AVS 67 Session MI-Contributed On Demand: Magnetic Interfaces Contributed On Demand Session

For the development of novel spintronic devices, it is important to understand the dynamic magnetic processes on the micro- and nanoscale [1]. In spintronics the generation and manipulation of pure spin currents is in the focus of research activities. Amongst the utilized fundamental effects is spin pumping where a precessing magnetization of a ferromagnet being at ferromagnetic resonance (FMR) transfers angular momentum to an adjacent nonferromagnetic layer [2], i.e. spin current. By using lithographically fabricated micro-resonators it is possible to measure FMR of the small samples with a detection sensitivity of down to 10⁶ spins [3]. These micro-resonators allow combining STXM with XMCD spectroscopy and FMR (STXM-FMR). The STXM-FMR setup enables the visualization of the high frequency magnetization dynamics in the GHz regime with a high lateral resolution of nominally 35 nm and a time resolution of 17.4 ps [4]. In this contribution we present the STXM-FMR results for thin magnetic Py (Ni₈₀Fe₂₀) microstrips with the lateral dimensions: 5x1 µm² measured individually and adjacently to the Co-doped ZnO thin film.

Two types of samples were investigated: single Py microstrips and a heterostructure consisting of Co-doped ZnO thin film with the Py microstrip placed on top (Co:ZnO/Py) in order to investigate the ac component of the pumped spin polarization directly inside the nonferromagnet with ultimate spatio-temporal resolution and elemental selectivity. For FMR and STXM-FMR measurements a static magnetic field was applied in the plane of the strips. Both FMR and STXM-FMR measurements confirm that quasi-uniform and spin-wave modes can be excited in the Py microstrips. The results for the single Py strip show, that with increasing the static magnetic field it is possible to observe the transition from one mode to another and additionally observe superposition of the modes in-between the FMR signals. When superposition of the modes occurs, a non-standing character of the spin-waves can be observed for the Py strip. For the Co:ZnO/Py sample we demonstrate the feasibility to investigate the lateral distribution of the pumped ac spin polarization inside the adjacent non-ferromagnet, when Py microstrip is driven into quasi-uniform main FMR excitation or a spin-wave excitation [5].

Financial support by the Austrian Science Fund (FWF), Project No. I-3050 is gratefully acknowledged.

References

[1] H. Stoll et al., Front. in Phys. 3, 26 (2015).

[2] Y. Tserkovnyak et al., Phys. Rev. Lett. 88, 117601 (2002).

[3] R. Narkowicz et al., J. Magn. Reson. 175, 275 (2005).

[4]S. Bonetti et al., Rev. Sci. Instrum. 86, 093703 (2015).

We report on an additional effect influencing the observed spin polarization in SARPES (spin- and angle-resolved photoemission) and SRIPE (spin-resolved inverse photoemission) experiments. We detected and analyzed a spin-dependent intensity asymmetry for spin-degenerate surface states at the M point in the unoccupied electronic structure of Tl/Ge(111)-(1x1) [1]. Approximating the initial state by a plane wave, we calculated the SRIPE process and obtained good agreement with the experimental data [2]. Our model reveals that this spin asymmetry at a point of time-reversal invariant momentum (TRIM) is of different origin than other effects discussed in the literature, which are based on the light detection geometry, photon energy, and experimental probing depth of the electrons. Instead, the spin asymmetry in this nonmagnetic material with spin-orbit interaction is caused by breaking the initial-state time-reversal symmetry in the experiment. Since SARPES and SRIPE are based on time-reversed processes, the effect is also expected for SARPES experiments where the outgoing photoelectrons break the time-reversal symmetry in the same way as the incoming electrons in SRIPE experiments. Understanding and considering such experimentally induced spin asymmetries is essential for correctly interpreting spin-resolved (inverse) photoemission data.

[1] P. Eickholt, P. Krüger, S. D. Stolwijk, A. B. Schmidt, and M. Donath, Phys. Rev. B 93, 085412 (2016).

