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

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.

Figures (6)

• Figure 1: Magnetization curves for MnSi and Mn$_{1-x}$Co$_x$Si.
• Figure 2: Magnetic susceptibility of MnSi and Mn$_{1-x}$Co$_x$Si as a function of the temperature at 0.03 T.
• Figure 3: Longitudinal and transverse magnetoresistance of MnSi as a function of the magnetic field at (a) 1.8 K and (b) 4 K.
• Figure 4: Magnetoresistance of MnSi as a function of the magnetic field at 7 K and 10 K (a) and at 20 K, 30 K, 50 K, 100 K (b). At temperatures above 20 K the longitudinal and transverse magnetoresistance curves can't be distinguished. Arrow indicates border between conical and field polarized phases.
• Figure 5: Magnetoresistance of (a) Mn$_{0.94}$Co$_{0.06}$Si and (b) Mn$_{0.83}$Co$_{0.17}$Si as a function of the magnetic field. The longitudinal and transverse magnetoresistance can not be distinguished in the present experiment.