Two-dimensional magnetic correlations and partial long-range order in geometrically frustrated CaOFeS with triangle lattice of Fe ions

We report the results on the structure, transport, and magnetic properties of a layered oxysulfide CaOFeS with a stacked triangle lattice of Fe ions. The susceptibility data show a broad maximum near 120 K, indicating the existence of two-dimensional (2D) short-range ordering in this compound. Features associated with long-range antiferromagnetic (AFM) phase transition are seen below 40 K. Meanwhile, a very small heat-capacity anomaly is detected around 35 K, and most of the measured magnetic entropy is lost during the 2D ordering process. Both crystal and magnetic structures were studied by neutron powder diffraction at 300, 125, 40, and 6 K. The structure was refined based on space group $P{6}_{3}mc$ with $a=3.75998\left(4\right)$ and $c=11.38351\left(16\right)\AA$ at ambient temperature. Low-temperature diffraction reveals 2D magnetic correlations between Fe moments without showing significant structural distortion. Warren peak shape analysis of the neutron-diffraction data at $2\theta$ near 18° is employed to characterize the correlation length in the 2D magnetic state with lowering temperature. The geometrically frustrated compound is found to gradually condense into a partial long-range ordered state with AFM coupled Fe layers between 40.6 and 26 K. The resulting partially ordered magnetic structure is a $G$-type Ising AFM with a propagation vector of $\mathbf{k}=(1/2,1/2,0)$ and an ordered magnetic moment of $2.59\left(3\right){\mu }_{\mathrm{B}}/\mathrm{Fe}$ along $c$ at 6 K.

Figure 1

(a) Room-temperature crystal structure of CaOFeS, (b) coordination environment of the $\infty \left[\mathrm{FeO}{\mathrm{S}}_3\right]$ plane, (c) powder neutron diffraction and result of Rietveld refinement of the CaOFeS sample at 300 K.

Figure 2

ZFC and FC $\chi \left(T\right)$ data taken in an applied field of 10 kOe. Inset: enlarged $\chi \left(T\right)$ data below 60 K.

Figure 3

(a) Temperature dependence of specific heat for CaOFeS between 200 and 2 K; the heat capacity of a diamagnetic isostructural material CaOZnS is measured to account for the lattice component. Left inset: The low-temperature specific-heat data in the plot of $C_p/T$ vs $T$ [2]. The solid lines are fitted using formula $C_p/T=\beta T^2+\alpha$; right inset: enlargement of the specific-heat data to emphasize a small peak contributed by magnetic entropy around 35 K; the orange curve represents a polynomial fit by neglecting this peak. (b) Temperature dependence of magnetic entropy and $C_{\mathrm{mag}}/T$ for CaOFeS, and the $C_{\mathrm{mag}}/T$ data show a very broad feature in the temperature range between 200 and 40 K and a relative sharp contribution to the magnetic entropy of ∼35 K.

Figure 4

Temperature dependence of the resistivity $\rho \left(T\right)$ of the CaOFeS with $H=0$. The inset shows the fitting results of $\rho \left(T\right)$ at zero field using a thermal activation model (red line) and a variable range hopping model (green line).

Figure 5

Neutron powder-diffraction pattern for CaOFeS taken at (a) 125 K, (b) 40 K, and (c) 6 K. Upper black cross: observed pattern; upper red curve: calculated pattern; tick marks: calculated peak positions; lower curve: difference between observed and calculated patterns. Magnetic stands for the magnetic Bragg peak.

Figure 6

(a) Normalized lattice parameters $a$ and $c$ of CaOFeS vs temperature and (b) O-Fe-S and S-Fe-S vs $T$. (c) Fe-S and Fe-O bond lengths vs temperature, respectively.

Figure 7

(a) Chemical and magnetic structures of CaOFeS at 6 K. (b) View of the Fe triangular coupling sheet; the magnetic structure is a $G$-type Ising AFM with a propagation vector of $\mathbf{k}=(1/2,1/2,0)$ and an ordered magnetic moment of $2.59\left(3\right){\mu }_{\mathrm{B}}/\mathrm{Fe}$ along $c$ at 6 K.

Figure 8

Enlarged neutron powder-diffraction pattern with the short-range ordering feature where the red solid line is the Warren line-shape fit.

Figure 9

Temperature dependence of the intensity at the (3/2,1/2,1) magnetic Bragg peak in CaOFeS. The intensity data were obtained by measuring counts at the Bragg position of (3/2,1/2,1) magnetic peak vs $T$. Phase transition from a paramagnetic phase to a partial AFM phase was roughly estimated between 40.6 and 26 K. The inset shows the respective neutron data collected at 6, 40, and 125 K, respectively.