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DOE OSTI2019

Half Metallic Ferromagnetic Character in ZnXP{sub 2} (X = Ge, Si) Chalcopyrites Doped with Mn

The electronic and the magnetic properties of ZnXP{sub 2} (X = Ge, Si) were studied using the Korring-Kohn-Rostoker (KKR) method combined with the coherent potential approximation (CPA). The total and partial density of state (DOS) are computed for different Mn concentrations. The total magnetic moment, FM and DLM energies, and their variation as well as Curie temperature are also given. It is shown that the substitution of X cations (X = Ge, Si) by Mn atoms in ZnXP{sub 2} chalcopyrites leads to half metallic ferromagnetic character with double exchange mechanism. Thus, the critical temperature can be controlled by varying the concentration of manganese impurity.

36 MATERIALS SCIENCE↗

DOE OSTI2019

Slow Magnetic Relaxation of Co( II ) Single Chains Embedded within Metal–Organic Superstructures

We report two coordination polymers of the type Co(BPDC)(N-ox), with BPDC being 4,4'-biphenyldicarboxylate and N-ox being pyridine N-oxide (PNO) or isoquinoline N-oxide (IQNO), have been synthesized and characterized. The compounds feature 2D and 3D metal–organic networks that encapsulate Co(II)-based chains in a rigid superstructure. The dc and ac magnetic properties of these Co(BPDC)(N-ox) materials have been investigated alongside those of a related Co(BDC)(PNO) compound (where BDC is 1,4-benzenedicarboxylate), which contains a smaller dicarboxylate linker. These Co(II)-containing coordination polymers exhibit slow magnetic relaxation, as observed by ac susceptibility measurements. The observed magnetic behavior of all compounds is consistent with an antiferromagnetic interaction between canted Co spins along the 1D skeleton, resulting in single-chain magnet behavior. In the case of Co(BPDC)(IQNO), weak interchain magnetic interactions yield 3D antiferromagnetic order while the inherent magnetic behavior stemming from the chain component is maintained. The combination of these effects in this material puts it at the frontier between single-chain magnets and classical bulk antiferromagnets. This work contributes to the limited group of materials featuring the organization of single-chain magnets within a coordination polymer superstructure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2026

Strain and Defect-Tailored Magnetotransport in NiCo 2 O 4 Thin Films and Freestanding Membranes

Magnetic spinel NiCo 2 O 4 is promising for developing spintronic applications due to its high magnetic Curie temperature, high spin polarization, fast spin dynamics, and strain-tunable magnetic anisotropy, while its electronic and magnetic properties depend sensitively on epitaxial strain and disorder. Here, in this study, we use epitaxial NiCo 2 O 4 thin films and freestanding NiCo 2 O 4 membranes as model systems to reveal the complex interplay of strain and defects in determining the metallicity and magnetotransport properties of the ferrimagnetic spinel. NiCo 2 O 4 on perovskite substrates and NiCo 2 O 4 membranes exhibit insulating behaviors and spin canting, in sharp contrast to the metallic NiCo 2 O 4 films on spinel substrates that possess strong perpendicular magnetic anisotropy. Anisotropic magnetoresistance studies provide critical information about disorder-induced spin scattering and strain-induced tetragonal magnetocrystalline anisotropy, which is corroborated by comprehensive electron microscopy characterizations. Our study presents a promising venue for designing flexible magnetic memory, sensor, and spintronic applications.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Ferromagnetic Exchange and Slow Magnetic Relaxation in Cobalt Bis(1,2-dithiolene)-Bridged Dilanthanide Complexes

