Session X19: Magnetic Nanoparticles and Films

Quantum-Spin-Liquid Corrections near a Transition to Strong Ferromagnetism

The Curie temperature of bulk and nanostructured Fe₅Si₃ is investigated using experiments, density-functional simulations, and many-body model calculations. The bulk intermetallic, which crystallizes in the hexagonal D8₈ structure, exhibits several intriguing features: it does not exist as a room-temperature equilibrium phase, is close to the onset of ferromagnetism, and exhibits two crystallographically very different Fe sites. The samples, produced by rapid quenching (bulk) and cluster deposition (nanoparticulate thin films), have Curie temperatures of about 400 K, with a pronounced dependence on the Si content. Calculations using the Vienna ab-initio simulation package (VASP) show that the nanostructuring triggers a transition from weak to strong ferromagnetism. Interatomic exchange constants are calculated using the Kohn-Korringa-Rostoker (KKR) method and used to solve the multisublattice mean-field problem for the system, VASP calculations are used to study the dependence of the Fe moment on the thermally induced spin misalignment, and a model calculation yields an estimate for quantum-spin-liquid corrections.   

Magnetism of CoSi Nanoparticles with B20-Type Structure

Magnetic nanoparticles or clusters often exhibit novel magnetic properties compared to isostructural bulk alloys due to low-dimensional and quantum-confinement effects.^1,2 Magnetic materials with noncentrosymmetric B20 cubic structure have attracted much interest due to skyrmion spin structures originating from Dzyaloshinskii-Moriya interactions.³ We have used a cluster-deposition method to fabricate CoSi nanoparticles having an average size of 10 nm with an rms standard deviation of σ/d ≈0.10, and investigated their magnetic properties by experiments and first-principle DFT calculations. The X-ray diffraction pattern shows the formation of a B20-type cubic structure in CoSi nanoparticles, similar to bulk alloy. In sharp contrast to bulk CoSi, which is paramagnetic down to 4 K , our experimental results and DFT calculations show a net magnetic moment of about 0.2 μ_B/Co for the nanoparticles. Nanoscale effects on the magnetism of CoSi, including complex DM interactions, will be presented.
References
¹ B. Balasubramanian et al. Appl. Phys. Lett. 108, 152406 (2016).
²B. Das et al. Nano Lett. 16, 1132 (2016).
³N. Nagaosa and Y. Tokura, Nat. Nanotech. 8, 899 (2013).   

Surface Magnetism and Proximity Effects in Hexaboride Thin Films

The unexpected ferromagnetism in alkaline hexaborides has attracted extensive research efforts to understand the exceptionally high Curie temperature sustained by low magnetic moments density. Here, we report the coexistence of ferromagnetic and superparamagnetic orders with 2D-type magnetic anisotropy in CaB₆ and SrB₆ thin films. Boron deficiency and closely correlated local oxidization were found at the film surface and its interface with the oxide substrate. These results suggest that the magnetism in hexaboride films is likely generated by two thin surface/interface layers with high concentration of boron vacancies which are energetically stabled by oxygen cation. Additionally, clear suppression of the superconductivity proximity effect in SmB₆ film was observed when SmB₆ was grown on top of the magnetic CaB₆. Such results demonstrate the interesting perspective of engineering multifunctional heterostructures by coupling the surface magnetic layer with the diversity of other hexaboride materials.   

Interfacing SiGe Technology with Nanoscale Magnetic Tunnel Junctions

Silicon germanium heterojunction bipolar transistors (SiGe HBTs) have been tested at cryogenic temperatures and a theory of operation has been developed for the purpose of interfacing with nanomagnets for rapid magnetic readout and data storage. Magnetic tunnel Junctions have been fabricated by electron beam lithography and shadow evaporation. The magnetization dynamics of these junctions with resistance near the resistance quantum have been measured with a SiGe HBT in a bench-top room temperature test, but these initial measurements were limited by a rise time of approximately 10 microseconds. The next step involves SiGe technology incorporated physically next to the magnetic tunnel junction to increase the measurement bandwidth by effectively transforming the high source impedance of the junction to a lower apparent value. A summary of the measurement results will be discussed including: tunnel magnetoresistance at room temperature and 1.8K, magnetic reversal times at room temperature, and single domain switching times and switching precursors at 1.8K.   

