- Email: [email protected]
FMR study of Co-substituted yttrium iron garnet films
Journal of Magnetism and Magnetic Materials 160 (1996) 367-369
Journalof
magnetism
, a ~ and
magnetic materials
ELSEVIER
FMR study of Co-substituted yttrium iron garnet films R. Jab/ofiski a, A. Maziewski b,. M. Tekielak b, J.M. Desvignes c ,
~ ITME, 133 W6lczal~ska, 01-919 Warsaw, Poland b Institute ~['Physics, Warsaw UnicersiO" Branch, 41 Lipon'a, 15-424 Biatw~tok, Poland c CNRS, L.M.M.M.. Meudon, France
Abstract Ferromagnetic resonance has been studied in Y3 :Ca:F% .... Co~Ge,Ot2 films grown on the (001) plane of a GGG substrate. Temperature-induced spin-reorientation phase transitions were observed. On cooling the sample, the easy magnetization axes orientations changed from directions near [111] to [100], and the [001] direction had the lowest energy. The FMR signal was practically non-measurable below 60 and 120 K when the magnetic field was applied along the [110] and [100] axes, respectively. Keywords: Ferromagnetic resonance: Garnet film; Magnetic anisotropy
Epitaxial yttrium iron garnet films doped with strongly amsotropic cobalt ions have been intensively investigated recently because of their interesting magnetic and magnetooptical properties. Domain structure shape memory, photomagnetism, huge magnetic after effect (observed also at room temperature) and complicated temperature-induced spin-reorientation phase transitions have been observed [1-4]. Interesting FMR results obtained for YIG:Co with low cobalt concentration have also been reported [5]. The purpose of this work is to extend the study of the temperature properties of these YIG:Co films using the FMR technique [6]. Y3_:Ca:F%_ ~_ ~Co~Ge,O]2 films were grown by liquid phase epitaxy on the (001) plane of a GGG substrate using a method similar to that described in Ref. [7]. The samples with high cobalt concentration (x-~ 0.08) were produced with different growth rates (see Table 1). The chemical composition (Co, C a - G e ) was determined by electron probe microanalysis. The FMR measurements were carried out with an X-band spectrometer (Bruker 300, equipped with a continuous flow cryostat, Oxford Instruments ESR-900) in the 4 300 K temperature range. The resonance field //1 amplitudes were measured for the external magnetic field applied in two characteristic planes: (001) and (110).
' Corresponding author. Email: [email protected]; fax: +48-85-420-272.
The standard resonance condition
y2
M:sin20
002 0d)2
/]
~ J
,
(1)
where F is the anisotropic part of the free energy, was used. In the coordinate system related to the main cubic axes, F can be expressed as the sum of the magnetic field energy F m - M H [ s i n 0 sin 0N cos($ - SH) + COS 0 cos OH], the demagnetizing energy F D = 2~-M 2 sin20, the uniaxial anisotropy F U = Ku~ sin20 + Ku2 sina0 and the cubic anisotropy F c = K~ s + K 2 P + K3s-, where s = c~c~ + ai-c~~ + a;_a3, p = c~i-o~c~3 and a], c~ and c~3 are directional cosines of the magnetization; y = g l e l / ( 2 m e C ) ; angles 0,~b and 0H,d~H denote the polar and azimuth angles of magnetization M and the field H vectors, respectively. The room temperature FMR data are well described using both uniaxial and cubic anisotropies in the simplest form; fitted H I = K I / M and Hue n. = 2(Kui -- 2 " r r M ) / M constants are shown in Table 1. Predominant tetrahedral Co ion contributions to the cubic anisotropy could be deduced at room temperature for all the samples, assuming the single-ion contribution [8] to the anisotropy constant K~ and comparing the pure YIG K l constant and values from Table 1. The FMR spectra, measured for different samples, are similar; the discussion below is focused on sample No. 9. The temperature changes inducing angular dependences of the resonance field amplitude Hr(OH,c~H) are shown in Fig. I A,B. These angular dependences become more corn-
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0304-8 853(96)00229-6
=
368
R. Jabtofiski et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 367-369
