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تسجيل مستخدم جديدLarge adiabatic temperature and magnetic entropy changes in EuTiO3
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We have investigated the magnetocaloric effect in single and polycrystalline samples of quantum paraelectric EuTiO3 by magnetization and heat capacity measurements. Single crystalline EuTiO3 shows antiferromagnetic ordering due to Eu2+ magnetic moments below TN = 5.6 K. This compound shows a giant magnetocaloric effect around its Neel temperature. The isothermal magnetic entropy change is 49 Jkg-1K-1, the adiabatic temperature change is 21 K and the refrigeration capacity is 500 JKg-1 for a field change of 7 T at TN. The single crystal and polycrystalline samples show similar values of the magnetic entropy change and adiabatic temperature changes. The large magnetocaloric effect is due to suppression of the spin entropy associated with localized 4f moment of Eu2+ ions. The giant magnetocaloric effect together with negligible hysteresis, suggest that EuTiO3 could be a potential material for magnetic refrigeration below 20 K.
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Magnetic resonance spectra of EuTiO3 in both bulk and thin film form were taken at temperatures from 3-350 K and microwave frequencies from 9.2-9.8 and 34 GHz. In the paramagnetic phase, magnetic resonance spectra are determined by magnetic dipole an
d exchange interactions between Eu2+ spins. In the film, a large contribution arises from the demagnetization field. From detailed analysis of the linewidth and its temperature dependence, the parameters of spin-spin interactions were determined: the exchange frequency is 15-15.5 GHz and the estimated critical exponent of the spin correlation length is ~ 0.5. In the bulk samples, the spectra exhibited a distinct minimum in the linewidth at the Neel temperature, T_N = 5.5 K, while the resonance field practically does not change even on cooling below T_N. This is indicative of a small magnetic anisotropy ~ 320 G in the antiferromagnetic phase. In the film, the magnetic resonance spectrum is split below T_N into several components due to excitation of the magnetostatic modes, corresponding to a non-uniform precession of magnetization. Moreover, the film was observed to degrade over two years. This was manifested by an increase of defects and a change in the domain structure. The saturated magnetization in the film, estimated from the magnetic resonance spectrum, was about 900 emu/cm3 or 5.5 mu_B/unit cell at T = 3.5 K.
Polycrystalline ceramic samples and a single crystal of EuTiO3 have been investigated by Raman spectroscopy in the temperature range 80-300 K. Although synchrotron XRD data clearly indicated the cubic to tetragonal phase transition around 282 K, no m
ode from the symmetry allowed Raman active phonons was found in the tetragonal phase, contrary to the case of the homologous SrTiO3. In order to study the evolution of this unique characteristic, ceramics of EuxSr1-xTiO3 (x=0.03-1.0) characterized by synchrotron XRD for the structural phase transition have been also investigated by Raman spectroscopy, verifying the very strong influence on the Raman yield by Eu substitution. By applying an external magnetic field or alternatively hydrostatic pressure modes are activated in the Raman spectra. The temperature dependence of the main mode that is activated shows remarkable agreement with theoretical predictions. We attribute the puzzling absence of the Raman modes to a mechanism related to strong spin-lattice interaction that drives the cubic to tetragonal structural phase transition and makes the Raman tensor antisymmetric. On the contrary, the external perturbations induce a symmetric Raman tensor allowing even symmetry modes to be present in the spectra. Previous EPR, muon scattering and magnetic measurements indicated the presence of small magnetic interactions deep inside the paramagnetic phase. In order to probe those magnetic interactions in our EuTiO3 polycrystalline sample and test our hypothesis, we have performed temperature dependant XAS/XMCD, which support the existence of magnetic nanodomains even close to room temperature.
The multiferroic properties of EuTiO3 are greatly enhanced when a sample is strained, signifying that coupling between strain and structural, magnetic or ferroelectric order parameters is extremely important. Here resonant ultrasound spectroscopy has
been used to investigate strain coupling effects, as well as possible additional phase transitions, through their influence on elastic and anelastic relaxations that occur as a function of temperature between 2 and 300 K and with applied magnetic field up to 14 T. Antiferromagnetic ordering is accompanied by acoustic loss and softening, and a weak magnetoelastic effect is also associated with the change in magnetization direction below ~2.8 K. Changes in loss due to the influence of magnetic field suggest the existence of magnetic defects which couple with strain and may play a role in pinning of ferroelastic twin walls.
A typical strategy of realizing an adiabatic change of a many-particle system is to vary parameters very slowly on a time scale $t_text{r}$ much larger than intrinsic equilibration time scales. In the ideal case of adiabatic state preparation, $t_tex
t{r} to infty$, the entropy production vanishes. In systems with conservation laws, the approach to the adiabatic limit is hampered by hydrodynamic long-time tails, arising from the algebraically slow relaxation of hydrodynamic fluctuations. We argue that the entropy production $Delta S$ of a diffusive system at finite temperature in one or two dimensions is governed by hydrodynamic modes resulting in $Delta S sim 1/sqrt{t_text{r}}$ in $d=1$ and $Delta S sim ln(t_text{r})/t_text{r}$ in $d=2$. In higher dimensions, entropy production is instead dominated by other high-energy modes with $Delta S sim 1/t_text{r}$. In order to verify the analytic prediction, we simulate the non-equilibrium dynamics of a classical two-component gas with point-like particles in one spatial dimension and examine the total entropy production as a function of $t_text{r}$.
We report the observation of large magnetocaloric effect near room temperature in antipervoskite SnCMn3. The maximal magnetic entropy change at the first-order ferrimagnetic-paramagnetic transition temperature (TC 279 K) is about 80.69mJ/cm3 K and 13
3mJ/cm3 K under the magnetic field of 20 kOe and 48 kOe, respectively. These values are close to those of typical magnetocaloric materials. The large magnetocaloric effect is associated with the sharp change of lattice, resistivity and magnetization in the vicinity of TC. Through the measurements of Seebeck coefficient and normal Hall effect, the title system is found to undergo a reconstruction of electronic structure at TC. Considering its low-cost and innocuous raw materials, Mn-based antiperovskite compounds are suggested to be appropriate for pursuing new materials with larger magnetocaloric effect.
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