Noncollinear Magnetic Structure Research Articles

Electronic and magnetic properties of the 2H-NbS2 intercalated by 3d transition metal atoms

Abstract The electronic structure and magnetic properties of the compound 2H-NbS2 intercalated by 3d elements from Cr to Ni, have been investigated using the Korringa–Kohn–Rostoker electronic structure method. Here, we consider the phases with 33% of intercalation within the ordered phase having a 3 × 3 $\sqrt 3 \times \sqrt 3 $ arrangement of the magnetic atoms. We analyze the relationship of the magnetic and electronic properties on the structural parameters dependent on the intercalant. The exchange coupling parameters calculated from first principles have been used for subsequent Monte Carlo simulations. Within these investigations, the FM order was found for the Cr and Mn intercalated phases as ground state configuration with a Curie temperature being in good agreement with the experiment. According to the Monte Carlo simulation, Fe1/3NbS2 has a complicated noncollinear magnetic structure with a noncompensated total magnetic moment, whereas Co1/3NbS2 and Ni1/3NbS2 are found to be antiferromagnetic, all in line with experimental observations.

Magnetoresistance in Bilayers of Heavy Metal and Non-collinear Antiferromagnet

We report on magnetoresistance measurements in a heavy metal/ Mn3Ir multilayers. After a post annealing process, we observed the magnetoresistance associated with the ordered crystalline structure of the Mn3Ir. The resistance change as a function of the strength as well as the direction of the applied field suggests that the magnetoresistance is partially related to a modification of the Néel order by the magnetic field. Our further detailed investigation revealed that there is an additional component of the resistance change, perhaps due to the non-collinear magnetic structure associated with the L12-ordered Mn3Ir, which cannot be accounted for by any conventional magnetoresistance effects.

Multi-Step Magnetization Process of Gd-Co/Co/Cu/Co Thermo-Sensitive Spin Valves

Magnetic and magnetoresistive properties of the Gd-Co/Co/Cu/Co magnetic type multilayered sensitive spin valve were studied as a function of temperature. It is shown that the appearance of a non-collinear magnetic structure significantly affects the shape of the magnetoresistive hysteresis loop. The characteristic values of the critical field related to the appearance of non-collinear structure depend on the temperature of the spin valve. The obtained results can serve as a basis for the improvements of functional properties and expanding the application areas of magnetic multilayered sensitive elements of the spin valve type; for example, for precise determination of the position of the object.

Supervised learning approach for recognizing magnetic skyrmion phases

We propose and apply simple machine learning approaches for recognition and classification of complex non-collinear magnetic structures in two-dimensional materials. The first approach is based on the implementation of the single-hidden-layer neural network that only relies on the z projections of the spins. In this setup one needs a limited set of magnetic configurations to distinguish ferromag- netic, skyrmion and spin spiral phases, as well as their different combinations in transitional areas of the phase diagram. The network trained on the configurations for square-lattice Heisenberg model with Dzyaloshinskii-Moriya interaction can classify the magnetic structures obtained from Monte Carlo calculations for triangular lattice and vice versa. The second approach we apply, a minimum distance method performs a fast and cheap classification in cases when a particular configuration is to be assigned to only one magnetic phase. The methods we propose are also easy to use for analysis of the numerous experimental data collected with spin-polarized scanning tunneling microscopy and Lorentz transmission electron microscopy experiments.

Negative Magnetoresistance in Nanotwinned NiMnGa Epitaxial Films

Magnetic shape memory alloys are under intensive investigation due to their unusual physical properties, such as magnetic shape memory effect, magnetic field induced superelasticity, direct and inverse magnetocaloric effect etc., promising for novel applications. One of the intriguing properties of these materials in a single phase state is a giant magnetoresistance. Here we report the remarkable results about the magnetoresistive properties of epitaxial films of Ni52.3Mn26.8Ga20.9 magnetic shape memory alloy in the temperature range of 100–370 K, well below the martensitic transformation temperature. It was found that the formation of non-collinear magnetic structure due to a nanotwinning of the film results in electron scattering on such a structure and noticeable negative magnetoresistance in the entire investigated temperature range.

