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Magnetic properties of Cu–Permalloy granular alloy
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1519}1521
Magnetic properties of Cu}Permalloy granular alloy JuH lio C. Cezar *, Marcelo Knobel, HeH lio C. N. Tolentino Laborato& rio Nacional de Luz Sn& ncrotron } LNLS, CP 6192, 13083-970 Campinas, Brazil Instituto de Fn& sica Gleb Wataghin, IFGW, UNICAMP, Campinas, Brazil
Abstract Magnetization measurements were performed on Cu Fe Ni melt-spun alloys thermally treated at several temperatures up to 6003C. The magnetization curves were "tted using the sum of paramagnetic and ferromagnetic contributions, where the paramagnetic component is given by a Langevin function weighted by a log} normal moment distribution. The average moment of superparamagnetic clusters was determined, ranging from 31 to 11 000 , corresponding to average cluster sizes between 0.9 and 6.2 nm. 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetic clusters; Granular systems; Magnetoresistance; Melt-spun ribbons; Permalloy
Granular alloys composed of a low concentration of magnetic material in a non-magnetic matrix attracted the interest of the magnetism and materials science community because they display giant magnetoresistance e!ect (GMR) comparable to the one found in multilayered systems [1}3]. Usually, the alloy components are imiscible at room temperature, but reach a metastable condition due to a very fast cooling. The concentrations of elements are chosen in such a way that an application of a heat treatment enables the coalescence of the magnetic material, originating magnetic clusters [4]. The properties of these magnetic clusters (e.g., average size, size distribution, concentration) determine the magnetic behavior of the alloy, and its characterization are of key importance to understand the GMR phenomena [5]. In this work, we performed magnetization measurements of melt-spun Copper-Permalloy ribbons and from the "tting of these data the average size and size distribution were determined. The samples are melt-spun ribbons of Permalloy diluted in copper, with nominal atomic concentrations Cu Fe Ni (produced at the IEN Galileo Ferraris,
* Corresponding author. Fax: 55}19}287}4632. E-mail address: [email protected] (J.C. Cezar).
Turin, Italy). The samples were heat treated in a resistive furnace at 3703C, 4003C, 4503C, 5003C and 6003C, for 2 h. The heating and cooling rates were controlled at 103C/min. Magnetization curves were measured for each sample at room temperature in Quantum Design MPMS SQUID magnetometer. The "rst quadrant of resulting curves is shown in Fig. 1. The annealed samples have a hysteretic behavior at very low "elds, displaying coercive "elds between 2 and 6 kA/m. One can see that, as the thermal treatment temperature increases, the magnetization curve approaches saturation faster. This indicates the presence of larger magnetic particles in the samples treated at higher temperatures, which is an evidence of the progressive clustering of magnetic elements. In order to quantify the average size and size distribution of these clusters, we have analyzed the magnetization curves by using a two-component magnetization function [6]: (a) a superparamagnetic component, with a log}normal size distribution function, and (b) a ferromagnetic component.
H M"M.+ ¸ f ()d 1 k¹
H$H S ! tan #M$+ tan\ 1 2 H 2 !
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 9 4 4 - 6
,
(1)
1520
J.C. Cezar et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1519}1521
Fig. 1. M;H curves for Cu Fe Ni system, for di!erent annealing temperatures, measured at 300 K.
Fig. 2. Example (for the sample treated at 4003C) of the results obtained in the "tting procedure.
where M.+ and M$+ are the saturation magnetizations 1 1 of the paramagnetic and ferromagnetic components respectively, is the particle magnetic moment, H is the magnetic "eld, k is the Boltzmann constant, ¹ the absolute temperature, H is the coercive "eld of the ferromag! netic component and S the ratio between the remanent and saturation magnetization of the ferromagnetic component. ¸(x) is the Langevin function ¸(x)"coth x!x\, and f () is the log}normal distribution function divided by [7],
paramagnetic component, and M.+, H and S for the 1 ! ferromagnetic component (Table 1). The average magnetic moment N was calculated from and by N " eN [7]. We used the bulk Fe Ni saturation magnetization (833 emu/cm [2]) to estimate the cluster diameter D from the average magnetic moment N (considering spherical particles). Both N and D are also listed in Table 1. The procedure assumes the nominal concentration of Permalloy (Fe Ni ) in the clusters. This may be incorrect and can a!ect the size determination. This point deserves further veri"cation. In conclusion, a rapid increase in the particle size with annealing temperature has been observed. The importance of ferromagnetic phase increases with the particles diameter (see the contribution of ferromagnetic component to the whole magnetization in Table 1). The next step in this work is the determination of the transport properties of this system and correlate them with the valuable structural information obtained through this magnetic granulometry.
