Search

Magnetization and resistivity were measured as a function of temperature in polycrystalline Ni–Mn–Sn Heusler alloys with a magnetocrystalline first order phase transition. At external magnetic fields

Tunneling current and thickness inhomogeneities of ultrathin aluminum oxide layers of magnetic tunnel junctions are studied by conducting atomic force microscopy (CAFM). The current inhomogeneities are attributed to thickness inhomogeneities on a nanometer scale. Thickness distributions are derived from the current distributions, using the Simmons’ tunneling model by assuming a constant and uniform barrier height. It is shown that dielectric breakdown at high voltages can cause modifications of the AlOx layer during CAFM measurements, characterized by the irreversibility of both current images and local I–V characteristics. Working at low voltage and low current is a necessary condition for applicability of the CAFM method for testing the insulator layers. The effect of the method of film preparation on the film dielectric strength is studied.

We have studied the shape of the distribution F(G) for the conductance G between points on the surface of a metal–insulator nanocomposite film and the conducting substrate by Conducting Atomic Force Microscopy and computer simulation. Random resistor networks with both metallic and tunneling bonds included are used to model the nanocomposite films. The shape of F(G) is determined mainly by the connectivity of metal particles and the maximum tunneling distance in the composite. By qualitatively comparing the simulation results with the experimental data on granular NiFe-SiO2 films near the metal–insulator transition, we find important implications for the understanding of microscopic conduction mechanisms near the metal–insulator transition.

Dielectric measurements were done on cosputtered metal-insulator nanocomposite films with metal volume fraction above the metal-insulator transition, in the frequency range between 20 Hz and 30 MHz. At intermediate and high frequency, the dielectric function can be qualitatively described in terms of the percolation theory. In the low-frequency region, we observe a sharp relaxation-type increase of the real part of dielectric constant with decreasing frequency, while the imaginary part is dominated by dc conductivity. We suggest that the low-frequency behavior may be due to a combination of metallic and tunneling conduction in the system.

Multilayers of Co and ZnO, with nominal layer thicknesses on the atomic scale with up to 25 bilayers, were deposited by ion beam sputtering on silicon and glass substrates at ambient temperature. Thick epitaxial CoxZn1−xO films on Al2O3(012) substrates were grown by metalorganic chemical vapor deposition using a liquid precursor delivery system. All were co-doped with Al. Comparative analysis of magnetization, resistivity, and magnetoresistance measurements, performed in the temperature range 2.5–300 K, is presented. At small thickness of Co layers in the multilayer samples, these structures are diluted magnetic semiconductor (DMS) superlattices, with properties close to the epitaxial films. A crossover from DMS to discontinuous magnetic metal/semiconductor multilayers is observed with increasing metal content in the multilayers. This leads to changes in conduction mechanisms, with increasing contribution of quasithree-dimensional or quasitwo-dimensional intergranular hopping, and superparamagnetism.

Multilayers of Co/ZnO, with varying nominal thickness of metal (2–10 A) and semiconductor (2–20 A), were prepared by ion-beam sputtering. Magnetic, transport, and magnetotransport measurements were carried out over a temperature range of 2.5 to 300 K. Upon decreasing Co thickness and increasing ZnO thickness in the multilayer stack, the properties of the samples undergo a crossover from those of granular metallic Co/semiconductor multilayers to a dilute magnetic semiconductor superlattice. We interpret ferromagnetism in the latter case as due to ordering in the Co-rich layers, mediated by carriers from lightly doped, high carrier concentration layers.

Ferromagnetic properties have been observed in CuO doped with 3.5–15 at. % of Mn. The transition from ferromagnetic to paramagnetic phase at TC=80 K is associated with the metal–insulator transition. Magnetoresistance is weakly negative in the vicinity of the transition, but positive in a wide range of temperatures below TC. The experimental results suggest a possibility of interpretation in terms of the Zener double-exchange mechanism and strong electron–phonon interactions.

Magnetization and magnetotransport were measured in CoxAg1−x granular composites as a function of temperature and applied magnetic field. A transition from blocked to superparamagnetic behavior with increasing temperatures can be observed in magnetization, giant magnetoresistance and the extraordinary Hall effect measurements. However, the blocking temperature determined from magnetotransport measurements is systematically lower than the one estimated from magnetic measurements. This is due to the selective magnetic scattering, which is enhanced for smaller particles, while the magnetization probes the whole particle size distribution.

We study the shape of distribution F(G) for the conductance G between a point on the surface of a metal–insulator nanocomposite film and the conducting substrate. Random resistor networks with both metallic and tunneling bonds included are used to model nanocomposite films. Our simulation results show explicitly that the shape of F(G) is determined mainly by the connectivity of metal particles and the maximum tunneling distance in the composite. By applying our results to the available experimental data on granular NiFe–SiO2, we find important implications for the understanding of microscopic conduction mechanisms near the metal–insulator transition.

$\mathrm{Co}$-doped $\mathrm{ZnO}({\mathrm{Co}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{O})$ is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial ${\mathrm{Co}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{O}$ films on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}(012)$ substrates by metalorganic chemical vapor deposition using a liquid precursor delivery system. High concentrations of $\mathrm{Co}(x\ensuremath{\leqslant}0.35)$ can be uniformly incorporated into the film without phase segregation. $\mathrm{Co}$ is found to be in the $+2$ oxidation state, independent of $x$, by both surface-sensitive core-level $x$-ray photoemission and bulk-sensitive optical absorption spectroscopies. This material can be grown $n$-type by the deliberate incorporation of oxygen vacancies, but not by inclusion of $\ensuremath{\sim}1$ at. % $\mathrm{Al}$. Semiconducting films remain ferromagnetic up to $350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In contrast films without oxygen vacancies are insulating and nonmagnetic, suggesting that exchange interaction is mediated by itinerant carriers. The saturation and remanent magnetization on a per $\mathrm{Co}$ basis was very small $(l0.1\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}∕\mathrm{Co})$, even in the best films. The dependence of saturation magnetization, as measured by optical magnetic circular dichroism, on magnetic field and temperature, agrees with the theoretical Brillouin function, demonstrating that the majority of the $\mathrm{Co}(\mathrm{II})$ ions behave as magnetically isolated $S=3∕2$ ions.

