Large magnetoelectric effects mediated by electric-field-driven nanoscale phase transformations in sputtered (nanoparticulate) and electrochemically dealloyed (nanoporous) Fe-Cu films

Large magnetoelectric effects are observed in as-sputtered (nanoparticulate-like) and electrochemically dealloyed (nanoporous) 200 nm thick Fe-Cu films. Large magnetoelectric effects are observed in as-sputtered (nanoparticulate-like) and electrochemically dealloyed (nanoporous) 200 nm thick Fe-Cu films. Application of positive voltages decreases both the saturation magnetization (M ) and coercivity (H ) of the films, while negative voltages cause the reverse effect (increase of M and H ). The relative variations are as high as 20% for M and beyond 100% for H , both for the as-sputtered and dealloyed states. These changes in magnetic properties are caused by controlled and reversible electric-field-driven nanoscale phase transformations between face-centered cubic (fcc) and body-centered cubic (bcc) structures. These phase transitions are in turn due to selective redox reactions induced by the applied voltage, which can be regarded as a "magnetoionic effect." The controlled tuning of H and M with the moderate values of applied voltage, together with the sustainable composition of the investigated alloys (not containing noble metals, as opposed to many previous works on magnetoelectric effects in thin films), pave the way towards the implementation of magnetic and spintronic devices with enhanced energy efficiency and functionalities

Magneto-ionic control of magnetism in two-oxide nanocomposite thin films comprising mesoporous cobalt ferrite conformally nanocoated with HfO2

Advances in nanotechnology require of robust methods to fabricate new types of nanostructured materials whose properties can be controlled at will using simple procedures. Nanoscale composites can benefit from actuation protocols that involve mutual interfacial interactions on the nanoscale. Herein, a method to create nanoscale composite thin films consisting of mesoporous cobalt ferrite (CFO) whose pore walls are nanocoated with HfO2 is presented. Porous CFO films are first prepared by sol-gel. Atomic layer deposition is subsequently used to conformally grow a HfO2 layer at the surface of the pore walls, throughout the thickness of the films. The magnetic properties of uncoated and HfO2-coated CFO mesoporous films are then modulated by applying external voltage, via magneto-ionic effects. The CFO-HfO2 composite films exhibit an enhanced magnetoelectric response. The magnetic moment at saturation of the composite increases 56% upon the application of −50 V (compared to 24% for CFO alone). Furthermore, dissimilar trends in coercivity are observed: after applying −50 V, the coercivity of the composite film increases by 69% while the coercivity of the CFO alone decreases by 25%. The effects can be reversed applying suitable positive voltages. This two-oxide nanocomposite material differs from archetypical magneto-ionic architectures, in which voltage-driven ion migration is induced between fully-metallic and oxide counterparts. The synthesized material is particularly appealing to develop new types of magnetoelectric devices with a highly tunable magnetic response

Secondary nucleating sequences affect kinetics and thermodynamics of tau aggregation

Tau protein was scanned for highly amyloidogenic sequences in amphiphilic motifs (X)nZ, Z(X)nZ (n≥2) or (XZ)n (n≥2), where X is a hydrophobic residue and Z is a charged or polar residue. N-acetyl peptides homologous to these sequences were used to study aggregation. Transmission electron microscopy (TEM) showed 7 peptides, in addition to well known primary nucleating sequences c275VQIINK (AcPHF6*) and Ac306VQIVYK (AcPHF6), formed fibers, tubes, ribbons or rolled sheets. Of the peptides shown by TEM to form amyloid, Ac10VME, AcPHF6*, Ac375KLTFR, and Ac393VYK were found to enhance the fraction of β-structure of AcPHF6 formed at equilibrium, and Ac375KLTFR was found to inhibit AcPHF6 and AcPHF6* aggregation kinetics in a dose-dependent manner, consistent with its participation in a hybrid steric zipper model. Single site mutants were generated which transformed predicted amyloidogenic sequences in tau into non-amyloidogenic ones. A M11K mutant had fewer filaments and showed a decrease in aggregation kinetics and an increased lag time compared to wild type tau, while a F378K mutant showed significantly more filaments. Our results infer that sequences throughout tau, in addition to PHF6 and PHF6*, can seed amyloid formation or affect aggregation kinetics or thermodynamics

Reversible, electric-field induced magneto-tonic control of magnetism in mesoporous cobalt ferrite thin films

