Spiral Magnetic Order Research Articles

Magnetotransport and spin dynamics in an electron gas formed at oxide interfaces

We investigate the spin-dependent transport properties of a two-dimensional electron gas formed at oxides' interface in the presence of a magnetic field. We consider several scenarios for the oxides' properties, including oxides with co-linear or spiral magnetic and ferroelectric order. For spiral multiferroic oxides, the magnetoelectric coupling and the topology of the localized magnetic moments introduce additional, electric field controlled spin-orbit coupling that affects the magneto-oscillation of the current. An interplay of this spin-orbit coupling, the exchange field, and of the applied magnetic field results in a quantum, gate-controlled spin and charge Hall conductance.

Local symmetry and magnetic anisotropy in multiferroicMnWO4and antiferromagneticCoWO4studied by soft x-ray absorption spectroscopy

Soft x-ray absorption experiments on the transition-metal ${L}_{2,3}$ edge of multiferroic ${\text{MnWO}}_{4}$ and antiferromagnetic ${\text{CoWO}}_{4}$ are presented. The observed linear polarization dependence, analyzed by full-multiplet calculations, is used to determine the ground-state wave function of the magnetic ${\text{Mn}}^{2+}$ and ${\text{Co}}^{2+}$ ions. The impact of the local structure and the spin-orbit coupling on the orbital moment is discussed in terms of the single-ion anisotropy. It is shown that the orbital moment in ${\text{CoWO}}_{4}$ is responsible for the collinear antiferromagnetism, while the small size of spin-orbit coupling effects make spiral magnetic order in ${\text{MnWO}}_{4}$ possible, enabling the material to be multiferroic.

Magnetic Phase Transitions in Cobalt Chromite Nanoparticles

Cobalt chromite (CoCr2O4), an insulating normal spinel compound, is a potential multiferroic material. We report that pure, nanosize CoCr2O4 particles are synthesized through a conventional coprecipiation technique by controlling the pH of the precipitation. Both single-crystal and polycrystalline samples develop long-range ferrimagnetic order below the Curie temperature, T c (97 K), and a sharp phase transition at T s∼31 K, attributed to the onset of long-range spiral magnetic order. However, we observed a transition from paramagnetic to superparamagnetic phase at T c. Further lowering the temperature below T c (97 K), the superparamagnetic phase transforms to ferrimagnetic phase at blocking temperature, T b, which is found to be between 50 and 60 K. This intermediate superparamagnetic phase in between paramagnetic and long-range ferrimagnetic phases is attributed to a nanosize effect.

Tunneling anisotropic magnetoresistance of helimagnet tunnel junctions

We theoretically investigate the angular and spin dependent transport in normal-metal/helical-multiferroic/ferromagnetic heterojunctions. We find a tunneling anisotropic magnetoresistance (TAMR) effect due to the spiral magnetic order in the tunnel junction and to an effective spin-orbit coupling induced by the topology of the localized magnetic moments in the multiferroic spacer. The predicted TAMR effect is efficiently controllable by an external electric field due to the magnetoelectric coupling.

High-field study of multiferroic BiFeO3

We studied the magnetization (M) and electric polarization (P) in single crystals of BiFeO3 with a unipolar ferroelectric domain in pulsed high magnetic fields up to 55 T. At low temperatures, the application of magnetic fields causes sharp changes in M and P at around 18 T, which can be ascribed to the magnetic transition from a cycloidal to a canted antiferromagnetic state. Measurements of P up to 600 K suggest that this transition disappears as the temperature approaches the Náel temperature. These results indicate that the field-induced change in P originates from the P component generated by the spiral magnetic ordering in BiFeO3. Furthermore, we found that the microscopic magnetoelectric coupling constant of BiFeO3 is greater than those of typical multiferroic materials such as TbMnO3. This fact indicates strong coupling between the magnetic and dielectric properties of BiFeO3.

