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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 Materials
Published in David Jiles, Introduction to Magnetism and Magnetic Materials, 2015
The needle-shaped particles are aligned by a magnetic field during the fabrication process. The final tapes of γ-Fe2O3 have coercivities typically of 20–24 kA m −1, and the acicular particles have lengths ranging from 0.1 to 0.7 μm [13], with length-to-diameter ratios from 3:1 to 10:1. Tapes made from CrO2 have coercivities of 36–44 kA m−1. The chromium dioxide particles have dimensions ranging from 0.5 × 0.03 μm to 0.2 × 0.02 μm, which are significantly smaller than the typical sizes of gamma iron oxide particles used in recording tapes. In all cases, the ferromagnetic particles used in magnetic recording are too small to contain a domain wall and we therefore have single-domain particles. Research into perpendicular recording media has been conducted [14], in particular much attention has been directed toward CoCr layers for this purpose.
Dispersions of Rigid (Spherical and Nonspherical) Magnetic Particles
Published in Rajinder Pal, Rheology of Particulate Dispersions and Composites, 2006
Suspensions of small (~10 nm) dipolar or magnetic nanoparticles, also referred to as ferrofluids or magnetic fluids, are of significant practical interest as they exhibit unique physical properties [1–35]. Owing to the small size of the magnetic particles, generally made up of magnetic iron oxide, the particles can be treated as single-domain permanent magnets unlike macroscopic objects of magnetic materials, which do not possess a permanent magnetic moment because of random orientation of many domains. Ferrofluids are “superparamagnetic” and, therefore, their flow and properties can be controlled using only moderate magnetic fields of the order of 10 to 100 mT. However, in the absence of an external magnetic field, ferrofluids have no net magnetization as Brownian motion orients the particles randomly. The magnetic nanoparticles of ferrofluids are often coated with a surfactant, made of long-chain organic molecules to prevent agglomeration of particles due to attractive Van der Waal’s forces.
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).
Simulation of magneto-mechanical response of ferrogel samples with various polymer structure
Published in Soft Materials, 2022
P. V. Melenev, A. V. Ryzhkov, M. Balasoiu
The magnetic inclusions are presented as equal rigid spheres with diameter . Magnetic particles have a single-domain structure; therefore, their magnetic moments have permanent magnitude . Then, they possess uniaxial anisotropy with energy and interact with each other as point dipoles. Thus, the magnetic part of the energy of k-th magnetic particle takes the following form.