Is it possible to use $ characters in field names?

[Mike Thomsen]

I am trying to build an avro schema for a NiFi flow that is going to insert
data into Mongo, and Mongo extended JSON requires the use of $ characters
in cases like this (to represent a date):

{
    "timestamp": {
        "$date": TIMESTAMP_LONG_HERE
    }
}

I tried building a schema with that, and it failed saying there was an
invalid character in the schema.  just wanted to check and see if there was
a work around for this or if I'll have to choose another option.

Thanks,

Mike