You must keep your .jmx file in jmeter bin folder and you can change your log file location anywhere you wish.
Just run this after placing the .jmx file in your jmeter bin folder ,
jmeter -n -t abc.jmx -l def.jtl
jmeter -n -t abc.jmx -l C:\users\desktop\def.jtl
jmeter -n -t abc.jmx -l C:\users\desktop\def.Csv
jmeter -n -t abc.jmx -l C:\users\desktop\def.Txt
jmeter -n -t abc.jmx -l C:\users\desktop\def.Xls
If the log file is not there in the location, it will create a new file while running the test.