CODE EXAMPLE FOR CSHARP

how to rotate object in unity only on one axis

// If you want the obj to rotate on only one axis do this

// Create vector3
Vector3 target_position = new Vector3(transform.position.x,
                                                target.position.y,
                                                transform.position.z);
// give the target_position to transform.LookAt

transform.LookAt(target_position);