Minato used the Death Reaper Seal to seal the Yin half the Kyuubi in him, then he used the Eight Trigram Seal combined with the 4 Element Seal to seal the Yang Half of the Kyuubi in Naruto.
From Naruto Wiki
During its attack on Konoha, Minato Namikaze sacrificed his life by
splitting the fox's chakra into two: sealing the Yang half within his
son and sealing the Yin half within himself.
Lower down in the page:
Since Kurama's chakra was too immense to be sealed within an infant
like Naruto, Minato first used the Dead Demon Consuming Seal to
separate and seal its Yin half within himself and then prepared the
Eight Trigrams Seal to imprison the Yang half within Naruto
The 4 Element Seal was used so that the chakra of the Nine Tailed Fox would slowly leak out from the Eight Trigrams Seal and merge with Naruto's own chakra.