Suppression of magnetic order in ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$ with Fe substitution: Magnetization, neutron diffraction, and x-ray diffraction studies of $\mathrm{Ca}{\left({\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x\right)}_y{\mathrm{As}}_2$

Magnetization, neutron diffraction, and high-energy x-ray diffraction results for Sn-flux grown single-crystal samples of $\mathrm{Ca}{\left({\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x\right)}_y{\mathrm{As}}_2$, $0\le x\le 1$, $1.86\le y\le 2$, are presented and reveal that A-type antiferromagnetic order, with ordered moments lying along the $c$ axis, persists for $x\lesssim 0.12\left(1\right)$. The antiferromagnetic order is smoothly suppressed with increasing $x$, with both the ordered moment and Néel temperature linearly decreasing. Stripe-type antiferromagnetic order does not occur for $x\le 0.25$, nor does ferromagnetic order for $x$ up to at least $x=0.104$, and a smooth crossover from the collapsed-tetragonal (cT) phase of ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$ to the tetragonal (T) phase of ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ occurs. These results suggest that hole doping ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$ has a less dramatic effect on the magnetism and structure than steric effects due to substituting Sr for Ca.

Figure 1

(a) The stripe-type magnetic order present in ${\mathrm{CaFe}}_2{\mathrm{As}}_2$, and (b) the A-type magnetic order present in ${\mathrm{CaCo}}_{1.86}{\mathrm{As}}_2$. Both compounds are shown with their ambient temperature and pressure tetragonal chemical unit cells. The diagrams were created with vesta [16].

Figure 2

(a) Temperature dependence of the magnetic susceptibility of the $x=0$ sample for magnetic fields $H$ applied within the basal plane $\left({\chi }_{\mathrm{ab}}\right)$ and along c $\left({\chi }_{\mathrm{c}}\right)$. These data also appear in Ref. [22]. (b) The derivatives with respect to temperature of the products ${\chi }_{ab}T$ and ${\chi }_{\mathrm{c}}T$. The dashed line indicates the value of $T_{\mathrm{N}}$ determined from the plots, as described in the text.

Figure 3

The magnetic susceptibilities versus temperature of the $x=0$ (a), 0.043 (b), 0.057 (c), 0.068 (d), 0.104 (e), 0.19 (f), 0.25 (g), 0.35 (h), 0.48 (i), and 0.67 (j) samples for magnetic fields $H$ applied within the basal plane. The insets show the derivatives with respect to temperature of ${\chi }_{ab}T$ for samples with an AFM transition. Red arrows and lines indicate the values for $T_{\mathrm{N}}$ determined from the data, whereas black arrows and lines indicate the values for $T_{\mathrm{N}}$ determined by neutron diffraction. The ${\chi }_{\mathrm{ab}}\left(T\right)$ data for $x=0$ also appear in Ref. [22].

Figure 4

Neutron diffraction data for $x=0$ from $\theta$ and $\theta \text{−}2\theta$ scans through the $\left(110\right)$ [(a), (b)], $\left(004\right)$ [(c), (d)], $\left(111\right)$ [(e), (f)], and $\left(113\right)$ [(g), (h)] Bragg peaks at $T=4$ K. Solid lines are fits to Gaussian line shapes.

Figure 5

Neutron diffraction data for $x=0.068$ from $\theta \text{−}2\theta$ scans through the $\left(110\right)$ (a) and $\left(004\right)$ (b) Bragg peaks at $T=4$ K, and $\theta$ and $\theta \text{−}2\theta$ scans through the $\left(111\right)$ [(c), (d)], and $\left(113\right)$ [(e), (f)] Bragg peaks at $T=4$ K. Solid lines are fits to Gaussian line shapes.

Figure 6

Neutron diffraction data from $\theta \text{−}2\theta$ scans through the $\left(111\right)$ Bragg peak and the temperature dependence of the integrated intensity (II) of the peak for $x=0.043$ [(a), (b)], $x=0.068$ [(c), (d)], and $x=0.104$ [(e), (f)]. Lines in (a), (c), and (e) are fits to Gaussian line shapes, and lines in (b), (d), and (f) are guides to the eye. Arrows indicate the values for $T_{\mathrm{N}}$ determined from the data.

Figure 7

Neutron diffraction data for $x=0.068$ from Q scans through the $\left(001\right)$ (a), $\left(003\right)$ (b), $\left(\frac{1}{2}\frac{1}{2}1\right)$ (c), and $\left(\frac{1}{2}\frac{1}{2}3\right)$ (d) reciprocal-lattice positions. In (a) and (c), data taken at $T=50$ K are shown in red and offset by 1.5 and 1 counts/s, respectively. Arrows in (a) and (b) indicate the positions for peaks that would correspond to A-type AFM order with a component of the ordered moment lying in the $ab$ plane. Arrows in (c) and (d) indicate the positions for peaks that would correspond to the stripe-type AFM order possessed by ${\mathrm{CaFe}}_2{\mathrm{As}}_2$. Data in (a), (b), and (d) are for a beam monitor value corresponding to a counting time of 30 s per point, and data in (c) are for a monitor value corresponding to 60 s per point.

