Intriguing magnetic and electronic behaviors in La and Ru doped Sr₂IrO₄

We report a detailed experimental study of the structural, magnetic and electrical properties of La and Ru doped (Sr_1-<i>x</i>La<i>_x</i>)₂Ir_1-<i>x</i>Ru<i>_x</i>O₄(<i>x</i>= 0.05, 0.15). X-ray diffraction analysis reveals that both samples crystallize in a tetragonal structure with a space group<i>I</i>4₁/<i>acd</i>without impurities. Substitution with La and Ru leads to an increase in the lattice parameter<i>a</i>and a decrease in<i>c</i>. 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⁺(5<i>d</i>⁶) and Ir⁴⁺(5<i>d</i>⁵) charge states, where the Ir⁴⁺charge state decreases with an increase in doping concentration. The<i>dc</i>magnetic susceptibility<i>χ</i>(<i>T</i>) of<i>x</i>= 0.05 reveals a transition from paramagnetic to weak ferromagnetic (wFM) at<i>T</i>_C∼ 229 K, arising from the canted antiferromagnetic (AFM) spin arrangement. The magnetic ordering temperature<i>T</i>_Cremains unaltered for higher doping, whereas the magnetic moment is significantly reduced. The analysis of real and imaginary components of<i>ac</i>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<i>x</i>= 0.05. On the other hand, for higher doping (<i>x</i>= 0.15) of hole and electron, the RSG phase vanishes, leaving only the wFM phase at the same temperature as observed in<i>x</i>= 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 (<i>ρ</i>) measurement analysis reveals that both the samples show semiconducting/insulating behavior across the temperature range. The<i>ρ</i>of the<i>x</i>= 0.05 sample is lower than that of pure sample Sr₂IrO₄(Bhatti<i>et al 2015 J. Phys.:Condens. Matter</i><b>27</b>016005), while<i>ρ</i>of<i>x</i>= 0.15 shows two orders of magnitude larger than the<i>x</i>= 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.