Perpendicular Magnetic Anisotropy and Hydrogen‑Induced Magnetization Modulation in Ta/Pd/CoFeMnSi/MgO/Pd Multilayers

Background

Hydrogen (H₂) is rapidly emerging as a clean, high‑energy‑density fuel, yet its safe handling demands reliable detection technologies. Conventional solid‑state conductometric sensors lack chemical selectivity and are hindered by humidity interference [1]. Magnetic sensors, particularly those incorporating palladium (Pd), have proven highly selective and sensitive to H₂ owing to Pd’s catalytic dissociation of hydrogen and the resultant lattice expansion [2,3,4].

In Pd‑rich ferromagnetic alloys and Pd/ferromagnet (FM) multilayers—such as Co₁₇Pd₈₃, Pd/Fe, [Co/Pd]₁₂, and Pd/Co/Pd—hydrogen absorption induces measurable magnetic changes by expanding the Pd lattice by 2–3 % [5–7]. This strain alters the interfacial d‑d orbital hybridization that governs perpendicular magnetic anisotropy (PMA). Because the interfacial strain is highly sensitive to hydrogen loading, PMA‑based films with Pd layers present a promising route to ultra‑sensitive hydrogen detection.

Perpendicular anisotropy in FM/Noble‑Metal or FM/Oxide interfaces arises from d‑d or d‑p hybridization and is strongly modulated by interfacial strain [8–12]. The quaternary Heusler alloy CoFeMnSi (CFMS) is a spin‑gapless semiconductor (SGS) that shows pronounced external‑field sensitivity, positioning it as an attractive FM layer for sensor applications [13–16]. Here, we engineered a Ta/Pd/CFMS/MgO/Pd structure to achieve strong PMA and investigate hydrogen‑induced magnetic modulation, leveraging the interfacial strain sensitivity of both the Pd/CFMS and CFMS/MgO interfaces.

Methods

Four film stacks were deposited on Si substrates by magnetron sputtering under a base pressure <2.6×10⁻⁵ Pa. The stacks were: (i) Ta (6 nm)/Pd (2.4 nm)/CFMS (2.3 nm)/MgO (t_MgO)/Pd (2 nm) (0.9–1.5 nm); (ii) Ta (6 nm)/Pd (2.4 nm)/CFMS (t_CFMS)/MgO (1.3 nm)/Pd (2 nm) (1.9–3.1 nm); (iii) Ta (6 nm)/Pd (2.4 nm)/CFMS (2.3 nm)/Pd (2 nm); and (iv) Ta (6 nm)/CFMS (2.3 nm)/MgO (1.3 nm)/Pd (2 nm). CFMS was sputtered at 0.9 Pa Ar, 40 W DC; MgO at 0.2 Pa Ar, 150 W RF; Ta at 0.3 Pa Ar, 50 W DC; Pd at 0.3 Pa Ar, 25 W DC. Post‑deposition anneals (250–450 °C, 30 min, <10⁻⁴ Pa) optimized interfacial ordering.

Magnetometry was performed with a Lakeshore 7404 VSM at room temperature. Hall resistivity under dynamic H₂ loading was measured with an ET9000 system. Hydrogen concentrations (0–5 %) were controlled by mixing H₂/Ar (5:95) and N₂ streams at a total flow of 3.5 L min⁻¹.

Results and Discussion

MgO thickness and PMA – Figure 1 shows M–H loops for Ta/Pd/CFMS/MgO/Pd films annealed at 300 °C. All samples display strong out‑of‑plane easy axes; the anisotropy peaks at t_MgO = 1.3 nm, achieving a loop squareness (M_r/M_s) ≈ 1. Excessively thin or thick MgO reduces Co–O bonding and weakens PMA, consistent with previous reports [11,12].

In‑plane (a) and out‑of‑plane (b–d) M–H loops for varying t_MgO at 300 °C.

Annealing temperature – Figure 2 demonstrates that the as‑deposited film exhibits in‑plane anisotropy (IMA). Annealing at 300 °C induces PMA; this anisotropy persists up to 350 °C but degrades beyond 400 °C due to interdiffusion that disrupts d‑d hybridization [9,12,17,18].

In‑plane and out‑of‑plane loops for varying annealing temperatures.

Interface contributions – Figure 3 compares stacks lacking MgO or Pd. Removing MgO eliminates PMA; omitting the bottom Pd weakens it, confirming that both Pd/CFMS and CFMS/MgO interfaces are essential, with the latter providing the dominant contribution through optimal Co–O bonding [12,17].

M–H loops for (a) Ta/Pd/CFMS/Pd, (b) Ta/CFMS/MgO/Pd, and (c) Ta/Pd/CFMS/MgO/Pd at 300 °C. (d) K_eff×t_CFMS vs. t_CFMS at various anneals.

From Eq. (1), the effective anisotropy is quantified. The largest K_eff (5.6×10⁵ erg cm⁻³) occurs for t_CFMS = 2.3 nm, confirming the strong interfacial contribution.

Hydrogen‑induced magnetic modulation – Under 5 % H₂, the out‑of‑plane loop shifts, increasing the saturation field from 5.5 to 18 Oe and reducing M_r by 75 % (Fig. 4). The effect is reversible: removal of H₂ restores the original loop, indicating that hydrogen absorption in Pd introduces tensile strain that modulates the CFMS magnetization. Hall resistivity measurements (Fig. 5) show a faster rise during H₂ uptake than during desorption, reflecting the interfacial strain dynamics.

Out‑of‑plane M–H loops under H₂ (a), after H₂ removal (b), and the dependence of M_r and H_k on H₂ concentration (c).

Time evolution of Hall resistivity during H₂ absorption and desorption.

Conclusions

We have demonstrated that Ta/Pd/CFMS/MgO/Pd multilayers, when annealed at 300 °C, exhibit exceptional PMA with K_eff = 5.6×10⁵ erg cm⁻³ and near‑unity loop squareness for t_CFMS = 2.3 nm and t_MgO = 1.3 nm. The films are highly responsive to hydrogen: a 5 % H₂ environment reduces residual magnetization by 75 % while increasing the saturation field by a factor of ~3. The reversible magnetic response, driven by Pd‑mediated lattice expansion and resultant interfacial strain, establishes this system as a promising platform for high‑sensitivity hydrogen sensing.

Abbreviations

CFMS

CoFeMnSi

IMA

In‑plane magnetic anisotropy

PMA

Perpendicular magnetic anisotropy