- Email: [email protected]
Temperature dependence of magnetic properties of Cu80Co19Ni1 thin microwires
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 316 (2007) e71–e73 www.elsevier.com/locate/jmmm
Temperature dependence of magnetic properties of Cu80Co19Ni1 thin microwires C. Garciaa,, A. Zhukova, V. Zhukovab, V. Larinb, J. Gonzaleza, J.J. del Vala, M. Knobelc a
Dpto. Fisica de Materiales, Fac. Quı´micas, UPV/EHU, 20018, San Sebastia´n, Spain TAMAG Ibe´rica S.L., Parque Tecnolo´gico de Miramo´n, Paseo Mikeletegi 56, 1a Planta, 20009 San Sebastia´n, Spain % c Instituto de Fisica ‘‘Gleb Watagin’’, Universidad Estadual de Campinas, Sao Paulo, Brazil
b
Available online 13 April 2007
Abstract In the present work, we report the studies of temperature dependence of magnetic properties in thin microwires with composition Cu80Co19Ni1. An extensive study of structural and magnetic characterization was realized. The structure was observed using X-ray diffraction with CuKa radiation. The magnetic measurements were carried out using a SQUID at temperatures between 5 and 300 K. The as-prepared Cu80Co19Ni1 microwire presents a coercivity of about 80 Oe. The variation of the coercivity and remanent magnetization at 5–300 K were obtained from the hysteresis loops. From the difference of the ZFC and FC curves below T ¼ 100 K, we can assume the presence of small superparamagnetic grains embedded in the Cu matrix. Those superparamagnetic grains should be blocked at temperatures below the maximum of the magnetization observed below 50 K. The measurements show an unusual temperature dependence of the coercive field, consequence of a coexistence of blocked and unblocked particles, and the typical decreasing behaviour of the remanence increasing temperature. r 2007 Published by Elsevier B.V. PACS: 75.50.Tt; 75.60.d; 75.75.+a Keywords: Microwire; Granular
1. Introduction Granular systems made of inmiscible (Co, Fe, Ni)–(Cu, Pt, Au, Ag) alloys gained special interest because of their unusual magnetic properties suitable for applications in magnetic recording and sensors, especially to the giant magneto-resistance effect (GMR) [1–3]. These systems can be generally obtained by different techniques such as sputtering, mechanical alloying, rapid solidification, etc. Recently, the Taylor–Ulitovsky technique [4,5] has been widely employed to obtain thin magnetic glass-coated microwires with mostly amorphous structure, achieved by the high-quenching rate due to the introduction of the molten alloy into a water jet during the drawing of the Corresponding author. Tel.: +34 943018611; fax: +34 943017130.
E-mail address: [email protected] (C. Garcia). 0304-8853/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.jmmm.2007.02.208
microwire. It is worth mentioning that in the last few years, this technique has been also employed to produce microwires with a granular structure [6,7] which opening the way to obtain novel magnetic and transport properties, such as GMR effect [2,3,6]. In this case, a metastable solid solution of immiscible elements has been obtained at room temperature through a rapid quenching. Particularly, the solubility of Co and Ni in Cu decreases as the temperature decreases and, at room temperature, it is quite small (less than 0.1% for Co). Consequently, these elements are almost immiscible at room temperature. Therefore, an unstable at room temperature single-phase solid solution decomposes after proper annealing. Fine crystalline structure containing mixture of small ferromagnetic particles in paramagnetic matrix obtained after such re-crystallization gives rise to the observed GMR.
ARTICLE IN PRESS e72
C. Garcia et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e71–e73
2. Experimental
3. Results and discussion Fig. 1 shows representative WAXS pattern for the sample studied showing clearly the existence of immiscible phases. The pattern corresponding to Cu80Co19Ni1 sample contains three phases [8]; the three peaks characteristic of the Cu phase (face-centered cubic FCC, lattice parameter: 3.61 A˚A), that correspond to atomic spacing of 2.09, 1.81 and 1.28 A˚, another three peaks characteristic of Co phase (hexagonal close packed HCP, lattice parameters; 2.51 and 4.07 A˚) with atomic spacing of 2.05, 1.77 and 1.25 A˚ and a third phase corresponding to Cu3.8Ni (with peaks associated to atomic spacing 2.08, 1.80, 1.27 A˚) is detected. Our sample shows only three raw peaks correspond to an overlapping of the three peaks of the Cu, Co and Cu3.8Ni and only a large maximum can easily appreciated with some shoulder in the best case. Pyrex contribution is observed at low angles with broad maximum at 2yffi22.
