Structural and magnetothermal properties of compounds: Yb5Si[subscript x]Ge4−[subscript x], Sm5Si[subscript x]Ge4−[subscript x], EuO, and Eu3O4

Ahn, Kyunghan
Major Professor
Vitalij K. Pecharsky
Committee Member
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Materials Science and Engineering

The family of R5Si xGe4-x alloys demonstrates a variety of unique physical phenomena related to magneto-structural transitions associated with reversible breaking and reforming of specific bonds that can be controlled by numerous external parameters such as chemical composition, magnetic field, temperature, and pressure. Therefore, R 5SixGe4-x systems have been extensively studied to uncover the mechanism of the extraordinary magneto-responsive properties including the giant magnetoresistance (GMR) and colossal magnetostriction, as well as giant magnetocaloric effect (GMCE). Here, we report on phase relationships and structural, magnetic, and thermodynamic properties in the Yb5SixGe4- x and Sm5SixGe 4-x pseudobinary systems, which may exhibit mixed valence states.;The crystallography, phase relationships, and physical properties of Yb5SixGe4- x alloys with 0 ≤ x ≤ 4 have been examined by using single crystal and powder x-ray diffraction at room temperature, and dc magnetization and heat capacity measurements between 1.8 K and 400 K in magnetic fields ranging from 0 to 7 T. Both the crystallographic and magnetic property data indicate that Yb5SixGe 4-x alloys are mixed valence systems, in which the majority (60%) of Yb atoms is divalent, while the minority (40%) is trivalent. This finding is supported by recent Mossbauer spectroscopy data.;The magnetic properties of the Sm5SixGe 4-x compounds can be well described by considering the temperature-independent Van Vleck term due to small energy separation between the ground state and the first excited state of Sm3+ ions. All Sm5SixGe4- x compounds have unusually high magnetic ordering temperatures. The change in both the magnetic and structural behaviors with the substitution of Ge by Si is similar to that observed in the Gd5Si xGe4-x system. The external magnetic field seems to have no effect on the magnetism of the Sm5Si xGe4-x alloys.;Europium oxides, EuO with the divalent state and Eu3O 4 with the mixed-valence state, may exhibit a strong magnetocaloric effect (MCE) and interesting magnetism because of the unique magnetic properties of Eu. Europium has two valence states: Eu2+ and Eu 3+. The Eu2+ ion is similar to the Gd3+ ion with 4f7 state (J = 7/2), while the Eu3+ ion has a 4f6 configuration (J = 0). Elemental Gd and many Gd-based compounds are good magnetic refrigerant materials due to their large magnetic moments, large available magnetic entropy, and low hysteresis. Thus, we report on magnetic behavior and the MCE of EuO and Eu3O4 as evaluated from both the heat capacity and magnetization measurements.;EuO was synthesized through the thermal reduction of Eu2O 3 by a stoichiometric quantity of metallic Eu. According to the heat capacity and magnetic measurements, EuO undergoes a second-order phase transformation at ∼69 K from the ferromagnetic to the paramagnetic state on heating. The magnetocaloric effect of EuO, both as the isothermal magnetic entropy change (DeltaSmag) and the adiabatic temperature change (DeltaTad), was obtained from the heat capacity data. Also, the magnetization isotherms were used to calculate Delta Smag. EuO exhibits the magnetocaloric effect with a peak in the vicinity of the magnetic phase transition temperature (∼69 K), the amplitude of which is comparable to other known magnetocaloric materials. The DeltaSmag calculated from the heat capacity data is in excellent agreement with that calculated from the magnetization data.;Mixed-valence compound Eu3O4 was prepared by heating EuO and Eu2O3 together at 1800 °C for 30 h in a sealed W crucible under a high vacuum. It was confirmed as a single phase Eu3O4 using the room temperature x-ray powder diffraction method. We characterized a polycrystalline Eu3O4 through the heat capacity and magnetic measurements. Our results from magnetic measurements are in good agreement with the references reported previously. As far as we are aware, heat capacity of Eu3O4 was not studied in the past. The magnetic entropy change (-DeltaSmag) in Eu3O4 near 6.5 K is around 12.7 J/kg K with the magnetic field change (DeltaB) of 5 T. The adiabatic temperature change (DeltaTad) in Eu3O4 near 7 K is around 7.0 K with the DeltaB of 5 T. Also, the magnetic entropy change (-DeltaSmag) calculated from magnetization data in Eu3O4 near 6.3 K is around 13.6 J/kg K with the magnetic field change (DeltaB) of 5 T, which is roughly same as that from heat capacity data. (Abstract shortened by UMI.)