Colossal magnetoresistance in the insulating ferromagnetic double perovskites Tl$_2$NiMnO$_6$: A neutron diffraction study

TL;DR

This study reports the discovery of colossal magnetoresistance in a ferromagnetic insulating double perovskite Tl$_2$NiMnO$_6$, elucidating the role of cation ordering and structural properties in its magnetic behavior.

Contribution

It introduces Tl$_2$NiMnO$_6$ as the first ferromagnetic insulating double perovskite exhibiting CMR, with detailed neutron diffraction and electronic structure analysis.

Findings

Tl$_2$NiMnO$_6$ shows CMR near Curie temperature.

Cation ordering (~70%) is crucial for ferromagnetic insulating behavior.

Disordered Tl$_2$NiMnO$_6$ exhibits spin glass behavior due to antisite defect clustering.

Abstract

In the family of double perovskites, colossal magnetoresistance (CMR) has been so far observed only in half-metallic ferrimagnets such as the known case Sr$_2$FeMoO$_6$ where it has been assigned to the tunneling MR at grain boundaries due to the half-metallic nature. Here we report a new material-Tl$_2$NiMnO$_6$, a relatively ordered double perovskite stablized by the high pressure and high temperature synthesis-showing CMR in the vicinity of its Curie temperature. We explain the origin of such effect with neutron diffraction experiment and electronic structure calculations that reveal the material is a ferromagnetic insulator. Hence the ordered Tl$_2$NiMnO$_6$ (~70% of Ni$^{2+}$/Mn$^{4+}$ cation ordering) represents the first realization of a ferromagnetic insulating double perovskite, showing CMR. The study of the relationship between structure and magnetic properties allows us to clarify the nature of spin glass behaviour in the disordered Tl$_2$NiMnO$_6$ (~31% of cation ordering), which is related to the clustering of antisite defects and associated with the short-range spin correlations. Our results highlight the key role of the cation ordering in establishing the long range magnetic ground state and lay out new avenues to exploit advanced magnetic materials in double perovskites.