Tunable perpendicular magnetic anisotropy (PMA) in thin film magnetic devices provides an isotropic in-plane permeability, necessary for integrated microwave and mm-wave devices and an opportunity to break inversion symmetry that allows implementations of non-reciprocal devices like circulators and isolators. In ferrimagnetic alloys like GdxCo1-x, this is usually achieved by changing the composition such that the magnetic properties lie on either side of the composition-dependent magnetic compensation at room temperature, as shown in Fig. 1. Different compositions achieved in this work by changing the sputtering power of the Gd target show that magnetic compensation window lies between x = 28-32, and the effective anisotropy diverges about the compensation point. 

One less explored pathway is the incorporation of oxygen, which until now has been ruled out as a detriment to the ferrimagnetic properties [1]. Here, we show that preferential oxygenation of GdxCo1-x through a reactive sputtering process, such that the cation (Gd and Co) to oxygen anion ratio is 1:1, increases the effective anisotropy energy density by two orders of magnitude compared to the corresponding alloys without oxygen. X-ray photoelectron spectroscopy reveals that strong PMA with an optimal oxygen flow results in a Co-deficient stoichiometry of Gd21Co28O51, which suggests that strong ferrimagnetic order likely arises from a superexchange-like coupling between Gd and Co via the non-magnetic O [2]. This can produce a spontaneous magnetization in the out-of-plane direction due to weaker exchange interactions within the alloy in a heavy-metal/ferrimagnet heterostructure for PMA. Even greater anisotropy fields on the order of 11 kOe are achieved in a super-lattice with 10 repetitions of the GdxCo1-x heterostructure unit-cell, which corresponds to an operating frequency of 30 GHz. Therefore, not only does oxygenation increase PMA and provide tunable anisotropy to amorphous Gd-Co films, but the oxygen incorporation itself also increases the resistivity of the film, making it even more suitable for that frequency range. 
**References:** [1] K. Ueda, A. J. Tan, and G. S. D. Beach, AIP Advances, vol. 8, no. 12, p. 125204, Dec. 2018. 
[2] R. J. Gambino and J. J. Cuomo, Journal of Vacuum Science and Technology, vol. 15, no. 2, pp. 296–301, Mar. 1978. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/0641d84a745d89c9edb1bbc17e16653a.png) 
Fig. 1. Saturation Magnetization and Effective Anisotropy Energy Density for varying GdxCo1-x compositions for x = 18-39.

Role of Composition and Oxygen in Ferrimagnetic GdxCo1 x Alloys for Microwave/mm

High-performance magneto-dielectrics with high permeability and permittivity, low magnetic and dielectric losses, and high cutoff frequency are promising candidate materials for next-generation communication applications, owing to their significant advantages for antenna miniaturization and bandwidth enhancement. Due to limited resonant frequencies and large natural resonance linewidths, conventional microwave ferrites and their composites, which are usually used over the frequency range from several hundred MHz to several GHz, do not meet the requirements for 5G communications.1-3 A unique type of hexaferrites named Mg-Zn 18H hexaferrite, whose crystal structure can be interpreted as the structure of Y-type hexaferrites with intercalation of a three-layered hexagonal barium titanate block into the middle of the T-block as shown in Fig. 1a, has been synthesized, and its microwave properties have been comprehensively investigated for the first time.4 The Mg-Zn 18H hexaferrite shows an extremely low damping coefficient of 0.1-0.2, indicating concentrated magnetic loss within a narrow frequency band and a narrow ferrimagnetic resonance linewidth of 486-660 Oe. Owing to this remarkably low damping coefficient, the Mg-Zn 18H hexaferrite possesses an excellent magnetic loss tangent as low as 0.06 and a small dielectric loss tangent less than 0.006 over 2-4 GHz, representing superior magneto-dielectric performance among the reported microwave ferrites for the S band applications as shown in Fig. 1b. Moreover, excellent thermal stability for the damping coefficient is observed to be 0.0004 K-1 over the temperature variation of most practical applications. When employed as the substrate of a 3.6-GHz patch antenna, the Mg-Zn 18H hexaferrite shows a miniaturization factor of ~5 and significant bandwidth improvement of ~50-110% over conventional dielectrics. These results imply the great technological potential and commercial value of the 18H hexaferrites for 5G communications. 
**References:** 1V. G. Harris, IEEE Trans. Mag. 48, 1075 (2012). 
2V. G. Harris, A. Geiler, Y. Chen, S. Yoon, M. Wu, A. Yang, Z. Chen, P. He, P. V. Parimi, X. Zuo, C. E. Patton, M. Abe, O. Acher, and C. Vittoria, J. Magn. Magn. Mater. 321, 2035 (2009). 
3Q. Li, Y. Chen, C. Yu, K. Qian, V. G. Harris, J. Am. Ceram. Soc. 103, 5076 (2020). 
4Q. Li, Y. Chen, C. Yu, L, Young, J. Spector, and V. G. Harris, Acta Mater. 231, 117854 (2022). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/b2b57c67782800adf088a5c437a9d3be.png) 
(a) Schematic illustration of the Mg 18H hexaferrite unit cell with the stacking subunit blocks denoted. (b) Performance of magneto-dielectric materials for microwave applications. The figure of merit (FOM) are indicated by the dashed lines.

