Well, I would say that this all depends how one defines a Mantra
in one's heart. In much of Buddhism and related religions of India (although something very similar can be found in about all religions really ... e.g., like "God Is Great/Allahu al-Akbar" in Islam, an orthodox Jew's reciting the sacred letters of Torah, or "Praise Jesus" in some corners of Christianity
), it is a sound, word or words that create transformation in some way.
Nichiren Buddhism (my wife's family are Nichiren Buddhists
) is a school of Buddhism which developed in Japan hundreds of years ago centered on the power of the Lotus Sutra ... on the power of faith and recital even in just the name of the Lotus Sutra. Thus, they recite "Nam-myoho-renge-kyo" (Hail the Wonderful Law of the Lotus Flower Sutra
). Many Tibetans chant "Om mani padme hum" (Om! Hail the Jewel in the Lotus!
). Another school of Buddhism in Asia is the Jodo (Pure Land) school(s), who worship or rely upon Amida Buddha, and thus chant "Namu Amida Butsu" (or its equivalent in Chinese etc., Homage to Amida, Buddha of Infinite Light
In my opinion, of course, seated Zazen is "complete, whole, the only thing needed to do
" in that moment of sitting. When we sit, it is very very vital to sit with the attitude sunk deep in one's bones that " there is no other place to be, nothing lacking, not one more thing to do
" than this. (We do so because in daily life, running here and there and always feeling some lacks or discontents in life, we rarely if ever undertake one action with total heart and completeness in such way! Thus we call this "non-doing".
However, rising from the cushion ... one must come to express Zazen all through daily life. All of daily life is also "Zazen" in its wider meaning. So, if a particular person wished to also chant "Nam-myoho-renge-kyo
" or "Namu Amida Butsu
" or "Allahu al-Akbar
" or "Kwan Seum Bosal
" or the Torah or "Praise Jesus
" (or "Praise Richard Dawkins" for our atheist members
8) ) ... that is fine. Up to each person in their heart. All Zazen in its wider meaning, as is everything from changing the baby to cooking dinner to sewing a Kesa.