From soft to hard magnetic Fe–Co–B by spontaneous strain: a combined first principles and thin film study

Reichel, Ludwig; Schultz, L.; Pohl, Darius; Oswald, Steffen; Fähler, S.; Werwiński, Mirosław; Edström, Alexander; Delczeg‐Czirjak, Erna K.; Rusz, Ján

doi:10.1088/0953-8984/27/47/476002

Cited by 37 publications

(48 citation statements)

References 74 publications

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“…Surface anisotropy contributions were finally evaluated by comparing anisotropy energies with a calculation for a bulk bct alloy with same lattice parameters and Brillouin zone integration on a grid of close to 100000 k-points (46x46x46). As shown in previous investigations 14,21 , epitaxially grown Fe-Co films with additions of 2 at% B or C exhibit spontaneous strain with c/a of approx. 1.03.…”

Section: Experimental and Theoretical Methodssupporting

confidence: 78%

“…Again, we argue, that the buffer induced strain relaxes during the film growth and only spontaneous strain remains. TEM observations confirm this statement 14,21 , since the c/a ratio measured close to the buffer interface does not vary significantly from the latter. It can be concluded that there is no influence of induced strain on the magnetic properties as no induced strain is left at the time of the magnetic ex situ measurements.…”

Section: Separating Volume and Faces Related Anisotropysupporting

confidence: 65%

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On the origin of perpendicular magnetic anisotropy in strained Fe–Co(–X) films

Reichel

Edström

Pohl

et al. 2017

J. Phys. D: Appl. Phys.

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Very high magnetic anisotropies have been theoretically predicted for strained Fe-Co(-X) and indeed several experiments on epitaxial thin films seemed to confirm strain induced anisotropy enhancement. This study presents a critical analysis of the different contributions to perpendicular anisotropy: volume, interface and surface anisotropies. Tracing these contributions, thickness series of single layer films as well as multilayers with Au-Cu buffers/interlayers of different lattice parameters have been prepared. The analysis of their magnetic anisotropy reveals a negligible influence of the lattice parameter of the buffer. Electronic effects, originating from both, the Au-Cu interface and the film surface, outrange the elastic effects. Surface anisotropy, however, exceeds the interface anisotropy by more than a factor of three. A comparison with results from Density Functional Theory suggests, that the experimentally observed strong perpendicular surface anisotropy originates from a deviation from an ideal oxide-free surface. Accordingly, tailored Fe-Co-X/oxide interfaces may open a route towards high anisotropy in rare-earth free materials. PACS: 75.30.Gw Magnetic anisotropy 75.70.Ak Magnetic properties of monolayers and thin films 75.50.Bb Fe and its alloys 75.50.Ww Permanent magnets MotivationNearly one hundred years ago, polycrystalline Co steels were state of the art permanent magnets 1,2 . With the rise of Al-Ni-Co and the rare earth based alloys, the interest on these so called KS or Honda steels was suppressed. Within the last decade, new interest has risen because new methods of computation allow for a deeper understanding of magnetic anisotropy in single crystalline Fe-Co. Thin films of Fe-Co are now in the focus of research since large magnetocrystalline anisotropies were predicted, when their lattice is strained tetragonally 3-5 . A common approach to achieve such strain is coherent growth on substrates or buffer layers with certain lattice parameters, which apply compressive stress in the films' plane. While the in-

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Section: Experimental and Theoretical Methodssupporting

confidence: 78%

Section: Separating Volume and Faces Related Anisotropysupporting

confidence: 65%

On the origin of perpendicular magnetic anisotropy in strained Fe–Co(–X) films

Reichel

Edström

Pohl

et al. 2017

J. Phys. D: Appl. Phys.

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“…For example, it was shown that a careful control of strain and alloy concentration allows for a large MAE in bct FeCo alloys [9][10][11][12] . The potential route to FeCo-based permanent magnets offered by that work inspired subsequent studies aiming to stabilize tetragonality in FeCo by B or C-impurities [13][14][15] . Also the tetragonal (Fe 1−x Co x ) 2 B compound has been carefully studied due to its tunable MAE as function of x [16][17][18][19][20] which, furthermore, has an intriguing temperature dependence 19,21,22 .…”

