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Results for “FERROMAGNETISM”

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DOE OSTI2022

Domain state exchange bias in a single layer FeRh thin film formed via low energy ion implantation

Modern spintronics relies heavily on the exchange bias effect to pin the orientation of ferromagnetic layers in magnetic tunnel junctions. The current implementations of exchange bias in magnetic tunnel junctions employ pristine interfaces between antiferromagnetic and ferromagnetic layers. Here we reveal an interfacial exchange bias introduced by a single-step, low-energy ion implantation process in a single layer FeRh thin film. Both 1 keV He + ions and 5 keV Fe + were investigated, their energy selected to ensure the ions would stop within the FeRh film. The ions reduce the metamagnetic transition temperature through defect generation to form a surface layer with ferromagnetic ordering. Temperature dependent magnetism measurements reveal a room temperature exchange bias between the ferromagnetic surface and the antiferromagnetic bulk of ~41 Oe in 5 keV Fe + implanted samples and ~36 Oe for 1 keV He + implanted samples. We directly scrutinize this exchange bias effect in magnetic depth profiles obtained by polarized neutron reflectometry which clearly show a pinned ferromagnetic layer adjacent to the disordered layer created by low energy Fe + ion implantation. These results reveal a novel method to implement exchange bias in an antiferromagnetic layer that can have direct application in the field of spintronics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2013

Metal to semiconductor transition in metallic transition metal dichalcogenides

We report on tuning the electronic and magnetic properties of metallic transition metal dichalcogenides (mTMDCs) by 2D to 1D size confinement. The stability of the mTMDC monolayers and nanoribbons is demonstrated by the larger binding energy compared to the experimentally available semiconducting TMDCs. The 2D MX{sub 2} (M = Nb, Ta; X = S, Se) monolayers are non-ferromagnetic metals and mechanically softer compared to their semiconducting TMDCs counterparts. Interestingly, mTMDCs undergo metal-to-semiconductor transition when the ribbon width approaches to ∼13 Å and ∼7 Å for zigzag and armchair edge terminations, respectively; then these ribbons convert back to metal when the ribbon widths further decrease. Zigzag terminated nanoribbons are ferromagnetic semiconductors, and their magnetic properties can also be tuned by hydrogen edge passivation, whereas the armchair nanoribbons are non-ferromagnetic semiconductors. Our results display that the mTMDCs offer a broad range of physical properties spanning from metallic to semiconducting and non-ferromagnetic to ferromagnetic that is ideal for applications where stable narrow bandgap semiconductors with different magnetic properties are desired.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Electronic and magnetic transitions in perovskite SrRu{sub 1-x}Ir{sub x}O{sub 3} thin films

We have investigated the electronic and magnetic properties of perovskite SrRu{sub 1−x}Ir{sub x}O{sub 3} (0.0≤ x ≤ 0.25) thin films grown by pulsed laser deposition on atomically flat (001) SrTiO{sub 3} substrates. SrRuO{sub 3} has the properties of a ferromagnetic metal (resistivity ρ ∼ 200 μΩ · cm at T = 300 K) with Curie temperature T{sub C} ∼ 150 K. Substituting Ir (5d{sup 5+}) for Ru (4d{sup 4+}) in SrRuO{sub 3}, films (0.0 ≤ x ≤ 0.20) showed fully metallic behavior and ferromagnetic ordering, although ρ increased and the ferromagnetic T{sub C} decreased. Films with x = 0.25 underwent the metal-to-insulator transition (T{sub MIT}∼75 K) in ρ, and spin-glass-like ordering (T{sub SG}∼45 K) with the elimination of ferromagnetic long-range ordering caused by the electron localization at the substitution sites. In ferromagnetic films (0.0 ≤ x ≤ 0.20), ρ increased near-linearly with T at T > T{sub C}, but in paramagnetic film (x = 0.25) ρ increased as T{sup 3/2} at T > T{sub MIT}. Moreover, observed spin-glass-like (T{sub SG}) ordering with the negative magnetoresistance at T < T{sub MIT} in film with x = 0.25 validates the hypothesis that (Anderson) localization favors glassy ordering at amply disorder limit. These observations provide a promising approach for future applications and of fundamental interest in 4d and 5d mixed perovskites.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2017

