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Magnetization enhancement in Fe–Co–B alloy nanoparticles
ARTICLE IN PRESS
Physica B 384 (2006) 274–276 www.elsevier.com/locate/physb
Magnetization enhancement in Fe–Co–B alloy nanoparticles B. Molina Conchaa,, R.D. Zyslera, H. Romerob a
Centro Ato´mico Bariloche, 8400 S. C. de Bariloche, R.N., Argentina Departamento de Fı´sica, Facultas de Ciencias, Universidad de los Andes, 5101 Me´rida, Venezuela
b
Abstract Co–Fe alloy may have magnetic moment higher than that of pure metallic Fe or Co. Particularly, bulk FexCo1x shows a broad maximum in the magnetic moment as a function of composition centred in x ¼ 0:65 at 0 K (theoretical calculation) and at x ¼ 0:6 for room temperature. We have synthesised 3-nm (FexCo1x)0.6B0.4 (0pxp1) amorphous nanoparticles by chemical route dispersed in a non-magnetic matrix (non-interacting nanoparticles). Similar to bulk alloy, this system exhibits a maximum of the saturation magnetization at 31% at Co at room temperature, but this maximum is sharp in composition. The powder samples (with interparticle interaction presence) show similar behaviour at RT. r 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Magnetization enhancement; Fe–Co alloy
1. Introduction Magnetic nanoclusters have attracted much attention since such clusters have novel properties and technological applications [1,2]. The effect of nanoscale confinement on these nanoclusters leads to magnetic behaviour different from the bulk one. For fine particles as the particle size decrease, finite-size effect dominates the magnetic properties due to surface effects (for example, for a particle of 3 nm radius, about 70% of the atoms lie on the surface). The picture of a magnetic single-domain particle where all spins are pointing in the same direction, thus leading to coherent relaxation process, is no longer valid if one considers the effect on the global magnetic properties of the particle of misaligned spins on the surface [3]. It is expected that the decrease in the coordination number induces a weakening in the exchange interactions of surface atoms with the surrounding ones. In metallic nanoparticles, band structure is also modified by the finite size, leading to a confinement of conduction electron band. Magnetic interactions between particles can also modify the surface configuration of spins and the problem become more complex. Corresponding author. Tel.: +54 2944 445158; fax: +54 2944 445299.
E-mail address: [email protected] (B.M. Concha). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.06.009
On the other hand, Fe–Co alloys have attracted theoretical and technological interest due to its unusual magnetic features. The atomic magnetic moment of FexCo1x shows a maximum for x ¼ 0:65 at T ¼ 0 K with a magnetic moment m 2:45mB per atom which is larger than that of pure metallic Fe or Co [4]. The maximum values at room temperature is attained at about x ¼ 0:60. It is expected that this behaviour may be modified at nanometric scale by confinement and surface effects. In this frame work we study the behaviour of saturation magnetization of (FexCo1x)0.6B0.4 (0pxp1) amorphous nanoparticles at room temperature dispersed in a nonmagnetic matrix (non-interacting system) and powder samples (strongly interacting system). 2. Experimental procedure Amorphous nanoparticles of Fe–Co–B alloy were obtained by chemical route by reduction of aqueous solutions of metallic salts [5,6]. The relative Fe/Co composition was determined by energy dispersive spectroscopy microanalysis (EDX) and the boron concentration by atomic absorption analysis. The iron–cobalt ratio resulted close to the nominal one, the boron concentration was 40 at%. The X-ray powder diffraction pattern showed a broad spectrum, confirming the amorphous nature of the
ARTICLE IN PRESS B.M. Concha et al. / Physica B 384 (2006) 274–276
particles. The mean diameter and the particle size were determined by TEM and light-scattering experiments. The mean diameter was 3 nm and the distribution was determined by light-scattering method of an aqueous suspension of the particles. This method yields a narrow lognormal distribution centred at 2.8 nm and a dispersion s ¼ 1 nm [6]. In order to have a system where interparticle interactions are negligible, particles were dispersed in a non-magnetic matrix (PVP) at about 1% w/w, leading to an average interparticle distance of 19 nm. As our nanoparticles are ferromagnetically ordered, single-domain behaviour is expected to be observed. Magnetization measurements were performed as a function
16 14
M (emu/g)
12 10 8 (Fe0.69Co0.31)0.6B0.4 6
Dispersed sample H=50 Oe
4 2
0
50
100
150 T (K)
200
250
300
Fig. 1. Magnetization vs. temperature measured applying a field H ¼ 50 Oe under ZFC (open circles) and FC conditions (solid circles) for dispersed nanoparticles and x ¼ 0:69.
