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Discovering element 113 Panelists: Yasushige Yano,1 Chief Scientist, Masayuki Kase,2 Senior Scientist, and Kosuke Morita,3 Senior Research Scientist 1RIKEN Discovery Research Institute, Cyclotron Center, Dr. Sci., 2Accelerator-based Research Group, Beam Technology Division, 3Accelerator -based Research Group, Beam Technology Division, Dr. Sci. |
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| Synthesizing superheavy elements that do not occur naturally | |||
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| -- How did you discover the element with atomic number*2 113? Yano: Elements with larger atomic numbers that are heavier than neptunium (Np)*3, which is element 93, are called superheavy elements. These elements do not exist in nature because they are unstable and rapidly decay into other stable elements. A new element is synthesized through a nuclear reaction in which we first have to prepare a target nucleus and then bombard it with an accelerated atomic beam. Morita: In synthesizing element 113, we used bismuth as a target nucleus. Bismuth (209Bi) is the element with atomic number 83 and mass number 209. We bombarded a bismuth target with a beam of zinc (70Zn), the element with atomic number 30 and mass number 70 (Figure 1). The zinc beam was accelerated by the RIKEN heavy ion linear accelerator (RILAC) until the speed of the zinc nuclei reached 10% of the speed of light. At this speed, 2.5 trillion atoms of zinc per second bombarded the target. The bombardment continued for as long as 80 days, giving a total bombardment time of 1,920 hours. It was a very tough experiment. The number of collisions between zinc nuclei and bismuth targets reached 1 x 1014. At the end of this experiment, an element with atomic number 113 and mass number 278 was successfully synthesized. -- How has the discovery of superheavy elements been developed? Morita: The first synthesized superheavy element was neptunium, with atomic number 93. Since neptunium was discovered in 1940, the US, Russia, and Germany have been in fierce competition with each other. Elements with atomic numbers from 93 to 103 were all discovered by the US teams. There was fierce competition between the US and Russian teams to synthesize elements with atomic numbers 104 to 106. Elements with atomic numbers 107 to 112 were created by German teams. Our discovery of element 113 this time has allowed Japan to jump up to the top position in the world.
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| It all started 20 years ago. | |||
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-- When did RIKEN start experiments on synthesizing superheavy elements? Yano: In fact we started preparing for the discovery of superheavy elements 20 years ago. The first person to suggest conducting superheavy element experiments was Toru Nomura, professor emeritus at the High Energy Accelerator Research Organization, who was then at the Cyclotron Laboratory. Here's a brochure for the Ring Cyclotron published in 1986, and in it can be found the question "Who will discover the superheavy elements?" Elements up to atomic number 109, meitnerium (Mt), had already been discovered at that time. Scientists around the world, including us, aimed at creating atomic number 114. While superheavy elements have an extremely short lifetime of about 1/1000 seconds, element 114 was predicted to be long-lived. Therefore, scientists believed it would be relatively easier to create the element. Morita started his research carrier in RIKEN to be in charge of the discovery of superheavy elements using the Ring Cyclotron. Morita: I had only a little knowledge about superheavy elements back then. I did not think that the discovery would be that difficult. We first developed a gas-filled recoil ion separator (GARIS). A semiconductor detector is used to detect a new nucleus that has been synthesized as a result of a collision of a target nucleus with a beam nucleus. It is important to isolate the nucleus of interest effectively and accurately from other nuclei. The synthesized nucleus will soon release neutrons or protons, which deviate from the orbit and do not fly straight. The bore of the separator therefore must be as large as possible. For this reason, the separator we designed has an unusual shape like a paramecium. GARIS is still the world's best nuclei separator, and has a superior performance. -- The Ring Cyclotron was completed in 1986 and in the following year, full scale experiments started. Did you achieve successful results in a short period of time? Kase: No. We had a difficult time. We intended to perform experiments on superheavy elements, but the accelerator was not designed for this purpose. At first, we installed GARIS downstream of the Ring Cyclotron in consideration of the accelerated energy. The beam we obtained in this configuration was too weak to generate new elements. Yano: If you are serious about