0 5 10 x 10 9 2.5 3 3.5 4 E  (V/m)  (rad) (a)E vs angle. −400 −200 0 200 400 0 10 20 30 40 50 k  (cm −1 ) /2 c  (cm −1 ) −A   A  −L L  SP  ’$_{m}$  ’$_{ey}$  ’$_{m}$  (b)Influence of E into Dispersion relation. 0 5 10 15 4 8 12  H  (T) $_{ }$θ  (10 −2  rad) (c)H o vs angle. −30 −10 10 30 2.2 2.6 3 3.4  k  (cm −1 ) /2  c  (cm −1 ) −A  −L  L  A  ’$_{m}$  (d)Influence of H into Dispersion relation. FIG. 4. Influence external field in the canting angle and dispersion relation In (a)the external electric field E increase the canting angle with small value. In (b) dispersion relation is shown with an external electric field along the spontaneous polarisation E = 5 × 10 8 V/m. In (c)the external electric field H$_{o}$ increase the canting angle. In (d), magnetic resonance frequency shift down when the external magnetic field, H$_{o}$ = 10T is applied. The dotted line is the magnetic frequency in the absence of magnetic field. between electric and magnetic resonance is smaller. VI. PARAMETER EFFECTS ON SURFACE MODE PROPERTIES Lastly, we identify the key parameters affecting surface mode frequencies. In the first case, changing the phonon mass to f = 9 × 10 $^{21}$statA$^{2}$s$^{2}$/gcm3 moves the magnetic resonance frequency to 1 cm − 1 above the the electric resonance frequency ω$_{ey}$ . The result on surface 14