4000
I (a.u)
Cu-based microwire with composition Cu80Co19Ni1 has been studied to evaluate the influence of a different atom in the behavior of CuXCo100X. The master alloys of the different compositions were prepared by arc melting of the pure elements in Ar atmosphere. Subsequently, when the metallic alloy and the Pyrex glass coating were molten, it was drawn and rolled onto a rotating cylinder and quenched to room temperature. The samples obtained were in the form of a thin metallic wire, d ¼ 34 mm, covered by a thin layer of Pyrex glass, so the total diameter is D ¼ 48.6 mm. These geometrical characteristic were controlled by the optical microscopy. The structural characteristics of the sample were determined by wide X-ray scattering (WAXS) in a powder diffractometer provided with an automatic divergence slit and graphite monochromator CuKa radiation (l ¼ 1.54 A˚). The measurements were carried out using the step scanning technique between 5 and 90 (2y) is step of 0.05 with accumulation times of 5 s in each point. All magnetic measurements were performed using a Quantum Design physical property measurement system model 6000. The hysteresis loops M(H) of these samples were measured in the temperature range from 10 to 350 K and maximum field of 22 kOe. The temperature dependence of the coercive field and remanent magnetization were determined from the hysteresis loops. The saturation magnetization was determined by extrapolation of M(1/H) for 1/H ¼ 0, at 10 K. ZFC curves were measured cooling the system in zero magnetic field and measuring during warming with an external field applied. The FC curves were measured during the cooling procedure with field.
2000
0 20
40
60
80
2θ
Fig. 1. WAXS patterns for Cu80Co19Ni1 microwires.
Structural information for the microwires is extracted once the crystalline peak of each patterns is identified. The grain size of the crystals formed in is derived from Scherrer’s equation D¼
Kl , cos ym
where 2ym is the scattering angle corresponding to maximum of the peak and e is its half height width [8]. The parameter K is assumed to be around 0.9 or closer to the unidity in certain cases [9]. Calculations show the existence of nanocrystalline grains of around 30 nm in grain size. The distribution of particle sizes causes a temperature dependence of the coexistence of both blocked and superparamagnetic particles. The influence of super-paramagnetic particles on the coercive field was explicily taken into account by Kneller and Luborsky [10]. Nunes et al. [11] have developed a generalized model for the description of the thermal dependence of Hc of granular magnetic systems. The M(H) curve shown in Fig. 2(a) exhibits a super-paramagnetic shape. A more detailed analysis of the hystersis loops at low magnetic fields shows (Fig. 2(b)) that while the remanence decay with temperature, the coercive field at 50 K is smaller than at 10 and 300 K. The temperature dependence of the coercive field Hc(T) is shown in Fig. 3. The Cu80Co19Ni1 sample exhibit an unusual Hc(T) with a sharp decrease up to 75 K, followed by a continuous increasing. The maximum was not observed in our range or temperature. The interesting behavior of Hc can be understood in terms of the superparamagnetic susceptibility, which has two contributions [11]. Initially, Hc decreases with temperature due to the unblocking of the small particles. Then, thermal fluctuations leads to a decrease to an increase of Hc with T until the large particles start to unblock and HC(T) decrease again. The lack of a maximum in our temperature range and the f(TB) distribution (Fig. 4(b)) can be attributed to the small size of the most of the particles
ARTICLE IN PRESS C. Garcia et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e71–e73
300 K
0.2
a
0.0006 d(MZFC-FC)/dT (e.m.u./K)
0.018
a
0.016
M (emu)
M (emu)
0.014 0.0
0.012
-20000
-10000
0
20000
10000
H (Oe)
0.0004 0.0003 0.0002 0.0001 0.0000 0
0.010
50
100
150
200
250
300
T (K)
ZFC-FC
0.006 0.004 0
b
50
100
150
200
250
300
T (K)
10 K 0.02
M (emu)
b
0.0005
0.008
-0.2
50 K
Fig. 4. (a) Zero field cooled and fiels cooled curves for the Cu80Co19Ni1 as cast measured at HDC ¼ 100 Oe up to 300 K. (b) f(TN) distribution.
300 K 0.00
the Fig. 4(b) can be inferred the existence of a large number of small particles with a diameter less than 35 nm and just a few large particles.
-0.02
-200
-100
0 H (Oe)
100
200
4. Conclusions
Fig. 2. (a) Hysteresis loop for the Cu80Co19Ni1 as cast sample at room temperature. (b) A detail of the narrow hysteresis at different temperatures.