Emerging Magneto Dielectric Materials for 5G Antenna Miniaturization: 18H Hexaferrites

The structurally coupled interaction between a piezoelectric and ferromagnetic magnetostrictive material can be exploited to develop magnetoelectric systems which have potential to rival current technologies with reduced power consumption and size. In a magnetoelectric multiferroic system the bilateral control of the magnetic and piezoelectric response is dictated by the material properties, where a soft magnetic material with large magnetostriction facilitates the large voltage-induced strain in the piezo/ferroelectric layer. Thin film deposition techniques utilized for device fabrication often complicate the optimization of the interaction between layers due to inherent stresses occurring during deposition which are often deleterious to the magnetoelectric functionality. While incorporation of metalloid substitutions, such as in (Fe0.5Co0.5)1-xCx, and (Fe0.5Co0.5)1-xBx are able to reduce the inherent coercive field and retain a high magnetostrictive and piezomagnetic properties the as-deposited stress states are rarely discussed [1,2]. To this end we have studied the incorporation of Hafnium in to an Fe50Co50 matrix with a focus on correlation between the film stress and magnetic properties. In these sputter-deposited thin films we find a relationship among the magnetic and stress properties as a function of doping percentage. With increasing Hf composition the system exhibits a minimum coercive field occurring at a crossover from a compressive to tensile stress state. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) of the (Fe0.5Co0.5)1-xHfx system reveal the magnetic softening is also correlated to emergence of an amorphous phase with reduced grain size for these sputter-deposited films. This ternary alloy presents an additional avenue for optimization of artificial magnetoelectric systems and has applications for multiple types of microelectromechanical devices. 
**References:** [1] J. Wang, Phys. Rev. Applied 12, 034011 (2019) 
[2] J. Lou, Appl. Phys. Lett. 91, 182504 (2007)