plicated at lower temperatures. Three cubic anisotropy constants and at least two uniaxial anisotropy constants were considered for their description. Analyzing Fig. 1A,B one can deduce the existence of temperature-induced spin-reorientation phase transitions. At room temperature the easy magnetization axes are oriented in the (110) plane near [l 1 l]-type crystallographic directions. On cooling the sample, these easy axes rotate and achieve [110] and [710] directions at about 140 K. The appearance of two additional easy magnetization axes oriented along the [100] and [010] directions could be deduced at 140 K. These new easy axes directions were characterized (at 140 K) by a higher magnetic anisotropy energy than [ l l 0 ] and [110]. The [100] type directions became energetically favorable with decreasing temperature. The temperature induced easy axis reorientation between directions near [111] and [100] is associated with the increasing contribution of Co 2+ octahedral ions to the cubic anisotropy. These ions are characterized by a very strong (in comparison with other magnetic ions expected in YIG:Co,Ge,Ca samples such as both Co ions in tetrahedral positions and Fe 3+ ions) increase of the single-ion contributions to the anisotropy constants at low temperature, see, e.g., Ref. [8]. The direction normal to the film plane [001] and the lowest energy was achieved with decreasing temperature. The FMR signal was practically non-measurable after the temperature had been lowered further. The critical temperature for the FMR signal to vanish, Tv, could be estimated at about 60 and 120 K for a magnetic field applied along the [110] and [100] directions, respectively (see Fig. 1C). No strong temperature dependence of the FMR linewidth was observed. The fact that the FMR signal was not detectable with the spectrometer used could be put down to the magnetic anisotropy changes estimated from torque anisometry [3]. In this investigation we determined the anisotropy, described by many constants (up to three for cubic and four for uniaxial anisotropy terms), at low temperature. The H r field was calculated using these constants in Eq. (1). A low temperature FMR signal is expected for magnetic fields much higher than the field
Table 1 Characterization of the investigated samples. Technological parameters: thickness (h), growth temperature (~), growth rate (w), chemical composition (Co, Ca-Ge); magnetic parameters: first cubic anisotropy field (H I ), effective anisotropy (HuoFt.) field Sample
~ (°C)
w (/~m/min)
Co
Ca-Ge
Hi (mY)
Hut, it. (mY)
9 l0 12 14 11
894.8 892.0 890.2 890.0 888.5
0.19 0.27 0.28 0.29 0.37
0.080 0.080 0.080 0.080 0.081
0.90 0.94 0.94 0.94 0.96
-79 -94 - 106 - 119 - 134
6 25 20 41 49
650 300 K 200 K .... ~,---- 170 K
.... o....
u.,l~
550
-- -O n
.,D O O
.... o - - - 1 4 0 K
.- O . " , .•,
" - ' - ( P - - 130 K
: ,",
(3 O°'"
~',.
[~]~y~,"-&.-A"
350
"'O...Q
250 150 -20 55O
20
q)H [deg] 60
lBlo%.
I O0 .... o---- 3 0 0 K .... o---- 2 0 0 K
- A- 150K • 140 K - --o-- = 130 K ,¢ 120 K
450
Y\ •.."~, .E.
350
£ 250.
[1101
[0011 I
150 -20
I 20
-,-o,9.rdeel so
'too
6O
-.., ,BI &
40,
•~
30.
" E
20.
50
1 O0
150
200
250
300
TIKI Fig. 1. Angular dependences of the resonance field amplitude H, measured at different temperatures in: (A) the (001) plane and (B) the (I 10) plane. (C) Temperature dependences of the amplitude measured in the [100] and [110] directions.
available with the spectrometer used. H r simulations show that, at low temperature, the signal amplitude is out of the spectrometer range, first for the field along [100] then for [110]. A similar effect was observed in our experiments [3]. Acknowledgement: This work was supported by Polish grant 2P03B 168 09. References
[l] M. Kisielewski, A. Maziewski and J.M. Desvignes, J. Magn. Magn. Mater. 140-144 (1995) 1923. [2] A. Maziewski, J. Magn. Magn. Mater. 88 (1990) 325.
R. Jabtohski et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 367-369 [3] M. Tekielak, W. Andr~, C. Kleint, A. Maziewski and J. Taubert, Presented at ICM 94: an extended version of the paper is being prepared for publication. [4] A.B. Chizhik, S.N. Lyakhimets, A. Maziewski and M. Tekielak, J. Magn. Magn. Mater. 140-144 (1995) 2111. [5] M. Mary~ko, IEEE Trans. Magn. 22 (1994) 978. [6] R. Jabtofiski and A. Maziewski, RAMIS 91.
369
[7] P. GiSrnert, M. Nev~iva, J. Sim~ova, W. Andr~i, W. Schiippel, P. Sum~al and B. Bub~.kow~., Phys. Status Solidi (a) 74 (1982) 107. [8] P. Hansen, in: Landolt-B~3mstein, Numerical Data and Functional Relationships in Science and Technology, Group III: Voh 12, eds. K. Enke and G. Winkler (Springer, Berlin, 1978).
