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At least 685 records · Page 38
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Effect of Controlled Artificial Disorder on the Magnetic Properties of EuFe 2 (As 1– x P x ) 2 Ferromagnetic Superconductor

Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, EuFe 2 ( As 1 – x P x ) 2 ( x = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, T c ( H ) , shows an extraordinarily large decrease. It starts at T c ( H = 0 ) ≈ 24 K in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Remarkably, after the irradiation not only T c moves significantly below the FM transition, its values differ drastically for measurements at different frequencies, ≈16 K in AC measurements and ≈12 K in a DC regime. We attribute such a large difference in T c to the appearance of the spontaneous internal magnetic field below the FM transition, so that the superconductivity develops directly into the mixed spontaneous vortex-antivortex state where the onset of diamagnetism is known to be frequency-dependent. We also examined the response to the applied DC magnetic fields and studied the annealing of irradiated samples, which almost completely restores the superconducting transition. Overall, our results suggest that in EuFe 2 ( As 1 – x P x ) 2 superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered Eu 2 + lattice act as efficient pairbreakers leading to a significant T c reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of T m (less than 1 K) after the irradiation drives T c to below T m . Our results suggest that FM and SC phases coexist microscopically in the same volume.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Insulating ferromagnetic oxide films: the controlling role of oxygen vacancy ordering

The origin of ferromagnetism in strained epitaxial LaCoO3 films has been a long-standing mystery. Here, we combine atomically resolved Z-contrast imaging, electron-energy-loss spectroscopy, and density-functional calculations to demonstrate that, in epitaxial LaCoO3 films, oxygen-vacancy superstructures release strain, control the film s electronic properties, and produce the observed ferromagnetism via the excess electrons in the Co d states. Although oxygen vacancies typically dope a material n-type, we find that ordered vacancies induce Peierls-like minigaps which, combined with strain relaxation, trigger a nonlinear rupture of the energy bands, resulting in insulating behavior.

Salafranca Laforga, Juan I↗
DOE OSTI2013

Oxygen vacancy induced photoluminescence and ferromagnetism in SrTiO{sub 3} thin films by molecular beam epitaxy

SrTiO{sub 3} thin films were epitaxially grown on (100) SrTiO{sub 3} substrates using molecular beam epitaxy. The temperature for growth of the films was optimized, which was indicated by x-ray diffraction and further confirmed by microstructural characterization. Photoluminescence spectra show that oxygen-vacancy contributes to red and blue luminescence of oxygen-deficient post-annealed films, and a red shift was observed in blue region. On the other hand, ferromagnetism in film form SrTiO{sub 3} was observed from 5 K to 400 K and could be further enhanced with decreasing oxygen plasma partial pressure in annealing processes, which might be explained by the theory involving d{sup 0} magnetism related to oxygen-vacancy. From the cooperative investigations of optical and magnetic properties, we conclude that intrinsic defects, especially oxygen-vacancy, can induce and enhance luminescence and magnetism in SrTiO{sub 3} films.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Thin-film topological insulator-ferromagnet heterostructures for terahertz detection

An atomically thin topological insulator is investigated theoretically for long-wavelength photodetection when it interacts with a magnetic material. Through the coupling between top and bottom surfaces as well as the exchange interaction with the proximate ferromagnet, the distribution of optically excited carriers exhibits unique patterns that depend sensitively on the frequency of the incoming light. This effect results in the generation of strong nonzero photocurrent, leading potentially to room-temperature detection of far-infrared/THz radiation with the advantage of low noise and fast response. The ease of frequency tuning by an external electrical bias offers an added versatility in the realistic implementation.

36 MATERIALS SCIENCE↗
DOE OSTI2014

New opportunities in microwave electronics with ferromagnetic nanowires

A series of monolithic tunable microwave band-stop filters and phase shifters have been designed, fabricated, and characterized on arrays of ferromagnetic nanowires (FMNWs) constituted of Ni, Co, CoFeB, and NiFe alloy embedded in anodic alumina templates (AAO). The aim of the present investigation is to significantly boost the operating frequency of devices in very low applied magnetic field. The stop-band response can cover from 8 to 40 GHz with an applied magnetic field up to 5 kOe using these materials. In addition, we observed a substantial increase in frequency tunability (>110%) for Ni and NiFe alloy based filter. Frequency bandwidth/linewidth can be tuned with the proper choice of magnetic material, broad band for Co nanowires to narrow band for CoFeB nanowires. The amorphous nature of CoFeB reduces linewidth considerably and hence increases resonance absorption. It is also observed that the frequency linewidth (Δf) decreases with increasing applied magnetic field. The same filter can be used as a tunable phase shifter. For Ni nanowire based phase shifter, a maximum shift of 75°/cm at 4 kOe was observed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Observations of thermally excited ferromagnetic resonance on spin torque oscillators having a perpendicularly magnetized free layer

