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Co–CoO nanoparticles prepared by reactive gas-phase aggregation
Abstract and Figures
The technique of gas-phase aggregation has been used to prepare partially oxidized Co nanoparticles films by allowing a controlled
flow of oxygen gas into the aggregation zone. This method differs from those previously reported, that is, the passivation
of a beam of preformed particles in a secondary chamber and the conventional (low Ar pressure) reactive sputtering of Co to
produce Co–CoO composite films. Transmission electron microscopy shows that the mean size of the particles is about 6nm.
For sufficiently high oxygen pressures, the nanoparticles films become super-paramagnetic at room temperature. X-ray diffraction
patterns display reflections corresponding to fcc Co and fcc CoO phases, with an increasing dominance of the latter upon increasing
the oxygen pressure in the aggregation zone, which is consistent with the observed reduction in saturation magnetization.
The cluster films assembled with particles grown under oxygen in the condensation zone exhibit exchange-bias fields (about
8kOe at 20K) systematically higher than those measured for Co–CoO core-shell nanoparticles prepared by oxidizing preformed
particles in the deposition chamber, which we attribute, in the light of results from annealing experiments, to a higher ferromagnetic–antiferromagnetic
(Co–CoO) interface density.
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... So the magnetic moments of the antiferromagnetic phase, which are adjacent to the ferromagnetic border, get an uncompensated magnetic moment parallel to that of ferromagnetic direction, that is saturated by the filed which leads to an enhancement in exchange coupling. It inserts a new unidirectional anisotropy in the direction of the cooling field and then causes a horizontal shift in the hysteresis loop [1,5,[13][14][15]. Exchange bias has been extensively studied in thin-film systems because of good control over the interface and thickness of the layers [1,[16][17][18], with reports demonstrating the sensitivity of exchange bias to ferromagnetic and antiferromagnetic thicknesses as well as, crucially, interface quality [1,12,14,19,20]. ...
... It inserts a new unidirectional anisotropy in the direction of the cooling field and then causes a horizontal shift in the hysteresis loop [1,5,[13][14][15]. Exchange bias has been extensively studied in thin-film systems because of good control over the interface and thickness of the layers [1,[16][17][18], with reports demonstrating the sensitivity of exchange bias to ferromagnetic and antiferromagnetic thicknesses as well as, crucially, interface quality [1,12,14,19,20]. Recently it has been shown that it is possible to induce exchange coupling in nanoparticles when they are milled together and compared to other synthesis methods, this is less costly and time-consuming [4][5][6]15,21]. ...
... Shifts of hysteresis loops in the ferromagnetic-antiferromagnetic exchange-coupled systems were reported in different researches [4,5,15]. By field cooling of an ferromagnetic-antiferromagnetic exchange-coupled system from above antiferromagnetic's Néel temperature to below it, because of extra coupling between ferromagnetic and antiferromagnetic adjacent moments, a horizontal shift happens in magnetization loop [1,5,[13][14][15]. CoO-0h has no significant exchange bias (16 Oe). ...
This study aimed to investigate the effect of milling time on the structural and exchange bias properties of two exchange-coupled nanocomposite systems that were prepared by mechanical milling. The first system consists of an antiferromagnet (CoO), named CoO-series and the second one consists of a mixture of a ferromagnet with an antiferromagnet (10 wt% Co + CoO), called MIX-series and both systems were milled for various times. Upon field cooling, going through low temperature (5 K), hysteresis loop shifts, coercivity enhancements, and saturation magnetization reductions were observed as milling time increases. Noticeable enhancements of exchange bias (270 Oe) and coercivity (1039 Oe) of the CoO-series after 10 h of milling, in comparison with those of unmilled ones (16 Oe) and (136 Oe) respectively, showed that the main structural changes occurred at the first milling hours. Introduction of large structural defects, the formation of cobalt and crystallite size reduction, ensuing from milling, can be the causes of an increase in exchange bias and coercivity. On the other hand, ferromagnet-antiferromagnet exchange coupling induces by milling is another origin for exchange bias. The results show that ferromagnet-antiferromagnet ratio and/or crystallite sizes play key roles in exchange bias enhancement. So that milled CoO-series with lower Co content in comparison with that of MIX-series and finer crystallites have considerable higher exchange bias.