Development of a Hafnium Iron

Enormous research activities to develop advanced superparamagnetic iron oxide nanoparticles (SPIONPs) enabling to generate higher AC heat induction temperature (TAC) or specific loss power (SLP) at the biologically safe and physiologically tolerable range of AC magnetic field (HAC) (HAC,safe < 1.8 × 109 A m-1 s-1, fappl < 120 kHz, Happl < 190 Oe (12.5 A m-1)) have been intensively made in the past two decades for magnetic hyperthermia application [1-3]. According to the reports made so far, a high magnetic softness (or high magnetic moment m) and a high initial susceptibility (χ0) at the DC magnetic field were revealed to be critical parameters in enhancing the TAC (SLP) due to the correspondingly enhanced Néel relaxation time constant (τN), which is the same as the effective relaxation time constant (τeff), τN ≈ τeff , in solid-state SPIONPs. However, MnFe2O4 SPIONPs with high magnetic moment usually show low TAC (SLP) compared to magnetically semi-hard CoFe2O4 SPIONPs with a relatively low m [4,5]. 
In this work, our research efforts primarily focused on exploring the physical and chemical reasons why MnFe2O4 SPIONPs with high DC magnetic softness show an extremely low TAC (SLP) by systematically investigating the physical role of magnetically semi-hard Co2+ cation addition in enhancing TAC (SLP) of solid (CoxMn1-x)Fe2O4 SPIONPs. According to the experimentally and theoretically analyzed results, it was clearly demonstrated that the enhancement of magnetic anisotropy (Ku)-dependent AC magnetic softness including the Néel relaxation time constant τN (≈ τeff), and its dependent out-of-phase susceptibility (χ″) are the most dominant parameters to enhance the TAC (SLP). This clarified result strongly suggests that the development of new design and synthesis methods enabling to significantly enhance the Ku by improving the crystalline, shape, stress (magnetoelastic), thermally-induced, and exchange anisotropies is the most critical to enhance the TAC (SLP) of SPIONPs at the HAC,safe (particularly at the lower fappl < 120 kHz) for clinically safe magnetic nanoparticle hyperthermia. 
**References:** [1] Joshi et al, J. Phys. Chem. C, 113, 17761-17767 (2009) 
[2] Lee et al, Nat. Nanotechnol., 6, 418-422 (2011) 
[3] Sathya et al, Chem. Mater., 28 1769-1780 (2016) 
[4] Mameli et al, Nanoscale, 8 10124-10137 (2016) 
[5] Albino et al, J. Phys. Chem. C, 123, 6148-6157 (2019)

The Role of Co2+ Cation Addition in Enhancing the AC Heat Induction Power of (CoxMn1 x)Fe2O4 Superparamagnetic Nanoparticles

Rare-earth iron garnets, a ferrimagnetic insulator, have received attention for their many properties including small damping and tunable magnetization and angular momentum compensation temperatures. Most research efforts have been focused on garnet thin films grown on the (111) plane because of their perpendicular magnetic anisotropy (PMA), and the in-plane anisotropy, however neglected, can be important for our understanding of the switching behavior and the energetics of magnetic textures. In-plane anisotropy can be conveniently explored in garnet thin films on (110) planes. Here we combine a theoretical and experimental analysis of the anisotropy landscape for Europium iron garnet (EuIG) epitaxially grown on (110) Gadolinium Gallium garnet (GGG) using pulsed laser deposition. 

The angular dependence of anisotropy is calculated by combining the individual contributions including magnetocrystalline anisotropy, magnetoelastic anisotropy, shape anisotropy and growth-induced anisotropy, which may contribute for non-ideal stoichiometry. This predicts an easy axis out of plane along [110], with hard and intermediate axes in plane along [001] and [1-10] respectively. The anisotropy landscape is probed with vibrating sample magnetometry (VSM) and spin-Hall magnetoresistance (SMR) measurements. The SMR result is simulated and fitted to a Stoner-Wohlfarth model. The experimental measurements yield the expected hard and easy axes but the model requires additional multi-domain consideration to capture the hysteresis behavior when the external field is swept along the intermediate in-plane axis. Understanding the in-plane anisotropy effect, especially the unexpected hysteresis in in-plane direction for a PMA film could have help with the interpretation of the switching behavior. 
**References:** 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/63c6aed2fe471bacc1d3440ce0a33059.png) 
Fig. 1 (a) VSM measurement along the in-plane easy (IP 35, [1-10]), in-plane hard (-55, [001]) and out-of-plane (OP, [110]) axes. (b) Left: Principal anisotropy axis definition in EuIG and setup for Stoner Wohlfarth simulation. Right: 3D polar plot for anisotropy landscape in EuIG. (c) SMR measurement for field swept along the hard anisotropy axis, with fit (red). (d) SMR measurement for field swept along the intermediate anisotropy axis.

