Magnetism behavior of $T^{\prime}$-type Eu$_2$CuO$_4$ revealed by muon spin rotation/relaxation measurements

TL;DR

This study uses muon spin rotation/relaxation to explore the magnetic phases of Eu$_2$CuO$_4$, revealing how oxygen-reduction annealing influences magnetic order and fluctuations in this parent compound of electron-doped cuprates.

Contribution

It provides detailed insights into the magnetic ordering sequence and the effects of annealing on magnetism in $T^{ extprime}$-type Eu$_2$CuO$_4$, highlighting the role of defects and doping.

Findings

Development of magnetic correlations below 265 K.

Coexistence of fluctuating and partially ordered spins in 150-265 K.

Long-range magnetic order established below 110 K.

Abstract

We performed muon spin rotation/relaxation measurements to investigate the magnetic behavior of $T^{\prime}$-type Eu$_2$CuO$_4$ (ECO), which is the parent compound of electron-doped cuprate superconductors, and the effects of oxygen-reduction annealing on its magnetism. In as-sintered (AS) ECO, we clarified the development of magnetic correlations upon cooling below $T_\mathrm{N1}$ (= 265 K) as well as the coexistence of a dominant fluctuating spin state and partially ordered spin state in the temperature range between $\sim$150 K and $T_\mathrm{N1}$. Upon further cooling, uniform long-range magnetic order was observed below $T_\mathrm{N2} = 110$ K, which is close to the ordering temperature of 115 K in $T^{\prime}$-type La$_2$CuO$_4$ (LCO) [Phys. Rev. B {\bf 82}, 180508(R) (2010)]. For oxygen-reduction-annealed ECO, a similar ordering sequence with the same $T_\mathrm{N2}$ was observed but without the partially ordered spin state. Therefore, the fluctuating spin state over a wide temperature range and a $T_\mathrm{N2}$ less than the N{\'e}el temperature ($T_\mathrm{N2} \approx T_\mathrm{N1}$) in $T$-type LCO are common features of the $T^{\prime}$-type parent $R_2$CuO$_4$ ($R$CO, $R$: rare-earth ion). The origin of the partially ordered spin state in AS ECO is discussed from the viewpoint of chemical defect. Furthermore, we discuss the roles of electron doping and repairing defect in the observed effect of annealing on the magnetism of $T^{\prime}$-type $R$CO.