Noncollinear Magnetic Structure Research Articles

De Haas-van Alphen effect, magnetization, magnetoresistance and magnetostriction of GdB4 as compared with TmB4

De Haas-van Alphen (dHvA) effect measurements were performed on GdB4 single crystal (R 300 K/R 4.2 K ≈ 12) in AFM state from 1.5 to 4.2 K in magnetic fields up to 14 T. 12 main branches of the oscillations were observed in the frequency range (0.1 ÷ 28) × 102 T. The angular dependence of the dHvA spectrum suggests a strongly anisotropic Fermi surface. Corresponding cyclotron masses m* c are light, ranging from 0.2 to 0.8m 0. We studied additionally the magnetization of GdB4, linear and volume magnetostriction, and magnetoresistance of GdB4 in comparison with the same properties of TmB4 single crystal above 1.5 K in the field of 0 ÷ 14 T. At 4.2 K and H || ⟨110⟩ collinear to the GdB4 easy magnetization axis the volume magnetostriction revealed the lattice contraction with the field rise excluding the range of H ≈ (11 ÷ 13) T, where the lattice expansion is accompanied by a sharp magnetization increase due to the destruction of its non-collinear magnetic structure. The magnetostricive behavior in GdB4 and TmB4 is different due to the CEF effect.

Antiferromagnet-ferromagnet transitions in cobaltites

Three series of oxygen-deficient cobaltites, La0.5Ba0.5CoO3−δ, LnBaCo2O5.5 and Sr2YCo4O10.5, have been studied. La0.5Ba0.5CoO3 is an insulating ferromagnet whereas La0.5Ba0.5CoO2.75 is a pure antiferromagnet in which the oxygen vacancies are disordered. The ordering of the oxygen vacancies leads to the appearance of a ferromagnetic component, apparently because of the formation of a noncollinear magnetic structure. The antiferromagnet-“ferromagnet” transition is accompanied by a giant magnetoresistance. It is suggested that in the ferromagnetic oxidized compounds, the Co3+ and Co4+ ions adopt intermediate spin states, whereas in the antiferromagnetic (Co4+-free) compositions, the Co3+ ions have a high-spin state (CoO5 pyramids) with a dominant low-spin state (CoO6 octahedra). In both the ferromagnetic and antiferromagnetic compounds, superexchange via oxygen plays an essential role in determining the magnetic properties.

Exchange coupling and helical spin order in the triangular lattice antiferromagnet CuCrO2 using first principles

The magnetic and electronic properties of the geometrically frustrated triangular antiferromagnet CuCrO2 are investigated by first principles through density functional theory calculations within the generalized gradient approximations (GGA)+U scheme. The spin exchange interactions up to the third nearest neighbours in the ab plane as well as the coupling between adjacent layers are calculated to examine the magnetism and spin frustration. It is found that CuCrO2 has a natural two-dimensional characteristic of the magnetic interaction. Using Monte—Carlo simulation, we obtain the Néel temperature to be 29.9 K, which accords well with the experimental value of 24 K. Based on non-collinear magnetic structure calculations, we verify that the incommensurate spiral-spin structure with (110) spiral plane is believable for the magnetic ground state, which is consistent with the experimental observations. Due to intra-layer geometric spin frustration, parallel helical-spin chains arise along the a, b, or a + b directions, each with a screw-rotation angle of about 120°. Our calculations of the density of states show that the spin frustration plays an important role in the change of d—p hybridization, while the spin-orbit coupling has a very limited influence on the electronic structure.

Structural chemistry and magnetic properties of R11M4In6 (R = Gd, Tb, Dy, Ho, Er, Y; M = Si, Ge) compounds

Crystal structure and magnetic properties of R11M4In6 compounds (R=Gd, Tb, Dy, Ho, Er, Y; M=Si, Ge) were investigated by means of X-ray diffraction and magnetometric measurements. The compounds crystallize in the tetragonal Sm11Ge4In6-type crystal structure (an ordered version of the Ho11Ge10-type; space group I4/mmm). Gd11Ge4In6, Tb11Ge4In6 and Tb11Si4In6 order antiferromagnetically at low temperatures. For the compounds with Dy, Ho and Er magnetic phase transitions to noncollinear magnetic structures with a ferromagnetic component were observed. The magnetocaloric effect, in terms of the isothermal magnetic entropy change ΔSm, was observed for Dy11Si4In6. Both yttrium compounds are Pauli paramagnets.

