Tatiana Eggers

Strong room-temperature ferromagnetism in VSe 2 monolayers on van der Waals substrates

Reduced dimensionality and interlayer coupling in van der Waals materials gives rise to fundamentally different electronic1, optical2 and many-body quantum3–5 properties in monolayers compared with the bulk. This layer-dependence permits the discovery of novel material properties in the monolayer regime. Ferromagnetic order in two-dimensional materials is a coveted property that would allow fundamental studies of spin behaviour in low dimensions and enable new spintronics applications6–8. Recent studies have shown that for the bulk-ferromagnetic layered materials CrI3 (ref. 9) and Cr2Ge2Te6 (ref. 10), ferromagnetic order is maintained down to the ultrathin limit at low temperatures. Contrary to these observations, we report the emergence of strong ferromagnetic ordering for monolayer VSe2, a material that is paramagnetic in the bulk11,12. Importantly, the ferromagnetic ordering with a large magnetic moment persists to above room temperature, making VSe2 an attractive material for van der Waals spintronics applications.Reducing the dimensionality of paramagnetic VSe2 results in the emergence of ferromagnetism that is observed in a monolayer and up to room temperature.

Relating surface roughness and magnetic domain structure to giant magneto-impedance of Co-rich melt-extracted microwires

Understanding the relationship between the surface conditions and giant magneto-impedance (GMI) in Co-rich melt-extracted microwires is key to optimizing their magnetic responses for magnetic sensor applications. The surface magnetic domain structure (SMDS) parameters of ~45 μm diameter Co69.25Fe4.25Si13B13.5-xZrx (xu2009=u20090, 1, 2, 3) microwires, including the magnetic domain period (d) and surface roughness (Rq) as extracted from the magnetic force microscopy (MFM) images, have been correlated with GMI in the range 1–1000u2009MHz. It was found that substitution of B with 1 at. % Zr increased d of the base alloy from 729 to 740u2009nm while retaining Rq from ~1u2009nm to ~3u2009nm. A tremendous impact on the GMI ratio was found, increasing the ratio from ~360% to ~490% at an operating frequency of 40u2009MHz. Further substitution with Zr decreased the high frequency GMI ratio, which can be understood by the significant increase in surface roughness evident by force microscopy. This study demonstrates the application of the domain period and surface roughness found by force microscopy to the interpretation of the GMI in Co-rich microwires.

Enhancement of Giant Magneto-Impedance in Series Co-Rich Microwires for Low-Field Sensing Applications

Two soft ferromagnetic Co68.25Fe4.25Si12.25B15.25 microwires with the same diameter of 50xa0±xa01xa0μm but different fabrication processes were placed in series and in parallel circuit configurations to investigate their giant magneto-impedance (GMI) responses in a frequency range of 1–100xa0MHz for low-field sensing applications. We show that, while the low-field GMI response is significantly reduced in the parallel configuration, it is greatly enhanced in the series connection. These results suggest that a highly sensitive GMI sensor can be designed by arranging multi-wires in a saw-shaped fashion to optimize the sensing area, and soldered together in series connection to maintain the excellent magnetic field sensitivity.

Tailoring circular magnetic domain structure and high frequency magneto-impedance of melt-extracted Co69.25Fe4.25Si13B13.5 microwires through Nb doping

The surface roughness, surface magnetic domain structure (SMDS), and high frequency magneto-impedance (MI) response of melt-extracted Co69.25Fe4.25Si13B13.5 microwires with 1 at.% Nb substitution for B have been studied by atomic force microscopy (AFM), magnetic force microscopy (MFM), and impedance analyzer, respectively. We show that the Nb doping significantly increases the domain width from 729 to 1028 nm, while preserving the low surface roughness (∼2 nm) of the base composition. As a result, a greater improvement of the high frequency MI response (∼300%/Oe at 20 MHz) is achieved in the Nb-doped microwire. A well-defined circumferential anisotropy formed with Nb-substitution is key to a highly sensitive MI field sensor.The surface roughness, surface magnetic domain structure (SMDS), and high frequency magneto-impedance (MI) response of melt-extracted Co69.25Fe4.25Si13B13.5 microwires with 1 at.% Nb substitution for B have been studied by atomic force microscopy (AFM), magnetic force microscopy (MFM), and impedance analyzer, respectively. We show that the Nb doping significantly increases the domain width from 729 to 1028 nm, while preserving the low surface roughness (∼2 nm) of the base composition. As a result, a greater improvement of the high frequency MI response (∼300%/Oe at 20 MHz) is achieved in the Nb-doped microwire. A well-defined circumferential anisotropy formed with Nb-substitution is key to a highly sensitive MI field sensor.