CoO Thickness Research Articles

Antiferromagnet thickness dependence and rotatable spins in exchange biased CoO/Fe films

The emergence of exchange bias and coercivity enhancement has been investigated in epitaxial CoO/Fe films with varied antiferromagnet (AF) thicknesses, even smaller than the critical value where the frozen CoO spins are detectable. Vector magnetometry and first-order reversal curve (FORC) measurements reveal different CoO thickness dependence of the exchange bias and coercivity enhancement, including the evolution of magnetization reversal from a high coercivity, low bias phase due to rotatable CoO moments to a high bias, low coercivity phase due to frozen CoO moments. The AF domain state is found to be metastable, which can be reoriented by external and exchange fields prior to the appearance of frozen spins, pointing to a generic origin of the training effect. Monte Carlo simulations show that the AF anisotropy energy barrier and the rotatable spins induced by magnetic field and exchange interaction at the interface are responsible for the observed effects.

Antiferromagnetism of CoO-NiO bilayers studied by XMLD spectroscopy

CoO-NiO epitaxial bilayer system grown on MgO(001) substrate is investigated using x-ray magnetic linear dichroism (XMLD) spectroscopy with varying CoO overlayer thickness. An analysis of the Ni L 2 edge XMLD spectra using anisotropic XMLD formulation within a two-domain model reveals that the Ni moments undergo a spin reorientation with increasing CoO thickness. Such a spin reorientation is attributed to the competing magnetic interactions at both the NiO film interfaces, suggesting the existence of a sharp horizontal domain wall separating the in-plane and out-of-plane NiO domains. Our study also demonstrates a possible way to investigate the spin-structure along the thickness within the same chemical structure using a model-based approach, in a noninvasive manner.

Formation of ultrathin cobalt ferrite films by interdiffusion ofFe3O4/CoObilayers

In this work an alternate pathway is demonstrated to form ultrathin cobalt ferrite (${\mathrm{Co}}_{x}{\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{O}}_{4}$) films by interdiffusion of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/CoO bilayers. Bilayer samples with different ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$/CoO thickness ratios have been prepared by reactive molecular beam epitaxy on Nb-doped ${\mathrm{SrTiO}}_{3}$(001) substrates to obtain cobalt ferrite films of varied stoichiometry. Subsequently, oxygen-assisted postdeposition annealing experiments for consecutive temperature steps between $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ and $600{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ have been conducted monitoring the interdiffusion process by means of high-resolution x-ray reflectivity, soft and angle-resolved hard x-ray photoelectron, and x-ray absorption spectroscopy. Magnetic properties were characterized using superconducting quantum interference device magnetometry. The interdiffusion process starts from $300{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ annealing temperature and is completed for temperatures above $500{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. For completely interdiffused films with Co:Fe ratios larger than 0.84:2 a thin segregated CoO layer on top of the ferrite is formed. This CoO segregation is attributed to surface and interface effects. In addition, multiplet calculations of x-ray absorption spectra are performed to determine the occupancy of different sublattices. These results are correlated with the magnetic properties of the ferrite films. A stoichiometric ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ film with partial inversion has been formed exhibiting homogeneously distributed ${\mathrm{Co}}^{2+}$ and mainly ${\mathrm{Fe}}^{3+}$ valence states if the initial Co:Fe content is 1.09:2. Thus, for the formation of stoichiometric cobalt ferrite by the proposed postdeposition annealing technique an initial Co excess has to be provided as the formation of a top CoO layer is inevitable.

Steering the magnetic properties of Ni/NiO/CoO core-shell nanoparticle films: The role of core-shell interface versus interparticle interactions

Supported core-shell Ni/NiO/CoO nanoparticle (NP) films were obtained by deposition of preformed and mass-selected Ni NPs on a buffer layer of CoO, followed by a top CoO layer. The resulting NPs have core/shell morphology, with a McKay icosahedral Ni core and a partially crystalline CoO shell. X-ray photoelectron spectroscopy evidenced the presence of a thin NiO layer, which was shown to be between the Ni core and the CoO shell by elemental TEM mapping. CoO and NiO shells with different thickness values were obtained, allowing us to investigate the evolution of the magnetic properties of the NP assemblies as a function of the oxide shell thickness. Both exchange-coupling and magnetostatic interactions significantly contribute to the magnetic behavior of Ni/NiO/CoO NP films. After the Ni/NiO/CoO NPs are cooled in a weak magnetic field, they have blocking temperature higher than room temperature because of strong magnetostatic interactions, which support the formation of a spin-glass-like state below $\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Exchange coupling dominates the magnetic behavior after the NPs are cooled in a strong magnetic field. The exchange bias (EB) is in the 0.17--2.35 kOe range and strongly depends on the CoO thickness (0.4--2.7 nm), showing the onset of the EB at the few-nanometer scale. The switching field distribution showed that the EB opposes the magnetization reversal from the direction along the cooling field but it does not significantly ease the opposite process. The EB depends on ${t}_{\mathrm{CoO}}$ only for ${t}_{\mathrm{NiO}}\ensuremath{\le}0.5\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$, but when NiO is 0.7 nm thick it strongly interacts with CoO and a large increase of the EB and coercivity is observed.

