Magnetic Short Range Research Articles

A New Spin on Material Properties: Local Magnetic Structure in Functional and Quantum Materials.

The past few decades have made clear that the properties and performances of emerging functional and quantum materials can depend strongly on their local atomic and/or magnetic structure, particularly when details of the local structure deviate from the long-range structure averaged over space and time. Traditional methods of structural refinement (e.g., Rietveld) are typically sensitive only to the average structure, creating a need for more advanced structural probes suitable for extracting information about structural correlations on short length- and time-scales. In this Perspective, we describe the importance of local magnetic structure in several classes of emerging materials and present the magnetic pair distribution function (mPDF) technique as a powerful tool for studying short-range magnetism from neutron total-scattering data. We then provide a selection of examples of mPDF analysis applied to magnetic materials of recent technological and fundamental interest, including the antiferromagnetic semiconductor MnTe, geometrically frustrated magnets, and iron-oxide magnetic nanoparticles. The rapid development of mPDF analysis since its formalization a decade ago puts this technique in a strong position for making continued impact in the study of local magnetism in emerging materials.

Resilience of the Aurivillius structure upon La and Cr doping in a Bi5Ti3FeO15 multiferroic.

Combining the experimental techniques of high-resolution X-ray diffraction, magnetometry, specific heat measurement, and X-ray photoelectron, Raman and dielectric spectroscopy techniques, we have studied the influence of La and Cr doping on the crystal structure and magnetism of the room temperature Aurivillius multiferroic Bi5Ti3FeO15 by investigating the physical properties of (Bi4La)Ti3FeO15 and Bi5Ti3 (Fe0.5Cr0.5)O15. The parent (Bi5Ti3FeO15) and the doped ((Bi4La)Ti3FeO15 and Bi5Ti3(Fe0.5Cr0.5)O15) compounds crystallize in the A21am space group, which is confirmed through our analysis of high-resolution synchrotron X-ray diffraction data obtained on phase-pure polycrystalline powders. We determined the oxidation states of the metal atoms in the studied compounds as Fe3+, Ti4+, Cr3+, and La3+ through the analysis of X-ray photoelectron spectroscopy data. The magnetic susceptibilities of the three compounds are marked by the absence of a long-range ordered ground state, but dominated by superparamagnetic clusters with dominant antiferromagnetic interactions. This signature of short-range magnetism is also seen in specific heat as a low temperature enhancement which is suppressed upon the application of external magnetic fields up to 8 T. Our dielectric spectroscopy experiments showed that the three studied compounds display similar features in the dielectric constant measured as a function of frequency. However, upon doping La at the Bi site, the width of the ferroelectric hysteresis loop increases for (Bi4La)Ti3FeO15 compared to that of the parent compound Bi5Ti3FeO15, and with Cr doping, Bi5Ti3(Fe0.5Cr0.5)O15 becomes a leaky dielectric. The resilience of the Aurivillius crystal structure towards doping of La at the Bi site and Cr at the Fe site is clearly seen in the bulk properties of magnetic susceptibility, specific heat and the average crystal structure. The relevance of changes in the local structure is evident from our Raman spectroscopy and X-ray pair distribution function studies.

Structure dependent and strain tunable magnetic ordering in ultrathin chromium telluride

Two-dimensional (2D) chromium tellurides have attracted considerable research interest for their high Curie temperatures. Their magnetic properties have been found diverse in various experiments, the understanding of which however remains limited. In this work, we report that the magnetic ordering of ultrathin chromium tellurides is structure dependent and can be tuned by external strain. Based on first-principles calculations and Monte Carlo simulations, we show long-range stable magnetism with high and low Curie temperature, and short-range magnetism in 2D Cr5Te8, CrTe2, and Cr2Te3 layers, respectively. We further find that ferromagnetic-to-antiferromagnetic transition can be realized by 2% compressive strain for CrTe2 and 2% tensile strain for Cr2Te3, and their magnetic easy axis is tuned from out-of-plane to in-plane by the medium tensile and compressive strain. This strain dependent magnetic coupling is found to be related to Cr-Cr direct exchange and the change of magnetic anisotropy is understood by the atom and orbital resolved magnetic anisotropy energy and second order perturbation theory. Our results reveal the important roles of the structure and strain in determining the magnetic ordering in 2D chromium telluride, shedding light on understanding of the diverse magnetic properties observed in experiments.

