Spiral Antiferromagnets Research Articles

Structural, magnetic, dielectric and $$^{57}\text {Fe}$$ Mössbauer spectroscopic studies on $$\text {Fe}_{1-x}\text {Ce}_x\text {VO}_4$$: a type-II multiferroic material

Here, we report detailed studies on magnetic phase transitions and magneto-electric (ME) coupling in $$\text {Fe}_{1-x}\text {Ce}_x\text {VO}_4$$ . X-ray diffraction (XRD) and Raman spectroscopic measurements confirm triclinic crystal structure (P-1) with small variation in lattice parameters and Ce incorporation into the $$\text {FeVO}_4$$ lattice up to 10% of Ce. The presence of local lattice distortions and electronic inductive effect in $$\text {FeVO}_4$$ between $$\text {Fe}^{3+}$$ and $$\text {V}^{5+}$$ ions were found in X-ray absorption near edge structure (XANES) studies due to the presence of $$\text {Fe}^{3+}$$ –O– $$\text {V}^{5+}$$ linkages. Two antiferromagnetic (AFM) transitions similar to $$\text {FeVO}_4$$ appear at 21.86 K and 16.03 K, and Ce doping has little effect on the magnetic transitions. $$^{57}\text {Fe}$$ Mossbauer spectroscopic results show the invariance of Fe valance. Low temperature high magnetic field Mossbauer data depicts the presence of spiral AFM order in all the samples. A sharp peak at 16.0 K in dielectric permittivity with minimal suppression due to applied magnetic field is observed indicating the presence of ferroelectricity and magneto-dielectric coupling. Magnetic and ME transitions are almost robust against Ce doping.

Superconducting Spin Valves Based on Spiral Magnets

Abstract—Structures with spiral antiferromagnets are proposed as superconducting memory elements. The first structure consists of one superconducting layer and one magnetic layer with a spiral magnetization. The latter is a Josephson junction with spiral magnet as a weak link. The schematic diagram of switching between logic states in proposed structures is described.

Increase in the Critical Temperature of the Superconducting Transition of a Hybrid Structure upon the Magnetization of Spiral Antiferromagnets

The spin-valve effect in superconductor hybrid structures based on magnetic materials with a noncollinear magnetic ordering is described theoretically. It is shown that the following effect is possible in such structures: when the sample is magnetized, the critical temperature of the superconducting transition increases. The origin is the suppression of long-range triplet superconducting correlations. Such structures can be used as newtype memory elements for superconducting spintronics.

A mean-field approach to Kondo-attractive-Hubbard model

With the purpose of investigating coexistence between magnetic order and superconductivity, we consider a model in which conduction electrons interact with each other, via an attractive Hubbard on-site coupling U, and with local moments on every site, via a Kondo-like coupling, J. The model is solved on a simple cubic lattice through a Hartree–Fock approximation, within a ‘semi-classical’ framework which allows spiral magnetic modes to be stabilized. For a fixed electronic density, nc, the small J region of the ground state (T = 0) phase diagram displays spiral antiferromagnetic (SAFM) states for small U. Upon increasing U, a state with coexistence between superconductivity (SC) and SAFM sets in; further increase in U turns the spiral mode into a Néel antiferromagnet. The large J region is a (singlet) Kondo phase. At finite temperatures, and in the region of coexistence, thermal fluctuations suppress the different ordered phases in succession: the SAFM phase at lower temperatures and SC at higher temperatures; also, reentrant behaviour is found to be induced by temperature. Our results provide a qualitative description of the competition between local moment magnetism and superconductivity in the borocarbides family.

Сверхпроводящие спиновые вентили на основе спиральных магнетиков

Abstract —Structures with spiral antiferromagnets are proposed as superconducting memory elements. The first structure consists of one superconducting layer and one magnetic layer with a spiral magnetization. The latter is a Josephson junction with spiral magnet as a weak link. The schematic diagram of switching between logic states in proposed structures is described.

