Magnetic order in the S=1/2 two-dimensional molecular antiferromagnet, copper pyrazine perchlorate Cu(Pz)_2(ClO_4)_2

TL;DR

This study investigates magnetic ordering in a 2D S=1/2 quantum magnet, revealing a transition to 3D long-range order at 4.21 K despite no specific heat anomaly, highlighting the material's excellent two-dimensional isolation.

Contribution

First detailed experimental evidence of 3D magnetic order in Cu(Pz)_2(ClO_4)_2 using muon-spin relaxation, confirming theoretical predictions about 2D quantum magnets.

Findings

Transition to 3D long-range order at 4.21 K

Absence of specific heat anomaly at T_N

High ratio of interlayer to intralayer exchange constants

Abstract

We present an investigation of magnetic ordering in the two-dimensional S=1/2 quantum magnet Cu(Pz)_2(ClO_4)_2 using specific heat and zero field muon-spin relaxation (\mu^+SR). The magnetic contribution to the specific heat is consistent with an exchange strength of 17.7(3) K. We find unambiguous evidence for a transition to a state of three-dimensional long range order below a critical temperature T_N=4.21(1) K using \mu^+SR even though there is no feature in the specific heat at that temperature. The absence of a specific heat anomaly at T_N is consistent with recent theoretical predictions. The ratio of T_N/J=0.24 corresponds to a ratio of intralayer to interlayer exchange constants of |J'/J|=6.8x10^-4, indicative of excellent two-dimensional isolation. The scaled magnetic specific heat of [Cu(Pz)_2(HF_2)]BF_4, a compound with an analogous structure, is essentially identical to that of Cu(Pz)_2(ClO_4)_2 although both differ slightly from the predicted value for an ideal 2D S=1/2 Heisenberg antiferromagnet.