Magnetic structure in square cupola compound Ba(TiO)Cu₄(PO₄)₄: ³¹P NMR Study

The magnetic structure of the antiferromagnetic square cupola compound Ba(TiO)Cu₄(PO₄)₄ single crystal is studied with ³¹P nuclear magnetic resonance techniques. The magnetic hyperfine shift K shows a clear splitting at the Neel temperature T_N = 9.5 K, where the resonance splits into 2 lines when the external field is oriented along c-axis and into 4 lines when the field is along a-axis. In paramagnetic region K(T) follows temperature dependence of the magnetic susceptibility χ(T). From K vs χ plot we determined hyperfine field values H^a_hf = 765 mT/μ_B and H^c_hf = 740 mT/μ_B for magnetic field oriented along a- and c-axis, respectively. From the rotation of the single crystal in external magnetic field we determined 8 different orientations of K-tensor in paramagnetic region. In antiferromagnetic state at T = 6 K we found that the local field at phosphorus is mainly due to dipolar field of coppers. Here the rotation of single crystal shows 8 different orientations of the local field B_int = 38 ± 2 mT. Experimental orientation of B_int corresponds to the calculation of dipolar fields at phosphorus assuming magnetic quadrupolar configuration of magnetic moments Γ₃(1).