succeeding, the beam needs to be intensified 1,000 times. In order to meet this requirement, we produced an ECR ion source to create high-energy ions at 18 GHz and an RFQ accelerator which allowed us to change the frequency of the high-frequency wave used to accelerate ions. With this instrument, the best in the world and an original RIKEN invention, we finally succeeded in generating a beam with high intensity that is essential to perform experiments on superheavy elements. Morita: Thanks to this success, we achieved some results, but it was not enough. What we created were radons (197Rn, 196Rn) and francium (200Fr), radioactive isotopes which do not occur naturally. This was in 1995. However, we still had a long way to go before discovering superheavy elements. |
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| Relocation of GARIS for the Japonium project | |||
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-- When were you fully prepared to engage in the discovery of superheavy elements? Yano: In 1999, scientists at the Lawrence Berkeley National Laboratory of the US announced the discovery of element 118. In response, we conducted a follow-up experiment but only to fail to confirm it. Other follow-up experiments were also conducted in Germany and France, but the results were the same as ours. The discovery turned out to have been fabricated, an unforgivable deed. Anyway, this was the first time that we performed a full-scale experiment on synthesizing a superheavy element. Mr. Kase still has the e-mail he received saying "we should start full-scale experiments on superheavy elements." This was in October 1999. We named the project "The Japonium Project" and mounted an extensive PR campaign. Despite our enthusiasm, the intensity of beams obtained downstream of the Ring Cyclotron remained low. We desperately needed to use the RILAC linear accelerator. By adding a CSM accelerating tank to RILAC with the cooperation of the Center for Nuclear Study (CNS) of the University of Tokyo, we were able to solve the problem in terms of accelerated energy. We finally perfected the apparatus for performing experiments on the synthesis of superheavy elements (Figure 2). Kase: It was really hard to relocate GARIS from the Ring Cyclotron to RILAC because there were many restrictions. GARIS weighs in at a heavy 60 tons. At the same time, a new accelerator was being constructed as part of the Radio Isotope Beam Factory (RIBF) Project. Drilling for the construction had started just in front of the place where GARIS was supposed to be relocated. Unless we had started the relocation immediately, we would have had to wait another three years until the hole had been filled in again. But, before relocation, we had to remodel the GARIS... It was a race against time.
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| Atomic number 113, a challenge to the RIKEN team | |||
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-- Which element did you aim at in the Japonium project? Morita: Number 113. The atomic number 112 had already been created in 1996. But you cannot jump into an experiment to generate the element. First of all, we conducted follow-up experiments based on data from the Gesellschaft für Schwerionenforschung (Laboratory for Heavy Ion Research, GSI) in Germany, which had discovered elements 107 to 112. In July 2002, we started an experiment on atomic number 108. We synthesized ten atoms of them within one week. We also synthesized element 110. While GSI had discovered six atoms of element 111 during a period of several years, we successfully synthesized 14 atoms in our 50-day experiment. We finally became confident that we were better than GSI. This was in the summer of last year. In September last year, we obtained information that GSI intended to synthesize atomic number 113. We therefore decided to skip 112 and go for 113. However our efforts, which lasted until the end of the year, were in vain. Although we were still confident in what we were doing, we thought we might have to restart from 112 again. This was in April of this year. On April 16, the day before the Wako Institute was opened to the public, we succeeded in generating an element with atomic number 112. A poster announcing this success was designed and printed in a rush in time for the opening of the institute. We have so far synthesized two atoms with atomic number 112, which is the same number that GSI have synthesized. Kase: According to our initial plan, the superheavy element experiment would be completed with the discovery of 112. There were other users who were planning to use beams obtained from RILAC to inject into the Ring Cyclotron. We had to let them use RILAC. Then the Ring Cyclotron broke down. Since RILAC was available, we thought we should not waste any time and decided to start the experiment on synthesizing element 113 ahead of schedule. Morita: On the night of July 23rd, we finally generated element 113. -- What do you