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The magnetic behaviour of an Fe-substituted Co—Ga alloy
Solid State Communications, Vol. 33, pp. 903-905. Pergamon Press Ltd. 1980. Printed in Great Britain. THE MAGNETIC BEHAVIOUR OF AN Fe-SUBSTITUTED Co-Ga ALLOY G.L. Whittle and P.E. Clark Department of Physics, Monash University, Clayton, Victoria, 3168, Australia
(Received 15 August 1979 by C. W. McCombie) The results of M6ssbauer experiments at low temperatures and low field d.c. magnetisation measurements are presented for the alloy Co2 Gal.9SFeo.0s. The presence of "frozen" magnetic clusters with different relaxation rates is verified. The magnetic behaviour of the alloy is best explained by a superparamagnetic model where the magnetic clusters freeze with a range of blocking temperatures. AT THE EQUIATOMIC COMPOSITION Co and Ga form a/3-phase compound with the B2 (CsC1) structure, which may be considered as consisting of two interpenetrating simple cubic sublattices, one occupied solely by Co atoms and the other by Ga. Small angle diffuse neutron scattering measurements on Co-Ga alloys [I] ( 5 2 - 6 0 at.% Co) confirm the presence of superparamagnetic assemblies formed by the magnetic clustering of moment bearing Co atoms on the Ga sublattice. It is believed that the percolation of these clusters leads to the onset of long range ferromagnetic order, although above the critical concentration some superparamagnetic assemblies remain uncoupled to behave as large independent spins. Recently Grover et al. [2] have concluded from NMR and a.c. susceptibility studies that Co-Ga alloys ( 5 0 - 6 0 at.% Co) belong to the cluster spin glass category similar to the typical cases AuFe and CuMn. Spin glass systems are well known for their unique characteristic freezing temperature, Tr, and corresponding sharp peak in magnetisation. However low field d.c. magnetisation measurements [3] on a Cos4Ga46 alloy tend to refute this idea in favour of a classical superparamagnetic model, where a range of cluster blocking temperatures exist. The Co (Ga, Fe) series of alloys, formed by partial substitution of Fe for Ga in the equiatomic CoGa, has been shown to be magnetically similar to the Co-Ga system [4], superparamagnetic clustering being related in this case to Fe substitutional atoms on Ga sites. In an attempt to clarify the conflicting ideas on the magnetic order in Co-Ga we have therefore performed M6ssbauer experiments at low temperatures and low field d.c. magnetisation measurements on Co2 Gal.gSFeo.os. This alloy is known to be below the percolation limit and therefore displays no long range ferromagnetic order. For the magnetisation measurements a 5 gm
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VeLocity (mm/sec) Fig. 1. M6ssbauer spectra of Co2 Gal.9s Feo.os in the temperature range 4.2-19 K in zero external field. ingot was prepared by melting the appropriate weights of 5N constituent metals in an argon arc furnace. A spherical magnetisation sample was formed by 903
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THE MAGNETIC BEHAVIOUR OF AN Fe-SUBSTITUTED C o - G a ALLOY
Vol. 33, No. 8
a.
