Thermal Activation in Permanent Magnets

Bance, Simon; Fischbacher, Johann; Kovacs, Alexander; Oezelt, Harald; Reichel, Franz; Schrefl, T.

doi:10.1007/s11837-015-1415-7

Cited by 37 publications

(22 citation statements)

References 37 publications

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“…All calculated coercive fi eld values are higher than the experimental coercive fi eld (Table 2 ). This can be related to the fact that (i) the real physical discontinuity that occur at stacking faults is less than that assumed, as explained above, (ii) numerical calculation overestimates the fraction of the surface area that is pinned, due to the small size of the considered model, and (iii) thermal activation has not been taken into account in the presented simulation and this may reduce the coercive fi eld by 10-15% at 300 K, which is in agreement with both theoretical modeling [ 25 ] and experiments. [ 26 ] The calculated coercive-fi eld value increases signifi cantly as the thickness of the defect region is reduced, which may be related to the "spring-magnet" effect.…”

Section: Discussionsupporting

confidence: 80%

Preparation, Characterization, and Modeling of Ultrahigh Coercivity Sm–Co Thin Films

Akdoğan

Sepehri-Amin

Dempsey

et al. 2015

Adv Elect Materials

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on the requirements of the specifi c application and the compatibility of processing conditions with fi lm integration. There have been many more reports on the fabrication of Nd-Fe-B based fi lms [3][4][5] compared to Sm-Co based fi lms, [6][7][8][9][10] which may be attributed to a drive towards maximizing the remanence and thus the energy product of the hard magnetic fi lm. However, in certain applications where the hard magnetic fi lm is exposed to high demagnetizing fi elds and/or high operating temperatures, Sm-Co fi lms would be a better choice, due to the much higher values of magneto-crystalline anisotropy and thus high coercivity achievable in Sm-Co based materials. [11][12][13][14] We previously reported on the fabrication of high coercivity Sm-Co thin fi lms ( µ 0 H c ≤ 5 T at 300 K) by triode sputtering. [ 6 ] In this paper we demonstrate that the coercivity of Sm-Co based fi lms can be signifi cantly increased through a modifi cation of the target composition. With the aim to gain a better understanding of the origin of the high coercivity values, we carried out high-resolution microstructural characterization by using scanning transmission electron microscopy and atom probe tomography, and simulated the effect of the observed microstructural defects on magnetization reversal. While the high value of coercivity that was achieved could be relevant for particular applications, combined exploitation of recent advances in sample preparation, characterization, and modeling should contribute to the engineering of better bulk magnets. ResultsThe GI-XRD patterns of samples #1, #2, and #3 are shown in Figure 1 (note that all samples are amorphous in the asdeposited state -data not shown). Comparison of experimental peak positions with powder diffraction data fi les indicates that the crystal structure of the annealed fi lms changes from SmCo 1:7 to 1:5 and then to 2:7 as the Sm content of the layer is increased. Hereafter, samples "#1", "#2", and "#3" will be identifi ed as fi lms of 1:7, 1:5, and 2:7, respectively. Corresponding c / a ratios of 0.82, 0.795, and 0.808 were calculated from XRD peak positions for 1:7, 1:5, and 2:7, respectively. These values are quite close to the actual values for 1:7, 1:5, and 2:7 phases, namely 0.816, 0.793, and 0.803, respectively. [ 15 ] The as-deposited fi lms show soft magnetic behavior ( Figure 2 a). Room temperature in-plane hysteresis loops of

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Section: Discussionsupporting

confidence: 80%

Preparation, Characterization, and Modeling of Ultrahigh Coercivity Sm–Co Thin Films

Akdoğan

Sepehri-Amin

Dempsey

et al. 2015

Adv Elect Materials

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“…The reduction of the coercive field owing to demagnetizing effects equates to N eff = 0.2. Finally, we take into account thermal activation by computing the energy barrier for the nucleation of reversed domains as function of field [38]. The decrease of coercivity by thermal activation is µ 0 H f = 0.23 T.…”

Section: Permanent Magnets and Intrinsic Magnetic Propertiesmentioning

confidence: 99%

“…The critical field value at which the energy barrier becomes 25k B is the temperature dependent coercive field. T = 450 K. Data taken from[38]. (b) Coercivity of a Nd 2 Fe 14 B particle as function of the percentage of coverage with a Tb-containing shell for the continuous coverage model and the percolation model.…”

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confidence: 99%

Micromagnetics of rare-earth efficient permanent magnets

Fischbacher

Kovacs

Gusenbauer

et al. 2018

J. Phys. D: Appl. Phys.

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The development of permanent magnets containing less or no rareearth elements is linked to profound knowledge of the coercivity mechanism. Prerequisites for a promising permanent magnet material are a high spontaneous magnetization and a sufficiently high magnetic anisotropy. In addition to the intrinsic magnetic properties the microstructure of the magnet plays a significant role in establishing coercivity. The influence of the microstructure on coercivity, remanence, and energy density product can be understood by using micromagnetic simulations. With advances in computer hardware and numerical methods, hysteresis curves of magnets can be computed quickly so that the simulations can readily provide guidance for the development of permanent magnets. The potential of rare-earth reduced and free permanent magnets is investigated using micromagnetic simulations. The results show excellent hard magnetic properties can be achieved in grain boundary engineered NdFeB, rare-earth magnets with a ThMn 12 structure, Co-based nano-wires, and L1 0 -FeNi provided that the magnet's microstructure is optimized.

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“…Finally, we discuss the external magnetic field H ext response of the energy barrier (activation energy) [20][21][22][23][24] which governs the probability of magnetization reversal via the thermal fluctuation of spins. If this response can be measured experimentally, 25 it would allow the magnetic coercivity mechanism to be predicted at finite temperatures.…”

Section: Energy Barriermentioning

confidence: 99%

Monte Carlo analysis for finite-temperature magnetism ofNd2Fe14Bpermanent magnet

et al. 2016

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We investigate the effects of magnetic inhomogeneities and thermal fluctuations on the magnetic properties of a rare earth intermetallic compound, Nd2Fe14B. The constrained Monte Carlo method is applied to a Nd2Fe14B bulk system to realize the experimentally observed spin reorientation and magnetic anisotropy constants K A m (m = 1, 2, 4) at finite temperatures. Subsequently, it is found that the temperature dependence of K A 1 deviates from the Callen-Callen law,3 , even above room temperature, TR ∼ 300 K, when the Fe (Nd) anisotropy terms are removed to leave only the Nd (Fe) anisotropy terms. This is because the exchange couplings between Nd moments and Fe spins are much smaller than those between Fe spins. It is also found that the exponent n in the external magnetic field Hext response of barrier height FB = F 0 B (1 − Hext/H0) n is less than 2 in the low-temperature region below TR, whereas n approaches 2 when T > TR, indicating the presence of Stoner-Wohlfarth-type magnetization rotation. This reflects the fact that the magnetic anisotropy is mainly governed by the K A 1 term in the T > TR region.

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Thermal Activation in Permanent Magnets

Cited by 37 publications

References 37 publications

Preparation, Characterization, and Modeling of Ultrahigh Coercivity Sm–Co Thin Films

Preparation, Characterization, and Modeling of Ultrahigh Coercivity Sm–Co Thin Films

Micromagnetics of rare-earth efficient permanent magnets

Monte Carlo analysis for finite-temperature magnetism ofNd2Fe14Bpermanent magnet

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