I am wondering if there are any differences in the usage of 自己
and 自分
, or are they simply synonyms?
It seems that the two are interchangeable with nouns stemming from them.
I.e, 非自己
and 非自分
also appear to be synonyms.
I'm pretty sure that the meaning is about 'self'. Now there are differences in the two words. I myself am not really sure about all the little differences in meaning of these two words. But I am pretty sure that in certain cases, it's not interchangeable. So to answer your question, I would like to give you some phrases about each one.
自分はそう思う。 Methinks
自分なりにやる。 Do something in one's own way
自分自身 Me, myself
両親のためではなく、自分のために勉強する To study for myself not for my parents
自分で決める Decide it by myself
自己紹介 Introducing oneself I'm pretty sure they don't use 自分紹介
自己主義 自己中心 Egoism, selfish
自己宣伝 To advertise oneself
自己流 Self-taught, one's own way
自己批判 Criticizing oneself
So in conclusion the two are a bit like myself vs oneself/self. And some sites said that it's the subjective self (自分 = I, me) vs objective self (自己, oneself).
Also there are other words that you might be interested in too.
[自我]{じが} (means ego)、 [自]{みずか}ら means himself/oneself.
In fact, [自]{みずか}ら is a bit like himself. Sample sentences from Weblio:
彼は自らを[省]{かえり}みる He reviews himself
社長自ら[指揮]{しき}をとる The CEO himself takes command of sth.
Synonyms. 自己 is a little bit formal/academic.