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Ferromagnetic resonance and antiresonance of amorphous and nanocrystalline Fe73.5Cu1Nb3Si16.5B6 alloy
Journal of Magnetism and Magnetic Materials 117 (1992) 353-355 North-Holland
AI41
Ferromagnetic resonance and antiresonance of amorphous and nanocrystalline Fe73.sCulNb3Si16.sB 6 alloy * Z. F r a i t a n d D. Fraitovfi Institute of Physics, CSAV, Na Slovance 2, CS-18040 Prague 8, Czechoslocakia
Received 5 August 1992
The first ferromagnetic resonance and antiresonance investigation was performed on a Metglas type alloy in the amorphous and nanocrystaUine state in a broad frequency interval (18 to 103 GHz). Data on magnetization, g-factor, relaxation constant, surface anisotropy and homogeneityof the samples are evaluated and discussed.
1. Introduction Recently a new type of ferromagnetic materials has been prepared, i.e. heterogeneous alloys, in which very fine crystalline grains (with diameter of about 10 nm) are embedded into an amorphous matrix. The iron-rich amorphous alloys FeSiB with an addition of a few percent of Cu and Nb metal, prepared by a fast quenching method in the form of thin ribbons, and subsequently annealed at temperatures above the crystallization point (around 550°C for about one hour) represent a typical case of so called nanocrystalline materials [1]. Especially, the alloys of the composition Fe73.sCulSbaSi16.sB6 [2] shows excellent soft magnetic properties and has been recently studied by several methods (measurements of permeability, anisotropy, magnetostriction and magnetization, domain and X-ray observation, M6ssbauer spectra, see e.g. refs. [25]). In this paper we present first data on these materials (magnetization M, g-factor, relaxation
constant L, surface anisotropy K S and surface stray fields), which were obtained by ferromagnetic resonance ( F M R ) and antiresonance ( F M A R ) methods [6] at room temperature and in a broad interval of microwave frequencies (18 to 103 GHz), both for as prepared (amorphous AP) and annealed (nanocrystalline - NN) samples.
2. Skin-depths, experimental By applying the F M R and F M A R methods to the studies of bulk samples of metals, which electromagnetic radiation penetrates only to a certain depth (skin-depth), one has to be well aware of which part of the sample volume is investigated. In F M R and F M A R regions the skin depths differ largely from the value for nonmagnetic material. In ferromagnetic metal the skin-depth is given by the formula [6,8]: = (2A/I~o)1/2(MZr)
Correspondence to: Prof. Z. Frait, Institute of Physics, CSAV, Na Slovance 2, CS 18040 Prague 8, Czechoslovakia. Tel.: +42-2-8152163; telefax: +42-2-8584569. Email: FZU59@ CSPGCS11. * Paper presented at the International Conference on Magnetism (ICM '91), Edinburgh, Scotland, 2-6 September 1991.
-',
(1)
where A is the exchange constant, Z r the real part of the relative surface impedance [6], depending mainly on the external field H, frequency f and resistivity p, further on L, K s and A. For our case we have computed the 6 values for all frequencies used in the experiment (values
0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
354
Z. Frait, D. Fraitord / FMR and FMAR of nanocrystalline alloy
of L, K S, M, g were taken from this experiment, p = 140 ~f~ cm [2], A was estimated as 4.7 or 7.0 p J / m for the AP or NN material, respectively, by using the Curie point data [3,5]). E.g. for NN at 96 G H z one finds a strong increase of 6 in F M A R region to about 8 Ixm and a pronounced decrease in F M R region to about 200 nm. From our computations it further follows, that 6 at the F M R point is only slightly (15%) frequency dependent and 3 at F M A R point is inversely proportional to f. Around F M R and F M A R points 6 varies sharply with H; by considering the line width values established in our measurements (tens of mT) this change amounts to 25% (20%) in the H region between the F M R ( F M A R ) line inflection points, respectively. As the theory [6] used for the evaluation of experiments assumes the sample thickness (in our case 22 Ixm) much larger than 6, only F M A R m e a s u r e m e n t s at f > 60 G H z were taken into account.
