Session X6: Nanoparticles, Nanowires, and Clusters

Using Magnetic Proximity Effects to Stabilize Magnetic Nanoparticles

The current miniaturization trend in magnetic applications has led to a quest to suppress spontaneous thermal fluctuations (superparamagnetism) in ever-smaller nanostructures, which constitutes a clear example of the fundamental efforts of condensed matter physics to meet technological challenges (e.g., the continued growth of magnetic recording density). We have demonstrated that ferromagnetic (FM) Co nanoparticles with blocking temperature below 70 K become magnetically stable above 400 K when embedded in a high Neel temperature antiferromagnetic (AFM) NiO matrix [1]. This remarkable stabilization is due to a magnetic proximity effect between a thin CoO shell (with low Neel temperature and high anisotropy) surrounding the Co nanoparticles and the NiO matrix (with high Neel temperature but low anisotropy). This proximity effect yields an effective antiferromagnet with an apparent ordering temperature beyond that of bulk CoO, and an enhanced anisotropy compared to NiO. In turn, the Co core FM moment is stabilized against thermal fluctuations via core-shell exchange-bias coupling, leading to the observed increase in blocking temperarature. A mean-field model, corrected for thermal activation effects, closely reproduces the experimental exchange-bias data, corroborating the above interpretation and providing a semi-quantitative understanding of the nature of the proposed proximity effect. The results presented in this study constitute a striking illustration of how a subtle combination of interactions may permit the occurrence of unique magnetic properties by exploiting proximity effects in magnetism. 1. J. A. De Toro, D. P. Marques, P. Muniz, V. Skumryev, J. Sort, D. Givord, and J. Nogues, Phys. Rev. Lett. 115, 057201 (2015).   

Emergent 1/f noise in collections of individually oscillating magnetic dots

We experimentally demonstrate an emergent $^1\!/_f$ spectrum from a superposition of the noise from random telegraph noise (RTN) oscillators. The system consists of individual square magnetic permalloy dots with dimensions on the order of 200nm x 200nm x 10nm that exhibit RTN in their magnetization at appropriate applied fields. The magnetization fluctuations are measured by the anisotropic magnetoresistance (AMR). AMR is used to find applied fields necessary to exhibit RTN, which result in Lorentizan spectra in the power spectral density of the measurement. A composite AMR measurement of multiple oscillating dots at once, however, shows an emergent $^1\!/_f$ spectrum in the power spectral density. This agrees with the prediction of Van Der Ziel [1] that, for an appropriate distribution of oscillators showing Lorentzian spectra, the composite spectrum will have a $^1\!/_f$ character. This experimental demonstration of $^1\!/_f$ noise from a system of two-state oscillators indicates a possible mechanism for the origin of $^1\!/_f$ spectra observed in both other magnetic systems, and potentially in other, more disparate systems. [1] A. van der Ziel, Physica $\textbf{16}$, 359 (1950).   

Magnetic and Structural Properties of Mn$_{\mathrm{5}}$Ge$_{\mathrm{3\thinspace }}$Nanoparticles

Magnetic nanoparticles have unique and interesting properties which are scientifically important and attractive for numerous advanced technologies. In this work, we have used the cluster-beam deposition technique to synthesize Mn$_{\mathrm{5}}$Ge$_{\mathrm{3}}$ nanoparticles with different size. The composition, crystal structure and magnetic properties of the nanoparticles have been characterized by energy dispersive x-ray spectroscopy (EDS), X-ray diffraction, high-resolution transmission electron microscopy (HR-TEM) and magnetometry, respectively. Particles made with 1.7 Torr Argon pressure, and power of 80 W had an average size of 14 nm. Selected area electron diffraction showed that the particles had a hexagonal Mn$_{\mathrm{5}}$Si$_{\mathrm{3}}$-type structure with space group P63/mcm which is the same as in bulk. Magnetic measurements showed that the nanoparticles are ferromagnetic with a Curie temperature near room temperature. The effects of particle size and temperature on the magnetic properties are currently being studied and the results will be reported and discussed.   

Computational Atomistic Modeling of Bi-Magnetic Core-Shell Nanoparticles

Since its discovery, there has been an increasing interest in the modeling of magnetic phenomena found in materials that present exchange bias. In particular, ferro-antiferromagnetic core-shell nanoparticles are an interesting case in which the magnetic properties of the nanostructure can be altered by adjusting their size, shape, and composition. Here we present a computational scheme that efficiently models the magnetic behavior of bi-magnetic core-shell nanostructures. Using a Heisenberg-Dirac-van Vleck Hamiltonian in combination with a continuous spin model, we simulate a wide range of hysteresis diagrams displaying exchange bias. Furthermore, we will demonstrate our efforts towards improving the efficiency of the simulation algorithms, aiming to afford magnetic atomistic simulations of large nanostructures by using a method based on a tessellated unit sphere to account for spin orientations. Our results allow for further semi-quantitative comparisons with existing experimental data and provide a means to discover new phenomena associated with these core-shell nanoparticles and other nanostructures.   

