M. Rubinstein

High-temperature thermopower in La2/3Ca1/3MnO3 films: Evidence for polaronic transport.

Thermoelectric power, electrical resistivity, and magnetization experiments, performed in the paramagnetic phase of La{sub 2/3}Ca{sub 1/3}MnO{sub 3}, provide evidence for polaron-dominated conduction in colossal magnetoresistance materials. At high temperatures, a large, nearly-field-independent difference between the activation energies for resistivity {rho} and thermopower {ital S}, a characteristic of Holstein polarons, is observed, and ln{rho} ceases to scale with the magnetization. On approaching {ital T}{sub {ital c}}, both energies become field dependent, indicating that the polarons are magnetically polarized. Below {ital T}{sub {ital c}}, the thermopower follows a law {ital S}({ital H}){approximately}1/{rho}({ital H}) as in nonsaturated ferromagnetic metals. {copyright} {ital 1996 The American Physical Society.}

High-temperature thermopower in La{sub {bold 2/3}}Ca{sub {bold 1/3}}MnO{sub {bold 3}} films: Evidence for polaronic transport

Thermoelectric power, electrical resistivity, and magnetization experiments, performed in the paramagnetic phase of La{sub 2/3}Ca{sub 1/3}MnO{sub 3}, provide evidence for polaron-dominated conduction in colossal magnetoresistance materials. At high temperatures, a large, nearly-field-independent difference between the activation energies for resistivity {rho} and thermopower {ital S}, a characteristic of Holstein polarons, is observed, and ln{rho} ceases to scale with the magnetization. On approaching {ital T}{sub {ital c}}, both energies become field dependent, indicating that the polarons are magnetically polarized. Below {ital T}{sub {ital c}}, the thermopower follows a law {ital S}({ital H}){approximately}1/{rho}({ital H}) as in nonsaturated ferromagnetic metals. {copyright} {ital 1996 The American Physical Society.}

Magnetothermopower in La0.67Ca0.33MnO3 thin films

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.

Magnetic properties of R2Fe14B single crystals

A whole new class of high‐performance permanent magnet materials is based on the ternary tetragonal structure R2Fe14B, where R is one of the rare‐earth elements. We have successfully grown single crystals of this structure with R=Y, Nd, and Tb. Y is a nonmagnetic rare‐earth substitute, while Nd and Tb couple ferro‐ and ferrimagnetically, respectively, relative to the iron moment. All three of the compounds have [001] easy axes at room temperature, although the Nd compound exhibits a spin reorientation away from the [001] below about 150 K. Nd2Fe14B has a saturation induction at room temperature of 16.2 kG, which places an upper limit of approximately 65.6 MGOe on the energy product obtainable by magnets based on that material. While Tb2Fe14B exhibits a smaller magnetization because of ferrimagnetic coupling of the rare earths and the iron, it also has an extremely large magnetic anisotropy which is nearly temperature independent between 4.2 and 300 K.

Coexistence of localized and itinerant carriers near TC in calcium-doped manganites

We explore the possibility that polaronic distortions in the paramagnetic phase of La{sub 0.67}Ca{sub 0.33}MnO{sub 3} manganites persist in the ferromagnetic phase by considering the observed electrical resistivity to arise from coexisting field- and temperature-dependent polaronic and band-electron fractions. We use an effective medium approach to compute the total resistivity of the two-component system, and find that a limit with low percolation threshold explains the data rather well. To test the validity of this model, we apply it to the thermoelectric coefficient. We propose a plausible mean-field model that reproduces the essential features of a microscopic model and provides a comparison with the experimental mixing fraction, as well as the magnetization and magnetic susceptibility. {copyright} {ital 1999} {ital The American Physical Society}

HALL-EFFECT SIGN ANOMALY AND SMALL-POLARON CONDUCTION IN (LA1-XGDX)0.67CA0.33MNO3

The Hall coefficient of Gd-doped La{sub 2/3}Ca{sub 1/3}MnO{sub 3} exhibits Arrhenius behavior over a temperature range from 2T{sub c} to 4T{sub c}, with an activation energy very close to (2)/(3) that of the electrical conductivity. Although both the doping level and thermoelectric coefficient indicate holelike conduction, the Hall coefficient is electronlike. This unusual result provides strong evidence in favor of small-polaronic conduction in the paramagnetic regime of the manganites. {copyright} {ital 1997} {ital The American Physical Society}

Properties of electrodeposited Co‐Cu multilayer structures

Alternate layers of Co and Cu of individual layer thicknesses from 1.5 to 8.0 nm and total thickness of about 100 layers and of Co bilayers separated by Cu have been electroplated from an electrolyte having a low concentration of Cu and a high concentration of Co atoms. The properties of the resulting structures have been analyzed using SEM, x‐ray diffraction, VSM, and FMR methods. Comparable structures have also been fabricated by e‐beam evaporation in high vacuum for comparison purposes. X‐ray diffraction patterns indicate only the fcc structure. Assuming the Co thickness to be that deduced from the plating charge, we infer 4πM values in the range 6.0–14.5 kG from VSM and FMR. Some uniaxial anisotropy is apparent in this system. These results are similar to those of earlier work on thin fcc Co layers in the same range of thicknesses, where the room‐temperature moment was reduced but depended only slightly on layer thickness, and where the uniaxial anisotropy was observed to be small.

Phase-separated Fe and Co particles in a BN matrix

Using co‐sputtering, 10–300 A particles of Fe and Co have been prepared in an insulating BN matrix. The Fe particles have the α‐Fe structure. The saturation magnetic moment of the iron particles per at. % Fe was found to be approximately independent of the Fe concentration and equal to the value of α‐Fe. The system undergoes a metal–nonmetal transition at approximately 40 vol % Fe. For concentrations of Fe particles above this threshold the temperature dependence of the resistivity is metallic and the room‐temperature coercivity is large (50–100 Oe). For the Co particles, the room‐temperature coercivity is about twice as large as Fe above the metal–nonmetal threshold. Below the metal–nonmetal threshold the particles behave as superparamagnets and the coercivity is approximately zero.

Magnetic properties of permalloy-coated organic tubules

We present an initial investigation of the ferromagnetic properties of a novel type of magnetic composite, viz., permalloy‐coated submicron diameter hollow cylinders or tubules. The tubules form spontaneously from an organic material, a diacetylenic phosopholipid, and were used as templates on which the ferromagnetic material was deposited by electroless deposition. The permalloy‐coated tubules were dispersed in an epoxy matrix to measure the magnetization and ferromagnetic resonance (FMR) properties of individual tubules. The nature of the magnetic anisotropy and the FMR spectra observed confirmed that the tubules are well aligned by a magnetic field during the epoxy curing. The FMR spectra are interpreted in terms of a powder pattern distribution of thin‐film spectra consistent with the large diameter‐to‐thickness ratio.

Studies of Hyperfine Fields in Iron Alloys

The 57Fe hyperfine field spectra of several bcc iron‐rich binary alloys have been studied by the Mossbauer effect, the spin‐echo technique, and by frequency‐swept nuclear magnetic resonance. The alloys contained low concentrations of Al, Si, Mn, Cr, V, Co, or Ni. By computer analysis, the experimental spectra have been decomposed into component lines, assuming a model where the effects of the individual neighbor shells are additive. The resulting shifts in hyperfine fields due to the three nearest‐neighbor shells obtained by the three techniques are compared and discussed.