The magnetic properties of mesoporous cobalt ferrite films can be largely tuned by the application of an electric field using a liquid dielectric electrolyte. By applying a negative voltage, the cobalt ferrite becomes reduced, leading to an increase in saturation magnetization of 15% (M) and reduction in coercivity (H) between 5-28%, depending on the voltage applied (−10 V to −50 V). These changes are mainly non-volatile so after removal of −10 V M remains 12% higher (and H 5% smaller) than the pristine sample. All changes can then be reversed with a positive voltage to recover the initial properties even after the application of −50 V. Similar studies were done on analogous films without induced porosity and the effects were much smaller, underscoring the importance of nanoporosity in our system. The different mechanisms possibly responsible for the observed effects are discussed and we conclude that our observations are compatible with voltage-driven oxygen migration (i.e., the magneto-ionic effect)

Structural and magnetic properties of FexCu1-x sputtered thin films electrochemically treated to create nanoporosity for high-surface-area magnetic components

Sputter deposition is a facile and widely used technique for fabricating thin-film materials. Electrochemical dealloying, on the other hand, is a promising method for creating nanoporosity, and therefore increasing surface area, in metallic materials. Surprisingly, little work has been done on the application of electrochemical dealloying to sputter-deposited thin films. Here, we prepare FexCu1-x thin films by sputter deposition to be then electrochemically treated to create porosity. We investigate the structural and magnetic properties of as-sputtered and electrochemically treated films. We find that the morphology, crystal structure, and magnetic properties are highly dependent on initial film composition. For high copper content films (Fe29Cu71), relative Cu content is found to decrease during the dealloying process. For these films, the crystal structure is not greatly affected by the induced porosity and the porous films show increased saturation magnetization. However, for the more Fe-rich compositions (Fe63Cu37), we find that Fe is preferentially lost and making the films nanoporous induces a crystal structure change from body-centered cubic (bcc) to a mixture of face-centered cubic (fcc) and bccphases. These same porous films show a decrease in saturation magnetization and a large increase in coercivity compared to the as-sputtered films. These films are attractive as high-surface-area magnetic components because of the tunability of their magnetic properties and their high surface area due to porosity. To the best of our knowledge, these results constitute the first example of nanoporous, magnetic thin films by prepared by sputtering and subsequent electrochemical treatment

Coercivity Modulation in Fe-Cu Pseudo-Ordered Porous Thin Films Controlled by an Applied Voltage : A Sustainable, Energy-Efficient Approach to Magnetoelectrically Driven Materials

Fe-Cu films with pseudo-ordered, hierarchical porosity are prepared by a simple, two-step procedure that combines colloidal templating (using sub-micrometer-sized polystyrene spheres) with electrodeposition. The porosity degree of these films, estimated by ellipsometry measurements, is as high as 65%. The resulting magnetic properties can be controlled at room temperature using an applied electric field generated through an electric double layer in an anhydrous electrolyte. This material shows a remarkable 25% voltage-driven coercivity reduction upon application of negative voltages, with excellent reversibility when a positive voltage is applied, and a short recovery time. The pronounced reduction of coercivity is mainly ascribed to electrostatic charge accumulation at the surface of the porous alloy, which occurs over a large fraction of the electrodeposited material due to its high surface-area-to-volume ratio. The emergence of a hierarchical porosity is found to be crucial because it promotes the infiltration of the electrolyte into the structure of the film. The observed effects make this material a promising candidate to boost energy efficiency in magnetoelectrically actuated devices

Tailoring the Magnetic and Magnetoelectric Properties of Nanostructured Materials Using Solution-Phase Methods

