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Magnetic and structural properties of magnetoresistive FexAu100−x alloys produced by mechanical alloying
Physica B 320 (2002) 149–152
Magnetic and structural properties of magnetoresistive FexAu100x alloys produced by mechanical alloying L.M. Socolovskya,*, F.H. Sa! ncheza, P.H. Shingub a
Depto. de F!ısica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC67, 1900 La Plata, Argentina b Department of Energy Science and Engineering, Kyoto University, Japan
Abstract FexAu100x alloys have been produced for the first time by mechanical alloying. X-ray diffractograms show FCC peaks. From the X-ray diffracion peak-widths we estimate the final grain size, which vary with x from 112 nm (for x ¼ 15) to 32 nm (for x ¼ 30). Lattice parameter decreases with concentration (minimum 0.401 nm at x ¼ 30), but above Vegard’s law values. Susceptibility measurements show cluster-glass behaviour. Critical temperatures are consistently lower than similar alloys produced by arc melting followed by fast quenching. A magnetic phase diagram is presented. Giant magnetoresistance is present in all samples, with a maximum at x ¼ 25: This effect is caused by the dispersion of small iron clusters produced by the mechanical work. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Fe–Au; Giant magnetoresistance; Mechanical alloying
Fe–Au alloys have attracted attention for the last 70 years. Several works concerning magnetic and transport properties were studied in this system, in which Fe was diluted in Au [1]. Dilute magnetic alloys were obtained by quenching from the liquid state. High-temperature annealing favours Fe miscibility in the Au matrix (more than 60 at% at 10001C), therefore, fast quenching retains high-temperature randomness of iron atoms distributions [2]. Granular Au–Fe alloys prepared by non-equilibrium techniques show giant magnetoresistivity effects [1], a negative, isotropic change of the electrical resistivity when a magnetic field is applied [3]. This property has not *Corresponding author. Tel.: +54-221-424-6062; fax: +54221-425-2006. E-mail address: leandro@venus.fisica.unlp.edu.ar (L.M. Socolovsky).
been investigated yet in Au–Fe alloys prepared by mechanical alloying (MA). MA has proven its ability to produce metastable materials such as Fe–Cu alloys. In this set of techniques, cold working would produce ‘‘atom pumping’’ which forces elements to mix [4]. We have used one of the MA techniques: ball milling, in which powders of the constitutive elements of the alloy are ground together using hardened stainless-steel balls in a closed vial. The vial is inserted into the milling device which shakes it at a given frequency. Relevant parameters in this technique are frequency, milling time, and ball-to-powder mass ratio. FexAu100x blends of pure elements (Au purity >99.9% and 100 mesh, Fe purity=99.9% and 200 mesh) with x ¼ 15; 20, 25, and 30 were loaded into a closed vial under Ar atmosphere, and ground together in a water-refrigerated vibratory
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 6 6 0 - 9
L.M. Socolovsky et al. / Physica B 320 (2002) 149–152
0.410
Lattice parameter [nm]
Pure Au 0.405
0.400
0.395
15
20
x
25
30
Fig. 1. Lattice parameter a vs. iron concentration (black circles). Vegard’s law line was plotted using a ¼ 0:40782 nm for Au and a ¼ 0:36467 nm for g-Fe.
0.5 0.0 -0.5 -1.0
∆ρ/ρ [%]
mill Nissin giken NEV MA8. Powder-to-ball mass ratio was 1:25. Small amount (0.3 cm3) of methanol was added in order to avoid excessive sticking to milling tools. Milling time was 20 h. X-ray diffractograms were taken with a Phillips PW1170 diffractometer. Susceptibility was measured in a LakeShore 7130 susceptometer, using different driving frecuencies from 5 to 9920 Hz. Magnetoresistivity was measured using the conventional four probe configuration, with the magnetic field applied perpendicular to the current. X-ray diffractograms show peaks only in positions near the reflections of pure Au. There is neither evidence of carbides nor BCC-Fe. Positions of all peaks were used to calculate lattice parameter a: Fig. 1 shows that a falls with Fe concentration, but remains always above Vegard’s law line. These results suggest that Fe atoms disperse substitutionally within the FCC gold lattice, but not in a complete random way. Final average grain sizes (calculated by the integral breadth method [5]) decrease with Fe concentration, from 112 nm for x ¼ 15 to 32 nm for x ¼ 30: Resultant powders were pressed into small ingots for transport measurements. Magnetoresistivity measurements done at low temperature (77 K) show GMR effect for all samples (Fig. 2). No saturation is reached even at the maximum applied field (1.4 T). In-phase susceptibility measurements show a changing behaviour with concentration (see
-1.5 -2.0
15 20 25 30
-2.5 -3.0 -3.5 -4.0 -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Applied field [T]
Fig. 2. Magnetoresistivity measured at 77 K. Numbers indicate iron concentration.
