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Interplay of atomic order and magnetic behaviour in Au Fe
Journal of Magnensm and MagnetLc Materials 31-34 (1983) 1337-1339
1337
INTERPLAY OF ATOMIC ORDER A N D MAGNETIC BEHAVIOUR IN AuFe G.L WHITTLE
a n d S.J C A M P B E L L
Department of Sohd State Phvst~s. Research S~hool of Phv~t~al S~wnce~, Au~trahan Natmnal Umeer~m', Canberra, A C T 2600, A ustraha
Room temperature Mossbauer spectra of 5 and 10% AuFe alloys reveal a tendency for clustenng of iron atoms for samples m the as-rolled state, whereas the opposite tendency towards atomic short range order is observed after heat treatment The effect of atomic order is also reflected m the spectra of magneucall~ ordered samples
Solid solutions of fcc g o l d - i r o n alloys are well known to exhibit complex magnetic behaviour with a variety of magnetic phases - spin glass, cluster glass, ferromagnetism and a critical region - occurring over the concentratlon range of 0-25 at% Fe [1]. However, one aspect which has received relatively little attention is the d e p e n d e n c e of the magnetic properties on atomic order F r o m ac susceptibility measurements, Crane and Claus [2] d e m o n s t r a t e d the sensitivity of magnetic behavlour on sample preparation near 15 at% Fe, but no systematic study has been reported in which a measure of the atomic order has been determined and correlated with the magnetic properties The Mossbauer effect enables detailed study of atomic environment and in the present investigation (1 5-300 K) we have subjected 5 and 10 at% Fe alloys to different heat treatments in order to appraise atomic order effects. The alloys were prepared as 200 mg ingots in the m a n n e r described previously [3] and then cold rolled to the o p t i m u m thickness Samples were examined in the as-rolled state (AR) and after various heat treatments, and spectra were obtained using a conventional Mossbauer spectrometer and a variable temperature helium cryostat In this paper we consider the room temperature (RT) and 4 2 K data obtained from specimens in the A R state and after heat treatment at 550°C for 1 day (HT) followed by a rapid quench into water A detailed study of the dependence of atomic order on heat treatment [4] has shown that this condition is representative of the effects of a n n e a h n g at temperatures above 150°C The effect of heat treatment on the 5% Fe sample is shown by the RT Mossbauer spectra in fig 1 These spectra and the corresponding ones for the 10% Fe sample, have been least-squares fitted using the most hkelv near nelghbour (NN) atomic configurations For r a n d o m fcc solid solutions of 5 and 10% Fe in Au, the main NN environments for the iron atoms are (1) 12 A u , ( 2 ) 11 A u - 1 F e a n d ( 3 ) 10Au 2 F e N N a t o m s w I t h (4) 9 A u - 3 Fe N N atoms also being significant for the 10% Fe sample Using Lorentzlan doublets to represent these N N configurations the best fits to the experimental data are shown as the full lines in fig 1 The indlvld0304-8853/83/0000-0000/$03.00
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Fig 1 Room temperature Mossbauer spectra for the 5 at% AuFe alloys m the AR and HT states (5VCo m Rh source) The full hne represents the fits to the data obtained from the spectral components also shown in the figure ual spectral c o m p o n e n t s comprising the composite fit are also shown As reported by previous authors [5,6] c o m p o n e n t (1) ( i e 12 Au NN) was found to be best represented by a closely spaced doublet rather than the expected single line; this is believed to be due to the effects of more distant Fe atom neighbours which have not been accomodated in the model. The Mossbauer parameters obtained for all components of the 5% and 10% Fe samples in the A R and HT states are given in table 1 The agreement between the hyperfine parameters and those published previously (e.g ref [6]) is extremely good Also shown in table 1 is the atomic order parameter, al, which was calculated from the area of c o m p o n e n t (1), A I, following the method outlined by Campbell and Hicks [7] Both the 5 and 10% Fe samples in the A R condition are found to possess a small posltlve aj indicative of clustering of iron atoms within the fcc gold matrix These values represent a significant deviation from the random solid solution which many workers believe exists in the A R
c,~ 1983 N o r t h - t t o l l a n d
1338
G L
Wluttle, S J Campbell / Atomt~ order and ma,gnetl~ beha~,mur m A ut e
