Long-range Spin Order Research Articles

Spin effect in the ferromagnetic organic photovoltaics cells

In organic photovoltaic devices, the separation and transport of photogenerated charges play crucial roles for power conversion efficiency. Magnetic doping in organic solar cells can effectively enhance the power conversion efficiency by introducing a static magnetic field. In this study, we observed that in pure organic magnetic solar cells, the spin-polarization-induced spin scattering effect can also efficiently modulate the photocurrent in solar cells. Compared to the demagnetized state, the short-circuit current of PTB7:nw-P3HT:PCBM solar cells increased by approximately 0.3% after magnetization. The dielectric constant only increased by about 0.05%. However, above the Curie temperature 310 K, the long-range spin order in PTB7:nw-P3HT:PCBM solar cells disappears, resulting in consistent circuit currents before and after magnetization. Therefore, magnetic doping can enhance the short-circuit current in organic solar cells by weakening the spin scattering effect and enhancing the charge carrier mobility.

Signatures of polarized chiral spin disproportionation in rare earth nickelates

In rare earth nickelates (RENiO3), electron-lattice coupling drives a concurrent metal-to-insulator and bond disproportionation phase transition whose microscopic origin has long been the subject of active debate. Of several proposed mechanisms, here we test the hypothesis that pairs of self-doped ligand holes spatially condense to provide local spin moments that are antiferromagnetically coupled to Ni spins. These singlet-like states provide a basis for long-range bond and spiral spin order. Using magnetic resonant X-ray scattering on NdNiO3 thin films, we observe the chiral nature of the spin-disproportionated state, with spin spirals propagating along the crystallographic (101)ortho direction. These spin spirals are found to preferentially couple to X-ray helicity, establishing the presence of a hitherto-unobserved macroscopic chirality. The presence of this chiral magnetic configuration suggests a potential multiferroic coupling between the noncollinear magnetic arrangement and improper ferroelectric behavior as observed in prior studies on NdNiO3 (101)ortho films and RENiO3 single crystals. Experimentally-constrained theoretical double-cluster calculations confirm the presence of an energetically stable spin-disproportionated state with Zhang-Rice singlet-like combinations of Ni and ligand moments.

(Invited) Manipulating the Exotic Excitons of Van Der Waals Correlated Antiferromagnet with Impurities and Size

A Van der Waals correlated antiferromagnet, metal phosphorus trichalcogenides (MPX3, M = Mn, Fe, Ni; X = S, Se), have emerged as promising candidates for next generation spintronics, and quantum information technologies. Nickel phosphorus trisulfide (NiPS3) is a most notable member of this family and has been extensively studied for its unique properties, including the emergence of an ultra-sharp photoluminescence (PL) peak at ~1.476 eV below Néel temperature of 150 Kelvin. While the highly anisotropic, linearly polarized emission of this PL peak has been shown to be directly correlated to the long-range spin order of NiPS3, its ultra-narrow linewidth of a few hundred meV has led to the speculation that it is originated from a coherent many-body exciton state. Observations of multiple phonon bounds states and formation of exciton-polaritons also creates more excitement as they present new possibilities for design and control of correlated electron systems.Aiming to achieve understanding and control of this spin-correlated exciton, we conduct temperature dependent, polarization resolved, PL studies on compounds of NiPS3 doped with different concentration of Mn (Ni1-xMnxPS3) and Se (NiPS3(1-x)Se3x). We observed systematic variation in characteristics of the spin-correlated exciton revealing their sensitive dependence on disturbance in the magnetic order of NiPS3.Next, to explore the effect of nanostructuring, we developed a novel chemical synthesis for highly crystalline NiPS3 nano flakes. We observed an activation of a new exciton state that share many exciting properties of bulk spin-correlated exciton.

(Ca0.5Mn0.5)2MnTeO6 - An Anomalously Stable High-Pressure Double Perovskite.

