Journal of Magnet ism and Magnetic Materials 123 (1993) 304-310 North-Holland
Magnetic properties of amorphous films of Bi203-CuO-Fe203: Studies on the effects of sputtering parameters
G. Sr in ivasan and B. U m a M a h e s h w a r R a o
Department of Physics, Oakland University, Rochester, MI 48309-4401, USA
Received 27 May 1992; in revised form 28 August 1992
Amorphous thin films of B i 2 0 3 - C u O - F e 2 0 3 are paramagnet ic in the as-deposited state and show a ferrimagnetic character when annealed in air at high temperatures. From magnetization and ferromagnetic resonance studies, the spontaneous moment in the annealed samples is attributed to the presence of ordered clusters with a composition and spin structure similar to that for crystalline copper ferrite. This study is concerned with the dependence of magnetic properties of sputtered films on preparation conditions such as the sputtering atmosphere and substrate temperature. Studies on 0.2 Bi~O3-0.3 CuO-0 .5 F e 2 0 3 prepared in a mixed oxygen-argon atmosphere and annealed at high temperatures show (i) that the saturation magnetization, Curie temperature, ferromagnetic resonance linewidth and the gyromagnetic ratio increase with increase in the oxygen partial pressure p(O2), and (ii) that the uniaxial anisotropy field decreases as the oxygen partial pressure is increased. The anticipated presence of divalent iron ions in films prepared in an argon-rich atmosphere may account for the observed variation of some magnetic parameters with p(O2). Attempts to initiate the formation of magnetically ordered amorphous clusters in situ by heating the substrates were unsuccessful. Films prepared at substrate temperatures T~ below 800 K are amorphous and are paramagnetic at room temperature. At higher T~, one observes the formation of crystalline magnetic phases in the samples.
1. Introduction
In spite of unsuccessful at tempts in the past to synthesize magnetically ordered amorphous ox- ides, several studies in recent years show evi- dence for a long-range order in such compounds. Notable among them are amorphous ferrites in phosphate glasses [1,2] and rapidly quenched or rf-sputtered B izO3-Fe20 3 substituted with mag- netic and nonmagnetic compounds such as CuO, CaO, ZnO, and ABO 3 (AB = BaTi, PbTi, PbZr) [3-12]. The room-temperature saturation induc- tion 4wM for the compounds is as high as 3.8 kG and the Curie tempera ture T c ranges from 400 to 800 K [3]. The spontaneous moment in the non- crystalline samples is suggested to arise from magnetically ordered clusters.
Correspondence to: Dr G. Srinivasan, Depar tment of Physics, Oakland University, Rochester, MI 48309-4401, USA.
We observed recently a ferrimagnetic behavior characterized by uniaxial anisotropy in rf- sputtered films of ( 0 . 5 - x ) Bi20 3 - x C u O - 0.5 Fe20 3 for x = 0 . 1 - 0 . 4 [11,12]. The most impor- tant features of the results of the studies are as follows: (i) as-sputtered samples with x = 0.1-0.4 are paramagnetic and samples annealed in air at temperatures above 700 K are ferrimagnetic with an uniaxial anisotropy; (ii) the largest 4rrM of 3 kG at room temperature is observed for the film with x = 0.4; and (iii) a positive identification of the nature and composition of magnetically or- dered clusters in annealed samples was possible from data on Curie temperature, low-tempera- ture magnetization, and g-values. The T c value of 725 K, the magnetization of 1.1-1.6 Bohr magne- ton (/x B) per copper ion, and the g-value of 2.1-2.3 are in excellent agreement with the val- ues expected for copper ferrite. Thus the ordered clusters in the amorphous oxides are likely to
G. Srinivasan, G. Uma Maheshwar Rao / Magnetic properties of Bi 20 3-CuO-Fe 20 3 films 305
have compositions and spin structures similar to those for crystalline CuFe204 [12].
