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Garnet films for storage applications: growth and properties
Journal of Magnetism and Magnetic Materials 193 (1999) 140—142
Garnet films for storage applications: growth and properties E. Rezlescu*, N. Rezlescu Institute of Technical Physics, Bd.D.Mangeron 47, 6600 Iasi, Romania
Abstract Based on the measurements of the domain structure only (domain period, p, thickness, h, bubble collapse field, H , and field at which bubbles run-out, H ,) a graphical method to determine the intrinsic length, l, and the saturation magnetization, 4pM , of magnetic bubble garnet films is described. The films of (YYbEuGdFe), (YLaTmGaFe) and (EuErFeGa) garnets are grown by the liquid-phase epitaxy technique (LPE) which consists of dipping a non-magnetic Gd Ga O garnet substrate into an isothermal supersaturate solution of the component garnet oxides in a PbO-B O solvent. Typical growth temperatures are &950°C and growth rates are &1 lm/min. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Thin film; Garnet; Bubble domains; Liquid-phase epitaxy; Intrinsic length; Saturation magnetization
The basic materials for storage application are yttrium iron garnet, with partial substitution of yttrium and/or iron for mobile cylindrical magnetic domain storage systems (bubble memory). The growth of the garnet films by the liquid-phase epitaxy (LPE) demands three requirements [1]: (1) low-growth temperature, in the range of 950°C and lower, appreciably lower than that for a normal flux growth; (2) high-growth rates: (3) very good fit of the lattice parameter of the substrate to that of the film to be grown (to within 0.01—0.02 As ). In this way very smooth epitaxial films are obtained which are suitable for device use. The gadolinium gallium garnet (GGG) is the favoured substrate for yttrium iron garnet. We used 11 1 12 oriented discs of Czochralski grown GGG of 8—10 mm in diameter. In this work the magnetic garnet films were grown by the liquid phase epitaxy technique (LPE) which consists of dipping a non-magnetic Gd Ga O substrate into an isothermal supersaturate solution of the component garnet oxides in a PbO-B O solvent [2]. Typical growth temperatures are &950°C and growth rates are &1 lm/min. During growth the substrate is rotated
* Corresponding author. Tel.: #40-32-130680; fax: #40-32231132; e-mail: [email protected].
in a horizontal plane to promote uniformity of properties across the film diameter. The film thickness was varied from 2 to 8 lm. The procedure is as follows. The oxides of the melt are mixed and premelted into the platinum crucible by heating within the LPE apparatus (Fig. 1) to 50°C above saturation temperature ¹ and stirred with a platinum paddle for homogenisation. After homogenisation the melt is kept on a temperature 20°C above ¹ . The sub strate, of thickness 0.4—0.5 mm, is brought for 5—10 min above the melt surface, before it is dipped into the melt. For the growth period, the temperature is lowered to the growth temperature ¹ and the substrate (GGG) was dipped in a vertical position. Uniformity of the thickness of the film was assured by rotating the substrate with a rate of 8 rpm during the growth (5—10 min), and by minimizing of the temperature gradient within the melt. All temperatures are monitored inside the melt with a Pt/Pt#10% Rh thermocouple with a relative accuracy of $0.5C. The growth time is adjusted, the growth rate was about 0.5 lm per minute. The relationships between the melt composition, growth conditions and resultant film composition are complex [3,4] and are illustrated schematically in Fig. 2. The compositions of the films grown by LPE are given in Table 1.
0304-8853/99/$ — see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 4 1 8 - 1
E. Rezlescu, N. Rezlescu / Journal of Magnetism and Magnetic Materials 193 (1999) 140—142
141
Fig. 2. The factors which control the LPE film parameters.
Fig. 1. Cross-section of the LPE furnace: (1) preheating electrical resistance; (2) growth zone electrical resistance; (3) alumina tube; (4) thermal insulation; (5) platinum crucible; (6) platinum lid; (7) melt; (8) thermocouple for automatic regulation of the temperature; (9) measurement thermocouple; (10) Pt—PtRh thermocouple; (11) substrate; (12) platinum support; (13) alumina rod; (14) adjustable support.
The static behaviour of thin magnetic films which support magnetic bubbles is characterised by some intrinsic parameters as thickness h, saturation magnetisation 4pM , intrinsic length l, uniaxial induced anisotropy constant K , exchange constant A, domain period p (twice the domain width) for stripe domains in zero field and the domain wall width, l . In this work a graphical method to determine two parameters, l and 4pM , from the measurements on the domain structure is described. The following experimental measurements were carried out: the thickness of the magnetic layer, h, the domain period, p, the bias field at which the magnetic bubble domains collapse, H , and the field at which the bubbles run-out, H . The other parameters were calculated using the relations: l"p /4pM, p "4(AK ) and l "p(A/K ) if the exchange constant A 4 ) 10\erg/cm for magnetic garnet is taken.