275
of the magnetic field up to 10 kOe at different temperatures in a commercial VSM, and magnetization measurements as a function of temperature (in the range 5–300 K) in zerofield cooling (ZFC) and field-cooling (FC) conditions were performed using a commercial superconducting quantum interference device magnetometer (SQUID). 3. Experimental results Fig. 1 shows the magnetization measurement vs. temperature under ZFC and FC conditions for (Fe0.69 Co0.31)0.6B0.4 dispersed samples. It is noticeable that the FC magnetization curve remains quasi-constant with decreasing temperature. This fact is unusual in dispersed nanoparticle and probably is an indicator of intraparticle interaction or magnetic order frustration al the particle surface. The observed behaviour shows that the blocking temperature is above room temperature. The results of magnetization measurements as a function of magnetic field for powder sample at room temperature are shown in Fig. 2, for x ¼ 0:62, 0.69, and 0.75. These measurements show hysteresis according to the blocked regime. This figure clearly shows an enhancement of the saturation magnetization for the x ¼ 0:69 sample. This feature is evidenced in Fig. 3 where we have plotted the saturation magnetization of dispersed samples as a function of Fe composition at room temperature. The bulk alloy saturation magnetization is also plotted in order to compare both cases. These results show that the saturation magnetization of nanoparticles, i.e. the magnetic moment of the particle, decreases with cobalt substitution except for x ffi 0:69. For this composition this system exhibits an enhancement of the saturation. Similar behaviour is
100 (FexCo1-x)0.6B0.4
80
Powder sample
60
x = 0.625 40
x = 0.69
M (emu/g)
20
x = 0.75 25 20 15 10 5 0 -5 -10 -15 -20 -25 -0.2
0 -20 -40 -60 -80
-0.1
0.0
0.1
0.2
-100 -10
-8
-6
-4
-2
0 H (kOe)
2
4
6
8
10
Fig. 2. Magnetization vs. magnetic field at room temperature for (FexCo1x)0.6B0.4 powder samples (x ¼ 0:625, 0.69, and 0.75).
ARTICLE IN PRESS B.M. Concha et al. / Physica B 384 (2006) 274–276
276
350
MS (emu/g)
100
300 50
250
particles have been performed. The saturation magnetization as a function of composition shows a maximum for x ¼ 0:69 similarly to bulk alloy. In the bulk case, Fe–Co alloy shows a broad maximum for x ¼ 0:65 but in the nanoparticle case, a sharp maximum very sensitive to composition is observed. This difference between both behaviours may be attributed to size effect, band confinement, and surface anisotropy effects. On the other hand, the value of saturation magnetization is lower than the magnetization of bulk alloy for the same composition but the relative increment is larger than the bulk alloy. The low value of the magnetization might be due to the misaligned spins on surface.
0 0
20
40
60
80
100
% Fe Fig. 3. Saturation magnetization as a function of composition at room temperature for dispersed samples (open circles) and for the bulk alloy (dashed line) for Ref. [4].
observed in the bulk alloy, where a broad maximum of the magnetic moment with composition is present at 65% Fe [4]. Moreover, for the bulk material the saturation magnetization is larger than for the nanoparticles of the same composition. The lower values of MS obtained for the nanoparticles are due to the misalignment of the spins at the particle surface (caused by the low coordination between spins and the surface anisotropy influence). The sharp maximum observed might be related with size effects, band confinement, surface anisotropy, etc. The powder samples shows a similar behaviour with composition at room temperature. 4. Conclusions Magnetization measurements on 3 nm (Fex Co1x)0.6B0.4 (0pxp1) ferromagnetic amorphous nano-
Acknowledgement This work has been accomplished with partial support by CONICET-CONICIT (Argentina–Venezuela) cooperation project.
References [1] J.L. Dorman, D. Fiorani (Eds.), Magnetic Properties of Fine Particles, North-Holland, Amsterdam, 1992. [2] G.C. Hadjipanayis, G.A. Prinz (Eds.), Science and Technology of Nanostructurated Materials, Plenum Press, New York, 1991. [3] D. Fiorani (Ed.), Surface Effects in Magnetic Nanoparticles, Springer, Berlin, 2005. [4] S. Chikazumi, Physics of Magnetism, Krieger, Malabar, Florida, 1964 Chapter 4. [5] R.D. Zysler, C.A. Ramos, H. Romero, A. Ortega, J. Mater. Sci. 36 (9) (2001) 2291. [6] B. Molina Concha, R.D. Zysler, H. Troiani, H. Romero, Phys. B. 354 (2004) 121.
