Fabrication and magnetic properties of FePt nanoparticle assemblies embedded in MgO-matrix systems

摘要

Hydrophilic FePt nanoparticles (NPs) have been embedded into the MgO-matrix systems via a sol-gel process to prevent FePt NPs from aggregating and sintering during the heat-treatment process required for the L1_0 ordering. The chemically ordered Ll_0-phase FePt can be obtained after annealing at 700 °C for 60 min in atmosphere containing H2. The effect of the pH value of MgO collosol and FePt nanocrystal loading amount on the structure, morphology, and magnetic properties of FePt/ MgO nanocomposites has been investigated. The neutral pH value of 7 in MgO sol is beneficial to stabilize FePt NPs and obtain higher chemical ordering parameter S for the face-centered tetragonal -FePt/MgO nanocomposites with larger coercivity. The FePt NPs loading amount also plays a key role in tuning the microstructure and magnetic properties of the nanocomposites. The relatively higher FePt NPs loading with FePt/MgO molar ratio (R_(fm)) of 1:2 leads to relatively perfect hexagonal assembly and pure L1_0 phase. When the R_(fm) is 1:5 and 1:10, the MgO-matrix in nanocomposites causes the Fe element loss in FePt NPs along with formation of secondary phases such as magne-sioferrite or Pt3Fe during the annealing process. Under optimal processing of neutral pH value of 7 and R_(fm) of 1:2, the presence of MgO matrix produces more homogeneous microstructures and better magnetic properties with higher room-temperature coercivity (H_C = 4.65 kOe).