0.016 Hc Mr 80
0.012
70
0.010
60
0.008
References
0.006
50
0.004 40 0
50
100
150
200 T (K)
250
300
350
We present novel results of thermal dependence of magnetic properties in granular microwires. Observed dependences can be related to a coexistence of blocked and unblocked grains. Experimental results show the size distribution, where small particles has predominate contribution.
0.014
Mr (emu)
90
Hc (A/m)
e73
0.002 400
Fig. 3. Coercive field Hc vs. temperature and remanent magnetization Mr vs. temperature for the sample investigated.
and, probably, a few very large particles with a large unblocking temperature. We used ZFC/FC curves (Fig. 4(a)) as usually [12] to determine the average blocking temperature and distribution f(TB). The obtained f(TB) confirms inhomogeneous magnetic structure observed by the others authors. From
[1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creucet, A. Friederich, J. Chazuelas, Phys. Rev. Lett. 161 (1988) 2742. [2] A.E. Berkowitz, J.R. Mitchel, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett 68 (1992) 3745. [3] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 61 (1992) 2745. [4] G.F. Taylor, Phys. Rev. 23 (1924) 665. [5] A.V. Ulitovski, I.M. Maianski, A.I. Avramenko, Patent No. 128–427,USSR, March 9, 1956. [6] A. Zhukov, J. Gonzalez, V. Zhukova, J. Magn. Magn. Mater. 294 (2005) 165. [7] J.J. del Val, J. Gonzalez, A. Zhukov, Physica B 299 (2001) 242. [8] R. Jemkins, R.L. Snyder, Introduction to X-ray Powder Driffractometry, Wiley, New York, 1996. [9] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystaline and Amorphpus Materials, Wiley, New York, 1974. [10] E.F. Kneller, F.E. Luborsky, J. Appl. Phys. 34 (1963) 656. [11] W.C. Nunes, W.S.D. Folly, J.P. Sinnecker, M.A. Novak, Phys. Rev. B 70 (2004) 014419. [12] N. Peleg, S. Shtrikman, G. Gorodetsky, I. Felner, J. Magn. Magn. Mater. 191 (1999) 349.
Journal of Magnetism and Magnetic Materials 316 (2007) e71–e73 www.elsevier.com/locate/jmmm
Temperature dependence of magnetic properties of Cu80Co19Ni1 thin microwires C. Garciaa,, A. Zhukova, V. Zhukovab, V. Larinb, J. Gonzaleza, J.J. del Vala, M. Knobelc a
Dpto. Fisica de Materiales, Fac. Quı´micas, UPV/EHU, 20018, San Sebastia´n, Spain TAMAG Ibe´rica S.L., Parque Tecnolo´gico de Miramo´n, Paseo Mikeletegi 56, 1a Planta, 20009 San Sebastia´n, Spain % c Instituto de Fisica ‘‘Gleb Watagin’’, Universidad Estadual de Campinas, Sao Paulo, Brazil
b
Available online 13 April 2007
Abstract In the present work, we report the studies of temperature dependence of magnetic properties in thin microwires with composition Cu80Co19Ni1. An extensive study of structural and magnetic characterization was realized. The structure was observed using X-ray diffraction with CuKa radiation. The magnetic measurements were carried out using a SQUID at temperatures between 5 and 300 K. The as-prepared Cu80Co19Ni1 microwire presents a coercivity of about 80 Oe. The variation of the coercivity and remanent magnetization at 5–300 K were obtained from the hysteresis loops. From the difference of the ZFC and FC curves below T ¼ 100 K, we can assume the presence of small superparamagnetic grains embedded in the Cu matrix. Those superparamagnetic grains should be blocked at temperatures below the maximum of the magnetization observed below 50 K. The measurements show an unusual temperature dependence of the coercive field, consequence of a coexistence of blocked and unblocked particles, and the typical decreasing behaviour of the remanence increasing temperature. r 2007 Published by Elsevier B.V. PACS: 75.50.Tt; 75.60.d; 75.75.+a Keywords: Microwire; Granular
1. Introduction Granular systems made of inmiscible (Co, Fe, Ni)–(Cu, Pt, Au, Ag) alloys gained special interest because of their unusual magnetic properties suitable for applications in magnetic recording and sensors, especially to the giant magneto-resistance effect (GMR) [1–3]. These systems can be generally obtained by different techniques such as sputtering, mechanical alloying, rapid solidification, etc. Recently, the Taylor–Ulitovsky technique [4,5] has been widely employed to obtain thin magnetic glass-coated microwires with mostly amorphous structure, achieved by the high-quenching rate due to the introduction of the molten alloy into a water jet during the drawing of the Corresponding author. Tel.: +34 943018611; fax: +34 943017130.
