The motion characteristics of the magnetic thin film have been studied under various forms of applied magnetic field(AMF):pulse

A.C. field and D.C.field

[1～6]

.In present paper

the dynamic behaviour of the magntooptic thin film under the composite fields has been measured experimentally.The formula of magnetooptic specimen is BiTm)

3

(FeGa)

5

O

12

which has two surface with thickness of 6 19×2μm.The AMF corresponding the saturation Faraday's rotation angle is 8000A/m.The domain wall(DW) can be considered as the 180 degree wall and the equivalent resistant field is the order of 1000A/m in the thin film specimen.There are three forms of AMF:type A

low frequency A.C.(the amplitude 140A/m) produced by a function generator;type B

uniform magnetic field sweeping(UMFS

d

H

/d

t

=83.6A/m/S in present paper

H is the magnetic field) produced by a D.C. power supply with a sweeping rate control system

[6]

;type C

the UMFS together with low frequency A.C.field.We remark the type C as the composite field.The data of DWs motion have been obtained by means of the computer with charge couple device(CCD) and of photography from the polarization microscope.The transformer put on the stage of the microscope was prepared a fair hole to lay up the thin film specimen and was connected to the D.C.power supply or/and to the function generator for carrying out AMF.The average distance of DWs from the photography for various experiment conditions of H could be plotted against the time.The percentage data of DWs area from the CCD for various experiment conditions of H do the time in the same grapher.After comparing the dynamic character of DWs with these curves corresponding to 0.1～10Hz A.C.fields or/and to

H

/d

t

=83.6A/m/S UMFS rate

it can be shown that: (1)The motion behaviors of DWs in the AMF of type C are not the same as that of simple composition of type A and type B

it would be an interaction term represented the force between AMF and DWs

[7]

.(2)Even if the amplitude of A.C.field value 140A/m1000A/m

the equivalent resistant field

[6]

and the value of D.C.field100A/m

the DWs of the thin film specimen can be moved.The motion of DWs is not synchronized with the AMF.It is supposed that the DWs may be easier to move and can be pushed forward or pulled back by a small D.C.field superposed a weak A.C.field.