In-field critical behavior and magnetocaloric effect in Ni5Al3/NiO nanoparticle compacts

Experimental evidence is presented for the transformation of the previously reported 'zero-field' para-magnetic (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 beta, gamma and delta 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 beta, gamma and delta not only satisfy the Widom scaling relation (beta + gamma) = beta delta 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, -Delta S-M, computed from the M(T, H) data, exhibits two peaks: a sharp peak at T-dagger(H) similar or equal to 10 K and a broad one at T-p(H) similar or equal to 140 K. The characteristic temperatures, T-dagger and T-p, shift to higher temperatures with increasing magnetic field, H, in accordance with the power laws H-1/2 and H-1/3, respectively. While the peak at T-p(H) signifies the PM-FM phase transition, the peak at T-dagger(H) is reminiscent of a transition to a reentrant spin glass state. Using the rescaled temperature axis, the reduced -Delta S-M(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 T-c. The presently determined 3D INN Heisenberg values for the exponents beta and gamma are shown to correctly describe the observed variation of -Delta S-M with H at the Curie temperature, T = T-c. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

Experimental evidence is presented for the transformation of 'zero-field' para-magnetic (PM)-chiral glass and PM-spin glass transitions into a single PM-ferromagnetic (FM) phase transition at applied magnetic fields in Ni5Al3/NiO nanoparticle compacts. The study accurately determines the critical exponents and characteristic temperatures of the system, showing that the nanoparticle system behaves as a three-dimensional isotropic nearest-neighbor Heisenberg ferromagnet.