Showing papers on "Antiferromagnetism published in 2009"

Spin-lattice coupling in frustrated antiferromagnets

Abstract: We review the mechanism of spin-lattice coupling in relieving the geometrical frustration of pyrochlore antiferromagnets, in particular spinel oxides. The tetrahedral unit, which is the building block of the pyrochlore lattice, undergoes a spin-driven Jahn-Teller instability when lattice degrees of freedom are coupled to the antiferromagnetism. By restricting our considerations to distortions which preserve the translational symmetries of the lattice, we present a general theory of the collective spin-Jahn-Teller effect in the pyrochlore lattice. One of the predicted lattice distortions breaks the inversion symmetry and gives rise to a chiral pyrochlore lattice, in which frustrated bonds form helices with a definite handedness. The chirality is transferred to the spin system through spin-orbit coupling, resulting in a long-period spiral state, as observed in spinel CdCr2O4. We discuss explicit models of spin-lattice coupling using local phonon modes, and their applications in other frustrated magnets.

First-order magnetic and structural phase transitions in Fe1+ySexTe1-x

Abstract: We use bulk magnetic susceptibility, electronic specific heat, and neutron scattering to study structural and magnetic phase transitions in Fe1+ySexTe1-x. Fe1.068Te exhibits a first-order phase transition near 67 K with a tetragonal-to-monoclinic structural transition and simultaneously develops a collinear antiferromagnetic (AF) order responsible for the entropy change across the transition. Systematic studies of the FeSe1-xTex system reveal that the AF structure and lattice distortion in these materials are different from those of FeAs-based pnictides. These results call into question the conclusions of present density-functional calculations, where FeSe1-xTex and FeAs-based pnictides are expected to have similar Fermi surfaces and therefore the same spin-density wave AF order.

Quantum spin Hall effect in a transition metal oxide Na2IrO3.

Abstract: We study theoretically the electronic states in a $5d$ transition metal oxide ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$, in which both the spin-orbit interaction and the electron correlation play crucial roles. A tight-binding model analysis together with the first-principles band structure calculation predicts that this material is a layered quantum spin Hall system. Because of the electron correlation, an antiferromagnetic order first develops at the edge, and later inside the bulk at low temperatures.

Ab initio investigation of the non-collinear magnetic structure and the lowest magnetic excitations in dysprosium triangles

Abstract: The unusual magnetism exhibited by dysprosium triangles [Dy3(μ3-OH)2L3Cl2(H2O)4][Dy3(μ3-OH)2L3Cl(H2O)5]Cl5·19H2O is explained using the recently developed ab initio methodology for the simulation of magnetic properties of complexes The local anisotropy axes on the dysprosium sites are found to lie in the plane of the Dy3 triangle and to make angles of ca 120° with each other The small antiferromagnetic exchange interaction between sites leads to a non-magnetic Kramers doublet in the ground state of the complex The arrangement of the local magnetization vectors in this state is close to toroidal By contrast, the lowest excited states are characterized by a huge magnetic moments of ca 20 μB and show very different behavior of magnetization for fields applied along and perpendicular to the plane of the Dy3 triangle

The electronic phase diagram of the LaO 1− x F x FeAs superconductor

Abstract: The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, for example in high-transition-temperature cuprates, heavy fermions and organic superconductors. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, for example charge-carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently, high-temperature superconductivity has been discovered in iron pnictides, providing a new class of unconventional superconductors. Similar to other unconventional superconductors, the parent compounds of the pnictides show a magnetic ground state and superconductivity is induced on charge-carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of X-ray scattering, muon spin relaxation and Mossbauer spectroscopy on the series LaO(1-x)F(x)FeAs. We find a discontinuous first-order-like change of the Neel temperature, the superconducting transition temperature and the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in iron pnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.

Spin waves and magnetic exchange interactions in CaFe 2 As 2

Abstract: It is likely that antiferromagnetism has a role in the superconductivity of iron arsenide. But is the magnetism local, as described by the Heisenberg model, or itinerant, which is more in agreement with the Stoner model? The answer is both.

First-principles calculations of the electronic structure of tetragonal alpha-FeTe and alpha-FeSe crystals: evidence for a bicollinear antiferromagnetic order.

