Conical Spin Research Articles

Spin Spiral State at a Ferromagnetic Gd Vacuum Interface.

Centrosymmetric bulk magnets made of layered Gd intermetallics had been discovered recently to exhibit helical spin spirals with a wavelength of ≈2 nm that transform into skyrmion lattices at certain magnetic fields. Here we report on the observation of a spin spiral state at the Gd(0001) surface. Spin-polarized scanning tunneling microscopy images show striped regions with a periodicity of about 2nm. These stripes rearrange upon application of an external magnetic field, thereby unambiguously confirming their magnetic origin. Density functional theory calculations explain that competing exchange interactions in the surface layer of Gd(0001) together with a magnetovolume fine-tuning of the exchange interaction to the next Gd layer favor a chiral 2nm conical spin spiral at the surface, arising as a general behavior of the Gd monolayer.

Dielectric, magnetic, and magnetodielectric change mechanisms in Y-type BaSrZn2Fe12O22 ceramics regulated by doping Al3+ and Ga3+ ions

We systematically investigated the dielectric, magnetic, and magnetodielectric (MD) properties of Y-type hexaferrite doped with Al3+ and Ga3+ ions. Al3+ ions preferentially enter the spin-up octahedral lattice sites while Ga3+ ions occupy the spin-down octahedral lattice sites first and then, enter the spin- octahedral sites with the increase in doping amount. The initial magnetization reaches saturation from outside of the hysteresis loops, exhibiting a non-collinear spin order. The step-like increase in magnetization with magnetic field indicates the ferroelectric changes in longitudinal conical spin order. The positive MD effect at low temperatures and the negative MD effect after warming for the doped samples are controlled by non-collinear spin ordering and electron hopping, respectively. The Al-doped and Ga-doped samples exhibit different ferroelectric behaviors driven by magnetic fields. Their ferroelectric phase diagrams were proposed on the basis of magnetic and MD properties. The results associate the physical properties with the doped ions in Y-type hexaferrite.

Effect of cation site occupancy on the structure and magnetic properties of SrCoxTixFe12–2xO19 thin films

Multiferroic materials have potential applications in multifunctional electronic devices, and M-type hexaferrite with conical spin order is a promising candidate. In this paper, a series of SrCoxTixFe12–2xO19 thin films were prepared by a sol-gel method, and x-dependent structure, cation site occupancy and magnetic properties were investigated to explore the origin of conical spin order in M-type hexaferrite. It was found that the introduction of Co2+ and Ti4+ ions decreases the uniaxial magnetocrystalline anisotropy and breaks the threefold symmetry of the lattice, inducing a conical magnetic structure. Moreover, the appearance temperature of the conical spin order (Tcone) increases first and then decreases with the increase of x, and the largest Tcone of about 324 K is obtained in SrCo1.8Ti1.8Fe8.4O19 thin film. The evolution of magnetization is closely related to the site occupancy of Co2+ and Ti4+ ions, and the net effective substitution ratio of spin-down Fe3+ is positive correlation to the Tcone of SrCoxTixFe12–2xO19 and thus considered to be responsible for the introduction of conical spin order. The results reveal the correlation between the structure, magnetic properties and cation site occupancy, pointing out an effective method to induce conical spin order in M-type hexaferrites.

Deformation theory of asymptotically conical Spin(7)-instantons

We develop the deformation theory of instantons on asymptotically conical Spin(7)-manifolds where the instanton is asymptotic to a fixed nearly G2-instanton at infinity. By relating the deformation complex with spinors, we identify the space of infinitesimal deformations with the kernel of the twisted negative Dirac operator on the asymptotically conical Spin(7)-manifold.Finally we apply this theory to describe the deformations of Fairlie–Nuyts–Fubini–Nicolai Spin(7)-instantons on R8, where R8 is considered to be an asymptotically conical Spin(7)-manifold asymptotic to the cone over S7. We calculate the virtual dimension of the moduli space using Atiyah–Patodi–Singer index theorem and the spectrum of the twisted Dirac operator.