High pressure high temperature treatments of the composition CaMnMnTeO6 are found to yield only an A2BB'O6-type double perovskite (Ca0.5Mn0.5)2MnTeO6, rather than a AA'BB'O6 double double perovskite with A- and B- site cation order as found in analogs CaMnMnReO6 and CaMnMnWO6 with similar cation sizes. Double perovskite (Ca0.5Mn0.5)2MnTeO6 adopts a monoclinic structure in space group P21/n with a framework of highly tilted MnO6 and TeO6 octahedra enclosing disordered Ca2+ and Mn2+ cations. Magnetic measurements show that (Ca0.5Mn0.5)2MnTeO6 is a highly frustrated spin glass with a freezing transition at 5 K, and no long-range spin order is apparent by neutron diffraction at 1.6 K.

An Optical Spectroscopic Study of Air-Degradation of van der Waals Magnetic Semiconductor Cr2Ge2Te6

Two-dimensional (2D) magnetic semiconductors exhibit unique combination of electronic and magnetic properties, holding great promise in potential applications such as spintronics and magneto-optics. However, many of them are air-sensitive, and their properties can be significantly altered upon exposure to air. Here, we showed an optical spectroscopic investigation of the effects of air-degradation on few-layered van der Waals (vdW) magnetic semiconductor Cr2Ge2Te6. It was found that although the partially degraded few-layered Cr2Ge2Te6 showed a significant Raman redshift and a split of Eg peak at room temperature, the magneto-optic Kerr hysteresis loop can remain largely unchanged below the Curie temperature. Temperature-dependent Raman measurements further revealed characteristic blueshifts of phonon energy, which were associated with the ferromagnetic phase transition in partially degraded Cr2Ge2Te6, in agreement with Kerr measurements. Our results provide an optical spectroscopic insight into the air-instability of 2D magnetic semiconductors, and contribute to a better understanding of the relationship between phonon modes and long-range spin order at the presence of defects in ultra-thin vdW magnetic semiconductors.

Нелинейность и гармоники магнитной восприимчивости моно-ионного магнита Co-=SUP=-2+-=/SUP=- в парамагнитной области выше температуры магнитного упорядочения

In monoionic complexes based on Co2+ ions, the second and third harmonics of the magnetic susceptibility were found at temperatures of 2–4 K, which exceed the Néel temperature. The maxima of the second and third harmonics of the magnetic susceptibility are observed at a frequency of ~1 Hz, at which the maximum of the first harmonic is observed in a field of 3.2 kOe. An analysis of the dependences of the second and third harmonics of the magnetic susceptibility on the field and temperature showed that the nonlinearity arises as a result of the formation of a spin glass state at temperatures slightly higher than the Néel temperature. In this state, there is no long-range spin order, but there are spin clusters in the spin glass state. The spin-glass state in a compound with a Co2+ ion with a high magnetic anisotropy is unusual in that the exchange interaction is much smaller than the single-ion anisotropy energy.

Nonlinearity and harmonics of the magnetic susceptibility of a Co-=SUP=-2+-=/SUP=- single-ion magnet in the paramagnetic region above the magnetic ordering temperature

In single-ion complexes based on Co2+ ions, the second and third harmonics of the magnetic susceptibility were found at temperatures of 2-4 K, which exceed the Neel temperature. The maxima of the second and third harmonics of the magnetic susceptibility are observed at a frequency of ~1 Hz, at which the maximum of the first harmonic is observed in a field of 3.2 kOe. An analysis of the dependences of the second and third harmonics of the magnetic susceptibility on the field and temperature showed that the nonlinearity arises as a result of the formation of a spin glass state at temperatures slightly higher than the Neel temperature. In this state, there is no long-range spin order, but there are spin clusters in the spin glass state. The spin-glass state in a compound with a Co2+ ion with a high magnetic anisotropy is unusual in that the exchange interaction is much smaller than the single-ion anisotropy energy. Keywords: molecular magnets, nonlinear magnetic susceptibility, nanostructures, spin dynamics, spin glass.

Carrier Doping Physics of Rare Earth Perovskite Nickelates RENiO3

The family of rare earth (RE) nickelate perovskites RENiO3 has emerged over the past two decades as an important platform for quantum matter physics and advanced applications. The parent compounds from this family are strongly correlated insulators or metals, in most cases with long-range spin order. In the past few years, carrier doping has been achieved using different approaches and has been proven to be a powerful tuning parameter for the microscopic properties and collective macroscopic states in RENiO3 compounds. In particular, a series of recent studies has shown that carrier doping can be responsible for dramatic but reversible changes in the long-range electronic and magnetic properties, underscoring the potential for use of nickelates in advanced functional devices. In this review, we discuss the recent advancements in our description, understanding and application of electron-doped rare earth nickelates. We conclude with a discussion of the developments and outlook for harnessing the quantum functional properties of nickelates in novel devices for sensing and neuromorphic computation.

