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Thermodynamics
Published in Harshad K. D. H. Bhadeshia, Theory of Transformations in Steels, 2021
Consider a ferromagnetic material containing unpaired electrons, at a temperature which is below its Curie temperature but above 0K. The spins due to the unpaired electrons will therefore be imperfectly aligned due to thermal agitation. Magnetic disordering of this kind does not happen by the exact reversal of the aligned spins on an increasing number of sites as the temperature increases, but by correlated deviations of the spin orientation. These are called spin waves or magnons (Figure 2.9). The increased disorder at high temperatures excites ever shorter wavelengths and thus gives the material an ability to absorb energy. This is the origin of the magnetic component of heat capacity. Unlike phonons, magnons are not vibrational waves but involve angular displacements between adjacent spin orientations.
Ferromagnetism
Published in Daniel D. Pollock, PHYSICAL PROPERTIES of MATERIALS for ENGINEERS 2ND EDITION, 2020
These spin waves are limited to certain wavelengths because of the boundary conditions in a manner similar to the normal modes of lattice vibrations. Quantized spin waves are known as magnons, and, as would be expected, behave in a manner corresponding to phonons. The possibility of the pairing of two reversed spins on adjacent ions is neglected. The magnons are completely degenerate because their total number is not fixed. They obey the Bose-Einstein statistics because the interchange of two opposite spins does not change the state of the system; this method is used to calculate their density of states and their probable number.
Magnon Spintronics
Published in Evgeny Y. Tsymbal, Žutić Igor, Spintronics Handbook: Spin Transport and Magnetism, Second Edition, 2019
The field of science that refers to information transport and processing by spin waves is known as magnonics [4, 17, 48, 49]. The utilization of magnonic approaches in the field of spintronics, hitherto addressing electron-based spin currents, gave birth to the field of magnon spintronics [18, 19, 50]. Magnon spintronics comprises magnon-based elements operating with analog and digital data as well as converters between the magnon subsystem and electron-based spin and charge currents.
Transfer of squeezing in a cavity magnomechanics system
Published in Journal of Modern Optics, 2023
Abdelkader Hidki, Abderrahim Lakhfif, Jamal El Qars, Mostafa Nassik
Achieving strong light-matter interaction via collective excitations of spin ensembles in ferrimagnetic systems, such as yttrium-iron-garnets (YIG), has made tremendous progress in recent years for studying macroscopic quantum effects due to their extraordinarily high spin density and low dissipation rate. This potentially leads to applications in quantum communication and quantum information processing. Magnons are, in fact, quasi-particles defined by the collective motion of a large number of spins in the YIG sphere; they can couple strongly to microwave (MW) cavity photons leading to cavity polaritons [1–5]. These properties have paved the way for the study of several other important phenomena in the cavity-magnon-polariton system, such as cavity-mediated manipulation of distant spin currents [6], magnon dark modes [7], and bistability [8].
Diamond quantum sensors: from physics to applications on condensed matter research
Published in Functional Diamond, 2022
Kin On Ho, Yang Shen, Yiu Yung Pang, Wai Kuen Leung, Nan Zhao, Sen Yang
Regarding spintronic devices made by YIG thin film, magnons are another exciting playground due to the long coherence length, extended lifetime, and low dissipation. Lee-Wong et al. [56] reported optical detection of magnons, by proximate NV centres, with a broad range of wavevectors in YIG. The magnons were generated by employing the nonlinear parametric excitation. By measuring the PL at different MW frequencies and external magnetic fields, they first demonstrated the intrinsic coupling between exchange spin waves with an NV qubit, as shown in Fig. 6(d). They then modified the magnon band structure by varying the thickness and dimensions of the YIG thin film, showcasing the universality of their technique. The enhanced dipolar interaction changes the magnon dispersion significantly. They further illustrate the discrete values of the magnon wavevectors by patterned the YIG thin film into a microdisk, and several spin-wave modes had been observed. Their experimental findings were supported by theoretical calculations, showing the powerfulness of NV imaging on spin systems.
Magnetic properties of spinels Co x Zn1−x Cr2O4 systems: Green’s functions, high-temperature series expansions technique and mean-field theory
Published in Phase Transitions, 2021
A. El Grini, R. Masrour, A. Jabar, S. Salmi, A. Azouaoui, M. El Haoua, K. Bouslykhane, A. Hourmatallah, M. Hamedoun, N. Benzakour, A. Rezzouk, J. Kharbach
Materials in the spinel structure (AB2O4) are still the subject of extensive experimental and theoretical studies because of their wide variety of physical properties and potential applications in nanoscience and technology. In particular, materials exhibiting geometric frustration due to the three-dimensional networks of corner-sharing tetrahedral. The Green function theory (GFT) theoretical approach is used to study the magnetic behavior of CoxZn1−xCr2O4 [1,2]. This technique makes use of the spectral theorem to connect correlation functions to the Green’s functions. This contribution deals with the reorientation of the magnetization of thin ferromagnetic (FM) Heisenberg films as a function of the temperature or an external magnetic field. It is of particular importance to take into account collective excitations (magnons), which influence the magnetic properties of films more strongly than those of bulk magnet. The field-induced reorientation of the magnetization is treated for all temperatures of interest. Since expectation values of all three components of the spin operator are considered, a corresponding set of Green’s functions must be defined. The method used for the calculation of the expectation values does not utilize only the eigenvalues but also the eigenvectors of the (non-symmetric) matrix governing the equations of motion for the Green’s functions with the random-phase approximation (RPA) [3–6]. The thermal magnetization and magnetic susceptibility are given for different values of magnetic field and dilution x. The values of critical temperatures have been deduced. The spin-1/2 anisotropic Heisenberg model with antiferromagnetic exchange interactions in the presence of a longitudinal external magnetic field and a Dzyaloshinskii–Moriya interaction is studied by employing the effective-field theory (EFT) and usual mean-field approximation [7,8]. On the other hand, the anomalous behavior in the phase diagram of the anisotropic Heisenberg antiferromagnetic model by using the EFT with finite cluster N = 2 spin EFT-two-site cluster with Dzyaloshinskii–Moriya interaction in a longitudinal magnetic field at low temperature is investigated [9] and the anisotropic two-dimensional nearest-neighbor Ising model with competitive interactions in both uniform longitudinal field and transverse magnetic field is investigated using EFT [10]. The thermodynamics properties of the quenched decorated Ising model with competitive interactions are studied using the EFT [11].