Magnetic structure ofFe1−xCoxSiin a magnetic field studied via small-angle polarized neutron diffraction

Abstract

The magnetic structure of Fe 1−x Co x Si single crystals with x = 0.10, 0.15, 0.20, 0.50 has been studied by small angle polarized neutron diffraction and superconducting quantum interference device measurements. Experiments have shown that in zero field the compounds with x = 0.1, 0.15 have a well-defined tendency to order in the one-handed spiral along ͗100͘ axes due to the anisotropic exchange, that, however, decreases with increasing Co concentration x. The magnetic structure of Fe 1−x Co x Si with x = 0.2, 0.5 consists of spiral domains with randomly oriented spiral wave vector k. The applied magnetic field produces a single domain helix oriented along the field. The process of the reorientation starts at the field H C1 . Further increase of the field leads to a magnetic phase transition from a conical to a ferromagnetic state near H C2 . In the critical range near T C the integral intensity of the Bragg reflection shows a well-pronounced minimum at H fl attributed to a k flop of the helix wave vector. On the basis of our experiments we built the H-T phase diagram for each compound. It is shown that the same set of the parameters governs the magnetic properties of these compounds k, H C1 , H fl , and H C2 . Our experimental findings are well interpreted in the framework of a recently developed theory ͓Phys. Rev. B 73, 174402 ͑2006͔͒ for cubic magnets with Dzyaloshinskii-Moriya ͑DM͒ interaction. In particular, the theory suggests an additional quantum term in the magnetic susceptibility caused by the DM interaction which is in good agreement with the experiment.

Confinement of chiral magnetic modulations in the precursor region of FeGe

“…Cubic iron monogermanide, FeGe, [1] together with the iso-structural MnSi [2] and MnGe [3] as well as the pseudo-binary compounds Fe 1−x Co x Si (0.1 ≤ x ≤ 0.7) [4,5,6,7], Mn 1−y Co y Si (0 ≤ y < 0.9) [5,8], and Mn 1−z Fe z Si (0 ≤ z < 0.13) [8,9] are non-centrosymmetric cubic helimagnets (space group P 2 1 3, B20-type structure) Table 1. Curie temperature T C , spontaneous magnetic moment µs(T → 0), lattice parameter a, helix period L D , and saturation field H D ≡ H c2 (T = 0) for some cubic helimagnets with B 20-type structure (space group P 2 1 3).…”

Pressure-induced quantum phase transition in Fe1−xCoxSi (Forthaus1, Hearne2, Manyala3 et al. 2011Phys. Rev. B

“…However, the anisotropy of the system decreases with increasing Co concentration for x ≥ 0.2 and results in a distribution of the helix wave vector. This distribution has been shown to be random for high Co concentration (x = 0.5) 10 . On the other hand, ab initio calculations on Fe 1−x Co x Si revealed that such local disorder at concentrations above x > 0.25 is responsible for the decrease of the polarization of the ferromagnetic half-metallic state.…”

“…From the crystal structure of FeSi, the EFG principal axis at the iron site is along 111 directions and the asymmetry parameter is zero 35 . At low Co concentration, the iron moments are along 100 directions but become random with increasing Co concentration 10 . Since the four possible 111 directions are all at an angle of arccos(1/ √ 3) to 100 , diagonal EFG terms drop out of the Hamiltonian, but non-diagonal terms do not (see for example Ref.…”

“…It has to be mentioned here that such local crystallographic disorder is on a much shorter length scale than that typical for x-ray diffraction and therefore it can not be detected on a long length scale. The most important consequence of such local crystallographic disorder in Fe 1−x Co x Si is that it strongly affects the magnetic structure 10 and is shown to weaken the polarization of the ferromagnetic half-metallic state for x > 0.25 6 . In the following, we would like to relate the observed pressure dependence of T M for the different concentrations to a corresponding change of the magnetic structure and character of the ferromagnetic half-metallic state.…”

“…In recent years, the disordered Fe 1−x Co x Si alloys, crystallizing in the cubic B20-type structure, have attracted significant interest due to their remarkable electronic and magnetic properties [1][2][3][4][5][6][7][8][9][10] . The undoped system FeSi is a paramagnetic metal at high temperatures (T 400 K).…”