Antiferromagnetic CoO Research Articles

Exchange bias in ferromagnetic nanoparticles on antiferromagnetic films

Magnetic properties were investigated for single layer films of closely packed FePt nanoparticles fabricated on nonmagnetic Si substrate and antiferromagnetic CoO films. Temperature dependence of magnetization study showed the enhancement of blocking temperature only for the case of the sample having the interface between ferromagnetic and antiferromagnetic materials. This result suggests that the exchange bias enhanced the anisotropy of nanoparticles.

Magnetic phase transition in a nanostructured antiferromagnet CoO embedded in porous glass

The onset of long-range magnetic order in a nanostructured antiferromagnet CoO embedded in porous glass is revealed by neutron diffraction. The Neel temperature and the volume-averaged magnetic moment/ion are found to be smaller than those in a bulk sample.

Magnetic anisotropy ofCoxPt1−xclusters embedded in a matrix: Influences of the cluster chemical composition and the matrix nature

We report on the magnetic properties of ${\mathrm{Co}}_{x}{\mathrm{Pt}}_{1\ensuremath{-}x}$ clusters embedded in various matrices. Using a careful analysis of magnetization curves and zero field cooled susceptibility measurements, we determine the clusters magnetic anisotropy energy (MAE) and separate the surface and volume contributions. By comparing different chemical compositions, we show that a small amount of Pt (15%) induces an important increase in the volume anisotropy with respect to pure Co clusters, even in chemically disordered fcc clusters. Comparing the measurements of clusters embedded in Nb and MgO matrices, we show that the oxide matrix induces an important increase of the surface MAE attributed to the formation of an antiferromagnetic CoO shell around the clusters.

Composite state control and magnetic properties of Co and Si cluster assemblies prepared with double-glow-discharge sources

Using a double-glow-discharge-cluster-source system, in which one glow discharge was a dc mode and the other an rf discharge mode, Co and Si clusters were independently produced and simultaneously deposited on a substrate. When a separation plate was inserted between two glow-discharge chambers, a mixture of Co and Si clusters was obtained: small Co clusters were distributed at random, while the Si clusters were aggregated to form large secondary particles. Without inserting the separation plate, on the other hand, core-shell clusters were obtained: a Co core was surrounded by small Si crystallites. The magnetization measurement indicated that the magnetic coercive force of Co∕Si core-shell cluster assemblies was much smaller than that of Co cluster assemblies in which Co clusters were covered with antiferromagnetic CoO shells, indicating that the Si shell prevented Co cluster surfaces from their oxidation. Therefore, the present double-cluster-source system is useful in fabricating various sorts of cluster composites, which cannot be prepared by conventional coevaporation or precipitation methods.

