Spin Superlattice Research Articles

Magnetic field driven dynamics in twisted bilayer artificial spin ice at superlattice angles

Geometrical designs of interacting nanomagnets have been studied extensively in the form of two-dimensional arrays called artificial spin ice. These systems are usually designed to create geometrical frustration and are of interest for the unusual and often surprising phenomena that can emerge. Advanced lithographic and element growth techniques have enabled the realization of complex designs that can involve elements arranged in three dimensions. Using numerical simulations employing the dumbbell approximation, we examine possible magnetic behaviors for bilayer artificial spin ice, in which the individual layers are rotated with respect to one another. The goal is to understand how magnetization dynamics are affected by long-range dipolar coupling that can be modified by varying the layer separation and layer alignment through rotation. We consider bilayers where the layers are both either square or pinwheel arrangements of islands. Magnetic reversal processes are studied and discussed in terms of domain and domain wall configurations of the magnetic islands. Unusual magnetic ordering is predicted for special angles that define lateral spin superlattices for the bilayer systems.

High thermoelectric power factor in ambient-stable semiconducting rare-earth ErN thin films

Erbium nitride (ErN) is an emerging semiconducting rare-earth pnictide with unique electronic and magnetic properties. ErN has attracted significant interest for spin superlattices and spintronic devices and as a second-stage regenerator for Gifford–McMahon cryo-coolers. Solid-solution alloys of ErN with III-nitride semiconductors such as GaN have been studied extensively for use in solid-state lasers, amplifiers, and light-emitting devices operating in the retina-safe and fiber-optic communication wavelength window of 1.54 μm. However, due to the high affinity of Er toward oxygen, ErN is prone to oxidation in ambient conditions. To date, no reports on the deposition of the high-quality ErN thin film and its thermoelectric properties have been published. In this Letter, semiconducting ErN thin films are deposited inside an ultrahigh-vacuum chamber and capped with thin (3 nm) AlN layers to stabilize it in ambient conditions. Structural, optical, and electronic characterization reveals that ErN thin films (a) grow with (111) and (002) orientations on (0001) Al2O3 and (001) MgO substrates with sharp and abrupt ErN–substrate interfaces, (b) demonstrate a direct bandgap of 1.9 eV, and (c) exhibit a high carrier concentration in the range of 4.3 × 1020 to 1.4 × 1021 cm−3. Thermoelectric measurements show a moderately high Seebeck coefficient of –72.6 μV/K at 640 K and a maximum power factor of 0.44 × 10−3 W/m K2 at 486 K. Demonstration of an ambient-stable semiconducting ErN thin film and its high thermoelectric power factor marks significant progress in rare-earth pnictide research and will help develop ErN-based spintronic and thermoelectric devices.

Topological spin excitations in Harper-Heisenberg spin chains

Many-body spin systems represent a paradigmatic platform for the realization of emergent states of matter in a strongly interacting regime. Spin models are commonly studied in one-dimensional periodic chains, whose lattice constant is on the order of the interatomic distance. However, in cold atomic setups or functionalized twisted van der Waals heterostructures, long-range modulations of the spin physics can be engineered. Here we show that such superlattice modulations in a many-body spin Hamiltonian can give rise to observable topological boundary modes in the excitation spectrum of the spin chain. In the case of an XY spin-$1/2$ chain, these boundary modes stem from a mathematical correspondence with the chiral edge modes of a two-dimensional quantum Hall state. Our results show that the addition of many-body interactions does not close some of the topological gaps in the excitation spectrum, and the topological boundary modes visibly persist in the isotropic Heisenberg limit. These observations carry through when the spin moment is increased and a large-spin limit of the phenomenon is established. Our results show that such spin superlattices provide a promising route to observe many-body topological boundary effects in cold atomic setups and functionalized twisted van der Waals materials.

The nature of spin excitations in the one-third magnetization plateau phase of Ba3CoSb2O9

Magnetization plateaus in quantum magnets—where bosonic quasiparticles crystallize into emergent spin superlattices—are spectacular yet simple examples of collective quantum phenomena escaping classical description. While magnetization plateaus have been observed in a number of spin-1/2 antiferromagnets, the description of their magnetic excitations remains an open theoretical and experimental challenge. Here, we investigate the dynamical properties of the triangular-lattice spin-1/2 antiferromagnet Ba3CoSb2O9 in its one-third magnetization plateau phase using a combination of nonlinear spin-wave theory and neutron scattering measurements. The agreement between our theoretical treatment and the experimental data demonstrates that magnons behave semiclassically in the plateau in spite of the purely quantum origin of the underlying magnetic structure. This allows for a quantitative determination of Ba3CoSb2O9 exchange parameters. We discuss the implication of our results to the deviations from semiclassical behavior observed in zero-field spin dynamics of the same material and conclude they must have an intrinsic origin.

