United States

Curvilinear magnetism is of great fundamental and practical interest whose rapid development is inspired by novel experimental technologies and wide potential applications [1]. A general approach for description of curvilinear ferromagnets [2] has been recently developed and used for thin wires and shells uncovering magnetochiral effects in statics and dynamics [1,3]. Besides intensive research of ferromagnetic materials, their antiferromagnetically ordered (AFM) counterparts are promising candidates for spintronics applications by their low sensitivity to external fields and ultra high eigenfrequencies [4]. Here, we present a general approach for description of AFM textures in curvilinear spin chains [5]. We show that the magnetic dipole-dipole interaction in these systems can be reduced to a hard-axis anisotropy along the chain. Lagrangian of the curvilinear AFM spin chain in continuum limit corresponds to the biaxial chiral helimagnet. Helix geometry shows existence of two equilibrium magnetic states depending on values of curvature and torsion: (i) homogeneous state in the local reference frame, it is typical for helices with the curvature larger than torsion; and (ii) periodic state is quasi-homogeneous in the laboratory reference frame. For specific case of the AFM flat chain there is the only ground state, with the order parameter being oriented perpendicular to the chain plane. We show that in curvilinear system transverse and longitudinal magnon modes in the AFM helix and ring are coupled due to geometry-induced Dzyaloshinskii–Moriya interaction.References: [1] R. Streubel, J. Lee, D. Makarov et al, J. Phys. D, 49, 363001, (2016); A. Fernández-Pacheco et al, Nat. Comm., Vol. 8, p. 15756 (2017). [2] Y. Gaididei, V. P. Kravchuk, D. D. Sheka, Phys. Rev. Lett. 112, 257203 (2014); D. D. Sheka, V. P. Kravchuk, Y. Gaididei, J. Phys. A, Vol. 48, p. 125202 (2015). [3] O. V. Pylypovskyi, D. D. Sheka, V. P. Kravchuk et al, Sci. Rep. Vol. 6, p. 23316 (2016); O. V. Pylypovskyi, D. Makarov, V. P. Kravchuk et al, Phys. Rev. Applied, Vol. 10, p. 064057 (2018) [4] V. Balz, A. Manchon, M. Tsoi et al, Rev. Mod. Phys., Vol. 90, p. 015005 (2018) [5] D. Y. Kononenko, O. V. Pylypovskyi, K. V. Yershov et al., arXiv:2005.05835 (2020)

MMM 2020

Effects of Geometry on Curvilinear Spin Chains

**Please view our Welcome to MH21 video.**

<center>
<figure class="video_container">
	 <iframe src="https://screencast-o-matic.com/watch/cr1UYHV1pcz?v=6&ff=1&title=0&controls=1" width=826 height=526 frameborder="0" allowfullscreen="true"> </iframe>
</figure>
	</center>


**Walkthrough Video**

Here is a Walkthrough showing how to navigate the conference and network with others 

<center>
<figure class="video_container">
	 <iframe src="https://screencast-o-matic.com/watch/cr1vnrV1paE?v=6&ff=1&title=0&controls=1" width=826 height=526 frameborder="0" allowfullscreen="true"> </iframe>
</figure>
	</center>




	 Organized by
 
<a href="https://mrs.org.sg/">
<img src="https://assets.underline.io/uploads/markdown_image/1/image/95d5789689d5f900b90e9fe54ea3cf0d.jpg"</a>

 
	
	 In association with
 
<a href="http://www.ntu.edu.sg/">
<img src="https://assets.underline.io/uploads/markdown_image/1/image/334cd171ad2fdee033c635b368e5e160.png"</a>
	<a href="https://www.nus.edu.sg/">
<img src="https://assets.underline.io/uploads/markdown_image/1/image/32e6f82c0440f7bb24c590f9f67821bd.png"</a>
		<a href="https://www.a-star.edu.sg/">
<img src="https://assets.underline.io/uploads/markdown_image/1/image/72557865b589cfa3d2a0243ffce4a270.png"</a>
	
 


	 Supported by
 
		 <a href="https://www.visitsingapore.com/mice/en/about-us/about-secb/">
<img src="https://assets.underline.io/uploads/markdown_image/1/image/6e679842ead62be522ba6a630b82d5c9.png"</a>

To access the MH 21 Conference, [please register](https://mh21.mrs.org.sg/event/da3f9651-429f-4763-81b2-ebd7311b6bc8/websitePage:206f7605-1731-41f9-8421-56d3890f398f)

MATERIALS FOR HUMANITY (MH 21)

MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

poster

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

We were reluctant to give up the benefits of meeting face-to-face, and know that shifting to a virtual conference is a significant change for this community. Nevertheless, we are first and foremost concerned about the health and safety of all our Conference participants and their loved ones on returning home after travelling.