Lattice Distortion in Antiferromagnetic CoO under High Magnetic Fields

Cobaltous monoxide, CoO, is an archetypal antiferromagnet, with a Neel temperature TN 1⁄4 290K, which has been studied for more than half a century. Tombs and Rooksby found that CoO exhibits a change from the high temperature NaCl type structure to a tetragonal one below room temperature. Greenwald and Smart pointed out that this structural change is related to the magnetic order. A detailed measurement showed that the lattice constant c contracts with decreasing temperature, T , up to about 1%. Later, the low temperature crystal structure was found to be monoclinic, rather than tetragonal. A lattice distortion associated with a magnetic ordering is called the magnetostriction. Magnetostriction depends on the direction of the magnetic moments and should be distinguished from an exchange striction which does not depend on the moment direction. The magnetic structure of CoO was determined by Shull et al. The structure is such that Co moments in a ð111Þ plane point in the same direction with an antiparallel arrangement of moments in the neighboring ð111Þ planes. Roth determined the direction of the magnetic moments from a detailed neutron diffraction measurement on a powder sample of CoO. The spin direction is parallel to 1⁄2 1 17 which makes an angle 11 300 with the 1⁄2001 axis. Kanamori studied theoretically the magnetic anisotropy and the magnetostriction in CoO. The essential point in the Kanamori’s theory is that the orbital degrees of freedom of Co2þ ion are not quenched in a cubic crystal field and magnetic ordering induces a lattice distortion along the spin direction through the spin–orbit interaction. From a microscopic calculation, Kanamori was successful in explaining the lattice contraction along the spin direction. Despite these extensive studies, the magnetism of CoO, in particular under applied magnetic fields, remains enigmatic. One reason for this is that the exchange interaction is very strong, judging from the Neel temperature, and a conventional magnetic field is insufficient strength to study the field induced phenomena in CoO. In this paper, we report the results of synchrotron X-ray diffraction measurements on a powder sample of CoO under strong magnetic fields. Using an X-ray diffractometer in conjunction with a pulsed magnetic field, we measured the magnetic field dependence of the lattice constants of CoO in the magnetically ordered phase. We find the lattice constants expand with magnetic field, H. Because magnetic ordering in CoO strongly influences the lattice, so we expect that an applied magnetic field will induce a lattice distortion. Nakamichi measured macroscopic deformation of a single crystal of CoO up to 1.1 T using a strain gauge and interpreted the results as due to a movement of the antiferromagnetic domain walls. The present X-ray diffraction measurement gives a direct evidence that the phenomenon takes place at an atomic level. The X-ray diffraction experiment was performed at the beamline BL19LXU at SPring-8. The experimental setup and the method are described in ref. 9. The pulsed magnetic field was applied perpendicular to the scattering plane. The magnetic field dependence of the d spacing at the ð002Þ and ð110Þ reflections obtained from the analysis of the diffraction data is shown in Figs. 1 and 2, respectively. Since the difference between the tetragonal and the monoclinic structures of CoO below TN is small, 4) we use the tetragonal structure to discuss our results. In the tetragonal structure, dð002Þ 1⁄4 c=2 and dð110Þ 1⁄4 a= ffiffiffi

Weak ferromagnetism and exchange biasing in cobalt oxide nanoparticle systems

Cobalt oxide nanoparticle systems have been prepared by wet chemical processing involving the encapsulation of the nanoparticles by an organic ligand shell (oleic acid and oleylamine). CoO nanoparticles were easily prepared by this method, while the synthesis of the CoPt∕CoO nanocomposites was achieved using a two step polyol process. CoPt nanoparticles were first obtained by simultaneous reduction of cobalt acetate and platinum acetylacetonate and then used as seeds for the growth upon them of cobalt oxide using a second polyol process. The antiferromagnetic CoO nanoparticles, when field cooled to temperatures below 200K, show displacement of the magnetization curves (along the magnetization axis) characteristic of weak ferromagnetism phenomena that are attributed to the uncompensated surface magnetic moments. The transition temperature of the particles is considerably lower than the Néel temperature of CoO and it is followed by an upswing at low temperatures, which is attributed to spins that are loosely coupled to the antiferromagnetic core. In the CoPt∕CoO nanocomposites, magnetic measurements show the appearance of increased coercivity with respect to the as-prepared CoPt particles and unidirectional anisotropy (loop shift of Heb=1125Oe) at temperatures below 20K, as a result of exchange coupling between CoO and CoPt.

X-ray diffractometer combining synchrotron radiation and pulsed magnetic fields up to 40 T

A synchrotron X-ray diffractometer incorporating a pulsed field magnet for high fields up to 40 T has been developed and a detailed description of this instrument is reported. The pulsed field magnet is composed of two coaxial coils with a gap of 3 mm at the mid-plane for passage of the X-rays. The pixel detector PILATUS 100K is used to store the diffracted X-rays. As a test of this instrument, X-ray diffraction by a powder sample of the antiferromagnet CoO is measured below the Néel temperature. A field-dependent lattice distortion of CoO due to magnetostriction is observed up to 38 T.

Raman spectroscopy and magnetic properties of bulk ZnO:Co single crystal

We report on the investigation of structural and magnetic properties of the cobalt-based diluted magnetic semiconductor with wurtzite structure: Zn 1− x Co x O. Single crystals were grown by chemical vapour transport. From the Raman measurements in the temperature range 10–300 K on samples with Co concentration of 5%, we detect the presence of very small quantities of hcp-Co and also the presence of anti-ferromagnetic CoO impurities. This result is in agreement with the behaviour of magnetisation measurements under pulsed magnetic fields. High frequency–high field ESR reveals clearly the large crystal field splitting of the energy levels of isolated Co 2+ ions.