Effects of dynamic stress in magnetic superlattice of a monoaxial chiral magnet Cr1/3NbS2

A monoaxial chiral magnet forms a kind of spin superlattice structure, termed chiral soliton lattice (CSL), by the application of a magnetic field H perpendicular to a helical chiral axis. It has been reported that the CSL accompanies the magnetoresistance effect as well as a discrete change in magnetization and magnetoresistance. In order to verify the effect of the structural modification on the CSL state, we measured the magnetoresistance under the dynamic stress (DS) with a frequency of the order of MHz, which was applied by a piezoelectric ceramic oscillator. The steady application of DS while decreasing H resulted in a suppression of the insertion of chiral soliton. On the other hand, the application of a pulse-like DS while H decreased assisted the insertion of chiral soliton. These results demonstrate that DS modifies the spin structure of the monoaxial chiral magnet, and we can therefore change the activation energy for the insertion of chiral soliton while H is decreased.

Quantum Monte Carlo study of the electric properties of a ferroelectric superlattice

Abstract By using quantum Monte Carlo (MC) simulation, the electric properties of an Ising spin superlattice formed by two ferroelectric slabs A and B with an antiferroelectric interfacial coupling was studied within the framework of the Transverse Ising Model (TIM). We have examined the effects of the temperature T and the transverse field Ω on the polarization properties. We have also examined the effects of the interfacial coupling JAB, T, and Ω on the hysteresis behavior. Our results are in good agreement with the previous theoretical results; we have found that the critical temperature Tc and the critical transverse field Ωc decrease with the increase of Ω and T respectively. In addition one or triple hysteresis loops can appear in the present system.

Crystallization of spin superlattices with pressure and field in the layered magnet SrCu2(BO3)2.

An exact mapping between quantum spins and boson gases provides fresh approaches to the creation of quantum condensates and crystals. Here we report on magnetization measurements on the dimerized quantum magnet SrCu2(BO3)2 at cryogenic temperatures and through a quantum-phase transition that demonstrate the emergence of fractionally filled bosonic crystals in mesoscopic patterns, specified by a sequence of magnetization plateaus. We apply tens of Teslas of magnetic field to tune the density of bosons and gigapascals of hydrostatic pressure to regulate the underlying interactions. Simulations help parse the balance between energy and geometry in the emergent spin superlattices. The magnetic crystallites are the end result of a progression from a direct product of singlet states in each short dimer at zero field to preferred filling fractions of spin-triplet bosons in each dimer at large magnetic field, enriching the known possibilities for collective states in both quantum spin and atomic systems.

Hexagonal spin structure of A-phase in MnSi: Densely packed skyrmion quasiparticles or two-dimensionally modulated spin superlattice?

We have studied in detail the A-phase region in the field-temperature (H-T) phase diagram of the cubic heli-magnet MnSi using small angle neutron diffraction. The A-phase revealed itself as a two-dimensional hexagonal pattern of Bragg spots with k h(1, 2, 3) ⊥ H. The directions and magnitudes of the wave vectors k h(1, 2, 3) are well preserved over the whole crystal of the size of 100 mm3, but in the small room of the (H-T) phase diagram just below T c = 29 K. The droplets of the orientationally disordered, presumably hexagonal, spin structure with k h ⊥ H are observed in the wide range beyond the A-phase boundaries in the field range from B T1 ≈ 0.1 T to B T2 ≈ 0.25 T at temperatures down to 15 K. No melting of these droplets into individual randomly located skyrmions is observed for all temperatures and magnetic fields. The wave vector of two-dimensional modulations k h is equal to the wave vector of the cone phase k c . We conclude that observable is a two dimensionally modulated hexagonal spin superlattice built on the same competition of interactions (ferromagnetic exchange and Dzyaloshinskii-Moriya interactions) similar to a case of one-dimensionally modulated simple spin spiral.

Electronic superlattice revealed by resonant scattering from random impurities in Sr3Ru2O7.

Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive tool for the study of subtle charge, orbital, and spin superlattice orders driven by the valence electrons, which therefore escape detection in conventional x-ray diffraction (XRD). Although the power of REXS has been demonstrated by numerous studies of complex oxides performed in the soft x-ray regime, the cross section and photon wavelength of the material-specific elemental absorption edges ultimately set the limit to the smallest superlattice amplitude and periodicity one can probe. Here we show – with simulations and REXS on Mn-substituted Sr3Ru2O7 – that these limitations can be overcome by performing resonant scattering experiments at the absorption edge of a suitably-chosen, dilute impurity. This establishes that – in analogy with impurity-based methods used in electron-spin-resonance, nuclear-magnetic resonance, and Mössbauer spectroscopy – randomly distributed impurities can serve as a non-invasive, but now momentum-dependent probe, greatly extending the applicability of resonant x-ray scattering techniques.

NMR Evidence for the Persistence of a Spin Superlattice Beyond the1/8Magnetization Plateau inSrCu2(BO3)2

We present 11B NMR studies of the 2D frustrated dimer spin system SrCu2(BO3)_{2} in the field range 27-31 T covering the upper phase boundary of the 1/8 magnetization plateau, identified at 28.4 T. Our data provide a clear evidence that above 28.4 T the spin superlattice of the 1/8 plateau is modified but does not melt even though the magnetization increases. Although this is precisely what is expected for a supersolid phase, the microscopic nature of this new phase is much more complex. We discuss the field-temperature phase diagram on the basis of our NMR data.

Electronic Spin Filter via Spin Superlattice

Recently there was a proposal for a spin filter by using the spin superlattice structure. In a certain energy range, the proposed structure exhibits a high spin filtering efficiency close to 100%. Unfortunately such energy range turns out to be narrow. In this paper, we report a method to widen the energy range by using an analogy to optical anti-reflection coating. In optics, it is well known that a stack of alternating layers of two dielectric materials can function as a highly transmissive or reflective filter for wide range of wavelength. Since electrons also have wave character as light, it would be possible to make an electronic analog of an optical filter. We demonstrate that alternating layers of two materials with different g-factors can function as a spin filter that allows electrons to be transmitted only when their spins point towards a certain particular direction. This spin-superlattice-based spin filter operates in wide energy ranges, curing the problem in the previous proposal.

The effective charge velocity of spin-¹/2 superlattices

We calculate the spin gap of homogeneous and inhomogeneous spin chains, using the White's density matrix renormalization group technique. We found that the spin gap is related to the ration between the spin velocity and the correlation exponent. We consider a spin superlattice, which is composed of a repeated pattern of two spin-1/2 XXZ chains with different anisotropy parameters. The behavior of the charge velocity as a function of the anisotropy parameter and the relative size of sub-chains was investigated. We found reasonable agreement between the bosonization results and the numerical ones.

The magnetization curve of spin chains with superlattice structure

The magnetization curve of the spin superlattices composed of repeat pattern of two spin- 1 2 XXZ chains with different anisotropy parameters was calculated using density matrix renormalization group. We observe a nontrivial plateau with magnetization value given by the relative sizes of the subchains.

Magneto-photoluminescence spectroscopy of ZnMnTe/ZnMgTe spin superlattice

In this work, we studied the magneto-optical properties of ZnMnTe/ZnMgTe spin superlattices by using photoluminescence (PL) spectroscopy at low temperature. The circularly polarized PL measurements in an increasing magnetic field were performed. Detailed characteristics of the level crossing behavior for the spin-polarized excitonic states due to the giant Zeeman splitting were observed. The results show that an increase of the magnetic field transfers the spin superlattices from direct band gap system to indirect band gap system, and spatially separates the electrons and the holes by changing type-I band alignment to type-II band alignment. These features are confirmed by the behavior of the PL intensity in magnetic field, and agree well with the theoretical expectation basing on the sp–d exchange interaction.

Quantum anisotropic Heisenberg chains with superlattice structure: A DMRG study

Using the density matrix renormalization group technique, we study spin superlattices composed of a repeated pattern of two spin-$\frac{1}{2}\phantom{\rule{0.3em}{0ex}}XXZ$ chains with different anisotropy parameters. The magnetization curve can exhibit two plateaus, a nontrivial plateau with the magnetization value given by the relative sizes of the subchains and another trivial plateau with zero magnetization. We find good agreement of the value and the width of the plateaus with the analytical results obtained previously. In the gapless regions away from the plateaus, we compare the finite-size spin gap with the predictions based on bosonization and find reasonable agreement. These results confirm the validity of the Tomonaga-Luttinger liquid superlattice description of these systems.

