Noncollinear Magnetic Structure Research Articles

Magnetoelectric Coupling Triggered by Noncollinear Magnetic Structure in M‐Type Hexaferrite

AbstractDirect magnetoelectric (ME) coupling between magnetic and ferroelectric orders, which can be realized in spiral magnets, is vital in technological applications of multifunctional materials. Multisusceptible BaFe12O19 is noteworthy for its excellent magnetic, dielectric properties and thus the application in high‐density information storage, while its collinear spin structure limits the emergence of ferroelectrics and ME coupling. In this work, nonzero Dzyaloshinskii–Moriya (DM) interaction is created by partial substitution of the spin‐down Fe3+ sites to induce conical spin structure in BaFe12O19. Larger In3+ ions are introduced and prefer to occupy the spin‐down 4f1 and 4f2 lattice sites. The evolution of magnetization versus temperature adduces evidence of conical magnetic structure. As a result, direct ME and magnetodielectric coupling are obtained in In‐doped BaFe12O19 ceramics. An interlayer DM interaction model is built to discuss the intrinsic relationship among crystal structure, noncollinear magnetic structure, and ionic substitution. Moreover, BaZn0.9Zr0.9Fe10.2O19 ceramics are also prepared, and exhibit noncollinear magnetic structure and direct ME coupling since Zn2+ and Zr4+ also possess larger radii than Fe3+ and prefer to enter the spin‐down sites. The present result provides a feasible avenue to develop multiferroic and magnetoelectric coupling in ubiquitous M‐type hexaferrite.

Giant topological Hall effect around room temperature in noncollinear ferromagnet NdMn2Ge2 single crystal

We report the observation of the giant topological Hall effect near room temperature in a complex noncollinear ferromagnet NdMn2Ge2 single crystal. Three successive magnetic transitions are observed below 400 K, including a spin reorientation transition at TSR = 215 K. The complex noncollinear magnetic structures give rise to anomalous transport behaviors. When the current flows along the a axis and the magnetic field is applied along the c axis, the anomalous Hall effect is observed, which is found to be dominated by the skew scattering mechanism. Strikingly, a giant topological Hall effect appears in a wide temperature range, which stems from the noncollinear spin configuration with finite scalar spin chirality. The topological Hall resistivity reaches the maximum of −1.35 μΩ cm at 300 K and drops slightly with temperature until below TSR. These results suggest that the NdMn2Ge2 single crystal would be a promising topological material for spintronic applications at room temperature.

Магнитокалорический эффект в окрестности температуры магнитной компенсации аморфных ферримагнитных пленок Gd-Co

The magnetic and magnetocaloric properties of amorphous ferrimagnetic Gd-Co films with perpendicular magnetic anisotropy were studied in a wide range of fields and temperatures. The change in the sign of the magnetocaloric effect near the compensation temperature occurs almost abruptly when the magnetic field is oriented perpendicular to the film plane. In a certain temperature range, it happens when the field is oriented along the plane of the sample. The most probable reason for the blurring of this transition is the appearance of a non-collinear magnetic structure.

Two-step magnetic ordering into a canted state in ferrimagnetic monoclinic Mn3As2

We report the magnetic structure of monoclinic Mn3As2 at 3 ​K and 250 ​K using neutron powder diffraction measurements. From magnetometry data, the Curie temperature of Mn3As2 was confirmed to be around 270 ​K. Calorimetry analysis showed the presence of another transition at 225 ​K. At 270 ​K, Mn3As2 undergoes a k=0 ferrimagnetic ordering in the magnetic space group C2/m (#12.58) with Mn moments pointing along b. Below 225 ​K, there is a canting of Mn moments in the ac plane which produces a multi-k non-collinear magnetic structure in space group C2/c (#15.85). The components of Mn moments along b follow k=0 ordering and the components along a and c have k=[0012] propagation vector. The change in the magnetic ground state with temperature provides a deeper insight into the factors that govern magnetic ordering in Mn–As compounds.

