Nicholas S. Bingham

Magnetocaloric effect and refrigerant capacity in Sm1−xSrxMnO3 (x = 0.42, 0.44, 0.46) manganites

Sm1−xSrxMnO3 (SSMO) with x ∼ 0.44 shows a multi-critical phase diagram which includes antiferromagnetic, charge ordering, and ferromagnetic transitions that give rise to large magnetoresistive and magnetocaloric effects. In this study, we systematically investigated the magnetic transitions, magnetocaloric effect (MCE), and refrigerant capacity (RC) in polycrystalline Sm1−xSrxMnO3 (x = 0.42, 0.44, 0.46) samples prepared by a standard ceramic method. Magnetization measurements and Arrott plot analyses reveal a first-order ferromagnetic transition for all samples investigated, with TC = 130 K, 134 K, and 133 K for x = 0.42, 0.44, and 0.46, respectively. The largest peak entropy change of −ΔSM = 4.61 J/kg K for a field change of μ0ΔH = 5 T is observed for the x = 0.44 composition around its Curie temperature of 134 K. After correcting for hysteretic losses of ∼15% – 30% engendered by the first order nature of the transition, sizable RC values of 151.42 J/kg, 140.15 J/kg, and 135.91 J/kg are still observed fo...

Nanocolumnar interfaces and enhanced magnetic coercivity in preferentially oriented cobalt ferrite thin films grown using oblique-angle pulsed laser deposition.

Highly textured cobalt ferrite (CFO) thin films were grown on Si (100) substrates using oblique-angle pulsed laser deposition (α-PLD). X-ray diffraction and in-depth strain analysis showed that the obliquely deposited CFO films had both enhanced orientation in the (111) crystal direction as well as tunable compressive strains as a function of the film thicknesses, in contrast to the almost strain-free polycrystalline CFO films grown using normal-incidence PLD under the same conditions. Using in situ optical plume diagnostics the growth parameters in the α-PLD process were optimized to achieve smoother film surfaces with roughness values as low as 1-2 nm as compared to the typical values of 10-12 nm in the normal-incidence PLD grown films. Cross-sectional high resolution transmission electron microscope images revealed nanocolumnar growth of single-crystals of CFO along the (111) crystallographic plane at the film-substrate interface. Magnetic measurements showed larger coercive fields (∼10 times) with similar saturation magnetization in the α-PLD-grown CFO thin films as compared to those deposited using normal-incidence PLD. Such significantly enhanced magnetic coercivity observed in CFO thin films make them ideally suited for magnetic data storage applications. A growth mechanism based on the atomic shadowing effect and strain compression-relaxation mechanism was proposed for the obliquely grown CFO thin films.

Influence of microstructure and interfacial strain on the magnetic properties of epitaxial Mn3O4/La0.7Sr0.3MnO3 layered-composite thin films

Epitaxial Mn3O4/La0.7Sr0.3MnO3 (Mn3O4/LSMO) bilayer thin films were grown on lattice-matched single crystal substrates of SrTiO3 (STO) (100) and MgO (100), with Mn3O4 as the top layer, using a pulsed laser deposition technique. X-ray diffraction (XRD) patterns revealed the single crystalline nature and epitaxial relationship between the layers. A detailed analysis of strains using XRD asymmetric/symmetric scans indicated an increasing in-plane compressive strain in the LSMO layer with increasing thicknesses of the Mn3O4 layer, resulting in a tetragonal distortion of the LSMO lattice in the Mn3O4/LSMO films in comparison to the tensile strains in LSMO single-layer films grown on both STO and MgO substrates. Cross-sectional high resolution transmission electron microscope (HRTEM) images showed atomically sharp interfaces in all films. However, as opposed to a flat interface between LSMO and STO, the Mn3O4 and LSMO interface was undulating and irregular in the bilayer films. Magnetic measurements revealed th...

Challenges in the stoichiometric growth of polycrystalline and epitaxial PbZr0.52Ti0.48O3/La0.7Sr0.3MnO3 multiferroic heterostructures using pulsed laser deposition

High quality polycrystalline and epitaxial PbZr0.52Ti0.48O3/La0.7Sr0.3MnO3 (PZT/LSMO) multiferroic thin films were deposited on single-crystal Si (100) and SrTiO3 (STO) (100) substrates using pulsed laser deposition (PLD) technique. The deposition conditions were optimized to overcome some of the challenges during the growth of stoichiometric PZT/LSMO thin films (with LSMO as the bottom layers). The major setback of the preferential evaporation of Pb during the ablation of PZT target, which leads to the growth of non-stoichiometric, Pb-deficient PZT thin films with poor ferroelectric properties, was investigated by studying the laser-target interaction sites and intensified charge-coupled detector (ICCD) imaging of the laser-ablated plumes. X-ray studies revealed that the PZT/LSMO heterostructures deposited under the optimum conditions were highly crystalline. Atomic force microscope images showed uniform grain growth with surface roughness values as low as 1.6 nm. In- and out-of-plane magnetization measu...

