JP2011190138A - Method for producing multiferroic single crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a large-size single crystal of BiFeO<SB>3</SB>which can be used for a multiferroic device, a ferroelectric device, a piezoelectric device and so on. <P>SOLUTION: A BiFeO<SB>3</SB>single crystal rod is fabricated as follows: using a high density BiFeO<SB>3</SB>raw material rod 3 supported on one of upper and lower two crystal-driving shafts, a seed crystal rod 4 supported on the other crystal-driving shaft, and a flux (melting agent) 5; heating the flux 5 by means of the floating zone method to form a molten zone 7; and growing the single crystal under an atmosphere of oxygen, an inert gas or a mixed gas thereof. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to a technique for producing an electromagnetic effect single crystal that can be used for an electromagnetic effect (multiferroic) device, a ferroelectric device, a piezoelectric device, and the like. More specifically, the present invention relates to a technology for producing a large single crystal of BiFeO _{3 that} can be expected to have an electromagnetic effect.

BiFeO ₃ coexists antiferromagnetic and ferroelectric (ferro electric) and Ferroelastic (ferro elastic) (anti ferro magnetic ), they are characterized in that it interacts. It is called a multiferroic material because a plurality of ferro properties are entangled. For example, it is possible to control electric polarization by an electric field and thereby indirectly control magnetism. Such an effect is called an electromagnetic effect or a cross correlation. To date, BiFeO ₃ is the only multiferroic material realized at room temperature. It is expected to be used for magnetic memory, hard disk writing, and the like by utilizing magnetic control by an electric field (see Non-Patent Document 1). In addition, it is known that BiFeO _{3 has} an extremely large electric polarization of 100 μC / cm ^2, and its utility value is great even with ferroelectricity and ferroelasticity alone. When used as a piezo device or the like, unlike PZT or the like, there is an advantage that it does not contain lead.

Regarding the synthesis of BiFeO ₃ , there are many reports on polycrystals and thin films. There are few reports on single crystals, and there are reports only on flake microcrystals by the flux method using a crucible (see Non-Patent Document 2). High-quality large single crystals are required for device development, but the BiFeO ₃ microcrystals produced by the flux method are not only small in size but also large in variation in shape and size. Since it is unavoidable that impurities such as these are mixed in, it cannot completely satisfy the demand for device formation.

In addition, sufficient insulation is required to use ferroelectricity, but BiFeO ₃ has a drawback that leakage current tends to occur, and there is a problem that it is difficult to use ferroelectricity. Possible causes of the leakage current include the presence of conductive heterogeneous phases and the presence of carriers due to the oxygen non-quantitative ratio (that is, deviation from the stoichiometric ratio of the oxygen composition in BiFeO ₃ ). In order to remove these causes, indirect measures such as substitution with an impurity element (see Patent Documents 1 and 2 and Non-Patent Documents 3 to 7) and treatment with dilute nitric acid (see Non-Patent Document 2 above) are taken. . In the thin film, a direct measure of controlling the oxygen amount is taken (see Patent Document 3), but no successful example of leakage current suppression by synthesis of a pure bulk sample with an oxygen quantitative ratio is known.

In order to solve the problems of the BiFeO ₃ single crystal production by the flux method as described above, it is conceivable to adopt the single crystal production method by the melting zone method (floating zone method), but the BiFeO ₃ single crystal is successful. There have never been any reports of examples.
In the process of growing a single crystal using a conventional lamp-heated floating zone furnace, the present inventors have developed a BiFeO ₃ single crystal for penetration of the melt into the raw material and dripping of the melt into the grown crystal. Recognized the need to solve the problems. After that, the present inventors developed a laser heating type floating zone furnace for the purpose of preventing melt penetration and dripping which became a problem in the lamp heating type floating zone furnace (see Patent Documents 4 and 5). The inventors of the present invention have continued to study the application of such a laser-heated floating zone furnace to BiFeO ₃ single crystal growth, but generally, such a laser-heated floating zone furnace and a BiFeO 3 ₃ The relationship with single crystal growth is not known, nor is there any known range of conditions such as raw material rod preparation method, growth rate, atmosphere, flux composition, etc. suitable for BiFeO ₃ single crystal production It was.

