Magnetic phase diagram ofY2CuO4: Weak ferromagnetism and metamagnetic transition

Abstract

We have studied the magnetic properties of ${\mathrm{Y}}_2$ ${\mathrm{CuO}}_4$, synthesized at high pressures. This cuprate crystallizes in the ${\mathrm{Nd}}_2$ ${\mathrm{CuO}}_4$-type structure (T’), characteristic of the electron-doped superconductors. Since the Y ions are nonmagnetic, this compound is very suitable to study the magnetism of the Cu lattice, without any interference from the rare earths. We have measured dc magnetization vs temperature in several magnetic fields up to 50 kOe, both after field cooling (FC) and zero-field cooling (ZFC) the samples, and we have made isothermal magnetization measurements vs field, from 5 to 340 K. We have also performed a detailed study of the ac-susceptibility dependence on the dc field, the ac field, and the frequency. All the measurements indicate a three-dimensional antiferromagnetic (AF) ordering of the Cu lattice below ${\mathit{T}}_{\mathit{N}}$=257(1) K, with the Cu spins slightly canted away from perfect AF alignment. This canting produces a weak ferromagnetic (WF) component for each ${\mathrm{CuO}}_2$ plane. However, at zero field, the AF coupling between different planes makes that the WF components remain almost compensated, and only after a metamagnetic transition can the weak ferromagnetic behavior be evidenced. Above ${\mathit{T}}_{\mathit{N}}$, the WF component is still induced by the field in a large temperature interval. Below ${\mathit{T}}_{\mathit{N}}$, an irreversibility line has been determined from hysteresis loops and magnetization vs temperature measurements in FC and ZFC conditions. It follows a de Almeida–Thouless law ${\mathit{H}}_{\mathrm{irr}}$∝(${\mathit{T}}_{\mathit{N}}$-${\mathit{T}}_{\mathrm{irr}}$ ${)}^{3/2}$. Below the irreversibility line, typical logarithmic relaxation processes of the ZFC magnetization are detected. Finally, an activated dynamic scaling describes the frequency dependence of the ac-susceptibility peak found below ${\mathit{T}}_{\mathit{N}}$ and associated with magnetic freezing processes. All these features are characteristic of the existence of finite-size weak ferromagnetic clusters.