Deformations of asymptotically conical Spin(7)-manifolds

Deformations of asymptotically conical Spin(7)-manifolds

Magnon squeezing in conical spin spirals

Magnon squeezing in conical spin spirals

Evidence for a conical spin spiral state in the Mn triple layer on W(001): Spin-polarized scanning tunneling microscopy and first-principles calculations

Evidence for a conical spin spiral state in the Mn triple layer on W(001): Spin-polarized scanning tunneling microscopy and first-principles calculations

Improved room-temperature magnetoelectric coupling properties for multiferroic Z-type hexaferrites

Here we present the results of a systematic investigation of the effect of Ba substitution on the spiral magnetic structure and magnetoelectric effect in polycrystalline BaxSr3−xCo2Fe24O41 (x = 0.0, 0.1, 0.3, 0.5, and 0.7). The magnetoelectric coupling coefficient was improved from 455 ps/m (x = 0.0) to 624 ps/m (x = 0.3). Furthermore, the magnetic field range at which the magnetoelectric signal appears extended from 0-∼0.4 T (x = 0.0) to 0-∼0.9 T (x = 0.5), demonstrating that the transverse conical spin structure was significantly strengthened by Ba substitution. These improvements are attributed to the decrease of Fe-O-Fe bond angle near the L and S blocks boundary, the reduction of cone angle in the transverse conical spin structure, and the change in the distribution of magnetic Co ions caused by the substitution of large Ba ions for small Sr ions. In general, we achieved the improved magnetoelectric coupling strength in polycrystalline Z-type hexaferrite for the first time.

Magnetoelectric coupling induced by Jahn–Teller Cu2+ in SrFe12O19 ceramics

Realizing conical spin order and magnetoelectric coupling by Jahn–Teller Cu2+ in SrFe12O19 ceramics.

Enhanced magnetoelectric effect by spin-orbit coupling in M-type hexaferrite

Magnetoelectric (ME) effect triggered by spiral spin orders in various hexaferrites develops both the theory and application of ME coupling. However, the strength of such ME effect is restricted by the small spin-orbit coupling (SOC) of $3d$ electrons in M-type hexaferrite. Here, we introduce the $5d$ element Ir with large SOC into $\mathrm{Sr}{\mathrm{Fe}}_{12}{\mathrm{O}}_{19}$. It is found that a low ${\mathrm{Ir}}^{4+}\text{\ensuremath{-}}\mathrm{doping}$ level (about 4.2% of total ${\mathrm{Fe}}^{3+}$ ions) can induce conical spin order in $\mathrm{Sr}{\mathrm{Mg}}_{x}{\mathrm{Ir}}_{x}{\mathrm{Fe}}_{12\ensuremath{-}2x}{\mathrm{O}}_{19}$ ceramics, mainly because ${\mathrm{Ir}}^{4+}$ ions prefer to enter a crucial $4{f}_{2}$ magnetic-interacted site and thus strengthen the local Dzyaloshinskii-Moriya interaction. Spin-order-induced polarization $({P}_{\mathrm{spin}})$ is achieved and displays a synchronous temperature-dependent behavior with the displacive polarization $({P}_{\mathrm{disp}})$. This interesting ME effect is discussed via $d\text{\ensuremath{-}}p$ hybridization emerging at $\mathrm{Fe}{\mathrm{O}}_{5}$ trigonal bipyramids. Moreover, ${P}_{\mathrm{spin}}$ response decreases step by step during the periodical ME test, reflecting the controllable helicity and the possible multilevel states. These results demonstrate the impact of the $5d$ element with strong SOC on the formation of conical spin order and the enhancement of ME effect in M-type hexaferrite, and also reveal the interconnectedness between ${P}_{\mathrm{spin}}$ and ${P}_{\mathrm{disp}}$.

Chiral surface spin textures in Cu2OSeO3 unveiled by soft X-ray scattering in specular reflection geometry

ABSTRACT Resonant elastic soft X-ray magnetic scattering (XRMS) is a powerful tool to explore long-periodic spin textures in single crystals. However, due to the limited momentum transfer range imposed by long wavelengths of photons in the soft x-ray region, Bragg diffraction is restricted to crystals with the large lattice parameters. Alternatively, small-angle X-ray scattering has been involved in the soft energy X-ray range which, however, brings in difficulties with the sample preparation that involves focused ion beam milling to thin down the crystal to below a few hundred nm thickness. We show how to circumvent these restrictions using XRMS in specular reflection from a sub-nanometer smooth crystal surface. The method allows observing diffraction peaks from the helical and conical spin modulations at the surface of a Cu 2 OSeO 3 single crystal and probing their corresponding chirality as contributions to the dichroic scattered intensity. The results suggest a promising way to carry out XRMS studies on a plethora of noncentrosymmetric systems hitherto unexplored with soft X-rays due to the absence of the commensurate Bragg peaks in the available momentum transfer range.

