Magnetic properties of the Itinerant helimagnets MnSi and FeGe

Abstract

This thesis describes high quality magnetisation measurements made on single crystals of MnSi, FeGe and ZrZn(_2) using a vibrating sample magnetometer. The measurements on MnSi have also been complemented with neutron scattering experiments. MnSi is a heavily investigated itinerant helimagnet which exhibits a variety of interesting phenomena associated with formation of a helical spin density wave propagating along < 111 > directions. Magnetisation measurements were performed as a function of magnetic field at fixed temperatures stepping through the magnetic transition observed at 29.1 ± 0.05K. These were found to be highly anisotropic and included observation of the so-called 'Phase A' consistent with measurements using other techniques and providing explanation of apparent anomalies in previous magnetisation data (Kadowaki et al. (1981)). Further investigation of 'Phase A' using small angle neutron scattering (SANS) was successful in determining the magnetic state of MnSi within this regime in terms of helix reorientation which is shown to be broadly consistent with the expression for the free energy derived by Bak and Jensen (1980) and Plumer and Walker (1981). Reorientation of the helical spin density wave as a function of magnetic field was also studied using SANS to complement the magnetisation measurements. The second order process observed is similar to that predicted by Plumer and Walker (1981) and the form of their model for the magnetisation of is compared with the experimental results. Finally, anomalous magnetisation measurements close to the magnetic transition were further explored through neutron scattering. The results suggest a possible isotropic phase pre-empting the helical spin density wave formation. Cubic FeGe is also capable of supporting a static helical spin density wave and has a critical temperature of 278.7K with helix propagating along < 100 > directions above 211K and along < 111 > directions below 211K (for decreasing temperatures). Magnetisation measurements were made on cubic FeGe with magnetic field applied parallel to the < 100 > direction and the magnetic phase diagram determined. Coupled with the SANS data of Lebech et al. (1989) it shows similar processes in terms of helix reorientation in an applied magnetic field occur for both FeGe and MnSi. The magnetic phase diagram is in good agreement with that predicted by Plumer (1990) for magnetic field applied parallel to the < 100 >