Annealing and doping effects on magnetism for T ^* -type (La, Eu, Sr) _2 CuO _4-y F _y cuprates

The magnetic behavior of T ^* -phase cuprates in the underdoped region has been studied through zero-field muon spin rotation/relaxation ( μ SR) measurements. By examining the manner in which the superconducting transition temperature changes with increased fluorine substitution, we demonstrate the possibility of achieving an underdoped region in T ^* -La _1-x/2 Eu _1-x/2 Sr _x CuO _4-y F _y (LESCOF). For as-grown (AS) samples, the onset temperature for development of spin correlation is determined by μ SR measurement. We observed a substantial enhancement from ∼ 7 K (x = 0.18, y = 0) to ∼ 80 K (x = 0.18, y = 0.15) with an appearance of a magnetic order below ∼ 55 K. Thus, the magnetic order can be stabilized by underdoping in AS T ^* -LESCOF, similar to T-phase La _2-x Sr _x CuO _4 . However, the damping of spin precession in the μ SR time spectra appears to be significant than in La _2 CuO _4 (LCO), suggesting the presence of inhomogeneous magnetism. In addition, the unrecovered normalized time spectrum to 1/3 along the time axis indicates a dynamical nature of magnetism. These magnetic behavior disappears with oxidation annealing, even in the lightly hole-doped sample. The absence of static magnetism in the oxidation-annealed T ^* -LESCOF could be attributed to the structural effect, as the physical properties of the T ^' -phase cuprates has been discussed in terms of oxygen coordination around the Cu ion.