Large orbital magnetic moments of small, free cobalt cluster ions Co$_n^+$ with $n \leq 9$

TL;DR

This study reveals that small free cobalt cluster ions exhibit exceptionally large orbital magnetic moments, especially at n=5, with a non-monotonic size dependence, and the spin moments remain nearly constant across sizes.

Contribution

First measurement of size-dependent electronic and magnetic moments in free Co$_n^+$ clusters using x-ray spectroscopy, highlighting large orbital moments at specific sizes.

Findings

Co$_5^+$ has an orbital magnetic moment of 1.4 μ_B per atom.

Large orbital moments (~1 μ_B) found in Co$_4^+$, Co$_6^+$, Co$_8^+$.

Orbital magnetic moments vary non-monotonically with cluster size.

Abstract

The size dependent electronic structure and separate spin and orbital magnetic moments of free Co$_n^+$ ($n = 4\;\textrm{-}\;9$) cluster ions have been investigated by x-ray absorption and x-ray magnetic circular dichroism spectroscopy in a cryogenic ion trap. A very large orbital magnetic moment of $1.4\pm0.1\;\mu_{\mathrm B}$ per atom was determined for Co$_5^+$, which is one order of magnitude larger than in the bulk metal. Large orbital magnetic moments per atom of $\approx 1\;\mu_B$ were also determined for Co$_4^+$, Co$_6^+$, and Co$_8^+$. The orbital contribution to the total magnetic moment shows a non-monotonic cluster size dependence: The orbital contribution increases from a local minimum at $n = 2$ to a local maximum at $n = 5$ and then decreases with increasing cluster size. The $3d$ spin magnetic moment per atom is nearly constant and is solely defined by the number of $3d$ holes which shows that the $3d$ majority spin states are fully occupied, that is, $3d$ hole spin polarization is 100%.