Alessandro Puri

Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias

Rational material design can accelerate the discovery of materials with improved functionalities. This approach can be implemented in Heusler compounds with tunable magnetic sublattices to demonstrate unprecedented magnetic properties. Here, we have designed a family of Heusler alloys with a compensated ferrimagnetic state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant exchange bias (EB) of more than 3 T and a large coercivity are established. The large exchange anisotropy originates from the exchange interaction between the compensated host and ferrimagnetic clusters that arise from intrinsic anti-site disorder. Our design approach is also demonstrated on a second material with a magnetic transition above room temperature, Mn-Fe-Ga, exemplifying the universality of the concept and the feasibility of room-temperature applications. These findings may lead to the development of magneto-electronic devices and rare-earth-free exchange-biased hard magnets, where the second quadrant magnetization can be stabilized by the exchange bias.

Optimum inhomogeneity of local lattice distortions in La2CuO4+y

Electronic functionalities in materials from silicon to transition metal oxides are, to a large extent, controlled by defects and their relative arrangement. Outstanding examples are the oxides of copper, where defect order is correlated with their high superconducting transition temperatures. The oxygen defect order can be highly inhomogeneous, even in optimal superconducting samples, which raises the question of the nature of the sample regions where the order does not exist but which nonetheless form the “glue” binding the ordered regions together. Here we use scanning X-ray microdiffraction (with a beam 300 nm in diameter) to show that for La2CuO4+y, the glue regions contain incommensurate modulated local lattice distortions, whose spatial extent is most pronounced for the best superconducting samples. For an underdoped single crystal with mobile oxygen interstitials in the spacer La2O2+y layers intercalated between the CuO2 layers, the incommensurate modulated local lattice distortions form droplets anticorrelated with the ordered oxygen interstitials, and whose spatial extent is most pronounced for the best superconducting samples. In this simplest of high temperature superconductors, there are therefore not one, but two networks of ordered defects which can be tuned to achieve optimal superconductivity. For a given stoichiometry, the highest transition temperature is obtained when both the ordered oxygen and lattice defects form fractal patterns, as opposed to appearing in isolated spots. We speculate that the relationship between material complexity and superconducting transition temperature Tc is actually underpinned by a fundamental relation between Tc and the distribution of ordered defect networks supported by the materials.

Evidence of local structural inhomogeneity in FeSe 1 − x Te x from extended x-ray absorption fine structure

Local structure of FeSe(1-x)Te(x) has been studied by extended x-ray absorption fine-structure (EXAFS) measurements as a function of temperature. Combination of Se and Fe K edge EXAFS has permitted to quantify the local interatomic distances and their mean-square relative displacements. The Fe-Se and Fe-Te bond lengths in the ternary system are found to be very different from the average crystallographic Fe-Se/Te distance, and almost identical to the Fe-Se and Fe-Te distances for the binary FeSe and FeTe systems, indicating distinct site occupation by the Se and Te atoms. The results provide a clear evidence of local inhomogeneities and coexisting electronic components in the FeSe1-xTex, characterized by different local structural configurations, with direct implication on the fundamental electronic structure of these superconductors.

The effect of internal pressure on the tetragonal to monoclinic structural phase transition in ReOFeAs: The case of NdOFeAs

We report the temperature dependent x-ray powder diffraction of the quaternary compound NdOFeAs (also called NdFeAsO) in the range between 300 and 95 K. We have detected the structural phase transition from the tetragonal phase, with P4/nmm space group, to the orthorhombic or monoclinic phase, with Cmma or P112/n (or P2/c) space group, over a broad temperature range from 150 to 120 K, centered at T0 ∼ 137 K. Therefore the temperature of this structural phase transition is strongly reduced, by about ∼30 K, by increasing the internal chemical pressure going from LaOFeAs to NdOFeAs. In contrast, the superconducting critical temperature increases from 27 to 51 K going from LaOFeAs to NdOFeAs doped samples. This result shows that the normal striped orthorhombic Cmma phase competes with the superconducting tetragonal phase. Therefore by controlling the internal chemical pressure in new materials it should be possible to push toward zero the critical temperature T0 of the structural phase transition, giving the striped phase, in order to get superconductors with higher Tc. (Some figures in this article are in colour only in the electronic version)

Large local disorder in superconducting K0.8Fe1.6Se2 studied by extended x-ray absorption fine structure

We have measured the local structure of superconducting K(0.8)Fe(1.6)Se(2) chalcogenide (T(c) = 31.8 K) by temperature dependent polarized extended x-ray absorption fine structure (EXAFS) at the Fe and Se K-edges. We find that the system is characterized by a large local disorder. The Fe-Se and Fe-Fe distances are found to be shorter than the distances measured by diffraction, while the corresponding mean square relative displacements reveal large Fe-site disorder and relatively large c-axis disorder. The local force constant for the Fe-Se bondlength (k ~ 5.8 eV Å(-2)) is similar to the one found in the binary FeSe superconductor, however, the Fe-Fe bondlength appears to be flexible (k ~ 2.1 eV Å(-2)) in comparison to the binary FeSe (k ~ 3.5 eV Å(-2)), an indication of partly relaxed Fe-Fe networks in K(0.8)Fe(1.6)Se(2). The results suggest a glassy nature for the title system, with the superconductivity being similar to that in the granular materials.

Random alloy-like local structure of Fe(Se, S)(1-x)Te(x) superconductors revealed by extended x-ray absorption fine structure.

