Chirality-driven intrinsic spin-glass ordering and field-induced ferromagnetism in Ni sub(3)Al nanoparticle aggregates

Weak itinerant-electron ferromagnet Ni sub(3)Al is driven to magnetic instability (quantum critical point, QCP, where the long-range ferromagnetic order of the bulk ceases to exist) by reducing the average crystallite size to d=50 nm. 'Zero-field' (H=0) linear and nonlinear ac-susceptibilities, measured on Ni sub(3)Al nanoparticle aggregates, with d=50 nm (S sub(1)) and d=5 nm (S sub(2)), provide strong evidence for two spin glass (SG)-like thermodynamic phase transitions: one at View the MathML source Ti(H=0)[sime]30K (View the MathML source Ti(H=0)[sime]230K) and the other at a lower temperature View the MathML source Tp(H=0)[sime]8K (View the MathML source Th(H=0)[sime]52K) in S sub(1) (S sub(2)). 'In-field' (H[ne]0H[ne]0) linear ac-susceptibility and dc magnetization demonstrate that the thermodynamic nature of these transitions is preserved in finite fields. The presently determined H-T phase diagrams for the samples S sub(1) and S sub(2) are compared with those predicted by the Kotliar-Sompolinsky and Gabay-Toulouse mean-field models and Monte Carlo simulations, based on the chirality-driven spin glass (SG) ordering scenario, for a three-dimensional nearest-neighbor Heisenberg SG system with or without weak random anisotropy. Such a detailed comparison permits us to unambiguously identify various 'zero-field' and 'in-field' SG phase transitions as: (i) the simultaneous paramagnetic (PM)-chiral glass (CG) and PM-SG phase transitions at T sub(i)(H ), (ii) the PM-CG transition at View the MathML source Ti(H), (iii) the replica symmetry-breaking SG transition at T sub(p)(H), and (iv) the continuous spin-rotation symmetry-breaking SG transition at T sub(h)(H). In the presence of random anisotropy, magnetization fails to saturate even at 90 kOe in S sub(1) whereas negligibly small anisotropy allows even fields as weak as 1 kOe to saturate magnetization and induce ferromagnetism in S sub(2). Due to the proximity to CG/SG-QCP, magnetization and susceptibility both exhibit non-Fermi liquid behavior over a wide range at low temperatures.