Use of a garnet thin film in a magnetostatic-wave device

Abstract: A material for magnetostatic wave devices comprising a ferrimagnetic thin film composed of the above material and epitaxially grown on a substrate of Gd₃Ga₂(GaO₄)₃. The material for the ferrimagnetic thin film has a composition expressed by the general formula: Y 3-x Bi x Fe₅O₁₂ where x is a mole fraction of Bi and takes a value in the following range: 0.01 < x ≦ 0.23. The composition of the general formula (I) may be modified by replacing a part of Fe with either Ga or Al.

Description: The present invention relates to a garnet thin film used in magnetostatic-wave devices and, more particularly, a ferrimagnetic material for magnetostatic wave devices comprising a garnet thin film of the ferrimagnetic material epitaxially grown on a substrate of gadolinium-gallium-garnet. Recently, study of magnetostatic wave devices has developed rapidly and its achievements have been reported in various papers. For example, some examples of magnetostatic wave devices are described by J.D. Adam, M.R. Daniel and D.K. Schroder in "Magnetostatic-wave devices move microwave design into gigaherts realm", Electronics, May, 1980, pp. 123-128. Also, a new configuration for a magnetostatic wave filter is discussed by T. Nishikawa, K. Wakino, H. Taraha, T.Okada, S. Shinmura and Y. Ishikawa in "a low loss magnetostatic wave filter using parallel strip transducers", Microwave and RF Engineering, September/October 1989, pp.63-68. Such magnetostatic wave devices generally comprise a thin film of a ferrimagnetic material, generally, yttrium-iron-garnet (YIG) of the chemical formula Y 3 Fe 2 (FeO 4 ) 3 , grown by liquid-phase epitaxy techniques on a substrate of gadolinium-gallium-garnet (GGG) of the chemical formula Gd 3 Ga 2 (GaO 4 ) 3 . The epitaxially grown YIG film makes it possible to reduce the difference between input and output signals of the magnetostatic wave devices because of its very small magnetic resonance half linewidth (ΔH). However, the YIG films have such a disadvantage that the change in saturation magnetization (4πM s ) with temperature is large and of the order of about 2100 ppm/°C. Thus, in order to improve temperature characteristics of magnetostatic-wave devices, it is required to choose a permanent magnet with a suitable temperature coefficient. However, it is difficult to minimize the temperature coefficient of the magnetostatic wave device because of limited kinds of permanent magnet materials and because of the fixed temperature coefficient of saturation magnetization of the YIG films. US-A-4,968,954 (Ryuo et al.) discloses materials for magnetostatic wave devices that comprise an epitaxial layer formed on a substrate of rare earth gallium garnet, having a chemical composition of the formula: Y 3-x Bi x Fe 5-y Ga y O 12 in which the subscript x takes a value in the range from 0.01 to 0.9, and the subscript y takes a value in the range from 0.49 to 1.0. It is therefore an object of the present invention to provide a material for magnetostatic-wave devices, which makes it possible to provide a ferrimagnetic thin film with any desired temperature coefficient of saturation magnetization (4πM s ), without causing increase in magnetic resonance half linewidth (ΔH). According to the present invention there is provided a material for magnetostatic wave devices comprising a thin film of the material epitaxially grown on a substrate of gadolinium-gallium-garnet (GGG), said material having a composition expressed by the general formula (I): Y 3-x Bi x Fe 5-y Ga y O 12 where x and y are mole fractions of Bi and Ga, 0 ≤ x ≤ 0.41, and 0 ≤ y less than 0,49, said composition having a set of x and y falling in the area encompassed by a polygon ABCDE defined by points A (0, 0), B (0.23, 0), C (0.41, 1), D (0.15, 1) and E (0, 0.1) in a phase diagram of a binary system with x as abscissa and y as ordinate, except for a set of x and y falling on the point A. The present invention makes it possible to produce a ferrimagnetic thin films with any desired temperature coefficient of saturation magnetization ranging from about 500 to about 2000 ppm/°C, without causing increase in the magnetic resonance half linewidth (ΔH). Accordingly, it