United States

Due to the expected increase of the energy consumption related to cooling, finding more energy efficient and environment friendly alternatives is of high interest for researchers. A promising alternative are cooling systems based on the magnetocaloric effect (MCE).1,2 Since the discovery of the giant MCE,3 several material systems, e.g. La-Fe-Si-,4 Fe2P-,5 and Heusler alloys,6 have been studied extensively as their MCE properties are tunable by the elemental composition. However, systematic studies on the effect of microstructure and intrinsic properties, e.g. anisotropy, are limited. We propose a methodology for calculating the MCE in the vicinity of TC in second-order-type materials by combining micromagnetic simulations with the Arrott-Noakes equation (ANE). Firstly, micromagnetic simulations are performed using the Object Oriented MicroMagnetic Framework (OOMMF)7 in which the microstructure and intrinsic properties can be varied. Secondly, the parameters of the ANE are determined from the simulations. Finally, the parameterized ANE is used to calculate the entropy change ΔST around TC according to the Maxwell relations.8 We choose Co2B nanograins as an example system. A simple microstructure of 4x4x4 cube-shaped Co2B nanograins is generated, for which we vary the grain size and boundary thickness. Fig. 1 shows the calculated ΔST in comparison with experimental results on a Co2B single crystal.5 It is found that decreasing the grain size results in increased ΔST, while the influence of the grain boundary thickness is negligible. The proposed methdology allows for investigating the effect of microstructure and intrinsic properties in micromagnetic simulations and extrapolating them to temperatures above TC. This allows for more systematic studies on the relation between microstructural and intrinsic properties and the MCE. The authors acknowledge the support from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation programme (Grant agreement No 743116, project CoolInnov) and from the CRC/TRR 270 HoMMage (DFG).References: 1. O. Gutfleisch, M. A. Willard, E. Brück, Adv. Mater., 23(7), 821–842, 2011. 2. V. Franco, J.S. S Blázquez, B. Ingale, Annu. Rev. Mater. Res., 42(1), 305–342, 2012. 3. V. K. Pecharsky, K. A. Gschneidner, Phys. Rev. Lett., 78(23):4494–4497, 1997. 4. J. Liu, J. D. Moore, K. P. Skokov, Scr. Mater., 67(6), 584–589, 2012. 5. M. Fries, L. Pfeuffer, E. Bruder, Acta Mater., 132, 222–229, 2017. 6. T. Gottschall, K. P. Skokov, D. Benke, Phys. Rev. B, 93(18), 2–7, 2016. 7. M. J. Donahue, D. G. Porter. OOMMF User Guide. xiii–xxii, 2012. 8. J. C. Maxwell, Phil. Trans. R. Soc., 155, 459–512, 1865.

MMM 2020

Calculating the magnetocaloric effect in second-order-type material by micromagnetic simulations

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.

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technical paper

The realization of energy efficient spin-current based electronics relies on the existence of low-loss magnetic insulator thin films capable of generating spin currents without an accompanying charge current. Currently, the number of such materials is extremely limited, with the only candidates being the garnets Y3Fe5O12 (YIG) [1] and Lu3Fe5O12 (LuIG) [2] along with the recently discovered spinels Ni0.65Zn0.35Al0.8Fe1.2O4 (NZAFO) [3] and MgAl0.5Fe1.5O4 (MAFO) [4,5]. Bulk phase Li0.5Fe2.5O4 (LFO) is well known to have the lowest Gilbert damping parameter (α ≈ 0.002) among the spinel ferrites, but thus far LFO thin films have not lived up to these expectations. In this talk, we demonstrate the realization of epitaxial ultra-thin (~3 nm) LFO thin films on MgAl2O4 substrates with an ultra-low damping parameter of α ≈ 0.0013 and an FMR linewidth of 1.17 mT at 20.6 GHz. Such a damping value rivals those of YIG and is comparable to the lowest damping previously reported in sub-10-nm magnetic films [2]. LFO also offers distinct advantages over YIG such as smaller external field requirements and thinner interfacial magnetic dead layers due to the low growth temperature, making it ideal for magnetic switching applications. Our results show the promise of LFO as a new candidate for spin-current based spintronics. This work was supported by the U.S. Department of Energy, Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Contract No. DESC0008505.References: [1] B. M. Howe, S. Emori, H. Jeon, IEEE Magnetics Letters., Vol. 6, p. 1-4 (2015) [2] C. L. Jermain, H. Paik, S. V. Aradhya, Applied Physics Letters., Vol. 109, p. 192408 (2016) [3] M. T. Gray, S. Emori, B. A. Gray, Physics Review Applied., Vol. 9, p. 064039 (2018) [4] S. Emori, D. Yi, S. Crossley, Nano Letters., Vol. 18, p. 4273-4278 (2018) [5] J. J. Wisser, S. Emori. L. Riddiford, Applied Physics Letters., Vol. 115, p. 132404 (2019) 


