C. Daboo

Bio: C. Daboo is an academic researcher from University of Cambridge. The author has contributed to research in topics: Anisotropy & Magnetization. The author has an hindex of 16, co-authored 39 publications receiving 1008 citations.

Continuous evolution of the in‐plane magnetic anisotropies with thickness in epitaxial Fe films

Abstract: We have studied the evolution of the magnetic in‐plane anisotropy in epitaxial Fe/GaAs films of both (001) and (110) orientation as a function of the Fe layer thickness using the longitudinal magneto‐optic Kerr effect and Brillouin light scattering. Magnetization curves which are recorded in situ during film growth reveal a continuous change of the net anisotropy axes with increasing film thickness. This behavior can be understood to arise from the combination of a uniaxial and a cubic in‐plane magnetic anisotropy which are both thickness dependent. Structural analysis of the substrate and Fe film surfaces provides insight into the contribution of atomic steps at the interfaces to the magnetic anisotropy. Changing the degree of crystalline order at the Fe–GaAs interface allows us to conclude that the magnetic anisotropies are determined by atomic scale order.

Anisotropy and orientational dependence of magnetization reversal processes in epitaxial ferromagnetic thin films

Abstract: We have undertaken a detailed study of the macroscopic and microscopic magnetization reversal processes in epitaxial ferromagnetic thin films with varying cubic and uniaxial magnetocrystalline anisotropy strengths. The macroscopic magnetization reversal processes were observed with in-plane magneto-optic Kerr effect (MOKE) vector magnetometry as a function of the relative anisotropy strengths and on the orientation of the applied field with respect to the anisotropy directions. Measurements of the component of magnetization in the plane of the sample and perpendicular to the applied field allow a precise determination of the relative orientation of the hard and easy in-plane anisotropy axes. This can be used to accurately determine the ratio of uniaxial to cubic anisotropy constants, when the ratio is less than one. The ratios obtained from MOKE agree well with those obtained by Brillouin light scattering (BLS). MOKE vector magnetometry reveals loop features that can be associated with either one or two irreversible jumps in the direction of the magnetization, depending sensitively on the anisotropy ratio and the orientation of the applied field. Minimum-energy calculations predict that the reversal process should proceed by a continuous rotation of the magnetization vector with either one or two irreversible jumps between single-domain states, depending on the applied field orientation and the nature of the anisotropy of the film. The calculations provide a good qualitative description of the observed reversal process, although the magnetic microstructure influences the exact values of the switching fields.

Size dependence of the magnetoresistance in submicron FeNi wires

Abstract: The field‐dependent magnetoresistance (MR) characteristic and magnetic hysteresis behavior has been studied in 300–500 A thickness Ni80Fe20 wires of variable width (w) in the range from 0.2 to 10 μm. As the width of the wire decreases, a marked increase in the easy axis coercive field is seen for fields applied along the wire axis and the form of the MR characteristic is markedly modified for the in‐plane perpendicular hard axis direction with a large field‐dependent MR response observed for applied field strengths exceeding the edge demagnetizing field. The low field hard axis results are discussed in terms of an inhomogeneous spin configuration across the width of the wires arising from the spatial variation of the demagnetizing field.

Optimized process for the fabrication of mesoscopic magnetic structures

Abstract: We have used the advantage of the high etch selectivity between metals in a wet etching process to develop an optimized technique for engineering magnetic materials. This method is based on electron beam lithography and optimized pattern transfer by a combination of dry and wet etching. The technique has been used in fabricating mesoscopic Ni80Fe20 dots and wires with lateral dimensions down to 0.2 μm. We have used scanning electron microscopy to verify the lateral sizes and edge acuity of the structures. The magnetic properties were characterized using magneto-optic Kerr effect and magnetoresistance measurements. A marked increase in the coercive field and the saturation field is seen as the width of the wire is decreased. The magnetoresistance change (∂R) is found to increase significantly as the width of the wire is decreased.