Universality of defect-skyrmion interaction profiles

Magnetic skyrmions are prime candidates for future spintronic devices. However, incorporating them as information carriers hinges on their interaction with defects ubiquitous in any device. Here we map from first-principles, the energy profile of single skyrmions interacting with single-atom impurities, establishing a generic shape as function of the defect’s electron filling. Depending on their chemical nature, foreign 3d and 4d transition metal adatoms or surface implanted defects can either repel or pin skyrmions in PdFe/Ir(111) thin films, which we relate to the degree of filling of bonding and anti-bonding electronic states inherent to the proximity of the non-collinear magnetic structure. Similarities with key concepts of bond theories in catalysis and surface sciences imbue the universality of the shape of the interaction profile and the potential of predicting its interaction. The resulting fundamental understanding may give guidance for the design of devices with surface implanted defects to generate and control skyrmions.

Magnetic anisotropy of single-crystal ferromagnetic Ce3RuSn6

• A single-crystal of Ce 3 RuSn 6 ferromagnet was grown by the Czochralski pulling method. • The anisotropic magnetic properties of Ce 3 RuSn 6 single-crystal have been studied. • The magnetic structure below the Curie temperature is not a simple ferromagnetic one. Using the Czochralski pulling method, we have grown a single-crystal of orthorhombic Ce 3 RuSn 6 that exhibits a ferromagnetic transition. Here we present electrical resistivity, specific heat, and magnetization measurements of this sample in the temperature range from 2 to 300 K. The electrical resistivity ρ displays a - ln T temperature dependence from 20 K to above the Curie temperature T C = 3 K. At T C , ρ decreases, and the specific heat C exhibits a sharp peak due to the magnetic transition. The magnetization measurements reveal that the magnetization M is highly anisotropic, and the magnetic structure below T C is probably not a simple ferromagnetic structure. When the magnetic field B is applied along the b-axis or the c-axis, M shows a ferromagnetic-like rapid increase or an antiferromagnetic-like cusp anomaly, respectively, at T C . Moreover, for B | | c , a metamagnetic-like anomaly appears in the magnetization curve at B = 2.5 T. However, for B | | a , M does not exhibit a distinct anomaly at T C . The results indicate that below T C , Ce 3 RuSn 6 has a non-collinear magnetic structure and that the magnetic moments are aligned in the bc-plane.

Novel multiferroicity in orthorhombic SmCrO3

This paper reports the ferroelectricity in antiferromagnetic SmCrO3 with a reasonably large value of spontaneous electric polarization (70 μC/cm2 mC cm−2 at 50 K) based on the measurement of pyroelectric current. The experimental results showed that the temperature of the onset of polar order was evidently higher (about 220 K) than the Neel temperature (TN =197 K) in the magnetodielectric studies. The calculated magnetodielectric effect (MDE) value was explicitly negative with maximum | MDE | values of about 3.6% around 230 K. The absence of a convincing signature in MDE at antiferromagnetic ordering temperature TN indicated the absence of magnetoelectric coupling around this temperature regime. The results further confirmed that the onset of polar order can be attributed to the structural distortion, rather than the canted noncollinear magnetic structure.

Influence of Ti atoms on the magnetic order in quaternary NiMnGe:Ti compounds

ABSTRACTThe NiMn0.95Ti0.05MnGe and Ni1–xTixMnGe (x = 0.05, 0.15 and 0.25) compounds have been studied by X-ray (at room temperature), neutron diffraction (2–360 K), DSC (130–390 K) and magnetic studies (2–350 K). The compounds with a low Ti concentration exhibit an orthorhombic crystal structure of NiTiSi-type (space group Pnma), whereas those with a large Ti content exhibit a hexagonal Ni2In-type structure (space group P63/mmc). The compounds with x = 0.05 are antiferromagnets with the helicoidal magnetic structure. The samples with the hexagonal crystal structure have a noncollinear magnetic structure with antiferro- along a-axis and ferro- along c-axis components. The obtained magnetic entropy change has the maximum value equal to 6.2 J/kgK at T = 200 K and H = 90 kOe for Ni0.85Ti0.15MnGe. For compounds with x = 0.05, 0.15 and 0.25, the magnetic (P, T) phase diagrams, under hydrostatic pressure up to 12 kbar, were determined.