[2] P. Eickholt, P. Krüger, S. D. Stolwijk, A. B. Schmidt, and M. Donath, Phys. Rev. B 101, 165411 (2020).

The influence of spin-orbit interaction on low-energy electron reflection from MoS₂ on Au(111) was studied by VLEED (very-low-energy electron diffraction) [1,2,3]. Maps of the electron reflectivity and the spin asymmetry of the reflected

electron intensities were measured for a wide range of electron incidence angles and kinetic energies. To account for an adlayer coverage of about 30%, maps of the Au(111) substrate and for a MoS₂ bulk sample were measured as well.

The adlayer and substrate signals were distinguished by a comparison of the maps.

For MoS₂/Au(111), we obtained a spin asymmetry of the reflected intensities, which shows a characteristic feature with alternating sign in the energy region of a VLEED fine structure [1]. The Au(111) substrate, in contrast, shows qualitatively

different spin-asymmetry features, partially with reversed sign compared with MoS₂/Au(111). The results of bulk MoS₂ resemble the single-layer data to a great extend. The influence of the substrate on the results will be discussed.

[1] Burgbacher et al., Phys. Rev. B 87, 195411 (2013).

[2] Thiede et al., Phys. Rev. Applied 1 , 054003 (2014).

[3] Angrick et al., J. Phys.: Condens. Matter 33, 115001 (2020).

Pure spin currents have been ubiquitous in contemporary spintronics research. Despite its pro­found physical and technological significance, the detection of pure spin current has largely remained indirect, which is usually achieved by probing spin-transfer torque effects or spin-to-charge conversions. By using scanning transmission X-ray microscopy, we report the direct detection and spatial mapping of spin accumulation in a nonmagnetic Cu layer without any direct charge current injection. Such a pure spin current is induced by spin pumping from a Ni₈₀Fe₂₀ layer and is not accompanied by concomitant charge motion. The observed frequency dependence indicates that the signal is dominated by a coherent, pure spin current, but the magnitude of the spin accumulation suggests also possible additional thermal contributions. Our technique takes advantage of the X-ray magnetic circular dichroism and the synchronization of microwave with X-ray pulses, which together provide a high sensitivity for probing transient magnetic moment. From the detected X-ray signals, we observe two distinct resonance modes induced by spin pumping, Based on micromagnetic simulations, we attribute these two resonances to non-linear microwave excitations. Our result provides a new pathway for detecting pure spin currents that originate from many spintronics phenomena, such as spin Hall and spin Seebeck effects, and which can be applied to both metal and insulator spin current sources [1].

Work at Argonne, including experimental design and measurement, sample fabrication, data analysis, and manuscript preparation, was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division under Contract No. DE-AC02-06CH11357. Use of the Stanford Synchrotron Radiation Light source, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. W.Z. acknowledges the DOE visiting FacultyProgram.

References:

[1] J. Ding et al, “Direct Observation of Spin Accumulation in Cu Induced by Spin Pumping”, Phys. Rev. Research Vol 2, art no. 013262 (2020)

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Scanning Transmission X-Ray Microscopy imaged Ferromagnetic Resonance (STXM-FMR) [1, 2] at the Fe L₃-edge was used for the first-time detection of the magnetization dynamics of a bi-segmented chain of 19 Fe₃O₄ nanoparticles (particle diameter: 40-50 nm), naturally grown inside a bacterium Magnetospirillum Magnetotacticum MS-1. A uniform resonant excitation within the chain segments was measured element specifically with a spatial resolution of about 50 nm and pico second time resolution. Micromagnetic simulations of a corresponding nano particle chain agree well to the observations made in the experiment. The combination of conventional Ferromagnetic Resonance, micromagnetic simulations and STXM-FMR allows the comprehensive characterization of magnetization dynamics on the nanoscale, necessary for the development of genetically engineered spin wave computing devices, as suggested in [3].