Loop currents in two-leg ladder cuprates

New phases with broken discrete Ising symmetries are uncovered in quantum materials with strong electronic correlations. The two-leg ladder cuprate Sr14−xCaxCu24O41 hosts a very rich phase diagram where, upon hole doping, the system exhibits a spin liquid state ending to an intriguing ordered magnetic state at larger Ca content. Using polarized neutron diffraction, we report here the existence of short range magnetism in this material for two Ca contents, whose origin cannot be ascribed to Cu spins. This magnetism develops exclusively within the two-leg ladders with a diffraction pattern at forbidden Bragg scattering, which is the hallmark of loop current-like magnetism breaking both time-reversal and parity symmetries. Our discovery shows local discrete symmetry breaking in a one dimensional spin liquid system as theoretically predicted. It further suggests that a loop current-like phase could trigger the long range magnetic order reported at larger doping in two-leg ladder cuprates.

Long- and short-range magnetism in the frustrated double perovskite Ba2MnWO6

The structural and magnetic properties of the face-centered cubic double perovskite Ba2MnWO6 were investigated using neutron powder diffraction, DC-magnetometry, muon spin relaxation and inelastic neutron scattering. Ba2MnWO6 undergoes Type II long-range antiferromagnetic ordering at a Neel temperature of 8(1) K with a frustration index, f = 8. Inelastic neutron scattering was used to identify the magnetic coupling constants J1 and J2, which were found to equal -0.080 meV and -0.076 meV respectively. This indicated that both of the magnetic coupling constants were antiferromagnetic with similar magnitudes, which is in contrast to other known 3d metal double perovskites Ba2MWO6. Above the Neel temperature, muon spin relaxation measurements and inelastic neutron scattering techniques identify a short-range correlated magnetic state that is similar to that observed in the archetypical face-centered cubic lattice antiferromagnet MnO.

Magnetic and atomic short range order in Fe1−xCrx alloys

We study the magnetic short range order (MSRO) in ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cr}}_{x}\phantom{\rule{4pt}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}0.15)$ where an inversion of atomic short range order (ASRO) occurs at ${x}_{C}=0.11(1)$. Our combination of neutron diffuse scattering and bulk magnetization measurements offers a comprehensive description of these local orders at a microscopic level. In the dilute alloys $(xl0.04)$, the Cr atoms bear a large moment ${\ensuremath{\mu}}_{\mathrm{Cr}}=\ensuremath{-}1.0(1)\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$, antiparallel to the Fe ones $({\ensuremath{\mu}}_{\mathrm{Fe}})$. They fully repel their Cr first and second neighbors, and perturb the surrounding Fe moments. With increasing $x$, near neighbor Cr-Cr pairs start to appear and the Cr moment magnitude decreases, while ${\ensuremath{\mu}}_{\mathrm{Fe}}$ shows a rounded maximum for ${x}_{1}=0.06(1)l{x}_{C}$. Above ${x}_{C}$, ASRO turns to local Cr segregation, thereby increasing magnetic frustration. First-principles calculations reproduce the observed moment variations but overestimate the magnitude of the Cr moment. In order to reconcile theory with experiment quantitatively, we propose that the magnetic moments start canting locally, already above ${x}_{1}$. This picture actually anticipates the spin glasslike behavior of Cr-rich alloys. The whole study points out the subtle interplay of MSRO and ASRO, yielding an increasing frustration as $x$ increases, due to competing Fe-Cr and Cr-Cr interactions and Cr clustering tendency.