Abstract: By the first-principles electronic structure calculations, we find that the ground state of PbO-type tetragonal alpha-FeTe is in a bicollinear antiferromagnetic order, in which the Fe local moments (similar to 2.5 mu(B)) align ferromagnetically along a diagonal direction and antiferromagnetically along the other diagonal direction on the Fe square lattice. This novel bicollinear order results from the interplay among the nearest, the next-nearest, and the next-next-nearest neighbor superexchange interactions, mediated by Te 5p band. In contrast, the ground state of alpha-FeSe is in a collinear antiferromagnetic order, similar to those in LaFeAsO and BaFe(2)As(2). This finding sheds new light on the origin of magnetic ordering in Fe-based superconductors.

Novel Magnetic Properties of Graphene: Presence of Both Ferromagnetic and Antiferromagnetic Features and Other Aspects

Abstract: Investigations of the magnetic properties of graphenes prepared by different methods reveal that dominant ferromagnetic interactions coexist along with antiferromagnetic interactions in all of the samples, somewhat like in frustrated or phase-separated systems. All of the graphene samples exhibit room-temperature magnetic hysteresis. The magnetic properties of the graphene samples depend on the number of layers and the sample area, small values of both favoring larger magnetization. Molecular charge-transfer affects the magnetic properties of graphene, interaction with a donor molecule such as tetrathiafulvalene having greater effect than with an electron-withdrawing molecule such as tetracyanoethylene.

Magnetic anisotropy of dysprosium(III) in a low-symmetry environment: a theoretical and experimental investigation.

Abstract: A mixed theoretical and experimental approach was used to determine the local magnetic anisotropy of the dysprosium(III) ion in a low-symmetry environment. The susceptibility tensor of the monomeric species having the formula [Dy(hfac)(3)(NIT-C(6)H(4)-OEt)(2)], which contains nitronyl nitroxide (NIT-R) radicals, was determined at various temperatures through angle-resolved magnetometry. These results are in agreement with ab initio calculations performed using the complete active space self-consistent field (CASSCF) method, validating the predictive power of this theoretical approach for complex systems containing rare-earth ions, even in low-symmetry environments. Susceptibility measurements performed with the applied field along the easy axis eventually permitted a detailed analysis of the temperature and field dependence of the magnetization, providing evidence that the Dy ion transmits an antiferromagnetic interaction between radicals but that the Dy-radical interaction is ferromagnetic.

Metal–Organic Perovskites: Synthesis, Structures, and Magnetic Properties of [C(NH2)3][MII(HCOO)3] (M=Mn, Fe, Co, Ni, Cu, and Zn; C(NH2)3= Guanidinium)

Abstract: We report the synthesis, crystal structures, and spectral, thermal, and magnetic properties of a family of metal-organic perovskite ABX(3), [C(NH(2))(3)][M(II)(HCOO)(3)], in which A = C(NH(2))(3) is guanidinium, B = M is a divalent metal ion (Mn, Fe, Co, Ni, Cu, or Zn), and X is the formate HCOO(-). The compounds could be synthesized by either diffusion or hydrothermal methods from water or water-rich solutions depending on the metal. The five members (Mn, Fe, Co, Ni, and Zn) are isostructural and crystallize in the orthorhombic space group Pnna, while the Cu member in Pna2(1). In the perovskite structures, the octahedrally coordinated metal ions are connected by the anti-anti formate bridges, thus forming the anionic NaCl-type [M(HCOO)(3)](-) frameworks, with the guanidinium in the nearly cubic cavities of the frameworks. The Jahn-Teller effect of Cu(2+) results in a distorted anionic Cu-formate framework that can be regarded as Cu-formate chains through short basal Cu-O bonds linked by the long axial Cu-O bonds. These materials show higher thermal stability than other metal-organic perovskite series of [AmineH][M(HCOO)(3)] templated by the organic monoammonium cations (AmineH(+)) as a result of the stronger hydrogen bonding between guanidinium and the formate of the framework. A magnetic study revealed that the five magnetic members (except Zn) display spin-canted antiferromagnetism, with a Neel temperature of 8.8 (Mn), 10.0 (Fe), 14.2 (Co), 34.2 (Ni), and 4.6 K (Cu). In addition to the general spin-canted antiferromagnetism, the Fe compound shows two isothermal transformations (a spin-flop and a spin-flip to the paramagnetic phase) within 50 kOe. The Co member possesses quite a large canting angle. The Cu member is a magnetic system with low dimensional character and shows slow magnetic relaxation that probably results from the domain dynamics.

