Leontii A. Hagemeister (or Gagemeister) was a Baltic German navigator of the Russian navy and briefly governor of the Russian-American company. The best work on him in English is by Richard Pierce, whose Builders of Alaska repackages 1970s work saying that Hagemeister's journals remained unstudied: "One difficulty for the scholar is that he kept his diaries in Russian, German, English, Spanish, French and Portuguese." Pierce did not mention where the documents were kept, and I haven't succeeded in finding out. Which archive holds Hagemeister's diaries?
Thanks to the author, Glenn Farris, I obtained a copy of "Report on a Project to seek out documents describing early 19th Century California housed in the Russian Naval Archives in St. Petersburg, Russia". It contains a translation of a 2005 letter from the director of the Russian Naval Archive (RGAVMF), Vladimir Semenovich Sobolev, who describes the archive's holdings: "There are original journals kept during the voyages of V.M. Golovnin and L.A. Hagemeister.". I believe that the latter is the document in question.