Noncollinear Magnetic Phase Research Articles

Macroscopic tunneling probe of Moiré spin textures in twisted CrI3

Various noncollinear spin textures and magnetic phases have been predicted in twisted two-dimensional CrI3 due to competing ferromagnetic (FM) and antiferromagnetic (AFM) interlayer exchange from moiré stacking—with potential spintronic applications even when the underlying material possesses a negligible Dzyaloshinskii–Moriya or dipole–dipole interaction. Recent measurements have shown evidence of coexisting FM and AFM layer order in small-twist-angle CrI3 bilayers and double bilayers. Yet, the nature of the magnetic textures remains unresolved and possibilities for their manipulation and electrical readout are unexplored. Here, we use tunneling magnetoresistance to investigate the collective spin states of twisted double-bilayer CrI3 under both out-of-plane and in-plane magnetic fields together with detailed micromagnetic simulations of domain dynamics based on magnetic circular dichroism. Our results capture hysteretic and anisotropic field evolutions of the magnetic states and we further uncover two distinct non-volatile spin textures (out-of-plane and in-plane domains) at ≈1° twist angle, with a different global tunneling resistance that can be switched by magnetic field.

New insight into tuning magnetic phases of RMn6Sn6 kagome metals

Predicting magnetic ordering in kagome compounds offers the possibility of harnessing topological or flat-band physical properties through tuning of the magnetism. Here, we examine the magnetic interactions and phases of ErMn6Sn6 which belongs to a family of RMn6Sn6, R = Sc, Y, Gd–Lu, compounds with magnetic kagome Mn layers, triangular R layers, and signatures of topological properties. Using results from single-crystal neutron diffraction and mean-field analysis, we find that ErMn6Sn6 sits close to the critical boundary separating the spiral-magnetic and ferrimagnetic ordered states typical for non-magnetic versus magnetic R layers, respectively. Finding interlayer magnetic interactions and easy-plane Mn magnetic anisotropy consistent with other members of the family, we predict the existence of a number of temperature and field dependent collinear, noncollinear, and noncoplanar magnetic phases. We show that thermal fluctuations of the Er magnetic moment, which act to weaken the Mn-Er interlayer magnetic interaction and quench the Er magnetic anisotropy, dictate magnetic phase stability. Our results provide a starting point and outline a multitude of possibilities for studying the behavior of Dirac fermions in RMn6Sn6 compounds with control of the Mn spin orientation and real-space spin chirality.

Magnon spin current from a non-collinear magnetic phase in a compensated rare earth ferrimagnet

The rare earth iron garnets have been commonly treated like ferromagnets due to the strong exchange coupling between the Fe3+ sublattices. However, the exchange coupling between the rare earth (RE) and transition metal (TM) in the RE-TM compounds is relatively weak and cannot hold the magnetic moments rigidly in a collinear state upon effective anisotropy energy. In this work, we show that a non-collinear magnetic phase is readily presented for a rare earth ferrimagnetic insulator, gadolinium iron garnet (GdIG), when a small magnetic field (H) is applied in the hard axis. We excite magnons from both the collinear and non-collinear magnetic configuration in GdIG via the spin Seebeck effect and detect these magnons in the adjacent normal metal. The comparison between the collinear and non-collinear magnetic configurations reveals a comparable magnon compensating temperature (TMM) for both cases. Moreover, as temperature decreases, the in-plane component of the magnon from the non-collinear case enhances and becomes comparable with that of the collinear case at TMM, representing the dominant role Gd3+ 4f spin plays at low T. Our study shows a wide magnetic tunability in the compensated ferrimagnetic insulator, which offers a myriad of opportunities for magnonic applications.

Magnetic tunability in tetragonal Mn–Rh–Ir–Sn inverse Heusler compounds

Gaining control over magnetic structure has been an ongoing challenge in materials that form complex, nanoscale, and non-collinear magnetic configurations. Recently, it was predicted that tuning the ratio of the Dzyaloshinskii–Moriya interaction to the uniaxial magnetic anisotropy in tetragonal inverse Heuslers through changes in composition could allow a range of interesting magnetic states to be accessed, from simple ferrimagnetism, to helical and antiskyrmionic phases. Here, we show tunability of the magnetic phase behavior in the Mn–Rh–Sn system through Ir substitution on the Rh substructure. Iridium substitution correlates to an increase in the strength of ferromagnetic exchange couplings, at the expense of antiferromagnetic exchange couplings. However, we do not observe the complex non-collinear magnetic phases proposed previously, likely due to the extremely narrow composition window where these phases are predicted to form in a bulk sample. This work highlights the sensitivity of complex magnetic structures to stoichiometry, which makes them difficult to discover empirically.