Baromagnetic Effect in the Hexagonal Mn3Sn System

The magnetic behaviors influenced by the hydrostatic pressure in the polycrystalline hexagonal Mn3Sn intermetallic compound are briefly reported. The crystalline structure of Mn3Sn is determined to be hexagonal with lattice parameters $a = b = 5.674$ (6) A and $c = 4.533$ (6) A by means of the powder X-ray diffraction and the subsequent Rietveld refinement. Two distinct transitions can be observed from the M(T) curves in the range of 150-450 K. During the cooling process, Mn3Sn sample first changes into a triangular antiferromagnetic (AFM) state with weak ferromagnetism at the Neel temperature of ~412 K, and then, it further transforms to a spiral AFM spin arrangement over a broad temperature range from 260 to 180 K at ambient pressure. With the application or the increase of hydrostatic pressures, this triangular to spiral AFM transition occurs at higher temperatures, while the magnetization of triangular AFM phase increases significantly. Such a phenomenon can be explained by the fact that the hydrostatic pressure compresses the sample and shortens the Mn–Mn separation, and thus, the moments of Mn will deviate from their ideal 120° configuration to their easy axis in the basal plane. The variations of transition temperature and maximum magnetization with applied hydrostatic pressure confirm the incremental instability of the triangular AFM arrangement under pressure.

Resistivity study of Dy0.93Y0.07 alloy

The electrical resistivity measurements of the polycrystalline rare earth alloy Dy0.93Y007 in the temperature range 2-300K at different magnetic fields are carried out. Two transitions from paramagnetic (PM) to spiral antiferromagnetic (S-AFM) and S-AFM to ferromagnetic (FM) states are clearly seen in the zero field resistivity data. The various contributions to the electrical resistivity of Dy and Dy093Y0 07 in three different magnetic states are studied. In the ferromagnetic region, the spin wave excitation energy term increases on decreasing the temperature due the enhanced magnetic anisotropy at lower temperatures.

Structural and magnetic properties of MnCo1−xFexSi alloys

The crystal structures, martensitic structural transitions and magnetic properties of MnCo1−xFexSi (0≤x≤0.50) alloys were studied by differential scanning calorimetry (DSC), X-ray powder diffraction (XRD) and magnetic measurements. In high-temperature paramagnetic state, the alloys undergo a martensitic structural transitions from the Ni2In-type hexagonal parent phase to the TiNiSi-type orthorhombic martensite. Both the martensitic transition temperature (TM) and Curie temperatures of martensite (TCM) decrease with increasing Fe content. The introduced Fe atoms establish ferromagnetic (FM) coupling within Fe-6Mn atom configurations and destroy the double spiral antiferromagnetic (AFM) coupling in MnCoSi compound, resulting in a magnetic change in the martensite phase from a spiral AFM state to an FM state. For the alloys with x=0.10, 0.15 and 0.20, a metamagnetic transition was observed in between the two magnetic states. A magnetostructural phase diagram of MnCo1−xFexSi (0≤x≤0.50) alloys was proposed.

Broken discrete and continuous symmetries in two-dimensional spiral antiferromagnets

We study the occurrence of symmetry breaking, at zero and finite temperatures, in the J1–J3 antiferromagnetic Heisenberg model on the square lattice using Schwinger boson mean field theory. For spin- the ground state always breaks the SU(2) symmetry with a continuous quasi-critical transition at J3/J1 ∼ 0.38, from Néel to spiral long range order, although local spin fluctuation considerations suggest an intermediate disordered regime around 0.35 ≲ J3/J1 ≲ 0.5, in qualitative agreement with recent numerical results. At low temperatures we find a Z2 broken symmetry region with short range spiral order characterized by an Ising-like nematic order parameter that compares qualitatively well with classical Monte Carlo results. At intermediate temperatures the phase diagram shows regions with collinear short range orders: for J3/J1 < 1 Néel (π,π) correlations and for J3/J1 > 1 a novel phase consisting of four decoupled third neighbour sublattices with Néel (π,π) correlations in each one. We conclude that the effect of quantum and thermal fluctuations is to favour collinear correlations even in the strongly frustrated regime.