This study is concerned with the variation in magnetic parameters that accompany changes in the sputtering conditions for films of Bi203- CuO-Fe203. In particular, studies were per- formed to understand the effects of two key pa- rameters: the sputtering atmosphere and sub- strate temperature. A systematic study on the influence of the oxygen partial pressure p(O 2) is important for several reasons. First, an oxygen deficiency in films sputtered in an argon-rich atmosphere will influence the valence state of Fe and, therefore, the sample magnetization. Sec- ond, one expects changes in the nature and strength of superexchange interactions in the presence of divalent iron in the ordered clusters. The other sputtering parameter of importance is the substrate temperature. Since it is clear from our investigations that high-temperature anneal- ing is a key ingredient for achieving a long-range order in rf-sputtered amorphous oxides, we made an attempt to obtain ordered clusters in situ by heating the substrates. The results of such investi- gations are presented in the following.
2. Experiment
Films of 0.2 Bi203-0.3 CuO-0.5 Fe203 were prepared by the technique of rf sputtering using single-target (Torus 2C, Kurt J. Lesker Co.) sput- tering system with provisions for deposition in controlled atmospheres. The most significant ad- vantage of the Torus sputter source is its ability to sputter targets of magnetic materials. For film preparations, a 2 inch diameter polycrystalline sintered target prepared by standard ceramic techniques from appropriate chemicals was at- tached to the magnetron gun. The system was evacuated to a base pressure of 10 -5 Torr. The sputtering was carried out in a mixed argon- oxygen atmosphere at a total pressure of 1 mTorr and at an oxygen partial p r e s s u r e p ( O 2) varying from 0 to 100%. An rf power of 100-200 W at 13.6 MHz was supplied to the magnetron gun. Films were deposited on Corning 7059 glass, sili- con, and silica substrates mounted on a rotating
,,.<
kr, d
<
Z
Ud
D
U3
300
200
100
I [ I T I I r I
20 40 60 80 I00
PERCENTAGE OXYGEN PARTIAL PRESSURE
Fig. 1. Data on sputtering rate versus percentage oxygen particle pressure p(O 2) for amorphous films of 0.2 Bi203-0.3 CuO-0.5 Fe203 sputtered at a total pressure of 1 mTorr in a
mixed oxygen-argon atmosphere.
unheated platform placed below the target. In studies involving substrate heating effects, a resis- tive heater was used to achieve desired substrate temperatures.
The film thickness was measured with a stylus profilometer. Figure 1 shows the dependence of the thickness on the sputtering atmosphere. A linear decrease in the sputtering rate is observed as the oxygen partial pressure is increased. Films with a thickness of about 1 ~m were used for structural and magnetic characterization. The chemical composition of the films was deter- mined by energy dispersive X-ray spectroscopy. Samples with the composition (0.25 + 0.05) Bi203-0.3 CUO-(0.45 +0.05) Fe203 were ob- tained. Thus the films were bismuth rich, but iron deficient. We did not observe any systematic vari- ation of the film composition with the oxygen partial pressure. In the discussion to follow, we designate the films by the target composition.
Studies on the crystal structures of as-sputtered and annealed samples were done by X-ray diffractometry (XRD). Structural data on an- nealed films are essential for an understanding of the magnetic properties discussed in the follow- ing. The as-deposited samples were found to be amorphous. Figure 2 shows XRD data for a rep- resentative sample annealed at a series of tem-
306 G. Srinivasan, G. Uma Maheshwar Rao / Magnetic properties of Bi203 CuO-Fe 20 ~ films
< talline CuO and Bi2Fe40 ~ are seen. The most = important inferences from data in fig. 2 are (i) [-
< sample crystallization occurs at 825-875 K, and (ii) crystallization does not lead to the formation of ferrimagnetic compounds such as Bi3FesO~> y - F e 2 0 3 , Fe304, o r C u F e 2 0 4 .
3. Results
1000
800
600
400
200
• 100%
• 60 %
• 2 0 %
./ 700
0 I I
600 800 900 1000
ANNEALING TEMPERATURE (K)
Fig. 3. Variation of room-temperature saturation induction 4-n-M with the annealing temperature T a for films sputtered in P (O2)= 1()0, 60 and 20%. The magnetization was meas-
ured with an in-plane static magnetic field of 5 kOe.
3.1. Effects of sputtering atmosphere
Saturation magnetization and ferromagnetic resonance measurements were carried out with a Faraday balance and an X-band ESR spectrome- ter, respectively. As-deposited samples were
~i,,~, I : ,-: it') 7- t-~ !
