Constraints of the initial spin period and the surface magnetic field of a putative magnetar engine from the LAT upper limit. Dotted lines, dashed lines, and solid lines represent cases of η = 0.01, 0.1, and 1, respectively. The allowed parameter space is above each curve for each case. The horizontal line indicates that the lower limit of the initial period of a magnetar is P₀ = 0.6 ms. The redshift of FRB 180814.J0422+73 in the top, middle, and bottom panel is assumed to be z = 0.11, 0.05, and 0.01, respectively.