The objective of this study on the iconic exchange-bias bilayer Permalloy/CoO has been to identify those elements of the interfacial microstructure and accompanying magnetic properties that are responsible for the exchange-bias and hysteretic properties of this bilayer. Both epitaxial and polycrystalline samples were examined. X-ray and neutron reflectometry established that there existed an interfacial region, of width ∼1 nm, whose magnetic properties differed from those of Py or CoO. A model was developed for the interfacial microstructure that predicts all the relevant properties of this system; namely; the temperature and Permalloy thickness dependence of the exchange-bias, HEX, and coercivity, HC; the much smaller measured values of HEX from what was nominally expected; the different behavior of HEX and HC in epitaxial and polycrystalline bilayers. A surprising result is that the exchange-bias does not involve direct exchange-coupling between Permalloy and CoO, but rather is mediated by CoFe2O4 nanoparticles in the interfacial region.

We have measured the interfacial magnetization depth profile in ferromagnet / antiferromagnet exchange-coupled NiFe / CoO bilayers. Both a polycrystalline and an epitaxial-(111) bilayer were examined. We find that the nonswitchable magnetization profile in the biased state is highly correlated with the magnetization profile in the unbiased state. The nonswitchable moment distributions are shown to be consistent with the predictions of a previously reported model for the magnetic and microstructural features of the interfacial region

We report a new 'spark erosion' technique for producing high-quality thermoelectric nanoparticles at a remarkably high rate and with enhanced thermoelectric properties. The technique was utilized to synthesize p-type Bi0.5Sb1.5Te3 nanoparticles with a production rate as high as 135 g h 1, using a relatively small laboratory apparatus and low energy consumption. The compacted nanocomposite samples made from these nanoparticles exhibit a well-defined, 20-50 nm size nanograin microstructure, and show an enhanced figure of merit, ZT, of 1.36 at 360 K. Such a technique is essential for providing inexpensive, oxidation-free nanoparticles which are required for the fabrication of high performance thermoelectric devices for power generation from waste heat, and for refrigeration.

Single phase CoO, NiO, and Ni0.5Co0.5O epitaxial films have been prepared by reactive sputtering onto 〈0001〉 α−Al2O3 substrates maintained at 373 K. Epitaxy was confirmed by x-ray diffraction (XRD) and high resolution electron microscopy (HREM) techniques. XRD experiments indicate that these monoxide films are cubic and contain rotation twins with the twin axis parallel to 〈111〉. Lattice parameters for the CoO and NiO films are 0.4254 ± 0.0001 nm and 0.4173 ± 0.0006 nm, respectively, and agree with published values for the corresponding bulk oxides. The lattice parameter 0.4220 ± 0.0001 nm for the Ni0.5Co0.5O film lies between those of CoO and NiO and suggests that the mixed oxide film is compositionally homogeneous. Cross-sectional HREM images of the Ni0.5Co0.5O specimen show Σ3(11̅2) twin boundaries perpendicular to the oxide-substrate interface. The twin regions are approximately 30 nm in size and are uniformly distributed throughout the film. The epitaxial orientation of the monoxide films with respect to the substrate can be summarized by the relationships [111] monoxide // [0001] α−Al2O3, [11̅0] monoxide // [11̅00] α−Al2O3, and [112̅] monoxide // [111̅0] α−Al2O3.

When two magnetic films are separated by a nonmagnetic film, pinholes in the nonmagnetic film can allow direct contact and, thereby, direct magnetic exchange coupling between the two magnetic films. We have studied this coupling by having one of the magnetic films pinned and leaving the other free to switch at low field. The pinning is accomplished with test structures based on exchange bias and synthetic antiferromagnetic layers. Since the pinning strength increases sharply at low temperatures but orange-peel coupling does not, low-temperature (77 K) measurements appear to identify whether an observed coupling arises primarily from magnetic coupling through pinholes or primarily from orange-peel roughness. Our measurements appear to indicate that the observed coupling arises primarily from magnetic coupling through pinholes for Cu films less than 2.1 nm thick and for Al2O3films less than 0.6 nm thick but primarily from roughness-induced (orange-peel) magnetostatic coupling for larger thicknesses.

More than 50 years of extensive research into exchange anisotropy in ferromagnetic-antiferromagnetic bilayers has not produced a convincing explanation for any given system of its principal manifestations, namely, a shift of the hysteresis loop along the field axis (exchange bias) and enhanced coercivity. We have examined this issue in the prototypical polycrystalline Permalloy-CoO bilayer system with samples whose Permalloy thicknesses ranged from 1 to 25 nm. The heterogeneous magnetic and chemical microstructure of the similar to 1-nm-thick interfacial region is responsible for the observed exchange bias and coercivity, and for their dependence on Permalloy thickness and on temperature. Approximately 75% of the interfacial moment is produced by magnetically hard particles which are exchange coupled to the CoO and are responsible for exchange bias and coercivity by virtue of their exchange coupling to the Permalloy. The remainder of the interfacial moment is produced by a magnetically soft phase that exhibits no exchange bias. The thickness dependence of the exchange bias agrees with the prediction of a random-field model in which the exchange coupling of the distributed hard particles provides a random field operating on the Permalloy. The coercivity is determined by the switching of the hard interfacial particles coupled to the Permalloy; it has a remarkably linear temperature dependence which can be explained by a simple thermal fluctuation model. The exchange bias exhibits the same temperature dependence as the CoO uncompensated spins and these uncompensated spins are on the interfacial {111} planes of the [111]-textured CoO. Finally, the kinetics of the chemical reactions responsible for the interfacial heterogeneity can contribute to the latent period during which the exchange bias can be substantially reversed by applying a field antiparallel to the cooling field.