The film thickness, h, was measured by optical interference. The stripe domain period, p, in zero field was measured with a polarising microscope. For higher precision, the period was determined from a measurement over many periods. The error involved can be reduced if the length of stripe domain is much larger than the domain period. By stroking the film with a thin permalloy wire a large area with stripe domains can be achieved in zero field. The bubble collapse and run-out fields were also measured optically. To evaluate l and 4pM , the curves given in Fig. 3 were used. These were obtained using equations established by Thiele [5] and Fowlis and Copeland [6]. Using curve A the ratio l/h was determined. The magnetization was determined as an average of the values corresponding to H and H (B and C curves). A summary for values of typical parameters as obtained from the present method (LPE) for three films of (YYbEu) (GaFe) O , (YLaTm) (GaFe) O and (EuEr) (FeGa) O grown by us is given in Table 1. Garnet thin films obtained by LPE have a firm adhesion between the film and the substrate and offer a potential improvement in wear and corrosion resistance compared to metallic films. The method discussed here offers the possibility to determine the important parameters which characterise
Table 1 Values of the typical parameters for three films Samples
h (mm)
Y Yb Eu 6.6 Fe Ga O Y La Tm Fe 2.2 Ga O Eu Er Fe Ga O 8.0 Calculated values.
p (mm)
H (Oe)
H (Oe)
l (mm)
4pM (Gs)
p S (erg/cm)
K (erg/c m)
l (mm)
18.6
44
29.3
1.18
101
0.096
5760
0.13
2.86
272
230
0.14
450
0.222
7700
0.22
7.8
325
280
0.24
442
0.373
8000
0.11
142
E. Rezlescu, N. Rezlescu / Journal of Magnetism and Magnetic Materials 193 (1999) 140—142
fields at which the magnetic bubble domains collapse and run-out.
References [1] [2] [3] [4]
L.G. Varnerin, IEEE Trans. Mag. MAG-7 (1971) 404. V. Kostishyn et al., J. Phys. IV 7 Coll. C1 (1997) 757. F.B. Stein, AIP Conf. Proc. 18 (1973) 48. B.S. Hewitt, R.D. Pierce, S.L. Blank, S. Knight, IEEE Trans. Mag. MAG-9 (1973) 366. [5] A.A. Thiele, Bell System Tech. J. 50 (1971) 743. [6] D.C. Fowlis, J.A. Copeland, AIP Conf. Proc. 5 (1972) 240. Fig. 3. The three curves to determine the l and 4pM para meters.
magnetic bubble domain materials from the four following experimental measurements: the thickness of the magnetic layer, the domain period(twice the domain width) for stripe domains in zero bias field and the bias
Garnet films for storage applications: growth and properties E. Rezlescu*, N. Rezlescu Institute of Technical Physics, Bd.D.Mangeron 47, 6600 Iasi, Romania
Abstract Based on the measurements of the domain structure only (domain period, p, thickness, h, bubble collapse field, H , and field at which bubbles run-out, H ,) a graphical method to determine the intrinsic length, l, and the saturation magnetization, 4pM , of magnetic bubble garnet films is described. The films of (YYbEuGdFe), (YLaTmGaFe) and (EuErFeGa) garnets are grown by the liquid-phase epitaxy technique (LPE) which consists of dipping a non-magnetic Gd Ga O garnet substrate into an isothermal supersaturate solution of the component garnet oxides in a PbO-B O solvent. Typical growth temperatures are &950°C and growth rates are &1 lm/min. 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Thin film; Garnet; Bubble domains; Liquid-phase epitaxy; Intrinsic length; Saturation magnetization
The basic materials for storage application are yttrium iron garnet, with partial substitution of yttrium and/or iron for mobile cylindrical magnetic domain storage systems (bubble memory). The growth of the garnet films by the liquid-phase epitaxy (LPE) demands three requirements [1]: (1) low-growth temperature, in the range of 950°C and lower, appreciably lower than that for a normal flux growth; (2) high-growth rates: (3) very good fit of the lattice parameter of the substrate to that of the film to be grown (to within 0.01—0.02 As ). In this way very smooth epitaxial films are obtained which are suitable for device use. The gadolinium gallium garnet (GGG) is the favoured substrate for yttrium iron garnet. We used 11 1 12 oriented discs of Czochralski grown GGG of 8—10 mm in diameter. In this work the magnetic garnet films were grown by the liquid phase epitaxy technique (LPE) which consists of dipping a non-magnetic Gd Ga O substrate into an isothermal supersaturate solution of the component garnet oxides in a PbO-B O solvent [2]. Typical growth temperatures are &950°C and growth rates are &1 lm/min. During growth the substrate is rotated
* Corresponding author. Tel.: #40-32-130680; fax: #40-32231132; e-mail: [email protected].
