After reading through the comments, the question should be updated.
This answer is Windows specific.
You can create a temporary file with exclusive read and exclusive write access AND set the file to be deleted when closed by your application.
See this link:
http://msdn.microsoft.com/en-us/library/windows/desktop/aa363858%28v=vs.85%29.aspx[
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Use zero for share mode (exclusive). Include FILE_FLAG_DELETE_ON_CLOSE for the flags parameter.
Leave the file open while your application is running. Just before you close the file, wipe the file contents.
This isn't 100% but covers most cases. If the device is powered off while the file exists, the data is still there.
A better solution (though not your original question) would be to decrypt only the portions needed and retain them in memory.