Experimental evidence is presented for the transformation of the previously reported ‘zero-field’ paramagnetic (PM)-chiral glass and PM-spin glass transitions into a single PM-ferromagnetic (FM) phase transition at applied magnetic fields H ≥ 3 kOe in Ni5Al3/NiO nanoparticle compacts. Accurate values of the asymptotic critical exponents β, γ and δ for spontaneous magnetization (order parameter), ‘zero-field’ magnetic susceptibility and the critical M − H isotherm, respectively, have been determined from the magnetization, M(T, H), data using the generalized scaling equation of state (SES) analysis. The critical exponents β, γ and δ not only satisfy the Widom scaling relation (β + γ) = βδ but also unambiguously establish that the nanoparticle system in question behaves as a three-dimensional (3D) isotropic nearest-neighbor (INN) Heisenberg ferromagnet in the critical region. As a function of temperature, the isothermal magnetic entropy change, −ΔSM, computed from the M(T, H) data, exhibits two peaks: a sharp peak at T†(H) ≃ 10 K and a broad one at Tp(H) ≃ 140 K. The characteristic temperatures, T† and Tp, shift to higher temperatures with increasing magnetic field, H, in accordance with the power laws H1∕2 and H1∕3, respectively. While the peak at Tp(H) signifies the PM-FM phase transition, the peak at T†(H) is reminiscent of a transition to a reentrant spin glass state. Using the rescaled temperature axis, the reduced -ΔSM(T) curves for different applied magnetic fields collapse onto a single universal curve characteristic of magnetic materials that exhibit a second-order phase transition at Tc. The presently determined 3D INN Heisenberg values for the exponents β and γ are shown to correctly describe the observed variation of -ΔSM with H at the Curie temperature, T = Tc.