"Resolving the puzzles of the magnetic properties of Nd and some of those in HTc materials" Abstract: Understanding the static and dynamic magnetic properties of light rare-earth metal Nd may shed light on the magnetic properties in the high temperature superconducting (HTc) oxides (YBCO, LSCO etc.). This brings Nd to the forefront of present day science. Since the total angular moment is S=9/2, the neutron scattering has much higher intensity than from the HTc (S=1/2) materials and therefore more information is available. Neutron diffraction investigations of the magnetic structure of Nd starting by Moon, Cable and Koehler over 50 years ago - and others (in particular at Risoe and ILL) are numerous. These revealed that Nd has the most complicated magnetic structures and dynamic properties known for any pure element. The reason for the found complexity has previously not been understood, basically because the magnetic interactions were not determined. This is done here - based on the recent diffuse scattering measurements by T. Chatterji et al1 - thereby resolving a large number of the puzzles found in Nd - particularly around the sinusoidal phase and the 2-q structure. A distorted spiral phase2 can account for neutron observations at T=10K better than a sinusoidal phase. An effective spiral spin phase ground state, as well, provides a new paradigm for the high temperature superconducting cuprates3. It accounts for the recent neutron scattering observations of spin excitations regarding both the energy dispersion and the intensities, including the ‘universal’ rotation by 45○ around the resonance energy, Eres. The intensity has 2D character even in a single twin crystal. The value of E¬res is related to the nesting properties of the Fermi surface. The excitations above Eres are shown to be due to in-plane spin fluctuations, a testable difference from the stripe model. The deduced exchange interaction function reveals effects of the Fermi surface, and the unique shape predicts large quantum spin fluctuations in the ground state. It would be highly interesting with an ab initio calculation of the exchange interaction. 1. Per-Anker Lindgård, Tapan Chatterji, Karel Prokes, Vadim Sikolenko and Jens-Uwe Hoffmann, J. Phys. Cond. Matter, 19, 286201 (2007). 2. Per-Anker Lindgård, J. Phys. Cond. Matter, 19, 286202, (2007). 3. Per-Anker Lindgård, Phys. Rev. Lett., 95, 217001 (2005).