15
0.0125
0.0100
χ ' [emu / Oe.gFe]
150
ω = 375 Hz 1 Oe
0.0075
20
0.02
ω = 375 Hz 1 Oe
0.01 0
50
100 150 200 250 300
0.20
25
0.15 0.10
ω = 825 Hz 1 Oe
0.05 0.00
0.03
0
50
100 150 200 250 300
0 0.20
50
100 150 200 250 300
30
0.15 0.10
ω = 825 Hz 1 Oe
0.05 0.00
0
50
100 150 200 250 300
T [K ]
Fig. 3. In-phase susceptibility. Iron concentration, driving frequency and field are indicated. Arrows indicate Tc (see text).
Fig. 3). For x ¼ 15 and 20, spin-glass-like cusps are clearly seen, located at 38 and 46 K, respectively. For x ¼ 25 a flattened cusp appears. In the region To50 K w0 behaves as the lower concentrations samples, with same convexity. This sample seems to be a transitional state between spin-glass type to a percolated mictomagnetic one [6]. In the region To50 K, Fe30Au70 displays a convexity different from the formers, which is seen as a ‘‘shoulder’’. A peak is located at 197 K, then, w0 decreases very sharply. The plateau centred at about 80 K for x ¼ 25 was not atributed to demagnetization effects since a peak occurring at
L.M. Socolovsky et al. / Physica B 320 (2002) 149–152
W ¼ DTc =½Tc D logðoÞ; which establishes a criterion for distinguishing between spin-glass or superparamagnetic behaviours. In all samples W was lower than 0.009, as is the case of canonical spin glasses [8]. In Fig. 4 a T2x phase diagram is presented, together with results compiled by Coles and Sarkissian [6]. Our Tc line is located under the critical line of other
350 300
Tf Sarkissian Tc Sarkissian Tc by ME Tc - this work
250
Tc [K]
similar susceptibility values could be observed under similar geometric conditions for x ¼ 30: Mechanical alloying causes iron atoms to disperse coherently in the Au matrix, shrinking in this way the lattice to a value lower than the parameter of pure Au, but higher than a simple dilution as indicated by the Vegard’s law. Positive heat of mixing of the Fe–Au system [7] may be the cause for this: it promotes the clustering of Fe. Final grain sizes fall below 150 nm. All samples show giant magnetoresistance effect, which indicates that iron is heterogeneously dispersed into the Au matrix forming magnetic clusters. Iron-rich clusters are randomly distributed in the grains and would have smaller sizes than these, so they would become magnetic monodomains. Superparamagnetic behaviour is expected. In-phase susceptibility behaves as in a mictomagnetic system, in which magnetic moments are randomly dispersed in a non-magnetic matrix. Very diluted Au–Fe alloys are canonical spin glasses, forming a system in which RKKY interaction dominates. With increasing iron content, magnetic atoms become clustered and each cluster acquires a macroscopic spin. Clusters interact with their neighbours in different ways when iron concentration increases. At the so-called ‘‘percolation threshold’’, the dominant magnetic interaction is not RKKY, but dipolar or exchange, and the w0 features of the samples can be explained. In this understanding, we have calculated the critical temperatures Tc from our data. For x ¼ 15 and 20, we have taken the temperature of the maximum (Tg ; glassy temperature in the literature) as Tc ; and for x ¼ 25 and 30 the inflection point of the right side of the curve. These critical temperatures slightly change with the driving frecuency o: This change can be described by the quantity
151
200
PM
150 FM-like
100 50 SG
0 0
5
10
15
20
25
30
x Fig. 4. Magnetic phase diagram. Critical temperatures defines . three zones. Tc obtained by Mossbauer (ME) were taken from Ref. [6].
