Low temperature incommensurately modulated and noncollinear spin structure in FeCr₂S₄

Abstract

FeCr(2)S(4) orders magnetically at T(N)≈ 170 K. According to neutron diffraction, the ordered state down to 4.2 K is a simple collinear ferrimagnet maintaining the cubic spinel structure. Later studies, however, claimed trigonal distortions below ∼ 60 K coupled to the formation of a spin glass type ground state. To obtain further insight, muon spin rotation/relaxation (μSR) spectroscopy was carried out between 5 and 200 K together with new (57)Fe Mössbauer measurements. Below ∼ 50 K, our data point to the formation of an incommensurately modulated noncollinear spin arrangement like a helical spin structure. Above 50 K, the spectra are compatible with collinear ferrimagnetism, albeit with a substantial spin disorder on the scale of a few lattice constants. These spin lattice distortions become very large at 150 K and the magnetic state is now better characterized as consisting of rapidly fluctuating short-range ordered spins. The Néel transition is of second order, but ill defined, extending over a range of ∼ 10 K. The Mössbauer data around 10 K confirm the onset of orbital freezing and are also compatible with the noncollinear order of iron. The absence of a major change in the quadrupole interaction around 50 K renders the distortion of crystal symmetry to be small.

Magnetic properties of spinel FeCr2S4 in high magnetic field

“…Irreversibility between field cooled and zero field cooled magnetization M(T ) is observed in the low magnetic field range. From temperature and magnetic field dependence of magnetization, M(T ) and M(B), we proposed the existence of the new magnetic phase, below ∼9 K and above ∼5.5 T. Assuming the incommensurate-noncollinear spin structure in T < 60 K predicted by recent µSR work [16], high magnetic field gives rise to stabilize for the commensurate-collinear spin structure. Because new phase boundary exists in orbital-ordered phase, the spin structure of FeCr 2 S 4 in T < T OO is strongly coupled with the lattice.…”

“…Recent µSR investigation of FeCr 2 S 4 predicted that the formation of incommensurately modulated noncollinear spin arrangement below ∼ 50 K [16], and a spin-reorientation due to this formation is cause of the cusp-like anomaly of M(T ) at T m ∼ 70 K in the low B range. Meanwhile, in the high B range, M(T ) in T < T m is recovered gradually by applying B.…”

Spin re-orientation in FeCr2S4

“…2. Its increase directly below T C is typical for a second order magnetic phase transition and then follows a completely smooth sublattice magnetization curve down to about 50 K. A kink in B hf around 130 K as indicated in [13] could not be reproduced.…”

“…Muon spin rotation experiments [13] clearly pick up the magnetic ordering around 165 K, yet with some partial order extending up to about 175 K. A single spontaneous rotation signal is indicative for the collinear ferromagnetic regime. Below about 50 K, however, other rotation frequencies are found which have been attributed to the appearance of an incommensurate modulated spin structure, probably of helical type.…”

“…Some preliminary data have been included in [13]. We will concentrate on a gross interpretation of the magnetic hyperfine interaction, a detailed discussion related to the behavior of the electric field gradients and the spin dynamics close to the magnetic and orbital transitions will be presented elsewhere.…”