copy ldf from one server to another
Take a step back and ask, why are we copying log files?  SQL Server LDF files contain transaction log data, but they're not terribly useful by themselves.  To work with a database, you need both the data files and log files.  In order to access those files directly via the file system, though, you'll either need to stop SQL Server or use snapshot technologies like VSS.

If you're trying to synchronize transactions across multiple databases, check out technologies like transactional replication.

If you're trying to back up the database, use the SQL Server BACKUP DATABASE command.