Session A23: Metals Magnetic

Bulk electronic structure of FeRh undergoing metamagnetic transition via hard x-ray photoemission

In this study changes in the electronic structure accompanying a temperature-induced metamagnetic transition from anti-ferromagnetic to ferromagnetic order are investigated in strained epitaxial FeRh thin films via valence-band and core-level hard x-ray photoelectron spectroscopy with a photon energy of 6 keV. At such high photon energy, the resulting inelastic mean-free paths of the photoemitted electrons and therefore the average probing depths are on the order of 60 {\AA}, corresponding to about 20 unit cells and ensuring truly bulk-sensitive measurement. Clear differences between the AFM and FM states are observed across the entire valence-band spectrum and these are well reproduced using density functional theory. Changes in the Fe 2p core-levels of Fe are also observed and interpreted using Anderson impurity model calculations. These results suggest that significant electronic structure changes are involved in this AFM-FM transition.   

Random magnet with competing anisotropies in Fe$_{x}$Ni$_{1-x}$F$_{2}$ alloys

A series of epitaxial (110) Fe$_{x}$Ni$_{1-x}$F$_{2}$ films were deposited on (110) MgF$_{2}$ via molecular beam epitaxy. The Fe concentration $x$ was determined by measuring the lattice parameter along the [110] direction using x-ray diffraction. The film thicknesses and the roughness of each interface were found by fitting of x-ray reflectivity data. The magnetic ordering as a function of x was analyzed by SQUID magnetometry. Enhancement of the N\'{e}el temperature in alloys as well as evidence of spontaneous magnetization along the c-axis after field-cooling were observed for samples with x$>$0.1; for samples with x$<$0.1 the magnetization was perpendicular to the c-axis. Two phase transitions were observed for alloy samples with x$>$0.1. The phase diagram of the upper transition was consistent with mean field theory of a system with competing anisotropies. The transition at lower temperatures was unaffected by the application of a magnetic field, whereas the upper temperature transition was broadened by the application of fields as small as 50 Oe. This suggests the presence of a spin-glass phase at lower temperatures, followed by melting of the spin glass prior to the main transition to paramagnetic behavior as the temperature is raised.   

Structural and electronic origin of large magnetostrictive $Fe_{1-x}Ga_x$ alloys

$Fe_{1-x}Ga_x$ binary alloys exhibiting large magnetostriction and excellent ductility have great potentials for various applications. The origin of large magnetostriction has not been thoroughly studied, especially at high Ga concentration ($x > 18.75 \%$). We conduct extensive ab initio molecular dynamics simulation of $Fe_{1-x}Ga_x$ alloys to generate atomic structures; and find that the alloys adopt the disordered A2 structure for $x < 18.75 \%$ and the A2+$D0_3$ mixed structures for $18.75 \% < x < 23.4 \%$, respectively. The formation of $D0_3$-like structure play a key role for the decrease of magnetostriction beyond $x = 19\%$. Interestingly, the magnetostriction may greatly enhance, up to $\lambda_{001} = 850$ ppm in ternary alloys with incorporation of $3 - 5 \%$ Cu and Zn.   

Thin film deposition of Mn$_2$Ga under various growth condition

The tetragonal \emph{DO}$_{22}$ phase of Mn$_{3-x}$Ga is a ferromagnetic Heusler with perpendicular magnetic anisotropy. It has high spin polarization, high Curie temperature and low magnetic moment, and thus becomes a good candidate for spin-transfer-torque magnetic random access memory. This work reports the epitaxial growth of tetragonal Mn$_2$Ga thin films using sputtering method. The effect of various substrates, buffer layers, and substrate temperature on the film roughness is presented and compared. The formation of Mn$_2$Ga islands are observed at the beginning of growth on most buffer layers by reflection-high-energy-electron-diffraction. The smoothest film with root-mean-square roughness 0.10nm is obtained by using Pt buffer layer. The magnetic properties are also compared.   

Magneto Caloric Effect in Ni-Mn-Ga alloys: First Principles and Experimental studies

Understanding the Magneto-Caloric Effect (MCE) in alloys with real technological potential is important to the development of viable MCE based products. We report results of computational and experimental investigation of a candidate MCE materials Ni-Mn-Ga alloys. The Wang-Landau statistical method is used in tandem with Locally Self-consistent Multiple Scattering (LSMS) method to explore magnetic states of the system. A classical Heisenberg Hamiltonian is parametrized based on these states and used in obtaining the density of magnetic states. The Currie temperature, isothermal entropy change, and adiabatic temperature change are then calculated from the density of states. Experiments to observe the structural and magnetic phase transformations were performed at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) on alloys of Ni-Mn-Ga and Fe-Ni-Mn-Ga-Cu. Data from the observations are discussed in comparison with the computational studies. This work was sponsored by the Laboratory Directed Research and Development Program (ORNL), by the Mathematical, Information, and Computational Sciences Division; Office of Advanced Scientific Computing Research (US DOE), and by the Materials Sciences and Engineering Division; Office of Basic Energy Sciences (US DOE).   