Enhancing Exchange Bias with Diluted Antiferromagnets

The exchange bias H(E) of coupled polycrystalline films of antiferromagnetic CoO and ferromagnetic Co was significantly enhanced by the systematic substitution of nonmagnetic Mg for Co in CoO. Samples in which either Co or Co(1-x)Mg(x)O were deposited first were investigated at temperatures from 10 to 300 K. With Co(1-x)Mg(x)O on the bottom, the increased interfacial uncompensated spin density of the single antiferromagnetic domain Co(1x)Mg(x)O crystallites produced the enhanced H(E). With Co on the bottom, a thin interfacial oxide layer was primarily responsible for the strongly increased H(E).

Reversing the Training Effect in Exchange BiasedCoO/CoBilayers

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) display an asymmetry in the first magnetization reversal process and training in the subsequent reversal processes. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in-plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.

Shining light on magnetic microstructures

The increasing interest in micro- and nano-magnetism requires new instruments to examine the collective behavior of laterally structured ferromagnetic (FM) thin films. We demonstrate the possibility to extend the established Bragg-MOKE technique, which measures the Kerr effect on off-specular diffracted polarized laser light to soft x-rays. The shorter wavelength, which allows investigating smaller structures, and the element selectivity of the resonant scattering make soft x-rays an attractive tool for studying patterned ferromagnetic heterostructures. We illustrate this technique on permalloy islands and on exchange biased iron islands on antiferromagnetic CoO as two model systems. The results so far are in agreement with other experimental techniques and show the feasibility of off-specular resonant magnetic soft x-ray scattering.

Positive Domain Wall Resistance of180°Néel Walls in Co Thin Films

We measured the intrinsic domain wall resistance (DWR) of 180 degrees Ne el walls in a polycrystalline Co thin film deposited on top of a patterned antiferromagnetic CoO template. After field cooling through the CoO blocking temperature, exchange bias induces a spatially modulated coercivity of the Co film, resulting in a periodic domain pattern with 180 degrees Ne el walls. The intrinsic DWR is determined unambiguously by using rotating magnetic fields that result in a reversible creation and annihilation of the Ne el walls. In contrast with earlier reports, the DWR is positive and in agreement with models based on the giant magnetoresistance mechanism. A reliable, quantitative determination of the DWR requires careful numerical evaluation of the anisotropic magnetoresistance effect.

Electrical transport study of exchange bias in patterned Co/CoO bilayers

A stripe pattern of antiferromagnetic CoO is used to locally exchange bias a ferromagnetic Co film. Magnetization and magnetoresistance measurements enable to investigate the resulting change in the magnetization and the magnetic hysteresis behavior. The results demonstrate that a regular antiparallel stripe domain pattern can be created. Moreover, it is possible to tune the shape of the magnetic hysteresis by properly choosing the geometry of the CoO template.

Magnetic structure of CoO studied by neutron and synchrotron x-ray diffraction

Below ${T}_{N}=289\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the antiferromagnet CoO exhibits magnetic long-range order described by a propagation vector along the trigonal axis, whereas the lattice is distorted monoclinicly along both the tetragonal and the trigonal axes. In order to elucidate this inconsistency, we performed neutron and synchrotron x-ray diffraction experiments on single crystals of CoO, and found a series of magnetic reflections represented by a propagation vector along the tetragonal axis below ${T}_{N}$. These results suggest an altered magnetic structure of CoO, which includes both the trigonal and the tetragonal propagation vectors, has a monoclinic symmetry, and which is in accordance with the distorted lattice symmetry.

Magneto-optical study of exchange-biased sputtered Co/CoO films

Abstract We investigate the magnetic properties of sputtered Co/CoO films. Field cooling the samples causes a negative shift of the hysteresis loops below a characteristic temperature, dependent on the growth conditions (choice of the substrate, bilayer thickness, oxidation procedure), which is sensibly lower than the Neel temperature of bulk antiferromagnetic CoO, indicating a granular structure of the oxide layer. However, the maximum observed value of the exchange bias field is larger than what obtained in well-controlled epitaxial films reported in the literature.

