Multiwall carbon nanotubes (MWCNTs) fabricated by chemical vapor deposition contain magnetic nanoparticles. While increasing frequency of electromagnetic field (EMF) exposure (up to <10 kHz) of MWCNTs resulted in slight induced magnetization decrease due to skin effect of the conducting carbon, we discovered that higher frequencies (>10 kHz) contained an exponential magnetization increase. We show that puzzling magnetization increase with decreasing magnetic field amplitude (less than 0.5 A/m for 512 kHz) is due to matching the field amplitudes of the magnetic nanoparticles inside nanotubes. This observation reveals a possibility of magnetic tunneling in MWCNTs (change of magnetic state of blocked magnetic moments). This interpretation is supported by observation of unblocking larger magnetic remanence (MR) portion from MWCNTs with progressively smaller amplitude of oscillating magnetic field.

Department of Applied Geophysics Charles Univ Albertov 6 Prague Czech Republic

Department of Electronics and Materials Science Shizuoka University 3 5 1 Johoku Naka ku Hamamatsu 432 8561 Japan

Geophysical Institute University of Alaska Fairbanks 903 N Koyukuk Drive Fairbanks AK USA

High School Karlovy Vary Czech Republic

Institute of Geology Czech Academy of Sciences Rozvojova 269 Prague Czech Republic

ZH Instruments Inc Brno Czech Republic

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