Can you get a whole row with a max on one column

I have a table with these columns (and more but irrelevant ones)

| magnetic_susceptibility          | float8
| spin_coupling                    | float8
| boundary_coupling                | float8

I want a query to give (spin_coupling, boundary_coupling) where for each
boundary_coupling it gives the spin_coupling for which magnetic_susceptibility is
maximum.

Kinda like: 
select boundary_coupling, max(magnetic_susceptibility) from run_averages group by 
boundary_coupling; 

except I don't want to see max(magnetic_susceptibility) but the spin_coupling which
corresponds to max(magnetic_susceptibility).

I know I can do this with a small program, but I would like to have a single query
so that I can use psql -c "query" in a script and pipe the results to a file.

Thanks,
-- 
Scott V. McGuire 
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