Anomalous Hall effect in a full Heusler Co2TiGe thin film

Weyl Semimetals (WSM) are interesting class of topological materials where Weyl fermions have been realized [1] with distinct chirality. Recent theoretical predictions have shown the time-reversal symmetry breaking in the full Heusler alloy Co2TiGe [2,3] which can create a new heterostructure spintronics devices that combine the diverse physical properties of Heusler compounds. We have presented the fabrication, magnetic and transport properties of Co2TiGe thin films prepared on Si (100) substrates by electron-beam evaporation in ultra-high vacuum. Thin films exhibit L21 crystal structure and saturation magnetic moment of 1.9 μ_B/f.u. at 10 K with a Curie temperature of 380 K. Magnetotransport properties on the thin films reveal semimetallic resistivity, small anomalous Hall resistivity and weak-localization. The anomalous Hall resistivity is proportional to square of longitudinal resistivity, which is due to the topological Berry curvature.
1) S. Y. Xu et. al, Science 349, 613 (2015).
2) G. Chang et. al, Sci. Rep. 6, 38839 (2016).
3) Z. Wang et. al, Phys. Rev. Lett. 117, 236401 (2016).36401 (2016).


  

Magnetic and Magnetocaloric Properties of NiFeMnGa0.5Sn0.5 Heusler Alloy

Heusler alloys have attracted recent attention because of their multiple interesting properties with prospects for spintronics, energy technology and magnetic refrigeration. Additionally, the magnetic properties of these materials can be tuned to fit specific applications by adjusting the elemental composition. Here, we present our investigation on how the structural order and magnetic properties of a Heusler alloy NiFeMnGa can be modified by replacing a fraction of Ga with Sn. We have investigated NiFeMnGa_0.5Sn_0.5 alloy prepared by arc melting and annealing. The samples show highly ordered cubic LiPdMnSn-type crystal structure. The samples are ferromagnetic at room temperature with the Curie temperature of 336 K. The high-field (3T) magnetization at 100 K is 61 emu/g. We will discuss the effect of nanostructuring and heat treatment on the structural, magnetic, and magnetocaloric properties of this material. We will also present the first-principle results of the band structure and magnetic properties and compare them with the observed experimental data.   

Thickness Dependent Ferromagnetism in Fe3GeTe2

Recent discoveries of intrinsic 2D ferromagnetism in van der Waals materials provide promising platforms for studying fundamental ferromagnetism in atomically thin limit. Most of these materials are found to be insulators/semiconductors and extremely sensitive to air. Here, we show that Fe₃GeTe₂ is a ferromagnetic metal down to monolayer limit with strong perpendicular anisotropy. Using both magneto-optical spectroscopy and electrical transport measurements, we establish the cross over from 3D to 2D Ising ferromagnetism as a function of layer thickness, which accompanies the fast drop of Curie temperature.   

Room-temperature ferromagnetism induced by UV-light irradiation in Ce-doped nanocrystals of TiO2

A UV-irradiation-induced magnetic transition from diamagnetism to room-temperature ferromagnetism has been experimentally observed in Ce-doped nanocrystals of TiO₂. As demonstrated by synchrotron-radiation x-ray absorption analyses, the concentration of oxygen vacancy defects was increased and the valence of Ce dopant atoms was effectively reduced as a result of UV-light irradiation, while the Ti valence remains largely unchanged. The physical mechanism underlying these effects has been theoretically investigated by using density functional theory (DFT) calculations. Based on a simple model, the experimentally observed effect of Ce valence reduction as a result of UV irradiation was theoretically reproduced. The increased Ce³⁺ concentration due to UV irradiation consequently incurs ferromagnetism in the samples as indicated by the DFT simulation. In addition, incorporation of Ce into the TiO₂ matrix results in band-gap narrowing due to the band tailing effect. The band gap width can be further decreased as the Ce³⁺ concentration is increased by UV irradiation. The irradiation induced band gap narrowing provides an ultraconvenient method for band gap engineering in TiO₂ for photocatalytic applications.   