Journalof
magnetism
, a ~ and
magnetic materials
ELSEVIER
FMR study of Co-substituted yttrium iron garnet films R. Jab/ofiski a, A. Maziewski b,. M. Tekielak b, J.M. Desvignes c ,
~ ITME, 133 W6lczal~ska, 01-919 Warsaw, Poland b Institute ~['Physics, Warsaw UnicersiO" Branch, 41 Lipon'a, 15-424 Biatw~tok, Poland c CNRS, L.M.M.M.. Meudon, France
Abstract Ferromagnetic resonance has been studied in Y3 :Ca:F% .... Co~Ge,Ot2 films grown on the (001) plane of a GGG substrate. Temperature-induced spin-reorientation phase transitions were observed. On cooling the sample, the easy magnetization axes orientations changed from directions near [111] to [100], and the [001] direction had the lowest energy. The FMR signal was practically non-measurable below 60 and 120 K when the magnetic field was applied along the [110] and [100] axes, respectively. Keywords: Ferromagnetic resonance: Garnet film; Magnetic anisotropy
Epitaxial yttrium iron garnet films doped with strongly amsotropic cobalt ions have been intensively investigated recently because of their interesting magnetic and magnetooptical properties. Domain structure shape memory, photomagnetism, huge magnetic after effect (observed also at room temperature) and complicated temperature-induced spin-reorientation phase transitions have been observed [1-4]. Interesting FMR results obtained for YIG:Co with low cobalt concentration have also been reported [5]. The purpose of this work is to extend the study of the temperature properties of these YIG:Co films using the FMR technique [6]. Y3_:Ca:F%_ ~_ ~Co~Ge,O]2 films were grown by liquid phase epitaxy on the (001) plane of a GGG substrate using a method similar to that described in Ref. [7]. The samples with high cobalt concentration (x-~ 0.08) were produced with different growth rates (see Table 1). The chemical composition (Co, C a - G e ) was determined by electron probe microanalysis. The FMR measurements were carried out with an X-band spectrometer (Bruker 300, equipped with a continuous flow cryostat, Oxford Instruments ESR-900) in the 4 300 K temperature range. The resonance field //1 amplitudes were measured for the external magnetic field applied in two characteristic planes: (001) and (110).
' Corresponding author. Email: [email protected]; fax: +48-85-420-272.
The standard resonance condition
y2
M:sin20
002 0d)2
/]
~ J
,
(1)
where F is the anisotropic part of the free energy, was used. In the coordinate system related to the main cubic axes, F can be expressed as the sum of the magnetic field energy F m - M H [ s i n 0 sin 0N cos($ - SH) + COS 0 cos OH], the demagnetizing energy F D = 2~-M 2 sin20, the uniaxial anisotropy F U = Ku~ sin20 + Ku2 sina0 and the cubic anisotropy F c = K~ s + K 2 P + K3s-, where s = c~c~ + ai-c~~ + a;_a3, p = c~i-o~c~3 and a], c~ and c~3 are directional cosines of the magnetization; y = g l e l / ( 2 m e C ) ; angles 0,~b and 0H,d~H denote the polar and azimuth angles of magnetization M and the field H vectors, respectively. The room temperature FMR data are well described using both uniaxial and cubic anisotropies in the simplest form; fitted H I = K I / M and Hue n. = 2(Kui -- 2 " r r M ) / M constants are shown in Table 1. Predominant tetrahedral Co ion contributions to the cubic anisotropy could be deduced at room temperature for all the samples, assuming the single-ion contribution [8] to the anisotropy constant K~ and comparing the pure YIG K l constant and values from Table 1. The FMR spectra, measured for different samples, are similar; the discussion below is focused on sample No. 9. The temperature changes inducing angular dependences of the resonance field amplitude Hr(OH,c~H) are shown in Fig. I A,B. These angular dependences become more corn-
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0304-8 853(96)00229-6
=
368
R. Jabtofiski et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 367-369
