V.S.R. de Sousa

The giant anisotropic magnetocaloric effect in DyAl2

We report on calculations of the anisotropic magnetocaloric effect in DyAl2 using a model Hamiltonian including crystalline electrical field effects. The anisotropic effect is produced by the rotation of a constant magnetic field from the easy to a hard magnetic direction in the crystal and is enhanced by the first order nature of the field induced spin reorientation transition. The calculated results indicate that for a field with modulus of 2 T rotating from a hard to the easy direction, the isothermal magnetic entropy (ΔSiso) and adiabatic temperature (ΔTad) changes present peak values higher than 60% the ones observed in the usual process, in which the field direction is kept constant and the modulus of the field is varied.

A comparative study of the magnetocaloric effect in RNi2 (R=Nd,Gd,Tb) intermetallic compounds

Conventional and anisotropic magnetocaloric effects were studied in cubic rare earth RNi2 (R=Nd,Gd,Tb) ferromagnetic intermetallic compounds. These three compounds are representative of small, null, and large magnetocrystalline anisotropy in the series, respectively. Magnetic measurements were performed in polycrystalline samples in order to obtain the isothermal magnetocaloric data, which were confronted with theoretical results based on mean field calculations. For the R=Tb case, we explore the crystalline electrical-field anisotropy to predict the anisotropic magnetocaloric behavior due to the rotation of an applied magnetic field of constant intensity. Our results suggest the possibility of using both conventional and anisotropic magnetic entropy changes to extend the range of temperatures for use in the magnetocaloric effect.

The influence of the magnetoelastic interaction on the magnetocaloric effect in ferrimagnetic systems: a theoretical investigation

In this work the magnetocaloric effect is theoretically investigated considering a microscopic model Hamiltonian, which describes a magnetic system formed by two sublattices of different magnetic ions coupled by exchange and magnetoelastic interactions. We analyze systematically several profiles of the ferrimagnetic arrangements that were studied earlier without the magnetoelastic interaction. The influence of changing the magnetoelastic parameters on the magnetization, isothermal entropy change and adiabatic temperature change curves are investigated. Depending on the model parameters, the magnetic system shows a first-order magnetic phase transition leading to high direct and inverse magnetocaloric effect, besides two simultaneous first-order magnetic phase transitions which were predicted. A constant ΔS(T) = 0.4 J mol(-1) K(-1) is obtained in the simulated system in a temperature interval of 50 K, around 110 K.

The influence of spontaneous and field induced spin reorientation transitions on the magnetocaloric properties in rare earth intermetallic compounds: Application to TbZn

We report a theoretical investigation on the magnetocaloric properties of the cubic CsCl-type TbZn compound. Two successive peaks in the magnetocaloric quantities are observed and attributed to different types of phase transitions. For the magnetic field applied in the ⟨110⟩ direction, the first peak is ascribed to a spontaneous first-order spin reorientation transition (SRT) at T1=63 K, and the second one to the ferroparamagnetic phase transition. The application of an external magnetic field of 2 T along this direction leads to a tablelike behavior in the magnetocaloric quantities (ΔST and ΔTS) as a consequence of two successive second-order SRTs at TSR1=71 K and at TSR2=160 K. Applying a magnetic field of 5 T suppress the flat behavior but a high refrigeration capacity of 352 J/kg is predicted in a wide temperature range from 62 to 258 K. When the magnetic field is applied along the ⟨100⟩ direction an inverse magnetocaloric effect is observed in the temperature range below T1. The system was studied th...

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.

Electric field triggering the spin reorientation and controlling the absorption and release of heat in the induced multiferroic compound EuTiO3

We report remarkable results due to the coupling between the magnetization and the electric field induced polarization in EuTiO3. Using a microscopic model Hamiltonian to describe the three coupled sublattices, Eu-(spin-up), Eu-(spin-down), and Ti-(moment), the spin flop and spin reorientation phase transitions were described with and without the electric-magnetic coupling interaction. The external electric field can be used to tune the temperature of the spin reorientation phase transition TSR = TSR(E). When the TSR is tuned around the EuTiO3—Neel temperature (TN = 5.5 K), an outstanding effect emerges in which EuTiO3 releases heat under magnetic field change. The electric field controlling the spin reorientation transition and the endo-exothermic processes are discussed through the microscopic interactions model parameters.

Theoretical investigation on the barocaloric and magnetocaloric properties in the Gd5Si2Ge2 compound

We report a theoretical microscopic model to discuss the barocaloric and magnetocaloric effects in the Gd5Si2Ge2 compound based on the recent experimental data by Yuce et al. [Appl. Phys. Lett. 101, 071906 (2012)]. For this purpose, our model Hamiltonian includes three interactions: Zeeman, magnetoelastic, and exchange interactions, considering the magnetic field dependence of phonons entropy. Using this model, the combined magnetocaloric and barocaloric effects were calculated in Gd5Si2Ge2 compound showing satisfactory agreement with the experimental data. Besides, a high entropy change was predicted for simultaneous changes in the applied magnetic field and pressure (the combined magnetocaloric and barocaloric effects).

Theoretical investigation on the magnetocaloric effect in garnets R3Fe5O12 where (R=Y and Dy)

In this work the magnetocaloric effect in ferrimagnetic rare-earth iron garnets R3Fe5O12 where R=Y and Dy was theoretically investigated considering a model Hamiltonian, which takes into account three coupled magnetic sublattices in the mean field approximation. In Dy3Fe5O12 the inverse magnetocaloric effect was predicted and associated with the compensation temperature. Our theoretical results for the adiabatic temperature change in both compounds are in good agreement with the experimental data.

Theoretical investigation on the magnetic and electric properties in TbSb compound through an anisotropic microscopic model

We report the strong correlations between the magnetoresistivity and the magnetic entropy change in the cubic antiferromagnetic TbSb compound. The theoretical investigation was performed through a microscopic model which takes into account the crystalline electrical field anisotropy, exchange coupling interactions between the up and down magnetic sublattices, and the Zeeman interaction. The easy magnetization directions changes from ⟨001⟩ to ⟨110⟩ and then to ⟨111⟩ observed experimentally was successfully theoretically described. Also, the calculation of the temperature dependence of electric resistivity showed good agreement with the experimental data. Theoretical predictions were calculated for the temperature dependence of the magnetic entropy and resistivity changes upon magnetic field variation. Besides, the difference in the spin up and down sublattices resistivity was investigated.