Preparation, magnetoresistance and magnetocaloric effect of two-layered perovskite La 2.5- x K 0.5+ x Mn 2 O 7+δ

Polycrystalline two-layered perovskite La_2.5-xK_0.5+xMn₂O _{7 + δ} (0 < x < 0.5) samples have been prepared by a modified sol-gel method and their magnetoresistance and magnetocaloric effects have been studied. A large deviation between the metal-insulator (MI) transition temperature (T _ρ ) and the magnetic transition temperature (T_C) is observed. Large magnetoresistance (MR) effects with Δρ/ρ of ∼40% at 12 kOe are obtained in wide temperature ranges. The maximum of the magnetic entropy change peaks at its Curie temperature (T_C), far above its MI transition temperature (T _ρ ). The large magnetic entropy change (∼1.4 J/kg.K) is obtained in the sample La_2.5-xK_0.5+xMn₂O _{7 + δ} (x = 0.35) upon 10 kOe applied magnetic field. Received 2 May 2002 / Received in final form 1st October 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: wzhong@ufp.nju.edu.cn     

Large magnetocaloric effect and magnetoresistance behavior in Gd4Co3

We report a large entropy change (ΔS) below 300 K, peaking near T _C = 220 K, due to isothermal change of magnetic field, for Gd₄Co₃, with a refrigeration capacity higher than that for, say, LaFe_11.4Si_1.6, ordering magnetically in the same temperature range. A noteworthy finding is that the isothermal magnetization is nonhysteretic — an important criterion for magnetic refrigeration without loss. ΔS behavior is also compared with that of magnetoresistance.     

Anomalous temperature dependence of resistivity in quasi-one-dimensional conductors in a strong magnetic field

We present a heuristic, semiphenomenological model of the anomalous temperature (T) dependence of resistivity recently observed experimentally in the quasi-one-dimensional (Q1D) organic conductors of the family in moderately strong magnetic fields. We suggest that a Q1D conductor behaves like an insulator (), when its effective dimensionality is one, and like a metal (), when its effective dimensionality is greater than one. Applying a magnetic field reduces the effective dimensionality of the system and switches the temperature dependence of resistivity between the insulating and metallic laws depending on the magnitude and orientation of the magnetic field. We critically analyze whether various microscopic models suggested in literature can produce such a behavior and find that none of the models is fully satisfactory. In particular, we perform detailed analytical and numerical calculations within the scenario of magnetic-field-induced spin-density-wave precursor effect suggested by Gor'kov and find that the theoretical results do not agree with the experimental observations. Received 20 October 1998     

Investigation of the first-order metamagnetic transitions and the colossal magnetocaloric effect using a Landau expansion applied to MnAs compound

We have explored a simple Landau model to calculate magnetization isotherms considering magnetic hysteresis. The model parameters have been chosen to fit the magnetic and magnetocaloric data of MnAs compound. Experimental data show that there is a great difference between the isothermal variation of the entropy (S_T) obtained from isotherms measured increasing and decreasing magnetic field. This great difference is reproduced theoretically. From the experimental and phenomenological isotherms, we calculated the S_T. From the theoretical entropy, we also obtained S_T, which does not present the colossal peak.     

Thermal expansion and magnetostriction in an anomalous Gd intermetallic compound

Thermal expansion and forced magnetostriction measurements are reported on two Gd intermetallic compounds which order magnetically below 10 K. The relative influence of the electronic, lattice and magnetic degrees of freedom was determined using results obtained on a non-magnetic isostructural compound. A Grüneisen analysis revealed that whilst the magnetic contribution to the specific heat is similar for both Pd₂GdIn and Cu₂GdIn the spontaneous magnetostriction was significantly smaller in the Pd compound. Forced magnetostriction measurements suggest that the thermal expansion in Pd₂GdIn is primarily associated with spin fluctuations in the Pd 4d band. It is suggested that these additional degrees of freedom give rise to the enhanced specific heat observed in Pd₂GdIn. Received 28 July 1999     

Can one hear the shape of an electrode? I. Numerical study of the active zone in Laplacian transfer

