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Effect of heat treatment on transport and magnetic properties of Co-based amorphous alloys
Journal of Non-Crystalline Solids 353 (2007) 869–871 www.elsevier.com/locate/jnoncrysol
Effect of heat treatment on transport and magnetic properties of Co-based amorphous alloys P.M. Sheverdyaeva a, V.N. Prudnikov a, N.S. Perov a, A.S. Konstantinova A.E. Yelsukova a, C.G. Kim b, A.B. Granovsky a b
a,*
,
a Faculty of Physics, MSU, Leninskie Gory, GSP-2, Moscow 119992, Russia Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
Available online 22 February 2007
Abstract Magnetotransport and magnetic properties of Co66Fe4B15Si15 amorphous ribbons, annealed in vacuum and in open air for various times, are studied. The air-annealed samples exhibit the presence of the near-surface hard magnetic layer. The anomalous Hall effect coefficient and the resistivity change simultaneously under heat treatment with slightly non-linear correlation between them. Positive magnetoresistance in low fields changes to negative magnetoresistance with increase of magnetic field. We explain both positive and negative magnetoresistance in terms of spin-dependent scattering. 2007 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 75.60.d Keywords: Alloys; Conductivity; Magnetic properties mechanical alloying; Crystal growth; X-ray diffraction; Magnetic properties; Nanocrystals; Calorimetry
1. Introduction
2. Experiment and discussion
As it has been shown recently [1,2], the air-annealed amorphous ribbons have inhomogeneous near-surface magnetic properties that are ascribed to the inhomogeneous nanocrystalline structure of the near-surface layer. The microstructure of the near-surface layer is very sensitive to the annealing conditions as it clearly seen from magneto-impedance measurements [3]. In this work we report on experimental investigation of the influence of this layer on magnetostatic and magnetotransport properties.
2.1. Magnetostatic properties
*
Corresponding author. Tel.: +7 4959391817; fax: +7 4959394787. E-mail address: [email protected] (A.S. Konstantinova).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.12.105
Magnetic properties were studied with the vibration sample magnetometer (VSM) at room temperatures. The shape of magnetization curves of vacuum-annealed ribbons does not change significantly with the increasing of the annealing temperature (Fig. 1(a)). On the other hand, the annealing in air influences the shape of curves very strongly that is due to surface oxidation. High annealing temperatures produce the high-field hysteresis part on the magnetization curves with the coercivity up to 300 Oe. The magnetic moment corresponding to this part of magnetization curve is about 10% of the magnetic moment of the sample (Fig. 1(b)). The change of the saturation magnetization MS during the annealing is difficult to reveal correctly because of the inhomogeneity of properties of the ribbon along its length
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P.M. Sheverdyaeva et al. / Journal of Non-Crystalline Solids 353 (2007) 869–871
Fig. 1. Magnetization curves: (a) vacuum-annealed, 200–450 C; (b) atmosphere-annealed, 450 C.
(more than 20% of mean value). The inhomogeneity increases with the annealing temperature, reaches maximum for the samples annealed at 350–380 C, and then decreases, also it was more pronounced for air-annealed samples. To achieve the most valid values all results were averaged over the 5 cm length of ribbons. 2.2. Magnetotransport properties Magnetotransport measurements were carried out at 77.4–300 K in magnetic fields up to 16.5 kOe. The electrical contacts were prepared by Wood solder. Samples were typically 10 · 2 · 2 · 103 cm3 in dimension. The Hall effect was measured using the 4-probe method. The anomalous Hall effect coefficient RS was determined from the field dependence of Hall resistivity qH = R0B + RSM, where R0 is the ordinary Hall effect coefficient. The electrical resistivity q and the anomalous Hall coefficient RS simultaneously change with the annealing temperature. As it follows from Fig. 2 the correlation between them is slightly non-linear, so no RS q neither RS q2 relations, typical for skew-scattering and sidejump mechanisms correspondingly, are not fulfilled for the samples under investigation. All samples show the metallic behavior q T.
