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Discovery of the giant magnetocaloric effect (MCE) in interrmetallic Gd5(Si2Ge2) [1] led to an increase the interest in the materials with large MCE due to the possibility of their practical application as a working body [2] in magnetic cooling devices. Recently so-called shape-memory ferromagnets based on Heusler alloys Ni-Mn-Z (Z = Ga, In, Sn, Sb) began to be considered as the promising materials. For example, for Ni50Mn35Sn15 the value of △S = 14,2 J/(kgK) at 187 K in a magnetic field of 50 kOe [3]. The aim of this work was to study the effect of variation in chemical composition (the relative concentration Ni/Mn) on the MCE in Ni43+xMn46-xSn11 (x = 0; 1) Heusler alloys. Ingots were prepared by arc melting in an inert atmosphere and subsequently subjected to annealing at 1100 K for 24 h followed by furnace cooling. The elemental analysis and X-ray diffraction studies were performed. The magnetic and galvanomagnetic properties were measure at the Atomistitut, TU Wien using an MPMS XL7 (Quantum Design) SQUID magnetometer. The magnetic properties were measured in magnetic fields of up to 70 kOe in the temperatures range 150-330 K. As a result of this work it was established that the variation in the chemical composition does not lead to a significant change of the maximum value of the magnetocaloric effect (△S = 44 J/(kgK) and 40 J/(kgK) for Ni43Mn46Sn11 and Ni44Mn45Sn11, respectively), but shift its maximum (≈ 32 K). It was found that in the investigated alloys a change in the relative concentration Ni/Mn (and hence the e/a) does not significantly influence on the value of Curie temperature TC. The results of this work were obtained within the state assignment of the Ministry of Education and Science of the Russion Federation (theme “Spin” No. AAAA-A18-118020290104-2) and supported in part by RFBR grant (project No. 18-02-00739).References: [1] V.K. Pecharsky, K.A. Gschneider, Physical Review Letters, Vol. 78, p. 4494 (1997) [2] V.K. Pecharsky, K.A. Gschneider, Journal of Magnetism and Magnetic Materials, Vol. 200, p. 44 (1999) [3] T. Krenke, E. Duman, M. Acet, E.F. Wassermann, X. Moya, L. Mañosa, A. Planes, Nature Materials, Vol. 4, p. 450 (2005)

MMM 2020

Influence of Ni/Mn Concentration on the Magnetocaloric Effect in Ni43+xMn46-xSn11 (x = 0; 1) Heusler Alloys

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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poster

We discuss localized spin modes in the model of monoaxial chiral helimagnet. Folllowing the idea suggested in Ref. [1] we address the so-called phase of forced ferromagnetism, where in the ground state all spins align with an external magnetic field applied along the chiral axis [2]. Our analysis shows that the intrinsic localized modes, or discrete breathers, exist and the Dzyaloshinskii-Moryia (DM) interaction does not hinder their emergence. We consider the spin Hamiltonian including the Heisenberg exchange coupling, the easy-plane anisotropy, the Zeeman coupling with an external magnetic field directed along the chiral axis, and the Dzyaloshinskii-Moryia interaction term along the chiral axis. It is supposed that the external field exceeds a critical value to stabilize the forced ferromagnetic phase. By using equations of motion for spin operators of a finite chain we find an approximate analytical solution of the discrete breather, establish necessary conditions for the breather frequency and the easy-plane anisotropy strength. The discrete breather accumulates not only magnon density but a topological charge. The topological charge depends on the balance between a scale of breather localization and a pitch of the spiral determined by the DM interaction. We calculate the energy of the discrete breather. It includes a part that may be obtained in the continuum limit and a pinning part that takes into account discreteness of the spin chain. The work is supported by the RFFI grant (project 20-02-00213)References: [1] R.F. Wallis, D.L. Mills and A.D. Boardman, Physical Review B, Vol. 52, p. R3828, (1995). [2] J. Kishine, A.S. Ovchinnikov, Solid State Physics, Vol. 66, Pp. 1-130, (2015).

