Properties of Nanocomposites With Different Concentrations of Magnetic Ions in an Insulating Matrix

Using (Co
₄₀
Fe
₄₀
B
₂₀
)
_x
(LiNbO
₃
)
_100-
x
(
x
= 6–55 at.%) nanocomposite films as an example, we have performed comparative investigations of granular systems properties with a high (∼10
²²
 cm
^–3
) and low (≤10
²¹
 cm
^–3
) content of dispersed atoms (Fe and Co) in an insulating nonstoichiometric matrix. The nanocomposite films were produced using ion-beam sputtering of the composite targets onto glass-ceramic substrates at different growth temperatures (∼40 and ∼80 °C). We show that magnetic ions dispersed in a matrix play a crucial role both in the ferromagnetic exchange between granules and in magnetic, transport, and memristive properties below the percolation threshold
x_p
≈ 50–55 at.%. The presence of a high content of dispersed magnetic ions induces intergranular interaction, shifts the critical concentration of the metal–insulator transition
x_c
from 48 to 43 at.%, leads to high-field positive magnetoresistance near
x_p
and superferromagnetic ordering and magnetic hysteresis below
x_c
(down to 33 at.%), and makes the stable resistive switching for capacitorlike structures with resistance ratio
R
_OFF
/
R
_ON
more than 10
²
at
x
≈ 10 at.%.