Emergent electrodynamics of skyrmions in a chiral magnet

When an electron moves in a smoothly varying non-collinear magnetic structure, its spin orientation adapts constantly, thereby inducing forces that act both on the magnetic structure and on the electron. These forces may be described by electric and magnetic fields of an emergent electrodynamics1,2,3,4. The topologically quantized winding number of so-called skyrmions—a type of magnetic whirl discovered recently in chiral magnets5,6,7—has been predicted to induce exactly one quantum of emergent magnetic flux per skyrmion. A moving skyrmion is therefore expected to induce an emergent electric field following Faraday’s law of induction, which inherits this topological quantization8. Here we report Hall-effect measurements that establish quantitatively the predicted emergent electrodynamics. We obtain quantitative evidence for the depinning of skyrmions from impurities (at current densities of only 106 A m−2) and their subsequent motion. The combination of exceptionally small current densities and simple transport measurements offers fundamental insights into the connection between the emergent and real electrodynamics of skyrmions in chiral magnets, and might, in the long term, be important for applications.

First-principles studies of complex magnetism in Mn nanostructures on the Fe(001) surface

The magnetic properties of Mn nanostructures on the Fe(001) surface have been studied using the noncollinear first-principles real space-linear muffin-tin orbital-atomic sphere approximation method within density-functional theory. We have considered a variety of nanostructures such as adsorbed wires, pyramids, and flat and intermixed clusters of sizes varying from two to nine atoms. Our calculations of interatomic exchange interactions reveal the long-range nature of exchange interactions between Mn-Mn and Mn-Fe atoms. We have found that the strong dependence of these interactions on the local environment, the magnetic frustration, and the effect of spin-orbit coupling lead to the possibility of realizing complex noncollinear magnetic structures such as helical spin spiral and half-skyrmion.

Magnetic properties of a GdMn6Sn6 single crystal

The magnetization of a GdMn6Sn6 single crystal has been measured in static magnetic fields up to 14T as well as in pulsed fields up to 60T. The easy magnetization direction has been confirmed to lie in the basal plane of the hexagonal crystal, the anisotropy within the plane being negligible. However, our data do not corroborate the earlier assertion that GdMn6Sn6 is a collinear ferrimagnet. This disagrees with the observed field dependence of magnetization along the easy direction as well as with the temperature dependence of spontaneous magnetization. A way out of the contradiction is to admit that GdMn6Sn6 has a more complex, non-collinear magnetic structure.

Preparation and characterization of nickel manganese ferrite

Arrays of Ni1-xMnxFe2O4 (x = 0.0, 0.25, 0.5, 0.75) nanowires with an average diameter of about 80 nm are prepared by porous anodic aluminum oxide membrane poured sol technique. X-ray diffraction analysis shows that the nickel manganese ferrites nanowires with cubic spinel structure are synthetized. Scanning electron microscopy and transmission electron microscope images indicate that the nanowire arrays are composed of prolate spheroids with different crystal orientations. Magnetic measurements show that saturation magnetization increases and then decreases with Mn increasing. The change is related to the location and the substitution of ion in spinel structure. Compared with of block material NiFe2O4, the saturation magnetization of nickel ferrite nanowire arrays is low. This is due to the fact that the noncollinear magnetic structure in nanowire arrays become predominant.

Balance of neutrons during their transport through magnetic noncollinear and noncoplanar layered systems

The violation of the detailed balance principle (DBP) for polarized neutrons during their mirror reflection and transport through a two-layered noncollinear magnetic structure in a magnetic field (noncoplanar system), and without it (noncollinear system), is revealed. Moreover, the existence of left-right non-polarized neutron transmission asymmetry for a noncoplanar layered system is uncovered.

Crystal and magnetic properties of Er 5Ni 2In 4 at low temperatures

The Er 5Ni 2In 4 compound belongs to a novel family of rare earth-based intermetallics crystallizing within the Lu 5Ni 2In 4-type structure ( Pbam Space group). In this paper we report results of structural as well as magnetic studies including X-ray diffraction (between 12 and 300 K); neutron diffraction (between 1.5 and 24 K), magnetometric and calorimetric data (in 2–300 K temperature range). The Er 3+ ions occupy three crystal positions within the unit cell: two 4g sites and the 2a site. According to results of neutron diffraction, the Er magnetic moments at 4g sites form a non-collinear magnetic structure below 18.5 K. Additional magnetic transition at about 2 K, originating in ordering of the 2a sublattice, occurs as well. In addition, magnetic entropy derived on the basis of specific heat studies is discussed yielding the doublet crystal field ground states for each of the sublattices.

