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At least 577 records · Page 32
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DOE OSTI2022

Light-induced ferromagnetism in moiré superlattices

Many-body interactions between carriers lie at the heart of correlated physics. The ability to tune such interactions would allow the possibility to access and control complex electronic phase diagrams. Recently, two-dimensional moiré superlattices have emerged as a promising platform for quantum engineering such phenomena. The power of the moiré system lies in the high tunability of its physical parameters by adjusting the layer twist angle, electrical field, moiré carrier filling and interlayer coupling. In this work, we report that optical excitation can highly tune the spin–spin interactions between moiré-trapped carriers, resulting in ferromagnetic order in WS 2 /WSe 2 moiré superlattices. Near the filling factor of v = 1/3 (that is, one hole per three moiré unit cells), as the excitation power at the exciton resonance increases, a well-developed hysteresis loop emerges in the reflective magnetic circular dichroism signal as a function of magnetic field, a hallmark of ferromagnetism. The hysteresis loop persists down to charge neutrality, and its shape evolves as the moiré superlattice is gradually filled, indicating changes of magnetic ground state properties. The observed phenomenon points to a mechanism in which itinerant photoexcited excitons mediate exchange coupling between moiré-trapped holes. This exciton-mediated interaction can be of longer range than direct coupling between moiré-trapped holes, and thus magnetic order arises even in the dilute hole regime. This discovery adds a dynamic tuning knob to the rich many-body Hamiltonian of moiré quantum matter.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Ferromagnetic-like behavior of Bi0.9La0.1FeO3–KBr nanocomposites

We studied magnetostatic response of the Bi0.9La0.1FeO3– KBr composites (BLFO-KBr) consisting of nanosized (≈100nm) ferrite Bi0.9La0.1FeO3 (BLFO) conjugated with fine grinded ionic conducting KBr. When the fraction of KBr is rather small (less than 15wt%)the magnetic response of the composite is very weak and similar to that observed for the BLFO (pure KBr matrix without Bi1-xLaxFeO3 has no magnetic response as anticipated). However, when the fraction of KBr increases above 15%,the magnetic response of the composite changes substantially and the field dependence of magnetization reveals ferromagnetic-like hysteresis loop with a remnant magnetization about 0.14emu/g and coercive field about 1.8Tesla (at room temperature). Nothing similar to the ferromagnetic-like hysteresis loop can be observed in Bi1-zLazFeO3 ceramics with z≤0.15, which magnetization quasi-linearly increases with magnetic field. Different physical mechanisms were considered to explain the unusual experimental results for BLFO-KBr nanocomposites, but only those among them, which are highly sensitive to the interaction of antiferromagnetic Bi0.9La0.1FeO3 with ionic conductor KBr, can be relevant.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Low energy electrodynamics of CrI 3 layered ferromagnet

We report on the optical properties from terahertz (THz) to Near-Infrared (NIR) of the layered magnetic compound CrI 3 at various temperatures, both in the paramagnetic and ferromagnetic phase. In the NIR spectral range, we observe an insulating electronic gap around 1.1 eV which strongly hardens with decreasing temperature. The blue shift observed represents a record in insulating materials and it is a fingerprint of a strong electron-phonon interaction. Moreover, a further gap hardening is observed below the Curie temperature, indicating the establishment of an effective interaction between electrons and magnetic degrees of freedom in the ferromagnetic phase. Similar interactions are confirmed by the disappearance of some phonon modes in the same phase, as expected from a spin-lattice interaction theory. Therefore, the optical properties of CrI 3 reveal a complex interaction among electronic, phononic and magnetic degrees of freedom, opening many possibilities for its use in 2-Dimensional heterostructures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Ultrafast optical observation of spin-pumping induced dynamic exchange coupling in ferromagnetic semiconductor/metal bilayer

Abstract Spin angular momentum transfer in magnetic bilayers offers the possibility of ultrafast and low-loss operation for next-generation spintronic devices. We report the field- and temperature- dependent measurements on the magnetization precessions in Co 2 FeAl/(Ga,Mn)As by time-resolved magneto-optical Kerr effect. Analysis of the effective Gilbert damping and phase shift indicates a clear signature of an enhanced dynamic exchange coupling between the two ferromagnetic (FM) layers due to the reinforced spin pumping at resonance. The temperature dependence of the dynamic exchange-coupling reveals a primary contribution from the ferromagnetism in (Ga,Mn)As.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2023

Unconventional surface state pairs in a high-symmetry lattice with anti-ferromagnetic band-folding