The Disorder-Free Non-BCS Superconductor Cs3C60 Emerges from an Antiferromagnetic Insulator Parent State

Abstract: The body-centered cubic A15-structured cesium fulleride Cs3C60 is not superconducting at ambient pressure and is free from disorder, unlike the well-studied face-centered cubic A3C60 alkali metal fulleride superconductors. We found that in Cs3C60, where the molecular valences are precisely assigned, the superconducting state at 38 kelvin emerges directly from a localized electron antiferromagnetic insulating state with the application of pressure. This transition maintains the threefold degeneracy of the active orbitals in both competing electronic states; it is thus a purely electronic transition to a superconducting state, with a dependence of the transition temperature on pressure-induced changes of anion packing density that is not explicable by Bardeen-Cooper-Schrieffer (BCS) theory.

Multiband magnetism and superconductivity in Fe-based compounds

Abstract: Recent discovery of superconductivity in Fe-based layered compounds may have opened a new pathway to the room temperature superconductivity. A model Hamiltonian describing FeAs layers is introduced, highlighting the crucial role of puckering of As atoms in promoting d electron itinerancy and warding off large local-moment magnetism of Fe ions, the main enemy of superconductivity. Quantum many-particle effects in charge, spin and multiband channels are explored and a nesting-induced spin density-wave order is found in the parent compund. We argue that this largely itinerant antiferromagnetism and high Tc itself are essentially tied to the multiband nature of the Fermi surface.

Quantum spin liquid in the spin-1/2 triangular antiferromagnet EtMe$_{3}$Sb[Pd(dmit)$_{2}$]$_{2}$

Abstract: The family of layered organic salts $X{[\\mathrm{Pd}{(\\text{dmit})}_{2}]}_{2}$ are Mott insulators and form scalene-triangular spin-$1∕2$ systems. Among them, $\\mathrm{Et}{\\mathrm{Me}}_{3}\\mathrm{Sb}{[\\mathrm{Pd}{(\\text{dmit})}_{2}]}_{2}$ has a nearly regular-triangular lattice. We have investigated the spin state of this salt by $^{13}\\mathrm{C}\\text{\\ensuremath{-}}\\mathrm{NMR}$ and static susceptibility measurements. The temperature dependence of the susceptibility is described as that of a regular-triangular antiferromagnetic spin-$1∕2$ system with an exchange interaction $J=220\\ensuremath{-}250\\phantom{\\rule{0.3em}{0ex}}\\mathrm{K}$. The $^{13}\\mathrm{C}\\text{\\ensuremath{-}}\\mathrm{NMR}$ measurements reveal that there is no indication of either spin ordering/freezing or an appreciable spin gap down to $1.37\\phantom{\\rule{0.3em}{0ex}}\\mathrm{K}$, which is lower than 1% of $J$. This result strongly suggests that this system is in the quantum spin-liquid state with no appreciable spin gap, which has been long sought after.

Anisotropy in the electrical resistivity and susceptibility of superconducting BaFe2As2 single crystals.

Abstract: We report that sizable single crystals of BaFe2As2 have been grown with the self-flux method. Measurements and anisotropy of intrinsic transport and magnetic properties from high-quality single crystal are first presented. The resistivity anisotropy (rho{c}/rho{ab}) is as large as 150 and independent of temperature. In contrast to the susceptibility behavior observed in polycrystalline samples, no Curie-Weiss behavior is observed, and a linear-T dependent susceptibility occurs from the spin-density-wave transition temperature, (T{s}), to 700 K. This result suggests that strong antiferromagnetic correlations are present well above T{s}. A twofold symmetry of susceptibility in the ab plane indicates a stripelike spin structure as observed by neutron scattering. The resistivity minimum is strongly dependent on the magnetic field, suggesting that the upturn of the resistivity at low temperatures should be related to spin fluctuation.

