Magnetization reversal and anisotropies in buffered transition-metal alloys thin films

TL;DR

This work demonstrates that buffer-layer engineering and magnetic-field-assisted deposition provide robust control over magnetic anisotropy and reversal mechanisms in NiFe- and CoFe-based thin films, with direct implications for Planar Hall Effect sensor performance. NiFe exhibits a growth-field–driven uniaxial anisotropy largely independent of buffer, while CoFe is dominated by the buffer, yielding either isotropic or biaxial responses depending on the underlayer. The angular dependence of coercivity and the remanent magnetization are successfully described by a mixed anisotropy model and a two-phase reversal framework, highlighting domain rotation as the dominant mechanism in most cases, except for specific NiFe-Ta(H) and some Cu/Ag-buffered CoFe regimes where domain-wall motion or near-coherent rotation appears. Practically, NiFe on Ta without growth field stands out as a favorable configuration for low-hysteresis, high-stability PHE sensing, with buffer and field conditions offering tunable anisotropy for sensor optimization.

Abstract

Interest in planar Hall effect (PHE) sensors has re-emerged in recent years due to their promising potential for a wide range of applications, particularly in biotechnology. Sensor sensitivity can be enhanced by lowering the effective anisotropy field; however, this favors magnetic domain formation during magnetization reversal, leading to hysteretic responses. Therefore, precise control of magnetic anisotropy and magnetization reversal is essential to balance sensitivity and stability in PHE sensors. In this work, we investigate the magnetic anisotropy and magnetization reversal mechanisms of Ni-Fe- and Co-Fe-based multilayers grown on various metallic buffer layers and deposited with and without an external magnetic field, in order to evaluate the effects of the buffer layers and field-assisted deposition on the resulting magnetic anisotropy. NiFe films exhibit a dominant uniaxial anisotropy mainly determined by the applied field during growth, with an anisotropy constant of approximately $3,\mathrm{kerg,cm^{-3}}$, largely independent of the buffer layer. In contrast, the magnetic anisotropy of CoFe films is dominated by the buffer layer, resulting in a biaxial magnetic response. In particular, Ag-buffered films deposited under an external magnetic field exhibit a biaxial anisotropy with values up to $14.88,\mathrm{kerg,cm^{-3}}$. The magnetization reversal mechanism of each system was deduced from the analysis of the angular dependence of the coercive field.

Figures (6)

• Figure 1: GIXRD patterns of 100nm-thick CoFe and NiFe films with Ag buffer and capping.
• Figure 2: (a) SEM image and (b) grain size distribution of a NiFe_Cu(H) bilayer; (c) Gaussian fit parameters, mean grain radius $r_c$ and variance $\sigma$, for samples grown under magnetic field (full symbols) and without (open symbols)
• Figure 3: In-plane magnetization loops of a NiFe_Ta (H) structure measured at T=300K, along both the uniaxial easy ($\color{blue}\bullet$, EA) and hard ($\color{orange}\blacktriangle$, HA) axes respectively.
• Figure 4: Schematic representation of the magnetization and magnetic field vectors in the description of the free energy with respect to the anisotropy axes.
• Figure 5: Polar plots of the normalized remanent magnetization (left) and coercive field (right) for NiFe-based films deposited with ($\blacksquare$) and without ($\bullet$) an applied magnetic field. The solid lines are fits to $m_r/m_s$ using Eqn.\ref{['eq:remanencia_modelo']} and to $H_c$ using Eqn. \ref{['eq:two-phase']}, respectively.