authors, except for x ¼ 15 which coincides. The upper line indicates a change of phase, from ferromagnetic-like or percolated mictomagnetic to paramagnetic. This could be explained by assuming that the effect of mechanical alloying is to create iron-rich zones. In that way our alloy has less randomness than those produced by fast quenching, better defined clusters are formed, and there are less iron atoms between clusters. So that, magnetic connections between the clusters are scarce, producing a shift of the percolation threshold. These clusters are responsible for the giant magnetoresisitive effect. Percolation is surpassed at x ¼ 30: At this concentration, some clusters remain to produce GMR effect. Mechanical alloying produces a dispersion of FCC iron-rich clusters. These clusters produce GMR effect in the whole concentration range surveyed. Mictomagnetic behaviour is seen in these samples. Critical temperatures appears shifted, compared with those produced by arc melting.
References [1] A.K. Nigam, A.K. Majumdar, Phys. Rev. B 27 (1) (1983) 495; N.M. Nakhimovitch, J. Phys. USSR 5 (1941) 141. [2] A.P. Murani, J. Phys.: Metal Phys. 4 (1974) 757.
152
L.M. Socolovsky et al. / Physica B 320 (2002) 149–152
[3] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 68 (1992) 3749. [4] R.B. Schwarz, Mater. Sci. Forum 269–272 (1998) 665. [5] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures, Wiley, New York, 1974, p. 661. [6] B.V.B. Sarkissian, J. Phys. F 208 (1981) 2191.
[7] F.R. de Boer, R. Boom, W. Mattens, A.R. Miedema, A. Niessen, Cohesion in Metals: Transition Metal Alloys, North-Holland, Amsterdam, 1989, p. 240. [8] J.A. Mydosh, Spin Glasses, Taylor & Francis, London, 1993 (Chapter 3); G.F. Goya, H.R. Rechenberg, M.R. Ibarra, Presentation to the ISMANAM 2001.
Magnetic and structural properties of magnetoresistive FexAu100x alloys produced by mechanical alloying L.M. Socolovskya,*, F.H. Sa! ncheza, P.H. Shingub a
Depto. de F!ısica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CC67, 1900 La Plata, Argentina b Department of Energy Science and Engineering, Kyoto University, Japan
Abstract FexAu100x alloys have been produced for the first time by mechanical alloying. X-ray diffractograms show FCC peaks. From the X-ray diffracion peak-widths we estimate the final grain size, which vary with x from 112 nm (for x ¼ 15) to 32 nm (for x ¼ 30). Lattice parameter decreases with concentration (minimum 0.401 nm at x ¼ 30), but above Vegard’s law values. Susceptibility measurements show cluster-glass behaviour. Critical temperatures are consistently lower than similar alloys produced by arc melting followed by fast quenching. A magnetic phase diagram is presented. Giant magnetoresistance is present in all samples, with a maximum at x ¼ 25: This effect is caused by the dispersion of small iron clusters produced by the mechanical work. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Fe–Au; Giant magnetoresistance; Mechanical alloying
Fe–Au alloys have attracted attention for the last 70 years. Several works concerning magnetic and transport properties were studied in this system, in which Fe was diluted in Au [1]. Dilute magnetic alloys were obtained by quenching from the liquid state. High-temperature annealing favours Fe miscibility in the Au matrix (more than 60 at% at 10001C), therefore, fast quenching retains high-temperature randomness of iron atoms distributions [2]. Granular Au–Fe alloys prepared by non-equilibrium techniques show giant magnetoresistivity effects [1], a negative, isotropic change of the electrical resistivity when a magnetic field is applied [3]. This property has not *Corresponding author. Tel.: +54-221-424-6062; fax: +54221-425-2006. E-mail address: leandro@venus.fisica.unlp.edu.ar (L.M. Socolovsky).
