Nanometer size Fe particles in an Ag matrix were prepared by pulsed laser deposition and were characterized from 4 to 300 K using ferromagnetic resonance (FMR) at 9.46 GHz and superconducting quantum interference device magnetometry. The magnetic behavior changes systematically from superparamagnetic to ferromagnetic with increasing Fe concentration, increasing deposition or postdeposition annealing temperature, or decreasing measurement temperature; FMR linewidths and anisotropy fields also increase. At low temperatures a dramatic further increase in linewidth occurs, accompanied by a strong downward shift in the resonance field for all orientations, and zero field cooled susceptibility decreases sharply. These phenomena indicate the development of much larger anisotropy than in bulk Fe. For samples annealed in H gas above 600 K, this anisotropy is largely absent, indicating that the anomalous behavior is induced by a surface oxide layer on the Fe, as in the exchange bias effect, despite the absence of oxygen in the nominal composition.

Dilute magnetic semiconductors exhibit unique magnetic and transport properties which arise from the coupling of carrier spins and local moments. We have studied the ferromagnetic resonance spectra in ferromagnetic Mn-doped GaAs (with Mn concentrations ∼4–6%), whose properties are strongly influenced by this coupling. The resonance fields were found to have an anomalous temperature dependence, which we attribute to the development of a “bottleneck” between the two types of spins. The gyromagnetic ratio of hole spins in this p-type ferromagnetic semiconductor was found to be negative, with g∼−0.5. At low temperatures, additional anisotropy appears, which we have attributed to the trapping of itinerant holes by the core spins.

Ferromagnetic resonance (FMR) was observed in epitaxial thin films of CrO2 grown on TiO2. FMR spectra were taken at 9.5 and 35 GHz and at temperatures from 4 K to above the ordering temperature of 393 K. The spectra of these films are generally complicated because of surface roughness and the distribution of stresses and anisotropies through their thickness. The thinnest films, however, display typical spin wave spectra, which could be approximated using uniform magnetic material analyses. The exchange constant at 300 K, D∼70 meV Å2, is consistent with values derived from the temperature dependence of M at low temperatures. The temperature dependence of D was extracted from the spin wave spacing and is similar to other magnetic metals. Our data indicate a room temperature Gilbert damping parameter of at most 0.0023 near 300 K, which is less than those of metallic magnetic materials except possibly Fe. The small damping parameter suggests that intrinsic losses seen in other magnetic metals, which may arise from electronic transitions between bands of different spin character, are small in CrO2.

The ferromagnetic resonance linewidth of Fe73.5CuNb3Si13.5B9 melt-spun ribbons has been investigated as a function of annealing temperature. Fe73.5CuNb3Si13.5B9 possesses an ultrafine grain structure which can be altered by suitable annealing to exhibit a combination of excellent soft magnetic characteristics and high saturation induction. It is a ferromagnetic metal consisting of crystallites whose anisotropy axes are randomly oriented and which may interact with each other via exchange or dipolar fields. When annealed at moderate temperatures, Fe73.5CuNb3Si13.5B9 is characterized by an extremely low coercive field, and is useful in magnetic cores and memories. At a critical temperature (∼600°C), the grain size rapidly increases with annealing temperature. This onset of crystallization is accompanied by a relatively abrupt increase in the FMR linewidth, the magnetic anisotropy field, and the coercive field. We extend the scaling arguments of Herzer to explain these obviously related phenomena.

The electron spin resonance (ESR) spectra of antiferromagnetic nanoparticle NiO specimens have been investigated as a function of temperature at x-band (microwave) frequencies. Below the nominal Néel temperature, the x-band resonances arising from the bulk antiferromagnets, including NiO particles with diameters greater than 100Å, all vanish due to the emergence of large molecular exchange fields. The ESR resonance signals of 60Å antiferromagnetic nanoparticles, however, persist to the lowest temperatures. These nanoparticle resonance lines shift to lower fields rapidly as the temperature is decreased, while the lineshapes broaden and distort.

A phenomenological two-component polaron model, originally formulated to describe the temperature-dependent transport properties of La2/3Ca1/3MnO3(LCMO), has been applied to treat the resistivity and Hall effect of EuO and ZrTe5.

We have investigated the ferromagnetic resonance spectra of an exchange-biased Ni80Fe20/CoO bilayer between room temperature and 4K. Primary attention has been paid to the effect of the antiferromagnetic CoO film on the temperature-dependent resonance field shift of the ferromagnetic Ni80Fe20 film with respect to that of an unbiased film. At low temperatures, the field shift with the magnetic field applied perpendicular to the plane was determined to be more than twice the magnitude of the parallel field shift, and of the same sign, while an unoxidized single ferromagnetic film has much smaller parallel and perpendicular low-temperature shifts (here defined with respect to room temperature) of opposite sign. This observation implies that the anisotropy axis can rotate with the applied field, provided that the primary cause of the anisotropy is the interaction between the adjacent ferromagnetic and antiferromagnetic films. Since the perpendicular shift is more than a factor of two larger than the parallel field shift, the rotatable anisotropy is, in fact, anisotropic in this bilayer.