Discrete Magnetic Breathers In Monoaxial Chiral Helimagnet

Thin films with high saturation magnetization deposited on semiconducting substrates have the potential to enable monolithic microwave integrated circuits (MMICs) operating at higher frequencies than conventional devices based on ferrites [1]. The ability to control the anisotropy in these films is a key requirement for use in high frequency devices. To demonstrate this ability, heterostructures of 2.5 nm Ru/50 nm FM/2.5 nm Ru, (FM = Co or Fe60Co40), were deposited by magnetron sputtering on native-oxide coated silicon substrates. The samples were deposited in the presence of a 20 mT in-plane bias field. The Ru underlayer is known to reduce the interfacial perpendicular anisotropy in transition metal films compared to SiO2 [2] and the Ru overlayer protects the film from oxidizing in air. In addition to quasistatic magnetization reversal curves broadband ferromagnetic resonance measurements (FMR) and in-plane angle dependent FMR at 20GHz were used to determine the magnetic properties of the samples. This includes the saturation magnetization, the gyromagnetic ratio and the field induced in-plane anisotropy. We find that the bias field produces an easy axis along the bias field direction characterized by a uniaxial anisotropy field of 2.9 ± 0.1 mT for Co and 3.1 ± 0.1 mT for Fe60Co40. We also find a low effective damping parameter of 9.8 × 10-3 in Co and 3.4× 10-3 in Fe60Co40.References: 1. Y. D. Zhang, S. H. Wang, J. I. Xiao et al., IEEE Trans. Mag. 37, 4 (2001). 2. J. B. Mohammado, G. Mankey, C. K. A. Mewes et al., J. Appl. Phys. 125, 023901 (2019).

Field Induced Uniaxial Anisotropy in Ferromagnetic Thin Films

Recently, there has been a significant amount of interest in the areas of ultrafast magnetism and spintronics due to the search of faster methods to process and magnetically record information [1]. One of the most promising materials in the field of ultrafast magnetism and spintronics are heterostructures consisting of ferromagnetic and antiferromagnetic layers. In such structures, spin-polarized current can be excited by femtosecond laser pulses [2]. One example of such materials is an intermetallic superlattice of exchange-bonded nanolayers TbCo2/FeCo, in which interlayer interaction shows ferrimagnetic pattern. Such structure possesses giant magnetostriction, a large coefficient of magnetomechanical coupling, controlled magnetic anisotropy and spin reorientation transitions, which can be generated by a magnetic field [3]. In this paper we investigated the spin-polarized photocurrent in a TbCo2/FeCo superlattice by the THz time domain spectroscopy. The sample was grown on a SiO2 substrate by cathode sputtering in magnetic field, which resulted in an in-plane anisotropy [4]. Optical pump beam illuminates the structure through the substrate and is focused on the superlattice. This allowed to excite spin-polarized currents in the superlattice, which generated an electromagnetic field in the THz range. The generated THz radiation was detected by electro-optical sampling [5]. The generation of THz radiation from a spintronic emitter based on TbCo2/FeCo in the range from 0.1 to 3 THz was shown. The total phase change of the THz signal after switching the magnetization of the spintronic emitter was observed (Fig.1). The change of THz phase took place at the application of coercive field with the value ~ 10 mT. The work is supported by State Task for Universities FSFZ-0706-2020-0022 and RFBR (#18-52-16021)References: 1. Stanciu C. D. et al., Physical review letters, Vol. 99, p. 047601. (2007) 2. Huisman T. J. et al. Applied physics letters, Vol. 110, p. 072402.(2017) 3. A. Klimov et al, IEEE Trans. Magn., Vol. 42, p. 3090-3092 (2006). 4. N.Tiercelin et al. Journ of Mag. & Mag. Mat., Vol 249, p. 519-523 (2002). 5. Kovalev, S. P., G. Kh Kitaeva., JOSA B, Vol.30, p. 2650-2656 (2013)