Fig. 2. Hall coefficients (left axis) and resistance (right axis) vs. annealing temperature; (a) vacuum annealing; (b) atmosphere annealing.
The transverse magnetoresistance (MR) Dq?/q = (q?(H) q(0))/q(0) · 100% was studied by applying magnetic field perpendicular to the ribbon plane with the current oriented along the long axis of the sample. Measurements were carried out at 77.4 and 300 K. MR is extremely small at 300 K but is noticeable at 77.4 K (Fig. 3). The absolute value of MR is about 0.02–0.04% and increases with the annealing temperature from 0.03% to 0.04% for air-annealed, and from 0.02% to 0.035% for vacuum-annealed ribbons. The saturation field is about 0.5–1.0 kOe and it also increases, so the peak becomes wider. The MR is negative for all samples, but for the 350–400 C annealing temperature the positive MR is observed in low fields (Fig. 3). It appears for both airand vacuum-annealed at 350–400 C ribbons. To explain the MR behavior we propose the mechanism based on spin-dependent scattering. In fact, nanocrystallites appear in amorphous matrix as a result of annealing. Nanocrystallites are small enough to be considered as single-domain magnetic grains. Therefore the structure of annealed ribbons resembles that for all-metallic granular alloys, for example Co–Cu or Co–Ag, exhibiting giant negative MR [4]. By the contrast to magnetic granular alloys resistivity of nanocrystallites is smaller than that for amorphous matrix and the volume fraction of nanocrystallites is
P.M. Sheverdyaeva et al. / Journal of Non-Crystalline Solids 353 (2007) 869–871
871
samples MR is negative both in weak and strong magnetic fields. If it is not the case and the magnetic moments of adjacent grains are antiparallel at non-zero magnetic field then resistivity has to increase with magnetic field, to reach maximum value at the field of complete disorder of orientations of nanocrystallites magnetic moments and decrease. That exactly corresponds to observed positive MR behavior and allows us to explain qualitatively both negative and positive MR in the framework of spin-dependent scattering mechanism. It follows from this model that all these features has to be more pronounced in the samples annealed in air because of the presence in these samples high-coercive layer enriched with nanocrystalline grains (see [2] and discussion of magnetization curves above) in addition to the nanocrystallites in the bulk. It is also consistent with experiment. 3. Conclusions
Fig. 3. Transverse magnetoresistance at 77.4 K: (a) 380 C annealing; (b) 450 C annealing.
not so large. That is why the main requirements for giant negative MR [4] are not fulfilled in our case and the observed negative MR in ribbons is rather small. The volume fraction of nanocrystallites increases with annealing temperature and it leads to increase of negative MR that is in agreement with experiment. Spin-dependent scattering provides negative MR only if in the initial state the magnetic moments of adjacent grains are antiparallel in average. To reach this completely disorder state the annealing temperature has to be high enough that in our case corresponds to the samples annealed at 450 C and for these
The air-annealed samples exhibited the presence of the near-surface hard magnetic layer with volume up to 10% and coercive force up to 300 Oe. The anomalous Hall effect coefficients and electrical resistivities as a function of the annealing temperature were presented and founded to have similar dependence on the annealing conditions. A positive MR change, observed at low fields for both vacuum and air-annealed at 350–400 C samples, and negative MR at high fields probably is caused by growth of nanocrystalline grains in amorphous matrix, which gives rise to spin-dependent scattering. In spite this hypothesis allows us to explain qualitatively the obtained results more experimental evidence supporting the proposed model is required. References [1] E.E. Shalyguina, E.A. Ganshina, Young-Woo Rheem, Cheol Gi Kim, Chong-Oh Kim, Physica B 327 (2003) 300. [2] Y.W. Rheem, C.G. Kim, B.S. Lee, L. Jin, C.O. Kim, E.E. Shalyguina, E.A. Gan’shina, J. Appl. Phys. 93 (2003) 7214. [3] Y.W. Rheem, C.G. Kim, C.O. Kim, Y. Choi, Korean. J. Magn. 6 (2001) 86. [4] S. Zhang, P.M. Levy, J. Appl. Phys. 73 (1993) 5315.
