Absence of ferromagnetism in Co and Mn substituted polycrystalline ZnO

TL;DR

This study investigates the magnetic properties of Mn and Co doped ZnO and finds no evidence of ferromagnetic ordering down to 2 K, despite high Weiss temperatures, highlighting the absence of ferromagnetism in these doped semiconductors.

Contribution

The paper provides detailed magnetic measurements and analysis showing the lack of ferromagnetism in Mn and Co doped ZnO, clarifying previous ambiguities about magnetic ordering in these materials.

Findings

No magnetic ordering observed down to 2 K.

Magnetization fits a Curie-Weiss plus Curie model.

High Weiss temperature indicates strong magnetic interactions without ordering.

Abstract

We discuss the properties of semiconducting bulk ZnO when substituted with the magnetic transition metal ions Mn and Co, with substituent fraction ranging from $x$ = 0.02 to $x$ = 0.15. The magnetic properties were measured as a function of magnetic field and temperature and we find no evidence for magnetic ordering in these systems down to $T$ = 2 K. The magnetization can be fit by the sum of a Curie-Weiss term with a Weiss temperature of $\Theta\gg$100 K and a Curie term. We attribute this behavior to contributions from both \textit{t}M ions with \textit{t}M nearest neighbors and from isolated spins. This particular functional form for the susceptibility is used to explain why no ordering is observed in \textit{t}M substituted ZnO samples despite the large values of the Weiss temperature. We also discuss in detail the methods we used to minimize any impurity contributions to the magnetic signal.