Anisotropy Landscape Analysis in (110) Rare earth Iron Garnet Films

Spin-orbit torque (SOT) arising from spin-orbit coupling promises efficient magnetization switching in spintronic devices. It is important for device applications that the SOT switches perpendicular magnetizations without an external magnetic field. In addition to field-free switching, reducing SOT switching current is another essential requirement for low energy consumption.
In this talk, we first demonstrate spin currents and associated SOT in ferromagnet/non-magnet/ferromagnet trilayers, in which the torque on the top magnetic layer can be controlled by changing the magnetization of the bottom ferromagnetic layer. In this structure, by aligning the magnetization of the bottom magnetization parallel to the current direction, spin current can carry out-of-plane spin polarization, enabling field-free SOT switching of the top perpendicular magnetization [1]. The out-of-plane SOT is generated by spin current precession at the interface due to the interfacial spin-orbit field. Second, we show that the spin current caused by the spin anomalous Hall effect can be additionally exploited in epitaxial Co-based magnetic trilayers (2). The large in-plane magnetic anisotropy of the epitaxial Co makes it possible to control the magnetic easy axis of the bottom magnetic layer away from the current direction. It is found that the critical current for field-free switching is reduced when the magnetization is at an angle between 30 and 45 degrees from the current direction where the spin anomalous Hall effect is non-negligible.
References (1) S-h. C. Baek, et al, Nat. Mater. 17, 509 (2018)
(2) J. Ryu, et al, Nat. Electron. 5, 217 (2022)

Efficient field free spin

The combination of inversion symmetry breaking and spin-orbit coupling (SOC) give rise to spin-orbit torque (SOT), charge pumping, and chiral magnetism. In conventional SOT studies, the inversion symmetry breaking usually comes from the structure or the geometry, e.g. crystal inversion assymmety in the bulk of diluted ferromagnet with zinc-blende structure (Fig. 1a) and space inversion assymmetry in a heavy metal/ferromagnet bilayer (Fig. 1b). It was known that the SOT strength can be controlled by the magnitude of the SOC, therefore, heavy metals with large SOC has been widely used for SOT-based magnetic random-access memory (MRAM).
In our study, we demonstrated that the vertical composition gradient in a single FePt layer acts as a new type of inversion symmetry breaking (Fig. 1c). The SOT efficiencies scale almost linearly with the composition gradient (G), which is defined to be the change of the Co/Pt ratio per 1 nm. We found that by changing G from 0.31 %/nm to 0.71 %, the SOT efficiency increases by 300 %. In experiments, the composition gradient can be easily controlled during sample growth. Therefore, our work provides a powerful tool to tune the SOT strength.
Then we demonstrated the current-induced magnetization switching in a FePt single layer (Fig. 1e). However, this switching requires an in-plane external magnetic field to break the symmetry. For real application, it is pursued to realize magnetic field-free switching. To achieve this goal, we move our attention from L10 FePt (P4/mmm) to fcc CoPt3. Both of them possess good perpendicular magnetic anisotropy (PMA) and hold promise in magnetic storage (media and memory). We found that the CoPt3 has two structural properties, as illustrated in Fig. 2(a). One is the composition gradient along the thickness direction which can break the inversion symmetry and allows for the generation of the in-plane damping-like torque. The other is the formation of the Co platelets in the Pt-rich matrix near the substrate during growth. We can make a symmetry analysis of the Co platelet/Pt structure, and we can easily find that the mirror symmetry is broken relative to the (11-2) plane and preserved relative to the (1-10) plane. Therefore, when the current is applied along the [1-10] direction (low-symmetry axis), the lateral mirror symmetry is broken so that the out-of-plane SOT can be allowed. Then we can achieve the field-free magnetization switching (0 deg in Fig. 2c). In contrast, when the current is applied along the [11-2] direction (high-symmetry axis), the lateral mirror symmetry is preserved and there is no field-free switching (90 deg in Fig. 2c). We also found that the current-induced switching exhibit a three-fold angular dependence on the current angle (θI), which is in perfect agreement with the three-fold rotational symmetry of the crystal structure in Fig. 2b.
To summarize, we observed the field-assisted magnetization switching in FePt single layer by introducing a new type of inversion symmetry breaking: composition gradient. Furthermore, we demonstrated the symmetry-dependent field-free magnetization switching in the CoPt3 single layer. The composition gradient along the film's normal direction gives rise to the in-plane damping-like torque while the low symmetry (3m1) property at the interface of Co platelet/Pt gives rise to the 3m torque in CoPt3. The cooperation of these two effects leads to a three-fold field-free switching in the CoPt3 single layer. Our result of the self-switching in CoPt3 has provided one of the most simplified structures for field-free switching of perpendicular magnetization. The good endurance and high thermal stability make it a good candidate for magnetic memory devices and other spintronic applications.

