Xiaoshan Zhang

Mechanically tunable magnetic properties of Fe81Ga19 films grown on flexible substrates

We investigated on magnetic properties of magnetostrictive Fe81Ga19 films grown on flexible polyethylene terephthalate (PET) substrates under various mechanical strains. The unstrained Fe81Ga19 films exhibit a significant uniaxial magnetic anisotropy due to a residual stress in PET substrates. It was found that the squareness of hysteresis loops can be tuned by an application of strains, inward/compressive or outward/tensile bending of the films. A modified Stoner-Wohlfarth model with considering a distribution of easy axes in polycrystalline films was developed to account for the mechanically tunable magnetic properties in flexible Fe81Ga19 films. These results provide an alternative way to tune mechanically magnetic properties, which is particularly important for developing flexible magnetoelectronic devices.

Positive temperature coefficient of magnetic anisotropy in polyvinylidene fluoride (PVDF)-based magnetic composites

The magnetic anisotropy is decreased with increasing temperature in normal magnetic materials, which is harmful to the thermal stability of magnetic devices. Here, we report the realization of positive temperature coefficient of magnetic anisotropy in a novel composite combining β-phase polyvinylidene fluoride (PVDF) with magnetostrictive materials (magnetostrictive film/PVDF bilayer structure). We ascribe the enhanced magnetic anisotropy of the magnetic film at elevated temperature to the strain-induced anisotropy resulting from the anisotropic thermal expansion of the β-phase PVDF. The simulation based on modified Stoner-Wohlfarth model and the ferromagnetic resonance measurements confirms our results. The positive temperature coefficient of magnetic anisotropy is estimated to be 1.1 × 102 J m−3 K−1. Preparing the composite at low temperature can enlarge the temperature range where it shows the positive temperature coefficient of magnetic anisotropy. The present results may help to design magnetic devices with improved thermal stability and enhanced performance.

Static and high frequency magnetic properties of FeGa thin films deposited on convex flexible substrates

In this study, we provided an approach of pre-strained growth to significantly enhance the FMR frequency of magnetic films. A series of 100 nm magnetostrictive FeGa films were grown on flexible polyethylene terephthalate (PET) substrates which were bowed onto the convex molds with different curvature radii. When the FeGa films were flatten to a plane, they were suffered a compressive strain arisen from the shaped substrates. As shown in Figs . 1(a) and 1(b), the FeGa films exhibit a remarkable uniaxial anisotropy perpendicular to the compressive strain. The strength of magnetic anisotropy is enhanced with increasing the compressive strain from 0 to 0 .78%. The permeability spectra measured in the frequency range from 0 .5 to 8 GHz are shown in Figs . 1(c) and 1(d). With the increase of the compressive-strain induced magnetic anisotropy, the initial permeability μi of the FeGa films inconspicuously decreases, while the FMR frequency shifts toward the higher values. In order to quantitatively understanding the effect of strain on the dynamic properties of the FeGa thin films, the experimentally obtained permeability spectra can be fitted by using the Landau-Lifshitz-Gilbert (LLG) theory.

Effect of buffer layer and external stress on magnetic properties of flexible FeGa films

We systematically investigated the effect of a Ta buffer layer and external stress on the magnetic properties of magnetostrictive Fe81Ga19 films deposited on flexible polyethylene terephthalate (PET) substrates. The Ta buffer layers could effectively smoothen the rough surface of PET. As a result, the FeGa films grown on Ta buffer layers exhibit a weaker uniaxial magnetic anisotropy and lower coercivity, as compared to those films directly grown on PET substrates. By inward and outward bending the FeGa/Ta/PET samples, external in-plane compressive and tensile stresses were applied to the magnetic films. Due to the inverse magnetostrictive effect of FeGa, both the coercivity and squareness of hysteresis loops for FeGa/Ta films could be well tuned under various strains.

Controllable strain-induced uniaxial anisotropy of Fe81Ga19 films deposited on flexible bowed-substrates

We propose a convenient method to induce a uniaxial anisotropy in magnetostrictive Fe81Ga19 films grown on flexible polyethylene terephthalate (PET) substrates by bending the substrate prior to deposition. A tensile/compressive stress is induced in the Fe81Ga19 films when PET substrates are shaped from concave/convex to flat after deposition. The stressed Fe81Ga19 films exhibit a significant uniaxial magnetic anisotropy due to the internal stress arising from changes in shape of PET substrates. The easy axis is along the tensile stress direction and the coercive field along easy axis is increased with increasing the internal tensile stress. The remanence of hard axis is decreased with increasing the compressive stress, while the coercive field is almost unchanged. A modified Stoner-Wohlfarth model with considering the distribution of easy axes in polycrystalline films is used to account for the magnetic properties tuned by the strain-controlled magnetoelastic anisotropy in flexible Fe81Ga19 films. Our inv...