We have measured element-specific magnetization depth profiles across the interface between a polycrystalline ferromagnet and an antiferromagnet in an exchange-biased bilayer of Py/CoO. Using soft x-ray resonant reflectivity we have identified a thin (0.5 nm) layer containing uncompensated Co magnetization at the interface with the Py. The majority of this magnetization follows the external field; however, similar to 10% of the magnetization in this interfacial layer is pinned antiparallel to the cooling field used when biasing the sample, consistent with the negative exchange bias in this bilayer system, provided that the pinned Co spins are antiferromagnetically coupled to the ferromagnetic layer.

By spark-eroding Fe_75Si_15B_10 in water/ethanol mixtures, spherical particles with nanostructured cores consisting of mixed amorphous and crystalline phases were produced. The relative volume fractions of the amorphous and crystalline phases were dependent on the water/ethanol ratio. In the same process, continuous oxide layers were formed on the particle surfaces. The basic mechanisms responsible for the formation of the surface oxide layers and the core nanostructures were modeled. At frequencies ranging from 1 to 100 MHz, the combination of the core nanostructures and the insulating oxide shells yielded exceptionally low-loss magnetic behavior.

We report the magnetic and microstructural properties of antiferromagnetic MnO nanoparticles with shells of ferrimagnetic ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$, which is opposite the usual arrangement of antiferromagnetically coated ferromagnetic nanoparticles. In addition, the antiferromagnetic MnO cores order at much higher temperature $({T}_{N}=118\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ than the ferrimagnetic ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ shells $({T}_{C}=43\phantom{\rule{0.3em}{0ex}}\mathrm{K})$---another reversal of the usual situation. The single crystal MnO cores, with rocksalt structure, are crystallographically aligned with the tetragonal spinel structure of the ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$ shells. Particles field cooled in $50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ have large coercive force and exchange bias below ${T}_{C}$, e.g., 5800 and $2950\phantom{\rule{0.3em}{0ex}}\mathrm{Oe}$, respectively, at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The spontaneous magnetization at ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$ is $\ensuremath{\sim}20%$ of its value at $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and remains finite for more than $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ above ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$. Hysteresis with exchange bias is present in this anomalous region. The MnO cores with their uncompensated spins are responsible for the behavior above ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$. The MnO cores have a blocking temperature of $95\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the hysteresis and exchange bias above ${T}_{C}({\mathrm{Mn}}_{3}{\mathrm{O}}_{4})$ results from the switching of the MnO spin lattices by their uncompensated spins. Analysis of the thermoremanent magnetization and field cooling and/or zero field cooling in $50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, and the dependence of exchange bias on the temperature at which the cooling field was applied support this model.

We report the microstructural and magnetic properties of monodispersed nanoparticles (NPs) of antiferromagnetic MnO (TN = 118 K), with epitaxial ferrimagnetic Mn3O4 (TC = 43 K) shells. Above TC, an unusually large magnetization is present, produced by the uncompensated spins (UCSs) on the surface of the MnO particles. These spins impart a net anisotropy to the MnO particles that is approximately three orders of magnitude larger than the bulk value. As a result, an anomalously high blocking temperature is exhibited by the MnO particles, and finite coercivity and exchange bias are present above TC. When field cooled below TC, a strong exchange bias was established in the Mn3O4 shells as a result of high net anisotropy of the MnO particles. A large coercivity was also observed. Models of several aspects of the behaviour of this unusual system emphasized the essential role of the UCSs on the surfaces of the MnO NPs.

We have used magnetometry and resonant soft x-ray magnetic reflectometry to determine the depth-dependent charge and magnetization density on an absolute scale across a Permalloy/CoO interface above the Neel temperature of CoO. A thin magnetic layer of 1.0 nm forms at the interface. This layer has larger magnetization density and different temperature dependence of magnetization than Permalloy.

We have analytically calculated and compared the total magnetic energies of remanent magnetization states of hollow particles to find conditions for single-domain behavior. The size of the particles and the thickness of the spheroidal shell have been systematically varied for different hard and soft magnetic materials and the corresponding phase diagrams of the lowest-energy configurations are derived.

In the present study, the atomic structure of Co1-xMgxO was modeled and the density of uncompensated spins due to the presence of Mg atoms was calculated as a function of Mg concentration, x. The results were also compared with experimental measurements of exchange bias field (Hex) of Co/Co1-xMgxO bilayers, and with the ther-moremanent moments (TRM) of Co1-xMgxO. The formation of uncompensated spins in AFM CoO with substitution of Co atoms with non-magnetic Mg atoms was calculated from a direct modeling of the atomic structure, and their effects on the exchange coupled bilayers were confirmed.