Magnetic ordering in the quaternary R2Fe2Si2C (R = Y, Gd - Tm) intermetallic compounds

The magnetic ordering of the quaternary R2Fe2Si2C (Y, Gd - Tm) compounds which crystallise in the monoclinic Dy2Fe2Si2C -type structure (C2/m space group), have been studied. The studies encompass the low temperature structural, thermal and magnetic properties of R2Fe2Si2C (Gd - Tm) and the determination of their magnetic structures. The fundamental goals of this study are to understand the interplay between the exchange interaction and the crystal field effects, relevant to the overall magnetic properties, and to determine whether or not the Fe sublattice is magnetically ordered in this series of compounds.The determination of the magnetic properties of these compounds reveals two important results: (i) Gd2Fe2Si2C orders at TN = 39 K, much higher than the previously reported value of TN ~ 9 K and (ii) Dy2Fe2Si2C exhibits two magnetic transitions at low temperatures. Systematic studies on the magnetic properties of this series of compounds show that the ordering temperatures of the R2Fe2Si2C (R = Gd - Tm) compounds deviate from the expected de Gennes scaling. This deviation can be described fully if both the exchange interaction and the crystal field effect are taken into account, indicating the significant influence of the crystal field in this series.Three types of magnetic structure were found to exist in this series of compounds. While the common magnetic structure of the R2Fe2Si2C (R = Gd - Ho) compounds involves ordering of the R sublattice along the b-axis with a propagation vector k1 = [0, 0, 1/2], the magnetic structures of Er2Fe2Si2C and Tm2Fe2Si2C are characterised by the propagation vectors k2 = [1/2 , 1/2 , 0] and k3 = [0.403(1), 1/2 ,0], respectively, with the R moments lying in the ac-plane. The origin of the evolution of magnetic structure across this series is discussed, revealing that crystal field effects play an important role in modifying the propagation vector, as well as defining the direction of the R moment.Neutron diffraction refinements alone were unable to answer the question of whether or not the Fe is magnetic in R2Fe2Si2C. However, 57Fe Mossbauer experiments provide the first unequivocal evidence that the Fe atom is non-magnetic in any member of the R2Fe2Si2C compounds