T :/-,. 2 K
b
T= 190 K
05 O
n-
~- 0 t
100
,
,
,
,
I
,
~
,
,
I
,
200 300 HYPERFINE FIELD. H (KOe)
Fig. 2. The distribution of hyperfine fields in the 4.2 K spectrum. remelting part of this master alloy. For the M6ssbauer sample, iron enriched to 90% in STFe was melted with Co and Ga in a 4 mm quartz ampoule. Both resulting ingots were heat treated at 830°C for 24 hr followed by a water quench. Weight losses were less than 0.1%. M6ssbauer absorbers were made by pressing a calculated optimum amount of the powdered alloy with the binder benzophenone. M6ssbauer spectra were obtained using a STCo in Rh M6ssbauer source at 4.2 K, and driven by a conventional constant acceleration drive system. The various absorber temperatures used were maintained to an accuracy of 0.1 K. Themagnetic measurements were made using a modified Foner balance system [5]. M6ssbauer spectra were recorded in the temperature range 4.2-300 K. The spectra shown in Fig. 1 are representative of the behaviour observed. The solid lines through the data have been obtained from a computer fitting procedure based on a programme developed by Window [6]. This method is applicable to systems displaying a range of hyperFme fields, and produces the corresponding hyperf'me field distribution [P(/-/) curve] based on the fit. Over the temperature range shown the spectrum changes from a broad magneticaUy split six-line pattern at 4.2 K to an apparent singlet at about 19 K. The spectrum is unchanged from 19 K to room temperature, except for the temperature dependent second .order Doppler shift. A low velocity spectrum taken at room temperature reveals the presence of more than one Fe site in the apparent single line spectrum. The different Fe near neighbour environments appear to arise due to vacancies, known to exist on the Co sublattice [7] and antistructure Fe atoms occupying Co sites instead of the normal Ga. Since the Fe atoms occupy mostly Ga sites, any antistructure Co atoms will have almost no influence on the spectrum. The collapse of the hyperf'me field is typical of many magnetically ordered systems, the apparent ferromagnetic transition occuring at 19 + 1 K for
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Ve(ocity (mm/sec) Fig. 3. MOssbauer spectrum of Co; Gal.9s Feo.os at (a) 4.2 K (b) 19 K in the presence of a 25 kOe external
magnetic field. this alloy. It is worthwhile noting that although Co2 Gal.95 Feo.os is well below the percolation limit the M6ssbauer measurements are clearly revealing some magnetic ordering of the spins at temperatures up to 19 K. In a system with a distribution of magnetic clusters, a wide range of anisotropies, and therefore cluster relaxation rates may exist. The broadness of the lines in the magnetically split spectra are believed to be indicative of a distribution of cluster relaxation rates present in the alloy. Different relaxation rates produce different hyperfine fields in the spectra. These rates are comparable to 10-Tsec, the characteristic time for a M6ssbauer measurement, so that the observed hyperfine fields are intermediate between H~t (the long relaxation time limit) and ((Sz)/S)H~t (the very short relaxation time limit). The distribution of hyperfine fields in the 4.2 K spectrum (Fig. 2) shows a main peak centred at about 215 kOe. A spectrum recorded at 4.2 K in the presence of an external 25 kOe magnetic field shows negligible change from the zero field spectrum, which suggests a "freezing" of clusters in random orientations. There is no visible line broadening or decrease in hyperfine field but the intensities of the second and fifth peaks are slightly reduced, as shown in Fig. 3(a). If a magnetic field is applied at temperatures of 19 K or above evidence of free magnetic clusters is observed. The spectrum recorded at 19 K in the presence of a 25 kOe external magnetic field is shown in Fig. 3(b). The M6ssbauer study therefore provides evidence
Vol. 33, No. 8
THE MAGNETIC BEHAVIOUR OF AN Fe-SUBSTITUTED C o - G a ALLOY
1.5 c
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,~ ~.0
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~E 0"5 0
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20 30 40 T E M P E R A T U R E (K )
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50
Fig. 4. Low field d.6. magnetisation measurements on Co2Gal.gsFeo.o5 (a) ×re (b) XzFc (c) TRM. of magnetic clusters with various relaxation rates which begin to freeze at about 19 K. To investigate whether the magnetic assemblies freeze co-operatively or individually block as in a classical superparamagnet, low field d.c. magnetisation measurements were performed. Magnetisation measurements were made in an applied field of 20 Oe with the sample in the zero field cooled (×z~c) and field cooled (XFc) conditions. In addition the thermoremanent magnetisation (TRM) was measured. The TRM is the frozen-in magnetisation resulting from cooling the sample in an applied field (20 Oe), the results, shown in Fig. 4, are similar to those obtained by Gibbs and Cywinski for Cos4Ga46. As they pointed out, the main features of these curves are a broad maximum in XZFC at a blocking temperature Tn, a deviation of Xrc from Xzvc at a temperature greater than Tn, and a TRM which persists to temperatures greater than TB. A unique characteristic freezing temperature, Tf, is expected for spin glass systems, and therefore the TRM will disappear as Tf is approached from below and all spins unfreeze. However in a superparamagnet a wide range of blocking temperatures can result in a broad peak in ×zFc at Tn. Above TB some clusters may remain blocked therefore contributing to the TRM and Xrc. We therefore consider the results of Fig. 4 to be representative of a superparamagnetic alloy rather than a spin glass system. The broad peak in XzFc
905
appears to be approximately at 19 +__1 K whereas the TRM obviously persists to temperatures near 30 K. The TB from magnetisation measurements is in complete agreement with the "magnetic ordering" temperature from the M6ssbauer studies. (Magnetisation and M6ssbauer experiments both measure the ordering temperature of large clusters and are therefore expected to be the same.) In conclusion, we are of the opinion that our results are best interpreted in terms of a superparamagnetic model. The MOssbauer experiments revealed the presence of frozen clusters with a range of relaxation times in the Co2Gal.gs Feo.os alloy, which is well below the percolation limit. The low field d.c. magnetisation measurements are in agreement with the M6ssbauer work and show that beyond the blocking temperature some clusters remain frozen, and so contribute to the TRM and Xrc. This work is currently being extended to investigations of the structural and magnetic properties of other alloys in the system, and will be reported in more detail shortly.