First-principles Dzyaloshinskii\u2013Moriya interaction in a non-collinear framework

We have derived an expression of the Dzyaloshinskii–Moriya interaction (DMI), where all the three components of the DMI vector can be calculated independently, for a general, non-collinear magnetic configuration. The formalism is implemented in a real space—linear muffin-tin orbital—atomic sphere approximation (RS-LMTO-ASA) method. We have chosen the Cr triangular trimer on Au(111) and Mn triangular trimers on Ag(111) and Au(111) surfaces as numerical examples. The results show that the DMI (module and direction) is drastically different between collinear and non-collinear states. Based on the relation between the spin and charge currents flowing in the system and their coupling to the non-collinear magnetic configuration of the triangular trimer, we demonstrate that the DMI interaction can be significant, even in the absence of spin-orbit coupling. This is shown to emanate from the non-collinear magnetic structure, that can induce significant spin and charge currents even with spin-orbit coupling is ignored.

Magnetic structure and phase transition at the surface region of Fe3O4(100)

The magnetic structure and phase transition of the near-surface region of Fe3O4(100) was investigated by 57Fe conversion electron Mössbauer spectroscopy (CEMS) and theoretical calculations. It is revealed that at 300 K the magnetization is in-plane in the surface region and cants from the in-plane to the 〈111〉 direction in a deeper region suggesting the presence of a noncollinear magnetic structure. The critical exponents for the tetrahedral and octahedral sites are estimated to be 0.24 ± 0.01 and 0.28 ± 0.01, respectively. Near the critical temperature, furthermore, the magnetization direction in the surface region was found to deviate from the in-plane direction.

Magnetic properties and electronic structure of CeFe2−xMnx and CeFe2Mnx compounds

The effect of manganese alloying on the structure and magnetic properties of CeFe2 has been studied on two isostructural series: quasibinary CeFe2−xMnx and nonstoichiometric CeFe2Mnx alloys. The single-phase bcc MgCu2-type structure is formed at x≤0.5 in both systems. At x=0.5, the lattice parameter is increased by ∼0.3%. The Mn alloying leads to a nonmonotonic variation of magnetic moment and gradual decrease in the Curie temperature from 230 K to 150 K and 167 K for CeFe1.5Mn0.5 and CeFe2Mn0.5, respectively. For x≥0.3, the magnetization data indicate the formation of noncollinear magnetic structure. The binary CeFe2 and nonstoichiometric CeFe2Mn0.15 have been studied using Mössbauer effect and X-ray photoelectron spectroscopy. The Ce valence state remains unchanged upon the Mn alloying. The parameters of hyperfine interactions have been determined in paramagnetic state at room temperature and in magnetically ordered state at 78 K. The Mössbauer data revealed a difference in the quadrupole coupling constants of CeFe2 and CeFe2Mn0.15, which is associated with the difference in the local distortions of the lattice.

Prospecting chiral multisite interactions in prototypical magnetic systems

Atomistic spin models have found enormous success in addressing the properties of magnetic materials, grounded on the identification of the relevant underlying magnetic interactions. The huge development in the field of magnetic skyrmions and other noncollinear magnetic structures is largely due to our understanding of the chiral Dzyaloshinskii-Moriya interaction. Recently, various works have proposed new types of chiral interactions, with seemingly different forms, but the big picture is still missing. Here, we present a systematic construction of a generalized spin model containing isotropic and chiral multi-site interactions. These are motivated by a microscopic model that incorporates local spin moments and the spin-orbit interaction, and their symmetry properties are established. We show that the chiral interactions arise solely from the spin-orbit interaction and that the multi-site interactions do not have to follow Moriya's rules, unlike the Dzyaloshinskii-Moriya and chiral biquadratic interactions. The chiral multi-site interactions do not vanish due to inversion symmetry, and comply with a generalized Moriya rule: If all sites connected by the interaction lie in the same mirror plane, the chiral interaction vector must be perpendicular to this plane. We then illustrate our theoretical considerations with density functional theory calculations for prototypical magnetic systems. These are triangular trimers built out of Cr, Mn, Fe and Co adatoms on the Re(0001), Pt(111) and Au(111) surfaces, for which $C_\mathrm{3v}$ symmetry applies, and Cr and Fe square tetramers on Pt(001) with $C_\mathrm{4v}$ symmetry. The multi-site interactions are substantial in magnitude and cannot be neglected when comparing the energy of different magnetic structures. Finally, we discuss the recent literature in light of our findings, and clarify several unclear or confusing points.

