The electromagnetic spectrum, in terms of increasing frequency is:
Radio waves, Microwaves, Infrared, Visible light, Ultraviolet, X-Rays, Gamma Rays
Thus, plausible acronyms would involve either R-M-I-V-U-X-G or G-X-U-V-I-M-R, if you wanted to go from increasing wavelength.
Mnemonics are the little tools and associations you use to individually remember things. They are highly user specific, since not everyone may associate your particular sentence or word with that topic. For example, you could use this mnemonic:
Raging Martians Invaded Venus Using X-ray Guns
Or you could create your own.