Correlation between magnetostriction and magnetic structure in pseudobinary compounds Tb(Co 1-x Fe x ) 2

Giant low-field magnetostriction has been achieved in pseudobinary Laves phase compounds RR’T 2 (R and R’: rare earth elements; T: transition metal elements) around the ferromagnetic - ferromagnetic (ferro.-ferro.) transition temperature. Evolution of the magnetic structure across such transition requires comprehensive investigation. In this work, pseudobinary system Tb(Co 1-x Fe x ) 2 is selected to investigate the evolution of local magnetic moment, for which two end terminals TbCo 2 and TbFe 2 possess the rhombohedral (R) structure at the ferromagnetic state but with different magnetic ordering temperatures (T C ). Magnetometry measurements reveal that a composition independent ferromagnetic - ferromagnetic transition occurs at ~100 K despite the increased Curie temperature T C with raised Fe concentration in the Tb(Co 1-x Fe x ) 2 . Synchrotron XRD data reveal that both the lattice parameter and the lattice strain along the <111> direction are abnormally temperature dependent, accompanied with experimentally observed magnetostriction abnormality at 100 K. In-situ neutron powder diffraction (NPD) results show that the local magnetic moments of T2 (9e site) atoms are also abnormally temperature dependent, which is larger at 100 K than that at a lower temperature 50 K. Such findings indicate close correlations between the magnetic structure and the magnetostrictive effect in the pseudobinary RT 2 compounds.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Computational search for itinerant n -type and p -type magnetic semiconductors: Arsenopyrites as bipolar magnetic semiconductors

We report a computational search for three-dimensional bulk ferromagnetic semiconductors based on itinerant Stoner physics. Stoner-based ferromagnetic semiconductors require an unusual combination of high band edge density of states and moderate transport effective mass. Nonetheless, several potential materials are found. Here, we identify arsenopyrites, exemplified by FeAsS, as materials that, with suitable doping, exhibit ferromagnetic semiconducting behavior for both n- and p-type dopants. The existence of a material that becomes ferromagnetic for both p and n types may enable device concepts, for example, ferromagnetic p–n junction devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Competing magnetic and nonmagnetic states in monolayer VSe 2 with charge density wave

The field of two-dimensional ferromagnets has been reinvigorated by the discovery of VSe 2 monolayer grown on van der Waals substrates, which is reported to be ferromagnetic with a Curie point higher than 330 K. However, the ferromagnetic and nonmagnetic states of pristine monolayer VSe 2 are highly debated. Here, employing density functional theory, Wannier function calculations, and the band unfolding method, we explore the electronic structure of monolayer VSe 2 with a $\sqrt{3} × \sqrt{7}$ charge density wave (CDW). Certain qualitative aspects of the calculated unfolded band dispersion and unfolded Fermi surface of monolayer VSe 2 with the $\sqrt{3} × \sqrt{7}$ CDW in the nonmagnetic state agree well with previous angle-resolved photoemission spectroscopy results, albeit with uncertainty about whether these experiments probed single or multiple domains. Specifically, we find that an isolated CDW domain naturally induces a strong breaking of the threefold symmetry of the electronic structure. In addition we find that, relative to the undistorted structure, the CDW structure shows a strong competition between nonmagnetic and various magnetic states, with an energy difference less than 5 meV/f.u. For the CDW structure in the antiferromagnetic state, the band dispersions and Fermi surface are similar to those in the nonmagnetic state, while the unfolded bands of the ferromagnetic CDW state display a sizable exchange splitting. These results indicate the possibility of various antiferromagnetic fluctuations in VSe 2 to coexist and compete with ferromagnetic order and the experimentally reported CDW order. In this work, our calculations build insights for exploring the interplay between magnetism and CDW behaviors more generally in transition metal dichalcogenides.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Local atomic and magnetic structure of dilute magnetic semiconductor ( Ba , K ) ( Zn , Mn ) 2 As 2