I ~ ~ z c
I I / I h , b
I I i i i i t l i I
0 1 0 2 0 3 0 ,1 0 5 0 6 0 7 0 8 0 9 0
, ' \ N G I , E 2 0 ~ d e g . )
Fig. 2. X-ray diffraction pat tern for samples sputtered in a 50% oxygen + 50% argon a tmosphere and annealed in air at (a) 775 K, (b) 825 K, and (c) 875 K for 60 min. The diffraction peaks corresponding to the crystalline phases BizFe409 and
CuO are indicated by 1 and 2, respectively.
paramagnetic at room temperature. The films develop a spontaneous moment on annealing in air at high temperatures. Figure 3 shows the room-tempera ture saturation induction 4 v M versus T, data for samples sputtered in a series of oxygen partial pressures ranging from 0 to 100%. The data were obtained for an in-plane static magnetic field of 5 kOe. Consider first the varia- tion of 4-rrM with T,. The magnetization remains small and Zd-independent for low annealing tem- peratures. The onset of a long-range magnetic order is clearly evident for T~, = 725 K. The mag- netization increases at higher T, and shows a maximum at about 800 K. Any further increase in T, leads to a decrease in 4~rM. One also notices a pronounced dependence of 4"rrM on the sput- tering atmosphere. The magnetization is the largest for the film sputtered in pure oxygen. As p(O 2) is decreased from 100 to 0%, a correspond- ing decrease in 4"rrM is evident in fig. 3.
The film magnetization corresponds to 15-50% of crystalline ferrimagnetic phases such as C u F e 2 0 4 , Bi3Fe5012, y - F % O 3, or F % O 4 and XRD is sensitive enough to detect their forma- tion. Since the data in fig. 2 do not show the presence of such phases, we propose that the spontaneous magnetization in annealed samples is due to ordered amorphous clusters [11,12].
G. Srinivasan, G. Urea Maheshwar Rao / Magnetic properties of Bi203-CuO-Fe20 3 films 307
Further, the observed decrease in 4rrM at high T a in fig. 3 is accompanied by the loss of Cu and Fe from the amorphous lattice in the form of crystalline CuO and Bi2FeaO 9 as seen in fig. 2. Thus, the ordered clusters are most likely to have chemical compositions and spin structures similar to those for ferrimagnetic copper ferrite. Addi- tional evidence for their possible presence was obtained from data on low-temperature magneti- zation, Curie temperature and g-values.
The variation of 4~rM with temperature for annealed samples is shown in fig. 4. The data are for films sputtered at various oxygen partial pres- sures. A definite increase in 4,a'M with p(O 2) is seen at all temperatures. The magnetization at liquid nitrogen temperature varies from 0.5/x B per formula unit for the film sputtered in pure oxygen to 0.2/x a for the argon sputtered sample. The Curie temperatures for the films were deter- mined by linear extrapolation of the high-temper- ature region of the data in fig. 4, and the varia- tion of T c with p(O 2) is shown in fig. 5. A linear increase in T c with increasing oxygen partial pressure is obvious from the data. The Curie temperatures are smaller than the crystallization temperature of 825-875 K. The T c values are either smaller or larger than the Curie tempera- ture for crystalline -/-Fe203, Fe304, Bi3FesO12 [13-151.
1200 ~ i ~ , . . . • 100% ~ * 60
Z ~ Z 600
(3 400
~" 200
r~ 0 ' ' 0 200 400 600 800
TEMPERATURE (K)
Fig. 4. Saturation induction 4wM versus temperature T data for samples sputtered in p(O 2) = 100, 60 and 20% and an-
nealed in air at 750-800 K.
800
G 780 k ; d
D 760
740
72O
D 700' U
68O 0
] T I I I I T I I
20 40 60 80 100
PERCENTAGE OXYGEN PARTIAL PRESSURE
Fig. 5. The dependence of the Curie temperature T¢ on the oxygen partial pressure p(O2) for annealed films. The Curie temperatures were determined from 4~rM versus T data as in
fig. 4.
