Conical Spin Order Research Articles

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.

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 ${P}_{\mathrm{disp}}$.

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.

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.

Conical order, magnetic compensation, and sign reversible exchange bias in spinel structured AB2O4 compounds: A Monte Carlo study

$A{B}_{2}{\mathrm{O}}_{4}$ are a special class of spinel structured compounds exhibiting unique multiferroic properties. Recent experimental observation confirms switchable polarization in $A{\text{Cr}}_{2}{\text{O}}_{4}$ $(A=\text{Co}, \mathrm{Mn}, \mathrm{Fe})$, arising out of noncollinear magnetic spin order. In this paper we unfold the microscopic origin behind such magnetic spin order, hysteresis, polarization, and the so-called magnetic compensation effect in $A{\text{Cr}}_{2}{\text{O}}_{4}$ $(A=\text{Co}, \mathrm{Mn}, \mathrm{Fe}, \mathrm{Ni})$ using Monte Carlo simulation. We have carefully constructed a set of exchange interaction parameters, which help to explain various experimental findings such as magnetization vs temperature $(T)$, conical stability, magnetic ordering, and polarization in a representative compound ${\text{CoCr}}_{2}{\text{O}}_{4}$. When substituted with Fe, ${\text{CoCr}}_{2}{\text{O}}_{4}$ shows few exotic phenomena such as sign reversible exchange bias effect and magnetic compensation. ${\text{CoMn}}_{2}{\text{O}}_{4}$ and ${\text{CoFe}}_{2}{\text{O}}_{4}$ are two other compounds in the spinel family where we found no conical spin order and polarization, and thus facilitate a distinct contrast compared to others.

Ni doping effect on the magnetic properties of polycrystalline Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12−xNixO22

The microstructure and magnetic properties of multiferrioc polycrystalline Y-type hexaferrite Ba0.5Sr1.5Zn2Fe12−xNixO22 (BSZFNO, x = 0, 0.05, 0.1, 0.2, 0.35 and 0.5) synthesized by the solid-state reaction method are investigated. It is found that the shrink of BSZFNO lattice constant and the increase of grain size appear after substituting Fe ion with Ni ion, and Ni doping depresses the formation of Z-type and M-type impurity phases. BSZFNO undergoes two main magnetic phase transitions. Near room temperature, the temperature of phase transition from proper screw spin phase to collinear ferromagnetic phase increases with Ni amount increasing, suggesting the enhancement of exchange interaction of Fe(4)–O–Fe(5). The magnetic phase transition at lower temperature is attributed to the spin transition from a longitudinal conical spin order to a proper screw spin order, whose transition temperature varies due to different magnetocrystalline anisotropy with introducing Ni into BSZFO lattices. Furthermore, Ni doping leads to the expansion of the range of field-driven magnetic phases, especially intermediate-II and intermediate-III helical spin phase. The saturation magnetization at 5 K increases with Ni doping and arrives at a maximum value with Ni content x = 0.2, finally decreases with x. The increasing of the pitch of spin helix in initial Ni-doping leads to the enhancement of the magnetization. However, the conical spin phase eventually collapses in higher-concentration Ni-doping.

Ni-substitution effect on the properties of Ba0.5Sr1.5Zn2-xNixFe12O22 powders

We consider the influence of the magnetic cation (Ni2+) substitution on the structural and magnetic properties of Ba0.5Sr1.5Zn2-xNixFe12O22 (x = 0.8, 1, 1.5) powders. The powders were synthesized by using the citric-acid sol–gel auto-combustion method. Their ac-magnetization was measured in an ac-magnetic field with an amplitude of 10Oe and a frequency of 1000 Hz to detect any magnetic phase transitions. A strong influence of the Ni-substitution on the magnetic properties was thus observed. A magnetic phase transition from a helicoidal to a ferrimagnetic spin order at 284 K was observed for the sample Ba0.5Sr1.5Zn0.5Ni1.5Fe12O22. The magnetic-phase transition from conical spin order to hellicoidal spin order was registered at 149 K and 177 K for x = 0.8 and x = 1, respectively. Triple hysteresis loops were observed for all samples at 4.2 K, which indicates the presence of two kinds of ferromagnetic states with different magnetization values.

Conical-to-ferromagnetic phase conversion induced by cation order–disorder transition in Hf1–xTixMnSb2

The relation between metal order-disorder and magnetic properties in the solid solution Hf1–xTixMnSb2 (0 ≤ x ≤ 1) with the NiAs structure is investigated. The synchrotron X-ray diffraction studies have revealed that two-dimensional (2D) metal-ordered structure in HfMnSb2 persists up to x = 0.15, though its degree of metal order decreases with x. Although the NiAs-like phase is not formed for 0.2 ≤ x ≤ 0.3, a metal disordered phase appears for x ≥ 0.4. A conical spin order is only found in the metal-ordered region and is fairly robust against partial cation disorder, while the completely metal-disordered region is a ferromagnetic state, providing firm evidence that the presence/absence of metal order essentially defines the ground state. Neutron depolarization measurement for HfMnSb2 suggests possible successive phase transitions.