plicated at lower temperatures. Three cubic anisotropy constants and at least two uniaxial anisotropy constants were considered for their description. Analyzing Fig. 1A,B one can deduce the existence of temperature-induced spin-reorientation phase transitions. At room temperature the easy magnetization axes are oriented in the (110) plane near [l 1 l]-type crystallographic directions. On cooling the sample, these easy axes rotate and achieve [110] and [710] directions at about 140 K. The appearance of two additional easy magnetization axes oriented along the [100] and [010] directions could be deduced at 140 K. These new easy axes directions were characterized (at 140 K) by a higher magnetic anisotropy energy than [ l l 0 ] and [110]. The [100] type directions became energetically favorable with decreasing temperature. The temperature induced easy axis reorientation between directions near [111] and [100] is associated with the increasing contribution of Co 2+ octahedral ions to the cubic anisotropy. These ions are characterized by a very strong (in comparison with other magnetic ions expected in YIG:Co,Ge,Ca samples such as both Co ions in tetrahedral positions and Fe 3+ ions) increase of the single-ion contributions to the anisotropy constants at low temperature, see, e.g., Ref. [8]. The direction normal to the film plane [001] and the lowest energy was achieved with decreasing temperature. The FMR signal was practically non-measurable after the temperature had been lowered further. The critical temperature for the FMR signal to vanish, Tv, could be estimated at about 60 and 120 K for a magnetic field applied along the [110] and [100] directions, respectively (see Fig. 1C). No strong temperature dependence of the FMR linewidth was observed. The fact that the FMR signal was not detectable with the spectrometer used could be put down to the magnetic anisotropy changes estimated from torque anisometry [3]. In this investigation we determined the anisotropy, described by many constants (up to three for cubic and four for uniaxial anisotropy terms), at low temperature. The H r field was calculated using these constants in Eq. (1). A low temperature FMR signal is expected for magnetic fields much higher than the field
Table 1 Characterization of the investigated samples. Technological parameters: thickness (h), growth temperature (~), growth rate (w), chemical composition (Co, Ca-Ge); magnetic parameters: first cubic anisotropy field (H I ), effective anisotropy (HuoFt.) field Sample
~ (°C)
w (/~m/min)
Co
Ca-Ge
Hi (mY)
Hut, it. (mY)
9 l0 12 14 11
894.8 892.0 890.2 890.0 888.5
0.19 0.27 0.28 0.29 0.37
0.080 0.080 0.080 0.080 0.081
0.90 0.94 0.94 0.94 0.96
-79 -94 - 106 - 119 - 134
6 25 20 41 49
650 300 K 200 K .... ~,---- 170 K
.... o....
u.,l~
550
-- -O n
.,D O O
.... o - - - 1 4 0 K
.- O . " , .•,
" - ' - ( P - - 130 K
: ,",
(3 O°'"
~',.
[~]~y~,"-&.-A"
350
"'O...Q
250 150 -20 55O
20
q)H [deg] 60
lBlo%.
I O0 .... o---- 3 0 0 K .... o---- 2 0 0 K
- A- 150K • 140 K - --o-- = 130 K ,¢ 120 K
450
Y\ •.."~, .E.
350
£ 250.
[1101
[0011 I
150 -20
I 20
-,-o,9.rdeel so
'too
6O
-.., ,BI &
40,
•~
30.
" E
20.
50
1 O0
150
200
250
300
TIKI Fig. 1. Angular dependences of the resonance field amplitude H, measured at different temperatures in: (A) the (001) plane and (B) the (I 10) plane. (C) Temperature dependences of the amplitude measured in the [100] and [110] directions.
available with the spectrometer used. H r simulations show that, at low temperature, the signal amplitude is out of the spectrometer range, first for the field along [100] then for [110]. A similar effect was observed in our experiments [3]. Acknowledgement: This work was supported by Polish grant 2P03B 168 09. References
[l] M. Kisielewski, A. Maziewski and J.M. Desvignes, J. Magn. Magn. Mater. 140-144 (1995) 1923. [2] A. Maziewski, J. Magn. Magn. Mater. 88 (1990) 325.
R. Jabtohski et al. / Journal of Magnetism and Magnetic Materials 160 (1996) 367-369 [3] M. Tekielak, W. Andr~, C. Kleint, A. Maziewski and J. Taubert, Presented at ICM 94: an extended version of the paper is being prepared for publication. [4] A.B. Chizhik, S.N. Lyakhimets, A. Maziewski and M. Tekielak, J. Magn. Magn. Mater. 140-144 (1995) 2111. [5] M. Mary~ko, IEEE Trans. Magn. 22 (1994) 978. [6] R. Jabtofiski and A. Maziewski, RAMIS 91.
369
[7] P. GiSrnert, M. Nev~iva, J. Sim~ova, W. Andr~i, W. Schiippel, P. Sum~al and B. Bub~.kow~., Phys. Status Solidi (a) 74 (1982) 107. [8] P. Hansen, in: Landolt-B~3mstein, Numerical Data and Functional Relationships in Science and Technology, Group III: Voh 12, eds. K. Enke and G. Winkler (Springer, Berlin, 1978).