First principles determination of the rhombohedral magnetostriction of $Fe_{100-x}Ga_x$ and $Fe_{100-x}Al_x$ ($x<20$) alloys

Extensive efforts have been dedicated to investigate the extraordinary magnetostriction of Galfenol (Fe$_{100-x}Ga_x$) and Alfenal (Fe$_{100-x}Al_x$) alloys, which are very promising magnetostrictive materials for various applications such as sensors, transducers and spintronic devices. In contrast to the strong response of the tetragonal magnetostriction ($\lambda_{100}$) to the Ga/Al concentration (x), the rhombohedral magnetostriction,$\lambda_{111}$, vs. x curves for both FeGa and FeAl alloys show negative and steady values for $0 [Preview Abstract]

Intrinsic Magnetic Properties of $\textit{L}1_{0}$ Derivates

It is investigated how atomic substitutions modify the magnetization, exchange and anisotropy of $\textit{L}1_{0}$-ordered ferromagnets. Emphasis is on properties of interest in permanent magnetism, including the reduction of raw-materials costs by substituting iron-series transition metals for expensive heavy (4$\textit{d}$/5$\textit{d}$) transition-metal elements. In particular, VASP calculations are used to determine the magnetizations of the Fe-Co-Pt, Mn-Al-C and Fe-Ni-S systems. We perform supercell calculations to determine the moments of Fe and Co in various $\textit{L}1_{0}$ derivates with chemical disorder. The local magnetic moments exhibit a subtle dependence on the environment, not only in each Fe-Co layer but also through alternating 4$\textit{d}$/5$\textit{d}$ layers. However, the magnitude of these spatial fluctuations is not very large, and after configurational averaging, the moments exhibit very simple dependencies on the concentrations of the involved atoms. The FeNi system is also interesting because $\textit{L}1_{0}$-ordered FeNi was originally discovered in meteorites (tetrataenite), formed with cooling times in excess of one million years.   

Growth of Mn$_{2}$TiSn alloy and its magnetic and structural characterizations

A ternary inter-metallic alloy Mn$_{2}$TiSn is one of the candidates predicted to be a Heusler compound with high spin polarization and Curie temperature well above room temperature, which is suitable for spintronic applications. Mn$_{2}$TiSn powder samples were prepared by arc melting and annealing under controlled environment to study magnetic and structural properties. Temperature and field dependent magnetization measurements show the sample is ferromagnetically ordered with a Curie temperature above 400 K. The low temperature magnetization at 10 K is about 2.5 emu/g under 100 Oe. The coercivity increases as temperature decreases from 1 kOe at room temperature to 2.4 kOe at 10 K. Theoretical calculation from the high field data shows the anisotropy energy is 4.0 $\times$ 10$^{5}$ ergs/cm$^{3}$ at 300 K but it becomes slightly more than doubled at 10 K. TEM and XRD characterizations reveal that the compound crystallizes in the hexagonal structure (D0$_{19}$, P6$_{3}$/mmc) rather than the theoretically proposed L2$_{1}$ cubic structure, which is supported by the first-principle structure calculations where the total energy per unit cell volume is preferred for the hexagonal structure.   

Unusual temperature dependence of the magnetic anisotropy constant in barium ferrite BaFe$_{12}$O$_{19}$

We report magnetic hysteresis loops in a wide temperature range (4-700 K) for silica-coated barium ferrite BaFe$_{12}$O$_{19}$ nanoparticles. The saturation magnetization $M_{s}$ and the first magnetic anisotropy constant $K$ are determined simultaneously from the magnetic hysteresis loop using the law of approach to saturation. It is remarkable that $K$ is linearly proportional to $M_{s}$ and varies precisely with temperature as $K(T) = K(0)[1-(T/T_{C})^{1.58}]$ in the whole temperature range below the Curie temperature $T_{C}$ (740 K). The unusual temperature dependence of the anisotropy constant and its linear relation with the saturation magnetization in BaFe$_{12}$O$_{19}$ are not predicted from the existing theoretical models.   

First principles study of magnetic properties of Zn-Sn substituted M-type Sr-hexaferrite

Site occupancy and magnetic properties of Zn-Sn substituted M-type Sr-hexaferrite SrFe$_{12-x}$(Zn$_{0.5}$Sn$_{0.5}$)$_x$O$_{19}$ with $x=1$ were studied using density functional theory and generalized gradient approximation (GGA). Using the GGA+U method the description of strongly correlated $3d$ electrons of Fe was improved. Our results show that Zn and Sn atoms prefer to occupy $4f_1$ and $4f_2$ sites respectively. Favorable Zn-Sn substituted configurations show an increase in saturation of magnetization (M$_s$), and a decrease in magnetic anisotropy energy (MAE), over the pure M-type Sr-hexaferrite ($x=0$). Experimental observations agree with the decrease of MAE and the increase of M$_s$ for Zn-Sn substituted Sr-hexaferrite.   

