L. Haupt

Intrinsic giant magnetoresistance of mixed valence La‐A‐Mn oxide (A=Ca,Sr,Ba) (invited)

A large intrinsic magnetoresistance has been found near the ferromagnetic transition of metallic manganese oxides with perovskite‐type crystal structure. The magnetic and transport properties were measured on bulk and thin‐film La1−xAxMnO3+δ with A=Ca,Sr,Ba. Assuming the double‐exchange model proposed by Zener [Phys. Rev. 81, 440 (1951); 82, 403 (1951)], the strong dependence of the transport properties on the magnetic field and also on the chemical composition is attributed to the mixed Mn3+/Mn4+ valence.

Metal-semiconductor-transition and spin-glass properties of La(2-x)/3Ba(1+x)/3Mn1-xCuxO3

Abstract Curie-temperature and saturation magnetization of the metallic ferromagnetic perovskite La 2 3 Ba 1 3 MnO 3 decreases, when manganese is substituted by copper. If the content of copper exceeds 20%, a transition to a semiconducting state occurs, which shows spin-glass-like properties at low temperatures. The semiconducting behaviour seems to be connected with the magnetic disorder, which leads to a localization of charge carriers.

Metal−semiconductor transition in the double exchange system La0.8Sr0.2Mn1−xCuxO3

Abstract Magnetization and resistivity of La 0.8 Sr 0.2 Mn 1− x Cu x O 3 compounds are measured for 10 T B Mn 3+ / Mn 4+ valences. Details of the dc and optical conductivity suggest spin scattering for the ferromagnetic metals and variable range hopping involving the spin disorder for the spin-glass like semiconductors.

Effect of Oxygen Content and Oxygen Vacancy Ordering on the Properties of TbBaCo2O6?? Perovskites

Using IR-spectroscopy, heat capacity experiments, neutron and X-ray diffraction, as well as magnetization and resistivity measurements, TbBaCo2O6—δ samples with different oxygen content and different thermal history are studied. It is shown that the structural, magnetic and electrical properties are strongly sensitive to oxygen content and oxygen vacancy ordering. We submit that the first order semiconductor-to-metal phase transition at 340 to 345 K in this compound is accompanied by oxygen vacancy ordering and short-range magnetic ordering, and only at 285 K spontaneous long-range ferrimagnetic ordering occurs.

Search for the ferromagnetic-superconducting transition in oxide based metallic compounds

Abstract The two end members La 0.8 Sr 0.2 MnO 3 and La 0.8 Sr 0.2 CuO 3 of a possible mixed crystals series have been prepared and their magnetization, resistivity, X-ray diffraction, Raman- and IR data were taken; the compound containing Mn constitutes a “double exchange” ferromagnet, the Cu-compound a high T c superconductor; preliminary experiments on Mn/Cu mixed samples suggest the existence of compounds which might throw some light on the question of coexistence of ferromagnetism and superconductivity in oxide based metallic compounds.

Raman spectra of ferromagnetic La0.8Sr0.2CuxMn1−xO3 mixed crystals

Abstract X-ray diffraction, neutron diffraction, resistivity, and magnetization versus composition and temperature have been used to characterize a magnetically dilute mixed crystal series of La 0.8 Sr 0.2 Cu x Mn 1−x O 3 ( O ≤ x ≤ 0.4). The Raman spectra of these compounds are presented for the first time; they show various phonon bands and an anomalous broad band at 650 cm -1 which we tentatively connect with magnetic or electronic excitations.

Magnetization of LaBa2(Cu1−xMnx)3O7−γ

Abstract Substituting copper by manganese in the high-temperature superconductor LaBa2Cu3O7 decreases both the critical temperature and the flux exclusion. While we could reproduce the known superconducting properties of LaBa2Cu3O7, the manganese containing superconductors (x = 3% and x = 5%) show a completely different behaviour, in particular, in magnetic fields well below Hc2, i.e. an increasing magnetic field reduces the flux-exclusion and induces a superconducting state with a positive magnetization.

Heat diffusivity of Nd1−xSrxMnO3−δ and La1−xCaxMnO3−δ compounds

The thermal diffusivity of bulk and epitaxial layer samples of the giant magnetoresistance compounds Nd0.67Sr0.33MnO3−δ and La0.67Ca0.33MnO3−δ is investigated in a comparative study for temperatures between 35 and 350 K in order to detect preparational differences and to correlate the thermal transport with magnetization M and resistivity ρ. We find for Nd–Sr an anomaly of the heat diffusivity D near 145 K (80 K La–Ca) which we attribute to a temperature-induced canted metal to AFM insulator transition and a second anomaly in D(T) near 250 K (300 K) which we assign to the transition to paramagnetism. The low-temperature anomaly can be a step of up to five times in the heat conductivity, which makes these compounds potential magnetic field activated heat switches, as this transition can also be induced by a magnetic field.

Comparison of the magnetic properties between Y3(Fe, Cr)29 and Y2Fe15Cr2 single crystals

A Y-3(Fe, Cr)(29) single crystal which has a Nd-3(Fe, Ti)(29)-type monoclinic structure has been prepared using the Czochralski method followed by heat treatment of an initial Y-2(Fe, Cr)(17) crystal. The magnetization curves were measured along the easy and hard magnetization directions in the range from 1.5 to 293 K for crystals of Y-3(Fe, Cr)(29) and Y-2(Fe, Cr)(17). The saturation magnetization of Y-3(Fe, Cr)(29) decreases with increasing temperature and is in good agreement with a T-3/2 spin-wave relation law. The magnetic properties of the two crystals are compared

Magnetic and electrical properties of Nd0.66(Sr1−yLiy)0.34MnO3 manganites

We present X-ray diffraction, magnetization, electrical resistivity, and magnetoresistance data for Nd 0.66 (Sr 1-y Li y ) 0.34 MnO 3 ceramics with y = 0, 0.05, 0.1, 0.2 and 0.3 in the temperature range 50 < T < 400 K and in fields up to 0.5 T. We find that Li substitution is possible up to y ≤ 0.3 and generally enhances the resistivity and the magnetic randomization. Nevertheless, the resistivity curves still show the characteristic hump-like structure indicative of a combined ferromagnetic-paramagnetic metal-insulator transition, and a significant magnetoresistance is observed. The increase of the resistivity with Li content is ascribed to a blocking of Zener bonds associated with the charge unbalancing effect of Li + .