Measurements of thermally excited ferromagnetic resonance were performed on spin torque oscillators having a perpendicularly magnetized free layer and in-plane magnetized reference layer (abbreviated as PMF-STO in the following) for the purpose of obtaining magnetic properties in the PMF-STO structure. The measured spectra clearly showed a large main peak and multiple smaller peaks on the high frequency side. A Lorentzian fit on the main peak yielded Gilbert damping factor of 0.0041. The observed peaks moved in proportion to the out-of-plane bias field. From the slope of the main peak frequency as a function of the bias field, Lande g factor was estimated to be about 2.13. The mode intervals showed a clear dependence on the diameter of the PMF-STOs, i.e., intervals are larger for a smaller diameter. These results suggest that the observed peaks should correspond to eigenmodes of lateral spin wave resonance in the perpendicularly magnetized free layer.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Identification of the interstitial Mn site in ferromagnetic (Ga,Mn)As

We determined the lattice location of Mn in ferromagnetic (Ga,Mn)As using the electron emission channeling technique. We show that interstitial Mn occupies the tetrahedral site with As nearest neighbors (T{sub As}) both before and after thermal annealing at 200 °C, whereas the occupancy of the tetrahedral site with Ga nearest neighbors (T{sub Ga}) is negligible. T{sub As} is therefore the energetically favorable site for interstitial Mn in isolated form as well as when forming complexes with substitutional Mn. These results shed new light on the long standing controversy regarding T{sub As} versus T{sub Ga} occupancy of interstitial Mn in (Ga,Mn)As.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Half metallic ferromagnetism in tri-layered perovskites Sr{sub 4}T{sub 3}O{sub 10}(T = Co, Rh)

First-principles density functional theory (DFT) is used to investigate the electronic and magnetic properties of Sr{sub 4}Rh{sub 3}O{sub 10}, a member of the Ruddlesden-Popper series. Based on the DFT calculations taking into account the co-operative effect of Coulomb interaction (U) and spin-orbit couplings (SOC), Sr{sub 4}Rh{sub 3}O{sub 10} is found to be a half metallic ferromagnet (HMF) with total magnetic moment μ{sub tot} = 12 μ{sub B} per unit cell. The material has almost 100% spin-polarization at the Fermi level despite of sizable SOC. Replacement of Rh atom by the isovalent Co atom is considered. Upon full-replacement of Co, a low-spin to intermediate spin transition happens resulting in a HMF state with the total magnetic moment three-time larger (i.e., μ{sub tot} = 36 μ{sub B} per unit cell), compared to Sr{sub 4}Rh{sub 3}O{sub 10}. We propose Sr{sub 4}Rh{sub 3}O{sub 10} and Sr{sub 4}Co{sub 3}O{sub 10} as candidates of half metals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Obtaining strong ferromagnetism in diluted Gd-doped ZnO thin films through controlled Gd-defect complexes

We demonstrate the fabrication of reproducible long-range ferromagnetism (FM) in highly crystalline Gd{sub x}Zn{sub 1−x}O thin films by controlling the defects. Films are grown on lattice-matched substrates by pulsed laser deposition at low oxygen pressures (≤25 mTorr) and low Gd concentrations (x ≤ 0.009). These films feature strong FM (10 μ{sub B} per Gd atom) at room temperature. While films deposited at higher oxygen pressure do not exhibit FM, FM is recovered by post-annealing these films under vacuum. These findings reveal the contribution of oxygen deficiency defects to the long-range FM. We demonstrate the possible FM mechanisms, which are confirmed by density functional theory study, and show that Gd dopants are essential for establishing FM that is induced by intrinsic defects in these films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Cell culture arrays using micron-sized ferromagnetic ring-shaped thin films

Cell patterning has become an important technology for tissue engineering. In this research, domain walls are formed at the two ends of a ferromagnetic ring thin film after applying a strong external magnetic field, which can effectively attract magnetically labeled cells and control the position for biological cell. Magnetophoresis experiment was conducted to quantify the magnetic nanoparticle inside the cells. A ring-shaped magnetic thin films array was fabricated through photolithography. It is observed that magnetically labeled cells can be successfully attracted to the two ends of the ring-shaped magnetic thin film structure and more cells were attracted and further attached to the structures. The cells are co-cultured with the structure and kept proliferating; therefore, such ring thin film can be an important candidate for in-vitro biomedical chips or tissue engineering.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2015

Real-space presentation of the magnetic anisotropy of metallic ferromagnets

A real-space representation of the uniaxial magnetic anisotropy (MA) of metallic ferromagnets is formulated using the full-orbital tight-binding model. By adopting a second-order perturbation for the spin-orbit interaction, numerical calculations of the uniaxial MA are performed for the L1{sub 0} FePt and Y{sub 2}Fe{sub 14}B compounds to evaluate the validity and applicability of the present method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetoresistive system with concentric ferromagnetic asymmetric nanorings

A structure consisting of two concentric asymmetric nanorings, each displaying vortex remanent states, is studied with micromagnetic calculations. By orienting in suitable directions, both the asymmetry of the rings and a uniform magnetic field, the vortices chiralities can be switched from parallel to antiparallel, obtaining in this way the analogue of the ferromagnetic and antiferromagnetic configurations found in bar magnets pairs. Conditions on the thickness of single rings to obtain vortex states, as well as formulas for their remanent magnetization are given. The concentric ring structure enables the creation of magnetoresistive systems comprising the qualities of magnetic nanorings, such as low stray fields and high stability. A possible application is as contacts in spin injection in semiconductors, and estimations obtained here of magnetoresistance change for a cylindrical spin injection based device show significant variations comparable to linear geometries.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Spin-dependent Seebeck effects in a graphene nanoribbon coupled to two square lattice ferromagnetic leads