**References:** 

[1] L. Liu, et.al., Electrical switching of perpendicular magnetization in a single ferromagnetic layer Physical Review B 101 (22), 220402 (2020). 

[2] L.Liu, et.al., Current-induced self-switching of perpendicular magnetization in CoPt single layer Nature Communication 13, 3539 (2022).

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/d17d241b8dd41a32df8b80ebc8770a92.jpg)

Spin orbit torque induced magnetization switching in single ferromagnetic layer

Due to their high spin polarization and easy manipulation, ferromagnetic materials dominate as the magnetically active element in spintronic devices but come with drawbacks such as large stray fields, limited scalability, and limited operational frequencies [1-3]. These shortcomings have recently inspired researchers to consider antiferromagnetic spintronics [4], as antiferromagnetic materials have no stray fields and possess ultrafast spin dynamics. However, antiferromagnets have no net magnetic moment, which leads to weak read-out signals and more difficult operation. Compensated ferrimagnets (FiMs) provide an alternative as they combine the properties of ferromagnets and antiferromagnets. In particular, FiMs can have near THz resonances as in antiferromagnets, while still having a net magnetic moment which can be used for strong read-out signals and frequency control, such as efficient tunable microwave signal output from FiM based nano-oscillators. Due to these unique properties, research in FiMs for spintronic applications is intensifying [5-7]. However, to use ferrimagnets in spintronic devices such as in spin Hall nano-oscillators (SHNOs), it is important to ensure that such films retain their advantageous properties in ultrathin films and in nano devices.
In the present study, ferrimagnetic GdFeCo thin films were grown using co-sputtering of Gd and Fe87.5Co12.5 on high resistance Si (100) substrates, and their magnetodynamic properties were studied using broadband ferromagnetic resonance measurements at room temperature. By tuning the stoichiometry of the GdFeCo films, a nearly compensated behavior is for the first time demonstrated in 2 nm thin film, as shown in Fig. 1. Values for the effective magnetization and effective Gilbert damping constant (α) of 0.02 Tesla and 0.0078±0.0002 are obtained for a 2 nm Gd24.4(FeCo)75.6 film [8], where α is comparable to the lowest value obtained in thick films [9]. The vertical dotted lines in Fig. 1 show literature values [6, 9-10] for the spin angular momentum (xa) and magnetic momentum (xm) compensation points in thick films. The composition of the films for the magnetic moment and angular momentum compensation in this work is very close to those reported in the literature.
In search of high-frequency auto-oscillations in spin Hall nano-oscillators (SHNOs), constrictions with 50-300 nm widths were also prepared using GdFeCo(2-10nm)/Pt(5nm) based stacks. Brillouing Light Scattering microscopy measurements on SHNOs show auto-oscillations in the 8 GHz range, as shown in Fig. 2 [11]. Very interestingly, different Gd compositions result in different signs of the threshold current, which indicates different signs of the effective spin-orbit torque as a function of the composition. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/0ec1329d0bf0522e3ffef8763deade00.png) 
Fig. 2. (a,b) Brillouin Light Scattering Spectroscopy (BLS) vs. current of two GdFeCo based SHNOs with different Gd content, believed to operate at opposite sides of the angular momentum conpensation point (SHNO temperature > RT due to Joule heating). While thermal SWs [ferromagnetic resonace (FMR)] are seen at all currents, the Gd rich SHNO only shows auto-oscillations (AO) at negative current, and the Gd poor SHNO only shows AO at positive current. (c,d) Log plots of the BLS counts from (a,b) for the thermal spin waves SWs (green) and the AO SWs (red). While the FMR show a slight current dependence due to heating, the AO intensity shows an initially exponential increase, as expected around threshold. Insets shows the BLS spectrum profile at the highest current.