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state By comparison both samples in the HT state are found to exhibit the opposite tendency towards atomic ~hort range order (SRO), as indicated by the negati,¢e values of cq To our knowledge, the only other mcasuiement of atomic order parameters for A u F e was obtained by X-ray diffuse scattering measurements on a 25 at% Fe sample [8] Their sample, which was quenched from 900°C, yielded a value for cq of - 0 . 0 6 1 , again indicating SRO in an annealed condition The effect of these different atomic ordermg states on the magnetic character of the A u F e alloys is shown by the 4 2 K spectra of the A R and H T 10~ Fe sample in fig 2 Both spectra consist essentially of two broad six-line subspectra as represented by the snck diagrams [4]. This is consistent with the magnetic phase diagram for A u F e [1] which indicates the coexistence of a spin glass and cluster glass phase in alloys below the percolation limit ( 1 5 7 at% Fe) The spectra for the 10~ Fe sample in the A R and H T states in fig 2, as well as the corresponding spectra for the 5% Fe sample at 4 2 K, show that the effect of the heat treatment is to decrease the fraction of the phase labelled cg and to sharpen the distribution labelled sg We interpret this to indicate that the SRO induced in the H T sample state decreases the fraction of cluster glass phase and sharpens the spin glass distribution, consistent with the formation of a more regular distribution of spins throughout the latUce In summary, Mossbauer measurements have provided a sensitive method of measuring the effect of heat t r e a t m e n t on the distribution of N N ~ron atoms m A u F e and have enabled the atomic order parameter, (~ to be determined C o m p a r i s o n of the 4 2 K spectra of alloys in the as rolled and annealed states shows that the effects on the magnetic behavlour are consistent with the atomic short range order induced by a n n e a h n g
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F i g 2 4 2 K M o s s b a u e r ~pectra for the 10 a t ~ A u F e a l l o ~ m the A R a n d H T states T h e two m a g n e t i c regHnes ~.g a n d sg are nndnc,~ted hv
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G,L Whtttle, S J Campbell / A t o m t c order and magnettc behavtour m AuFe
References [1] B V B Sarklsslan, J Phys F 11 (1981) 2191 [2] S. Crane and H Claus, Sohd State Commun 36 (1980) 461 [3] G L Whittle, R Cywmskl and P E Clark, J Phys F 10 (1980) L31
[4] [5] [6] [7] [8]
1339
G L Whittle and S.J Campbell, to be pubhshed. B. Window, Phys. Rev B 6 (1972) 2013 M S. Radout, J Phys C 2 (1969) 1258 S J Campbell and T.J Hicks, J Phys F 5 (1975) 27 R Sundahl, J Swertsen and T Chen, J Appl Phys 14 (1963) 122
1337
INTERPLAY OF ATOMIC ORDER A N D MAGNETIC BEHAVIOUR IN AuFe G.L WHITTLE
a n d S.J C A M P B E L L
Department of Sohd State Phvst~s. Research S~hool of Phv~t~al S~wnce~, Au~trahan Natmnal Umeer~m', Canberra, A C T 2600, A ustraha
Room temperature Mossbauer spectra of 5 and 10% AuFe alloys reveal a tendency for clustenng of iron atoms for samples m the as-rolled state, whereas the opposite tendency towards atomic short range order is observed after heat treatment The effect of atomic order is also reflected m the spectra of magneucall~ ordered samples
Solid solutions of fcc g o l d - i r o n alloys are well known to exhibit complex magnetic behaviour with a variety of magnetic phases - spin glass, cluster glass, ferromagnetism and a critical region - occurring over the concentratlon range of 0-25 at% Fe [1]. However, one aspect which has received relatively little attention is the d e p e n d e n c e of the magnetic properties on atomic order F r o m ac susceptibility measurements, Crane and Claus [2] d e m o n s t r a t e d the sensitivity of magnetic behavlour on sample preparation near 15 at% Fe, but no systematic study has been reported in which a measure of the atomic order has been determined and correlated with the magnetic properties The Mossbauer effect enables detailed study of atomic environment and in the present investigation (1 5-300 K) we have subjected 5 and 10 at% Fe alloys to different heat treatments in order to appraise atomic order effects. The alloys were prepared as 200 mg ingots in the m a n n e r described previously [3] and then cold rolled to the o p t i m u m thickness Samples were examined in the as-rolled state (AR) and after various heat treatments, and spectra were obtained using a conventional Mossbauer spectrometer and a variable temperature helium cryostat In this paper we consider the room temperature (RT) and 4 2 K data obtained from specimens in the A R state and after heat treatment at 550°C for 1 day (HT) followed by a rapid quench into water A detailed study of the dependence of atomic order on heat treatment [4] has shown that this condition is representative of the effects of a n n e a h n g at temperatures above 150°C The effect of heat treatment on the 5% Fe sample is shown by the RT Mossbauer spectra in fig 1 These spectra and the corresponding ones for the 10% Fe sample, have been least-squares fitted using the most hkelv near nelghbour (NN) atomic configurations For r a n d o m fcc solid solutions of 5 and 10% Fe in Au, the main NN environments for the iron atoms are (1) 12 A u , ( 2 ) 11 A u - 1 F e a n d ( 3 ) 10Au 2 F e N N a t o m s w I t h (4) 9 A u - 3 Fe N N atoms also being significant for the 10% Fe sample Using Lorentzlan doublets to represent these N N configurations the best fits to the experimental data are shown as the full lines in fig 1 The indlvld0304-8853/83/0000-0000/$03.00
