Uniform FePt nanoparticles were synthesized through nanostructuring of pulsed laser deposited FePt thin films by single shot H+ ion irradiation using a plasma focus device. The annealing temperature required for phase transition from low Ku face-centred cubic to high Ku face-centred tetragonal, for ion irradiated samples, is simultaneously lowered down to 400 °C. The energetic H+ ion irradiation significantly reduces the activation energy for atomic ordering by increasing the number of vacancies. The advantage of using a plasma focus device is that it can achieve nanostructuring in much shorter time, in single shot ion exposure with pulse duration of typically about a few hundreds of nanoseconds, as compared with much longer duration required by continuous ion sources.
A repetitive NX2 dense plasma focus (DPF) device, operating at a low voltage of 8 kV with a stored energy of capacitor bank in the sub-kilojoule range (∼880 J), was successfully used to deposit nanostructured magnetic CoPt thin films. The samples were synthesized at different filling hydrogen gas pressures and using different numbers of plasma focus deposition shots. The size of agglomerates/nanoparticles and the thickness of the CoPt thin films depend strongly on the filling gas pressure and the number of plasma focus deposition shots; hence it provides a possibility to control the CoPt agglomerates/nanoparticles size and the deposition rate by simply changing the operating parameters of the DPF system. The typical deposition rate of nanostructured CoPt thin film in the DPF device is much higher as compared with that of conventional PLD. The as-deposited CoPt nanoparticles are in the magnetically soft fcc phase and an annealing temperature of about 600 °C is required for phase transition to the magnetically hard fct phase, which may find possible applications in high density data storage.
FePt : Al2O3 nanocomposite thin films synthesized by magnetic trapping (MT) assisted pulsed laser deposition (PLD) were found to have lower transition temperature for L10 face-centred-tetragonal (fct) phase due to higher concentration of defects. The low phase transition temperature together with non-magnetic matrix materials helps to reduce grain growth and agglomeration during annealing. Small remanence ratio and coercive squareness for nanocomposite thin films annealed at 300 °C to fct phase confirm that the main intergranular interaction is magnetostatic interaction rather than exchange coupling. The MT assisted PLD can synthesize fct-FePt : Al2O3 nanocomposite thin films with reduced intergranular exchange coupling.
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