Cobalt‑Doped FeMn₂O₄ Spinel Nanoparticles: Size‑Controlled Synthesis and Magnetic Behavior

Abstract

Mixed‑metal oxide nanoparticles are of high scientific and technological interest due to their versatile applications. However, controlling size and composition remains a major challenge. In this work, we synthesized Co‑doped FeMn₂O₄ nanoparticles by a solvothermal route at 190 °C for 24 h under autogenous pressure. X‑ray diffraction (XRD) shows that the crystallite size decreases from 9.1 nm to 4.4 nm as Co content increases, consistent with TEM observations. Magnetometry reveals that the saturation magnetization (M_S) rises to a maximum of 48.4 emu g⁻¹ at x = 0.4 and then declines for higher Co levels. The trend is attributed to cation redistribution between tetrahedral and octahedral sites.

Introduction

Spinel oxides (AB₂O₄) exhibit unique magnetic, electrical, and optical properties, enabling applications in spintronics, data storage, supercapacitors, biomedicine, and environmental remediation. Their properties are governed by the distribution of cations among tetrahedral (A) and octahedral (B) sites, yielding normal, inverse, or mixed spinels. Cation substitution further tailors these characteristics. The Mn_xFe_3−xO₄ system is a prominent example; its structure changes from cubic to tetragonal at x ≈ 1.9 due to Jahn–Teller distortion of Mn³⁺O₆ octahedra. Although the Fe‑rich regime (x ≤ 1) has been extensively studied, the Mn‑rich side remains underexplored. Here, we present the first systematic investigation of Co‑doped FeMn₂O₄ nanoparticles, focusing on how Co incorporation influences structural and magnetic properties.

Methods

Synthesis of Co‑Doped FeMn₂O₄ Nanoparticles

Fe(Mn_1−xCo_x)₂O₄ spinel nanoparticles were prepared by a solvothermal method (Scheme 1). Analytical‑grade Fe(acac)₃, Mn(acac)₂ and Co(acac)₂ (Table 1) were dissolved in benzyl alcohol, stirred, and transferred to a 50 mL Teflon‑lined autoclave (50 % fill). Crystallization proceeded at 190 °C for 24 h under autogenous pressure. The resulting powders were magnetically separated, washed with ethanol, and vacuum‑dried at room temperature.

Characterization

Structural analysis was performed by XRD (Bruker D8 Advance, Cu Kα, λ = 1.5418 Å). Morphology and size were examined by TEM (JEOL JEM‑1230, 80 kV). Elemental composition was confirmed by ICP‑MS (Thermo Scientific ELEMENT XR). Raman spectra were recorded with a 533‑nm He–Ne laser on a Shamrock 750 spectrograph. Magnetic measurements employed a Lakeshore 7400 VSM in fields up to ±17 kOe.

Results and Discussions

Figure 1a shows XRD patterns for varying Co content. Increasing Co narrows the diffraction peaks, indicating improved crystallinity and a gradual decrease in crystallite size from 9.1 nm (x = 0) to 4.4 nm (x = 0.9). The dominant peaks at 29.4°, 34.9°, 42.4°, 56.4°, 61.7°, and 73.1° correspond to (220), (311), (400), (511), (440), and (533) planes, matching the cubic ferrite JCPDS 10–0319 (Fd–3m). Although bulk FeMn₂O₄ is tetragonal above x = 1.9, the nanoparticles adopt a cubic phase, suggesting a size‑dependent phase transition.

Using Scherrer’s equation, the lattice parameter ‘a’ decreases from 8.52 Å to 8.37 Å as Co increases, reflecting substitution of smaller Co²⁺ (0.545 Å) for Mn²⁺ (0.645 Å) on octahedral sites. ICP‑MS confirms the intended compositions for 0 ≤ x ≤ 0.4; for x > 0.4 a slight Co loss occurs (Table 2).

Transmission electron microscopy shows uniform, spherical or quasi‑spherical particles that tend to agglomerate due to van der Waals forces at the nanoscale. Size distributions are Gaussian, with averages of 10.5 ± 2 nm (x = 0) and 5.3 ± 1.5 nm (x = 0.9), matching XRD estimates.

Raman spectra reveal three prominent modes: ~634 cm⁻¹ (A_1g, symmetric tetrahedral stretching), ~479 cm⁻¹ (T_2g(2), octahedral vibrations), and ~321 cm⁻¹ (E_g). Peak broadening with Co addition indicates redistribution of Mn/Co–O bonds. A ~457 cm⁻¹ band, attributed to benzyl alcohol, is minor and does not affect magnetic behavior.

All samples exhibit S‑shaped hysteresis loops with negligible remanence and coercivity, confirming superparamagnetic behavior at room temperature. Saturation magnetization peaks at 48.4 emu g⁻¹ (x = 0.4) and declines to 31.6 emu g⁻¹ (x = 0.9). This trend aligns with the magnetic moments of Co²⁺ (3 µ_B) being lower than Mn²⁺ and Fe³⁺ (5 µ_B). For low Co levels (x ≤ 0.4), Co²⁺ promotes Fe³⁺ migration from tetrahedral to octahedral sites, enhancing the net magnetization according to Néel’s two‑sublattice model (M_S = M_B − M_A). Beyond x = 0.4, the reduced Co moment dominates, leading to a net decrease.

Conclusions

We demonstrated that solvothermal synthesis yields Co‑doped FeMn₂O₄ nanoparticles with uniform, spherical morphology and a cubic spinel structure, despite bulk FeMn₂O₄ being tetragonal. Particle size decreases from 10.5 ± 2 nm (x = 0) to 5.3 ± 1.5 nm (x = 0.9) with increasing Co. All samples are superparamagnetic at 300 K. Saturation magnetization first increases to 48.4 emu g⁻¹ at x = 0.4, then declines for higher Co, reflecting cation redistribution. These findings advance the design of size‑controlled spinel nanoparticles for magnetic applications.

Availability of Data and Materials

The raw data underpinning these results are not publicly available at this time, as they are part of an ongoing study. However, processed data and additional details can be requested via email at aleksandr.a.spivakov@gmail.com.