5)o AuFe
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Fig 1 Room temperature Mossbauer spectra for the 5 at% AuFe alloys m the AR and HT states (5VCo m Rh source) The full hne represents the fits to the data obtained from the spectral components also shown in the figure ual spectral c o m p o n e n t s comprising the composite fit are also shown As reported by previous authors [5,6] c o m p o n e n t (1) ( i e 12 Au NN) was found to be best represented by a closely spaced doublet rather than the expected single line; this is believed to be due to the effects of more distant Fe atom neighbours which have not been accomodated in the model. The Mossbauer parameters obtained for all components of the 5% and 10% Fe samples in the A R and HT states are given in table 1 The agreement between the hyperfine parameters and those published previously (e.g ref [6]) is extremely good Also shown in table 1 is the atomic order parameter, al, which was calculated from the area of c o m p o n e n t (1), A I, following the method outlined by Campbell and Hicks [7] Both the 5 and 10% Fe samples in the A R condition are found to possess a small posltlve aj indicative of clustering of iron atoms within the fcc gold matrix These values represent a significant deviation from the random solid solution which many workers believe exists in the A R
c,~ 1983 N o r t h - t t o l l a n d
1338
G L
Wluttle, S J Campbell / Atomt~ order and ma,gnetl~ beha~,mur m A ut e
+1
+ 1 + 1
>
~7
0
I
0 ~ 0 ~ ~ 0 0 ~
+
state By comparison both samples in the HT state are found to exhibit the opposite tendency towards atomic ~hort range order (SRO), as indicated by the negati,¢e values of cq To our knowledge, the only other mcasuiement of atomic order parameters for A u F e was obtained by X-ray diffuse scattering measurements on a 25 at% Fe sample [8] Their sample, which was quenched from 900°C, yielded a value for cq of - 0 . 0 6 1 , again indicating SRO in an annealed condition The effect of these different atomic ordermg states on the magnetic character of the A u F e alloys is shown by the 4 2 K spectra of the A R and H T 10~ Fe sample in fig 2 Both spectra consist essentially of two broad six-line subspectra as represented by the snck diagrams [4]. This is consistent with the magnetic phase diagram for A u F e [1] which indicates the coexistence of a spin glass and cluster glass phase in alloys below the percolation limit ( 1 5 7 at% Fe) The spectra for the 10~ Fe sample in the A R and H T states in fig 2, as well as the corresponding spectra for the 5% Fe sample at 4 2 K, show that the effect of the heat treatment is to decrease the fraction of the phase labelled cg and to sharpen the distribution labelled sg We interpret this to indicate that the SRO induced in the H T sample state decreases the fraction of cluster glass phase and sharpens the spin glass distribution, consistent with the formation of a more regular distribution of spins throughout the latUce In summary, Mossbauer measurements have provided a sensitive method of measuring the effect of heat t r e a t m e n t on the distribution of N N ~ron atoms m A u F e and have enabled the atomic order parameter, (~ to be determined C o m p a r i s o n of the 4 2 K spectra of alloys in the as rolled and annealed states shows that the effects on the magnetic behavlour are consistent with the atomic short range order induced by a n n e a h n g
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F i g 2 4 2 K M o s s b a u e r ~pectra for the 10 a t ~ A u F e a l l o ~ m the A R a n d H T states T h e two m a g n e t i c regHnes ~.g a n d sg are nndnc,~ted hv
the t'~n ,.l×-Ime
,.tv(-k ch.l,ar.arn~.
G,L Whtttle, S J Campbell / A t o m t c order and magnettc behavtour m AuFe
References [1] B V B Sarklsslan, J Phys F 11 (1981) 2191 [2] S. Crane and H Claus, Sohd State Commun 36 (1980) 461 [3] G L Whittle, R Cywmskl and P E Clark, J Phys F 10 (1980) L31
[4] [5] [6] [7] [8]
1339
G L Whittle and S.J Campbell, to be pubhshed. B. Window, Phys. Rev B 6 (1972) 2013 M S. Radout, J Phys C 2 (1969) 1258 S J Campbell and T.J Hicks, J Phys F 5 (1975) 27 R Sundahl, J Swertsen and T Chen, J Appl Phys 14 (1963) 122