Many complex magnetic structures in a high-symmetry lattice can arise from a superposition of well-defined magnetic wave vectors. These “multi-q” structures have garnered much attention because of interesting real-space spin textures such as skyrmions. However, the role multi-q structures play in the topology of electronic bands in momentum space has remained rather elusive. Here we show that the type-I anti-ferromagnetic 1q, 2q and 3q structures in an face-centered cubic sublattice with band inversion, such as NdBi, can induce unconventional surface state pairs inside the band-folding hybridization bulk gap. Our density functional theory calculations match well with the recent experimental observation of unconventional surface states with hole Fermi arc-like features and electron pockets below the Neel temperature. We further show that these multi-q structures have Dirac and Weyl nodes. Our work reveals the special role that band-folding from anti-ferromagnetism and multi-q structures can play in developing new types of surface states.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Minority-spin t 2 g states and the degree of spin polarization in ferromagnetic metallic La 2-2x Sr 1+2x Mn 2 O 7 (x = 0.38)

In this paper, a half-metal is a material with conductive electrons of one spin orientation. This type of substance has been extensively searched for due to the fascinating physics as well as the potential applications for spintronics. Ferromagnetic manganites are considered to be good candidates, though there is no conclusive evidence for this notion. Here we show that the ferromagnet La 2–2x Sr 1+2x Mn 2 O 7 (x = 0.38) possesses minority-spin states, challenging whether any of the manganites may be true half-metals. However, when electron transport properties are taken into account on the basis of the electronic band structure, we found that the La 2–2x Sr 1+2x Mn 2 O 7 (x = 0.38) can essentially behave like a complete half metal.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites

We report an unusual robust ferromagnetic order above room temperature upon amorphization of perovskite [YCrO3] in pulsed laser deposited thin films. This is contrary to the usual expected formation of a spin glass magnetic state in the resulting disordered structure. To understand the underlying physics of this phenomenon, we combine advanced spectroscopic techniques and first-principles calculations. We find that the observed order-disorder transformation is accompanied by an insulator-metal transition arising from a wide distribution of Cr-O-Cr bond angles and the consequent metallization through free carriers. Similar results also found in YbCrO3-films suggest that the observed phenomenon is more general and should, in principle, apply to a wider range of oxide systems. The ability to tailor ferromagnetic order above room temperature in oxide materials opens up many possibilities for novel technological applications of this counter intuitive effect.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Stable room-temperature ferromagnetic phase at the FeRh(100) surface

Interfaces and low dimensionality are sources of strong modifications of electronic, structural, and magnetic properties of materials. FeRh alloys are an excellent example because of the first-order phase transition taking place at ~400 K from an antiferromagnetic phase at room temperature to a high temperature ferromagnetic one. It is accompanied by a resistance change and volume expansion of about 1%. We have investigated the electronic and magnetic properties of FeRh(100) epitaxially grown on MgO by combining spectroscopies characterized by different probing depths, namely X-ray magnetic circular dichroism and photoelectron spectroscopy. Furthermore, we find that the symmetry breaking induced at the Rh-terminated surface stabilizes a surface ferromagnetic layer involving five planes of Fe and Rh atoms in the nominally antiferromagnetic phase at room temperature. First-principles calculations provide a microscopic description of the structural relaxation and the electron spin-density distribution that support the experimental findings.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Search for ferromagnetic order in overdoped copper-oxide superconductors

In copper-oxides that show high-temperature superconductivity (HTS), the critical temperature (Tc) has a dome-shaped doping dependence. The cause of demise of both Tc and superfluid density ns on the overdoped side is a major puzzle. A recent study of transport and diamagnetism in a large number of overdoped La2-xSrxCuO4 (LSCO) films shows that this cannot be accounted for by disorder within the conventional Bardeen-Cooper-Schrieffer theory. This brings to focus an alternative explanation — competition of HTS with ferromagnetic order, fluctuating in superconducting samples and static beyond the superconductor-to-metal transition. Here, we examine this proposal by growing single-crystal LSCO thin films with doping on both sides of the transition by molecular beam epitaxy, and using polarized neutron reflectometry to measure their magnetic moments. In a heavily overdoped, metallic but non-superconducting LSCO (x = 0.35) film, the spin asymmetry of reflectivity shows a very small static magnetic moment (~2 emu/cm3). Less-doped, superconducting LSCO films show no magnetic moment in neutron reflectivity, both above and below Tc. Therefore, the collapse of HTS with overdoping is not caused by competing ferromagnetic order.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Boronated holey graphene: a case of 2D ferromagnetic metal

