A.M. Gomes

Magnetocaloric properties of Ni2Mn1−xCuxGa

We present the magnetocaloric properties of the substituted Heusler alloy Ni2Mn1−xCuxGa, which shows a maximum magnetic entropy change of ΔSM≈−64J∕KgK=−532mJ∕cm3K at 308K for a magnetic field change ΔH=5T. The dependence of ΔSM on ΔH is approximately linear and does not reach saturation at 5T. It is demonstrated that the temperature at which ΔSM occurs can be tuned through subtle variations in composition.

Magnetocaloric effect in manganites: Metamagnetic transitions for magnetic refrigeration

We present a study of the magnetocaloric effect in La5/8-yPryCa3/8MnO3 (y=0.3) and Pr0.5Ca0.09Sr0.41MnO3 manganites. The low temperature state of both ystems is the result of a competition between the antiferromagnetic and ferromagnetic phases. The samples display magnetocaloric effect evidenced in an adiabatic temperature change during a metamagnetic transition from an antiferromagnetic to a ferromagnetic phase . As additional features, La5/8-yPryCa3/8MnO3 exhibits phase separation characterized by the coexistence of antiferromagnetic and ferromagnetic phases and Pr0.5Ca0.09Sr0.41MnO3 displays inverse magnetocaloric effect in which temperature decreases while applying an external magnetic field. In both cases, a significant part of the magnetocaloric effect appears from non-reversible processes. As the traditional thermodynamic description of the effect usually deals with reversible transitions, we developed an alternative way to calculate the adiabatic temperature change in terms of the change of the relative ferromagnetic fraction induced by magnetic field. To evaluate our model, we performed direct measurement of the samples adiabatic temperature change by means of a differential thermal analysis. An excellent agreement has been obtained between experimental and calculated data. These results show that metamagnetic transition in manganites play an important role in the study of magnetic refrigeration.

Field-tuned magnetocaloric effect in metamagnetic manganite system

We have investigated the origin of the huge magnetocaloric effect in a manganite system with ferro–antiferromagnetic phase mixture at low temperatures. We carried out magnetic measurements in fields up to 100 kOe in order to show that both a high hysteretic behavior and a metamagnetic transition in the antiferromagnetic state are responsible for the large magnetic entropy change. The temperature where the maximum of the magnetic entropy change occurs can be tuned by varying the maximum value of the applied magnetic field to the system. This last procedure can open a new perspective for charge-ordered manganite applications to magnetic refrigeration at low temperatures.

Magnetocaloric properties of (La, RE)Fe11.4Si1.6 compounds (RE=Y,Gd)

The mechanosynthesis process has been applied in the LaFe11.4Si1.6 compound to reduce the undesirable segregated rich-Fe phases that impair its application as a solid magnetic refrigerant. The influence of La substitution (5u2002at.u2009%) by Y or Gd atoms on the magnetic and magnetocaloric properties has been also studied. Y- and Gd-substituted compounds have a magnetic ordering temperature higher than the pure La compound. While the Y-substituted compound keeps a first-order-like magnetic transition feature, the Gd-substituted one seems to suppress it. The maximum value of the magnetic entropy change of the Y compound is roughly the same as the La compound (−18u2002J∕kgu2009K) but with a magnetic entropy change peak significantly broader. For the Gd-compound case a drastic reduction of the magnetic entropy change (−7u2002J∕kgu2009K) is found.

Magnetocaloric properties of the Ni2Mn1−x(Cu,Co)xGa Heusler alloys

We have investigated the magnetocaloric properties on the Ni2Mn1−xAxGa Heusler alloys with partial substitution of Mn by A=Co (x=0.10, 0.20, and 0.30) and Cu (x=0.15 and 0.20) in the vicinity of the martensitic transition by measuring magnetization curves at magnetic field up to 20kOe and in the temperature range of 250–300K. The changes of the magnetic part of entropy dependence on magnetic field and temperature have been evaluated.We have investigated the magnetocaloric properties on the Ni2Mn1−xAxGa Heusler alloys with partial substitution of Mn by A=Co (x=0.10, 0.20, and 0.30) and Cu (x=0.15 and 0.20) in the vicinity of the martensitic transition by measuring magnetization curves at magnetic field up to 20kOe and in the temperature range of 250–300K. The changes of the magnetic part of entropy dependence on magnetic field and temperature have been evaluated.

Synthesis and characterization of magnetic mesoporous particles.

Magnetic mesoporous particles were synthesized and their magnetic and structural properties are reported. The synthesis procedure consists of four steps: (i) preparation of magnetite colloidal nanoparticles; (ii) growth of a silica layer; (iii) development of the mesoporous structure and (iv) template removal. Two different methods for the template removal were studied and their effectiveness was discussed. Magnetization and Mössbauer spectroscopy measurements showed superparamagnetic behavior for the particles at room temperature. X-ray diffraction and nitrogen adsorption measurements showed a mesoporous MCM-41 structure with 2.48nm pore diameter and 1023m(2)/g total area.

La(Fe1−xCox)11.44Al1.56: A composite system for Ericsson-cycle-based magnetic refrigerators

The La(Fe1−xCox)11.44Al1.56 system, with x values of 0, 0.04, 0.08, and 0.12, was investigated for its magnetocaloric potential. For selected values of Co doping, it was possible to cover a wide magnetic ordering temperature range, from 200 up to 370K, and reduce substantially the metamagnetic critical fields. At applied magnetic fields up to 5T, the maximum magnetic entropy change approaches a nearly constant value of about 5J∕kgK for Co-doped compounds, with x varying from 0.04 to 0.12. The magnetic entropy change of a prototype composite was calculated in order to obtain a constant value in a wide temperature span. The results indicate that this material can be a good candidate for magnetic refrigeration using the Ericsson cycle.

Magnetocaloric effect in (Er, Tb)Co2

Abstract A study of the magnetocaloric effect in the serie Er 1− x Tb x Co 2 ( x =0.8 and 1) of polycrystalline samples was performed. From magnetization messurements under different applied fields, it was possible to calculate the change of magnetic entropy of each sample due to a change in magnetic field. We obtained a large value for the magnetocaloric potential in such compounds, comparable to those of rare-earth alloys.

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.

Low temperature specific heat of molecular nanomagnets and the contribution of the internal fields

Abstract We present the specific heat measurements at very low temperatures of the single molecule magnets Mn12ac and Fe8. Below 3xa0K, both systems present superparamagnetic blocking effects in zero external magnetic field and an excess specific heat contribution around 1xa0K. The experimental results are explained considering a distribution of internal fields of dipolar and hyperfine origin as well as quantum tunneling of the magnetization.