think brought you this success? Yano: First of all, RILAC has the world's best performance. Morita also played an important role. He found the optimal incident energy for the beam nuclei. If the incident energy is wrong, a target nuclear reaction will not occur. Morita: Once you've selected the incident energy, you cannot change it during the experiment. You don't have a second chance. We were searching for the optimal incident energy for synthesizing element 113 while performing the follow-up experiments on the synthesis of elements 110 and 111 by carefully changing incident energies. Since GARIS has background reduction 100 times higher than other institutes' recoil ion separators, we didn't have room for mistakes. Yano: And we were lucky. We had been lucky and we did not let the opportunity pass us by. Morita: We did everything we could. Nothing was left to chance. We did visit many shrines too, always throwing 113 yen into the offertory box for good luck. <making smile> Kase: The last bit of good luck for us was the trouble with the Ring Cyclotron on June 25, which allowed us to continue our experiment. Of course there is no "if" in history, but if the Ring Cyclotron had not broken down right then, our experiment was to start on September 1. Considering that the related construction work might have been delayed, we might not have been able to start the experiment before the end of September. If so, we might have lost in the race against GSI. We should not forget the contribution of the team who were involved in the RILAC operation. In experiments on superheavy elements, we need to supply a beam not only at an extremely high intensity but in a stable manner for a long period of time. In order to supply the beam continuously for 80 days, the team members had to be always highly alert for more than 100 days so that they could adjust the ion source and deal with any failure of the accelerator. The discovery of element 113 was therefore the outcome of efforts by the many people who were involved in, ranging from developing the instrument, to adjusting it and operating the accelerator. |
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| Is Japonium to be the name for element 113? | |||
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-- There is a report claiming that element 113 was discovered by a Russian team before you did it. Morita: A paper was published in February this year, which claims that scientists at the Flerov Laboratory of Nuclear Reactions in Russia synthesized element 115. When element 115 alpha-decays, it becomes element 113. According to their way of thinking, the Russians claim to have discovered both elements 115 and 113. We admit the facts resulting from their experiments. I think they are almost right, but at the end of the decay process they had an unknown nucleus, and it is difficult to identify the atomic and mass numbers of the synthesized element on an experimental basis. This is their weak point. The element we synthesized reached a known nucleus after spontaneous fission, which followed four alpha decays (Figure 3). By tracing back the chain of decay, we can prove on an experimental basis the atomic and mass numbers of the synthesized element. For these reasons, we claim to be the first to discover a new element with atomic number 113. A Russian team also claimed to have discovered atomic number 114, but again, the end of its decay process was an unknown nucleus. Therefore, the heaviest element whose discovery is supported by experimental evidence is the element 113 we have created. -- Isn't the team who discovers a new element entitled to name the element? Yano: We think there is a high possibility that we will be allowed to name element 113. Since no Japanese has ever named an element, we would like to name it if we possibly can. One of the candidate names is Japonium and another is Rikenium. -- What is your future strategy for experiments on superheavy elements? Morita: Above all things, we want to complete the one on 113, hoping to create more atoms of them. We will then move on from 114 to 115 step by step. Having discovered one new element so far, we are still at the starting point. -- RIBF is under construction. When is it scheduled to be completed? Yano: According to our plans, the first beam will be produced at 15:34 on December 16, 2006. Exactly at the same time on the same day twenty years ago, the first beam was produced at the ring cyclotron that is now operating. On that day a new era started for scientists of my generation. On December 16, 2006, another new era will start for the younger generation of scientists. The rest of the world is now frantically trying to catch us up. We will however be moving further ahead, which makes us feel good. Theory predicts that no elements heavier than atomic number 173 can exist. I want to ascertain where the limit is for the existence of elements. But this will not be our mission; it will be left to younger scientists. 
