Chunhui Dong

Observation of rotatable stripe domain in permalloy films with oblique sputtering

Stripe domain (SD) in obliquely sputtered permalloy films were investigated by comparing with normally sputtered ones. The critical thickness for SD formation of obliquely sputtered films was about 100 nm thinner than that of normally sputtered films. The hysteresis loops of obliquely sputtered films showed a peculiar shape. A rotation of SD towards easy axis was observed in the obliquely sputtered films, which was confirmed by permeability spectra under a bias field. The origin of the rotation could result from in-plane uniaxial anisotropy, which is induced by the shape effect of the oblique columnar growth of permalloy grains.

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.

Hydrothermal epitaxial growth and nonvolatile bipolar resistive switching behavior of LaFeO3-PbTiO3 films on Nb:SrTiO3(001) substrate

Epitaxial LaFeO3-PbTiO3 (LFPTO) thin films were hydrothermally grown on the Nb-SrTiO3 (100) (NSTO) substrates with a thickness about 250 nm. As fabricated Pt/LFPTO/NSTO/Pt devices exhibit reversible bipolar resistive switching behavior. The resistance ratios between high resistance state and low resistance state exceed three orders of magnitude, which can be maintained over 6 h without observable degradation. It indicates that the Pt/LFPTO/NSTO/Pt devices reveal excellent data retention and endurance characteristics. The resistive switching mechanism of the device could be attributed to the trap-controlled space-charge-limited current conduction which is controlled by the localized oxygen vacancies in the films. Furthermore, variation of Pt/LFPTO Schottky junction depletion thickness and barriers height modulated by oxygen vacancies at Pt/LFPTO interface was suggested to be responsible for the resistance switching behaviors of the devices.

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 field tuning of non-volatile three-state magnetoelectric memory in FeCo-NiFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures

We demonstrate electric field impulse-induced reversible tristable magnetization switching in FeCo-NiFe2O4/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) heterostructures at room temperature. The magnetic properties of the FeCo-NiFe2O4 film can be changed reversibly by the strain-mediated magnetoelectric coupling effect. Three piezostrain-mediated reversible and stable electric resistance states were obtained in the FeCo-NiFe2O4 film when different electric field impulses were applied, including large positive and negative fields and an impulse that was smaller than the electric coercive field. Consequently, reversible electric field impulse tuning of the tristable resistance state, which is related to the different magnetization switching properties of the materials, was realized. These results provide a promising approach for low loss multistate magnetoelectric memory devices for information storage applications.

Growth, structure, morphology, and magnetic properties of Ni ferrite films

The morphology, structure, and magnetic properties of nickel ferrite (NiFe2O4) films fabricated by radio frequency magnetron sputtering on Si(111) substrate have been investigated as functions of film thickness. Prepared films that have not undergone post-annealing show the better spinel crystal structure with increasing growth time. Meanwhile, the size of grain also increases, which induces the change of magnetic properties: saturation magnetization increased and coercivity increased at first and then decreased. Note that the sample of 10-nm thickness is the superparamagnetic property. Transmission electron microscopy displays that the film grew with a disorder structure at initial growth, then forms spinel crystal structure as its thickness increases, which is relative to lattice matching between substrate Si and NiFe2O4.

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

Polarization-induced resistive switching behaviors in complex oxide heterostructures

Complex oxide heterostructures are fabricated by growing La0.67Ca0.33MnO3 films on ferroelectric 0.71Pb(Mg1/3Nb2/3)O3-0.29PbTiO3 (011) single-crystal substrates. The nonvolatile or pulsed resistive switching behaviors induced by an electric field are achieved simultaneously. Further analyses indicate that the different resistive switching behaviors are resulted from co-control of piezostrain and polarization current effects. With decreasing in-plane read current from 0.1 mA to 0.001 mA, the polarization current effect gradually begins to play a more important role than the piezostrain effect. Consequently, the nonvolatile resistive switching behavior is converted to pulse resistive switching behavior. The results further enhance the application of complex oxides in multifunctional memory devices.

The resistive switching memory of CoFe2O4 thin film using nanoporous alumina template

A novel conductive process for resistive random access memory cells is investigated based on nanoporous anodized aluminum oxide template. Bipolar resistive switching characteristic is clearly observed in CoFe2O4 thin film. Stable and repeatable resistive switching behavior is acquired at the same time. On the basis of conductive filament model, possible generation mechanisms for the resistive switching behaviors are discussed intensively. Besides, the magnetic properties of samples (before and after the annealing process) are characterized, and the distinct changes of magnetic anisotropy and coercive field are detected. The present results provide a new perspective to comprehend the underlying physical origin of the resistive switching effect.PACS68.37.-d; 73.40.Rw; 73.61.-r

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.