UDC 538.69.083.2:539.124

PHYSICS

Academician of the Academy of Sciences of the Ukrainian SSR A. A. Galkin, A. D. Prokhorov,
G. A. Tsintsadze, V. A. Shapovalov

ISOTOPIC HYPERFINE STRUCTURE OF THE EPR SPECTRUM OF $\mathrm{Cu}^{2+}$ IN $\mathrm{ZnWO}_4$

In a single crystal of $\mathrm{ZnWO}_4$ doped with copper (0.1 wt.%), a sharply resolved hyperfine structure of the EPR spectrum ($^1$) was observed, caused by the interaction of the electron spin of the $\mathrm{Cu}^{2+}$ ion ($S = 1/2$) with the nuclear spins of two copper isotopes, $\mathrm{Cu}^{63}$ ($I = 3/2$) and $\mathrm{Cu}^{65}$ ($I = 3/2$), only at helium temperature. The spectrum consists of two groups of quartet lines, whose intensities ($I^{65}/I^{63}$) are in the ratio $3:5$.

Two Cu ions substituting for Zn in the unit cell are magnetically equivalent. A study of the angular dependence of the spectrum showed that the $\mathrm{Cu}^{2+}$ ions are acted upon by an intracrystalline field whose symmetry is no higher than rhombic. The corresponding spin Hamiltonian ($^2$) in the principal $z$ orientation has the form:

$$ \mathcal{H} = g_z \beta H_z S_z + A S_z I_z . $$

Measurements of the spectrum at frequencies of $\sim 10$ and 40 GHz at a temperature of 4.2°K showed that:

$$ \begin{aligned} g_z &= 2.0023 \pm 0.005,\\ A_z^{63} &= 228.7 \pm 0.2\ \text{MHz},\\ A_z^{65} &= 243.0 \pm 0.2\ \text{MHz} \end{aligned} \qquad \left\} \begin{array}{l} \text{at a frequency of } 40\ \text{GHz} \end{array} \right. $$

$$ \mu^{65}/\mu^{63} = A_z^{65}/A_z^{63} = 1.063 $$

$$ \begin{aligned} g_z &= 2.0019 \pm 0.002,\\ A^{63} &= 227.6 \pm 0.2\ \text{MHz},\\ A^{65} &= 247.4 \pm 0.2\ \text{MHz} \end{aligned} \qquad \left\} \begin{array}{l} \text{at a frequency of } 10\ \text{GHz} \end{array} \right. $$

The isotopic hyperfine structure at a temperature of 77°K disappears because of line broadening, while the constants measured at a frequency of 10 GHz at 77°K change to the values:

$$ g_z = 2.014 \pm 0.005;\qquad A_z = 218.2 \pm 0.2\ \text{MHz}. $$

Such a change apparently indicates that the intracrystalline field varies owing to a change in the volume of the crystal at different temperatures.

The authors express their deep gratitude to M. M. Zaripov and V. G. Stepanov for valuable assistance and discussions.

Donetsk Physicotechnical Institute

Received
25 VI 1966

REFERENCES

1. Z. Šroubek, K. Zdansky, E. Simanek, Phys. Stat. Sol., 12, K149 (1964).
2. M. M. Zaripov, L. Ya. Shekun, in: Paramagnetic Resonance, 1964, p. 9.