United States

Magnetization states in conducting nanostructures have attracted renewed attention because they give rise to a Berry phase and to an anomalous Hall contribution commonly known as topological Hall effect (THE). The effect requires spin configurations that are not only noncollinear but also noncoplanar, while spins in small spherical Stoner-Wohlfarth particles are parallel and the THE is therefore zero. However, metallic nanoparticles are never perfectly spherical, which creates magnetization inhomogeneities. Examples are stray-field effects near corners and edges, such as the flower state in cubes [1] and the spin configuration shown in Fig. 1(a). When the particles form in conducting thin films or granular nanostructures, then these inhomogeneities yield skyrmionic magnetization features [2] and THE contributions [3]. Our focus is on nanoparticles whose radius R is smaller than the coherence radius Rcoh above which a reverse magnetic field causes magnetization curling. We estimate the Berry curvature Φ by considering the triple product M1.(M2 × M3), which describes the degree of noncoplanarity. When the three vectors are symmetrically arranged and form an angle θ with the symmetry axis, then the triple product is equal to (27/4)1/2 sin2 θcosθ Ms3. The angle θ depends on the radius of the nanoparticles and on the external magnetic field. In very small nanoparticles, the exchange energy ~ A/R2 dominates, and θ approaches zero. Figure 1(b) shows the zero-field Berry curvature as a function of R. An external magnetic field reduces Φ, and the THE is obtained by areal integration. For very small nanoparticles, Φ decreases as R4. This research is supported by DOE BES (DE-FG02-04ER46152).References: [1] E. Pinilla-Cienfuegos, S. Mannas-Valero, A. Forment-Aliaga, and E. Coronado, ACS Nano 10, 1764 (2016). [2] B. Das, B. Balasubramanian, R. Skomski, P. Mukherjee, S. R. Valloppilly, G. C. Hadjipanayis, and D. J. Sellmyer, Nanoscale 10, 9504 (2018). [3] W. Zhang, B. Balasubramanian, A. Ullah, R. Pahari, X. Li, L. Yue, S. R. Valloppilly, A. Sokolov, R. Skomski, and D. J. Sellmyer, Appl. Phys. Lett. 115, 172404 (2019).

MMM 2020

Berry Curvature and Topological Hall Effect in Quasi-Coherent Nanoparticles

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 current density required for current induced magnetization switching in magnetic devices such as Magnetoresistive Random Access Memory (MRAM) is proportional to Gilbert damping constant. The lower damping would enable more energy-efficient excitations and thus less current needed. The Gilbert damping constant of the Co2FeAl film, a half-metal material important for spintronics with its 100% spin polarization at Fermi level, has shown the lowest Gilbert damping constant among the Heusler alloys. However, the increased Gilbert damping constant in ultra-thin Co2FeAl films is undesirable for low power consumption applications such as STT-MRAM. Being able to manipulate the Gilbert damping on demand is crucial for spintronic device engineering and optimization. Here, we report that the Gilbert damping of ultra-thin Co2FeAl1+x films of nanometer thicknesses can be effectively tuned by delicately controlling the stoichiometric ratio during the growth. The Gilbert damping has been found to be the lowest of 0.065 in Co2FeAl1+0.1, which is deduced by ~50% compared to that in Co2FeAl. We have further found that the damping constant of the ultra-thin Co2FeAl1+0.1 film is restored to the value of 0.062 of the nominal stoichiometric Co2FeAl by compensating the Al composition-deficiency- as supported by the 10.37% of composition change from the scanning transmission electron microscope energy-dispersive-spectroscopy. This work offers a unique path to manipulate the Gilbert damping constant in ultra-thin Co2FeAl films by Al concentration control.

