Magnetic ground state of the $M_{n+1}A{X}_n$-phase nitride Cr${}_2$GaN

Aiming to find magnetic states in the $M_{n+1}A{X}_n$ phases, we measured the magnetic susceptibility, the resistivity, and nuclear magnetic resonance of Cr${}_2$Ga$X$ ($X=\text{N}$ and C). Zero-field and longitudinal-field muon spin relaxation experiments were also applied to Cr${}_2$GaN. A magnetic phase transition, most probably to the spin-density-wave (SDW) state, has been found in Cr${}_2$GaN at $T_{\mathrm{N}}=170$ K in contrast to the Pauli paramagnetism in the carbide counterpart Cr${}_2$GaC. The origin of the SDW transition is interpreted in terms of possible Fermi-surface nesting in the two-dimensional-like electronic structure.

Figure 1

Crystal structure of $M_{2}AX$ (Cr${}_2$AlC type, $3\times 3\times 1$ unit cells), where $M_{6}X$ nets are interleaved with $A$-element layers.

Figure 2

Temperature dependences of the susceptibility for Cr${}_2$GaN and Cr${}_2$GaC measured under a field of 1 T. The arrow indicates the anomaly corresponding to $T_{\mathrm{N}}$.

Figure 3

Temperature dependences of the resistivity for Cr${}_2$GaN and Cr${}_2$GaC. The arrow indicates the temperature at which the susceptibility shows the anomaly.

Figure 4

Field-swept ${}^{69}$Ga and ${}^{71}$Ga NMR spectra for Cr${}_2$GaN and Cr${}_2$GaC measured at a frequency of 38.5 MHz and at 4.2 K. Arrows indicate zero shift positions for ${}^{69}$Ga and ${}^{71}$Ga. Simulated paramagnetic powder-pattern spectra are also presented for comparison. Nuclear quadrupole frequencies, 25.1 and 15.8 MHz, were assumed for ${}^{69}$Ga and ${}^{71}$Ga, respectively. The hyperfine shift was neglected for simplicity.

Figure 5

Typical examples of ZF-$\mu$SR spectra (at 22.3, 150, 160, 200, and 300 K) of Cr${}_2$GaN. Solid curves indicate the fit by the damped Kubo-Toyabe function.

Figure 6

Temperature dependence of the initial asymmetry of the ZF-$\mu$SR spectra for Cr${}_2$GaN.

Figure 7

Temperature dependence of the muon-spin relaxation rate $\lambda$ in Cr${}_2$GaN. The solid curve is the guide for eyes. Inset shows the temperature dependence of the stretched exponent $\beta$.

Figure 8

Typical LF-$\mu$SR spectra of Cr${}_2$GaN measured at 25 K. Solid curves are the best fits to the stretched exponential function.

Figure 9

Longitudinal field dependence of muon spin polarization for Cr${}_2$GaN at 25 K. The dotted and the solid curves represent the best fit to Eq. (3) with $B_{\mathrm{loc}}=1.2$ kOe and the calculation assuming Eq. (5) with $B_{\min }=0.1$ and $B_{\max }=2$ kOe, respectively.

Figure 10

Total and partial density of states of spin-unpolarized Cr${}_2$GaN (left) and Cr${}_2$GaC(right).

Figure 11

Cross sections of Fermi surfaces consist of the 21st band for spin-unpolarized Cr${}_2$GaN (top) and Cr${}_2$GaC (bottom). The arrow indicates one of possible nesting vectors, $(0.56,0,0)$, in the hexagonal Brillouin zone.