Neutron diffraction study of magnetically ordered Tm2Fe3Si5.

Abstract

The magnetic ordering of ${\mathrm{Er}}_{2}$${\mathrm{Fe}}_{3}$${\mathrm{Si}}_{5}$ has been studied by neutron-diffraction techniques at low temperatures. At 4.3 K, the crystal structure is tetragonal (space group $\frac{P4}{\mathrm{mnc}}$, $a=10.393$ \AA{}, $c=5.428$ \AA{}). The atomic positional parameters were determined by Rietveld refinement of the data. Magnetic scattering peaks at positions incommensurate with the chemical cell are observed below 2.85 K. Below about 2.55 K, a second set of magnetic peaks appears, at positions commensurate with the chemical cell, but with the tetragonal $a$ axis doubled. Between 2.55 and 2.45 K, the two sets of magnetic peaks coexist, and hysteresis of about 0.01 K is observed. In this temperature region, peaks from the incommensurate phase steadily decrease in intensity as temperature is reduced until, below 2.45 K, only commensurate peaks are present. The commensurate peaks increase in intensity as the temperature is reduced down to the lowest temperatures studied. Rietveld refinement of data taken at 1.83 K shows the magnetic intensities to be well accounted for by a noncollinear magnetic structure whose principal features are the following: (1) ordering occurs only at Er-atom sites, the moment being $8.2{\ensuremath{\mu}}_{B}$ at 1.83 K, and (2) the moments are ordered in the (001) planes with a noncollinear arrangement along the $〈110〉$ set of directions, the $c$ glide planes of the chemical structure being retained in the magnetic cell (space group ${C}_{p}\mathrm{cc}{m}^{\ensuremath{'}}$)). For the incommensurate model, all elements of the commensurate arrangement are retained, but a sinusoidal modulation along [001] with a wavelength of about $5.7c$ is introduced. This wavelength is independent of temperature to within \ifmmode\pm\else\textpm\fi{}3%. Specific-heat measurements were also performed on the sample. These measurements show three peaks, one at 2.75 K corresponding to the onset of magnetic order and two others at 2.45 and 2.43 K in the coexistence region of the incommensurate and commensurate phases.