mnemonic vs device

mnemonic and device both are nouns.

Word	Noun	Adjective	Verb	Adverb
mnemonic	Yes	Yes	No	No
device	Yes	No	No	No

As nouns, device is a hypernym of mnemonic; that is, device is a word with a broader meaning than mnemonic:

Other hypernyms of mnemonic include gimmick, twist.

mnemonic (noun)	device (noun)
a device (such as a rhyme or acronym) used to aid recall	any clever maneuver
	an instrumentality invented for a particular purpose
	an emblematic design (especially in heraldry)
	any ornamental pattern or design (as in embroidery)
	something in an artistic work designed to achieve a particular effect

mnemonic (adjective)	device (adjective)
of or relating to or involved the practice of aiding the memory