K. Bärner

Transport properties of manganates with giant magnetoresistance

Abstract A short review of transport properties related to the giant magnetoresistance in perovskite-type double-exchange ferromagnets is presented. The paper deals with the essential experimental results for the understanding of the large MR effect, which is based on the correlation of electric and magnetic properties. These experiments are mainly the dependence of the resistivity on temperature and magnetic field, and also on the chemical composition. In the region of the highest magnetoresistance, which usually occurs close to a magnetic transition, a peak in resistivity indicates the strong spin disorder scattering. Even localization of charge carriers and thus a metal-insulator transition occurs in some cases.

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.

Low temperature resistivity minimum in MnAs1−xPx compounds☆

Abstract The conductivity and the magnetic moment of MnAs 1− x P x single crystals have been measured in the temperature range from 5 to 300 K and for compositions between x = 0.03 and x = 0.12. Comparison of the temperature dependence of the magnetic moment with that of the electrical resistivity yields a connection between a low temperature resistivity minimum which resembles the Kondo-Nagaoka type anomaly and a sharp decrease of the magnetic moment. In this temperature range the magneto-resistance is relatively large (∽ 5%) and negative. A small temperature hysteresis and a slow response to magnetic fields is observed. The occupation of quasi-bound states at the cation sites by the conduction electrons would explain qualitatively the observations.

Magnetic phase transitions and magnetoresistance in the La0.7Ba0.3(Mn1−xMex)O3 (Me=Fe, Al) single crystals and La0.7Sr0.3(Mn1−xMex)O3 (Me=Fe, Cr, Al) ceramics

Abstract The single crystals of La 0.7 Ba 0.3 (Mn 1− x Fe x )O 3 ( x ⩽0.28) and La 0.7 Ba 0.3 (Mn 1− x Al x )O 3 ( x ⩽0.15) compositions were grown using flux method and characterized by X-ray, electrical and magnetization measurements. The Fe-doping above x =0.2 destroys a long range ferromagnetic order thus leading to a spin glass state. It is found that insulating spin glasses exhibit a large magnetoresistance in the paramagnetic region which is comparable to that for ferromagnetic crystals showing metal–insulator transition close to T C . The magnetic behavior of La 0.7 Ba 0.3 (Mn 1− x Me x )O 3 (Me=Fe, Cr, Al) ceramics is in agreement with superexchange magnetic interactions via oxygen.

Magnetic susceptibility anomaly connected with a displacive phase transition in MnAs1−xPx and CrxMn1−xas compounds

Abstract With the second order B31 [rlhar2] B81 phase transition Tt in MnAs1-xPx and CrxMn1-xAs compounds a high-spi low-spin transition occurs simultaneously with a drastic change in the magnetic coupling. An interpolation scheme is proposed which yields C and Tc for each temperature below Tt separately.

Elastic anomalies and phonon damping in a metallic high spin-low spin system

Abstract The elastic constants and the sound attenuation in single crystals of the metallic high spin (hs)-low spin (ls) system MnAs 1− x P x have been measured for temperaturres between 10 and 500 K. Elastic anomalies and damping maxima have been found for the second-order displacive (B8 1 ⇌B31) phase transition, the hs-ls transition and for the magnetic order-disorder transition. The phenomena near the hs-ls transition, in particular, are interpreted in terms of a condensation of a soft static phonon at the ls (hs) site in a hs (ls) matrix.

Magnetic and electrical properties of Cu substituted La1−xCaxCoO3 crystals

Abstract La 1− x Ca x Co 1− y Cu y O 3-δ polycrystals have been prepared in the compositional range 0 y x y = 0, a transition to metallic (impurity band) conduction is observed at x c = 0.45. All other compounds are semiconducting ( ∂ρ / ∂T ρ ∼ T − 1 4 ) and a spin-glass-like behaviour except for T = 4.2 K. A field-induced antiferromagnetic low spin/spin-glass-like mixed spin transition is observed, yielding 0.12 meV as an estimate for the ls/hs energy gap.

Specific heat capacity of mn1+δas1−xsbx single crystals

Abstract The specific heat capacity C p of Mn 1+δ As 1−x Sb x single crystals with 0≦ x ≦1, 0 at. % ≦ δ ≦ 2 at. % is presented for 110 K T x − T phase diagram and compared with those in the literature. From the analysis of C p ( T ) we find evidence for a transition from a ferromagnetic to a spin-glass-like state (at T u ) and only at T f > T u is paramagnetism attained. Temperatureinduced “in-layer randomization” of the manganese displacements might occur for mixed crystals with small x and for vacancy-disordered MnAs.

Modulation of the hall resistance in CrxMn1−xAs compounds☆

Abstract The Hall resistance ϱ H of Cr x Mn 1− x As polycrystals has been measured for x = 0.5, 0.4, 0.33, 0.2, 0, for magnetic inductions up to B = 0.4 T and for temperatures between 20 and 300 K. Near the transition from antiferromagnetic (ferromagnetic) spin order to a canted spin order, for x = 0.2 and 0.4, we observe a modulation of the Hall resistance when the external field is increased. We relate this to modulations of the anomalous Hall constant R s which arise because discontinuous in the density of states at the Fermi-level occur when the spin-dependent electron transfer changes with the cant angle.

Magnetic properties of CrxMn1−xAs mixed crystals

Abstract The magnetic moment of Cr x Mn 1− x As mixed crystals has been measured for temperatures between 3 and 400 K and for ten different compositions between x = 0 and x = 1. Measurements of the electrical resistivity have been added in order to achieve an unambiguous identification of the various magnetic ordering temperatures. The combination of all the data enabled us to construct the complete magnetic phase diagram. It is found that the transition from antiferromagnetic CrAs to ferromagnetic MnAs is very complex. We propose to explain the transition as an interplay of two competing double exchange-superexchange coupling mechanisms.