[Paper Review] Tailoring anomalous Nernst effect in stressed magnetostrictive film grown onto flexible substrate

M.A. Corrêa, M. Gamino+4 more|arXiv (Cornell University)|Apr 5, 2019

Magnetic properties of thin films67 references5 citations

TL;DR

This study demonstrates that the anomalous Nernst effect (ANE) in flexible CoFeB magnetostrictive films can be precisely tailored by applying external stress and tuning the magnetic field. By combining a theoretical model incorporating uniaxial and magnetoelastic anisotropy with experimental measurements, the authors show quantitative agreement between predicted and observed thermoelectric voltage responses, proving that stress-induced modification of effective magnetic anisotropy enables dynamic control of ANE for use in flexible, energy-harvesting spintronic devices.

ABSTRACT

The anomalous Nernst effect in nanostructured magnetic materials is a key phenomenon to optimally control and employ the internal energy dissipated in electronic devices, being dependent on for instance the magnetic anisotropy of the active element. Thereby, here we report a theoretical and experimental investigation of the magnetic properties and anomalous Nernst effect in a flexible magnetostrictive film with induced uniaxial magnetic anisotropy and under external stress. Specifically, we calculate the magnetization behavior and the thermoelectric voltage response from a theoretical approach for a planar geometry and with a magnetic free energy density which takes into account the induced uniaxial and magnetoelastic anisotropy contributions. Experimentally, we verify modifications of the effective magnetic anisotropy and thermoelectric voltage with the stress and explore the possibility of tailoring the anomalous Nernst effect in a flexible magnetostrictive film by modifying both, the magnetic field and external stress. We find quantitative agreement between experiment and numerical calculations, thus elucidating the magnetic and thermoelectric voltage behaviors, as well as providing evidence to confirm the validity of the theoretical approach to describe the magnetic properties and anomalous Nernst effect in ferromagnetic magnetostrictive films having uniaxial magnetic anisotropy and submitted to external stress. Hence, the results place flexible magnetostrictive systems as a promising candidate for active elements in functionalized touch electronic devices.

Motivation & Objective

• To investigate the interplay between external stress, magnetic anisotropy, and thermoelectric voltage in flexible magnetostrictive films.
• To develop a theoretical model that accounts for uniaxial and magnetoelastic anisotropy contributions to predict magnetization and ANE response.
• To experimentally verify the stress-dependent modulation of effective magnetic anisotropy and ANE voltage in CoFeB films on flexible substrates.
• To demonstrate the feasibility of tailoring the anomalous Nernst effect through concurrent control of magnetic field and mechanical stress.
• To validate the theoretical framework against experimental data for ferromagnetic magnetostrictive films under stress.

Proposed method

• Theoretical modeling of magnetization and ANE voltage response using a free energy density functional including uniaxial and magnetoelastic anisotropy terms.
• Numerical calculation of magnetization and thermoelectric voltage using the derived free energy expression under varying magnetic field and stress conditions.
• Experimental measurement of thermoelectric voltage (VANE) using a custom setup with Peltier cooling/heating, differential thermocouple for ∆T, and nanovoltmeter for voltage detection.
• Application of controlled tensile stress via substrate bending, with stress magnitude calculated using established mechanical bending models.
• Use of a vibrating sample magnetometer (VSM) to measure quasi-static magnetization curves at various field angles to assess magnetic anisotropy.
• Estimation of effective temperature difference ∆Tf using thermal conductivity and thickness ratios of film and substrate (Eq. 6), and determination of Nernst coefficient λN via linear fitting of V Smax ANE vs. ∆T.

Experimental results

Research questions

• RQ1 How does external stress influence the effective magnetic anisotropy in flexible CoFeB films with induced uniaxial anisotropy?
• RQ2 To what extent can the anomalous Nernst voltage be tuned by varying both magnetic field orientation and applied stress?
• RQ3 How well does the theoretical model incorporating uniaxial and magnetoelastic anisotropy reproduce the experimental ANE response?
• RQ4 What is the quantitative relationship between applied stress and the Nernst coefficient λN in flexible magnetostrictive films?
• RQ5 Can the combination of magnetic field and mechanical stress enable dynamic, reversible control of the anomalous Nernst effect in flexible spintronic systems?

Key findings

• The theoretical model accurately predicts the magnetization behavior and ANE voltage response in CoFeB films under varying magnetic fields and stress, showing quantitative agreement with experimental data.
• Applied tensile stress of 1400 MPa reduces the Nernst coefficient λN to 13.9×10⁻⁶ V/KT, indicating a significant stress-dependent modulation of the ANE response.
• Stress application induces a measurable change in effective magnetic anisotropy, confirmed by both magnetization measurements and ANE voltage variation.
• The Nernst coefficient λN decreases from 16.7×10⁻⁶ V/KT (no stress) to 13.9×10⁻⁶ V/KT (1400 MPa), demonstrating tunability of the ANE via mechanical stress.
• The experimental and theoretical results confirm that the interplay between uniaxial magnetic anisotropy and magnetoelastic effects governs the ANE in flexible magnetostrictive films.
• Flexible CoFeB films on Kapton substrates exhibit stable, measurable ANE under stress, positioning them as viable candidates for functionalized touch electronic devices.