Slow magnetic fluctuations and critical slowing down in Sr2Ir1−xRhxO4

Abstract

Hidden magnetic order in the correlated iridate Sr 2 Ir 1−x Rh x O 4 , x = 0.05 and 0.1, has been studied using muon spin relaxation spectroscopy. In zero field (ZF) and weak longitudinal fields (LFs) ( 2 mT), the muon spin relaxation data indicate that static and dynamic local fields coexist at each muon site, and can be well described by exponentially damped static Lorentzian Kubo-Toyabe functions. The ZF relaxation rate is dominated by the static-field distribution, and a broad relaxation rate maximum at 175 K for x = 0.1 in ZF is attributed to muon diffusion and trapping. For LF 2 mT the static rate is completely decoupled, and the exponential decay is due to dynamic spin fluctuations. The temperature dependencies of the relaxation rates exhibit maxima at 215 K (x = 0.05) and 175 K (x = 0.1), in agreement with previous second harmonic generation and polarized neutron diffraction determinations of transition temperatures to a hidden-order state. The maxima are most likely due to critical slowing down of electronic spin fluctuations. The field dependencies of the dynamic spin fluctuation rates can be well described by the Redfield relation, from which the rms width B rms loc and correlation time τ c of the fluctuating field are obtained. Values of τ c are in the range of 1.5-4 ns for x = 0.1 and shorter than 2 ns for x = 0.05, suggesting an increase with increasing Rh concentration. Values of B rms loc are on the order of 1 mT, consistent with the polarized neutron diffraction cross section.

Spin-orbit-enhanced magnetic surface second-harmonic generation in Sr2IrO4

“…Here we report field-dependent SHG rotational anisotropy (RA), magnetic susceptibility, as well as overlapping atomic force and SHG microscopy measurements on parent Sr 2 IrO 4 . While we do not rule out the presence of hidden order in Sr 2 IrO 4 [8][9][10], our data show that the C 1 SHG signal arises from an unexpectedly strong surface-magnetization-induced ED process in the − + +− state, rather than from laser-induced or intrinsic hidden orders.…”

“…Recent polarized neutron diffraction [8] and torque magnetometry [10] measurements have suggested the presence of a bulk magnetoelectric loop-current order in Sr 2 IrO 4 . There is also evidence that it survives and onsets above T N in Rhdoped samples [7][8][9][10]. However, these works point toward a magnetoelectric loop-current order that is ferroically stacked along the c axis, which is incompatible with our observation of an SHG signal that switches sign every two layers.…”

“…Recently, optical second-harmonic generation (SHG) experiments reported evidence of unexpected broken spatial symmetries in Sr 2 IrO 4 [7], suggestive of a hidden order. However, both the existence and the microscopic origin of a hidden order in Sr 2 IrO 4 remain under intensive experimental [8][9][10] and theoretical investigation [11][12][13][14][15][16].…”

Muon spin relaxation and fluctuating magnetism in the pseudogap phase of YBa2Cu3Oy

“…It is intriguing that similar unusual magnetic fluctuations have been observed via the Redfield field dependence of μ + dynamic relaxation in the "hidden-order" phase of Sr 2 Ir 1−x Rh x O 4 [58]. The hole-doped cuprates and the Rhdoped iridates share similar crystal symmetry, and there is a similarity in electronic structure and magnetic order geometry as well [18,65].…”

“…For more general fluctuation spectra, λ L (H L ) decreases with increasing H L as the μ + Zeeman frequency passes through a high-frequency cutoff. The Redfield relation has been widely applied in μSR to characterize dynamic fluctuating magnetic fields in strongly correlated electron systems [56][57][58].…”