In search of new candidates for two-dimensional ferromagnets, we consider boronated monolayer holey graphene (C 2 B), akin to recently synthesized and extensively studied nitrogenated monolayer holey graphene (C 2 N). In contrast to C 2 N which is semiconducting and nonmagnetic, our first-principles calculations show that C 2 B is metallic and ferromagnetic. The microscopic origin of this interesting behavior is found to be related to the hole doping of π–π* network of C–B which produces metallicity while the unpaired electron on the dangling bond of sp 2 hybridized state of two-coordinated B produces magnetism. Calculated cohesive energy of boronated holey graphene indicates that the formation of this structure is energetically feasible as is the case with its nitrogenated counterpart. The dynamic and thermal stability of the predicted boronated holey graphene are checked in terms of phonon calculations and finite temperature molecular dynamics simulations. We further investigate the electronic and magnetic properties of embedded transition-metal single atom and pairs on C 2 B and C 2 N structures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Diffusive ferromagnetic roller gas

An ensemble of actively rotating ferromagnetic particles is used to realize an active roller gas. Here, we investigate the diffusive properties of such a gas in experiments and simulations. We reveal that ferromagnetic rollers demonstrate a normal (Fickian) diffusion with a characteristic linear growth of the mean-squared displacement, while statistics of displacements stay non-Gaussian. At short times the system has a bimodal distribution of the displacements that transitions with time to a quasi-Gaussian distribution (Gaussian core with overpopulated tails) for a range of studied particle number densities. Inert particles introduced into the active roller gas exhibit similar diffusive behavior. Here, the results provide insights into diffusive properties of active colloidal systems with activity originating from spinning degrees of freedom.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Re-entrance to a ferromagnetic insulator with oxygen-vacancy ordering in the La 0.7 Sr 0.3 MnO 3 /SrTiO 3 superlattice

The charge-density wave is correlated with the properties in transition metal oxides, and regulating this state is important to explore the potential of oxides. Defects could modulate the local environment, in which the effect is subject to the concentration and arrangement, and an artificial arrangement of defect-ordering is highly expected to design the material properties. In this work, the ordering of oxygen vacancies with a preferred location on the (001) La/SrO plane by self-structural optimization was induced in La 0.7 Sr 0.3 MnO 3 /SrTiO 3 superlattices with medium oxygen pressure during SrTiO 3 deposition, and then the ferromagnetic charge-ordering (CO)/charge-density-wave (CDW) state was formed in La 0.7 Sr 0.3 MnO 3 with susceptibility to magnetic field, temperature, and superlattice configuration. In addition, re-entrance to the ferromagnetic insulator CO/CDW could be induced by a high magnetic field. The selective occupancy of oxygen vacancies on the (001) La/SrO plane and the phase fluctuation around the CO/CDW-to-ferromagnetic metal phase transition were critical to the above properties. The interfacial diffusion with atomic-level resolution to control oxygen vacancies could be used in other magnetoelectric systems and corresponding device applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Magnetoresistance and electron-hole exchange interaction in (Co{sub 1-x}In{sub x}){sub 2}O{sub 3-v} concentrated ferromagnetic semiconductors

Systematic studies of electrical transport properties of (Co{sub 1-x}In{sub x}){sub 2}O{sub 3-v} concentrated ferromagnetic semiconductors were performed. Quantitative analysis demonstrated that spin dependent variable range hopping dominated the low temperature transport behavior. Moreover, it was found that ferromagnetic or antiferromagnetic coupling of electron-hole pair generated during the variable range hopping process is responsible for the negative or positive magnetoresistance, respectively. Our results not only illustrate the connection between magnetoresistance and electron-hole exchange interaction but also provide a unique method to detect the exchange interaction of electron-hole pair.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Long-range ferromagnetic order induced by a donor impurity band exchange in SnO{sub 2}:Er{sup 3+} nanoparticles

In this work, the structural and magnetic properties of Er-doped SnO{sub 2} (SnO{sub 2}:Er) nanoparticles are reported. The SnO{sub 2}:Er nanoparticles have been synthesized by a polymer precursor method with Er content from 1.0% to 10.0%. X-ray diffraction results indicate the formation of only the rutile-type structure in all samples. The estimated mean crystallite size shows a decrease from ∼10 to ∼4 nm when the Er content is increased from 1.0% to 10.0%. The particle size values have been corroborated by transmission electron microscopy technique. The thermal dependence of the magnetization is consistent with the 3+ oxidation state of erbium ions for all samples. A strong paramagnetic-like behavior coexisting with a ferromagnetic phase has been determined for samples with Er content below 5.0%. Above this concentration, only a paramagnetic behavior has been determined. Isothermal magnetization curves are consistent with the occurrence of long-range ferromagnetic order mediated by donor electrons forming bound magnetic polarons which overlap to produce a spin-split impurity band.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Ordering ferromagnetic In{sub 1−x}Mn{sub x}As quantum dots

