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Oxidation of permalloy thin films
Thin Solid Film& 34 (1976) 95-98
© Elsevier Sequoia S.A., Lausanne-Printed in Switzerland
95
OXIDATION OF PERMALLOY THIN FILMS* R. L. COREN
Electrical Engineering Department, Drexel University, Philadelphia, PaL (U.S.A.)
M. H. FRANCOMBE Wesffnghouse Research Laboratories, Pittsburgh, Pc- (U.S.A.)
(Received August 25, 1975)
1. INTRODUCTION The recent proposals 1'2 for bubble memories which involve magnetoresistive sensing of the bubble place stringent requirements on the Permalloy film detection dements because of the small amount of stray flux. While the fractional saturation magnetoresistance R = A p / p is high in Permalloy, it varies significantly with composition3: in bulk at room temperature d R / d C = ~% per % nickel at C = 80% Ni composition, where R = 4%. As a result, attention has been focussed 4 on magnetoresistance variations and degradation arising from compositional changes caused by ambient temperature oxidation of the film sensors. In previously reported investigations s of sputtered Permalloy Films, we concluded from magnetostrictive measurements that the presence of small amounts of oxygen in the vacuum system reduced the metallic iron content of the Ni-Fe alloy Films, Le. that the iron is preferentially oxidized. In the present study, electron diffraction and the optical Faraday effect have been used to examine further the low temperature oxidation behavior of 81% Ni: 19% Fe alloy Films. The results suggest that strong preferential oxidation of iron does in fact occur during initial stages of oxidation and that large changes might consequently be expected in the magnetoresistive coefficient for the fdms. 2. EXPERIMENTAL The PermaUoy films were prepared by d.c. sputtering from vacuum-melted alloy targets of nominal composition 81% Ni : 19% Fe using experimental conditions described previously s. The Films were grown to a thickness of about 500 A on vitreous silica substrates. Oxidation was carried out in flowing oxygen at temperatures between 350 ° and 500 °C and for times ranging up to 2 h. Electron diffraction studies were performed in a Siemens Elmiskop electron microscope in the transmission mode at 80 kV. The Faraday rotation measurements were made using an experimental arrangement described previously by the authors in a study of ferrite films6. Polarized light was passed through holes in the poles of an electromagnet, normally through the film and through an analyzer to a photomultiplier tube. * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 5-02. This study was made while the authors were at the Philco Scientific Laboratory, Blue Bell, Pa., U.S.A.
96
R.L. COREN, M.H. FRANCOMBE
The output of the tube was fed to the Y axis of an X Y recorder, while the X axis recorded the applied magnetic field. With the polarizer and analyzer at 45 ° , small Faraday rotations produced proportional phototube signals. With the field off, the analyzer could be rotated through a known angle to calibrate the vertical recorder scale. As the magnetic field is increased, the magnetization rises out of the film plane. Since the ferromagnetic Faraday effect varies linearly with the component of M along the light path, the rotation 0 increases until M stands perpendicular to the film. This saturation occurs when the applied field equals 4riM, the demagnetizing field, so that the mean saturation value of 47rM can be determined. In addition, from the height of the curve the magnitude of the Faraday effect can be measured in each film*. 3. RESULTS AND DISCUSSION