Frustrated quantum dots, the experimental validation of a computational model

The onset of spin frustration with increasing iron incorporation into wurtzite ZnSe is computationally predicted and experimentally tested. Density functional theory (DFT) models indicate magnetic spin frustration arises due to formation of local Fe-Fe spin lattices within the ZnSe lattice. Comparison of the DFT-GGA spin models to the experimentally prepared 1.8 nm Fe_0.1Zn_0.9Se QD reveal the predicted spin frustration within the QD is observed by comparison to field and temperature dependent Mossbauer and magnetic susceptibility measurements. The use of computational methods that can anticipate the onset of desirable properties apriori in QDs prior to the synthetic preparation will enhance the toolset of the experimentalist when searching for specific magnetic properties.
  

Non-Equilibrium Theory of Orbital Magnetization in Periodically Driven Solids

Berry phases play an important role in the orbital magnetization of crystals with broken time reversal symmetry. Although the theory of orbital magnetization was now well established in equilibrium with extension to interacting systems, the influence of optical driving field on the orbital magnetization is largely unexplored. In this work, we have developed a theory of orbital magnetization in a periodically driven solid by a strong optical field. Using the Keldysh-Floquet Green’s function formalism, we find the non-perturbative photon-induced corrections to the time-averaged orbital magnetization. Our theory highlights the combined influence of Berry phases and multiphoton processes on the magnetization.   

Coexistence of Ferromagnetism and Giant Paramagnetism in Small Carbon Coated Copper Nanoparticles

Systematic measurements were performed using XRD, STEM, EPR and PPMS for structural and magnetic properties of different size copper nanoparticles embedded in carbon matrix syntesized by solid phase pyrolysis of polycrystalline copper phthalocyanine (CuPc, Pc= C₃₂N₈H₁₆). Our results on magnetization carried out by vibrational magnetometer in wide range of temperature for average sizes of copper nanoparticles in range of 5-7 nm provide a strong evidence on coexistence of ferromagnetism and giant paramagnetism. At low temperatures we observe a giant paramagnetism, apparently due to the conduction electrons with ballistic mean free path (large orbital magnetism). The values of the specific susceptibility at T= 10K show a record high giant paramagnetism with magnetic specific susceptibility of order ≈1.5×10^-4 emu/gOe while ferromagnetic behavior (M_s ≈ 0.5 emu/g_Cu) with hysteresis was observed from helium up to the room temperature.   

Electronic State and Concentration of Fe in CuAl1-xFexO2 Determined by Magnetic Investigations

CuAlO₂ is among several ternary delafossites in which the electronic bandgap (2.68 eV) is less than the optical bandgap (3.5 eV), due to Laporte selection rules. Because alloying is expected to provide band engineering in delafossites, we are investigating Fe-doped CuAlO₂. Here, magnetic characterization of the CuAl_1-xFe_xO₂ system (x = 0, 1, 5, and 10%) is reported. The samples were prepared by a solid-state reaction at 1100 °C. X-ray diffraction of the powder samples showed an expansion of the rhombohedral unit cell with increasing x, in accordance with Vegard’s Law (Fe³⁺ has a larger ionic radius than Al³⁺). Analysis of the magnetization (M) vs. temperature data (T = 5 to 300 K) and magnetic field (up to H = 90 kOe) verifies Fe³⁺ as the electronic state of Fe. M vs. T is fitted to the Curie-Weiss Law: M ∝ H/(T-θ) and, using the isothermal data of M vs. H, the variation of M vs. H/(T-θ) is fitted to a modified Brillouin function to determine x. The CuAlO₂ (x=0) sample also shows significant paramagnetism probably due to defects or Fe³⁺ impurities. The Curie-Weiss fits require a negative θ, which signifies antiferromagnetic Fe³⁺-Fe³⁺ exchange coupling. Optical investigations are now in progress.   