Multiple antiferromagnetic phases and magnetic anisotropy in exfoliated CrBr3 multilayers

In twisted two-dimensional (2D) magnets, the stacking dependence of the magnetic exchange interaction can lead to regions of ferromagnetic and antiferromagnetic interlayer order, separated by non-collinear, skyrmion-like spin textures. Recent experimental searches for these textures have focused on CrI3, known to exhibit either ferromagnetic or antiferromagnetic interlayer order, depending on layer stacking. However, the very strong uniaxial anisotropy of CrI3 disfavors smooth non-collinear phases in twisted bilayers. Here, we report the experimental observation of three distinct magnetic phases—one ferromagnetic and two antiferromagnetic—in exfoliated CrBr3 multilayers, and reveal that the uniaxial anisotropy is significantly smaller than in CrI3. These results are obtained by magnetoconductance measurements on CrBr3 tunnel barriers and Raman spectroscopy, in conjunction with density functional theory calculations, which enable us to identify the stackings responsible for the different interlayer magnetic couplings. The detection of all locally stable magnetic states predicted to exist in CrBr3 and the excellent agreement found between theory and experiments, provide complete information on the stacking-dependent interlayer exchange energy and establish twisted bilayer CrBr3 as an ideal system to deterministically create non-collinear magnetic phases.

Magneto-Optical Visualization of Magnetic Phases in an Epitaxial Film of Ferrite–Garnet Near the Compensation Point

A method for the magneto-optical visualization of magnetic phases in ferrimagnets in the vicinity of the compensation point in the lateral temperature gradient is described. On the example of films of bismuth–gadolinium ferrite garnet, zones corresponding to collinear and non-collinear magnetic phases were observed, depending on the temperature and the magnitude of the magnetic field in the range from 0 to 10 T.

Thickness-Dependent Topological Hall Effect in 2D Cr5 Si3 Nanosheets with Noncollinear Magnetic Phase.

Antiferromagnets with noncollinear spin order are expected to exhibit unconventional electromagnetic response, such as spin Hall effects, chiral abnormal, quantum Hall effect, and topological Hall effect. Here, 2D thickness-controlled and high-quality Cr5 Si3 nanosheets that are compatible with the complementary metal-oxide-semiconductor technology are synthesized by chemical vapor deposition method. The angular dependence of electromagnetic transport properties of Cr5 Si3 nanosheets is investigated using a physical property measurement system, and an obvious topological Hall effect (THE) appears at a large tilted magnetic field, which results from the noncollinear magnetic structure of the Cr5 Si3 nanosheet. The Cr5 Si3 nanosheets exhibit distinct thickness-dependent perpendicular magnetic anisotropy (PMA), and the THE only emerges in the specific thickness range with moderate PMA. This work provides opportunities for exploring fundamental spin-related physical mechanisms of noncollinear antiferromagnet in ultrathin limit.

Phase Transitions in Magneto-Electric Hexaferrites.

Hexaferrites have long been the object of extensive studies because of their great possibility for applications-permanent magnets, high-density recording media, microwave devices, in biomedicine, to name but a few. Lately, many researchers' efforts have been focused on the existence of the magneto-electric effect in some hexaferrite systems and the appealing possibility of them being used as single-phase multiferroic and magneto-electric materials. As indicated by theoretical analyses, the origin of the large magneto-electric effect can be sought in the strong interaction between the magnetization and the electric polarization that coexist in insulators with noncollinear magnetic structures. The hexaferrites' magnetic structure and, particularly, the specific magnetic spin ordering are the key factors in observing magneto-electric phases in hexaferrites. Some of these phases are metastable, which hampers their direct practical use. However, as the hexaferrites' phase diagrams reveal, chemical doping can be used to prepare a number of noncollinear stable magnetic phases. Since the magneto-electric effect has to do with the magnetic moments ordering, it seems only logical that one should study the cation substitutions' influence on the magnetic phase transition temperature. In this paper, we summarize recent examples of advances in the exploration of magnetic phase transitions in Y-type hexaferrites. In particular, the effect is emphasized by substituting in Y-type hexaferrites the nonmagnetic Me2+ cations with magnetic ones and of the magnetic Fe3+ cations with nonmagnetic ones on their magnetic properties and magnetic phase transitions. The work deals with the structural properties of and the magnetic phase transitions in a specific Y-type hexaferrite, namely, Ba(Sr)2Me2Fe12O22.