Acknowledgements - The authors are indebted to Dr. R. Cywinski for many useful discussions, and to Mr. H. Pencak for help with sample preparation. One of us (GLW) acknowledges receipt of an Australian Commonwealth Postgraduate Research Award. This work is supported by a grant from the Australian Research Grants Committee. REFERENCES 1. 2. 3. 4. 5. 6. 7.
R. Cywinski, J.G. Booth & B.D. Rainford, J. Phys. F." Metal Phys. 7, 2567 (1977). A.K. Grover, L.C. Gupta, R. Vijayaragharan, M. Matsumura, M. Nakano & K. Asayama, Solid State Commun. 30,457 (1979). P. Gibbs & R. Cywinski, Solid State Commun (to be published). R. Cywinski, J.G. Booth & B.D. Rainford, J. Appl. OTst. 11,641 (1978). E.M. Gray, Ph.D. thesis, Monash University (1979). B. Window, J. Phys. E: Sci. Instrum. 4, 401 (1971). E. Wachtel, V. Linse & V. Gerold, J. Phys. Chem. Solids 34, 1461 (1973).
(Received 15 August 1979 by C. W. McCombie) The results of M6ssbauer experiments at low temperatures and low field d.c. magnetisation measurements are presented for the alloy Co2 Gal.9SFeo.0s. The presence of "frozen" magnetic clusters with different relaxation rates is verified. The magnetic behaviour of the alloy is best explained by a superparamagnetic model where the magnetic clusters freeze with a range of blocking temperatures. AT THE EQUIATOMIC COMPOSITION Co and Ga form a/3-phase compound with the B2 (CsC1) structure, which may be considered as consisting of two interpenetrating simple cubic sublattices, one occupied solely by Co atoms and the other by Ga. Small angle diffuse neutron scattering measurements on Co-Ga alloys [I] ( 5 2 - 6 0 at.% Co) confirm the presence of superparamagnetic assemblies formed by the magnetic clustering of moment bearing Co atoms on the Ga sublattice. It is believed that the percolation of these clusters leads to the onset of long range ferromagnetic order, although above the critical concentration some superparamagnetic assemblies remain uncoupled to behave as large independent spins. Recently Grover et al. [2] have concluded from NMR and a.c. susceptibility studies that Co-Ga alloys ( 5 0 - 6 0 at.% Co) belong to the cluster spin glass category similar to the typical cases AuFe and CuMn. Spin glass systems are well known for their unique characteristic freezing temperature, Tr, and corresponding sharp peak in magnetisation. However low field d.c. magnetisation measurements [3] on a Cos4Ga46 alloy tend to refute this idea in favour of a classical superparamagnetic model, where a range of cluster blocking temperatures exist. The Co (Ga, Fe) series of alloys, formed by partial substitution of Fe for Ga in the equiatomic CoGa, has been shown to be magnetically similar to the Co-Ga system [4], superparamagnetic clustering being related in this case to Fe substitutional atoms on Ga sites. In an attempt to clarify the conflicting ideas on the magnetic order in Co-Ga we have therefore performed M6ssbauer experiments at low temperatures and low field d.c. magnetisation measurements on Co2 Gal.gSFeo.os. This alloy is known to be below the percolation limit and therefore displays no long range ferromagnetic order. For the magnetisation measurements a 5 gm
1
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I
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I
-2
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VeLocity (mm/sec) Fig. 1. M6ssbauer spectra of Co2 Gal.9s Feo.os in the temperature range 4.2-19 K in zero external field. ingot was prepared by melting the appropriate weights of 5N constituent metals in an argon arc furnace. A spherical magnetisation sample was formed by 903
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THE MAGNETIC BEHAVIOUR OF AN Fe-SUBSTITUTED C o - G a ALLOY
Vol. 33, No. 8
a.
