Magnetic and electrical properties of ferromagnetic nanocomposites based on Co nanoparticles in Al₂O₃ matrix

Abstract

Magnetic, structural, and electrical properties are reported for ferromagnetic nanocomposites (FMNC) consisting of nanometer size Co ferromagnetic nanoparticles (NP) embedded in Al 2 O 3 dielectric matrix. Co x (Al 2 O 3 ) 100Àx FMNC with x ¼ 7.8, 28.2, and 42.2 at.% were obtained in the form of (1-5) mm thick films by modified electron beam physical vapor deposition of Al 2 O 3 and Co vapors condensing on policor substrate at temperatures 100-400 8C. Magnetic properties of these FMNC studied by SQUID magnetometry and ferromagnetic resonance (FMR) methods revealed paramagnetic properties for layers with x ¼ 7.8 at.%, superparamagnetic behavior for x ¼ 28.2 at.%, and ferromagnetic state for the highest Co concentration studied x ¼ 42.2 at.%. Temperature dependence of electrical conductivity in FMNC with low Co content was found to follow the Mott law for variable range hopping mechanism. Magnetic field dependence of resistivity revealed a negative magnetoresistance effect. These results confirm the effectiveness of the applied technology for the preparation of Co x (Al 2 O 3 ) 100Àx FMNC.

Electronic Transport and Magnetic Properties of Co/SiO2Magnetic Nanocomposites

“…This is manifested by the shape of M ZFC ( T ) and M FC ( T ) curves with the average blocking temperature T B of particles corresponding to the temperature of the maximum in M ZFC ( T ) curve as well as reversible room temperature M ( H ) curve with a Langevine‐like shape. These results are in good agreement with those ones obtained for a nanocomposite system 28.2 at.% Co in Al 2 O 3 matrix (Co/Al 2 O 3 nanocomposite) …”

Giant Thermoelectric Power in Co/Al2O3 Ferromagnetic Nanocomposites Below Percolation Threshold

“…The transition from SP to ferromagnetic state is due to the magneto-dipole interaction between FMNP's what is expressed in the appearance the spontaneous magnetization and hysteresis loop at low temperatures. [9] The "magnetic exchange bias" (MEB) was observed by us for Co/Al 2 O 3 . [10] It is associated with the magnetic exchange interaction between the ferromagnetic metallic Co core and antiferromagnetic (AFM) CoO shell formed on the surface of Co NP's.…”

“…This temperature is about: 200 K for NP's dimensions d ∼ 10–20 nm, 50 K for d ∼ 3–5 nm, and 12 K for d < 3 nm. The transition from SP to ferromagnetic state is due to the magneto‐dipole interaction between FMNP's what is expressed in the appearance the spontaneous magnetization and hysteresis loop at low temperatures …”

The phenomenon of magnetic exchange bias in ferromagnetic nanocomposites grown by electron beam evaporation

“…It is interesting that at T = 300 K, the hysteresis loop does not disappear. This behavior is associated with the transition from the state of a spin glass at temperatures below the blocking one (T B = 200 K) [2] to the region of ferromagnetic ordering, obliged to the effect of the magnetostatic interaction between Co/CoO NPs. At T = 300 K instead of the usual superparamagnetic state there is an intermediate region of magnetic ordering for the ensemble of NPs [4].…”

“…Thus, the exchange energy at the interface grows, and giant magnetic anisotropy θ − = cos a a k E appears (θ is the angle between orientation of the Co magnetic moments to the interface (in the nearest AFM layer) and magnetic field H). The value of anisotropy constant k a reaches ~10 5 J/m 3 [2]. This leads to a shift of the center of hysteresis loop toward the negative magnetic field (relatively to the direction of the first applied one).…”