setting sga max size on asm

i have logged in as sysasm and would like to increase the size of sga size to 512M.see the error below.Please advise

SQL> sho parameters  sga_max_size

NAME                                 TYPE        VALUE

------------------------------------ ----------- ------------------------------

sga_max_size                         big integer 272M

SQL>

SQL> alter system set sga_max_size=512m scope=spfile;

alter system set sga_max_size=512m scope=spfile

*

ERROR at line 1:

ORA-32001: write to SPFILE requested but no SPFILE is in use