Signatures of low-dimensional magnetism and short-range magnetic order in Co-based trirutiles

Features of low dimensional magnetism resulting from a square-net arrangement of Co atoms in trirutile CoTa$_2$O$_6$ is studied in the present work by means of density functional theory and is compared with the experimental results of specific heat and neutron diffraction. The small total energy differences between the ferromagnetic (FM) and antiferromagnetic (AFM) configuration of CoTa$_2$O$_6$ shows that competing magnetic ground states exist, with the possibility of transition from FM to AFM phase at low temperature. Our calculation further suggests the semi-conducting behavior for CoTa$_2$O$_6$ with a band gap of $\sim$0.41 eV. The calculated magnetic anisotropy energy is $\sim$2.5 meV with its easy axis along the [100] (in-plane) direction. Studying the evolution of magnetism in Co$_{1-x}$Mg$_x$Ta$_2$O$_6$ (x = 0, 0.1, 0.3, 0.5, 0.7 and 1). it is found that the sharp AFM transition exhibited by CoTa$_2$O$_6$ at $T_N$ = 6.2 K in its heat capacity vanishes with Mg-dilution, indicating the obvious effect of weakening the superexchange pathways of Co. The current specific heat study reveals the robust nature of $T_N$ for CoTa$_2$O$_6$ in applied magnetic fields. Clear indication of short-range magnetism is obtained from the magnetic entropy, however, diffuse components are absent in neutron diffraction data. At $T_N$, CoTa$_2$O$_6$ enters a long-range ordered magnetic state which can be described using a propagation vector, (1/4, 1/4, 0). Upon Mg-dilution at $x \geq$0.1, the long-range ordered magnetism is destroyed. The present results should motivate an investigation of magnetic excitations in this low-dimensional anisotropic magnet.

Theoretical study of impurity-induced magnetism in FeSe

Experimental evidence suggests that FeSe is close to a magnetic instability, and recent scanning tunneling microscopy (STM) measurements on FeSe multilayer films have revealed stripe order locally pinned near defect sites. Motivated by these findings, we perform a theoretical study of locally induced magnetic order near nonmagnetic impurities in a model relevant for FeSe. We find that relatively weak repulsive impurities indeed are capable of generating short-range magnetism, and explain the driving mechanism for the local order by resonant eg-orbital states. In addition, we investigate the importance of orbital-selective self-energy effects relevant for Hund's metals, and show how the structure of the induced magnetization cloud gets modified by orbital selectivity. Finally, we make concrete connection to STM measurements of iron-based superconductors by symmetry arguments of the induced magnetic order, and the basic properties of the Fe Wannier functions relevant for tunneling spectroscopy.

Quantum critical nature of the short-range magnetic order in Sr2−xLaxIrO4

We have investigated the nature of the short-range magnetic phase region of ${\mathrm{Sr}}_{2\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{IrO}}_{4}$ ($0\ensuremath{\le}x\ensuremath{\le}0.14$). This so-called spin-orbit Mott insulator's predicted transition to superconductivity with electron doping has remained elusive. Instead, short-range magnetism prevails over an extensive doping range up to the La solubility limit. We find evidence for quantum critical-like fluctuations of the short-range magnetic phase, in both a scaling relation of the ac susceptibility $\ensuremath{\chi}{T}^{\ensuremath{\alpha}}$ and the magnetization $M{T}^{\ensuremath{\alpha}\ensuremath{-}1}$ with $\ensuremath{\alpha}=0.4--0.5$, as well as in the time-field scaling ($t/{H}^{\ensuremath{\gamma}}$) of the $\ensuremath{\mu}\mathrm{SR}$ asymmetry function. Our combined study of ac susceptibility and $\ensuremath{\mu}\mathrm{SR}$, on the one hand, uncovers a two-stage weak symmetry breaking to spin freezing or proximate spin glass and to a subsequent spin glass within the short-range magnetic phase. On the other hand, we demonstrate that magnetic correlations become more static above $x=0.06$ in spite of an apparent dilution of magnetic moments, indicative of enhanced inhomogeneities in a higher La-doping regime.