Effect of heat treatment on transport and magnetic properties of Co-based amorphous alloys P.M. Sheverdyaeva a, V.N. Prudnikov a, N.S. Perov a, A.S. Konstantinova A.E. Yelsukova a, C.G. Kim b, A.B. Granovsky a b
a,*
,
a Faculty of Physics, MSU, Leninskie Gory, GSP-2, Moscow 119992, Russia Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Republic of Korea
Available online 22 February 2007
Abstract Magnetotransport and magnetic properties of Co66Fe4B15Si15 amorphous ribbons, annealed in vacuum and in open air for various times, are studied. The air-annealed samples exhibit the presence of the near-surface hard magnetic layer. The anomalous Hall effect coefficient and the resistivity change simultaneously under heat treatment with slightly non-linear correlation between them. Positive magnetoresistance in low fields changes to negative magnetoresistance with increase of magnetic field. We explain both positive and negative magnetoresistance in terms of spin-dependent scattering. 2007 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 75.60.d Keywords: Alloys; Conductivity; Magnetic properties mechanical alloying; Crystal growth; X-ray diffraction; Magnetic properties; Nanocrystals; Calorimetry
1. Introduction
2. Experiment and discussion
As it has been shown recently [1,2], the air-annealed amorphous ribbons have inhomogeneous near-surface magnetic properties that are ascribed to the inhomogeneous nanocrystalline structure of the near-surface layer. The microstructure of the near-surface layer is very sensitive to the annealing conditions as it clearly seen from magneto-impedance measurements [3]. In this work we report on experimental investigation of the influence of this layer on magnetostatic and magnetotransport properties.
2.1. Magnetostatic properties
*
Corresponding author. Tel.: +7 4959391817; fax: +7 4959394787. E-mail address: [email protected] (A.S. Konstantinova).
0022-3093/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.12.105
Magnetic properties were studied with the vibration sample magnetometer (VSM) at room temperatures. The shape of magnetization curves of vacuum-annealed ribbons does not change significantly with the increasing of the annealing temperature (Fig. 1(a)). On the other hand, the annealing in air influences the shape of curves very strongly that is due to surface oxidation. High annealing temperatures produce the high-field hysteresis part on the magnetization curves with the coercivity up to 300 Oe. The magnetic moment corresponding to this part of magnetization curve is about 10% of the magnetic moment of the sample (Fig. 1(b)). The change of the saturation magnetization MS during the annealing is difficult to reveal correctly because of the inhomogeneity of properties of the ribbon along its length
870
P.M. Sheverdyaeva et al. / Journal of Non-Crystalline Solids 353 (2007) 869–871
Fig. 1. Magnetization curves: (a) vacuum-annealed, 200–450 C; (b) atmosphere-annealed, 450 C.
(more than 20% of mean value). The inhomogeneity increases with the annealing temperature, reaches maximum for the samples annealed at 350–380 C, and then decreases, also it was more pronounced for air-annealed samples. To achieve the most valid values all results were averaged over the 5 cm length of ribbons. 2.2. Magnetotransport properties Magnetotransport measurements were carried out at 77.4–300 K in magnetic fields up to 16.5 kOe. The electrical contacts were prepared by Wood solder. Samples were typically 10 · 2 · 2 · 103 cm3 in dimension. The Hall effect was measured using the 4-probe method. The anomalous Hall effect coefficient RS was determined from the field dependence of Hall resistivity qH = R0B + RSM, where R0 is the ordinary Hall effect coefficient. The electrical resistivity q and the anomalous Hall coefficient RS simultaneously change with the annealing temperature. As it follows from Fig. 2 the correlation between them is slightly non-linear, so no RS q neither RS q2 relations, typical for skew-scattering and sidejump mechanisms correspondingly, are not fulfilled for the samples under investigation. All samples show the metallic behavior q T.
Fig. 2. Hall coefficients (left axis) and resistance (right axis) vs. annealing temperature; (a) vacuum annealing; (b) atmosphere annealing.