Pressure Collapse of the Magnetic Ordering in MnSi via Thermal Expansion

The itinerant quasi-ferromagnetic metal MnSi has been studied by detailed thermal expansion measurements under pressures and magnetic fields. A sudden decrease of the volume at the critical pressure P c ∼1.6 GPa has been observed and is in good agreement with the pressure variation of the volume fraction of the spiral magnetic ordering. This confirms that the magnetic order disappears by a first order phase transition. The energy change estimated by the volume discontinuity on crossing P c is of similar order as the Zeeman energy of the transition from the spiral ground state to a polarized paramagnetic one under magnetic field. In contrast to the strong pressure dependence of the transition temperature, the characteristic fields are weakly pressure dependent, indicating that the strength of the ferromagnetic and the Dzyaloshinskii–Moriya interactions do not change drastically around P c . The evaluated results of the thermal expansion coefficient and the magnetostriction are analyzed thermodynamically. The Sommerfeld coefficient of the linear temperature term of the specific heat is enhanced just below P c . The magnetic field-temperature phase diagrams in the ordered and paramagnetic phases are also compared. Comparison is made with other heavy fermion compounds with first order phase transition at 0 K.

Changes of spin dynamics in multiferroic

Abstract We report the results of a series of μ SR measurements performed on polycrystalline and single crystalline Tb 1 - x Ca x MnO 3 , with x = 0 , 0.05 and 0.1. Analysis of the data indicates that the Neel transition temperature around 40 K for the undoped sample is in agreement with the previous result obtained by susceptibility measurement. We further find a peculiar behavior related to the relaxor ferroelectric of magnetic origin for 5% Ca doping. This type of relaxor is suggested to be governed by spin fluctuations due to inhomogeneity in the spiral magnetic ordering induced by the A-site Ca doping. It is shown that the 5% Ca doping appears to induce faster reduction of spin fluctuation with decreasing temperature. Our μ SR data in applied longitudinal fields (LF) also indicate the possible occurrence of inhomogeneous internal field although a more detailed study is required for its clarification and verification.

Re-entrant spiral magnetic order and ferroelectricity in Mn1−xFexWO4 (x=0.035)

We report the observation of a re-entrant spiral (noncollinear) magnetic order and ferroelectricity below 6 K in Mn0.965Fe0.035WO4 under applied magnetic fields above 3.6 T. At zero field, this compound shows a transition into the spiral magnetic and ferroelectric phase at 12 K, and it becomes paraelectric at 10.5 K with the entrance into the commensurate magnetic phase, which is the ground state at H=0. Under magnetic field above 3.6 T, however, the spiral magnetic phases with a larger FE polarization reappears below 6 K. The re-entrant magnetic/ferroelectric phase behavior is further studied by single crystal neutron scattering and the H-T phase diagram is completely resolved from neutron, dielectric constant, polarization, and magnetic measurements.

Magnetic ordering of weakly coupled frustrated quantum spin chains

The ordering temperature of a quasi-one-dimensional system, consisting of weakly interacting quantum spin-1/2 chains with antiferromagnetic spin-frustrating couplings (or zigzag ladder), is calculated. The results show that a quantum critical point between two phases of the one-dimensional subsystem plays a crucial role. If the one-dimensional subsystem is in the antiferromagneticlike phase in the ground state, similar to the phase of a spin chain without frustration, weak couplings yield magnetic ordering of the N\'eel type. For intrachain spin-frustrating interactions larger than the critical one (at which the quantum phase transition takes place), the quasi-one-dimensional spin system manifests a spiral magnetic incommensurate ordering. The obtained results of our quantum theory are compared with the quasiclassical approximations. The calculated features of magnetic ordering are expected to be generic for weakly-coupled quantum spin chains with gapless excitations and spin-frustrating nearest- and next-nearest-neighbor interactions.

