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Effect of oxygen on the ferromagnetism of Pd-Fe alloys
1061
Journal of Magnetism and Magnetic Materials 54-57 (1986) 1061-1062
EFFECT OF O X Y G E N O N T H E F E R R O M A G N E T I S M OF P d - F e A L L O Y S F.A. VOLKENING,
G. G R I F F I T H
a n d H. C L A U S
Department of Physics, University of Illinois at Chicago, Chicago, IL 60680, USA
Low field ac susceptibility measurements are reported for a ferromagnetic Pd-0.4 at%Fe sample sublected to various oxygen anneals. Two very different effects are observed. Small amounts of oxygen, which seem to go quite readily into solution, dramatically increase the Curie temperature, but prolonged anneals at high oxygen pressure causes some form of internal oxidation making the sample paramagnetic. The two competing effects can be studied separately because they occur on very different time scales.
There has been much interest in the interaction of Pd with various gases. The ferromagnetism of P d - F e alloys [1] has been successfully used as probe to study hydrogen absorption in Pd. It was shown that the addition of hydrogen to P d - F e alloys leads to a large decrease in the Curie temperature T~ and possible spin glass behavior [2,3]. There has also been considerable interest in the P d - O system. It has been shown that the P d - O system has a rich structure and that small amounts of oxygen can exist in solid solution in Pd [4-7]; however, little is known about exactly how much oxygen can be absorbed and how it affects the bulk electronic properties of Pd. In this paper we present the first study of how oxygen absorption affects the ferromagnetic state of P d - F e alloys. For this purpose we prepared a Pd-0.4 at%Fe sample of dimension 2 x 1 x 0.3 mm 3 as previously described [8]. To remove any possible oxygen contamination, occurring during sample preparation, the sample was heated at 1000°C for 12 h in an atmosphere of Ar ÷ 3% H 2. To prevent hydrogen absorption the annealing chamber was evacuated to 10 -7 Torr for about half an hour before the sample was allowed to cool to room temperature. A C susceptibility measurements were performed at 35 Hz and magnetic field amplitude of 0.2 Oe as described elsewhere [9]. Fig. I demonstrates what happens if the sample is exposed to small pressures of oxygen. In the oxygen free state (closed circles in fig. 1) the sample is ferromagnetic with a Curie temperature of about 3 K. The susceptibility has a fairly sharp cusp at Tc. This type of cusp has been observed in other ferromagnetic transition m e t a l alloys and is due to anisotropy and hysteresis below T~ [10]. As the oxygen content increases Tc increases and the cusps become less pronounced (fig. 1). The number labeling the various susceptibility curves is the total time (in min) that the sample was exposed to an 02 pressure of 50 rtm Hg at 1000°C [11]. After about 2 h at this 02 pressure the oxygen uptake is saturated, the Curie temperature remaining at 9.7 K. At a higher oxygen pressure of 0.2 atm, the same saturated state 0304-8853/86/$03.50
_~ ~-~-o-~~ ~i" I
l.O
I
10
I
25
40
I
150
I
I
I
t i m e (rain.) a t 1000 °('
so. 0~
0.8
0.6
0.4
•
\
\ 0 0
I 2
10~'~,4-~ 4 6
o
a
o
\ aJ D-~-__k D 8 10
T(K)
o_ 0',~ 1~
I 14
I 16
Fig. 1. AC susceptibility vs. temperature for a Pd-0.4 at%Fe sample. The number labeling each curve is the total anneal time (in min) at 1000°C in an 02 pressure of about 50 ~m Hg. with T~ = 9.7 K is reached after only 15 min at 1000°C. This uptake of oxygen is quite reversible i.e., we can obtain the original state with a T~ = 3 K by annealing the sample at 1000°C for several hours in a A r - 3 % H 2 atmosphere. Similar annealing experiments performed with an alloy with 3 at%Fe changed its Curie temperature from 30 to 110 K. This huge variation in T~ may explain the large spread of Curie temperatures reported for P d - F e alloys [1]. It is important to point out that all susceptibility curves of fig. 1 remain fairly sharp and reach essentially the same demagnetization limit. This clearly indicates that we are dealing with a homogeneous bulk effect, i.e. the oxygen is uniformly absorbed and desorbed throughout the sample. Additional experiments at lower temperatures revealed that below about 600°C both processes i.e. incorporation and removal of oxygen from the bulk are inhibited. There are two possible explanations of how the oxygen uptake increases the Curie temperature of the P d - F e alloys. One is that some oxygen goes into solid solution covalently binding some of the Pd d-electrons. This would increase the density of states at the fermi
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
1062
