Effects of interfacial Fe electronic structures on magnetic and electronic transport properties in oxide/NiFe/oxide heterostructures
Introduction
When a ferromagnetic metal (FM) layer is in direct contact with an oxide layer, rich physical and chemical behaviors such as charge transfer, stress, diffusion and hybridization may occur at FM/oxide interface [1], [2], [3], [4], which play important roles in magnetic and transport properties of the FM/oxide heterostructure [5], [6], [7]. Previous researches on FM/oxide interfaces mainly focused on the effects of interfacial diffusion, roughness, electron scattering and lattice matching on the properties of FM films [8], [9], [10], [11]. However, some studies indicate that redox reactions at FM/oxide interfaces also have deep impacts on properties of FM films. For example, interface oxidation could be driven by external field [12], [13], [14]. And reversible control of interfacial oxidation state has been successfully achieved via applying electric field [15], [16]. Moreover, oxygen vacancies vary significantly in different oxides [17], thus both the external field and the attribute of oxide affect the oxidation of FM at FM/oxide interface. High energy, which is generated in the annealing process, may cause not only interface diffusion but also interfacial oxidation reaction or reduction reaction. Whether different sort of oxide could alter the interfacial oxidation state in a single annealing process is still not clear. The work is aimed to investigate the interfacial electronic structures of NiFe/oxide (oxide = SiO2, MgO, HfO2) heterostructures and the interface effects on the magnetic and electronic transport properties of ultrathin NiFe films sandwiched by oxide with different elemental electronegativity. Our results provide direct evidence of different changes of interfacial electronic structures and the considerable influence on properties of heterostructures via different oxide candidates. This work suggests a thermodynamic approach to select alternative oxide interlayer and provides further insight into control of interfacial structure in NiFe-based magnetic sensors, spin valves and magnetic random access memory (MRAM).
Section snippets
Experimental
All samples were deposited on Si/SiO2 substrates by magnetron sputtering. The sample structure is substrate/oxide(4 nm)/NiFe(2 nm)/oxide(3 nm)/Ta(2 nm), where oxide refers to SiO2, MgO or HfO2. The oxides were chosen according to their different elemental electronegativity. The values of electronegativity for Si, Mg and Hf are 1.90, 1.31 and 1.30, respectively [18]. The base pressure of the sputtering system was better than 1.0 × 10−5 Pa and the working argon pressure was 0.4 Pa. During deposition, a
Results and discussion
Fig. 1 shows the MR curves of oxide(4 nm)/NiFe(2 nm)/oxide(3 nm)/Ta(2 nm) films. The MR ratio of the NiFe film sandwiched by SiO2 is almost zero for either the as-deposited state or the annealed state. In comparison, the MR ratio of the MgO sandwiched film is 0.26% for the as-deposited state and increases to 0.35% for the annealed state. For the HfO2 sandwiched film, before annealing the MR ratio is 0.55%, and after annealing the MR ratio has an almost 60% enhancement to 0.88%. Fig. 2 shows the
Conclusion
In summary, the changed Fe electronic structures in ultrathin oxide/NiFe(2 nm)/oxide/Ta films through annealing were successfully revealed. Oxidation, reduction and no reaction occur at different ultrathin NiFe/oxide interfaces, respectively. The elemental electronegativity and formation enthalpy of the oxides play important roles. Different redox reactions induce different changes in the electronic structures of interfacial Fe, resulting in significant difference in the magnetic and MR
Acknowledgments
The present work was supported by the National Key Science Research Projects of China (No. 2015CB921502), by the Natural Science Foundation of China (Grant Nos. 51331002, 51371027, and 51461014), by the Beijing Nova program (Grant No. Z141103001814039), and by the Fundamental Research Funds for the Central Universities (Nos. FRF-SD-12-031A and FRF-TP-14-002C1).
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