3. Results and discussion
The investigated alloy was p r e p a r e d by the one-roller quenching method [2] in the form of thin ribbons 10 to 15 m m wide [5]. The samples were cut out to the form of squares or discs, annealing took place in the dry Ar or N 2 atmosphere at 550°C for one hour. The wheel side of the ribbon adjoining the roller (WS) is of a dull appearance (pointing to microscopic surface defects), the free side (FS) is shiny, with a certain waviness (mean wavelength around 0.5 mm, surface unevenness less than 1 txm). F M R and F M A R m e a s u r e m e n t s were performed at 18, 36, 49, 86, 96 and 103 G H z in the parallel (PC, M lies in the sample plane) and in the normal configuration (NC, M parallel to the sample normal), at RT, on both sample surfaces (WS and FS) of AP and NN samples; field derivative of microwave absorption A ' was registered, for details see ref. [9]. The following results were obtained from the F M R and F M A R field values, line widths and line shapes and their frequency dependence by using the resonance equations in refs. [6,9]. Generally no pronounced differences between mea-
I'~
......... .
q
a
/ K "p
25
.... k- ....... . A
2;6
2.7 'k'~'
2.8 HITI
2.9
Fig. 1. Plots of field derivative (A') of FMR (a) and FMAR (b) absorption for the nanocrystalline (NN) and amorphous (AP) samples, respectively. Full lines: measurements at the free side, broken lines: at the wheel side of the samples. Measurements at RT and at 95.787 GHz. surements on AP and NN samples were found, see fig. 1. At both surfaces of AP F M R and F M A R yield /z0M = 1.14 T ( + 2 % ) ; in N N / . % M = 1.13 T (-+3%) was established at both presurface regions by F M R and /z0M = 1.095 T (-+3%) in the bulk of the material by F M A R . These values are in a good agreement with static measurements on the same material [2-5]. Moreover, on NN a small subsidiary F M R peak is observed (more pronounced at FS), see fig. 1, pointing to the presence of some different magnetic phase near the surface, with / z 0 M = 1.6 T. No such effect was found by F M A R . The g-factors were found as g = 2.085 (2.075) (-+0.5%) for the AP (NN) samples, respectively. The decrease of g in NN is probably connected with the large Si content in the crystalline component [5] (g-factor of iron decreases with Si alloying [6]). The F M R line width values in PC increase linearly with frequency from 16(22) m T at 18 G H z to 37(55) m T at 103 G H z for the AP(NN) samples, respectively. The frequency independent part of the line widths, 12(16) m T for AP(NN), is most probably caused by surface imperfections; by means of the E P R of a free radical layer precipitated at the sample surface [9] we have established a mean value of stray surface fields as approx. 13 m T (-+30%) for both AP and N N samples. The values of damping p a r a m e t e r L
Z. Frait, D. FraitoL'd / FMR and FMAR of nanocrystalline alloy
were computed from the frequency dependent part of the F M R line width [10] as L = 0.0048 (0.0075) ( + 15%) for both surfaces of the AP(NN) samples. L obtained from F M A R in PC for AP samples ( L = 0.0056 + 20%)agrees with the F M R value (considering the accuracy), which points to (together w i t h / z 0 M data) a good magnetic homogeneity of the AP samples. For NN in PC F M A R yields L = 0.0049(+20%). The discrepancy in F M R and F M A R L values in NN, together with differences in /x0M , and with the occurrence of the subsidiary F M R mode, suggests a presence of a more pronounced magnetic inhomogeneity near the NN surface region (about hundreds of nm thick). Most probably, the surface crystallization in the annealing process is different from the bulk, e.g. the growth of nanocrystals may be not fully stopped by the Cu and Nb atoms. Relatively low F M A R L value points to a good homogeneity inside the bulk of NN material. The line widths in NC are found about two times larger than in PC; this is again caused by surface imperfections, which cause surface nonhomogeneous demagnetizing fields, and which in NC largely influence the F M R and F M A R resonance conditions [6,11]. At the end of this section we would like to stress one important fact, that both the line width and the L values in the homogeneous AP and in the heterogeneous NN samples were found comparable between themselves and with most of the values for amorphous Metglas type alloys [6]. This is in agreement with other authors' conclusions [2-5] that crystallization in NN occurs on the 10 to 20 nm scale (much smaller than the wavelengths of F M R magnetic excitations spin waves). The value of the surface anisotropy K S (uniaxial type, symmetry axis normal to the sample surface) can be evaluated by observing the F M R line shape (its asymmetry or the occurrence of exchange-dominated surface modes) [6]. By com-
355
paring the observed line shapes with spectra computed from the formula for Z r [6], in which K s values were changed in the interval K s = - 2 to + 2 m J / m 2, one can conclude, that both in AP and NN samples K s satisfies the relation - 0 . 3 m J / m 2 < K S < + 0.2 m J / m 2. Such small K s values have an undetectable influence on the F M R effect in these materials. Finally, by rotating the samples in the ribbon plane we investigated the effective fields of in-plane magnetic anisotropy; in AP (NN) samples these fields were found less than 6(3) mT, respectively, i.e. the samples are practically isotropic.
