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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.

MMM 2020

Multi-shaped individual nanoparticles investigated by nanoSQUID magnetometry at variable temperature INVITED

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

Mn2RuxGa (MRG) is a half-metallic compensated ferrimagnet crystallising in a cubic inverse Heusler structure (XA) with a substrate-induced biaxial strain of about 1% leading to perpendicular magnetic anisotropy of the net moment. The Mn atoms occupy two different crystallographic sites, Wyckoff positions 4a and 4c. [1] From band structure calculations we find that the states originating from the Mn4c sites dominate the transport. Anomalous Hall effect (AHE) resistance is proportional to the z-component of the magnetisation of the Mn4c sublattice. [2] The AHE measurements show loops with high remanence and only one magnetic phase, with coercivity diverging close to compensation. SQUID magnetometry measures the net moment, i.e. the sum of Mn4a and Mn4c sublattice moments. SQUID-measured magnetisation, in the same geometry as AHE, exhibits a combination of an apparent soft magnetic response in addition to a component with the coercivity observed by AHE. We infer that the net magnetic moment is canted due to non-collinearity of the sublattice moments. Our DFT calculations suggest that the non-collinearity may be due to competing exchange on the Mn4c sites as bulk antisymmetric exchange averages to zero in the -43m point group [3]. The Mn4c sublattice and net magnetisations differ by an order of magnitude, so a small canting of the Mn4c moment can result in a large canting of the net moment. X-ray magnetic circular dichroism measurements were performed to map sublattice magnetisations in the z-x plane. Both sublattices can be distinguished by L-edge absorption due to charge-transfer separation of the Mn absorption edges at the two sites by ≈ 1 eV. [2] Analysis of the mz-mx moments shows that they are indeed non-collinear (Fig.1). From balancing of torques, anisotropy fields at the Mn4a and Mn4c sublattices were found to be easy-plane -1.8 T and easy-axis 3 T, respectively (Fig.2.).References: [1] H. Kurt, K. Rode, P. Stamenov, et al., “Cubic Mn2Ga thin films: crossing the spin gap with ruthenium”, Phys. Rev. Lett., 112, p. 27201, 2014. [2] D. Betto, N. Thiyagarajah, Y.-C. Lau, et al., “Site-specific magnetism of half-metallic Mn2RuxGa thin films determined by x-ray absorption spectroscopy”, Phys. Rev. B, 91, p. 094410, 2015. [3] O. Meshcheriakova, S. Chadov, A.K. Nayak, et al. "Large Noncollinearity and Spin Reorientation in the Novel Mn2RhSn Heusler Magnet", Phys. Rev. Lett. 113, p. 087203, 2014.

Investigation of non-collinearity in Mn2RuxGa compensated ferrimagnetic half-metallic thin films

Co2- based intermetallic Heusler alloys are among the most attractive half-metallic systems due to their high Curie temperatures, high spin polarization and the structural similarity to binary semiconductors [1,2]. The Co2FeGe system is one such candidate of interest for spintronic applications, but experimentally it does not form a single phase in bulk form [3,4,5]. In this work, we have introduced Cr doping to stabilize the Co2FeGe system, and examined the structural, magnetic, transport, and mechanical properties of quaternary Heusler alloys Co2-xCrxFeGe with 0 ≤ x ≤ 1, both experimentally and theoretically. Single phase microstructures are observed for Cr compositions from x = 0.25 to x = 1. The samples with Cr concentrations below x = 0.25 are multi-phase. XRD patterns at room temperature show the presence of ordered Y structure (space group F-43m, # 216) for all single-phase samples. The low-temperature saturation magnetic moments agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a noted prerequisite for half metallicity. The Density functional theory calculations performed using VASP code with projector augmented wave method (PAW) and the Perdew-Burke-Enzerhof (PBE) functional predict half-metallic character in the alloys after Cr substitution. All alloys are observed to have high Curie temperatures, and show a linear dependence with the saturation magnetic moment which is expected in half metallic Co-based Heusler alloys [6]. Transport measurements were performed to explain the electronic structure of the alloys. High mechanical hardness values are also observed which makes these materials suitable for industrial applications requiring repetitive thermal cycling and resist cracking from vibrations.References: 1. I. Galanakis, P. Dederichs, and N. Papanikolaou, Physical Review B 66, 174429 (2002). 2. C. Felser, G. H. Fecher, and B. Balke, Angewandte Chemie International Edition 46, 668(2007). 3. K. R. Kumar, K. K. Bharathi, J. A. Chelvane, IEEE Transactionson Magnetics 45, 3997 (2009). 4. S. KC, R. Mahat, P. LeClair, Phys. Rev. Materials 3 (2019) 114406. 5. R. Mahat, S. KC, D. Wines, P. LeClair, Bulletin of the American Physical Society (2020). 6. G. H. Fecher, H. C. Kandpal, S. Wurmehl, C. Felser, Journal of applied Physics 99, 08J106 (2006).

