U. Khan

Electrical control over perpendicular magnetization switching driven by spin-orbit torques

Flexible control of magnetization switching by electrical manners is crucial for applications of spin-orbitronics. Besides of a switching current that is parallel to an applied field, a bias current that is normal to the switching current is introduced to tune the magnitude of effective damping-like and field-like torques and further to electrically control magnetization switching. Symmetrical and asymmetrical control over the critical switching current by the bias current with opposite polarities is both realized in Pt/Co/MgO and

Structural and Magnetic Response in Bimetallic Core/Shell Magnetic Nanoparticles

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Temperature-Dependent Magnetic Response of Antiferromagnetic Doping in Cobalt Ferrite Nanostructures

-Ta/CoFeB/MgO systems, respectively. This research not only identifies the influences of field-like and damping-like torques on switching process but also demonstrates an electrical method to control it.

Structural and Magnetic Properties of Mn/Fe co-Doped ZnO Thin Films Prepared by Sol–Gel Technique

Bimagnetic monodisperse CoFe2O4/Fe3O4 core/shell nanoparticles have been prepared by solution evaporation route. To demonstrate preferential coating of iron oxide onto the surface of ferrite nanoparticles X-ray diffraction (XRD), High resolution transmission electron microscope (HR-TEM) and Raman spectroscopy have been performed. XRD analysis using Rietveld refinement technique confirms single phase nanoparticles with average seed size of about 18 nm and thickness of shell is 3 nm, which corroborates with transmission electron microscopy (TEM) analysis. Low temperature magnetic hysteresis loops showed interesting behavior. We have observed large coercivity 15.8 kOe at T = 5 K, whereas maximum saturation magnetization (125 emu/g) is attained at T = 100 K for CoFe2O4/Fe3O4 core/shell nanoparticles. Saturation magnetization decreases due to structural distortions at the surface of shell below 100 K. Zero field cooled (ZFC) and Field cooled (FC) plots show that synthesized nanoparticles are ferromagnetic till room temperature and it has been noticed that core/shell sample possess high blocking temperature than Cobalt Ferrite. Results indicate that presence of iron oxide shell significantly increases magnetic parameters as compared to the simple cobalt ferrite.

Magnetic fingerprint of interfacial coupling between CoFe and nanoscale ferroelectric domain walls

In this work MnxCo1−xFe2O4 nanoparticles (NPs) were synthesized using a chemical co-precipitation method. Phase purity and structural analyses of synthesized NPs were performed by X-ray diffractometer (XRD). Transmission electron microscopy (TEM) reveals the presence of highly crystalline and narrowly-dispersed NPs with average diameter of 14 nm. The Fourier transform infrared (FTIR) spectrum was measured in the range of 400–4000 cm−1 which confirmed the formation of vibrational frequency bands associated with the entire spinel structure. Temperature-dependent magnetic properties in anti-ferromagnet (AFM) and ferromagnet (FM) structure were investigated with the aid of a physical property measurement system (PPMS). It was observed that magnetic interactions between the AFM (Mn) and FM (CoFe2O4) material arise below the Neel temperature of the dopant. Furthermore, hysteresis response was clearly pronounced for the enhancement in magnetic parameters by varying temperature towards absolute zero. It is shown that magnetic properties have been tuned as a function of temperature and an externally-applied field.

Experimental investigation and micromagnetic simulations of hybrid CoCr2O4/Ni coaxial nanostructures

Mn/Fe co-doped ZnO thin films are prepared by simple sol-gel and spin coating method. Five different sols with the change in concentration (1-5 wt%) of both Mn and Fe are synthesized. Molar ratio of Mn and Fe is kept constant, i.e., 1:1. Sols are spun onto glass and copper substrates by spin coating method followed by the post magnetic field annealing at 300 °C for 1 h. Effect of Mn and Fe codoping on ferromagnetic properties of ZnO is reported in this paper. Structural and magnetic properties of as prepared and annealed samples are investigated by X-ray diffractometer (XRD) and vibrating sample magnetometer (VSM). Scanning electron microscope is used to study the surface morphology of co-doped films. XRD results show incorporation of Mn and Fe in the host lattice upto a dopant concentration of 4 wt%. However, small crystallites of Mn and Fe2O3 are observed by increasing the dopant concentration to 5 wt%. VSM results indicate room temperature ferromagnetism in all samples without the presence of any secondary phases. Low value of shape anisotropy is observed in the case of Mn doped ZnO. However, no shape anisotropy is observed in the case of co-doped thin films. Moreover, Mn/Fe co-doped thin films show magnetic hysteresis equivalent to that of multilayered structure, indicating that such complex structures used in spintronic devices can be replaced by a single ZnO layer with codoping of Mn and Fe.

