Direct Evidence for the Source of Reported Magnetic Behavior in "CoTe"

TL;DR

This study uses high-quality epitaxial CoTe films and advanced characterization to demonstrate that reported ferromagnetism in Co-Te systems originates from metallic Co inclusions, not the CoTe compound itself.

Contribution

It provides direct evidence linking ferromagnetism in Co-Te films to metallic Co impurities, clarifying previous ambiguities about the magnetic source.

Findings

Metallic Co inclusions are present in the films.

No metallic Co phase detected by XRD.

Ferromagnetism is due to Co inclusions, not CoTe.

Abstract

In order to unambiguously identify the source of magnetism reported in recent studies of the Co-Te system, two sets of high-quality, epitaxial CoTe$_x$ films (thickness $\simeq$ 300 nm) were prepared by pulse laser deposition (PLD). X-ray diffraction (XRD) shows that all of the films are epitaxial along the [001] direction and have the hexagonal NiAs structure. There is no indication of any second phase metallic Co peaks (either $fcc$ or $hcp$) in the XRD patterns. The two sets of CoTe$_x$ films were grown on various substrates with PLD targets having Co:Te in the atomic ratio of 50:50 and 35:65. From the measured lattice parameters $c = 5.396 \AA$ for the former and $c = 5.402\AA$ for the latter, the compositions CoTe$_{1.71}$ (63.1% Te) and CoTe$_{1.76}$ (63.8% Te), respectively, are assigned to the principal phase. Although XRD shows no trace of metallic Co second phase, the magnetic measurements do show a ferromagnetic contribution for both sets of films with the saturation magnetization values for the CoTe$_{1.71}$ films being approximately four times the values for the CoTe$_{1.76}$ films. $^{59}$Co spin-echo nuclear magnetic resonance (NMR) clearly shows the existence of metallic Co inclusions in the films. The source of weak ferromagnetism reported in several recent studies is due to the presence of metallic Co, since the stoichiometric composition "CoTe" does not exist.