Now we discuss two important features of the data in figs. 3-5: the origin of the spontaneous moment in annealed samples and the variation of magnetic parameters with the sputtering atmo- sphere. The low-temperature moment for sam- ples sputtered in an oxygen-rich atmosphere is 0.3-0.5~z B per formula unit. If the moment arises from ferrimagnetic clusters in which the iron mo- ments are antiparallel, then one estimates a mo- ment of 1-1.6#B per Cu ion in the films, and the magnetization is in reasonable agreement with the value of 1.3/~ B for crystalline copper ferrite [13]. In addition, the measured T c of 740-790 K for the samples are in good agreement with the Curie temperature of 725 for copper ferrite [13]. Thus films made in oxygen-rich atmospheres are likely to contain clusters with a ferrimagnetic spin structure similar to crystalline copper ferrite. Samples sputtered in argon-rich atmospheres show a relatively small 4"rrM and T c. Studies on amorphous BiFeO3-PbZrO 3 [5] also show a simi- lar reductions in the magnetization for samples deposited in argon-rich atmospheres. Under such sputtering conditions, films are expected to de- velop an oxygen deficiency leading to the forma- tion of divalent iron ions. The low-temperature moment for divalent and trivalent iron ions are 4 and 5/XB, respectively. Therefore, the presence of
308 G. Srinivasan, G. Uma Maheshwar Rao / Magnetic properties of Bi eO 3-CuO-Fe 20 ~ films
Fe z+ could result in a reduced sample magneti- zation. In addition, in ferrimagnets the dominant antiferromagnetic superexchange interaction be- tween trivalent iron ions is stronger than either Fe3+-Fe e+ or FeZ+-Fe e+ superexchange inter- actions. Thus the observed decrease in T c with decreasing p (O e) could be understood in terms of a reduction in the strength of superexchange interactions when divalent Fe is present in the films.
The possibility of ferrimagnetic order in amor- phous oxides was examined recently by Kaneyoshi [16]. A system consisting of even-membered rings of s p i n - l / 2 and spin-1 ions was considered in this work. This simple model predicts a canted ferro- magnetic structure, with a random distribution for the canting angle. The estimated 4"rrM versus T show features expected for ferrimagnetic or- dering.
Results of the present study are in qualitative agreement with a recent report on magnetic or- dering in rapidly quenched amorphous samples of B i z O 3 - C u O - F e 2 0 3 [10]. Mossbauer measure- ments showed evidence for coexisting paramag- netic and ferrimagnetic phases in the amorphous samples. A room-tempera ture moment of 2-5
Fig. 6. Temperature dependence of the uniaxial anisotropy field H u perpendicular to the sample plane for films sput- tered at p(O 2) = 100, 50 and 0% and annealed at 750-800 K. The anisotropy values were determined from the static mag- netization data as in fig. 4 and from ferromagnetic resonance
Fig. 7. Variation of the gyromagnetic ratio 7 with T for films sputtered with P(O2)= 100, 50, and 0% and annealed at
750-800 K.
e m u / g in the samples was attributed to clusters of copper ferrite. A cluster size of 10-30 nm was estimated from small angle X-ray scattering stud- ies.
In the present study, the if-sputtered films show a uniaxial anisotropic character. The anisotropy field H u perpendicular to the film plane was estimated from the saturation magneti- zation 4rrM and the effective magnetization 4rrMef f = 4~rM - Hu. Values of 4~rMeff were de- termined from ferromagnetic resonance fields for in-plane and out-of-plane static field orientations. In fig. 6, H u versus T data are shown for an- nealed films sputtered in oxygen, argon, and 50% oxygen+ 50% argon atmospheres. As T de- creases, a general decrease in H~ is seen for all samples. The anisotropy is the largest for the argon sputtered film. For the oxygen sputtered sample, a planar anisotropy characterized by a negative H u value is observed for temperatures below 275 K.