JP 2009-224668 JP 2009-267363 A JP 2007-294986 JP JP2009-040626 Japanese Patent Application No. 2010-006894

Nature Materials 7, 425 (2008), Physics 2, 105 (2009) Physical Review B 76, 024116 (2007) Journal of Applied Physics 100, 074111 (2006) Solid State Communications 135, 133 (2005) Applied Physics Letters 91, 112913 (2007) Journal of Applied Physics 102, 074107 (2007) Journal of Applied Physics 101, 014108 (2007)

In the present invention, the first problem is to solve the above-mentioned problems in the prior art and develop a BiFeO ₃ large single crystal growth method.
In order to reduce the leakage current of BiFeO _3, the second challenge is to develop a single crystal growth method that can eliminate the direct causes of heterogeneous contamination and oxygen indefinite ratio.

In the course of research for developing a BiFeO ₃ large single crystal growth method against the background of the above-described conventional technology, the present inventors have obtained the following knowledge (a) to (c).
(A) The above-mentioned laser heating type floating zone furnace developed by the present inventors is applied to BiFeO ₃ single crystal growth, and the conventional lamp is adjusted by adjusting the growth conditions such as increasing the density of the raw material rod. The BiFeO ₃ single crystal can be grown by solving the problems of the penetration of the melt into the raw material and the dripping of the melt into the grown crystal when the heating type floating zone furnace is used.
(B) By adjusting various conditions such as the raw material rod preparation method, growth rate, atmosphere, and flux composition, it becomes possible to grow BiFeO ₃ large single crystals with higher quality.
(C) When growing the single crystal, the leakage current of the BiFeO ₃ single crystal can be reduced by adjusting the oxygen quantitative ratio in the atmospheric gas.

The present invention is based on the above findings and is characterized by the following matters.
(1) BiFeO ₃ high-density raw material rod supported on one of the upper and lower two crystal drive shafts, a seed crystal rod supported on the other crystal drive shaft, and a flux ( Using a melting agent, the flux is heated by the laser heating melting zone method (floating zone method) to form a melting zone, and a single crystal is grown in an atmosphere of oxygen, inert gas, or a mixed gas thereof. And manufacturing a BiFeO ₃ single crystal rod.
(2) The method for producing an electromagnetism single crystal according to (1), wherein the laser beam used for the laser heating has a substantially rectangular shape in radial and / or axial intensity distribution.
(3) The high density raw material rod of BiFeO ₃ is
A step of preparing a raw material rod by mixing and sintering a composition ratio of Fe ₂ O ₃ and Bi ₂ O ₃ in a molar ratio of 1: 1 to 1:
A step of melting and solidifying with high-speed movement in an atmosphere of oxygen, an inert gas, or a mixed gas thereof by a melting zone method using a flux placed between the raw material rod and the seed crystal rod When,
The method for producing an electromagnetic effect single crystal according to (1), which is prepared by a preparation method comprising:
(4) The flux uses Fe ₂ O ₃ and Bi ₂ O ₃ as raw materials, and the ratio R of Fe ₂ O _{3 in} the total raw materials is
0 mol% ≤ R ≤ 30 mol%
(1) The method for producing an electromagnetic effect single crystal according to (1), which is prepared by a step of sintering into a lump.
(5) Growth rate V of the BiFeO ₃ single crystal rod
V ≦ 1.0mm / h
BiFeO ₃ single crystal is grown as a method for producing an electromagnetic effect single crystal according to (1).
(6) Oxygen content I in the atmosphere
0% ≦ I ≦ 100%
BiFeO ₃ single crystal is grown as a method for producing an electromagnetism single crystal according to (1) or (3).
(7) Oxygen content I in the atmosphere
0% ≦ I ≦ 1%
(6) The method for producing an electromagnetic effect single crystal according to (6), wherein leakage current of the grown crystal is reduced.
(8) The method for producing an electromagnetism single crystal according to any one of (1), (3), (6), and (7), wherein the inert gas is argon gas.
(9) A perovskite oxide BiFeO ₃ single crystal produced by the method for producing an electromagnetic effect single crystal according to any one of (1) to (8).

In the case of the conventional lamp heating type, the penetration of the melt into the raw material rod is intense at the time of growth, the solid-liquid boundary is unclear, the dripping of the melt occurs, the diameter is not uniform, and the growth is unstable. In addition, the impurity phase also precipitates to produce a collection of fine crystals that cannot be said to be a single crystal, whereas in the laser heating method of the present invention, almost no penetration of the melt into the raw material rod is recognized. Thus, the solid-liquid boundary is very clear, the melt does not sag, the diameter is uniform, and even if there is no precipitation of the impurity phase, stable single crystals can be grown.
In addition to being able to stabilize the growth using laser heating, we succeeded in growing large crystals for the first time by optimizing the growth conditions (densification of raw materials, flux composition, growth rate, atmosphere).
Furthermore, crystal growth using precise oxygen control in a reducing atmosphere makes it possible to sufficiently suppress leakage current (leakage current) at room temperature using bulk materials that have not undergone special processing (impurity element substitution or acid treatment). It was.

