Thomas A. Lograsso

Iowa State University, Ames, Iowa, United States

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Publications (176)487.83 Total impact

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    ABSTRACT: Fe1−xMx (M = Ga, Ge, Si, Al, Mo and x ∼ 0.18) alloys offer an extraordinary combination of magnetoelasticity and mechanical properties. They are rare-earth-free, can be processed using conventional deformation techniques, have high magnetic permeability, low hysteresis, and low magnetic saturation fields, making them attractive for device applications such as actuators and energy harvesters. Starting with Fe-Ga as a reference and using a rigid-band-filling argument, Zhang et al. predicted that lowering the Fermi level by reducing the total number of electrons could enhance magnetoelasticity. To provide a direct experimental validation for Zhang's hypothesis, elemental additions with lower-than-Ga valence are needed. Of the possible candidates, only Be and Zn have sufficient solubility. Single crystals of bcc Fe-Ga-Zn have been grown with up to 4.6 at. % Zn in a Bridgman furnace under elevated pressure (15 bars) in order to overcome the high vapor pressure of Zn and obtain homogeneous crystals. Single-crystal measurements of magnetostriction and elastic constants allow for the direct comparison of the magnetoelastic coupling constants of Fe-Ga-Zn with those of other magnetoelastic alloys in its class. The partial substitution of Ga with Zn yields values for the magnetoelastic coupling factor, −b 1, comparable to those of the binary Fe-Ga alloy.
    Journal of Applied Physics 05/2015; 117(17):17E701. DOI:10.1063/1.4907181 · 2.19 Impact Factor
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    ABSTRACT: Inelastic neutron scattering was employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at $T_{{\rm S}}$, sets in well above the stripe antiferromagnetic ordering at $T_{{\rm N}}$. We find that the temperature-dependent dynamic susceptibility displays an anomaly at $T_{{\rm S}}$ followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. Our findings can be consistently described by a model that attributes the structural/nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides.
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    ABSTRACT: Inelastic neutron scattering is employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe_{0.953}Co_{0.047})_{2}As_{2}. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at T_{S}, sets in well above the stripe antiferromagnetic ordering at T_{N}. We find that the temperature-dependent dynamic susceptibility displays an anomaly at T_{S} followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. Our findings can be consistently described by a model that attributes the structural or nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides.
    Physical Review Letters 02/2015; 114(5):057001. · 7.73 Impact Factor
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    ABSTRACT: Magnetic refrigeration based on the magnetocaloric effect is one of the best alternatives to compete with vapor-compression technology. Despite being already in its technology transfer stage, there is still room for optimization, namely, on the magnetic responses of the magnetocaloric material. In parallel, the demand for different magnetostrictive materials has been greatly enhanced due to the wide and innovative range of technologies that emerged in the last years (from structural evaluation to straintronics fields). In particular, the Gd5(Six Ge 1−x)4 compounds are a family of well-known alloys that present both giant magnetocaloric and colossal magnetostriction effects. Despite their remarkable properties, very few reports have been dedicated to the nanostructuring of these materials: here, we report a ∼800 nm Gd5Si2.7 Ge 1.3 thin film. The magnetic and structural investigation revealed that the film undergoes a first order magnetostructural transition and as a consequence exhibits large magnetocaloric effect (−ΔSmMAX ∼ 8.83 J kg−1 K−1, ΔH = 5T) and giant thermal expansion (12000 p.p.m). The thin film presents a broader magnetic response in comparison with the bulk compound, which results in a beneficial magnetic hysteresis reduction. The ΔSmMAX exhibited by the Gd5(Si,Ge)4 thin film makes it a promising candidate for micro/nano magnetic refrigeration area.
