Laravel 5.8 foreign key

I'm trying to connect my tables via foreign key.So I updated my students table as below

public function up()
    {
        Schema::create('students', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->string('name');
            $table->date('birthday');
            $table->string('address');
            $table->string('school');
            $table->string('student_email');
            $table->string('student_mobile');
            $table->string('mother_name');
            $table->string('mother_mobile');
            $table->string('mother_email');
            $table->integer('user_id')->unsigned();
            $table->foreign('user_id')->references('id')->on('users');
            $table->timestamps();
        });
    }

Then I have updated store function in studentcontroller as below.

            $newStudent->name = $request->name;
            $newStudent->birthday = $request->birthday;
            $newStudent->address = $request->address;
            $newStudent->school = $request->school;
            $newStudent->student_email = $request->student_email;
            $newStudent->student_mobile = $request->student_mobile;
            $newStudent->mother_name= $request->mother_name;
            $newStudent->mother_mobile = $request->mother_mobile;
            $newStudent->mother_email = $request->mother_email;
        

        $newStudent->save();
        return redirect()->route('student.index')->withStatus(__('User successfully created.'));

When I try to store data it generate a error saying that SQLSTATE[HY000]: General error: 1364 Field 'user_id' doesn't have a default value . I know that I haven't mentioned ** user_id** at studentcontroller. Also I know that user_id can be added as follow $newStudent->user_id = Auth::User()->id;

Could anyone please tell me how to use foreign key in laravel for store data. Any how should relevant controller or model to be updated. Please help