Foreign Key makes Problems in Laravel 5.6

I would like to program an inventory system and need 3 tables for it. These can also be generated via artisan without a foreign key. But as soon as I want to add a foreign key, I get the following error message.

SQLSTATE[HY000]: General error: 1005 Can't create table `inventar`.`#sql-fd4_141` (errno: 150 "Foreign key constraint is incorrectly formed") 

(SQL: alter table items add constraint items_lend_foreign foreign key (lend) references lending (id))

Here my Code:

Item Table

Schema::create('items', function (Blueprint $table) {
        $table->string('barcode');
        $table->primary('barcode');

        $table->string('name');
        $table->string('description')->nullable();
        $table->string('room')->nullable();
        $table->string('status')->nullable();
        $table->string('annotation')->nullable();
        $table->string('image')->nullable();
        $table->integer('lend')->unsigned()->nullable();
        $table->string('manufactor')->nullable();



        $table->timestamps();
    });

    Schema::table('items',function ($table){
        $table->foreign('lend')->references('id')->on('lending');

    });

Lending Table

Schema::create('lending', function (Blueprint $table) {
        $table->increments('id')->unsigned();


        $table->integer('personid')->unsigned();

        $table->dateTime('startdate');
        $table->dateTime('enddate');
        $table->timestamps();
    });

    Schema::table('lending',function ($table){
        $table->foreign('barcode')->references('barcode')->on('items');
        $table->foreign('personid')->references('personid')->on('persons');

    });

Persons-Table

 Schema::create('persons', function (Blueprint $table) {
        $table->integer('personid')->unsignd()->primary();

        $table->string('firstname');
        $table->string('lastname');
        $table->string('email');
        $table->string('annotation')->nullable();
        $table->timestamps();
    });

I've also googled, but found no solution that works for me.

Is it a problem that my primary key is a string?

via Chebli Mohamed