Figure 8

Neutron diffraction data for $x=0.068$ from $\theta \text{−}2\theta$ scans through the $\left(110\right)$ (a) and $\left(004\right)$ (b) Bragg peaks at various temperatures. Lines are fits to Gaussian line shapes. The integrated intensities (II) determined from the fits are plotted versus temperature in (c) $\left[\right(110\left)\right]$ and (d) $\left[\right(004\left)\right]$.

Figure 9

Neutron diffraction data from scans across possible magnetic Bragg peak positions at $T=1.5$ K for $x=0.043$ and 0.104. (a), (b) Data from $\theta \text{−}2\theta$ scans of the $\left(111\right)$ magnetic Bragg peak for $x=0.043$ (a) and $x=0.104$ (b). Lines are fits to Gaussian line shapes. (c), (d) Data from longitudinal scans through the $\left(001\right)$ reciprocal-lattice position for $x=0.043$ (c), and through the $\left(003\right)$ position for $x=0.104$ (d). (e), (f) Data from scans through the $\left(\frac{1}{2}\frac{1}{2}1\right)$ position for $x=0.043$ (e) and 0.104 (f). Arrows indicate either the expected positions for peaks arising due to a component of the ordered moment lying in the $ab$ plane [(c), (d)] or for peaks corresponding to the stripe-type AFM order possessed by ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ [(e), (f)]. Data in (c) and (e) are for a beam monitor value corresponding to a counting time of 30 s per point, and data in (d) and (f) are for a monitor value corresponding to 180 s per point. The small peaks in (f) are ruled out as being magnetic Bragg peaks by results from complimentary measurements made above $T_{\mathrm{N}}$, or are ruled out as being due to the single-crystal sample by results from $\theta$ scans across their positions.

Figure 10

Neutron diffraction data from $\theta \text{−}2\theta$ scans through the $\left(110\right)$ [(a) and (c)] and $\left(004\right)$ [(b) and (d)] Bragg peaks for $x=0.043$ and 0.104 at the temperatures indicated. Data for $x=0.043$ are shown in (a) and (b), and data for $x=0.104$ are shown in (c) and (d).

Figure 11

Neutron diffraction data from $\theta$ scans through the $\left(111\right)$ [(a), (b)], $\left(113\right)$ [(c), (d)], $\left(\frac{1}{2}\frac{1}{2}1\right)$ [(e), (f)], and $\left(\frac{1}{2}\frac{1}{2}3\right)$ [(g), (h)] reciprocal-lattice positions for $x=0.19$ [(a), (c), (e), (g)] and $x=0.25$ [(b), (d), (f), (h)] at $T=1.5$ K. All of the data shown are for a beam monitor value corresponding to a counting time of 120 s per point.

Figure 12

Integrated intensities of the measured structural (left axis, open symbols) and magnetic (right axis, closed symbols) Bragg peaks for the $x=0$, 0.043, 0.068, and 0.104 samples plotted versus the square of their calculated structure factors. The bottom axis corresponds to the square of the chemical structure factor, whereas the top axis corresponds to the square of the magnetic structure factor calculated for A-type AFM order with an ordered moment of $\mu =1{\mu }_{\mathrm{B}}$ laying along c. The integrated intensities have been normalized to the sample mass and corrected by the Lorentz factor. Solid (dashed) lines show linear fits to the structural (magnetic) data.

Figure 13

Magnetic phase diagram and ordered magnetic moment $\mu$ for low values of $x$. Lines are fits to the data as described in the text.

Figure 14

Temperature evolution of the $a$ (a) and $c$ (b) lattice parameters, the ratio $c/a$ (c), and $V_{\mathrm{cell}}$ (d) for $x=0$, 0.18, and 0.35. Data shown are from either neutron or high-energy x-ray diffraction experiments. The uncertainty in the data is either within the symbol size or indicated by a representative error bar. Data for $x=0$ are reproduced from Ref. [21].

Figure 15

Evolution of the $a$ (a) and $c$ (b) lattice parameters, the ratio $c/a$ (c), and $V_{\mathrm{cell}}$ (d) with Fe doping concentration $x$. Filled symbols denote data at $T=300$ K and open symbols denote data at base temperature ($4\le T\le 10$ K). Data are shown from neutron diffraction, high-energy x-ray diffraction, and laboratory-based x-ray diffraction experiments.

Figure 16

Phase diagram for $\mathrm{Ca}{\left({\mathrm{Co}}_{1-x}{\mathrm{Fe}}_x\right)}_y{\mathrm{As}}_2$. Points and lines for $x\ge 0.91$ are from Fig. 4(a) in Ref. [12], which includes data from Ref. [11]. The A-type (A) and stripe-type (st) antiferromagnetic structures are illustrated with the collapsed-tetragonal (cT) and tetragonal (T) chemical unit cells, respectively, although the chemical unit cell is orthorhombic (O) for temperatures at which the stripe-type order occurs. SC labels the superconducting region.