The construction of multinuclear lanthanide-based molecules with significant magnetic exchange interactions represents a key challenge in the realization of single-molecule magnets with high operating temperatures. Here, we report the synthesis and magnetic characterization of two series of heterobimetallic compounds, (Cp* 2 Ln) 2 (μ-Co(pdt) 2 ) (Ln = Y 3+ , Gd 3+ , Dy 3+ ; pdt 2– = 1,2-diphenylethylenedithiolate) and [K(18-crown-6)][(Cp* 2 Ln) 2 (μ-Co(pdt) 2 )] (Ln = Y 3+ , Gd 3+ ), featuring two lanthanide centers bridged by a cobalt bis(1,2-dithiolene) complex. Dc magnetic susceptibility data collected for the Gd congeners indicate significant Gd–Co ferromagnetic exchange interactions with fits affording J = +11.5 and +7.33 cm –1 , respectively. Magnetization decay and ac magnetic susceptibility measurements carried out on the single-molecule magnet (Cp* 2 Dy) 2 (μ-Co(pdt) 2 ) reveal full suppression of quantum tunneling and open-loop hysteresis persisting up to 3.5 K. These results, along with those of high-field EPR spectroscopy, suggest that transition metalloligands can enforce strong exchange interactions with adjacent lanthanide centers while maintaining a geometry that preserves molecular anisotropy. Furthermore, the magnetic properties of [K(18-crown-6)][(Cp* 2 Gd) 2 (μ-Co(pdt) 2 )] show that increasing the spin of the ground state of the bridging complex may be a viable alternative to increasing J in obtaining well-isolated, strongly coupled magnetic ground states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Effect of RF Power on Structural, Magnetic, and Optical Properties of CoFe{sub 2}O{sub 4} Thin Films

The thin films of CoFe{sub 2}O{sub 4} were deposited on glass substrates by radio-frequency (RF) sputtering. The effect of RF power variation from 60–120 W on the structure, magnetic, and optical properties of the as-deposited films and films annealed at 500 °C was studied. The magnetic parameters of the film, i.e., in-plane coercivity (H{sub c}), saturation magnetization (M{sub s}), and remanence (M{sub r}) increase as RF power is increased from 60 to 80 W followed by a decreasing trend with further increase in RF power to 120 W. Annealing of the film significantly improves the magnetic properties. The largest grain size and the best crystallinity of the film are obtained for the film deposited at 80 W of RF power. The thin films exhibited 70–80% optical transmittance. Optical band gap (E{sub g}) depends on the RF power and grain size, and it varies between 2.08 and 2.16 eV for the as-deposited films and between 2.16 and 2.30 eV for the annealed films. The study shows that RF power used for depositing the film has strong effect on the microstructure and properties of the film.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Donor–Acceptor Conjugated Copolymers Containing Transition-Metal Complex: Intrachain Magnetic Exchange Interactions and Magneto-Optical Activity

A donor-acceptor conjugated copolymer containing thienothiophene (TT) (Donor) units and Co(II) complexes (Acceptor) has been synthesized and characterized. The studies of magnetic properties of the polymers by SQUID and EPR measurements showed antiferromagnetic exchange interaction between neighboring spin centers along with the polymer backbone through intrachain in-teractions. It is the first time that D-A conjugated polymer backbone serving as bridging ligands for magnetic couplings, making this type of polymers promising as magnetic materials. Moreover, the magneto-optical activity (MOA) of polymer LCP1 was char-acterized using magnetic circular dichroism (MCD) spectroscopy. It was found that the material exhibits a large magneto-optic rotation with a peak Verdet constant of 2980 deg*T -1 *m -1 at 590 nm.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Candidate spin-liquid ground state in CsNdSe 2 with an effective spin-1/2 triangular lattice

