What is the agreement allowing free movement between northern European countries including Finland, Sweden and Denmark?
Finland–Sweden border:  Because of the Schengen treaty and the Nordic Passport Union, the border can be crossed mostly freely. Nordic Passport Union: The Nordic Passport Union allows citizens of the Nordic countriesIceland, Denmark, Norway, Sweden, and Finlandto travel and reside in another Nordic country (excluding Greenland and Svalbard) without any travel documentation (e.g. a passport or national identity card) or a residence permit.
Nordic Passport Union