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Register a new userTitanium Nitride as a Seed Layer for Heusler Compounds
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Titanium nitride (TiN) shows low resistivity at room temperature, high thermal stability and thus has the potential to serve as seed layer in magnetic tunnel junctions. High quality TiN thin films with regard to the crystallographic and electrical properties were grown and characterized by X-ray diffraction and 4-terminal transport measurements. Element specific X-ray absorption spectroscopy revealed pure TiN in the bulk. To investigate the influence of a TiN seed layer on a ferro(i)magnetic bottom electrode, an out-of-plane magnetized Mn2.45Ga as well as in- and out-of-plane magnetized Co2FeAl thin films were deposited on a TiN buffer, respectively. The magnetic properties were investigated using a superconducting quantum interference device (SQUID) and anomalous Hall effect (AHE) for Mn2.45Ga. Magneto optical Kerr effect (MOKE) measurements were carried out to investigate the magnetic properties of Co2FeAl. TiN buffered Mn2.45Ga thin films showed higher coercivity and squareness ratio compared to unbuffered samples. The Heusler compound Co2FeAl showed already good crystallinity when grown at room temperature.
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Todays supercapacitor energy storages are typically discrete devices aimed for printed boards and power applications. The development of autonomous sensor networks and wearable electronics and the miniaturisation of mobile devices would benefit substantially from solutions in which the energy storage is integrated with the active device. Nanostructures based on porous silicon (PS) provide a route towards integration due to the very high inherent surface area to volume ratio and compatibility with microelectronics fabrication processes. Unfortunately, pristine PS has limited wettability and poor chemical stability in electrolytes and the high resistance of the PS matrix severely limits the power efficiency. In this work, we demonstrate that excellent wettability and electro-chemical properties in aqueous and organic electrolytes can be obtained by coating the PS matrix with an ultra-thin layer of titanium nitride by atomic layer deposition. Our approach leads to very high specific capacitance (15 F/cm$^3$), energy density (1.3 mWh/cm$^3$), power density (up to 214 W/cm$^3$) and excellent stability (more than 13,000 cycles). Furthermore, we show that the PS-TiN nanomaterial can be integrated inside a silicon chip monolithically by combining MEMS and nanofabrication techniques. This leads to realisation of in-chip supercapacitor, i.e., it opens a new way to exploit the otherwise inactive volume of a silicon chip to store energy.
X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) were used to probe the oxidation state and element specific magnetic moments of Mn in Heusler compounds with different crystallographic structure. The results were compared with theoretical calculations, and it was found that in full Heusler alloys, Mn is metallic (oxidation state near 0) on both sublattices. The magnetic moment is large and localized when octahedrally coordinated by the main group element, consistent with previous theoretical work, and reduced when the main group coordination is tetrahedral. By contrast, in the half Heusler compounds the magnetic moment of the Mn atoms is large and the oxidation state is +1 or +2. The magnetic and electronic properties of Mn in full and half Heusler compounds are strongly dependent on the structure and sublattice, a fact that can be exploited to design new materials.
Based on high throughput density functional theory calculations, we performed systematic screening for spin-gapless semiconductors (SGSs) in quaternary Heusler alloys XX 0 YZ (X, X 0 , and Y are transition metal elements without Tc, and Z is one of B, Al, Ga, In, Si, Ge, Sn, Pb, P, As, Sb, and Bi). Following the empirical rule, we focused on compounds with 21, 26, or 28 valence electrons, resulting in 12, 000 possible chemical compositions. After systematically evaluating the thermodynamic, mechanical, and dynamical stabilities, we successfully identified 70 stable SGSs, confirmed by explicit electronic structure calculations with proper magnetic ground states. It is demonstrated that all four types of SGSs can be realized, defined based on the spin characters of the bands around the Fermi energy, and the type-II SGSs show promising transport properties for spintronic applications. The effect of spin-orbit coupling is investigated, resulting in large anisotropic magnetoresistance and anomalous Nernst effects.
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The XYZ half-Heusler crystal structure can conveniently be described as a tetrahedral zinc blende YZ structure which is stuffed by a slightly ionic X species. This description is well suited to understand the electronic structure of semiconducting 8-electron compounds such as LiAlSi (formulated Li$^+$[AlSi]$^-$) or semiconducting 18-electron compounds such as TiCoSb (formulated Ti$^{4+}$[CoSb]$^{4-}$). The basis for this is that [AlSi]$^-$ (with the same electron count as Si$_2$) and [CoSb]$^{4-}$ (the same electron count as GaSb), are both structurally and electronically, zinc-blende semiconductors. The electronic structure of half-metallic ferromagnets in this structure type can then be described as semiconductors with stuffing magnetic ions which have a local moment: For example, 22 electron MnNiSb can be written Mn$^{3+}$[NiSb]$^{3-}$. The tendency in the 18 electron compound for a semiconducting gap -- believed to arise from strong covalency -- is carried over in MnNiSb to a tendency for a gap in one spin direction. Here we similarly propose the systematic examination of 18-electron hexagonal compounds for semiconducting gaps; these would be the stuffed wurtzite analogues of the stuffed zinc blende half-Heusler compounds. These semiconductors could then serve as the basis for possibly new families of half-metallic compounds, attained through appropriate replacement of non-magnetic ions by magnetic ones. These semiconductors and semimetals with tunable charge carrier concentrations could also be interesting in the context of magnetoresistive and thermoelectric materials.
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