Object rotate and slowly stop to toward

I want to rotate object by z axis.

Here is my code.

[SerializeField] GameObject obj = default(GameObject);
float _lotationSpeed = 50f;
float _stopAngleZ = default(float);

// determine stop angle z (range: 0 ~ 360)
void StopRotate()
    _stopAngleZ = 270f; // for example..
    _status = GameStatus.Stopping;        

void Update() 
      case GameStatus.Rotate:
        obj.transform.Rotate(Vector3.back * _lotationSpeed);
      case GameStatus.Stopping:
        // TO-DO: rotate speed deceleration
        // TO-DO: change status Stopping -> Stopped
      case GameStatus.Stopped:
        // game end..

I already search same name. But I can’t find a solution.
How can I slowly stop to toward?
(! important, I don’t want to reverse rotation, keep a same direction)

You should look into Time.DeltaTime and multiply that in the obj.transform.Rotate like this:
obj.transform.Rotate(Vector3.back * Time.DeltaTime * _lotationSpeed);