The transverse magnetoresistance (MR) Dq?/q = (q?(H) q(0))/q(0) · 100% was studied by applying magnetic field perpendicular to the ribbon plane with the current oriented along the long axis of the sample. Measurements were carried out at 77.4 and 300 K. MR is extremely small at 300 K but is noticeable at 77.4 K (Fig. 3). The absolute value of MR is about 0.02–0.04% and increases with the annealing temperature from 0.03% to 0.04% for air-annealed, and from 0.02% to 0.035% for vacuum-annealed ribbons. The saturation field is about 0.5–1.0 kOe and it also increases, so the peak becomes wider. The MR is negative for all samples, but for the 350–400 C annealing temperature the positive MR is observed in low fields (Fig. 3). It appears for both airand vacuum-annealed at 350–400 C ribbons. To explain the MR behavior we propose the mechanism based on spin-dependent scattering. In fact, nanocrystallites appear in amorphous matrix as a result of annealing. Nanocrystallites are small enough to be considered as single-domain magnetic grains. Therefore the structure of annealed ribbons resembles that for all-metallic granular alloys, for example Co–Cu or Co–Ag, exhibiting giant negative MR [4]. By the contrast to magnetic granular alloys resistivity of nanocrystallites is smaller than that for amorphous matrix and the volume fraction of nanocrystallites is
P.M. Sheverdyaeva et al. / Journal of Non-Crystalline Solids 353 (2007) 869–871
871
samples MR is negative both in weak and strong magnetic fields. If it is not the case and the magnetic moments of adjacent grains are antiparallel at non-zero magnetic field then resistivity has to increase with magnetic field, to reach maximum value at the field of complete disorder of orientations of nanocrystallites magnetic moments and decrease. That exactly corresponds to observed positive MR behavior and allows us to explain qualitatively both negative and positive MR in the framework of spin-dependent scattering mechanism. It follows from this model that all these features has to be more pronounced in the samples annealed in air because of the presence in these samples high-coercive layer enriched with nanocrystalline grains (see [2] and discussion of magnetization curves above) in addition to the nanocrystallites in the bulk. It is also consistent with experiment. 3. Conclusions
Fig. 3. Transverse magnetoresistance at 77.4 K: (a) 380 C annealing; (b) 450 C annealing.
not so large. That is why the main requirements for giant negative MR [4] are not fulfilled in our case and the observed negative MR in ribbons is rather small. The volume fraction of nanocrystallites increases with annealing temperature and it leads to increase of negative MR that is in agreement with experiment. Spin-dependent scattering provides negative MR only if in the initial state the magnetic moments of adjacent grains are antiparallel in average. To reach this completely disorder state the annealing temperature has to be high enough that in our case corresponds to the samples annealed at 450 C and for these
The air-annealed samples exhibited the presence of the near-surface hard magnetic layer with volume up to 10% and coercive force up to 300 Oe. The anomalous Hall effect coefficients and electrical resistivities as a function of the annealing temperature were presented and founded to have similar dependence on the annealing conditions. A positive MR change, observed at low fields for both vacuum and air-annealed at 350–400 C samples, and negative MR at high fields probably is caused by growth of nanocrystalline grains in amorphous matrix, which gives rise to spin-dependent scattering. In spite this hypothesis allows us to explain qualitatively the obtained results more experimental evidence supporting the proposed model is required. References [1] E.E. Shalyguina, E.A. Ganshina, Young-Woo Rheem, Cheol Gi Kim, Chong-Oh Kim, Physica B 327 (2003) 300. [2] Y.W. Rheem, C.G. Kim, B.S. Lee, L. Jin, C.O. Kim, E.E. Shalyguina, E.A. Gan’shina, J. Appl. Phys. 93 (2003) 7214. [3] Y.W. Rheem, C.G. Kim, C.O. Kim, Y. Choi, Korean. J. Magn. 6 (2001) 86. [4] S. Zhang, P.M. Levy, J. Appl. Phys. 73 (1993) 5315.