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Curie temperatures and anisotropy fields in very thin garnet films
925
Journal of Magnetism and Magnetic Materials 31-34 (1983) 925-926 CURIE TEMPERATURES T. T A K E U C H I ,
AND ANISOTROPY
FIELDS IN VERY THIN GARNET
FILMS
N . O H T A a n d Y. S U G I T A
Central Research Laboratory, Hitachi, Ltd., Kokubunfi, Tokyo 185, J a p a n
Curie temperature T~ and effective anisotropy field H k - 4 ~ r M s have been measured for epitaxial magnetic garnet films with thicknesses ranging from 20 ,A to 2.8 pm. It was found that T~ and H k -4~rM s were unchanged down to 60 A for YaFesOi2 (YIG) and down to 600 ,A for mixed garnet. 1. Introduction
Y3FesOj2 (YIG) and mixed rare-earth ion garnets are used in microwave devices like isolators and magnetic bubble devices. These garnets are of bulk or of films with thicknesses around 1 # m or larger. Magnetic properties of garnet films with such thicknesses have been widely investigated [1-3], whereas those of garnet films with much smaller thicknesses have not been reported. We prepared Y I G and ( L a L u S m G d ) 3 (FeGa)5012 films with thicknesses less than 1000 .A and measured their Curie temperature T~ and effective anisotropy field H k - 4 ~ ' M s. Details will be described below. 2. Experimental procedure
Films of Y I G and of (LaLuSmGd)3(FeGa)5Ol2 [2] were grown on ( l l l) oriented G d a G a s O i 2 ( G G G ) substrates by liquid phase epitaxy (LPE). Substrates were rotated at 60 rpm in the melt changing rotation direction. In order to obtain extremely thin films, special growth conditions were employed. For instance, the thinnest Y I G film was grown under such a condition that melt temperature was just at the saturation temperature or higher and the growth time was only 5 s. In this case, the film was grown because the substrate temperature was lower than the melt temperature. Film thickness was adjusted by controlling growth temperature and growth time for Y I G and mainly by etching for (LaLuSmGd)a(FeGa)~ O12. Film thickness was measured using three techniques: reflectance interference, fluorescent X-ray analysis, and absorbance analysis according to thickness range. Fluorescent X-ray analysis, which is superior in precision to reflectance interference method, does not give thicknesses less than 100 ,~ due to much larger background intensity. Thickness less than 100 ,A was measured by absorbance which is the logarithm of the ratio of incident light intensity to transmitted light intensity and proportional to film thickness. Absorbance was measured with a conventional spectrophotometer in the wavelength region of 0.30-0.36 # m where its magnitude was appropriate. Effective anisotropy field H k - 4 ~ r M s was obtained 0304-8853/83/0000-0000/$03.00
by X-band ferromagnetic resonance of perpendicular geometry using the well known formula H k --
47rMs =
~/V
- H,
where ~0 is microwave angular frequency, 7 the gyromagnetic ratio and H the resonance field. As temperature increases, H k - 4~-Ms tends to zero. Curie temperature T~ was obtained as the temperature at which H k - 4 ~ r M s became zero. 3. Results and discussion
Fig. 1 shows T~ and H , - 4~rMs (at room temperature) of Y I G as a function of film thickness. T¢ and H k - 4 ~ r M s are almost unchanged in the thickness range of 60 A to 2.8/~m. In this range, T~ is about 290°C and Hk -- 4~rM~ is about - 1.6 kOe. The value of Tc is almost the same as that of bulk [l]. H k - 4~rM~ of - 1.6 kOe agrees with the calculated value obtained with the following values: (a) anisotropy field of 100 Oe induced by the stress due to lattice mismatch through magnetostriction; (b) anisotropy field of 60 Oe related to cubic anisotropy; and (c) saturation induction 4~rM~ of about 1800 G for bulk Ill. Thus, films are very homogeneous for thickness down to 60 ,~. It is seen that T¢ decreases by 25°C and H k - 4~rM~ increases by 0.3 kOe at about 20 .A. A decrease in T~ indicates that average molecular field is reduced due to thickness reduction or Fe molar fraction is reduced due to Ga diffusion from G G G . Both cases lead to a reduction of 4~rM~. The 4~rMs reduction is in accord with the H k - 4~rM~ increase if we assume that the H k change is smaller.
•~. -,
Tc
f-'~
-1 .E
1o
• .300~ 280
'-3' -1.C. - * ~ - - ~ '
~-2
•
YIG ,,~
e ~ m Hk_41tM~i
16o
1ooo'
~60 ~ Z40
10600
Film Thickness (~)
Fig. 1. Curie tem[~erature and effective anisotropy field vs. film thickness for Y1G.
© 1983 N o r t h - H o l l a n d
926
T. Takeuchi et aL / Curie temperatures and anisotropy fields in garnet films =
1
Te • • 1200 .{"F . . . . . . i,80._. \ (LaLuSm )a(F-e~)s% , ~ ,...,,_.,~,___,, 160 i-~ o~o--
asL.~
Hk-41lMs
100
1000
10600
l
Film Thickness (/~) Fig. 2. Curie temperature and effective anisotropy field vs. film thickness for substituted YIG.