    Applied Physics Letters 01/2015; 5(106):032402. DOI:10.1063/1.4906056 · 3.52 Impact Factor
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    Yong Liu, Thomas A. Lograsso
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    ABSTRACT: We report the doping evolution of magnetic susceptibility $\chi (T)$ and Hall coefficient $R_H$ in high quality Ba$_{1-x}$K$_x$Fe$_2$As$_2$ ($0.13 \leq x \leq 1$) single crystals. It is found that the normal-state magnetic susceptibility of Ba$_{1-x}$K$_x$Fe$_2$As$_2$ compounds undergoes a crossover from linear-$T$ dependence in the undoped and underdoped samples into KFe$_2$As$_2$-type magnetic response in the overdoped samples with increasing K content. Hall coefficient $R_H$ of underdoped sample $x$=0.22 shows a rapid increase above spin-density-wave transition temperature $T_{SDW}$. Below $T_{SDW}$, it increases slowly. $R_H$ of optimally doped and slightly overdoped samples ($0.34 \leq x \leq 0.65$) shows relative weak temperature dependence and saturation tendency below 150 K. However, $R_H$ of K heavily overdoped samples ($0.80 \leq x \leq 1$) increases rapidly below 150 K. Meanwhile, Hall angle $cot \theta _H$ displays a concave temperature dependence within the doping range $0.22 \leq x \leq 0.55$, whereas it changes to a convex temperature dependence within the doping range $0.65 \leq x \leq 1$. The dramatic change coincides with Lifshitz transition occurred around the critical doping $x$=0.80, where ARPES measurements had confirmed that electron pocket disappears with excess hole doping in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ system. It is suggested that the characteristic temperature $T$* at around $120 \sim 150$ K observed in susceptibility and Hall coefficient, as well as previously reported resistivity data, may indicate an incoherence-coherence crossover in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ system.
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    ABSTRACT: A study of the formation of Gd11M4In9 (M = Ni, Pd, Pt) and R11Ni4In9 (R = rare earth) compounds revealed a unique and peculiar property, which is to naturally crystallize in a bundle of self-assembled fibers when cooled from the melt. The fibers, which are nano- to millimeters in cross-section and ≈1–40 mm long, grow unidirectionally along a temperature gradient. These compounds adopt the orthorhombic Nd11Pd4In9 structure type (oC48-Cmmm). This structure is layered, with slabs of R atoms alternating with slabs of Ni/In atoms along a short c-axis (much shorter than either the a- or b-axis). The growth direction of the fibers is along the crystallographic c-axis, orthogonal to the a–b plane. Two strong and short In–In bonds lie in the a–b plane, which are even shorter than in In metal. Integrated crystal orbital Hamilton population calculations show that the In–In bonds create isolated “R8Ni4In9” rods growing along the c-axis, with the In–In bonds being part of the rods. This appears to be an important factor explaining the microfibrous nature of these phases. Some physical properties have been measured on the Gd11Ni4In9 homolog. The compound orders ferrimagnetically at TC ≈ 88 K, and at lower temperatures (46 and 10 K), two other magnetic anomalies were observed, probably due to spin reorientations. As expected from the bonding features, the mechanical, magnetic and electrical properties are strongly anisotropic.
    Acta Materialia 07/2014; 73:27–36. DOI:10.1016/j.actamat.2014.03.061 · 3.94 Impact Factor
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    ABSTRACT: We report on the different roles of two orbital-active Fe$^{2+}$ at the A site and V$^{3+}$ at the B site in the magnetic excitations and on the anomalous spin wave broadening in FeV$_{2}$O$_{4}$. FeV$_{2}$O$_{4}$ exhibits three structural transitions and successive paramagnetic (PM)-collinear ferrimagnetic (CFI)-noncollinear ferrimagnetic (NCFI) transitions. The high-temperature tetragonal/PM -orthorhombic/CFI transition is accompanied by the appearance of an energy gap with a high magnitude in the magnetic excitations due to strong spin-orbit coupling induced anisotropy at the Fe$^{2+}$ site. While there is no measurable increase in the energy gap from the orbital ordering of V$^{3+}$ at the orthorhombic/CFI-tetragonal/NCFI transition, anomalous spin wave broadening is observed in the orthorhombic/CFI state due to V$^{3+}$ spin fluctuations at the B site. The spin wave broadening is also observed at the zone boundary without softening, which is discussed in terms of magnon-phonon coupling.