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Chemical biology leads to clarifying biological processes and new molecular targeting drugs
RIKEN Discovery Research Institute |
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| Chemical biology | |||
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The 60 trillion cells that make up our bodies grow through repeated cell division from a single fertilized egg. Even as adults, our cells are dividing constantly. "Cell division is among the most fundamental phenomena of life and yet, the details still hold many mysteries. That's what makes this subject so fascinating. Of innumerable life phenomena, we pursue cell division most intently because we are passionately eager to know how the system works, how it occurs." Speaking is Chief Scientist Osada, who inherited and further developed the Antibiotics Laboratory in RIKEN for fifty years. The most common methods for investigating the mechanisms of life phenomena belong to the molecular biologists, who approach through the genes. Of course, molecular biology is indispensable in his laboratory as well, but Osada's research team attempts to elucidate the mechanisms of life phenomena through another method, namely, chemical biology, the fusion of chemistry and biology. Osada explains. "It is a way of using the powers of chemistry to reveal complicated phenomena of life. Numerous biological or chemical projects are underway at RIKEN as well, but chemical biology fills the space between those disciplines, and we expect it to open whole new vistas in life science. It holds enormous potential for the creation of important new medicines." |
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| Bioprobes - analytical tools for life process | |||
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Research at Osada Antibiotics Laboratory begins with the search for compounds that stopped cell division. "We just call it a search for compounds, but our approach is highly original," says Osada. First, they isolate and culture actinomycetes, filamentus, or other microorganisms from soil samples (Figure 1). Then, the cultured solution from each microorganism is tested against dividing cells to observe its effect. The key to this search for new compounds that stop cell division lies in the words "from a microorganism." When most people think about compounds created by microorganisms, they think of penicillin, streptomycin and other antibiotics used to treat bacterial infections. However, in addition to antibiotics, we have found compounds that inhibit enzyme activity, stop cell division, cause spontaneous cell death (apoptosis), and regulate a variety of other cell functions. When these compounds bind to specific proteins, they inhibit their activities to regulate cell functions. When a compound stops cell division, we know that the target protein for that compound plays a vital role in the process. Using such compounds as tools, we can find proteins that regulate cell division. Each such discovery teaches us more about the mechanism. Seven years ago, Osada coined the term "bioprobe" to refer to compounds derived from microorganisms, which can regulate life phenomena. Today, that term has established itself in the classification of antibiotics used in basic research. A "probe" is usually a needle or other device used for exploration. 
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| Pironetin destroys tubulin | |||
Researchers at Osada Antibiotics Laboratory have found more than 40 types of compounds that stop cell division. Already, many are commercially available worldwide as reagents for basic research science. The compound now receiving most attention is pironetin. The target protein for pironetin is tubulin, which is involved in cell movement and molecular transport during cell division. When cells divide, especially when the replicated chromosomes divide into individual pairs, tubulin plays a key role. Tubulin is composed of two protein units ( -tubulin and  -tubulin) and its dimer forms a micro tube called "microtubule." However, if pironetin is present, the microtubule fall apart into dimeric units. As a result, the chromosomes fail to separate and cell division stops. Several compounds that target tubulin have already been found, but they all bind with -tubulin. Pironetin is the first compound that binds with -tubulin (Figure 2). "Pironetin is receiving attention because it has potential as an anticancer drug. In the development of anticancer drugs, compounds that bind to different sites ever unknown are extremely important," explains Osada. Anticancer drugs exhibit large individual differences in terms of effectiveness and side effects. A compound with the same target protein but a different binding site could prove beneficial for patients unable to use the previous drug. Since pironetin binds near the GTP-GDP binding site (Figure 2) which is supposed to be important for the polymerization of tubulin, pironetin could be a highly effective anticancer drug. |
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| Developing anticancer drugs | |||
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Normal cells divide in the presence of sufficient nutrition and an appropriate environment for cell proliferation. Cancer cells divide and proliferate without limit, totally ignoring the environment. Most anticancer drugs now in use stop cell division. However, Osada points out, "Anticancer drugs are far from thorough understanding in terms of target proteins and the mechanisms underlying cell division." Therefore, chemical biology is the field awaited with great expectation. Bioprobes are used like hooks, thrown into the system to fish for compounds, often "catching" target proteins. They make it possible to find the proteins involved in cell division with certainty and ease. Once we know the proteins, we can identify the genes. Osada's Laboratory is analyzing the structure of target proteins to clarify exactly which part of the protein is involved and how the proteins bind to each compound. As a result, they will soon be able to produce anticancer drugs for which the pharmacological mechanisms are thoroughly understood. Modification of the compound structures opens the possibility of designing more effective anticancer drugs. "Looking back," Osada says, "cancer cells were simply tools for me." In studying the mechanisms of biological phenomena, the easiest method is to compare normal and abnormal phenomena. Dr. Osada had been engaged in basic researches on cell division using cancer cells. "However, as I grew older and lost close friends and relatives caused by cancer, my approach changed. Rather than using cancer cells as research tools, I became determined to cure cancer. In my laboratory, we do perform basic research, but we are also working intently to find new drugs to cure cancer." |