... Figure 10a indicates that the variation in coercivity with respect to the oxidation time is not uniform in the OMO-series and has a maximum for 40 min oxidation (90.9 kA/m) and a minimum for 20 and 60 min oxidation times (75.3 kA/m). The range of coercivity values reported in the literature on Co/Co-oxide nanoparticles varies from 16 to 800 kA/m [31,32,[56][57][58][59][60][61][62]. It was shown that the coercivity is highly influenced by the particle size, domain structure, shape of the particle, and the thickness of each layer [31,32,42,56,57,62,63]. ...
... The extra Co-oxide shell causes an increase in the exchange bias. Compared to the range of exchange bias values reported in the literature on Co/Co-oxide nanoparticles, the value achieved here is small [10,58,59,62,70]. Moreover, the reported exchange biases in the Co-oxide/Co nanostructure with 6 and 4 nm sizes are 64 and 200 kA/m, respectively [31,32]. ...
In this study, we investigate the enhancement of exchange bias in core/shell/shell structures by synthesizing single inverted core/shell (Co-oxide/Co) and core/shell/shell (Co-oxide/Co/Co-oxide) nanostructures through a two-step reduction and oxidation method. We evaluate the magnetic properties of the structures and study the effect of shell thickness on the exchange bias by synthesizing various shell thicknesses of Co-oxide/Co/Co-oxide nanostructures. The extra exchange coupling formed at the shell-shell interface in the core/shell/shell structure leads to a remarkable increase in the coercivity and the strength of the exchange bias by three and four orders, respectively. The strongest exchange bias is achieved for the sample comprising the thinnest outer Co-oxide shell. Despite the general declining trend of the exchange bias with Co-oxide shell thickness, we also observe a nonmonotonic behavior in which the exchange bias oscillates slightly as the shell thickness increases. This phenomenon is ascribed to the dependence of the antiferromagnetic outer shell thickness variation at the expense of the simultaneous opposite variation in the ferromagnetic inner shell.
... 1d and S1b of ESI, a large orientated nanocrystal was observed in the TEM images after the annealing of S2. This result indicates a kinetic growth mechanism enhanced by the heating rate as reported in previous works 27,30,45,46 . Specifically, the growth of CoO nanoparticles is dependent on two mechanisms: a) Co diffusion into the remaining organic coating, into the lattice and intra lattice, and, principally, b) by coalescence and orientated attachment of CoO nanoparticles. ...
We herein report a comprehensive investigation on the magnetic, structural, and electric properties of CoO nanoparticles with different sizes by local inspection through hyperfine interactions measured in a wide range of temperatures (10–670 K) by using radioactive 111\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{111}$$\end{document}In(111\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{111}$$\end{document}Cd) tracers with the perturbed angular correlations technique. Small cobalt oxide nanoparticles with the characteristic size of 6.5 nm have been prepared by the wet chemical route that turned out to be essential to incorporate radioactivity tracers during nucleation and growth of the particles. Nanocrystalline samples with 22.1 nm size were obtained by thermal treatments under low pressure of helium at 670 K. The hyperfine data were correlated with X-ray diffraction, ZFC–FC magnetic measurements, and transmission electron microscopy to describe the structure, magnetic properties, size, and shape of samples. An analysis of the temperature evolution of hyperfine parameters revealed that the structural distortion and the magnetic disorder in the core and on the surface layer play an important role in the magnetic behavior of CoO nanoparticles.
... This means they have more chances to collide with the surrounding sputtered material, therefore growing larger [34,35]. 2) Sputtering power, it affects the amount of sputtered atoms, the temperature and the ionization of the gas, which in turn influences the cluster growth and charging state [33,34,[36][37][38]. 3) Reactive gas, enhancing the formation of dimers, which act as seeds in the cluster growth process [37,39,40]. 4) Inert gas composition and flow rate, they influence not only the pressure but also the ejected target atom thermalization efficiency as well as plasma properties which eventually affects the particle charge state [36]. ...