Magnetoresistance and magnetization reversal process of Co nanowires covered with Pt

The magnetoresistance (MR) of single Co nanowires of various width is investigated at low temperatures in magnetic fields up to 5 T. The in-plane longitudinal MR shows pronounced features at coercive fields indicating the magnetization reversal process. The nanowires are prepared by electron beam lithography. Some of the wires are covered with a 2 nm thick Pt layer, the others are uncovered. The MR behavior of the Pt covered Co nanowires shows significantly different behavior from that of the uncovered nanowires. This is interpreted mainly as arising from an oxidation of the Co surface leading to a formation of an antiferromagnetic CoO layer on top of the uncovered Co nanowires. The CoO layer hinders the magnetization reversal process by domain wall pinning, which is reflected by the MR behavior of these samples. In contrast, the MR behavior of the Pt covered Co nanowires shows no pinning effects. Thus, we conclude that covering the Co nanowires in situ with a 2 nm thick Pt layer prevents oxidation effects.

Domain formation in exchange biased Co/CoO bilayers

The magnetic behavior of exchange biased Co/CoO polycrystalline thin films has been investigated using magnetometry and magneto-optical (MO) imaging. For CoO layer thicknesses of about 30 Å, these films exhibit a strong training effect below the blocking temperature of 130 K. A sharp initial reversal of the magnetization of the FM Co layer after field-cooling is followed by S-shaped magnetization loops with reduced coercive fields. The MO images show that during the initial magnetization reversal the remagnetization front moves from the edge of the sample into the homogenously magnetized film, leaving behind an irregular pattern of domains of the order of 10 μm. These domains, once generated during the first reversal, do not expand or move on subsequent magnetization loops and can be erased only by heating above the blocking temperature. This suggests that the domains are related to domains in the antiferromagnetic CoO layer.

Anomalous magnetic properties of antiferromagnetic CoO nanoparticles

Antiferromagnetic CoO nanoparticles ranging from 10 to 80 nm were prepared by the sol–gel process. The morphology, size and structure of the particle were characterized by TEM microscopy, electron diffraction and X-ray diffraction. Magnetic properties were measured using VSM and MPMS magnetometers. The nanoparticles has an FCC structure with a lattice parameter of 4.258 Å. Anomalous magnetic properties, such as hysteresis, were observed in CoO nanoparticles comparing to the course grain materials. The coercive force increases as the particle size is reduced, but decreases when the size is less than 20 nm. The magnetization increases below 100 K for the nanoparticles. However, no significant shifts of Neel temperature were observed. This magnetic behavior could be interpreted in terms of a core-shell model, in combination with the magnetic interaction between particles.

Finite size effects on the moment and ordering temperature in antiferromagnetic CoO layers

The relationship between magnetic properties and microstructure of thin antiferromagnetic CoO layers in {CoO(X)/SiO 2 (50 A)} 2 5 multilayers has been investigated. The temperature decay of the thermoremanent moment, zero-field-cooled/field-cooled magnetization measurements, and specific heat were evaluated as indicators of the magnetic ordering temperature. The temperatures associated with each decreased slightly with decreasing CoO layer thickness from 100 to 30 A, but then exhibited a sharp decrease for CoO layer thickness below 20 A. This decrease has been previously observed, and was attributed to intrinsic finite size effects associated with broken magnetic bonds at the surfaces. In the present investigation, it was determined that the CoO layer was amorphous in these thinner layers, accounting for the dramatic drop in Neel temperature. For thicker CoO layers, all measures of magnetic ordering coincide, indicating a true Neel temperature, whereas they do not for the thinner films. The structural change of CoO from crystalline to amorphous also causes a significant change in the temperature dependence of the magnetization, due to an increased number of weakly coupled uncompensated spins.

Multi-scale analysis of the nanostructured granular solid CoO–Ag by TEM and EELS

Nanostructured granular solid consisting of antiferromagnetic CoO particles embedded in a metallic medium (Ag) has been investigated by both transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) on a multiple scale. Quantitative composition determination was carried out by energy dispersive X-ray spectroscopy (EDXS) for Ag and Co, and by EELS for oxygen and Co quantification. Because of the complex morphology of the indicated CoO–Ag granular system, only a detailed analysis of the spatial variation of the EELS signals can provide information on the particle size and distribution. A mean size of 5 nm for CoO particles has been evaluated. The oxidation state of Co has also been studied using fine structures on EELS spectra. CoO phase is clearly identified as the oxide but appears mixed with small metallic Co area.