Session F16: Focus Session: High Magnetic Anisotropy Materials

Growth, structure and magnetism of self-organized epitaxial nano-alloys on a metallic substrate

The CoPt alloy is one of the most studied bimetallic compounds, due to its potential application for magnetic recording. We report here on Co$_{x}$Pt$_{1-x}$ nano-alloys deposited on the well-known Au(111) reconstructed surface since it has been recognized as a powerful substrate in order to investigate the magnetic properties of metallic nano-clusters [1,2]. The growth of Co$_{x}$Pt$_{1-x}$ clusters on the Au(111) surface observed by STM revealed a morphological transition from single layer to bilayer islands with the Co concentration x. Using molecular dynamics calculations, we show that this transition is driven by the local strain due to Co atoms. These results are interpreted by a competition between the interface energy, the mixing energy and the elastic energy. Using X-ray Magnetic Circular Dichroism, we have studied the magnetic properties of these nano-alloys. The out-of-plane anisotropy of pure Co clusters strongly decreases, until it goes in-plane for 40\% of Pt. This spin reorientation transition is interpreted by a phenomenological pair model for magnetic anisotropy. \textbf{References} [1] N. Weiss et al., Phys. Rev. Lett. 95, 157204 (2005) [2] S. Rohart et al., Phys. Rev Lett. 104, 104, 137202 (2010).   

Tailoring anisotropy in (001) oriented (Fe$_{\mathrm{1-x}}$Cu$_{\mathrm{x}})_{55}$Pt$_{45}$ films

High anisotropy magnetic materials are central to future spintronic and recording technologies. Binary alloy FePt in its fct-L1$_{0}$ phase offers ideal magnetic properties, but usually requires high temperature annealing. Alloying with Cu has been suggested to lower the annealing temperature. However, it has been difficult to grow oriented films and prior studies have often focused on non-ideal compositions. In this work we investigate (Fe$_{\mathrm{1-x}}$Cu$_{\mathrm{x}})_{55}$Pt$_{45}$ films -- an ideal ratio for the L1$_{0}$ phase. Fully ordered films with a strong (001) texture were grown by an atomic-scale multilayer sputtering method and rapid thermal annealing at 400 $^{\circ}$C. The room-temperature deposition, low annealing temperature, and lack of a seed layer shows the strength of this technique. An increase in the tetragonal lattice distortion and fragmentation of the microstructure [while retaining the (001) texture] were observed with added Cu. Magnetic properties were evaluated and show a strong perpendicular anisotropy. The Cu inclusion is demonstrated to decrease T$_{\mathrm{C}}$ without hefty sacrifices to M$_{\mathrm{S}}$ and K$_{\mathrm{U}}$, making such films ideal for heat-assisted magnetic recording.   

Direct Chemical synthesis of L1$_{\mathrm{0}}$ FePt Nanoparticles with High Coercivity

FePt particles with tetragonal L1$_{\mathrm{0}}$ structure have been of great interest as one of the most promising candidate for ultra-high density recording media. Chemical synthesis is one of the two major methods to fabricate FePt nanoparticles because it can lead to high uniformity and patterned assembly. However, traditional approaches require post annealing above 500${^\circ}$ to transform the FePt nanoparticles from the disordered face-centered cubic (fcc) to the ordered L1$_{\mathrm{0}}$ phase which introduces undesirable agglomeration and sintering. In this study, we have fabricated ordered L1$_{\mathrm{0}}$ FePt nanoparticles using one-step chemical synthesis without post annealing. The traditional synthesis method of reduction of Pt(acac)$_{\mathrm{2}}$ and Fe(CO)$_{\mathrm{5}}$ was used at higher temperatures in the range of 300 to 400${^\circ}$ . Monodispersed Au nanoparticles with average size of 10 nm were used as catalysts. X-ray diffraction (XRD) spectra and selected area electron diffraction (SAED) patterns revealed that the FePt nanoparticles are in L1$_{\mathrm{0}}$ phase. The highest coercivity obtained was 8 kOe at room temperature and 11 kOe at 50 K and is achieved at the reaction temperature of 400${^\circ}$. Transmission electron microscopy (TEM) images showed that FePt nanoparticles are partially agglomerated which needs further improvement.   

