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Magnetic anisotropy of LaCo-substituted SrFe12O19 ferrites
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e1845–E1846
Magnetic anisotropy of LaCo-substituted SrFe12O19 ferrites G. Astia,*, F. Bolzonib, J.M. Le Bretonc, M. Ghidinia, A. Moreld, M. Solzia, F. Koolsd, P. Tenaudd INFM-Dip.to di Fisica, Parco Area delle Scienze 7/A, Universita" di Parma 43100, Italy b IMEM-CNR Institute, Parco Area delle Scienze 37/A, 43010 Parma, Italy c GPM-UMR CNRS 6634, 76801 Universit!e de Rouen, St. Etienne du Rouvray, France d Carbone Lorraine-Ferrites, 27016 Evreux and 38830 St. Pierre d’Allevard Cedex, France a
Abstract The magnetic anisotropy of LaCo-hexaferrites has been investigated by the use of the singular point detection and the second harmonic techniques in the low-temperature range. Below 200 K a steep rise of the anisotropy field has been observed together with a strong distortion of the magnetization curve which is interpreted as due to a negative K2 and a large positive K3 anisotropy constants. The results are analyzed in terms of an extended Stoner–Wohlfarth model and the obtained values of the anisotropy coefficients w2;0 ; w4;0 ; w6;0 are reported versus temperature. r 2003 Elsevier B.V. All rights reserved. PACS: 75.30Gw; 75.50Vv; 75.50Ww Keywords: Ferrites; Magnetic anisotropy; Permanent magnets
La and Co substitutions in both SrFe12 O19 and BaFe12 O19 [1,2] have proved to lead, for the first time in 40 years, to significant improvements in the magnetic properties of ceramic permanent magnets. The obtained increase in both remanence and magnetic anisotropy is of particular importance in this respect. Several investigations have been performed on these ferrimagnetic oxides by various techniques in order to clarify the mechanisms responsible of the observed phenomena. In particular, the cationic distribution of Co2þ in Sr1z Laz Fe12z Coz O19 hexagonal ferrite was investi. gated by Mossbauer spectrometry [3]. A detailed study of the magnetic anisotropy and its temperature dependence is fundamental in gaining some insight into the complex interplay of different effects that typically take part in a complex hexaferrite structure. As a matter of fact, in addition to the distinctive magnetocrystalline anisotropy of the M-type structure, based on crystal field and dipolar interactions of Fe3þ ; important contributions from Co2þ and Fe2þ in different lattice *Corresponding author. Tel.: +39-0521-905265; fax: +390521-905223. E-mail address: asti@fis.unipr.it (G. Asti).
sites are expected, taking into account a special role of La [4]. In the present work, we report on the results of anisotropy field measurements by the SPD technique both in pulsed fields and by the second harmonic method [5] at low temperatures, on Sr1z Laz Fe12z Coz O19 ðz ¼ 0:1–0.4) compounds. At room temperature the anisotropy field is already higher in substituted compounds with respect to pure SrFe12 O19 ferrite: it can be described by a linear increase of the anisotropy field Ha ðzÞ ¼ Ha ð0Þð1 þ 0:88zÞ up to z ¼ 0:3; for higher x values saturation-type effects occur. The measured anisotropy fields are in agreement with previous measurements [1] down to 200 K: However, below this temperature the measurements become more and more difficult because the SPD peak amplitude decreases rapidly and disappears below 100 K in all compounds (Fig. 1). Another remarkable effect at low temperature is the onset of a strong distortion of the magnetization curve that produces a strong wide peak in the SPD signal well below the sharp anisotropy field peak. Such a behavior reveals that contributions from higher-order anisotropy constants certainly come into play. It is interpreted as due to a negative K2 and a large positive
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1275
ARTICLE IN PRESS G. Asti et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e1845–E1846
e1846 3.5
2e+05 x10
3 0
2.5 χ j,0 (J/m^3)
0 1.5
1
0.5
Arbitrary units
µ0 Ha (T)
x100
2
-5 230 K
-10
−2e+05
−4e+05
x1
150 K
-15 -20
0
z=0 z=0.1 z=0.2 z=0.3 z=0.4
µ0H (T)
0
100
1
2 150
3
−6e+05 150
4 200
250
300
200
250
300
Temperature (K)
Temperature (K)
Fig. 1. Temperature dependence of the anisotropy fields of the compounds Sr1z Laz Fe12z Coz O19 for z ¼ 0; 0.1, 0.2, 0.3, 0.4 measured by the SPD technique both in pulsed fields and by the second harmonic method. The inset gives simulations of the pulsed SPD signal at 150 and 230 K:
K3 anisotropy constants. According to the SPD theory in fact the peak amplitude should be proportional to ð1 þ 2x þ 3yÞ3=2 =ð1 þ 6x þ 15yÞ3=2 ; where x ¼ K2 =K1 and y ¼ K3 =K1 : These peculiarities have never been observed in other substituted M-type ferrites. In most cases the presence of Co ions in M-, W-, X- or Z-type ferrites was known to be responsible of high-order terms in the magnetocrystalline anisotropy, but always in favor of planar or conical easy directions. Again as a consequence of high-order anisotropy constants Co substitution has been observed to give rise in some cases to FOMP type transitions [6] both along the c-axis direction and perpendicular to it in different temperature intervals. The samples utilized in the present experiments were prepared with a high degree of orientation and the distribution function has been determined by a special version of the SPD technique [7]. The overall analysis of the obtained results was performed taking into account several features of the magnetization curve, namely the position and amplitude of the anisotropy field peak, the differential susceptibility at remanence and the peculiar broad peak observed at low field in the
Fig. 2. Measured anisotropy coefficients wðTÞ2;0 ; wðTÞ4;0 and wðTÞ6;0 versus T for the compound with z ¼ 0:2: For wðTÞ4;0 and wðTÞ6;0 the vertical scale should be multiplied by 10 and 100, respectively.