Ultra-low Magnetic Damping in Epitaxial Li0.5Fe2.5O4 Thin Films

High magnetostriction and low damping are two crucial properties for voltage tunable high frequency devices using magnetostrictive ferromagnet-piezoelectric hybrid structures to achieve low microwave loss [1, 2]. FeGa is a promising candidate for such devices as it is free of rare earth elements and has a large magnetostriction [3]. We grew epitaxial FeGa films on MgO (100) substrates using off-axis sputtering technique. Broadband ferromagnetic resonance (FMR) is used to investigate the magnetization dynamics of the epitaxial FeGa thin films. The resonance field Hres versus frequency f data is fitted using the Kittel equation to extract the reduced gyromagnetic ratio γ′, the effective magnetization Meff, and the four-fold magnetic anisotropy H4. The frequency dependence of the linewidth data is used to extract the effective damping αeff and residual linewidth ΔH0 due to inhomogeneities in the film. We report ultra-low residual linewidth, ΔH0 = 13±1 [Oe] – indicative of the high quality of the epitaxial films- significantly lower than reported previously [4]. The effective damping parameter αeff is found to be as low as 0.0065(+0.0005 -0.0001), as shown in figure 1, which is one order magnitude lower than previously reported values [5, 6]. In-plane angular dependence of resonance field Hres clearly demonstrates a four-fold in-plane anisotropy arising from the crystal symmetry. The four-fold anisotropy extracted from the Kittel plot H4 = -786±15 [Oe] is in good agreement with the value determined from fitting of in-plane rotation data H4 = -795±1 [Oe] as shown in figure 2.References: 1) V. Novosad et al., Journal of applied physics 87, 6400 (2000). 2) J. Lou et al., Applied Physics Letters 92, 262502 (2008). 3) A. E. Clark et al., Materials transactions 43, 881 (2002). 4) B. K. Kuanr et al., Journal of Applied Physics 115, 17C112 (2014). 5) D. Parkes et al., Scientific reports 3, 2220 (2013). 6) D. B. Gopman et al., IEEE transactions on magnetics 53, 1 (2017). 


Low damping and linewidth in magnetostrictive epitaxial FeGa thin films using ferromagnetic resonance