**References:** 

[1] M. Romera et al., Nature 2018, 563, 230. 
[2] M. Zahedinejad et al., Nat. Nanotechnol. 15, 47 (2020). 
[3] B. Dieny et al., Nat. Elec. 3, 446 (2020). 
[4] T. Jungwirth et al., Nat. Nanotechnol. 11, 231 (2016). 
[5] S. K. Kim et al., Nat. Mat. 21, 24 (2022). 
[6] C. Stanciu et al., Phys. Rev. B 73, 220402 (2006). 
[7] D. Cespedes-Berrocal et al., Adv. Mat. 33, 2007047 (2021). 
[8] Lakhan Bainsla et al., Adv. Funct. Mater. 32, 2111693 (2022). 
[9] D.H. Kim et al., Phys. Rev. L 122, 127203 (2019). 
[10] Y. Hirata et al., Phys. Rev. B 97, 220403 (R) (2018). 
[11] L. Bainsla, A. A. Awad, J. Åkerman et al., unpublished.

Composition dependent threshold current polarity in ferrimagnetic spin Hall nano oscillators based on GdFeCo

Symmetry plays a central role in determining the form of electrically-generated spin torques in magnetic devices. Here, we show that an unconventional out-of-plane damping-like torque can be generated in ruthenium oxide (RuO2)/permalloy devices when the Néel vector of the collinear antiferromagnet RuO2 is canted relative to the sample plane [1]. By measuring characteristic changes in all three components of the electric-field-induced torque vector as a function of the angle of the electric field relative to the crystal axes, we find that the RuO2 generates a spin current with a well-defined tilted spin orientation that is approximately parallel to the Néel vector. This dependence is the signature of an antiferromagnetic spin-Hall effect predicted to arise from momentum-dependent spin splitting within the bandstructure of RuO2, rather than from spin-orbit coupling [2]. The unconventional components are absent in the isostructural but non-magnetic rutile oxide IrO2[3]. The out-of-plane antidamping component of the spin torque from RuO2 is among the strongest measured in any material even with the antiferromagnetic domain structure uncontrolled, suggesting that high efficiencies are useful for switching magnetic devices with perpendicular magnetic anisotropy might be achieved by controlling the domain structure. 

**References:** 

[1] A. Bose, N. J. Schreiber, R. Jain, et. al. Nature Electronics 5, 267 (2022). doi.org/10.1038/s41928-022-00744-8 
[2] R. González-Hernández, L. Šmejkal, K. Výborný et al., Phys. Rev. Lett. 126, 127701 (2021). 
[3] A. Bose, J. Nelson, X. S. Zhang. et al. Effects of anisotropic strain on spin-orbit torque produced by the Dirac nodal line semimetal IrO2. ACS Appl. Mater. Interfaces 12, 55411–55416 (2020). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/6a48eb9c2f8c65dbb2616e8a12630299.png) 

(a) Schematic representation of the tilted spin-current generated from RuO2 from the spin-split bands. Angular dependence of the generated torques in Py/(101)RuO2 bilayers originating from the y-component of the spins (a), z-component spins (b), and x-component spins (c), exhibiting angular dependence of cos2Φ, cosΦ, and sin2Φ respectively.

