Frustration of square cupola in Sr(TiO)Cu$_{4}$(PO$_{4}$)$_{4}$

TL;DR

This study investigates the structural and magnetic properties of Sr(TiO)Cu$_{4}$(PO$_{4}$)$_{4}$, revealing strong magnetic frustration and antiferromagnetic order driven by in-plane anisotropy and next-nearest-neighbor interactions.

Contribution

The paper provides a comprehensive experimental and theoretical analysis of the frustrated square cupola antiferromagnet Sr(TiO)Cu$_{4}$(PO$_{4}$)$_{4}$, highlighting its magnetic anisotropy and frustration mechanisms.

Findings

Magnetic long-range order occurs below 6.2 K.

High-field magnetization shows a 1/3 saturation kink.

Strong in-plane magnetic anisotropy and frustration are confirmed.

Abstract

The structural and magnetic properties of the square-cupola antiferromagnet Sr(TiO)Cu$_{4}$(PO$_{4}$)$_{4}$ are investigated via x-ray diffraction, magnetization, heat capacity, and $^{31}$P nuclear magnetic resonance experiments on polycrystalline samples, as well as density-functional band-structure calculations. The temperature-dependent unit cell volume could be described well using the Debye approximation with the Debye temperature of $\theta_{\rm D} \simeq $ 550~K. Magnetic response reveals a pronounced two-dimensionality with a magnetic long-range-order below $T_{\rm N} \simeq 6.2$~K. High-field magnetization exhibits a kink at $1/3$ of the saturation magnetization. Asymmetric $^{31}$P NMR spectra clearly suggest strong in-plane anisotropy in the magnetic susceptibility, as anticipated from the crystal structure. From the $^{31}$P NMR shift vs bulk susceptibility plot, the isotropic and axial parts of the hyperfine coupling between $^{31}$P nuclei and the Cu$^{2+}$ spins are calculated to be $A_{\rm hf}^{\rm iso} \simeq 6539$ and $A_{\rm hf}^{\rm ax} \simeq 952$~Oe/$\mu_{\rm B}$, respectively. The low-temperature and low-field $^{31}$P NMR spectra indicate a commensurate antiferromagnetic ordering. Frustrated nature of the compound is inferred from the temperature-dependent $^{31}$P NMR spin-lattice relaxation rate and confirmed by our microscopic analysis that reveals strong frustration of the square cupola by next-nearest-neighbor exchange couplings.