The local structure of Fe(Se, S)(1-x)Te(x) ternary (11-type) chalcogenides has been studied by temperature dependent Fe K-edge extended x-ray absorption fine structure measurements. We find that the Fe-Se and Fe-Te distances in ternary FeSe(1-x)Te(x) are closer to the respective distances in the binary systems, revealing significant divergence of the local structure from the average one. The mean square relative displacements show a systematic change with Te content, consistent with bond relaxation in the inhomogeneous ternary phases. Also, the Fe-Te and Fe-S distances in the FeS(0.2)Te(0.8) ternary system are found to be different in the crystallographically homogeneous structure. The observed features are characteristic of ternary random alloys, suggesting that a proper consideration should be given to the atomic distribution for describing the complex electronic structure of these multi-band Fe-based chalcogenides.

Flux Dynamics in Iron-Based Superconductors

The newly discovered iron-based superconductors share a common layered structure. After the iron pnictide (1111) family, other iron-based superconductors such as the iron chalcogenides FeTe1 - xSex (11) family have been synthesized. The latter system, characterized by a simple crystal structure, represents an interesting reference system. Indeed, all iron-based superconductors have a stacked structure composed of a layer of iron atoms linked by tetrahedrally coordinated pnictogens (P, As) or a chalcogen (Se, Te) anion: the active layer. The latter is either simply stacked together, as in the FeSe (11), or separated by spacer layers with alkali (e.g., Li), alkaline earth (e.g., Ba), or rare earth oxides/fluorides. In this contribution, we present an ac multiharmonic susceptibility study of the flux dynamic in representative 1111 and 11 iron-based superconductors. Data analysis has been performed in the glass-weak pinning scenario. In particular, the comparison of the third harmonic components vs. temperature and magnetic field returned information on pinning strength and dimensionality. Although in the presence of large thermal fluctuations, because of the high Tc, susceptibility data points out a three-dimensional flux dynamic and, unexpectedly, an increase of the pinning amplitude in the Fe-based superconductor systems where the spacer layer is present.

Controlling mesoscopic phase separation near electronic topological transitions via quenched disorder in ternary diborides

A phase separation driven by the negative compressibility of the electron gas, near electronic topological transitions (ETT), could drive the system at the verge of a catastrophe. We show here that the metastable phases very close to the ETT transition are observed in a mesoscopic phase separation (MePhS) driven by the quenched lattice disorder. By using high resolution synchrotron radiation x-ray powder diffraction we have identified the MePhS for the intermetallic ternary Mg1−x Alx B2 in the proximity of two ETTs: the first at x1 = 0. 1a nd the second at x2 = 0.3. We have identified the competition between a first ‘relaxed’ (R) hole poor and a second ‘tense’ (T) hole rich phase, and by micro-Raman we observe the splitting of the in-plane phonon E2g mode in the proximity of the first ETT at x = 0.1. The anisotropic quenched disorder due to a random distribution of Al 3+ and Mg 2+ ions both in the axial (c axis) direction and planar (ab plane) direction, probed by x-ray diffraction and Raman data, is proposed to be the physical variable that allows the formation of metastable phases near the critical points of electronic topological transitions, where Feshbach shape resonances in interband pairing amplifies the superconducting critical temperature. (Some figures in this article are in colour only in the electronic version) Q.1

Competing exchange interactions in multiferroic and ferrimagnetic CaBaCo4O7

Competing exchange interactions can produce complex magnetic states together with spin-induced electric polarizations. With competing interactions on alternating triangular and kagome layers, the swedenborgite CaBaCo4O7 may have one of the largest measured spin-induced polarizations of ∼1700 nC/cm2 below its ferrimagnetic transition temperature at 70 K. Upon rotating our sample about c=[0,0,1] while the magnetic field is fixed along [1,0,0], the threefold splitting of the spin-wave frequencies indicates that our sample is hexagonally twinned. Magnetization measurements then suggest that roughly 20% of the sample is in a domain with the a axis along [1,0,0] and that 80% of the sample is in one of two other domains with the a axis along either [−1/2,√3/2,0] or [−1/2,−√3/2,0]. Powder neutron-diffraction data, magnetization measurements, and terahertz (THz) absorption spectroscopy reveal that the complex spin order in each domain can be described as a triangular array of bitetrahedral c-axis chains ferrimagnetically coupled to each other in the ab plane. The electric-field dependence of bonds coupling those chains produces the large spin-induced polarization of CaBaCo4O7.

Local Noncentrosymmetric Structure of Bi 2 Sr 2 CaCu 2 O 8+ y by X-ray Magnetic Circular Dichroism at Cu K -Edge XANES

The two-dimensional Bi2Sr2CaCu2O8+y (Bi2212), the most studied prototype cuprate superconductor, is a lamellar system made of a stack of two-dimensional corrugated CuO2 bilayers separated by Bi2O2 + ySr2 O2 layers. While the large majority of theories, proposed to interpret unconventional high Tc superconductivity in Bi2Sr2CaCu2O8+y, assume a centrosymmetric tetragonal CuO2 lattice for the [CuO2]Ca[CuO2] bilayer, here, we report new compelling results providing evidence for local noncentrosymmetric symmetry at the Cu atom. We have measured polarized Cu K-edge XANES (X-ray absorption near-edge structure) and the K-edge X-ray magnetic circular dichroism (XMCD) of a Bi2212 single-crystal near-optimum doping. The Cu K-edge XMCD signal was measured at ID12 beamline of ESRF with the k-vector of X-ray beam parallel to c-axis, i.e., with the electric field of X-ray beam E//ab, using a 17-T magnetic field parallel to the c-axis of a Bi2212 single crystal. Numerical simulations of the XMCD signal of Bi2212 by multiple scattering theory have shown agreement with the experimental XMCD signal only for the local structure with noncentrosymmetric Bb2b space group of Bi2Sr2CaCu2O8+y.