The MMM 2020 Steering Committee was working exceptionally hard, along with our sponsors, the IEEE Magnetics Society and AIP Publishing, to make this event serve the needs of our participants, who live in time zones all around the globe, in the best possible way. 

You need a ticket to explore this event.

If you have already registered for MMM 2020, and you have already created an account, then you
just need to login (above).

If you have already registered for MMM 2020, but you have not yet created an account, then you
need to click on &quot;create an account&quot; (above) and then login.

Forgot your password? Go to
https://www.underline.io/log-in

If you have not yet registered for MMM 2020 but would like to do so, go to
https://magnetism.org/register

Still need help? Contact our team!


YbPtBi is a half Heusler alloy with claimed properties of topological Weyl nodes and chiral anomaly; antiferromagnetism from the RKKY coupling of localized Yb f-electrons at low temperatures; and heavy fermion behaviour as those localized f-electrons interact with the conduction bands [1,2]. So far only bulk crystals have been grown of YbPtBi with no published work on thin films. By using c-plane sapphire the half Heusler alloy has an epitaxially relationship of YbPtBi(111)||Al2O3(0001) and YbPtBi(110)||Al2O3(1100). With thin films we could have to option of induced strain, device fabrication and many other thin films manipulation techniques. In this work, we present a study of epitaxially grown YbPtBi thin films that grew as triangular islands with matched faceted edge orientations. The structure is confirmed to be epitaxial from x-ray diffraction and cross-sectional transmission electron microscopy. We also measure the electrical transport properties and find a non-metallic ρ(T) similar to other semimetals RePtBi (Re=Gd, Ce, Sm) half Heusler Alloy, although different from bulk YbPtBi. We measured a negative longitudinal magnetoresistance and the temperature dependence of an apparent chiral anomaly (Fig: a,b). Both of which are indicators of a possible Weyl semimetal state. We also obversed a large hysteretic MR at very low temperatures, 250 mK, not previously seen in YbPtBi.References: [1] C. Y. Guo, F. Wu, Z. Z. Wu, M. Smidman, C. Cao, A. Bostwick, C. Jozwiak, E. Rotenberg,Y. Liu, F. Steglich, and H. Q. Yuan, Nature Communications. 9, 4622 (2018) [2] B. G. Ueland, A. Kreyssig, E. D. Mun, J. W. Lynn, L. W. Harriger, D. K. Pratt, K. Prokeš, Z. Hüsges, R. Toft-Petersen, S. Sauerbrei, S. M. Saunders, Y. Furukawa, S. L. Bud’ko, R. J.McQueeney, P. C. Canfield, and A. I. Goldman, Phys. Rev. B. 99, 184431 (2019)

Epitaxial Growth Of The Weyl Semimetal YbPtBi (111) On C-plane Sapphire By Co-Sputtering

The magnetic properties of van der Waals magnetic topological insulators MnBi2Te4 and MnBi4Te7 are investigated by magneto-transport measurements. We evidence that the relative strength of the inter-layer exchange coupling J to the uniaxial anisotropy K controls a transition from an A-type antiferromagnetic order to a ferromagnetic-like metamagnetic state. A bi-layer Stoner-Wohlfarth model allows us to describe this evolution, as well as the typical angular dependence of specific signatures, such as the spin-flop transition of the uniaxial antiferromagnet and the switching field of the metamagnet. In micron-size magnets, the single-domain switching-field astroid are however partly truncated by the nucleation of domain walls along the easy-axis direction.References: A. Tan, V. Labracherie, N. Kunchur, A. U. B. Wolter, J. Cornejo, J. Dufouleur, B. Büchner, A. Isaeva and R. Giraud Metamagnetism of Weakly Coupled Antiferromagnetic Topological Insulators. Phys. Rev. Lett. 124, 197201. (2020)