!ln(/ ) 1 , exp f ()" 2 (2
(2)
with the most probable value of the magnetic moment and the distribution width. The superparamagnetic and ferromagnetic behaviors are clearly separated (Fig. 2; with the ferromagnetic component detailed in the inset). The as-spun sample does not display a ferromagnetic contribution, and just the superparamagnetic component has been used. The parameters coming out from the "tting procedure are M.+, and for the super1
This work is supported by the Brazilian agencies FAPESP (98/16329-0 and 98/03774-5) and CNPq.
Table 1 Resulting parameters obtained from the analysis procedure. The last two columns show the cluster average magnetic moment and respective cluster diameter. and are given in Bohr magnetons
As spun 3703C 4003C 4503C 6003C
M.+ 1 (J/T kg)
( )
M.+ 1 (J/T kg)
H ! (kA/m)
S
( )
D (nm)
M$+ 1 M$+#M.+ 1 1
8.0 10.5 9.1 7.5 8.5
21 23 33 173 6000
0.89 0.90 0.83 0.66 1.11
* 0.047 0.084 0.102 0.309
* 10.2 12.6 6.8 14.0
* 0.12 0.12 0.07 0.19
31 35 47 215 11110
0.9 0.9 1.0 1.7 6.2
* 0.0045 0.0091 0.0134 0.0351
J.C. Cezar et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1519}1521
References [1] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 25 (1992) 7345. [2] Y.K. Yang, L.H. Chen, Y.H. Chang, Y.D. Yao, J. Magn. Magn. Mater. 189 (1998) 195. [3] C.S. Martins, F.P. Missell, J. Magn. Magn. Mater. 205 (1999) 275.
1521
[4] P. Allia, P. Tiberto, M. Baricco, F. Vinai, Rev. Sci. Instrum. 64 (1993) 1053. [5] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, D. Rao, A. Starr, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, J. Appl. Phys. 73 (1993) 5320. [6] M.B. Stearns, Y. Cheng, J. Appl. Phys. 75 (1994) 6894. [7] E.F. Ferrari, F.C.S. da Silva, M. Knobel, Phys. Rev. B. 56 (1997) 6086.
Magnetic properties of Cu}Permalloy granular alloy JuH lio C. Cezar *, Marcelo Knobel, HeH lio C. N. Tolentino Laborato& rio Nacional de Luz Sn& ncrotron } LNLS, CP 6192, 13083-970 Campinas, Brazil Instituto de Fn& sica Gleb Wataghin, IFGW, UNICAMP, Campinas, Brazil
Abstract Magnetization measurements were performed on Cu Fe Ni melt-spun alloys thermally treated at several temperatures up to 6003C. The magnetization curves were "tted using the sum of paramagnetic and ferromagnetic contributions, where the paramagnetic component is given by a Langevin function weighted by a log} normal moment distribution. The average moment of superparamagnetic clusters was determined, ranging from 31 to 11 000 , corresponding to average cluster sizes between 0.9 and 6.2 nm. 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetic clusters; Granular systems; Magnetoresistance; Melt-spun ribbons; Permalloy
Granular alloys composed of a low concentration of magnetic material in a non-magnetic matrix attracted the interest of the magnetism and materials science community because they display giant magnetoresistance e!ect (GMR) comparable to the one found in multilayered systems [1}3]. Usually, the alloy components are imiscible at room temperature, but reach a metastable condition due to a very fast cooling. The concentrations of elements are chosen in such a way that an application of a heat treatment enables the coalescence of the magnetic material, originating magnetic clusters [4]. The properties of these magnetic clusters (e.g., average size, size distribution, concentration) determine the magnetic behavior of the alloy, and its characterization are of key importance to understand the GMR phenomena [5]. In this work, we performed magnetization measurements of melt-spun Copper-Permalloy ribbons and from the "tting of these data the average size and size distribution were determined. The samples are melt-spun ribbons of Permalloy diluted in copper, with nominal atomic concentrations Cu Fe Ni (produced at the IEN Galileo Ferraris,
* Corresponding author. Fax: 55}19}287}4632. E-mail address: [email protected] (J.C. Cezar).
