Contacting professor for PhD in different research area than past experience: do I need to prepare a research proposal before first contact? I've done my masters in CS/Artificial Intelligence. My thesis work is related to Human-Robot interaction, that involves robotics and psychological theories. 

I did that project because it seemed to me as more interesting, I'm still interested but during my masters studies I have realized that I am more interested in machine learning, deep learning and its applications in so many domains. Which is why I'm highly motivated to pursue my PhD in machine learning, specifically deep learning. 

I do have theoretical concepts about machine learning, image processing and good knowledge about research process (working as research assistant currently). I'm quick learner and have good CGPA as well, few conference in different research area. What I don't have is any research project or publication in this area and hence I don't have any proposal prepared so far. 

Now I want to contact professor showing my interest in Deep learning. My question is since I don't have a prepared proposal, how do I approach him? Do all professor expect the proposal attached at first email or Can I prepare proposal while being in touch with him with little bit of his guidance. 

Should I spend like a month and prepare proposal or would it be better if I just contact professor and with his guidance I can be more focused and save my time?
the answer depends a bit on the culture of your country and even on the individual professor you are adressing. If you would contact me, sending an e-mail containing more or less the question posted would be sufficient to get an invitation to talk about potential research topics and about how to proceed. 

At least in my position, the answer is more dependent on whether I have an idea on how to finance your position - so if you have ideas about this (grants, ...), you should include such information.