Tilted spin current generated by the collinear antiferromagnet RuO2

Spin orientation of LTP-MnBi can be controlled by partially substituting Bi of hexagonal LTP-MnBi with either the third alloying element or temperature. The spin reorientation occurs at about 90 K from ab-plane (0 - 90 K) to c-axis (T > 90 K) of LTP MnBi [1]. Sakuma et al. have performed first-principles calculations on Sn-doped Mn (Bi1-xSnx) and found that magnetocrystalline anisotropy constant (Ku) increases to about 3 MJ/m3 (x = 0.1) from -0.5 MJ/m3 (x = 0.0) at 0 K and then remains unchanged up to x = 0.3 [2]. However, Stutius et al. reported -0.15 MJ/m3 (x = 0.0) of the measured Ku at 4.3 K [3]. Therefore, Sakuma’s calculated Ku (x = 0.0) does not agree with the experimental Ku in [3]. This may be attributed to 300 K lattice constants in [2] used in the calculation. Sakuma did not relax the MnBi formula unit to obtain lattice constants at the ground state. 
In this study, we have performed first-principles calculations on Mn0.5Bi0.5, Mn0.5Bi0.25Sn0.25 (substitutional doping: SD), and (Mn0.5Bi0.5)66.7Sn33.3 (interstitial doping: ID) in Fig. 1 and used the relaxed lattice constants. Both a and c decrease from 4.287 and 6.118 Å to 3.954 and 5.456 Å, respectively, when Sn substitutes for Bi of LTP-MnBi. However, the a of ID increases to 4.611 Å, while the c decreases to 5.683 Å. 
Figure 2 shows density of states for MnBi-Sn. The net magnetic moment is 7.224 μB/f.u. for LTP-MnBi, 5.034 μB/f.u. for SD, and 6.609 μB/f.u for ID. The Ku is changed from in-plane (-0.202 MJ/m3) to out-of-plane (1.711 MJ/m3) by substitutionally doping Sn and maintains in-plane with interstitially doping Sn-MnBi (-0.043 MJ/m3). Our results agree with experimental one [3], but Sakuma’s. The Curie temperature decreases from 716 K for LTP-MnBi to 445 K for SD and 285 K for ID. 
* This work was supported in part by the National Science Foundation (NSF) under Grant No. 2137275. 
**References:** [1] Michael A. McGuire, Huibo Cao, Bryan C. Chakoumakos, and Brian C. Sales, Phys. Rev. B 90, 174425 (2014). 
[2] Akimasa Sakuma, Yuki Manabe, and Yohei Kota, Journal of the Physical Society of Japan 82, 073704 (2013). 
[3] W.E. Stutius, T. Chen, and T.R. Sandin, AIP Conf. Proc. 18, 1222 (1974) 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/081c10dda94527f46067ae6a0ecd1f71.png) 
Figure 1. Magnetic structure for (a) LTP-MnBi (in-plane), (b) substitutionally (out-of-plane), and (c) interstitially (in-plane) Sn-doped MnBi. 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/228c6b8e39f3866599951f9ce67548c7.png) 
Figure 2. Density of states for (a) LTP-MnBi (Mn0.5Bi0.5), (b) substitutionally Sn-doped MnBi (Mn0.5Bi0.25Sn0.25), and (c) interstitially Sn-doped MnBi ((Mn0.5Bi0.5)66.7Sn33.3).

Tuning Magnetocrystalline Anisotropy of LTP MnBi*

The resource criticality of rare-earth (RE) elements on the global market has triggered significant research into more resource-efficient alternative hard-magnetic materials[1]. The utilization of the RE-lean ThMn12 materials system in combination with the abundant RE element Ce is a promising strategy. One of the main challenges for the Ce-based hard-magnetic materials is the formation of detrimental Laves phases next to the ThMn12-type compound CeFe11Ti[2]. In this contribution, we present an ab initio-based approach to modify the stability of these phases in the Ce-Fe-Ti system by additions of 3d and 4d-elements. We start with Cu and Ga substitutions and employ state-of-the-art approaches for vibrational, electronic, and magnetic entropy contributions[3,4] to calculate the Helmholtz free energy F(T,V) for all relevant phases. The results are used to provide two fundamental methodological insights. One of them is our new modeling concept of partial decomposition, which considers the enrichment of the added solutes in phases that would at the considered temperature not be stable in a full decomposition. In the case of Cu, this effect suppresses the formation of CeFe11Ti, whereas in the case of Ga this ensures the presence of a small phase fraction of CeFe11Ti. The second conclusion is the dominant impact of 0 K formation enthalpies on the solute-enhances phase stability compared to finite temperature entropy terms. Based on this a screening approach, considering the substitution of all 3d and 4d-elements, is developed. We show that substituted elements with more than a half-filled 3d-shell or with less than a half-filled 4d-shell mainly reduce the formation temperature of the 1:12 phase. According to these thermodynamic considerations, especially Zn and Tc turn out to be promising substitution candidates. Therefore, more detailed finite temperature ab initio evaluations of the free energies have been performed for these elements. The difference between the screening determination of the critical temperature and a fully temperature-dependent determination of free energies is within the error bar of the other approximations. This demonstrates the robustness and efficiency of our developed screening approach. 
**References:** [1] O. Gutfleisch, et al.: Magnetic materials and devices for the 21st century: Stronger, lighter, and more energy efficient. Advanced Materials, 23, (2011) 821. 
[2] H. I. Sözen, et al.: Ab initio phase stabilities of Ce-based hard magnetic materials and comparison with experimental phase diagrams. Physical Review Materials, 3 (2019) 084407. 
[3] H. I. Sözen, et al.: Impact of magnetism on the phase stability modeling for rare-earth based hard magnetic materials. CALPHAD: Computer Coupling of Phase Diagrams and Thermochemistry, 68 (2020) 101731. 
[4] H. I. Sözen, et al.: Ab initio phase stabilities of rare-earth lean Nd-based hard magnets. Journal of Magnetism and Magnetic Materials, 559 (2022) 169529. 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/b3c664197ef98be7d78fc805536f1286.png) 
Figure 1