Large tunable anomalous Hall effect in the kagome antiferromagnet U3Ru4Al12

The Berry curvature in magnetic systems is attracting interest due to the potential tunability of topological features via the magnetic structure. $f$-electrons, with their large spin-orbit coupling, abundance of non-collinear magnetic structures and high electronic tunability, are attractive candidates to search for tunable topological properties. In this study, we measure anomalous Hall effect (AHE) in the distorted kagom$\acute{e}$ heavy fermion antiferromagnet U$_3$Ru$_4$Al$_{12}$. A large intrinsic AHE in high fields reveals the presence of a large Berry curvature. Moreover, the fields required to obtain the large Berry curvature are significantly different between $B \parallel a$ and $B \parallel a^*$, providing a mechanism to control the topological response in this system. Theoretical calculations illustrate that this sensitivity may be due to the heavy fermion character of the electronic structure. These results shed light on the Berry curvature of a strongly correlated band structure in magnetically frustrated heavy fermion materials, but also emphasize 5$f$-electrons as an ideal playground for studying field-tuned topological states.

Non-collinear Order and Spin–Orbit Coupling in Sr3ZnIrO6

Sr$_{3}$ZnIrO$_{6}$ is an effective spin one-half Mott insulating iridate belonging to a family of magnets which includes a number of quasi-one dimensional systems as well as materials exhibiting three dimensional order. Here we present the results of an extensive investigation into the magnetism including heat capacity, a.c. susceptibility, muon spin rotation ($\mu$SR), neutron diffraction and inelastic neutron scattering on the same sample. It is established that the material exhibits a transition at about $17$ K into a three-dimensional antiferromagnetic structure with propagation vector $\boldsymbol{k}=(0,\frac{1}{2},1)$ in the hexagonal setting of R$\bar{3}$c and non-collinear moments of $0.87$$\mu_B$ on Ir$^{4+}$ ions. Further we have observed a well defined powder averaged spin wave spectrum with zone boundary energy of $\sim 5$ meV at $5$ K. We stress that a theoretical analysis shows that the observed non-collinear magnetic structure arises from anisotropic inter- and intra- chain exchange which has its origin in significant spin-orbit coupling. The model can satisfactorily explain the observed spin wave excitations.

Tailoring the anomalous Hall effect in the noncollinear antiperovskite Mn3GaN

Based on first-principles calculations, we investigated the topological transport properties of Mn$_3$GaN with coplanar noncollinear magnetic structures. The intrinsic anomalous Hall conductivity (IAHC) displays a significant dependence with respect to the in-plane magnetization direction between the $\Gamma_{5g}$ and $\Gamma_{4g}$ magnetic configurations, where large anomalous Nernst effect (ANE) can be induced by tailoring the magnetization direction. Moreover, we observed strong piezospintronic effect in Mn$_3$GaN, where large IAHC can be induced by moderate epitaxial strain. Symmetry analysis reveals that for both cases, the nonzero IAHC is originated from the spin-orbit coupling instead of the noncollinear magnetic configurations

Element- and orbital-selective magnetic coherent rotation at the first-order phase transition of a hard uniaxial ferrimagnet

$3d\text{\ensuremath{-}}4f$ intermetalic compounds with heavy rare-earth elements show first-order phase transitions in high magnetic fields due to the competition between the exchange interaction and the magnetocrystalline anisotropy. However, the microscopic picture of the field-induced noncollinear magnetic structures remains elusive. Here we report the direct experimental observation of the coherent stepwise rotation of the $3d$ and $4f$ magnetic moments of the uniaxial hard ferrimagnet ${\mathrm{TmFe}}_{5}{\mathrm{Al}}_{7}$ by using soft x-ray magnetic circular dichroism in pulsed magnetic fields up to 25 T. The element- and shell-selective moments show a transition from the collinear ferrimagnet toward the forced ferromagnetic state via a canted phase, which is explained by a two-sublattice model.

