Rittmar Von Helmolt

Structure and magnetoresistive properties in La–manganite thin films

This study investigates the structure of perovskite thin films and its influence on their colossal magnetoresistance (CMR) properties. Epitaxial thin films of perovskite manganites La1−xBxMnO3−δ (B=Ca,Sr) were prepared on SrTiO3 (100) substrates using on- and off-axis pulsed laser deposition (PLD) techniques. X-ray diffraction, resistance and magnetoresistance measurements, as well as high-resolution transmission electron microscopy (HRTEM) investigations were carried out. HRTEM observations reveal epitaxial growth for the first few layers of all prepared samples. Thicker on-axis prepared films grow with a large number of defects, whereas off-axis prepared samples grow in a columnar structure. Since the magnetic properties in systems with double-exchange interaction are very sensitive to the local structure it has great influence on the electronic properties.

Heat diffusivity of La1−xCaxMnO3 epitaxial layers

The Seebeck effect and the thermal diffusivity of La0.67Ca0.33MnO3−δ were investigated in the temperature range 35<T<300 K and in particular in the regime of the high magnetoresistance and maximum resistivity ρmax. We find a sign change of the thermoelectric power S(T) and a step in heat conductivity near ρmax which we attribute to a temperature‐induced metal–insulator transition at about 80 K, suggesting that the large giant magnetoresistance found in these substances is due to a field‐induced metal–insulator transition.

Anomalies of Hall effect and magnetoresistance in oxygen deficient La2/3A1/3MnO3-δ epitaxial layers

The effect of oxygen disorder on the magnetic moment, resistivity, Hall effect and magnetoresistance of oxygen deficient La 2/3 Al 1/3 InO 3-δ epitaxial thin films has been studied in magnetic fields up to 20 T and temperatures down to 4.2 K. Our results show, that?(T, H) of these thin films depends on δ but differs in detail from that of equivalent nonstoichiometric bulk samples. In particular, changes in sign and magnitude of the magnetoresistance ΔQ/Q occur which suggest positive and negative shifts of a magnetic order-oder transition temperature in a magnetic field, dT 1 /dH, depending on δ. The Hall measurements give a hole carrier concentration of about 3 x 10 21 cm -3 for near stoichiometric compounds.

Magnetic properties and stability of melt spun Nd/sub 5/Fe/sub 17/

Nd/sub 5/Fe/sub 17/ (also referred to as A/sub 2/-phase) is prepared by rapid-quenching and subsequent annealing. Due to the fine, homogeneous distribution of the elements in the melt-spun ribbons, only a short reaction time is required in the annealing treatment. The samples are almost single-phase, as revealed by X-ray and magnetization measurements. Unlike Nd/sub 2/Fe/sub 17/, the A/sub 2/-phase does not interstitially dissolve nitrogen, but decomposes into NdN and alpha -iron. >

Saturation of the photoresponse in Nd0.67Sr0.37MnO3−δ epitaxial layers close to the metal–insulator transition

Abstract Internal electrical fields of different origin are observed in the semiconducting regime of Nd2/3Sr1/3MnO3−δ epitaxial layers and are assigned to thermoelectric effects and to an ambipolar charge distribution in trap states, following a photogeneration of electron–hole pairs. In particular, if the resistivity shows a double peak, the photoresponse shows saturation in the regime of the low-temperature peak, suggesting the existence of a gap in the transitional regime of the metal–insulator transition.

Evidence of photogenerated carriers in the semiconducting state of Nd0.67Sr0.37MnO3-δ epitaxial layers

Time-resolved thermoelectric power measurements on Nd2/3Sr1/3MnO3-delta epitaxial layers show a voltage transient which is superimposed to the usual quasistatic thermovoltage decay which is due to heat diffusion, but only in the activated regime. The characteristic temperature of the separation is 140 K. The extra transients are ascribed to the decay of an ambipolar charge distribution in trap states

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.

Magnetoresistance of granular Cu‐(Co,Fe) and Cu‐Co‐B (abstract)

Giant magnetoresistance (GMR) is known to occur in alloys consisting of superparamagnetic precipitates in a metallic matrix. This had been demonstrated for the first time for Cu‐Co prepared as co‐sputtered thin films and later for rapidly quenched alloys. For Cu‐Fe no GMR has been reported even in multilayer systems. In order to study the transition from Cu‐Co to Cu‐Fe in more detail we prepared quasibinary Cu90Co10−xFex ribbons by conventional melt spinning followed by an annealing treatment to precipitate Co‐Fe clusters in the Cu matrix. The particle sizes of the ferromagnetic phase have been determined by fitting the magnetization and magnetoresistance curves with a Langevin function. The saturation magnetization increases with increasing Fe content x for x≤4 as expected from the Slater–Pauling curve. However, a strong decrease of the magnetization is observed at high Fe contents. The GMR effect continuously decreases with increasing Fe content and for x≳0.7 no GMR is observed, similar to Cu‐Fe multila...

Ti-substituted hard magnetic mechanically alloyed Sm/sub 2/Fe/sub 17/N/sub x/

Sm/sub 12.6/Fe/sub 87.4-z/Ti/sub z/ with z=2, 3 and 4 elemental powder blends are mechanically alloyed and subsequently two-step heat treated to form hard magnetic microcrystalline Sm-(Fe,Ti)-N. The optimum constitution, i.e., the formation of mainly Sm/sub 2/(Fe, Ti)/sub 17/N/sub x/, accompanied by a small fraction of Sm(F,Ti)/sub 9/N/sub x/, and, particularly, the suppression of alpha -Fe precipitation, is achieved for 3 at.% Ti. Such material possesses an almost completely step-free hysteresis loop, a remanence of 0.73 T and a coercivity of 16 kA/cm (20 kOe), resulting in a maximum energy product of 90 kJ/m/sup 3/ (11.2 MGOe) in the powder. >