Jamming Behavior of Domains in a Spiral Antiferromagnetic System

Using resonant magnetic x-ray photon correlation spectroscopy, we show that the domains of a spiral antiferromagnet enter a jammed state at the onset of long-range order. We find that the slow thermal fluctuations of the domain walls exhibit a compressed exponential relaxation with an exponent of 1.5 found in a wide variety of solidlike jammed systems and can be qualitatively explained in terms of stress release in a stressed network. As the temperature decreases, the energy barrier for fluctuations becomes large enough to arrest further domain wall fluctuations, and the domains freeze into a spatial configuration within 10K of the Néel temperature. The relaxation times can be fitted with the Vogel-Fulcher law as observed in polymers, glasses, and colloids, thereby indicating that the dynamics of domain walls in an ordered antiferromagnet exhibit some of the universal features associated with jamming behavior.

Colloquium: Spontaneous magnon decays

A theoretical overview of the phenomenon of spontaneous magnon decays in quantum antiferromagnets is presented. The intrinsic zero-temperature damping of magnons in quantum spin systems is a fascinating many-body effect, which has recently attracted significant attention in view of its possible observation in neutron-scattering experiments. An introduction to the theory of magnon interactions and a discussion of necessary symmetry and kinematic conditions for spontaneous decays are provided. Various parallels with the decays of anharmonic phonons and excitations in superfluid 4He are extensively used. Three principal cases of spontaneous magnon decays are considered: field-induced decays in Heisenberg antiferromagnets, zero-field decays in spiral antiferromagnets, and triplon decays in quantum-disordered magnets. Analytical results are compared with available numerical data and prospective materials for experimental observation of the decay-related effects are briefly discussed.

Magnetic structure of the quasi-one-dimensional frustrated antiferromagnet LiCu2O2 with Spin S = 1/2

The magnetic properties of LiCu2O2 single-crystal samples without twinning are investigated using electron spin resonance and nuclear magnetic resonance spectroscopy. The experimental results obtained are described in terms of the model of a planar spiral antiferromagnet for the orientation of the magnetic field H ‖ b or H ‖ c and the model of a collinear spin-modulated antiferromagnet for the orientation of the static magnetic field H ‖ a.

Magnetic property of Ba 2CoGe 2O 7

A two-dimensional antiferromagnet (S= 3 2 ) , Ba 2CoGe 2O 7, was studied, which is isostructural to S=1/2 spiral antiferromagnet, Ba 2CuGe 2O 7. A single crystal of Ba 2CoGe 2O 7 was grown and its magnetization was measured. A weak ferromagnetism was observed at 6.7 K , while the spiral antiferromagnetism was reported in Ba 2CuGe 2O 7. The origin of the ordered states in these isostructural materials are Dzyaloshinskii–Moriya interaction and the change of the direction of D vector results in the different spin configurations. We have also synthesized polycrystalline samples of Ba 2Cu 1− x Co x Ge 2O 7, and observed the change of the ordered state with x.

Intermediate phase in the spiral antiferromagnetBa2CuGe2O7

The magnetic compound Ba_2CuGe_2O_7 has recently been shown to be an essentially two-dimensional spiral antiferromagnet that exhibits an incommensurate-to-commensurate phase transition when a magnetic field applied along the c-axis exceeds a certain critical value H_c. The T=0 dynamics is described here in terms of a continuum field theory in the form of a nonlinear sigma model. We are thus in a position to carry out a complete calculation of the low-energy magnon spectrum for any strength of the applied field throughout the phase transition. In particular, our spin-wave analysis reveals field-induced instabilities at two distinct critical fields H_1 and H_2 such that H_1 < H_c < H_2. Hence we predict the existence of an intermediate phase whose detailed nature is also studied to some extent in the present paper.

Magnetic Phase Transitions in Substituted Spinels of Zn1−x Cu χ Cr2 Se4; 0.0 ≤ x ≤ 0.3