    Physical review. B, Condensed matter 06/2014; 89,:224416. DOI:10.1103/PhysRevB.89.224416 · 3.66 Impact Factor
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    ABSTRACT: We report in-plane thermoelectric power measurements on single crystals of (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (0.44 $\leq x \leq$ 1). We observe a minimum in the S$|_{T=const}$ versus x at x ~ 0.55 that can be associated with the change in the topology of the Fermi surface, a Lifshitz transition, related to the electron pockets at the center of M point crossing the Fermi level. This feature is clearly observable below ~ 75 K. Thermoelectric power also shows a change in the x ~ 0.8 - 0.9 range, where maximum in the thermoelectric power collapses into a plateau. This Lifshitz transition is most likely related to the reconstruction of the Fermi surface associated with the transformation of the hole pockets at the M point into four blades as observed by ARPES measurements.
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    ABSTRACT: The low electron concentration region (e/a < 7.75) of the magnetic phase diagram of the off-stoichiometric Ni–Mn–Sn Heusler alloys was investigated in detail by DSC and magnetization measurements of the Ni40+xMn39−xSn21(x = 0, 2, 4, 6 and 8 at.%) alloys. The alloys show a stable austenitic phase without any martensitic transformation down to 5 K even after heat treatment. The Curie temperature exhibits a broad maximum over a large composition range. The evolution of the magnetic moment with the electron concentration fits the data of previous studies and confirms the peak-like dependence in the extended range of e/a values predicted by ab initio calculations. The explored part of the moment versus e/a curve can be explained in terms of a localized magnetic moment model and full atomic order in the alloys.
    Journal of Alloys and Compounds 05/2014; 594:171–174. DOI:10.1016/j.jallcom.2014.01.142 · 2.73 Impact Factor
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    ABSTRACT: We report on the different roles of two orbital-active Fe$^{2+}$ at the A site and V$^{3+}$ at the B site in the magnetic excitations and on the anomalous spin wave broadening in FeV$_{2}$O$_{4}$. FeV$_{2}$O$_{4}$ exhibits three structural transitions and successive paramagnetic (PM)-collinear ferrimagnetic (CFI)-noncollinear ferrimagnetic (NCFI) transitions. The high-temperature tetragonal/PM -orthorhombic/CFI transition is accompanied by the appearance of an energy gap with a high magnitude in the magnetic excitations due to strong spin-orbit coupling induced anisotropy at the Fe$^{2+}$ site. While there is no measurable increase in the energy gap from the orbital ordering of V$^{3+}$ at the orthorhombic/CFI-tetragonal/NCFI transition, anomalous spin wave broadening is observed in the orthorhombic/CFI state due to V$^{3+}$ spin fluctuations at the B site. The spin wave broadening is also observed at the zone boundary without softening, which is discussed in terms of magnon-phonon coupling.
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    ABSTRACT: Self-organized molecular films with long-range quasiperiodic order have been grown by using the complex potential energy landscape of quasicrystalline surfaces as templates. The long-range order arises from a specific subset of quasilattice sites acting as preferred adsorption sites for the molecules, thus enforcing a quasiperiodic structure in the film. These adsorption sites exhibit a local five-fold symmetry resulting from the cut by the surface plane through the cluster units identified in the bulk solid. Symmetry matching between the C60 fullerene and the substrate leads to a preferred adsorption configuration of the molecules with a pentagonal face down, a feature unique to quasicrystalline surfaces, enabling efficient chemical bonding at the molecule-substrate interface. This finding offers opportunities to investigate the physical properties of model 2D quasiperiodic systems as the molecules can be functionalized to yield architectures with tailor-made properties.
    ACS Nano 03/2014; 8(4). DOI:10.1021/nn500234j · 12.03 Impact Factor
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    ABSTRACT: We report measurements of electrical resistivity under pressure up to 5.8 GPa, magnetization up to 6.7 GPa and ac susceptibility up to 7.1 GPa in KFe2As2. At a pressure pc= 1.8 GPa, there is a change of slope in the pressure dependence of the superconducting transition temperature Tc(p) as previously reported. Above pc, Tc is almost constant up to 7 GPa. The T-p phase diagram is very sensitive to the pressure conditions as a consequence of anisotropic uniaxial pressure dependence of Tc. In addition, we observe a change in the upper critical field behavior across pc and provide a quantitative measure of this change. We show how this can be used to investigate a change of the superconducting gap structure and suggest the appearance of a kz modulation of the superconducting gap as a possible explanation.