A multi-component Fermi surface in the vortex state of an underdoped high-Tc superconductor

To understand the origin of superconductivity, it is crucial to ascertain the nature and origin of the primary carriers available to participate in pairing. Recent quantum oscillation experiments on high-transition-temperature (high-T(c)) copper oxide superconductors have revealed the existence of a Fermi surface akin to that in normal metals, comprising fermionic carriers that undergo orbital quantization. The unexpectedly small size of the observed carrier pocket, however, leaves open a variety of possibilities for the existence or form of any underlying magnetic order, and its relation to d-wave superconductivity. Here we report experiments on quantum oscillations in the magnetization (the de Haas-van Alphen effect) in superconducting YBa(2)Cu(3)O(6.51) that reveal more than one carrier pocket. In particular, we find evidence for the existence of a much larger pocket of heavier mass carriers playing a thermodynamically dominant role in this hole-doped superconductor. Importantly, characteristics of the multiple pockets within this more complete Fermi surface impose constraints on the wavevector of any underlying order and the location of the carriers in momentum space. These constraints enable us to construct a possible density-wave model with spiral or related modulated magnetic order, consistent with experimental observations.

Magnetic properties and magnetic structure of the Mn5Si3-type Tb5Si3 compound

Abstract Neutron diffraction and magnetization measurements have been performed on the Tb5Si3 compound (hexagonal Mn5Si3-type, hP16, P63/mcm) to understand its magnetic structure and magnetic properties. The temperature-dependent neutron diffraction results prove that this intermetallic phase shows a complex flat spiral magnetic ordering, presenting three subsequent changes in magnetization at T m 1 Neu ∼ 100 K , T m 2 Neu = 62 ( 4 ) K and T m 3 Neu = 54 ( 4 ) K on cooling. However, the magnetization data depict two transitions at 72 K (TN1) and 55 K (TN2). The extended temperature range between T m 1 Neu and T m 2 Neu over which the neutron diffraction patterns slowly evolve might correspond to the high-temperature antiferromagnetic transition at TN1 and low-temperature antiferromagnetic transition at TN2 of the magnetic data. Between T m 1 Neu ∼ 100 K and T m 2 Neu = 62 ( 4 ) K Tb5Si3 shows a flat spiral antiferromagnetic ordering with a propagation vector K1 = [0,0, ±1/4]; then, between T m 2 Neu = 62 ( 4 ) K and T m 3 Neu = 54 ( 4 ) K the flat spiral type ordering is conserved, but by two coexisting propagation vectors K1 = [0,0, ±1/4] and K2 = [0,0, ±0.4644(3)]. The terbium magnetic moments arrange in the XY(ab) plane of the unit cell. Below T m 3 Neu = 54 ( 4 ) K the magnetic component with K1 = [0,0, ±1/4] vanishes and magnetic structure of Tb5Si3 is a flat spiral with K2 = [0,0, ±0.4644(3)], only. Low field magnetization measurements confirm the occurrence of complex, multiple magnetic transitions. The field dependence of the magnetization indicates a metamagnetic transition at a critical field of ∼3 T.

Hubbard Model on the Triangular Lattice: Spiral Order and Spin Liquid

We investigate the half-filled Hubbard model on an isotropic triangular lattice with the variational cluster approximation. By decreasing the on-site repulsion U (or equivalently increasing pressure) we go from a phase with long-range, three-sublattice, spiral magnetic order, to a nonmagnetic Mott insulating phase--a spin liquid--and then, for U less or similar to 6.7t, to a metallic phase. Clusters of sizes 3, 6, and 15 with open boundary conditions are used in these calculations, and an extrapolation to infinite size is argued to lead to a disordered phase at U = 8t, but to a spiral order at U greater or similar to 12.