Spin-Density Wave as a Superposition of Two Magnetic States of Opposite Chirality and Its Implications.

A magnetic solid with weak spin frustration tends to adopt a noncollinear magnetic structure such as a cycloidal structure below a certain temperature and a spin-density wave (SDW) slightly above this temperature. The causes for these observations were explored by studying the magnetic structure of BaYFeO4, which undergoes a SDW and a cycloidal phase transition below 48 and 36 K, respectively, in terms of the density functional theory calculations. We show that a SDW structure arises from a superposition of two magnetic states of opposite chirality, an SDW state precedes a chiral magnetic state because of the lattice relaxation, and whether a SDW is transversal or longitudinal is governed by the magnetic anisotropy of magnetic ions.

Heusler, Weyl and Berry

Heusler materials, initially discovered by Fritz Heusler more than a century ago, have grown into a family of more than 1000 compounds, synthesized from combinations of more than 40 elements. These materials show a wide range of properties, but new properties are constantly being found. Most recently, by incorporating heavy elements that can give rise to strong spin-orbit coupling (SOC), non-trivial topological phases of matter, such as topological insulators (TIs), have been discovered in Heusler materials. Moreover, the interplay of symmetry, SOC and magnetic structure allows for the realization of a wide variety of topological phases through Berry curvature design. Weyl points and nodal lines can be manipulated by various external perturbations, which results in exotic properties such as the chiral anomaly, and large anomalous spin and topological Hall effects. The combination of a non-collinear magnetic structure and Berry curvature gives rise a non-zero anomalous Hall effect, which was first observed in the antiferromagnets Mn3Sn and Mn3Ge. Besides this k-space Berry curvature, Heusler compounds with non-collinear magnetic structures also possess real-space topological states in the form of magnetic antiskyrmions, which have not yet been observed in other materials. The possibility of directly manipulating the Berry curvature shows the importance of understanding both the electronic and magnetic structures of Heusler compounds. Together, with the new topological viewpoint and the high tunability, novel physical properties and phenomena await discovery in Heusler compounds.

Complex magnetic order in nickelate slabs

Magnetic ordering phenomena have a profound influence on the macroscopic properties of correlated-electron materials, but their realistic prediction remains a formidable challenge. An archetypical example is the ternary nickel oxide system RNiO3 (R = rare earth), where the period-four magnetic order with proposals of collinear and non-collinear structures and the amplitude of magnetic moments on different Ni sublattices have been subjects of debate for decades. Here we introduce an elementary model system - NdNiO3 slabs embedded in a non-magnetic NdGaO3 matrix - and use polarized resonant x-ray scattering (RXS) to show that both collinear and non-collinear magnetic structures can be realized, depending on the slab thickness. The crossover between both spin structures is correctly predicted by density functional theory and can be qualitatively understood in a low-energy spin model. We further demonstrate that the amplitude ratio of magnetic moments in neighboring NiO6 octahedra can be accurately determined by RXS in combination with a correlated double cluster model. Targeted synthesis of model systems with controlled thickness and synergistic application of polarized RXS and ab-initio theory thus provide new perspectives for research on complex magnetism, in analogy to two-dimensional materials created by exfoliation.

Spin Hall effect emerging from a noncollinear magnetic lattice without spin–orbit coupling

The spin Hall effect (SHE), which converts a charge current into a transverse spin current, has long been believed to be a phenomenon induced by spin–orbit coupling. Here, we identify an alternative mechanism to realize the intrinsic SHE through a noncollinear magnetic structure that breaks the spin rotation symmetry. No spin–orbit coupling is needed even when the scalar spin chirality vanishes, different from the case of the topological Hall effect and topological SHE reported previously. In known noncollinear antiferromagnetic compounds Mn3X (X = Ga, Ge, and Sn), for example, we indeed obtain large spin Hall conductivities based on ab initio calculations.