Exchange biasing and interlayer coupling in Co/Pt/Co/CoO multilayers with perpendicular anisotropy

Investigations have been performed on exchange biasing and interlayer coupling in the Co/Pt/Co/CoO multilayers with perpendicular anisotropy. We have observed the asymmetry of the magnetization reversal of the top Co layer in contact with CoO layer and the shifting EHE loops of the bottom Co layer, which could be ascribed to the competition between the interfacial exchange biasing of the top Co layer and interlayer coupling between Co layers across the Pt layer. Due to other oxide phases observed in CoO layer, non-stoichiometric structure of CoO lowers the blocking temperature for the exchange biasing systems. The blocking temperature also depends on the CoO thickness and we observed lower blocking temperature for the thinner CoO layer.

Variation of blocking temperatures for exchange biased CoO/Co/Ge(100) films

Variations of the blocking temperature and related structures for CoO/Co/Ge(100) films are investigated by employing reflection high energy electron diffraction, Auger electron spectroscopy, and surface magneto-optic Kerr effect measurements. By increasing the CoO thickness, the blocking temperature is smaller than the Neel temperature of CoO. The monotonous increase of the blocking temperature is mainly attributed to the increasing thermal stability of the antiferromagnetic grains by way of increasing the antiferromagnetic thickness. The deviation of the blocking temperature from the linear relation and the full widths at half maximum of the diffraction spots show a similar trend. The minimums appear around 25 monolayer of CoO and are related to the formation of larger grains.

Self-organized nano-structuring of CoO islands on Fe(001)

The realization of nanometer-scale structures through bottom-up strategies can be accomplished by exploiting a buried network of dislocations. We show that, by following appropriate growth steps in ultra-high vacuum molecular beam epitaxy, it is possible to grow nano-structured films of CoO coupled to Fe(001) substrates, with tunable sizes (both the lateral size and the maximum height scale linearly with coverage). The growth mode is discussed in terms of the evolution of surface morphology and chemical interactions as a function of the CoO thickness. Scanning tunneling microscopy measurements reveal that square mounds of CoO with lateral dimensions of less than 25nm and heights below 10 atomic layers are obtained by growing few-nanometers-thick CoO films on a pre-oxidized Fe(001) surface covered by an ultra-thin Co buffer layer. In the early stages of growth, a network of misfit dislocations develops, which works as a template for the CoO nano-structuring. From a chemical point of view, at variance with typical CoO/Fe interfaces, neither Fe segregation at the surface nor Fe oxidation at the buried interface are observed, as seen by Auger electron spectroscopy and X-ray Photoemission Spectroscopy, respectively.

Activation of antiferromagnetic domain switching in exchange-coupled Fe/CoO/MgO(001) systems

In contrast to the extensive study of domain reversal in ferromagnetic materials, the domain switching process in antiferromagnets is much less studied due to the difficulty of probing antiferromagnetic spins. Using a combination of hysteresis loop, Kerr microscope, and x-ray magnetic linear dichroism measurements, we investigated the antiferromagnetic (AFM) domain switching process in single crystalline Fe/CoO bilayers on MgO(001). We demonstrate that the CoO AFM switching is a Kolmogorov-Avrami process in which the thermal activation energy creates AFM domain nucleation centers which further expand by domain wall propagation. From the temperature- and thickness-dependent measurements, we are able to retrieve quantitatively the important parameter of the CoO AFM activation energy, which is shown to increase linearly with CoO thickness.

Compositions and Magnetic Properties of CoO/Co/Ge(111) Films

On the top of Co/Ge(111) films, cobalt oxides are prepared by evaporating Co atoms in an oxygen atmosphere. Depth profiling measurements show a layered structure of a pure Co layer covered by a CoO overlayer. As the CoO thickness increases, the Auger intensity ratio of O KL2L2 and Co L3M45M45 Auger signals increases until reaching a saturated value which shows a CoO layer with a concentration ratio of Co and O close to 1:1. After introduction of CoO overlayers on Co/Ge(111), hysteresis occurs in the longitudinal configuration and CoO/Co/Ge(111) exhibits in-plane anisotropy. A slight reduction of the Kerr intensity occurs due to the oxidation of cobalt at the CoO/Co interface while an enhanced coercive force is observed owing to the imperfection introduced by oxygen to impede the magnetization reversal. Under conditions of cooling in a magnetic field, exchange bias field increases as the sample temperature decreases resulting from the formation of an antiferromagnetic/ferromagnetic interface.

