We have measured the thermopower S(T,H) and resistivity ρ(T,H) of laser ablated La0.67Ca0.33MnO3 films, as a function of temperature and external magnetic field. On heating, a metal–insulator transition occurs at temperatures below the resistivity peak, observed via a jump between a low T regime, where S∝T, and a high T regime, where S∝1/T. An applied magnetic field shifts the transition in a postannealed sample to higher temperatures, causing a giant magnetothermopower effect 100×(S8T−S0T)/S8T=−1400%. Both S(T) and ρ(T) are activated at high temperatures, but with significantly different activation energies. We interpret this as an evidence of small polarons at high temperatures.

The giant magnetoresistance ͑GMR͒ effect in granular and multilayer thin films has been widely investigated because of possible device applications. Despite this intensive effort, the underlying mechanisms responsible for the effect have not been identified. We present measurements of the thermoelectric power ͑TEP͒ and thermal conductivity on a wide variety of granular and multilayer GMR systems. The strong magnetic field dependences of both the TEP and the thermal conductivity are found to be closely related to the magnetoresistance. The TEP measurements require that the high density of states in the ferromagnetic materials play a major role in the GMR effect. The thermal conductivity measurements indicate that the scattering mechanisms in granular samples are elastic while multilayer samples have a significant inelastic, spin-flip component. ͓S0163-1829͑96͒09745-7͔

We have grown high quality superlattices of Dy with nonmagnetic Lu and find that the 2.4% epitaxial compression nearly doubles the ferromagnetic Tc of Dy with little change in the Neel temperature. A helimagnetic phase exists over a narrow temperature range. Below Tc in superlattices, 300 A orthorhombic domains form despite epitaxial constraints, each with a magnetostrictive distortion comparable to that of bulk Dy. For the thinnest intervening Lu layers, individual ferromagnetic Dy blocks have parallel alignment; the remaining samples show antiparallel alignment, coherent over many bilayer periods.PACS numbers: 75.70.Fr, 68.55.Bd, 75.25,+z, 75.70.Kw A new regime in the study of rare-earth magnetism began with the discovery [1,2] that 3D, long-range order can occur in rare-earth/yttrium superlattice (SL) structures. Two important effects were found in the Dy/Y system: (i) Helimagnetic order propagates through nonmagnetic Y layers as thick as 100 A and (ii) coherency strain and clamping to the substrate caused by the epitaxial constraint completely suppress the low-temperature ferromagnetic phase of dysprosium. In this Letter, we report significant new effects in the first Dy/Lu superlattices and films ever prepared. In the present case, the 2.4% epitaxial compression of the Dy lattice (opposed to the 1.6% expansion in the Dy/Y case) enhances the ferromagnetic transition temperature by nearly 100%. An enhancement [3] of 25% was found previously for Dy grown on Er (1% compression). Despite epitaxial constraints in the Dy/Lu samples, the onset of ferromagnetic order is accompanied by the appearance of microscopic, orthorhombically distorted domains. Such distortion necessarily induces long-range coherence of the easy axis along the growth direction. Our further results indicate that this mechanism, aided by local dipolar coupling, results in antiparallel stacking of the individual Dy ferromagnetic blocks for thick Lu interlayers.Samples were grown by molecular beam epitaxy using methods described elsewhere [4]. An hep Lu-(000l) buffer layer was grown on (110) Nb, followed by the growth of either Dy films ranging in thickness from 40 to 400 A, or of Dy/Lu superlattices. In the superlattices, the Dy layer thickness was maintained at approximately 15 atomic planes (40 A) per layer while the Lu layers were varied between 5 and 29 atomic planes (15-80 A). Room-temperature x-ray diffraction confirmed good crystal quality, with coherence lengths of 700 A along the c axis (growth direction) and mosaic widths <0.3°. Analysis of SL harmonics around the (00*2) reflection shows that interdiffusion involves at most 3-4 atomic layers at the interfaces.In Fig. 1 we show both field-cooled (FC) and zerofield-cooled (ZFC) easy-axis magnetization data at 200 Oe and 1 kOe for a SL comprised of 60 bilayers, denoted [Dyi6|Lu2ol60. Each period contains 16 atomic planes of Dy and 20 of Lu. Shown for comparison is the magnetization of bulk Dy, measured [5] at 1 kOe. Not discernible on this scale is a small cusp in the bulk data at 178 K ...

We report clear manifestations of biquadratic exchange in the magnetoresistance ͑MR͒ of ͑110͒ singlecrystal NiFe/Cu multilayers. Magnetoresistance curves show a low-field MR minimum which results from an asymmetric canting of the moments away from the applied magnetic field. The stability of these magnetic configurations indicates that the biquadratic coupling can be significantly stronger than the bilinear coupling. In samples with ferromagnetic ͑F͒ bilinear coupling the strength of both the bilinear and biquadratic coupling decays in the temperature range from 200 to 400 K. In samples that are coupled antiferromagnetically ͑AF͒ at room temperature, we observe a crossover from AF to F bilinear coupling as the temperature is lowered below 200 K. We attribute the strong biquadratic coupling and temperature dependence of the bilinear coupling to pinholes in the Cu spacer layers.