Proton Irradiation Induces Magnetic Ordering in Graphite
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absence of intermixing between the lanthanum and cerium phosphates in nanoparticles. They noted that the core–shell structure of nanoparticles is also confirmed by high quantum yields of 80%, while phosphors prepared by conventional solid-state methods have much lower quantum yields at grain sizes smaller than 0.5 µm. Haase said, “Quantum yields so close to those obtained for macrocrystalline materials were, until recently, considered unattainable with nanocrystalline emitters.” The researchers believe that they will be able to further increase the quantum yield by adjusting the shell thickness and by using metal salts of very high purity. EKATERINA A. LITVINOVA
Proton Irradiation Induces Magnetic Ordering in Graphite Macroscopic magnetic ordering phenomena found in highly oriented pyrolytic graphite (HOPG) are of great interest to the scientific community. A group of researchers from the University of Leipzig has focused on the fact that the saturation magnetization of carbon prepared from
hydrogen-rich materials increases with an increase of hydrogen concentration. They note that the origin of magnetization might be related to a mixture of carbon atoms with sp2 and sp3 bonds. As reported in the November 28, 2003, issue of Physical Review Letters, P. Esquinazi and his colleagues used highly oriented pyrolytic graphite to show that implanted protons in HOPG triggers ferromagnetic (or ferrimagnetic) ordering with a Curie temperature above room temperature. The researchers irradiated HOPG samples, containing a total content of magnetic metal impurities below 1 ppm, with a 2.25 MeV proton microbeam. They demonstrated that, depending on the irradiation dose, the saturation magnetization of HOPG increases up to 5 × 10-6 emu, almost a factor of 100 larger than that of the non-irradiated sample. The coercive field was found to be nearly the same for all irradiation doses (~100 Oe) and was only weakly temperaturedependent. The researchers measured the maximum contribution of Fe impurities to the magnetic moment of the samples to be less than 6.1 × 10-8 emu and con-
cluded that ferromagnetic ordering of HOPG is not correlated with magnetic metals. According to the temperaturedependent studies, the researchers indicated a Curie temperature in HOPG above 400 K. The researchers also performed magnetic force microscopy studies of the irradiated and non-irradiated areas. They showed the presence of homogeneously distributed magnetic domains with a periodicity of 2–4 µm, depending on the irradiation dose. However, because of the small coercive field of the magnetic surface and the influence of the magnetic tip, the magnetic domain distribution depended on the distance between tip and surface. By measuring the phase shift as a function of tip–surface distance, the research team estimated the maximum magnetic moment observed by the tip to be ~ 3 × 10-15 A m2, and the total saturation magnetic moment to be 6 × 10-6 emu. The researchers concluded that their findings “open a new research field in magnetism with possible application
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