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At least 1,747 records · Page 97
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Density functional study of CaN mono and bilayer on Cu(001)

Density functional - pseudopotential calculations are performed to provide first-principles insights into magnetic behaviour of bulk CaN and CaN monolayers on Cu(001) in the rock-salt (RS) and zinc-blende (ZB) structures. Our results indicate that both RS- and ZB-CaN exhibit half-metallic ferromagnetism originated from the incomplete 2p shell of the nitrogen ion. In contrast to the bulk CaN, the CaN monolayers on Cu(001) generally favor ZB structure. We argue that the more stable ZB-CaN thin films on Cu(001) are nonmagnetic, because of strong Cu-N bonding at the interface, while the less stable Ca terminated ZB-CaN thin films exhibit half-metallic ferromagnetism. The transition path between the high energy ferromagnetic and the stable nonmagnetic configurations of the ZB-CaN monolayer on Cu(001) are studied by using the nudged elastic band method. We observe a two stages transition and an activation barrier of about 1.18 eV in the minimum energy path of this transition.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Magnetic and dielectric properties of Aurivillius phase Bi{sub 6}Fe{sub 2}Ti{sub 3−2x}Nb{sub x}Co{sub x}O{sub 18} (0 ≤ x ≤ 0.4)

We investigate the structural, magnetic, and dielectric properties of Bi{sub 6}Fe{sub 2}Ti{sub 3−2x}Nb{sub x}Co{sub x}O{sub 18} (0 ≤ x ≤ 0.4). The room-temperature ferromagnetism is observed in the Nb and Co co-doped samples compared with the paramagnetic behavior in Bi{sub 6}Fe{sub 2}Ti{sub 3}O{sub 18}. The ferromagnetism in Bi{sub 6}Fe{sub 2}Ti{sub 3−2x}Nb{sub x}Co{sub x}O{sub 18} can be understood in terms of spin canting of the antiferromagnetic coupling of the Fe-based and Co-based sublattices via Dzyaloshinsky-Moriya interaction. Moreover, doping Co at Ti sites can significantly enhance the ferromagnetic Curie temperature compared with the substitution of Co for Fe in the Aurivillius compounds. The dielectric loss of Bi{sub 6}Fe{sub 2}Ti{sub 3−2x}Nb{sub x}Co{sub x}O{sub 18} (0.1 ≤ x ≤ 0.4) exhibits a relaxation process. The rather large activation energy in the 0.1 ≤ x ≤ 0.3 samples implies that the relaxation process is not due to the thermal motion of oxygen vacancies inside ceramics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnetic fields and fluctuations in weakly Mn doped ZnGeP{sub 2}

We report on our measurements of local and bulk magnetic features in weakly Mn doped ZnGeP{sub 2}. Utilizing muon spin rotation and relaxation measurements, we identify local ferromagnetic order and fluctuations in the local fields as sampled by an implanted muon (μ{sup +}). We also report on field induced ferromagnetism occurring above the claimed paramagnetic to ferromagnetic transition temperature (T{sub c} = 312 K)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Effect of size reduction on magnetic ordering in Bi{sub 0.2}Sr{sub 0.8}MnO{sub 3}

We present a comparative study of the temperature dependent magnetic properties and electron paramagnetic resonance parameters of nano and bulk samples of Bi{sub 0.2}Sr{sub 0.8}MnO{sub 3} (BSMO). Bulk BSMO is known to have a high T{sub N} ∼ 260 K and robust charge ordering (T{sub CO} ∼ 360 K). We confirm that the bulk sample shows an antiferromagnetic transition around ∼260 K and a spin-glass transition ∼40 K. For the nano sample, we see a clear ferromagnetic transition at around ∼120 K. We conclude that spin glass state, which is present due to the co-existence of antiferromagnetic and ferromagnetic states in the bulk sample, is suppressed in the nano sample and ferromagnetism is induced instead.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Enhanced magnetoresistance induced by oxygen deficiency in La{sub 0.4}Ca{sub 0.6}MnO{sub 3-δ} oxides

