Intrinsic chiral-spin glass and spin glass transitions in compacted Ni5Al3/NiO core/shell nanoparticles

We report the first synthesis of the intermetallic compound Ni5Al3 in the nanocrystalline form. At the nanometer length scale, a stable Ni5Al3 compound (core) does not form without a NiO shell. Detailed structural and microstructural characterization as well as the compositional analysis indicate the presence of Ni5Al3/NiO core/shell nanoparticles and log-normal crystallite size distribution with mean size d similar or equal to 6 nm. "Zero-field" (dc magnetic field, H = 0) linear (chi(1)) and nonlinear (chi(n) with n = 2, 3, 4, 5) ac-magnetic susceptibilities have been measured as functions of temperature at various ac driving field amplitudes over three decades of frequency on the nanocrystalline samples S-1 and S-2 of composition Ni5+xAl3-x/NiO (x = 0.014) and Ni5-xAl3+x/NiO (x = 0.038), respectively. chi(1) (T) and chi(n) (T) are shown to provide conclusive evidence for the existence of two spin glass (SG) thermodynamic phase transitions: one at T-i (H = 0) similar or equal to 138 K and the other at a lower temperature T-p (H = 0) similar or equal to 108 K in both S-1 and S-2. Linear ac-susceptibility in superposed dc magnetic fields demonstrates that the thermodynamic nature of these transitions is preserved in finite fields and the H-T phase diagrams for the samples S-1 and S-2 conform very well only with the H-T phase diagram predicted by the chirality-driven spin glass (SG) ordering model for a three-dimensional nearest-neighbor Heisenberg SG system with weak random anisotropy. A detailed comparison between theory and experiment permits us to unambiguously identify various "zero-field" and "in-field" SG phase transitions as (i) the simultaneous paramagnetic (PM): chiral-spin glass (CG) and PM-SG phase transitions at T-i (H) and (ii) the replica symmetry-breaking SG transition at T-p(H).