Many research works have demonstrated that the combination of atomically precise cluster deposition and theoretical calculations is able to address fundamental aspects of size-effects, cluster-support interactions, and reaction mechanisms of cluster materials. Although the wet chemistry method has been widely used to synthesize nanoparticles, the gas-phase synthesis and size-selected strategy was the only method to prepare supported metal clusters with precise numbers of atoms for a long time. However, the low throughput of the physical synthesis method has severely constrained its wider adoption for catalysis applications. In this review, we introduce the latest progress on three types of cluster source which have the most promising potential for scale-up, including sputtering gas aggregation source, pulsed microplasma cluster source, and matrix assembly cluster source. While the sputtering gas aggregation source is leading ahead with a production rate of ∼20 mg·h−1, the pulsed microplasma source has the smallest physical dimensions which makes it possible to compact multiple such devices into a small volume for multiplied production rate. The matrix assembly source has the shortest development history, but already show an impressive deposition rate of ∼10 mg·h−1. At the end of the review, the possible routes for further throughput scale-up are envisaged.
... As shown in Fig. 1d and Fig.S2b of ESI, a large orientated nanocrystal was observed by TEM images after annealing in sample S2. This result indicates a kinetic grown mechanism enhanced by the heating rate as reported in previous works 30,35,42,43 . Specifically, the growth of CoO nanoparticles is dependent on two mechanisms: a) Co diffusion into the remained organic coating, into the lattice and intra lattice, and, principally, b) by coalescence and orientated attachment of CoO nanoparticles. ...
We herein report a comprehensive investigation on the magnetic, structural, and electric properties of CoO nanoparticles with different sizes by local inspection through hyperfine interactions measured in a wide range of temperatures (10-670 K) by using radioactive 111In( 111Cd) tracers with the perturbed angular correlations (PAC) technique. Small cobalt oxide nanoparticles with 6.5 nm have been prepared by the wet chemical route, which has shown essential to incorporate radioactivity tracers during nucleation and grown syntheses. Nanocrystalline samples with 22 nm size were obtained by thermal treatments under low pressure of helium at 670 K. The hyperfine data were correlated with X-ray diffraction (XRD), ZFC-FC magnetic measurements and transmission electron microscopy (TEM) to describe the structure, magnetic properties, size, and shape of samples. An analysis of the temperature evolution of hyperfine parameters revealed that the structural distortion and the magnetic disorder in the core and on the surface layer play an important role in the magnetic behavior of CoO nanoparticles.
... Advances in synthesis methods have allowed the fabrication of bi-magnetic nanostructures such as thin films and core/shell NPs, in which exchange coupling between the magnetic phases brings out novel or improved properties [2]. The properties of these heterostructures are different from those of each phase, leading to innovative applications [2,[11][12][13]. Different combinations of exchange-coupled hard (with high H c ) and soft (providing high saturation magnetization) magnetic materials are used to design permanent magnets with enhanced energy products [2,14]. ...
Two series of Co/Co-oxide nanostructures have been synthesized by the co-precipitation method followed by different reduction and oxidation processes in an attempt to optimize their exchange bias (EB) properties. The samples are characterized by X-ray diffraction, scanning and transmission electron microscopy, and SQUID (superconducting quantum interference device) magnetometry. The two series differ with respect to their average Co core grain sizes: in one (the l-series), the size is ≈100 nm, and in the other (the s-series, obtained using lower synthesis temperatures than the l-series), it is ≈10 nm. In the l-series, progressive oxidation yields an increase in the EB field together with a reduction in Co core size. In contrast, progressive oxidation in the s-series results in growth of the Co-oxide fraction at the expense of the Co core upon oxidation, which is accompanied by a decrease in the EB effect that is attributed to an ordering of the ferromagnetic–antiferromagnetic interface and therefore a reduction of uncompensated spins density. These results illustrate how the interface details become relevant only for small enough ferromagnetic cores.
... However, a reduction of the ordering temperature required for L1 0 phase formation can be obtained via suitable modulation of the stoichiometry, for example by adding other elements to the composition. In the case of the FePt system, noble metals such as Au, Ag or Nb have been considered as suitable additions, having the role of promoting early ordering by segregation to the (Fe,Co)Pt grain boundaries [7-14] while a glass-forming element such as boron has been added to FePt to allow synthesis by out-of-equilibrium techniques such as rapid solidification from the melt [15][16][17][18][19][20][21]. ...