Electric Field Switching of the Magnetic Anisotropy of a Ferromagnetic Layer Exchange Coupled to the Multiferroic Compound BiFeO 3

Abstract: We report here that a Permalloy layer deposited on top of a multiferroic ${\mathrm{BiFeO}}_{3}$ single crystal acquires an easy magnetic direction along the propagation vector of the cycloidal arrangement of antiferromagnetic moments in ${\mathrm{BiFeO}}_{3}$. This anisotropy originates from a direct magnetic coupling with the canted spins forming the cycloid. Moreover, we show that an electric field-induced change of electric polarization is able to toggle the direction of anisotropy in the ferromagnet through the magnetoelectric effect, which links the antiferromagnetic spins to the local polarization in ${\mathrm{BiFeO}}_{3}$.

Magnetocaloric effect and refrigerant capacity in charge-ordered manganites

Abstract: The influence of first- and second-order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity (RC) of charge-ordered Pr0.5Sr0.5MnO3 has been investigated. The system undergoes a paramagnetic to ferromagnetic transition at TC∼255 K followed by a ferromagnetic charge-disordered to antiferromagnetic charge-ordered transition at TCO∼165 K. While the first-order magnetic transition (FOMT) at TCO induces a larger MCE (6.8 J/kg K) limited to a narrower temperature range resulting in a smaller RC (168 J/kg), the second-order magnetic transition at TC induces a smaller MCE (3.2 J/kg K) but spreads over a broader temperature range resulting in a larger RC (215 J/kg). In addition, large magnetic and thermal hysteretic losses associated with the FOMT below TCO are detrimental to an efficient magnetic RC, whereas these effects are negligible below TC because of the second-order nature of this transition. These results are of practical importance in assessing the usefulness of charge-o...

Critical thickness for itinerant ferromagnetism in ultrathin films of SrRuO3

Abstract: Ultrathin films of the itinerant ferromagnet ${\text{SrRuO}}_{3}$ were studied using the transport and magneto-optic polar Kerr effect. We find that below 4 monolayers, the films become insulating and their magnetic character changes as they loose their simple ferromagnetic behavior. We observe a strong reduction in the magnetic moment which for 3 monolayers and below lies in the plane of the film. Exchange-bias behavior is observed below the critical thickness and may point to induced antiferromagnetism in contact with ferromagnetic regions.

Vesignieite BaCu3V2O8(OH)2 as a Candidate Spin-1/2 Kagome Antiferromagnet

Abstract: A polycrystalline sample of vesignieite BaCu 3 V 2 O 8 (OH) 2 comprising a nearly ideal kagome lattice composed of Cu 2+ ions carrying spin 1/2 has been synthesized and studied by magnetization and heat capacity measurements. Magnetic susceptibility shows a neither long range order, a spin glass transition nor a spin gap down to 2 K, in spite of a moderately strong antiferromagnetic interaction of J / k B =53 K between nearest-neighbor spins. A broad peak observed at a temperature corresponding to 0.4 J in intrinsic magnetic susceptibility indicates a marked development of the short-range order. The ground state of vesignieite is probably a gapless spin liquid or is accompanied by a very small gap less than ∼ J /30.

Roles of multiband effects and electron-hole asymmetry in the superconductivity and normal-state properties of Ba(Fe 1-x Co x ) 2 As 2

Abstract: We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe1-xCox)(2)As-2 over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons in excess of one order of magnitude in the low-doping, low-temperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between antiferromagnetism and superconductivity, while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing but also in the normal transport.

A large iron isotope effect in SmFeAsO(1 - x)F(x) and Ba(1 - x)K(x)Fe(2)As(2).

Abstract: The recent discovery of superconductivity in oxypnictides with a critical transition temperature (T-C) higher than the McMillan limit of 39K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer theory) has generated great excitement(1-5). Theoretical calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures(6), but strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible(7-9). Superconductivity and magnetism in pnictide superconductors, however, show a strong sensitivity to the crystal lattice, suggesting the possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotope substitution on T-C and the spin-density wave (SDW) transition temperature (T-SDW) in the SmFeAsO1-xFx and Ba1-xKxFe2As2 systems. The oxygen isotope effect on T-C and T-SDW is very small, while the iron isotope exponent alpha(C) =-dlnT(C)/dlnM is about 0.35 (0.5 corresponds to the full isotope effect). Surprisingly, the iron isotope exchange shows the same effect on T-SDW as T-C. This indicates that electron-phonon interaction plays some role in the superconducting mechanism, but a simple electron-phonon coupling mechanism seems unlikely because a strong magnon-phonon coupling is included.

