Rare earth metals and new physics
It so happened that a generation of physicists who developed the solid-state physics in the 1960s, and believed in the priority the three "classical" NOT principles (1 – superconductivity and ferromagnetism are not compatible; 2 – ferromagnetism is a characteristic of only metals and their alloys; 3 – magnetism can be observed only in 3D structures), were courageous enough to review their current assumptions, taking into consideration the effects discovered in those years, mostly having a quantum nature. Primarily, it refers to Josephson tunneling effect (1961) – moving of Cooper pair between two superconductors separated by a dielectric barrier, which is a discovery that marked the start of the rapid development of weak superconductivity research and practical development of quantum cryoelectronic devices, or squids.
Furthermore, the ferromagnetism of classical REM-based semiconductor, europium monoxide (EuO), was discovered in 1961, when the era of magnetic semiconductors and practical realization of Heisenberg’s magnetism started. Slightly later (in 1967), Nobel Laureate L.Esaki et al. carried out the classical single-particle electron tunneling between two normal metals separated by a layer of magnetic dielectric of EuS and EuTe. In fact, it was the first observation of spin-polarized tunnel current flowing in this type of contact because the mentioned barrier layers have spontaneous magnetization at helium temperatures and serve as a spin filter for the current carriers of different spinning directions. This was evidenced by a marked difference in conductance of forward and reverse voltages of contact displacement. However, further development of this area in solid-state physics was not as successful as the first one because this period witnessed the rise of research in the field of semiconductor electronics based on Si-technology. Although silicon-based transistor devices worked under normal conditions at a room temperature, either all magnetic semiconductors created by the time were made from cryogenic materials, or their Curie temperature, at best, matched the temperatures of liquid nitrogen. Therefore, they were mainly of theoretical interest as models for research.
Due to the lack of apparent technical uses, this area of the solid-state science experienced falling interest from practitioners. This happened first in the early 1970s, when a group of scientists of the Institute for Physical Problems of the USSR Academy of Sciences failed to carry out Josephson tunneling through a ferromagnetic barrier (despite the fact that it was made of Fe with a thickness of only 0.05 nm). This result only confirmed the assumptions existing then in the physical science relating to antagonistic effects and the impossibility of concurrence between superconductivity and ferromagnetism, inter alia, in contact systems.
The studies of magnetic semiconductors continued mainly thanks to the theoretical research by T.Kasuya and E.Nagaev, who predicted the possibility of formation of the so-called magnetic impurity states – "ferromagnetic quasimolecules" – in doped magnetic semiconductors. It was experimentally established in the early 1980s by teams of American and Soviet (Russian) researchers that the emergence of such quasimolecules in solid solutions of Eu1-xRxO (R-La, Gd, Ho, Sm) resulted in an increase of the Curie temperature to 120-130°K, which inspired hopes. However, by 1985, the studies of magnetic semiconductors in the West practically ceased due to the lack of their technical application. The same situation happened in the Soviet Union, which is confirmed by the special report of a scientific mission to one of the French research centers by a famous Soviet physicist – Professor of the Leningrad Physical-Technical Institute of the USSR Academy of Sciences. The governmental funding of research in this area was suspended, but applied research related to a possible use of thin-film magnetic semiconductors in cryoelectronic devices continued in the United States by a group of physicists from MIT (Cambridge) and in the Soviet Union by a group of scientists from the Institute of Solid State Chemistry of the USSR Academy of Sciences (Yekaterinburg). In 1982-83, they carried out Josephson tunneling of Cooper pairs through a barrier layer made of EuS and EuO, respectively. In fact, these works first implemented the triplet mechanism of Cooper electrons pairing, or, in today’s terminology, a spin current transportation in superconducting tunnel structures. This type of research was widely developed only in the last decade, as well as the theoretical proof of a triplet mechanism of Cooper pairing of electrons in superconducting alloys and structures containing ferromagnetic ordered fields. In those same years, a group of scientists from Saint-Petersburg Ferrite Domen Scientific Research Institute and Yekaterinburg Institute of Metal Physics of the USSR Academy of Sciences started the contact studies of ferromagnetic semiconductors with non-magnetic semiconductors, which in many respects predetermined the emergence of the modern area in semiconductor magnetoelectronics – spin electronics (spintronics), which is still rapidly developing.
Finally, we are observing the failure of the third "NOT" postulate in the possibility of magnetic ordering, including ferromagnetism in thin (nano-thick) films of magnetic materials or graphenes, which are in fact 2D systems. Devices using many of them, in particular, multilayer metallic structures are already widely used in the technological art, and magnetically ordered semiconductors are supposed to contribute to the development of both existing and spin nanoelectronics.
Thus, the discovery of ferromagnetism in europium monoxide in 1961, which also has semiconductor conductivity, changed the ideas relating to the possibility of ferromagnetic manifestation in metals. In fact, the whole theory of ferromagnetism collapsed being based on band assumptions and indirect exchange interaction via charge carriers.
Almost concurrently with EuO, the Curie temperature of which is TC = 69.4°K, the scientists synthesized and researched the magnetic properties of the related monochalcogenides of divalent europium – EuS, EuSe, EuTe. A ferromagnet in them was only monosulfide (TC = 16.5 K); EuSe was a metamagnet, and EuTe was a typical antiferromagnet. All other electrical parameters were close to dielectrics. The strangeness of this monochalcogenide series is that the europium ion is in the oxidation degree of R2+ being the lowest in rare earth metals, which is abnormal and less chemically stable under normal conditions than typical oxidation degree of R3+. This, in turn, aroused a subsequent interest for a synthesis and study of compounds from rare-earth and transitional elements at abnormal oxidation states (valences). Currently, this area is known as "physics of magnetic (in particular, ferromagnetic) semiconductors", and it is one of the most important and widely developed in the science of spin current transport in solid-state structures, or semiconductor spin electronics. This area of research forms the basis for the emerging quantum electronic devices – quantum computers. Structures that contain europium monoxide with its outstanding physical properties, in particular, the saturation magnetization, which is the highest among ferromagnets (magnetic moment) M = 2.4 T at T = 4.2°K, and nearly 100 % spin polarization of charge carriers, can play here an important role.
The well-studied image of exchange interactions makes monochalcogenides europium related to typical Heisenberg magnets and research objects of S.Vonsovsky’s s-d/d-f exchange model. They are still the most debated models in the theory of magnetism and serve as a "touchstone" to test new ideas in this area of condensed-state physics and to improve the methods for relativistic quantum calculations of their electronic band structures. It is important that the group of Russian researchers were able to synthesize a composite alloy on the basis of this monoxide, containing the solid solution of Eu-Fe-O in its composition with TC = 480°K, which has ferromagnetism and semiconducting properties at room temperatures. As for the degree of spin polarization of current carriers (around 60%), this composite is a champion among today’s well-known spintronic materials. Its use in semiconductor spin electronic devices will undoubtedly contribute to the successful development of the 21st century’s electronics as a branch of solid-state physics.