Half-field and normal electron spin resonance study of the quasi-two dimensional magnetic crystal Mn(CHOO)2·2H2O

We have studied the normal electron spin resonance at the Larmor frequency as well as the half-field resonance of the quasi two dimensional magnetic crystal Mn(CHOO)₂·2H₂O at 35 GHz and room temperature. There are two groups of Mn²⁺ ions referred as A and B in this compound. The A ions lie in parallel (100) planes and have a strong exchange interaction within the plane. On the other hand, the B ions which lie in the adjacent parallel planes have no exchange interaction in the plane. We have measured the line width of the normal resonance from the A ions as a function of θ and φ where θ is the angle between the d.c. magnetic field H₀ and the a-axis of the crystal, which is very nearly perpendicular to the (100) plane and φ is the azimuthal angle of H₀ in the bc plane with respect to the b-axis. We also measured the intensity of the half-field resonance as a function of φ. We have calculated the second moments of the normal resonance line as a function of θ and φ. There is a satisfactory agreement between the calculated and the observed θ dependence of the line widths. The intensity of the half-field resonance has a maximum at φ = 40° whereas in an ideal 2d magnet, the maximum intensity is expected at φ = 45°. The ESR line width, because of the spindynamics in a 2 dimensional magnetic crystal, has an angular dependence of the form (3 cos²θ ? 1)². The observed dependence of the line width in Mn(COOH)₂·2H₂O seems to follow the same angular dependence when corrected for the contribution from the nearest neighbor B ions.