Rare-earth-based triangular lattice materials are extremely attractive for studying unconventional magnetism. Here, we report the magnetic properties of layered CsNdSe 2 based on direct current (DC) and alternating current (AC) susceptibility measurements down to 0.04 K. While the AC susceptibility at the zero DC field shows a broad hump below 0.5 K, there is no sign of any long-range magnetic ordering. Quantitative analysis of the DC magnetic susceptibility gives the negative Curie-Weiss (CW) temperature θ CW < 0 in all directions, indicating antiferromagnetic interaction between Nd ions. Of particular interest is the low temperature magnetic susceptibility, which reflects the effective spin-1/2 state with $\theta^a_{\text {cw}}/\theta^c_{\text {cw}}$ > 3. The estimated exchange interactions are J a /k B = 1.42 K (in-plane) and J c /k B = 0.44 K (out-of-plane), pointing to the anisotropic magnetism. First-principles calculations that include spin-orbit coupling and Coulomb correlations reveal multiple states with zero net magnetization for CsNdSe 2 . Both experiment and simulation strongly suggest CsNdSe 2 has the spin liquid ground state with effective spin-1/2. Application of a magnetic field can induce long-range antiferromagnetic ordering with the maximum transition temperature around 0.3 K, in further support of the zero-field spin liquid state.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Water-dispersible and quasi-superparamagnetic magnetite nanoparticles prepared in a weakly basic solution at the low synthetic temperature

Highlights: • Magnetite nanoparticles were prepared in a weakly basic solution at the low reaction temperature. • The double layer may be steadily formed on the surfaces of magnetite nanoparticles at the high pH value. • Magnetite nanoparticles present a quasi-superparamagnetic property and a good dispersion in the aqueous solution. - Abstract: Magnetite nanoparticles were prepared in a weakly basic solution at the low reaction temperature by the co-precipitation method. As a comparison, the oxidative precipitation method was also applied in this study. The structure, morphology, and other properties of the obtained samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), transmission electron microscope (TEM), and thermogravimetric analysis (TGA). The above characterization data indicate that small size and narrow size distribution are found for magnetite nanoparticles prepared by the co-precipitation method. Further magnetic property and Zeta potential results illuminate that magnetite nanoparticles prepared by this method display a quasi-superparamagnetic property and a good dispersion in the aqueous solution. Based on the investigation results, the magnetite nanoparticles with a quasi-superparamagnetic property and a fine dispersion can be facilely prepared in a weakly basic solution at the low reaction temperature by the co-precipitation method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Magnetic Anisotropy of Graphene-Coated Thin Iron Films

The ab initio calculations of the magnetic anisotropy of thin (to six monolayers) nickel films and iron films with unusual fcc structures formed upon intercalation of graphene with Fe atoms are presented. The data have been obtained for both the pure-surface films and the films coated with graphene. The density functional theory and the pseudopotential method are used to calculate the magnetic moments of atoms of all the layers and to determine the total energies of the structures with different orientations of magnetic moments of iron and nickel atoms with respect to the film surface. A strong influence of graphene on the magnetic properties contacting iron films is demonstrated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Ubiquitous spin freezing in the superconducting state of UTe 2

In most superconductors electrons form Cooper pairs in a spin-singlet state mediated by either phonons or by long-range interactions such as spin fluctuations. The superconductor UTe 2 is a rare material wherein electrons are believed to form pairs in a unique spin-triplet state with potential topological properties. While spin-triplet pairing may be mediated by ferromagnetic or antiferromagnetic fluctuations, experimentally, the magnetic properties of UTe 2 are unclear. By way of muon spin rotation/relaxation (μSR) measurements on independently grown UTe 2 single crystals we demonstrate the existence of magnetic clusters that gradually freeze into a disordered spin frozen state at low temperatures. Our findings suggest that inhomogeneous freezing of magnetic clusters is linked to the ubiquitous residual linear term in the temperature dependence of the specific heat (C) and the low-temperature upturn in C/T versus T. The omnipresent magnetic inhomogeneity has potential implications for experiments aimed at establishing the intrinsic low-temperature properties of UTe 2 .