d2 M=dH 2 curve. Moreover, the process was compared with computer simulations based on an extended Stoner–Wohlfarth-type model (including K1 ; K2 and K3 anisotropy constants and orientation distribution functions of the crystallites). Typical simulated SPD signals are reported in the inset of Fig. 1 for the z ¼ 0:2 sample at 150 and 230 K: The anisotropy coefficients wðTÞ2;0 ; wðTÞ4;0 ; wðTÞ6;0 have been deduced in the temperature interval from 150 to 300 K and reported in Fig. 2 for the compound with z ¼ 0:2:
References [1] F. Kools, et al., J. Magn. Magn. Mater. 242–245 (2002) 1270. [2] G. Wiesinger, et al., Phys. Stat. Sol. a 189 (2002) 499. [3] J.M. Le Breton, et al., IEEE Trans. Mag. 38 (2002) 2952. [4] F. Kools, et al., in: M. Abe, Y. Yamazaki (Eds.), Proceedings of ICF8 on Ferrites, Kyoto, Japan. 2000, p. 437. [5] G. Asti, et al., IEEE Trans. Magn. 36 (2000) 3505. [6] G. Asti, F. Bolzoni, F. Licci, M. Canali, IEEE Trans. Magn. 14 (1978) 883. [7] G. Asti, F. Bolzoni, R. Cabassi, M. Ghidini, J. Magn. Magn. Mater. 128 (1993) 58.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e1845–E1846
Magnetic anisotropy of LaCo-substituted SrFe12O19 ferrites G. Astia,*, F. Bolzonib, J.M. Le Bretonc, M. Ghidinia, A. Moreld, M. Solzia, F. Koolsd, P. Tenaudd INFM-Dip.to di Fisica, Parco Area delle Scienze 7/A, Universita" di Parma 43100, Italy b IMEM-CNR Institute, Parco Area delle Scienze 37/A, 43010 Parma, Italy c GPM-UMR CNRS 6634, 76801 Universit!e de Rouen, St. Etienne du Rouvray, France d Carbone Lorraine-Ferrites, 27016 Evreux and 38830 St. Pierre d’Allevard Cedex, France a
Abstract The magnetic anisotropy of LaCo-hexaferrites has been investigated by the use of the singular point detection and the second harmonic techniques in the low-temperature range. Below 200 K a steep rise of the anisotropy field has been observed together with a strong distortion of the magnetization curve which is interpreted as due to a negative K2 and a large positive K3 anisotropy constants. The results are analyzed in terms of an extended Stoner–Wohlfarth model and the obtained values of the anisotropy coefficients w2;0 ; w4;0 ; w6;0 are reported versus temperature. r 2003 Elsevier B.V. All rights reserved. PACS: 75.30Gw; 75.50Vv; 75.50Ww Keywords: Ferrites; Magnetic anisotropy; Permanent magnets
La and Co substitutions in both SrFe12 O19 and BaFe12 O19 [1,2] have proved to lead, for the first time in 40 years, to significant improvements in the magnetic properties of ceramic permanent magnets. The obtained increase in both remanence and magnetic anisotropy is of particular importance in this respect. Several investigations have been performed on these ferrimagnetic oxides by various techniques in order to clarify the mechanisms responsible of the observed phenomena. In particular, the cationic distribution of Co2þ in Sr1z Laz Fe12z Coz O19 hexagonal ferrite was investi. gated by Mossbauer spectrometry [3]. A detailed study of the magnetic anisotropy and its temperature dependence is fundamental in gaining some insight into the complex interplay of different effects that typically take part in a complex hexaferrite structure. As a matter of fact, in addition to the distinctive magnetocrystalline anisotropy of the M-type structure, based on crystal field and dipolar interactions of Fe3þ ; important contributions from Co2þ and Fe2þ in different lattice *Corresponding author. Tel.: +39-0521-905265; fax: +390521-905223. E-mail address: asti@fis.unipr.it (G. Asti).
