Magnetic order in the quasi-one-dimensional spin-1/2 molecular chain compound copper pyrazine dinitrate

We present evidence of magnetic order in the quasi-one-dimensional spin-1/2 molecular chain compound, copper pyrazine dinitrate Cu(C4H4N2)(NO3)2. Zero field muon-spin relaxation measurements made at dilution refrigerator temperatures show oscillations in the measured asymmetry characteristic of a quasistatic magnetic field at the muon sites. Our measurements provide convincing evidence for long-range magnetic order below a temperature TN=107(1) mK. This leads to an estimate of the interchain coupling constant of │J'│ lkB =0.046 K and to a ratio of │J'lJ│=4.4x10-3.