Sweden's ruling party have opposed military alignment in the past, but pressure from Finland, a key ally, helped sway their position on Sunday.
Marin is younger than New Zealand's Jacinda Ardern, 39, and Ukraine's Oleksiy Honcharuk, 35.
Socialists candidate Rumen Radev won Sunday's presidential election by a wide margin.
The former opposition leader has pledged to act as a neutral "refugee" in the country's heated political scene.
Netherlands increases spending for first time in decade after MH17 and Russian ties worsen
Independent candidate wins Vezprem by-election as PM warns that government 'cannot sit on its laurels'
The coordinated trip comes as rebels advanced on a railway hub held by Ukrainian troops.
Berlusconi's petition to have the sentence reduced was filed last month and approved on Monday.
The Greek government began negotiations with the head of the eurozone finance ministers in Athens.