Magnetic resonance in iron-doped magnesium oxide

A linewidth analysis of the ½ ↔ -½ transition of Fe(^3+) in Magnesium Oxide crystals has been made using electron spin resonance techniques at x-band and at room and nitrogen temperatures. The values of linewidth at 6=0 (i.e. magnetic field along fourfold axis of the crystal) lay between 7 and 9 Gauss for specimens containing from 140 to 11900 p.p.m. Fe. The experimental linewidths measured for a range of samples show that the linewidth is concentration independent, as opposed to the (concentration)(^½) dependence expected from Van Vleck's second moment theory. The discrepancy is explained on the grounds of exchange narrowing interaction between the ferric ions and the idea is supported by the values of the ratio of the fourth to second moments obtained by numerical integration of the absorption3+derivative line. The linewidth data indicates that the Fe(^3+) enters the lattice substitutionally, occupying Mg sites, at concentrations of up to11,900 p.p.m. Fe. Preliminary spin-lattice relaxation time measurements have also been made using the pulse-saturation method at 35.5 GHz over the temperature range 4.2 - 27 K. Experimental evidence is presented for a fast relaxing process taking place in all the samples studied; this is explained in terms of spin exchange-interaction (Fe(^3+) –Fe(^3+) by correlation with the proposed linewidth model. It also shows angular and concentration dependence and it is suggested that cross-relaxation between different parts of the spin system may account for variations observed. As regards the concentration dependence it is tentatively proposed that the spin-lattice relaxation time obeys a (concentration) (^-1) law and comparison is made with results published for other ions in the same host lattice. Using 9 GHz results available in the literature it is shown that Mattuck and Strandberg's (resonance frequency)(^-2) dependence for T(_1) holds for Fe(^3+) in magnesia; furthermore there is approximate agreement of the experimental points with T(^-1)(direct) and T(^-7) (Raman) lines for the temperature variation which suggests that the slower relaxation process proceeds as for a single ion relaxing to the lattice