The Two-Dimensional Square-Lattice S=1/2 Antiferromagnet Cu(pz)$_2$(ClO$_4$)$_2$

TL;DR

This study investigates the magnetic properties and quantum fluctuations of the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)$_2$(ClO$_4$)$_2$ using various experimental techniques, revealing small spin anisotropies and field-induced suppression of quantum fluctuations.

Contribution

It provides detailed experimental insights into the magnetic structure, excitations, and quantum fluctuations of Cu(pz)$_2$(ClO$_4$)$_2$, highlighting the effects of magnetic fields on quantum behavior.

Findings

Magnetic field dependence of ordering temperature shows identical phase diagrams for different orientations.

Ordered magnetic moment is significantly smaller than the single-ion moment, indicating strong quantum fluctuations.

Magnetic fields increase the ordered moment, suggesting suppression of quantum fluctuations.

Abstract

We present an experimental study of the two-dimensional S=1/2 square-lattice antiferromagnet Cu(pz)$_2$(ClO$_4$)$_2$ (pz denotes pyrazine - $C_4H_4N_2$) using specific heat measurements, neutron diffraction and cold-neutron spectroscopy. The magnetic field dependence of the magnetic ordering temperature was determined from specific heat measurements for fields perpendicular and parallel to the square-lattice planes, showing identical field-temperature phase diagrams. This suggest that spin anisotropies in Cu(pz)$_2$(ClO$_4$)$_2$ are small. The ordered antiferromagnetic structure is a collinear arrangement with the magnetic moments along either the crystallographic b- or c-axis. The estimated ordered magnetic moment at zero field is m_0=0.47(5)mu_B and thus much smaller than the available single-ion magnetic moment. This is evidence for strong quantum fluctuations in the ordered magnetic phase of Cu(pz)$_2$(ClO$_4$)$_2$. Magnetic fields applied perpendicular to the square-lattice planes lead to an increase of the antiferromagnetically ordered moment to m_0=0.93(5)mu_B at mu_0H=13.5T - evidence that magnetic fields quench quantum fluctuations. Neutron spectroscopy reveals the presence of a gapped spin excitations at the antiferromagnetic zone center, and it can be explained with a slightly anisotropic nearest neighbor exchange coupling described by J_1^{xy}=1.563(13)meV and J_1^z=0.9979(2)J_1^{xy}.