Effect of mechanical strain on magnetic properties of flexible exchange biased FeGa/IrMn heterostructures

We have fabricated flexible exchange biased heterostructures with magnetostrictive Fe81Ga19 alloy as the ferromagnetic layer and Ir20Mn80 as the antiferromagnetic layer on polyethylene terephthalate substrates. The mechanical strain can modify both the strength and the orientation of the uniaxial anisotropy, giving rise to the switching between the easy and hard magnetization directions. Different from the previously reported works on rigid exchange biased systems, a drastic decrease in exchange bias field was observed under a compressive strain with magnetic field parallel to the pinning direction, but only a slightly decrease was shown under a tensile strain. Based on a Stoner-Wohlfarth model calculation, we suggested that the distributions of both ferromagnetic and antiferromagnetic anisotropies be the key to induce the mechanically tunable exchange bias.

Fieldlike spin-orbit torque in ultrathin polycrystalline FeMn films

Field-like spin orbit torque in FeMn/Pt bilayers with ultra-thin polycrystalline FeMn has been characterized through planar Hall effect measurements. A large effective field is obtained for FeMn in the thickness range of 2 to 5 nm. The experimental observations can be reasonably accounted for by using a macro-spin model under the assumption that the FeMn layer is composed of two spin sublattices with unequal magnetizations. The large effective field corroborates the spin Hall origin of the effective field considering the much smaller uncompensated net moments in FeMn as compared to NiFe. The effective absorption of spin current by FeMn is further confirmed by the fact that spin current generated by Pt in NiFe/FeMn/Pt trilayers can only travel through the FeMn layer with a thickness of 1 to 4 nm. By quantifying the field-like effective field induced in NiFe, a spin diffusion length of 2 nm is estimated in FeMn, in consistence with values reported in literature by ferromagnetic resonance and spin-pumping experiments.

In-plane anisotropic converse magnetoelectric coupling effect in FeGa/polyvinylidene fluoride heterostructure films

We investigated the converse magnetoelectric (CME) effect in the Fe81Ga19/polyvinylidene fluoride (PVDF) heterostructure films. A weak in-plane uniaxial magnetic anisotropy was observed in the as-deposited magnetostrictive FeGa films. When a positive (negative) electric field is applied on the ferroelectric PVDF substrates, both the coercivity and the squareness of magnetic hysteresis loops of FeGa films for the magnetic field parallel to the easy axis become larger (smaller), but for the magnetic field parallel to the hard axis the coercivity and the remanence get smaller (larger), indicating an anisotropic CME effect in FeGa/PVDF heterostructure films.

Thermally assisted electric field control of magnetism in flexible multiferroic heterostructures.

Thermal and electrical control of magnetic anisotropy were investigated in flexible Fe81Ga19 (FeGa)/Polyvinylidene fluoride (PVDF) multiferroic heterostructures. Due to the large anisotropic thermal deformation of PVDF (α1 = −13 × 10−6 K−1 and α2 = −145 × 10−6 K−1), the in-plane uniaxial magnetic anisotropy (UMA) of FeGa can be reoriented 90° by changing the temperature across 295 K where the films are magnetically isotropic. Thus, the magnetization of FeGa can be reversed by the thermal cycling between 280 and 320 K under a constant magnetic field lower than coercivity. Moreover, under the assistance of thermal deformation with slightly heating the samples to the critical temperature, the electric field of ± 267 kV cm−1 can well align the UMA along the two orthogonal directions. The new route of combining thermal and electrical control of magnetic properties realized in PVDF-based flexible multiferroic materials shows good prospects in application of flexible thermal spintronic devices and flexible microwave magnetic materials.

Instability analysis of a programmed hydrogel plate under swelling

We investigate the bifurcation instabilities of swelling circular hydrogel plates programed with a space-varying cross-linking density along the radial direction. Depending on the distribution of the cross-linking density and the aspect ratio of the plate, different buckling patterns are observed. When the cross-linking density is lower at the center and increases monotonically outwards, a global buckling occurs, and the hydrogel plate forms an axisymmetric bowl-shaped container. When the cross-linking density is higher at the center and decreases monotonically outwards, edge wrinkling occurs. These wrinkles can be either localized near the edge or penetrate deep into the plate, depending on the detailed distribution of the cross-linking density. The rich post-buckling morphologies exhibited by the programed hydrogel plate provide a powerful route to fabricate smart hydrogel components, such as, self-adjusting containers and microlenses.