Multiple conical spin order in spinel structure stabilized by magnetic anisotropy

Conical spin order, where the spin components along the conical axis form magnetization while the spiral parts induce ferroelectric polarization, possesses multiferroicity with inherent magnetoelectric coupling. A Monte Carlo simulation performed using a classical Heisenberg spinel (AB2O4) model reveals a multiple conical spin order, i.e., three modulations with different cone angles and wavelengths on A sites and two alternate B sites. The spin order not only exists as the ground state but also survives locally stably in a larger parameter region. The whole existence range can be effectively expanded by anisotropy to cover the cases of CoCr2O4 and MnCr2O4. The multiple conical spin order is well maintained and finely tuned by frustration and anisotropy over the whole existence range, and the magnetic and ferroelectric properties are influenced correspondingly.

HfMnSb2: A Metal‐Ordered NiAs‐type Pnictide with a Conical Spin Order

AbstractThe NiAs‐type structure is one of the most common structures in solids, but metal order has been almost exclusively limited to chalcogenides. The synthesis of HfMnSb2 is reported with a novel metal‐ordered NiAs‐type structure. HfMnSb2 undergoes a conical spin order below 270 K, in marked contrast to conventional magnetic order observed in NiAs‐type pnictides. We argue that the layered arrangement of Hf and Mn makes it a quasi 2D magnet, where the Mn layers with localized magnetic moments (Mn2+; S=5/2) can interact only through RKKY interactions, instead of metal–metal bonding that is otherwise dominant for typical NiAs‐type pnictides. This result suggests that controlling order–disorder in NiAs‐type pnictides enables a study of 2D‐to‐3D crossover behavior in itinerant magnetic system.

HfMnSb2 : A Metal-Ordered NiAs-type Pnictide with a Conical Spin Order.

The NiAs-type structure is one of the most common structures in solids, but metal order has been almost exclusively limited to chalcogenides. The synthesis of HfMnSb2 is reported with a novel metal-ordered NiAs-type structure. HfMnSb2 undergoes a conical spin order below 270 K, in marked contrast to conventional magnetic order observed in NiAs-type pnictides. We argue that the layered arrangement of Hf and Mn makes it a quasi 2D magnet, where the Mn layers with localized magnetic moments (Mn(2+) ; S=5/2) can interact only through RKKY interactions, instead of metal-metal bonding that is otherwise dominant for typical NiAs-type pnictides. This result suggests that controlling order-disorder in NiAs-type pnictides enables a study of 2D-to-3D crossover behavior in itinerant magnetic system.

Magnetoelectric effects in the skyrmion host material Cu2OSeO3.

Insulating helimagnetic Cu2OSeO3 shows sizeable magnetoelectric effects in its skyrmion phase. Using magnetization measurements, magneto-current analysis and dielectric spectroscopy, we provide a thorough investigation of magnetoelectric coupling, polarization and dielectric constants of the ordered magnetic and polar phases of single-crystalline Cu2OSeO3 in external magnetic fields up to 150 mT and at temperatures below 60 K. From these measurements we construct a detailed phase diagram. Especially, the skyrmion phase and the metamagnetic transition of helical to conical spin order are characterized in detail. Finally we address the question if there is any signature of polar order that can be switched by an external electric field, which would imply multiferroic behaviour of Cu2OSeO3.

Effects of aluminum substitution on the crystal structure and magnetic properties in Zn2Y-type hexaferrites

Crystal structure and magnetic properties of multiferroic Y-type hexaferrites Ba0.5Sr1.5Zn2(Fe1−xAlx)12O22 (x = 0, 0.04, 0.08, and 0.12) were investigated. The Z- and M-type impurity phases decrease with increasing Al content, and the pure phase samples can be obtained by modulating Al-doping. Lattice distortion exists in Al-doped samples due to the different radius of Al ion (0.535 Å) and Fe ion (0.645 Å). The microstructural morphologies show that the hexagonal shape grains can be observed in all the samples, and grain size decreases with increasing Al content. As for magnetic properties of Ba0.5Sr1.5Zn2(Fe1−xAlx)12O22, there exist rich thermal- and field-driven magnetic phase transitions. Temperature dependence of zero-field cooling magnetization curves from 5 K to 800 K exhibit three magnetic phase transitions involving conical spin phase, proper-screw spin phase, ferromagnetic phase, and paramagnetic phase, which can be found in all the samples. Furthermore, the phase-transition temperatures can be modulated by varying Al content. In addition, four kinds of typical hysteresis loops are observed in pure phase sample at different temperatures, which reveal different magnetization processes of above-motioned magnetic spin structures. Typically, triple hysteresis loops in low magnetic field range from 0 to 0.5 T can be observed at 5 K, which suggests low-field driven magnetic phase transitions from conical spin order to proper-screw spin order and further to ferrimagnetic spin order occur. Furthermore, the coercive field (HC) and the saturation magnetization (MS) enhance with increasing Al content from x = 0 to 0.08, and drop rapidly at x = 0.12, which could be attribute to that in initial Al-doped process the pitch of spin helix increases and therefore magnetization enhances, but conical spin phase eventually collapses in higher-concentration Al-doping.