    Physical Review B 02/2014; 89(22). DOI:10.1103/PhysRevB.89.220509 · 3.66 Impact Factor
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    ABSTRACT: We measured the Raman spectra of ferromagnetic nearly half metal CoS2 in a broad temperature range. All five Raman active modes Ag, Eg, Tg(1), Tg(2) and Tg(3) were observed. The magnetic ordering is indicated by a change of the temperature dependences of the frequency and the line width of Ag and T g(2) modes at the Curie point. The temperature dependence of the frequencies and linewidths of the Ag, Eg, Tg(1), T g(2) modes in the paramagnetic phase can be described in the framework of the Klemens approach. Hardening of the Tg(2), Tg(1) and A g modes on cooling can be unambiguously seen in the ferromagnetic phase. The linewidths of Tg(2) and Ag modes behave a natural way at low exciting laser power (decrease with decreasing temperature) in the ferromagnetic phase. At high exciting laser power the corresponding linewidths increase at temperature decreasing below the Curie temperature. Then as can be seen the line width of Ag mode reaches a maxima at about 80K. This intriging feature probably signifies a specific channel of the optical phonon decay in the ferromagnetic phase of CoS2. Tentative explanations of some of the observed effects are given, taking into account the nearly half metallic nature of CoS2.
    Journal of Physics Condensed Matter 02/2014; 26(39). DOI:10.1088/0953-8984/26/39/396001 · 2.22 Impact Factor
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    ABSTRACT: Neutron and synchrotron resonant X-ray magnetic scattering (RXMS) complemented by heat capacity and resistivity measurements reveal the evolution of the magnetic structures of Fe and Ce sublattices in single crystal CeFeAsO. The RXMS of magnetic re ections at the Ce LII-edge shows a magnetic transition that is speci�c to the Ce antiferromagnetic long-range ordering at TCe � 4 K with short-range Ce ordering above TCe, whereas neutron di�raction measurements of a few magnetic re ections indicate a transition at T� � 12 K with unusual order parameter. Detailed order parameter measurements on several magnetic re ections by neutrons show a weak anomaly at 4 K which we associate with the Ce ordering. The successive transitions at TCe and T� can also be clearly identi�ed by two anomalies in heat capacity and resistivity measurements. The higher transition temperature at T� � 12 K is mainly ascribed to Fe spin reorientation transition, below which Fe spins rotate uniformly and gradually in the ab plane. The Fe spin reorientation transition and short-range Ce ordering above TCe re ect the strong Fe-Ce couplings prior to long-range ordering of the Ce. The evolution of the intricate magnetic structures in CeFeAsO going through T� and TCe is proposed.
    Physical Review B 11/2013; 88:174517. DOI:10.1103/PhysRevB.88.174517 · 3.66 Impact Factor
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    ABSTRACT: We observe a distinct excitation-fluence-dependent transient carrier relaxation in a strongly correlated colossal magnetoresistive manganite that correlates with photoinduced magnetic and electronic phase transitions characterized by nonlinear photoexcitation behaviors.
