Baohe Li

Electric field control of deterministic current-induced magnetization switching in a hybrid ferromagnetic/ferroelectric structure

All-electrical and programmable manipulations of ferromagnetic bits are highly pursued for the aim of high integration and low energy consumption in modern information technology. Methods based on the spin-orbit torque switching in heavy metal/ferromagnet structures have been proposed with magnetic field, and are heading toward deterministic switching without external magnetic field. Here we demonstrate that an in-plane effective magnetic field can be induced by an electric field without breaking the symmetry of the structure of the thin film, and realize the deterministic magnetization switching in a hybrid ferromagnetic/ferroelectric structure with Pt/Co/Ni/Co/Pt layers on PMN-PT substrate. The effective magnetic field can be reversed by changing the direction of the applied electric field on the PMN-PT substrate, which fully replaces the controllability function of the external magnetic field. The electric field is found to generate an additional spin-orbit torque on the CoNiCo magnets, which is confirmed by macrospin calculations and micromagnetic simulations.

Spin-orbit torque in Pt/CoNiCo/Pt symmetric devices

Current induced magnetization switching by spin-orbit torques offers an energy-efficient means of writing information in heavy metal/ferromagnet (FM) multilayer systems. The relative contributions of field-like torques and damping-like torques to the magnetization switching induced by the electrical current are still under debate. Here, we describe a device based on a symmetric Pt/FM/Pt structure, in which we demonstrate a strong damping-like torque from the spin Hall effect and unmeasurable field-like torque from Rashba effect. The spin-orbit effective fields due to the spin Hall effect were investigated quantitatively and were found to be consistent with the switching effective fields after accounting for the switching current reduction due to thermal fluctuations from the current pulse. A non-linear dependence of deterministic switching of average Mz on the in-plane magnetic field was revealed, which could be explained and understood by micromagnetic simulation.

Electromigration induced fast L10 ordering phase transition in perpendicular FePt films

Realizing fast L10 ordering phase transition (LOPT) in L10 structured magnetic materials without heat treatment is crucial for their applications in spintronic devices. This article reports on the electromigration controlled momentum transfer and rapid ordering of Fe and Pt atoms in the as-deposited FePt films. Lattice defects in the films provide enough diffusion pathways and allow the Fe and Pt atoms rearranging. Through the current driven atomic motion and rearrangement, fast LOPT can result in the establishment of perpendicular magnetic anisotropy of the FePt films at room temperature. This effect is expected to work with other L10 typed magnetic materials for spintronic devices development.

Magnetic properties and microstructure of L10-FePt/AlN perpendicular nanocomposite films

Based on interfacial manipulation of a MgO (100) substrate and non-magnetic AlN compound, L10-FePt/AlN perpendicular nanocomposite films were designed and prepared. Systematic studies on magnetic properties and microstructure of the films show that the MgO substrate controls crystal orientation of the FePt lattice and induces perpendicular magnetic anisotropy (PMA). The AlN compound helps to control the island growth mode and acts as isolators of FePt islands to pin the sites of FePt domains, resulting in manipulation of coercivity and magnetic exchange interaction of the films. Moreover, PMA of the film was optimized by appropriately decreasing film thickness or increasing substrate temperature.

Enhancement of anisotropic magnetoresistance in MgO/NiFe/MgO trilayers via NiFe nanoparticles in MgO layers

MgO/NiFe/MgO trilayers, the new development in highly sensitive anisotropic magnetoresistance (AMR) sensor film materials, exhibit severely reduced magnetoresistance ratios at small NiFe thicknesses. By inserting ultrathin NiFe(І) layers into the top and bottom MgO layers of MgO/NiFe/MgO trilayers, films with a structure of MgO/NiFe(І)/MgO/NiFe/MgO/NiFe(І)/MgO were designed and synthesized. The AMR value can be significantly enhanced for thin NiFe films due to the improved specular reflections of electrons at both NiFe/MgO interfaces. For a thin NiFe film with the structure of MgO/NiFe(І)(1.5 nm)/MgO/NiFe(5 nm)/MgO/NiFe(І)(1.5 nm)/MgO, the AMR value was greatly enhanced to as high as 2.71%, an increase of 37% over MgO/NiFe(5 nm)/MgO film.

