The thermomagnetic writing was studied for (BiSmEr)
3 (GaFe)
5O
12 films. Since the optical absorption coefficient (α) of this kind of films is relatively low, the writing threshold power (
Pw) becomes larger than those of other materials.
In order to lower
Pw, the effective optical-path length was enhanced by increasing the film thickness (
h) and/or by coating them with reflecting layers. Firstly,
Pw was minimized by choosing
h=1/α to 2/α; i. e.
Pw=20mW at
h=2.2μm, 13 mW at 2.2μm and 12 mW at 1.0μm, for the laser irradiation with the duration of 10μs, a spot radius of 2.5μm, and with λ=514.5, 488.0 and 457.9 nm, respectively. Analytical calculation of heat conduction for this system indicates that the optimum thickness for which the temperature elevation on the film-substrate boundary reaches maximum lies also between 1/α and 2/α. Since, however, the selection of the film thickness is limited by restriction in the wall coercivity (
Hw), some reflecting layers were put on these films. The preliminary results for a 0.55μm thick film with a 0.12 μm thick Al reflector are
Pw=26mW for λ=514.5nm, and 20 mW for 488.0 nm. These values of
Pw were 52% and 69% of those without reflector, respectively.
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