Low-energy magnetic excitations in the spin-orbital Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$

We report a high-field electron spin resonance study in the sub-THz frequency domain of a single crystal of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ that has been recently proposed as a prototypical spin-orbital Mott insulator. In the antiferromagnetically ordered state with noncollinear spin structure that occurs in this material at $T_{\mathrm{N}}$ $\approx$ 240 K we observe both the “low” frequency mode due to the precession of weak ferromagnetic moments arising from a spin canting, and the “high” frequency modes due to the precession of the antiferromagnetic (AFM) sublattices. Surprisingly, the energy gap for the AFM excitations appears to be very small, amounting to only 0.83 meV. This suggests a rather isotropic Heisenberg dynamics of interacting Ir${}^{4+}$ effective spins despite the spin-orbital entanglement in the ground state.

Figure 1

Left: crystallographic unit cell of ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. Right: the ordering pattern of effective Ir spins $J_{\mathrm{eff}}=1/2$ in the $ab$ crystallographic plane [3]. The spin in the center and its four neighbors belong to the two AFM sublattices $A$ and $B$, respectively, canted by an angle $\varphi$.

Figure 2

(a) FMR signal at $\nu =9.5$ GHz at selected temperatures below $T_{\mathrm{N}}$ for the in-plane orientation of the magnetic field; (b) orientational dependence of the FMR resonance field at $\nu =9.5$ GHz and $T=220$ K (circles). Solid line shows $1/sin\theta$ fit (see text).

Figure 3

High-frequency AFMR mode for the in-plane orientation of the magnetic field. Upper panel: Characteristic frequency dependence at $T=180$ K. The $\nu \left(H\right)$ AFMR resonance branch shown in the inset reveals a gap $\Delta \approx 150$ GHz. Bottom panel: Selected spectra at several temperatures for a fixed frequency $\nu =474$ GHz. Dashed lines are the Lorentzian fits.

Figure 4

$\nu \left(H\right)$ diagram of the AFMR excitations in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ at $T=4$ K together with characteristic spectra for two directions of the magnetic field [symbols: experimental data points; solid lines: result of the modeling according to Eqs. (2) and (3)]. Thick dashed lines show the corresponding FMR modes. The symbol ($☆$) represents the FMR mode at 9.5 GHz for $\mathbf{H}\parallel \left[110\right]$ (see the text).