We studied the atomic and magnetic structure of the dilute ferromagnetic semiconductor system (Ba,K)(Zn,Mn) 2 As 2 through atomic and magnetic pair distribution function analysis of temperature-dependent x-ray and neutron total scattering data. Furthermore, we detected a change in curvature of the temperature-dependent unit cell volume of the average tetragonal crystallographic structure at a temperature coinciding with the onset of ferromagnetic order. We also observed the existence of a well-defined local orthorhombic structure on a short length scale of ≲5Å, resulting in a rather asymmetrical local environment of the Mn and As ions. Finally, the magnetic PDF revealed ferromagnetic alignment of Mn spins along the crystallographic c axis, with robust nearest-neighbor ferromagnetic correlations that exist even above the ferromagnetic ordering temperature. Finally, we discuss these results in the context of other experiments and theoretical studies on this system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Hole doping and pressure effects on the II-II-V-based diluted magnetic semiconductor ( B a 1 - x K x ) ( Z n 1 - y M n y ) 2 A s 2

We investigate doping- and pressure-induced changes in the electronic state of Mn 3d and As 4p orbitals in II-II-V based diluted magnetic semiconductor (Ba 1-x ,K x )(Zn 1-y ,Mn y ) 2 As 2 to shed light into the mechanism of indirect exchange interactions leading to high ferromagnetic ordering temperature (T c = 230 K in optimally doped samples). A suite of x-ray spectroscopy experiments (emission, absorption and dichroism) show that the emergence, and further enhancement of ferromagnetic interactions with increased hole doping into the As 4p band is accompanied by a decrease in local 3d spin density at Mn sites. This is a result of increasing Mn 3d - As 4p hybridization with hole doping which enhances indirect exchange interactions between Mn dopants and gives rise to induced magnetic polarization in As 4p states. On the contrary, application of pressure suppresses exchange interactions. While Mn Kβ emission spectra show a weak response of 3d state to pressure, clear As 4p band broadening (hole delocalization) is observed under pressure ultimately leading to loss of ferromagnetism concomitant with a semiconductor to metal transition. The pressure response of As 4p and Mn 3d states is intimately connected with the evolution of the As-As interlayer distance and the geometry of the MnAs 4 tetrahedral units, which we probed with X-ray diffraction. Our results indicate that hole doping increases the degree of covalency between the anion (As) p states and cation (Mn) d states in the MnA s4 tetrahedron, a crucial ingredient to promote indirect exchange interactions between Mn dopants and high T c ferromagnetism. As a result, the instability of ferromagnetism and semiconducting state against pressure is mainly dictated by delocalization of anion p states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Electron Accumulation and Emergent Magnetism in LaMnO 3 /SrTiO 3 Heterostructures

Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here in this paper we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched LaMnO$_{3}$/SrTiO$_{3}$ (001) heterostructures. Using a combination of element-specific x-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation, and ferromagnetism have been observed within the polar, antiferromagnetic insulator LaMnO$_{3}$. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron overaccumulation. In turn, by controlling the doping of the LaMnO$_{3}$, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin LaMnO$_{3}$ films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2019

Magnetic-Competition-Induced Colossal Magnetoresistance in $n$-Type $\mathrm{HgCr_2Se_4}$ Under High Pressure

The $n$-type HgCr 2 $\mathrm{Se_4}$ exhibits a sharp semiconductor-to-metal transition (SMT) in resistivity accompanying the ferromagnetic order at ${T_C}$ = 106 K . Here, we investigate the effects of pressure and magnetic field on the concomitant SMT and ferromagnetic order by measuring resistivity, dc and ac magnetic susceptibility, as well as single-crystal neutron diffraction under various pressures up to 8 GPa and magnetic fields up to 8 T. Our results demonstrate that the ferromagnetic metallic ground state of $n$-type HgCr 2 $\mathrm{Se_4}$ is destabilized and gradually replaced by an antiferromagnetic, most likely a spiral magnetic, and insulating ground state upon the application of high pressure. On the other hand, the application of external magnetic fields can restore the ferromagnetic metallic state again at high pressures, resulting in a colossal magnetoresistance (CMR) as high as $\mathrm{~ 3 × 10^{11}}$% under 5 T and 2 K at 4 GPa. The present study demonstrates that $n$-type HgCr 2 $\mathrm{Se_4}$ is located at a peculiar critical point where the balance of competition between ferromagnetic and antiferromagnetic interactions can be easily tipped by external stimuli, providing a new platform for achieving CMR in a single-valent system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Electronic structure and exchange interactions in altermagnetic MnGeP 2 in the quasiparticle self-consistent G W approach

