by Alexander MELNIKOV, Dr. Sc. (Phys. & Math.), Head of the Laboratory of Mesoscopic Systems Theory, Institute of Microstructure Physics, RAS (Nizhni Novgorod)
The interaction between two antagonistic phenomena, i.e. superconductivity and ferromagnetism, has long been drawing attention of researchers. The importance and nontrivial character of this problem were discussed as far back as in the 1950s in a pioneer work of Acad. Vitaly Ginzburg, Nobel Prize Winner of 2003. Moreover, competition of superconducting and magnetic states of substance is not only an intriguing and fine physical problem, it can also practically be used in a new class of cryoelectronic devices, namely, solid state devices (superconducting spin valves, phase inverters, etc.), which operate at cryogenic temperatures (below 100 K). International cooperation played a decisive role in shaping one of the key subjects of the superconductivity department of the Institute of Microstructure Physics, which were focused on these problems.
The most important works, which stimulated inception of a topical research line at our institute, i.e. physics of interaction between superconductivity and ferromagnetism, were implemented in the 1990s, long before the establishment of international contacts. I mean here theoretical studies of Gennady Genkin, losif Tokman and Vladimir Skuzovatkin related to changes of a domain structure* and initiation of
* Domain structure is a set of microscopic regions (domains) of a magnetically ordered substance. - Ed.
vortexes in the superconductor-ferromagnetic layered systems, and in the experimental works of Yuri Noz-drin in induction of a vortex state in a superconducting film by a magnetic particle field. However, in many ways due to weak international contacts they remained unnoticed in the superconducting community, failed to obtain marked scientometric characteristics and were repeated independently by other researchers later.
The next outburst of activity of the superconductivity-ferromagnetism problem at the Institute of Microstruc-ture Physics was caused just by development of international contacts. In 2003 in cooperation with professor of the University of Bordeaux (France) Alexander Buzdin, Dr. Sc. (Phys. & Math.), we wrote a theoretical paper on origination of superconductivity near domain walls in ferromagnetic superconductors and the super-
conductor-ferromagnetic hybrid layered structures. On the one hand, it was directed to interpretation of the experiments (carried out in the French Grenoble) with a quite new and unusual class of compounds of the UGe2 type in which superconductivity manifested itself against a well-developed magnetic ordering. With account of destructive effect of an exchange field of ferromagnetic on the Cooper pairs* with opposite spins** of electrons (singlet pairing) experimental detection of such unusual compounds caused serious interest of the superconducting community. But surprisingly our theoretical paper served as an incentive for absolutely different experiments in measuring of phase diagrams of the superconductor-ferromagnetic bilayers with a domain structure. Here we are dealing with a rather simple and at the same time intriguing possibility of creating submicron superconducting channels in the region of ferromagnetic domain walls: in this case the control of the boundary configuration provides control of a superconducting channel position thus changing the system conductivity. The experimental realization of this problem started in the same years in the group of Viktor Moshchalkov, Dr. Sc. (Phys. & Math.) at
* Cooper pairs are named after American physicist Leon Cooper. In 1956 he proved theoretically that near the absolute zero conductivity electrons were capable of creating pairs which combined particles with equal and opposite pulses.-Ed.
**Spin is a fundamental quantum property of electrons.-Ed.
the Catholic University of Leuven (Belgium), and in a rather short time its staff members obtained very encouraging results.
The theoretical activity which thus originated at our laboratory did not die out mainly due to the fact that we felt the experimenters' interest in the research subject. The important point was that we managed to recruit active young postgraduates Denis Ryzhov and Alexei Aladyshkin. In the course of the year we generalized the results on domain superconductivity and also revealed very curious manifestations in hybrid systems of the Little-Parks effect. The effect was discovered in 1962 by British scientists Malcolm Little and Gordon Parks; it is one of the important confirmations of a quantum nature of the superconductivity phenomenon and is connected with fluctuations of a critical temperature value of transition to a superconducting state. It is also interesting that a part of this work was implemented by the authors in the course of their business trip to the US Argonne National Laboratory, which can be regarded as a confirmation of the thesis on importance of international contacts.
The following steps in the development of the SF subject-matter at our institute were taken in the direction of experiments. The group headed by Andrei Fraerman, Dr. Sc. (Phys. & Math.), carried out successful works connected with the control of the Jo-
sephson transport (current transportation)* by means of a state of change in magnetic particles placed in the region of the Josephson contact (solid-state superconducting nanoelement). Later on they transformed into a task of creating a controlled pi-contact** in transition with a magnetic particle, which was solved in cooperation with a team headed by Valery Ryazanov, Dr. Sc. (Phys. & Math.), from the Institute of Solid State Physics (Chernogolovka, Moscow Region). In my opinion, the quite substantial theoretical assistance was provided by Alexei Samokhvalov, Cand. Sc. (Phys. & Math.), a senior researcher of our laboratory.