Ziz-zag interface and strain-influenced ferromagnetism in epitaxial Mn3O4/La0.7Sr0.3MnO3 thin films grown on SrTiO3 (100) substrates

Epitaxial Mn3O4/La0.7Sr0.3MnO3 (Mn3O4/LSMO) bilayer thin films were grown on single crystal SrTiO3 (STO) (100) substrates, with Mn3O4 as the top layer, using pulsed laser deposition technique. X-ray diffraction revealed the single crystalline nature and epitaxial relationship between the layers, as well as increased lattice distortion of LSMO in the bilayer structure relative to a single layer film of LSMO. Cross-sectional high resolution transmission electron microscope images showed atomically sharp interfaces, but in contrast to the almost flat interface between LSMO and STO substrate, the Mn3O4 and LSMO interface showed a zig-zag structure. Magnetic measurements indicated that the high temperature in situ deposition of a Mn3O4 layer on top of the LSMO film significantly reduced the magnetization of the LSMO film, probably due to the formed ziz-zag interface between LSMO and Mn3O4.

Emergent dynamic chirality in a thermally driven artificial spin ratchet

Modern nanofabrication techniques have opened the possibility to create novel functional materials, whose properties transcend those of their constituent elements. In particular, tuning the magnetostatic interactions in geometrically frustrated arrangements of nanoelements called artificial spin ice can lead to specific collective behaviour, including emergent magnetic monopoles, charge screening and transport, as well as magnonic response. Here, we demonstrate a spin-ice-based active material in which energy is converted into unidirectional dynamics. Using X-ray photoemission electron microscopy we show that the collective rotation of the average magnetization proceeds in a unique sense during thermal relaxation. Our simulations demonstrate that this emergent chiral behaviour is driven by the topology of the magnetostatic field at the edges of the nanomagnet array, resulting in an asymmetric energy landscape. In addition, a bias field can be used to modify the sense of rotation of the average magnetization. This opens the possibility of implementing a magnetic Brownian ratchet, which may find applications in novel nanoscale devices, such as magnetic nanomotors, actuators, sensors or memory cells.

Magnetic properties of strained multiferroic CoCr2O4: A soft x-ray study

Using resonant soft x-ray techniques we follow the magnetic behavior of a strained epitaxial film of CoCr2O4, a type-II multiferroic. The film is [ 110] oriented, such that both the ferroelectric and ferromagnetic moments can coexist in-plane. X-ray magnetic circular dichroism (XMCD) is used in scattering and in transmission modes to probe the magnetization of Co and Cr separately. The transmission measurements utilized x-ray excited optical luminescence from the substrate. Resonant soft x-ray diffraction (RXD) was used to study the magnetic order of the low temperature phase. The XMCD signals of Co and Cr appear at the same ordering temperature TC approximate to 90 K, and are always opposite in sign. The coercive field of the Co and of Cr moments is the same, and is approximately two orders of magnitude higher than in bulk. Through sum rules analysis an enlarged Co2+ orbital moment (m(L)) is found, which can explain this hardening. The RXD signal of the (q q 0) reflection appears below T-S, the same ordering temperature as the conical magnetic structure in bulk, indicating that this phase remains multiferroic under strain. To describe the azimuthal dependence of this reflection, a slight modification is required to the spin model proposed by the conventional Lyons-Kaplan-Dwight-Menyuk theory for magnetic spinels.

Computational logic with square rings of nanomagnets

Nanomagnets are a promising low-power alternative to traditional computing. However, the successful implementation of nanomagnets in logic gates has been hindered so far by a lack of reliability. Here, we present a novel design with dipolar-coupled nanomagnets arranged on a square lattice to (i) support transfer of information and (ii) perform logic operations. We introduce a thermal protocol, using thermally active nanomagnets as a means to perform computation. Within this scheme, the nanomagnets are initialized by a global magnetic field and thermally relax on raising the temperature with a resistive heater. We demonstrate error-free transfer of information in chains of up to 19 square rings and we show a high level of reliability with successful gate operations of ∼94% across more than 2000 logic gates. Finally, we present a functionally complete prototype NAND/NOR logic gate that could be implemented for advanced logic operations. Here we support our experiments with simulations of the thermally averaged output and determine the optimal gate parameters. Our approach provides a new pathway to a long standing problem concerning reliability in the use of nanomagnets for computation.