Noncollinear magnetism at interfaces in iron-chromium alloys: The ground states and finite-temperature configurations

Noncollinear configurations of local magnetic moments at Fe/Cr interfaces in Fe-Cr alloys are explored using a combination of density functional theory (DFT) and magnetic cluster expansion (MCE) simulations. We show that magnetic frustration at Fe/Cr interfaces can be partially resolved through the formation of noncollinear magnetic structures, which occur not only at stepped but also at smooth interfaces, for example at the (110) interface where magnetic noncollinearity predicted by simulations is observed experimentally. Both DFT and MCE simulations predict that the magnetically frustrated (110) interface has the highest formation energy in the low-temperature limit. Using MCE and kinetic Monte Carlo simulations, we investigate the effect of temperature on magnetic order at interfaces and on interface energies. We find that while the low-temperature noncollinear bulk magnetic configurations of Cr remain stable up to the N\'eel temperature, the chromium atomic layers close to the interfaces retain their magnetic order well above this temperature. We also show that above the Curie temperature the (110) interface is the lowest energy interface, in agreement with DFT simulations of interfaces separating ferromagnetic Fe and nonmagnetic Cr.

Effects of interfacial noncollinear magnetic structures on spin-dependent conductance in Co2MnSi/MgO/Co2MnSi magnetic tunnel junctions: A first-principles study

We investigate the effects of spin-flip scattering on tunneling magnetoresistance (TMR) in magnetic tunnel junctions (MTJs) with half-metallic Co${}_{2}$MnSi (CMS) and MgO on the basis of the first-principles calculations. We found that noncollinear magnetic structures of interfacial Co spin moments resulting from the thermal fluctuations cause spin-flip scattering, leading to a significant reduction of the TMR. Interface states originating from a projection of the majority-spin ${\ensuremath{\Delta}}_{1}$ states of CMS in the minority-spin half-metallic gap because of the interfacial noncollinear magnetic structures play an important role in the spin-flip process. From these results, together with an estimated interfacial exchange stiffness constant, we conclude that the TMR ratio at room temperature in MTJs with half-metallic Co-based full-Heusler alloys can be attributed to the spin-flip scattering by the interfacial noncollinear magnetic structures as a result of the thermal fluctuation.

Geometric and magnetic properties of Pt clusters supported on graphene: Relativistic density-functional calculations

The geometric and magnetic structures of small Pt(n) clusters (n = 1 - 5) supported on a graphene layer have been investigated using ab initio density functional calculations including spin-orbit coupling. Pt-Pt interactions were found to be much stronger than the Pt-C interactions promoting the binding to the support. As a consequence, the equilibrium structure of the gas-phase clusters is preserved if they are deposited on graphene. However, the clusters bind to graphene only via at most two Pt-C bonds: A Pt(2) dumbbell prefers an upright position, the larger clusters are bound to graphene only via one edge of the planar cluster (Pt(3) and Pt(5)) or via two terminal Pt atoms of a bent Pt(4) rhombus. Evidently, the strong buckling of the graphene layer induced by the Pt-C bonds prevents the formation of a larger number of cluster-support bonds. As the local spin and orbital magnetic moments are quenched on the Pt atoms forming Pt-C bonds, the magnetic structure of the supported clusters is much more inhomogeneous as in the gas-phase. This leads to noncollinear magnetic structures and a strongly reduced magnetic anisotropy energy.

Crystal field and magnetic structure of UO2

The properties of UO$_{2}$ result from rich $f$-electron physics, including electronic Coulomb interactions, spin-orbit and crystal field effects, as well as inter-ionic multipolar coupling. We present a comprehensive theoretical study of the electronic structure of UO$_{2}$ using a combined application of self-consistent DFT+$U$ calculations and a model Hamiltonian. The $\Gamma_{5}$ ground state of U$^{4+}$ and the energies of crystal field excitations $\Gamma_{5} \rightarrow \Gamma_{3,4,1}$ are reproduced in very good agreement with experiment. We also investigate competing non-collinear magnetic structures and confirm 3-k as the T=0 K ground state magnetic structure of UO$_2$.

First-principles investigation of higher oxides of uranium and neptunium:U3O8and Np2O5

A computational study is presented of the structural, electronic, and magnetic properties of ${\mathrm{U}}_{3}$O${}_{8}$ and Np${}_{2}$O${}_{5}$, which are actinide oxides in a higher oxidation state than the tetravalent state of the common dioxide phases, UO${}_{2}$ and NpO${}_{2}$. The calculations are based on the density functional theory+$U$ approach, in which additional Coulomb correlations on the actinide atom are taken into account. The calculated properties of these two higher oxidized actinide oxides are analyzed and compared to those of their tetravalent analogs. The optimized structural parameters of these noncubic oxides are found to be in reasonable agreement with available experimental data. ${\mathrm{U}}_{3}$O${}_{8}$ is predicted to be a magnetic insulator, having one U atom in a hexavalent oxidation state and two U atoms in a pentavalent oxidation state. For Np${}_{2}$O${}_{5}$, which is also predicted to be an insulator, a complicated noncollinear magnetic structure is computed, leading to a nonzero overall magnetization with a slight antiferromagnetic canting. The calculated electronic structures are presented and the variation of the U $5f$ or Np $5f$--O $2p$ hybridization with the oxidation state is analyzed. With increasing oxygen content, the nearly localized 5$f$ electrons of the actinide elements are more positioned near the Fermi level and the hybridization between 5$f$ and 2$p$ states is markedly increased.