Nonthermal breaking of magnetic order via photogenerated spin defects in the spin-orbit coupled insulator Sr3Ir2O7

In Mott insulators the evolution of antiferromagnetic order to superconducting or charge-density-wave-like states upon chemical doping underpins the control of quantum phases. Photo-doping can induce similar transitions on the ultrafast timescale, however the response of the spin system has remained elusive. Here, we use 4D-ultrafast optical spectroscopy to extract quantitative magnetic dynamics in the spin-orbit coupled Mott insulator Sr3Ir2O7. We demonstrate that light can non-thermally melt long-range spin order. At low fluences magnetic order recovers within 1 ps despite demagnetization of roughly 50%. However, high fluences induce a crossover to a long-lived demagnetized state without increasing the lattice temperature. We show that the generation of photo-induced spin defects enables a mechanism that stabilizes the demagnetized state which could help expose new transient phases.

Multi- k spin ordering in CaFe3Ti4O12 stabilized by spin-orbit coupling and further-neighbor exchange

Orthogonal spin ordering is rarely observed in magnetic oxides because nearest-neighbor symmetric Heisenberg superexchange interactions usually dominate. We have discovered that in the quadruple perovskite $\mathrm{Ca}{\mathrm{Fe}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$, where only the $S=2$ ${\mathrm{Fe}}^{2+}$ ion is magnetic, long-range magnetic order consisting of an unusual arrangement of three interpenetrating orthogonal sublattices is stabilized. Each magnetic sublattice corresponds to a set of $\mathrm{Fe}{\mathrm{O}}_{4}$ square planes sharing a common orientation. This multi-$k$ magnetic spin ordering is the result of fourth-neighbor spin couplings with a strong easy-axis anisotropy. In an applied magnetic field, each sublattice tends towards ferromagnetic alignment, but remains polarized by internal magnetic fields generated by the others, thus stabilizing in a noncollinear canted ferromagnetic structure. $\mathrm{Ca}{\mathrm{Fe}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ provides a rare example of how nontrivial long-range spin order can arise when near-neighbor Heisenberg superexchange is quenched.

Non-classical photonic spin texture of quantum structured light

Classical structured light with controlled polarization and orbital angular momentum (OAM) of electromagnetic waves has varied applications in optical trapping, bio-sensing, optical communications and quantum simulations. However, quantum noise and photon statistics of three-dimensional photonic angular momentum are relatively less explored. Here, we develop a quantum framework and put forth the concept of quantum structured light for space-time wavepackets at the single-photon level. Our work deals with three-dimensional angular momentum observables for twisted quantum pulses beyond scalar-field theory as well as the paraxial approximation. We show that the spin density generates modulated helical texture and exhibits distinct photon statistics for Fock-state vs. coherent-state twisted pulses. We introduce the quantum correlator of photon spin density to characterize nonlocal spin noise providing a rigorous parallel with electronic spin noise. Our work can lead to quantum spin-OAM physics in twisted single-photon pulses and opens explorations for phases of light with long-range spin order.

Revealing the A1g-type strain effect on superconductivity and nematicity in FeSe thin flake* *Project supported by the National Key R&D Program of China (Grant Nos. 2017YFA0303000 and 2016YFA0300201), the National Natural Science Foundation of China (Grant No. 11888101), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB25000000), and the Anhui Initiative in Quantum Information Technologies (Grant