been investigated yet in Au–Fe alloys prepared by mechanical alloying (MA). MA has proven its ability to produce metastable materials such as Fe–Cu alloys. In this set of techniques, cold working would produce ‘‘atom pumping’’ which forces elements to mix [4]. We have used one of the MA techniques: ball milling, in which powders of the constitutive elements of the alloy are ground together using hardened stainless-steel balls in a closed vial. The vial is inserted into the milling device which shakes it at a given frequency. Relevant parameters in this technique are frequency, milling time, and ball-to-powder mass ratio. FexAu100x blends of pure elements (Au purity >99.9% and 100 mesh, Fe purity=99.9% and 200 mesh) with x ¼ 15; 20, 25, and 30 were loaded into a closed vial under Ar atmosphere, and ground together in a water-refrigerated vibratory
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 0 6 6 0 - 9
L.M. Socolovsky et al. / Physica B 320 (2002) 149–152
0.410
Lattice parameter [nm]
Pure Au 0.405
0.400
0.395
15
20
x
25
30
Fig. 1. Lattice parameter a vs. iron concentration (black circles). Vegard’s law line was plotted using a ¼ 0:40782 nm for Au and a ¼ 0:36467 nm for g-Fe.
0.5 0.0 -0.5 -1.0
∆ρ/ρ [%]
mill Nissin giken NEV MA8. Powder-to-ball mass ratio was 1:25. Small amount (0.3 cm3) of methanol was added in order to avoid excessive sticking to milling tools. Milling time was 20 h. X-ray diffractograms were taken with a Phillips PW1170 diffractometer. Susceptibility was measured in a LakeShore 7130 susceptometer, using different driving frecuencies from 5 to 9920 Hz. Magnetoresistivity was measured using the conventional four probe configuration, with the magnetic field applied perpendicular to the current. X-ray diffractograms show peaks only in positions near the reflections of pure Au. There is neither evidence of carbides nor BCC-Fe. Positions of all peaks were used to calculate lattice parameter a: Fig. 1 shows that a falls with Fe concentration, but remains always above Vegard’s law line. These results suggest that Fe atoms disperse substitutionally within the FCC gold lattice, but not in a complete random way. Final average grain sizes (calculated by the integral breadth method [5]) decrease with Fe concentration, from 112 nm for x ¼ 15 to 32 nm for x ¼ 30: Resultant powders were pressed into small ingots for transport measurements. Magnetoresistivity measurements done at low temperature (77 K) show GMR effect for all samples (Fig. 2). No saturation is reached even at the maximum applied field (1.4 T). In-phase susceptibility measurements show a changing behaviour with concentration (see
-1.5 -2.0
15 20 25 30
-2.5 -3.0 -3.5 -4.0 -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Applied field [T]
Fig. 2. Magnetoresistivity measured at 77 K. Numbers indicate iron concentration.
15
0.0125
0.0100
χ ' [emu / Oe.gFe]
150
ω = 375 Hz 1 Oe
0.0075
20
0.02
ω = 375 Hz 1 Oe
0.01 0
50
100 150 200 250 300
0.20
25
0.15 0.10
ω = 825 Hz 1 Oe
0.05 0.00
0.03
0
50
100 150 200 250 300
0 0.20
50
100 150 200 250 300
30
0.15 0.10
ω = 825 Hz 1 Oe
0.05 0.00
0
50
100 150 200 250 300
T [K ]
Fig. 3. In-phase susceptibility. Iron concentration, driving frequency and field are indicated. Arrows indicate Tc (see text).
