Why many research results are presented as paid articles?
Professors who publish their research results normally do not earn money from these paid articles. It is the publishing journal who earns. There are even journals who ask the author to pay for publishing the articles.

Contact the authors of the paid article directly and ask them for the copy of the article. Be polite, explain why do you need it for. In most cases, you will get that copy for free.

Also, try put the article name and authors into Google search. Another similar article by the same authors may exist and be free to download.