${\text{CaMn}}_2{\text{Al}}_{10}$: Itinerant Mn magnetism on the verge of magnetic order

We report the discovery of ${\text{CaMn}}_2{\text{Al}}_{10}$, a metal with strong magnetic anisotropy and moderate electronic correlations. Magnetization measurements find a Curie-Weiss moment of $0.83{\mu }_{\mathrm{B}}/\mathrm{Mn}$, significantly reduced from the Hund's rule value, and the magnetic entropy obtained from specific heat measurements is correspondingly small, only $\approx 9\%$ of $R\ln 2$. These results imply that the Mn magnetism is highly itinerant, a conclusion supported by density functional theory calculations that find strong Mn-Al hybridization. Consistent with the layered nature of the crystal structure, the magnetic susceptibility $\chi$ is anisotropic below 20 K, with a maximum ratio of ${\chi }_{\left[010\right]}/{\chi }_{\left[001\right]}\approx 3.5$. A strong power-law divergence $\chi \left(T\right)\sim T^{-1.2}$ below 20 K implies incipient ferromagnetic order with a low Curie temperature $T_C<2\mathrm{K}$. Our experiments indicate that ${\text{CaMn}}_2{\text{Al}}_{10}$ is a rare example of a system where the weak and itinerant Mn-based magnetism is poised on the verge of order.

Figure 1

(a) A simplified picture of the crystal structure of ${\mathrm{CaMn}}_2{\mathrm{Al}}_{10}$ that shows only the Mn lattice. Square plaquettes of Mn atoms (red) form in two planes (red and blue) separated by a distance of $c/2$ along the [001] direction. (b) The coordination polyhedron of Mn (red) with Al (teal) and Ca (gray) as indicated.

Figure 2

(a) The $T$ dependence of ${(\chi -{\chi }_0)}^{-1}$ measured with a 1000 Oe dc field $\mathbf{H}\parallel \mathbf{c}$ (red filled circles) and $\mathbf{H}\perp \mathbf{c}$ (blue open squares). The solid black line is a fit to the CW law. (b) The $T$ dependence of ${\chi }^{\prime }$ with a 4.17 Oe ac field and a dc field $\mathbf{H}\parallel \mathbf{c}$ of 0 T (red filled circles) and also $\mathbf{H}\perp \mathbf{c}$ of 0 T (blue open squares) and 0.5 T (blue filled squares). The inset shows a close-up of the maximum in the $\mathbf{H}\perp \mathbf{c}=0$ T data. The solid lines serve as guides for the eye. (c) The temperature dependence of the imaginary part of the ac magnetic susceptibility ${\chi }^{\prime \prime }$; colors as in (b). (d) The $H$ dependence of the dc magnetization $M$; colors as indicated. The solid lines are guides for the eye.

Figure 3

(a) The temperature dependence of the specific heat $C$. The lines correspond to fits to the Debye model with ${\gamma }_{HT}=40$ mJ/mol ${\mathrm{K}}^2$ (orange solid) and ${\gamma }_{LT}=5$ mJ/mol ${\mathrm{K}}^2$ (blue dotted) as described in the text. (b) Plots of $C/T$ (open black circles) and $C_M/T$ (filled black circles; see text) versus $T$. The nuclear Schottky contribution $C_N$ (green dashed line) and Debye contributions with ${\gamma }_{HT}$ (orange solid line) and ${\gamma }_{LT}$ (blue dotted line) are overplotted. The entropy $S$ (see text) is shaded gray. (c) The temperature dependence of the resistivity $\rho$, with current flowing along the $c$ axis. (d) A semilogarithmic plot of $\rho \left(T\right)$.

Figure 4

(a) The temperature dependence of the real part of the ac susceptibility ${\chi }^{\prime }$ (blue open squares). The solid red line is a fit to ${\chi }^{\prime }\sim {(T-T_C)}^{-\gamma }$ as described in the text. The dashed black line corresponds to the CW fit in Fig. 2. (b) The mean square deviation ${\chi }^2$ for fit parameters $\gamma$ and $T_C$.

Figure 5

(a) The total (black), Mn (red), Al (teal), and Ca (gray) density of states $N\left(E\right)$ as indicated. (b) A Rhodes-Wohlfarth plot of ${\mu }_{CW}/{\mu }_S$ versus $T_C$ for FM materials, with Mn-based compounds shown in red. The placement of ${\text{CaMn}}_2{\text{Al}}_{10}$ on the dashed line (as a guide for the eye) is described in the text. Data from [10, 11, 17, 19, 25, 26, 27, 31].