Identifying the origins of ferromagnetic resonance (FMR) linewidth broadening is essential for the fundamental understanding of magnetic relaxation, often discussed in terms of “damping”. It is not yet well understood how inhomogeneity and structural disorder contribute to magnetic relaxation in polycrystalline thin films, even though they are widely used in device applications. In this presentation, we systematically examine the impact of inhomogeneity and disorder on the FMR linewidth of sputter-grown polycrystalline Fe and Fe80V20 films with thickness 2-25 nm and different seed layers. A highly nonlinear frequency dependence of FMR linewidth is observed for Fe films on Cu/Ti seed layers. As shown in Fig. 1, all linewidth vs frequency data are quantitatively reproduced with a two-magnon scattering model [1, 2, 3] that accounts for a random distribution of the effective anisotropy field among grains in the polycrystalline film. The fit results indicate that a larger grain size is correlated with stronger two-magnon scattering, which – somewhat counterintuitively – may be accompanied by a lower Gilbert damping parameter. Films with the strongest two-magnon scattering contribution (cf. 10-nm-thick Fe/Cu/Ti, Fig. 1) exhibit the highest coercivity, pointing to a connection between magnetic inhomogeneity and the observed non-Gilbert relaxation. In addition, it is found that two-magnon scattering is reduced by changing the seed layer or alloying Fe with V (Fig. 2). We will discuss the possible mechanisms that lead to narrower FMR linewidths in polycrystalline Fe(V) films. The fundamental understanding gained from our study enables a coherent approach to develop low-loss polycrystalline magnetic media for power-efficient spintronic devices.References: [1] R. McMichael and P. Krivosik, IEEE Trans. Mag., Vol. 40, p.2 (2004) [2] S. Kalarickal, P. Krivosik and J. Das, Phys. Rev. B, Vol. 77, p.054427 (2008) [3] W. Peria, T. Peterson and A. McFadden, Phys. Rev. B, Vol. 101, p.134430 (2020) 


Two-magnon Scattering in Polycrystalline Fe and FeV Alloy Thin Films

Magnetic materials showing perpendicular magnetic anisotropy (PMA) and low damping are of great importance in spintronics. Co/Ni multilayers have been known to exhibit large tunable PMA [1], high spin polarization [2] and relatively low intrinsic damping [3]. In this study we investigated a series of Pt/[Co/Ni]N/Ir multilayers grown on Si/SiO2 with a Ta/Pt seed adhesion layer and the multilayer stack was capped with an Ir/Pt/Ta capping layer. Broadband ferromagnetic resonance (FMR) and polar angle dependent FMR measurements reveal the presence of a strong fourth-order contribution to the perpendicular anisotropy. Polar angle dependent data are fitted using the Smit-Beljers relation [4] to extract both the second and fourth-order anisotropy constants (shown in Fig.1). This model explains the data qualitatively and we observe systematic deviations for samples near or in the easy cone state of the phase diagram. We find that the effective Gilbert damping scales with the inverse multilayer thickness, indicating a significant interfacial contribution to the damping process.References: 1. G. Daalderop, P. Kelly and F. D. Broeder, Physical Review Letters., vol 68, p:682-685 (1992) 2. S. Mangin, D. Ravelsona, J. A. Katine, et.al., Nature Materials., vol 5, p:210-215 (2006) 3. J. M. Shaw, H. T. Nembach and T. J. Silva, Journal of Applied Physics., vol 108, p: 093922 (2010) 4. J. Smith and H. Beljers, Philips Research Reports., vol 10, p:113 (1955) 



Higher-order contribution to perpendicular anisotropy and interfacial damping of Co/Ni multilayers

Insulator / 3d ferromagnet / 5d metal trilayers with broken inversion symmetry along the film normal direction is of great interest, since the broken symmetry is known to contribute to the perpendicular magnetic anisotropy (PMA) and the conversion of in-plane charge current to perpendicular spin current through the Rashba spin-orbit coupling [1, 2]. Here, we report on the PMA and magnetization dynamics of the MgO/Fe/Au trilayer with the Fe layer thickness from 0.4 nm to 1.2 nm, and compare the results with the Au/Fe/Au system with a symmetric structure along the film normal. These trilayers were grown on MgO(001) substrate by molecular beam epitaxy method. The PMA and dynamics were investigated by the time-resolved magneto-optical Kerr effect (TRMOKE) measurements. Figure 1 shows Fe thickness dependence of (a) effective anisotropy field Hkeff, (b) g-factor, (c) anisotropy distribution ΔHkeff, and (d) damping constant α of MgO/Fe/Au and Au/Fe/Au trilayers. Both MgO/Fe/Au and Au/Fe/Au showed an increase of Hkeff with decreasing the Fe thickness, and MgO/Fe/Au was confirmed to exhibit slightly larger Hkeff than Au/Fe/Au due to the Rashba effect. The g-factor of both trilayers decreased from the bulk value with decreasing the Fe thickness. ΔHkeff increased at Fe thickness of 0.8 nm for both trilayers which is considered to be due to the relaxation of lattice mismatch around the Fe thickness of 1 nm. The damping constant α of both trilayers increased with decreasing the Fe thickness from 0.8 nm to 0.4 nm due to the effect of spin pumping. The reduction of α at Fe thickness of 0.8 nm is explained by the effect of two-magnon scattering which is enhanced by the increase of ΔHkeff. Interestingly, MgO/Fe/Au trilayers show larger damping constant than Au/Fe/Au with the same Fe thickness, which may reflect the Rashba spin-orbit coupling and/or the variation in the density of states at the Fermi level.References: [1] S. E. Barnes et. al., Sci. Rep. 4, 4105 (2014). [2] I. M. Miron et al., Nat. Mater. 9, 230 (2010).

