He said: "It will be great for my own personal development as a player and I am particularly looking forward to working with Arsene Wenger.
"I am thrilled to be joining a club of the stature of Arsenal and am looking forward to playing in the Premier League.
"It is clear the club has huge ambition and I look forward to being part of an exciting future."
He added the deal came as something of a surprise.
"At the weekend, I was certain that I would stay at Real Madrid, but afterwards I realised that I did not have the faith from the coach or the bosses. I am a player who needs this faith and that is what I have felt from Arsenal, which is why I have joined.
"I am really looking forward to it because I have the faith of the coach. I had spoken to him at length on the telephone, he explained his plans and that he has faith in me - that is what I need as a player."
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