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We present an investigation of magnetic ordering in the two-dimensional S=1/2 quantum magnet Cu(Pz)(2)(ClO4)(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 parallel to J(')/J parallel to=6.8x10(-4), indicative of excellent two-dimensional isolation. The scaled magnetic specific heat of [Cu(Pz)(2)(HF2)]BF4, a compound with an analogous structure, is very similar to that of Cu(Pz)(2)(ClO4)(2) although both differ slightly from the predicted value for an ideal 2D S=1/2 Heisenberg antiferromagnet.

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Physical Review B





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