Influence of nonmagnetic Ga ions on the magnetoelectric coupling in CuFe1−xGaxO2

The magnetic properties and dielectric polarization of CuFe1−xGaxO2 with various nonmagnetic Ga3+ doping levels have been measured in a wide magnetic field region, in which the observed experiment results have been explained by the consequence of the change in exchange interaction, dilution effect, and partial reduction of spin frustration. Compared with pure CuFeO2, doped nonmagnetic Ga3+ ions have an important effect on magnetization and polarization behavior and lead to an enhanced magnetoelectric coupling, especially, an enhanced spontaneous polarization has been observed in the doping amount of 0.02 and 0.05. This magnetoelectric effect appears mainly in the ferroelectric incommensurate magnetic phase region due to the noncollinear magnetic structure with a proper helical order in the CuFeO2. Based on the magnetization and polarization data, a magnetic phase diagram of temperature, magnetic field, and the doping amount are obtained.

Magnetic Resonance in [(CoP)soft/NiP/(CoP)hard/NiP]n Multilayer Magnetic Springs

The magnetic resonance properties of [(CoP)soft/NiP/(CoP)hard/NiP]n multilayer films with the properties of magnetic springs have been experimentally studied. It has been found that the deposition of a NiP nonmagnetic amorphous layer on a (CoP)soft magnetic layer induces the appearance of perpendicular interface anisotropy. The increase in the number of blocks n in the multilayer structure leads to the appearance of the third absorption peak, which is explained by the formation of a noncollinear three-sublattice magnetic structure.

Plastic deformation inducing ferromagnetism in Fe2MnAl: Probing Fe magnetism

Structural and magnetic properties of the disordered and ordered Fe2MnAl Heusler alloys have systematically been investigated by local and bulk experimental methods. Plastic deformation, induced by cold-work, leads to a structural phase transformation from the ordered L21 to a disordered A2-structure, concomitantly with a change from paramagnetic PM to ferromagnetic FM state; an effect attributed to the strain-induced ferromagnetism. This transition is explained assuming a gradual transformation of the L21-structure to the anti-phase boundary APB tubes, which result in Fe-cluster formation. Plastic deformed ribbon annealed at low temperatures (Tan < 673 K) favors stabilization of L21-structure with (4 0 0) preferential orientation and magnetic ordering temperature of about 120 K. Considering that Fe atoms order magnetically at 120 K, as seen by Mössbauer, and the measured reduction of total magnetization for temperatures below 50 K, one proposes that Mn and Fe sublattices interact magnetically in a non-collinear magnetic structure that is established due to Fe-Fe, Mn-Mn and Fe-Mn frustrated magnetic couplings. High temperature annealing (Tan > 673 K) favors a strong Mn segregation, concomitantly with formation of disordered Fe-Mn-Al alloys that also have magnetic ordering temperature approaching to 300 K.

Stabilizing spin spirals and isolated skyrmions at low magnetic field exploiting vanishing magnetic anisotropy

Skyrmions are topologically protected non-collinear magnetic structures. Their stability is ideally suited to carry information in, e.g., racetrack memories. The success of such a memory critically depends on the ability to stabilize and manipulate skyrmions at low magnetic fields. The non-collinear Dzyaloshinskii-Moriya interaction originating from spin-orbit coupling drives skyrmion formation. It competes with Heisenberg exchange and magnetic anisotropy favoring collinear states. Isolated skyrmions in ultra-thin films so far required magnetic fields as high as several Tesla. Here, we show that isolated skyrmions in a monolayer of Co/Ru(0001) can be stabilized down to vanishing fields. Even with the weak spin-orbit coupling of the 4d element Ru, homochiral spin spirals and isolated skyrmions were detected with spin-sensitive scanning tunneling microscopy. Density functional theory calculations explain the stability of the chiral magnetic features by the absence of magnetic anisotropy energy.