T :/-,. 2 K
b
T= 190 K
05 O
n-
~- 0 t
100
,
,
,
,
I
,
~
,
,
I
,
200 300 HYPERFINE FIELD. H (KOe)
Fig. 2. The distribution of hyperfine fields in the 4.2 K spectrum. remelting part of this master alloy. For the M6ssbauer sample, iron enriched to 90% in STFe was melted with Co and Ga in a 4 mm quartz ampoule. Both resulting ingots were heat treated at 830°C for 24 hr followed by a water quench. Weight losses were less than 0.1%. M6ssbauer absorbers were made by pressing a calculated optimum amount of the powdered alloy with the binder benzophenone. M6ssbauer spectra were obtained using a STCo in Rh M6ssbauer source at 4.2 K, and driven by a conventional constant acceleration drive system. The various absorber temperatures used were maintained to an accuracy of 0.1 K. Themagnetic measurements were made using a modified Foner balance system [5]. M6ssbauer spectra were recorded in the temperature range 4.2-300 K. The spectra shown in Fig. 1 are representative of the behaviour observed. The solid lines through the data have been obtained from a computer fitting procedure based on a programme developed by Window [6]. This method is applicable to systems displaying a range of hyperFme fields, and produces the corresponding hyperf'me field distribution [P(/-/) curve] based on the fit. Over the temperature range shown the spectrum changes from a broad magneticaUy split six-line pattern at 4.2 K to an apparent singlet at about 19 K. The spectrum is unchanged from 19 K to room temperature, except for the temperature dependent second .order Doppler shift. A low velocity spectrum taken at room temperature reveals the presence of more than one Fe site in the apparent single line spectrum. The different Fe near neighbour environments appear to arise due to vacancies, known to exist on the Co sublattice [7] and antistructure Fe atoms occupying Co sites instead of the normal Ga. Since the Fe atoms occupy mostly Ga sites, any antistructure Co atoms will have almost no influence on the spectrum. The collapse of the hyperf'me field is typical of many magnetically ordered systems, the apparent ferromagnetic transition occuring at 19 + 1 K for
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-4
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I
-2
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0
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Ve(ocity (mm/sec) Fig. 3. MOssbauer spectrum of Co; Gal.9s Feo.os at (a) 4.2 K (b) 19 K in the presence of a 25 kOe external
magnetic field. this alloy. It is worthwhile noting that although Co2 Gal.95 Feo.os is well below the percolation limit the M6ssbauer measurements are clearly revealing some magnetic ordering of the spins at temperatures up to 19 K. In a system with a distribution of magnetic clusters, a wide range of anisotropies, and therefore cluster relaxation rates may exist. The broadness of the lines in the magnetically split spectra are believed to be indicative of a distribution of cluster relaxation rates present in the alloy. Different relaxation rates produce different hyperfine fields in the spectra. These rates are comparable to 10-Tsec, the characteristic time for a M6ssbauer measurement, so that the observed hyperfine fields are intermediate between H~t (the long relaxation time limit) and ((Sz)/S)H~t (the very short relaxation time limit). The distribution of hyperfine fields in the 4.2 K spectrum (Fig. 2) shows a main peak centred at about 215 kOe. A spectrum recorded at 4.2 K in the presence of an external 25 kOe magnetic field shows negligible change from the zero field spectrum, which suggests a "freezing" of clusters in random orientations. There is no visible line broadening or decrease in hyperfine field but the intensities of the second and fifth peaks are slightly reduced, as shown in Fig. 3(a). If a magnetic field is applied at temperatures of 19 K or above evidence of free magnetic clusters is observed. The spectrum recorded at 19 K in the presence of a 25 kOe external magnetic field is shown in Fig. 3(b). The M6ssbauer study therefore provides evidence