The driving mechanism of nematicity and its twist with superconductivity in iron-based superconductors are still under debate. Recently, a dominant B1g-type strain effect on superconductivity is observed in underdoped iron-pnictides superconductors Ba(Fe1–x Co x )2As2, suggesting a strong interplay between nematicity and superconductivity. Since the long-range spin order is absent in FeSe superconductor, whether a similar strain effect could be also observed or not is an interesting question. Here, by utilizing a flexible film as substrate, we successfully achieve a wide-range-strain tuning of FeSe thin flake, in which both the tensile and compressive strain could reach up to ∼0.7%, and systematically study the strain effect on both superconducting and nematic transition (T c and T s) in the FeSe thin flake. Our results reveal a predominant A1g-type strain effect on T c. Meanwhile, T s exhibits a monotonic anti-correlation with T c and the maximum T c reaches to 12 K when T s is strongly suppressed under the maximum compressive strain. Finally, in comparison with the results in the underdoped Ba(Fe1–x Co x )2As2, the absence of B1g-type strain effect in FeSe further supports the role of stripe-type spin fluctuations on superconductivity. In addition, our work also supports that the orbital degree of freedom plays a key role to drive the nematic transition in FeSe.

Controlling the anisotropy of a van der Waals antiferromagnet with light.

Van der Waals magnets provide an ideal playground to explore the fundamentals of low-dimensional magnetism and open opportunities for ultrathin spin-processing devices. The Mermin-Wagner theorem dictates that as in reduced dimensions isotropic spin interactions cannot retain long-range correlations, the long-range spin order is stabilized by magnetic anisotropy. Here, using ultrashort pulses of light, we control magnetic anisotropy in the two-dimensional van der Waals antiferromagnet NiPS3 Tuning the photon energy in resonance with an orbital transition between crystal field split levels of the nickel ions, we demonstrate the selective activation of a subterahertz magnon mode with markedly two-dimensional behavior. The pump polarization control of the magnon amplitude confirms that the activation is governed by the photoinduced magnetic anisotropy axis emerging in response to photoexcitation of ground state electrons to states with a lower orbital symmetry. Our results establish pumping of orbital resonances as a promising route for manipulating magnetic order in low-dimensional (anti)ferromagnets.

Magnetism in Mixed Valence, Defect, Cubic Perovskites: BaIn1-x Fe x O2.5+δ, x = 0.25, 0.50, and 0.75. Local and Average Structures.

The series BaIn1–xFexO2.5+δ, x = 0.25, 0.50, and 0.75, has been prepared under air-fired and argon-fired conditions and studied using X-ray diffraction, d.c. and a.c. susceptibility, Mössbauer spectroscopy, neutron diffraction, X-ray near edge absorption spectroscopy (XANES), and X-ray pair distribution (PDF) methods. While Ba2In2O5 (BaInO2.5) crystallizes in an ordered brownmillerite structure, Ibm2, and Ba2Fe2O5 (BaFeO2.5) crystallizes in a complex monoclinic structure, P21/c, showing seven Fe3+ sites with tetrahedral, square planar, and octahedral environments, all phases studied here crystallize in the cubic perovskite structure, Pm3̅m, with long-range disorder on the small cation and oxygen sites. 57Fe Mössbauer studies indicate a mixed valency, Fe4+/Fe3+, for both the air-fired and argon-fired samples. The increased Fe3+ content for the argon-fired samples is reflected in increased cubic cell constants and in the increased Mössbauer fraction. It appears that the Pm3̅m phases are only metastable when fired in argon. From a slightly modified percolation theory for a primitive cubic lattice (taking into account the presence of random O atom vacancies), long-range spin order is permitted for the x = 0.50 and 0.75 phases. Instead, the d.c. susceptibility shows only zero-field-cooled (ZFC) and field-cooled (FC) divergences at ∼6 K [5 K] for x = 0.50 and at ∼22 K [21 K] for x = 0.75, with values for the argon-fired samples in [ ]. Neutron diffraction data for the air-fired samples confirm the absence of long-range magnetic order at any studied temperature. For the air-fired x = 0.50, a.c. susceptibility data show a frequency-dependent χ′(max) and spin glass behavior, while for x = 0.75, χ′(max) is invariant with frequency, ruling out either a spin glass or a superparamagnetic ground state. These behaviors are discussed in terms of competing Fe3+–Fe3+ antiferromagnetic exchange and ferromagnetic Fe3+–Fe4+ exchange. The PDF and 57Fe Mössbauer data indicate a local structure at short interatomic distances, which deviates strongly from the average Pm3̅m model. Fe Mössbauer, PDF, and XANES data show a systematic dependence on x and indicate that the Fe3+ sites are largely fourfold-coordinated and Fe4+ sites are fivefold- or sixfold-coordinated.