Meta-Magnetism of Weakly-Coupled Antiferromagnetic Topological Insulators

The rare earth nitride series of intrinsic ferromagnetic semiconductors offers a rich selection of spin-orbit systems in which to study electrical transport and magnetic phenomenon, the various Hall effects being one of the many meeting places of these[1,2]. The central member of the series GdN (8S7/2) has a strong spin only magnetisation and relatively simple band structure, dominated by the Gd 5d conduction band and N 2p valence band. Electron doping is routinely achieved via the inclusion of nitrogen vacancies. Here we report on 'anomalous' Hall effect data in heavily doped GdN, which show an apparent change of sign of the dominant charge carrier as the sample is cooled through the ferromagnetic transition, while the anomalous Hall effect retains the negative sign of electron doped GdN. We will discuss these in the context of the calculated band structures of stoichiometric and heavily doped GdN.References: [1] W. F. Holmes-Hewett, R. G. Buckley, B. J. Ruck, F. Natali and H. J. Trodahl, 4f conduction in the magnetic semiconductor NdN, Phys. Rev. B. 100, 195119 (2019) [2] W. F. Holmes-Hewett, F. H. Ullstad, B. J. Ruck, F. Natali and H. J. Trodahl, Anomalous Hall effect in SmN: Influence of orbital magnetism and 4f band conduction, Phys. Rev. B. 98, 235201 (2018)

Anomalous Hall effects in rare earth nitrides

Spin polarization in nonmagnetic materials originates from the break of inversion crystalline symmetry that could occur globally or locally. Centrosymmetric (CS) nonmagnetic materials with hidden spin polarization induced by non-CS site symmetries and spin-orbit coupling are promising candidates for spintronic applications, in light of the zero net spin polarization and modulatable spin effects hidden in the local structures. There is, however, an open issue regarding the possible spin splitting induced by broken inversion symmetry at the sample surface. Therefore, it is critical to perform combined experimental and theoretical studies on centrosymmetric TMDC materials such as 2H-MoTe2 to explore the mechanism of the hidden spin polarization. 2H-MoTe2 crystallizes in a trigonal prismatic arrangement with an in-plane lattice constant of 3.517 Å and an out-of-plane lattice constant of 13.962 Å. The unit cell of it is centrosymmetry, but the monolayer has non-centrosymmetric structure. With the virtue of low detection depth of spin-ARPES (<6 Å with He-I), a large spin splitting of 236 meV and opposite spin polarizations up to 80% along out-of-plane direction (z axis) in K and K′ valleys were observed, consist with the density functional theory (DFT) calculations. We further found from the DFT calculations that a medium dipole field mimicked the surface symmetry breaking in ARPES measurements induces negligible variation of spin polarization, confirming that the measured spin effects originate from the intrinsic hidden spin polarization in 2H-MoTe2. The large spin splitting and net spin polarization found in spin-ARPES experiments also suggest that the hidden spin effects in inversion symmetric layered compounds can be used to generate large spin splitting on the surfaces in the absence of strong dipole field. Our combinatorial experimental and theoretical studies clarify the existence of hidden spin polarization in the centrosymmetric materials and opens the way of designing novel functional materials with coexisting hidden spin polarization and other hidden effects, such as hidden orbital polarization and hidden Berry curvature, for the energy efficient spintronics applications.