Possible Half-Metallic Behavior of Co2-xCrxFeGe Heusler Alloys: Theory and Experiment

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

The ferromagnetic Heusler compound Co2MnGa is a promising Weyl semimetal with the Fermi energy in the vicinity of the Weyl nodes. Bulk Co2MnGa recently attracted attention because of an exceptionally large anomalous Nernst effect [1,2] and exotic surface states [3]. Simultaneously, Co2MnGa thin films were successfully prepared and showed interesting transport properties [4]. Beside others, the non-trivial topology of the band structure of Weyl semimetals leads to unexpected magneto-thermoelectic transport phenomena which will be discussed in this talk. We report on our observation of a record large anomalous Nernst coefficient in Co2MnGa thin films - up to -3µV/K [5, 6], and discuss the procedure for the quantitative determination of the anomalous Nernst coefficient. Several samples with comparable saturated magnetization were studied, yielding significantly different anomalous Nernst responses. We conclude that the microscopic origin of the anomalous Nernst effect in Co2MnGa is complex and contains contributions from the intrinsic Berry phase and surface states. We further employ a series of Co2MnGa thin films to study the validity of the Mott relation by measuring all four transport coefficients - the longitudinal resistivity, anomalous Hall, Seebeck and anomalous Nernst coefficients [6]. We show that the measured anomalous Nernst signal can be connected to the remaining three magneto-thermal transport coefficients in the whole sample series. This is strong evidence that the Mott relation holds also in Weyl semimetals, where the intrinsic Berry curvature dominates the transport. In the last part of the talk we focus on another magneto-transport phenomena and its thermal counterpart – anisotropic magnetoresistance and anisotropic magneto-thermopower. We show that due to high epitaxial quality of the Co2MnGa films both non-crystalline and crystalline contributions to the anisotropic magnetoresistance can be measured. Interestingly, we do not observe the same crystalline contributions in the magneto-thermopower measurements [7].References: [1] S. N. Guin et al., NPG Asia Materials 11, 16 (2019) [2] A. Sakai et al., Nat. Phys. 13, 1119-1124 (2018) [3] I. Belopolski et al., Science 365, 1278-1281 (2019) [4] A. Markou et al., PRB 100, 054422 (2019) [5] H. Reichlova et al., APL 113, 212405 (2018) [6] G. H. Park, H.R. et al. PRB 101, 060406(R) (2020) [7] P. Ritzinger, H.R. et al., in preparation

Magneto-Thermal Transport in Weyl Semimetal Co2MnGa Thin Films INVITED

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

Glass-ceramic (GC) materials have formability advantages of glasses controlled crystallization of functional ceramic phases. Here, we obtain GCs containing the hard magnetic phases Ba- or Sr-hexaferrite from a borate glass. Twelve compositions were explored, varying B2O3 and Ba/Fe or Sr/Fe ratio. Selected compositions were heat treated from 30○C to 700-900○C. Resulting phases were identified using X-ray diffraction (XRD) as α-Fe2O3 plus SrB2O4 and SrFe12O19 or their Ba equivalents. Vibrating sample magnetometry (VSM) also identified Fe3O4. The best compositions for hexaferrite GCs contained 40-42 mol% B2O3 and Fe/(Sr,Ba) ratios of 0.5-1. Heat treatments for one Sr glass were performed at 625○C or 675○C to assess phase development at the peak crystallization temperatures determined by thermal analysis, but both treatments yielded only SrB2O4 by XRD; however, coercivity increased from the quenched glass (395 Oe) to 625○C (460 Oe) to 675○C treatment (520 Oe). Selected glasses were studied with in-situ crystallization, measuring magnetic properties during heating to 800○C in the VSM. Hysteresis measurements on initial heating showed predominantly paramagnetism. After cooling and reheating, wasp-waisted loops were observed with Hc > 650 Oe, dropping to 0 at 650○C, with changes occurring 290○C, 490○C (TC of SrFe12O19) and 550○C (TC of Fe3O4 suggesting oxidation despite Ar heating gas). First order reversal curve (FORC) measurements confirmed low and high coercivity phases. These results suggest that crystallization of magnetic phases is most significant on cooling, and that this glass might allow alignment of the anisotropic hexaferrite crystals while heating and cooling with an applied field.

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Multi-shaped individual nanoparticles investigated by nanoSQUID magnetometry at variable temperature INVITED

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Investigation of non-collinearity in Mn2RuxGa compensated ferrimagnetic half-metallic thin films

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