Influence of microstructure on the tunnel magnetoresistance of Co–Cr₂O₃granular films

Abstract

A series of Co x (Cr 2 O 3 ) 1−x granular films with various Co volume fractions x (x = 0.15-0.55) have been fabricated by RF co-sputtering. X-ray diffraction, transmission electron microscopy, energy dispersive x-ray and x-ray photoelectron spectrometer have been employed to characterize the structure, composition and element valence. VSM and the conventional four-point resistance measurement method have been used to investigate the magnetic and electrical properties. It is found that the films consist of Co particles with an average size of about 5 nm distributed in an amorphous Cr 2 O 3 matrix for x = 0.35. The isotropic tunnelling magnetoresistance (TMR) is obtained. TMR increases with decrease in x from 0.55, reaches a maximum of 5.1% at x = 0.35, then decreases with further decrease in Co volume fraction. This behaviour can be explained by the film microstructure variation and conductive mechanism. What is interesting is that TMR becomes negative, −3.5%, for the x = 0.15 sample. This behaviour was also observed in Fe-Cr-O granular films with the lowest Fe volume fraction (x = 0.15). This change in the sign of TMR implies that the barrier thickness of the metal-insulator-metal junction plays an important role in the determination of the spin polarization of tunnelling electrons.

Magnetoresistance of granular Pt–C nanostructures close to the metal–insulator transition

“…For the samples investigated in this work the correlation function < P i P j > is always negative and it decreases by increasing intergrain coupling strength. Note that a similar trend has been observed in magnetic granular metals with Co or F e nanograins embedded in a crystalline insulating Cr 2 O 3 matrix [42]. In particular, a change from positive to negative values of < P i P j > was measured by increasing the volume fraction of the magnetic grains.…”

“…Whether the inversion takes place or not, depends on the details of the spin-polarized DOS of the surface states, on the electronic structure and on the thickness of the tunneling barrier [41]. For example, in magnetic granular metals with Co or Fe nanograins embedded in a crystalline insulating Cr 2 O 3 matrix [42] a change was observed from positive to negative values of P i P j on increasing the volume fraction of the magnetic grains. For the samples investigated in this work the correlation function P i P j is always negative, indicating a preferred anti-parallel alignment between neighboring Pt particle moments.…”

“…The correlation function P i P j depends on the electronic density of states (DOS), the applied bias voltage and the hopping distance. It can be either positive or negative [24,27]. By defining the MR in % as 100 × (R(H, T )/R(0, T ) − 1), from equation (2) one finds:…”