Direct observation of hidden spin polarization in 2H-MoTe2

Magnetic impurities in metals spin polarize their surroundings. This Ruderman-Kittel-Kasuya-Yosida (RKKY) polarization has been fundamental in the understanding of interactions in magnetic atoms and layers. When the impurity is heavy and magnetic, its large atomic spin-orbit coupling unquenches the orbital momentum. This is the case of lanthanide atoms, which have a significant orbital momentum responsible for their strong magnetocrystalline anisotropy. Recent utilization of this atomic spin-orbit coupling in spintronics include the perpendicular magnetic anisotropy of thulium iron garnet (TIG) films [1-4], the voltage-controlled magnetic anisotropy of interfacial rare-earth ions [5-6], and double compensation points of ferrimagnets [7]. In this presentation, we study lanthanide impurities in simple metals. The lanthanide orbital momentum couples to conduction electrons via the so-called skew scattering and induces a rotational current [8]. The predicted current has an orbital momentum density very similar to the RKKY one, with an algebraic decay and oscillations at twice the Fermi wavenumber. In addition to the charge density profile, we compute the observable effects, namely the Oersted magnetic field and Knight shift.References: [1] M. Kubota, A. Tsukazaki, F. Kagawa, K. Shibuya, Y. Tokunaga, M. Kawasaki, and Y. Tokura, APEX 5, 103002 (2012). [2] C. Tang, P. Sellappan, Y. Liu, Y. Xu, J. E. Garay, and J. Shi, Phys. Rev. B 94, 140403(R) (2016). [3] C. N. Wu, C. C. Tseng, Y. T. Fanchiang, C. K. Cheng, K. Y. Lin, S. L. Yeh, S. R. Yang, C. T. Wu, T. Liu, M. Wu, M. Hong, and J. Kwo, Sci. Rep. 8, 11087 (2018). [4] C. O. Avci, A. Quindeau, C.-F. Pai, M. Mann, L. Caretta, A. S. Tang, M. C. Onbasli, C. A. Ross, and G. S. D. Beach, , Nat. Mat. 16, 309 (2017). [5] A. O. Leon, A. B. Cahaya, and G. E. W. Bauer, Phys. Rev. Lett. 120, 027201 (2018). [6] AO Leon, GEW Bauer, accepted by Journal of Physics: Condensed Matter. [7] M. Imai, H. Chudo, M. Ono, K. Harii, M. Matsuo, Y. Ohnuma, S. Maekawa, and E. Saitoh, Appl. Phys. Lett. 114, 162402 (2019). [8] A. B. Cahaya, A. O. Leon, M. Rahimi Aliabad, G. E. W. Bauer, arXiv:2006.13024

Rotational currents around lanthanide dopants in simple metals

The signature of magnetism without having a ferromagnet in the sample stack is both novel and fascinating from fundamental research point of view. It is possible to alter the electronic states of non-ferromagnetic materials (Cu, Mn, Sc, Pt) to overcome the Stoner criterion and make them ferromagnetic at room temperature1,2. In this context it is desired to quantify the magnetic moment at the interface of such non-FM/OSC. We have studied the Cu/C60 heterostructure and quantified the magnetic moment induced in Cu using X-ray magnetic circular dichroism (XMCD) sum rules. The multilayer of Cu/C60 were prepared on Si (100) substrate without breaking the vacuum using DC magnetron sputtering and thermal evaporation for Cu and C60, respectively. SQUID magnetometry on sample S1:(Si(100)/Ta(5nm)/C60(8nm)/Cu(1.8nm)/C60(4nm)/Ta(2nm)) and S2:(Si(100)/Ta(5nm)/C60(16nm)/[Cu(1.8nm)/C60(8nm)]x2/Ta(2nm)) exhibit a clear ferromagnetic hysteresis even in the absence of any ferromagnetic element in the sample stack. The induced magnetic moment in the samples is corroborated to the charge transfer and the molecular coupling between the metal (Cu) and carbon. The origin of the charge state of C60 to a reconstructed interface is due to (4 4) 7-atom vacancy holes in the surface3. The possible reason of this induced magnetic moment is hybridization between dcu and pC60 orbitals1. In order to evaluate the spinterfacial magnetic moment, XMCD has been performed on the samples. Figure1 shows the XAS and XMCD spectra of the samples. The XAS spectra do not exactly match with Cu. The reason behind such misalignment is the hybridization between Cu and C60. Due to the charge transfer at the reconstructed Cu/C60 interface the density of state of Cu has been modified and exhibits ~ 0.01 μB /atom magnetic moment4. Our understanding of the interface between the NM/OSC heterostructures may bring the new insights to the field of organic spintronics.References: 1. F. Al Ma'Mari et.al., Nature 524, 69 (2015). 2. F. Al Ma’Mari et al., PNAS, 1620216114 (2017). 3. W.W.Pai et.al., PRL 104, 036103 (2010). 4. P. Sharangi et al., (unpublished)

