Anomalous low-temperature magnetoelastic properties of nanogranular (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$

TL;DR

This study investigates the unusual low-temperature magnetoelastic behavior of nanogranular CoFeB-SiO2 films, revealing a temperature-dependent coercivity anomaly linked to individual granule properties and thermal expansion effects.

Contribution

It provides new insights into the low-temperature magnetic anomalies in nanogranular films and links these effects to thermal expansion and magnetostriction of individual granules.

Findings

Coercive field deviates from Neel-Brown law below 100 K

Sharp increase in coercivity at low temperatures

Effect diminishes as granules merge into conglomerates

Abstract

We report magnetostatic measurements for granulated films (CoFeB)$_{x}$-(SiO$_{2}$)$_{1-x}$ with fabrication induced intraplanar anisotropy. The measurements have been performed in the film plane in the wide temperature interval 4.5$\div$300 K. They demonstrate that above films have low-temperature anomaly below the percolation threshold for conductivity. The essence of the above peculiarity is that below 100 K the temperature dependence of coercive field for magnetization along easy direction deviates strongly from Neel-Brown law. At temperature lowering, the sharp increase of coercivity is observed, accompanied by the appearance of coercive field for magnetization along hard direction in the film plane. We establish that observed effect is related to the properties of individual ferromagnetic granules. The effect weakens as granules merge into conglomerates at $x$ higher then percolation threshold and disappears completely at $x>1$. We explain the above effect as a consequence of the difference in thermal expansion coefficients of granule and cover material. At temperature lowering this difference weakens the envelopment of an individual granule by the cover matrix material, thus permitting to realize the spontaneous magnetostriction of a granule. The latter induces an additional anisotropy with new easy axis of a granule magnetization along the external magnetic field direction. Our explanation is tested and corroborated by the ferromagnetic resonance measurements in the films at $T$ = 300 K and $T$ = 77 K.