It is a schematic plan view of the apparatus used for the manufacturing method of the electromagnetic effect single crystal of this invention. It is a figure which shows radial direction condensing intensity distribution of the rectangular distribution heating system used for the manufacturing method of the electromagnetic effect single crystal of this invention. It is a figure which shows the axial direction condensing intensity distribution of the rectangular distribution heating system used for the manufacturing method of the electromagnetic effect single crystal of this invention. BiFeO ₃ taken by a television camera provided in the furnace body of the present method is a real-time image of the melting zone near the single crystal growth. It is a real-time image of the vicinity of the melting zone during BiFeO ₃ single crystal growth in a conventional lamp furnace. The Fe ₂ O ₃ and Bi ₂ O ₃ is a phase diagram showing the equilibrium of the mixture to the composition. It is a single crystal rod photograph of a comparative example. 2 is a single crystal rod photograph of Example 1. FIG. 2 is a photograph of a single crystal rod of Example 2 and a remaining raw material rod. 4 is a single crystal rod photograph of Example 3. 4 is a single crystal rod photograph of Example 4. FIG. It is a photograph of the single crystal rod of Example 5 and the remaining raw material rod. It is a Laue photograph of the crystal bar cut surface of a comparative example. 2 is a Laue photograph of a crystal rod cut surface of Example 1. FIG. 3 is a Laue photograph of a crystal rod cut surface of Example 2. FIG. 4 is a Laue photograph of a crystal bar cut surface of Example 3. FIG. 6 is a Laue photograph of a crystal bar cut surface of Example 4. FIG. 6 is a Laue photograph of a crystal rod cut surface of Example 5. FIG. 3 is a graph showing the relationship between oxygen content I in the atmosphere during production and leakage current under an electric field of 1 kV / cm.

[Apparatus / Experiment Method]
Laser heating floating zone device for growing magneto-electric effect single crystal, main body with single crystal growth space for growing single crystal from raw material rod and seed crystal rod, personal computer, liquid crystal display, power supply unit, earth leakage breaker, control The control device is configured to control the output of the fiber coupling type high-power semiconductor laser or the moving speed and rotating speed of the upper and lower crystal drive shafts.

  The main body is equipped with a TV camera, and the heated part can be observed in real time. The fiber-coupled high-power semiconductor laser supplied with power from a constant-voltage / constant-current DC power source is connected to the main body via an optical fiber, and laser is applied to a sample disposed in a single crystal growth space in a transparent quartz tube. Irradiate light. The fiber coupling type high-power semiconductor laser is incorporated on the Peltier element, and the temperature of the Peltier element is controlled by a Peltier controller. Further, a gas controller for flowing atmospheric gas into the single crystal growth space and a circulating liquid cooling device for cooling the heated portion of the main body and the Peltier element are connected to the main body.

Next, the main part near the melting zone of the laser heating floating zone apparatus will be described with reference to FIGS.
FIG. 1 is a schematic plan view of an apparatus used in the method for producing an electromagnetic effect single crystal of the present invention, and FIG. FIG. 3 shows the axial light collection intensity distribution of the rectangular distribution heating method used in the method for producing an electromagnetic effect single crystal of the present invention.
The apparatus can sufficiently achieve the intended purpose by arranging an odd number of fiber-coupled high-power semiconductor lasers as a heating source, but FIG. 1 has five fiber-coupled high-power semiconductor lasers arranged. The case is illustrated.

This apparatus uses a plurality of fiber coupled high-power semiconductor lasers 1-1 to 1-5 having the same irradiation intensity, and a rectangular surface light source by reflecting light multiple times on the sides of a polygonal cylinder or a polygonal pyramid. Light pipes (optical elements) 2-1 to 2-5 to be formed, an upper crystal drive shaft, a lower crystal drive shaft, and the like not shown. The two upper and lower crystal drive shafts are arranged at an interval so as to be on the same vertical line. The light pipes 2-1 to 2-5 can use a collimator lens or the like in addition to the light pipe, and are arranged at equal angles around the drive shaft in the same plane in the circumferential direction of the both drive shafts. The laser beam is irradiated perpendicularly to both drive axes. As shown in FIG. 2, the raw material rod 3 is held on an upper crystal drive shaft (not shown), and the seed crystal rod 4 is held on a lower crystal drive shaft (not shown). A predetermined amount of flux (melting agent) 5 that is a mixture of Fe ₂ O ₃ and Bi ₂ O ₃ is placed on the upper end surface of the seed crystal rod 4. A rod-shaped sample 6 is formed by the raw material rod 3, the seed crystal rod 4 and the flux 5. The rod-shaped sample 6 is disposed in a transparent quartz tube (not shown), and an atmosphere gas suitable for single crystal growth is flowed into the quartz tube and filled with a predetermined pressure. The upper crystal drive shaft and the lower crystal drive shaft are rotated in opposite directions. The upper crystal drive shaft is moved downward at a predetermined raw material supply speed, and the lower crystal drive shaft is moved downward at a predetermined crystal growth speed.