Physica B 384 (2006) 274–276 www.elsevier.com/locate/physb
Magnetization enhancement in Fe–Co–B alloy nanoparticles B. Molina Conchaa,, R.D. Zyslera, H. Romerob a
Centro Ato´mico Bariloche, 8400 S. C. de Bariloche, R.N., Argentina Departamento de Fı´sica, Facultas de Ciencias, Universidad de los Andes, 5101 Me´rida, Venezuela
b
Abstract Co–Fe alloy may have magnetic moment higher than that of pure metallic Fe or Co. Particularly, bulk FexCo1x shows a broad maximum in the magnetic moment as a function of composition centred in x ¼ 0:65 at 0 K (theoretical calculation) and at x ¼ 0:6 for room temperature. We have synthesised 3-nm (FexCo1x)0.6B0.4 (0pxp1) amorphous nanoparticles by chemical route dispersed in a non-magnetic matrix (non-interacting nanoparticles). Similar to bulk alloy, this system exhibits a maximum of the saturation magnetization at 31% at Co at room temperature, but this maximum is sharp in composition. The powder samples (with interparticle interaction presence) show similar behaviour at RT. r 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Magnetization enhancement; Fe–Co alloy
1. Introduction Magnetic nanoclusters have attracted much attention since such clusters have novel properties and technological applications [1,2]. The effect of nanoscale confinement on these nanoclusters leads to magnetic behaviour different from the bulk one. For fine particles as the particle size decrease, finite-size effect dominates the magnetic properties due to surface effects (for example, for a particle of 3 nm radius, about 70% of the atoms lie on the surface). The picture of a magnetic single-domain particle where all spins are pointing in the same direction, thus leading to coherent relaxation process, is no longer valid if one considers the effect on the global magnetic properties of the particle of misaligned spins on the surface [3]. It is expected that the decrease in the coordination number induces a weakening in the exchange interactions of surface atoms with the surrounding ones. In metallic nanoparticles, band structure is also modified by the finite size, leading to a confinement of conduction electron band. Magnetic interactions between particles can also modify the surface configuration of spins and the problem become more complex. Corresponding author. Tel.: +54 2944 445158; fax: +54 2944 445299.
E-mail address: [email protected] (B.M. Concha). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.06.009
On the other hand, Fe–Co alloys have attracted theoretical and technological interest due to its unusual magnetic features. The atomic magnetic moment of FexCo1x shows a maximum for x ¼ 0:65 at T ¼ 0 K with a magnetic moment m 2:45mB per atom which is larger than that of pure metallic Fe or Co [4]. The maximum values at room temperature is attained at about x ¼ 0:60. It is expected that this behaviour may be modified at nanometric scale by confinement and surface effects. In this frame work we study the behaviour of saturation magnetization of (FexCo1x)0.6B0.4 (0pxp1) amorphous nanoparticles at room temperature dispersed in a nonmagnetic matrix (non-interacting system) and powder samples (strongly interacting system). 2. Experimental procedure Amorphous nanoparticles of Fe–Co–B alloy were obtained by chemical route by reduction of aqueous solutions of metallic salts [5,6]. The relative Fe/Co composition was determined by energy dispersive spectroscopy microanalysis (EDX) and the boron concentration by atomic absorption analysis. The iron–cobalt ratio resulted close to the nominal one, the boron concentration was 40 at%. The X-ray powder diffraction pattern showed a broad spectrum, confirming the amorphous nature of the
ARTICLE IN PRESS B.M. Concha et al. / Physica B 384 (2006) 274–276
particles. The mean diameter and the particle size were determined by TEM and light-scattering experiments. The mean diameter was 3 nm and the distribution was determined by light-scattering method of an aqueous suspension of the particles. This method yields a narrow lognormal distribution centred at 2.8 nm and a dispersion s ¼ 1 nm [6]. In order to have a system where interparticle interactions are negligible, particles were dispersed in a non-magnetic matrix (PVP) at about 1% w/w, leading to an average interparticle distance of 19 nm. As our nanoparticles are ferromagnetically ordered, single-domain behaviour is expected to be observed. Magnetization measurements were performed as a function
16 14
M (emu/g)
12 10 8 (Fe0.69Co0.31)0.6B0.4 6
Dispersed sample H=50 Oe
4 2
0
50
100
150 T (K)
200
250
300
Fig. 1. Magnetization vs. temperature measured applying a field H ¼ 50 Oe under ZFC (open circles) and FC conditions (solid circles) for dispersed nanoparticles and x ¼ 0:69.