Fig. 2 shows T¢ and H k - 4 ~ r M s (at room temperature) of (LaLuSmGd)3(FeGa)5Ol2 as a function of film thickness. It is seen that H k - 4 ~ ' M s of this garnet is positive in contrast with that of YIG. This is because the magnetic easy axis is perpendicular to the film plane due to growth-induced anisotropy resulting from site ordering of the rare-earth ions. T¢ and H k -4~'Ms are almost unchanged (190°C and 1.0 kOe, respectively) in the thickness range of 600 ,~ to 2/~m. Though a gradual change in H k - 4~rMs is found in this range, the variation is +5% for the magnitude of H k - 4 * r M s and is negligibly small. The fact that T~ is unchanged indicates that the Ga to Fe molar ratio [1], which determines 4*rMs, is unchanged. Consequently, the fact that the variation in H k -4*rM~ is small leads to the fact that the variation in Hk, which is related to rare-earth ions molar ratio, is also small. Thus, the films are homogeneous down to 600 ~,.
Films thinner t h ~ this value show an abrupt increase in H k - 4*rMs: The thickness at which H k - 4~rM~ begins to vary for this garnet is one order of magnitude larger than that for YIG. This difference is presumably due to the difference in the number of elements contained in a site. When more than two elements are contained in a site, the composition depends on growth rate through the segregation effect [4]. This brings about the composition variation in the initially deposited region because of rapid growth prior to steady state growth. Magnetic properties for this altered composition appear in the thinner (LaLuSmGd)a(FeGa)5Ol2 films. The change in T~ for this garnet at thicknesses less than 600 A is not dear. This is because the resonance linewidth was large due to large relaxation of Sm and the resonances for the thinner films were concealed behind G G G paramagnetic resonance. However, even if T¢ changes, the amount is less than 15°C at thicknesses down to 200 ,~. The authors thank Professor S. Chikazumi for helpful discussions. References [1] P. Hansen, P. Roschmann and W. Tolksdorf, J. Appl. Phys. 45 (1974) 2728. [2] N. Ohta, T. Ikeda, F. Ishida and Y. Sugita, IEEE Trans. Magn. MAG-16 (1980) 610. [3] H.A. Algra and J.M. Robertson, J. Appl. Phys. 50 (1979) 4295. [4] E.A. Giess, D.C. Cronemeyer, R. Ghez, E. Klokholm and J.D. Kuptsis, J. Am. Ceram. Soc. 56 (1973) 593.
Journal of Magnetism and Magnetic Materials 31-34 (1983) 925-926 CURIE TEMPERATURES T. T A K E U C H I ,
AND ANISOTROPY
FIELDS IN VERY THIN GARNET
FILMS
N . O H T A a n d Y. S U G I T A
Central Research Laboratory, Hitachi, Ltd., Kokubunfi, Tokyo 185, J a p a n
Curie temperature T~ and effective anisotropy field H k - 4 ~ r M s have been measured for epitaxial magnetic garnet films with thicknesses ranging from 20 ,A to 2.8 pm. It was found that T~ and H k -4~rM s were unchanged down to 60 A for YaFesOi2 (YIG) and down to 600 ,A for mixed garnet. 1. Introduction
Y3FesOj2 (YIG) and mixed rare-earth ion garnets are used in microwave devices like isolators and magnetic bubble devices. These garnets are of bulk or of films with thicknesses around 1 # m or larger. Magnetic properties of garnet films with such thicknesses have been widely investigated [1-3], whereas those of garnet films with much smaller thicknesses have not been reported. We prepared Y I G and ( L a L u S m G d ) 3 (FeGa)5012 films with thicknesses less than 1000 .A and measured their Curie temperature T~ and effective anisotropy field H k - 4 ~ ' M s. Details will be described below. 2. Experimental procedure
Films of Y I G and of (LaLuSmGd)3(FeGa)5Ol2 [2] were grown on ( l l l) oriented G d a G a s O i 2 ( G G G ) substrates by liquid phase epitaxy (LPE). Substrates were rotated at 60 rpm in the melt changing rotation direction. In order to obtain extremely thin films, special growth conditions were employed. For instance, the thinnest Y I G film was grown under such a condition that melt temperature was just at the saturation temperature or higher and the growth time was only 5 s. In this case, the film was grown because the substrate temperature was lower than the melt temperature. Film thickness was adjusted by controlling growth temperature and growth time for Y I G and mainly by etching for (LaLuSmGd)a(FeGa)~ O12. Film thickness was measured using three techniques: reflectance interference, fluorescent X-ray analysis, and absorbance analysis according to thickness range. Fluorescent X-ray analysis, which is superior in precision to reflectance interference method, does not give thicknesses less than 100 ,~ due to much larger background intensity. Thickness less than 100 ,A was measured by absorbance which is the logarithm of the ratio of incident light intensity to transmitted light intensity and proportional to film thickness. Absorbance was measured with a conventional spectrophotometer in the wavelength region of 0.30-0.36 # m where its magnitude was appropriate. Effective anisotropy field H k - 4 ~ r M s was obtained 0304-8853/83/0000-0000/$03.00