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| Reveromycin and osteoporosis | |||
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Of the compounds discovered by Osada's Laboratory, the one closest to be an anticancer drug is reveromycin, but this compound has already traveled a long and winding road until the present great assessment. It was known from the moment of its discovery that reveromycin was extremely powerful in its ability to stop cell division. Everyone at the lab thought, "This one will be a great anticancer drug." "Research fellow Hidetoshi Takahashi, who was working with me when we discovered reveromycin, thought we needed to gather data about the side effects that emerge when reveromycin is in the blood stream. He started his observations with mice. We had no previous experience in creating drugs, so we thought we would have a product right away. When I think of it now, I realize how ridiculously ahead of ourselves we were," Osada laughs. However, that initial study led to an unanticipated discovery. Ordinarily, the concentration of calcium in the blood increases dramatically in the final stages of cancer. However, when mice were treated with reveromycin, this increase in calcium concentration failed to appear. On further investigation, it was learned that reveromycin inhibits the bone resorption and the number of osteoclasts releasing calcium into the blood stream. Reveromycin induced apoptosis (programmed cell death) in osteoclasts. Researchers at the Osada Lab then thought reveromycin might be used to fight osteoporosis. They continued their research with that possibility in mind. Osteoporosis is a disease that reduces bone density by calcium deficiency, creating the bones much more breakable. In animal experiments, the therapeutic effect of reveromycin was found to be high, but there was a serious obstacle to using it as a treatment for osteoporosis. Reveromycin did not enter the blood after being ingested. It had to be injected intravenously. Because osteoporosis is not an immediately life-threatening disease, it requires a drug that can be taken orally at home. Reveromycin hit a wall. |
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| Suppressing cancer metastasis to bone | |||
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Reveromycin research took another turn. Breast cancer, prostate cancer, and lung cancer metastasize easily to bone. Osteoclasts melt the bones, allowing the cancer cells to move in. When that happens, the late stages of the cancer are accompanied by a great deal of pain, which becomes a serious problem for treatment. "We discovered that even small amounts of reveromycin are highly effective in controlling metastasis to bone," says Osada. "At that point, we thought we could use reveromycin to induce apoptosis and reduce the number of osteoclasts." However, the structure of reveromycin is too complicated. The protein target is isoleucine-tRNA synthetase, but they had no idea how it was bound. Also, when manufacturing a medicine, it is preferable to synthesize it chemically, but reveromycin is so complex it was extremely difficult to synthesize perfectly. Osada: "First, we had to learn how reveromycin was bound to its target protein. If we could find that out, we thought we could possibly avoid synthesizing the whole thing. We might be able to obtain the effect more easily by synthesizing just the critical part. We began using computer to design a compound that would bind perfectly to the target protein." This approach was effective. Animal testing is underway with a clinical physician and a drug company taking part in the joint research. |
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| Developing the small molecule microarray | |||
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It is said that more than a hundred million microorganisms can inhabit a single gram of soil, and only 1% of those microorganisms have been isolated. These microorganisms will undoubtedly produce vast numbers of new compounds. However, as Osada points out, "the question is, how can we efficiently ferret out the useful ones from among all these compounds?" Until now, a certain compound was applied to a group of cells to see if it would stop cell division. Then, the next compound was applied and observed. At this rate, a dizzying number of operations would be required. To solve this problem, Osada Antibiotics Laboratory has developed a "Small Molecule Microarray" (Figure 3). Thousands of compounds with low molecular weights are fixed on a glass slide. If, for example, a protein involved in cell division bound to a fluorescent dye is applied to the microarray, all compounds that target the protein can be detected quite efficiently. Those compounds, then, become antitumor candidates. Small molecule microarray applies DNA microarray technology to small molecular weight compounds. However, the structures of these compounds are all different and diverse, and multiple types could not be fixed on the same slide at the same time. This problem was overcome by "the cross-linking method." A functional group called aryldiaziridine generates a highly reactive compound known as carbene when irradiated with ultraviolet ray at 360 nm. With this, numerous types of small molecules can be fixed on the slide simultaneously. The Osada Lab is developing a small molecule microarray that fixes 2000 types of compounds. There are about 20,000 genes in human genomes. There should be compounds that bind specifically to each of the proteins created by these 20,000 genes. Osada intends to find them all. The more he finds, the greater his chances of finding a useful medicine. "No one has ever thought about those proteins or searched for them. However, if we look, we'll find them. Small molecule microarray is a powerful technique in that effort." 