Manipulation of Gilbert damping in ultrathin half-metallic Co2FeAl1+x by composition-deficiency-compensation

While the half-metallic Heusler alloys are considered ideal materials to be used in magnetic tunnel junctions, spin-filters, and spin valves, the search for new candidate half-metal remains an active field of research [1]. One of the ways of rational material design is by doping, which is useful not only to tailor the electric and magnetic properties but also to stabilize a single-phase compound [2, 3]. In this work, we show how the introduction of Mn helps to stabilize a single-phase compound and to tune half-metallic character in the Co2FeGe alloy, which otherwise is a multi-phased normal ferromagnet. We have investigated three substitutional series viz; (Co2-αMnα)FeGe, (Co2-αFeα)MnGe, Co2(FeαMn1-α)Ge, that can be generated from the intermixing of Mn, Fe, and Co by keeping the Ge content constant in the Co2FeGe alloy. These series cover all possible site doping that can be made for a magnetic atom in a full Heusler alloy and allow us to directly compare their magnetic and electric properties. First principles calculations performed using the generalized gradient approximation (GGA) predict that a half-metal is obtained in Co2(FeαMn1-α)Ge series while the other two series generate “near half-metallic” alloys. The experimental investigation of bulk samples of the (Co2-αMnα)FeGe series shows that a single-phase compound can be achieved up to α = 1.25. For all the single-phase compounds, the magnetic moment agrees reasonably well with the Slater-Pauling moment and a very high Curie temperature (~ 1000 K) is measured for low α. The experimental investigation of other series is currently underway; however, the preliminary results show that these series also generate single-phase compounds. Hence by doping Mn, Fe, and Co for one another, many single-phase compounds can be designed from Co2FeGe, with potential half-metallic behavior that can have actual use in spintronic devicesReferences: 1. Z. Bai, L. Shen, G. Han, Spin, Vol. 02, No. 04, 1230006 (2012) 2. R. Umetsu, K. Kobayashi, A. Fujita, Phys. Rev. B, 72, 214412 (2005) 3. S. KC, R. Mahat, S. Regmi, Phys. Rev. Materials, 03, 114406 (2019)

Interplay Between Mn, Fe, and Co in the Co2FeGe Alloy to Design Novel Singe-Phase Compounds with Potential Spintronic Application

Magnetic two-dimensional (2D) materials [1, 2] opened unprecedented opportunities for nanoscale spintronic devices in miniaturized form factor. With the aggressive pursuit for dimensional scaling is another paramount endeavor, persistently ongoing for high power efficiency [3]. Under this energy consumption umbrella, high spin polarization is one critical prerequisite. In this talk, I will introduce two schemes [4, 5] to create half-metallicity in layered material systems. In scheme I [4], electric field will be applied across the bilayer A-type antiferromagnetic 2D materials. By electrostatically changing the relative energy levels of the two spin-polarized bands in the two layers, one spin-polarized band from both layers will be merged whereas the other spin-polarized band will be opened with a gap, leading to half-metallicity (i.e., 100% spin polarized conductive electrons). In scheme II [5], a multiferroic superlattice consisting of alternative ferroelectrics and A-type antiferromagnets is constructed. In ferroelectric phase, the two interfaces on the opposite sides of the ferroelectric layer will be made half metallic, with 2D electron gas and 2D hole gas of the same pure spin polarization created at the alternative interfaces. With the opposite ferroelectric polarization, the half metallicity of the opposite spin polarization will be produced, leading to ferroelectric switching of the interfacial half metallicity. Both schemes are generic and experimentally practical, and will provide valuable contributions to the developments of miniaturized, high-efficiency spintronics.References: [1] C. Gong et al., Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals, Nature 546, 265-269 (2017). [2] C. Gong, X. Zhang, Two-dimensional magnetic crystals and emergent heterostructure devices, Science 363, eaav4450 (2019). [3] C. Gong et al., Multiferroicity in atomic van der Waals heterostructures, Nat. Commun. 10, 2657 (2019). [4] S.-J. Gong, et al., Electrically induced 2D half-metallic antiferromagnets and spin field effect transistors, Proc. Natl. Acad. Sci. 115, 8511-8516 (2018). [5] E.-W. Du, et al., Ferroelectric switching of pure spin polarization in two-dimensional electron gas, (2020, unpublished).

