Anisotropic Magnetocaloric Effect Research Articles

Magnetocaloric Effect of RCo<sub>5</sub> Single Crystals in the Region of Spin-Reorientation Transitions

Direct measurements of the magnetocaloric effect (MCE) in RCo5 single crystals in the region of spin-reorientation transitions (SRT) were carried out. It was shown that the values of MCE in the region of SRTs are comparable to the values of MCE in the vicinity of the Curie temperature. The anisotropy constants of the compounds were measured by the torque method. The anisotropic MCE calculation on the basis of anisotropy constants showed a good accordance with the experimental data.

Giant and anisotropic magnetocaloric effect in antiferromagnetic single crystalline DySb

Magnetocaloric effect has been studied for antiferromagnetic single crystalline DySb, by measuring the temperature ( T ) and magnetic field ( H ) dependences of magnetization in the range 2 ≤ T ≤ 300 K and 0 ≤ H ≤ 7 T with field along the [100] and the [110] direction. The magnetic entropy change caused by a field change of 5 T is as larger as −36.4 J kg −1 K −1 for H //[110] around 11 K, which is a value much greater than that reported for the polycrystalline sample. The magnetocaloric effect is further found to be anisotropic. Above T N (=10 K), the entropy change is much larger for H //[110] than for H //[100]. In contrast, the entropy change is positive below T N , and it is significantly lower for H //[110] than for H //[100]. Our results indicate that DySb is a typical material possessing giant and highly anisotropic magnetocaloric effect, and suggest that magnetic orientation could be used to enhance magnetic entropy change.

A comparative study of the magnetocaloric effect in RNi 2 (R = Dy, Ho, Er) intermetallic compounds

In this work we study the conventional and anisotropic magnetocaloric effects in the cubic rare-earth RNi 2 (R = Dy, Ho, Er) ferromagnetic intermetallic compounds. For each of these compounds we have explored the crystalline electrical-field anisotropy to predict the anisotropic magnetocaloric quantities under rotations of an applied magnetic field of constant intensity. The calculations were performed using proper parameters reported in the literature. Our results for the anisotropic magnetocaloric effect reveal some peculiarities of the spin reorientation which can be considered in designing new magnetocaloric materials for magnetic refrigerators.

The anisotropic magnetocaloric effect described by Maxwell formulation: Application to DyAl 2 and TbNi 2

In this work we present the Maxwell-like formulation of the anisotropic magnetocaloric effect for materials in single crystalline form subjected to a rotating applied magnetic field of constant intensity. This effect is a new topic in the magnetocaloric research area and can highlight some relevant mechanisms on spin reorientation. Application of the model to the ferromagnetic compounds DyAl 2 and TbNi 2, which are subjected to cubic crystalline electric field anisotropy and isotropic exchange interaction, are performed. We obtain continuous and discontinuous reorientation transitions for the Cartesian components of magnetization which revealed some characteristics of the anisotropic magnetocaloric curves.

Investigation on the magnetocaloric effect in DyNi 2, DyAl 2 and Tb 1−nGd nAl 2 ( n=0, 0.4, 0.6) compounds

The magnetocaloric effect (MCE) in the DyNi 2, DyAl 2 and Tb 1− n Gd n Al 2 ( n=0, 0.4, 0.6) was theoretically investigated in this work. The DyNi 2 and DyAl 2 compounds are described considering a model Hamiltonian which includes the crystalline electrical field anisotropy. The anisotropic MCE was calculated changing the magnetic field direction from 〈1 1 1〉 to 〈0 0 1〉 in DyNi 2 and from 〈1 0 0〉 to 〈0 1 1〉 in DyAl 2. The influence of the second- and first-order spin-reorientation phase transitions on the MCE that occurs in these systems is discussed. For the calculations of the MCE thermodynamic quantities in the Tb 1− n Gd n Al 2 systems we take into account a two sites magnetic model, and good agreement with the available experimental data was obtained.

Anisotropy of the magnetocaloric effect in DyNiAl

We present study of the anisotropic magnetocaloric effect in DyNiAl. This compound crystallizes in the hexagonal ZrNiAl-type structure, orders magnetically below T C = 31 K and undergoes a further magnetic phase transition at T 1 = 15 K . The Dy-moments are aligned ferromagnetically along the hexagonal c -axis below T C , the additional antiferromagnetic component develops within the basal plane below T 1 . The magnetocaloric effect was evaluated from the magnetization measurements with field applied along the c -axis and perpendicular to it. Our data reveal a strong anisotropy of the magnetocaloric effect. The large effect occurs for field applied along the c -axis whereas the entropy change is small for the perpendicular field direction.

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.

Theoretical investigation on the anisotropic magnetocaloric effect: Application to DyAl 2

Using a simple model Hamiltonian, the formulation of the anisotropic MCE was introduced and the calculations performed for DyAl 2 leads to quantitative values of Δ S iso[0 0 1][1 0 0]. An anomalous MCE was also predicted when the magnetic field is applied along the non-easy magnetization direction [0 0 1].

Magnetic field induced phase transitions in Gd5(Si1.95Ge2.05) single crystal and the anisotropic magnetocaloric effect

Magnetization measurements using a Gd5(Si1.95Ge2.05) single crystal with the magnetic field applied along three crystallographic directions, [001], [010] and [100], were carried out as a function of the applied field (0–56 kOe) at various temperatures (∼5–320 K). The magnetic field (H)–temperature (T) phase diagrams were constructed for the Gd5(Si1.95Ge2.05) single crystal with field along the three directions. A small anisotropy was observed. The magnetocaloric effect was calculated from isothermal magnetization data, and the observed anisotropy correlates with the H–T phase diagrams. The results are discussed in connection with the magnetic field induced martensitic-like structural transition observed in Gd5(Si2Ge2)-type compounds.

Anisotropic magnetocaloric effect in nanostructured magnetic clusters.

We report the first experimental observation of anisotropic magnetocaloric effect (MCE) in the Fe8 clusters. It is found that the magnetic anisotropy plays a very important role in the determination of the magnetocaloric effect. The maximum and minimum MCE's are observed when the applied magnetic fields are parallel and perpendicular to the easy axis, respectively. The quantum spin Hamiltonian of a Fe8 cluster is used to calculate the partition function and the magnetization in a range of temperature and magnetic field. Excellent quantitative agreement between the experimental data and calculation is observed.