The Origin of Secondary Hematite Phase in Non-stoichiometric Co-ferrite Prepared by Ceramic Method

Surprising results have shown the formation of a secondary phase of hematite in co-ferrite. Co-ferrite based materials have been increasingly studied for magnetoelastic and magnetoelectric applications. Enhancement of the properties for such applications can be made by cation substitution, heat treatment or processing. Often, samples whose compositions deviate from stoichiometry or targeted compositions are made. It is not yet known, how far from stoichiometric composition one would go to create other phases; which would also affect the properties. This study shows that, when samples are made by the ceramic method, a secondary hematite phase forms with the spinel Co-ferrite phase. The origin of the hematite phase is related to the processing temperature, sintering environment and deviation from stoichiometric composition. Consequently, deviation from targeted or stoichiometric compositions may explain why properties of co-ferrite reported in literature vary.   

Magnetic Characterization of heavily proton-irradiated MnAl Hall bars

$\tau -$MnAl has L1$_{0}$ structure with the perpendicular magnetic anisotropy and it attracts some interests due to the potential application in magnetic recording media and spintronics. We fabricate $\sim $2um wide Hall bar devices from 40 nm MnAl thin film to study the transport properties. Anomalous Hall Effect (AHE) resistance loops of the devices and out-of-plane magnetization loops of unpatterned films mimic one another. This correlation shows that the electrical transport of the material is strongly spin dependent at room temperature. Crystallinity and chemical ordering characterization are performed using X-ray diffraction on unpatterned films. Both patterned and unpatterned films are then exposed to 2 MeV-energy protons for 2 hours at Naval Research Laboratory for displacement damages study. The magnetic and magneto-transport properties were not degraded after the irradiation, which shows promising radiation hardness for future spintronics devices. The heavier, more energetic beam sources and larger dose radiations will be performed and discussed.   

Structural and magnetic properties of high anisotropy HfCo$_{7-x}$Fe$_{x}$ alloys

An increasing demand of rare-earth elements in recent years intensifies the search for rare-earth free permanent magnetic materials with magnetocrystalline anisotropy $K_{1}$ = 1 MJ/m$^{3}$. The present study reports melt-spun HfCo$_{7}$ alloys with a high $K_{1}$ of 1.3 MJ/m$^{3}$ along with an appreciable saturation-magnetic polarization ($J_{s})$ of 8.9 kG. A substitution of Fe for Co in HfCo$_{7-x}$Fe$_{x}$ further improves $K_{1}$ and $J_{s}$ to 1.5 MJ/m$^{3}$ and 10.4 kG, respectively. XRD studies of HfCo$_{7}$ are in agreement with an orthorhombic structure and also reveal a lattice expansion on substituting Fe for Co. These results show that HfCo$_{7-x}$Fe$_{x}$ can be a promising candidate for permanent-magnet and other significant applications. In brief, the structural and magnetic properties of HfCo$_{7-x}$Fe$_{x}$ alloys (0 $\le $ x $\le $ 1) investigated using XRD, EDX, TEM, and SQUID magnetometer will be presented.   

Calciothermic Preparation of Sm-Co From Nanostructured Precursor Oxides

A calciothermic reduction/diffusion process has been developed for the preparation of Sm-Co alloys with good magnetic properties using nanostructured Co- and SmCo-oxide powders as precursors. The precursor oxides were obtained from an aqueous solution of Sm(NO$_{3})_{3}$\textbullet 6H$_{2}$O, Co(NO$_{3})_{2}$\textbullet 6H$_{2}$O, and citric acid which, after the removal of excess water, spontaneously reacts to form a fine mixture of Co$_{3}$O$_{4}$ and SmCoO$_{3}$ nanoparticles. These nanoparticles were then mixed with metallic Ca granules, sealed under inert gas in Nb tubes, and heated to temperatures between 850 and 1000 $^{o}$C for various lengths of time. The products of the reduction/diffusion reaction were studied by scanning electron microscopy with elemental analysis, vibrating sample magnetometry, and x-ray diffraction. These measurements revealed that (in addition to the oxidation product CaO) the products of the reduction/diffusion process were micron-sized particles of essentially single phase SmCo$_{5 }$with a coercivity in excess of 40 kOe and a Curie temperature of about 950 $^{o}$C.   

Theoretical search for new permanent magnets with no rare earth atoms

We use the density functional theory and Quasiparticle Self-Consistent GW approximation to investigate the crystal and electronic structure, magnetic moment, anisotropy, and exchange coupling of Fe$_{16}$N$_2$, Fe$_{13}$Al$_3$, Co$_7$Hf and Zr$_2$Co$_{11}$. Both methods show similar results for magnetization and electronic structure. The experimentally unknown crystal structures of Co$_7$Hf and Zr$_2$Co$_{11}$ are obtained using structural optimization. We also discuss possible usage of these materials as permanent magnets.