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| Eternal and new in turn | |||
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The motto at Osada Antibiotics Laboratory is, "Fueki Ryuko (Eternal and New in Turns)." In the elegant haiku method taught by Basho Matsuo, fueki means unchanging, while ryuko means to change with the times. The motto refers to the changes that arise from the unchanging, and the eternal truths that emerge from change. "The unchanging at our lab is the use of microorganisms. The actual research topics and methods change to adapt to the times. My predecessors made agricultural chemicals and drugs to fight tuberculosis. Now, one of our goals is a cure for cancer, and we are also targeting osteoporosis and diseases related to blood vessels, which will be ever greater problems in our aging society." Osada's dream is to spread chemical biology throughout Japan. "I've always known that the US is advanced in chemical biology. However, I recently went to South Korea and was quite surprised by what I found. Not only are South Koreans, like the Japanese, talking about the chemistry of natural products, they have already incorporated the American interdisciplinary system and is ahead of Japan in chemical biology. Meanwhile, Japan has no sense of crisis. We are so determined to insist on originality that we have to feel risk being left behind the global trend." US universities offer courses in chemical biology. Here in Japan, we still struggle with the thick wall between the disciplines of chemistry and biology. The need to fuse disciplines for research purposes is given by lip service, but has yet to affect research style. Osada insists, "If chemistry and biology were fused throughout this country, it would change life science and drug discovery in Japan. I want to spread the concept of chemical biology through RIKEN and beyond to the rest of Japan." 
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Spin-flow manipulations Establishing a technological foundation for spintronics
RIKEN Frontier Research System |
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| What is spintronics? | |||
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"When I was a senior at university, I joined the research laboratory on magnetism. Magnetic substances are very mysterious. What are magnets? Why do alloys based on iron, nickel, and cobalt become magnetic and attract each other? I became very interested in this topic," says Laboratory Head Otani, looking back. The attractive power of magnets is associated with a property of electrons, known as electron "spin." Spin is angular momentum similar to the earth's rotation. A magnetic force is generated when charged electrons rotate on their own axis at a certain speed. Electrons have magnetic properties intrinsically. Why then do ordinary substances not become magnets? There are two spin directions, up and down. In ordinary substances, there are the same number of up-spin and down-spin electrons, in which the magnetization is cancelled out. In magnets, however, the densities of up-spin and down-spin electrons are different. This state is called spin-polarization. In magnets, electronic spins are polarized where spins of many electrons point in the same preferential direction, causing a magnetization. Iron is attracted by a magnet because the spins of iron electrons are polarized. When a magnet is moved closer to iron, all the spins of the iron electrons are aligned in the same direction. The iron itself then becomes magnetized, attracting other iron materials. Magnetic substances such as magnets and iron are called ferromagnets. They are widely used as recording media for information, such as prepaid cards, train tickets, cassette tapes, video tapes, MDs, and hard discs for PCs. Information is recorded on these media according to the direction of magnetization. For example, the direction of pole S is assigned as "1" and pole N as "0," or vice versa. "Currently available information devices, however, process information that has been magnetically recorded only by converting it to electrical signals," Otani points out. For instance, whether or not an electric current passes through a semiconductor transistor corresponds to "1" or "0" respectively. How much you can minimize the size of storage elements, such as transistors, will decide the amount of information that an integrated circuit can process as well as its processing speed. The smallest line widths of currently available circuitry elements are less than 100 nm (1 nano meter is 1/1,000,000,000 meters). As micronization advances, the effects of quantum mechanics, which dominates the microscopic world, will become stronger, eventually hindering the functioning of elements. There will be a limit waiting for us in future advancements of the electronics technology. "We are attempting to process information that has been stored in the form of spin, or magnetically, without converting it to other forms. By applying spins to information processing, we are aiming at overcoming the limit of currently available technologies," says Otani. Spin-based electronics is called spintronics. We need to study at least two major research themes to develop spintronics: one for manipulating externally the direction of magnetization of magnets (ferromagnets) and the other for studying the behavior of spins inside magnets. |
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| Reversal of magnetization | |||