We theoretically investigate spin-dependent Seebeck effects for a system consisting of a narrow graphene nanoribbon (GNR) contacted to square lattice ferromagnetic (FM) electrodes with noncollinear magnetic moments. Both zigzag-edge graphene nanoribbons (ZGNRs) and armchair-edge graphene nanoribbons (AGNRs) were considered. Compared with our previous work with two-dimensional honeycomb-lattice FM leads, a more realistic model of two-dimensional square-lattice FM electrodes is adopted here. Using the nonequilibrium Green's function method combining with the tight-binding Hamiltonian, it is demonstrated that both the charge Seebeck coefficient S{sub C} and the spin-dependent Seebeck coefficient S{sub S} strongly depend on the geometrical contact between the GNR and the leads. In our previous work, S{sub C} for a semiconducting 15-AGNR system near the Dirac point is two orders of magnitude larger than that of a metallic 17-AGNR system. However, S{sub C} is the same order of magnitude for both metallic 17-AGNR and semiconducting 15-AGNR systems in the present paper because of the lack of a transmission energy gap for the 15-AGNR system. Furthermore, the spin-dependent Seebeck coefficient S{sub S} for the systems with 20-ZGNR, 17-AGNR, and 15-AGNR is of the same order of magnitude and its maximum absolute value can reach 8 μV/K. The spin-dependent Seebeck effects are not very pronounced because the transmission coefficient weakly depends on spin orientation. Moreover, the spin-dependent Seebeck coefficient is further suppressed with increasing angle between the relative alignments of magnetization directions of the two leads. Additionally, the spin-dependent Seebeck coefficient can be strongly suppressed for larger disorder strength. The results obtained here may provide valuable theoretical guidance in the experimental design of heat spintronic devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Layer thickness dependence of current induced effective fields in ferromagnetic multilayers

We report the relation between current induced effective fields and ferromagnetic layer thickness. Hall measurements with rotating magnetic field show that the transverse and perpendicular effective fields have linear relations to the inverse of the magnetic moment m per area S. The results imply that both of these effective fields may originate from spin angular momentum transferring. However, the non-zero intercept of the transverse field at m/S = 0 implies that magnetization independent effects, such as Rashba effect, may contribute to transverse field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spin wave isolator based on frequency displacement nonreciprocity in ferromagnetic bilayer

We demonstrated the spin wave isolator using bilayer ferromagnetic media comprising single crystalline and poly-crystalline yttrium iron garnet films, which can control the propagation frequency of magnetostatic waves by the direction of applied magnetic field. This isolator's property does not depend on their thickness then this can be downsized and integrated for nano-scale magnonic circuits. Calculated dispersion relationship shows good agreement with measured one.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetometry of single ferromagnetic nanoparticles using magneto-optical indicator films with spatial amplification

We present a magneto-optical technique to spatially amplify and image fringe fields from single ferromagnetic nanorods. The fringe fields nucleate magnetic domains in a low-coercivity, perpendicularly magnetized indicator film, which are expanded by an applied out-of-plane field from the nanoscale to the microscale for measurement with polar Kerr microscopy. The nucleation location and therefore magnetic orientation of the sample nanorod are detected as spatially dependent field biases in locally measured hysteresis loops of the indicator film. We first discuss our method to fabricate the high-sensitivity indicator film with low energy argon ion irradiation. We then present a map of the amplified signal produced from a single nanorod as measured by the indicator film and compare it with a simultaneously obtained, unamplified fringe field map. The comparison demonstrates the advantage of the amplification mechanism and the capability of the technique to be performed with single-spot magneto-optical Kerr effect magnetometers. Our signal-to-noise ratio determines a minimum measureable particle diameter of tens of nanometers for typical transition metals. We finally use our method to obtain hysteresis loops from multiple nanorods in parallel. Our technique is unperturbed by applied in-plane fields for magnetic manipulation of nanoparticles, is robust against many common noise sources, and is applicable in a variety of test environments. We conclude with a discussion of the future optimization and application of our indicator film technique.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Gilbert damping of ferromagnetic metals incorporating inhomogeneous spin dynamics

The effects of inhomogeneous spin dynamics on magnetic damping in ferromagnetic metals are studied. On the basis of linear response theory, we derive the microscopic expression for the Gilbert damping term in a two-dimensional electron gas interacting with the magnetization via exchange coupling in the presence of Rashba spin-orbit coupling (SOC). In the spin wave propagating with the wave vector, q, the behavior of q-dependent damping can be explained in terms of both inter- and intra-band spin excitations. The spatially dependent damping torques originating from Rashba SOC that cancel out in a uniform precession system distort the circular orbit of a magnetization-precession trajectory in the presence of inhomogeneous spin dynamics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