Turin, Italy). The samples were heat treated in a resistive furnace at 3703C, 4003C, 4503C, 5003C and 6003C, for 2 h. The heating and cooling rates were controlled at 103C/min. Magnetization curves were measured for each sample at room temperature in Quantum Design MPMS SQUID magnetometer. The "rst quadrant of resulting curves is shown in Fig. 1. The annealed samples have a hysteretic behavior at very low "elds, displaying coercive "elds between 2 and 6 kA/m. One can see that, as the thermal treatment temperature increases, the magnetization curve approaches saturation faster. This indicates the presence of larger magnetic particles in the samples treated at higher temperatures, which is an evidence of the progressive clustering of magnetic elements. In order to quantify the average size and size distribution of these clusters, we have analyzed the magnetization curves by using a two-component magnetization function [6]: (a) a superparamagnetic component, with a log}normal size distribution function, and (b) a ferromagnetic component.
H M"M.+ ¸ f ()d 1 k¹
H$H S ! tan #M$+ tan\ 1 2 H 2 !
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 9 4 4 - 6
,
(1)
1520
J.C. Cezar et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1519}1521
Fig. 1. M;H curves for Cu Fe Ni system, for di!erent annealing temperatures, measured at 300 K.
Fig. 2. Example (for the sample treated at 4003C) of the results obtained in the "tting procedure.
where M.+ and M$+ are the saturation magnetizations 1 1 of the paramagnetic and ferromagnetic components respectively, is the particle magnetic moment, H is the magnetic "eld, k is the Boltzmann constant, ¹ the absolute temperature, H is the coercive "eld of the ferromag! netic component and S the ratio between the remanent and saturation magnetization of the ferromagnetic component. ¸(x) is the Langevin function ¸(x)"coth x!x\, and f () is the log}normal distribution function divided by [7],
paramagnetic component, and M.+, H and S for the 1 ! ferromagnetic component (Table 1). The average magnetic moment N was calculated from and by N " eN [7]. We used the bulk Fe Ni saturation magnetization (833 emu/cm [2]) to estimate the cluster diameter D from the average magnetic moment N (considering spherical particles). Both N and D are also listed in Table 1. The procedure assumes the nominal concentration of Permalloy (Fe Ni ) in the clusters. This may be incorrect and can a!ect the size determination. This point deserves further veri"cation. In conclusion, a rapid increase in the particle size with annealing temperature has been observed. The importance of ferromagnetic phase increases with the particles diameter (see the contribution of ferromagnetic component to the whole magnetization in Table 1). The next step in this work is the determination of the transport properties of this system and correlate them with the valuable structural information obtained through this magnetic granulometry.
!ln(/ ) 1 , exp f ()" 2 (2
(2)
with the most probable value of the magnetic moment and the distribution width. The superparamagnetic and ferromagnetic behaviors are clearly separated (Fig. 2; with the ferromagnetic component detailed in the inset). The as-spun sample does not display a ferromagnetic contribution, and just the superparamagnetic component has been used. The parameters coming out from the "tting procedure are M.+, and for the super1
This work is supported by the Brazilian agencies FAPESP (98/16329-0 and 98/03774-5) and CNPq.
Table 1 Resulting parameters obtained from the analysis procedure. The last two columns show the cluster average magnetic moment and respective cluster diameter. and are given in Bohr magnetons
As spun 3703C 4003C 4503C 6003C
M.+ 1 (J/T kg)
( )
M.+ 1 (J/T kg)
H ! (kA/m)
S
( )
D (nm)
M$+ 1 M$+#M.+ 1 1
8.0 10.5 9.1 7.5 8.5
21 23 33 173 6000
0.89 0.90 0.83 0.66 1.11
* 0.047 0.084 0.102 0.309
* 10.2 12.6 6.8 14.0
* 0.12 0.12 0.07 0.19
31 35 47 215 11110
0.9 0.9 1.0 1.7 6.2
* 0.0045 0.0091 0.0134 0.0351
J.C. Cezar et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1519}1521
References [1] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 25 (1992) 7345. [2] Y.K. Yang, L.H. Chen, Y.H. Chang, Y.D. Yao, J. Magn. Magn. Mater. 189 (1998) 195. [3] C.S. Martins, F.P. Missell, J. Magn. Magn. Mater. 205 (1999) 275.
1521
[4] P. Allia, P. Tiberto, M. Baricco, F. Vinai, Rev. Sci. Instrum. 64 (1993) 1053. [5] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, D. Rao, A. Starr, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, J. Appl. Phys. 73 (1993) 5320. [6] M.B. Stearns, Y. Cheng, J. Appl. Phys. 75 (1994) 6894. [7] E.F. Ferrari, F.C.S. da Silva, M. Knobel, Phys. Rev. B. 56 (1997) 6086.