Electron diffraction studies showed that, typically for films oxidized at about 400 °C, an oxide phase could be readily detected after about 5 min. Complete oxidation of the 500 A Film required times ranging up to several hours. At all stages of oxidation the oxide comprised a single NaCl-type monoxide structure with a lattice parameter close to those of the cubic WiJstite phases of NiO and FeO. The close similarity of the lattice parameters for NiO and FeO made it impossible to infer the composition of the monoxide phase from lattice parameter measurements. However, the fact that these monoxides are non-ferromagnetic meant that Faraday rotation effects measured during stages of oxidation were influenced only by the metal alloy constituent remaining. Figure 1 shows the measured values of 4riM and of saturation Faraday rotation as a function of oxidation time. After each stage of oxidation the translation from the rising linear portion of the 0 versus H Faraday curve to the region of saturation was quite sharp, indicating a fairly uniform residual alloy composition throughout the Film. During the first 20 min both 4ztM and 0 decreased rapidly, this being followed by a much more gradual change. The change in 4zrM is from about 10 000 to 7800 G. Since we have found no sensible thickness dependence of magnetization in our alloy Films, down to about 100 A, this change is attributed to a compositional variation and corresponds to a change of composition from 81% Ni : 19% Fe to 92% Ni : 8% Fe. Using the known Verdet constantt 7 for this final composition and the fact that the Faraday rotation is directly proportional to the metal thickness (see also Fig. 2), the observed rotation corresponds to a final effective alloy thickness of about 200 A. By carrying out an analysis similar to the above for several points taken during oxidation, we have derived curves showing the variations, with oxidation time, of alloy composition and of apparent metal thickness. It should be noted that these curves, shown in Fig. 3, are derived using rather uncertain values of the Verdet constants 7. This, together with the scatter of experimental points, makes the results presented in Fig. 3 rather imprecise, so that they should be taken to represent only qualitative trends. Several significant conclusions can be drawn, however. Since the residual alloy becomes * Simple graphical correction must be made for the ordinary Faraday effect which is proportional to H rather than M and which is very much smaller than the ferromagnetic effect. t In ferromagnetiesit is actually more appropriate to use the Kundt constant K, i.e. the rotation per centimeter at saturation.
OXIDATION OF PERMALLOY THIN FILMS
97
1.3
6
12
12
~o
~0
II~.
4Tru ^
&
X
1.0
4.M*
IOkg
(d*grees)
8 :
z" ocus
!
e •
•
•
e
Ni Fe2 04
~o, I
30
I
I
60 TIME(MINUTES)
12o
90
0.1
0.2 O.S 0.4 FILM THICKNESS I microns)
0.5
06
Fig. 1. Variation of saturation magnetization 4rcM and saturation Faraday rotation 0, for Permalloy f'dms, with oxidation time at 400 °C. Fig. 2. Dependence of saturation Faraday rotation on f'dm thickness for sputtered films of Permalloy, 81% Ni : 19% Fe, and nickel ferrite, NiFeEO 4. IO0
3~
95
400 % NICKEL 300
ALLOY 9 0 COMP~SRION
ALLOT THICKNESS
(II
1%16}
ALLOY THICKNESS
20o S3
*00
L
I EO
I
I 40
I
I
&O TIME I miflutes)
i
I SO
I
I I00
i
80
Fig. 3. Variation of the calculated residual alloy composition and thickness with oxidation time at 400 °C.
richer in nickel, the films must experience a preferential oxidation o f the iron. Taken together with the decreasing effective thickness o f the alloy, this suggests that the oxide phase which first appears is FeO, while the nickel oxidizes more slowly and combines with the iron oxide to form a monoxide solid solution o f which the final constitution is near Nio.81 Feo.190. The slowly varying part o f the thickness curve is probably indicative o f the nickel oxidation. Films oxidized at higher temperatures oxidized more rapidly and completely, b u t in a similar manner. REFERENCES 1 W. Strauss, Prec. IEEE, 58 (1970) 1386;./. Appl. Phys., 42 (1971) 1251. 2 G. S. Almasi, G. E. Keels, Y. S. Lin and D. A. Thomson, J. Appl. Phys., 42 (1971) 1268. 3 R. Bozorth, Ferromagnetism, Van Nostrand, Princeton, N.J., 1951, p. 758.
98
R.L. COREN, M. H. FRANCOMBE
4 C. H. Bajorek and A. F. Mayadas, Magnetism and Magnetic Materials- 1972, AlP Conf. Proc., Vol. 8, in the press. 5 M. H. Francombe and A. J. Noreika, Electric and Magnetic Properties o f Thin Metallic Layers, AWLSK, Palais der Academi~n, Brussels, 1961, p. 193. 6 R. L. Coren and M. H. Franeombe, J. Phys. (Paris}, 25 (1964) 233. 7 W. Schutze, Handbuch der Experimental Physik, Band 16, Magnetooptik, Akademische Verlagsgessellschaft M.B.H., Leipzig, 1936.
