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Magnetic Anisotropy of Nanocomposites Made of Magnetic Nanoparticles Dispersed in Solid Matrices
Published in Mahmood Aliofkhazraei, Advances in Nanostructured Composites, 2019
The magnetic anisotropy originates in the existence of the spins (magnetic moments) from the crystalline network and the coupling between them and crystalline network, through the spin-orbit and orbit-network interactions under the crystalline field action, leading ultimately to a preferred direction of the spontaneous magnetization within the crystal, corresponding to a position of minimum energy with respect to the crystallographic axes. This causes a magnetic anisotropy. The echillibrum position of the spontaneous magnetization is in most cases very close to a direction in which the magnetization is the easiest to achieve in an external magnetic field, called the easy magnetization axis (e.m.a.). For expressing the anisotropy energy, a component of the crystal’s free energy will be introduced, taking into account the symmetry conditions of the crystal, which will depend on the orientation of the spontaneous magnetization vector relative to the crystallographic axes of the crystal (Akulov 1928, Becker and Döring 1939). The anisotropy energy in this case, known as the magnetocrystalline anisotropy energy, will depend both on the crystal’s symmetry, i.e., cubic, hexagonal, etc., and the nature of the material (Kneller 1962, Herpin 1968, Vonsovski 1971). The magnetocrystalline anisotropy depends also on the temperature, that usually being maximum at 0 K (where the entropy is minimal) and minimum at the Curie temperature, where the spontaneous magnetization does not exist.
Magnetic Nanoparticle for Information Storage Applications
Published in Claudia Altavilla, Enrico Ciliberto, Inorganic Nanoparticles: Synthesis, Applications, and Perspectives, 2017
Materials used in magnetic information storage are chosen based on their values of the parameters described above. The magnetic components of a disk drive are the read head, the write head, and the media. The media, which is the focus of this chapter, stores data based on the two stable magnetic configurations corresponding to Mr and –Mr on the hysteresis loop. The stability of the media can be evaluated based on the magnetic anisotropy of the material used. The anisotropy energy, EA, describes the amount of energy required to reverse the magnetization from one stable state to the other. This depends on several factors, most importantly the effective anisotropy constant, K. The magnetic anisotropy is positively correlated with the coercivity, which is often used to provide a qualitative estimate of the anisotropy of the material, since direct measurements of K can be difficult. In terms of magnetic stability, the more energy that is required to randomize the electronic spins either thermally or via an applied field, the less likely that the information stored in the media will be erased due to stray magnetic fields or excess heat. Therefore, the quest to find better-performing materials to make up hard disk media is centered upon the tailoring of the anisotropy, and thus, the coercivity of the candidate materials.
Magnetic Materials for Nuclear Magnetic Resonance and Magnetic Resonance Imaging
Published in Sam Zhang, Dongliang Zhao, Advances in Magnetic Materials, 2017
Elizaveta Motovilova, Shaoying Huang
The formation of domains, alignment of magnetic moments of atoms in a domain, and formation of flux closure domains where the field lines are allowed to form a closed loop crossing the domains, are the results of energy minimization. The formation of a domain is done in order to reduce magnetostatic energy. Within a domain, when two nearby atoms both have unpaired electrons, it is favorable for the electrons to have their spins aligned because in this way they occupy different orbitals and thus the Coulomb repulsion is smaller and the exchange energy is minimized. The flux closure domains can only be formed when the magnetostatic energy saved is greater than the energy cost for changing the local net magnetization. It takes two types of energy to form a loop of field lines. One is called magnetocrystalline anisotropy energy that is the energy which magnetizes a material in directions other than the favorable “easy axis.” The other type is magnetoelastic anisotropy energy that is the energy needed due to magnetostriction, the energy for overcoming the mechanical stresses due to the change of the orientation of the molecule in the process of magnetization.
Effects of Pd- and Ta-buffer layer on magnetic and interfacial perpendicular properties of sputtered Co2FeSi/MgO heterostructures
Published in Surface Engineering, 2021
Ke Wang, Yongming Tang, Xiaopeng Xiao, Jian Liu
To investigate the contribution of the CFS/MgO interface to PMA, 4-nm-Pd-buffered CFS(4)/MgO(d) heterostructures with variable thickness of MgO layer were studied. The anisotropy energy Kµ can be given by the equation Kµ=MsHK/2, where HK is anisotropy field. The Ms is obtained from the M-H curves and the anisotropy field HK is decided from the intersection between the saturated parts of the in-plane and out-of-plane M-H loops. Figure 2 shows MgO thickness dependence of Kµ for the Pd(4)/CFS(4)/MgO(d) samples annealed at 350 °C. The structure without MgO layer exhibits in-plane anisotropy and negative Kµ. For the stack with 0.5 nm-thick MgO layer, PMA appears and Kµ rapidly increases to 0.75×106 erg/cm3. The largest Kµ of 1.05×106 erg/cm3 is achieved in the structures when MgO thickness reaches 1 nm, which can be attributed to the proper oxidation of CFS. The initially sharp increase in PMA with MgO thickness indicates that dominant contribution on the PMA comes from the interface of CFS/MgO heterostructure. The hybridization between ferromagnetic Co/Fe-3d and O-2p orbitals at the heterostructure interface accounts for the developed PMA [35]. With further increasing MgO thickness, the PMA decreases due to excessive oxidation of CFS. This confirms proper oxidation at the heterostructure interface is necessary for achieving strong PMA [30].
Influence of Zn incorporation on the microstructural and magnetic properties of La0.67Sr0.33Mn1−xZnxO3 nanoparticles synthesised by the sol–gel method
Published in Philosophical Magazine Letters, 2018
Hilal Ahmed, Shakeel Khan, Wasi Khan, M Ashiq, Swaleha Naseem
Figure 4 shows the field-dependent isothermal dc magnetization at room temperature for the Z0, Z10 and Z20 samples up to the maximum available field of 1.8 Tesla. The hysteresis loops for all the investigated samples measured at room temperature are closed and show almost no coercivity. This may be considered as typical superparamagnetic behaviour of the samples. Superparamagnetism arises on account of the tiny nanoscale size of the particles. Thermal energy is sufficient to modify the direction of magnetization of the whole crystal even though the temperature is less than the Curie or Néel temperatures. These fluctuations in the direction of magnetization are responsible for zero net magnetic field. However, sample shows a paramagnetic nature, as every individual atom comes under the influence of the magnetic field forcing the magnetic moment of the whole crystal to align with the applied magnetic field [23]. The energy required to transform the direction of magnetization of a crystallite is known as crystalline anisotropy energy. It is dependent on the properties of the material and the size of the crystallites. Hence, owing to the small crystallite size, this energy decreases, resulting in a lowering of the temperature at which the sample has a superparamagnetic nature [24,25]. Complete saturation behaviour is not detected even at the highest value of magnetic field (~1.8 Tesla) for all the samples, which can be explained by canting of the surface spins of the nanoparticles. The value of slope of the M-H curve increases with Zn doping up to 10%, but the slope of the curve is lower for 20% Zn doping.
A micromechanical constitutive model for porous ferromagnetic shape memory alloys considering magneto-thermo-mechanical coupling
Published in Advanced Composite Materials, 2023
The magnetocrystalline anisotropy energy represents the energy required to move the magnetization vector away from the magnetically easy axis. When the magnetization vector is aligned along the easy axis, this energy is minimum (or zero), and when rotating 90° away from the easy axis, this energy is the maximum. Thus, this energy can be described as