Stripe Phase: Analytical Results for Weakly Coupled Repulsive Hubbard Model

Motivated by the stripe developments in cuprates, we review analytical results for the spin-charge solitonic superstructures derived in the framework of the Hubbard model in our studies of weakly coupled (quasi) one-dimensional repulsive electron systems on a lattice. These results demonstrate that close to half filling, in the high temperature regime above the mean field transition temperature, short range repulsions favor charge density fluctuations with wavevectors bearing special relations with those of the spin density fluctuations. In the low temperature regime, besides the wavevectors, mutual phases of the charge and spin densities also become coupled due to a quantum interference phenomenon, leading to the stripe phase instability. It is shown that away from half filling, periodic lattice potential causes cooperative condensation of the spin and charge superlattices. "Switching off" this potential leads to the vanishing of the stripe order. The leading spin-charge coupling term in the effective Landau functional is derived microscopically. Results of the 1D renormalization group ("parquet") analysis away from half filling are also presented. They reveal transient-scale correlations resembling the mean-field pattern. Possible correspondence of our theory with the experimental data on stripe phase in high Tc cuprates is discussed.

Perovskite oxide tricolor superlattices with artificially broken inversion symmetry by interface effects

We have designed and fabricated spin superlattices composed of three kinds of perovskite oxides: ferromagnetic La0.6Sr0.4MnO3 (LSMO) sandwiched by two nonmagnetic insulators, SrTiO3 (STO), and LaAlO3 (LAO). Conventional superlattices, structured as alternating stack of two components, preserve centrosymmetry. By stacking three compounds repeatedly in an asymmetric sequence such as (LAO/LSMO/STO)n, one can break the inversion symmetry. Magnetic and transport properties of the superlattices clearly indicate that the STO and LAO layers modulate the ferromagnetic spin order in LSMO layers adjacent to the interfaces in a different manner, giving rise to broken inversion symmetry of magnetic structure in the (LAO/LSMO/STO)n superlattices.

Spin-Dependent Carrier Confinements and Radiative Recombinations in Zn1-xMnxTe/Zn1-yMgyTe Spin Superlattices

The properties of carrier confinement in Zn 0.950 Mn 0.050 Te/Zn 0.965 Mg 0.035 Te spin superlattices have been studied by transient photoluminescence in magnetic field at various temperatures. The polarization characteristics of asymmetric Zeeman splitting, the increased photoluminescence intensity and the enhanced exciton lifetime are assigned to the evidence of spin superlattices. The type-I optical transition for the electron and hole spin states is realized by the formation of spin superlattices due to the magnetic-field-induced confinement of the band potential. The radiative recombination process dominates at 4.2-30 K in the present spin superlattice structure.

Magnetization plateaus and Luttinger liquid behavior inXXZchains with superlattice structure

We study spin superlattices composed of a repeated pattern of two long spin-$\frac{1}{2}$ XXZ chains with different anisotropy parameters. They can be viewed as the limit of p-merized chains when the number of sites per cluster is very large. Magnetization plateaus are found, with magnetization values that depend on the relative sizes of the subchains. In certain regions of parameter space, the low-energy properties are described in terms of a Luttinger liquid superlattice parametrized by an effective velocity and an effective compactification radius.

Low-Dimensional Structures of Diluted Magnetic (Semimagnetic) Semiconductors - a Subjective Review

A review on selected areas of research on low-dimensional structures consisting of diluted magnetic (semimagnetic) semiconductors is attempted. In the introductory part, after short historical preliminaries, we describe some early ideas and speculations concerning properties of such structures and we confront them with real results obtained in the course of their experimental investigations. Several issues are addressed to but, in view of the richness of the field, the review is by no means not pretending to be an exhaustive one. We begin with early studies of metal-insulator-semiconductor structures and progress to structures grown by molecular beam epitaxy. We discuss, in particular, the idea of a spin superlattice, type-I-to-type-II transition, wave functions mapping, and detailed determination of the profiles of realistic quantum wells. We put special emphasis on digital growth and, specifically, parabolic and half-parabolic quantum wells, use of tunable g-factor in studies of trions, self-assembled quantum dots, spin injection and, finally, on hybrid structures of ferromagnets and diluted magnetic semiconductor structures. The focus of the present paper is on work clone in the Institute of Physics of the Polish Academy of Sciences, mostly in the Laboratory of Growth and Physics of Low-Dimensional Crystals.