Collapse and control of theMnAu2spin-spiral state through pressure and doping

Abstract

MnAu2 is a spin-spiral material with in-plane ferromagnetic Mn layers that form a screw-type pattern around a tetragonal c axis. The spiral angle θ was shown using neutron diffraction experiments to decrease with pressure, and in later studies it was found to suffer a collapse to a ferromagnetic state above a critical pressure, although the two separate experiments did not agree on whether this phase transition is first or second order. To resolve this contradiction, we use density functional theory calculations to investigate the spiral state as a function of pressure, charge doping, and also electronic correlations via a Hubbard-like U . We fit the results to the one-dimensional J1−J2−J3−J4 Heisenberg model, which predicts either a first-or second-order spiral-to-ferromagnetic phase transition for different regions of parameter space. At ambient pressure, MnAu2 sits close in parameter space to a dividing line separating first-and second-order transitions, and a combination of pressure and electron doping shifts the system from the first-order region into the second-order region. Our findings demonstrate that the contradiction in pressure experiments regarding the kind of phase transition are likely due to variations in sample quality. Our results also suggest that MnAu2 is amenable to engineering via chemical doping and to controlling θ using pressure and gate voltages, which holds potential for integration in spintronic devices.

Pressure tuning of collapse of helimagnetic structure in Au2Mn

“…We present measurements of the temperature-and pressure-dependence of the helical spin angle, and the pressure dependence of the critical field of the SS-FM transition through magnetization and magnetoresistance (MR) measurements. Pressure drives Au 2 Mn through a second order phase transition, which agrees with recent band structure calculations using DFT within the local spin density approximation (LSDA) 9,13 . An anomalous magnetic peak in positive MR at low temperature and step-like magnetization track the SS-FM transition, and interestingly, in the pressure-induced FM phase, we did not see evidence of the expected Anomalous Hall Effect (AHE) 14,15 .…”

“…These observations support the notion that competing exchange terms in Au 2 Mn put the SS-FM transition on the boundary between first-and second-order character 9 . Considering the dependence of φ on competing exchange terms 13 , for angles smaller than 30 degrees, the range of parameter space is limited, and even if φ smoothly goes to zero, it may occur over a narrow pressure range. Our results show that this question should be addressed by a neutron diffraction experiment using a pressure cell capable of exceeding 10 kbar.…”