Acknowledgements The authors would like to thank Dr. J. Schneider and his coworkers from the Central Institute for Solid State Physics and Material Research (Dresden, Germany) for the kind supply of the ribbons.
References [1] R. Birringer, Mater. Sci. Eng. A l l 7 (1989) 33. [2] Y. Yoshizawa, S. Oguma and K. Yamanchi, J. Appl. Phys. 63 (1988) 6044. [3] H.R. Hilzinger, Mater. Sci. Forum 62-64 (1990) 515. [4] G. Herzer, IEEE Trans. Magn. MAG-25 (1989) 3327. [5] T. Zem~ik, Y. Jir~skov~, K. Z~iv~ta, D. Eckert, J. Schneider, N. Mattern and D. Hesske, Mater. Lett. 10 (1991) 313. [6] Z. Frait and D. Fraitovfi, in: Spin Waves and Magnetic Excitations, vol. 2, eds. A.S. Borovik-Romanov and S.K. Sinha (North-Holland, Amsterdam, 1988) p. 1. [7] D. FraitovA and Z. Frait, J. Magn. Magn. Mater. 101 (1991) 29. [8] D. Fraitov~, Phys. Stat. Sol. (b) 120 (1983) 659. [9] Z. Frait, J. Magn. Magn. Mater. 35 (1983) 37. [10] J.F. Cochran, R.W. Qiao and B. Heinrich, Phys. Rev. B 32 (1989) 4399. [11] Z. Frait and M. Ondris, Czech. J. Phys. B 12 (1962) 485.
AI41
Ferromagnetic resonance and antiresonance of amorphous and nanocrystalline Fe73.sCulNb3Si16.sB 6 alloy * Z. F r a i t a n d D. Fraitovfi Institute of Physics, CSAV, Na Slovance 2, CS-18040 Prague 8, Czechoslocakia
Received 5 August 1992
The first ferromagnetic resonance and antiresonance investigation was performed on a Metglas type alloy in the amorphous and nanocrystaUine state in a broad frequency interval (18 to 103 GHz). Data on magnetization, g-factor, relaxation constant, surface anisotropy and homogeneityof the samples are evaluated and discussed.
1. Introduction Recently a new type of ferromagnetic materials has been prepared, i.e. heterogeneous alloys, in which very fine crystalline grains (with diameter of about 10 nm) are embedded into an amorphous matrix. The iron-rich amorphous alloys FeSiB with an addition of a few percent of Cu and Nb metal, prepared by a fast quenching method in the form of thin ribbons, and subsequently annealed at temperatures above the crystallization point (around 550°C for about one hour) represent a typical case of so called nanocrystalline materials [1]. Especially, the alloys of the composition Fe73.sCulSbaSi16.sB6 [2] shows excellent soft magnetic properties and has been recently studied by several methods (measurements of permeability, anisotropy, magnetostriction and magnetization, domain and X-ray observation, M6ssbauer spectra, see e.g. refs. [25]). In this paper we present first data on these materials (magnetization M, g-factor, relaxation
constant L, surface anisotropy K S and surface stray fields), which were obtained by ferromagnetic resonance ( F M R ) and antiresonance ( F M A R ) methods [6] at room temperature and in a broad interval of microwave frequencies (18 to 103 GHz), both for as prepared (amorphous AP) and annealed (nanocrystalline - NN) samples.
