How to get the first column of a pandas DataFrame as a Series?

This will get first column of dataframe pandas as a series:

 import pandas as pd

 df = pd.DataFrame({'x' : [1, 2, 3, 4], 'y' : [4, 5, 6, 7]})

 df

   x  y

0  1 4

1  2 5

2  3 6

3  4 7

 s = df.ix[:,0]

 type(s)