Observation of magnetic field-induced second magnetic ordering and peculiar ferroelectric polarization in L-type ferrimagnetic Fe2(MoO4)3

${\mathrm{Fe}}_{2}{({\mathrm{MoO}}_{4})}_{3}$ was synthesized and characterized as a prototype of L-type ferrimagnets (L-FiM) with an ordering temperature ${T}_{\mathrm{N}1}\ensuremath{\sim}12\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ using magnetic susceptibility $(\ensuremath{\chi})$, specific heat $({C}_{\mathrm{p}})$, and dielectric $({\ensuremath{\varepsilon}}^{\ensuremath{'}})$ anomaly. Two remarkable findings are magnetic field $(H)$ induced (i) an additional magnetic phase transition at ${T}_{\mathrm{N}2}$ below ${T}_{\mathrm{N}1}$; and (ii) the emergence of flexible ferroelectric polarization $(P)$ with the tuning parameters $H$ and $T$ below ${T}_{\mathrm{N}2}$. Thus, the H-T phase diagram for spin-induced type-II multiferroics was established. In contrast to some known multiferroics with a critical field-induced spin-flip $P$ below ${T}_{\mathrm{N}1}$, the observed multiferroic nature is exotic. The origin of the profound multiferroic nature may be hidden in the complex $T$-and $H$-dependent spin and lattice structures. Consequently, the schematic picture of $H$-induced possible change of lattice symmetry and conical spin structure has been proposed. More experimental and theoretical works providing solid evidence and interpretations have been suggested to explore these peculiar multiferroic phenomena.

Chiral magnetic structure of Y-type Ba0.6Sr1.4Zn2Fe12O22 hexaferrite in intermediate phase

Chiral magnetic structure of Y-type Ba0.6Sr1.4Zn2Fe12O22 hexaferrite in intermediate phase. / G. Khazaradze, A. Shengelaya. / Nano Studies. – 2020. – # 20. – pp. 131-136. – eng. Magnetic structure in intermediate-II and intermediate-III phases of Y-type hexaferrite single crystal Ba0.6Sr1.4Zn2Fe12O22 was investigated using neutron diffraction with polarization analysis. It was found that both phases have chiral magnetic structures. This shows that in contrast to previous assumptions, intermediate-II phase has transverse conical spin structure similar to the intermediate-III phase. Therefore, magnetic field induced ferroelectric state may exist also in the intermediate-II phase of Y-type Ba0.6Sr1.4Zn2Fe12O22 hexaferrite. Obtained results provide important insight into the recent observations of room temperature magnetoelectric coupling in Y-type Ba0.6Sr1.4Zn2Fe12O22 hexaferrite. Fig. 3, Ref. 12.

Effect of cation substitutions in Y-type Ba0.5Sr1.5Me2Fe12O22 hexaferrites on the magnetic phase transitions

We investigated the magnetic properties and magnetic phase transition in Y-type Ba0.5Sr1.5NiMgFe12O22 hexaferrite powder prepared by citrate sol-gel spontaneous combustion. The saturation magnetisation value of 32 emu/g at 4.2 K was lowered to 24 emu/g at 300 K. The magnetisations curves did not saturate even at a magnetic field of 50 kOe for both temperatures - 4.2 K and 300 K. A step-like behaviour appeared in the initial magnetisation curve at 4.2 K. A magnetic phase transformation from a spiral magnetic ordering to a conical spin one was observed at 40 K.