Spin-polarized Photocurrent in TbCo/FeCo Superlattice Observed via THz Time Domain Spectroscopy

Strontium Ferrite magnetic nano-powder is being studied to be applied in magnetic 3D printer filament. Here we report on a study of the time dependence of its magnetic properties to better understand the permanent magnetic moment per unit volume of a nanocomposite containing the material [1,2,3]. Samples were made by loading up to 30 mg of OP-71 from Dowa Electronics Materials Company in a cylindrical custom design powder sample holder. Isotropic and anisotropic samples were made by loading the material in 0 field or in a field of 22,000 Oe. After loading the powder, the samples were compacted by compressing the powder along the cylindrical access. The typical samples were cylinders with a diameter of 2.5 mm and a length of 3 mm. The magnetic hysteresis curves of the samples were measured with a biaxial MicroSense Vibrating Sample Magnetometer (VSM) as a function of the field angle. For the isotropic samples, the average coercivity and the Mr/Ms were approximately constant as a function of the field angle for the Mx signal and were respectively 3402.35 Oe and 0.49. The shape of the Mx-hysteresis curves was independent of the field angle. For the anisotropic samples, the coercivities were around 3455 Oe (Mx curves). The Mr/Ms of the anisotropic samples depends on the field angle and varies from 0.41 along the hard axis to 0.64 along the easy axis. The coercivity of the My curves is around 4373 Oe. The coercivity of the anisotropic data seems to be larger than that of the isotropic sample due to the orientation of the crystallites. The time dependence of the Mx /Ms and My/Ms signals after a field step from saturation were measured for both the isotropic and anisotropic samples. The time derivative of the magnetization normalized on the Ms (S) depend on the field magnitude and shows a maximum. For the Mx signal this maximum coincides with the sample’s coercivity. The S(H) for the anisotropic curve for the My signal is shifted to larger fields as shown in the graph below. This work was supported in part by NSF through an DMR-MRI Grant under Award 1726970.References: [1] reference of strontium ferrite, maybe original philips patent. [2] R. Street, J. C. Wooley, Proc. Phys. Soc. London, Sect. A 62 (1949) 562. [3] L. Neel , Ann. Geophys. 5 (1949) 99.

Time Dependence of Magnetic Moment of Strontium-Ferrite Powder Measured by Biaxial VSM

Recently, to achieve faster, smaller, and more energy efficient devices for information and communications technologies, much attention have been focused on Fe-Ga films. Until now, we investigated the magnetization dynamics of Fe-Ga polycrystalline films, and suggested that further study to appreciably reduce damping constant (α) for these highly magnetostrictive films are strongly desired from technological standpoints because of wide FMR widths and high α (1) for these films. Herein, to improve the soft magnetic properties and high-frequency magnetic properties of Fe-Ga films, we prepare 10-nm-thick B-doped Fe-Ga films with different Ga composition (x) and discuss the structures and magnetic properties of films in detail. As for static magnetic properties of every thin film, the uniaxial anisotropy appears and coercivity (Hc) are much lower than those of Fe-Ga films(1). As shown in Fig. 1(a), the saturation magnetostriction (λs) increases for x ≤ 21.8 at.%, decreases for 21.8 at.% ≤ x ≤ 24.5 at.%, and further increases again for x ≥ 24.5 at.%. Their values becomes approximately 49 - 67 ppm, and are higher than that of amorphous (Fe80Ga20)0.85B15 films (50 ppm)(2). On the other hand, as for high-frequency magnetic properties of these films, the damping constant (α) increases slightly for x ≤ 20.8 at.%, decreases for 20.8 at.% ≤ x ≤ 24.5 at.%, and further increases for x ≥ 24.5 at.% (Fig. 1(b)). This trend is similar to that of λs. The α values becomes 0.006 - 0.010 and are much lower than those of Fe-Ga binary alloys films (1), (3), which might be attribute to both the structural characteristics and the static magnetic properties. Therefore, these results demonstrate that B-doped Fe-Ga thin films possess good soft magnetic properties and high-frequency magnetic properties compared to the conventional Fe-Ga films.References: (1)Y. Kawabe, Y. Endo and T. Miyazaki, T.Magn. Soc. Jpn. (Special Issues), 3, 34-38 (2019). (2)J. Lou, R. E. Insignares and Z. Cai, APPLIED PHYSICS LETTERS 91, 182504 (2007). (3)Daniel B. Gopman, Vimal Sampath and Hasnain Ahmad, IEEE TRANSACTIONS, VOL. 53, No.11, (2017).