Ab initio based Thermodynamic Modeling for Sustainable Hard Magnetic Materials

Nd-Fe-B magnets are widely used in electric vehicles (EVs), wind turbines, and electronic devices in civilian and defence sectors due to their high magnetic performance. Alternate to the sintering route, the hot-deformation route does not require heavy rare elements (HRE) such as Dy and Tb and is a promising route to produce Nd-Fe-B magnets at low cost [1]. Hot deformed magnets are produced through melt spinning and hot pressing/deformation techniques [2]. Nd-Fe-B powder prepared from the melt-spun ribbon is used as precursors and hence optimization of melt spinning parameters is important to obtain desirable grain size and hard magnetic properties [3]. Hence a study was undertaken to prepare Nd-Fe-B melt-spun ribbons to tailor the microstructure and hard magnetic properties.
Melt spun ribbons with a nominal composition of Nd13.6Fe73.6Co6.6Ga0.6B5.6 alloy were prepared at various wheel speeds from 17 to 25 m/s. While the structure of the ribbons was obtained using X-Ray diffraction, the microstructure was obtained by transmission electron microscope (TEM). The magnetic properties and thermo-magnetic properties were obtained using a vibrating sample magnetometer (VSM). XRD analysis showed Nd2Fe14B phase with strong (00l) texture in all ribbons and texture fades away at higher wheel speeds. Thermal stability studied by calorimetry showed that the Nd2Fe14B phase was stable up to 500oC, beyond which precipitation of the α-Fe phase takes place. TEM analysis (Fig.1) showed that the Nd2Fe14B phase was distributed uniformly throughout the sample with a grain size of 50-150 nm. The average grain size was found to decrease with an increase in the wheel speed. The remanence (Br) and coercivity (Hc) values were found to increase with the increase in wheel speed up to 23 m/s and beyond which it starts decreasing gradually (Fig.2). Ribbons with the highest magnetic properties having a Br value of 6.6 kG and Hc value of 12.3 kOe were obtained at 23 m/s and may be best suited for the fabrication of hot deformed Nd-Fe-B magnets. 
**References:** 1 N.J. Yu, M.X. Pan, P.Y. Zhang, and H.L. Ge, J. Magn. 18, 235 (2013). 
2 K. Hioki, Sci. Technol. Adv. Mater. 22, 72 (2021). 
3 C. Rong and B. Shen, Chinese Phys. B 27, 117502 (2018). 

![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/0eba546af9c83328c8a792d6bbcd2171.png) 
Fig.1: TEM image showing the microstructure of Nd-Fe-B ribbon prepared at a wheel speed of 23 m/s 
![](https://s3.eu-west-1.amazonaws.com/underline.prod/uploads/markdown_image/1/image/cb92d75e28c54dad0e4f00e8be180f65.png) 
Fig.2: Magnetic properties (Br and Hc) of Nd-Fe-B ribbon prepared at different wheel speeds

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