In this work, we present a method to order low temperature (LT) self-assembled ferromagnetic In{sub 1−x}Mn{sub x}As quantum dots (QDs) grown by molecular beam epitaxy (MBE). The ordered In{sub 1−x}Mn{sub x}As QDs were grown on top of a non-magnetic In{sub 0.4}Ga{sub 0.6}As/GaAs(100) QDs multi-layered structure. The modulation of the chemical potential, due to the stacking, provides a nucleation center for the LT In{sub 1−x}Mn{sub x}As QDs. For particular conditions, such as surface morphology and growth conditions, the In{sub 1−x}Mn{sub x}As QDs align along lines like chains. For comparison purposes, we also report the study of QDs grown on plain GaAs(100) substrates. Ferromagnetic behavior was observed for all structures at 2 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Ferromagnetic ordering of Cr and Fe doped p-type diamond: An ab initio study

Ferromagnetic ordering of transition metal dopants in semiconductors holds the prospect of combining the capabilities of semiconductors and magnetic systems in single hybrid devices for spintronic applications. Various semiconductors have so far been considered for spintronic applications, but low Curie temperatures have hindered room temperature applications. We report ab initio DFT calculations on the stability and magnetic properties of Fe and Cr impurities in diamond, and show that their ground state magnetic ordering and stabilization energies depend strongly on the charge state and type of co-doping. We predict that divacancy Cr{sup +2} and substitutional Fe{sup +1} order ferromagnetically in p-type diamond, with magnetic stabilization energies (and magnetic moment per impurity ion) of 16.9 meV (2.5 μ{sub B}) and 33.3 meV (1.0 μ{sub B}), respectively. These magnetic stabilization energies are much larger than what has been achieved in other semiconductors at comparable impurity concentrations, including the archetypal dilute magnetic semiconductor GaAs:Mn. In addition, substitutional Fe{sup +1} exhibits a strong half-metallic character, with the Fermi level crossing bands in only the spin down channel. These results, combined with diamond’s extreme properties, demonstrate that Cr or Fe dopedp-type diamond may successfully be considered in the search for room temperature spintronic materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Static critical phenomena in Co-Ni-Ga ferromagnetic shape memory alloy

Ferromagnetic shape memory alloys are smart materials because they exhibit temperature driven shape memory effect and magnetic field induced strain. Thus two types of energy, i.e. thermal and magnetic, are used to control their shape memory behaviour. Study of critical phenomenon in such materials has received increased experimental and theoretical attention for better understanding of the magnetic phase transition behavior as well as further development of ferromagnetic shape memory materials. In the present study we report the preparation and characterization of bulk Co{sub 45}Ni{sub 25}Ga{sub 30} alloy, prepared by a sequence of arc melting technique followed by homogenization at 1150 °C for 24 hours and ice-water quenching. Structural and magnetic properties of the alloys were studied by means of X-ray diffraction and vibrating sample magnetometer in an applied field range of ±18 kOe equipped with a high temperature oven. We have determined the critical temperature T{sub C} (∼375.5 K) and the critical exponents viz; β=0.40, γ=1.68 and δ=5.2. Asymptotic critical exponents β, γ, and δ obey Widom scaling relation, γ+β=βδ, and the magnetization data satisfy the scaling equation of state for second-order phase transition in the asymptotic critical region.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Room-temperature spin-polarized organic light-emitting diodes with a single ferromagnetic electrode

In this paper, we demonstrate the concept of a room-temperature spin-polarized organic light-emitting diode (Spin-OLED) structure based on (i) the deposition of an ultra-thin p-type organic buffer layer on the surface of the ferromagnetic electrode of the Spin-OLED and (ii) the use of oxygen plasma treatment to modify the surface of that electrode. Experimental results demonstrate that the brightness of the developed Spin-OLED can be increased by 110% and that a magneto-electroluminescence of 12% can be attained for a 150 mT in-plane magnetic field, at room temperature. This is attributed to enhanced hole and room-temperature spin-polarized injection from the ferromagnetic electrode, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