Magnetocaloric effect: From materials research to refrigeration devices

Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

https://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004197.pdf

Zinc Vacancy induced magnetism in ZnO thin films and nanowires

We report here enhancement of ferromagnetism in pure ZnO upon thermal annealing with the ferromagnetic transition temperature Tc above room temperature. We observe a finite coercive field upto 300K and a finite thermoremanent magnetization upto 340K for the annealed sample. We propose that magnetic moments can form at anionic vacancy clusters. Ferromagnetism can occur due to either superexchange between vacancy clusters via isolated F+ centers, or through a limited electron delocalization between vacancy clusters. Isolated vacancy clusters or isolated F+ centers give rise to a strong paramagnetic like behaviour below 10K.

Enhancement of ferromagnetism upon thermal annealing in pure ZnO

First‐principles electronic structure calculations on wurtzite AlN, GaN, and InN reveal crystal‐field splitting parameters ΔCF of −217, 42, and 41 meV, respectively, and spin–orbit splitting parameters Δ0 of 19, 13, and 1 meV, respectively. In the zinc blende structure ΔCF≡0 and Δ0 are 19, 15, and 6 meV, respectively. The unstrained AlN/GaN, GaN/InN, and AlN/InN valence band offsets for the wurtzite (zinc blende) materials are 0.81 (0.84), 0.48 (0.26), and 1.25 (1.04) eV, respectively. The trends in these spectroscopic quantities are discussed and recent experimental findings are analyzed in light of these predictions.

Valence band splittings and band offsets of AlN, GaN and InN.

Author(s): Ahn, KH; Wu, XW; Liu, K; Chien, CL | Abstract: The effect of Fe doping (l20%) on the Mn site in the ferromagnetic ((Formula presented)) and the antiferromagnetic ((Formula presented)) phases of (Formula presented) has been studied. The same ionic radii of (Formula presented) and (Formula presented) cause no structure change in either series, yet conduction and ferromagnetism have been consistently suppressed by Fe doping. Colossal magnetoresistance has been shifted to lower temperatures, and in some cases enhanced by Fe doping. Doping with Fe bypasses the usually dominant lattice effects, but depopulates the hopping electrons and thus weakens the double exchange. © 1996 The American Physical Society.

Magnetic properties and colossal magnetoresistance of LaÑCaÖMnO3 materials doped with Fe

Though ZnO is known as a diamagnetic material, recent studies have revealed that its nanostructures can be ferromagnetic (FM). The FM origin has been ascribed to intrinsic defects. This work shines light on an alternate method based on mechanical milling to induce defect-related ferromagnetism in ZnO nanoparticles (NPs) from initial diamagnetic ZnO powders. Our idea is motivated by the fact that mechanical milling introduces more defects to a ground material. We point out that the FM order increases with increasing the density of defects in ZnO NPs. The experimental results obtained from analyzing X-ray absorption, electron spin resonance, and Raman scattering spectra demonstrate that the ferromagnetism in ZnO NPs is due to intrinsic defects mainly related to oxygen and zinc vacancies. Among these, zinc vacancies play a decisive role in introducing a high FM order in ZnO NPs.

Defect-induced ferromagnetism in ZnO nanoparticles prepared by mechanical milling

We prepared a polycrystalline sample of orthorhombic La0.7Ca0.2Sr0.1MnO3 and then investigated its magnetic properties. Detailed studies and analyses in the vicinity of the ferromagnetic (FM)-paramagnetic phase-transition temperature prove the sample undergoing a second-order phase transition, where variations of the saturation magnetization and the initial susceptibility versus temperature obey asymptotic relations. Basing on the modified Arrott plot, we have determined the critical parameters (TC, β, γ, and δ) in the two characteristic regions of low- and high-magnetic fields, and in the cases of no-subtracting and subtracting the demagnetization field (Hd). The experimental results reveal that values of the critical parameters depend remarkably on analysis routes of isothermal-magnetization data. Intrinsically, β keeps close to the value expected for the 3D Heisenberg ferromagnets with ferromagnetic short-range interactions, while γ is close to the value of the mean-field theory. These results are diff...

Critical behavior and magnetic-entropy change of orthorhombic La0.7Ca0.2Sr0.1MnO3

We have systematically investigated the compositional dependence of the magnetic and magnetocaloric properties of La0.7-xPrxSr0.3MnO3 (LPSMO). Polycrystalline samples of LPSMO with 0.02 ≤ x ≤ 0.30 were prepared by a standard solid-state reaction method with phase purity and structure confirmed using x-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal second order ferromagnetic transitions in each sample with Curie temperature decreasing progressively with increasing Pr content from ∼350 K for x = 0.02 to ∼295 K for x = 0.30. Magnetic entropy change (ΔSM) was calculated by applying the thermodynamic Maxwell equation to a series of isothermal field dependent magnetization curves. In the sample with x = 0.30, the maximum value of −ΔSM reaches ∼2.08 J/kg K at 295 K for a field change of 1.5 T. Reduced Pr content corresponds to larger values of entropy change, reaching −ΔSM ∼2.79 J/kg K for the x = 0.02 doping. The refrigeration capacity for each composition reached s...

Tunable magnetocaloric effect near room temperature in La0.7-xPrxSr0.3MnO3 (0.02 ≤ x ≤ 0.30) manganites

We report a systematic study of the magnetic and magnetocaloric properties of La0.7Ca0.3Mn1-xFexO3, where trivalent Fe is substituted for Mn on the perovskite B-site in the doping range 0.00≤x≤0.07. The polycrystalline materials were prepared by a standard solid state reaction method and X-ray diffraction confirmed that Fe was incorporated without significant structural changes. Magnetic measurements indicate that the ferromagnetic double exchange interaction is weakened with increasing Fe concentration, resulting in a shift in TC from 263 K to 114 K as x varied between 0.00 and 0.07. The large entropy change of 7.7 J/kg K found in the sample with x=0 for a field change of 3 T is initially enhanced to 8.6 J/kg K for x = 0.01, which is likely due to oxygen deficiency. As Fe content increased further, peak entropy values were seen to decline. However, a simultaneous broadening of the ΔSM(T) peaks led to enhanced refrigerant capacity (RC) in the Fe-doped samples of up to 30% over that of La0.7Ca0.3MnO3. Arro...

Y. D. Zhang

Papers

Defect-induced ferromagnetism in ZnO nanoparticles prepared by mechanical milling

Critical behavior and magnetic-entropy change of orthorhombic La0.7Ca0.2Sr0.1MnO3

Tunable magnetocaloric effect near room temperature in La0.7-xPrxSr0.3MnO3 (0.02 ≤ x ≤ 0.30) manganites

Magnetic phase transitions and magnetocaloric effect in La0.7Ca0.3Mn1-xFexO3 0.00 ≤ x ≤ 0.07 manganites

Local structure and magnetocaloric effect for La0.7Sr0.3Mn1−xNixO3