Fig. 3). For x ¼ 15 and 20, spin-glass-like cusps are clearly seen, located at 38 and 46 K, respectively. For x ¼ 25 a flattened cusp appears. In the region To50 K w0 behaves as the lower concentrations samples, with same convexity. This sample seems to be a transitional state between spin-glass type to a percolated mictomagnetic one [6]. In the region To50 K, Fe30Au70 displays a convexity different from the formers, which is seen as a ‘‘shoulder’’. A peak is located at 197 K, then, w0 decreases very sharply. The plateau centred at about 80 K for x ¼ 25 was not atributed to demagnetization effects since a peak occurring at
L.M. Socolovsky et al. / Physica B 320 (2002) 149–152
W ¼ DTc =½Tc D logðoÞ; which establishes a criterion for distinguishing between spin-glass or superparamagnetic behaviours. In all samples W was lower than 0.009, as is the case of canonical spin glasses [8]. In Fig. 4 a T2x phase diagram is presented, together with results compiled by Coles and Sarkissian [6]. Our Tc line is located under the critical line of other
350 300
Tf Sarkissian Tc Sarkissian Tc by ME Tc - this work
250
Tc [K]
similar susceptibility values could be observed under similar geometric conditions for x ¼ 30: Mechanical alloying causes iron atoms to disperse coherently in the Au matrix, shrinking in this way the lattice to a value lower than the parameter of pure Au, but higher than a simple dilution as indicated by the Vegard’s law. Positive heat of mixing of the Fe–Au system [7] may be the cause for this: it promotes the clustering of Fe. Final grain sizes fall below 150 nm. All samples show giant magnetoresistance effect, which indicates that iron is heterogeneously dispersed into the Au matrix forming magnetic clusters. Iron-rich clusters are randomly distributed in the grains and would have smaller sizes than these, so they would become magnetic monodomains. Superparamagnetic behaviour is expected. In-phase susceptibility behaves as in a mictomagnetic system, in which magnetic moments are randomly dispersed in a non-magnetic matrix. Very diluted Au–Fe alloys are canonical spin glasses, forming a system in which RKKY interaction dominates. With increasing iron content, magnetic atoms become clustered and each cluster acquires a macroscopic spin. Clusters interact with their neighbours in different ways when iron concentration increases. At the so-called ‘‘percolation threshold’’, the dominant magnetic interaction is not RKKY, but dipolar or exchange, and the w0 features of the samples can be explained. In this understanding, we have calculated the critical temperatures Tc from our data. For x ¼ 15 and 20, we have taken the temperature of the maximum (Tg ; glassy temperature in the literature) as Tc ; and for x ¼ 25 and 30 the inflection point of the right side of the curve. These critical temperatures slightly change with the driving frecuency o: This change can be described by the quantity
151
200
PM
150 FM-like
100 50 SG
0 0
5
10
15
20
25
30
x Fig. 4. Magnetic phase diagram. Critical temperatures defines . three zones. Tc obtained by Mossbauer (ME) were taken from Ref. [6].
authors, except for x ¼ 15 which coincides. The upper line indicates a change of phase, from ferromagnetic-like or percolated mictomagnetic to paramagnetic. This could be explained by assuming that the effect of mechanical alloying is to create iron-rich zones. In that way our alloy has less randomness than those produced by fast quenching, better defined clusters are formed, and there are less iron atoms between clusters. So that, magnetic connections between the clusters are scarce, producing a shift of the percolation threshold. These clusters are responsible for the giant magnetoresisitive effect. Percolation is surpassed at x ¼ 30: At this concentration, some clusters remain to produce GMR effect. Mechanical alloying produces a dispersion of FCC iron-rich clusters. These clusters produce GMR effect in the whole concentration range surveyed. Mictomagnetic behaviour is seen in these samples. Critical temperatures appears shifted, compared with those produced by arc melting.
References [1] A.K. Nigam, A.K. Majumdar, Phys. Rev. B 27 (1) (1983) 495; N.M. Nakhimovitch, J. Phys. USSR 5 (1941) 141. [2] A.P. Murani, J. Phys.: Metal Phys. 4 (1974) 757.
152
L.M. Socolovsky et al. / Physica B 320 (2002) 149–152
[3] J.Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 68 (1992) 3749. [4] R.B. Schwarz, Mater. Sci. Forum 269–272 (1998) 665. [5] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures, Wiley, New York, 1974, p. 661. [6] B.V.B. Sarkissian, J. Phys. F 208 (1981) 2191.
[7] F.R. de Boer, R. Boom, W. Mattens, A.R. Miedema, A. Niessen, Cohesion in Metals: Transition Metal Alloys, North-Holland, Amsterdam, 1989, p. 240. [8] J.A. Mydosh, Spin Glasses, Taylor & Francis, London, 1993 (Chapter 3); G.F. Goya, H.R. Rechenberg, M.R. Ibarra, Presentation to the ISMANAM 2001.