Richardson-Schottky transport mechanism in ZnS nanoparticles

Magnetoelectric coupling in ferromagnetic/multiferroic systems is often manifested in the exchange bias effect, which may have combined contributions from multiple sources, such as domain walls, chemical defects, or strain. In this study we magnetically “fingerprint” the coupling behavior of CoFe grown on epitaxial BiFeO3 (BFO) thin films by magnetometry and the first-order-reversal-curves (FORC). The contribution to exchange bias from 71°, 109° and charged ferroelectric domain walls (DWs) was elucidated by the FORC distribution. CoFe samples grown on BFO with 71° DWs only exhibit an enhancement of the coercivity, but little exchange bias. Samples grown on BFO with 109° DWs and mosaic DWs exhibit a much larger exchange bias, with the main enhancement attributed to 109° and charged DWs. Based on the Malozemoff random field model, a varying-anisotropy model is proposed to account for the exchange bias enhancement. This work sheds light on the relationship between the exchange bias effect of the CoFe/BFO heterointerface and the ferroelectric DWs, and provides a path for multiferroic device analysis and design.

Fabrication, structural and magnetic properties of electrodeposited Fe 80 Pt 20 nanowires and nanotubes

Multiphase CoCr2O4/Ni core-shell nanowires (NWs) have been synthesized within anodic aluminum oxide membranes by the combination of the sol-gel method with electrodeposition techniques. X-ray diffraction and x-ray photoemission spectroscopy results confirmed the formation of a cubic spinel structure of CoCr2O4 shell with space group Fd-3m (227). The morphology and composition of the as-grown NWs were studied by field emission scanning electron microscopy, as well as transmission electron microscopy. The magnetic properties of the CoCr2O4 NT shell and hybrid CoCr2O4/Ni NWs were measured at low temperature using a physical property measurement system. The temperature dependence of the magnetization curves showed that CoCr2O4 NTs undergo a transition from a paramagnetic state to a ferrimagnetic state at about 90 K and a spiral ordering transition temperature near 22 K. An enhanced coercivity and saturation field were observed for the CoCr2O4/Ni core-shell NWs compared to the single-phase Ni NWs. Micromagnetic simulation results indicated that there is a strong coupling between the shell and core layers during the magnetization reversal process. The combination of hard CoCr2O4 and soft Ni in a single NW structure may have potential applications in future multifunctional devices.

Influence of manganese substitution on structural and magnetic properties of CoFe2O4 nanoparticles

We report the synthesis and electrical transport mechanism in ZnS semiconductor nanoparticles. Temperature dependent direct current transport measurements on the compacts of ZnS have been performed to investigate the transport mechanism for temperature ranging from 300 K to 400 K. High frequency dielectric constant has been used to obtain the theoretical values of Richardson-Schottky and Poole-Frenkel barrier lowering coefficients. Experimental value of the barrier lowering coefficient has been calculated from conductance-voltage characteristics. The experimental value of barrier lowering coefficient βexp lies close to the theoretical value of Richardson-Schottky barrier lowering coefficient βth,RS showing Richardson-Schottky emission has been responsible for conduction in ZnS nanoparticles for the temperature range studied.

Influence of cobalt doping on structural and magnetic properties of BiFeO3 nanoparticles

Summary form only given. Magnetic nanostructures have attracted a great deal of attention during the last decade because of their prospective applications not only in microwave absorption, high density recording media, and biosensor applications but also in functional micro and nanodevices. Recently, significant investigations have been motivated to get excess of 1Tbit/in2 in magnetic storage devices but at smaller bit size superparamagnetic limit arises. The process of DC electrodeposition has been employed to synthesize highly ordered Fe80Pt20 nanowires (NWs) and nanotubes (NTs) in anodic aluminum oxide (AAO) templates with average pore diameter of about 200 nm. The structural and magnetic properties of nanostructures has been investigated before and after simple and magnetic field annealing. A significant influence of magnetic field annealing with field strength of 1 T has been observed in the direction parallel and perpendicular to NWs and NTs axis respectively. X-Ray Diffraction analysis showed face centered cubic (fcc) as the dominant phase for Fe80Pt20 NWs and post annealing led to better crystallinity with retained fcc phase. On the other hand, Fe80Pt20 NTs shows amorphous behavior before and after simple and magnetic field annealing. Furthermore, magnetic properties, including saturation magnetization (Ms), squareness (Mr/Ms), and coercivity (Hc), have been investigated as a function of annealing temperature by Vibrating Sample Magnetometer. In conclusion, behavior of coercivity (Hc) was based on prolate ellipsoid chain model. Microstructural and magnetic properties strongly correlate with each other.