A novel class of quaternary intermetallic alloys based on CoPt is investigated in view of their interesting magnetic properties induced by the presence of hard magnetic L10 phase. A Co48Pt28Ag6B18 alloy has been prepared by rapid solidification from the melt and subjected to various isothermal annealing procedures. The structure and magnetism of both as-cast and annealed samples as well as the phase evolution with temperature are investigated by means of thermal analysis, X-ray, and selected area electron diffraction, scanning and high-resolution electron microscopy, and magnetic measurements. The X-ray diffraction (XRD) analysis shows that both the as-cast alloy and the sample annealed at 400 °C (673 K) have a nanocrystalline structure where fcc CoPt phase predominates. Annealing at 473 °C promotes the formation of L10 phase triggered by the disorder-order phase transformation as documented in the differential scanning calorimetry results. The sample annealed at 670 °C (943 K) shows full formation of L10 CoPt as revealed by XRD. Magnetic measurements showed coercivity values ten times increased compared to the as-cast state. This confirms the full formation of L10 CoPt in the annealed samples. Moreover, detailed atomic resolution HREM images and SAED patterns show the occurrence of the rarely seen (003) superlattice peaks, which translated into a high ordering of the L10 CoPt superlattice. Such results spur more interest in finding novel classes of nanocomposite magnets based on L10 phase.
In order to prove the usefulness of having a structurally disordered precursor to the formation of FePt L10 phase and to facilitate the co-existence of exchange coupled hard and soft magnetic phases with optimized magnetic properties in various conditions of annealing, a Fe-Pt-Zr-B melt spun alloy has been synthesized and detailed structural and magnetic investigations have been undertaken to probe its phase evolution during annealing. The dynamics of formation of the hard magnetic L10 phase during the gradual disorder–order phase transformation has been monitored by using a complex combination of X-ray diffraction methods and 57Fe Mössbauer spectroscopy methods, over a wide range of annealing temperatures. Multiple phases co-existing in the annealed sample microstructures, observed in XRD, have been reconfirmed by the Mössbauer spectra analysis and, moreover, accurate quantitative data have been acquired in what concerns the relative abundance of each of the observed crystalline phases in every stage of annealing. It is shown that the formation of the hard magnetic phase, emerging from the chemically disordered precursor, is gradual and occurs via complex mechanisms, involving the presence of a disordered Fe-Zr-B-rich intergranular region which contributes to an increase in the abundance of the L10 phase for higher annealing temperatures. Magnetic measurements have confirmed the good performances of these alloys in terms of coercivity and remanence. These results contribute to the development of these alloys as the next generation of rare earth, free permanent magnets.
We propose a concept of hybrid nanoelectronic-magnetic device made of magnetic Fe–C core–shell nanoparticles deposited onto prepatterned Si (111) substrate with basic circuitry made of metallic conductive lines. The synthesis of magnetic material and the creation of nanoelectronic prepatterned interdigitated die are reported and to prove the effectiveness in devices, their magnetotransport properties are investigated. Magnetic Fe/FeC nanoparticles, 11 nm diameter, with a core–shell structure have been prepared by laser pyrolysis. Two different layouts of prepatterned interdigitated die, have been conceived using e-beam lithography, with various geometries. A range of microscopy techniques, transmission electron, scanning and optical, were employed for morphological characterization of the as-obtained structures. Magnetic and magnetotransport characterization using SQUID magnetometry has been performed onto both the core–shell nanoparticles and onto the hybrid device obtained by depositing centrifugated and dispersed core–shell nanoparticles from liquid carrier solutions. From magnetotransport measurements, it has been revealed that the hybrid device made of Fe/FeC nanosized materials on prepatterned interdigitated die exhibit a large giant magnetoresistive (GMR) effect of about 8% at 300 K. This result is promising in view of the use of such devices as arrays of nanosensors and in spintronic applications.
Nanoparticle engineering for catalytic applications requires both a synthesis technique for production of well-defined nanoparticles as well as measurements of their catalytic performance. In this paper, we present a new approach to rationally engineering highly active Ni-Mo-S nanoparticle catalysts for hydrodesulfurization (HDS), i.e. removal of sulfur from fossil fuels. Nanoparticle catalysts are synthesized by sputtering of a Mo75Ni25 metal target in a reactive atmosphere of Ar and H2S, followed by gas aggregation of the sputtered material into nanoparticles. The nanoparticles are filtered by a quadrupole mass filter and subsequently deposited on a planar substrate, such as a grid for electron microscopy or a microreactor. By varying the mass of the deposited nanoparticles, it is demonstrated that the Ni-Mo-S nanoparticles can be tuned into fullerene-like particles, flat-lying platelets and up-right oriented platelets. The nanoparticle morphologies provide different abundances of Ni-Mo-S edge sites, which are commonly considered as the catalytically important sites. Using a micro-reactor system, we assess the catalytic activity of the Ni-Mo-S nanoparticles for HDS of dibenzothiophene. The measurements show that platelets are twice as active as the fullerene-like particles, demonstrating that the Ni-Mo-S edges are more active than basal planes for the HDS. Furthermore, the up-right standing orientation of platelets show an activity that is six times higher than the fullerene-like particles, demonstrating the importance of the edge sites number and accessibility to reduce e.g. sterical hindrance for the reacting molecules.