Effect of the Ga Composition on the Soft and High-frequency Magnetic Properties of Fe85.1-xGaxB14.9 Thin Films

Physics in nanometer-thick metallic films has been attracting many researchers because of abundant spintronics nature. In ultrathin films, some physical properties, such as the spin-charge conversion [1] and ferromagnetism [2], are gate-tunable. To extend further potential of ultrathin metallic films in spintronics, we demonstrate ferromagnetic resonance (FMR) in a 1 nm-thick ultrathin Co film [3], which exhibits sufficiently small FMR resonance field and narrow FMR linewidth simultaneously, for future realization of tunable magnon excitation. We prepared SiO2 (capping layer)/Co (tCo )/SiO2 (Sample A), where tCo was set to be 1, 2, 3 and 5 nm, and SiO2 (capping layer)/Co (1 nm)/Ta (3 nm)/SiO2 (Sample B). The Co was grown by using magnetron sputtering. The sample was placed in the center of an electron spin resonance (ESR) cavity (TE011) and FMR spectra were measured. The external dc magnetic field (Hex) and ac magnetic field (hrf) were applied along the in-plane direction of the film as shown in Fig. 1. Figure 2 shows comparison of the FMR spectra between Sample A (tCo = 1 nm) and Sample B. Whereas the thickness of the Co of both samples are the same, the clear FMR signal was observed only from Sample B, where the half-width at half-maximum and the resonance field were comparable to that of the bulk Co system. The results directly indicate that the insertion of 3 nm-thick Ta plays a dominant role for generating FMR in the ultrathin Co film. Furthermore, the narrow linewidth and the low resonance field indicate that the ultrathin Co film simultaneously possesses uniform and strong magnetization, which has not been achieved previously [4,5]. The successful detection of FMR from ultrathin Co enables efficient magnon excitation that is gate-tunable. More detailed discussion will be given in the presentation.References: [1] S. Dushenko, M. Shiraishi et al., Nat. Commun. 9, 3118 (2018). [2] D. Chiba et al., Nat. Mater. 10, 853 (2011). [3] S. Yoshii, M. Shiraishi et al., submitted. [4] S. T. Purcell et al., J. Appl. Phys. 69, 5640 (1991) [5] R. Gieniusz et al., J. Magn. Magn. Mater. 272-276, E911 (2004)