Coupled ferroelectric polarization and magnetization in spinel FeCr2S4.

One of the core issues for multiferroicity is the strongly coupled ferroelectric polarization and magnetization, while so far most multiferroics have antiferromagnetic order with nearly zero magnetization. Magnetic spinel compounds with ferrimagnetic order may be alternative candidates offering large magnetization when ferroelectricity can be activated simultaneously. In this work, we investigate the ferroelectricity and magnetism of spinel FeCr2S4 in which the Fe2+ sublattice and Cr3+ sublattice are coupled in antiparallel alignment. Well defined ferroelectric transitions below the Fe2+ orbital ordering termperature Too = 8.5 K are demonstrated. The ferroelectric polarization has two components. One component arises mainly from the noncollinear conical spin order associated with the spin-orbit coupling, which is thus magnetic field sensitive. The other is probably attributed to the Jahn-Teller distortion induced lattice symmetry breaking, occuring below the orbital ordering of Fe2+. Furthermore, the coupled ferroelectric polarization and magnetization in response to magnetic field are observed. The present work suggests that spinel FeCr2S4 is a multiferroic offering both ferroelectricity and ferrimagnetism with large net magnetization.

Stable and locally stable conditions for a conical spin state in the spinel structure

A conical spin state generates the multiferroicity with both spontaneous magnetization and ferroelectric polarization, offering a great promise for the mutual control of magnetism and ferroelectricity. To clarify the stable and locally stable conditions for the conical spin order, a Monte Carlo simulation is performed on a three-dimensional spinel lattice with classical Heisenberg spins to explore the possible ground states in different parameter spaces. The simulation confirms the locally stable range of the conical spin state, which was suggested by the LKDM (Lyons, Kaplan, Dwight, and Menyuk) theory. Furthermore, it is revealed that the anisotropy plays an important role in stabilizing the conical spin order and expanding the parameter range of its existence, whereas the nearest-neighboring antiferromagnetic A-A exchange interaction enhances frustration and thereby suppresses the conical spin order. Thus, our simulation gives a necessary supplement and a reasonable improvement to the LKDM theory.

Gigantic terahertz magnetochromism via electromagnons in the hexaferrite magnet Ba2Mg2Fe12O22

Effects of temperature (6--225 K) and magnetic field (0--7 T) on the low-energy (1.2--5 meV) electrodynamics of the electromagnon, the magnetic resonance driven by the light electric field, have been investigated for a hexaferrite magnet Ba$_2$Mg$_2$Fe$_{12}$O$_{22}$ by using terahertz time-domain spectroscopy. We find the gigantic terahertz magnetochromism via electromagnons; the magnetochromic change, as defined by the difference of the absorption intensity with and without magnetic field, exceeds 500% even at 0.6 T. The results arise from the fact that the spectral intensity of the electromagnon critically depends on the magnetic structure. With changing the conical spin structures in terms of the conical angle $\theta$ from the proper screw ($\theta=0^\circ$) to the ferrimagnetic ($\theta=90^\circ$) through the conical spin-ordered phases ($0^\circ<\theta<90^\circ$) by external magnetic fields, we identify the maximal magnetochromism around $\theta\approx45^\circ$. On the contrary, there is no remarkable signature of the electromagnon in the proper screw and spin-collinear (ferrimagnetic) phases, clearly indicating the important role of the conical spin order to produce the magnetically-controllable electromagnons. The possible origin of this electromagnon is argued in terms of the exchange-striction mechanism.

The influence of magnetic anisotropy on magnetoelectric behavior in conical spin ordered multiferroic state

For the magnetism-driven multiferroic materials, the magnetic anisotropy plays an essential role in the magnetoelectric behavior. To understand the influence of magnetic anisotropy on multiferroic state resulting from the conical spin order, we have performed Monte Carlo simulation on a three-dimensional classical Heisenberg model in spinel lattice. The single-ion anisotropy from the easy-axis type to the easy-plane type is considered in the system, and the corresponding magnetoelectric behavior is investigated under a rotating external magnetic field (h). It is revealed that the magnetic anisotropy drags the orientation of conical spin structure slightly away from the direction of h, and distorts the conical spin structure as well. The balance between h and the anisotropy results in the anisotropic magnetoelectric properties during the rotation of h.

Multiferroicity and magnetoelectric coupling in polycrystalline Y-doped EuMnO 3

The effects of Y-doping on the magnetic, dielectric and ferroelectric properties of EuMnO 3 were carefully investigated through a series of polycrystalline Eu 1 − x Y x MnO 3 ( x = 0, 0.15, 0.25, 0.3 and 0.5) samples prepared by solid state reaction. The Y-doping reduces the A-site ionic size and then the Mn–O–Mn bond angle. It is postulated that upon the doping, the magnetic ground state firstly changes to a conical spin order at x = 0.15 and then a spiral spin order for higher doping. The ferroelectric transitions appear at the conical and spiral spin order transition points ∼ 25 K, and a polarization ∼ 90 μC/m 2 is achieved in sample with x = 0.25.