© Elsevier Sequoia S.A., Lausanne-Printed in Switzerland
95
OXIDATION OF PERMALLOY THIN FILMS* R. L. COREN
Electrical Engineering Department, Drexel University, Philadelphia, PaL (U.S.A.)
M. H. FRANCOMBE Wesffnghouse Research Laboratories, Pittsburgh, Pc- (U.S.A.)
(Received August 25, 1975)
1. INTRODUCTION The recent proposals 1'2 for bubble memories which involve magnetoresistive sensing of the bubble place stringent requirements on the Permalloy film detection dements because of the small amount of stray flux. While the fractional saturation magnetoresistance R = A p / p is high in Permalloy, it varies significantly with composition3: in bulk at room temperature d R / d C = ~% per % nickel at C = 80% Ni composition, where R = 4%. As a result, attention has been focussed 4 on magnetoresistance variations and degradation arising from compositional changes caused by ambient temperature oxidation of the film sensors. In previously reported investigations s of sputtered Permalloy Films, we concluded from magnetostrictive measurements that the presence of small amounts of oxygen in the vacuum system reduced the metallic iron content of the Ni-Fe alloy Films, Le. that the iron is preferentially oxidized. In the present study, electron diffraction and the optical Faraday effect have been used to examine further the low temperature oxidation behavior of 81% Ni: 19% Fe alloy Films. The results suggest that strong preferential oxidation of iron does in fact occur during initial stages of oxidation and that large changes might consequently be expected in the magnetoresistive coefficient for the fdms. 2. EXPERIMENTAL The PermaUoy films were prepared by d.c. sputtering from vacuum-melted alloy targets of nominal composition 81% Ni : 19% Fe using experimental conditions described previously s. The Films were grown to a thickness of about 500 A on vitreous silica substrates. Oxidation was carried out in flowing oxygen at temperatures between 350 ° and 500 °C and for times ranging up to 2 h. Electron diffraction studies were performed in a Siemens Elmiskop electron microscope in the transmission mode at 80 kV. The Faraday rotation measurements were made using an experimental arrangement described previously by the authors in a study of ferrite films6. Polarized light was passed through holes in the poles of an electromagnet, normally through the film and through an analyzer to a photomultiplier tube. * Paper presented at the Third International Conference on Thin Films, "Basic Problems, Applications and Trends", Budapest, Hungary, August 25-29, 1975; Paper 5-02. This study was made while the authors were at the Philco Scientific Laboratory, Blue Bell, Pa., U.S.A.
96
R.L. COREN, M.H. FRANCOMBE
The output of the tube was fed to the Y axis of an X Y recorder, while the X axis recorded the applied magnetic field. With the polarizer and analyzer at 45 ° , small Faraday rotations produced proportional phototube signals. With the field off, the analyzer could be rotated through a known angle to calibrate the vertical recorder scale. As the magnetic field is increased, the magnetization rises out of the film plane. Since the ferromagnetic Faraday effect varies linearly with the component of M along the light path, the rotation 0 increases until M stands perpendicular to the film. This saturation occurs when the applied field equals 4riM, the demagnetizing field, so that the mean saturation value of 47rM can be determined. In addition, from the height of the curve the magnitude of the Faraday effect can be measured in each film*. 3. RESULTS AND DISCUSSION