by X-band ferromagnetic resonance of perpendicular geometry using the well known formula H k --
47rMs =
~/V
- H,
where ~0 is microwave angular frequency, 7 the gyromagnetic ratio and H the resonance field. As temperature increases, H k - 4~-Ms tends to zero. Curie temperature T~ was obtained as the temperature at which H k - 4 ~ r M s became zero. 3. Results and discussion
Fig. 1 shows T~ and H , - 4~rMs (at room temperature) of Y I G as a function of film thickness. T¢ and H k - 4 ~ r M s are almost unchanged in the thickness range of 60 A to 2.8/~m. In this range, T~ is about 290°C and Hk -- 4~rM~ is about - 1.6 kOe. The value of Tc is almost the same as that of bulk [l]. H k - 4~rM~ of - 1.6 kOe agrees with the calculated value obtained with the following values: (a) anisotropy field of 100 Oe induced by the stress due to lattice mismatch through magnetostriction; (b) anisotropy field of 60 Oe related to cubic anisotropy; and (c) saturation induction 4~rM~ of about 1800 G for bulk Ill. Thus, films are very homogeneous for thickness down to 60 ,~. It is seen that T¢ decreases by 25°C and H k - 4~rM~ increases by 0.3 kOe at about 20 .A. A decrease in T~ indicates that average molecular field is reduced due to thickness reduction or Fe molar fraction is reduced due to Ga diffusion from G G G . Both cases lead to a reduction of 4~rM~. The 4~rMs reduction is in accord with the H k - 4~rM~ increase if we assume that the H k change is smaller.
•~. -,
Tc
f-'~
-1 .E
1o
• .300~ 280
'-3' -1.C. - * ~ - - ~ '
~-2
•
YIG ,,~
e ~ m Hk_41tM~i
16o
1ooo'
~60 ~ Z40
10600
Film Thickness (~)
Fig. 1. Curie tem[~erature and effective anisotropy field vs. film thickness for Y1G.
© 1983 N o r t h - H o l l a n d
926
T. Takeuchi et aL / Curie temperatures and anisotropy fields in garnet films =
1
Te • • 1200 .{"F . . . . . . i,80._. \ (LaLuSm )a(F-e~)s% , ~ ,...,,_.,~,___,, 160 i-~ o~o--
asL.~
Hk-41lMs
100
1000
10600
l
Film Thickness (/~) Fig. 2. Curie temperature and effective anisotropy field vs. film thickness for substituted YIG.
Fig. 2 shows T¢ and H k - 4 ~ r M s (at room temperature) of (LaLuSmGd)3(FeGa)5Ol2 as a function of film thickness. It is seen that H k - 4 ~ ' M s of this garnet is positive in contrast with that of YIG. This is because the magnetic easy axis is perpendicular to the film plane due to growth-induced anisotropy resulting from site ordering of the rare-earth ions. T¢ and H k -4~'Ms are almost unchanged (190°C and 1.0 kOe, respectively) in the thickness range of 600 ,~ to 2/~m. Though a gradual change in H k - 4~rMs is found in this range, the variation is +5% for the magnitude of H k - 4 * r M s and is negligibly small. The fact that T~ is unchanged indicates that the Ga to Fe molar ratio [1], which determines 4*rMs, is unchanged. Consequently, the fact that the variation in H k -4*rM~ is small leads to the fact that the variation in Hk, which is related to rare-earth ions molar ratio, is also small. Thus, the films are homogeneous down to 600 ~,.
Films thinner t h ~ this value show an abrupt increase in H k - 4*rMs: The thickness at which H k - 4~rM~ begins to vary for this garnet is one order of magnitude larger than that for YIG. This difference is presumably due to the difference in the number of elements contained in a site. When more than two elements are contained in a site, the composition depends on growth rate through the segregation effect [4]. This brings about the composition variation in the initially deposited region because of rapid growth prior to steady state growth. Magnetic properties for this altered composition appear in the thinner (LaLuSmGd)a(FeGa)5Ol2 films. The change in T~ for this garnet at thicknesses less than 600 A is not dear. This is because the resonance linewidth was large due to large relaxation of Sm and the resonances for the thinner films were concealed behind G G G paramagnetic resonance. However, even if T¢ changes, the amount is less than 15°C at thicknesses down to 200 ,~. The authors thank Professor S. Chikazumi for helpful discussions. References [1] P. Hansen, P. Roschmann and W. Tolksdorf, J. Appl. Phys. 45 (1974) 2728. [2] N. Ohta, T. Ikeda, F. Ishida and Y. Sugita, IEEE Trans. Magn. MAG-16 (1980) 610. [3] H.A. Algra and J.M. Robertson, J. Appl. Phys. 50 (1979) 4295. [4] E.A. Giess, D.C. Cronemeyer, R. Ghez, E. Klokholm and J.D. Kuptsis, J. Am. Ceram. Soc. 56 (1973) 593.