Figure 7 shows the dependence of the gyro- magnetic ratio y on temperature for annealed samples. The values were obtained from FMR measurements at 9.2 GHz. The room tempera- ture 3' = 2.9-3.0 G H z / k O e corresponds to a g- value of 2.07-2.14, and is in very good agreement with the reported g of 2.05-2.2 for crystalline copper ferrite. A sharp increase in 3' with a
b.. r~
"7
Z
e,,
r:_
1600
1400
1200
1000
800
600
400 5O
n 41
G. Srinivasan, G. Uma Maheshwar Rao / Magnetic properties of Bi 2 0 3-CuO-Fe 20 3 films
• 100%
,t, 50%
• • u 0%
I I
m
o • • 4 I=
309
0 O 0 0
T , I i I i , I
100 150 200 250 300
TEMPERATURE (K)
Fig. 8. Ferromagnetic resonance linewidth AH versus tem- perature data for films sputtered with p(O 2) = 100, 50 and 0% and annealed at 750-800 K. The linewidth were measured at 9.2 GHz for static magnetic fields perpendicular to the film
plane.
800
Z 600 O E-
C~ 400 Z
Z (3
200 < r.,
[.-. <
0 5O0 600 700 800
I
900
SUBSTIL-kTE TEMPERATURE (K)
Fig. 9. Room-temperature saturation induction 4~rM versus substrate temperature T s data for films of 0.2 Bi203-0.3 CuO-0.5 Fe203 sputtered in a 50% oxygen+50% argon
atmosphere.
decrease in T is observed for all samples. Such a strong dependence is often seen at temperatures close to the compensation temperature in ferri- magnetic rare earth iron garnets [17]. Theoretical works are essential for an understanding of the observed variation of 3' with T.
The peak-to-peak FMR linewidth AH versus T for the three films is shown in fig. 8 for static fields perpendicular to the film plane. The linewidth for in-plane static fields shows a similar behavior. One observes broadening of the reso- nance absorption with decreasing T. The absence of Fe 2+ in films sputtered in oxygen-rich atmo- spheres is expected to result in a relatively small A H since conductivity assisted line-broadening does not occur in the absence of Fe 2+. However, the oxygen sputtered samples shows the largest A H and may be due to structural defects such as pinholes.
3.2. Substrate heating effects
The studies were conducted on films of 0.2 Bi203-0.3 CuO-0.5 Fe203 sputtered in a 50% oxygen + 50% argon atmosphere. Desired sub- strate tempera tures T s = 450-900 K were
achieved using a resistive heater. The saturation induction 4"rrM was measured at room tempera- ture for a in-plane static field of 5 kOe. Figure 9 shows the observed changes in 4~rM with T~ for as-sputtered samples. For T s < 675 K, the films are paramagnetic and the moment remains small. The sample develops a large magnetization for
-=
[. . , Z
I I [ I [ [ I I'
10 20 30 40 50 60 70 80
ANGLE 20 (deg.)
Fig. 10. X-ray diffraction pattern for a 0.2 Bi203-0.3 CuO-0.5 Fe203 film sputtered with p(O 2) = 50% and T s = 775 K. The intensity peaks are due to crystalline Bi2 Fe409 and CuFe204.
310 G. Srinivasan, G. Uma Maheshwar Rao / Magnetic properties Of Bi 20.¢-CuO-Fe 203 filrns
T, = 775 K. X-ray diffraction data were obtained for the films in order to understand the origin of the moment. Such data are shown in fig. 10 for T~ = 775 K. Films sputtered at T, = 675 K were amorphous. Figure 10 shows crystalline phases for the film sputtered at 775 K. The phases are identified as antiferromagnetic B i z F e 4 0 9 and ferrimagnetic CuFe204. The Curie temperature for the film was 740 K, in excellent agreement with T c for crystalline copper ferrite. Thus the moment in the sample sputtered at 775 K ap- pears to arise due to the crystallization of ferri- magnetic phases in the sample. Recall that, as seen in figs. 2 and 3, a film sputtered on an unheated substrate and annealed at 775 K also exhibits a large moment even though X R D data do not indicate any crystalline phases. Thus sput- tering on unheated substrates fol lowed by high- temperature annealing appear to be essential for the formation of magnetically ordered amorphous clusters.
Acknowledgements
We are grateful to Ms Uma Saligram for as- sistance with magnetic measurements. The work was supported by a grant from the Petroleum Research Fund, administered by the American Chemical Society.
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