Xu-Jing Li

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.

Perpendicularizing magnetic anisotropy of full-Heusler Co2feAl films by cosputtering with terbium

In this letter, we fabricated Co2FeAl films with perpendicular-to-plane magnetic anisotropy by cosputtering with terbium (Tb). The as-prepared (Tb+Co2FeAl) films (TCFA) consists of nanocrystalline L21 Co2FeAl and amorphous alloy of Tb(Co, Fe, and Al). The coercivity field (Hc) of the TCFA films is adjustable from 200 to 800 Oe. After annealing, the Hc decreases to 70 Oe. A perpendicularly magnetized spin valve with the TCFA films as free and reference layers shows a current-perpendicular-to-plane magnetoresistance of 1.8% at room temperature. Our result opens a way to fabricate perpendicularly magnetized full-Heusler alloys and makes it possible to realize faster and simple structured magnetic storage bits in the future.

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.

Synthetic antiferromagnet with Heusler alloy Co2FeAl ferromagnetic layers

Heusler alloy Co2FeAl was employed as ferromagnetic layers in Co2FeAl (3nm)∕Ru (xnm)∕Co2FeAl (5nm) synthetic antiferromagnet structures. The experimental results show that the structure with a Ru thickness of 0.45nm is strongly antiferromagnetic coupled, which is maintained after annealing at 150°C for 1h. The structure has a very low saturation magnetization Ms of 425emu∕cm3, a low switching field Hsw of 4.3Oe, and a high saturation field Hs of 5257Oe at room temperature, which are favorable for application in ultrahigh density magnetic read heads or other magnetic memory devices. Crystal structure study testifies that the as-deposited Co2FeAl film is in the B2 phase. Therefore, Heusler alloys can be used to fabricate synthetic antiferromagnetic and it is possible to make “all-Heusler” spin valves or magnetic tunneling junctions with better magnetic switching properties and high magnetoresistance.

Effect of Ta Underlayer on Perpendicular Anisotropy of TbFeCo Films

We study Ta underlayer thickness dependence of perpendicular magnetic anisotropy (PMA) in TbFeCo films. The experiment shows that perpendicular coercivity, saturation magnetization, and remanent squareness ratio of the TbFeCo films change significantly with the Ta underlayer thickness. For example, the perpendicular coercivity (Hc) of the films increases from 671 to 2305 Oe. The reason is that the Ta underlayer affects the number of magnetic pinning sites and the oxidation of the films by residual oxygen atoms in the experimental environment.

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.

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.

High post-annealing stability for perpendicular [Co/Ni] n multilayers by preventing interfacial diffusion

This paper reports that by introducing an appropriate thickness of Cu spacer at a Co/Ni interface, the perpendicular magnetic anisotropy of [Co/Cu/Ni] n multilayers can be maintained at the annealing temperature as high as 400 °C, implying high post-annealing stability. X-ray reflectivity results demonstrate that the multilayers with Cu spacer exhibit good multilayer structure, indicating the weak intermixing of Co and Ni, which is one important reason for the enhanced post-annealing stability of perpendicular magnetic anisotropy. The result is of great importance for out-of-plane magnetized spintronic devices which need to be combined with complementary metal-oxide semiconductors.

Co2FeAl films with perpendicular magnetic anisotropy in multilayer structure

We have fabricated Co2FeAl (CFA) films with perpendicular magnetic anisotropy (PMA) in a (Co2FeAl/Ni)6 multilayer structure. The effects of underlayer Cu thickness (tCu), Co2FeAl thickness (tCFA) and Ni thickness (tNi) on the magnetic properties have been studied. The PMA is realized with a large anisotropy energy density K = 3.7×106 ergs/cm3, a high squareness Mr/Ms = 1 and a small perpendicular coercivity Hc = 60 Oe, while tCu, tCFA and tNi are 9 nm, 0.2 nm and 0.6 nm respectively. The PMA remains after 300 °C annealing, which demonstrates better thermal stability of the (Co2FeAl/Ni)6 multilayer than that of (Co/Ni)n.