Monodispersed Co/CoO cluster assemblies with the mean cluster sizes of 6
and 13 nm have been prepared by a plasma-gas condensation type cluster
beam deposition apparatus. We measured the effects of the oxygen gas
flow rate during deposition, temperature, and cluster size on the
coercivity and hysteresis loop shift induced by field cooling. The large
exchange bias field (10.2 kOe) and coercivity (5 kOe) were observed at 5
K for the monodispersed Co/CoO cluster assembly with d=6 nm. The
correlations between unidirectional anisotropy and uniaxial anisotropy,
training effect and magnetic relaxation can be interpreted by the
hypothesis of a spin disorder in the interfacial layer between the
antiferromagnetic CoO shell and the ferromagnetic Co core.
Cr clusters have been produced by a plasma-gas-condensation type cluster deposition apparatus, and studied using a time-of-flight mass spectrometer and a transmission electron microscope. The Cr clusters formed in high pressure inert (Ar and/or lie) gas atmosphere are of an A15-type structure. When an O-2. Ps is mixed with the inert gases in the source (sputtering) chamber, a bcc phase is formed together with Cr2O3. The O-2 gas introduction leads to an increase in the gas temperature of the source chamber probably due to release of the formation enthalpy of the oxide. The A15 phase is annealed by such excess heat and becomes the equilibrium bee phase. The sizes of bee clusters are smaller than those of the A15-clusters, probably due to the heterogeneous nucleation promoted by the oxide formation.
A nanogranular Co–CoO∕Ag system (with core-shell∕matrix structure), synthesized by reactive cosputtering, has been subjected to thermal treatments at progressively higher temperatures. The exchange-bias onset temperature increases in close correlation with the average superparamagnetic blocking temperature of the magnetic particles. This behavior, together with the nonmonotonic evolution of the exchange-bias field, is explained in terms of two structural relaxation processes previously characterized in simple Co∕Ag granular alloys: (i) segregation of Co solute atoms from the Ag-rich matrix, and (ii) particle coarsening.
We have studied the magnetic properties of [Ag(tAg)∕Co(1.2 nm)]60 multilayers grown in an oxygen atmosphere. Partial oxidation of the Co layers results in the appearance of tAg dependent exchange-bias properties. A strong increase in the exchange-bias field HE together with a significant reduction in the coercivity HC are observed when tAg is decreased below tAg*=4 nm, whereas these two parameters adopt approximately constant values for tAg>tAg*. At tAg*, a transition from continuous to islandlike silver layers (on reducing tAg) is signaled by electrical resistivity and x-ray reflectivity results. From magnetic hysteresis loops recorded at room temperature and magnetization curves, it is concluded that this transition induces a granular morphology in the Co regions, which are previously (tAg>tAg*) plateletlike, and enhances the oxidized fraction of Co (fCoO). The increase (decrease) in HE (HC) with reducing tAg below tAg* is correlated to the increase in both the electrical resistivity and fCoO. From the latter correlation, we infer that the higher degree of oxidation in the granular Co layers is associated with effectively thicker antiferromagnetic (CoO) regions than in the tAg>tAg* (continuous multilayer) case—with correspondingly higher magnetic anisotropy energies—which may account for both the enhancement in HE and the reduction in HC. In addition, our study provides information on the surfactant effect of O2 in Ag sputter growth since the continuity thickness value (tAg*=4 nm) is found to be lower than those previously reported in nonreactive sputtering of Ag.
Exchange anisotropy was studied by SQUID magnetometry on an array of monodisperse colloidal nanoparticles consisting of a 7–8 nm diameter FCC Co core covered with a 2–2.5 nm thick FCC CoO shell. Temperature-dependent measurements of the exchange bias field show that the exchange anisotropy vanishes when a magnetic field was applied during cooling below 150 K. The suppression of exchange anisotropy is due to uncompensated interfacial antiferromagnetic spins.