Vol. 33, No. 8
THE MAGNETIC BEHAVIOUR OF AN Fe-SUBSTITUTED C o - G a ALLOY
1.5 c
,q
,~ ~.0
0
~E 0"5 0
,
10
,
20 30 40 T E M P E R A T U R E (K )
I
50
Fig. 4. Low field d.6. magnetisation measurements on Co2Gal.gsFeo.o5 (a) ×re (b) XzFc (c) TRM. of magnetic clusters with various relaxation rates which begin to freeze at about 19 K. To investigate whether the magnetic assemblies freeze co-operatively or individually block as in a classical superparamagnet, low field d.c. magnetisation measurements were performed. Magnetisation measurements were made in an applied field of 20 Oe with the sample in the zero field cooled (×z~c) and field cooled (XFc) conditions. In addition the thermoremanent magnetisation (TRM) was measured. The TRM is the frozen-in magnetisation resulting from cooling the sample in an applied field (20 Oe), the results, shown in Fig. 4, are similar to those obtained by Gibbs and Cywinski for Cos4Ga46. As they pointed out, the main features of these curves are a broad maximum in XZFC at a blocking temperature Tn, a deviation of Xrc from Xzvc at a temperature greater than Tn, and a TRM which persists to temperatures greater than TB. A unique characteristic freezing temperature, Tf, is expected for spin glass systems, and therefore the TRM will disappear as Tf is approached from below and all spins unfreeze. However in a superparamagnet a wide range of blocking temperatures can result in a broad peak in ×zFc at Tn. Above TB some clusters may remain blocked therefore contributing to the TRM and Xrc. We therefore consider the results of Fig. 4 to be representative of a superparamagnetic alloy rather than a spin glass system. The broad peak in XzFc
905
appears to be approximately at 19 +__1 K whereas the TRM obviously persists to temperatures near 30 K. The TB from magnetisation measurements is in complete agreement with the "magnetic ordering" temperature from the M6ssbauer studies. (Magnetisation and M6ssbauer experiments both measure the ordering temperature of large clusters and are therefore expected to be the same.) In conclusion, we are of the opinion that our results are best interpreted in terms of a superparamagnetic model. The MOssbauer experiments revealed the presence of frozen clusters with a range of relaxation times in the Co2Gal.gs Feo.os alloy, which is well below the percolation limit. The low field d.c. magnetisation measurements are in agreement with the M6ssbauer work and show that beyond the blocking temperature some clusters remain frozen, and so contribute to the TRM and Xrc. This work is currently being extended to investigations of the structural and magnetic properties of other alloys in the system, and will be reported in more detail shortly.
Acknowledgements - The authors are indebted to Dr. R. Cywinski for many useful discussions, and to Mr. H. Pencak for help with sample preparation. One of us (GLW) acknowledges receipt of an Australian Commonwealth Postgraduate Research Award. This work is supported by a grant from the Australian Research Grants Committee. REFERENCES 1. 2. 3. 4. 5. 6. 7.
R. Cywinski, J.G. Booth & B.D. Rainford, J. Phys. F." Metal Phys. 7, 2567 (1977). A.K. Grover, L.C. Gupta, R. Vijayaragharan, M. Matsumura, M. Nakano & K. Asayama, Solid State Commun. 30,457 (1979). P. Gibbs & R. Cywinski, Solid State Commun (to be published). R. Cywinski, J.G. Booth & B.D. Rainford, J. Appl. OTst. 11,641 (1978). E.M. Gray, Ph.D. thesis, Monash University (1979). B. Window, J. Phys. E: Sci. Instrum. 4, 401 (1971). E. Wachtel, V. Linse & V. Gerold, J. Phys. Chem. Solids 34, 1461 (1973).