Ferromagnetic ordering of linearly coordinated Co ions in ${\mathrm{LiSr}}_2\left[{\mathrm{CoN}}_2\right]$

$\mathrm{Li}{\mathbf{Sr}}_2\left[{\mathrm{CoN}}_2\right]$ single crystals were successfully grown out of Li-rich flux. Temperature- and field-dependent measurements of the magnetization in the range of $T=2–300\mathrm{K}$ and up to ${\mu }_0\mathit{H}=7\mathrm{T}$ as well as measurements of the heat capacity are presented. Ferromagnetic ordering emerges below $T_C=44\mathrm{K}$ and comparatively large coercivity fields of ${\mu }_{0}H=0.3\mathrm{T}$ as well as pronounced anisotropy are observed upon cooling. Polycrystalline samples of the Ca analog $\mathrm{Li}{\mathbf{Ca}}_2\left[{\mathrm{CoN}}_2\right]$ were obtained and investigated in a similar way. In both compounds Co manifests orbital contributions to the magnetic moment and large single-ion anisotropy caused by second-order spin-orbit couplings. Quantum chemistry calculations reveal a magnetic anisotropy energy of 7 meV, twice as large as the values reported for similar Co $d^8$ systems.

Figure 1

Crystal structure of $\mathrm{Li}{AE}_2\left[{\mathrm{CoN}}_2\right]$ (AE = Sr, Ca). The effective N-Co-N linear molecules form the basic magnetic units and are oriented along $\left[110\right]$ and $\left[1\overline{1}0\right]$.

Figure 2

(a) Laue-back-reflection pattern of $\mathrm{Li}{\mathbf{Sr}}_2\left[{\mathrm{CoN}}_2\right]$. The beam was oriented parallel to the crystallographic $c$ axis and the fourfold symmetry along the tetragonal $c$ axis is clearly visible. (b) Photo of the corresponding single crystal on a millimeter grid. The sample was embedded in vacuum grease in order to prevent oxidation.

Figure 3

Magnetic susceptibility $\chi =M/H$ per mol Co as a function of temperature for an applied magnetic field of ${\mu }_0\mathit{H}=0.1\mathrm{T}$. The solid and dashed lines show the measurements on plate-like $\mathrm{Li}{\mathbf{Sr}}_2\left[{\mathrm{CoN}}_2\right]$ with field applied perpendicular and parallel to the crystallographic $c$ axis. Ferromagnetic ordering is observed at $T_{\mathrm{C}}=44\mathrm{K}$. The dotted line shows the measurement on polycrystalline $\mathrm{Li}{\mathbf{Ca}}_2\left[{\mathrm{CoN}}_2\right]$, that orders ferromagnetically at $T_{\mathrm{C}}=76\mathrm{K}$.

Figure 4

Inverse magnetic susceptibility, $1/({\chi }_{\mathrm{mol}}-{\chi }_0)\left(T\right)$, per mol Co as a function of temperature for an applied magnetic field of ${\mu }_0\mathit{H}=7\mathrm{T}$. The extracted Weiss temperatures of ${\mathrm{\Theta }}_{\mathrm{W}}=46\mathrm{K}$ and ${\mathrm{\Theta }}_{\mathrm{W}}=42\mathrm{K}$ for $\mathrm{Li}{\mathbf{Sr}}_2\left[{\mathrm{CoN}}_2\right]$ with $H\perp c$ and $H\parallel c$, respectively, are in good agreement with the $T_{\mathrm{C}}$ inferred from $\chi \left(T\right)$ in small applied fields. For polycrystalline $\mathrm{Li}{\mathbf{Ca}}_2\left[{\mathrm{CoN}}_2\right]{\mathrm{\Theta }}_{\mathrm{W}}$ amounts to 78 K.

Figure 5

Isothermal magnetization at $T$ = 2 K in $\frac{{\mu }_{\mathrm{B}}}{\mathrm{Co}}$ as a function of an applied magnetic field. The observed coercivity fields are ${\mu }_0{H}_{\mathrm{c}}=0.1\mathrm{T}$ and ${\mu }_0{H}_{\mathrm{c}}=0.3\mathrm{T}$ for $H\perp c$ and $H\parallel c$, respectively, for $\mathrm{Li}{\mathbf{Sr}}_2\left[{\mathrm{CoN}}_2\right]$ and ${\mu }_0{H}_{\mathrm{c}}=0.4\mathrm{T}$ for $\mathrm{Li}{\mathbf{Ca}}_2\left[{\mathrm{CoN}}_2\right]$.

Figure 6

Specific heat of $\mathrm{Li}{AE}_2\left[{\mathrm{CoN}}_2\right]$ without (filled circles) and with an applied magnetic field of ${\mu }_{0}H=7\mathrm{T}$ (open circles). (a) AE = Sr. The transition temperature is $T_{\mathrm{C}}=45\mathrm{K}$. (b) AE = Ca. The transition occurs at $T_{\mathrm{C}}=77\mathrm{K}$. In both materials the peak is suppressed in an applied magnetic field of ${\mu }_{0}H=7\mathrm{T}$.