Magnetoresistance in FeCoZr–Al₂O₃ nanocomposite films containing ‘metal core–oxide shell’ nanogranules

Abstract

Temperature and magnetic field dependences of electrical conductivity are systematically studied in granular films (Fe45Co45Zr10) x (Al2O3)100−x (28 ⩽ x ⩽ 64) containing crystalline metallic α-FeCo-based nanoalloy cores encapsulated in an amorphous oxide shell embedded in an amorphous Al2O3 matrix. Formation of ‘metallic core–oxide shell’ nanogranules is confirmed by transmission electron microscopy (TEM) and HRTEM. The structure of core and shell is governed with the difference in the oxidation states of Fe and Co ions investigated with EXAFS, XANES and Mössbauer spectroscopy. A considerable negative magnetoresistance (MR) effect of spin-dependent nature is observed in the whole range of x values. Its increase with decreasing temperature is correlated with the magnetic saturation of superparamagnetic metallic nanogranules. The enhanced MR effect in ‘core–shell’ granular films is related to the percolation of oxide shells and their influence through spin filtering processes. A considerable high field MR at low temperatures and the resulting deviation of MR and squared magnetization are attributed to a magnetic randomness and/or strong magnetic anisotropy of the magnetic oxide shell.

Tailored Magnetic and Electric States in 3d-Metal-Insulator Films: Characterization and Applications

“…Above the percolation threshold, at x > 50, stripe-like magnetic contrast (see Fig. 2d) conrms the formation of magnetically coupled net of the metallic granules [4,12,15]. 3d) and quadrupole doublet on the Mössbauer spectrum at RT (see Fig.…”

“…It is well established that magnetic properties of granular nanocomposites are fully dened with the size of metallic nanogranules (or their agglomerates) and, consequently, depend on metallic contribution x [4,12,13]. This is well illustrated in Fig.…”

“…Above magnetic percolation, at x > 50, formation of magnetically ordered net of the metallic granules is proved with resolved magnetic sextet on the Mössbauer spectrum (see Fig. 3i) [12,15,16].…”

“…5) [12]. Formation of such a structure composed of crystalline metallic bcc α-FeCo-based nanosized cores encapsulated in an amorphous oxide shell, all embedded in the amorphous Al 2 O 3 matrix, is revealed with TEM and HRTEM (see Fig.…”

“…nanoparticles that mean almost full oxidation of iron, while cobalt is found to be oxidized only partially [12]. …”