Origin and dynamics of umbrella states in rare-earth iron garnets

Rare-earth iron garnets R3Fe5O12 are fascinating insulators with very diverse magnetic phases. Their strong potential in spintronic devices has encouraged a renewal of interest in the study of their low temperature spin structures and spin wave dynamics. A striking feature of rare-earth garnets with R-ions exhibiting strong crystal-field effects like Tb-, Dy-, Ho-, Er-, Tm- and Yb-ions is the observation of low-temperature non-collinear magnetic structures featuring “umbrella-like” arrangements of the rare-earth magnetic moments. In this study, we demonstrate that such umbrella magnetic states are naturally emerging from the crystal-field anisotropies of the rare-earth ions. By means of a general model endowed with only the necessary elements from the crystal structure, we show how umbrella-like spin structures can take place and calculate the canting-angle as a result of the competition between the exchange-interaction of the rare-earth and the iron ions as well as the crystal-field anisotropy. Our results are compared to experimental data, and a study of the polarised spin wave dynamics is presented. Our study improves the understanding of umbrella-like spin structures and paves the way for more complex spin wave calculations in rare-earth iron garnets with non-collinear magnetic phases.

Influence of the Chemical Pressure on the Magnetic Properties of the Mixed Anion Cuprates Cu2OX2 (X = Cl, Br, I)

In this study, we theoretically investigate the structural, electronic and magnetic properties of the Cu2OX2 (X = Cl, Br, I) compounds. Previous studies reported potential spin-driven ferroelectricity in Cu2OCl2, originating from a non-collinear magnetic phase existing below TN∼70 K. However, the nature of this low-temperature magnetic phase is still under debate. Here, we focus on the calculation of J exchange couplings and enhance knowledge in the field by (i) characterizing the low-temperature magnetic order for Cu2OCl2 and (ii) evaluating the impact of the chemical pressure on the magnetic interactions, which leads us to consider the two new phases Cu2OBr2 and Cu2OI2. Our ab initio simulations notably demonstrate the coexistence of strong antiferromagnetic and ferromagnetic interactions, leading to spin frustration. The TN Néel temperatures were estimated on the basis of a quasi-1D AFM model using the abinitioJ couplings. It nicely reproduces the TN value for Cu2OCl2 and allows us to predict an increase of TN under chemical pressure, with TN = 120 K for the dynamically stable phase Cu2OBr2. This investigation suggests that chemical pressure is an effective key factor to open the door of room-temperature multiferroicity.

Exploration of trivial and nontrivial electronic phases and of collinear and noncollinear magnetic phases in low-spin d5 perovskites

The $4d$ and $5d$ transition metal oxides have become important members of the emerging quantum materials family due to competition between onsite Coulomb repulsion ($U$) and spin-orbit coupling (SOC). Specifically, the systems with $d^5$ electronic configuration in an octahedral environment are found to be capable of posessing invariant semimetallic state and perturbations can lead to diverse magnetic phases. In this work, by formulating a multi-band Hubbard model and performing SOC tunable DFT+$U$ calculations on a prototype SrIrO$_3$ and extending the analysis to other iso-structural and isovalent compounds, we present eight possible electronic and magnetic configurations in the $U$-SOC phase diagram that can be observed in the family of low-spin $d^5$ perovskites. They include the protected Dirac semimetal state, metal and insulator regimes, collinear and noncollinear spin ordering. The latter is explained through connecting hopping interactions to the rotation and tilting of the octahedra as observed in GdFeO$_3$. Presence of several soft phase boundaries makes the family of $d^5$ perovskites an ideal platform to study electronic and magnetic phase transitions under external stimuli.

Moiré Skyrmions and Chiral Magnetic Phases in Twisted CrX3 (X = I, Br, and Cl) Bilayers.

