Evidence of Formation of Superdense Nonmagnetic Cobalt

Nasrin Banu,Hema C P Movva,V Lauter,B N Dev,S Basu,B Satpati,P Chakraborty,A Roy,Surendra Singh

doi:10.1038/srep41856

Abstract

Because of the presence of 3d transition metals in the Earth’s core, magnetism of these materials in their dense phases has been a topic of great interest. Theory predicts a dense face-centred-cubic phase of cobalt, which would be nonmagnetic. However, this dense nonmagnetic cobalt has not yet been observed. Recent investigations in thin film polycrystalline materials have shown the formation of compressive stress, which can increase the density of materials. We have discovered the existence of ultrathin superdense nonmagnetic cobalt layers in a polycrystalline cobalt thin film. The densities of these layers are about 1.2–1.4 times the normal density of Co. This has been revealed by X-ray reflectometry experiments, and corroborated by polarized neutron reflectometry (PNR) experiments. Transmission electron microscopy provides further evidence. The magnetic depth profile, obtained by PNR, shows that the superdense Co layers near the top of the film and at the film-substrate interface are nonmagnetic. The major part of the Co film has the usual density and magnetic moment. These results indicate the possibility of existence of nonmagnetic Co in the earth’s core under high pressure.

Highlights

Interest in magnetism in 3d transition metals, iron, nickel and cobalt, has continued beyond the traditional bulk materials and thin films
Insets show electron scattering length density (ESLD) depth profiles used for fitting the X-ray reflectivity (XRR) data
Depth profiles of electron scattering length density (ESLD) obtained from XRR, and nuclear scattering length density (NSLD) from polarized neutron reflectometry (PNR), in a polycrystalline cobalt thin film on silicon have revealed regions of cobalt with much higher density compared to normal cobalt

Summary

Introduction

Interest in magnetism in 3d transition metals, iron, nickel and cobalt, has continued beyond the traditional bulk materials and thin films. In a high pressure experiment on Co a hcp to fcc phase transition has been observed around this atomic volume corresponding to a density of ~1.4 times the normal Co density[3]. We report on the observation of a nonmagnetic fcc phase of cobalt, the density of which is about 1.2–1.4 times the density of normal hcp cobalt. We observe such a HD nonmagnetic phase in an electron-beam deposited polycrystalline Co thin (~25 nm) film on a Si(111) substrate. The magnetic and the chemical depth profile analysis[7,8,9] from polarized neutron reflectometry (PNR) experiments corroborate the high density and show that this superdense cobalt layer is nonmagnetic

Methods

Results

Conclusion