Speaker
Description
Intensive studies on the magnetic and magneto-transport properties of ferromagnetic metal-insulator granular thin films and discontinuous metal-insulator multilayers have been mainly driven by improved parameters compared to GMR structures. At the same time, the issue of the electrophysical properties of composite materials, which differ significantly from those of bulk metal samples and bulk homogeneous films, also remains relevant.
[Ni$_{80}$Fe$_{20}$($d_{Fe}$)/HfO$_2$(3)]$_{10}$/Sub discontinuous multilayer systems (DMS) were prepared by sequential magnetron sputtering on the sapphire substrate. The base pressure in the vacuum chamber was $1\cdot10^{-7}$ Torr. During sputtering, the chamber was filled with Ar ($99.999 \%$, Messer, flow rate $18$ $\mathrm{cm^3\cdot min^{-1}}$) at a constant pressure of $3$ mTorr. For the formation of HfO$_2$ layers, radio frequency (RF) magnetron sputtering with a power of 100 W and a deposition rate of $1$ $\mathrm{nm\cdot min^{-1}}$ have been used. For the Ni$_{80}$Fe$_{20}$ layer formation, DC magnetron sputtering with a power of $100$ W and a deposition rate of $5$ $\mathrm{nm\cdot min^{-1}}$ have been used. The effective thickness of the insulator layer HfO$_2$ was $3$ nm and remained unchanged. The effective thickness of the ferromagnetic layer Ni$_{80}$Fe$_{20}$ changed within the range from $1$ to $3$ nm. The electrical resistance experiments were carried out using the four-point method. The distances between the electrical contacts were $1.27$ and $3$ mm. The deposited films were annealed at the temperature range from $300$ to $573$ K. After that, the low-temperature resistivity measurements from $300$ K down to $10$ K were performed.
It was demonstrated that the structure of the samples consists of magnetic granules separated by insulator channels. The exponential nature of electrical resistance under heat treatment up to $573$ K is observed for all investigated discontinuous multilayer systems. In general, the behavior of electrical resistance as a function of the temperature is described by the equation:
$$R=R_0 \exp(T_0⁄T)^{\alpha}$$
where $T_0$ depends on the concentration of the ferromagnetic metal; $\alpha$ takes values of $1/2$ in the case of thermally activated hopping [1] or $1/4$ in the case of variable range hopping [2]. Linear fitting to $\ln R$ vs. $T^{−1/4}$ and $\ln R$ vs. $T^{−1/2}$ data points for DMS demonstrated that the variable range hopping better represents experimental results. The influence of ferromagnetic layer thickness on the $T_0$ value was shown.
Acknowledgements
This work was funded by the NATO Program "Science for Peace and Security" (project Nr. G6131).
References
[1] P. Sheng et al., “Hopping Conductivity in Granular Metals,” Physical Review Letters, vol. 31, no. 1. American Physical Society (APS), pp. 44–47, Jul. 02, 1973. https://doi.org/10.1103/physrevlett.31.44
[2] R. Rosenbaum et al., “A useful Mott - Efros - Shklovskii resistivity crossover formulation for three-dimensional films,” Journal of Physics: Condensed Matter, vol. 9, no. 29. IOP Publishing, pp. 6247–6256, Jul. 21, 1997. https://doi.org/10.1088/0953-8984/9/29/010
