  The understanding of coercivity mechanism in high performance Nd-Fe-B
permanent magnets relies on the analysis of the magnetic properties of all
phases present in the magnets. By adding Cu in such compounds, a new Nd6Fe13Cu
grain boundary phase is formed, however, the magnetic properties of this phase
and its role in the magnetic decoupling of the matrix Nd2Fe14B grains are still
insufficiently studied. In this work, we have grown Nd6Fe13Cu single crystals
by the reactive flux method and studied their magnetic properties in detail. It
is observed that below the N\'eel temperature (TN = 410 K), the Nd6Fe13Cu is
antiferromagnetic in zero magnetic field; whereas when a magnetic field is
applied along the a-axis, a spin-flop transition occurs at approx. 6 T,
indicating a strong competition between antiferromagnetic and ferromagnetic
interactions in two Nd layers below and above the Cu layers. Our atomistic spin
dynamics simulation confirms that an increase in temperature and/or magnetic
field can significantly change the antiferromagnetic coupling between the two
Nd layers below and above the Cu layers, which, in turn, is the reason for the
observed spin-flop transition. These results suggest that the role of
antiferromagnetic Nd6Fe13Cu grain boundary phase in the coercivity enhancement
of Nd-Fe-B-Cu magnets is more complex than previously thought, mainly due to
the competition between its antiferro- and ferro-magnetic exchange
interactions.