Electron diffraction studies showed that, typically for films oxidized at about 400 °C, an oxide phase could be readily detected after about 5 min. Complete oxidation of the 500 A Film required times ranging up to several hours. At all stages of oxidation the oxide comprised a single NaCl-type monoxide structure with a lattice parameter close to those of the cubic WiJstite phases of NiO and FeO. The close similarity of the lattice parameters for NiO and FeO made it impossible to infer the composition of the monoxide phase from lattice parameter measurements. However, the fact that these monoxides are non-ferromagnetic meant that Faraday rotation effects measured during stages of oxidation were influenced only by the metal alloy constituent remaining. Figure 1 shows the measured values of 4riM and of saturation Faraday rotation as a function of oxidation time. After each stage of oxidation the translation from the rising linear portion of the 0 versus H Faraday curve to the region of saturation was quite sharp, indicating a fairly uniform residual alloy composition throughout the Film. During the first 20 min both 4ztM and 0 decreased rapidly, this being followed by a much more gradual change. The change in 4zrM is from about 10 000 to 7800 G. Since we have found no sensible thickness dependence of magnetization in our alloy Films, down to about 100 A, this change is attributed to a compositional variation and corresponds to a change of composition from 81% Ni : 19% Fe to 92% Ni : 8% Fe. Using the known Verdet constantt 7 for this final composition and the fact that the Faraday rotation is directly proportional to the metal thickness (see also Fig. 2), the observed rotation corresponds to a final effective alloy thickness of about 200 A. By carrying out an analysis similar to the above for several points taken during oxidation, we have derived curves showing the variations, with oxidation time, of alloy composition and of apparent metal thickness. It should be noted that these curves, shown in Fig. 3, are derived using rather uncertain values of the Verdet constants 7. This, together with the scatter of experimental points, makes the results presented in Fig. 3 rather imprecise, so that they should be taken to represent only qualitative trends. Several significant conclusions can be drawn, however. Since the residual alloy becomes * Simple graphical correction must be made for the ordinary Faraday effect which is proportional to H rather than M and which is very much smaller than the ferromagnetic effect. t In ferromagnetiesit is actually more appropriate to use the Kundt constant K, i.e. the rotation per centimeter at saturation.
OXIDATION OF PERMALLOY THIN FILMS
97
1.3
6
12
12
~o
~0
II~.
4Tru ^
&
X
1.0
4.M*
IOkg
(d*grees)
8 :
z" ocus
!
e •
•
•
e
Ni Fe2 04
~o, I
30
I
I
60 TIME(MINUTES)
12o
90
0.1
0.2 O.S 0.4 FILM THICKNESS I microns)
0.5
06
Fig. 1. Variation of saturation magnetization 4rcM and saturation Faraday rotation 0, for Permalloy f'dms, with oxidation time at 400 °C. Fig. 2. Dependence of saturation Faraday rotation on f'dm thickness for sputtered films of Permalloy, 81% Ni : 19% Fe, and nickel ferrite, NiFeEO 4. IO0
3~
95
400 % NICKEL 300
ALLOY 9 0 COMP~SRION
ALLOT THICKNESS
(II
1%16}
ALLOY THICKNESS
20o S3
*00
L
I EO
I
I 40
I
I
&O TIME I miflutes)
i
I SO
I
I I00
i
80
Fig. 3. Variation of the calculated residual alloy composition and thickness with oxidation time at 400 °C.
richer in nickel, the films must experience a preferential oxidation o f the iron. Taken together with the decreasing effective thickness o f the alloy, this suggests that the oxide phase which first appears is FeO, while the nickel oxidizes more slowly and combines with the iron oxide to form a monoxide solid solution o f which the final constitution is near Nio.81 Feo.190. The slowly varying part o f the thickness curve is probably indicative o f the nickel oxidation. Films oxidized at higher temperatures oxidized more rapidly and completely, b u t in a similar manner. REFERENCES 1 W. Strauss, Prec. IEEE, 58 (1970) 1386;./. Appl. Phys., 42 (1971) 1251. 2 G. S. Almasi, G. E. Keels, Y. S. Lin and D. A. Thomson, J. Appl. Phys., 42 (1971) 1268. 3 R. Bozorth, Ferromagnetism, Van Nostrand, Princeton, N.J., 1951, p. 758.
98
R.L. COREN, M. H. FRANCOMBE
4 C. H. Bajorek and A. F. Mayadas, Magnetism and Magnetic Materials- 1972, AlP Conf. Proc., Vol. 8, in the press. 5 M. H. Francombe and A. J. Noreika, Electric and Magnetic Properties o f Thin Metallic Layers, AWLSK, Palais der Academi~n, Brussels, 1961, p. 193. 6 R. L. Coren and M. H. Franeombe, J. Phys. (Paris}, 25 (1964) 233. 7 W. Schutze, Handbuch der Experimental Physik, Band 16, Magnetooptik, Akademische Verlagsgessellschaft M.B.H., Leipzig, 1936.