I've heard many armchair managers offer their opinions about how to deal with the situation. Most prescribe a confrontational strategy that has little chance of success. Although you should abandon any search for quick fixes, here are a few approaches that may produce results. First, assess whether the antagonism that exists between senior and junior staff revolves around technical or nontechnical issues. Often, resentment arises over issues like salary or input on decisions.
Many organizations have to pay premium salaries to attract new staff.
When these salaries are out of line with those of established employees, many of whom are constrained in their mobility for various reasons, you're asking for trouble. If you're willing to pay a premium for new talent, be prepared to raise veterans' salaries as well. Second, take the time to understand the opinions of all staff, old and new. You may walk away feeling you haven't achieved the closure you were seeking, but there is value in the interaction. Likewise, take an hour to hear the views of new employees, even if it only takes you five minutes to conclude that their position is poorly conceived.