Magnetic and crystallographic properties have been studied by neutron powder diffraction and measurements of magnetization and magnetization hysteresis-loops for substituted spinels of Zn1−xCuxCr2Se4 with 0.0≤x≤0.3. It is found that the Zn0.85Cu0.15Cr2Se4 spinel has two magnetic phase transitions at 23.0 K (Néel temperature; T N) and 410 K (Curie temperature; T C) and that the Zn0.70Cu0.30Cr2Se4 spinel has magnetic transitions at 24.5 K (T N) and 415 K (T C) on heating. The low-temperature magnetic phase transition is from a spiral antiferromagnet to a ferromagnet, and the high-temperature magnetic phase transition is from a ferromagnet to a paramagnet, while ZnCr2Se4 shows a magnetic phase transition only from a spiral antiferromagnet to a paramagnet at about 21.0 K. From neutron powder diffraction, it is also found that the spinels of Zn1−x Cu x Cr2Se4; 0.0 ≤ x ≤ 0.3. show satellite-like magnetic reflection having indexes (h ± Q, k, l) with Q = 0.470 below T N and short-range order of spins (spin glass-like) above T N. The incommensurate antiferromagnetic phase below T N results from a spiral long-range order of the spins of Cr3+. The intermediate ferromagnetic phase between T N and T C is related not to the spiral spin order but to double-exchange magnetic interaction among Cr3+ and Cr4+ mediated by current carriers, positive holes, which is made by the substitution of Zn2+ ions with Cu1+ ions in Zn1−x Cu x Cr2Se4.

Field-Induced Commensurate-Incommensurate Phase Transition in a Dzyaloshinskii-Moriya Spiral Antiferromagnet

We report an observation of a commensurate-incommensurate phase transition in a Dzyaloshinskii-Moriya spiral magnet Ba_2CuGe_2O_7. The transition is induced by applying a magnetic field in the plane of spin rotation. In this experiment we have direct control over the strength of the commensurate potential, while the preferred incommensurate period of the spin system remains unchanged. Experimental results for the period of the soliton lattice and bulk magnetization as a function of external magnetic field are in quantitative agreement with theory.

The magnetic phases of Gd-Y alloys

Measurements are presented of the temperature dependence of the elastic properties of two Gd-Y alloys. Both the 68.9 and the 70.3 atomic %Gd alloys exhibit at least two magnetic phase changes. However, temperature hysteresis in the 68.9% sample indicates that the high-temperature ordered phase is a spiral spin antiferromagnet, while for the 70.3% sample there is no hysteresis, indicating a ferromagnetic component even in the high-temperature phase.

A method for indexing neutron powder diffraction peaks from spiral spin antiferromagnets

A method is presented for the indexing of satellite diffraction peaks which arise in neutron powder diffraction patterns of spiral spin antiferromagnets with general propagation vectors. It has been successfully used to determine the propagation vector in Cu1/2Ga1/2Cr2S4 and also to confirm the direction of the propagation vectors in other materials with known structures.

Anomalous Thermal Expansion and Magnetostriction of Holmium Single Crystals

The $a$-, $b$-, and $c$-axis linear strains of single-crystal Ho have been measured from room temperature to 4\ifmmode^\circ\else\textdegree\fi{}K in zero field and in applied fields up to 30 kOe. Pronounced bumps appeared in the thermal-expansion coefficient at the magnetic ordering temperatures of 132 and 20\ifmmode^\circ\else\textdegree\fi{}K. The exchange magnetostriction contribution to the total $c$-axis strain was isolated and compared to the predictions of the molecular-field theory applied to spiral antiferromagnets. Strain data taken with an applied field show the effects of "fanning" of the moment about the applied-field direction and of the onset of basal-plane anisotropy below 75\ifmmode^\circ\else\textdegree\fi{}K. The dependence of the basal-plane magnetostriction on field angle was measured in the temperature range of magnetic order. Saturation values of the second-order magnetostriction constant were compared with the results of single-ion magnetoelastic theory. A value of 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ at $T=0\ifmmode^\circ\else\textdegree\fi{}$K was calculated.

Image of the Fermi Surface in Spin-Wave Spectra of Rare-Earth Metals

Calculations of spin-wave spectra in rare earth metals were carried out to find whether images of the electronic Fermi surface might be observable. In the space of spin-wave vectors q there should occur surfaces on which the frequencies have an infinite gradient with respect to q, the location of such abrupt changes, "kinks" in the dispersion curves, being determined by the shape of the Fermi surface. The spin-wave spectrum is found by assuming that the coupling between ionic spins takes place primarily through exchange scattering of conduction electrons, paralleling the calculation on the coupling of nuclear spins by Ruderman and Kittel. Spin-wave dispersion curves in two directions of high symmetry are computed numerically. The sought-for kinks in the dispersion curves are found to amount to about 2% of the maximum excitation frequency. The development is for ferromagnets, but extension to spiral antiferromagnets is taken up briefly.