High resolution magnetic field energy imaging of the magnetic recording head by A-MFM with Co-GdOx super-paramagnetic tip

In this letter, the concept of a high-resolution magnetic field energy imaging technique is demonstrated by a high susceptibility superparamagnetic Co-GdOx magnetic force microscopy (MFM) tip for a perpendicular magnetic recording head with alternating magnetic force microscopy (A-MFM). The distribution of the magnetic energy gradient from the perpendicular recording head is imaged by the Co-GdOx superparamagnetic tip and compared with magnetic field imaging by the FePt-MgO hard magnetic tip. The Fourier analysis of the A-MFM amplitude images revealed enhancement in a spatial resolution of 13 nm by the Co-GdOx superparamagnetic tip as compared to 17 nm by the state-of-the-art FePt-MgO hard magnetic tip. The magnetic dipolar nature and short range force character of magnetic energy imaging by the Co-GdOx superparamagnetic tip showed high performance, confirmed by the tip transfer function analysis as compared to the monopole type FePt-MgO hard magnetic tip. The proposed technique opens an opportunity for the development of advanced high-resolution magnetic energy based imaging methods and development of the high-resolution MFM tips.

Cu2IrO3: A New Magnetically Frustrated Honeycomb Iridate.

We present the first copper iridium binary metal oxide with the chemical formula Cu2IrO3. The material is synthesized from the parent compound Na2IrO3 by a topotactic reaction where sodium is exchanged with copper under mild conditions. Cu2IrO3 has the same monoclinic space group (C2/c) as Na2IrO3 with a layered honeycomb structure. The parent compound Na2IrO3 is proposed to be relevant to the Kitaev spin liquid on the basis of having Ir4+ with an effective spin of 1/2 on a honeycomb lattice. Remarkably, whereas Na2IrO3 shows a long-range magnetic order at 15 K and fails to become a true spin liquid, Cu2IrO3 remains disordered until 2.7 K, at which point a short-range order develops. Rietveld analysis shows less distortions in the honeycomb structure of Cu2IrO3 with bond angles closer to 120° compared to Na2IrO3. Thus, the weak short-range magnetism combined with the nearly ideal honeycomb structure places Cu2IrO3 closer to a Kitaev spin liquid than its predecessors.

Influence of electron doping on the ground state of(Sr1−xLax)2IrO4

The evolution of the electronic properties of electron-doped (Sr{1-x}La{x})2IrO4 is experimentally explored as the doping limit of La is approached. As electrons are introduced, the electronic ground state transitions from a spin-orbit Mott phase into an electronically phase separated state, where long-range magnetic order vanishes beyond x = 0.02 and charge transport remains percolative up to the limit of La substitution (x~0.06). In particular, the electronic ground state remains inhomogeneous even beyond the collapse of the parent state's long-range antiferromagnetic order, while persistent short-range magnetism survives up to the highest La-substitution levels. Furthermore, as electrons are doped into Sr2IrO4, we observe the appearance of a low temperature magnetic glass-like state intermediate to the complete suppression of antiferromagnetic order. Universalities and differences in the electron-doped phase diagrams of single layer and bilayer Ruddlesden-Popper strontium iridates are discussed.

Competing magnetic interactions and low temperature magnetic phase transitions in composite multiferroics

Novel magnetic properties and magnetic interactions in composite multiferroic oxides Pb[(Zr0.52Ti0.48)0.60(Fe0.67W0.33).40]O3]0.80–[CoFe2O4]0.20 (PZTFW–CFO) have been studied from 50 to 1000 Oe field cooled (FC) and zero field cooled (ZFC) probing conditions, and over a wide range of temperatures (4–350 K). Crystal structure analysis, surface morphology, and high resolution transmission electron microscopy images revealed the presence of two distinct phases, where micro- and nano-size spinel CFO were embedded in tetragonal PZTFW matrix and applied a significant built-in compressive strain (∼0.4–0.8%). Three distinct magnetic phase transitions were observed with the subtle effect of CFO magnetic phase on PZTFW magnetic phase transitions below the blocking temperature (TB). Temperature dependence magnetic property m(T) shows a clear evidence of spin freezing in magnetic order with lowering in thermal vibration. Chemical inhomogeneity and confinement of nanoscale ferrimagnetic phase in paramagnetic/antiferromagnetic matrix restrict the long range interaction of spin which in turn develop a giant spin frustration. A large divergence in the FC and ZFC data and broad hump in ZFC data near 200 (±10) K were observed which suggests that large magnetic anisotropy and short range order magnetic dipoles lead to the development of superparamagnetic states in composite.

