Exchange Bias in Bulk α-Fe/γ-Fe₇₀Mn₃₀ Nanocomposites for Permanent Magnet Applications

Abstract

Here we report on the microstructural factors influencing the formation of the interfacial exchange bias effect in three-dimensional transition-metal-based nanocomposite systems, with relevance to permanent magnet applications. Bulk phase-separated nanocomposites consisting of the ferromagnetic α-Fe and metastable antiferromagnetic γ-Fe 70 Mn 30 phases exhibit a notable low-temperature exchange bias and substantial coercivity (H ex = 24.6 kA/m, H C = 95.7 kA/m) as well as a near room-temperature blocking temperature. Structural investigation by synchrotron X-ray diffraction, neutron scattering, and transmission electron microscopy confirm that the ferromagnetic α-Fe phase nucleates as small precipitates (d ≈ 50 nm) at the grain boundaries of the antiferromagnetic γ-Fe 70 Mn 30 grains (d = 360−740 nm) and grows anisotropically upon heat treatment, resulting in an elliptical geometry. These results indicate that optimization of the exchange bias effect in bulk nanocomposite systems may be achieved through maximizing the surface-to-volume ratio of ferromagnetic precipitates in an antiferromagnetic matrix, enhancing the magnetocrystalline anisotropy of the antiferromagnetic phase to facilitate interfacial pinning and ensuring a balanced distribution of the ferromagnetic and antiferromagnetic phases. This work further clarifies critical factors influencing the formation of an exchange bias in an inexpensive transition-metal-based bulk nanocomposite system with potential for scalable production.

Spin glass behavior and exchange bias effect in GaFeO3‐type iron oxide

“…The GFO presents complex magnetic behaviors, which depend significantly on the measured temperature and the strength of the applied field 33–35 . Because the SG behavior is usually observed in the coexistent system of AFM and FM phases, the GFO potentially exhibits the EB effect.…”

“…As expected from the conventional EB effect, the shift of the hysteresis loop depends on the coupling mechanism at the interface that is in the same direction as the cooling field for AFM coupling and opposite to the cooling field direction for FM coupling. Generally, the interface between the SG and FM phases leads to the exchange bias effect, 35 and most of the SG phase magnetic moments are arranged antiparallel to the FM phase at the interface. Therefore, the generation of the EB effect may also be related to the SG behavior in GFO single crystal.…”

Advanced polarization analysis capability on the very small-angle neutron scattering instrument at the NIST Center for Neutron Research

“…At the NCNR, polarization analysis in SANS was initially developed on the NG-3 SANS instrument and later transitioned to the NG-7 SANS instrument [15]. Over the last decade, SANS polarization analysis capability at the NCNR has been utilized for a great deal of research in magnetism and magnetic materials including magnetic nanoparticles [3,4,[16][17][18], exchanged-biased systems [19,20], ferroelectric material [21], multiferroics [22], a giant magnetostrictor [23], magnetic nanoparticles for biomedical hyperthermia application [24], and segmented magnetic nanowire arrays [25]. A comprehensive description of SANS polarization analysis for soft matter applications was given in a recent article [26].…”

Unraveling Exchange Coupling in Ferrites Nano‐Heterostructures

“…Magnetic interactions play a crucial role in the effectiveness of magnetic nanomaterials in industrial applications, such as magnetic recording, permanent magnets, and spintronics. [1][2][3][4][5] Functional oxide nanocomposites (NCs) have demonstrated great potential in this regard, based on the combination of DOI: 10.1002/smll.202304152 two or more phases to create functional materials with tunable magnetic properties. [6][7][8][9][10] In this context, multiphase nano-heterostructures have attracted a huge interest in developing a new class of materials with enhanced magnetic properties.…”