E.P. Nóbrega

Understanding the table-like magnetocaloric effect

Abstract In this work, we explain the table-like magnetocaloric effect as a consequence of multiple magnetic phase transitions for different temperatures. Using a Hamiltonian that takes into account the dipolar and quadrupolar interactions and the crystalline electric field, it was possible to study the magnetocaloric effect of the Γ 3 –Γ 5 reduced magnetic system. We analyze the table-like magnetocaloric effect for different λ and K , dipolar and quadrupolar parameters, and for a suitable energy gap between Γ 3 and Γ 5 that permits successive magnetic orderings at different temperatures.

The magnetocaloric effect in R5Si4 (R = Gd, Tb): a Monte Carlo calculation

In this work we calculate the magnetocaloric effect in the compounds Gd5Si4 and Tb5Si4. We use a model Hamiltonian of interacting spins, and treat the spin–spin interaction in the Monte Carlo simulation. The theoretically calculated isothermal entropy change and the adiabatic temperature change upon variation of the magnetic field are in good agreement with the available experimental data.

The anomalous magnetocaloric effect in HoNi2

Abstract In this work, we report a theoretical investigation of the magnetocaloric effect in the ferromagnet HoNi 2 . To carry out this investigation, we have used a model Hamiltonian that takes into account the crystalline electric field (CEF) and the exchange interaction. Using the proper experimental CEF and exchange parameter, ascribed for HoNi 2 , a change of the easy magnetization direction, from 〈110〉 to 〈001〉 was predicted at T =1.5 K for the critical magnetic field H ∼2.4 T. The anomalous peak in the isothermal magnetic entropy change and in the adiabatic temperature change with magnetic field was calculated and analysed for the three main crystallographic directions.

Theoretical investigation on the magnetocaloric effect in amorphous systems, application to: Gd80Au20 and Gd70Ni30

The temperature dependence of the magnetocaloric effect in Gd80Au20 and Gd70Ni30 amorphous alloys were investigated, using the Handrich-Kobe model with a modified Brillouin function that takes an additional exchange fluctuation term. The exchange fluctuation parameters were determined to give better fits to magnetic entropy changes and adiabatic temperature changes. The magnetic entropy changes of 2.20 Jmol−1K−1 and 1.50 Jmol−1K−1 and the refrigerant capacity values of 135 Jmol−1 (ΔB=5 T) and 146 Jmol−1 (ΔB=7 T) are obtained for Gd80Au20 and Gd70Ni30, respectively. In addition, the influence of phase changes between crystalline and amorphous states on the isothermal entropy change was investigated.

The influence of the quadrupolar interaction in the magnetocaloric effect

Using a model Hamiltonian, which takes into account the dipolar and quadrupolar interaction within the molecular field approximation, it was possible to investigate the influence of a magnetic phase transition on the magnetocaloric potential. The results obtained show that discontinuous changes in the order parameters associated with the first-order phase transition involve a strong enhancement in magnetocaloric potential (which has practical importance for magnetic refrigeration efficiency) at the critical temperature. These results were fully discussed and correlated with the Maxwell thermodynamic relations.

Monte Carlo calculations of the magnetocaloric effect in (Gd0.6Tb0.4)5Si4

In this work we calculate the magnetocaloric effect in the compound (Gd0.6Tb0.4)5Si4(Gd0.6Tb0.4)5Si4. We use a model Hamiltonian of interacting 4f spins and treat the 4f spin–spin interaction in the Monte Carlo simulation. The theoretically calculated isothermal entropy changes upon variations of the magnetic field are in good agreement with the available experimental data.

Monte Carlo calculations of the magnetocaloric effect in RAl2 (R=Dy,Er)

In this work we calculate the magnetocaloric effect in the Laves phase compound RAl2 by using a model Hamiltonian of interacting spins where the spin-spin interaction is treated in the Monte Carlo simulation. The isothermal entropy change and the adiabatic temperature change upon magnetic field variations, for the compounds DyAl2 and ErAl2 are in good agreement with the available experimental data.

Theoretical investigation on the existence of inverse and direct magnetocaloric effect in perovskite EuZrO3

We report on the magnetic and magnetocaloric effect calculations in antiferromagnetic perovskite-type EuZrO3. The theoretical investigation was carried out using a model Hamiltonian including the exchange interactions between nearest-neighbor and next-nearest-neighbor for the antiferromagnetic ideal G-type structure (the tolerance factor for EuZrO3 is t = 0.983, which characterizes a small deformation from an ideal cubic perovskite). The molecular field approximation and Monte Carlo simulation were considered and compared. The calculated magnetic susceptibility is in good agreement with the available experimental data. For a magnetic field change from zero to 2 T a normal magnetocaloric effect was calculated and for a magnetic field change from zero to 1 T, an inverse magnetocaloric effect was predicted to occur below T = 3.6 K.

Calculations of the magnetic entropy change in amorphous through a microscopic anisotropic model: Applications to Dy70Zr30 and DyCo3.4 alloys

We report theoretical investigations on the magnetocaloric effect, described by the magnetic entropy change in rare earth—transition metal amorphous systems. The model includes the local anisotropy on the rare earth ions in Harris-Plischke-Zuckermann assumptions. The transition metals ions are treated in terms of itinerant electron ferromagnetism and the magnetic moment of rare earth ions is coupled to the polarized d-band by a local exchange interaction. The magnetocaloric effect was calculated in DyCo3.4 system, which presents amorphous sperimagnetic configuration. The calculations predict higher refrigerant capacity in the amorphous DyCo3.4 than in DyCo2 crystal, highlighting the importance of amorphous magnetocaloric materials. Our calculation of the magnetocaloric effect in Dy70Zr30, which presents amorphous asperomagnetic configuration, is in good agreement with the experimental result. Furthermore, magnetic entropy changes associated with crystal-amorphous configurations change are estimated.

Spin reorientation and the magnetocaloric effect in HoyEr(1−y)N

We report on the magnetic and magnetocaloric effect calculations in rare earth mononitrides HoyEr(1−y)N (y = 0, 0.5, 0.75, and 1). The magnetic Hamiltonian includes the crystalline electrical field in both magnetic sublattices; disorder in exchange interactions among Ho-Ho, Er-Er, and Ho-Er magnetic ions and the Zeeman effect. The theoretical results for the magnetic entropy change and adiabatic temperature change are in good agreement with the available experimental data. Besides, ferrimagnetic arrangement, inverse magnetocaloric effect, and spin reorientation transition (spin flop process) were predicted and quantitatively described.