Structure and Magnetic Properties of Sm-Co Particles Synthesized From Nanostructured Precursor Oxides

Sm-Co particles have been synthesized by a calciothermic reduction/diffusion reaction from a Sm-Co-O precursor. The precursor oxide was prepared by an autocombustion reaction and was subsequently milled with CaO, which served as a grain-growth inhibitor and thermal ballast. The effects of varying the amount of Ca metal between 2 and 6 times the amount needed for complete reduction of the available Sm-Co-O precursor, the reaction temperature between 850 and 1000 C, and reaction time between 15 minutes and 24 hours were studied. The structural and magnetic properties of the Sm-Co reaction products were studied by x-ray diffraction, scanning electron microscopy, and vibrating sample magnetometry measurements. The results of these measurements indicated that magnetically hard SmCo$_{\mathrm{5}}$ was preferentially formed in samples synthesized with high Ca amounts and short diffusion times, or low Ca amounts and long diffusion times. Particle sizes were also observed to increase with both increasing diffusion time and increasing Ca amount. The influence of changes in both reaction temperature and size of the CaO additive on the final product were also studied.   

Magnon softening in exchange-coupled hard-soft nanocomposites

We study spin excitations of the fully aligned state for three-dimensional nanocomposites of exchange coupled hard (SmFeN) and soft (FeCo) phases. When the amount of soft phase is increased the energy of low-lying spin excitation is considerably softened and contains a contribution proportional to the anisotropy constant of the soft phase. The dipolar interaction further lowers the magnon energy and controls the spin wave gap at ${\bf k}= 0$, which closes when the amount of soft phase exceeds a critical value. With the addition of soft phase or increasing the temperature the system moves to another ground state characterized by a magnetization mismatch between spins of hard and soft phases.   

Atomic structure of high-coercivity cobalt-carbide nanoparticles ensembles

Permanent magnets are increasingly important in numerous applications, including the quickly expanding area of green technologies ($e.g.$ high efficiency electric car motors and wind turbine power systems). We present studies of novel permanent magnet materials based on cobalt carbide nanoparticles (NPs), where the energy product ($BH_{max}$) exceeds 20 kJ / m$^3$ [1]. The NPs are synthesized via a polyol process, which offers a flexible approach to modify the Co-carbide phase (Co$_2$C and Co$_3$C), and NP morphology, size and size dispersion. The Co$_2$C and Co$_3$C phases have unique magnetic properties, and the combination exhibits the high $BH_{max}$. We present a detailed assessment of the structure of mixtures of Co$_2$C and Co$_3$ NPs, measured by high-resolution, synchrotron based powder x-ray diffraction (p-XRD). Both the Co$_2$C and Co$_3$ phases exhibit an orthorhombic structure (Pnnm and Pnma space groups, respectively). The high-resolution p-XRD facilitates identification of mixed phase samples, enabling detailed comparisons of the atomic structure with the magnetic properties, measured by both lab-based magnetometry and x-ray spectroscopy (soft x-ray XAS \& XMCD). \newline [1] V. G. Harris et al., J. Phys. D: Appl. Phys. 43, 165003 (2010).   

Magnetic and electronic structure of high-coercivity cobalt-carbide nanoparticles for permanent magnet applications

Permanent magnets are important in numerous technological applications. However, those with the largest energy product (\textit{BH}$_{max})$ contain rare earth elements, which increase costs and introduce volatility into the supply chain. Recently, rare-earth free Co$_{\mathrm{2}}$C and Co$_{\mathrm{3}}$C nanoparticles (NPs) with large magnetic coercivity and \textit{BH}$_{max}$ have been synthesized using a polyol process [1]. Optimal \textit{BH}$_{max} $is found in a mixture of the two phases. In this system, the nature of the magnetic interparticle interactions and the origins of intrinsic magnetic properties of the Co-carbide phases are not fully understood. We have investigated the origins of the magnetic properties of Co$_{\mathrm{2}}$C and Co$_{\mathrm{3}}$C NPs using x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements at the Co $L$-edge and C $K$-edge. From differences in the electronic structures of the two Co-carbide phases, as determined by XAS, the nature of their unique magnetic properties can be deduced. Furthermore, the role of the spin and orbital moments in determining magnetic anisotropy and \textit{BH}$_{max}$ in these materials is examined with XMCD. [1] V. G. Harris et al. J. Phys. D: Appl. Phys. \textbf{43}, 165003 (2010).   

