K. Dörr

Influence of the substrate on growth and magnetoresistance of La0.7Ca0.3MnOz thin films deposited by magnetron sputtering

Off-axis radio frequency magnetron sputtering was employed to grow La0.7Ca0.3MnOz (LCMO) thin films onto three different types of substrates. The substrate strongly influences the structure and the colossal magnetoresistance effect of the obtained films. Single-crystalline thin films were prepared on LaAlO3 (100) substrates, showing a low value of residual resistivity and a metal–insulator transition at a temperature of up to Tpeak=290 K. The latter value of the transition temperature is one of the highest reported so far on thin films of the La–Ca–Mn–O system. Films deposited onto Y-stabilized ZrO2 substrates and onto MgO substrates are polycrystalline and less textured. These films are characterized by a large negative magnetoresistance ratio MR=[R(H)−R(0)]/R(0) measured for small values of the magnetic field H. For H=1.5 kOe, the MR was found to be approximately −30%, −20%, and −8% at temperatures of 20, 77, and 180 K, respectively. The magnetoresistance of polycrystalline LCMO films shows two contribu...

Electrical characterization of PMN–28%PT(001) crystals used as thin-film substrates

Ferroelectric and piezoelectric properties of (001) 0.72PbMg1/3Nb2/3O3–0.28PbTiO3 (PMN–28%PT) single crystals have been investigated from cryogenic temperatures to 475 K. PMN–28%PT is used as piezoelectric substrate, e.g., in multiferroic heterostructures. Electric field-induced phase transformations have been examined by electrical characterization including measurements of polarization loops, dielectric permitivitty, and the resistance change in La0.7Sr0.3MnO3 films deposited on the (001) face. The relaxor ferroelectric transition behavior was studied by means of time-dependent current measurements. A phase diagram is set up. PMN–28%PT is found to be at the border of the appearance of the monoclinc phase (MC) bridging the rhombohedral-tetragonal (R-T) transformation at higher PbTiO3 contents. Measurements of the lattice expansion reveal that a high piezoelectric effect persists down to low temperatures. Therefore, PMN–28%PT single crystals are found to be appropriate substrates for application of piezoe...

Thickness dependent phase separation in La0.7Ca0.3MnO3 films

The complex dielectric function of La0.7Ca0.3MnO3 films with various thicknesses and on different substrates was determined by the analysis of measured ellipsometric parameters. The temperature-dependent redistribution of low-energy spectral weight shows that the metallicity is developed gradually below an onset temperature that is found to depend strongly on global and local strain properties, not on film thickness alone. Importantly, the 2 nm film shows no metallic conduction but an optical charge-carrier response below 240 K. This suggests that ultrathin films are characterized by phase separation and a percolation-type transition.

Dependence of the high-field grain-boundary magnetoresistance of ferromagnetic manganites on Curie temperature

We analyze the high-field magnetoresistance of polycrystalline ferromagnetic manganites of varied composition in magnetic fields up to μ0H=47T. Small to medium deviations from a linear field dependence of the conductance {as discovered for a La0.7Sr0.3MnO3 film recently, [N. Kozlova et al., J. Magn. Magn. Mater. 261, 48 (2003)]} are typical. The conductance is always well described by a quadratic polynomial G(H)=a+bH+cH2 with temperature-dependent coefficients a, b, and c. Both b and c increase with decreasing Curie temperature (TC) of the samples. The conductance slope 1∕G0 dG∕dH is related to an average susceptibility of the magnetically disturbed layer at the grain boundaries. The nonsaturated magnetic order of grain-boundary (GB) spins at 5K and 47T means the presence of strong antiferromagnetic interactions. The observation of a systematic correlation between GB magnetoresistance and bulk TC indicates that magnetic order at GBs is governed by parameters such as doping and average ionic size (electron...

Charge state modification in Mn site substituted CMR manganites: strong deleterious influence on the ferromagnetic?metallic state

The effect of charge state modification at the Mn site on the physical properties of CMR manganites is reported. With a view to avoiding additional complexity of local spin coupling effects, Mn site substitution of La0.67Ca0.33MnO3 is carried out with appropriate diamagnetic ions—Zn2+, Zr4+, Ta5+ and W6+—of different valence states. The substitution results in size changes of the unit cell and enhanced local structural distortions, which increase in the order Zn, Zr, Ta and W. The ground state is ferromagnetic–metallic below a certain critical concentration xc of the substituents, beyond which the magnetic ground state shows a glassy behaviour. The phase transition temperatures (TMI and Tc) decrease with substitution, but to different extents. The observed suppression rates of the Curie temperature, Tc, of ~39 K/at.% and ~45 K/at.% respectively for Ta5+ and W6+ substituted compounds are the highest reported in the Mn site substituted CMR manganites. Besides the modification of majority carrier concentration due to the increased (decreased) Mn3+ concentration and enhanced local structural effects, the local electrostatic potential of the substituents seems to contribute to the unusually strong reduction in the itinerant ferromagnetism and the observed glassy states.

