Magnetoresistance in the helical itinerant magnets MnSi and Mn$_{1-x}$Co$_x$Si

TL;DR

This study investigates the magnetoresistance behavior of helical magnets MnSi and Mn$_{1-x}$Co$_x$Si across various temperatures and magnetic fields, revealing unique suppression of spin fluctuations and quantum critical properties.

Contribution

It provides new insights into the magnetoresistance phenomena in helical magnets, highlighting differences between pure and doped MnSi and their relation to magnetic fluctuations.

Findings

Negative magnetoresistance above 4 K due to spin fluctuation suppression

Positive, anisotropic magnetoresistance in pure MnSi at low temperatures

Isotropic magnetoresistance at higher temperatures linked to scattering effects

Abstract

We studied the longitudinal and transverse magnetoresistance of helical magnets, MnSi and Mn$_{1-x}$Co$_x$Si, at temperatures between 1.8 and 100~K and in magnetic fields up to 9 Tesla. All substances exhibited negative longitudinal and transverse magnetoresistance at temperatures above 4~K, which is most likely related to the suppression of spin fluctuations by the magnetic field. Note that in contrast to our finding, the longitudinal magnetoresistance of ferromagnetic metals was found to be positive. The unique positive and anisotropic magnetoresistance of pure MnSi at low temperatures (1.8 and 4~K) in the induced ferromagnetic phase shows effective suppression of fluctuations by the magnetic field. The significant difference in behavior between pure MnSi and doped MnSi lies in the specifics of the latter material, which forms a sort of helical fluctuation cloud and reveals quantum critical properties at low temperatures. The observed isotropic magnetoresistance in MnSi and Mn$_{1-x}$Co$_x$Si at higher temperatures can tentatively be attributed to the shortening of the mean free path of electrical carriers due to scattering on magnetic fluctuations and impurities, which results in a suppression of Lorentz force effects.