Co/CoO nanocomposite thin films have been deposited by RF sputtering with different oxygen partial pressures in the working atmosphere. It has been found that the magnetic properties (exchange bias (EB), coercivity and remanence) of the films are strongly dependent on the oxygen partial pressure. Film deposited with an O2 pressure of 0.75×10−5Torr shows a relatively high coercivity and high remanence ratio. Enhanced asymmetry of magnetization reversal appears in the film deposited with 1.5×10−5Torr O2 partial pressure. When the O2 partial pressure is increased to 3.0×10−5Torr, a strong reduction in EB and coercivity have been obtained. The investigation of the microstructure using high-resolution transmission electron microscope reveals that the change in EB, remanence and coercivity is probably related to the competition between dipolar interaction and exchange coupling. Magnetoresistance and extraordinary Hall effect (EHE) of the Co/CoO nanocomposite films is also investigated in dependence on oxygen partial pressure. The film deposited under 2.3×10−5Torr of oxygen exhibits strongly enhanced asymmetric magnetization reversal and unusually high EHE with a high coercivity.
The phenomenology of exchange bias and related effects in nanostructures is reviewed. The types of systems discussed include: lithographically fabricated ferromagnetic (FM)—antiferromagnetic (AFM) nanostructures, chemically surface modified FM particles, FM particles embedded in an AFM matrix, controlled core–shell particles, nanoparticles with surface effects and coupled AFM–AFM systems. The main applications of exchange biased nanostructures are summarized. Finally, the implications of the nanometer dimensions on some of the existing exchange bias theories are briefly discussed.
A Co-CoO system was studied to obtain high coercive materials. Co-CoO thin films were sputtered at various oxygen gas flow rates by dual ion-beam sputtering. The Co was deposited onto glass substrates at room temperature and 650 K by argon ion-beam sputtering and simultaneously oxidized by oxygen gas from an assist ion gun. As the oxygen gas flow rate increases, there appear Co, the Co-CoO mixed phase, CoO, and Co2O3 in a stepwise manner. The coercive force increases sharply in the Co-CoO mixed phase region. However, the maximum coercive force at room temperature is small, about 600 Oe. At low temperatures, the addition of the CoO phase to Co causes a significant increase of the coercive force. The perpendicular coercive force of the Co-CoO film is larger than the in-plane coercive force. The perpendicular coercive force is about 17000 Oe at about 10 K. This is due to the magnetic anisotropy of the antiferromagnetic microcrystalline CoO below the Néel temperature of about 220 K. The CoO (111) plane is preferably oriented and the spin axis of CoO stands against the plane, which leads to a strong perpendicular magnetic anisotropy. There is a possibility that the coexistence of the ferromagnetic phase and the antiferromagnetic phase significantly increases the coercive force.
Exchange bias and coercivity have been studied in nanometric-size cobalt clusters as a function of oxygen exposure under high vacuum. This technique allows looking at the early stages of oxidation, as well as studying exchange bias in small clusters before they are completely transformed into oxide. It is found that oxygen induces at first an increase in the coercive field and in the superparamagnetic blocking temperature, before any substantial loss in the magnetization, which indicates that this effect is due to adsorbed oxygen rather than to exchange coupling with an oxide overlayer. © 2004 American Institute of Physics.
The magnetization reversal in exchange coupled polycrystalline Co/CoO bilayers has been investigated as a function of CoO thickness using anisotropic magnetoresistance as a probe. The anisotropic magnetoresistance (AMR) was measured during the magnetization reversal and it was used to determine the orientation of the magnetization. For thin CoO layers large training effects were present; ergo the first hysteresis loop after field cooling was not the same as the second. The magnitude of the observed training was found to decrease with increasing CoO thickness. In the samples where substantial training was observed, the first magnetization reversal was dominated by nucleation of reversed domains. For the reversal from the antiparallel state back to the parallel direction, the AMR is consistent with a rotation process. In thicker CoO films where the training was less, the asymmetry was drastically reduced. A simple model that couples the antiferromagnetic grains to the ferromagnetic layer simulates qualitatively the observed magnetoresistance. © 2002 American Institute of Physics.