Perpendicular anisotropy and damping of MgO/Fe/Au trilayers with broken inversion symmetry

Recently, antiferromagnetic spintronics have attracted great attention, with extensive research devoted to spin transport between ferromagnetic (FM) and antiferromagnetic (AFM) materials in FM/AFM heterostructures [1-6]. In order to enable a deeper understanding of the mechanisms behind this effect, it would be invaluable to perform time resolved studies of GHz spin dynamics in systems with antiferromagnetic order with sensitivity to each absorber site individually, however, to date experimental techniques that offer these capabilities are lacking. Here, we present the first experimental observation of dynamic X-ray magnetic linear dichroism (XMLD) from GHz spin precessions driven by ferromagnetic resonance (FMR). We explore the capabilities of this new technique in two model systems, a ferromagnetic metal, i.e., Ni80Fe20 (Py), and a ferrimagnetic insulator, i.e., Ni0.65Zn0.35Al0.8Fe1.2O4 (NZAFO). Both systems yield dynamic XMLD signals consistent with our models. Under experimental geometries with oblique x-ray polarization (i.e., when the angle between static magnetization and x-ray polarization is near 45°) the dynamic XMLD response occurs primarily at the fundamental frequency of the driving RF excitation (see Fig. 1). This effect is well suited for in-plane magnetic anisotropy studies conducted in normal incidence geometry. For studies that require complementary geometries (e.g., vertical or horizontal polarization), the dynamic signal can be extracted from the 2nd harmonic response. This also works for grazing incidence measurements that are sensitive to in- vs. out-of-plane magnetic anisotropy. The presented results are a breakthrough achievement in detecting dynamic manifestations of the XMLD effect at technologically relevant, GHz frequencies. By utilizing the sensitivity of XMLD to the spin axis rather than the spin direction, one can expand conventional x-ray detected FMR towards the direct dynamic study of antiferromagnetic order.References: [1] C. Hahn et al., EPL 108, 57005 (2014) [2] H. Wang et al., Phys. Rev. Lett. 113, 097202 (2014) [3] W. Lin et al., Phys. Rev. Lett. 116, 186601 (2016) [4] Z. Qiu et al., Nat. Commun. 7, 12670 (2016) [5] Q. Li et al., Nat. Commun. 10, 5265 (2019) [6] M. Dabrowski et al., Phys. Rev. Lett. 124, 217201 (2020)

Experimental realization of linearly polarized x-ray detected ferromagnetic resonance

