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Crystal Structures and Properties of Nanomagnetic Materials
Published in Ram K. Gupta, Sanjay R. Mishra, Tuan Anh Nguyen, Fundamentals of Low Dimensional Magnets, 2023
Mirza H. K. Rubel, M. Khalid Hossain
The examination of the magnetization behavior of NPs in the context of a single domain or monodomain is being performed using a paramagnetic analogy known as the superparamagnetic theory proposed by Bean and Livingston [46]. Domains are the batches of spins that are arranged in the same direction and are separated by domain walls, which have a characteristic width and formation energy. When the particle size is reduced in magnetic particles (multidomain materials), the generation of a single-domain structure is observed, which induces the phenomenon of superparamagnetism as well. Every particle acts like a paramagnetic atom, but each nanoparticle has a very large moment with a definite magnetic state [47]. Weak interparticle magnetic interactions yield superparamagnetic (SPM) behavior also. However, superparamagnetic materials are intrinsically nonmagnetic and are magnetized under an external field. The motion of domain walls is the basic reason for the reversal magnetization in superparamagnetic nanomaterials. The superparamagnetic theory deals with the assumption that all the magnetic moments of the particles will move coherently, with a magnitude of µ = µatN; here µat indicates the atomic moment and N denotes the number of magnetic atoms that compose such a particle [48].
Magnetic Nanomaterials
Published in Rajendra Kumar Goyal, Nanomaterials and Nanocomposites, 2017
In a single domain particle, all the magnetic moments within the particles are pointing toward the same direction. A colloidal solution of such particles has no net magnetization above the Bloch temperature in absence of external magnetic field due to thermal agitation. For these very small magnetic nanoparticles (i.e., <20 nm), the energy barrier (KV) becomes small (due to small volume, V, of particle) and comparable to thermal energy (kBT), where K the anisotropy energy density, kB Boltzmann's constant, and T the absolute temperature. So for the magnetic particles smaller than critical size (Dc), the energy barrier can no longer pin the direction of magnetization to the timescale of observation and hence, the rotation of the direction of magnetization occurs due to thermal fluctuations. In other words, thermal energy leads to flipping of the magnetic moment of small magnetic particles. Such particles are said to be superparamagnetic. The coercivity of a superparamagnetic particle is zero because thermal fluctuations prevent the existence of a stable magnetization. This leads to the anhysteretic but still sigmoidal M–H curve (Figure 9.2c).
Magnetic-Particle-Based Microfluidics
Published in Sushanta K. Mitra, Suman Chakraborty, Fabrication, Implementation, and Applications, 2016
Ranjan Ganguly, Ashok Sinha, Ishwar K. Puri
The most common magnetic particles used for microfluidic applications are single domain iron oxide nanoparticles of 5–15 nm diameter. At this size, the particles are superparamagnetic so that that the particle magnetization curves do not show any hysteresis, although their magnetization is comparable with ferri- or ferromagnetic particles. When these particles are coated with an adsorbed surfactant layer and stably suspended in a nonmagnetic liquid carrier, the resulting suspension is called a ferrofluid (Rosensweig, 1985). Although ferrofluids are nonmagnetic in the absence of a magnetic field, the thermally disoriented magnetic moments of the particles are readily aligned with an externally imposed magnetic field, making the fluid magnetically responsive. The superparamagnetic iron oxide nanoparticles (SPIONs) can also be embedded within biodegradable polymers, such as dextran, in the form of 50- to 500-nm aggregates (Kantor et al., 1998) or encapsulated inside 500-nm to 10-μm-sized polystyrene, latex, or silica microspheres that are often termed “magnetic beads” (Rida and Gijs, 2004). The SPION aggregates and magnetic beads possess larger magnetic moments than an isolated nanoparticle but still retain their superparamagnetic nature. The transport of ferrofluids, SPION aggregates, and magnetic beads can be controlled using a suitably tailored magnetic field. Similar to isolated nanoparticles, ferrofluids, SPIONs, and magnetic beads can also be surface functionalized with chemical and biological ligands to accomplish specific bioanalytical tasks.
Magneto-ionic suppression of magnetic vortices
Published in Science and Technology of Advanced Materials, 2021
Yu Chen, Aliona Nicolenco, Pau Molet, Agustin Mihi, Eva Pellicer, Jordi Sort
Magnetic vortex is one of fundamental magnetization states that occurs in micro-/nano sized ferromagnetic structures, e.g. disks, ellipses or nanowires, and so on, due to geometrical spin confinement [1–4]. This state is highly appealing for non-volatile magnetic memories and spintronic devices since the magnetic information can be stored by encoding both (i) the vortex chirality, i.e. the direction of in-plane magnetization rotation (clockwise or counter-clockwise), and (ii) the polarity, i.e. the out-of-plane magnetization of the nanoscale vortex core (up or down) [5]. Hence, each magnetic vortex can store four bits of information. Nonetheless, the applications of magnetic vortices extend far beyond memory applications, and also include oscillators for effective microwave generation [6], energy storage [7], or biomedical applications (theragnostics) where the magnetic vortices are capable of damaging the membrane of cancer cells or promote drug delivery [8,9]. In turn, single-domain magnetic nanoparticles have found applications in, e.g., high-density recording media, magnetic biosensing, magnetic imaging, or hyperthermia therapy, among others [10].
Structural, magnetic, and microwave absorption properties of SrCexFe12-xO19/PVP composites
Published in Journal of Microwave Power and Electromagnetic Energy, 2020
Zahra Rasouli, Mohammad Yousefi, Maryam Bikhof Torbati, Susan Samadi, Khadijeh Kalateh
However, the magnetic properties can be adjusted by replacing Ba2+, Fe3+ and Sr2+ in M-type ferrites. Recently, too much attention has been paid to the effect of rare earth elements on SrM magnetic properties, such as Nd3+ (Almessiere et al. 2018a), Y3+ (Irfan et al. 2016; Shekhawat and Roy 2018), Sm3+, Er3+(Luo et al. 2015), Ce3+(Kang 2015; Yasmin et al. 2019), and Tb3+(Ali et al. 2013), which the effect of their magnetic properties has been different and cerium is the best candidate due to its low price. In recent decades, in order to improve the magnetic properties, various synthesis techniques have been developed to obtain ferrite particles with the single domain such as: sol-gel, hydrothermal, pyrolysis aerosol, oxidation techniques using nitric acid, high-temperature synthesis, citrate precursor, microemulsions, and salt melting methods, etc. For the synthesis of hexaferrites with a single domain which has high saturation magnetism with optimum coercivity, the ratio of Fe/Sr, Ba, calcination temperature, dopant concentration, and other synthetic parameters should be optimized (Chang et al. 2012; Cao et al. 2017). Mosleh et al. used the Ce-substituted M-type barium ferrite to increase the absorption of microwave waves. Their experiments showed that with a dopping of Cerium to barium hexaferrite, the absorption properties improve (Mosleh et al. 2015).