E-mail address: [email protected] (C. Garcia). 0304-8853/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.jmmm.2007.02.208
microwire. It is worth mentioning that in the last few years, this technique has been also employed to produce microwires with a granular structure [6,7] which opening the way to obtain novel magnetic and transport properties, such as GMR effect [2,3,6]. In this case, a metastable solid solution of immiscible elements has been obtained at room temperature through a rapid quenching. Particularly, the solubility of Co and Ni in Cu decreases as the temperature decreases and, at room temperature, it is quite small (less than 0.1% for Co). Consequently, these elements are almost immiscible at room temperature. Therefore, an unstable at room temperature single-phase solid solution decomposes after proper annealing. Fine crystalline structure containing mixture of small ferromagnetic particles in paramagnetic matrix obtained after such re-crystallization gives rise to the observed GMR.
ARTICLE IN PRESS e72
C. Garcia et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e71–e73
2. Experimental
3. Results and discussion Fig. 1 shows representative WAXS pattern for the sample studied showing clearly the existence of immiscible phases. The pattern corresponding to Cu80Co19Ni1 sample contains three phases [8]; the three peaks characteristic of the Cu phase (face-centered cubic FCC, lattice parameter: 3.61 A˚A), that correspond to atomic spacing of 2.09, 1.81 and 1.28 A˚, another three peaks characteristic of Co phase (hexagonal close packed HCP, lattice parameters; 2.51 and 4.07 A˚) with atomic spacing of 2.05, 1.77 and 1.25 A˚ and a third phase corresponding to Cu3.8Ni (with peaks associated to atomic spacing 2.08, 1.80, 1.27 A˚) is detected. Our sample shows only three raw peaks correspond to an overlapping of the three peaks of the Cu, Co and Cu3.8Ni and only a large maximum can easily appreciated with some shoulder in the best case. Pyrex contribution is observed at low angles with broad maximum at 2yffi22.
4000
I (a.u)
Cu-based microwire with composition Cu80Co19Ni1 has been studied to evaluate the influence of a different atom in the behavior of CuXCo100X. The master alloys of the different compositions were prepared by arc melting of the pure elements in Ar atmosphere. Subsequently, when the metallic alloy and the Pyrex glass coating were molten, it was drawn and rolled onto a rotating cylinder and quenched to room temperature. The samples obtained were in the form of a thin metallic wire, d ¼ 34 mm, covered by a thin layer of Pyrex glass, so the total diameter is D ¼ 48.6 mm. These geometrical characteristic were controlled by the optical microscopy. The structural characteristics of the sample were determined by wide X-ray scattering (WAXS) in a powder diffractometer provided with an automatic divergence slit and graphite monochromator CuKa radiation (l ¼ 1.54 A˚). The measurements were carried out using the step scanning technique between 5 and 90 (2y) is step of 0.05 with accumulation times of 5 s in each point. All magnetic measurements were performed using a Quantum Design physical property measurement system model 6000. The hysteresis loops M(H) of these samples were measured in the temperature range from 10 to 350 K and maximum field of 22 kOe. The temperature dependence of the coercive field and remanent magnetization were determined from the hysteresis loops. The saturation magnetization was determined by extrapolation of M(1/H) for 1/H ¼ 0, at 10 K. ZFC curves were measured cooling the system in zero magnetic field and measuring during warming with an external field applied. The FC curves were measured during the cooling procedure with field.
2000
0 20
40
60
80
2θ
Fig. 1. WAXS patterns for Cu80Co19Ni1 microwires.