Ballistic spin transport in exciton gases

Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. This brings several complications: the Pauli principle prevents the carriers from moving with the same speed; Coulomb repulsion leads to rapid dephasing of electron flows. Spin-optronics is a valuable alternative to traditional spintronics. In spin-optronic devices the spin currents are carried by electrically neutral bosonic quasi-particles: excitons or exciton-polaritons. They can form highly coherent quantum liquids and carry spins over macroscopic distances. The price to pay is a finite life-time of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems where bosonic spin transport has been reported, in particular, to indirect excitons in coupled GaAs/AlGaAs quantum wells. We describe the effect of spin-orbit interaction of electrons and holes on the exciton spin, account for the Zeeman effect induced by external magnetic fields, long range and short range exchange splittings of the exciton resonances. We also consider exciton transport in the non-linear regime and discuss the definitions of exciton spin current, polarization current and spin conductivity.

Structural properties and phase diagram of the La(Fe1−xRux)AsO system

Structural refinement, lattice micro-strain and spontaneous strain analyses have been carried out on selected members of the La(Fe1−xRux)AsO system using high-resolution neutron and synchrotron powder diffraction data. The obtained results indicate that the character of the tetragonal to orthorhombic structural transition changes from first order for x = 0.10, possibly to tricritical for x = 0.20, up to second order for x = 0.30; for x ≥ 0.40 symmetry breaking is suppressed, even though a notable increase of the lattice micro-strain develops at low temperature. By combining structural findings with previous muon spin rotation data, a phase diagram of the La(Fe1−xRux)AsO system has been drawn. Long-range ordered magnetism occurs within the orthorhombic phase (x ≤ 0.30), whereas short-range magnetism appears to be confined within the lattice strained region of the tetragonal phase up to x < 0.60. Direct comparison between the magnetic and structural properties indicates that the magnetic transition is always associated with structural symmetry breaking, although confined to a local scale at high Ru contents.

Crystal Structure and Physical Properties of π–dSystem κ-(BDH-TTP)2FeBr4

The structure and physical properties of metallic magnetic conductor κ-(BDH-TTP)2FeBr4 (BDH-TTP: 2,5-bis(1,3-dithiolan-2-ylidene)-1,3,4,6-tetrathiapentalene) are investigated. It has a layered structure of BDH-TTP donor sheets extended along the \(ac\)-plane, which are sandwiched by FeBr4 anion sheets. The π electrons on the BDH-TTP sheets show simple metallic behavior down to 30 mK. The magnetic susceptibility obeys the Curie–Weiss law mainly associated with the Fe3+ \(d\) spins (\(S = 5/2\)), and indicates the presence of an anitiferromagnetic (AF) transition at \(T_{\text{N}} = 3.9\) K. In the AF state, a steep S-shaped increase of the magnetization at 1.5 T (\(H_{\text{SF}}\)) in the field parallel to the \(a\)-axis (\(H\parallel a\)) is found, which is ascribed to a spin-flop transition. In addition, the magnetization curves for \(H\parallel b\) and \(H\parallel c\) show an inflection point at \(H_{\text{c}} = 3.1\) T, suggesting the spin canting configuration in the \(bc\)-plane. A possible AF spin structure based on the magnetization data and molecular orbital calculations features triangular lattice consisting of the Fe \(d\) electron spins and the donor π electron spins. Possible origins responsible for the spin canting, Dzyaloshinskii–Moriya interaction, spin frustration effect, and anisotropy of the ligand field, are discussed. A steep decrease of the magnetoresistance in the AF state observed at \(H_{\text{SF}}\) for \(H\parallel a\) proves that the strong π–\(d\) interaction seriously affects the electron transport in the donor system. An anomalous broadening of the ESR linewidth in the critical region above \(T_{\text{N}}\) is suggestive of the development of a magnetic short range order, for which the low-dimensionality in the spin system is responsible.

