Yes, your employer owns all work done by you for the employer, in the course of employment, and/or using their equipment and facility. In deleting the work, you destroyed their property. It is unlikely you would face criminal charges unless they can show you had a criminal intent (e.g. you knew it was theirs, but determined to destroy it anyway). They could theoretically sue you for the value of the notes, work, etc. if it has some value and/or if they'd need to pay someone else to redo it. And of course they could mention this to any prospetive employers calling for a recommendation.