Strong anisotropic Hall effect in single-crystalline CeMn2Ge2 with helical spin order

The topological Hall effect (THE) is an emergent magnetotransport phenomena in noncollinear itinerant magnets, and has been extensively studied in magnetic materials hosting skyrmion lattices and geometrically frustrated lattices. To explore THE in materials with new nontrivial spin textures, we investigate the anisotropic Hall effect in a longitudinal conical-ordered magnet ${\mathrm{CeMn}}_{2}{\mathrm{Ge}}_{2}$ crystallized in ${\mathrm{ThCr}}_{2}{\mathrm{Si}}_{2}$-type structure, which has a spiral magnetic propagation vector along the $c$ axis and a helical component in the $ab$ plane. Magnetic susceptibility reveals it undergoes two magnetic transitions at \ensuremath{\sim}394 and \ensuremath{\sim}318 K; the latter corresponds to the antiferromagnetic to conical spin order transition in the magnetic Mn lattices. For applied field $({\ensuremath{\mu}}_{0}H)$ along the $c$ axis, ${\mathrm{CeMn}}_{2}{\mathrm{Ge}}_{2}$ exhibits conventional anomalous Hall effect with large intrinsic Hall conductivity ${\ensuremath{\sigma}}_{xy}^{\mathrm{int}}\ensuremath{\sim}276\phantom{\rule{0.28em}{0ex}}{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ but no THE responses. A large THE is realized as ${\ensuremath{\mu}}_{0}H$ is oriented within the $ab$ plane due to a field-induced noncoplanar spin texture with scalar spin chirality. Moreover, room temperature (RT) angle-dependent Hall effect measurements reveal the THE emerges when the field is applied nonparallel to the $c$ axis and it reaches maximal value for ${\ensuremath{\mu}}_{0}H$ when it is tilted close to the $ab$ plane with a canting angle of \ensuremath{\sim}15\ifmmode^\circ\else\textdegree\fi{}, at which large topological Hall conductivity of $\ensuremath{\sim}43\phantom{\rule{0.28em}{0ex}}{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ is detected. Our results demonstrate a RT anisotropic THE and its tunability by the application of a tilted magnetic field in conical-ordered magnets, making a step forward for designing spintronic devices based on THE.

Noncollinear spin state and unusual magnetoresistance in ferrimagnet Co-Gd

Rare-earth transition-metal ferrimagnetic materials are promising candidates for next-generation spintronic devices owing to their small magnetic moments and fast dynamics. However, only a few studies have been conducted thus far on the transport properties of noncollinear ferrimagnetic spin textures. In this study, we investigate the transverse and longitudinal resistances in a ferrimagnet/heavy-metal heterostructure composed of Co-Gd and Pt. The transition-metal sublattice arrangement and magnetic field--temperature phase diagram are obtained based on unconventional hysteresis loops observed near the compensation point. In the collinear regime, the spin Hall magnetoresistance (SMR) and anisotropic magnetoresistance (AMR) exhibit normal oscillation curves. In the noncollinear regime, the SMR and AMR signals are inverted for the existence of a 90\ifmmode^\circ\else\textdegree\fi{} phase shift between magnetization and magnetic fields. Owing to the conical spin textures in the spin-flop state, sharp peaks emerge when the field is perpendicular to the film plane. The abnormal magnetoresistance in our results can be used to analyze complex magnetic structures, which plays an important role in the all-electrical readout of noncollinear spintronics devices.

Structural, microstructural and temperature dependent magnetic properties of Mg–Ni doped CoCr2O4 ceramics

