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Self-reversed magnetic hysteresis (SRMH) is a unique hysteresis phenomenon containing an antiparallel magnetisation to an applied magnetic field. This leads to an inverted hysteresis loop with a ‘negative’ net area. A couple of Chinese researchers have developed a scalable and versatile method to produce iron oxide particles that can absorb metal cations by utilising the ‘negative’ area, leading to unique SRMH properties being discovered for iron oxide particles.
Iron-based materials with a SRMH loop typically possess diamagnetic features, but they shape like ferromagnetic loops. In ferromagnetism, the saturation of the loop is located within the first and third quadrants, but SRMH saturation is positioned within the second and fourth quadrants. It is often thought that the negative area violates the thermo-mechanical second law and has been proposed that SRMH arises from inappropriate or asymmetric sample positioning, or misinterpretation of data. As such, it is a phenomenon that is often ignored.
Despite these non-existent assumptions, SRMH can be found in ferrous-containing rocks, but as two magnetic phases- a stable phase and a metastable “x” phase. The “x” phase becomes weakly magnetised in the presence of an applied field, during a cooling process. The “x” phase becomes negatively coupled and strongly magnetised with the stable phase at low temperatures. This is found to be the basic mechanism as to why SRMH occurs.
SRMH is an important phenomenon in both academia and industry as it possesses interdisciplinary features. SRMH has been found in naturally occurring rocks and is used to reinforce the theory of reverse magnetisation at the north and south poles throughout Earth’s history. However, whilst observable in nature, there has been great problems to produce simple and effective methods to synthetically produce materials with tuneable SRMH features. Previously, this has been due to lack of the understanding towards the internal mechanism of SRMH.
In a new and major development in SMRH research, this research team has managed to produce a facile, repeatable and versatile method to produce SRMH iron oxides. The researchers studied the logical results in-depth to provide an effective and tailored approach to tune the desired SMRH characteristics of the materials.
To produce such tuneable and potentially large scale materials, the researchers tested various iron oxide-based nanoparticles that have naturally-occurring negative zeta potentials. Magnetite, maghemite and haematite, of different dimensions, were soaked in FeCl3 and the Fe3+ ions were electrostatically adsorbed onto surface of the iron-oxide materials, due to a negative surface zeta potential. Such particles investigated, ranged from 0D magnetite/maghemite nanoparticles to 2D pine-like and 3D flower-like Fe2O3 nanoparticles.
The cation absorption produces an iron core-shell nanoparticle. Such nanoparticles were shown to exhibit well defined SRMH features, displaying both ferromagnetic and diamagnetic characteristics. The SRMH phenomenon arises in the core-shell nanoparticles through negatively magnetic exchange coupling of the pre-magnetised Fe3+ particles in the paramagnetic shell and the post-magnetised iron oxide in the superparamagentic core. Under no field, the orientation of the magnetic moment in the Fe3+ shell is random, but becomes aligned towards the direction of the field, yielding a weak paramagnetism. The magnetic couplings are highly frustrated which degrades the ferromagnetism in the ions, producing essentially ‘dead’ nanoparticles. It is these nanoparticles that negatively couple and bind with the core.
The utilisation of multiple architectures alongside controlling the amount of Fe3+ ions in the solution can be used to change the core size of the nanoparticle and thus, easily and controllably tune its properties.
The development of synthetically-made, SRMH tuneable materials is a big step forward from previous research and it is anticipated that these materials will be used to facilitate the improvement of spintronics, magnetic recording and other magnetically-related fields.
Ma J., Chen K., Modulated self-reversed magnetic hysteresis in iron oxides, Scientific Reports, 7, 42312