in a horizontal plane to promote uniformity of properties across the film diameter. The film thickness was varied from 2 to 8 lm. The procedure is as follows. The oxides of the melt are mixed and premelted into the platinum crucible by heating within the LPE apparatus (Fig. 1) to 50°C above saturation temperature ¹ and stirred with a platinum paddle for homogenisation. After homogenisation the melt is kept on a temperature 20°C above ¹ . The sub strate, of thickness 0.4—0.5 mm, is brought for 5—10 min above the melt surface, before it is dipped into the melt. For the growth period, the temperature is lowered to the growth temperature ¹ and the substrate (GGG) was dipped in a vertical position. Uniformity of the thickness of the film was assured by rotating the substrate with a rate of 8 rpm during the growth (5—10 min), and by minimizing of the temperature gradient within the melt. All temperatures are monitored inside the melt with a Pt/Pt#10% Rh thermocouple with a relative accuracy of $0.5C. The growth time is adjusted, the growth rate was about 0.5 lm per minute. The relationships between the melt composition, growth conditions and resultant film composition are complex [3,4] and are illustrated schematically in Fig. 2. The compositions of the films grown by LPE are given in Table 1.
0304-8853/99/$ — see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 4 1 8 - 1
E. Rezlescu, N. Rezlescu / Journal of Magnetism and Magnetic Materials 193 (1999) 140—142
141
Fig. 2. The factors which control the LPE film parameters.
Fig. 1. Cross-section of the LPE furnace: (1) preheating electrical resistance; (2) growth zone electrical resistance; (3) alumina tube; (4) thermal insulation; (5) platinum crucible; (6) platinum lid; (7) melt; (8) thermocouple for automatic regulation of the temperature; (9) measurement thermocouple; (10) Pt—PtRh thermocouple; (11) substrate; (12) platinum support; (13) alumina rod; (14) adjustable support.
The static behaviour of thin magnetic films which support magnetic bubbles is characterised by some intrinsic parameters as thickness h, saturation magnetisation 4pM , intrinsic length l, uniaxial induced anisotropy constant K , exchange constant A, domain period p (twice the domain width) for stripe domains in zero field and the domain wall width, l . In this work a graphical method to determine two parameters, l and 4pM , from the measurements on the domain structure is described. The following experimental measurements were carried out: the thickness of the magnetic layer, h, the domain period, p, the bias field at which the magnetic bubble domains collapse, H , and the field at which the bubbles run-out, H . The other parameters were calculated using the relations: l"p /4pM, p "4(AK ) and l "p(A/K ) if the exchange constant A 4 ) 10\erg/cm for magnetic garnet is taken.
The film thickness, h, was measured by optical interference. The stripe domain period, p, in zero field was measured with a polarising microscope. For higher precision, the period was determined from a measurement over many periods. The error involved can be reduced if the length of stripe domain is much larger than the domain period. By stroking the film with a thin permalloy wire a large area with stripe domains can be achieved in zero field. The bubble collapse and run-out fields were also measured optically. To evaluate l and 4pM , the curves given in Fig. 3 were used. These were obtained using equations established by Thiele [5] and Fowlis and Copeland [6]. Using curve A the ratio l/h was determined. The magnetization was determined as an average of the values corresponding to H and H (B and C curves). A summary for values of typical parameters as obtained from the present method (LPE) for three films of (YYbEu) (GaFe) O , (YLaTm) (GaFe) O and (EuEr) (FeGa) O grown by us is given in Table 1. Garnet thin films obtained by LPE have a firm adhesion between the film and the substrate and offer a potential improvement in wear and corrosion resistance compared to metallic films. The method discussed here offers the possibility to determine the important parameters which characterise
Table 1 Values of the typical parameters for three films Samples
h (mm)
Y Yb Eu 6.6 Fe Ga O Y La Tm Fe 2.2 Ga O Eu Er Fe Ga O 8.0 Calculated values.
p (mm)
H (Oe)
H (Oe)
l (mm)
4pM (Gs)
p S (erg/cm)
K (erg/c m)
l (mm)
18.6
44
29.3
1.18
101
0.096
5760
0.13
2.86
272
230
0.14
450
0.222
7700
0.22
7.8
325
280
0.24
442
0.373
8000
0.11
142
E. Rezlescu, N. Rezlescu / Journal of Magnetism and Magnetic Materials 193 (1999) 140—142
fields at which the magnetic bubble domains collapse and run-out.
References [1] [2] [3] [4]
L.G. Varnerin, IEEE Trans. Mag. MAG-7 (1971) 404. V. Kostishyn et al., J. Phys. IV 7 Coll. C1 (1997) 757. F.B. Stein, AIP Conf. Proc. 18 (1973) 48. B.S. Hewitt, R.D. Pierce, S.L. Blank, S. Knight, IEEE Trans. Mag. MAG-9 (1973) 366. [5] A.A. Thiele, Bell System Tech. J. 50 (1971) 743. [6] D.C. Fowlis, J.A. Copeland, AIP Conf. Proc. 5 (1972) 240. Fig. 3. The three curves to determine the l and 4pM para meters.
magnetic bubble domain materials from the four following experimental measurements: the thickness of the magnetic layer, the domain period(twice the domain width) for stripe domains in zero bias field and the bias