  A single molecule magnet (SMM) can maintain its magnetization direction over
a long period of time [1,2]. It consists in a low number of atoms that
facilitates the understanding and control of the ground state, which is
essential in future applications such as high-density information storage or
quantum computers [3,4]. Endohedral fullerenes realize robust, nanometer sized,
and chemically protected magnetic clusters that are not found as free species
in nature. Here we demonstrate how adding one, two, or three dysprosium atoms
to the carbon cage results in three distinct magnetic ground states. The
significantly different hysteresis curves demonstrate the decisive influence of
the number of magnetic moments and their interactions. At zero field the
comparison relates tunneling of the magnetization, with remanence, and
frustration. The ground state of the tridysprosium species turns out to be one
of the simplest realizations of a frustrated, ferromagnetically coupled
magnetic system.