Quantifying induced moment in non-magnetic Cu – C60 interfaces via x-ray magnetic circular dichroism

Mn-Ga alloys show promise for spintronic applications based on their tunable saturation magnetization (Ms) and goldilocks region perpendicular magnetic anisotropy (Ku).1 Thin films with perpendicular magnetic anisotropy have potential as low-dimensional highly reliable spintronic devises.2-5 The task remains to optimize magnetic properties for applications as a function of composition and dimensionality. In this study we attempt to understand the impact of composition on magnetic properties of Mn-Ga systems in the D022 structure. Density Functional Theory (DFT) calculations are used to study the role of vacancies on Ms and Ku of Mn-Ga family of alloys. While the DFT calculations capture the general trends seen in experiments, they are unable to quantitatively predict Ms and Ku with sufficient accuracy. This is attributed to the inherent complexities in the thin film growth of Mn-Ga systems, where the variations in Ms and Ku show a strong dependence on the growth variables. Machine Learning (ML) is implemented as a low-cost alternative to DFT, where we train and validate data-driven models based on Ms and Ku data compiled from surveying experimental literature. The ML models enable rapid predictions of Ms and Ku for previously unexplored compositions. New compositions are identified with implications in emerging spintronics applications.References: 1) H. Kurt, K. Rode, M. Venkatesan, P. Stamenov, PHYSICAL REVIEW B 83, 020405(R) (2011) 2) S. Iwasaki and K. Ouchi, IEEE Trans. Magn. 14, 849 (1978) 3) P. F. Carcia, J. Appl. Phys. 63, 5066 (1988) 4) T. Seki, T. Shima, K. Takanashi, Y. Takanashi, E. 82, 2461 (2003) 5) S. N. Piramanayagam, J. Appl. Phys. 102, 011301 (2007).

Density Functional Theory and Machine Learning Guided Study of Mn-Ga Alloys with Targeted Magnetic Properties

How strong correlations drive quantum states of matter with nontrivial topology is an outstanding issue in contemporary condensed matter physics. This talk will provide an overview and discuss the latest development on Weyl-Kondo semimetal, which has been concurrently advanced by theoretical [1,2] and experimental [3,4,5] studies. The theoretical work addressed a nonsymmorphic Anderson lattice model that breaks the inversion symmetry while preserving the time-reversal symmetry, whereas the experimental effort focused on the non-magnetic non-centrosymmetric and non-symmorphic heavy fermion semimetal Ce3Bi4Pd3. The salient signatures of the Weyl-Kondo semimetal include a correlation enhanced specific heat C=ΓT3 [3], and a giant zero-magnetic-field Hall response and even-in-field Hall effects [4]. The latest high magnetic field experiments observed a two-stage topological phase transition, from Weyl-Kondo semimetal to Kondo insulator to heavy-fermion metal [5]. To bridge the latest experiments and our theoretical work, we investigated the aforementioned Anderson lattice model in the presence of a Zeeman coupling. Tuning both the Zeeman coupling and the inversion-symmetry-breaking potential controls the position and number of Weyl nodes in the Brillouin zone, and several topologically distinct phases emerge in this model.References: [1] H.-H. Lai, S. E. Grefe, S. Paschen, and Q. Si, Proc. Natl. Acad. Sci. U.S.A. 115, 93 (2018). [2] S. E. Grefe, H.-H. Lai, S. Paschen, and Q. Si, Phys. Rev. B, 101, 075138 (2020). [3] S. Dzsaber, L. Prochaska, A. Sidorenko, G. Eguchi, R. Svagera, M. Waas, A. Prokofiev, Q. Si, and S. Paschen, Phys. Rev. Lett. 118, 246601 (2017). [4] S. Dzsaber, X. Yan, G. Eguchi, A. Prokofiev, T. Shiroka, P. Blaha, O. Rubel, S. E. Grefe, H.-H. Lai, Q. Si, and S. Paschen., arXiv preprint arXiv:1811.02819, (2018). [5] S. Dzsaber, D. A. Zocco, A. McCollam, F. Weickert, R. McDonald, M. Taupin, X. Yan, A. Prokofiev, L. M. K. Tang, B. Vlaar, et al., arXiv preprint arXiv:1906.01182 (2019).

