at TA = 582oCand the rf-sidebands almost disappear (Fig. 6i). Thus, the rf-Mössbauer spectra strongly suggest that the nanocrystalline alloy with x = 40 reveals much better soft magnetic properties (small anisotropy and vanishing magnetostriction) as compared with the alloys with lower Ni-content (x = 10-30).

The magnetic anisotropy of the x = 40 alloy annealed at high temperatures (exceeding 700oC) significantly increases due to the formation of the fcc-FeNi phases. The rf-Mössbauer spectrum is only partly collapsed and the strongly broadened overlapping magnetic hypefine structure is observed (Fig. 6j).

The rf-Mössbauer measurements were performed as a function of the rf-field intensity for all as-quenched and annealed alloys. The results strongly depend on the Ni-content in the system. The most interesting dependence of the rf-collapse and r-side-band effects on the rf-field intensity was observed for the Fe41Ni40Zr7B12 alloy. The Mössbauer spectra recorded during the exposure of the Fe41Ni40Zr7B12 as-quenched and annealed at 520oC and 582oC samples to the rf-field of 61 MHz at various rf-field intensities are shown in Fig. 7. A gradual rf-induced narrowing of the magnetic hyperfine structure is observed for the as-quenched amorphous alloy (Figs. 7a-7f). A complete rf-collapse of the mhfs to a quadrupole doublet is observed for the rf-field of 12 Oe (Fig. 7e). However, at 6 Oe the rf-induced narrowing is only marginal (Fig. 7c). This means that the effective anisotropy field in the amorphous state is larger than 6 Oe but considerably smaller than 12 Oe. The formation of the nanocrystalline phase at TA = 520oC causes clear changes of the rf-Mössbauer spectra (Figs. 7g-7l). The spectra are significantly more collapsed as compared with the amorphous alloy (compare the relevant spectra in Figs. 7i and 7j with those in Figs. 7c and 7d). Substantial rf-induced narrowing of the mhfs occurs already at 6 Oe (Fig. 7i) and at 8 Oe the rf-collapsed central component accompanied by a partly collapsed magnetic “wings” appears (Fig. 7j). The shapes of the spectra in Figs. 7i and 7j suggest the existence of a two-phase structure consisting of magnetically very soft nanocrystalline grains which produce a collapsed central part of the spectra, and the matrix with a somewhat larger anisotropy, related to the partly collapsed magnetic component. A further increase in the rf-field intensity leads to an almost complete collapse of the mfs of the magnetic component. The central fully collapsed single line and rf-sidebands appears in the spectrum (Fig. 7k).

The rf-field of 12 Oe is sufficiently strong to overcome the anisotropy fields both in the residual amorphous matrix and in the nanocrystalline bcc-grains and to average the hyperfine fields in both phases. The magnetostriction of the alloy is still significant, as shown by the fairly strong rf-sidebands (Fig. 7l).

Very interesting results were obtained for the Fe41Ni40Zr7B12 alloy annealed at 582oC. The rf-Mössbauer spectrum reveals a substantial collapse effect for the rf-field

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intensity of only 4 Oe (Fig. 7n) at which no narrowing of the magnetic hyperfine structure was observed for the amorphous alloy (Fig. 7b). At 6 Oe the mhfs strongly narrows (Fig. 7o) and 8 Oe rf-field is sufficient for an almost complete collapse of the mhfs to a single line that originates from the nanocrystalline bcc grains accompanied by small magnetic “wings”, most probably related to the still incomplete collapse of the mhfs of the residual amorphous phase (Fig. 7p). An 12 Oe rf-field induces a complete collapse of mhfs of the entire nanocrystalline alloy. The weak sidebands still appear in the spectrum (Fig. 7r). Such an rf-field dependence of the rf-Mössbauer spectra reveals that the effective anisotropy of the Fe41Ni40Zr7B12 alloy annealed at 582oC is particularly small, considerably smaller than in the amorphous precursor and in the samples annealed at lower temperatures. Up to now in all rf-Mössbauer studies performed for various NANOPERM alloys (e.g., [7-11]), the amorphous precursor and residual amorphous matrix had a smaller anisotropy than the nanocrystalline bcc-Fe grains.

In the present case of Fe41Ni40Zr7B12 alloy annealed at optimal temperature, the crystalline Ni-containing nanostructure reveals substantially smaller anisotropy than the residual amorphous matrix and the amorphous precursor.

Figure 7. The rf-Mössbauer spectra recorded as a function of the 61 MHz rf-field intensity for the as-quenched (a-f), annealed at 520oC (g-l) and at 582oC (m-s) Fe41Ni40Zr7B12 alloy, (Refs. 12 and 13)

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Striking differences as regards the rf-sidebands effect could be noticed in the rf-Mössbauer spectra recorded for 16 Oe rf-field (Figs. 7f, 7l and 7s). Whereas the rf-collapse is complete for all alloys in these figures, the rf-sideband intensities strongly decrease in the nanocrystalline alloys and are the smallest for the alloy annealed at 582oC. Thus the alloy annealed at 582oC has the best soft magnetic properties: the smallest anisotropy and almost vanishing magnetostriction.

Annealing the Fe41Ni40Zr7B12 alloy at temperatures exceeding the second crystalliza-tion step (TA 730oC) leads to the formation of the phase which is magnetically much harder.

4. CONCLUSIONS

Conventional Mössbauer measurements allow the identification and estimation of the relative abundance of phases formed due to annealing of the amorphous precursors but do not provide information regarding either the magnetic properties (anisotropy fields and magnetostriction), or the size of the grains. The unconventional rf-Mössbauer technique, in which the rf-collapse and sideband effects are exploited provides information regarding magnetic properties of the alloys (anisotropy fields, magnetostric-tion, etc.) and permits the distinction of the magnetically soft nanocrystalline phase from magnetically harder microcrystalline one. Qualitative information concerning the distribution of anisotropy fields related to the distribution of the size of the nano-grains can be inferred from the dependence of the rf-collapsed spectra on the rf-field intensity. The rf-Mössbauer results show that the nanocrystalline phases, being magneti-cally very soft have, however, in most cases, anisotropy markedly larger than that of the parent amorphous phase. The complete rf-collapse of the magnetic hyperfine structure occurs only in the amorphous precursor or in the retained amorphous fraction in the nanocrystalline alloy. However, differently to the results obtained for most NANOPERM-type alloys, the rf-Mössbauer measurements have shown that the Fe41Ni40Zr7B12 nanocrystalline alloy annealed at 580oC has exceptional soft magnetic properties, i.e., very small anisotropy accompanied by the vanishing magnetostriction. For this alloy, a complete rf-collapse effect was observed for the first time both for the residual amorphous phase and the nanocrystalline one.

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