The quasiparticle self-consistent 𝐺⁡𝑊 method is used to study the electronic band structure, optical dielectric function, and exchange interactions in chalcopyrite, 𝐼⁢ ‾ 4 ⁢2⁢𝑑, structure MnGeP 2 . The material is found to be an antiferromagnetic semiconductor with a lowest direct gap of 2.44 eV at the Γ point and a lower indirect gap of 1.87 eV from Γ to 𝑀. The material is an altermagnet because the two magnetic atoms of opposite spin are related by a twofold rotation operation perpendicular to the main fourfold rotation inversion axis. The spin splittings along a low symmetry line like 𝑃⁢𝑁 is sizable while at k points on the diagonal mirror planes or on the twofold symmetry axes the spin splitting is zero. The exchange interactions are calculated using a linear response approach. The antiferromagnetic exchange interaction between nearest neighbors in the primitive unit cell is dominating and found to be slightly decreasing upon carrier doping but not sufficiently to change the interaction to become ferromagnetic. The bare (noninteracting) and interacting transverse spin susceptibilities, which provide interatomic site exchange interactions after averaging over the muffin-tin spheres, are calculated from the 𝐺⁡𝑊 band structure and wave functions. From these exchange interactions, the spin wave spectra are obtained along the high symmetry lines and the Néel temperature is calculated using the mean-field and Tyablikov estimations. The dielectric function and the optical absorption spectra are calculated including excitonic effects using the Bethe Salpeter equation. The exchange interactions around Mn Ge defect sites is also studied. While we find it can generate ferromagnetic interactions with neighboring spins, we did not find direct evidence of producing an overall ferromagnetic phase. First, if Mn antisites are introduced by exchanging Mn with a nearby Ge, the interactions stay largely antiferromagnetic. Second, when we add additional Mn, in other words in Mn-rich stoichiometry, the Mn Ge antisites produce a strong ferromagnetic interaction primarily with the Mn in the same basal plane but weaker ferromagnetic interaction with adjacent plane Mn. The interactions between regular lattice Mn stay antiferromagnetic as before and thus favor keeping the antiferromagnetic order along the [001] direction. Adding Mn antisites, however, does lead to a metallic band structure.

36 MATERIALS SCIENCE↗

DOE OSTI2018

Pressure-Induced Decrease in the Curie Temperature of Gd{sub 2}Fe{sub 17}: LSDA+U Calculations

To explain the magnetic properties of advanced ferromagnetic intermetallic compounds of the R{sub 2}Fe{sub 17} (R is a rare-earth element) class, experimentalists often use the hypothesis of competition between ferromagnetic exchange and antiferromagnetic exchange between four types of the nearest iron atoms in nonequivalent lattice sites. For the rhombohedral Gd{sub 2}Fe{sub 17} ferromagnet, we calculate the magnetic moments of iron and gadolinium ions, the parameters of exchange between Fe atoms, and Curie temperature T{sub C} at a zero pressure and during hydrostatic lattice compression. The magnetic moment of the unit cell of Gd{sub 2}Fe{sub 17} is shown to decrease under pressure, and this decrease is almost completely associated with a decrease in the magnetic moments of Fe rather than Gd ions, the pressure dependence of the magnetic moments of which is weaker by an order of magnitude. In contrast to the hypothesis regarding the competition of exchange interactions between different kinds of Fe atoms, the parameters of exchange between the nearest iron atoms in different crystallographic sites are found to be positive ferromagnetic (at a zero pressure and during compression), and a ferromagnetic character of interaction is shown to remain unchanged under pressure even for Fe atoms in the so-called dumbbell sites with the nearest interatomic distances. The Curie temperature T{sub C} of Gd{sub 2}Fe{sub 17} is shown to decrease with increasing pressure. The changes in the exchange parameters and the magnetic moments of Gd{sub 2}Fe{sub 17} during compression are found to be mainly related to a change in the position of energy spectrum branches with respect to each other and the Fermi level ϵ{sub F} rather than to a change in the overlapping of wavefunctions, which play a minor role.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2015

Pressure sensor

Disclosed is a passive, in-situ pressure sensor. The sensor includes a sensing element having a ferromagnetic metal and a tension inducing mechanism coupled to the ferromagnetic metal. The tension inducing mechanism is operable to change a tensile stress upon the ferromagnetic metal based on a change in pressure in the sensing element. Changes in pressure are detected based on changes in the magnetic switching characteristics of the ferromagnetic metal when subjected to an alternating magnetic field caused by the change in the tensile stress. The sensing element is embeddable in a closed system for detecting pressure changes without the need for any penetrations of the system for power or data acquisition by detecting changes in the magnetic switching characteristics of the ferromagnetic metal caused by the tensile stress.