Heuser alloys present a rich phase diagram to engineer new materials and alter tunability of the desired material properties for potential technological applications in spintronic devices. We investigate the structural, static, and dynamic properties of Co1.5Ti0.5FeAl Heusler alloy thin films, with variable thickness (t = 5 nm, 10 nm, 20 nm, 35 nm, 50 nm, and 70 nm) grown by sputter beam epitaxy on cubic MgO (100), MAO (100) (Fig. 1), and hexagonal Al2O3 (110) substrates. By partial substitution of Ti for Co in parent alloy Co2FeAl i.e Co1.5Ti0.5FeAl, we demonstrate a very low total damping parameter of 0.00156 ± 0.00003 and a low linewidth term 17 ±1 Oe (Fig. 2) for 20 nm film grown on MAO (100) substrate using ferromagnetic resonance spectroscopy comparable to elemental Fe1,2. A pronounced fourfold anisotropy is consistent with the fourfold crystalline symmetry in the epitaxial thin films grown on cubic substrates. A saturation magnetization of ~ 5.0 μB/f.u. (where f.u. represents formula unit) was obtained at room temperature, nearly double the value based on the Slater-Pauling rule. Co1.5Ti0.5FeAl films simultaneously exhibit a very low damping and approximately 40 % lower moment than that of elemental Fe3. Taken together, the smaller value of damping, saturation moment, and linewidth for Co1.5Ti0.5FeAl films can significantly improve the efficiency of STT-switching and makes it a promising metallic ferromagnet to engineer low-loss spintronic devices.References: 1) B. Khodadadi, A. Rai, A. Sapkota, A. Srivastava, B. Nepal, Y. Lim, D.A. Smith, C. Mewes, S. 16 Budhathoki, A.J. Hauser, M. Gao, J.-F. Li, D.D. Viehland, Z. Jiang, J.J. Heremans, P. V Balachandran, T. Mewes, and S. Emori, Phys. Rev. Lett. 124, (2020). 2) M. Oogane, T. Wakitani, S. Yakata, R. Yilgin, Y. Ando, A. Sakuma, and T. Miyazaki, Jpn. J. Appl. Phys. 45, 3889 (2006) 3) J. M. D. Coey, Magnetism and magnetic materials. Cambridge, UK: Cambridge University Press, 2012.

Structural and magnetic properties of Co1.5Ti0.5FeAl Heusler alloy epitaxial thin films

Tailoring the electromagnetic properties of synthetic magnetic structures is advantageous for high frequency applications, such as engineering shielding materials in magnetic recording where control of properties such as permeability is essential. Design of suitable components requires the characterisation of materials in the gigahertz frequency range to determine the parameters necessary for simulations. We present a systematic study of the dependence of the ferromagnetic resonance (FMR) and spin wave properties of NixFe100-x alloy thin films with varied Ni composition and deposition conditions using a vector network analyser (VNA) and coplanar waveguide [1][2][3][4]. The quality of the FMR response and the effective permeability in single layer films is particularly affected by the NiFe crystal structure, most notably over the fcc-bcc boundary at x=40, Figure 1. The FMR was also improved by proper choice of seed layer and lowering deposition pressure. Increasing substrate temperature was observed to have an actively detrimental effect on ferromagnetic resonance. The effect on FMR of modulating the composition in NiFe thin films was also studied using repeated bilayer structures with a constant total thickness of the form n[NixFe100-x/NiyFe100-y], where n is the number of bilayers and determines the period of modulation. Compared to single layers, the compositionally modulated structures were generally seen to enhance the FMR signal and increase the frequency of resonance, Figure 2.References: [1] Y. Ding et al. J. Appl. Phys. 96, 2969 (2004) [2] C. Kittel, Phys Rev 73, 2 (1948) [3] Y.V. Khivintsev. J. Appl. Phys. 108, 023907 (2010) [4] Kalarickal et al. J. Appl. Phys. 99, 093909 (2006)

A Systematic Ferromagnetic Resonance Study of Deposition Conditions in NiFe Thin Films for Shielding Applications.