36 MATERIALS SCIENCE↗

DOE OSTI2026

Frustrated Magnetism in FeGe 3 O 4 with a Chiral Trillium Network

The discovery of new magnetic ground states in geometrically frustrated lattices remains a central challenge in materials science. Here, we report the synthesis, structural characterization, and frustrated magnetic properties of FeGe 3 O 4 , a newly identified compound that crystallizes in the noncentrosymmetric cubic space group P 2 1 3. In this structure, Fe atoms form an intricate double-trillium lattice with nearest-neighbor Fe−Fe distances of ∼4.2 Å, while Ge 2+ ions mediate magnetic interactions through Fe− Ge−Fe pathways. Field-dependent magnetization at 2 K shows a pronounced nonlinearity, reaching a maximum moment of 2.55(3) μ B /Fe 2+ at 70 kOe without evidence of saturation. Magnetic susceptibility, heat capacity, and neutron scattering collectively reveal the onset of short-range magnetic interactions near 5 K, with no longrange ordering detected down to 0.06 K. Specific heat measurements demonstrate strong frustration: only ∼34% of the expected magnetic entropy is recovered at 2.4 K. Taken together, these results establish FeGe 3 O 4 as a rare example of a geometrically frustrated trillium lattice magnet, offering a promising platform for exploring exotic quantum magnetic phenomena.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2018

Spin Excitations in a 4f−3d Heterodimer on MgO

We report on the magnetic properties of HoCo dimers as a model system for the smallest intermetallic compound of a lanthanide and a transition metal atom. The dimers are adsorbed on ultrathin MgO(100) films grown on Ag(100). New for 4f elements, we detect inelastic excitations with scanning tunneling spectroscopy and prove their magnetic origin by applying an external magnetic field. In combination with density functional theory and spin Hamiltonian analysis, we determine the magnetic level distribution, as well as sign and magnitude of the exchange interaction between the two atoms. In contrast to typical 4f-3d bulk compounds, we find ferromagnetic coupling in the dimer. © 2018 American Physical Society.

60 APPLIED LIFE SCIENCES↗

DOE OSTI2021

Tuning spin excitations in magnetic films by confinement

Spin excitations of magnetic thin films are the founding element for novel transport concepts in spintronics, magnonics, and magnetic devices in general. While spin dynamics have been extensively studied in bulk materials, their behaviour in mesoscopic films is less known due to experimental limitations. Here, we employ Resonant Inelastic X-Ray Scattering to investigate the spin excitation spectrum in mesoscopic Fe films, from bulk-like down to 3 unit cells thick. In bulk-like samples, we find isotropic, dispersive ferromagnons consistent with the dispersion observed by neutron scattering in bulk single crystals. As the thickness is reduced, these ferromagnons survive and evolve anisotropically: renormalising to lower energies along the out-of-plane direction while retaining their dispersion in the in-plane direction. This thickness dependence is captured by simple Heisenberg model calculations accounting for the confinement in the out-of-plane direction through the loss of Fe bonds. Overall, our findings highlight the effects of mesoscopic scaling on spin dynamics and identify thickness as a knob for fine-tuning and controlling magnetic properties in films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Possible Origin of Ferromagnetism in Transition Metal Doped Zirconia

We have studied the electronic structure and magnetic properties of cubic zirconia (c-ZrO{sub 2}) with cobalt (Co) or nickel (Ni) doping using density functional theory (DFT) calculations. The pure c-ZrO{sub 2} is a nonmagnetic insulator with a wide bandgap. The calculated results reveal that isolated Co or Ni atom can both produce the local magnetic moment in c-ZrO{sub 2}. And the isolated Co atom can introduce a magnetic moment of about 0.98 μ{sub B}, while the Ni atom is 2.85 μ{sub B}. The impurity peaks can be formed in the bandgap. Our studies show that the magnetic moments mainly result from d orbitals of the impurity atoms. And the spin-up electrons will be arranged in t{sub 2g} orbitals under the ligand field of O{sub h} group in Co- or Ni-doped c-ZrO{sub 2}. Obviously, this will lead to a high-spin state (S = 1/2 or 1). The studies of magnetic coupling reveal that the two Co atoms in c-ZrO{sub 2} are not always coupled ferromagnetically at different distances. And the system will be in a spin singlet state (S = 0) when the distance is 6.209 or 7.170 Å between two Co atoms. However, the two Ni atoms in c-ZrO{sub 2} are always coupled ferromagnetically at all distances. So we can conclude that the Ni-doped c-ZrO{sub 2} is more suitable for spintronic material than Co doping. These results are significant for spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Electronic structures and magnetic performance related to spintronics of Sr{sub 0.875}Ti{sub 0.125}S