References

[1] S. Bonetti, R. Kukreja, et al.,Rev. Sci. Instrum. 86, 093703 (2015).

[2] S. Pile, T. Feggeler, et al., Appl. Phys. Lett. 116, 072401 (2020).

[3] B. W. Zingsem, T. Feggeler, et al., Nat. Commun., 10, 1 (2019).

Partial financial support by DFG Project: OL513/1-1 is acknowledged.

Helpful discussions with A. Ney and his team are acknowledged.

Spintronic device performance is compromised when thermal effects impact magnetic properties and spin polarization of the materials [1]. One way of mitigating this problem is to find materials having a very high saturation moment (MS) and high Curie temperature (TC). The full Heusler alloy Co2FeGe theoretically has promise to meet this demand. However, its inability to form a single-phase compound makes it problematic for application. The addition of a fourth element is sometimes useful in stabilizing a single-phase compound [2], but it can be detrimental to some properties, particularly due to the increased probability of chemical disorder. In this talk, we present the successful synthesis of a single-phase compound based on Co2FeGe, viz Co2Fe1.25Ge0.75, by altering the stoichiometry rather than adding a fourth element. For the single-phase compound, MS as high as 6.7 ± 0.1 μB/f.u. at 5K and a Curie temperature of 1135 ± 5 K are measured – to our knowledge the highest reported for full Heusler alloys. In addition, thin films of Co2Fe1.25Ge0.75 deposited on Al2O3(110) and MgAl2O4(100) substrates are highly epitaxial. The structural quality of the films can be argued to be among the best reported for Heusler films to date. The thin film MS values also agree with the bulk MS. First principle calculations suggest the system exhibits total energy minimum at the experimental lattice parameter. These calculations also predict an enhancement in magnetization with a value closer to the experimental value. Hence, the highest moment, highest Curie temperature, and the ability to grow excellent quality single crystal films make Co2Fe1.25Ge0.75 a strong candidate for many device applications.

1. Wurmehl et al., Phys. Rev. B 72, 184434 (2005).

2. KC et al., Phys. Rev. Materials 3, 114406 (2019).

Co3Sn2S2 is a magnetic Weyl semimetal with kagome-lattice. In such samples, the existence of bulk Weyl nodes, which are formed under broken inversion or time-reversal symmetry, creates nontrivial topological properties like robust Giant anomalous hall effect. The surface–bulk correspondence ensures the bulk bands related topological “Fermi arc” surface bands dispersion. In this presentation, we use low temperature high magnetic field scanning tunneling microscope, spin polarized STM, and quasiparticle interference (QPI) to study the influence of local defects to the Weyl nodes movement, including magnetic and nonmagnetic vacancies and adatoms. S, Co and Sn vacancies and adatoms are identified, their behavior under magnetic field are studied. The interplay among topology, defects and magnetism are discussed for the understanding of the involved quantum phenomena.

This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Magnetoelectric (ME) materials provide the ability to efficiently control magnetism with electric fields, offering the potential to circumvent the size and efficiency limitations of traditional electronic devices. One barrier towards enhancing the efficiency in ME devices is the need for ferromagnetic materials that exhibit both a large magnetostriction and soft magnetic properties to yield a high magneto-elastic coupling. Additionally, for microwave ME applications, a low Gilbert damping is needed for low loss operation whereas materials with large magnetostriction typically exhibit high Gilbert damping coefficients due to their large spin lattice coupling (α ~ 0.2 vs ~ 0.03 for most ferromagnetic metals).

