We present a detailed study of the structural and magnetic phase transitions of the Heusler compound Co 2 NbSn. This material undergoes a structural transition at T S ϭ235 K from the cubic Heusler Fm3m hightemperature phase into an orthorhombic low-temperature lattice of Pmma symmetry. Further, the system exhibits a magnetic transition at T C ϭ116 K from para-to ferromagnetism. While on a macro-and mesoscopic scale Co 2 NbSn appears to be fully Fm3m ordered, from microscopic studies we find that on an atomic scale site disorder is present. We discuss the implications of these findings for the anomalous macroscopic physical properties and domain formation in the ferromagnetic state in Co 2 NbSn.

We present a detailed study of the electronic transport properties on a single crystalline specimen of the moderately disordered heavy fermion system URh2Ge2. For this material, we find glassy electronic transport in a single crystalline compound. We derive the temperature dependence of the electrical conductivity and establish metallicity by means of optical conductivity and Hall effect measurements. The overall behavior of the electronic transport properties closely resembles that of metallic glasses, with at low temperatures an additional minor spin disorder contribution. We argue that this glassy electronic behavior in a crystalline compound reflects the enhancement of disorder effects as consequence of strong electronic correlations.

Abstract:We present a detailed structural investigation via neutron diffraction of differently heat treated samples Fe 2 VAl and Fe 2+x V 1−x Al. Moreover, the magnetic behaviour of these materials is studied by means of µSR and Mössbauer-experiments. Our structural investigation indicates that quenched Fe 2 VAl, exhibiting the previously reported "Kondo insulating like" behaviour, is off-stoichiometric (6% ) in itsAl content. Slowly cooled Fe 2 VAl is structurally better ordered and stoichiometric, and the microscopic magnetic probes establish long range ferromagnetic order below T C = 13K, consistent with results from bulk experiments. The magnetic state can be modelled as being generated by diluted magnetic ions in a non-magnetic matrix. Quantitatively, the required number of magnetic ions is too large as to be explained by a model of Fe/V site exchange. We discuss the implications of our findings for the ground state properties of Fe 2 VAl, in particular with respect to the role of crystallographic disorder. ) IntroductionRecently, the magnetic phase diagram of the alloying series (Fe 1−x V x ) 3 Al has been the focus of various detailed studies [1,2]. In particular, Heuslertype Fe 2 VAl has been reported to exhibit a very unusual behaviour for an intermetallic compound, namely a semiconductor-like resistivity close to a magnetic instability [1]. This was interpreted in terms of Kondo-insulating behaviour, analogous to the system FeSi [3,4]. In contrast, optical conductivity studies provided evidence for a pseudogap in the density of states of 1 Fe 2 VAl of 0.1-0.2eV [5], a view supported by various band structure calculations [6][7][8]. Notably, no temperature dependence of the gap features has been detected in these studies, apparently contradicting a Kondo insulator scenario for Fe 2 VAl. However, the pseudogap scenario itself does not account for the unusual resistivity of Fe 2 VAl, as in the absence of magnetic correlations it should predict no or a positive metallic magnetoresistance, in conflict with experimental observations [2,9]. Therefore, in Ref. 5 it has been speculated that the (magneto)resistivity of Fe 2 VAl reflects a mixture of electron excitation processes over the pseudogap and spin dependent scattering from impurities.Independently, on basis of specific heat and NMR-experiments it has been demonstrated that in Fe 2 VAl crystallographic disorder, assumed to be present in form of atomic site exchange between Fe and V atoms, substantially affects the ground state properties of this compound [10,11]. In particular, the anomalous low temperature specific heat has been attributed to ferromagnetic clusters with a density of 0.003-0.004/unit cell, consistent with the results from NMR experiments. These works, as well, are in broad agreement with the results from band structure calculations, which predict that via Fe/V site exchange or crystallographic superstructure formation ferromagnetic clusters or long-range order might be generated in Fe 2 VAl [6][7][8]. Recently, it has been claimed that s...

We report a study of the structure and pressure effects on magnetic and transport properties of a lanthanum manganite, La0.7Ca0.3−xSrxMnO3 (0⩽x⩽0.3, Δx=0.03). The pressure coefficient of the Curie temperature (Tc), dTc/dP, is shown to depend on x. The temperature Tc increases approximately linearly under applied pressure as dTc/dP≈14 K/GPa in the orthorhombic (Pbnm) phase and as dTc/dP≈7.5 K/GPa in the rhombohedral (R3̄c). The value of dTc/dP shows a minimum (≈3.5 K/GPa) and the temperature of the resistance maximum, Tp(x), shows a change of slope at x=0.15, corresponding to a concentration structural phase transition. Differences between the values of dTc/dP and the slopes of Tp vs x in Pbnm and R3̄c phases are explained by the different effect of external pressure on the Mn–O bond length and the Mn–O–Mn bond angle, and by the different internal pressure effect (depending on the value of x) in those phases, respectively.