Creating half-metallicity in two-dimensional magnetoelectric materials

The Anomalous Nernst Effect (ANE) is defined as the generation of a transverse electric current (Jc) when an in plane magnetised material is subjected to an out of plane temperature gradient (ΔT) (Figure 1). Over recent years a vast number of materials have been shown to exhibit an ANE response to a temperature gradient. From thin films to bulk single crystals, dilute magnetic semiconductors to topological insulators, the landscape has been reenergised by the enhancement of the ANE, intrinsically derived from the exotic band structures of materials which exhibit Weyl-like transport phenomena 1,2. The family of Heusler alloys contain proposed (and confirmed) magnetic Weyl semimetals most notably Co2MnGa. However, another Heusler which is of interest, particularly in the magnetic recording field is Co2MnSi, identified for its 100% spin polarisation3 and high magnetisation and Curie temperature. We present a series of Co2MnSi thin films where there is dependence of the lattice ordering upon annealing temperature (TAnn) (Figure 2)4. We show that polycrystalline thin films of Co2MnSi exhibit ANE of the order of S = 0.114 µVK-1 for a fully ordered L21 phase and S = 0.662 µVK-1 for a disordered A2 phase. A similar trend with the disorder is seen in the anomalous Hall resistivity (which is expected to scale with the ANE). Whilst Co2MnSi in the L21 phase does not exhibit any form of non-trivial band structure (and hence the low ANE), the increase in the ANE and AHE counter to the decrease in magnetisation with increasing disorder is suggestive of an intrinsic difference in the band structures between the two phases.References: 1 H. Reichlova, R. Schlitz, S. Beckert et. al., Appl. Phys. Lett. 113, (2018). 2 S.N. Guin, P. Vir, Y. Zhang et. al., Adv. Mater. 1806622, 1806622 (2019). 3 M. Jourdan, J. Minár, J. Braun et. al., Nat. Commun. 5, 3974 (2014). 4 C.D.W. Cox, A.J. Caruana, M.D. Cropper et. al., J. Phys. D. Appl. Phys. 53, 035005 (2020).

Anomalous Nernst Effect in Co2MnSi thin films

Using first-principles calculations, we demonstrated that the perpendicular magnetic anisotropy (PMA) of NiI2 monolayer can be effectively enhanced by decreasing interlayer distance of graphene/NiI2 (Gr/NiI2) heterostructure. Furthermore, by analyzing atomic-resolved magnetocrystalline anisotropy energy (MAE), orbital-hybridization-resolved MAE and density of states, we clarify that the magnetic anisotropy enhancement originates from the electronic states of interfacial I atoms. At the same time, the NiI2 substrate induces magnetic proximity effects on graphene, which can lead to the quantum anomalous hall effect (QAHE). Our findings demonstrate the enhancement of PMA via tuning interlayer distance of vdW heterostructure and provides a possible vdW system for realizing QAHE.References: Q. Cui, J. Liang, B. Yang, Z. Wang, P. Li, P. Cui, and H. Yang, Phys. Rev. B 101, 214439 (2020)

Giant enhancement of perpendicular magnetic anisotropy and induced quantum anomalous Hall effect in the graphene/NiI2 heterostructure via tuning the van der Waals interlayer distance