In the present work, [Co(1-(x + y)) MgxNiy][Cr2]O4 (CMNCr)(where, x = y = 0, 0.10, 0.20) NPs are examine in detail by using experimental results. Solution combustion method is adopted to prepare the CMNCr NPs. Experimental results were studied to understand structure (using XRD), microstructure (using SEM) and magnetic behaviour (using SQUID). The structural analysis was confirmed a single-phase spinel cubic structure with singular structural distortions associated with Mg–Ni doping and the generation of metal vacancies. SEM micrographs of all samples reveal their highly porous nature. EDS was utilized to examine the elemental analysis of the samples. The formation of spinel cubic structure without impurity were also confirmed by Fourier-transform infrared (FTIR) spectroscopy and the same tool is used to investigate the absorption bands related to the A-site and B-site. For all samples, paramagnetic phase to ferrimagnetic phase transition observed at TC and conical spiral spin order transition observed at TS. The curie temperature is found to be 98 K, 89 K, and 85 K for [Co][Cr2]O4, [Co0.8 Mg0.1Ni0.1][Cr2]O4 and [Co0.6 Mg0.2Ni0.2][Cr2]O4 samples, respectively. The spiral transition temperature is found to be 26 K, 14 K and 8 K for [Co][Cr2]O4, [Co0.8 Mg0.1Ni0.1][Cr2]O4 and [Co0.6 Mg0.2Ni0.2][Cr2]O4 samples, respectively. All of the samples exhibit paramagnetic behaviour above TC. The negative magnetization effect were observed in [Co0.6 Mg0.2Ni0.2][Cr2]O4 NPs. 20% of Mg and 20% of Ni substitution to Co site in [Co][Cr2]O4 leads to the evident magnetization reversal at the compensation temperature with an applied magnetic field of 100 Oe. The M − H loop at 10 K exhibits large coercivity due to the spiral spin ordering. This phenomenon of negative magnetization in the material results in a stable state of magnetization in the material, which has significant applications in the field of magnetic storage devices.

Unveiling the shape-selective CoCr2-yScyO4 nanomagnetism

In this study CoCr2-yScyO4 (y = 0, 0.01, 0.02, and 0.03) nanoparticles (NPs) have been synthesized following the solution combustion method and characterized by combining experimental techniques with high-throughput Density Functional Theory calculations. The structural analysis confirmed the crystalline nature with spinel cubic structure exhibiting an average particle size between 7 and 10 nm. The morphological analysis confirmed the exposure of the (220), (311), (400), and (422) planes in agreement with theoretical results based on Wulff Construction. Magnetic analysis indicated the existence of paramagnetic to ferrimagnetic phase transition at the critical temperature (Tc) and a conical spiral spin phase was found at the spiral transition temperature (Ts). In both the cases, the transition temperature was seen to decrease with increase in the Sc content in agreement with the exchange-coupling constants calculated by DFT, where a singular magneto-structural behavior was observed. Theoretical results for shape-oriented magnetic properties indicate the anisotropic spin density distribution and surface exposure, providing a general picture of shape-selective control of nanomagnetism.

Ferroelectric polarization reversal in multiferroic MnWO4 via a rotating magnetic field up to 52 T

A long-standing issue in multiferroic ${\mathrm{MnWO}}_{4}$ is the electric polarization reversal when magnetic field is applied along the magnetic easy axis $(H||u)$. In this work, we performed comprehensive ferroelectric polarization measurements on ${\mathrm{MnWO}}_{4}$ in fields up to 52 T and with rotating field direction in the $ac$ plane. When the field is rotated from $H||c$ to $H||u$, the induced high-field phase $(\ensuremath{-}P||b)$ favors a sign reversal relative to the low-field $+P||b$ phase, which is attributed to the vector spin chirality changing from one side of the spin connecting vector to the other. Particularly, further slight deviation of the field from the magnetic easy axis towards $H||a$ causes an abrupt reversal of the high-field phase from $\ensuremath{-}P||b$ to $+P||b$. We assume that this unusual angular dependence of polarization behavior can be understood by magnetic control of the helicity of the chiral plane with opposite conical spin structures. In addition, we also propose electric-field control of the chiral plane close to the ferroelectric phase transition at high fields. This study results in an overall understanding of this important multiferroic material and sheds light on field-induced polarization switching in related compounds.

Room-temperature multiferroic properties of Al-doped hexaferrites sintered at high oxygen atmospheric concentrations

The room-temperature magnetoelectric effects of Al-doped polycrystalline hexaferrites Sr3Co2(Fe1-xAlx)24O41(x = 0.00, 0.02, and 0.04) sintered under high oxygen atmospheric concentrations were comprehensively investigated. The magnetic phases and magnetic structures were modulated by high sintering oxygen atmospheric concentrations as demonstrated by M-T tests and magnetic hysteresis loops. Compared with previous specimens sintered under low oxygen atmospheric concentrations, the transverse conical spin structures of the Sr3Co2(Fe1-xAlx)24O41(x = 0.00, 0.02, and 0.04) were obviously strengthened by high sintering oxygen atmospheric concentrations, exhibiting magnetoelectric effects in high magnetic field ranges (>1 T).