Realization of Ferromagnetic Resonance in 1nm Co ultrathin film

There is increased interest in conductive ferromagnetic materials and structures due to their higher moments and compatibility with semiconductor fabrication methods and technology. They offer enhancement in resonance frequencies and higher permeabilities in composites and nano-structures for applications in telecommunications and wave absorption [1,2]. The electromagnetic wave interaction with conductive ferromagnetic structures in these applications is complex due to coupling of both electric and magnetic fields with the magnetic material. This leads to non-uniform electromagnetic fields with different skin depths (both non-magnetic and magnetic). Early theoretical work focused on simple and saturated magnetic structures, excited in the linear region with uniform fields and involved solving the linearised system of Maxwell's equations and the Landau-Liftshitz equation for simplicity. This included planar structures [3], thin-films [4], and infinite cylinders [5]. Practical confined 2-D and 3-D magnetic structures found, for example, in composites have complex ground states and magnetic pinning, multi-layered with dielectrics, and experience non-uniform internal electromagnetic fields that yield complex precession and resonance modes that are not well understood. In this work, we solved the coupled system of Maxwell's and Landau-Lifshitz-Gilbert equations using a stable algorithm based on the finite-difference time-domain (FDTD) method to study the transient electromagnetic wave propagation and resonance in infinitely long cobalt nano-prisms with square cross-section of sides 50-1000 nm, excited by a 70 GHz plane wave [6]. The simulations revealed a curling exchange resonance mode excited by the induced axial current for prism sizes less than 100 nm. Larger prisms exhibited confined edge and corner resonance modes due to local power absorption and skin effects. The magnetic skin depth at resonance was estimated to be 50 nm from the calculated local power absorption spectra (see Fig. 1).References: [1] Y. Shirakata; N. Hidaka, M. Ishitsuka, A. Teramoto, and T. Ohmi, IEEE Trans. Magn. 44, 2100 (2008). [2] Y-B. Feng, T. Qiu, C-Y. Shen, and X-Y. Li, IEEE Trans. Magn. 42, 363 (2006). [3] W. S. Ament and G. T. Rado, Phys. Rev. 97, 1558 (1955). [4] J. W. Hartwell, Proc. IEEE 56, 23 (1968). [5] L. Kraus, Czech. J. Phys. B 32, 1264 (1982). [6] M. M. Aziz and C. McKeever, Phys. Rev. Applied 13, 034073 (2020).

Electromagnetic Wave Absorption and Resonance in Infinite Cobalt Nano-Prisms

A novel magnetic cooling device (MCD) that comprises thermoelectric (TE) and magnetocaloric (MC) materials is proposed, partially based on the previous work [1]. A sandwich structure, as shown in Fig. 1, has a MC material (MCM) in the center and two TE materials at the outer parts. The presence of the TE materials in the MCD guides the heat flow direction and enhances heat pulsation. In this case, the usage of a TE material that combines large TE with small MC responses within a similar temperature region enhances not only magnetic flux density but also heat exchange efficiency. In this study, we demonstrate that MnP thin films with integrated TE and MC functionalities are an excellent candidate for the proposed MCD. The MnP films were grown on Si substrates at 300, 400 and 500°C [2], and both the MC and TE effects were investigated systematically. The 400 oC sample shows the optimal TE and MC responses. A large TE effect is observed in the temperature region 250 – 400 K. The large power factor of 24.06 μW m-1 K-2 is achieved at room temperature. In this temperature region, the film shows a small MC effect (~1.23 J/kgK at 5T) but a strong magnetic response that gives rise to the enhanced TE effect. These properties would enable the MCD to operate at high frequency.References: [1] A. Kitanovski, and P. W. Egolf, Int. J. Refrig. 33.3,449464(2010). [2] A. T. Duong, T. M. H.Nguyen, , D. L. Nguyen, R. Das, H. T. Nguyen, B. T. Phan, and S. Cho, J. Magn. Magn. Mater. 482,287291(2019).

Novel MnP thin films with integrated thermoelectric and thermomagnetic functionalities for energy-efficient solid state refrigeration