* In 1962 British physicist Brian Josephson proved in theory that the tunnel superconducting current passing through dielectric (insulator) possessed unique quantum properties. In 1973 he was awarded Nobel Prize for discovery of this phenomenon which became known as the "Josephson effect". Nowadays the effect is widely used-in particular, ultrasensitive superconducting magnetometers and other devices work on its basis.-Ed.
** Pi-contact is a special type of the Josephson transition when wave-function phases of the Cooper pairs shift between superconducting electrodes to a value of the pi character (hence its name).-Ed.
Another important experimental step was taken by Alexei Aladyshkin working in the team headed by Viktor Mosh-chalkov at the Catholic University of Leuven. Besides, both in Belgium and after his return to Afonino (a village in Nizhni Novgorod Region-the address of the Institute of Microstructure Physics, RAS), Alexei displayed an exceptional purposefulness concentrating on an experimental search of effects of domain superconductivity. His efforts were crowned with good results largely due to cooperation first with Belgian and then German scientific teams. Today he is engaged in experimental studies of domain superconductivity with the aid of a new low-temperature scanning tunneling microscopy device now at the disposal of our institute. One cannot but note also a contribution to the development of the said problem by the theoretician Denis Vodolazov, who came to the Institute of Microstructure Physics after his postdoctoral studies at the University of Antwerp (Belgium).
The discussed research works are based mainly on one of the possible interaction mechanisms of super-
conducting and magnetic types of ordering, namely, the electrodynamic (orbital) mechanism. However, despite diversity of possible distribution of magnetic fields in domain structures, the problem of such type becomes compiled sooner or later, and there arises an urgent need to attract new physics. And such class of problems connected with interaction of the magnetic moment and superconductor electrons sprang up in our team due to cooperation with Alexander Buzdin and trips from Afonino to Bordeaux supported by the ENS-Landau scientific exchange program. It was initiated in 1990 by the Ecole Normale Superieure (ENS, Paris) and the Landau Institute of Theoretical Physics (Moscow). In case of hybrid SF structures, the influence of exchange interaction on superconducting electrons proves to be essential under conditions of the so-called proximity effect, i.e. under not too high superconductor-ferromagnetic interfacial resistance. Besides, by changing distribution of the magnetic moment by an external magnetic field we obtain a possibility of direct control also of electron spins in the superconductor. The quantum-mechanical pattern of their motion causes a rather nontrivial range of phenomena which can be combined by the term "superconducting spintronics" (SPIN TRansport electrONICS). Among the most well-known spintronic devices one can name the Josephson contacts with reshapable relationships between superconducting current and phase difference of electron wave functions and also spin valves in which a critical temperature of transition is checked by mutual orientation of magnetic moments in magnetic subsystems (layers). The engineering of current-phase relationship can be used in the design of cubits (base elements for information storage in quantum computer) and for creation of low-noise SQUID magnetometers (Superconducting Quantum Interference Device), i.e. high-frequency devices for measuring of the magnetic flux.
At present works in superconducting spintronics have become really a burning issue. Discussions of the current state of this and other problems at working conferences in Bordeaux united a number of key experimenters and theoreticians and thus provided them with an opportunity to exchange updated scientific information. In recent years our cooperation with the local university was based on a financial support of the European program. We had an opportunity to send young staff members to France, which undoubtedly extended a circle of problems. They are handled by a new generation of our theoreticians including Mikhail Silayev, Ivan Khaimovich, Sergei Mironov and Anton Bespalov.
The contacts with the European research teams and also the obtained joint results and publications had a positive effect on contacts of our laboratory members with other research teams in our country, for example, those headed by Valery Ryazanov (Institute of Solid State Physics, Chernogolovka, Moscow Region), Mikhail Kupriyanov (Skobeltsyn Research Institute of Nuclear Physics, Moscow), Ilgiz Garifullin (Zavoysky Physico-Technical Institute, Kazan) and Lenar Tagirov (Kazan Federal University).
The successful work on the SF subject-matter and other research trends during a considerable period of time, publications in high-rating journals and conference activities cleared the way for three young and active members of our laboratory to continue their professional career abroad, namely, postdoctoral studies. This fact is undoubtedly an evidence of recognition of our team's proficiency and in this context is a step forward on the way to integration into the world community. But an inverse flow of scientific personnel from abroad could have also a positive effect on national science. However, the recent destructive developments in the Russian Academy of Sciences make these hopes very illusory. Therefore today, as in the 1990s, the main cause for optimism can be provided by the senior personnel of our laboratory, for whom it is far more difficult to find employment abroad for age limits. The established international contacts will help us retain at least a part of our staff and realize a number of challenging plans of the SF subject-matter optimistically orienting ourselves to the ancient motto of chivalry: "Do as you should, and come what may."
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