Brilliant CoC nanomagnets: highly magnetocrystalline anistropy for potential applications

No doubt that the development of novel materials and their understanding on a smaller size scale is still a challenging issue at the basis of progress in many areas of materials science. This is almost entirely true in the development of new magnetic materials for a various types of vital applications. Recently the focus has moved from the microcrystalline to the nanocrystalline magnetic regime. The most common amorphous and nanocrystalline magnetic materials are classified to be one of either magnetically hard (a quite larger coercivity) or soft (a material with a very smaller coercivity) materials. In the work at hand we are concerning the hard magnetic CoC nanopareticles. This material shows a mono-dispersed, stable against air environment and larger magnetocrystalline anisotropy as well as larger coercivty. In addition, the mono-dispersed and small particle size led to getting a Curie temperature much smaller than the related bulk materials. Based on the relation between the Curie temperature and the number of atoms, the shape of the particles can be determined. Therefore, the CoC nanomagnes with impressive magnetic properties open the root for various essential applications such as permanent magnets, magnetic sensors and contrast agent for MRI diagnostic tools.   

Structural stability of HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ magnetic nanoclusters

The gas-aggregation-type cluster deposition has emerged as an attractive method to create uniaxially aligned nanoparticle building-blocks of metastable and new permanent-magnet materials such as HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ with appreciable coercivities ($H_{c} \approx $ 5.0 kOe), magnetocrystalline anisotropies ($K_{1} \approx $ 10 Mergs/cm$^{3})$, and magnetic polarization ($J_{s} \approx $ 10 kG) at 300 K. In comparison, bulk HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ alloys form only at ideal stoichiometries and high temperatures above 1000 $^{\circ}$C at thermal equiliburium conditions. We have investigated the structural stability of HfCo$_{7}$ and Zr$_{2}$Co$_{11}$ phases on varying their stoichiometries from ideal values in HfCo$_{7\pm \delta }$ and Zr$_{2}$Co$_{11\pm \delta}$ nanoclusters (0$\le \delta \le $1), respectively and compared these results with the corresponding bulk phase diagrams. This study provides new insights to understand the underlying crystal structure and magnetic properties of the nanoclusters and to explore them for significant applications.   

Growth and Properties of Mn$_{\mathrm{x}}$Ga Magnetic Nanostructures

Rare-Earth (RE) magnets are becoming more expensive and less available for current applications in technology. Mn$_{\mathrm{x}}$Ga (x$=$2-3) has previously shown coercivity of \textgreater\ 2.5 T, close to that of RE magnets.\footnote{T.J. Nummy, S.P Bennett, T. Cardinal, and D. Heiman, \textit{Large Coercivity in Nanostructured Rare-earth-free Mn}$_{x}$\textit{Ga Films}, Appl. Phys. Lett. \textbf{99}, 252506 (2011).} In this project, the vapor-liquid-solid (VLS) method was used to grow nanoparticles of Mn$_{\mathrm{x}}$Ga (x$=$1-3) with MBE. The goal was to study the magnetic properties as a function of reduced dimensionality. The samples were prepared by depositing a 3-6 nm layer of Au on Si, GaAs, and glass. It was found that the miscibility of Ga and Au is high, but for Mn and Au it is much lower. Therefore, during the growth process Ga was deposited on the gold catalyst followed by Mn deposition. The samples were then annealed at temperatures 100-500 $^{\circ}$C. Nanostructures, including regions of nanowires, were found using scanning electron microscopy on all samples. The magnetic properties of the nanostructured samples were studied with SQUID magnetometry and found to have a magnetization of 200 emu/cm$^{3}$.   

