Nanoscale Magnetic Arrays through Block Copolymer Templating of Polyoxometalates.

Magnetic nanoarrays promise to enable new energy-efficient computations based on spintronics or magnonics. In this work, we present a block copolymer-assisted strategy for fabricating ordered magnetic nanostructures on silicon and permalloy substrates. Block copolymer micelle-like structures were used as a template in which polyoxometalate (POM) clusters could assemble in an opal-like structure. A combination of microscopy and scattering techniques was used to confirm the structural and organizational features of the fabricated materials. The magnetic properties of these materials were investigated by polarized neutron reflectometry, nuclear magnetic resonance, and magnetometry measurements. The data show that a magnetic structural design was achieved and that a thin layer of patterned POMs strongly influenced an underlying permalloy layer. This work demonstrates that the bottom-up pathway is a potentially viable method for patterning magnetic substrates on a sub-100 nm scale, toward the magnetic nanostructures needed for spintronic or magnonic crystal devices.

Gigantic Anisotropy of Self-Induced Spin-Orbit Torque in Weyl Ferromagnet Co2MnGa.

Spin-orbit torque (SOT) is receiving tremendous attention from both fundamental and application-oriented aspects. Co2MnGa, a Weyl ferromagnet that is in a class of topological quantum materials, possesses cubic-based high structural symmetry, the L21 crystal ordering, which should be incapable of hosting anisotropic SOT in conventional understanding. Here we show the discovery of a gigantic anisotropy of self-induced SOT in Co2MnGa. The magnitude of the SOT is comparable to that of heavy metal/ferromagnet bilayer systems, despite the high inversion symmetry of the Co2MnGa structure. More surprisingly, a sign inversion of the self-induced SOT is observed for different crystal axes. This finding stems from the interplay of the topological nature of the electronic states and their strong modulation by external strain. Our research enriches the understanding of the physics of self-induced SOT and demonstrates a versatile method for tuning SOT efficiencies in a wide range of materials for topological and spintronic devices.

Shaping Perpendicular Magnetic Anisotropy of Co2MnGa Heusler Alloy Using Ion Irradiation for Magnetic Sensor Applications.

Magnetic sensors are key elements in many industrial, security, military, and biomedical applications. Heusler alloys are promising materials for magnetic sensor applications due to their high spin polarization and tunable magnetic properties. The dynamic field range of magnetic sensors is strongly related to the perpendicular magnetic anisotropy (PMA). By tuning the PMA, it is possible to modify the sensing direction, sensitivity and even the accuracy of the magnetic sensors. Here, we report the tuning of PMA in a Co2MnGa Heusler alloy film via argon (Ar) ion irradiation. MgO/Co2MnGa/Pd films with an initial PMA were irradiated with 30 keV 40Ar+ ions with fluences (ions·cm-2) between 1 × 1013 and 1 × 1015 Ar·cm-2, which corresponds to displacement per atom values between 0.17 and 17, estimated from Monte-Carlo-based simulations. The magneto optical and magnetization results showed that the effective anisotropy energy (Keff) decreased from ~153 kJ·m-3 for the un-irradiated film to ~14 kJ·m-3 for the 1 × 1014 Ar·cm-2 irradiated film. The reduced Keff and PMA are attributed to ion-irradiation-induced interface intermixing that decreased the interfacial anisotropy. These results demonstrate that ion irradiation is a promising technique for shaping the PMA of Co2MnGa Heusler alloy for magnetic sensor applications.

Geometric Frustration and Long-Range Ordering Induced by Surface Pressure Oscillation in a Langmuir-Blodgett Monolayer of Magnetic Soft Spheres.

As a step toward the bottom-up construction of magnonic systems, this paper demonstrates the use of a large-amplitude surface-pressure annealing technique to generate 2-D order in a Langmuir-Blodgett monolayer of magnetic soft spheres comprising a surfactant-encapsulated polyoxometalate. The films show a distorted square lattice interpreted as due to geometric frustration caused by 2-D confinement between soft walls, one being the air interface and the other the aqueous subphase. Hysteresis and relaxation phenomena in the 2-D layers are suggested to be due to folding and time-dependent interpenetration of surfactant chains.

Hybrid magnetic iron oxide nanoparticles with tunable field-directed self-assembly.

We describe the synthesis of hybrid magnetic ellipsoidal nanoparticles that consist of a mixture of two different iron oxide phases, hematite (α-Fe2O3) and maghemite (γ-Fe2O3), and characterize their magnetic field-driven self-assembly. We demonstrate that the relative amount of the two phases can be adjusted in a continuous way by varying the reaction time during the synthesis, leading to strongly varying magnetic properties of the particles. Not only does the saturation magnetization increase dramatically as the composition of the spindles changes from hematite to maghemite, but also the direction of the induced magnetic moment changes from being parallel to the short axis of the spindle to being perpendicular to it. The magnetic dipolar interaction between the particles can be further tuned by adding a screening silica shell. Small-angle X-ray scattering (SAXS) experiments reveal that at high magnetic field, magnetic dipole-dipole interaction forces the silica coated particles to self-assemble into a distorted hexagonal crystal structure at high maghemite content. However, in the case of uncoated maghemite particles, the crystal structure is not very prominent. We interpret this as a consequence of the strong dipolar interaction between uncoated spindles that then become arrested during field-induced self-assembly into a structure riddled with defects.

Isolation and Characterization of a Bismuth(II) Radical.

More than 80 years after Paneth's report of dimethyl bismuth, the first monomeric Bi(II) radical that is stable in the solid state has been isolated and characterized. Reduction of the diamidobismuth(III) chloride Bi(NON(Ar))Cl (NON(Ar)=[O(SiMe2NAr)2](2-); Ar=2,6-iPr2C6H3) with magnesium affords the Bi(II) radical ˙Bi(NON(Ar)). X-ray crystallographic measurements are consistent with a two-coordinate bismuth in the +2 oxidation state with no short intermolecular contacts, and solid-state SQUID magnetic measurements indicate a paramagnetic compound with a single unpaired electron. EPR and density functional calculations show a metal-centered radical with >90% spin density in a p-type orbital on bismuth.