In this study, we have used the first-principle approaches of density functional theory to determine the electronic and magnetic performances related to spintronics applications of titanium (Ti)-substituted SrS such as Sr{sub 1−x}Ti{sub x}S at concentration x = 0.125. The results of electronic and magnetic properties by using TB–mBJ potential revealed that Sr{sub 0.875}Ti{sub 0.125}S compound is half-metallic ferromagnetic with a half-metallic gap and a spin polarization of 100%. The ferromagnetic nature of Sr{sub 0.875}Ti{sub 0.125}S material is confirmed by the ferromagnetic coupling between magnetic moments of Ti, Sr and S atoms. The total magnetic moment of integer Bohr magneton of 2 μ{sub B} and the large half-metallic gap of 1.245 suggest that this compound is a true half-metallic ferromagnet.

36 MATERIALS SCIENCE↗

DOE OSTI2026

Influence of minor Ce additions on magnetic and electrical properties in Fe–6.5 wt. % Si alloy

Fe–6.5 wt. % Si alloys exhibit lower core loss at higher frequency than the dominating electrical steel containing 3.2% Si due to its lower magneto-crystalline anisotropy energy (MAE) and higher electrical resistivity. However, compared to nanocrystalline and amorphous soft-magnetic materials, 6.5% Si steel, although being cost effective and having high saturation magnetization, still has much to improve, especially with respect to MAE and resistivity. To explore further improvement, the effects of minor Ce additions (due to its mixed valence 4f-electron configuration) on the microstructure, electrical and magnetic behaviors of Fe–6.5 wt. % Si alloys were investigated. Due to its limited solubility in Fe, Ce exhibits condition-dependent effects on electrical resistivity, showing only marginal improvement under rapid solidification. In contrast, Ce addition degrades magnetic performance, reducing saturation magnetization (from 1.86 to 1.76 T) and increasing coercivity (from 88 to 171 A/m), which results in an 83.4% increase in core loss (W 10/1000 condition). Overall, Ce addition provides limited benefit in electrical resistivity while deteriorating magnetic properties, making it ineffective for enhancing the soft magnetic performance of Fe–6.5 wt. % Si alloys.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Integrated ab initio modelling of atomic ordering and magnetic anisotropy for design of FeNi-based magnets

We describe an integrated modelling approach to accelerate the search for novel, single-phase, multicomponent materials with high magnetocrystalline anisotropy (MCA). For a given system we predict the nature of atomic ordering, its dependence on the magnetic state, and then proceed to describe the consequent MCA, magnetisation, and magnetic critical temperature (Curie temperature). Crucially, within our modelling framework, the same ab initio description of a material’s electronic structure determines all aspects. We demonstrate this holistic method by studying the effects of alloying additions in FeNi, examining systems with the general stoichiometries Fe 4 Ni 3 X and Fe 3 Ni 4 X, for additives including X = Pt, Pd, Al, and Co. The atomic ordering behaviour predicted on adding these elements, fundamental for determining a material’s MCA, is rich and varied. Equiatomic FeNi has been reported to require ferromagnetic order to establish the tetragonal L1 0 order suited for significant MCA. Our results show that when alloying additions are included in this material, annealing in an applied magnetic field and/or below a material’s Curie temperature may also promote tetragonal order, along with an appreciable effect on the predicted hard magnetic properties.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Data for "Integrated ab initio modelling of atomic order and magnetic anisotropy for rare-earth-free magnet design: effects of alloying additions in L1 0 FeNi."

36 MATERIALS SCIENCE↗