275
of the magnetic field up to 10 kOe at different temperatures in a commercial VSM, and magnetization measurements as a function of temperature (in the range 5–300 K) in zerofield cooling (ZFC) and field-cooling (FC) conditions were performed using a commercial superconducting quantum interference device magnetometer (SQUID). 3. Experimental results Fig. 1 shows the magnetization measurement vs. temperature under ZFC and FC conditions for (Fe0.69 Co0.31)0.6B0.4 dispersed samples. It is noticeable that the FC magnetization curve remains quasi-constant with decreasing temperature. This fact is unusual in dispersed nanoparticle and probably is an indicator of intraparticle interaction or magnetic order frustration al the particle surface. The observed behaviour shows that the blocking temperature is above room temperature. The results of magnetization measurements as a function of magnetic field for powder sample at room temperature are shown in Fig. 2, for x ¼ 0:62, 0.69, and 0.75. These measurements show hysteresis according to the blocked regime. This figure clearly shows an enhancement of the saturation magnetization for the x ¼ 0:69 sample. This feature is evidenced in Fig. 3 where we have plotted the saturation magnetization of dispersed samples as a function of Fe composition at room temperature. The bulk alloy saturation magnetization is also plotted in order to compare both cases. These results show that the saturation magnetization of nanoparticles, i.e. the magnetic moment of the particle, decreases with cobalt substitution except for x ffi 0:69. For this composition this system exhibits an enhancement of the saturation. Similar behaviour is
100 (FexCo1-x)0.6B0.4
80
Powder sample
60
x = 0.625 40
x = 0.69
M (emu/g)
20
x = 0.75 25 20 15 10 5 0 -5 -10 -15 -20 -25 -0.2
0 -20 -40 -60 -80
-0.1
0.0
0.1
0.2
-100 -10
-8
-6
-4
-2
0 H (kOe)
2
4
6
8
10
Fig. 2. Magnetization vs. magnetic field at room temperature for (FexCo1x)0.6B0.4 powder samples (x ¼ 0:625, 0.69, and 0.75).
ARTICLE IN PRESS B.M. Concha et al. / Physica B 384 (2006) 274–276
276
350
MS (emu/g)
100
300 50
250
particles have been performed. The saturation magnetization as a function of composition shows a maximum for x ¼ 0:69 similarly to bulk alloy. In the bulk case, Fe–Co alloy shows a broad maximum for x ¼ 0:65 but in the nanoparticle case, a sharp maximum very sensitive to composition is observed. This difference between both behaviours may be attributed to size effect, band confinement, and surface anisotropy effects. On the other hand, the value of saturation magnetization is lower than the magnetization of bulk alloy for the same composition but the relative increment is larger than the bulk alloy. The low value of the magnetization might be due to the misaligned spins on surface.
0 0
20
40
60
80
100
% Fe Fig. 3. Saturation magnetization as a function of composition at room temperature for dispersed samples (open circles) and for the bulk alloy (dashed line) for Ref. [4].
observed in the bulk alloy, where a broad maximum of the magnetic moment with composition is present at 65% Fe [4]. Moreover, for the bulk material the saturation magnetization is larger than for the nanoparticles of the same composition. The lower values of MS obtained for the nanoparticles are due to the misalignment of the spins at the particle surface (caused by the low coordination between spins and the surface anisotropy influence). The sharp maximum observed might be related with size effects, band confinement, surface anisotropy, etc. The powder samples shows a similar behaviour with composition at room temperature. 4. Conclusions Magnetization measurements on 3 nm (Fex Co1x)0.6B0.4 (0pxp1) ferromagnetic amorphous nano-
Acknowledgement This work has been accomplished with partial support by CONICET-CONICIT (Argentina–Venezuela) cooperation project.
References [1] J.L. Dorman, D. Fiorani (Eds.), Magnetic Properties of Fine Particles, North-Holland, Amsterdam, 1992. [2] G.C. Hadjipanayis, G.A. Prinz (Eds.), Science and Technology of Nanostructurated Materials, Plenum Press, New York, 1991. [3] D. Fiorani (Ed.), Surface Effects in Magnetic Nanoparticles, Springer, Berlin, 2005. [4] S. Chikazumi, Physics of Magnetism, Krieger, Malabar, Florida, 1964 Chapter 4. [5] R.D. Zysler, C.A. Ramos, H. Romero, A. Ortega, J. Mater. Sci. 36 (9) (2001) 2291. [6] B. Molina Concha, R.D. Zysler, H. Troiani, H. Romero, Phys. B. 354 (2004) 121.