2. Skin-depths, experimental By applying the F M R and F M A R methods to the studies of bulk samples of metals, which electromagnetic radiation penetrates only to a certain depth (skin-depth), one has to be well aware of which part of the sample volume is investigated. In F M R and F M A R regions the skin depths differ largely from the value for nonmagnetic material. In ferromagnetic metal the skin-depth is given by the formula [6,8]: = (2A/I~o)1/2(MZr)
Correspondence to: Prof. Z. Frait, Institute of Physics, CSAV, Na Slovance 2, CS 18040 Prague 8, Czechoslovakia. Tel.: +42-2-8152163; telefax: +42-2-8584569. Email: FZU59@ CSPGCS11. * Paper presented at the International Conference on Magnetism (ICM '91), Edinburgh, Scotland, 2-6 September 1991.
-',
(1)
where A is the exchange constant, Z r the real part of the relative surface impedance [6], depending mainly on the external field H, frequency f and resistivity p, further on L, K s and A. For our case we have computed the 6 values for all frequencies used in the experiment (values
0304-8853/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
354
Z. Frait, D. Fraitord / FMR and FMAR of nanocrystalline alloy
of L, K S, M, g were taken from this experiment, p = 140 ~f~ cm [2], A was estimated as 4.7 or 7.0 p J / m for the AP or NN material, respectively, by using the Curie point data [3,5]). E.g. for NN at 96 G H z one finds a strong increase of 6 in F M A R region to about 8 Ixm and a pronounced decrease in F M R region to about 200 nm. From our computations it further follows, that 6 at the F M R point is only slightly (15%) frequency dependent and 3 at F M A R point is inversely proportional to f. Around F M R and F M A R points 6 varies sharply with H; by considering the line width values established in our measurements (tens of mT) this change amounts to 25% (20%) in the H region between the F M R ( F M A R ) line inflection points, respectively. As the theory [6] used for the evaluation of experiments assumes the sample thickness (in our case 22 Ixm) much larger than 6, only F M A R m e a s u r e m e n t s at f > 60 G H z were taken into account.
3. Results and discussion
The investigated alloy was p r e p a r e d by the one-roller quenching method [2] in the form of thin ribbons 10 to 15 m m wide [5]. The samples were cut out to the form of squares or discs, annealing took place in the dry Ar or N 2 atmosphere at 550°C for one hour. The wheel side of the ribbon adjoining the roller (WS) is of a dull appearance (pointing to microscopic surface defects), the free side (FS) is shiny, with a certain waviness (mean wavelength around 0.5 mm, surface unevenness less than 1 txm). F M R and F M A R m e a s u r e m e n t s were performed at 18, 36, 49, 86, 96 and 103 G H z in the parallel (PC, M lies in the sample plane) and in the normal configuration (NC, M parallel to the sample normal), at RT, on both sample surfaces (WS and FS) of AP and NN samples; field derivative of microwave absorption A ' was registered, for details see ref. [9]. The following results were obtained from the F M R and F M A R field values, line widths and line shapes and their frequency dependence by using the resonance equations in refs. [6,9]. Generally no pronounced differences between mea-
I'~
......... .
q
a
/ K "p
25
.... k- ....... . A
2;6
2.7 'k'~'
2.8 HITI
2.9
Fig. 1. Plots of field derivative (A') of FMR (a) and FMAR (b) absorption for the nanocrystalline (NN) and amorphous (AP) samples, respectively. Full lines: measurements at the free side, broken lines: at the wheel side of the samples. Measurements at RT and at 95.787 GHz. surements on AP and NN samples were found, see fig. 1. At both surfaces of AP F M R and F M A R yield /z0M = 1.14 T ( + 2 % ) ; in N N / . % M = 1.13 T (-+3%) was established at both presurface regions by F M R and /z0M = 1.095 T (-+3%) in the bulk of the material by F M A R . These values are in a good agreement with static measurements on the same material [2-5]. Moreover, on NN a small subsidiary F M R peak is observed (more pronounced at FS), see fig. 1, pointing to the presence of some different magnetic phase near the surface, with / z 0 M = 1.6 T. No such effect was found by F M A R . The g-factors were found as g = 2.085 (2.075) (-+0.5%) for the AP (NN) samples, respectively. The decrease of g in NN is probably connected with the large Si content in the crystalline component [5] (g-factor of iron decreases with Si alloying [6]). The F M R line width values in PC increase linearly with frequency from 16(22) m T at 18 G H z to 37(55) m T at 103 G H z for the AP(NN) samples, respectively. The frequency independent part of the line widths, 12(16) m T for AP(NN), is most probably caused by surface imperfections; by means of the E P R of a free radical layer precipitated at the sample surface [9] we have established a mean value of stray surface fields as approx. 