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Published October 31, 2022 | Version v1
Journal article Open

Control of magnetoelastic coupling in Ni/Fe multilayers using He+ ion irradiation

  • 1. Institute of Physics, Johannes Gutenberg University Mainz, Staudingerweg 7, 55099 Mainz, Germany and Sensitec GmbH, Walter-Hallstein-Straße 24, 55130 Mainz, Germany
  • 2. Spin-Ion Technologies, 10 Boulevard Thomas Gobert, 91120 Palaiseau, France and Université Paris-Saclay, 3 rue Juliot Curie, 91190 Gif-sur-Yvette, France
  • 3. Institute of Physics, Johannes Gutenberg University Mainz, Staudingerweg 7, 55099 Mainz, Germany and Singulus Technologies AG, Hanauer Landstrasse 107, 63796 Kahl am Main, Germany
  • 4. CNR-IMM, UoS Agrate Brianza, Via Olivetti 2, 20864 Agrate Brianza, Italy
  • 5. Faculty of Engineering, Institute for Material Science, Synthesis and Real Structure, Kiel University, Kaiserstraße 2, 24143 Kiel, Germany
  • 6. Singulus Technologies AG, Hanauer Landstrasse 107, 63796 Kahl am Main, Germany
  • 7. Institute of Physics, Johannes Gutenberg University Mainz, Staudingerweg 7, 55099 Mainz, Germany
  • 8. Spin-Ion Technologies, 10 Boulevard Thomas Gobert, 91120 Palaiseau, France
  • 9. Sensitec GmbH, Walter-Hallstein-Straße 24, 55130 Mainz, Germany
  • 10. Spin-Ion Technologies, 10 Boulevard Thomas Gobert, 91120 Palaiseau, France and C2N, CNRS, Université Paris-Saclay, 10 boulevard Thomas Gobert, 91120 Palaiseau, France

Description

This study reports the effects of post-growth He+ irradiation on the magneto-elastic properties of a Ni/Fe multi-layered stack. The progressive intermixing caused by He+irradiation at the interfaces of the multilayer allows us to tune the saturation magnetostriction value with increasing He+ fluences and even to induce a reversal of the sign of the magnetostrictive effect. Additionally, the critical fluence at which the absolute value of the magnetostriction is dramatically reduced is identified. Therefore, insensitivity to strain of the magnetic stack is nearly reached, as required for many applications. All the above-mentioned effects are attributed to the combination of the negative saturation magnetostriction of sputtered Ni and Fe layers and the positive magnetostriction of the NixFe1−x alloy at the intermixed interfaces, whose contribution is gradually increased with irradiation. Importantly, the irradiation does not alter the layer polycrystalline structure, confirming that post-growth He+ ion irradiation is an excellent tool to tune the magneto-elastic properties of multilayer samples. An alternative class of spintronic devices can be envisioned with a material treatment able to arbitrary change the magnetostriction with ion-induced “magnetic patterning.”

The magnetoelastic properties of thin films are of major interest for technological use as well as for scientific investigations. The requirements for the magnetoelastic coefficient (λs) strongly depend on the application. Magnetic sensors often need, for example, strain immunity,1 i.e., zero magnetostriction, to reduce strain cross-sensitivity, while actuators require giant strain effects, achieved in materials such as TbFe2 (terfenol).2 One way to obtain the optimal value of the magnetostriction for a specific application is to use the combination of two or more materials with different magnetic and magnetoelastic properties. Multilayer systems have widely been investigated exploiting the combination of different parameters λs to achieve a target value.3–6

A widely used technique to modify magnetic properties7–9 and induce mixing at interfaces10,11is ion irradiation. Specifically, the use of light ions such as He+ at energies in the range of 10−3010−30 keV induces short range atomic displacements without generation of surface defects in the material, which instead is more prevailing for heavy atoms,12 such as Ar+ or Ga+. If compared to alternative techniques to promote atomic diffusion, e.g., annealing, the use of ion irradiation confines the intermixing to the magnetic layer boundaries and avoids mixing with the nonmagnetic seed layers (for details, see Fig. S1 of the supplementary material). In addition, annealing is a uniform process while the local nature of irradiation interaction can be applied to the magnetic patterning of the multilayer film system. For these reasons, ion irradiation is an excellent candidate to obtain a desired value of the magnetostriction in a multilayer, by controlling the vertical extension of the intermixed part. A previous work13reported intermixing induced magnetostriction changes using heavy ions and high energies (700 MeV). However, the use of these type of atoms can be harmful for thin magnetic layers,14whose magnetic properties, such as saturation magnetization or perpendicular magnetic anisotropy, can be easily degraded. Moreover, the presence of cascade collisions in the material and long-range atomic displacements15 makes the precise control of magnetic properties a difficult task.

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Funding

MagnEFi – Magnetism and the effects of Electric Field 860060
European Commission