We report electrical features and magnetoresistance behavior of the oxygen deficient La{sub 0.4}Ca{sub 0.6}MnO{sub 3-δ} perovskites (δ = 0, 0.15, and 0.2). These samples will be referred to as S0, S15, and S20, respectively. The dependence of electrical transport on temperature and magnetic field is systematically investigated between 2 K and 400 K in magnetic field ranging up to 5 T. The parent compound shows a stable charge ordering/antiferromagnetic state with a semiconductor-like behavior in all considered temperature range. The variable range hopping and thermally activated hopping models are found to fit well with the electrical resistivity data at low and high temperatures, respectively. Oxygen deficiency tends to weaken the charge ordering and induce ferromagnetism and metallicity at low temperature. Metal insulator transition appears at higher fields for lower oxygen deficit (S15 sample) and without field for the S20 sample. The resistivity data for S15 sample are discussed in the framework of the variable-range hopping model. Abnormal transport properties were observed in the S20 sample, characterized by the double metal-insulator transitions and low minimum behavior. These results are discussed in terms of phenomenological percolation model, based on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. While the parent compound shows no magnetoresistance, a large magnetoresistance is observed in the deficient samples at low temperature reaching 90% and 75% at 2 T for S15 and S20 samples, respectively. Noticeably, these values reached 98% and 91% at 5 T. The appearance of colossal magnetoresistance is attributed to the spin dependent hopping between spin clusters and/or ferromagnetic domains.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Investigation of cation distribution, electrical, magnetic properties and their correlation in Mn{sub 2-x}Co{sub 2x}Ni{sub 1-x}O{sub 4} films

X-ray photoelectron spectroscopy was performed on Mn{sub 2-x}Co{sub 2x}Ni{sub 1-x}O{sub 4} (x = 0.2, 0.4, 0.5, 0.6, and 0.8) series spinel films to determine their cation distributions. The results show that both Ni and Co ions exhibit bivalence, while Mn ions exhibit coexisting multivalence. The hopping frequency ν{sub 0} of each sample was obtained by combining cation distribution results and electrical measurement data. Analysis of the divergence at Neel temperature between field cooling and zero-field cooling magnetization curves revealed that the ferromagnetic coupling between octahedral site ions first leads to spontaneous magnetization at T{sub C}, after which a second spontaneous magnetization arises from the antiferromagnetic couple between tetrahedral and octahedral sites at a lower temperature T{sub N}. The correlation between hopping conductivity and magnetic transition temperature is discussed. We propose that the strength of the ferromagnetic couple between octahedral sites is subject to the product term of Nc(1 − c)ν{sub 0}, which determines the ferromagnetic transition temperature (T{sub C}) of the system. Our results provide an avenue to understand the origin of magnetic interaction in small-polaron hopping semiconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Role of oxygen vacancies in magnetic properties of LaAlO{sub 3}/SrTiO{sub 3} interface

The interface of two band-insulators SrTiO{sub 3} and LaAlO{sub 3} exhibits a highly conducting two-dimensional electron gas which shows low-temperature superconductivity, ferromagnetism and many other novel properties. Several studies reveal that the oxygen vacancies at the interface have strong influence on the electronic and magnetic properties of the interface electrons. Here, we develop an insight into the effects of oxygen vacancies on the magnetic properties of the system; the oxygen vacancies help in establishing ferromagnetism which competes with superconductivity at low temperatures. Simulated Monte-Carlo annealing suggests an ordering of the oxygen vacant sites and sheds light on the experimental observation of the quenching of magnetic moment on annealing. A possible explanation for the coexistence of superconductivity and ferromagnetism is also discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnetic properties of Fe doped SmCrO{sub 3} perovskite

The compound SmCr{sub 1−x} Fe{sub x}O{sub 3} perovskites were prepared by citric acid route. the samples were characterized by XRD and SEM. The temperature and field dependent magnetization measurements were carried out in the temperature range of 5K −400 K at 0.01T field and −5T to 5T field at 2K. SmCrO3 compound has shown two magnetic transition temperatures at 197 K and 38 K. The observed behavior at 197 K is the characteristic of anti-ferromagnetic ordering of Cr{sup 3+} moments with weak ferromagnetism. The drop in magnetization below 38 K is due to the spin reorientation of Sm{sup 3+} in anti ferromagnetic arrangement and Cr{sup 3+}spins. the doping of Fe in SmCrO{sub 3} compound has shown a decrease in T{sub N1} and also the two magnetization reversals at 177K and 57K. The magnetic behavior at low temperatures is (T<T{sub N1}) explained in the context of competition among moments of rare earth (Sm) and transition metal ion(Cr/Fe). The existence of the two magnetization reversals offers the characteristic switching of magnetization without changing the direction of the applied magnetic field.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Magnetism in undoped ZnS studied from density functional theory