First, let me talk about the research into external manipulation of the direction of magnetization. This is being conducted by the Quantum Nano-Scale Magnetics Laboratory. The research is also related to MRAM (magnetoresistive random access memory), which is regarded as a next-generation magnetic medium. Scientists are competing with each other fiercely to develop MRAMs.One of the advantages of magnetic recording is nonvolatility, the ability to store data even after the power is switched off by maintaining the direction of magnetization. This property of magnetic recording is used in hard discs for PCs, which are a kind of magnetic storage medium, on which are stored the operating system, applications, and data. The speed at which currently available hard discs can write and read data is not very fast. Therefore, when we process large amounts of data, we transfer the data from a hard disc into semiconductor memory called RAM (random access memory) which performs processing tasks at a higher speed. The data is then processed by RAM and CPU (central processing unit), between which information is exchanged at high speeds. Since RAM is volatile, the data will be lost when the power is turned off. We therefore have to rewrite the processed information back to the hard disc. If we can combine a magnetic storage medium and RAM, it should be possible to produce ultra-compact, high-performance PCs only requiring extremely small amounts of electrical power. MRAM provides this type of magnetic RAM. MRAM has a lattice structure. At the lattice points are pillar-shaped devices called TMR (tunnel magneto-resistive) elements (Figure 1). A TMR element consists of two layers of ferromagnet with a thin, non-ferromagnetic insulating layer in between. With the direction of magnetization of one ferromagnetic layer being fixed, the direction of the other layer's can be changed. When the magnetization directions of the two layers are the same (parallel), tunnel currents easily flow in large numbers, jumping over the insulating layer due to the low resistance. When the two directions are opposite (anti-parallel), tunnel currents do not flow easily due to the high resistance. With the low and high resistances corresponding to "0" and "1" respectively, MRAM writes and reads data rapidly. How do we then reverse the direction of spin in a ferromagnetic layer? "The current method being developed uses a magnetic field generated by an electric current. When electric currents are applied through the upper and lower wires in the direction towards the TMR element that you want to reverse, a strong magnetic field only acts on the TMR element located on the lattice point where the currents cross, enabling the spin to be reversed. Otani continues, saying that this method has a problem. "In order to increase memory density, the lattice structure needs to be as fine as possible. However, if we use a very small lattice, the magnetic field will also be exerted on neighboring TMR elements because the influence of a magnetic field extends over a certain distance. This will result in malfunctioning." "This problem can be solved by using a method of magnetization reversal caused by spin injection." Spins generally point in different directions. In the spin-injection magnetization reversal method, we send an electric current with spins pointing in one direction (polarized) into the ferromagnetic layers so that the spins are reversed by colliding with the magnetization. Otani and his laboratory members produced for their research a nanostructure called a spin-injection magnetization reversal (Figure 2). In producing the nanostructure, they made use of the giant magnetoresistance effect (GMR). The GMR is a phenomenon in which the electrical resistance of a substance changes according to the direction of magnetization in the ferromagnetic layers. This GMR effect is currently being applied to hard discs for PCs. "Elements we are currently using consist of ferromagnetic and non-magnetic metals. This structure passes electric currents more easily than TMR elements. The challenge for us is to replace these elements with nanoscale TMR elements and to see how much we can lower the tunnel current to reverse the magnetization. We are using 5 mA of current for our elements. Although seemingly very small, this level of electric current produces extremely high current density when used in nanometer-size elements. Repeated manipulation will destroy the elements." 
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| Spin current generation | |||
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Otani and his laboratory members have started an experiment on the phenomenon of "spin current," which may possibly lead to magnetization reversal without directly passing electric currents through elements. As shown in Figure 3, we first of all make a structure of two ferromagnets connected with copper wire. The populations of up-spins and down-spins are different in ferromagnets. Although copper wire is non-magnetic, the ratio between up- and down-spins is also different in the parts from the points connected to the ferromagnets up to a certain distance (spin diffusion length). The polarization will disappear at a sufficient distance away from the spin diffusion length. A polarized electrical current is sent from the ferromagnets on one side into the copper wire of this structure. For example, a large number of up-spin electrons are injected into the copper wire toward the left but no electrical current flows on the right side of the copper wire as shown in Figure 3. The excessive up-spin electrons at the junction will flow to the right of the copper wire as far as the spin diffusion length. In order to cancel the electrical charge which has been increased due to the flow of up-spin electrons, down-spin electrons will start moving in the opposite direction. The difference between the up-spin current and down-spin current is the spin current. "Spin current is created on the right side of the copper wire even without sending an electrical current. It has only been discussed theoretically as one phenomenon, but we can now measure it by way of experiment. We are hoping to conduct an experiment to cause magnetization reversal by using a collision of spin currents with a magnet." 