F.A. Volkening et at / Ferromagnetism of P d - Fe alloys
energy. Recent observation of a negatively charged "subsurface" oxygen in Pd [7] seem to support this conjecture. An alternative explanation would be that oxygen internally oxidizes impurities which in their unoxidized form have a detrimental effect on the magnetism (the starting material was 99.9% pure). More work is needed to investigate this effect. As can be seen in fig. 1, after more than 2 h at 1000°C at low 02 pressures, the susceptibility develops a long tail extending to fairly high temperatures. This is due to an Fe enriched surface layer with increased Curie temperature formed because of the high Pd evaporation rate [12]. This layer has a gradient in Fe concentration and thus a very smeared out magnetic transition. After oxygen saturation was reached (fig. 1) we subjected the sample to additional 1000°C anneals, but with increased O 2 pressure of 0.2 atm. The results are shown in fig. 2; the number labeling each curve is the total anneal time, in hours [11]; The curve labeled zero is the same one as in fig. 1 with the highest Tc. A totally new effect is observed (fig. 2); with increasing anneal time a larger and larger part of the sample looses its ferromagnetism, the remainder keeping its Curie temperature at 9.7 K. The sharpening of the transition after the first hour is due to the fact that the inhomogeneous surface layer is first to be magnetically neutralized. Thus, with increasing anneal time a fairly sharp boundary moves from the surface towards the inside of the sample. Behind this interface the sample is paramagnetic. Eventually the interface becomes less well defined, giving rise to a smeared ferromagnetic transition; finally the whole sample becomes paramagnetic. We were able to restore complete ferromagnetism after annealing the sample for 10 min at 1200°C in a small Ar pressure.
1.0
[ I I I I t i m e (hrs.) a t 1000 °C in 0,2 arm. 02
I
I
I
0 ~0--0~0--0~0~ 0.8
0.6
__--O--E2----C]-~O~
1 ,
.~ o.4 0.2
~ ~ 2
,o_+O~o.a..o.o._~o~ ~ 4
6
8
tO
~o-~o12
14
16
T(K) Fig. 2. AC susceptibility vs. temperature for a Pd-0.4 at%Fe sample. The number labeling each curve is the total anneal time (in h) at 1000°C in an 02 pressure of 0.2 atrn.
The effect of oxygen as demonstrated in fig. 2 looks like some form of internal oxidation magnetically neutralizing the Fe-moments [13]. More work is needed to identify the nature of this internal oxidation process. Preliminary Secondary Ion Mass Spectroscopy (SIMS) investigation performed on some of our samples seems to confirm this interpretation. A sample partly oxidized for 2 h at 1000°C in 0.2 atm 02 showed, indeed, considerably more oxygen near the surface as compared to its center. Also, the sharp boundary between the two regions has been confirmed. In summary, we have discovered two separate effects of oxygen interacting with P d - F e alloys: (i) small amounts of oxygen are quite readily absorbed enhancing ferromagnetism, (ii) prolonged annealing at high 02 pressures causes internal oxidation of the Fe destroying ferromagnetism. These two opposite effects can be studied separately because they occur on very different time ~eales. This work was supported by the N S F grant no. DMR8101857. We acknowledge helpful discussions with C.J. Alstetter, S. Bader and H.K. Birnbaum. We especially are grateful to C.J. Alstetter and Judy Baker for the help with the SIMS analysis of our samples at the Materials Research Laboratory, University of Illinois at Urbana-Champaign. [1] See for example G.J. Nieuwenhuys, Advan. Phys. 24 (1975) 515. [2] J.A. Mydosh, Phys. Rev. Lett. 33 (1974) 1562. [3] B. Souffache and J.P. Burger, Nato Intern. Symp. on the Electronic Structure and Properties of Hydrogen in Metals, eds. P. Jena and C.B. Satterthwaite (1983) 361. [4] C.T. Campbell, D.C. Foyt and J.M. White, J. Phys. Chem. 81 (1977) 491. [5] H. Conrad, G. Ertl, J. Kuppers and E.E, Lana, Surface Sci. 65 (1977) 245. [6] D.L. Weissman, M.L. Shek and W.E. Spicer, Surface Sci. 92 (1980) L59. [7] D.L. Weissman-Wenour, M.L Shek, P.M. Stefan, 1. Lindan and W.E. Spicer, Surface Sci. 127 (1983) 513. [8] G. Griffith, F.A. Volkening and H. Claus, J. Appl. Phys. 57 (1985) 3392. [9] D.W. Carnegie, C.J. Tranchita and H. Claus, J. Appl. Phys. 50 (1979) 7318. [10] S. Crane, D.W. Carnegie, Jr. and H. Claus, J. Appl. Phys. 53 (1982) 2179. [11] Before cooling the sample to room temperature, the 02 was pumped out to less than 10 6 Torr to prevent formation of PdO which is stable below 900°C. [12] This tail can be removed by grinding off a 0.01 mm surface layer of the sample. [13] K. Hauffe, Oxidation of Metals (Plenum Press, New York, 1965) p. 335.