We present a comprehensive theory of the magnetic phases in twisted bilayer chromium trihalides through a combination of first-principles calculations and atomistic simulations. We show that the stacking-dependent interlayer exchange leads to an effective moiré field that is mostly ferromagnetic with antiferromagnetic patches. A wide range of noncollinear magnetic phases can be stabilized as a function of the twist angle and Dzyaloshinskii-Moriya interaction as a result of the competing interlayer antiferromagnetic coupling and the energy cost for forming domain walls. In particular, we demonstrate that for small twist angles various skyrmion crystal phases can be stabilized in both CrI3 and CrBr3. Our results provide an interpretation for the recent observation of noncollinear magnetic phases in twisted bilayer CrI3 and demonstrate the possibility of engineering further nontrivial magnetic ground states in twisted bilayer chromium trihalides.

Metamagnetism of few-layer topological antiferromagnets

MnBi$_2$Te$_4$ (MBT) is a promising antiferromagnetic topological insulator whose films provide access to novel and technologically important topological phases, including quantum anomalous Hall states and axion insulators. MBT device behavior is expected to be sensitive to the various collinear and non-collinear magnetic phases that are accessible in applied magnetic fields. Here, we use classical Monte Carlo simulations and electronic structure models to calculate the ground state magnetic phase diagram as well as topological and optical properties for few layer films with thicknesses up to six septuple layers. Using magnetic interaction parameters appropriate for MBT, we find that it is possible to prepare a variety of different magnetic stacking sequences, some of which have sufficient symmetry to disallow non-reciprocal optical response and Hall transport coefficients. Other stacking arrangements do yield large Faraday and Kerr signals, even when the ground state Chern number vanishes.

Topological Hall effect in the Shastry-Sutherland lattice

We study the classical Heisenberg model on the geometrically frustrated Shastry-Sutherland (SS) lattice with additional Dzyaloshinskii-Moriya (DM) interaction in the presence of an external magnetic field. We show that several noncollinear and noncoplanar magnetic phases, such as the flux, all-in/all-out, 3-in--1-out/3-out--1-in, and canted-flux phases are stabilized over wide ranges of parameters in the presence of the DM interaction. We discuss the role of DM interaction in stabilizing these complex magnetic phases. When coupled to these noncoplanar magnetic phases, itinerant electrons experience a finite Berry phase, which manifests in the form of topological Hall effect, whereby a nonzero transverse conductivity is observed even in the absence of a magnetic field. We study this anomalous magnetotransport by calculating the electron band structure and transverse conductivity for a wide range of parameter values, and demonstrate the existence of topological Hall effect in the SS lattice. We explore the role of the strength of itinerant electron-local moment coupling on electron transport and show that the topological Hall features evolve significantly from strong to intermediate values of the coupling strength, and are accompanied by the appearance of a finite spin Hall conductivity.

Magnetic properties of single-crystalline terbium and holmium – Experiment and modeling

The trend in modern experimental physics of magnetic phenomena is the increasing complexity of experimental methods, or a combination of methods to study even well studied “classical” magnetic materials. The development of related sciences (materials science, chemistry, nanotechnology), as well as the emergence of modern highly informative laboratory and research complexes allows simultaneous analysis of both bulk and local magnetic properties on the same samples. At the same time, the samples are subject to increased requirements in terms of chemical purity and degree of structural perfection. The combination of these factors leads to the discovery of new properties of well-known magnetic materials. This is particularly noticeable in the case of heavy rare earth metals, where a high degree of purity and absence of impurities can lead to the appearance of new magnetic phases and phase transitions. It is important to note that the complexity of the experimental detection of some non-collinear magnetic phases causes the need for theoretical analysis and mathematical modelling of the fundamental possibility of the existence of a type of magnetic ordering in rare-earth metals in order to exclude the experimental error. In this paper we present the modeling of the experimental results of high purity single-crystalline samples of Tb and Ho [5,6] trying to find theoretical and phenomenological patters which explain the existence of certain magnetic phases in the series of heavy lanthanides.

Ultrafast spin dynamics in ferrimagnets with compensation point

We derive an effective Lagrangian that facilitates the modeling of magnetization dynamics in a ferrimagnet with magnetization compensation point. The model is able to explain the earlier reported magnetization dynamics in the noncollinear magnetic phase triggered by a femtosecond laser pulse in GdFeCo amorphous alloy in the vicinity of spin-flop transition. Moreover, the described approach can be easily extended and applied to other cases of ultrafast magnetism in uniaxial f -d (rare-earth—transition metal) ferrimagnet near the magnetization compensation point in high magnetic fields.We assume that the primary effect of the femtosecond laser pulse is the ultrafast demagnetization of the ferrimagnet. We show that in the noncollinear magnetic phase, which can be prepared by applying external magnetic field above the spin-flop transition, such a demagnetization results in a torque acting on the magnetizations of both sublattices. It is shown that, similarly to the experiment, the amplitude and timescales of the dynamics strongly depend on temperature and applied magnetic field. In particular, in the vicinity of the spin-flop phase transition the amplitude dramatically increases while the dynamics exhibit a critical slowdown.We expect that the developed theoretical framework will boost further research of ultrafast magnetism of noncollinear spin systems.

