Intriguing magnetic and electronic behaviors in La and Ru doped Sr<sub>2</sub>IrO<sub>4</sub>

Abstract

We report a detailed experimental study of the structural, magnetic and electrical properties of La and Ru doped (Sr1-x Lax)2Ir1-xRuxO4 (x= 0.05, 0.15). X-ray diffraction analysis reveals that both samples crystallize in a tetragonal structure with a space group I41/acd without impurities. Substitution with La and Ru leads to an increase in the lattice parameter a and a decrease in c. With increasing doping concentration, the Ir-O-Ir bond angle increases while the Ir-O bond length decreases. X-ray photoelectron spectroscopy (XPS) shows that Ir has Ir3+ (5d6) and Ir4+ (5d5) charge states, where the Ir4+ charge state decreases with an increase in doping concentration. The dc magnetic susceptibility chi (T) of x= 0.05 reveals a transition from paramagnetic to weak ferromagnetic (wFM) at TC similar to 229 K, arising from the canted antiferromagnetic (AFM) spin arrangement. The magnetic ordering temperature TC remains unaltered for higher doping, whereas the magnetic moment is significantly reduced. The analysis of real and imaginary components of ac susceptibility data, based on conventional critical slow model, frequency shift per decade and Vogel-Fulcher law, unanimously evidences the existence of reentrant spin-glass behavior (RSG), i.e. the coexistence of weak ferromagnetism and spin glass phases for the lowest doping of x= 0.05. On the other hand, for higher doping (x = 0.15) of hole and electron, the RSG phase vanishes, leaving only the wFM phase at the same temperature as observed in x = 0.05. This suggests the higher doping of La and Ru does not affect the magnetic order, but removes the disorder between FM and AFM phases. The electrical resistivity (rho) measurement analysis reveals that both the samples show semiconducting/insulating behavior across the temperature range. The rho of the x= 0.05 sample is lower than that of pure sample Sr2IrO4 (Bhatti et al 2015 J. Phys.:Condens. Matter 27 016005), while rho of x = 0.15 shows two orders of magnitude larger than the x = 0.05 sample at low temperatures. The conduction mechanism of both samples is described by the 2D Mott's variable-range hopping model. Our results demonstrate that co-doping of two cation sites generates intriguing, competing hopping and magnetic processes.