The ferromagnetic resonance of an ultrathin 7.5nm Pr0.67Sr0.33MnO3 film which possesses a large low-field magnetoresistive effect was investigated with a view to determine the orientation of the film's magnetization vector. By comparing resonance fields oriented parallel and perpendicular to the plane of the film, we determined that the easy axis of magnetization is perpendicular to the plane. The low-temperature perpendicular anisotropy field was estimated to be ∼10kOe. A possible link between the perpendicular anisotropy and the magnetoresistance is briefly discussed.

An expression for the interaction strength between two magnetic films separated by an insulating antiferromagnet spacer has been derived as a function of temperature and thickness. We consider the mechanism wherein the magnetic interaction between the ferromagnetic layers is mediated by the intervening antiferromagnetic insulator via the Suhl–Nakamura (SN) interaction. The interaction energy per unit area, σSN, is derived as σSN=18(JC2/JAF)(δ/a)exp(−t/δ). Here, JAF is the magnetic coupling constant between nearest-neighbor antiferromagnetic spins in the spacer, JC is the effective coupling constant (which is greatly reduced from the Heisenberg exchange constant), between the spins in the ferromagnetic film and the nearest-neighbor spins in the antiferromagnetic spacer, t is the separation of the two ferromagnetic plates, and δ is the width of an antiferromagnetic domain wall. This mechanism is the antiferromagnetic analog of the Ruderman–Kittel oscillatory coupling between two magnetic films separated by a normal metal.

A new method for determining the transport spin polarization, point contact tunneling from a low temperature superconductor into a ferromagnet, is used to determine the spin polarization of several LaSrMnO thin films and crystals. The Andreev process and its utility in measurements of spin-polarization are described. Preliminary results for the spin polarization of LSMO are presented.

We have studied Andreev reflection in point contacts formed between several low T/sub c/ superconductors and various spin polarized normal metals. We have also developed a theoretical model for the phenomenon. There is good accordance between theory and experiment.

We explore the possibility that polaronic distortions in the paramagnetic phase of La0.67Ca0.33MnO3 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.

Ceramic samples of composition (La1−xGdx)2/3Ca1/3MnO3 were prepared and used as targets to grow films onto LaAlO3 substrates by pulsed laser deposition. Comparison of the electrical resistance and thermopower, measured vs temperature and applied magnetic fields, indicates transport dominated by positively charged small polarons in the high temperature paramagnetic state. Nonetheless, the Hall effect, measured in 0.5 μm thick films of composition x=0 and x=0.25, has the sign that is normally ascribed to negatively charged free carriers. The magnitude of the Hall coefficient decreases exponentially with increasing temperature with an activation energy that differs from that of the resistivity. This behavior and its anomalous negative sign are interpreted to result from face-diagonal hopping of small polarons in the Mn sublattice.

A Sm-poor mixture of Fe-substituted Sm2Co17, having the nominal stoichiometry of Sm0.080Co0.645Fe0.276, was ball-milled to explore the possibility of enhancing its remanence through direct microstructural refinement. With milling, the Sm2(Co0.7Fe0.3)17 compound disassociates to a body-centered-cubic supersaturated SmCoFe solid solution and a residual SmCoFe amorphous phase. Correspondingly, the coercive field values first increase, peaking at 0.83 kOe after 180 min of milling, then decrease with continued milling to <0.1 kOe after 1200 min. The remanence, as Mr/Ms, is measured to track closely the coercive field behavior, experiencing modest increases to 0.26 after just 120 min of milling. Magnetization values are found to increase first with the dissociation of the 2:17 phase, and again with the ejection of Sm from the bcc component.

The Hall coefficient of Gd-doped La2/3Ca1/3MnO3 exhibits Arrhenius behavior over a temperature range from 2T(c) to 4T(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.

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 effect of substrate temperature and oxygen deposition pressure on the structure and properties of thin films of LaxCa1-xMnO(delta ) has been investigated. Thin films (approximately 1000 angstroms) of La0.67Ca0.33MnO(delta ) were deposited onto LaAlO3 (100) substrates by pulsed laser deposition at a substrate temperature of 600 and 700 degree(s)C. A series of films were grown on different oxygen pressures, between 15 and 400 mTorr, which systematically changed the oxygen concentrations in the films. As-deposited films exhibited an oriented orthorhombic structure. At low oxygen deposition pressures films were preferentially (202) oriented. At high pressures deposited films had a (040) preferred orientation. A 900 degree(s)C anneal in flowing oxygen of a film deposited at low oxygen pressure resulted in a decrease in the a lattice parameter and a change in the preferred orientation from (202) to (040). Vacuum annealing at 550 degree(s)C resulted in an increase in the a lattice parameter. The resistivity as a function of temperature showed a significant variation as a function of growth conditions. The peak in the resistivity curve (Tm) varied between 73 and 150 K depending upon the growth conditions. The activation energy associated with the semiconducting phase was approximately the same for all films (approximately 100 meV).