Magnetic and electrical transport signatures of uncompensated moments in epitaxial thin films of the noncollinear antiferromagnet Mn3Ir

Noncollinear antiferromagnets, with either an L12 cubic crystal lattice (e.g., Mn3Ir and Mn3Pt) or a D019 hexagonal structure (e.g., Mn3Sn and Mn3Ge), exhibit a number of phenomena of interest to topological spintronics. Among the cubic systems, for example, tetragonally distorted Mn3Pt exhibits an intrinsic anomalous Hall effect (AHE). However, Mn3Pt only enters a noncollinear magnetic phase close to the stoichiometric composition and at suitably large thicknesses. Therefore, we turn our attention to Mn3Ir, the material of choice for use in exchange bias heterostructures. In this letter, we investigate the magnetic and electrical transport properties of epitaxially grown, face-centered-cubic γ-Mn3Ir thin films with (111) crystal orientation. Relaxed films of 10 nm thickness exhibit an ordinary Hall effect, with a hole-type carrier concentration of (1.500 ± 0.002) × 1023 cm−3. On the other hand, TEM characterization demonstrates that ultrathin 3 nm films grow with significant in-plane tensile strain. This may explain a small net magnetic moment, observed at low temperatures, shown by X-ray magnetic circular dichroism spectroscopy to arise from uncompensated Mn spins. Being of the order of 0.02 μB/atom, this dominates electrical transport behavior, leading to a small AHE and negative magnetoresistance. These results are discussed in terms of crystal microstructure and chiral domain behavior, with spatially resolved XML(C)D-PEEM supporting the conclusion that small antiferromagnetic domains, <20 nm in size, with differing chirality account for the absence of observed Berry curvature driven magnetotransport effects.

Spin Excitations in the Noncollinear Magnetic Phase of LaMn2O5

Spin dynamics of the LaMn2O5 compound, which contains manganese ions with different charges in two crystallographically different positions, has been investigated. The description of the dynamics is based on the equations of motion for the irreducible spin operators. It has been shown that the presence of a noncollinear antiferromagnetic ordering of manganese ions in the different crystallographic positions leads to the necessity of not only the determination of the frequencies of spin excitations, but also the formulation of conditions for the stability of the magnetic state relative to which the dynamic processes are considered.

Firefly Algorithm Applied to Noncollinear Magnetic Phase Materials Prediction.

In most noncollinear crystal magnets, the number of metastable states is quite large and any calculation that tries to predict the ground state can fall into one of the possible metastable phases. In this work, we generalize the population based meta-heuristic firefly algorithm to the problem of the noncollinear magnetic phase ground state prediction within density functional theory (DFT). We extend the different steps in the firefly algorithm to this specific problem by using polarized constrained DFT calculations, whereby using Lagrange multipliers the directions of the atom magnetic moments remain fixed. By locking the directions of the magnetic moments at each search iteration, the method allows one to explore the entire Born-Oppenheimer energy surface of existing and physically plausible noncollinear configurations present in a crystal. We demonstrate that the number of minima can be large, which restrains the use of exhaustive searches.

Generation and control of noncollinear magnetism by supercurrent

When superconductivity couples with noncollinear spin textures, rich physics arises; for instance, singlet Cooper pairs can be converted to triplet pairs, and topological superconductors can be realized. For their applications, the controllability of noncollinear magnetism is a crucial issue. Here, we propose that a supercurrent can induce and control noncollinear magnetic orders in a correlated metal on top of a singlet superconductor. We show that the magnetic instability in the correlated metal is enhanced by the proximity effect of supercurrents, which leads to phase transitions from a paramagnetic state to noncollinear magnetic phases with helical or vortexlike spin textures. Furthermore, these magnetic orders can be switched by the direction of the supercurrent. We also discuss the effect of the Rashba spin-orbit coupling and the experimental realization.