FeGa is a material of interest due to its large magnetostriction (~275 ppm in polycrystalline bulk) and large piezomagnetic coefficient (>2 ppm/Oe) but is highly lossy at high frequencies and exhibits a large magnetic hysteresis. Recent studies of FeGa/NiFe (1:1 volume fraction) multilayer (ML) thin films [1-2] have found that these nanolaminates can achieve a low coercivity (<10 Oe) and can reach a >4x increase in magnetoelectric coefficient compared to single phase FeGa films. However, prior work on FeCo/NiFe suggests that only a thin interlayer of NiFe is required to achieve a significant enhancement in soft magnetic properties [3]. In this work, we explore a multilayering strategy of FeGa with NiFe – (100 nm FeGa / N)/ (2.5 nm NiFe) – where N represents the total number of bilayers. It was found that 2 bilayers optimized the lowest in-plane coercivity (7 Oe) and Gilbert damping coefficient (0.014). Compared to multilayers with an insulating layer insertion, this is less than an order of magnitude lower than reported FeGaB(25 nm)/Al2O3/FeGaB(25 nm) (α = 0.24) [4] and within the same order of magnitude for MLs we have previously studied of FeGa(10 nm)/NiFe(10 nm)/FeGa(10nm) with an Al2O3 insertion (α = 0.006).

Furthermore, it is expected that the magnetostriction increases linearly with the number of bilayers as the contribution from interfacial magnetostriction also increases. XRD and AFM imaging are used to correlate the coercivity, gilbert damping, and magnetostriction with the grain size and surface roughness of the composite film with increasing number of bilayers. This ML strategy shown here can be used to help design materials with a higher magneto-elastic coefficient for strain-mediated magnetoelectric devices.

References:

1. Rementer, Colin R., et al. Appl. Phys. Lett. 110.24 (2017): 242403

2. Shi, Jiaxing, et al. J. Alloys Compd. 806 (2019): 1465-1468

3. Rengarajan, S., et al. J. Appl. Phys. 81.8 (1997): 4761-4763

4. Imran, Shahid, et al. Rare Metal Mat. Eng. 47.7 (2018): 1951-1957

The ferrimagnetic spinels are known for anomalous magnetoresponsive behaviors which stem from strong spin-lattice coupling and orbital ordering effects. Single crystals of these materials (Mn3O4, MnV2O4, FeV2O4) display a nanometer length-scale patterning of magnetostructural domains which is strongly correlated with these effects. The domain patterns are remarkably sensitive to environmental stress and controllable by applied magnetic fields. In this talk, we present reciprocal-space characterizations of these mesoscale patterns using small-angle neutron scattering, which allows us to correlate various response functions including inverse magnetoelastic effect and magnetodielectric effects with the onset and field response of domains. Meanwhile, neutron diffraction and small-angle X-ray scattering reveal a strong intertwining of these effects with lattice degrees of freedom, collectively establishing strain as an important tuning parameter for anomalous behavior in these materials.

The intrinsic magnetic topological insulator MnBi₂Te₄ provides a great platform to explore quantum phenomena, such as quantum anomalous Hall effect and axion insulators, as reported earlier. However, the search of interesting electromagnetic effects in this material was hindered by the difficulty of preparing its high-quality films with well-controlled composition and thickness. In this study, we compare three different types of Mn-Bi-Te samples grown by molecular beam epitaxy: a heterostructure of MnBi₂Te₄ and Bi₂Te₃, a single MnBi₂Te₄ layer, and a heterostructure of MnBi₂Te₄, Bi₂Te₃, and MnTe. We measured the anomalous Hall effect (AHE) in each case and demonstrate its evolution from a two-component AHE contain the contribution of the MnTe and MnBi₂Te₄, to single component AHE resulting from few-layer MnBi₂Te₄ scattered out in a Bi₂Te₃ matrix, and finally to an antiferromagnetic AHE, characteristic of the pure phase of MnBi₂Te₄. Our work provides an understanding of the AHE for all three possible heterostructure compositions and paves the way for the realization of new quantum phenomena in pure MnBi₂Te₄ thin films.

Single layer FeX (X=Te, Se, S) epitaxially grown on SrTiO₃ (STO) substrate represents a model system for probing a host of quantum phenomena due to the interplay of topology, magnetism and superconductivity.