First-principles based thermodynamic model of phase equilibria in bcc Fe-Cr alloys

The complete and accurate thermodynamic and kinetic description of any systemis crucialfor understanding and predicting its properties. A particular interest is in systemsthat are used for some practical applications and have to be constantly improved usingmodification of their composition and structure. This task can be quite accuratelysolved at a fundamental level by density functional theory methods. Thesemethods areapplied to two practically important systems Fe-Cr and TiC-ZrC.The elastic properties of pure iron and substitutionally disordered Fe-Cr alloy are investigatedas a function of temperature and concentration using first-principles electronicstructurecalculations by the exact muffin-tin orbitals method. The temperature effectson the elastic properties are included via the electronic, magnetic, and lattice expansioncontributions. It is shown that the degree of magnetic order in both pure iron andFe90Cr10 alloy mainly determines the dramatic change of the elastic anisotropy of thesematerials at elevated temperatures. A peculiarity in the concentration dependence ofthe elastic constants in Fe-rich alloys is demonstrated and related to a change in theFermi surface topology.A thermodynamic model for the magnetic alloys is developed from first principles andapplied to the calculation of bcc Fe-Cr phase diagram. Various contributions to the freeenergy (magnetic, electronic, and phonon) are estimated and included in the model. Inparticular, it is found that magnetic short range order effects are important just abovethe Curie temperature. The model is applied for calculating phase equilibria in disorderedbcc Fe-Cr alloys. Model calculations reproduce a feature known as a Nishizawahorn for the Fe-rich high-temperature part of the phase diagram.The investigation of the TiC-ZrC system includes a detailed study of the defect formationenergies and migration barriers of point defects and defect complexes involvedin the diffusion process. It is found, using ab initio atomistic simulations of vacancymediateddiffusion processes in TiC and ZrC, that a special self-diffusion mechanism isoperative for metal atom diffusion in sub-stoichiometric carbides. It involves a noveltype of a stable point defect, a metal vacancy ”dressed” in a shell of carbon vacancies.It is shown that this vacancy cluster is strongly bound and can propagate through thelattice without dissociating.

Gauge equivalence and inverse scattering for long-range magnetic potentials

For Schrodinger operators with long-range magnetic vector potentials and short range electric scalar potentials in an exterior domain Ω in Rn with n ⩾ 2, we show that there is a one-to-one correspondence between the gauge equivalent classes of Hamiltonians and those of S-matrices if Ω is exterior to a bounded convex obstacle.

Thermomagnetic irreversibility and magnetic short range ordering in Mn2.5Co0.5O4 tetragonal spinel thin films

Mn 2.5 Co 0.5 O 4 films are grown on MgO(001) and (011) substrate by molecular beam epitaxy. Detailed structural and magnetic analyses are carried out by x-ray diffraction and magnetization measurements. The reduction of the lattice parameter c/a* ratio and the enhancement of the Curie temperature (from 43 to 66 K), due to the weakening of the Jahn–Teller effect, are observed. In addition, thermomagnetic reversibility is observed at lower temperature and low external filed indicating the existence of local canting states. The randomness of Co cation and uniaxial anisotropy further enhance the local canting effect. An H-T phase diagram is mapped out, which indicates a short rang order state in the temperature 66–85 K separating the ferrimagentic and paramagnetic phases.

Magnetoresistance of CuCrO2-based delafossite films

Oxides with the delafossite structure (space group R3̄m) are of interest as wide-bandgap semiconductors, and as multiferroics. Well oriented c-axis films of CuCrO2 and CuCr0.9Mg0.1O2 have been produced on c-cut sapphire by PLD at elevated temperatures (650 °C). The lattice parameters for CuCrO2 are a = 299.6 pm and c = 1719.7 pm. The end member is insulating at room temperature with ρ ≳ 1000 Ω m whereas the Mg-doped oxide is a degenerate p-type semiconductor with ρ ≈ 1.5 x 10−4 Ω m. There is a resistivity anomaly at TN. A large negative magnetoresistance, which reaches its maximum value (≈ 300 %) in 14 T at about 2 K, persists throughout the temperature range where magnetic short range order is significant.