W. Sano et al., ELECTRON-PARAMAGNETIC-RESONANCE SPECTRA OF NI2-DOPED ZN(BF4)2-CENTER-DOT-6H2O - TEMPERATURE AND CONCENTRATION-DEPENDENCE(), Physical review. B, Condensed matter, 50(5), 1994, pp. 2958-2962
The single-ion spin Hamiltonian parameters have been determined for zi
nc fluoroborate hexahydrate doped with Ni2+ (3d8, S = 1) and compared
with those obtained for nickel fluoroborate hexahydrate. To account fo
r the differences observed, EPR for several concentrations (x = 0.005
to 1) was studied from room to liquid-helium temperature. Between 4.2
and 1.5 K, for low doping, they are constant with g(parallel-to) = 2.2
29 +/- 0.006, g(perpendicular-to) = 2.261 +/- 0.025, and D = - (0.1291
+/- 0.0008) cm-1. Appropriate values for nickel fluoroborate are near
ly the same but they are weakly temperature dependent, attributed to a
ferromagnetic interaction. At higher temperatures these parameters ar
e remarkably concentration dependent. Two main lines arise in parallel
orientation. For concentrations up to x almost-equal-to 0.04 and at 7
7 K, they show asymmetrically distributed satellite lines outside the
more intense central portion. The satellite lines seem to increase wit
h concentration but only those assigned to a ferromagnetic-coupled fir
st-neighbor pair survive the simultaneous broadening effect. As the br
oadening is asymmetric, the lines' centers shift smoothly outwards. At
x almost-equal-to 0.7, we observe maximum broadening; for larger x no
structural detail is seen and the spectra are progressively exchange
narrowed. At the same time, the two lines are drawn together. The line
width dependence on concentration indicates that the Ni ions have rand
omly occupied the Zn sites as expected, and that the contribution from
spin-lattice relaxation practically does not change with concentratio
n. We conclude that the differences in the spin Hamiltonian parameters
are due essentially to magnetic interactions throughout the whole tem
perature range.