Oracle impdp/ expdp

When I try to export a table in Oracle database using the following command,

expdp hr/hr DIRECTORY=exp_dir DUMPFILE=tables_exp.dmp TABLES=employees

I got the following error

ORA-39002: invalid operation
ORA-39070: Unable to open the log file.
ORA-39087: directory name exp_dir is invalid

You need to create a directory by loging to sqlplus from ad hr

for windows...
create directory exp_dir as 'D:directory';

then exit from sqlplus and issue the previous command