47 OTHER INSTRUMENTATION↗

DOE OSTI2018

Sensor and methods of detecting target materials and situations in closed systems

47 OTHER INSTRUMENTATION↗

DOE OSTI2020

STRUCTURAL STABILITY OF MAGNETIC TUNNEL JUNCTION BASED MOLECULAR SPINTRONICS DEVICES (MTJMSD)

Harnessing the exotic properties of molecular level nanostructures to produce novel sensors, metamaterials, and futuristic computer devices can be technologically transformative. In addition, connecting the molecular nanostructures to ferromagnetic electrodes bring the unprecedented opportunity of making spin property based molecular devices. We have demonstrated that magnetic tunnel junction based molecular spintronics device (MTJMSD) approach to address numerous technological hurdles that have been inhibiting this field for decades. MTJMSD approach is based on producing a capacitor like a testbed where two metal electrodes are separated by an ultrathin insulator and subsequently bridging the molecule nanostructure across the insulator to transform a capacitor into a molecular device. Our prior work showed that MTJMSDs produced extremely intriguing phenomenon such as room temperature current suppression by six orders, spin photovoltaic effect, and evolution of new forms of magnetic metamaterials arising due to the interaction of the magnetic a molecule with two ferromagnetic thin films. However, making robust and reproducible electrical connections with exotic molecules with ferromagnetic electrodes is full of challenges and requires attention to MTJMSD structural stability. This paper focuses on MTJMSD stability by describing the overall fabrication protocol and the associated potential threat to reliability. MTJMSD is based on microfabrication methods such as (a) photolithography for patterning the ferromagnetic electrodes, (b) sputtering of metallic thin films and insulator, and (c) at the end electrochemical process for bridging the molecules between two ferromagnetic films separated by ~ 2nm insulating gap.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Structural and magnetic properties of irradiated SiC

We present a comprehensive structural characterization of ferromagnetic SiC single crystals induced by Ne ion irradiation. The ferromagnetism has been confirmed by electron spin resonance, and possible transition metal impurities can be excluded to be the origin of the observed ferromagnetism. Using X-ray diffraction and Rutherford backscattering/channeling spectroscopy, we estimate the damage to the crystallinity of SiC, which mutually influences the ferromagnetism in SiC.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Multi-state magnetoresistance in ferromagnet/organic-ferromagnet/ferromagnet junctions

Spin-dependent transport through a ferromagnetic metal/organic-ferromagnet/ferromagnet metal junction is investigated theoretically. It is demonstrated that the current through the device strongly depends on the alignment of the magnetization orientations of the electrodes and interlayer. The spin-related electron tunnelling between the ferromagnetic electrodes suffers a further spin selection induced by the spin-polarized states of the central organic ferromagnet. This work indicates an intriguing prospect of organic ferromagnets in spintronic devices, such as four-state magnetoresistance manipulated by a magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2014

Electronic structure, magnetic properties, and magnetostructural transformations in rare earth dialuminides

We report electronic structure, magnetic properties, and magnetostructural transformations of selected rare earth dialuminides calculated by using local spin density approximation (LSDA), including the Hubbard U parameter (LSDA + U) approach. Total energy calculations show that CeAl{sub 2} and EuAl{sub 2} adopt antiferromagnetic (AFM) ground states, while dialuminides formed by other magnetic lanthanides have ferromagnetic (FM) ground states. The comparison of theoretical and experimental magnetic moments of CeAl{sub 2} indicates that the 4f orbital moment of Ce in CeAl{sub 2} is quenched. Theoretical calculations confirm that Eu in EuAl{sub 2} and Yb in YbAl{sub 2} are divalent. PrAl{sub 2} exhibits a tetragonal distortion near FM transition. HoAl{sub 2} shows a first order magnetostructural transition while DyAl{sub 2} shows a second order transformation below magnetic transition. The dialuminides formed by Nd, Tb, and Er are simple ferromagnets without additional anomalies in the FM state.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