M. Balli

Anisotropy-enhanced giant reversible rotating magnetocaloric effect in HoMn2O5 single crystals

Magnetic and magnetocaloric properties of HoMn2O5 single crystals were investigated. HoMn2O5 undergoes a large conventional magnetocaloric effect around 10 K. The magnetocaloric effect was found to present a giant anisotropy. Consequently, a large magnetocaloric effect (−ΔSR,max= 12.43 J/kg K for 7 T) can be obtained simply by rotating the single crystal HoMn2O5 within the cb plane in constant magnetic field instead of moving it in and out of the magnetic field zone. This can open the way for the implementation of compact, simplified, and efficient rotary magnetic refrigerators.

Advanced materials for magnetic cooling: Fundamentals and practical aspects

Over the last two decades, the research activities on magnetocalorics have been exponentially increased, leading to the discovery of a wide category of materials including intermetallics and oxides. Even though the reported materials were found to show excellent magnetocaloric properties on a laboratory scale, only a restricted family among them could be upscaled toward industrial levels and implemented as refrigerants in magnetic cooling devices. On the other hand, in the most of the reported reviews, the magnetocaloric materials are usually discussed in terms of their adiabatic temperature and entropy changes (ΔTad and ΔS), which is not enough to get more insight about their large scale applicability. In this review, not only the fundamental properties of the recently reported magnetocaloric materials but also their thermodynamic performance in functional devices are discussed. The reviewed families particularly include Gd1-xRx alloys, LaFe13-xSix, MnFeP1-xAsx, and R1-xAxMnO3 (R = lanthanide and A = div...

Giant rotating magnetocaloric effect at low magnetic fields in multiferroic TbMn2O5 single crystals

In conventional magnetocaloric refrigeration systems, the magnetocaloric effect is exploited by moving the active material in and out of the magnetic field source. Here, we demonstrate that a large and reversible magnetocaloric effect (−ΔSR, max = 6.4 J/kg K and ΔTR, max = 8 K under 2 T) can be generated simply by rotating the multiferroic TbMn2O5 single crystal around its b axis in a relatively low constant magnetic field applied in the ac plane. Our results should inspire and open ways toward the implementation of compact, efficient and embedded magnetocaloric devices for low temperature and space applications.

A study of the phase transition and magnetocaloric effect in multiferroic La2MnNiO6 single crystals

Magnetic and magnetocaloric properties of single crystal double perovskite La2MnNiO6 have been investigated in details. Its ordered phase with a high Curie temperature (TC = 280 K) exhibits a significant refrigerant capacity around room temperature. A model based on the mean field theory approximation has been used in order to quantify the magnetic and magnetocaloric properties in the ordered La2MnNiO6. The magnetization and entropy changes were satisfactorily simulated as a function of temperature and magnetic field. On the other hand, the presence of cationic disorder in La2MnNiO6 phases allows to shift the Curie point to low temperature without a significant change in the magnetocaloric performance.

Observation of large refrigerant capacity in the HoVO3 vanadate single crystal

The HoVO3 orthovanadate undergoes a large negative and conventional magnetocaloric effects around 4 K and 15 K, respectively. The partly overlapping of the magnetic transition at 15 K and the structural transition occurring at 40 K, as well as the large magnetization, give rise to a giant refrigerant capacity without hysteresis loss. For a magnetic field variation of 7 T, the refrigerant capacity is evaluated to be 620 J/kg, which is larger than that for any known RMnO3 manganite. These results should inspire and open new ways for the improvement of magnetocaloric properties of ABO3 type-oxides.

Analysis of the phase transition and magneto-thermal properties in La2CoMnO6 single crystals

Structural, magnetic, and magnetocaloric properties of the double perovskite La2CoMnO6 single crystals are investigated. The nature of the magnetic phase transition is analysed in terms of magnetic measurements and the Landau theory. The proportion of cationic order and the Co/Mn oxidation states were identified from the effective magnetic moment and Raman spectroscopy studies. La2CoMnO6 single crystals exhibit a significant refrigerant capacity. For a magnetic field change of 5 T, a refrigerant capacity of 222 J/kg is obtained. The magnetocaloric performance of La2CoMnO6 single crystals in the vicinity of the ferromagnetic-paramagnetic transition region (TC = 170 K) is discussed and compared with that of a polycrystalline sample.

Micro-Raman and infrared studies of multiferroic TbMn2O5

We have studied the Raman and infrared spectral response of TbMn2O5 under an applied magnetic field parallel to the easy magnetic a-axis at 4.2 K. Strong spin-lattice coupling in TbMn2O5 is evidenced by a frequency shift of Raman and infrared phonons as a function of magnetic field compared to the phonon response of BiMn2O5 that remains unaffected. The magnetic field behavior of the highest frequency phonons retraces the polarization switching in TbMn2O5 and shows an important frequency softening below 3 T that is modulated by the J 3 and J 4 exchange parameters. The role of the Tb(3+) spin alignment with H is interpreted in terms of a local lattice striction and the contribution of the charge transfer mechanism to the magnetoelectric process is evaluated.

First-principles study of electronic, electrical and optical properties of HoMn2O5

In this work, we have studied the electronic, electrical and optical properties of HoMn2O5 using first-principles density functional theory within the generalized gradient approximation (GGA). The electrical conductivity decreases with increasing temperature and it exhibits metal-like behavior. The maximum conductivity values as a function of relaxation time reach 11,44.1022 (Q-1m-1s-1) at 300 K. The optical calculation presents this material a good absorbing light in the visible region which is good for many optoelectronic and photovoltaic applications.

Raman and infrared study of 4f electron-phonon coupling in HoVO3

First-order Raman scattering and multiphonons are studied in RVO3 (R  =  Ho and Y) as a function of temperature in the orthorhombic and monoclinic phases. Raman spectra of HoVO3 and YVO3 unveil similar features since both compounds have nearly identical R-radii. However, the most important difference lies in the transition temperature involving the V(3+) orbitals, the V(3+) magnetic moments as well as the crystallographic structure. Particularly, the magnetic and orbital reorientations occur at T N2  =  40 K for HoVO3 instead T N2  =77 K in the case of YVO3. For both systems, anomalous phonon shifts which are related to spin-phonon coupling are observed below the V(3+) magnetic ordering temperature (T N1  ≈  110 K) while additional phonon anomalies are exclusively observed in HoVO3 around T (*)  ≈  15 K. On the other hand, infrared (IR) transmittance measurements as a function of temperature reveal Ho(3+5)I8  →  (5)I7 excitations and additional excitations assigned as vibronics. These latter combined with drastic changes in Ho(3+5)I8  →  (5)I7 excitations at T N2, are indicative of a strong coupling between the Ho(3+) ions and the ligand field. This could explain the large magnetocaloric capacity shown by HoVO3.