Effects of Reducing Heat Treatment on the Structural and the Magnetic Properties of Mn:ZnO Ceramics

TL;DR

This study investigates how reducing heat treatment affects the structural and magnetic properties of Mn-doped ZnO ceramics, revealing phase transformations and paramagnetic behavior despite defect induction.

Contribution

It provides new insights into the effects of heat treatment on secondary phase evolution and magnetic properties in Mn:ZnO ceramics.

Findings

Secondary phases transform from ZnMn2O4 to Mn1-xZnxO after heat treatment

Heat treatment induces oxygen vacancies and phase changes in Mn:ZnO

Magnetic behavior remains paramagnetic despite defect formation

Abstract

Polycrystalline bulk Mn:ZnO ceramics with Mn nominal concentrations of 6, 11, 17 and 22 at.% were prepared trough solid-state reaction method and subjected to a heat treatment in reducing atmosphere (Ar (95%) and H2 (5%)). The samples were studied with particular emphasis on their compositions, structural, and magnetic properties. A detailed microstructural and chemical analysis confirms the Mn doping of the wurtzite ZnO structure mainly at the surface of the ZnO grains. For the samples with higher Mn content, the secondary phases ZnMn2O4 and Mn1-xZnxO (Zn-doped MnO) were detected for the as prepared and the heat treated samples, respectively. The structural change of the secondary phases under heat treatment, from ZnMn2O4 to Mn1-xZnxO, confirms the effectiveness of the heat treatment in to reduce the valence of the metallic ions and in the formation of oxygen vacancies into the system. In spite of the induced defects, the magnetic analysis present only a paramagnetic behavior with an antiferromagnetic coupling between the Mn ions. In the context of the bound magnetic polaron theory, it is concluded that oxygen vacancies are not the necessary defect to promote the desired ferromagnetic order at room temperature.