Angular dependence of magnetization in single crystalline cobalt nanowires

In this work, the magnetization behavior of Co nanowires has been investigated by applying the Stoner-Wohlfarth model. The single crystalline cobalt nanowires with a diameter of about 15 nm and a mean length of 200 nm were synthesized via a solvothermal chemical process that have high coercivity up to 12.5 kOe. It is found that the c-axis (002) or the easy magnetization direction of the single-crystalline wires is along the long axis of the nanowires. Particular attention has been paid to the angular dependence of magnetic properties on the applied magnetic field orientation with respect to the c-axis. The angular dependence of coercivity has been modeled and it was revealed that the coherent mode rotation gives the best fitting with the experimental observations. In addition, surface oxidized Co nanowires have also been studied that provided us a unique opportunity to understand the exchange bias in the aligned Co/CoO core-shell nanostructures. Ferromagnetic nanowires of this type are ideal building blocks for future bonded, consolidated and thin film magnets with high energy density and high thermal stability.   

Chalcogenide Cobalt telluride nanotubes

Cobalt telluride nanotubes are grown using wet chemical and hydrothermal syntheses. Wet chemical synthesized nanotubes display nearly 1: 1 Co to Te ratio. On the other hand, CoTe nanotubes synthesized using hydrothermal method show excess Co content leading to the compound Co$_{\mathrm{58}}$Te$_{\mathrm{42}}$. Both CoTe and Co$_{\mathrm{58}}$Te$_{\mathrm{42}}$ display magnetic properties, but with totally different characteristics. The Curie temperature of CoTe is higher than 400 K. However, the T$_{\mathrm{c}}$ of Co$_{\mathrm{58}}$Te$_{\mathrm{42}}$ is below 50 K. Transport properties of cobalt telluride (CoTe) nanotube devices show that they exhibit p-type semiconducting behavior. The magnetoresistance measured at 10 K show a magnetoresistance of 54{\%}. .   

Effect of Bi Substitution on the FCC to L1$_{\mathrm{\mathbf{0}}}$ Phase Transformation in CoPt(Bi) Nanoparticles

The transformation from the fcc to fct structure L1$_{\mathrm{0}}$ in CoPt requires annealing at temperatures over 600$^{\mathrm{0}}$ C, as compared to FePt which can occur at 550$^{\mathrm{0}}$ C. In the past, similar attempts to lower the transformation temperature in CoPt have been unsuccessful. In this work, we report for the first time a decrease in the phase transformation temperature of chemically synthesized CoPt nanoparticles by the addition of a small amount of bismuth. Our studies have shown that the phase transformation occurs in as-made CoPt(Bi) nanoparticles at refluxing temperatures as low as 330 $^{\mathrm{0}}$C, which is significantly lower than previously reported values in CoPt nanoparticles and thin films. The as-made CoPt nanoparticles with 5{\%} atomic weight Bi show partial L1$_{\mathrm{0}}$ ordering with an average size of 11.7 nm, as shown by TEM imaging, and have a coercivity of 1 kOe and saturation magnetization of 32 emu/g. Annealing of the CoPt(Bi) nanoparticles produced maximum coercivities of 12.4 kOe when annealed at 700 $^{\mathrm{0}}$C for 1 hour. The effect of amount of Bi addition on the formation and ordering of L1$_{\mathrm{0}}$ structure will be discussed.   

Magnetic and Structural Properties of Co5Ge3 Nanoparticles.

Magnetic semiconductor alloy nanostructures play a crucial role in advanced technologies due to their tunable band gaps and electronic properties. Among these magnetic semiconductor alloys, Co-Ge is important both scientifically and technologically. In this work, we studied the magnetic and transport properties of Co$_{5}$Ge$_{3\, }$nanoparticles(NPs) fabricated by cluster-beam deposition. The NPs were characterized by X-ray powder diffraction and the results demonstrated that they had the same hexagonal structure P63/mm-type as in bulk.Transmission-electron-microscope observations revealed that the particles have a single crystalline structure with an average size of 8nm. Selected-area electron diffraction(SAED) confirmed the XRD data, showing clearly that the particles have the hexagonal structure mentioned above. High-resolution electron microscopy images show lattice fringes with spacing of 1.99A and 2.02A which correspond to the (102) and (110) superlattice reflections of the hexagonal ordered Co$_{5}$Ge$_{3}$ structure. Magnetic properties showed that these nanoparticles are ferromagnetic at room temperature as-compared to bulk samples that are paramagnetic at all temperatures. This magnetic behavior in Co-Ge nanoparticles indicates new size-controlled spin structures in confined nanosize systems.   

Evolution of Magnetic Moments in Cobalt and Nickel Clusters

Ferromagnetism in transition-metal clusters has attracted much interest owing to their enhanced magnetic moments as compared to those of bulk phases. Here, we investigate the stability and the magnetism of Co and Ni clusters with various structures using a real-space formalism of pseudopotentials within the spin-polarized density-functional theory, i.e., the PARSEC code. We will discuss how the calculated magnetic moments evolve as a function of cluster size and compare them to experiment.   

