Jing-Yan Zhang

Mechanism of magnetoresistance ratio enhancement in MgO/NiFe/MgO heterostructure by rapid thermal annealing

To reveal thermal effects on the film quality/microstructure evolution and the resulted magnetoresistance (MR) ratio in MgO/NiFe/MgO heterostructures, positron annihilation spectroscopy studies have been performed. It is found that the ionic interstitials in the MgO layers recombine with the nearby vacancies at lower annealing temperatures (200-300 degrees C) and lead to a slow increase in sample MR. Meanwhile, vacancy defects agglomeration/removal and ordering acceleration in MgO will occur at higher annealing temperatures (450-550 degrees C) and the improved MgO and MgO/NiFe interfaces microstructure are responsible for the observed significant MR enhancement

Anomalous Hall effect engineering via interface modification in Co/Pt multilayers

An enhanced anomalous Hall effect (AHE) is observed via interface modification in MgO/[Co/Pt]2/Co/MgO multilayers, due to the insertion of a Hf metal layer at the Co/MgO interface. It is shown that the saturation anomalous Hall resistivity is 215% larger than that in the multilayers without Hf insertion. More importantly, thermally stable AHE and perpendicular magnetic anisotropy features are obtained in MgO/[Co/Pt]2/Co/Hf/MgO multilayers. The AHE is improved for sample MgO/[Co/Pt]2/Co/Hf/MgO by annealing, which is attributed to the enhancement of the side jump contribution.

Ultrahigh Anomalous Hall Sensitivity in Co/Pt Multilayers by Interfacial Modification

A large enhancement of anomalous Hall sensitivity (S-v) by metal/oxide interfacial modification is obtained in a MgO/[Co/Pt](3)/MgO multilayered structure. The values of S-v are increased to 8363 V/A.T by CoO insertion and 2261 V/A.T by Pt insertion, while S-v is approximately 1000 V/A.T in normal Hall semiconductor sensors. Microstructural analysis shows that the crystal orientation of [Co/Pt](3) multilayers is improved, and the oxygen concentration is largely changed by introducing an ultrathin CoO layer at metal/oxide interfaces, leading to a large increment of S-v

Research progress in anisotropic magnetoresistance

Anisotropic magnetoresistance (AMR) is an important physical phenomenon that has broad application potential in many relevant fields. Thus, AMR is one of the most attractive research directions in material science to date. In this article, we summarize the recent advances in AMR, including traditional permalloy AMR, tunnel AMR, ballistic AMR, Coulomb blockade AMR, anomalous AMR, and antiferromagnetic AMR. The existing problems and possible challenges in developing more advanced AMR were briefly discussed, and future development trends and prospects were also speculated.

Chemical reaction at ferromagnet/oxide interface and its influence on anomalous Hall effect

Chemical reactions at the ferromagnet/oxide interface in [Pt/Fe]3/MgO and [Pt/Fe]3/SiO2 multilayers before and after annealing were investigated by X-ray photoelectron spectroscopy. The results show that Fe atoms at the Fe/MgO interface were completely oxidized in the as-grown state and significantly deoxidized after vacuum annealing. However, only some of the Fe atoms at the Fe/SiO2 interface were oxidized and rarely deoxidized after annealing. The anomalous Hall effect was modified by this interfacial chemical reaction. The saturation anomalous Hall resistance (Rxy) was greatly increased in the [Pt/Fe]3/MgO multilayers after annealing and was 350% higher than that in the as-deposited film, while Rxy of the [Pt/Fe]3/SiO2 multilayer only increased 10% after annealing.

Ru Catalyst-Induced Perpendicular Magnetic Anisotropy in MgO/CoFeB/Ta/MgO Multilayered Films

The high oxygen storage/release capability of the catalyst Ru is used to manipulate the interfacial electronic structure in spintronic materials to obtain perpendicular magnetic anisotropy (PMA). Insertion of an ultrathin Ru layer between the CoFeB and Ta layers in MgO/CoFeB/Ta/MgO films effectively induces PMA without annealing. Ru plays a catalytic role in Fe-O-Ta bonding and isolation at the metal-oxide interface to achieve moderate interface oxidation. In contrast, PMA cannot be obtained in the sample with a Mg insertion layer or without an insertion layer because of the lack of a catalyst. Our work would provide a new approach toward catalyst-induced PMA for future CoFeB-based spintronic device applications.

Effect of annealing on microstructure evolution in CoFeB/MgO/CoFeB heterostructures by positron annihilation

As one of the most powerful tools for investigation of defects of materials, positron annihilation spectroscopy was employed to explore the thermal effects on the film microstructure evolution in CoFeB/MgO/CoFeB heterostructures. It is found that high annealing temperature can drive vacancy defects agglomeration and ordering acceleration in the MgO barrier. Meanwhile, another important type of defects, vacancy clusters, which are formed via the agglomeration of vacancy defects in the MgO barrier after annealing, still exists inside the MgO barrier. All these behaviors in the MgO barrier could potentially impact the overall performance in MgO based magnetic tunnel junctions.

Corrigendum: Nonvolatile modulation of electronic structure and correlative magnetism of L10-FePt films using significant strain induced by shape memory substrates.

Tuning the lattice strain (eL) is a novel approach to manipulate the magnetic, electronic, and transport properties of spintronic materials. Achievable eL in thin film samples induced by traditional ferroelectric or flexible substrates is usually volatile and well below 1%. Such limits in the tuning capability cannot meet the requirements for nonvolatile applications of spintronic materials. This study answers to the challenge of introducing significant amount of elastic strain in deposited thin films so that noticeable tuning of the spintronic characteristics can be realized. Based on subtle elastic strain engineering of depositing L10-FePt films on pre-stretched NiTi(Nb) shape memory alloy substrates, steerable and nonvolatile lattice strain up to 2.18% has been achieved in the L10-FePt films by thermally controlling the shape memory effect of the substrates. Introduced strains at this level significantly modify the electronic density of state, orbital overlap, and spin-orbit coupling (SOC) strength in the FePt film, leading to nonvolatile modulation of magnetic anisotropy and magnetization reversal characteristics. This finding not only opens an efficient avenue for the nonvolatile tuning of SOC based magnetism and spintronic effects, but also helps to clarify the physical nature of pure strain effect.

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.

Progress in oxygen behaviors in two-dimensional thin films

With the development of spintronics, the investigation on the behavior of oxygen in two-dimensional materials has never ceased. On account of its lively nature, oxygen is hard to exist alone in the system. However, it will interact with other atoms and produce complex orbital hybridization effect, which has influenced the performance of the material. Especially for materials in nanoscale, it is inevitable to introduce the oxygen atoms, no matter what in the process of preparation or employ. Therefore, it is necessary to carry on the research about the effect of oxygen behaviors in the two-dimensional thin films. In this paper, it will mainly introduce the effect of oxygen behaviors on the magnetic properties, electrical properties, phase transition, spin-dependent properties and thermal stability, summarize several factors which influence the oxygen behaviors, and generalize the research progress of the mechanism behind the oxygen behaviors.