LookRotation causes object keep walking in circle

Hello,

I am a beginner and trying to make First person game.

When I use "Quaternion.LookRotation", when the left, right or down button is pressed, the character keep walking in circle, instead of turn to the direction of the button.

Thank you for advice!

 public CharacterController characterController;
 private Vector3 prev;

 void Start()
 {
     anim = gameObject.GetComponentInChildren<Animator>();
     rb = GetComponent<Rigidbody>();
     Player_pos = GetComponent<Transform>().position;
     m_AudioSource = GetComponent<AudioSource>();
     prev = GetComponent<Transform>().position;
     
 }


 void Update()
 {
     bool isWalking;
     if (Input.GetAxis("Horizontal") != 0 || Input.GetKey("Vertical"))
     {
         isWalking = true;
     }
     else
     {
         isWalking = false;
     }
     anim.SetBool("IsWalking", isWalking);

     float horizontal = Input.GetAxis("Horizontal");
     float vertical = Input.GetAxis("Vertical");

     rb.velocity = transform.right * horizontal + transform.forward * vertical;

     if (characterController.isGrounded)
     {
         if (Input.GetAxis("Jump") > 0)
         {
             rb.velocity += new Vector3(0, 10 / jumpSpeed, 0);
         }
     }
     rb.velocity += new Vector3(0, Physics.gravity.y * Time.deltaTime, 0);
     characterController.Move(rb.velocity * walkSpeed);

     Vector3 diff = transform.position - prev;
     diff.y = 0;
     if (diff.magnitude > 0.01)
     {
         transform.rotation = Quaternion.LookRotation(diff);

     }
     prev = transform.position;
     

     if (isWalking)
     {
         if (!m_AudioSource.isPlaying)
         {
             m_AudioSource.Play();
         }
     }
     else
     {
         m_AudioSource.Stop();
     }
 }

}