  The laser light emitted from the fiber coupling type high-power semiconductor lasers 1-1 to 1-5 shown in FIG. 1 is reflected a plurality of times on the side surfaces of the polygonal cylinders or polygonal cones of the light pipes 2-1 to 2-5. Then, a laser beam spot 8 having a substantially rectangular shape as shown by a dotted line in FIGS. The flux 5 is melted by such laser irradiation to form a melting zone 7.

  FIG. 2 shows the intensity distribution of the laser beam in the radial direction. Concentration distribution of the laser beam in the radial direction of the rod-shaped sample 6 is the same as that described in detail in Japanese Patent Application No. 2007-206005 already filed. In particular, when the number of laser light sources is an odd number, the intensity distribution of the laser beam is smoother than when the number of laser light sources is an even number.

  FIG. 3 shows the intensity distribution of the laser beam in the axial direction. As shown in the drawing, the intensity distribution in the axial direction of the laser beam in the vicinity of the melting zone 7 becomes a substantially rectangular shape, and the rising (9a) and falling (9b) in the axial direction can be made steep. As a result, it is possible to grow a high-quality single crystal with very little penetration into the solid portion and no dripping into the solidified portion. The intensity distribution of the laser beam is preferably substantially rectangular in both the radial direction and the axial direction. However, from the viewpoint of preventing the penetration and sagging, only the axial direction can be substantially rectangular. In the present invention, the intensity distribution in the radial direction or the axial direction of the laser beam is substantially rectangular. In the graph of the intensity distribution in each direction, at least 70% (preferably at least 80%, preferably This means that the difference between the maximum value and the minimum value of the laser beam intensity is preferably within 25% (preferably 15%, more preferably 10%) of the maximum value.

4 is a real-time image of the vicinity of the melting zone during BiFeO ₃ single crystal growth taken with a TV camera provided on the main body. For comparison with FIG. 5, when BiFeO ₃ single crystal is grown in a conventional lamp furnace. A real-time image of the vicinity of the melting zone is shown. In the former, the penetration of the melt into the raw material is hardly observed, and the solid-liquid boundary is clearly in a straight line, whereas in the latter, the penetration of the melt into the raw material occurs and the solid-liquid boundary is complicated.

[Principle of crystal growth / leakage current and summary of experimental results]
FIG. 6 is a phase diagram showing an equilibrium state of a mixture of Fe ₂ O ₃ and Bi ₂ O ₃ . (Quoted from Journal of Solid State Chemistry 9, 139 (1974), ACerS-NIST PHASE EQUILIBRIA DIAGRAMS Version 3.2 (2008), The American Ceramic Society, Fig. 02357). This phase diagram shows the solid phase BiFeO _{3 in} the temperature range of 930 ° C. to 785 ° C. when the ratio R of the total Fe ₂ O ₃ in the liquid phase is 10 mol% ≦ R ≦ 30 mol%. Indicates that it can coexist with the liquid phase and produce BiFeO ₃ in the solid phase. Therefore, if an R flux (melting agent) in this range is used, the melting zone can be formed stably. When R> 30 mol%, Bi ₂ Fe ₄ O ₉ in the solid phase or a compound (solid phase) having a larger ratio of Fe coexists with the liquid phase and is generated, so that it is not suitable for BiFeO ₃ crystal growth. When R <10 mol%, the solid phase coexisting as the liquid phase is Bi ₄₀ Fe ₂ O ₆₃ or Bi ₂ O ₃ , but the composition of the flux is 10 mol% ≦ R ≦ when the raw material BiFeO ₃ is eluted. Since it may settle in the range of 30 mol%, it can be used for BiFeO ₃ crystal growth. Actually, since the composition of the flux changes as the crystal growth proceeds, there is no problem as long as the composition of the flux to be placed can stabilize the growth at an early stage. When a single crystal was grown using this apparatus, the composition was Fe ₂ O ₃ and Bi ₂ O ₃ as the flux used, and the growth was performed by changing the ratio R of Fe ₂ O ₃ in the whole. However, when R was 0 mol%, 10 mol%, and 30 mol% in molar ratio, good crystals were obtained in all cases.

