The temperature and field dependencies of magnetization and electrical resistance of bulk samples of multi-walled carbon nanotubes modified with cobalt have been studied. Samples of modified multi-walled carbon nanotubes with 30% mass fraction of cobalt have been obtained by the method of metal reduction from aqueous salt solution. The hysteresis loops and temperature dependence of mass magnetization under field cooling and zero field cooling protocols have been measured for bulk samples of pressed modified multi-walled carbon nanotubes in a temperature interval from 3 to 293 K. The magnetic field dependence of resistance across a range of temperatures 4.2–300 K in two distinct sample orientations (in-plane and out-of-plane) has been studied in a pulsed magnetic field up to 20 T. The study of the temperature dependence of magnetization in field cooling and zero field cooling modes revealed that the blocking temperature for the studied samples of carbon nanotubes modified with cobalt is ≈ 7 K. It is shown that above the blocking temperature, nanosized single-domain cobalt particles are in a superparamagnetic state. Below the blocking temperature, magnetoresistance is determined by a combination of two mechanisms. Relatively large cobalt particles form ferromagnetic regions, causing a giant magnetoresistive effect to appear. Meanwhile, smaller cobalt particles remain superparamagnetic and form a long tail that does not saturate up to the maximum magnetic field.
Ovsiienko et al. (Sun,) studied this question.