Chemically-synthesized magnetic nanoparticles (MNPs) consisting of different combinations of spinel ferrites (γ-Fe2O3, Fe3O4, MnFe2O4, CoFe2O4) provide a platform for developing ensembles of nanostructures whose magnetic properties are distinct from their bulk counterparts. We report on studies of MNPs composed of high-anisotropy CoFe2O4 combined with relatively high-moment, low anisotropy Fe3O4. We examined two variations of MNPs: a CoFe2O4 core with a Fe3O4 shell and the inverted structure (Fe3O4 core / CoFe2O4 shell). Both variants had a core diameter of 3-3.5 nm and an overall diameter of ∼10 nm and we investigated their properties using magnetometry, small-angle neutron scattering (SANS), and Monte Carlo simulations. The magnetometry indicates that both MNP variants are superparamagnetic at room temperature with a considerable coercivity enhancement below the blocking temperature (~250 K) accompanied by a multi-step path towards magnetization reversal (Fig 1-b). Fully spin-polarized SANS measurements show that magnetic order perpendicular to the applied field direction is strong at remanence; this spin configuration is quenched upon application of a small field (Fig. 1-a). Spin alignment parallel to the field develops at higher fields. Monte Carlo simulations indicate that the emergence of dipolar-coupled long range order may be associated with the steps observed in the field hysteresis loops. Modelling of the SANS distributions, informed by the Monte Carlo simulations, to investigate the spin alignment in the core and shell, as demonstrated for metallic Fe core / Fe oxide shell MNPs [1], for both MNP variants is currently underway. The results demonstrate the value of combining modeling of fully-spin polarized SANS data with numerical simulations of ensembles of macrospins to illuminate the complex spin ordering across a range of length scales. The authors acknowledge the support of Univ. of South Florida Nexus Initiative (UNI) Award, US Department of Energy, Office of Basic Energy Sciences (Award DE-FG02-07ER46438) and the Knut and Alice Wallenberg Foundation (Grant 2018.0060). Access to spin filters on NG-7 SANS was provided by CHRNS, a partnership between NIST and the NSF (No. DMR-1508249).References: [1] C. Kons, D. A. Arena, et al., “Investigating spin coupling across a three-dimensional interface in core/shell magnetic nanoparticles,” Phys. Rev. Mater., 4 034408 (2020).

Spin Ordering in Bi-Magnetic (Core / Shell) Nanoparticles With Varying Anisotropy

Thanks to advances in synthesis that enable control over size, structure, and properties, magnetic nanoparticles (NPs) present opportunities in data storage, cancer treatment, biomedical imaging, etc. While superparamagnetism (SP) dominates the properties of magnetic NPs, quantitative understanding of SP blocking in NP assemblies remains elusive. We address this here via comprehensive magnetic characterization and analysis of soft ferromagnetic Ni NP ensembles. NPs were synthesized by hot injection of a Ni-oleylamine (OAm) complex into trioctylphosphine (TOP), with size control achieved via TOP:OAm ratio, reaction time, and differential centrifugation.1 X-ray diffraction, transmission electron microscopy (TEM), and various spectroscopies reveal polycrystalline/twinned FCC Ni NPs with mean diameters <D> from 4 to 22 nm, dispersity down to 10%, and TOP/OAm ligands.1 SP blocking temperatures (TB) are carefully determined, quantitatively accounting for the substantial yet frequently ignored effects of dispersity, yielding mean TB (<TB>) from 5 to 300 K.1 Quantitative understanding of the size-dependent <TB> is achieved (Fig. 1), with no adjustable parameters, but only by accounting for the size distribution, effective ferromagnetic volume (Vmag), temperature-dependent magnetocrystalline anisotropy (MCA), and average shape anisotropy due to random deviations from spherical.1 The reduced saturation magnetization (Ms) and Vmag deduced from magnetometry were investigated by small-angle neutron scattering (SANS) (Fig. 2) and high-resolution TEM, evidencing a thin Ni-P shell arising from reactions with TOP. We thus achieve fully quantitative understanding of SP blocking in model NP assemblies. Work supported by 3M and DOE Center for Quantum Materials.References: [1] J.T. Batley, M. Nguyen, I. Kamboj, C. Korostynski, E.S. Aydil and C. Leighton, submitted (2020).