As the raw material rod, a BiFeO ₃ sintered rod can be used. However, since the sintered rod is a porous body, some penetration of the melt is inevitable. Using the melting zone method, the raw material rod prepared by the process of melting and solidifying with higher speed movement than the normal crystal growth has a higher density, can suppress the penetration of the melt, and crystal growth Desirable to use. Here, high-speed movement means a movement speed exceeding 1.0 mm / h, preferably 2.0 mm / h or more, more preferably 5.0 mm / hr or more. Practically, for example, 6 mm Movement speeds such as / hr and 10 mm / hr can be employed. Further, in the present invention, the high-density raw material rod means that the apparent density is 80% or more (preferably 90% or more) of the BiFeO ₃ true density.
If a BiFeO ₃ single crystal is used as the seed crystal, it can be grown in a desired crystal orientation from the initial growth stage, which is desirable. However, even if a polycrystal is used, the direction of crystal grains is gradually aligned, so that single crystal growth is finally possible. Even if a polycrystalline seed crystal was actually used, the growth rod became a single crystal after the growth of about 20 mm in length.

  Since the composition of the raw materials and crystals and the composition of the flux are different, the equilibrium cannot be maintained if the crystal growth rate is too high. Therefore, the growth rate needs to be slower than a predetermined rate, and the slower the growth rate, the better the crystal growth. Since a slow growth rate is not efficient from the viewpoint of crystal production, a realistic growth rate is used. The growth was carried out at various crystal growth speeds, but in the case of 1.0 mm / h, different phases were mixed in the crystal, whereas in the case of 0.5 mm / h and 0.7 mm / h, almost a single phase was good. Crystals were obtained.

The oxygen content in the atmosphere during growth relative to oxides varies the valence of bismuth and iron, and the indefinite ratio of oxygen content (ie, deviation from the stoichiometric ratio of oxygen composition in BiFeO ₃ ). It becomes a factor. Since they also affect the stability of the compound, it is necessary to adjust the atmosphere to be suitable for growth. When growth was carried out with various mixing ratios of argon and oxygen, crystals having almost no heterogeneous phase were obtained in the entire region where the oxygen content I was 0% ≦ I ≦ 100%. However, when I = 100%, a large number of crystal domains existed, and the entire rod did not become a single crystal, and the crystal growth conditions tended to be slightly inferior.

The leakage current has been thought to be due to the presence of a heterogeneous phase and the unquantified ratio of oxygen content. The former is the idea that a different phase having conductivity (to some extent) contributes to the leakage current. There was no particular correlation between the abundance of foreign phases and the leakage current. The latter is based on the idea that carriers (electrons or holes) are generated by an indefinite ratio of oxygen content and contribute to leakage current. There was a tendency that the smaller the oxygen content in the atmosphere during growth, that is, the smaller the oxygen content in the crystal, the smaller the leakage current. This agrees well with the idea that in BiFeO ₃ , holes are likely to be generated by excess oxygen, and by reducing the oxygen content in the atmosphere, generation of excess oxygen, ie, holes, in the crystal can be suppressed.

[Growth crystal evaluation method]
The following evaluation was performed on the obtained grown crystal rod. Substances contained in the crystals were identified by a powder X-ray diffractometer (manufactured by Mac Science Co., Ltd.). The growth of crystal grains was evaluated by Laue photographs using an X-ray generator for Laue photographs (manufactured by Rigaku Corporation). Since imaging was performed using an imaging plate, a horizontal line due to an afterimage sometimes appears as shown in FIG. 17, but this does not interfere with crystal evaluation. A thin piece was prepared from the grown crystal rod, two electrodes were placed on both sides of the thin piece with silver paste, and the leakage current of the crystal was measured with a ferroelectric physical property evaluation apparatus (manufactured by Toyo Technica).

[Comparative example]
A case where a lamp heating type floating zone furnace is used is shown here as a comparative example.
A raw material rod and a flux (R = 0 mol%) are prepared in the same manner as in the examples described later, and a high-density raw material rod is prepared by primary melting.
The growth was performed at a growth rate V of 1.0 mm / h and an oxygen content I in the atmosphere of 0.2. FIG. 5 shows the state during the training. The melt penetrates into the raw material rod, and the solid-liquid boundary is not clear. As can be seen from the crystal photograph in FIG. 7, the melt dripped onto the growing portion, and the thickness became uneven. The breeding was not stable. In powder X-ray diffraction, about 5% of Bi ₂ Fe ₄ O ₉ is precipitated as an impurity phase, which is considered to be the result of unstable growth. In the Laue photograph of FIG. 13, fine spots are irregularly observed, indicating that it is not a single crystal but a collection of fine crystals. The lamp heating type is not suitable for the purpose of producing a desired single crystal.

