Vacancy Tuned Magnetism in LaMn$_x$Sb$_2$

TL;DR

This study explores how intrinsic Mn vacancies in LaMn$_x$Sb$_2$ influence its magnetic phases, revealing a complex temperature-composition phase diagram with six distinct magnetic states and structural transitions.

Contribution

It provides the first detailed characterization of LaMn$_x$Sb$_2$, demonstrating synthetic control of Mn occupancy and revealing its rich, tunable magnetic and structural phase behavior.

Findings

LaMn$_x$Sb$_2$ exhibits a range of magnetic phases depending on temperature and Mn content.

Structural transition from P4/nmm to I$ar{4}$2m occurs below x < 0.79.

Neutron diffraction shows a transition from G-type AFM to a non-collinear magnetic structure at high x.

Abstract

The layered ATMPn$_2$ (A = alkali earth or rare earth atom, TM = transition metal, Pn = Sb, Bi) compounds are widely studied for their rich magnetism and electronic structure topology. Here, we characterize the physical properties of LaMn$_x$Sb$_2$, an understudied member of the ATMPn$_2$ family. LaMn$_x$Sb$_2$ forms with intrinsic Mn vacancies, and we demonstrate synthetic control of the Mn occupancy to produce single crystals with x = 0.74-0.97. Magnetization and transport measurements indicate LaMn$_x$Sb$_2$ has a rich temperature-composition (T-x) magnetic phase diagram with physical properties strongly influenced by the Mn occupancy. LaMn$_x$Sb$_2$ orders antiferromagnetically at T$_{1}$ = 130--180 K, where T$_{1}$ increases with x. Below T$_{1}$, the T-x phase diagram is complicated. At high x, there is a second transition T$_2$ that decreases in temperature as x is lowered, vanishing below x $\leq$ 0.85. A third, first-order, transition T$_3$ is detected at x $\approx$ 0.92, and the transition temperature increases as x is lowered, crossing above T$_2$ near x $\approx$ 0.9. On moving below x $<$ 0.79, we find the crystal structure changes from the P4/nmm arrangement to a I$\bar{4}$2m structure with partially ordered Mn vacancies. The change in crystal structure results in the appearance of two new low temperature phases and a crossover between regimes of negative and positive magnetoresistance. Finally, we provide neutron diffraction for x = 0.93, and find that the high x compositions first adopt a G-type AFM structure with the Mn moments aligned within the ab-plane which is followed on cooling by a second transition to a different, non-collinear structure where the moments are rotated within the basal plane. Our results demonstrate that LaMn$_x$Sb$_2$ is a highly tunable material with six unique magnetically ordered phases, depending on T and x.