Pressure tuning of the anomalous Hall effect in the chiral antiferromagnet Mn3Ge

We report on the pressure evolution of the giant anomalous Hall effect (AHE) in the chiral antiferromagnet Mn$_3$Ge. The AHE originating from the non-vanishing Berry curvature in Mn$_3$Ge can be continuously tuned by application of hydrostatic pressure. At room temperature, the Hall signal changes sign as a function of pressure and vanishes completely at $p=1.53$ GPa. Even though the Hall conductivity changes sign upon increasing pressure, the room-temperature saturation value of 23 ${\rm \Omega^{-1}cm^{-1}}$ at 2.85 GPa is remarkably high and comparable to the saturation value at ambient pressure of about 40 ${\rm \Omega^{-1}cm^{-1}}$. The change in the Hall conductivity can be directly linked to a gradual change of the size of the in-plane components of the Mn moments in the non-collinear triangular magnetic structure. Our findings, therefore, provide a route for tuning of the AHE in the chiral antiferromagnetic Mn$_3$Ge.

A review on crystal structure and properties of 3d transition metal (II) orthophosphates M3(PO4)2

Abstract A family of anhydrous 3d transition metal (II) orthophosphates M3(PO4)2 (M = Cr - Zn) counts fourteen members adopting amazing variety of eight structural types. Even more diverse are structural types in mixed cation orthophosphates A3-nMn(PO4)2 (A = Sr, Ca, Ba, Pb). Rich crystal chemistry of these species is associated with flexibility of basic structures of sarcopside Fe3(PO4)2, graftonite (Fe,Mn,Ca)3(PO4)2 and farringtonite Mg3(PO4)2 minerals. The low temperature magnetic properties of these compounds are quite peculiar showing either spin gap behavior due to the presence of even spin clusters in the crystal structure or formation of non-collinear magnetic structures with temperature driven succession of magnetic order phase transitions, multiple magnetization reversal and well defined plateaus in magnetization at low temperatures. The correspondence between crystal structure and physical properties in the vast family of transition metal orthophosphates is reviewed. In addition, the structural relationships between different members of the title family are discussed.

Spin-glass mediated large exchange bias and coercivity in hexagonal Mn1+xCu1−xGa (x = 0.3–0.5) alloys

Ni2In-type MnMX alloys possess interesting noncollinear magnetic structures that may embed a variety of physical phenomena. In this work, magnetic and electrical transport properties of pure Ni2In-type structure Mn1 +xCu1−xGa (x = 0.3–0.5) alloys are investigated systematically. A cluster spin-glass phase is identified below the Curie temperature by magnetization and ac susceptibility measurements in x = 0.5 sample. Mediated by the exchange interaction between Mn-Mn clusters, significant exchange bias effect and coercivity are established, in which a giant exchange bias field of up to 0.323 T is observed in x = 0.5 sample. The exchange bias effect and coercivity, as well as the anomalous Hall effect are found to be greatly tuned via field cooling process. The large EB and coercivity identified in Mn–Cu–Ga alloys make them potentially applied in rare-earth-free magnets and spintronic devices.

Magnetic properties and magnetic structures in R3Ni2In4 (R = Tb–Tm)

The R 3 Ni 2 In 4 (R = Tb–Tm) ternary indides have been investigated by X-ray and neutron powder diffraction as well as magnetometric measurements. The compounds crystallize in the hexagonal Lu 3 Co 2 In 4 -type structure (a lower-symmetry derivative of the ZrNiAl-type one). Er 3 Ni 2 In 4 has been found paramagnetic down to 1.6 K while all other samples order antiferromagnetically below 23.3 K (R = Tb), 9.7 K (R = Dy), 3.3 K (R = Ho) and 3.2 K (R = Tm). The magnetic unit cell in R 3 Ni 2 In 4 (R = Tb, Dy) is doubled along the c-axis with respect to the crystal one (propagation vector k → = [ 0,0 , 1 2 ] ). The magnetic moments lie in the basal plane and form a “triangular” magnetic structure as often found in geometrically frustrated antiferromagnets. The magnetic order in Ho 3 Ni 2 In 4 consists of two components: a commensurate one which is described above ( k → = [ 0,0 , 1 2 ] ) and an incommensurate one ( k → = [ 1 3 , 1 3 , k z ] where k z = 0.271 ( 11 ) at 1.6 K). The magnetic moments of the incommensurate component are inclined at a small angle with respect to the basal plane and form a non-collinear magnetic structure. The magnetic structure in Tm 3 Ni 2 In 4 is a commensurate one ( k → = [ 1 3 , 1 3 , 1 2 ] ) with magnetic moments within the basal plane forming a non-collinear “triangular” structure. A determination of magnetic structures is supported by detailed symmetry analysis. • R 3 Ni 2 In 4 (R = Tb–Tm), except Er 3 Ni 2 In 4 , are found antiferromagnetic at low temperatures. • Er 3 Ni 2 In 4 remains paramagnetic down to 1.6 K. • Only rare earth atoms carry magnetic moments. • In most cases the magnetic moments are confined to the basal plane. • “Triangular” magnetic structures, typical of geometrical frustration, are observed.