sites are expected, taking into account a special role of La [4]. In the present work, we report on the results of anisotropy field measurements by the SPD technique both in pulsed fields and by the second harmonic method [5] at low temperatures, on Sr1z Laz Fe12z Coz O19 ðz ¼ 0:1–0.4) compounds. At room temperature the anisotropy field is already higher in substituted compounds with respect to pure SrFe12 O19 ferrite: it can be described by a linear increase of the anisotropy field Ha ðzÞ ¼ Ha ð0Þð1 þ 0:88zÞ up to z ¼ 0:3; for higher x values saturation-type effects occur. The measured anisotropy fields are in agreement with previous measurements [1] down to 200 K: However, below this temperature the measurements become more and more difficult because the SPD peak amplitude decreases rapidly and disappears below 100 K in all compounds (Fig. 1). Another remarkable effect at low temperature is the onset of a strong distortion of the magnetization curve that produces a strong wide peak in the SPD signal well below the sharp anisotropy field peak. Such a behavior reveals that contributions from higher-order anisotropy constants certainly come into play. It is interpreted as due to a negative K2 and a large positive
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1275
ARTICLE IN PRESS G. Asti et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e1845–E1846
e1846 3.5
2e+05 x10
3 0
2.5 χ j,0 (J/m^3)
0 1.5
1
0.5
Arbitrary units
µ0 Ha (T)
x100
2
-5 230 K
-10
−2e+05
−4e+05
x1
150 K
-15 -20
0
z=0 z=0.1 z=0.2 z=0.3 z=0.4
µ0H (T)
0
100
1
2 150
3
−6e+05 150
4 200
250
300
200
250
300
Temperature (K)
Temperature (K)
Fig. 1. Temperature dependence of the anisotropy fields of the compounds Sr1z Laz Fe12z Coz O19 for z ¼ 0; 0.1, 0.2, 0.3, 0.4 measured by the SPD technique both in pulsed fields and by the second harmonic method. The inset gives simulations of the pulsed SPD signal at 150 and 230 K:
K3 anisotropy constants. According to the SPD theory in fact the peak amplitude should be proportional to ð1 þ 2x þ 3yÞ3=2 =ð1 þ 6x þ 15yÞ3=2 ; where x ¼ K2 =K1 and y ¼ K3 =K1 : These peculiarities have never been observed in other substituted M-type ferrites. In most cases the presence of Co ions in M-, W-, X- or Z-type ferrites was known to be responsible of high-order terms in the magnetocrystalline anisotropy, but always in favor of planar or conical easy directions. Again as a consequence of high-order anisotropy constants Co substitution has been observed to give rise in some cases to FOMP type transitions [6] both along the c-axis direction and perpendicular to it in different temperature intervals. The samples utilized in the present experiments were prepared with a high degree of orientation and the distribution function has been determined by a special version of the SPD technique [7]. The overall analysis of the obtained results was performed taking into account several features of the magnetization curve, namely the position and amplitude of the anisotropy field peak, the differential susceptibility at remanence and the peculiar broad peak observed at low field in the
Fig. 2. Measured anisotropy coefficients wðTÞ2;0 ; wðTÞ4;0 and wðTÞ6;0 versus T for the compound with z ¼ 0:2: For wðTÞ4;0 and wðTÞ6;0 the vertical scale should be multiplied by 10 and 100, respectively.
d2 M=dH 2 curve. Moreover, the process was compared with computer simulations based on an extended Stoner–Wohlfarth-type model (including K1 ; K2 and K3 anisotropy constants and orientation distribution functions of the crystallites). Typical simulated SPD signals are reported in the inset of Fig. 1 for the z ¼ 0:2 sample at 150 and 230 K: The anisotropy coefficients wðTÞ2;0 ; wðTÞ4;0 ; wðTÞ6;0 have been deduced in the temperature interval from 150 to 300 K and reported in Fig. 2 for the compound with z ¼ 0:2:
References [1] F. Kools, et al., J. Magn. Magn. Mater. 242–245 (2002) 1270. [2] G. Wiesinger, et al., Phys. Stat. Sol. a 189 (2002) 499. [3] J.M. Le Breton, et al., IEEE Trans. Mag. 38 (2002) 2952. [4] F. Kools, et al., in: M. Abe, Y. Yamazaki (Eds.), Proceedings of ICF8 on Ferrites, Kyoto, Japan. 2000, p. 437. [5] G. Asti, et al., IEEE Trans. Magn. 36 (2000) 3505. [6] G. Asti, F. Bolzoni, F. Licci, M. Canali, IEEE Trans. Magn. 14 (1978) 883. [7] G. Asti, F. Bolzoni, R. Cabassi, M. Ghidini, J. Magn. Magn. Mater. 128 (1993) 58.