The magnetic property induced by the native defects in ZnS bulk, thin film, and quantum dots are investigated comprehensively based on density functional theory within the generalized gradient approximation + Hubbard U (GGA + U) approach. We find the origin of magnetism is closely related to the introduction of hole into ZnS systems. The relative localization of S-3p orbitals is another key to resulting in unpaired p-electron, due to Hund's rule. For almost all the ZnS systems under study, the magnetic moment arises from the S-dangling bonds generated by Zn vacancies. The charge-neutral Zn vacancy, Zn vacancy in 1− charge sate, and S vacancy in the 1+ charge sate produce a local magnetic moment of 2.0, 1.0, and 1.0 μ{sub B}, respectively. The Zn vacancy in the neutral and 1− charge sates are the important cause for the ferromagnetism in ZnS bulk, with a Curie temperature (T{sub C}) above room temperature. For ZnS thin film with clean (111) surfaces, the spins on each surface are ferromagnetically coupled but antiferromagnetically coupled between two surfaces, which is attributable to the internal electric field between the two polar (111) surfaces of the thin film. Only surface Zn vacancies can yield local magnetic moment for ZnS thin film and quantum dot, which is ascribed to the surface effect. Interactions between magnetic moments on S-3p states induced by hole-doping are responsible for the ferromagnetism observed experimentally in various ZnS samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Optimization of excess Bi doping to enhance ferroic orders of spin casted BiFeO{sub 3} thin film

Multiferroic Bismuth Ferrite (BiFeO{sub 3}) thin films with varying excess bismuth (Bi) concentration were grown by chemical solution deposition technique. Room temperature multiferroic properties (ferromagnetism, ferroelectricity, and piezoelectricity) of the deposited BiFeO{sub 3} thin films have been studied. High resolution X-ray diffraction and Raman spectroscopy studies reveal that the dominant phases formed in the prepared samples change continuously from a mixture of BiFeO{sub 3} and Fe{sub 2}O{sub 3} to pure BiFeO{sub 3} phase and, subsequently, to a mixture of BiFeO{sub 3} and Bi{sub 2}O{sub 3} with increase in the concentration of excess Bi from 0% to 15%. BiFeO{sub 3} thin films having low content (0% and 2%) of excess Bi showed the traces of ferromagnetic phase (γ-Fe{sub 2}O{sub 3}). Deterioration in ferroic properties of BiFeO{sub 3} thin films is also observed when prepared with higher content (15%) of excess Bi. Single-phased BiFeO{sub 3} thin film prepared with 5% excess Bi concentration exhibited the soft ferromagnetic hysteresis loops and ferroelectric characteristics with remnant polarization 4.2 μC/cm{sup 2} and saturation magnetization 11.66 emu/g. The switching of fine spontaneous domains with applied dc bias has been observed using piezoresponse force microscopy in BiFeO{sub 3} thin films having 5% excess Bi. The results are important to identify optimum excess Bi concentration needed for the formation of single phase BiFeO{sub 3} thin films exhibiting the improved multiferroic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Magnetic anisotropy in ultrathin Fe films on GaAs, ZnSe, and Ge (001) substrates