Structural information for the microwires is extracted once the crystalline peak of each patterns is identified. The grain size of the crystals formed in is derived from Scherrer’s equation D¼
Kl , cos ym
where 2ym is the scattering angle corresponding to maximum of the peak and e is its half height width [8]. The parameter K is assumed to be around 0.9 or closer to the unidity in certain cases [9]. Calculations show the existence of nanocrystalline grains of around 30 nm in grain size. The distribution of particle sizes causes a temperature dependence of the coexistence of both blocked and superparamagnetic particles. The influence of super-paramagnetic particles on the coercive field was explicily taken into account by Kneller and Luborsky [10]. Nunes et al. [11] have developed a generalized model for the description of the thermal dependence of Hc of granular magnetic systems. The M(H) curve shown in Fig. 2(a) exhibits a super-paramagnetic shape. A more detailed analysis of the hystersis loops at low magnetic fields shows (Fig. 2(b)) that while the remanence decay with temperature, the coercive field at 50 K is smaller than at 10 and 300 K. The temperature dependence of the coercive field Hc(T) is shown in Fig. 3. The Cu80Co19Ni1 sample exhibit an unusual Hc(T) with a sharp decrease up to 75 K, followed by a continuous increasing. The maximum was not observed in our range or temperature. The interesting behavior of Hc can be understood in terms of the superparamagnetic susceptibility, which has two contributions [11]. Initially, Hc decreases with temperature due to the unblocking of the small particles. Then, thermal fluctuations leads to a decrease to an increase of Hc with T until the large particles start to unblock and HC(T) decrease again. The lack of a maximum in our temperature range and the f(TB) distribution (Fig. 4(b)) can be attributed to the small size of the most of the particles
ARTICLE IN PRESS C. Garcia et al. / Journal of Magnetism and Magnetic Materials 316 (2007) e71–e73
300 K
0.2
a
0.0006 d(MZFC-FC)/dT (e.m.u./K)
0.018
a
0.016
M (emu)
M (emu)
0.014 0.0
0.012
-20000
-10000
0
20000
10000
H (Oe)
0.0004 0.0003 0.0002 0.0001 0.0000 0
0.010
50
100
150
200
250
300
T (K)
ZFC-FC
0.006 0.004 0
b
50
100
150
200
250
300
T (K)
10 K 0.02
M (emu)
b
0.0005
0.008
-0.2
50 K
Fig. 4. (a) Zero field cooled and fiels cooled curves for the Cu80Co19Ni1 as cast measured at HDC ¼ 100 Oe up to 300 K. (b) f(TN) distribution.
300 K 0.00
the Fig. 4(b) can be inferred the existence of a large number of small particles with a diameter less than 35 nm and just a few large particles.
-0.02
-200
-100
0 H (Oe)
100
200
4. Conclusions
Fig. 2. (a) Hysteresis loop for the Cu80Co19Ni1 as cast sample at room temperature. (b) A detail of the narrow hysteresis at different temperatures.
0.016 Hc Mr 80
0.012
70
0.010
60
0.008
References
0.006
50
0.004 40 0
50
100
150
200 T (K)
250
300
350
We present novel results of thermal dependence of magnetic properties in granular microwires. Observed dependences can be related to a coexistence of blocked and unblocked grains. Experimental results show the size distribution, where small particles has predominate contribution.
0.014
Mr (emu)
90
Hc (A/m)
e73
0.002 400
Fig. 3. Coercive field Hc vs. temperature and remanent magnetization Mr vs. temperature for the sample investigated.
and, probably, a few very large particles with a large unblocking temperature. We used ZFC/FC curves (Fig. 4(a)) as usually [12] to determine the average blocking temperature and distribution f(TB). The obtained f(TB) confirms inhomogeneous magnetic structure observed by the others authors. From
[1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creucet, A. Friederich, J. Chazuelas, Phys. Rev. Lett. 161 (1988) 2742. [2] A.E. Berkowitz, J.R. Mitchel, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett 68 (1992) 3745. [3] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 61 (1992) 2745. [4] G.F. Taylor, Phys. Rev. 23 (1924) 665. [5] A.V. Ulitovski, I.M. Maianski, A.I. Avramenko, Patent No. 128–427,USSR, March 9, 1956. [6] A. Zhukov, J. Gonzalez, V. Zhukova, J. Magn. Magn. Mater. 294 (2005) 165. [7] J.J. del Val, J. Gonzalez, A. Zhukov, Physica B 299 (2001) 242. [8] R. Jemkins, R.L. Snyder, Introduction to X-ray Powder Driffractometry, Wiley, New York, 1996. [9] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystaline and Amorphpus Materials, Wiley, New York, 1974. [10] E.F. Kneller, F.E. Luborsky, J. Appl. Phys. 34 (1963) 656. [11] W.C. Nunes, W.S.D. Folly, J.P. Sinnecker, M.A. Novak, Phys. Rev. B 70 (2004) 014419. [12] N. Peleg, S. Shtrikman, G. Gorodetsky, I. Felner, J. Magn. Magn. Mater. 191 (1999) 349.