Magnetic nanomaterials are an important and widely studied class of materials with a wide variety of applications. The work presented here is aimed at both developing techniques to control the nanoscale structure of these materials and understanding the relationship between that structure and the overall material properties. The techniques used here are primarily solution-phase methods which offer a high degree of control and versatility. The first part of this work is focused on thin films of magnetic oxide materials which are particularly applicable to radio frequency (RF) devices. Here, both sol-gel and nanocrystal precursors are used to create thin films where the film composition, grain size and porosity are controllably tuned. We then investigated both the static and dynamic magnetic properties of the films to better understand how the nanoscale structure impacts the overall properties. These investigations provide valuable insights that can allow us to design materials with properties tailored to meet the requirements of individual devices. Importantly, these insights are applicable to a wide variety of magnetic materials and are not limited to the specific materials studied here.The second part of this work is focused on metallic alloy nanocrystals which have potential applications as elements in high density data storage devices. First, in chapter 5, the magnetoelectric properties of FePd nanocrystals is investigated. FePd is a good candidate for use in magnetoelectric memory devices which are highly energy efficient. By using nanocrystals of FePd, we hope to find a route to potentially reducing bit size in those devices which can lead to increased data storage densities. Then, in chapter 6, we move on to explore shape effects by looking at FePt nanorods. FePt has a very high magnetic anisotropy which in memory devices translates to increased bit stability and potentially allows for smaller bit sizes. In nanorods, shape anisotropy can enhance the already high magnetic anisotropy to create even stronger nanomagnets

Magneto-ionic control of magnetism in two-oxide nanocomposite thin films comprising mesoporous cobalt ferrite conformally nanocoated with HfO2

Advances in nanotechnology require of robust methods to fabricate new types of nanostructured materials whose properties can be controlled at will using simple procedures. Nanoscale composites can benefit from actuation protocols that involve mutual interfacial interactions on the nanoscale. Herein, a method to create nanoscale composite thin films consisting of mesoporous cobalt ferrite (CFO) whose pore walls are nanocoated with HfO2 is presented. Porous CFO films are first prepared by sol–gel. Atomic layer deposition is subsequently used to conformally grow a HfO2 layer at the surface of the pore walls, throughout the thickness of the films. The magnetic properties of uncoated and HfO2-coated CFO mesoporous films are then modulated by applying external voltage, via magneto-ionic effects. The CFO–HfO2 composite films exhibit an enhanced magnetoelectric response. The magnetic moment at saturation of the composite increases 56% upon the application of −50 V (compared to 24% for CFO alone). Furthermore, dissimilar trends in coercivity are observed: after applying −50 V, the coercivity of the composite film increases by 69% while the coercivity of the CFO alone decreases by 25%. The effects can be reversed applying suitable positive voltages. This two-oxide nanocomposite material differs from archetypical magneto-ionic architectures, in which voltage-driven ion migration is induced between fully-metallic and oxide counterparts. The synthesized material is particularly appealing to develop new types of magnetoelectric devices with a highly tunable magnetic response.This work was supported by the European Research Council under the SPIN-PORICS 2014-Consolidator Grant, Agreement No. 648454, and MAGIC-SWITCH 2019-Proof of Concept Grant, Agreement No. 875018, the Generalitat de Catalunya (2017-SGR-292, 2017-SGR-1519) and the Spanish Government (MAT2017-86357-C3-1-R, MAT 2017-83169-R associated FEDER). P.Y acknowledges China Scholarship Council for CSC fellowship No. 201606920073.Peer reviewe

Magneto-ionic control of magnetism in two-oxide nanocomposite thin films comprising mesoporous cobalt ferrite conformally nanocoated with HfO2

Advances in nanotechnology require of robust methods to fabricate new types of nanostructured materials whose properties can be controlled at will using simple procedures. Nanoscale composites can benefit from actuation protocols that involve mutual interfacial interactions on the nanoscale. Herein, a method to create nanoscale composite thin films consisting of mesoporous cobalt ferrite (CFO) whose pore walls are nanocoated with HfO2 is presented. Porous CFO films are first prepared by sol-gel. Atomic layer deposition is subsequently used to conformally grow a HfO2 layer at the surface of the pore walls, throughout the thickness of the films. The magnetic properties of uncoated and HfO2-coated CFO mesoporous films are then modulated by applying external voltage, via magneto-ionic effects. The CFO-HfO2 composite films exhibit an enhanced magnetoelectric response. The magnetic moment at saturation of the composite increases 56% upon the application of −50 V (compared to 24% for CFO alone). Furthermore, dissimilar trends in coercivity are observed: after applying −50 V, the coercivity of the composite film increases by 69% while the coercivity of the CFO alone decreases by 25%. The effects can be reversed applying suitable positive voltages. This two-oxide nanocomposite material differs from archetypical magneto-ionic architectures, in which voltage-driven ion migration is induced between fully-metallic and oxide counterparts. The synthesized material is particularly appealing to develop new types of magnetoelectric devices with a highly tunable magnetic response