The concept of active zone in the Laplacian transport to and across irregular interfaces is rigorously introduced. It applies to arbitrary geometries and uses the coarse-graining method proposed by Sapoval to compute the flux across an irregular interface from its geometry without solving the general Laplace problem. Such transport play a dominant role in electrochemistry, heterogeneous catalysis and physiological diffusion processes. In the field of electrochemistry, the method permits one to predict the impedance of an electrode of arbitrary geometry for any value of the frequency. It shows that, for systems with aspect ratios of the order of a few times unity or less, impedance spectroscopy yields in principle a reliable approximate measure of the length of the chord corresponding to a perimeter length inversely proportional to the interface capacitance and frequency. For these cases, impedance spectroscopy can determine the shape of an electrode to the extent that the knowledge of the average chord length as a function of the perimeter determines the shape. For systems of arbitrary geometry, it is shown that impedance spectroscopy permits a measure of the size of the active zone. These results can be transposed to several problems related to Laplacian transfer, such as etching of irregular solids and catalysis in the Eley-Rideal regime. Received 3 November 1998 and Received in final form 8 December 1998     

Giant entropy change at the co-occurrence of structural and magnetic transitions in the Ni Mn Ga Heusler alloy

We have studied the isothermal entropy change around a first-order structural transformation and in correspondence to the second-order Curie transition in the ferromagnetic Heusler alloy Ni_2.15Mn_0.85Ga. The results have been compared with those obtained for the composition Ni_2.19Mn_0.81Ga, in which the martensitic structural transformation and the magnetic transition occur simultaneously. With a magnetic field span from 0 to 1.6 T, the magnetic entropy change reaches the value of 20 J/kg K when transitions are co-occurring, while 5 J/kg K is found when the only structural transition occurs. Received 27 September 2002 / Received in final form 17 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: solzi@fis.unipr.it     

Magnetic interactions in the monoclinic ludwigite Cu FeO BO

The system Cu₂FeO₂BO₃ is an oxyborate belonging to the family of the ludwigites. In this paper we present AC susceptibility, magnetization measurements and M?ssbauer spectroscopy on this material which allows for a complete characterization of its complex magnetic behavior. We find an hierarchy of interactions which clearly defines three regimes with decreasing temperature. These are associated with, the freezing of the Fe moments, the antiferromagnetic ordering of the Cu sub-lattice and finally the coupling between both systems. Received 25 September 1998     

Ga-NMR study of the low-energy excitations in

We report the results of ⁶⁹Ga- and ⁷¹Ga-NMR measurements on NdGa₂ at temperatures between 0.1 and and in applied magnetic fields between zero and 74 kOe. NdGa₂ orders antiferromagnetically below and undergoes several metamagnetic transitions in external magnetic fields. In zero applied magnetic field and below the temperature dependence of the spin-lattice relaxation rate T1 ^-1 ( T ) shows a large linear-in-T term, about two orders of magnitude higher than for the reference compound LaGa₂. This strong enhancement confirms the presence of low-energy excitations in the antiferromagnetic phase of NdGa₂ as was previously indicated by specific heat data. Above , T1 ^-1 ( T ) is dominated by an exponential term, which we associate with excitations between the lowest energy levels of the f-electron system. The separation of these energy levels is determined by exchange, crystal-field and Zeeman interactions. Received 3 September 1998 and Received in final form 3 November 1998     

Nuclear spin conversion in CH F induced by an alternating electric field

The nuclear spin conversion in CH₃F molecules subjected to an alternating electric field was investigated experimentally. The conversion rate was found to be almost unaffected by low electric fields ( V/cm) but sharply increased tenfold when the electric field amplitude exceeds the values ( V/cm) sufficiently high to produce crossings of the ortho and para states of the molecule. A theoretical model for the molecular conversion in alternating electric field was developed. The results of the experiment were found to be in a good agreement with the theory. Received 23 July 1999 and Received in final form 7 September 1999     

Plateau transitions in fractional quantum Hall liquids

Effects of backward scattering between fractional quantum Hall (FQH) edge modes are studied. Based on the edge-state picture for hierarchical FQH liquids, we discuss the possibility of the transitions between different plateaux of the tunneling conductance G. We find a selection rule for the sequence which begins with a conductance (m: integer, p: even integer) in units of e ²/h. The shot-noise spectrum as well as the scaling behavior of the tunneling current is calculated explicitly. Received 5 October 1999 and Received in final form 19 November 1999