Two-magnon scattering (TMS) is a well-known extrinsic scattering process in ferromagnets, which is usually observed through its broadening effect on ferromagnetic resonance (FMR) lineshapes for in-plane magnetic field [1]. A lesser known effect is frequency pulling on FMR as a result of the hybridization between interacting magnon modes. The frequency pulling is not independent of the linewidth broadening effect and can be calculated if the two-magnon linewidth is known. We present broadband ferromagnetic resonance measurements for in-plane (IP) and perpendicular-to-plane (PP) directions of applied field in order to demonstrate the two-magnon frequency pulling effect in Fe0.7Ga0.3 thin films. We first show that a satisfactory fit of the resonance frequencies cannot be obtained when the magnetization is IP, and further, that the fit parameters which are obtained differ greatly from the fit parameters obtained for PP magnetization. We address these problems by first fitting the linewidths (where the two-magnon effect is very pronounced) to a combined two-magnon scattering and Gilbert damping model, and with those fit parameters calculate the corresponding two-magnon resonance line shifts (see figure). The fit to the two-magnon model yields a defect correlation length which we corroborate with structural characterization. We then show that this leads to a much better fit of the IP field-dependent dispersion. In addition, the effective demagnetizing field can be fixed to the value obtained for PP magnetization, and a good fit is still obtained. Thus the inconsistencies between IP and PP dispersions are eliminated by accounting for the frequency pulling effect of two-magnon scattering, and we emphasize the importance of this effect when using FMR to characterize static magnetic properties of thin films. This work was supported by SMART, a center funded by nCORE, a Semiconductor Research Corporation program sponsored by NIST.References: [1] R. D. McMichael and P. Krivosik, IEEE Trans. Mag. 40, 2 (2004).

Two-Magnon Frequency Pulling Effect in Ferromagnetic Resonance

Current magnetocaloric materials exhibit large adiabatic changes in temperature (ΔTad~10 °K) in applied magnetic fields (H) with the strength in several tens of kilo-oersted (kOe). To achieve an appreciable magnetocaloric effect (MCE) with moderate H, new magnetic materials with tailored magnetocaloric properties must be synthesized [1]. We explore the MCE in a layered system composed of a ferromagnet, NixCu1-x (x~0.7), with a relatively low Curie temperature (TC~300 °K), which is sandwiched by stronger ferromagnets, Ni80Fe20 and Co90Fe10, and exchange coupled to them across the interfaces. Switching of magnetization in the softer ferromagnetic surrounding, Ni80Fe20, exposed to H~0.1 kOe provides the isothermal magnetic entropy change (magnetocaloric potential) in the system because of magnetization redistribution in the spacer. In the limit, in which the NixCu1-x spacer can be magnetized at the interfaces up to its saturation magnetization of 4πms≈2.5 kG, the magnetocaloric potential is [2] Δs=ms2/C≈105 erg/cm3/°K (1) with C=0.4 °K being the Curie constant, whereas a separate NixCu1-x film with the same composition x exhibits such an MCE at H~100 kOe only. In real stacks with the spacer thickness ranging between 5 nm and 20 nm, we have found a significant (up to a factor of ~20) enhancement of MCE by comparison to a separate NixCu1-x film [3, 4]. However, the MCE obtained experimentally is still much lower than that expected in accordance with Eq. (1). To understand a reason for this inconsistency, we have performed simulations of the magnetization distributon across the spacer upon the basis of the phenomenological Landau theory for second-order phase transitions [4]. We find that the results of these simulations are compatible with the experimental data at realistic values of the interfacial exchange coupling constant.To further enhance the MCE in such systems, one should choose a material for the spacer, which would have a higher ms but still a low TC not far from room temperatures. This work was supported by the Russian Foundation for Basic Research (Grant No. 20-02-00356) and by the Ministry of Education and Science of Russian Federation (Grant No. 0035-2019-0022-C-01).References: [1] T. Mukherjee, et al., Magnetocaloric properties of Co/Cr superlattices, Phys. Rev. B, Vol. 79, 144406 (2009) [2] A. A. Fraerman, I. A. Shereshevskii, Magnetocaloric effect in ferromagnet/ paramagnet multilayer structures, JETP Lett., Vol. 101, p.618 (2015) [3] S.N. Vdovichev, et al., High magnetocaloric efficiency of a NiFe/NiCu/CoFe/MnIr multilayer in a small magnetic field, Phys. Rev. B, Vol. 98, 014428 (2018) [4] M.A. Kuznetsov, et al., Magnetocaloric effect in exchange-coupled strong/weak/ strong ferromagnet stacks, J. Appl. Phys., Vol. 127, 183904 (2020)

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Calculating the magnetocaloric effect in second-order-type material by micromagnetic simulations

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