Unusual rotating magnetocaloric effect in the hexagonal ErMnO3 single crystal

Abstract

It is known that orthorhombic RMnO3 multiferroics (R = magnetic rare earth) with low symmetry exhibit a large rotating magnetocaloric effect because of their strong magnetocrystalline anisotropy. In this paper, we demonstrate that the hexagonal ErMnO3 single crystals also unveils a giant rotating magnetocaloric effect that can be obtained by spinning them in constant magnetic fields around their a or b axes. When the ErMnO3 crystal is rotated with the magnetic field initially parallel to the c-axis, the resulting entropy change reaches maximum values of 7, 17 and 20 J/kg K under a constant magnetic field of 2, 5 and 7 T, respectively. These values are comparable or even larger than those shown by some of the best orthorhombic phases. More interestingly, the generated anisotropic thermal effect is about three times larger than that exhibited by the hexagonal HoMnO3 single crystal.The enhancement of the rotating magnetocaloric effect in the hexagonal ErMnO3 compound arises from the unique features of Er 3+ magnetic sublattice. In fact, the Er 3+ magnetic moments located at 2a sites experience a first-order metamagnetic transition close to 3 K along the c-axis resulting in a peaked magnetocaloric effect over a narrower temperature range. In contrast, the "paramagnetic" behaviour of Er 3+ magnetic moments within the ab-plane, produces a larger magnetocaloric effect over a wider temperature range. Therefore, the magnetocaloric effect anisotropy is maximized between the c and the ab-directions, leading to a giant rotating magnetocaloric effect. *Mohamed.balli@usherbrooke.ca I. IntroductionBased on the well-known magnetocaloric effect (MCE), magnetic cooling is a trending technology that continues to attract a worldwide interest due to its potential high thermodynamic efficiency as well as its ecofriendly character [1][2][3][4][5][6][7][8]. The implementation of this emergent technology in our daily life would enable to fully suppress the harmful synthetic refrigerants such as fluorinated fluids, usually present in standard refrigerators and air-conditioners [1][2][3][4][5][6][7][8], thus allowing to meet the requirements of several treaties that were universally adopted by the international community aiming to reduce the utilization of CFCs, HCFCs and HFCs gases and, green house gases (GHG) emissions [1]. However, the search for advanced magnetocaloric materials with outstanding thermal, chemical and mechanical properties is necessary for the transfer of magnetic refrigeration technology towards the market place. In this context, a large MCE has been pointed out in a wide variety of magnetocaloric materials including both intermetallics and oxides [1-8]. Some of them such as LaFe13-xSix compounds, Fe2P-type materials and ABO3-based oxides have been successfully tested in room-temperature devices unveiling the bright future of magnetic refrigeration [1]. On the other hand, magnetic materials with excellent caloric effects at the temperature range below 30 K are of great importance in several low-temperature applications...

Comment on “Giant anisotropy of magnetocaloric effect in TbMnO3 single crystals”

“…Similarly to the standard MCE, the isothermal entropy change associated with the rotation of TbVO4 single crystals between two different crystallographic directions can be also determined from magnetic isotherms. Considering the magnetic field initially applied along the c direction, the obtained rotating magnetocaloric effect (RMCE) in term of ΔST when spinning the crystals between the hard and easy axes by an angle of 90 ° in a constant magnetic field H is simply given by [1,14,[16][17][18]:…”

“…The latter have used the magnetocaloric garnet Dy2.4Gd0.6Al5O12 as refrigerant, resulting in the decrease of the hydrogen temperature at the cold side down to 20 K. However, the large specific heat usually shown by the garnet compounds 3 lower drastically their MCE in term of the adiabatic temperature change that reaches only 1 to 2 K in the magnetic field change of 1 T [8,9]. In order to deal with these drawbacks, interesting materials with better magnetocaloric performance in the temperature range below 30 K were recently reported including both intermetallics [10][11][12][13] and oxides such as RMO3 (M = metal) [14][15][16][17][18][19][20][21]. On the other hand, the RMn2O5 multiferroics are also of great importance in the cryogenic temperature range not only because of their strong MCEs that are usually obtained by varying magnetic fields along their easy-axes, but also due to their strong magnetocrystalline anisotropy.…”

“…The enhancement of the isothermal entropy change in the TbVO4 single crystal following the a axis can be partly attributed to the possibility to fully order the Tb 3+ magnetic moments by applying low magnetic fields along this direction as shown in figures 4 and 5. Unlike the manganite compounds such as RMnO3 and RMn2O5 [14][15][16][17][18][19][20][21][22][23], the absence of highly frustrated magnetism would contribute to this enhancement. On the other hand, the TbVO4 crystals unveil isothermal entropy changes (under 2 T) larger than those reported in a similar family of orthovanadates such as GdVO4 (about 10 J/kg K for 2 T) [31,33].…”

Effect of magnetocrystalline anisotropy on magnetocaloric properties of AlFe$_{2}$B$_{2}$ compound

“…6,7 The application of magnetocrystalline anisotropy is the rotating magnetocaloric effect. [8][9][10][11][12][13][14] The temperature and heat exchange of the anisotropic magnetic material can be controlled by rotating the sample (or changing the relative direction of the external magnetic field) at a fixed strength of the applied magnetic field. The origin of this effect is the modification of the magnetocrystalline anisotropy on the magnetization when changing the relative direction of the external magnetic field.…”

“…However, several experimental studies have found that the isothermal magnetic entropy change is modified not only on the low-temperature side but also on the high-temperature side of the main peak when a magnetic field is applied in different directions. [8][9][10][11][12][13][14] The magnetocrystalline anisotropy energy affects the isothermal magnetic entropy change even above the Curie temperature in the anisotropic magnetocaloric effect.…”