Memory target in Oracle RAC

Hello All,
I created an Oracle RAC database 11.2.0.3. in DBCA I checked the enable AMM and specified my memory target.

But after I created my database I saw that my memory_target is 0 while sga_target and pga_aggregate_target were set.
Why this behavior?  is there any disadvantage if I enabled AMM?

Can I use the below steps?

ALTER SYSTEM SET MEMORY_TARGET=20G SCOPE=SPFILE sid=*;
ALTER SYSTEM SET PGA_AGGREGATE_TARGET=0 SCOPE=SPFILE sid=*;
ALTER SYSTEM SET SGA_TARGET=0 SCOPE=SPFILE sid=*;

restart the database.

Regards,