Weyl-Kondo Semimetals: Correlated Topology and Control of Nodes INVITED

Magnon spin Nernst effect (SNE) has recently been proposed as a bulk effect in antiferromagnetic insulators without considering diffusions and boundaries1,2. However, real experiments3 deal with situations far from the ideal bulk picture in that: 1) the system is finite and shares imperfect boundaries with the detecting leads; 2) both the electrons in the leads and the magnons are diffusive; 3) the thermal power rather than the temperature gradient is calibrated. To properly accommodate these complexities, we formulate a diffusive theory of the SNE of antiferromagnetic magnons under a realistic device geometry. By solving the coupled spin diffusion equations of magnons and electrons with a finite spin transmission across the boundaries, we find that the detectable signal can be much larger than that predicted by the bulk picture. The diffusive effects also lead to a different phase diagram of the SNE compared to the bulk theory without diffusion. Counterintuitively, we also find that the diffusive magnon SNE exhibits an even symmetry with respect to a perpendicular magnetic field.References: 1. R. Cheng, S. Okamoto and D. Xiao, Phys. Rev. Lett., Vol. 117, 217202 (2016) 2. V. A. Zyuzin and A. A. Kovalev, Phys. Rev. Lett., Vol. 117, 217203 (2016) 3. Y. Shiomi, R. Takashima and E. Saitoh, Phys. Rev. B, Vol. 96, 134425 (2017)

Diffusive spin Nernst effect of antiferromagnetic magnons

Topological spin textures, such as skyrmions, have attracted intense interest in recent years. Magnetic skyrmions have been studied almost exclusively in bulk ferromagnets (FM) or FM multilayers. Antiferromagnets (AF) have also been predicted to harbor topological spin textures such as AF skyrmions. Topological Hall effect has been a primary indicator of spin textures in magnetic materials. We observe unambiguous evidence of topological Hall effect in Pt/Cr2O3 bilayers grown on Al2O3(0001) and Al2O3(11-20), where Cr2O3 epitaxial film is an antiferromagnetic insulator[1]. Although Cr2O3 is insulating, the topological magnetic textures can be detected through a novel phenomenon: “spin-Hall topological Hall effect” (SH-THE), similar to what recently observed topological Hall effect in Pt/Tm3Fe5O12 bilayers[2]. The Pt/Cr2O3 bilayers exhibit clear topological Hall resistivity for Cr2O3 thicknesses below 6 nm near and above room temperature, which is above the Néel temperature of Cr2O3, revealing the key role of thermal fluctuations in the formation of spin textures[3]. The similarity of topological Hall signals in (0001) and (11-20)-oriented Cr2O3 films indicates that the emergence of spin textures is insensitive to crystalline orientation. This first observational evidence of topological Hall effect in HM/AFI bilayers significantly expands our materials base to include the large family of AF insulators for the exploration of AF-based skyrmion technology.References: [1]. Yang Cheng, Sisheng Yu, Menglin Zhu, Jinwoo Hwang and Fengyuan Yang, “Evidence of Topological Hall Effect in Pt/Antiferromagnetic-Insulator Bilayers,” Phys. Rev. Lett. 123, 237206. [2]. A. S. Ahmed, A. J. Lee, N. Bagués, B. A. McCullian, A. M. A. Thabt, A. Perrine, P.-K. Wu, J. R. Rowland, M. Randeria, P. C. Hammel, D. W. McComb, and F.. Y. Yang, “Spin-Hall topological Hall effect in highly tunable Pt/ferrimagnetic-insulator bilayers,” Nano Lett., 19, 5683 (2019). [3]. Wenbo Wang, Matthew W Daniels, Zhaoliang Liao, Yifan Zhao, Jun Wang, Gertjan Koster, Guus Rijnders, Cui-Zu Chang, Di Xiao and Weida Wu, "Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular magnetic anisotropy" Nat. Mater. 2019, 18, 1054.

Premium content

Effects of Geometry on Curvilinear Spin Chains

Downloads

Next from MMM 2020

Epitaxial Growth Of The Weyl Semimetal YbPtBi (111) On C-plane Sapphire By Co-Sputtering

Similar lecture

Enhancing magnetism in Fe-Al alloys by increasing chemical disorder