Quantitative understanding of superparamagnetism in Ni nanoparticle assemblies

Magnetic nanoparticles are increasingly used in biomedical applications such as drug-delivery, gene delivery, hyperthermia, or contrast agents for MRI. Magnetite (Fe3O4) nanoparticles are good candidates for these applications due to their non-toxicity and long-life in the bloodstream. While the structural and magnetic properties of bulk Fe3O4 has been widely studied [1], knowledge is still lacking regarding the magnetic behaviour of collections of Fe3O4 nanoparticles. In particular, when the particles are small and the assembly is superparamagnetic, it is useful to understand the nature of the magnetic correlations within the assemblies. Here we show how inter-particle magnetic correlations can be probed at the nanoscale via x-ray resonant magnetic scattering (XRMS) [2] as illustrated in Figure 1. By tuning the energy of the x-rays to resonant edges of Fe and comparing opposite polarization helicities, we extract information about the local inter-particle magnetic orders within the nanoparticle assemblies. We show the dependence on particle size, varying from 5 to 11 nm, suggesting an enhancement of magnetic couplings for bigger particles. [3] Additionally, we show a model based on chains of nanoparticles, which we use to fit the XRMS data. The data fitting suggests the occurrence of ferromagnetic ordering when an external magnetic field is applied, but when the external field is brought to zero (remanence), the fit suggest a mix of antiferromagnetic ordering and randomness. [4]References: [1] Verwey, Nature 144, 327 (1939); Fleet, Acta Cryst. B 37, 919-929 (1981) [2] Kortright et al., Phys. Rev. B 71, 012402 (2005) [3] Chesnel et al., Magnetochemistry 4, 42-58 (2018) [4] J. Rackham et al, AIP Advances 9, 035033 (2019)

Magnetic correlations and time fluctuations in assemblies of Fe3O4 nanoparticles

Heusler compounds are exciting materials for future spintronics applications because they display a wide range of tunable electronic and magnetic interactions. Here, we use a femtosecond laser to directly transfer spin polarization from one element to another in a half-metallic Heusler material, Co2MnGe. This spin transfer initiates as soon as light is incident on the material, demonstrating spatial transfer of angular momentum between neighboring atomic sites on time scales < 10 fs. Using ultrafast high harmonic pulses to simultaneously and independently probe the magnetic state of two elements during laser excitation, we find that the magnetization of Co is enhanced, while that of Mn rapidly quenches. Density functional theory calculations show that the optical excitation directly transfers spin from one magnetic sublattice to another through preferred spin-polarized excitation pathways. This direct manipulation of spins via light provides a path toward spintronic devices that can operate on few-femtosecond or faster time scales.

Direct light–induced spin transfer between different elements in a spintronic Heusler material via femtosecond laser excitation INVITED

Performing magnetization studies on individual nanoparticles is a highly demanding task, especially when measurements need to be carried out under large sweeping magnetic fields or variable temperature. Yet, characterization under varying ambient conditions is paramount in order to fully understand the magnetic behavior of these objects, e.g., the formation of non-uniform states, the mechanisms leading to magnetization reversal, thermal stability or damping processes. This, in turn, is necessary for the integration of magnetic nanoparticles and nanowires into useful devices, e.g., spin-valves, racetrack memories or magnetic tip probes. Here we show that YBa2Cu3O7 nano Superconducting QUantum Interference Devices (YBCO nanoSQUIDs, see Fig.) are particularly well suited for this task. We have successfully characterized a number of individual nanoparticles of soft magnetic materials with different shapes (nanodots, nanodiscs and nanowires, see Figure). Samples are transported to the surface of the sensor with nanometric resolution, achieving sufficiently strong particle-SQUID coupling. Magnetization measurements (see Figure) performed under sweeping magnetic fields (up to ~ 500 mT) and variable temperature (1.4 - 80 K) underscore the intrinsic differences between samples owing to their shape and the presence (or absence) of magnetocrystalline anisotropy. Our measurements also allow us to distinguish the mechanisms leading to magnetization reversal mediated by, i.e., nucleation/propagation of domain walls or nucleation/annihilation of magnetic vortices. These studies serve to shed light on the nature and magnitude of the energy barriers separating different magnetic states, nucleation/annihilation fields and switching times.

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Berry Curvature and Topological Hall Effect in Quasi-Coherent Nanoparticles

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Manipulation of Gilbert damping in ultrathin half-metallic Co2FeAl1+x by composition-deficiency-compensation

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