For example, single layer FeTe exhibits long-range bi-collinear antiferromagnetic (AFM) order, which is not superconducting. We found that the AFM order, however, can be suppressed by alloying with Se at >10% concentrations, where enhanced superconductivity emerges [1]. Our spin spiral calculations further showed that the ground state is magnetically disordered paramagnetic, where local checkerboard (CB) AFM fluctuations is critical to the high Tc superconductivity in FeSe [2].

In this work, the impact of S incorporation into single layer FeSe is investigated. Our calculations indicate that the energy difference between CB and co-linear (CL) AFM order increases with increasing S. Fits of the spin-spiral dispersion to spin models leads to Heisenberg parameters J2/J1 of 0.55 for FeSe and 0.72 for FeS, placing the FeS system closer to the CL phase boundary and possibly decreasing the importance of CB fluctuations [3]. Experimentally, we synthesize high quality single layer FeSe_1-xS_x films on STO(001) substrates by molecular beam epitaxy, and probe their electronic properties using scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy [3]. We observe that with increasing S concentration x, the Fermi surface at M point becomes more anisotropic and the effective mass decreases from 4.3 to 1.2 m_e, while the hole pocket at Γ point stays at 60 meV below the Fermi level. More importantly, no Tc enhancement is observed for films with x > 0.7. Our findings demonstrate that the CB AFM fluctuations plays an essential role in the enhanced superconductivity in epitaxial single layeriron chalcogenidesuperconductors on STO.

1. Zhuozhi Ge, Qiang Zou, Huimin Zhang, Chenhui Yan, Daniel Agterberg, Michael Weinert, and Lian Li. "Superconductivity on Edge: Evidence of a One-Dimensional Superconducting Channel at the Edges of Single-Layer FeTeSe Antiferromagnetic Nanoribbons." ACS Nano14, 6539 (2020)
2. T. Shishidou, D. F. Agterberg, and M. Weinert. "Magnetic fluctuations in single-layer FeSe." Commun Phys1, 8 (2018)
3. Qiang Zou, Huimin Zhang, Tatsuya Shishidou, Michael Weinert, and Lian Li. "The critical role of checkerboard spin fluctuations in high-Tc single layer FeSe_1-xS_x superconductors. " (to be published)

This research is supported by DOE (DE-SC0017632).

Antiferromagnetic materials promise to show magnetization dynamics, including switching, at ultra-high frequency and thus are of immense interest for next generation memory and logic applications. Besides, spin waves in an antiferromagnetic insulators propagate very efficiently. However, electrically reading the states of the antiferromagnetic materials is not easy. Recently, spin Hall magnetoresistance (SMR) has been identified as one of the promising ways to access the surface states of the antiferromagnetic insulators. In this work, we focus on α-Fe₂O₃ insulator. We grow a 200-nm thick α-Fe₂O₃ film on (11-20) Al₂O₃ by reactive sputtering at 5 mTorr. A 6­nm thick Pt film is deposited in-situ on the Fe₂O₃ film. We measure the SMR of the patterned Hall bar structure of Pt at low temperatures and room temperatures. Below the Morin transition, we observe positive and negative magnetoresistance in the Pt/Fe₂O₃ bilayers depending on the direction of the current flow at the spin flop transition. This identifies that SMR can clearly determine the anisotropy of a thin film antiferromagnetic insulator. Moreover, we observe the spin-flop field of α-Fe₂O₃ using the Hall measurement in addition to the SMR measurement. The Hall magnetoresistance is 100x higher than the SMR at the spin-flop field. We will show a detailed study of the dependence of magnetoresistance on angular fields at different temperatures. Our study reveals important physical phenomena in the antiferromagnetic Fe₂O₃ thin film, which in turn will help to design energy efficient antiferromagnetic devices.

This work was supported by the Illinois MRSEC funded by NSF and by the US DOE BES Materials Science and Engineering Division.