Cupric oxide as an induced-multiferroic with high-TC

Materials that combine coupled electric and magnetic dipole order are termed 'magnetoelectric multiferroics'. In the past few years, a new class of such materials, 'induced-multiferroics', has been discovered, wherein non-collinear spiral magnetic order breaks inversion symmetry, thus inducing ferroelectricity. Spiral magnetic order often arises from the existence of competing magnetic interactions that reduce the ordering temperature of a more conventional collinear phase. Hence, spiral-phase-induced ferroelectricity tends to exist only at temperatures lower than approximately 40 K. Here, we propose that copper(II) oxides (containing Cu2+ ions) having large magnetic superexchange interactions can be good candidates for induced-multiferroics with high Curie temperature (T(C)). In fact, we demonstrate ferroelectricity with T(C)=230 K in cupric oxide, CuO (tenorite), which is known as a starting material for the synthesis of high-T(c) (critical temperature) superconductors. Our result provides an important contribution to the search for high-temperature magnetoelectric multiferroics.

Absence of a spiral magnetic order inLi2CuO2containing one-dimensionalCuO2ribbon chains

On the basis of first-principles density functional theory electronic structure calculations as well as classical spin analysis, we explored why the magnetic oxide ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$, consisting of ${\mathrm{CuO}}_{2}$ ribbon chains made up of edge-sharing ${\mathrm{CuO}}_{4}$ squares, does not exhibit a spiral-magnetic order. Our work shows that, due to the next-nearest-neighbor interchain interactions, the observed collinear magnetic structure becomes only slightly less stable than the spin-spiral ground state and many states become nearly degenerate in energy with the observed collinear structure. This suggests that the collinear magnetic structure of ${\mathrm{Li}}_{2}{\mathrm{CuO}}_{2}$ is a consequence of order by disorder induced by next-nearest-neighbor interchain interactions.

Spin orbit scattering effect on long-range odd frequency triplet pairing in ferromagnet/superconductor bilayers

A formulation of triplet proximity effect in a ferromagnet/superconductor bilayer based on the de Gennes correlation function method is presented by extending the Takahashi–Tachiki theory. The inhomogeneous in-plane spiral magnetic order is employed in this approach the superconducting order parameter is a conventional spin singlet s-wave pairing, and the induced triplet pair amplitude arises as a result of the broken time-reversal invariance due to the presence of the rotating exchange field. A new type of triplet condensate still possesses the s-wave state but the symmetry requires an odd frequency parity. We find that the inhomogeneous exchange interaction leads to the appearance of the long-range odd frequency triplet pair amplitude, and the inclusion of the spin orbit scattering affects both short-range and long-range triplet components. The superconducting critical temperature of the diffusive ferromagnet/superconductor sandwiches is determined from the secular equation of the linearized gap equation in order to investigate the behavior of the induced triplet pairing. It is found that the induced long-range triplet component exists which arises from the modulation of the in-plane pair amplitudes in the transverse direction. Moreover, the possibility of the cryptoferromagnetic state is demonstrated in favor of superconductivity and this may explain a possible origin of the magnetically dead layer.

Ferroelectricity in anS=1/2Chain Cuprate

We report our discovery of ferroelectricity in the spiral-magnetic state in the quantum quasi-one-dimensional (1D) S=1/2 magnet of LiCu2O2. Electric polarization (P) emerges along the c direction below the spiral-magnetic order temperature, but changes from the c to a axis when magnetic fields (H) are applied along the b direction. We also found that P(c) increases with H(c), and P(a) appears with H(a). LiCu2O2 in zero field appears to be the first ferroelectric cuprate and also a prototypical example of the "1D spiral-magnetic ferroelectrics." However, the unexpected behavior in H may demonstrate the complexity of the ordered spin configuration, inherent in the 1D S=1/2 magnet of LiCu2O2.

スピンカイラリティの電場制御と中性子による検出

A polarized-neutron-diffraction study has been performed on a single crystal sample of TbMnO3, where the ferroelectricity and spiral magnetic order coexist. The spin helicity, detected as the change in intensities of magnetic satellites with the spin direction of incident neutrons, can be controlled by the poling electric field alone. The temperature dependence of the change well agrees with that of the electric polarization. These results establish that an inverse effect of the Dzyaloshinskii-Moriya interaction is the origin of spontaneous electric polarization in the spiral ordered phase of TbMnO3.