Volume contribution of exchange-coupling-induced uniaxial anisotropy in Fe/CoO/MgO(001) system

An unusual volume contribution of exchange-coupling-induced uniaxial anisotropy in a single-crystalline Fe/CoO/MgO(001) system was discovered and measured using the magneto-optical Kerr effect. The observed volume contribution emerges with the establishment of CoO antiferromagnetic order below the CoO blocking temperature or above a critical CoO thickness. It decays with decreasing exchange coupling strength tuned by inserting a MgO layer between the Fe and CoO layers. The volume anisotropy of the Fe layer is attributed to the strain transferred from the CoO layer induced by the magnetostriction effect through a field cooling process. Our results indicate that the strain in antiferromagnetic film can be applied to control the exchange coupling effect in the future spintronics devices.

Probing Exchange Bias Effects in CoO/Co Bilayers with Pillar-Like CoO Structures

Exchange bias effects in CoO/Co bilayers fabricated by ion-assisted deposition were studied as a function of CoO thickness. During the deposition of the top CoO layer, pillar-like CoO structures were embedded in the underlying Co layer due to implantation of oxygen ions. The enhanced coercivity was attributed to the changes in the magnetic reversal mechanism in the ferromagnetic Co layer due to the penetration of pillar-like structures of antiferromagnetic CoO. At low temperature, we found a strong exchange bias field. Our measurements indicate that the exchange bias effect can exist in a nanocomposite system that has a disordered mixture of columnar and planar Co/CoO interfaces.

Probing Exchange Bias Effects in CoO/Co Bilayers with Pillar-Like CoO Structures

Exchange bias effects in CoO/Co bilayers fabricated by ion-assisted deposition were studied as a function of CoO thickness. During the deposition of the top CoO layer, pillar-like CoO structures were embedded in the underlying Co layer due to implantation of oxygen ions. The enhanced coercivity was attributed to the changes in the magnetic reversal mechanism in the ferromagnetic Co layer due to the penetration of pillar-like structures of antiferromagnetic CoO. At low temperature, we found a strong exchange bias field. Our measurements indicate that the exchange bias effect can exist in a nanocomposite system that has a disordered mixture of columnar and planar Co/CoO interfaces.

Determination of the Fe magnetic anisotropies and the CoO frozen spins in epitaxial CoO/Fe/Ag(001)

CoO/Fe/Ag(001) films were grown epitaxially and studied by X-ray Magnetic Circular Dichroism (XMCD) and X-ray Magnetic Linear Dichroism (XMLD). After field cooling along the Fe[100] axis to 80 K, exchange bias, uniaxial anisotropy, and 4-fold anisotropy of the films were determined by hysteresis loop and XMCD measurements by rotating the Fe magnetization within the film plane. The CoO frozen spins were determined by XMLD measurement as a function of CoO thickness. We find that among the exchange bias, uniaxial anisotropy, and 4-fold anisotropy, only the uniaxial magnetic anisotropy follows thickness dependence of the CoO frozen spins.

Element-specific study of epitaxial NiO/Ag/CoO/Fe films grown on vicinal Ag(001) using photoemission electron microscopy

NiO/Ag/CoO/Fe single crystalline films are grown epitaxially on a vicinal Ag(001) substrate using molecular beam epitaxy and investigated by photoemission electron microscopy. We find that after zero-field cooling, the in-plane Fe magnetization switches from parallel to perpendicular direction of the atomic steps of the vicinal surface at thinner CoO thickness but remains in its original direction parallel to the steps at thicker CoO thickness. CoO and NiO domain imaging result shows that both CoO/Fe and NiO/CoO spins are perpendicularly coupled, suggesting that the Fe magnetization switching may be associated with the rotatable-frozen spin transition of the CoO film.

Frustration-driven micromagnetic structure in Fe/CoO/Fe thin film layered systems

We have investigated the micromagnetic structure of magnetic domains in Fe/CoO/Fe trilayer systems and the magnetization coupling between the iron layers. We observe very small magnetic domains separated by nanometer-sized domain walls in the top Fe layer for a narrow CoO thickness range. Such domains have lateral dimensions as low as 30 nm and present topologies which are very similar to those observed in the top layer of Fe/NiO/Fe trilayers. Both magnetic domain structure and Fe interlayer coupling dramatically change with the CoO thickness. The role of magnetocrystalline anisotropy and magnetic frustrations on the observed phenomenology is discussed.

