C. Dupas

Transport properties and magnetic behaviour of La1-xSrxMnO3 single crystals

A semiconductor-metal transition occurring just below the Curie temperature TC was observed in single crystals of perovskite La1-xSrxMnO3 for x>or=0.175. For x or=0.13 whereas huge MR persists well below TC for the x=0.10 sample, which is probably due to a non-collinear spin configuration.

Effect of a magnetic field on the 3d ordering of the linear antiferromagnet TMMC

Abstract We have studied by PMR experiments the effect of magnetic fields of 0–10 kOe parallel and perpendicular to the chains on the 3d ordering of TMMC. It is found that TN increases as H2 with H perpendicular to c; the experimental values of ΔT N T N are much greater than the theoretical ones. With H parallel to c, we only observed a broad transition to the 3d order.

Inverse giant magnetoresistance (invited)

Inverse giant magnetoresistance (GMR) is obtained in multilayers alternating two ferromagnetic layers F1 and F2 with different asymmetry of spin scattering, α1≳1 and α2<1. This is clearly demonstrated in the simple spin‐valve system with perpendicular magnetization Fe1−xVx/Au/Co. With respect to Fe, the FeV alloys with x=0.18 and 0.29 exhibit an inversion of the spin scattering coefficients (αFe≳1, αFeV<1) due to the change of the densities of states at the Fermi level. The inverse MR of FeV/Au/Co is studied as a function of FeV layer thickness and temperature and compared to calculations based on the Camley–Barnas model. The data show that the inverse GMR is due to the bulk scattering within the FeV layer, which coexists with a substantial interface scattering favoring normal GMR.

Jahn-Teller effect and ferromagnetic ordering in La0.875Sr0.125MnO3: A reentrant behaviour

Abstract The compound La0.875Sr0.125MnO3 undergoes a series of structural phase transitions in which different physical effects are involved. The compound is rhombohedral above TRO ∼ 400 K. On cooling, a phase transition to an orthorhombic Pbnm O-phase takes place at TRO, no significant orbital ordering is observed. At Too′ ∼ 270 K a structural phase transition to another orthorhombic (O′) form takes place, and an orbital ordering appears due to a cooperative JT effect. For this temperature range the compound is a paramagnetic semiconductor. Upon cooling it becomes ferromagnetic at TC ∼ 180 K and the orbital ordering is partially suppressed. Below a temperature To′o″ ∼ 150 K at which the average Mn magnetic moment is about 2μB the orbital ordering is completely suppressed by a first order structural transition to another orthorhombic phase (O″) similar to the O-phase. Between TC and TC′o″ the compound is metallic and becomes a ferromagnetic insulator below To′o″ where a canting of the ferromagnetic order appears.

Colossal magnetoresistance in La2−2xSr1+2xMn2O7 single crystals

Abstract We performed magnetisation (M) and transport measurements along the ab plane and the c direction in La2−2xSr1+2xMn2O7 single crystals for x=0.4. In addition to a sharp para-ferromagnetic transition at T c =125 K , a weak maximum in M was found around 265xa0K. A sharp drop of resistivity at Tc associated with a semi-conductor to metal transition was found for both current orientations. The colossal magnetoresistance of the compound CMR=−(ρH−ρ0)/ρ0 was found to be ∼100% in a field of 8xa0T near Tc. In contrast with the well-known La1−xSrxMnO3 type manganites, a CMR effect larger than 10% was present at all temperatures ranging between 5 and 300xa0K.

Magnetoresistance of cobaltgold multilayers with an increasing number of periods

Abstract Coue5f8Au multilayers, eight in all, with between two and nine cobalt layers, were produced together in UHV by thermal evaporation. All cobalt layers were identically thick (0.8 nm) with easy magnetisation axis perpendicular to the film plane. The (111) gold interlayer thickness of 2.2 nm corresponds to an antiferromagnetic coupling [1]. Kerr rotation magneto-optics and magnetoresistance measurements under perpendicular magnetic field revealed separate coercive fields for each of the cobalt layers. The plateaux between two coercive fields represent different magnetisation direction configurations that could be identified by the magnetoresistance measurements. An adapted Camley—Barnas modelling of the multilayers with up to six cobalt layers was conducted which agreed well with experimental results and further confirmed the magnetisation reversal order deduced by the magnetoresistance measurements.

Magnetoresistance study of granular behaviour in Co / Au(111) ultrathin films

Abstract A study of the magnetoresistance (MR) of Au/Co( t Co )/Au and Au/Co( t Co )Au/Co/Au layers grown in UHV with one variable very small Co layer thickness, t Co , is reported. Large low-temperature MR for both sample structures was observed for t Co around the transition thickness between granular and continuous layers. A local minimum of MR was detected at t Co = 0.16 nm, corresponding to 1 atomic layer (AL) of Co epitaxially grown on Au(111).

Colossal magnetoresistance and re-entrant charge ordering in single crystalline two layer Mn perovskite LaSr2Mn2O7

Abstract We report on the resistivity (ρab), magnetisation (M), and X-ray synchrotron measurements on a single crystal of the two-layer manganite LaSr2Mn2O7. ρab showed a maximum at 180xa0K and increased rapidly for T K . In a field of 8xa0T, colossal magnetoresistance (CMR) data showed two maxima at 150 and 40xa0K. In low fields, M (H∥ab) showed also two maxima at these temperatures. X-ray investigations revealed satellite reflections whose intensity showed a strong maximum at 170xa0K followed by a re-entrant behaviour below about 40xa0K. The data seem to reflect the structural, charge and orbital degrees of freedom present in this compound.

Giant magnetoresistance of dissymmetrical Co/Au multilayers

Abstract Results are presented for the magnetoresistance (MR) of sapphire/Nb 3 /Cu 3 /Au 8 /(Co/Au 8 ) n dissymmetrical multilayers built by alternating a 0.3 nm discontinuous Co layer with a 0.7 nm continuous one. The observed enhanced MR is related to a higher spin scattering asymmetry for the granular Co layers.

Aimantation a basse temperature de deux sels ferromagnetiques uniaxes. Cu(NH4)2Br4, 2H2O ET CuRb2Br4, 2H2O

Resume La courbe theorique daimantation spontanee a basse temperature dun ferromagnetique de Heisenberg a ete calculee par la methode de renormalisation des ondes de spin de Bloch en tenant compte dune faible anisotropie dechange. La confrontation avec les courbes experimentales des sels Cu(NH4)2Br4, 2H2O et CuRb2Br4, 2H2O montre un tres bon accord pour 0