RKKY Ferromagnetism with Ising-Like Spin States in Intercalated ${\mathrm{Fe}}_{1/4}{\mathrm{TaS}}_2$

We investigated the magnetic nature of ${\mathrm{Fe}}_{1/4}{\mathrm{TaS}}_2$ using x-ray absorption spectroscopy, photoemission spectroscopy, and first principles band calculations. The results show a large unquenched orbital magnetic moment ($\sim 1.0{\mu }_B/\mathrm{Fe}$) at intercalated Fe sites, resulting in a gigantic magnetic anisotropy ($H_A\simeq 60\mathrm{T}$). The magnetic coupling is well understood in terms of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, suggesting a novel RKKY ferromagnet with Ising-type spin states. We also found that this indirect exchange coupling between the neighboring Fe spins is ferromagnetic and maximized at the Fe-Fe distance of $2\times 2$ superstructure.

Figure 1

(a) Crystal structure of ${\mathrm{Fe}}_{1/4}{\mathrm{TaS}}_2$. The lower panel shows the $2\times 2$ superstructure formed by the intercalated Fe. (b) Field cooled (FC) and zero field cooled (ZFC) $M$-T curves under $H=0.2\mathrm{T}$ along the $c$ axis. The in-plane ($H\parallel a$) and out-of-plane ($H\parallel c$) $M\mathrm{\text{−}}H$ hysteresis curves at 2 K are presented in the inset.

Figure 2

(a) Fe $L$-edge XMCD results of ${\mathrm{Fe}}_{1/4}{\mathrm{TaS}}_2$ at 80 K. $p$ and $q$ correspond to the integration values over the $L_3$ and $L_{2,3}$ regions of the MCD spectrum $\Delta \rho$, respectively. (b) The polarization dependent XAS spectra (top) compared with the cluster model calculation results (bottom). The thick arrows point the transition to the in-plane orbitals. (c) Schematic diagrams of the orbital level splittings by the crystal field (left) and the spin-orbit coupling (right). The large unquenched $m_o$ resides in the lowest state ($d_2$, $d_{-1}$) split from the low lying $e_g^{\pi }$ orbital.

Figure 3

(a)–(c) Fe $3d$ PDOS for majority (red $\uparrow$) and minority (blue $\downarrow$) spins from $\mathrm{GGA}+U+\mathrm{SOC}$. The occupied $d_{yz}/d_{zx}$ and $d_{xy}/d_{x^2\mathrm{\text{−}}{y}^2}$ correspond to the atomic $d_{-1}$ and $d_2$, respectively. (d)–(e) Ta $5d_{3z^2\mathrm{\text{−}}{r}^2}$ PDOS of Ta1 and Ta2 sites (see text). (f) Real space spin density in the (100) slice possessing the intercalated Fe, which visualizes the AFM coupling between Fe and Ta spin moments.

Figure 4

(a) ARPES intensity map at $E_F$ of ${\mathrm{Fe}}_{1/4}{\mathrm{TaS}}_2$, which shows hole pockets around $\Gamma$ and $K$-points in the 1st Brillouin zone (bold solid). (b) ARPES results along $\overline{\Gamma M}$ compared with the theoretical band dispersion (solid lines) of 2H-${\mathrm{TaS}}_2$ with -0.2 eV shift (1.4 electrons/Fe doping). (c) Real space RKKY oscillation curve in the (001) slice as in Fig. 3f with $k_F=0.53{\AA }^{-1}$.