Epitaxial growth and characterization of CoO/Fe(001) thin film layered structures

By means of X-ray photoemission spectroscopy and low energy electron diffraction, we show that it is possible to grow good quality thin epitaxial CoO films on Fe(001) substrates, through deposition in oxygen atmosphere. In particular, the composition and the structure of CoO(001)/Fe(001) bilayer systems and Fe(001)/CoO(001)/Fe(001) trilayer systems have been investigated by monitoring the evolution of the chemical interactions at the interfaces as a function of CoO thickness and growth temperature. We observe the presence of Fe oxides at the CoO/Fe interface and of a thin layer of metallic cobalt at the upper Fe/CoO interface of trilayer systems.

Tungsten atomic layer deposition on cobalt nanoparticles

Tungsten (W) atomic layer deposition (ALD) was performed on cobalt (Co) nanoparticles using WF6 and Si2H6 as reactants. A variety of techniques were then applied to analyze both the Co nanoparticles and flat Co substrates after W ALD. Analysis of the W ALD-coated Co nanoparticles is complicated because a CoO layer may exist on the Co nanoparticles and a WO3 layer may be present on the W ALD coating. LECO measurements quantified the oxygen weight percent in the W ALD-coated Co nanoparticles. The oxygen weight percent decreased with increasing number of W ALD AB cycles. To determine the location of this oxygen, x-ray reflectivity (XRR) investigations measured the WO3 film thickness on flat W ALD films. The XRR measurements yielded a WO3 film thickness on flat W ALD films of ∼20Å. X-ray photoelectron spectroscopy (XPS) studies also quantified the relative oxygen abundance at the W∕Co interface for W ALD on flat Co films. The XPS measurements revealed that nearly all the oxygen was in the WO3 layer on the W ALD film. Only an immeasurably small amount of oxygen was bonded as CoO at the W∕Co interface. To determine the thickness of W ALD film on the Co nanoparticle, surface profilometry of W ALD on flat Co substrates measured a W ALD growth rate of 3.9Å per AB cycle. A geometric model was then constructed to incorporate the information from all the measurements on Co nanoparticles and flat Co substrates. Excellent agreement between the geometrical model and the oxygen weight percent versus the number of W ALD cycles was obtained when the CoO thickness was negligible and the WO3 thickness on the W ALD layer on the Co nanoparticles was 28.5Å. This agreement indicates that the details of ALD on nanoparticles can be unraveled by a concert of techniques even when interfacial layers can form due to the high reactivity of nanoparticles. The W ALD-coated Co nanoparticles may be useful in fabricating WC–Co hardmetals with enhanced mechanical properties.

Contribution of low-temperature degrees of freedom to the anisotropy in Co/CoO exchange coupled bilayers

Measurements of the transverse magnetic AC-susceptibility χ(T) of exchange coupled polycrystalline Co/CoO bilayers have been made via the anisotropic magnetoresistance using a small AC field of 2–5Oe and frequencies from 40 to 400Hz. A step increase in Reχ is observed at 50–100K, with an accompanying dissipation peak in Imχ. This feature is independent of the CoO thickness, but its dispersion with frequency suggests a thermally activated degree of freedom with an activation energy Ea∼150K. This is consistent with a contribution to the anisotropy field due to the dynamics of the ferromagnetic rather than the antiferromagnetic grains.

Magnetization reversal in exchange biased Co/CoO probed with anisotropic magnetoresistance

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.

Magnetic properties of yttrium iron garnet thin films changed through surface modification by CoO overlayer growth for magneto-optic recording applications

Significant changes in the saturation magnetization, M s and coercivity, H c of yttrium iron garnet (YIG) thin films have been achieved through surface modification by an overlayer growth of CoO. Low pressure chemical vapor deposition of a CoO overlayer over extended (∼45 min) time at slow (∼10 nm/min) rate over a crystallized YIG film resulted in out of plane H c and M s values of 282.7 and 85.7 kA/m, respectively. For an unmodified YIG film corresponding M s and H c values are much smaller at 7.5 and 21.3 kA/m, respectively. Study of critical effects of CoO thickness and deposition rate, thermal diffusion of CoO and temperature dependence of coercivity have been described which indicate formation of a Co-rich YIG interface layer during low pressure chemical vapor deposition (LPCVD) growth though interaction between CoO and YIG and generation of interfacial stress are responsible for this unusual changes in the magnetic properties. Magnetization studies of CoO/YIG bilayer structures after sequential etching of surface and interface regions by energetic Ar + ion beam suggest that Co-rich YIG interface layer is caused by Co 2+ infiltration into YIG surface. Realization of high M s and H c values in bilayer CoO/YIG films without uniform doping of YIG by Co 2+ and inclusion of tetravalent non-magnetic compensating Ce 4+ or Ge 4+ ions as described in this paper are attractive for application as high-density magneto-optic recording media.