Noncollinear magnetic order in theS=12magnetSr3ZnRhO6

The compound ${\mathrm{Sr}}_{3}{\mathrm{ZnRhO}}_{6}$ with $S=1/2$ belongs to a family of pseudo-one-dimensional (1D) spin-chain systems with general formula ${\mathrm{A}}_{3}{\mathrm{A}}^{\ensuremath{'}}{\mathrm{BO}}_{6}$. The nature of the ground state of ${\mathrm{Sr}}_{3}{\mathrm{ZnRhO}}_{6}$ is investigated using microscopic techniques such as muon spin relaxation ($\ensuremath{\mu}$SR) and powder neutron diffraction in addition to bulk characterization using magnetization, ac-susceptibility, and heat capacity. Our $\ensuremath{\mu}$SR study clearly reveals the presence of three frequencies, below 16 K, whose temperature dependence follow a mean-field order parameter indicating long-range magnetic ordering of the ${\mathrm{Rh}}^{4+}$ moment. Powder neutron diffraction reveals the presence of four weak magnetic Bragg peaks below 16 K, indexed by the propagation vector $\mathbf{k}=(0,\frac{1}{2},1)$ in the hexagonal setting of the space group $R\stackrel{\underline{}}{3}c$. Analysis of the magnetic diffraction pattern constrained to symmetry-adapted magnetic modes reveals a noncollinear magnetic structure with an ordered magnetic moment of ${\mathrm{Rh}}^{4+}=0.7(1)$ ${\ensuremath{\mu}}_{B}$. This is the first compound in this spin chain family which exhibits noncollinear magnetic order with the moment tilting away from the $c$ axis, highlighting the important role of interchain and intrachain interactions.

Electronic structure of spiral magnetic manganite phases

An approach has been developed using the tight-binding and double-exchange Hamiltonian methods for calculating the E(k) spectrum of eg electrons in noncollinear (spiral) magnetic structures of R1 − xAxMnO3 (R = La, Pr, Nd, Sm; A = Ca, Sr, Ba) manganites. A magnetic structure in the form of a flat cycloid observed in TbMnO3 below 28 K has been considered for undoped manganites (x = 0).

Magnetic Properties of Non-Stoichiometric <i>R</i>Ni<sub>2</sub>Mn<sub>x</sub> (<i>R</i> = Tb, Dy) Compounds

Structure and magnetic properties of non-stoichiometric rare earth intermetallic compounds TbNi2Mnx and DyNi2Mnx x have been studied. It was found that for the compositions with x ≤ 1 the alloys retain the single-phase cubic structure despite the fact that the rare earth-to-3d metal ratio changes from 1:2 to 1:3. Magnetic measurements revealed that the Mn alloying leads to a substantial growth of the Curie temperature with a maximum at x ~ 0.5. At low temperatures the magnetization reversal of the Mn-containing samples is characterized by a considerable hysteresis caused by pinning of narrow domain walls by the structure defects. The observed experimental data are discussed within the assumption on a local distortion of the crystal electric field and the formation of a non-collinear magnetic structure of the rare earth sublattice.

Neutron diffraction study on the two-dimensional Ising systemKEr(MoO4)2

The magnetic properties of the two-dimensional Ising antiferromagnet KEr(MoO{sub 4}){sub 2} have been investigated below and above transition temperature T{sub N}{approx}0.95 K in zero field and in fields up to 6.5 T by means of elastic neutron-diffraction, heat-capacity, and magnetization measurements. The low-temperature signal recorded at 0.34 K by neutron diffraction is explained within a noncollinear magnetic structure model. However, additional contribution is also present when applying the external magnetic field along the c axis even at temperatures well above the magnetic transition temperature T{sub N}. Various explanations are discussed.

Special electronic structures of inverse spinels LiMVO4(M = Ni and Cu): a first-principles study

The special behaviours of a random distribution of ions in LiNiVO4 and noncollinear magnetic structures in LiCuVO4 are studied based on density functional theory. For LiNiVO4, the random distribution of Li/Ni ions is ascribed to the cubic ground-state structure of the compound. When Li and Ni ions are distributed alternately and homogeneously, an antiferromagnetic structure is found to be the ground state. Both on-site Coulomb interaction and spin-orbit coupling are found to be essential for LiCuVO4 to acquire the correct noncollinear magnetic structure. Semiconductor bands with a gap of 1.4 eV are obtained for the compound with the noncollinear magnetic structure. The reason for the spin on Cu2+ rotating only in the xy plane is discussed.