Sahar Keshavarz

Computational investigation of half-Heusler compounds for spintronics applications

The authors investigate the properties of 378 half-Heusler compounds using density functional theory with the goal of identifying promising candidates for spintronic applications, e.g. half-metals. Although DFT has often been applied to the search for half-metals, this study may be the most comprehensive attempt to identify which of the compounds predicted by DFT to be half-metals are likely to be fabricated. The calculated formation energy of each of the 378 potential half Heuslers was compared to that of all competing phases and combination of phases in the Open Quantum Materials Database. Those semiconductors, half-metals, and near half-metals within an empirically determined 0.1 eV/atom hull distance margin for neglected effects were deemed of interest for further experimental investigation.

Relaxation of Polymer Coated

Magnetic nanoparticles have shown great capability of sensing biological molecules in solution by correlating the magnetic susceptibility with the nanoparticle surface coatings. Here, we report on a new experimental approach of measuring magnetic nanoparticle relaxation. It uses a broadband ferromagnetic resonance setup specifically designed for samples in solution, enabling measurements of nanoparticles at variable frequencies up to 40 GHz. This design enables precise determination of the g-factor of the particles, by determining the resonance field for each microwave frequency. In addition this also enables us to determine the frequency dependence of the resonance linewidth, which provides information about the relaxation mechanisms of the magnetic nanoparticles. Polymer coated Fe3O4 nanoparticles were synthesized using a modified co-precipitation method by introducing the polymer coatings (e.g., polyacrylic acid) during the synthesis. Thereby water soluble nanoparticles with a narrow size distribution were produced in a single step. The relaxation measurement of these nanoparticles showed only a slight increase in linewidth over the entire frequency range suggesting that extrinsic relaxation mechanisms are dominant.

{\rm Fe}_{3}{\rm O}_{4}

Spinel ferrite NiFe2 O4 thin films have been grown on three isostructural substrates, MgAl2 O4 , MgGa2 O4 , and CoGa2 O4 using pulsed laser deposition. These substrates have lattice mismatches of 3.1%, 0.8%, and 0.2%, respectively, with NiFe2 O4 . As expected, the films grown on MgAl2 O4 substrate show the presence of the antiphase boundary defects. However, no antiphase boundaries (APBs) are observed for films grown on near-lattice-matched substrates MgGa2 O4 and CoGa2 O4 . This demonstrates that by using isostructural and lattice-matched substrates, the formation of APBs can be avoided in NiFe2 O4 thin films. Consequently, static and dynamic magnetic properties comparable with the bulk can be realized. Initial results indicate similar improvements in film quality and magnetic properties due to the elimination of APBs in other members of the spinel ferrite family, such as Fe3 O4 and CoFe2 O4 , which have similar crystallographic structure and lattice constants as NiFe2 O4 .

Magnetic Nanoparticles in Aqueous Solution

The crystallization process and the magnetization of Cr diluted CoFeB were investigated in both ribbon samples and thin film samples with Cr content up to 30 at. %. A primary crystallization of bcc phase from an amorphous precursor in ribbon samples was observed when the annealing temperature rose between 421 °C and 456 °C, followed by boron segregation at temperatures between 518 °C and 573 °C. The two onset crystallization temperatures showed strong dependences on both Cr and B concentrations. The impact of Cr concentration on the magnetic properties including a reduced saturation magnetization and an enhanced coercive field was also observed. The magnetizations of both ribbon samples and thin film samples were well fitted using the generalized Slater-Pauling curve with modified moments for B (−0.94 μB) and Cr (−3.6 μB). Possible origins of the enhanced coercive field were also discussed. We also achieved a damping parameter in CoFeCrB thin films at the same level as Co40Fe40B20, much lower than the val...

Bulk Single Crystal‐Like Structural and Magnetic Characteristics of Epitaxial Spinel Ferrite Thin Films with Elimination of Antiphase Boundaries

Ferromagnetic materials exhibiting large spin polarization at room temperature have been actively pursued in recent years for the development of next-generation spintronic devices. Chromium-based chalcospinels are the only ternary chalcogenide-containing magnetic materials with Curie temperatures above room temperature. However, the magnetic and electronic properties of chromium-based chalcospinels at the nanoscale level are not well understood. We have developed a facile colloidal method for the synthesis of CuCr2S4−xSex (0 ≤ x ≤ 4) nanocrystals over the entire composition range. Systematic changes in the lattice parameter and elemental composition confirm formation of CuCr2S4−xSex (0 ≤ x ≤ 4) nanocrystals. The dimensions of the nanocrystals, as determined from TEM images, vary from 12 ± 1.4 nm to 21 ± 1.4 nm. The Curie temperature (TC) shows a systematic increase with increasing selenium content. Saturation magnetization and coercivity values of CuCr2S4−xSex (0 ≤ x ≤ 4) nanocrystals at 5 K are found to steadily increase up to x = 3. Electronic structure calculations as a function of composition and size using density functional theory suggest ferromagnetic ordering over the entire composition range with partial spin polarization for the bulk materials. Furthermore, the calculations predict complete opening-up of a gap at the Fermi level in the minority spin channel at reduced dimensions to render them completely spin polarized, i.e., display half-metallic characteristics.

Structural and magnetic properties of Cr-diluted CoFeB

We have investigated (110) CrO2/natural barrier/Co magnetic tunnel junctions for their barrier and magneto-transport properties. A negative tunnel magnetoresistance (TMR) of over 5% was observed in micro-fabricated devices at 4.2 K, which is comparable to TMR values obtained with (100) CrO2. Both transport and cross-sectional transmission electron microscopy analysis reveal a natural barrier thickness 3.5 ± 0.5 nm. However, we obtain a low effective barrier height of 0.4 eV from transport measurements. The inelastic electron tunneling spectroscopy showed significant bias dependence with peak positions showing vibrational modes, which deviate from stoichiometric Cr2O3. We conclude that the transport characteristics are controlled by defects within the natural barrier, consistent with recent theoretical reports.

Nanocrystals of CuCr2S4−xSex chalcospinels with tunable magnetic properties

We experimentally investigate the structural, magnetic, and electrical transport properties of La0.67 Sr0.33MnO3 based magnetic tunnel junctions with a SrSnO3 barrier. Our results show that despite the high density of defects in the strontium stannate barrier, due to the large lattice mismatch, the observed tunnel magnetoresistance (TMR) is comparable to tunnel junctions with a better lattice matched SrTiO3 barrier, reaching values of up to 350% at T=5 K. Further analysis of the current-voltage characteristics of the junction and the bias voltage dependence of the observed tunnel magnetoresistance show a decrease of the TMR with increasing bias voltage. In addition, the observed TMR vanishes for T>200 K. Our results suggest that by employing a better lattice matched ferromagnetic electrode, and thus reducing the structural defects in the strontium stannate barrier, even larger TMR ratios might be possible in the future.