Hi Shinshou,
There are Mantra and Dharani used in traditional Soto Zen and throughout Buddhism, but I tend to not do so here as I believe that they have been traditionally used as "abracadabra" magic spells wishing for this worldly (wealth, health and preventing earthquakes) and other worldly payoffs. If you want, you can read more about my skepticism on such practices here ...
https://www.treeleaf.org/forums/show...l=1#post194904
Some people try to say that Mantra had other rolls, such as just "mnemonic" devices as memory tags for longer doctrines, but I don't really think so. Mostly, they were "presto chango" spells recited to keep evil spirits and the doctor away.
In fact, through the centuries, the Heart Sutra itself (in its entirety, as well as the "Mantra" at the end) has been used also as a protective talisman or magical formula by people who did not understanding the content beyond the "power" said to be there. It is like people who don't read the Lotus Sutra or the Bible or Koran, and just carry a copy for protection.
However, if one reads the Heart Sutra itself, it clearly states that ...
In other words ... PRAJNA PARAMITA (the Perfection of Wisdom, i.e., Emptiness) is the mantra, the saving Truth, not the words at the end which just proclaim the outcome of Wisdom. ... Simple as that.
Gassho, J
STLah