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This is one case where you have to come down on the team like a ton of bricks.
I am assuming that the unpopular team member does a good job and is not actively or deliberately unpleasant. In such a case the reason for the unpopularity is essentially a function of group dynamics, and it is your role as team manager to change these dynamics before they do any harm.
How you do it depends, as always, on circumstances. Remember not to single out or inadvertently victimise the unpopular person - they are having a hard enough time already. A fierce lecture on how the team is the most important thing may be appropriate.
Remind team members that if they dislike somebody in the team - which is certain to happen from time to time - then they are obliged, for the benefit of the team and therefore themselves, to make a point of being particularly kind and helpful to the person they dislike. Being unpleasant or unhelpful to somebody in the group who you dislike is not acceptable behaviour in the workplace, and will lead very swiftly to disciplinary procedures.
You will find, in such cases, that if there is a problem person it is probably not the unpopular one, but somebody else. If so, that person must be watched very carefully in case they create other problems in the future.
Communication and by assigning team tasks by shuffling group members each time with in team so that every one should have an opportunity to work and explore to that particular member.
