Electronic Band Structure and Helical Spin Density Wave of MnSi

Abstract

The intermetallic compounds with the B20 type of structure have been received continual attention and studied in detail in the past decades [1–3]. This is because their electrical and magnetic properties show a variety, depending on the constituent elements. Among them, MnSi has been studied most extensively and revealed to be a helical magnet whose Néel temperature T_N is 29 K [4]. According to the neutron diffraction experiments performed by ISHIKAWA et al., this helical spin density wave (HSDW) has the peculiar properties as follows [4,5]: (i) The period is very long (180 Å). (ii) The equilibrium direction of the wave vector $\vec{Q}$ is <111>. When an external magnetic field is applied, the direction of $\vec{Q}$ follows that of the magnetic field, (iii) The period does not depend on the direction of $\vec{Q}$, but depends weakly on temperature. (iv) The HSDW is changed into an induced ferromagnetic state (IFMS) by a weak external magnetic field (6.2 K0e). This means that the energy difference between the HSDW and the ferromagnetic state (FMS) is much small. The IFMS is supposed to be a typical example of itinerant weak ferromagnetism on the basis of the following experimental facts: (i) The induced ferromagnetic moment 0.4 µ_B/Mn is much less than 1.4 µ_B/Mn obtained by using the Curie-Weiss constant in the paramagnetic phase [2,6]. (ii) The magnetization curve does not saturate even at 150 K0e [7]. (iii) The spin-lattice relaxation time T_l of ⁵⁵Mn in MnSi is independent of temperature at temperatures much higher than T_N[8,9].