Microscopic model and phase diagrams of the multiferroic perovskite manganites

Abstract: Orthorhombically distorted perovskite manganites, $R\mathrm{Mn}{\mathrm{O}}_{3}$ with $R$ being a trivalent rare-earth ion, exhibit a variety of magnetic and electric phases including multiferroic (i.e., concurrently magnetic and ferroelectric) phases and fascinating magnetoelectric phenomena. We theoretically study the phase diagram of $R\mathrm{Mn}{\mathrm{O}}_{3}$ by constructing a microscopic spin model, which includes not only the superexchange interaction but also the single-ion anisotropy (SIA) and the Dzyaloshinsky-Moriya interaction (DMI). Analysis of this model using the Monte Carlo method reproduces the experimental phase diagrams as functions of the $R$-ion radius, which contain two different multiferroic states, i.e., the $ab$-plane spin cycloid with ferroelectric polarization $P\ensuremath{\parallel}a$ and the $bc$-plane spin cycloid with $P\ensuremath{\parallel}c$. The orthorhombic lattice distortion or the second-neighbor spin exchanges enhanced by this distortion exquisitely controls the keen competition between these two phases through tuning the SIA and DMI energies. This leads to a lattice-distortion-induced reorientation of $P$ from $a$ to $c$ in agreement with the experiments. We also discuss spin structures in the $A$-type antiferromagnetic state, those in the cycloidal spin states, origin and nature of the sinusoidal collinear spin state, and many other issues.

Lattice collapse and quenching of magnetism in CaFe 2 As 2 under pressure: A single-crystal neutron and x-ray diffraction investigation

Abstract: Single-crystal neutron and high-energy x-ray diffraction measurements have identified the phase lines corresponding to transitions among the ambient-pressure paramagnetic tetragonal (T), the antiferromagnetic orthorhombic (O), and the nonmagnetic collapsed tetragonal (cT) phases of ${\text{CaFe}}_{2}{\text{As}}_{2}$. We find no evidence of additional structures for pressures of up to 2.5 GPa (at 300 K). Both the T-cT and O-cT transitions exhibit significant hysteresis effects, and we demonstrate that coexistence of the O and cT phases can occur if a nonhydrostatic component of pressure is present. Measurements of the magnetic diffraction peaks show no change in the magnetic structure or ordered moment as a function of pressure in the O phase, and we find no evidence of magnetic ordering in the cT phase. Band-structure calculations show that the transition into the cT phase results in a strong decrease in the iron $3d$ density of states at the Fermi energy, consistent with a loss of the magnetic moment.

Decoupling of the superconducting and magnetic/structural phase transitions in electron-doped BaFe 2 As 2

Abstract: Study and comparison of over 30 examples of electron-doped ${\text{BaFe}}_{2}{\text{As}}_{2}$ for transition metal $(\text{TM})=\text{Co}$, Ni, Cu, and (Co/Cu mixtures) have led to an understanding that the suppression of the structural/antiferromagnetic phase transition to low-enough temperature in these compounds is a necessary condition for superconductivity but not a sufficient one. Whereas the structural/antiferromagnetic transitions are suppressed by the number of TM dopant ions (or changes in the $c$ axis) the superconducting dome exists over a limited range of values of the number of valence electrons added by doping (or values of the $a/c$ ratio). By choosing which combination of dopants is used we can change the relative positions of the upper phase lines and the superconducting dome, even to the extreme limit of suppressing the upper structural and magnetic phase transitions without the stabilization of a lower-temperature superconducting dome.

Enhanced ordering temperatures in antiferromagnetic manganite superlattices

Abstract: The disorder inherent to doping by cation substitution in the complex oxides can have profound effects on collective-ordered states. Here, we demonstrate that cation-site ordering achieved through digital-synthesis techniques can dramatically enhance the antiferromagnetic ordering temperatures of manganite films. Cation-ordered (LaMnO3)m/(SrMnO3)2m superlattices show Neel temperatures (TN) that are the highest of any La(1-x)Sr(x)MnO3 compound, approximately 70 K greater than compositionally equivalent randomly doped La(1/3)Sr(2/3)MnO3. The antiferromagnetic order is A-type, consisting of in-plane double-exchange-mediated ferromagnetic sheets coupled antiferromagnetically along the out-of-plane direction. Through synchrotron X-ray scattering, we have discovered an in-plane structural modulation that reduces the charge itinerancy and hence the ordering temperature within the ferromagnetic sheets, thereby limiting TN. This modulation is mitigated and driven to long wavelengths by cation ordering, enabling the higher TN values of the superlattices. These results provide insight into how cation-site ordering can enhance cooperative behaviour in oxides through subtle structural phenomena.

