Multi-state electromagnetic phase modulations in NiCo2O4 through cation disorder and hydrogenation

Technical Summary: Multi-state electromagnetic phase modulations in NiCo2O4 through cation disorder and hydrogenation

Problem Statement
The development of low-power, scalable all-oxide spintronic devices is hindered by a scarcity of intrinsic oxide materials that simultaneously possess room-temperature ferromagnetic (or ferrimagnetic) order, strong perpendicular magnetic anisotropy (PMA), and electronic conductivity. While spinel nickel cobaltite (NiCo2O4, NCO) is a promising candidate due to its robust PMA and metallic ground state, a significant challenge remains in the reversible control of its electromagnetic states within a robust energy landscape. Furthermore, the inherent cation disorder in NCO, while introducing competing electronic and magnetic states, has not been fully exploited as a control parameter for tuning proton evolution kinetics and electromagnetic phase transitions.

Methodology
The authors employed laser molecular beam epitaxy (LMBE) to grow high-quality epitaxial NiCo2O4 thin films on single-crystalline MgAl2O4 (MAO) (001) substrates. To investigate the role of cation disorder, films were deposited at varying temperatures ranging from 325 °C to 425 °C. Structural properties were characterized using X-ray diffraction (XRD), reciprocal space mapping (RSM), and atomic force microscopy (AFM).

To achieve reversible electromagnetic modulation, the authors utilized a hydrogen spillover strategy. Platinum (Pt) dots were sputtered onto the NCO films to catalyze the dissociation of H2 molecules into protons and electrons at the triple phase boundary. Hydrogenation was performed at varying temperatures (70 °C and 100 °C) and durations to induce different degrees of proton incorporation.

The study employed a multi-modal characterization approach to probe the resulting phase transitions:

• Structural: XRD and Raman spectroscopy to track lattice expansion and phase transformation.
• Electronic: Temperature-dependent resistivity ($\rho$-$T$) measurements and soft X-ray absorption spectroscopy (sXAS) at the Shanghai Synchrotron Radiation Facility (SSRF) to analyze valence states (Ni and Co L-edges).
• Magnetic: Magnetization hysteresis ($M$-$H$) loops measured via SQUID and Anomalous Hall Effect (AHE) measurements to determine magnetic ordering and transport mechanisms.
• Scaling Analysis: The relationship between anomalous Hall conductivity ($\sigma_{xy}$) and longitudinal conductivity ($\sigma_{xx}$) was analyzed to distinguish between intrinsic (Berry curvature) and extrinsic (scattering) contributions.

Key Results

1. Cation Disorder as a Tuning Knob: Increasing the deposition temperature (from 325 °C to 425 °C) induced greater cation distribution disorder and oxygen vacancies. This resulted in a structural expansion along the out-of-plane direction and a transition in the pristine state from a ferrimagnetic metal (at 325 °C) to a semiconductor (at 425 °C). The higher disorder degraded the ferrimagnetic metallic ground state and PMA.
2. Hydrogen-Induced Phase Modulations: Hydrogenation drove a reversible structural transformation characterized by lattice expansion along the out-of-plane direction while maintaining the in-plane epitaxial lock (topotactic transformation).
   • Intermediate States: Mild hydrogenation (70 °C, 10 min) created intermediate states where the material remained metallic with preserved AHE, though PMA was reduced.
   • Fully Hydrogenated States: Intense hydrogenation (100 °C, 1 h) drove a transition to an antiferromagnetic insulator (or semiconductor) state, characterized by the quenching of AHE and PMA.
3. Electronic and Magnetic Mechanisms: sXAS revealed that hydrogen incorporation reduced the valence states of transition metals, converting high-spin Ni$^{3+}$ to Ni$^{2+}$ and Co$^{3+}$ to Co$^{2+}$. This suppressed the Ni$^{3+}$-O$^{2-}$-Ni$^{2+}$ double exchange interaction responsible for metallic conductivity and weakened the ferrimagnetic ordering.
4. Scattering and Berry Curvature: AHE scaling analysis indicated that hydrogenation significantly reduced the intrinsic contribution (Berry curvature) to the anomalous Hall conductivity, shifting the transport mechanism toward a disorder-dominated scattering regime.
5. Kinetic Control via Disorder: The study demonstrated that cation disorder acts as a kinetic barrier to hydrogen incorporation. Films with higher initial cation disorder (deposited at 425 °C) showed suppressed hydrogen-driven phase transitions compared to those with lower disorder (325 °C).
6. Stability and Reversibility: The hydrogenated states exhibited chemical stability in air for up to 39 days. Upon prolonged exposure (52 days), the films showed spontaneous dehydrogenation, recovering the pristine metallic and ferrimagnetic properties, confirming the reversibility of the magneto-ionic control.

Significance and Claims
The authors claim that this work establishes NiCo2O4 as a versatile platform for discovering spin-dependent physical functionalities by leveraging the interplay between inherent cation disorder and magneto-ionic control. Key contributions include:

• Expanding the Phase Diagram: The work extensively expands the electromagnetic phase diagram of NCO by identifying multi-state modulations (intermediate and fully hydrogenated states) with robust chemical stability and reversibility.
• Decoupling of Properties: The study uncovers a decoupling between electron itinerancy and ferrimagnetic ordering in specific hydrogenated states, offering new insights into correlated physics.
• Control Mechanism: It identifies cation disorder not just as a defect but as a critical control parameter for kinetically adjusting proton evolution, thereby enabling the rational design of exotic quantum states.
• Device Potential: The findings suggest that NCO systems, through the combination of magneto-ionic control and inherent cation disorder, offer a pathway for developing state-of-the-art, low-power spintronic devices. The work clarifies how incorporated hydrogens affect spin-dependent transport behaviors, specifically by reducing intrinsic Berry curvature contributions and enhancing scattering mechanisms.