Magnetoresistance effects of La0.7M0.3MnO3−δ far below the Curie temperature (M=Ca, Pb)

Single-crystalline and nontextured polycrystalline thin films of La0.7Pb0.3MnO3−δ (LPMO) and La0.7Ca0.3MnO3−δ (LCMO) have been prepared by pulsed laser deposition and by magnetron sputtering, respectively. The oxygen deficiency δ of the samples was controlled by heat treatments under defined oxygen pressure. For δ≈0, the polycrystalline as well as the single-crystalline films are ferromagnetic metals with Curie temperatures Tc of 220–270 K. At low temperatures T≪Tc, the polycrystalline LCMO films show a negative magnetoresistance ratio (MR) at T=20, 77, and 180 K of −30%, −20%, and −8%, respectively, for a field of 1.5 kOe. At 20 K, around 200 Oe, the field sensitivity is as high as −0.5% (Oe)−1. A possible mechanism for this phenomenon is the spin-polarized tunneling through grain boundaries. For the single-crystalline films, the change of δ from 0 to about 0.1 has a similar effect as changing the doping level x from 0.3 to 0.1, namely, a transition from a ferromagnetic metal to a ferromagnetic insulator...

Strain dependence of antiferromagnetic interface coupling inLa0.7Sr0.3MnO3/SrRuO3superlattices

We have investigated the magnetic response of La0.7Sr0.3MnO3/SrRuO3 superlattices to biaxial in-plane strain applied in-situ. Superlattices grown on piezoelectric substrates of 0.72PbMg1/3Nb2/3O3-0.28PbTiO3(001) (PMN-PT) show strong antiferromagnetic coupling of the two ferromagnetic components. The coupling field of mu0HAF = 2.8 T is found to decrease by deltaHAF/delta epsilon ~ -520 mT %-1 under reversible biaxial strain mu0HAF at 80 K in a [La0.7Sr0.3MnO3(22)/SrRuO3(55)]15 superlattice. This reveals a significant strain effect on interfacial coupling. The applied in-plane compression enhances the ferromagnetic order in the manganite layers which are under as-grown tensile strain. It is thus difficult to disentangle the contributions from strain-dependent antiferromagnetic Mn-O-Ru interface coupling and Mn-O-Mn ferromagnetic double exchange near the interface, since the enhanced magnetic order of Mn spins leads to a larger net coupling of SrRuO3 layers at the interface. Strain-dependent orbital occupation in a single-ion picture cannot explain the sign of the observed strain dependence, whereas the enhanced Mn order at the interface is qualitatively in line with it.

Materials science: Two steps for a magnetoelectric switch

Magnetoelectric materials allow magnetism to be controlled by an electric field. The discovery of an indirect path for switching electrical polarization in one such material brings this idea close to practical use. See Letter p.370 The attraction of multiferroic materials rests with the fact that their magnetic properties can be controlled using with electrical fields. But there is a practical problem: thermodynamic arguments imply that the most technologically desirable form of magnetization switching — a full 180° reversal — is forbidden. John Heron and colleagues now show both theoretically and experimentally that this apparently fundamental obstacle can be overcome by considering the kinetics of the switching process. In particular, they demonstrate that a full magnetization reversal can be induced electrically using a two-step sequence of partial switching events. This finding suggest ways in which magnetoelectric switching can be engineered into technologically useful nanometre-scale, low-energy-consumption, non-volatile magnetoelectronics.

Characterization of Organic Thin Films on Ferromagnetic Substrates by Spectroscopic Ellipsometry and Magneto-Optical Kerr Effect Spectroscopy

We report the characterization of metal-phthalocyanine (CuPc and CoPc) thin films prepared on magnetic substrates by spectroscopic ellipsometry (SE) and magneto-optical Kerr effect (MOKE) spectroscopy. CuPc films were prepared on lanthanum strontium manganite (LSMO) substrates while CoPc was deposited onto Co/Pt layer systems. We address the challenge to distinguish between the dominating magneto-optical response of the magnetic substrate and the contribution of the phthalocyanine layers. The procedure is demonstrated for CuPc on LSMO and also applied to CoPc on Co/Pt. The resulting spectral contribution of the organic films is compared to modeled spectra deduced from optical model calculations based on data previously obtained from phthalocyanines on silicon substrates. For CoPc on Co/Pt the average tilt angle of the molecular plane with respect to the substrate plane is estimated to be 73 ° from the ellipsometry data evaluation.

Effect of spacer layer thickness on the magnetic and electrical properties of La/sub 0.7/Sr/sub 0.3/MnO/sub 3//SrTiO/sub 3/ superlattices

Summary form only given. Heteroepitaxial structures of colossal magnetoresistive manganites are of high interest for designing new devices with specific properties. SrTiO/sub 3/ has been found to be a promising interlayer in the fabrication of tunneling devices. This motivated us to study the effect of spacer layer thickness on the electrical and magnetic properties of La/sub 0.7/Sr/sub 0.3/MnO/sub 3/ (LSMO)/SrTiO/sub 3/ (STO) superlattices. The temperature dependence of magnetization showed a ferromagnetic transition for all studied films. However, the transition temperature did not show a monotonic dependence on d/sub STO/. The same behavior was observed for the saturation magnetization. The temperature dependence of electrical resistivity measurements showed metallic behavior below 300 K. Magnetoresistance measured at 20 K was found to depend on the d/sub STO/ and correlates with the magnetic measurements. The results are discussed in the light of surface corrugation due to epitaxial strain, the surface dead layer concept and interlayer coupling.