Role of atomic-scale thermal fluctuations in the coercivity

Toga, Yuta; Miyashita, Seiji; Sakuma, Atsushi; Miyake, Takashi

doi:10.1038/s41524-020-0325-6

Cited by 29 publications

(29 citation statements)

References 41 publications

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“…( 25 , 3.0, and 3.0, respectively, and we find that coercivity is c 3T. h (26) This value is close to the estimation c 3.3 h obtained by the MC method [40], which will be reviewed in the next subsection. The dashed line in Fig.…”

Section: Dynamics Of Magnetizationsupporting

confidence: 86%

“…s r s r ss (40) has a relevant effect on breaking down the uniform ordering. We have studied the effect of DDI for a thin film of the present material.…”

Section: Coercivity Of Large Grainsmentioning

confidence: 99%

“…However, at finite temperatures, the effect of thermal agitation plays an important role [ 20 ]. To study this effect quantitatively, we adopted two complementary methods, that is, a direct SLLG simulation of the dynamics of magnetization [ 38 ] and an analysis of free energy as a function of magnetization obtained by a MC simulation [ 40 ].…”

Section: Coercivity Of Nanoparticlesmentioning

confidence: 99%

“…Thus far, energy functionals of magnetization with temperature-dependent parameters have been used for this purpose, which may not appropriately express thermal fluctuations. On the other hand, we have developed a method of studying the metastable situation quantitatively at finite temperatures [ 40 ], for which we obtained the free energy as a function of magnetization from the atomistic Hamiltonian at a given temperature by using of the Wang–Landau method [ 41 ]. Using the explicit form of , we estimated coercivity with and without the activation effect, and also clarified the concept of the activation volume introduced in the literature [ 32–34 , 42 ].…”

Section: Introductionmentioning

confidence: 99%

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Atomistic theory of thermally activated magnetization processes in Nd₂Fe₁₄B permanent magnet

Miyashita

Nishino

Toga

et al. 2021

Science and Technology of Advanced Materials

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For the practical use of magnets, particularly at high temperatures, the temperature dependence of magnetic properties is an important ingredient. To study the temperature dependence, methods of treating the thermal fluctuation causing the so-called activation phenomena must be established. To study finite-temperature properties quantitatively, we need atomistic energy information to calculate the canonical distribution. In the present review, we report our recent studies on the thermal properties of the Nd 2 Fe 14 B magnet and the methods of studying them. We first propose an atomistic Hamiltonian and show various thermodynamic properties, e.g., the temperature dependences of the magnetization showing a spin reorientation transition, the magnetic anisotropy energy, the domain wall profiles, the anisotropy of the exchange stiffness constant, and the spectrum of ferromagnetic resonance. The effects of the dipole-dipole interaction (DDI) in large grains are also presented. In addition to these equilibrium properties, we also study coercivity, which is the most important issue for magnets. The temperature dependence of the coercivity of a single grain was studied using the stochastic Landau-Lifshitz-Gilbert equation and also by the analysis of the free energy landscape, which was obtained by Monte Carlo simulation. It was found that the upper limit of coercivity at room temperature is about 3T, which is significantly lower than the so-called theoretical coercivity given by a simple coherent rotation model. The coercivity of a polycrystalline magnet, i.e., an ensemble of grains, is expected to be reduced further by the effects of the grain boundary phase, which is also studied. Surface nucleation is a key ingredient in the domain wall depinning process. Finally, we study the effect of DDI among grains and also discuss the distribution of properties of grains from the viewpoint of first order reversal curve.

