H. L. Ju

Giant magnetoresistance in La1−xSrxMnOz films near room temperature

We report the observation of giant magnetoresistance near room temperature in ferromagnetic films of La1−xSrxMnOz for 0.16≤x≤0.33. For B=5 T, the maximum magnetoresistance ratio [R(0)−R(B)]/R(0) of an annealed film is 60% at 260 K for x=0.2, and 35% at 330 K for x=0.33. Annealed films have higher Curie temperature (Tc), a larger saturation moment and a larger magnetoresistance effect near Tc than do as‐grown films. The temperature dependence of resistivity for all the samples investigated is unusual, activated above Tc and metallic below Tc. This and the giant magnetoresistance are possibly explained by scattering from magnetic polarons which dominate the transport near Tc.

Giant magnetoresistance in epitaxial Nd0.7Sr0.3MnO3−δ thin films

The magnetoresistance of epitaxial Nd0.7Sr0.3MnOδ thin films has been studied. A giant magnetoresistance, with more than 4 orders of magnitude change in resistance (−ΔR/RH≳106%), was obtained at ∼60 K and a magnetic field of 8 T. This giant magnetoresistance (GMR) ratio is about one order of magnitude larger than the highest value reported previously which was observed in La–Ca–Mn–O film. We have also obtained a large GMR ratio with −ΔR/RH≳3000% for H=5 T in an in situ Nd0.7Sr0.3MnOδ thin film, a much larger effect than the previous results in doped manganese oxide films in which a large GMR ratio was obtained only in postannealed samples. Our results also show that the GMR effect in these films can be strongly influenced by the thin‐film preparation conditions.

Influence of preparation on resistivity behavior of epitaxial Nd0.7Sr0.3MnO3−δ and La0.67Ba0.33MnO3−δ thin films

The influence of sample preparation on the resistivity and magnetoresistance (MR) behavior of doped manganese oxides has been studied. Two systems, Nd0.7Sr0.3MnO3−δ and La0.67Ba0.33MnO3−δ thin films, have been chosen for this study. The former has the largest reported MR ratio at 60 K and the latter has a large MR ratio at room temperature. Two processes, deposition at a high temperature and annealing at a high temperature, have opposite influences on the resistivity behavior of these films. The results suggest that oxygen stoichiometry and diffusion are important factors in causing the behavior observed in doped manganese oxide films.

Ferromagnetic resonance and intrinsic properties of La0.67Ba0.33MnOz

Systematic magnetization and ferromagnetic resonance (FMR) measurements have been performed on thin‐film samples of La0.67Ba0.33MnOz (TC∼340 K). It is found from the FMR data that as‐ grown films show broad lines whose width Γ increases to nearly 1 kOe as T reduced to 77 K. The increase roughly follows the increase in the magnetization suggesting magnetic inhomogeneity as the main cause for the rise in Γ. However, when films were properly annealed in O2, we obtained narrow lines (Γ∼200 Oe at 10 GHz). Γ was effectively independent of temperature and linearly dependent upon microwave frequency, both of which are strong signs that, we have succeeded in obtaining a uniform, homogeneous manganite sample. Having gotten a uniform material, we have measured the intrinsic properties of this system.

Giant magnetoresistive memory effect in Nd0.7Sr0.3MnOz films

A novel giant magnetoresistance memory effect has been observed in epitaxial Nd0.7Sr0.3MnOz thin films which have previously been found to exhibit a linear increase in conductivity on first application of magnetic field B. The resistivity of the films depends not only on the instantaneous applied field but also on the magnetic history of the sample. At T well below the temperature Tp, where the zero‐field resistivity has a peak, the film enters a high‐conductivity state (upon application of a magnetic field) which persists even when B is reduced to zero. The original ‘‘zero’’ field state is not recovered until the sample is warmed to T∼Tp. Surprisingly, the dc magnetization exhibits only a weak irreversibility while the magnetoconductivity is markedly hysteretic. A possible explanation is proposed.

Superparamagnetic behavior and giant magnetoresistance in oxygen deficient R0.67Sr0.33MnOz (R=Nd, Pr) epitaxial films (abstract)

We have studied the magnetic and magnetotransport properties of oxygen deficient R0.67Sr0.33MnOz (R=Nd, Pr) films and have observed superparamagnetic behavior in these films. The superparamagnetism is indicated by (i) the increasing spread between zero field cooled (ZFC) and field cooled (FC) magnetization curves as temperature decreases, (ii) a sharp drop of the ZFC magnetization at low temperature, (iii) enhanced Curie–Weiss constant above the ferromagnetic temperature, and (iv) at low temperature a sharp drop of coercivity (Hc) with increasing temperature. These findings strongly suggest that all oxygen deficient manganites are magnetically inhomogeneous; i.e., ferromagnetic clusters exist. We estimate the spin cluster diameter to be 7–10 nm at low temperature (T<30 K). We discuss a possible origin of the giant magnetoresistance effect based on the existence of spin clusters.

Giant magnetoresistive memory effect in Nd0.7Sr0.3MnOz (abstract)

A novel giant magnetoresistance memory effect has been observed in epitaxial Nd0.7Sr0.3MnOz thin films which have previously been found to exhibit a linear increase in conductivity on first application of a magnetic field (B). Here we reported that the conductivity of the films depends not only on the applied field but also on the magnetic history. At T well below the temperature Tp where the zero‐field resistivity has a peak, the film enters a high conductivity state [(ΔR/RB)≳103] upon application of a magnetic field which persists even when B is reduced to zero. The original ‘‘zero’’ field state is not recovered until the sample is warmed to T∼Tp. Surprisingly, the dc magnetization exhibits only a weak irreversibility while the magnetoconductivity is markedly hysteretic. That is, while the remanent magnetization is small the remanent magnetic resistivity is 10−3 times the initial zero‐field‐cooled resistivity. A possible explanation based on a two‐component model of semiconducting matrix with embedded s...

Preparation and properties of high T/sub c/ superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ and ferromagnetic oxide La/sub 0.67/Ba/sub 0.33/MnO/sub 3/ multilayers

We have prepared superconducting YBa/sub 2/Cu/sub 3/O/sub 7/ and ferromagnetic oxide La/sub 0.67/Ba/sub 0.33/MnO/sub 3/ hetero-epitaxial multilayer samples by pulsed laser deposition. X-ray diffraction and ion channeling results indicate that epitaxial growth of ferromagnetic La/sub 0.67/Ba/sub 0.33/MnO/sub 3/ films has been achieved on (100) LaAlO/sub 3/ substrates. In situ epitaxial La/sub 0.67/Ba/sub 0.33/MnO/sub 3/ thin films show giant negative magnetoresistance around 150 K and a magnetoresistance ratio, -(R/sub H/-R/sub 0/)/R/sub 0/, of more than 90%. A superconducting transition and the magnetoresistance effect were observed in YBa/sub 2/Cu/sub 3/O/sub 7//La/sub 0.67/Ba/sub 0.33/MnO/sub 3/ multilayers simultaneously. The results suggest the possibility of an interface effect, which affects the resistivity and the superconductivity of the multilayers.<<ETX>>