Fenglong Wang

Piezoelectric control of magnetic anisotropy in the Ni0.46Zn0.54Fe2O4/Pb(Mg1/3Nb2/3)O3-PbTiO3 composite

A gate-controllable in-plane magnetic anisotropy with C2v symmetry was observed in a Ni0.46Zn0.54Fe2O4/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure. Detailed amplitude analysis reveals a linearly electric modulation in anisotropy energy that arises from a strain-mediated magnetoelectric coupling across the interface. In particular, an electrically-driven rotational in-plane magnetic easy axis and anisotropic-isotropic transition in NiZn ferrite film, respectively, enable possibilities for magnetization control in multiferroic devices.

Electric field controlled reversible magnetic anisotropy switching studied by spin rectification

In this letter, spin rectification was used to study the electric field controlled dynamic magnetic properties of the multiferroic composite which is a Co stripe with induced in-plane anisotropy deposited onto a Pb(Mg1∕3Nb2∕3)O3-PbTiO3 substrate. Due to the coupling between piezoelectric and magnetoelastic effects, a reversible in-plane anisotropy switching has been realized by varying the history of the applied electric field. This merit results from the electric hysteresis of the polarization in the nonlinear piezoelectric regime, which has been proved by a butterfly type electric field dependence of the in-plane anisotropy field. Moreover, the electric field dependent effective demagnetization field and linewidth have been observed at the same time.

Piezoelectric control of magnetic dynamics in Co/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure

A microstrip method with vector network analyzer was used to investigate electric field control of magnetic dynamic properties in Co/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure at room temperature. Under external electric field, the natural resonance frequency and permeability of the Co film were found to modulate between 1.8–2.8 GHz and 50–150, respectively. In addition, the in-plane uniaxial magnetic anisotropy field can also be electrically tuned from 54 to 170 Oe, while the ferromagnetic resonance field was substantially enhanced about 350 Oe as well. Such an improvement of magnetic anisotropy is desirable for effectively electric control of resonance frequency and permeability in low energy microwave devices.

Electric tuning of magnetization dynamics and electric field-induced negative magnetic permeability in nanoscale composite multiferroics

Steering magnetism by electric fields upon interfacing ferromagnetic (FM) and ferroelectric (FE) materials to achieve an emergent multiferroic response bears a great potential for nano-scale devices with novel functionalities. FM/FE heterostructures allow, for instance, the electrical manipulation of magnetic anisotropy via interfacial magnetoelectric (ME) couplings. A charge-mediated ME effect is believed to be generally weak and active in only a few angstroms. Here we present an experimental evidence uncovering a new magnon-driven, strong ME effect acting on the nanometer range. For Co92Zr8 (20 nm) film deposited on ferroelectric PMN-PT we show via ferromagnetic resonance (FMR) that this type of linear ME allows for electrical control of simultaneously the magnetization precession and its damping, both of which are key elements for magnetic switching and spintronics. The experiments unravel further an electric-field-induced negative magnetic permeability effect.

A non-volatile four-state magnetic memory in a Co/(011)Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure

A non-volatile four-state magnetic memory is achieved in a Co/(011)Pb(Mg1/3Nb2/3)O-3-PbTiO3 heterostructure. The in-plane magnetization of ferromagnetic Co film in the heterostructure can be controlled both electrically and magnetically. Electric field mediated magnetism is caused by piezostrain effect, which displays a stable non-volatile remnant magnetization vs electric field looplike behavior. In-plane strain-electric field (S-E) behavior under different temperatures reveals a nonvolatile strain switching effect, which is responsible for the non-volatile remnant magnetization switching through piezostrain mediated magnetoelectric effect. Further investigations on temperature dependence of S-E behavior suggest that the absent of the second non-180 degrees domain switching may be responsible for the asymmetry in strain curves that causes the non-volatile strain switching, and therefore causes the non-volatile remanent magnetization switching, which is crucial for the four-state magnetoelectric memory