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Section: Dynamics Of Magnetizationsupporting

confidence: 86%

“…s r s r ss (40) has a relevant effect on breaking down the uniform ordering. We have studied the effect of DDI for a thin film of the present material.…”

Section: Coercivity Of Large Grainsmentioning

confidence: 99%

Section: Coercivity Of Nanoparticlesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Atomistic theory of thermally activated magnetization processes in Nd₂Fe₁₄B permanent magnet

Miyashita

Nishino

Toga

et al. 2021

Science and Technology of Advanced Materials

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“…One is a sufficiently large magnet body compared with the exchange length, and the second is a sufficiently slow magnetization reversal compared with the relaxation of magnetization [ 47 ]. Very recently, Toga rigorously verified this picture using the energy landscape calculation based on the atomistic spin model [ 23 ]. Obviously, these two conditions are quite reasonable for magnetization reversal in permanent magnets irrespective of the magnetization reversal processes, i.e., nucleation or domain wall depinning.…”

Section: Experimental Magnetization Reversal Analyses For Permanent Mmentioning

confidence: 96%

Experimental approaches for micromagnetic coercivity analysis of advanced permanent magnet materials

Okamoto

2021

Science and Technology of Advanced Materials

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Although coercivity is one of the fundamental properties of permanent magnets, it has not been well understood. In this paper, micromagnetics and thermal activation magnetization reversal theories are briefly reviewed, and then our recent macroscopic and microscopic experimental approaches for thermally activated magnetization reversal in advanced Nd-Fe-B hot-deformed magnets are explained. Our experimental results are well supported by the recent atomistic spin model calculations. Moreover, the systematic micromagnetics simulation study makes much clearer the physical picture of the thermally activated magnetization reversal process in permanent magnets.

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Dynamical Simulation for Long‐Time Relaxation From Metastable States: Quantitative Estimation of Coercive Field and Relaxation Time

Nishino,

Miyashita

2024

Eur J Inorg Chem

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The bistability of spin‐transition materials is the origin of their multifunctional properties. It causes hysteresis phenomena, i.e., relaxation from a metastable state, of the spin (electronic) state, magnetization, etc. Collapse of a strong metastable state is a long‐time relaxation phenomenon. To study such nonequilibrium dynamical phenomenon, time evolution dynamics analyses are important. However, it is difficult to estimate long‐time relaxation phenomena studying time evolution dynamics simulations due to the limitation of the simulation time. Furthermore, because the relaxation occurs in a stochastic process, a wide distribution of the relaxation time has to be considered in the analysis of the relaxation. To overcome these difficulties, we recently developed two methods of quantitative estimation of the relaxation time from a metastable magnetic state and of the coercive field. These methods are applicable to the estimation of the relaxation time and coercive field of any magnetic particles. In this paper, staring with the Stoner‐Wohlfarth model, the difference in characteristic features of magnetization reversal dynamics between zero and finite temperatures is discussed. Then, the methods of quantitative estimation of the coercive field and relaxation time are presented. The methodological features and the validity of the estimation were discussed.

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Role of atomic-scale thermal fluctuations in the coercivity

Cited by 29 publications

References 41 publications

Atomistic theory of thermally activated magnetization processes in Nd₂Fe₁₄B permanent magnet

Atomistic theory of thermally activated magnetization processes in Nd₂Fe₁₄B permanent magnet

Experimental approaches for micromagnetic coercivity analysis of advanced permanent magnet materials

Dynamical Simulation for Long‐Time Relaxation From Metastable States: Quantitative Estimation of Coercive Field and Relaxation Time

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Role of atomic-scale thermal fluctuations in the coercivity

Cited by 29 publications

References 41 publications

Atomistic theory of thermally activated magnetization processes in Nd2Fe14B permanent magnet

Atomistic theory of thermally activated magnetization processes in Nd2Fe14B permanent magnet

Experimental approaches for micromagnetic coercivity analysis of advanced permanent magnet materials

Dynamical Simulation for Long‐Time Relaxation From Metastable States: Quantitative Estimation of Coercive Field and Relaxation Time

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Product

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Atomistic theory of thermally activated magnetization processes in Nd₂Fe₁₄B permanent magnet

Atomistic theory of thermally activated magnetization processes in Nd₂Fe₁₄B permanent magnet