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| Using magnets to create arithmetic circuits | |||
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Let me talk about the second study on the behavior of spins inside magnets, another spintronics-related issue for us to address. Not all the magnetizations in a magnet point in the same direction. The directions of magnetization are divided into smaller regions. Each small region is called a magnetic domain. A smaller magnet may have only one magnetic domain. Magnets in a disc shape are called magnetic discs. While spins in thick magnetic discs are aligned in the vertical direction (region 1 in Figure 4), spins in thin ones are aligned in the horizontal direction (region 2 in Figure 4). Otani and his laboratory members are considering the possibility of spin-based information processing using logic consisting of nanoscale thin magnetic discs. When thin magnetic discs are aligned as shown in Figure 5, magnetic forces influence each other, deciding the directions of spin. This is because, when magnets are moved closer each other, two S poles or two N poles repel each other, but S and N poles attract each other. In theory, magnetic needles in aligned compasses should point in the directions as shown in Figure 5. For example, assume that a spin pointing to the right is "1" and one pointing to the left is "0." As shown in Figure 5-left, when inputs A and B are both "0," the output is "1." When C is "1," an arithmetic circuit called a NAND gate is created. When the alignment of magnetic discs remains the same and the spin of C is reversed to be "0," it creates an arithmetic circuit called a NOR gate (Figure 5-right). "We should be able to manipulate the input into A and B externally by using spin currents. We are planning to conduct demonstration experiments of logic circuits using spins."
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| Creating artificial molecules using magnets | |||
Magnetic discs with larger diameters and thicknesses generally have very interesting properties as shown in Figure 4 (The magnetic disc in the reion 3 is larger than the regions 1 and 2 in diameter and thickness). "The magnetization swirls and spins in the center stand in a direction perpendicular to the disc."When a magnetic field is applied from outside this magnetic vortex, the position of its center deviates from the center of the disc, when S and N poles appear on the rim of the disc (Figure 6). When a magnetic field with a certain frequency is applied using microwaves and other electromagnetic waves, the disc absorbs its energy, making the vortex center start moving around. When two of these discs are placed side by side, a magnetic force attracts them to each other. The two discs interact with each other, making their magnetic vortices rotate collectively. "This state is similar to that of a molecule consisting of two atoms. The properties of a magnetic vortex depend on the size of a magnet. By placing magnetic discs of different sizes, we should be able to create artificial molecules that absorb microwaves at a specific frequency corresponding to each disc." |
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| Pursuing the possibilities of magnetic metals | |||
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"I used to be involved in research on permanent magnets. The purpose of the research was to produce powerful permanent magnets that would not reverse their magnetization even if a strong external magnetic field acted on them. Scientists usually apply a trial-and-error method to this kind of research. However, I preferred a more theoretical approach. I first made minute magnets so as to investigate the mechanism of magnetization reversal in more detail." Otani points to the two major magnetism-related discoveries at the end of the 1980s, which have led to the current spintronics. "One is the giant magnetoresistive effect discovered in 1988 in France." The second discovery was made by Japanese researchers. "At around the same time, Hiroo Munakata (currently professor at Tokyo Institute of Technology), Hideo Ohno (currently professor at Tohoku University) and other researchers at an IBM laboratory discovered a semiconductor exhibiting ferromagnetism at a low temperature (magnetic semiconductor). These discoveries raised the possibility of using spins in semiconductor technology. They inspired the idea to produce devices using spins. I developed research into magnetization reversal in 1991 when I studied at the Louis Neel Laboratory of Magnetism, starting research into what is now called spintronics." Otani, however, says that his aim is to pursue what we can do with magnetic metals rather than with magnetic semiconductors. "Production of semiconductors requires tremendous time and effort. In addition, there have been very few reports on semiconductors that exhibit ferromagnetism at room temperatures. On the other hand, metals are easy to process and iron, which is a typical magnetic metal, is an abundant resource. If we succeed in producing high-performance devices from magnetic metals that can be produced at a lower cost, they will rapidly become widespread." Technologies created by the Quantum Nano-Scale Magnetics Laboratory will someday allow magnetic metals to play a leading role in the electronics field, ranking with semiconductors.
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