Stripe domain and enhanced resonance frequency in ferrite doped FeNi films

Stripe domain (SD) structures of FeNi films were controlled by doping different concentrations of ferrite. With increasing concentration of doped ferrite, SD width decreases, which indicated that perpendicular magnetic anisotropy (H⊥) increases. SD disappears and bubble-like features are observed with doping ferrite at 32% due to large H⊥. The origin for the enhancement of H⊥ is exchange coupling between ferrimagnet and ferromagnet. Moreover, different doping concentrations of ferrite realize an increase of resonant frequency from 1.3 to 2.3 GHz. (Some figures may appear in colour only in the online journal)

Piezostrain tuning non-volatile 90 degrees magnetic easy axis rotation in Co2FeAl Heusler alloy film grown on Pb(Mg1/3Nb2/3) O-3-PbTiO3 heterostructures

Non-volatile electric field-based control of magnetic anisotropy in Co2FeAl/Pb(Mg1/3Nb2/3) O-3-PbTiO3 (CFA/PMN-PT) heterostructures is investigated at room temperature. The remnant magnetization response under different electric fields shows a asymmetric butterfly-like behavior; specifically, this behavior is consistent with the asymmetric butterfly-like piezostrain versus applied electric field curve. Thus electric field-induced non-volatile 90 degrees magnetic easy axis rotation can be attributed to the piezostrain effect. Further, the result measured by rotating-angle ferromagnetic resonance demonstrates piezostrain-mediated non-volatile 90 degrees magnetic easy axis rotation at the initial state and the two remnant polarization states after application of the poling fields of 10 and -10 kV cm(-1) turned off. The angular dependence of magnetic damping also indicates a 90 degrees phase shift at the above mentioned three different states. Additionally, the piezostrain-mediated non-volatile stable magnetization reversal in the two directions of easy and hard magnetization axes are observed under positive and negative pulsed electric fields, which can be used to improve the performance of low-loss multiple-state memory devices.

Electric field induced natural resonance and magnetic damping in FeCo/ Pb (Mg1/3Nb2/3) O3-PbTiO3 heterostructures

We demonstrated that magnetic damping and natural resonance frequency could be modulated by the electric field in an FeCo layer on Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) heterostructures by the microstrip method with a vector network analyzer. Strain mediated natural resonance increased from 0.8 to 2 GHz with increasing the electric field from 0 to 12.5 kV cm−1. Furthermore, the magnetic damping factor increased from 0.008 to 0.225, which resulted from magnetization precession under a microwave field and interface charge mediated spin current due to the spin screening effect. The results were also validated by the measurements of ferromagnetic resonance, which was also clear from analysis of the intrinsic and extrinsic contributions to the magnetic damping. In addition, in-plane uniaxial static magnetic anisotropy field was also modulated from 8 to 200 Oe with a different electric field.

Electric-field-induced angular dependence of magnetic anisotropy in a FeCo/Pb(Mg1/3Nb2/3)O3-PbTiO3 heterostructure

To understand the distribution of the in-plane magnetic anisotropy under a dc electric field, FeCo films deposited onto Pb(Mg1/3Nb2/3)O3-PbTiO3 (011)-orientated substrates by RF-magnetron sputtering were investigated. Vibrating sample magnetometer was performed and the occurrence of switching was demonstrated of the magnetization easy axis in FeCo films upon applying solely a dc electric field. A theoretical calculation was performed to provide a simplified account of the magnetoelastic contribution to the magnetic anisotropy. Quantification of the angular distribution of the magnetic anisotropy field under various electric fields was obtained, which can contribute to realizing low-loss electric-field-turning devices.

Enhanced microwave absorption in columnar structured magnetic materials

CoZr columnar structured magnetic films were fabricated by oblique sputtering onto porous aluminum oxide substrates with different oblique angles. The scanning electron microscope images showed the formation of columnar structure and nanoporous structure was disappeared with increasing oblique angle. The static magnetic properties showed larger coercivity and lower magnetization squareness due to columnar structure resulted from nanopores. Dynamic magnetic properties were determined by effect of holes and oblique sputtering. Enhanced microwave absorption was obtained by complex permeability measurement, which line width of columnar film increased 1.4 GHz comparing with 0.5 GHz of continuous film. Therefore, it is an effective way to get the adjustable anisotropy field and line width, which is desirable for obtaining the high resonance and high permeability ferromagnetic film materials for high frequency application.