    CLEO: QELS_Fundamental Science; 06/2013
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    ABSTRACT: The technological demand to push the gigahertz (10(9) hertz) switching speed limit of today's magnetic memory and logic devices into the terahertz (10(12) hertz) regime underlies the entire field of spin-electronics and integrated multi-functional devices. This challenge is met by all-optical magnetic switching based on coherent spin manipulation. By analogy to femtosecond chemistry and photosynthetic dynamics--in which photoproducts of chemical and biochemical reactions can be influenced by creating suitable superpositions of molecular states--femtosecond-laser-excited coherence between electronic states can switch magnetic order by 'suddenly' breaking the delicate balance between competing phases of correlated materials: for example, manganites exhibiting colossal magneto-resistance suitable for applications. Here we show femtosecond (10(-15) seconds) photo-induced switching from antiferromagnetic to ferromagnetic ordering in Pr0.7Ca0.3MnO3, by observing the establishment (within about 120 femtoseconds) of a huge temperature-dependent magnetization with photo-excitation threshold behaviour absent in the optical reflectivity. The development of ferromagnetic correlations during the femtosecond laser pulse reveals an initial quantum coherent regime of magnetism, distinguished from the picosecond (10(-12) seconds) lattice-heating regime characterized by phase separation without threshold behaviour. Our simulations reproduce the nonlinear femtosecond spin generation and underpin fast quantum spin-flip fluctuations correlated with coherent superpositions of electronic states to initiate local ferromagnetic correlations. These results merge two fields, femtosecond magnetism in metals and band insulators, and non-equilibrium phase transitions of strongly correlated electrons, in which local interactions exceeding the kinetic energy produce a complex balance of competing orders.
    Nature 04/2013; 496(7443):69-73. DOI:10.1038/nature11934 · 42.35 Impact Factor
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    ABSTRACT: Formation energies of native defects in Bi2(TexSe3−x), with comparison to ideal Bi2Te2S, are calculated in density-functional theory to assess transport properties. Bi2Se3 is found to be n type for both Bi- and Se-rich growth conditions, while Bi2Te3 changes from n to p type going from Te- to Bi-rich conditions, as observed. Bi2Te2Se and Bi2Te2S are generally n type, explaining observed heavily doped n-type behavior in most samples. A (0/−) transition level at 16 meV above valence-band maximum for Bi on Te antisites in Bi2Te2Se is related to the observed thermally active transport gap causing a p-to-n transition at low temperature. Bi2(TexSe3−x) with x>2 are predicted to have high bulk resistivity due to effective carrier compensation when approaching the n-to-p crossover. Predicted behaviors are confirmed from characterization of our grown single crystals.
    Physical review. B, Condensed matter 03/2013; 87(12). DOI:10.1103/PhysRevB.87.125303 · 3.66 Impact Factor
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    ABSTRACT: Spin valves have revolutionized the field of magnetic recording and memory devices. Spin valves are generally realized in thin film heterostructures, where two ferromagnetic (FM) layers are separated by a nonmagnetic conducting layer. Here, we demonstrate spin-valve-like magnetoresistance at room temperature in a bulk ferrimagnetic material that exhibits a magnetic shape memory effect. The origin of this unexpected behavior in Mn2NiGa has been investigated by neutron diffraction, magnetization, and ab initio theoretical calculations. The refinement of the neutron diffraction pattern shows the presence of antisite disorder where about 13% of the Ga sites are occupied by Mn atoms. On the basis of the magnetic structure obtained from neutron diffraction and theoretical calculations, we establish that these antisite defects cause the formation of FM nanoclusters with parallel alignment of Mn spin moments in a Mn2NiGa bulk lattice that has antiparallel Mn spin moments. The direction of the Mn moments in the soft FM cluster reverses with the external magnetic field. This causes a rotation or tilt in the antiparallel Mn moments at the cluster-lattice interface resulting in the observed asymmetry in magnetoresistance.
    Physical Review Letters 12/2012; 109(50):246601. DOI:10.1103/PhysRevLett.109.246601 · 7.73 Impact Factor

Publication Stats

3k Citations
487.83 Total Impact Points

Institutions

  • 1992–2015
    • Iowa State University
      • • Department of Materials Science and Engineering
      • • Ames Laboratory
      • • Department of Physics and Astronomy
      • • Department of Chemistry
      Ames, Iowa, United States
  • 2006
    • Massachusetts Institute of Technology
      • Department of Materials Science and Engineering
      Cambridge, Massachusetts, United States
    • U.S. Department of Energy
      • Ames Lab
      Washington, Washington, D.C., United States
  • 2004–2005
    • The Ohio State University
      • Department of Mechanical and Aerospace Engineering
      Columbus, Ohio, United States