Using complementary experiments we show that the room temperature ferromagnetism observed in anatase $\mathrm{Co}\ensuremath{\mathbin:}{\mathrm{TiO}}_{2}$ films is not carrier mediated, but coexists with the dielectric state. TEM and x-ray absorption spectroscopy reveal a solid solution of Co in anatase, where Co is not metallic but in the $+2$ state substituting for Ti. Measurements at 300 K yield a ${M}_{\mathrm{S}}$ of $1.1\text{ }\text{ }{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Co}$ atom, while all films are highly insulating. The evidence of intrinsic ferromagnetism in the dielectric ground state of $\mathrm{Co}\ensuremath{\mathbin:}{\mathrm{TiO}}_{2}$ leads to new considerations for the origin of ferromagnetism in transition metal doped oxides.

Measurements of the spontaneous magnetic moment and conductance of oxygen-saturated ZnO:Cr thin films and tunnel junctions show an exponential drop with increasing thickness. All films are in the insulating state. The type and concentration of dominant point defects in the bulk do not provide conditions for magnetic ordering, while surfaces play an important role in magnetism. We suggest that in film thicknesses below a characteristic length (∼30nm), surface states or enhanced concentration of point defects near the surface (oxygen vacancies or incorporated hydrogen) lead to surface magnetism. Ballistic tunneling is observed in junctions

We explore the effects of codoping with hydrogen on magnetism, conductivity, and spin polarization of carriers in Cr-doped ZnO. Zn0.99Cr0.01O:H films sputter deposited on sapphire show a correlation between magnetization and conductivity when H is introduced. In the first method, dielectric and weakly magnetic films grown in pure Ar are subsequently annealed at 400°C in a 5% H2 95% Ar 1atm flowing tube furnace. These films show increases in conductivity and saturation and remnant magnetization postanneal. In the second method, conducting ferromagnetic films are grown in the H∕Ar mixture. They are magnetic as grown but show a small decrease in saturation and remnant magnetization and conductivity post-H∕Ar anneal. Ferromagnetic CrO2 with TC=390K or antiferromagnetic phases are not detected in hydrogenated films. We studied spin polarization of carriers using anomalous Hall effect; however, initial experiments show no such signs, hence spin polarization is not yet confirmed. Hydrogen doped in dielectric Cr:...

Using high-resolution, aberration-corrected, scanning transmission electron microscopy and electron-energy-loss spectroscopy we show that in films of single-phase anatase Co:TiO2, the Co distribution and magnetic properties are strongly dependent both on the overall crystalline quality and postgrowth vacuum annealing process. The Co:TiO2 films are coherent, epitaxial anatase with no secondary phases or metallic Co. Films of lower crystalline quality reveal a relatively homogeneous Co concentration, while films of higher crystalline quality show a tendency for Co enrichment near the surface region, around grain boundaries, and the substrate interface. Both uniform and nonuniform samples show a notable enhancement in the saturation magnetization with annealing, while the coercive field is considerably higher in the samples with uniform Co distribution. These experiments confirm that films of single-phase anatase Co:TiO2 with both uniform and nonuniform Co distributions exhibit room-temperature ferromagnetism in the insulating ground state, while the compositional uniformity and film microstructure play a role in the bulk magnetic properties of the material and the mechanisms for ferromagnetic ordering.

We experimentally investigate the room temperature ferromagnetism observed in insulating Co doped anatase TiO2 thin films grown by sputter deposition. The Co was uniformly incorporated in the lattice as Co(II) with no evidence of Co metal. A series of annealing treatments were carried out to optimize the ferromagnetic ordering and a saturation moment of 1.1μB∕Co atom at 300 K was obtained with UHV annealing at 450 °C. Both as-deposited and annealed films were highly insulating at room temperature. Results show that the ferromagnetism is strongly dependent on the number and distribution of oxygen vacancies in the Co:TiO2 lattice.

We present a systematic approach to fabricate a variety of magnetic nanoparticles with desirable structure and controlled magnetic properties based on our studies of the process kinetics. The morphology of binary alloy particles is dependent on their bulk thermodynamics—for immiscible heterogeneous systems (Co–Au) core-shell structures are obtained while miscible systems (Fe–Pt) lead to alloy nanoparticles. The annealing effects on FePt nanoparticles show that the coercivity and magnetic anisotropy increase dramatically after annealing at temperatures above 650°C. Studies of Co–Au core-shell structure show that the core is magnetic, but the Au shell does not significantly affect its magnetic properties.

The Hall effect in granular co‐sputtered ferromagnetic metal–insulator films was found to increase dramatically as the magnetic volume fraction decreases toward the metal–insulator transition. The saturated Hall resistivity is up to 160 μΩ cm at T=5 K, that is almost four orders of magnitude greater than that in a pure magnetic metal sample. Close to the metal–insulator transition, both magnetoresistivity and the saturated Hall resistivity decrease with increasing temperature. Correlations of the Hall resistivity with resistivity and magnetoresistivity are discussed.