Spin Moments and Stability of VCun$+$ Clusters: The curious case of VCu$_{\mathrm{4}}^{\mathrm{+}}$, VCu$_{\mathrm{8}}^{\mathrm{+}}$, and VCu$_{\mathrm{12}}^{\mathrm{+}}$

The atomic structures, bonding characteristics, magnetic spin moments, and stability of VCu$_{\mathrm{n}}^{\mathrm{+}}$ clusters have been examined within density functional theory. Our studies show that at small sizes, the spin moments of the vanadium atom (3d$^{\mathrm{3}}$ 4s$^{\mathrm{2}})$ due to 3d electrons are unquenched as the bonding is primarily through 4s electrons. As the cluster grows, the 3d orbitals of the vanadium atom start to participate in hybridized bonding with the copper atoms, resulting in a quenching of the magnetic moment. Upon closer examination of the electronic structures, we observe shell closure at VCu$_{\mathrm{5}}^{\mathrm{+}}$, VCu$_{\mathrm{7}}^{\mathrm{+}}$, and VCu$_{\mathrm{14}}^{\mathrm{+}}$. However, the observed abundances in the photofragmentation profile do not correspond to these shell closures and the subsequent electronic stability they provide. Instead, the enhanced abundances of VCu$_{\mathrm{4}}^{\mathrm{+}}$, VCu$_{\mathrm{8}}^{\mathrm{+}}$, and VCu$_{\mathrm{12}}^{\mathrm{+}}$ seen in the mass spectrum are justified through geometric means and a cluster growth mechanism is proposed. Through synergetic theoretical and experimental efforts, the unusual enhanced stability of VCu$_{\mathrm{4}}^{\mathrm{+}}$, VCu$_{\mathrm{8}}^{\mathrm{+}}$, VCu$_{\mathrm{12}}^{\mathrm{+}}$, and their magnetic properties are probed and explained.   

Model for ferromagnetic behavior of metal cluster-fullerene superatomic solids.

Recent work has explored the precise assembly of binary superatomic solids from metal clusters and fullerene [1] as well as experimentally demonstrated ferromagnetic behavior in such assemblies at low temperatures (less than 10K). However, the origin of this behavior is not yet completely understood and modeled rigorously. We report theoretical analyses and simulations that explain the origin of ferromagnetic behavior from super exchange mechanism and model the temperature dependent magnetic behavior of these superatomic solids. [1] X. Roy et al., Science, 341, 157, 2013. [2] C.H. Lee et al., J. Am. Chem. Soc. 136, 16926, 2014.   

Transition metal doped semiconductor quantum dots: Optical and magnetic properties

We study optical and magnetic properties of CdSe and Cd-Mn-Se quantum dots (QD). We find that there are two luminescence lines, one is fast and another is slow (\textasciitilde 1ms). With the increase of a QD diameter the slow luminescence disappears at some critical QD size, thus only one line (fast) remains. Using the SAC SI computational method we find that D $=$ 3.2 nm and D $=$ 2.7 nm if the Mn impurity is located inside a QD or on a QD surface, respectively. For two or four Mn atoms in the quantum dot, now absorption takes place because the transition is spin-allowed. The DFT calculations of the magnetic state reveal that it is an antiferromagnet. We also study other quantum dots such as Cd-Mn-Se, Zn-Mn-S, and Zn-Mn-Se, doped and undoped. We find the slow luminescence energies for low concentrations of Mn impurities for each QD type. The calculations indicate that two luminescence lines, fast and slow, should always take place. However for Pb-Mn-S quantum dots there are now Mn levels inside a HOMO-LUMO gap, i.e., the Mn-levels are located in a PbS conduction band. The presence of Mn dopants increases the band gap and also removes the exciton peak. This effect is different to the other quantum dots.   

Anomalous Hall Effect and Electron Transport in Co$_{\mathrm{2}}$Si Nanocluster Films

Magnetic nanoparticles or clusters are of fundamental and technological importance, since they exhibit entirely different and/or improved magnetic and electronic properties as compared to bulk alloys. Our recent research shows large average magnetic moments of up to 0.70$\mu_{\mathrm{B}}$/Co at 10K and 0.49$\mu_{\mathrm{B}}$/Co at 300K for cluster-deposited Co$_{\mathrm{2}}$Si nanoparticles, in sharp contrast to the nearly vanishing bulk magnetization. In this talk, we present interesting electron-transport properties in Co$_{\mathrm{2}}$Si nanoparticle films. The film shows a room-temperature negative magnetoresistance (MR) of 0.14{\%} at to kOe, which become as high as 1.8{\%} at low temperatures. We also observed anomalous field-dependent Hall resistivities ($\rho_{\mathrm{xy}})$ in the nanoparticle film, which corroborate the magnetic hysteresis loops. Interestingly, the longitudinal metallic resistivity ($\rho _{\mathrm{xx}})$ shows a resistivity minimum at around 10K, similar to Kondo effects observed in the case of non-magnetic metals due to dilute magnetic impurities. The transport properties will be discussed in terms of the underlying spin correlations in the Co$_{\mathrm{2}}$Si nanoparticle films. This work is supported by the U.S. DOE-BES-DMSE (Grant No. DE-FG02-04ER46152) and NCMN.