We discuss magnetic anisotropy parameters of ferromagnetic body-centered cubic (bcc) Fe films grown by molecular beam epitaxy (MBE) on (001) substrates of face-centered cubic (fcc) GaAs, ZnSe, and Ge. High-quality MBE growth of these metal/semiconductor combinations is made possible by the fortuitous atomic relationship between the bcc Fe and the underlying fcc semiconductor surfaces, resulting in excellent lattice match. Magnetization measurements by superconducting quantum interference device (SQUID) indicate that the Fe films grown on (001) GaAs surfaces are characterized by a very strong uniaxial in-plane anisotropy; those grown on (001) Ge surfaces have a fully cubic anisotropy; and Fe films grown on ZnSe represent an intermediate case between the preceding two combinations. Ferromagnetic resonance measurements carried out on these three systems provide a strikingly clear quantitative picture of the anisotropy parameters, in excellent agreement with the SQUID results. Based on these results, we propose that the observed anisotropy of cubic Fe films grown in this way results from the surface reconstruction of the specific semiconductor substrate on which the Fe film is deposited. These results suggest that, by controlling surface reconstruction of the substrate during the MBE growth, one may be able to engineer the magnetic anisotropy in Fe, and possibly also in other MBE-grown ferromagnetic films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Local polar structure and multiferroic properties of (1−x)Bi{sub 0.9}Dy{sub 0.1}FeO{sub 3}−xPbTiO{sub 3} solid solution

The multiferroic (1–x)[0.9BiFeO{sub 3}–0.1DyFeO{sub 3}]–xPbTiO{sub 3} (BDF–xPT) solid solution with compositions around the morphotropic phase boundary (x = 0.25, 0.28, 0.31, 0.34, and 0.37) has been synthesized in the form of ceramics. The phase symmetry, microstructure, ferroelectricity, piezoresponse, and ferromagnetic properties have been characterized by various techniques. It is found that, with increasing content of lead titanate from x = 0.25 to x = 0.37, the grain size increases and the ferroelectric property is improved. The ferroelectric performance is further enhanced by the introduction of an excess amount (2%) of TiO{sub 2} or by sintering in oxygen atmosphere, which reduces the leakage. The local polar structure is imaged by piezoresponse force microscopy. Both the out-of-plane and in-plane images reveal distinct ferroelectric domain structures, with the amplitude and the average domain size decreasing with the increase of lead titanate amount. Compared with the (1–x)BiFeO{sub 3}–xPbTiO{sub 3} binary solid solution of about the same concentration of PT, the magnetic properties are enhanced in BDF–xPT due to the presence of dysprosium, as demonstrated by the ferromagnetic hysteresis loops displayed at room temperature and at 10 K, but the remnant magnetization decreases with increasing PT content. The simultaneous presence of ferroelectricity and ferromagnetism entitles the BDF–xPT solid solution a room-temperature multiferroic material.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Surface driven effects on magnetic properties of antiferromagnetic LaFeO{sub 3} nanocrystalline ferrite

LaFeO{sub 3} nanocrystalline ferrites were synthesized through sol-gel method in different size distributions and the effect of finite size on magnetic properties is investigated. Results of magnetization and Mössbauer measurements show that superparamagnetism and weak ferromagnetic behavior in some of the size distributions. The origin of the superparamagnetism is from fine particles similar to ferromagnetic single domains and the weak ferromagnetism comes from surface spin disorder caused by Dzyaloshinskii-Moriya interaction. The magnetic ground state of LaFeO{sub 3} nanoparticles differs from that of bulk, and the ground state is dictated by the finite size effect because density of states depends on the dimensionality of the sample.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Oxygen-vacancy-induced room-temperature magnetization in lamellar V{sub 2}O{sub 5} thin films

In this work, we study the local atomic and electronic structures as well as oxygen-vacancy-induced magnetic properties of electrodeposited V{sub 2}O{sub 5} films. Unlike stoichiometric V{sub 2}O{sub 5}, which is a diamagnetic lamellar semiconductor, our oxygen-defective V{sub 2}O{sub 5} films are ferromagnetic at room-temperature and their saturation magnetization decreases with air exposure time. X-ray absorption spectroscopy was used to monitor the aging effect on these films, revealing that freshly-made samples exhibit only local crystalline order, whereas the aged ones undoubtedly show an enhancement of crystallinity and coordination symmetry. The mean number of oxygen atoms around V tends to increase, indicating a decrease of oxygen vacancies with time. Concurrently with the decrease of oxygen vacancies, a loss of saturation magnetization is also observed. Hence, it can be concluded that the ferromagnetism of the V{sub 2}O{sub 5} films originates from a vacancy-induced mechanism, confirming the universality of this class of ferromagnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Anomalous magnetic configuration of Mn{sub 2}NiAl ribbon and the role of hybridization in the martensitic transformation of Mn{sub 50}Ni{sub 50−x}Al{sub x} ribbons

