リレーション先のデータをアローで取得したい。Property [departments] does not exist on this collection instance.

user,departments,user_departmentテーブルを作成し、
こちらを参考にそれぞれのモデル内でリレーションを定義しています。

ログインユーザーの情報から部署名を取得したいのですが、下記のエラーが出ます。
どうしたらdepartmentsテーブルの部署名を取得できるでしょうか？

Property [departments] does not exist on this collection instance.

UserController.php

namespace App\Http\Controllers\User;

use Illuminate\Support\Facades\Auth;

class AgentController extends Controller
{
    public function show()
    {
        $department = Auth::user()->corp_user_department->departments->department_name;
     
        return view('user.show')
            ->with('department',$department);
    }

User.php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\SoftDeletes;

class User extends Model 
{

    protected $table = 'users';

    use SoftDeletes,

    protected $fillable = [
        'name',
        'email',
        'tel',
        'password',
    ];

    protected $dates = ['created_at', 'updated_at', 'deleted_at'];

    public function user_department()
    {
        return $this->hasMany(UserDepartment::class);
    }
}

UserDepartment.php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\SoftDeletes;

class UserDepartment extends Model
{

    protected $table = 'user_department';

    use SoftDeletes;

    public $timestamps = true;

    protected $dates =  ['deleted_at', 'created_at', 'updated_at'];

    protected $fillable = [
        'user_id',
        'department_id',
        'role'
    ];

    public function user()
    {
        return $this->belongsTo(User::class);
    }

    public function department()
    {
        return $this->belongsTo(Department::class);
    }
}

Department.php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;
use Illuminate\Database\Eloquent\SoftDeletes;

class Department extends Model
{

    protected $table = 'departments';

    use SoftDeletes;

    public $timestamps = true;

    protected $dates =  ['deleted_at', 'created_at', 'updated_at'];

    protected $fillable = [
        'corporation_id',
        'branch_office_id',
        'department_name',
    ];

    public function corporation()
    {
        return $this->belongsTo(Corporation::class);
    }
    public function branch_office()
    {
        return $this->belongsTo(BranchOffice::class);
    }
    public function user_department()
    {
        return $this->hasMany(UserDepartment::class);
    }
}