Towards magnetic alumina: uncovering the roles of transition metal doping and electron hybridization in spin delocalization

Abstract

Judicious doping of normally diamagnetic alumina (Al2O3) could lead to bulk magnetism that would enable the usage of cutting edge technology, such as magnetoforming, to create advanced systems that take advantage of the high chemical and physical resilience of alumina. This study builds upon initial results (Nykwest et al 2018 J. Phys.: Condens. Matter 30 395801) which have shown that alumina doped with magnetic elements such as Fe and Ni should exhibit heightened magnetic activity. Here we expand the analysis to several additional transition metals that are otherwise non-magnetic (Sc, Ti, V, Mn, and Co) and use density functional theory to understand the origin of the spin delocalization, as well as to predict the structural, electronic, energetic, and magnetic properties of doped -alumina. The results indicate that adding small concentrations of such elements to -alumina may increase magnetic activity by generating coordination environments with magnetic moments. Our findings show conclusively that significant spin delocalization can only occur when there are unpaired electrons in the transition metal eg states.

Surface localized magnetism in transition metal doped alumina

“…2 . The trend is in agreement with our past work on bulk doped alumina 13 , with scandium and titanium being the most energetically favorable dopants introduced to the system overall, and nickel and copper the least favorable dopants for each respective aluminum substitutional site. This can be explained by the preference of Sc ([Ar]3d 4s ) and Ti ([Ar]3d 4s ) to donate 3 electrons to the octahedrally coordinated oxygen atoms, bringing their valency closer to Ar than the other considered dopants.…”

“…It has been demonstrated that substituting a transition metal dopant could produce alumina that possesses both unpaired electrons and a net magnetic moment 12 . In addition, it was also determined that the delocalization of these unpaired electrons varied greatly between dopants 13 , which may affect the final magnetic properties of the doped system 14 . Previous work in metallic systems 15 – 17 has pointed toward enhanced magnetism near surfaces, where a transition in spin density from the bulk to the surface region was observed.…”

“…In particular, the surface energy was converged to better than 0.002 J/m , and the interlayer spacing and bulk-like environment aluminum bond lengths better than 1%. The settings used in these calculations are similar to the ones that were rigorously tested and used in our previous studies 12 , 13 which involved an examination of bulk alumina doped with transition metals (Sc-Cu). First principles studies were performed within the general gradient approximation 36 (GGA) with Perdew–Burke–Ernzerhof 37 , (PBE) parameterization, utilizing the projector-augmented plane wave (PAW) pseudopotentials, as implemented in the Vienna Ab Initio Simulation Package (VASP) 38 – 40 , using 0.005 Gaussian smearing.…”

“…This can be explained by the preference of Sc ([Ar]3d 4s ) and Ti ([Ar]3d 4s ) to donate 3 electrons to the octahedrally coordinated oxygen atoms, bringing their valency closer to Ar than the other considered dopants. More details on the trends of defect formation energy can be found in 13 .…”

“…1 ). Position 6 shows the defect formation energy of bulk doped -alumina reported in references 12 , 13 .

Figure 3 Defect formation energy, in eV, relative to bulk-like doping (position 5) for each respective dopant.

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