gameobject still rotating over z axis

Hi guys, sorry for my english. i'm realy stack. i'm new with unity and physics. in my project i want to rotate gameobject over z axis when it pass 0.1 angle. The problem is that the gameobject still rotating even after reseting it rotation. One more question, i don't understand why i can't rotated the gameobject after reseting rotation (this code work one time before reseting rotation) and this is my code.

 void FixedUpdate()
 {

     if (this.gameObject.transform.rotation.z > 0.1 || this.gameObject.transform.rotation.z < -0.1) {
         
         if (this.GetComponent<Rigidbody> ().constraints == RigidbodyConstraints.None) {
             this.GetComponent<Rigidbody> ().constraints = RigidbodyConstraints.FreezeRotationZ;
         }
     }
         if (myInput > 0.5) {
             Debug.Log("Rotate left");
             rotateLeft ();
             
         } else if (myInput < -0.5) {
             Debug.Log ("Rotate right");
             rotateRight ();
         }

             else {
                 if (this.gameObject.GetComponent<Rigidbody> ().constraints != RigidbodyConstraints.None) {
                     if(this.gameObject.transform.rotation.z == 0)
                         this.gameObject.GetComponent<Rigidbody> ().constraints = RigidbodyConstraints.None;
                     if (this.gameObject.transform.rotation.z != 0)
                         this.gameObject.transform.rotation = new Quaternion (transform.rotation.x, transform.rotation.y, 0, transform.rotation.w);
                 
     }
             }
 }
 
 void rotateLeft()
 {
     if (this.gameObject.transform.rotation.z > /*amount of degrees*/ 0.4f)
         return;
     //rotate more
     transform.rotation = Quaternion.Slerp(transform.rotation, new Quaternion(transform.rotation.x,transform.rotation.y,0.4f,transform.rotation.w), Time.deltaTime*50f);
     Debug.Log("0.4f atteind");
 }
 void rotateRight()
 {

     if (this.gameObject.transform.rotation.z <  -0.4f) {    
         return;
     }
     //rotate more
     transform.rotation = Quaternion.Slerp(transform.rotation, new Quaternion(transform.rotation.x,transform.rotation.y,-0.4f,transform.rotation.w), Time.deltaTime*50f);
     Debug.Log("-0.4f atteind");
 }

is the problem of rigidbody or what? and for the best parctice do i use transform.rotate and transform.rotation or rotate the gameobject with rigidbody. thanks