Journal of Magnetism and Magnetic Materials 54-57 (1986) 1061-1062
EFFECT OF O X Y G E N O N T H E F E R R O M A G N E T I S M OF P d - F e A L L O Y S F.A. VOLKENING,
G. G R I F F I T H
a n d H. C L A U S
Department of Physics, University of Illinois at Chicago, Chicago, IL 60680, USA
Low field ac susceptibility measurements are reported for a ferromagnetic Pd-0.4 at%Fe sample sublected to various oxygen anneals. Two very different effects are observed. Small amounts of oxygen, which seem to go quite readily into solution, dramatically increase the Curie temperature, but prolonged anneals at high oxygen pressure causes some form of internal oxidation making the sample paramagnetic. The two competing effects can be studied separately because they occur on very different time scales.
There has been much interest in the interaction of Pd with various gases. The ferromagnetism of P d - F e alloys [1] has been successfully used as probe to study hydrogen absorption in Pd. It was shown that the addition of hydrogen to P d - F e alloys leads to a large decrease in the Curie temperature T~ and possible spin glass behavior [2,3]. There has also been considerable interest in the P d - O system. It has been shown that the P d - O system has a rich structure and that small amounts of oxygen can exist in solid solution in Pd [4-7]; however, little is known about exactly how much oxygen can be absorbed and how it affects the bulk electronic properties of Pd. In this paper we present the first study of how oxygen absorption affects the ferromagnetic state of P d - F e alloys. For this purpose we prepared a Pd-0.4 at%Fe sample of dimension 2 x 1 x 0.3 mm 3 as previously described [8]. To remove any possible oxygen contamination, occurring during sample preparation, the sample was heated at 1000°C for 12 h in an atmosphere of Ar ÷ 3% H 2. To prevent hydrogen absorption the annealing chamber was evacuated to 10 -7 Torr for about half an hour before the sample was allowed to cool to room temperature. A C susceptibility measurements were performed at 35 Hz and magnetic field amplitude of 0.2 Oe as described elsewhere [9]. Fig. I demonstrates what happens if the sample is exposed to small pressures of oxygen. In the oxygen free state (closed circles in fig. 1) the sample is ferromagnetic with a Curie temperature of about 3 K. The susceptibility has a fairly sharp cusp at Tc. This type of cusp has been observed in other ferromagnetic transition m e t a l alloys and is due to anisotropy and hysteresis below T~ [10]. As the oxygen content increases Tc increases and the cusps become less pronounced (fig. 1). The number labeling the various susceptibility curves is the total time (in min) that the sample was exposed to an 02 pressure of 50 rtm Hg at 1000°C [11]. After about 2 h at this 02 pressure the oxygen uptake is saturated, the Curie temperature remaining at 9.7 K. At a higher oxygen pressure of 0.2 atm, the same saturated state 0304-8853/86/$03.50
_~ ~-~-o-~~ ~i" I
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I
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I
25
40
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150
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I
t i m e (rain.) a t 1000 °('
so. 0~
0.8
0.6
0.4
•
\
\ 0 0
I 2
10~'~,4-~ 4 6
o
a
o
\ aJ D-~-__k D 8 10
T(K)
o_ 0',~ 1~
I 14
I 16
Fig. 1. AC susceptibility vs. temperature for a Pd-0.4 at%Fe sample. The number labeling each curve is the total anneal time (in min) at 1000°C in an 02 pressure of about 50 ~m Hg. with T~ = 9.7 K is reached after only 15 min at 1000°C. This uptake of oxygen is quite reversible i.e., we can obtain the original state with a T~ = 3 K by annealing the sample at 1000°C for several hours in a A r - 3 % H 2 atmosphere. Similar annealing experiments performed with an alloy with 3 at%Fe changed its Curie temperature from 30 to 110 K. This huge variation in T~ may explain the large spread of Curie temperatures reported for P d - F e alloys [1]. It is important to point out that all susceptibility curves of fig. 1 remain fairly sharp and reach essentially the same demagnetization limit. This clearly indicates that we are dealing with a homogeneous bulk effect, i.e. the oxygen is uniformly absorbed and desorbed throughout the sample. Additional experiments at lower temperatures revealed that below about 600°C both processes i.e. incorporation and removal of oxygen from the bulk are inhibited. There are two possible explanations of how the oxygen uptake increases the Curie temperature of the P d - F e alloys. One is that some oxygen goes into solid solution covalently binding some of the Pd d-electrons. This would increase the density of states at the fermi
© E l s e v i e r S c i e n c e P u b l i s h e r s B.V.