Section: Introductionmentioning

confidence: 99%

Magnetocrystalline anisotropy ofFe5PB2and its alloys with Co and5delements: A combined first-principles and experimental study

et al. 2018

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The Fe 5 PB 2 compound offers tunable magnetic properties via the possibility of various combinations of substitutions on the Fe and P-sites. Here, we present a combined computational and experimental study of the magnetic properties of (Fe 1−x Cox) 5 PB 2 . Computationally, we are able to explore the full concentration range, while the real samples were only obtained for 0 ≤ x ≤ 0.7. The calculated magnetic moments, Curie temperatures, and magnetocrystalline anisotropy energies (MAEs) are found to decrease with increasing Co concentration. Co substitution allows for tuning the Curie temperature in a wide range of values, from about six hundred to zero kelvins. As the MAE depends on the electronic structure in the vicinity of Fermi energy, the geometry of the Fermi surface of Fe 5 PB 2 and the k-resolved contributions to the MAE are discussed. Low temperature measurements of an effective anisotropy constant for a series of (Fe 1−x Cox) 5 PB 2 samples determined the highest value of 0.94 MJ m −3 for the terminal Fe 5 PB 2 composition, which then decreases with increasing Co concentration, thus confirming the computational result that Co alloying of Fe 5 PB 2 is not a good strategy to increase the MAE of the system. However, the relativistic version of the fixed spin moment method reveals that a reduction in the magnetic moment of Fe 5 PB 2 , by about 25%, produces a fourfold increase of the MAE. Furthermore, calculations for (Fe 0.95 X 0.05 ) 5 PB 2 (X = 5d element) indicate that 5% doping of Fe 5 PB 2 with W or Re should double the MAE. These are results of high interest for, e.g., permanent magnet applications, where a large MAE is crucial.

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“…This is in accordance with the typical doping concentrations experimentally accomplishable, e.g., 12.5 at% content of N in Fe and 9.6 at% of B in Fe 0.38 Co 0.62 . [10,11] Like Fe-Co alloys, we find light interstitials can indeed cause stable tetragonal distortion to cubic full Heusler alloys, which is quantizated by the c/a ratio between the c-axis and in-plane lattice constants. As shown in Table 1, with N interstitials, Fe 2 NiAl has the a tetragonal distortion as large as c/a=1.57.…”

Section: Resultsmentioning

confidence: 87%

“…Nevertheless, due to the strong tendency for the FeCo alloys to relax, it is difficult to maintain the tetragonal distortion induced by the underlying substrates for thin films thicker than 2 nm [6,7,8]. Recently, following the prediction based on DFT calculations [9,10], systematic studies have been performed on FeCo+X (X= C and B), where spontaneous tetragonal distortions with c/a=1.04 can be induced by a few atomic percent interstitial doping of C or B atoms occupying the octahedral interstitial sites. The resulting MAE can be as large as 0.5 MJ/m 3 with B concentration up to 4 at%, where the tetragonal strain reaches 5%.…”

Section: Introductionmentioning

confidence: 99%

Designing Rare-Earth Free Permanent Magnets in Heusler Alloys Via Interstitial Doping

et al. 2019

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Based on high-throughput density functional theory calculations, we investigated the effects of light interstitial H, B, C, and N atoms on the magnetic properties of cubic Heusler alloys, with the aim to design new rare-earth free permanent magnets. It is observed that the interstitial atoms induce significant tetragonal distortions, leading to 32 candidates with large (> 0.4 MJ/m 3 ) uniaxial magneto-crystalline anisotropy energies (MAEs) and 10 cases with large in-plane MAEs. Detailed analysis following the the perturbation theory and chemical bonding reveals the strong MAE originates from the local crystalline distortions and thus the changes of the chemical bonding around the interstitials. This provides a valuable way to tailor the MAEs to obtain competitive permanent magnets, filling the gap between high performance Sm-Co/Nd-Fe-B and widely used ferrite/AlNiCo materials.

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From soft to hard magnetic Fe–Co–B by spontaneous strain: a combined first principles and thin film study

Cited by 37 publications

References 74 publications

On the origin of perpendicular magnetic anisotropy in strained Fe–Co(–X) films

On the origin of perpendicular magnetic anisotropy in strained Fe–Co(–X) films

Magnetocrystalline anisotropy ofFe5PB2and its alloys with Co and5delements: A combined first-principles and experimental study

Designing Rare-Earth Free Permanent Magnets in Heusler Alloys Via Interstitial Doping

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