Magnetorheological elastomers are smart materials that can deform and change shape based on the application of a magnetic field. These magnetic elastomers perform best in soft robotics and biomedical applications when they have increased anisotropy, or different properties along different axes. To increase this anisotropy, one can utilize hard magnetic particulate, magnetic annealing, and fused deposition modeling (FDM), also known as 3D printing. Hard magnets can be oriented using an external alignment field, and strontium ferrite is specifically interesting because it costs less than other rare-earth magnets (or materials). FDM is usually a final step in creating anisotropy in a sample (1), and it allows magnetorheological elastomers components to be more accessible for other applications. Our thermoplastic magnetic elastomer composite is created using solvent-casting techniques with 15 vol% SrFe12O19 particulate. The material is then extruded into FDM filaments using a Filastuder. During the extrusion process, some filament is magnetically annealed in an axial applied field created by NdFeB magnets, while other filament extruded without magnetic annealing acts as the control. Mechanical stress vs. strain curves of the extruded filaments are acquired using an MTS tensile testing machine. Magnetic hysteresis loops are measured using a vibrating sample magnetometer (VSM). In order to determine the relative level of magnetic anisotropy within the samples due to these factors, hysteresis loops with the field applied parallel and perpendicular to the axis of extrusion are compared to one another. Magnetoactive testing is used to measure the mechanical deflection angle as a function of the strength of a transverse applied magnetic field. Understanding the mechanical, magnetic, and magnetoactive properties of this magnetorheological elastomer provides insight into how magnetic anisotropy can be controlled by a combination of hard magnetic particulate, magnetic annealing, and FDM for optimized performance.References: 1. Patton, et. al., “Manipulating magnetic anisotropy in fused filament fabricated parts via macroscopic shape, mesoscopic infill orientation, and infill percentage.” Additive Manufacturing, Vol. 27, pp. 482-488 (2019).

Thermoplastic Elastomers with Strontium Ferrite Particulate and Magnetic Annealing used for Fused Deposition Modeling

The design of the materials showing first-order magnetic transitions (FOMTs) with smallest possible thermomagnetic hysteresis stimulates considerable research interest because hysteresis can be detrimental for certain applications, such as solid-state caloric cooling [1]. The overwhelming majority of materials with a reasonably small hysteresis contain transition metals, and their behavior is closely associated with the itinerant-electron metamagnetism of the transition metals present in the system [2,3]. We observed anhysteretic FOMT in a lanthanide-based compound Pr2In with the classical RKKY-type magnetic interaction between 4f electrons of trivalent Pr. Although such behavior is phenomenologically similar to Eu2In, another lanthanide system showing hysteresis-free FOMT [4], the crystal structure and magnetic interactions in Eu2In are markedly different from Pr2In due to the presence of divalent Eu [4,5]. In the vicinity of ferromagnetic to paramagnetic transition temperature (TC ~57 K) Pr2In exhibits a large magnetic field-induced entropy change with the maximum value of -ΔSM ~ 23 J/kg K for ΔH = 50 kOe, which is comparable to a number of other potential magnetocaloric materials in the temperature range of 40-80 K, reported in the literature [6]. The temperature and magnetic field dependence of ΔSM can be expressed as ΔSM ~ Hn , where the local exponent n varies with both H and T, showing a clear maximum with n>>2 , a characteristic feature of FOMT [7]. The observation of anhysteretic FOMT and a large MCE in Pr2In with conventional RKKY-type magnetic exchange promises further advancements in developing novel functional caloric materials by manipulating electronic structure of 4f-based materials. Acknowledgement: This work was performed at Ames Laboratory and was supported by DMSE, BES, Office of Science of the U.S.DOE. Ames Laboratory is operated for U.S DOE by Iowa State University under Contract No. DE-AC02-07CH11358.References: 1. N. A. Zarkevich et al., Journal of Alloys and Compounds 802, 712-722 (2019). 2. F. X. Hu et al., Applied Physics Letters 78, 3675 (2001) 3. A. Fujita et al. , Physical Review B 67, 104416 (2003) 4. F. Guillou et al., Nature Communications 9, 2925 (2018). 5. A. Biswas et al., Physical Review B 101, 224402 (2020). 6. L. Li et al., Journal of Alloys and Compounds 823, 153810 (2020). 7. J. Law et al., Nature Communication 9, 2680 (2018).

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Influence of Ni/Mn Concentration on the Magnetocaloric Effect in Ni43+xMn46-xSn11 (x = 0; 1) Heusler Alloys

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