Experimental determination of the sign of the nuclear quadrupole interaction of ¹⁶⁵Ho is ferromagnetic gadolinium

The seven-line quadrupole-split NMR spectrum of ¹⁶⁵Ho in the ferromagnetic alloy Ho_0.01Gd_0.99 has been studied by the spin-echo method at temperatures between 0.5 and 4.2K. The temperature dependence of the echo amplitudes from the two outermost lines shows that the total quadrupole interaction of the nucleus with its environment is positive. Re-analysis of earlier NMR measurements on ¹⁶⁵Ho in terbium- and thulium-doped gadolinium alloys, using more recent data on the nuclear quadrupole moment gives V_cc=+(5.3+or-0.2)*10¹⁹Vm^-2 for the contribution of the host medium to the axial component of the electric field gradient (EFG) at the holmium site. This figure indicates that the conduction electron contribution to the EFG is nearly three times larger than the contribution of the tripositive ions. Similar conclusions are derived from a re-analysis of NMR data on ¹⁶⁵Ho in orthorhombic terbium- and dysprosium-rich alloys at liquid helium temperatures.