Anomalous phase transformation behavior and hyperfine ferromagnetism inCo; results from perturbed angular correlation spectroscopy corroborated byab-initio calculations

Abstract Temperature dependent phase transformation behavior in Co from hcp to fcc has been found to be contradictory to that reported earlier. It is found that hcp phase stabilizes at both low and high temperature (∼873 K)while fcc phase is stabilized at ∼500 K. At 298 K, hcp Co has been found to be predominant (∼70%) where hcp magnetic phase is ∼60%. At 973 K, hcp phase is again predominant (∼73%), but it is mainly the non-magnetic phase (∼67%). Contrary to present results, it was found earlier that fcc phase was stabilized at high temperature and hcp to fcc transformation occured at ∼700 K. Theoretical models show that non-magneticfcc phase is more stable than non-magnetic hcp phase and presence of magnetism destabilizes the fcc phaseat low temperature. Present results from perturbed angular correlation measurements, therefore, requires anew theoretical interpretation for Co phase transformation. From present measurements, hyperfine magneticfields in Co at room temperature for the hcp and fcc phases have been found to be 18.7(6) and 12.8(3) T, much lower than earlier reported results. The hyperfine magnetic fields at 181Ta impurity atom have been calculated by density functional theory (DFT) employing the full potential (linearized) augmented plane wave method (FP-LAPW). Present calculated results for both hcp and fcc phases corroborate our experimental results.