slack off vs slacken off

slack off

verb
  • To decrease in intensity; to ease off; to diminish; to die down. 

  • To be deliberately unproductive in one's work or study. 

slacken off

verb
  • To slacken or lessen; to become less intense. 

  • To do (something) less intensely. 

How often have the words slack off and slacken off occurred in a corpus of books? (source: Google Ngram Viewer )