Ferromagnetism and Giant Paramagnetism in Cu@C nanocomposites

M. Estiphanos, E. Sharoyan, H. Gyulasaryan, A. Manukyan, A. Mirzakhanyan, O. Bernal, A. Kocharian
California State University, Los Angeles,
United States

Keywords: giant paramagnetism, ferromagnetism, nanocomposites, copper nanoparticles

Summary:

Different size copper nanoparticles coated by carbon are synthesized by solid phase pyrolysis of polycrystalline phthalocyanine (CuPc, Pc= C32N8H16). The structural properties of obtained powder products are characterized by XRD and Raman spectroscopy. The morphologies of obtained Cu@C nanocomposites studied by TEM, HRSEM, and HRTEM suggest a core shell structure. The unpaired electron states and electron transfer by changes in g-values, hyperfine splitting constants at various copper concentration are also analyzed by EPR and FMR spectra. Measurements of magnetization carried out by vibrational PPMS magnetometer provide strong evidence for coexistence of ferromagnetism and giant paramagnetism in wide range of temperatures. Magnetization study reveals that at low concentration of Cu nanoparticles provide giant enhancement of magnetization. At low temperatures a giant paramagnetism is observed for small average sizes of copper anoparticles in the range of 2-6 nm, apparently due to the (ballistic) conduction electron (large orbital magnetism). The values of the specific susceptibility at T= 10K show a record high giant paramagnetism with magnetic specific susceptibility of ≈1.5×10-4 emu/gOe order while ferromagnetic behavior with hysteresis is preserved up to the room temperature. Magnetic properties of Cu nanoparticles and carbon matrix are affected by quantum confinement of the conduction electrons and 3d-electrons. Magnetic properties of coated NPs are different from their bare form. In the bulk form copper is known to be weakly diamagnetic, because of diamagnetic response from the completely filled ions core and 3d-band while the ultra small Cu nanoparticles overcome the diamagnetic responses from the inner core by exhibiting giant paramagnetic and ferromagnetic responses at low temperatures. The nanoparticle size and concentration dependences of Cu nanoparticles on the magnetic properties of nanoparticles are analyzed. This work was supported by the RA MES State Committee of Science, in the frames of the research project № 15T-1C249. The work at California State University was supported by the National Science Foundation-Partnerships for Research and Education in Materials under Grant DMR-1523588.