Magnetic structure of EuFe 2 As 2 determined by single-crystal neutron diffraction

Abstract: Among various parent compounds of iron pnictide superconductors, EuFe2As2 stands out due to the presence of both spin density wave of Fe and antiferromagnetic ordering (AFM) of the localized Eu2+ moment. Single crystal neutron diffraction studies have been carried out to determine the magnetic structure of this compound and to investigate the coupling of two magnetic sublattices. Long range AFM ordering of Fe and Eu spins was observed below 190 K and 19 K, respectively. The ordering of Fe2+ moments is associated with the wave vector k = (1,0,1) and it takes place at the same temperature as the tetragonal to orthorhombic structural phase transition, which indicates the strong coupling between structural and magnetic components. The ordering of Eu moment is associated with the wave vector k = (0,0,1). While both Fe and Eu spins are aligned along the long a axis as experimentally determined, our studies suggest a weak coupling between the Fe and Eu magnetism.

Training-induced positive exchange bias in NiFe/IrMn bilayers.

Abstract: Positive exchange bias has been observed in the Ni 81 Fe 19/ Ir 20 Mn 80 bilayer system via soft-x-ray resonant magnetic scattering. After field cooling of the system through the blocking temperature of the antiferromagnet, an initial conventional negative exchange bias is removed after training, i.e., successive magnetization reversals, resulting in a positive exchange bias for a temperature range down to 30 K below the blocking temperature (450 K). This new manifestation of magnetic training is discussed in terms of metastable magnetic disorder at the magnetically frustrated interface during magnetization reversal.

Exchange bias driven by the Dzyaloshinskii-Moriya interaction and ferroelectric polarization at G-type antiferromagnetic perovskite interfaces.

Abstract: Exchange bias is usually rationalized invoking spin pinning effects caused by uncompensated antiferromagnetic interfaces. However, for compensated antiferromagnets other extrinsic factors, such as interface roughness or spin canting, have to be considered to produce a small uncompensation. As an alternative, here we propose two (related) possible mechanisms, driven by the intrinsic Dzyaloshinskii-Moriya interaction and ferroelectric polarization, for the explanation of exchange bias effects in perovskites with compensated G-type antiferromagnetism. One of the mechanisms is only active when a multiferroic material is involved and it is controllable by electric fields.

Vesignieite BaCu3V2O8(OH)2 as a Candidate Spin-1/2 Kagome Antiferromagnet

Abstract: A polycrystalline sample of vesignieite BaCu3V2O8(OH)2 comprising a nearly ideal kagome lattice composed of Cu2+ ions carrying spin 1/2 has been synthesized and studied by magnetization and heat capacity measurements. Magnetic susceptibility shows a neither long range order, a spin glass transition nor a spin gap down to 2 K, in spite of a moderately strong antiferromagnetic interaction of J/kB = 53 K between nearest-neighbor spins. A broad peak observed at a temperature corresponding to 0.4J in intrinsic magnetic susceptibility indicates a marked development of the short-range order. The ground state of vesignieite is probably a gapless spin liquid or is accompanied by a very small gap less than J/30.

Possible origin of ferromagnetism in undoped anatase TiO2

Abstract: Using first-principles electronic structure calculations we find that the titanium vacancy and divacancy may be responsible for the unexpected ferromagnetism in undoped anatase TiO2. An isolated titanium vacancy produces a magnetic moment of 3.5 mu(B), and an isolated titanium divacancy produces a magnetic moment of 2.0 mu(B). The origin of the collective magnetic moments is the holes introduced by the titanium vacancy or divacancy in the narrow nonbonding oxygen 2p(pi) band. At the center of the divacancy, an O-2 dimer forms during the relaxation, which lowers the total energy of the system and leads to the decrease in the total magnetic moment due to a hole compensation mechanism. For both the two native defects, the ferromagnetic state is more stable than the antiferromagnetic state.