Dynamical mechanism for coercivity tunability in the electrically controlled FePt perpendicular films with small grain size

This article reports property manipulations and related dynamical evolution in electromigration controlled FePt perpendicular films. Through altering voltage and treatment time of the power supply applied on the films, electronic momentum was fleetly controlled to manipulate the kinetic energy of Fe and Pt atoms based on momentum exchanges. The electromigration control behavior was proven to cause steerable ordering degree and grain growth in the films without thermal treatment. Processed FePt films with small grain size, high magnetocrystalline anisotropy, and controllable coercivity can be easily obtained. The results provide a novel method for tuning magnetic properties of other L10 structured films.

Synthesis of L10-FePt perpendicular films with controllable coercivity and intergranular exchange coupling by interfacial microstructure control

A series of FePtBi/Au multilayers were fabricated by magnetron sputtering. The interfacial microstructure control of Bi and Au atoms and its effect on comprehensive properties of L10-FePt perpendicular films were carefully studied. Results show that: perpendicular magnetic anisotropy of the L10-FePt film can be remarkably enhanced with the epitaxial inducement of Au atoms. On the other hand, intergranular exchange coupling (IEC) of the film is greatly decreased due to the isolation of FePt particles by nonmagnetic Au particles. Moreover, the controllable coercivity of the film can be realized by adjusting ordering degree of the film through diffusion of Bi atoms. Thus, an L10-FePt perpendicular film with controllable coercivity and no IEC is realized with the interfacial microstructure control of surfactant Bi and Au atoms.

Current-Induced Fast-Ordering of L1

Current-induced fast-ordering method (CIFOM) was designed to achieve an ordered L10 -FePt film with small grain size under low energy consumption by applying direct current (DC) in the FePt film. The grain size (~ 8 nm) of the L10-FePt film prepared by CIFOM method is about one-third of that produced by the post-annealing method (PAM). The fast ordering of the L10-FePt film by CIFOM is probably attributed to the momentum exchange between the FePt atoms and the conductive electrons. The short ordering time by CIFOM contributes to the smaller grain size of FePt film.

_{0}

ABSTRACT The Co/Pt multilayers with Pt and MgO/Pt underlayer were prepared by magnetron sputtering and the perpendicular magnetic anisotropy(PMA) of the multilayers were researched by the means of Anomalous Hall effect (AHE). The annealing effect study of Pt(1.0)/[Co(0.4)/Pt(0.8)]3(unit: nm) shows that the sample has pretty good thermal stability but the coercivity(HC) has little change. The experimental results show that the PMA and Hc of Co/Pt multilayers was improved greatly by the additional scattering of electrons induce from the amorphous insulator-metal interface of MgO-Pt. When MgO layer thickness(tMgO) is 4nm, the Hall resistance (RHall) of MgO(4)/Pt(1.0)/[Co(0.4)/Pt(0.8)]3 multilayer has a maximum value of 2.11Ω which is increased by 16%. Furthermore the Hc increases to 172Oe which is more than 1.5 times to Pt(1.0)/[Co(0.4)/Pt(0.8)]3 multilaye and the Hc meets the maximum value of 204Oe when tMgO is 2 nm. Annealing effects of MgO(4)/Pt(2)/Co(0.2)/Ni(0.4)/Co(0.2)/Pt(2) show that it has good thermal stability and the Hc reaches its maximum value of 438Oe at annealing temperature(TA) of 300°C. The multilayer begins to lose the PMA property at higher TA. The Hysteresis loop of MgO(4)/Pt(1.0)/[Co(0.4)/Pt(0.8)]3 multilayer annealed at 300°C was tested and the anisotropy constant(Keff) was obtained as 8.9 × 106 erg/cm3, which is about 4.45 times the Pt(1.0)/ [Co(0.4)/Pt(0.8)]3 multilayer. The PMA of Co/Pt multilayers is greatly enhanced by MgO/Pt interface and annealing treatment.

-FePt Films With Small Grain Size

[Co/Ni]N and [Co/Pt]N multilayers are ideal perpendicular magnetic anisotropy materials due to their high spin polarization. There are strong perpendicular magnetic anisotropy(PMA) energy between Co and Ni or Pt layer, and if the energy is strong enough to overcome the demagnetizing effect the easy magnetization axis can be perpendicular to the film[1-3]. Perpendicularly magnetized spin valve structure can be prepared by these properties and it has significant implication in high density magnetoresistive random access memory(MRAM) and other aspects[4]. In this work, a perpendicularly magnetized spin valve structure was developed, consisted the ferromagnetic [Co/ Ni] and [Co/Pt] multilayers and separated by a Au spacer. The magnetoresistance(MR) of the samples with the thickness of Au and the layer repetition number of N was studied.