已 is more literary and formal than 已经 (yǐjīng). In spoken Chinese, 已经 is far more common for 'already.' 已 appears mainly in written Chinese and set phrases.
grammar
Common in four-character idioms: 已知 (yǐ zhī, already known), 已故 (yǐ gù, deceased), 已久 (yǐ jiǔ, for a long time).
