Km Rubi

Large adiabatic temperature and magnetic entropy changes in EuTi O3

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.

Giant magnetocaloric effect in magnetoelectric Eu1-xBaxTiO3

We report the magnetic entropy change (ΔSm) in magnetoelectric Eu1-xBaxTiO3 for 0.1 ≤ x ≤ 0.9. We find −ΔSm = 11 (40) J/kg·K in x = 0.1 for a field change of 1 (5) T, respectively, which is the largest value among all Eu-based oxides. ΔSm arises from the field-induced suppression of the spin entropy of Eu2+:4f7 localized moments. While ∣−ΔSm∣ decreases with increasing x, ∣−ΔSm∣ = 6.58 J/kg·K observed in the high spin diluted composition x = 0.9 is larger than that in many manganites. Our results indicate that these magnetoelectrics are potential candidates for cryogenic magnetic refrigeration.

Large magnetoresistance over a wide temperature range in Eu0.99La0.01TiO3

We report the magnetization (M ), electrical resistivity and magnetoresistance (MR ) in the electron-doped antiferromagnet Eu0.99 La0.01 TiO3 . While M (T ) measured upon cooling indicates the occurrence of a paramagnetic to antiferromagnetic transition at , zero field goes through a broad maximum at . The application of an external magnetic field raises the value of T p and decreases the magnitude of ρ at T p leading to a negative magnetoresistance (MR ) effect. A large MR of for is observed at 2.5 K with a remarkable change occurring in sub-tesla magnetic fields ( for ). In addition, significant MR prevails even up to 10 T N ( at 50 K). While MR over a wide field range for can be satisfactorily described by the equation , MR scales with M below T N . Unlike the resistivity, thermopower is insensitive to magnetic fields. Our results indicate that electrons doped into the Ti-3d conduction band are strongly coupled to localized 4f 7 spins of Eu2+ ions via the exchange interaction. We suggest that the observed MR is most likely caused by the field-induced suppression of 4f spin fluctuations and the subsequent reduction of the scattering of 3d electrons. This is a unique example in perovskite oxides where the magnetoresistance of 3d electrons is controlled by spin fluctuations associated with 4f localized electrons of a rare-earth ion.

Magnetocaloric properties of Eu1−xLaxTiO3 (0.01 ≤ x ≤ 0.2) for cryogenic magnetic cooling

We report magnetic and magnetocaloric (MCE) properties of polycrystalline Eu1−xLaxTiO3 samples over a wide composition range (0.01 ≤ x ≤ 0.20). It is found that the ground state changes from antiferromagnetic for x = 0.01 (TN = 5.2 K) to ferromagnetic for x ≥ 0.03 and the ferromagnetic Curie temperature increases from TC = 5.7 K for x = 0.03 to TC = 7.9 K for x = 0.20. The x = 0.01 sample shows a large reversible isothermal magnetic entropy change of −ΔSm = 23 (41.5) J/kg K and adiabatic temperature change of ΔTad = 9 (17.2) K around 6.7 K for a field change of μ0ΔH = 2 (5) Tesla. Although the peak value of −ΔSm decreases as La content increases, it is impressive in x = 0.2(−ΔSm = 31.41 J/kg K at T = 7.5 K for μ0ΔH = 5 T). The large value of MCE arises from suppression of the spin entropy associated with the localized moment (J = 7/2) of Eu2+:4f7 ions. This large MCE over a wide compositional range suggests that the Eu1−xLaxTiO3 series could be useful for magnetic cooling below 40 K.

Room temperature giant magnetoimpedance in polycrystalline La0.75Ba0.25MnO3

We report the magnetic field dependence of electrical impedance Z in polycrystalline La0.75Ba0.25MnO3 a function of frequency from f = 1 MHz to 1 GHz at room temperature. The magnetoimpedance [MI = ΔZ/Z(0) where ΔZ = Z(H)-Z(0)] is −37.5 % (-58.5%) at 1(10) MHz for H = 1.2 kOe, which far exceeds mere -1.15% dc magnetoresistance. As f increases above 10 MHz, MI decreases in magnitude and changes sign from negative to positive. The change in sign of MI results from the transition of a single peak at H = 0 to two peaks at H = ± Hp accompanied by a minimum at the origin. Hp shifts towards higher field with increasing frequency. The occurrence of giant radio-frequency magnetoimpedance is promising for low-field sensor applications.

Strong electron–phonon interaction and colossal magnetoresistance in EuTiO3

Abstract At low temperatures, EuTiO3 system has very large resistivities and exhibits colossal magnetoresistance. Based on a first principle calculation and the dynamical mean-field theory for small polaron we have calculated the transport properties of EuTiO3. It is found that due to electron–phonon interaction the conduction band may form a tiny polaronic subband which is close to the Fermi level. The tiny subband is responsible for the large resistivity. Besides, EuTiO3 is a weak antiferromagnetic material and its magnetization would slightly shift the subband via exchange interaction between conduction electrons and magnetic atoms. Since the subband is close to the Fermi level, a slight shift of its position gives colossal magnetoresistance.

Solution-processed 2-dimensional hole-doped ionic graphene compounds

Contacts with suitable work functions are important not only for ohmic injection of carriers but also to set up the built-in potential required for various semiconductor processes including current rectification, light emission and photovoltaic generation. For two-dimensional (2D) materials, one way to shift the work function is by intercalation doping with suitable donors or acceptors. Here, using an atomic sheet transfer methodology, we report layer-by-layer assembly of centimeter-square sizes of graphene–fluorofullerene multilayers through directed stacking of chemical-vapor-deposited (CVD) graphene sheets and self-assembly of fluorofullerene acceptors, for example C60F48, to give a 1 eV increase in work function to 5.7 eV, which is unprecedented for a well-defined compound. These assemblies exhibit an unusual motif of fully-ionized large dopants in open packing with the graphene sheets. As a consequence, they show a sizeable electrostatic dipole to give ultrahigh work function at acceptor-terminated surfaces even for a moderate hole doping level of 1.6 × 1013 cm−2 per sheet. They exhibit little additional carrier scattering and a remarkable chemical stability. Hall measurements reveal unity doping efficiency with temperature-independent hole density, mobility and electrical conductivity down to 2.5 K, which are atypical of conventional graphite intercalation compounds. These materials provide the first examples of a novel domain of doped 2D assemblies where large ions are incorporated through room-temperature solution processing, which opens new opportunities beyond van der Waals semiconductor heterostructures.

Magnetic Field Dependence of Dielectric Constant and Resistivity of Eu 0.98 Ba 0.02 TiO 3

Polycrystalline perovskite Eu_0.98Ba_0.02TiO₃, in which Eu²⁺: 4f⁷ spins order antiferromagnetically below T_N = 4.6 K, shows a large positive magnetodielectric effect (MDE) and a negative magnetoresistance (MR) in the paramagnetic state. The MDE ~ +120% and MR ~ -91% for f = 1 kHz at T = 10 K and under μ₀H = 7 T. The field dependence of the MDE and the MR is distinct. While the magnitude of the MR increases rapidly for μ₀H <; 1 T and nearly saturates for μ₀H > 3 T, the MDE shows a small value for μ₀H <; 1 T and increases smoothly without saturation above 1 T. It is suggested that a bound magnetic polaron forms around oxygen defects in zero field for T ≤ 50 K. The negative MR is suggested due to the increase in the size of the magnetic polaron and their percolation with increasing magnetic field. On the other hand, the positive MDE arises due to spin-phonon coupling possibly driven by the hybridization of Eu-4f and O-2p orbitals.