Manipulating the Magnetic Structure of Co Core/CoO Shell Nanoparticles: Implications for Controlling the Exchange Bias

Abstract

We present an experimental study of the effects of oxidation on the magnetic and crystal structures of exchange biased epsilon-Co/CoO core-shell nanoparticles. Transmission electron microscopy measurements reveal that oxidation creates a Co-CoO interface which is highly directional and epitaxial in quality. Neutron diffraction measurements find that below a Néel temperature TN of approximately 235 K the magnetization of the CoO shell is modulated by two wave vectors, q1=(1/2 1/2 1/2)2pi/a and q2=(100)2pi/a. Oxidation affects the q1 component of the magnetization very little, but hugely enhances the q2 component, resulting in the magnetic decompensation of the core-shell interface. We propose that the large exchange bias effect results from the highly ordered interface between the Co core and CoO shell, and from enhanced core-shell coupling by the uncompensated interface moment.

Co nanoparticles inserted into a porous carbon amorphous matrix: the role of cooling field and temperature on the exchange bias effect

“…2 we show the room temperature XRD pattern of the Co-NPs embedded in the AC matrix, where poorly defined crystalline diffraction peaks superimposed to large haloes coming from the predominant (90 %) amorphous carbon matrix are seen. This feature was previously found in other 35 magnetic AC composites. 9,15 All the observed peaks can be indexed as the Bragg reflections of metallic cobalt with face centered cubic (fcc) crystal structure ( The large contribution of the carbon matrix and the low intensity ratio between the diffraction peaks and the background, makes the Rietveld analysis of the XRD pattern very difficult (see Fig.…”

“…[7][8][9] From the fundamental point of view, magnetic NPs serve as model systems for investigating the Stoner-Wohlfarth, and the Néel-Brown models, to study finite-size effects or magnetic proximity effects. [10][11][12][13] In particular, most of the core-shell 35 systems for potential applications have been prepared from oxidation of transition metal NPs, leading to interface exchange interactions between the core (typically ferromagnetic, FM) and the shell (antiferromagnetic, AFM or ferrimagnetic FIM). 14,15 This core-shell morphology often 40 gives rise to striking and independent mechanisms like the hysteresis-loop shift (the so called exchange bias, EB, effect) and the coercivity (H c ) enhancement.…”

“…36, 37 Therefore, the maximum seen in the FC and the EB effect could be 25 associated with the transition into a frozen state of the spinglass (SG) Co-oxide shell. 38 In contrast, at higher temperatures, the curve exhibits a broad maximum centered at 220 K, which could be a signature of a 35 progressive unfrozen process of the magnetic moments in the oxide SG shell. 39 The fact that the M ZFC (T) and M FC (T) curves do not overlap in the whole temperature range of the measurements, together with the existence of hysteresis in the M(H), curves at 340 K, 40 imply that the system has not reached the fully SPM regime.…”

Low-energy magnetic excitations in Co/CoO core/shell nanoparticles

“…The range of correlations is further shortened by the nanoparticle form factor [44][45][46] , and in many cases by the disordering of the nanoparticle surface, sometimes accompanied by the formation of long ranged defect structures 42 . When these considerations are extended to the magnetic structure, the net result is that magnetic Bragg peaks are substantially wider than their bulk analogs and the diffuse scattering background is enhanced 47 . The Scherrer formula is useful for approximating an average length scale from a diffraction peak for nanoparticles 48 , but to obtain more detailed information about their structural modifications requires a more sophisticated treatment, such as the Pair Distribution Function approach 49 .…”

Core-Shell Magnetic Morphology of Structurally Uniform Magnetite Nanoparticles

“…While it is tempting to assume that the moments are uniformly aligned within each nanoparticle, their spin configurations are typically far more complex [1]. When the particles contain both ferromagnetic and antiferromagnetic phases, exchange bias effects may occur [2][3][4][5]. Surface disorder can lead to a magnetically dead layer or a spin glasslike phase [6][7][8][9].…”