Micromagnetism of Epitaxial Fe(001) Elements on the Mesoscale

Abstract: The size and orientation dependent micromagnetic structures of epitaxial Fe(001) thin film elements with in-plane anisotropy are reported. A transition from single domain to multidomain remanent states is observed upon reducing the element size beneath $\ensuremath{\sim}50\ensuremath{\mu}\mathrm{m}$, indicating that the in-plane dipolar field becomes competitive with the magnetocrystalline anisotropy at this size. Because of this competition, distinct micromagnetic structures arise according to the orientation of the element edges. The epitaxial elements are of high structural quality allowing the micromagnetic behavior to be controllably modified.

Breathing: Rhythmicity, plasticity, chemosensitivity

Abstract: ▪ Abstract Breathing is a vital behavior that is particularly amenable to experimental investigation. We review recent progress on three problems of broad interest. (i) Where and how is respiratory rhythm generated? The preBotzinger Complex is a critical site, whereas pacemaker neurons may not be essential. The possibility that coupled oscillators are involved is considered. (ii) What are the mechanisms that underlie the plasticity necessary for adaptive changes in breathing? Serotonin-dependent long-term facilitation following intermittent hypoxia is an important example of such plasticity, and a model that can account for this adaptive behavior is discussed. (iii) Where and how are the regulated variables CO2 and pH sensed? These sensors are essential if breathing is to be appropriate for metabolism. Neurons with appropriate chemosensitivity are spread throughout the brainstem; their individual properties and collective role are just beginning to be understood.

Magnetism in ultrathin film structures

Abstract: In this paper, we review some of the key concepts in ultrathin film magnetism which underpin nanomagnetism. We survey the results of recent experimental and theoretical studies of well characterized epitaxial structures based on Fe, Co and Ni to illustrate how intrinsic fundamental properties such as the magnetic exchange interactions, magnetic moment and magnetic anisotropies change markedly in ultrathin films as compared with their bulk counterparts, and to emphasize the role of atomic scale structure, strain and crystallinity in determining the magnetic properties. After introducing the key length scales in magnetism, we describe the 2D magnetic phase transition and survey studies of the thickness dependent Curie temperature and the critical exponents which characterize the paramagnetic–ferromagnetic phase transition. We next discuss recent experimental and theoretical results on the determination of the exchange constant, followed by an overview of measurements of the magnetic moment in the elemental 3d transition metal thin films in the various crystal phases that have been successfully stabilized, thereby illustrating the sensitivity of the magnetic moment to the local symmetry and to the atomic environment. Finally, we discuss briefly the magnetic anisotropies of Fe, Co and Ni in the fcc crystalline phase, to emphasize the role of structure and the details of the interface in influencing the magnetic properties. The dramatic effect that adsorbates can have on the magnetic anisotropies of thin magnetic films is also discussed. Our survey demonstrates that the fundamental properties, namely, the magnetic moment and magnetic anisotropies of ultrathin films have dramatically different behaviour compared with those of the bulk while the comparable size of the structural and magnetic contributions to the total energy of ultrathin structures results in an exquisitely sensitive dependence of the magnetic properties on the film structure.

Property variation with shape in magnetic nanoelements

Abstract: Nanometre scale magnetic particles (`nanoelements' or `nanomagnets') form a rich and rapidly growing new area in condensed matter physics, with many potential applications in data storage technology and magnetic field sensing. This paper reviews an extensive study into the influence of shape on the properties of nanomagnets in the size range 35-500 nm. Elliptical, triangular, square, pentagonal and circular geometries have all been considered. It is shown that the size, thickness and geometric shape of nanomagnets all play a vital role in determining the magnetic properties. The shape, size and thickness of a nanomagnet are shown to be linked to its magnetic properties by two distinct phenomena. The first is called configurational anisotropy and describes the role played by small deviations from uniformity in the magnetization field within the nanostructures, which allow unexpected higher-order anisotropy terms to appear. These anisotropies can often dominate the magnetic properties. The second is the competition which exists between exchange energy and magnetostatic energy. This competition determines whether the nanomagnets exhibit single domain or incoherent magnetization and also controls the non-uniformities in magnetization which lead to configurational anisotropy. Understanding the influence of shape opens the way to designing new nanostructured magnetic materials where the magnetic properties can be tailored to a particular application with a very high degree of precision.