Strong Ferromagnetism Achieved via Breathing Lattices in Atomically Thin Cobaltites.

Low-dimensional quantum materials that remain strongly ferromagnetic down to monolayer thickness are highly desired for spintronic applications. Although oxide materials are important candidates for the next generation of spintronics, ferromagnetism decays severely when the thickness is scaled to the nanometer regime, leading to deterioration of device performance. Here, a methodology is reported for maintaining strong ferromagnetism in insulating LaCoO3 (LCO) layers down to the thickness of a single unit cell. It is found that the magnetic and electronic states of LCO are linked intimately to the structural parameters of adjacent "breathing lattice" SrCuO2 (SCO). As the dimensionality of SCO is reduced, the lattice constant elongates over 10% along the growth direction, leading to a significant distortion of the CoO6 octahedra, and promoting a higher spin state and long-range spin ordering. For atomically thin LCO layers, surprisingly large magnetic moment (0.5 μB /Co) and Curie temperature (75 K), values larger than previously reported for any monolayer oxides are observed. The results demonstrate a strategy for creating ultrathin ferromagnetic oxides by exploiting atomic heterointerface engineering, confinement-driven structural transformation, and spin-lattice entanglement in strongly correlated materials.

Experimental study of multiple magnetic transitions in micrometer and nano-grain sized Ni3TeO6-type oxide

We report the experimental results of dc and ac magnetic susceptibility (χdc and χac) and specific heat (CP) measurements of the powdered Ni3TeO6-type oxide. Upon decreasing the grain size from micrometer (bulk) to nanometer range, the magnetic measurements have indicated multiple transitions, viz., a cusp at paramagnetic to ferrimagnetic (FIM) transition temperature (TC) ∼ 78 K, an antiferromagnetic (AFM) transition temperature (TN) ∼ 52 K, and a re-entrant spin glass (RSG) transition at Tm2 ∼ 7.49 K. The ferrimagnetic transition temperature TC is found to be highly sensitive to dc magnetic fields, where superposition of a small dc field (10 Oe) in the ac susceptibility measurement or the application of a dc magnetic field of up to 100 Oe in the dc magnetic measurement is enough to suppress the observed TC at 78 K. The CP(T) measurement also revealed a RSG state at lower temperatures for the sample with nano-sized grains. The absence of any sharp peak at TC in the CP(T) curves implies the absence of a true long-range FIM spin order in the samples. The coexistence of a RSG state with multiple magnetic transitions is understood as an effect of competitive AFM and FIM phases in the Ni3TeO6-type structure.

Spin reorientation by Ni doping in Cu1−xNixCr2O4 spinels with x = 0 and 0.1, and evidence for canted magnetic Cr order above the onset of a ferromagnetic Cu

In ferrimagnetic spinels AB2O4 the magnetic structure is strongly influenced by lattice distortions, geometric frustration and the electronic properties of the cations at the A and B sites. Here, we report a comprehensive study on the temperature dependence of the magnetic structure of CuCr2O4 and Cu0.9Ni0.1Cr2O4 using neutron diffraction. CuCr2O4 undergoes a first continuous magnetic transition around 155 K into a canted long-range spin order on the Cr sublattice established by an antiferromagnetic and a ferromagnetic mode. Below 130 K a second transition occurs into a ferromagnetic order on the Cu sublattice resulting in a ferrimagnetic spin arrangement. Correlations between the appearance of magnetic modes and changes in the lattice geometry at different temperatures are discussed giving insight to magnetoelastic coupling. The occurrence of a ferromagnetic Cr mode above 130 K questions the common interpretation that a strong antiferromagnetic coupling between A and B spins in ferrimagnetic AB2O4 spinels is responsible for spin canting. From our neutron diffraction measurements of Cu1−xNixCr2O4 we identify a spin reorientation for the ferromagnetic modes with a nickel content between x = 0 and 0.1.