13 m T (-+30%) for both AP and N N samples. The values of damping p a r a m e t e r L
Z. Frait, D. FraitoL'd / FMR and FMAR of nanocrystalline alloy
were computed from the frequency dependent part of the F M R line width [10] as L = 0.0048 (0.0075) ( + 15%) for both surfaces of the AP(NN) samples. L obtained from F M A R in PC for AP samples ( L = 0.0056 + 20%)agrees with the F M R value (considering the accuracy), which points to (together w i t h / z 0 M data) a good magnetic homogeneity of the AP samples. For NN in PC F M A R yields L = 0.0049(+20%). The discrepancy in F M R and F M A R L values in NN, together with differences in /x0M , and with the occurrence of the subsidiary F M R mode, suggests a presence of a more pronounced magnetic inhomogeneity near the NN surface region (about hundreds of nm thick). Most probably, the surface crystallization in the annealing process is different from the bulk, e.g. the growth of nanocrystals may be not fully stopped by the Cu and Nb atoms. Relatively low F M A R L value points to a good homogeneity inside the bulk of NN material. The line widths in NC are found about two times larger than in PC; this is again caused by surface imperfections, which cause surface nonhomogeneous demagnetizing fields, and which in NC largely influence the F M R and F M A R resonance conditions [6,11]. At the end of this section we would like to stress one important fact, that both the line width and the L values in the homogeneous AP and in the heterogeneous NN samples were found comparable between themselves and with most of the values for amorphous Metglas type alloys [6]. This is in agreement with other authors' conclusions [2-5] that crystallization in NN occurs on the 10 to 20 nm scale (much smaller than the wavelengths of F M R magnetic excitations spin waves). The value of the surface anisotropy K S (uniaxial type, symmetry axis normal to the sample surface) can be evaluated by observing the F M R line shape (its asymmetry or the occurrence of exchange-dominated surface modes) [6]. By com-
355
paring the observed line shapes with spectra computed from the formula for Z r [6], in which K s values were changed in the interval K s = - 2 to + 2 m J / m 2, one can conclude, that both in AP and NN samples K s satisfies the relation - 0 . 3 m J / m 2 < K S < + 0.2 m J / m 2. Such small K s values have an undetectable influence on the F M R effect in these materials. Finally, by rotating the samples in the ribbon plane we investigated the effective fields of in-plane magnetic anisotropy; in AP (NN) samples these fields were found less than 6(3) mT, respectively, i.e. the samples are practically isotropic.
Acknowledgements The authors would like to thank Dr. J. Schneider and his coworkers from the Central Institute for Solid State Physics and Material Research (Dresden, Germany) for the kind supply of the ribbons.
References [1] R. Birringer, Mater. Sci. Eng. A l l 7 (1989) 33. [2] Y. Yoshizawa, S. Oguma and K. Yamanchi, J. Appl. Phys. 63 (1988) 6044. [3] H.R. Hilzinger, Mater. Sci. Forum 62-64 (1990) 515. [4] G. Herzer, IEEE Trans. Magn. MAG-25 (1989) 3327. [5] T. Zem~ik, Y. Jir~skov~, K. Z~iv~ta, D. Eckert, J. Schneider, N. Mattern and D. Hesske, Mater. Lett. 10 (1991) 313. [6] Z. Frait and D. Fraitovfi, in: Spin Waves and Magnetic Excitations, vol. 2, eds. A.S. Borovik-Romanov and S.K. Sinha (North-Holland, Amsterdam, 1988) p. 1. [7] D. FraitovA and Z. Frait, J. Magn. Magn. Mater. 101 (1991) 29. [8] D. Fraitov~, Phys. Stat. Sol. (b) 120 (1983) 659. [9] Z. Frait, J. Magn. Magn. Mater. 35 (1983) 37. [10] J.F. Cochran, R.W. Qiao and B. Heinrich, Phys. Rev. B 32 (1989) 4399. [11] Z. Frait and M. Ondris, Czech. J. Phys. B 12 (1962) 485.