Magnetic field tunable superconducting transition in Nb/Co/Py/Nb exchange spring multilayers

Over the last decade, it has been shown that magnetic non-collinearity at an s-wave superconductor/ferromagnet interface is a key ingredient for spin-singlet to spin-triplet pair conversion. This has been verified in several synthetic non-collinear magnetic structures. A magnetically soft and hard ferromagnetic layer combination in a bilayer structure can function as a field tunable non-collinear magnetic structure, which may offer magnetic field tunability of singlet-to-triplet pair conversion. From magnetization measurements of Nb/Co/Py/Nb multilayers, we demonstrate a reversible enhancement of the superconducting critical temperature of 400 mK by measuring Tc with and without a non-collinear magnetic structure between Co and Py. The sensitivity of Tc in these structures offers the potential for realizing magnetic field tunable Josephson junctions in which pair conversion and Josephson critical currents are controllable using modest magnetic fields.

Nuclear forward scattering application to the spiral magnetic structure study in ε−Fe2O3

The $\ensuremath{\epsilon}\ensuremath{-}{\text{Fe}}_{2}{\text{O}}_{3}$ magnetic structure has been analyzed using the synchrotron radiation source. Time spectra of nuclear forward scattering for isolated nanoparticles with an average size of 8 nm immobilized in a xerogel matrix have been recorded in the temperature range of $4--300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in applied magnetic fields of $0--4\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ in the longitudinal direction at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). It has been found that the external magnetic field does not qualitatively change the ${H}_{\mathrm{hf}}(\mathrm{T})$ behavior, but makes a strong opposite impact on the hyperfine fields in the nonequivalent iron sites, leading to the divergence of ${H}_{\mathrm{hf}}$ polar angle dependences below 80 K. A complete diagram of the $\ensuremath{\epsilon}\ensuremath{-}{\text{Fe}}_{2}{\text{O}}_{3}$ magnetic structure in the temperature range of $4--300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is proposed. At 300 K, the $\ensuremath{\epsilon}\ensuremath{-}{\text{Fe}}_{2}{\text{O}}_{3}$ compound is confirmed to be a collinear ferrimagnet. The experimental results show that the magnetic transition at $150--80\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ leads to the formation of a noncollinear magnetic structure. Furthermore, in the range of the 80--4 K, the ground state of a magnetic spiral is established. The experimental results are supplemented by the analysis of the exchange interactions and temperature dependence of the magnetization in a magnetic field of 7 T.

Noncollinear magnetic structure in U2Pd2In at high magnetic fields

We report an unexpected magnetic-field-driven magnetic structure in the 5f-electron Shastry- Sutherland system U2Pd2In. This phase develops at low temperatures from a noncollinear antiferromagnetic ground state above the critical field of 25.8 T applied along the a-axis. All U moments have a net magnetic moment in the direction of the applied field, described by a ferromagnetic propagation vector qF = (0 0 0) and an antiferromagnetic component described by a propagation vector qAF = (0 0.30 1/2 ) due to a modulation in the direction perpendicular to the applied field. We conclude that this surprising noncollinear magnetic structure is due to a competition between the single-ion anisotropy trying to keep moments, similar to the ground state, along the [110]-type directions, Dzyaloshinskii-Moryia interaction forcing them to be perpendicular to each other and application of the external magnetic field attempting to align them along the field direction.

Magnetic tunnel junctions: An efficient way for electrical skyrmion detection investigated by ab initio theory

In our proof-of-principle study we examine the influence of skyrmions on magnetoresistive transport. In particular, we show that magnetic tunnel junctions are a technologically appealing and promising way for electrical detection of non-collinear magnetic structures. The calculated effect is shown to originate from scattering between different k-states and cannot be identified through densities of states alone. Our results suggest that the detection efficiency strongly depends on the utilized materials.