1062
F.A. Volkening et at / Ferromagnetism of P d - Fe alloys
energy. Recent observation of a negatively charged "subsurface" oxygen in Pd [7] seem to support this conjecture. An alternative explanation would be that oxygen internally oxidizes impurities which in their unoxidized form have a detrimental effect on the magnetism (the starting material was 99.9% pure). More work is needed to investigate this effect. As can be seen in fig. 1, after more than 2 h at 1000°C at low 02 pressures, the susceptibility develops a long tail extending to fairly high temperatures. This is due to an Fe enriched surface layer with increased Curie temperature formed because of the high Pd evaporation rate [12]. This layer has a gradient in Fe concentration and thus a very smeared out magnetic transition. After oxygen saturation was reached (fig. 1) we subjected the sample to additional 1000°C anneals, but with increased O 2 pressure of 0.2 atm. The results are shown in fig. 2; the number labeling each curve is the total anneal time, in hours [11]; The curve labeled zero is the same one as in fig. 1 with the highest Tc. A totally new effect is observed (fig. 2); with increasing anneal time a larger and larger part of the sample looses its ferromagnetism, the remainder keeping its Curie temperature at 9.7 K. The sharpening of the transition after the first hour is due to the fact that the inhomogeneous surface layer is first to be magnetically neutralized. Thus, with increasing anneal time a fairly sharp boundary moves from the surface towards the inside of the sample. Behind this interface the sample is paramagnetic. Eventually the interface becomes less well defined, giving rise to a smeared ferromagnetic transition; finally the whole sample becomes paramagnetic. We were able to restore complete ferromagnetism after annealing the sample for 10 min at 1200°C in a small Ar pressure.
1.0
[ I I I I t i m e (hrs.) a t 1000 °C in 0,2 arm. 02
I
I
I
0 ~0--0~0--0~0~ 0.8
0.6
__--O--E2----C]-~O~
1 ,
.~ o.4 0.2
~ ~ 2
,o_+O~o.a..o.o._~o~ ~ 4
6
8
tO
~o-~o12
14
16
T(K) Fig. 2. AC susceptibility vs. temperature for a Pd-0.4 at%Fe sample. The number labeling each curve is the total anneal time (in h) at 1000°C in an 02 pressure of 0.2 atrn.
The effect of oxygen as demonstrated in fig. 2 looks like some form of internal oxidation magnetically neutralizing the Fe-moments [13]. More work is needed to identify the nature of this internal oxidation process. Preliminary Secondary Ion Mass Spectroscopy (SIMS) investigation performed on some of our samples seems to confirm this interpretation. A sample partly oxidized for 2 h at 1000°C in 0.2 atm 02 showed, indeed, considerably more oxygen near the surface as compared to its center. Also, the sharp boundary between the two regions has been confirmed. In summary, we have discovered two separate effects of oxygen interacting with P d - F e alloys: (i) small amounts of oxygen are quite readily absorbed enhancing ferromagnetism, (ii) prolonged annealing at high 02 pressures causes internal oxidation of the Fe destroying ferromagnetism. These two opposite effects can be studied separately because they occur on very different time ~eales. This work was supported by the N S F grant no. DMR8101857. We acknowledge helpful discussions with C.J. Alstetter, S. Bader and H.K. Birnbaum. We especially are grateful to C.J. Alstetter and Judy Baker for the help with the SIMS analysis of our samples at the Materials Research Laboratory, University of Illinois at Urbana-Champaign. [1] See for example G.J. Nieuwenhuys, Advan. Phys. 24 (1975) 515. [2] J.A. Mydosh, Phys. Rev. Lett. 33 (1974) 1562. [3] B. Souffache and J.P. Burger, Nato Intern. Symp. on the Electronic Structure and Properties of Hydrogen in Metals, eds. P. Jena and C.B. Satterthwaite (1983) 361. [4] C.T. Campbell, D.C. Foyt and J.M. White, J. Phys. Chem. 81 (1977) 491. [5] H. Conrad, G. Ertl, J. Kuppers and E.E, Lana, Surface Sci. 65 (1977) 245. [6] D.L. Weissman, M.L. Shek and W.E. Spicer, Surface Sci. 92 (1980) L59. [7] D.L. Weissman-Wenour, M.L Shek, P.M. Stefan, 1. Lindan and W.E. Spicer, Surface Sci. 127 (1983) 513. [8] G. Griffith, F.A. Volkening and H. Claus, J. Appl. Phys. 57 (1985) 3392. [9] D.W. Carnegie, C.J. Tranchita and H. Claus, J. Appl. Phys. 50 (1979) 7318. [10] S. Crane, D.W. Carnegie, Jr. and H. Claus, J. Appl. Phys. 53 (1982) 2179. [11] Before cooling the sample to room temperature, the 02 was pumped out to less than 10 6 Torr to prevent formation of PdO which is stable below 900°C. [12] This tail can be removed by grinding off a 0.01 mm surface layer of the sample. [13] K. Hauffe, Oxidation of Metals (Plenum Press, New York, 1965) p. 335.