Examples 1 to 5 in the case of using a laser heating type floating zone furnace are shown below. Table 1 summarizes the growth conditions and the evaluation results.

[Example 1]
The ratio of Fe ₂ O ₃ and Bi ₂ O ₃ is weighed 1: 1 by molar ratio, and ground in a mortar until uniform. This is put in an alumina container, raised to 720 ° C in 2 hours and 30 minutes, and calcined while maintaining 720 ° C for 12 hours. Then, it is molded by rubber pressing with hydrostatic pressure, raised to 720 ° C. in 2 hours and 30 minutes, and gradually sintered to 750 ° C. over 5 hours to form a round bar having a diameter of 6 mm. In this way, two round bars are produced and used as the raw material bar 3 and the seed crystal bar 4.
Further, the ratio of Fe ₂ O ₃ and Bi ₂ O ₃ is weighed to a molar ratio of 0:10 (R = 0 mol%) and ground in a mortar until uniform. This is put into an alumina container and calcined at 650 ° C. for 12 hours. Then, it is molded by rubber pressing under hydrostatic pressure and sintered at 680 ° C. for 12 hours to form a round bar having a diameter of 6 mm. This round bar is cut, and a lump cut piece is used as the flux 5.
The crystal growth of the present invention is performed in two steps. The first stage is performed to increase the density of the raw material, and is a process of melting and solidifying the raw material with high-speed movement and is called primary melting. The second stage is for crystal growth and is called secondary melting. Although it is possible to omit primary melting, in that case, slight penetration of the melt into the raw material occurred, and crystal growth became somewhat unstable. Therefore, the final melting was performed without omitting the primary melting as follows.
First, primary melting will be described.
The raw material rod 3 and the flux 5 having a weight of 0.55 g are placed on the upper end of the seed crystal rod 4. The raw material rod 3 and the seed crystal rod 4 on which the flux 5 is placed on the upper end are attached to the upper crystal drive shaft and the lower crystal drive shaft, respectively. An atmosphere gas of 99% Ar and 1% O ₂ is flowed into the quartz tube at a flow rate of 0.15 to 0.17 L / min. As the laser output is increased, the melting zone is created and stabilized, and finally the laser output is 26.0%, the laser current is 9.82 A, the moving speed of the upper crystal drive shaft and the lower crystal drive shaft is 6 mm / hr, A high-density material rod of BiFeO ₃ is stably grown at 10 mm / hr and a rotational speed of 30 rpm on both drive shafts. The apparent density of the obtained material bar was about 90% of the true density of BiFeO ₃ .
Next, secondary melting will be described.
A high-density BiFeO ₃ high-density material rod with a diameter of 4 mmφ grown by primary melting is newly used as the raw material rod 3, and the remainder of the flux 5 used in the primary melting is deleted, and a new flux 5 weighing 0.47 g is then added. It is placed on the upper end of the seed crystal rod 4. The raw material rod 3 and the seed crystal rod 4 on which the flux 5 is placed at the upper end are attached to the upper crystal drive shaft and the lower crystal drive shaft, respectively.
An atmosphere gas (I = 10 ⁻² ) containing 99% Ar and 1% O ₂ is flowed into the quartz tube at a flow rate of 0.15 to 0.17 L / min. As the laser output is increased, the melting zone is created and stabilized, and finally the laser output is 24.3%, the laser current is 9.16 A, and the moving speed of the upper crystal drive shaft and the lower crystal drive shaft is 0.4 mm / hr, A single crystal is stably grown without fluctuation over a long period of time at 0.7 mm / hr (growth speed V) and a rotational speed of both drive shafts of 30 rpm. The penetration of the melt into the raw material rod and the dripping of the melt into the grown crystal were hardly observed.
A photograph of the single crystal rod grown in this way is shown in FIG. 8, and a Laue photograph by a Laue photographic X-ray generator is shown in FIG. Single crystal rods have a flat cut surface and a cleaved surface. Laue patterns generally show good crystallinity with sharp spots and lines. Further, as a result of the powder X-ray diffraction measurement, the Bi ₂ O ₃ peak was only 0.4%, indicating that the crystal was a BiFeO ₃ single-phase crystal rod.
In order to measure the leakage current, a crystal was cut into a cross-sectional area of 0.23 mm ² and a thickness of 0.35 mm, and electrodes were placed on both sides with silver paste. Measurement was performed with a ferroelectric physical property evaluation apparatus, and a leakage current of 2900 μA / cm ² was observed when an electric field of 1 kV / cm was applied.