Magnetism of MnGa-based nanostructures

Materials with high magnetic anisotropy and Curie temperature well above room temperature have potential for a range of applications including high-density recording, nonvolatile memory and permanent-magnet materials. Mn$_{\mathrm{y}}$Ga (1 $\le $ y $\le $ 2) is one such compounds that can be synthesized in the tetragonal L1$_{0}$ or D0$_{22}$ structures based on the value of y in Mn$_{\mathrm{y}}$Ga. Our experimental investigation of the rapidly quenched nanostructured ribbons shows that the material with y $=$1.2, 1.4 and 1.6 prefers the L1$_{0}$ structure and that with y $=$1.9 and 2.1 prefers D0$_{22}$ structure. We have found a maximum saturation magnetization of 88 emu/g in Mn$_{1.2}$Ga which decreases monotonically to 50 emu/g as y reaches 2.1. Although both the L1$_{0}$ and D0$_{22}$Mn$_{\mathrm{y}}$Ga samples show a high Curie temperature (T$_{\mathrm{c}})$ well above room temperature, the value of T$_{\mathrm{c}}$ decreases almost linearly from 740 K for Mn$_{2.1}$Ga to 550 K for Mn$_{1.2}$Ga. We will also discuss the effect of boron doping on the structural and magnetic properties of this material.   

MnBi: a better magnet via computational design

Using DFT-based methods, we study the magnetic properties of MnBi in the technologically important low-temperature phase. We identify the origin and behavior of the magnetoanisotropy and magnetism versus structure and doping. We perform high-throughput screening for dopants that improve magnetoanisotropy (larger, $c$-axis only -- no reorientations) and magnetization, and chemical and structural stability. We also assess the best-in-class materials for exchange-spring coupled magnet, without the use of rare-earth elements. Experimental assessment of the predictions is also provided. Work was supported by the U.S. Department of Energy, ARPA-E under REACT (0472-1526), using methods develop under support by the Office of Basic Energy Science, Division of Materials Science and Engineering (DE-FG02-03ER46026 and DE-AC02-07CH11358). Ames Laboratory is operated for the U.S. DOE by Iowa State University under contract DE-AC02-07CH11358.   

DFT high-throughput screening of transition metal dopant in MnBi for better magnetic properties

To improve the magnetic properties of MnBi, especially magnetic energy product, we use density functional theory (DFT) calculations to study alloying effects on MnBi properties with transition metals (TMs), both as dopant and soft phase in an exchange-spring magnet composite. We have considered various defects in the NiAs-type structure with interstitial and substitutional sites. Via high-throughput screening for dopants from groups 3-16, we have DFT trends in impurity formation energy, magnetization, structural parameters, magnetoanisotropy, etc. Early and late TMs prefer to occupy the Mn sites, while mid-TMs are not stable. Early and late TMs are antiferromagnetically coupled, while mid-TMs are ferromagnetically coupled to MnBi. For 3$d$ mid-TMs, there is no increase in magnetization. However, energetically favorable mid-late-TM substitutes at Mn sites can improve the magnetic anisotropy. To investigate improving the magnetic energy product of a composite alloy system, we also detail the coupling between MnBi thin films with soft magnetic materials having a high magnetization.   

First Principles~Studies of the Magnetic Properties of Alnico Permanent Magnet Materials

Until the advent of rare earth based magnets Alnico was one of the highest energy product hard magnets available. Recently, interest in this system has been rekindled as system whose properties and utility may be further enhanced but does not contain rare earth elements. Recent experiments on Alnico alloy suggest that there is no sharp interface between the disordered bcc FeCo magnetic phase and the ordered B2 NiAl non-magnetic phase; thereby undermining our understanding of the large coercivity of this material. By utilizing several electronic structure methods we first study the issue of the effect of substitutions of additional elements into B2 NiAl phase. We also calculate the magnetic moment distribution across the interface and examine the magnetic ground state. These calculations suggest that the magnetic structure of the B2-phase as well as the interface in much more complex than previously thought.