Ferroelectricity in the cycloidal spiral magnetic phase ofMnWO4

We investigate the relationships among magnetic, dielectric, and ferroelectric properties of a frustrated spin system $\mathrm{Mn}\mathrm{W}{\mathrm{O}}_{4}$, which undergoes several magnetic phase transitions including a commensurate-incommensurate and a collinear-noncollinear transition. Dielectric and pyroelectric measurements show that the transition into a spiral magnetic ordered phase produces a ferroelectric state. The direction of the electric polarization is perpendicular to the spin rotation axis and the propagation vector of the spiral. These observations agree well with recent theoretical predictions that a cycloidal spiral magnetic ordering can result in electric polarization. In a material where ferroelectricity is induced by magnetic order, we can magnetically tune the ferroelectric transitions by exploiting the difference of the net magnetization between the ferroelectric and neighboring paraelectric phases

Dielectric anomalies and spiral magnetic order inCoCr2O4

We have investigated the structural, magnetic, thermodynamic, and dielectric properties of polycrystalline CoCr$_2$O$_4$, an insulating spinel exhibiting both ferrimagnetic and spiral magnetic structures. Below $T_c$ = 94 K the sample develops long-range ferrimagnetic order, and we attribute a sharp phase transition at $T_N$ $\approx$ 25 K with the onset of long-range spiral magnetic order. Neutron measurements confirm that while the structure remains cubic at 80 K and at 11 K; there is complex magnetic ordering by 11 K. Density functional theory supports the view of a ferrimagnetic semiconductor with magnetic interactions consistent with non-collinear ordering. Capacitance measurements on CoCr$_2$O$_4$, show a sharp decrease in the dielectric constant at $T_N$, but also an anomaly showing thermal hysteresis falling between approximately $T$ = 50 K and $T$ = 57 K. We tentatively attribute the appearance of this higher temperature dielectric anomaly to the development of \textit{short-range} spiral magnetic order, and discuss these results in the context of utilizing dielectric spectroscopy to investigate non-collinear short-range magnetic structures.

Conventional magnetic superconductors: coexistence of singlet superconductivity and magnetic order

The basic physics of bulk magnetic superconductors (MS), related to the problem of the coexistence of singlet superconductivity (SC) and magnetic order, is reviewed. The interplay between the exchange (EX) and electromagnetic (EM) interaction is discussed. It is argued that the singlet SC and uniform ferromagnetic (F) order practically never coexist. In the case of their mutual coexistence the F order is modified into a domain-like or spiral structure depending on magnetic anisotropy. It turns out that this situation occurs in several superconductors such as ErRh 4B 4, HoMo 6S 8, HoMo 6Se 8 with electronic and in AuIn 2 with nuclear magnetic order. The latter problem is briefly discussed. The coexistence of SC with antiferromagnetism (AF) is more favorable than with the modified F order. Very interesting physics is realized in AF systems with SC and weak-ferromagnetism which results in an very rich phase diagram. A number of properties of magnetic superconductors in magnetic field are very peculiar, especially near the (ferro)magnetic transition temperature, where the upper critical field becomes smaller than the thermodynamical critical field. The interesting physics of Josephson junctions based on MS with spiral magnetic order is also discussed. The existence of the triplet pairing amplitude F ↑ ↑ ( F ↓ ↓ ) in MS with rotating magnetization (the effect recently rediscovered in SFS junctions) gives rise to the so called π-contact. Furthermore, the interplay of the superconducting and magnetic phase in such a contact renders possible a new type of coupled Josephson-qubits in a single Josephson junction. To cite this article: M.L. Kulić, C. R. Physique 7 (2006).