Since the experimental example below is the same as the experimental procedure in Example 1, the main points such as experimental conditions and differences in evaluation results are shown.
[Example 2]
The growth was carried out at a Fe ₂ O ₃ ratio R in the flux of 10 mol%, a growth rate V of 1.0 mm / h, and an oxygen content I in the atmosphere of 10 ⁻² . The cultivation was carried out stably for a long time. The remaining photographs of the grown crystal and the raw material bar are shown in FIG. (The upper part is the rest of the raw material rod, and there is almost no evidence of melt penetration. The two underneath are grown crystals, which are broken in the middle when taken out.) About 4% of Bi ₂ O ₃ is precipitated as a phase. As shown in FIG. 15, the Laue pattern generally indicates that the crystallinity is good with the spots and lines sharp. A leakage current of 20000 μA / cm ² was observed when an electric field of 1 kV / cm was applied.

[Example 3]
The growth was performed with the Fe ₂ O ₃ ratio R in the flux being 10 mol%, the growth rate V being 0.5 mm / h, and the oxygen content I in the atmosphere being 1 (ie, in oxygen). FIG. 4 shows the state of the training. Little penetration of the melt into the raw material rod was observed, the solid-liquid boundary was clear, and the growth was carried out stably for a long time. Facets (crystal planes) are seen in the grown crystal, suggesting good crystal orientation. A photograph of the grown crystal is shown in FIG. In powder X-ray diffraction, about 1% of Bi ₂ O ₃ is precipitated as an impurity phase, which is almost single-phase BiFeO ₃ . As shown in FIG. 16, the Laue pattern shows that the spot is broken and consists of a plurality of substantially oriented crystals. A leakage current of 300000 μA / cm ² was observed when an electric field of 1 kV / cm was applied, which is the largest of the examples.

[Example 4]
The growth was carried out at a Fe ₂ O ₃ ratio R of 10 mol% in the flux, a growth rate V of 0.5 mm / h, and an oxygen content I in the atmosphere of less than 10 ⁻⁷ (argon gas G1 grade). The cultivation was carried out stably for a long time. A photograph of the grown crystal is shown in FIG. In powder X-ray diffraction, about 0.2% of Bi ₂ O ₃ is precipitated as an impurity phase, which is almost single-phase BiFeO ₃ . As shown in FIG. 17, the Laue pattern generally indicates that the crystallinity is good with the spots and lines sharp. A leakage current of 1.2 μA / cm ² was observed when an electric field of 1 kV / cm was applied.

[Example 5]
The growth was carried out at a Fe ₂ O ₃ ratio R in the flux of 30 mol%, a growth rate V of 0.5 mm / h, and an oxygen content I in the atmosphere of 10 ⁻⁴ . The cultivation was carried out stably for a long time. The remaining photographs of the grown crystal and the raw material bar are shown in FIG. (The upper part is the rest of the raw material bar and there is almost no evidence of melt penetration. Below it is the grown crystal.) In powder X-ray diffraction, about 1% of Bi ₂ O ₃ is precipitated as an impurity phase. It is almost single-phase BiFeO ₃ . As shown in FIG. 18, the Laue pattern generally indicates that the crystallinity is good with the spots and lines sharp. A leakage current of 2.6 μA / cm ² was observed when an electric field of 1 kV / cm was applied.