Spin dynamics and magnetoelectric coupling mechanism of Co4Nb2O9

Neutron powder diffraction experiments reveal that Co4Nb2O9 forms a noncollinear in-plane magnetic structure with Co2+ moments lying in the ab plane. The spin-wave excitations of this magnet were measured by using inelastic neutron scattering and soundly simulated by a dynamic model involving nearest and next-nearest neighbour exchange interactions, in-plane anisotropy and the Dzyaloshinskii-Moriya interaction. The in-plane magnetic structure of Co4Nb2O9 is attributed to the large in-plane anisotropy while the noncollinearity of the spin configuration is attributed to the Dzyaloshinskii-Moriya interaction. The high magnetoelectric coupling effect of Co4Nb2O9 in fields can be explained by its special in-plane magnetic structure.

Prediction of a magnetic Weyl semimetal without spin-orbit coupling and strong anomalous Hall effect in the Heusler compensated ferrimagnet Ti2MnAl

We predict a magnetic Weyl semimetal in the inverse Heusler Ti2MnAl, a compensated ferrimagnet with a vanishing net magnetic moment and a Curie temperature of over 650 K. Despite the vanishing net magnetic moment, we calculate a large intrinsic anomalous Hall effect (AHE) of about 300 S/cm. It derives from the Berry curvature distribution of the Weyl points, which are only 14 meV away from the Fermi level and isolated from trivial bands. Different from antiferromagnets Mn3X (X= Ge, Sn, Ga, Ir, Rh, and Pt), where the AHE originates from the non-collinear magnetic structure, the AHE in Ti2MnAl stems directly from the Weyl points and is topologically protected. The large anomalous Hall conductivity (AHC) together with a low charge carrier concentration should give rise to a large anomalous Hall angle. In contrast to the Co-based ferromagnetic Heusler compounds, the Weyl nodes in Ti2MnAl do not derive from nodal lines due to the lack of mirror symmetries in the inverse Heusler structure. Since the magnetic structure breaks spin-rotation symmetry, the Weyl nodes are stable without SOC. Moreover, because of the large separation between Weyl points of opposite topological charge, the Fermi arcs extent up to 75% of the reciprocal lattice vectors in length. This makes Ti2MnAl an excellent candidate for the comprehensive study of magnetic Weyl semimetals. It is the first example of a material with Weyl points, large anomalous Hall effect and angle despite a vanishing net magnetic moment.

First principles study of the structural phase stability and magnetic order in various structural phases of Mn2FeGa

We investigate the structural and magnetic properties of Mn2FeGa for different phases (cubic, hexagonal and tetragonal) reported experimentally using density functional theory. The relative structural stabilities, and the possible phase transformation mechanisms are discussed using results for total energy, electronic structure and elastic constants. We find that the phase transformation form hexagonal to ground state tetragonal structure would take place through a Heusler-like phase which has a pronounced electronic instability. The electronic structures, the elastic constants and the supplementary phonon dispersions indicate that the transition from the Heusler-like to the tetragonal phase is of pure Jahn-Teller origin. We also describe the ground state magnetic structures in each phase by computations of the exchange interactions. For Heusler-like and tetragonal phases, the ferromagnetic exchange interactions associated with the Fe atoms balance the dominating antiferromagnetic interactions between the Mn atoms leading to collinear magnetic structures. In the hexagonal phase, the directions of atomic moments are completely in the planes with a collinear like structure, in stark contrast to the well known non-collinear magnetic structure in the hexagonal phase of Mn3Ga, another material with similar structural properties. The overwhelmingly large exchange interactions of Fe with other magnetic atoms destroy the possibility of magnetic frustration in the hexagonal phase of Mn2FeGa. This comprehensive study provides significant insights into the microscopic physics associated with the structural and magnetic orders in this compound.