The magnetic configuration of Mn{sub 2}NiAl ribbon has been investigated. In contrast to Ni{sub 2}MnAl, the compound Mn{sub 2}NiAl with considerable disorder does exhibit ferromagnetism and, due to exchange interaction competition, both ferromagnetic and antiferromagnetic moment orientations can coexist between nearest neighbor Mn atoms. This is unexpected in Heusler alloys. Regarding the mechanism of the martensitic transformation in Mn{sub 50}Ni{sub 50−x}Al{sub x}, it is found that increasing the Al content results in an unusual change in the lattice constant, a decrease of the transformation entropy change, and enhancement of the calculated electron localization. These results indicate that the p-d covalent hybridization between Mn (or Ni) and Al atoms gradually increases at the expense of the d-d hybridization between Ni and Mn atoms. This leads to an increased stability of the austenite phase and a decrease of the martensitic transformation temperature. For 11 ≤ x ≤ 14, Mn{sub 50}Ni{sub 50−x}Al{sub x} ferromagnetic shape memory alloys are obtained.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effect of the oxide layer on current-induced spin-orbit torques in Hf|CoFeB|MgO and Hf|CoFeB|TaO{sub x} structures

We study the effect of the oxide layer on the current-induced spin-orbit torques (SOTs) in perpendicularly magnetized Hf|CoFeB|MgO (MgO-capped) or Hf|CoFeB|TaO{sub x} (TaO{sub x}-capped) structures. The effective fields corresponding to both the field-like and damping-like current-induced SOTs are characterized using electric transport measurements. Both torques are found to be significantly stronger in MgO-capped structures than those in TaO{sub x}-capped structures. The difference in field-like and damping-like SOTs in the different structures may be attributed to the different Rashba-like Hamiltonian, arising from the difference in the electric potential profiles across the oxide|ferromagnet interfaces in the two cases, as well as possible structural and oxidation differences in the underlying CoFeB and Hf layers. Our results show that the oxide layer in heavy-metal|ferromagnet|oxide trilayer structures has a very significant effect on the generated SOTs for manipulation of ferromagnetic layers. These findings could potentially be used to engineer SOT devices with enhanced current-induced switching efficiency.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Exponentially decaying magnetic coupling in sputtered thin film FeNi/Cu/FeCo trilayers

Magnetic coupling in trilayer films of FeNi/Cu/FeCo deposited on Si/SiO{sub 2} substrates have been studied. While the thicknesses of the FeNi and FeCo layers were kept constant at 100 Å, the thickness of the Cu spacer was varied from 5 to 50 Å. Both hysteresis loop and ferromagnetic resonance results indicate that all films are ferromagnetically coupled. Micromagnetic simulations well reproduce the ferromagnetic resonance mode positions measured by experiments, enabling the extraction of the coupling constants. Films with a thin Cu spacer are found to be strongly coupled, with an effective coupling constant of 3 erg/cm{sup 2} for the sample with a 5 Å Cu spacer. The strong coupling strength is qualitatively understood within the framework of a combined effect of Ruderman-Kittel-Kasuya-Yosida and pinhole coupling, which is evidenced by transmission electron microscopy analysis. The magnetic coupling constant surprisingly decreases exponentially with increasing Cu spacer thickness, without showing an oscillatory thickness dependence. This is partially connected to the substantial interfacial roughness that washes away the oscillation. The results have implications on the design of multilayers for spintronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Magnetic phase transitions and monopole excitations in spin ice under uniaxial pressure: A Monte Carlo simulation

In this work, we explore the spin ice model under uniaxial pressure using the Monte Carlo simulation method. For the known spin ices, the interaction correction (δ) introduced by the uniaxial pressure varies in quite a wide range from positive to negative. When δ is positive, the ground state characterized by the ferromagnetic spin chains is quite unstable, and in real materials it serves as intermediate state connecting the ice state and the long range ordered dipolar spin ice ground state. In the case of negative δ, the system relaxes from highly degenerate ice state to ordered ferromagnetic state via a first order phase transition. Furthermore, the domain walls in such ferromagnetic state are the hotbed of the excitations of magnetic monopoles, thus indicating that the uniaxial pressure can greatly increase the monopole density.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