is possible to improve the temperature characteristics of magnetostatic wave devices by the suitable choice of the permanent magnet material. Further, the present invention makes it possible to produce excellent magnetic garnet films as the difference of the lattice constant between the garnet film and the GGG substrate can be minimized to not more than 0.01 Å. The above and other objects, features and advantages of the present invention will be understood clearly from the following explanations with reference to several examples. Example 1 Powdered materials of Fe 2 O 3 , Y 2 O 3 , Bi 2 O 3 and Ga 2 O 3 were weighed and mixed so that the product has a composition of the general formula (II): Y 3-x Bi x Fe 5-y Ga y O 12 where x and y are mole fractions of Bi and Ga and take values as shown in Table 1. Using the resultant mixtures, there were prepared magnetic garnet films on substrates of Gd 3 Ga 5 O 12 by the liquid-phase epitaxy (LPE) in the same manner as Example 1. For each garnet film produced, measurements were made on the lattice constant, saturation magnetization (4πM s ), temperature coefficient (TC) of 4πM s , and magnetic resonance half linewidth (ΔH). Results are shown in Table 2. In the table, the specimens with an asterisk (*) are those out of the scope of the present invention. The compositions of the resultant garnet films are plotted in a binary phase diagram of Fig. 1 with x in the general formula (I) as abscissa and y in that formula as ordinate, along with the area of the composition falling in the scope of the present invention. The numbers in Fig. 1 correspond to the respective specimen numbers. No. x y 4πM s (G) ΔH (Oe) TC of 4πM s (ppm/°C) lattice constant (A) 1* 0 0 1800 0.6 -2100 12.376 2 0.23 0 1740 1.3 -1300 12.393 3 0 0.1 1500 0.9 -2200 12.373 4* 0 0.5 Good films were never obtained 5 0.2 0.5 950 1.4 -1700 12.381 6* 0.4 0.5 Good films were never obtained 7 0.16 1 680 2.0 -2000 12.373 8 0.41 1 630 2.5 -1500 12.393 9* 0.3 1.1 570 5.5 -1400 12.385 As can be seen from the data for specimen No. 4 or 6, if the composition has a set of x and y falling in the area out of the line BC or the line DE, the difference in the lattice constant between the GGG substrate and the magnetic garnet film becomes more than 0.01A, causing cracking of the film, thus making it impossible to obtain garnet films with good quality. Further, as can be seen from the data for specimen No. 9, if the composition has a set of x and y falling in the area out of the line CD, the magnetic resonance half linewidth (ΔH) becomes more than 5 Oe, thus making it impossible to use it as a material for film in the magnetostatic wave devices. As can be seen from the data for specimen No. 1, this composition, i.e., YIG, possesses a large temperature coefficient of the saturation magnetization (4πM s ). The composition with x = 0 and y = 0 is of course excluded from the scope of the present invention. For the reasons mentioned above, the composition of the general formula (I) has been limited to those having a set of values of x and y falling in the area encompassed by the polygon ABCDE defined by the points A (0, 0), B (0.23), C (0.41, 1), D (0.15, 1) and E (0, 0.1) in Fig. 1, except for a set of x and y falling on the point A.

Claims: A material of garnet thin films used in magnetostatic wave devices comprising a garnet thin film of a ferrimagnetic material epitaxially grown on a substrate of Gd 3 Ga 2 (Ga O4 ) 3 , said material having a composition expressed by the general formula (I): Y 3-x Bi x Fe 5-y Ga y O 12 where x and y are mole fractions of Bi and Ga, 0 ≤ x ≤ 0.41, and 0 ≤ y less than 0,49, said composition having a set of x and y falling in the area encompassed by a polygon ABCDE defined by points A (0, 0), B (0.23, 0), C (0.41, 1), D (0.15, 1) and E (0, 0.1) in a phase diagram of a binary system with x as abscissa and y as ordinate, except for a set of x and y falling on the point A.

Inventors: AOTA MITSUHIRO, FUJINO MASARU, KUMATORIYA MAKOTO, MIZUNO SHINOBU, TAKAGI HIROSHI, AOTA, MITSUHIRO, FUJINO, MASARU, KUMATORIYA, MAKOTO, MIZUNO, SHINOBU, TAKAGI, HIROSHI
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1992
Main classification: H03H2
IPC codes: H03H2, H01F10