We present the analysis of Fermi Large Area Telescope (LAT) $gamma$-ray observations of HB~21 (G89.0+4.7). We detect significant $gamma$-ray emission associated with the remnant: the flux >100 MeV is $9.4pm0.8(stat)pm1.6(syst)times10^{-11}$ erg cm$^{
-2}$ s$^{-1}$. HB 21 is well modeled by a uniform disk centered at $l= 88{deg}.75pm 0{deg}.04$, $b = +4{deg}.65 pm 0{deg}.06$ with a radius of $1{deg}.19 pm 0{deg}.06$. The $gamma$-ray spectrum shows clear evidence of curvature, suggesting a cutoff or break in the underlying particle population at an energy of a few GeV. We complement $gamma$-ray observations with the analysis of the WMAP 7-year data from 23 to 93 GHz, achieving the first detection of HB 21 at these frequencies. In combination with archival radio data, the radio spectrum shows a spectral break which helps to constrain the relativistic electron spectrum, hence parameters of simple non-thermal radiation models. In one-zone models multiwavelength data favor the origin of $gamma$ rays from nucleon-nucleon collisions. A single population of electrons cannot produce both $gamma$ rays through bremsstrahlung and radio emission through synchrotron radiation. A predominantly inverse-Compton origin of the $gamma$-ray emission is disfavored because it requires lower interstellar densities than are inferred for HB 21. In the hadronic-dominated scenarios accelerated nuclei contribute a total energy of $sim 3 times10^{49}$ ergs, while in a two-zone bremsstrahlung-dominated scenario the total energy in accelerated particles is $sim1times10^{49}$ ergs.
Single crystal magnetic studies combined with a theoretical analysis show that cancellation of the magnetic moments in the trinuclear Dy3+ cluster [Dy3(OH)2L3Cl(H2O)5]Cl3, resulting in a non-magnetic ground doublet, originates from the non-collineari
ty of the single ion easy axes of magnetization of the Dy3+ ions that lie in the plane of the triangle at 120 (deg.) one from each other. This gives rise to a peculiar chiral nature of the ground non-magnetic doublet and to slow relaxation of the magnetization with abrupt accelerations at the crossings of the discrete energy levels.