Molecular Oxygen-Induced Ferromagnetism and Half-Metallicity in α-BaNaO4: A First-Principles Study.

Molecular oxygen resembles 3d and 4f metals in exhibiting long-range spin ordering and strong electron correlation behaviors in compounds. Ferromagnetic spin ordering and half-metallicity, however, are quite elusive and have not been well acknowledged. In this Article, we address this issue by studying how spins will interact with each other if the oxygen dimers are arranged in a different way from that in the known superoxides and peroxides by first-principles calculations. Based on the results of a structure search, thermodynamic studies, and lattice dynamics, we show that tetragonal α-BaNaO4 is a stable half-metal with a Curie temperature at 120 K, the first example in this class of compounds. Like 3d and 4f metals, the O2 dimer carries a local magnetic moment of 0.5 μB due to the unpaired electrons in its π* orbitals. This compound can be regarded as forming from the O2 dimer layers stacking in a head-to-head way. In contrast to the arrangement in AO2 (A = K, Rb, Cs), the spins are ferromagnetically coupled both within and between the layers. Spin polarization occurs in π* orbitals, with spin-up electrons fully occupying the valence band and spin-down electrons partially occupying the conduction band, forming semiconducting and metallic channels, respectively. Our results highlight the importance of geometric arrangement of O2 dimers in inducing ferromagnetism and other novel properties in O2-dimer-containing compounds.

Structural studies of magnetic C60/Cu multilayers

We report on x-ray and neutron scattering studies that reveal the structure of interfaces of C60 layers with adjacent transition metal layers, in this instance, Cu. Such interfaces produce room-temperature long-range spin order that is not described by conventional theories of metallic magnetism. We use a combination of hard x-ray reflectivity and neutron scattering to investigate the interfacial structure of two C60/Cu layered samples: a superlattice with multiple C60/Cu repeats and a simpler tri-layer structure. For both structures, we develop a consistent structural model for the two scattering techniques, which details the critical interfacial roughness between the layers. We find that while x-ray reflectivity provides a strong contrast between the C60 and Cu layers, the similar neutron scattering length density of the two materials severely reduces the neutron scattering contrast. Our results can be used to design material systems that permit studies of the magnetism of the C60/transition metal interfaces with spin-sensitive scattering probes such as polarized neutron reflectometry.

Crystal Growth and Magnetic Properties of Topological Nodal-Line Semimetal GdSbTe with Antiferromagnetic Spin Ordering

We report crystal growth, AC and DC magnetic susceptibilities [χ(T, H)], magnetization [M(T, H)], and heat capacity [CP(T, H)] measurement results of GdSbTe single crystal. GdSbTe is isostructural to the confirmed nonmagnetic nodal-line semimetal ZrSiS of noncentrosymmetric tetragonal crystal structure in space group P4/nmm (No. 129), but it shows additional long-range antiferromagnetic spin ordering for the Gd spins of S = 7/2 below TN. Both χ(T, H) and CP(T, H) measurements confirm the existence of a long-range antiferromagnetic (AFM) spin ordering of Gd spins below TN ∼ 12 K, and an additional spin reorientation/recovery (sr) behavior is identified from the change of on-site spin anisotropy between Tsr1 ∼ 7 and Tsr2 ∼ 4 K. The anisotropic magnetic susceptibilities of χ(T, H) below TN clearly demonstrate that the AFM long-range spin ordering of GdSbTe has an easy axis parallel to the ab-plane direction. The field- and orientation-dependent magnetization of M(T, H) at 2 K shows two plateaus to indicate the spin-flop transition for H||ab near ∼2.1 T and a metamagnetic state near ∼5.9 T having ∼1/3 of the fully polarized magnetization by the applied field. The heat capacity measurement results yield Sommerfeld coefficient of γ ∼ 7.6(4) mJ/mol K2 and θD ∼ 195(2) K being less than half of that for the nonmagnetic ZrSiS. A three-dimensional (3D) AFM spin structure is supported by the ab initio calculations for Gd having magnetic moment of 7.1 μB and the calculated AFM band structure indicates that GdSbTe is a semimetal with bare density of states (0.36 states/eV fu) at the Fermi level, which is 10 times smaller than the measured one to suggest strong spin-fluctuation.