Ferrimagnetism in EuFe4Sb12 due to the interplay of f-electron moments and a nearly ferromagnetic host

Non-hysteretic first-order phase transition with large latent heat and giant low-field magnetocaloric effect

First-order magnetic transitions (FOMTs) with a large discontinuity in magnetization are highly sought in the development of advanced functional magnetic materials. Isosymmetric magnetoelastic FOMTs that do not perturb crystal symmetry are especially rare, and only a handful of material families, almost exclusively transition metal-based, are known to exhibit them. Yet, here we report a surprising isosymmetric FOMT in a rare-earth intermetallic, Eu₂In. What makes this transition in Eu₂In even more remarkable is that it is associated with a large latent heat and an exceptionally high magnetocaloric effect in low magnetic fields, but with tiny lattice discontinuities and negligible hysteresis. An active role of the Eu-5d and In-4p states and a rather unique electronic structure borne by In to Eu charge transfer, altogether result in an unusual exchange mechanism that both sets the transition in motion and unveils an approach toward developing specific magnetic functionalities ad libitum.

Magnetism in Na-filled Fe-based skutterudites

The interplay of superconductivity and magnetism is a subject of ongoing interest, stimulated most recently by the discovery of Fe-based superconductivity and the recognition that spin-fluctuations near a magnetic quantum critical point may provide an explanation for the superconductivity and the order parameter. Here we investigate magnetism in the Na filled Fe-based skutterudites using first principles calculations. NaFe4Sb12 is a known ferromagnet near a quantum critical point. We find a ferromagnetic metallic state for this compound driven by a Stoner type instability, consistent with prior work. In accord with prior work, the magnetization is overestimated, as expected for a material near an itinerant ferromagnetic quantum critical point. NaFe4P12 also shows a ferromagnetic instability at the density functional level, but this instability is much weaker than that of NaFe4Sb12, possibly placing it on the paramagnetic side of the quantum critical point. NaFe4As12 shows intermediate behavior. We also present results for skutterudite FeSb3, which is a metastable phase that has been reported in thin film form.

Direct evidence for significant spin-polarization of EuS in Co/EuS multilayers at room temperature

The new era of spintronics promises the development of nanodevices, where the electron spin will be used to store information and charge currents will be replaced by spin currents. For this, ferromagnetic semiconductors at room temperature are needed. We report on significant room-temperature spin polarization of EuS in Co/EuS multilayers recorded by x-ray magnetic circular dichroism (XMCD). The films were found to contain a mixture of divalent and trivalent europium, but only Eu⁺⁺ is responsible for the ferromagnetic behavior of EuS. The magnetic XMCD signal of Eu at room temperature could unambiguously be assigned to magnetic ordering of EuS and was found to be only one order of magnitude smaller than that at 2.5 K. The room temperature magnetic moment of EuS is as large as the one of bulk ferromagnetic Ni. Our findings pave the path for fabrication of room–temperature spintronic devices using spin polarized EuS layers.

Effect of disorder on the magnetic properties of a partially filled skutterudite

The magnetic properties of the partially filled skutterudite Eu(0.5)Co(4)Sb(12) are investigated by a model Hamiltonian, with special emphasis on the effect of ordering and disordering occupancy of the filler atoms Eu on the magnetic properties. The magnetization, magnetic specific heat and entropy are calculated within the mean-field approximation. By introducing the position disorder of the filler atoms, the critical temperature T(C) above which the magnetization disappears is changed. The magnetization curve near T(C) also becomes concave compared to the convex magnetization of the ordered system. The filler disordering also leads to a kink in the isothermal magnetization curve and a valley in the magnetic specific heat near T(C). The effective magnetic field acting on the localized spin of the filler atoms has a competitive effect with the disorder and therefore makes the valley disappear.

Investigation of the emission mechanism in milled SrAl₂O₄:Eu, Dy using optical and synchrotron X-ray spectroscopy

There currently exists much debate as to the active state related to the "long afterglow" effect in europium doped oxide materials. Redox couples that consist of Eu(+/2+) and Eu(2+/3+) are discussed, but no common answer is currently accepted. Here, we present a comparison of the optical properties of a commercially available SrAl(2)O(4):Eu, Dy phosphor, as a function of nanoparticle size reduction via dry mechanical milling. X-ray and optical spectroscopic data indicate a significant decrease in phosphorescence efficiency and an increase in laser stimulated emission efficiency as near surface Eu(2+) ions are oxidized to Eu(3+) as a consequence of increased exposure during the milling process. These results show evidence only for Eu(2+/3+) oxidation states, suggesting the mechanism related to long afterglow effect does not arise from Eu(+) species. We also suggest that size reduction, as a rule, cannot be universally applied to improve optical properties of nanostructures.

On the oxidation of EuFe4Sb12 and EuRu4Sb12

Rare-earth-filled transition-metal pnictides having the skutterudite-type structure have been proposed for use as high-temperature thermoelectric materials to recover waste heat from vehicle exhaust, among other applications. A previous investigation by this research group of one of the most studied skutterudites, CeFe(4)Sb(12), found that, when exposed to air, this material oxidized at temperatures that are considerably below the proposed maximum operating temperature. Here, by the combined use of TGA, powder XRD, and XANES, it has been found that the substitution of Ce(3+) and Fe(2+) for larger rare-earth and transition-metal elements (Eu(2+) and Ru(2+)) results in a significantly higher oxidation temperature compared to that of CeFe(4)Sb(12). This increase can be related to the increased orbital overlap provided by these larger atoms (Eu(2+) and Ru(2+) vs Ce(3+) and Fe(2+)), enabling the development of stronger bonds. These results show how selective substitution of the constituent elements can significantly improve the thermal stability of materials.