The above comparative examples and examples revealed various conditions suitable for BiFeO ₃ crystal growth.
As for the heating method, as is clear from the comparative example, in the lamp method, the penetration of the melt into the raw material rod and the dripping of the melt into the grown crystal are serious, making stable crystal growth difficult, and BiFeO ₃ Not suitable for the production of crystals. The laser system avoids these problems and is suitable for the production of BiFeO ₃ crystals. In particular, the laser system used in the present invention is optimal, but it goes without saying that other laser systems have a sufficient effect due to good light condensing performance.
With respect to the raw material rod, crystal growth is possible without primary melting, but since growth tends to be unstable, it is desirable that the raw material rod is first melted and densified. As a seed crystal, even a polycrystal is gradually crystallized so that it can be single-crystallized without any problem. However, if a single-crystal seed crystal is used, it can be grown in a desired crystal orientation from the initial growth stage, which is more desirable.
If the ratio R of Fe ₂ O _{3 in} the flux is in the range of 0 to 30 mol%, crystal growth is possible. As shown in FIG. 6, when R> 30 mol%, Bi ₂ Fe ₄ O _{9 in} the solid phase or a compound having a larger Fe ratio coexists with the liquid phase and is generated, and therefore, it is not suitable for BiFeO ₃ crystal growth. Is clear. In reality, the flux composition changes and is optimized as the crystal grows, so R may be in the range of 0 to 30 mol%.
As is apparent from Example 2, the growth rate V is set to 0.7 mm / h or less because 1 mm / h is too early and there is a tendency that different phases are incorporated into the crystal without sufficiently maintaining the phase equilibrium. Is desirable. (Because crystal orientation is sufficient even at 1 mm / h, it can be used for crystal growth if heterogeneous phase is not a problem.) In principle, the slower the crystal growth, the better the crystal growth. Since slow growth speed is not efficient, realistic growth speeds such as 0.7 mm / h and 0.5 mm / h are used. The crystal can be pulled up by reversing the upper and lower sides of the embodiment, attaching the raw material rod to the lower crystal drive shaft, and attaching the seed crystal to the upper crystal drive shaft.
With respect to the oxygen content I in the atmospheric gas, crystal growth is possible in the entire range of 0% to 100%. However, as is clear from Example 3, in the case of 100%, there is a tendency that the crystal domains are not easily aligned.
A method for reducing leakage current has also been clarified. There was no correlation between the amount of mixed foreign phase and leakage current. As shown in FIG. 19 (excluded because the growth rate V was too high for Example 2), it became clear that the leakage current was smaller as the oxygen content I decreased. This is thought to be because the generation of holes due to excess oxygen in the crystal is suppressed and electrical conduction is reduced. In order to suppress the leakage current, I is preferably 10 ⁻² or less, and is preferably as low as possible.

  The electro-magnetic effect single crystal manufactured by the manufacturing method of the present invention can be made into a high-quality and large-sized one that can be made into a device, and further, the leakage current can be reduced. Ferroic) devices, ferroelectric devices, piezo devices and the like.

1-1 to 1-5 Fiber coupling type high power semiconductor laser 2-1 to 2-5 Light pipe 3 Raw material rod 4 Seed crystal rod 5 Flux 6 Rod-shaped sample 7 Melting zone 8 Laser beam spot 9a Rising portion 9b Falling portion

Claims (9)

BiFeO ₃ high-density raw material rod supported on one of the upper and lower crystal drive shafts, seed crystal rod supported on the other crystal drive shaft, and flux (melting agent) placed between the two rods , A flux zone is heated by a laser heating type melting zone method (floating zone method) to form a melting zone, and a single crystal is grown in an atmosphere of oxygen, inert gas, or a mixed gas thereof to form BiFeO. _3. A method for producing an electromagnetic effect single crystal, comprising producing a single crystal rod.   2. The method for producing an electromagnetism single crystal according to claim 1, wherein the laser light used for the laser heating has a substantially rectangular shape in intensity distribution in a radial direction and / or an axial direction. BiFeO ₃ high-density raw material rod,
A step of preparing a raw material rod by mixing and sintering a composition ratio of Fe ₂ O ₃ and Bi ₂ O ₃ in a molar ratio of 1: 1 to 1:
A step of melting and solidifying with high-speed movement in an atmosphere of oxygen, an inert gas, or a mixed gas thereof by a melting zone method using a flux placed between the raw material rod and the seed crystal rod When,
The method for producing an electromagnetic effect single crystal according to claim 1, which is prepared by a preparation method comprising The flux uses Fe ₂ O ₃ and Bi ₂ O ₃ as raw materials, and the proportion R of the total amount of Fe ₂ O _{3 in} the raw materials is
0 mol% ≤ R ≤ 30 mol%
The method for producing an electromagnetic effect single crystal according to claim 1, which is prepared by a step of sintering into a lump. Growth rate V of the BiFeO ₃ single crystal rod
V ≦ 1.0mm / h
The method for producing an electromagnetism single crystal according to claim 1, wherein a BiFeO ₃ single crystal is grown as: The oxygen content I in the atmosphere is
0% ≦ I ≦ 100%
4. The method for producing an electromagnetism single crystal according to claim 1 or 3, wherein a BiFeO ₃ single crystal is grown. The oxygen content I in the atmosphere is
0% ≦ I ≦ 1%
The method for producing an electromagnetic effect single crystal according to claim 6, wherein leakage current of the grown crystal is reduced.   The method for producing an electromagnetism single crystal according to any one of claims 1, 3, 6, and 7, wherein the inert gas is an argon gas. A perovskite oxide BiFeO ₃ single crystal produced by the method for producing an electromagnetic effect single crystal according to claim 1.