Fabrication Of Pure,well Protected And Crystallized Bi And Tri-metallic Sm(Co,X;X=Ni,Cu)nanoclusters

Magnetic materials have variety of applications ranging from nano level,such as drug delivery and high density information storage media,to macro level which includes motors and wind turbines.SmCo₅ with hexagonal CaCu₅ crystal structure possesses the highest magnetocrystalline anisotropy??1 x 10⁸ erg/cm³?and Curie temperature?1020 K?which makes it feasible specifically for high density recording media and also for permanent magnets.The high anisotropy values help to stabilize magnetism against thermal fluctuations.But,the synthesis of rare-earth based alloys is challenging due to their highly oxidative nature.Chemical methods face size and purity control issues so physical methods are preferred.Among physical methods,cluster beam deposition?CBD?method;a non-equilibrium synthesis technique,has been widely used for the synthesis of nanostructures in last few decades.This technique is advantageous to overcome issues in chemical methods as well as to stabilize certain phases in nanostructures which were not possible in bulk.CBD has been effectively used for the synthesis of size controlled SmCo₅ NCs in previous works.But,the purity and protection of such clusters have been scarcely studied so these issues need to be explored.Moreover,the previous studies reveal that use of metal additives like Ni,Cu,Zr in Sm-Co can reduce the crystallization temperature as compared to the virgin SmCo nanostructures.Effects of addition of these metals to SmCo have been widely investigated for bulk nanostructures and thin films.However,the synthesis of these trimetallic NCs by using CBD system has scarcely been studied so far.The clusters fabricated by CBD can be considered to possess their structural properties after deposition so they provide an efficient rout to investigate the structural properties of a single nanoparticle.The evolution of structural properties of single nanoparticle is vital because bulk nanocrystals comprises of an aggregate of nanoclusters.So once the properties and underlying physics of single nanoclusters is fully understood the idea can be scaled up.Hence,we attempted the structural evolution of single nanocluster of high anisotropy rare earth materials with 1:5 phase.In the first step,high purity SmCo nanoclusters having size less than 20 nm were successfully synthesized by cluster beam deposition method.The oxygen contents were found below the detectable limit of EDS in a TEM.Carbon layer with thickness 3-5 nm was used to protect the clusters from oxidation and it was found effective.As produced clusters were amorphous so they were annealed before deposition as well as after deposition.Pre-deposition annealing could not crystallize SmCo NCs effectively even at 820°C.However,Post-deposition annealing at500°C transformed the amorphous nanoclusters to single crystal SmCo₅ having hexagonal CaCu₅ symmetry.Increasing post-annealing temperature above 600°C distorts the crystal structure.This distortion is attributed to reaction between C and SmCo nanoclusters and this reaction is a nanoscale effect.Such reaction was not observed for C in bulk counterparts till 900°C so far.In the next step,composite targets with Ni and Cu addition in SmCo were prepared and used to produce nanoclusters.Line scan profiles showed that the constituent elements were homogenously distributed in the individual nanoclusters.Furthermore addition of Ni and Cu reduces the crystallization temperature from 500°C to 450°C.These clusters were also crystallized by pre-deposition annealing even at 750°C or below.Achievement of crystalline structure by pre-deposition annealing can be of real scientific interest as post-deposition annealing may cause certain unwanted effects.Furthermore,the coercivity of Sm-Co-Cu nanoclusters embedded under a Cr cover-layer was found to improve with annealing temperature upto 450°C and achieved 7.8 kOe for a thick film.The increase in coercivity with temperature was attributed to a combined effect of exchange interactions at the clusters surface and increased crystallization fraction of the clusters.Contrarily,thick film of Sm-Co-Ni nanoclusters showed a decrease in coercivity with temperature and the maximum coercivity of 6.1 kOe was achieved for an unannealed sample.Further investigation with different composition of Ni and Cu is suggested.