Modeling of ZnTe Quantum Dots (QDs) embedded in a ZnMnSe matrix

The redshift of the Photoluminescence (PL) peak from ZnTe Quantum Dots embedded in a ZnMnSe matrix in an external magnetic field is dependent on the wavelength of the photon energy of the exciting laser. When the laser wavelength is 488 nm (the photon energy below the matrix gap but above the ZnTe QDs) we find a significantly higher red shift in the PL than the shift when a laser with a wavelength of 405 nm (the photon energy is above the ZnMnSe matrix) is used. The redshift in the PL is enhanced further with increasing Mn concentration in the matrix. Our theoretical studies show that 488 nm excitation results in hole wave functions which extends more into the ZnMnSe matrix compared to excitation with 405 nm. Using the linear variational method, we numerically diagonalize the QD Hamiltonian and find that 488 nm excitation results in a QD occupied with two holes whose delocalization from the QD center increases with increasing Mn concentration.   

Structural identification and ultrafast spin manipulation of [Co3Ni]+EtOH

Laser-induced spin dynamics on ligand-attached magnetic structures can be exploited to create spin-logic elemenents [1-3]. In this joint experimental and theoretical study we characterize the heterotetranuclear magnetic cluster [Co₃Ni]⁺EtOH, and suggest ultrafast magnetic switching scenarios on the Co atoms, as well as a cyclic spin-SHIFT register.

We structurally identify the cluster, which is synthesized using a molecular beam experiment, by comparing the experimentally measured and the theoretically computed O-H stretching frequencies of both the intact and dissociated EtOH ligand. To this end, we use the state-of-the-art equation-of-motion quantum chemical method to compute the energy along the O-H vibrational normal mode, and thus we can also extract phonon-spin coupling constants. Our results show that out of many possible stereoscopic isomers, the energetically prefered geometry is a pyramidal one. Our findings can help pave the way towards realization of nanospintronic devices.

[1] W. Jin, M. Becherer, D. Bellaire, G. Lefkidis, M. Gerhards, and W. Hübner, Phys. Rev. B. 89, 144409 (2014)
[2] D. Chaudhuri, W. Jin, G. Lefkidis, and W. Hübner, J. Chem. Phys. 143, 174303 (2015)
[3] W. Jin, D. Chaudhuri, C. Li, G. Lefkidis, and W. Hübner, J. Supercond. Nov. Magn. 30, 801 (2017)   

Molecular Mechanisms of Spin Crossover in the {Fe(pz)[Pt(CN)4]} MOF upon Guest Adsorption

Metal-organic frameworks (MOFs) that exhibit spin-crossover behavior have attracted much interest because of their potential technological applications. A key parameter that characterizes these materials is the transition temperature, defined as the temperature at which equal populations of low-spin and high-spin species are present. It was experimentally reported that the transition temperature of [Fe(pz)Pt(CN)4] can be controlled by the adsorption of guest molecules [1]. In this study, we report on Monte Carlo/molecular dynamics simulations aimed at characterizing the spin-crossover properties of [Fe(pz)Pt(CN)4] upon adsorption of chemically and structurally different guest molecules. We develop a molecular-level picture of spin-crossover behavior in [Fe(pz)Pt(CN)4] [2,3] and determine the physical mechanisms responsible for the stabilization of the low-spin state [4]. We also demonstrate that the variation of the spin-crossover temperature of [Fe(pz)Pt(CN)4] upon guest adsorption can be used to detect toxic gases.
[1] M. Ohba et al., Angew. Chem. Int. Ed. 48, 4767 (2009).
[2] C.H. Pham, J. Cirera, F. Paesani, J. Am. Chem. Soc. 138, 6123 (2016)
[3] C.H. Pham, F. Paesani, J. Phys. Chem. Lett. 7, 4022 (2016)
[4] C.H. Pham, F. Paesani, to be submitted.