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With the discovery of two-dimensional (2D) ferroelectricity and ferromagnetism in van der Waals (vdW) materials, there has been significant interest in 2D multiferroics. Herein, we report the occurrence of weak ferromagnetism and magnetoelectricity in vdW antiferromagnet MnPSe3 single crystals. Our results demonstrate that MnPSe3 undergoes an antiferromagnetic transition at the Néel temperature TN = 70 K with weak ferromagnetism along the [1–10] direction. Detailed magnetoelectric (ME) data show that MnPSe3 exhibits a linear ME tensor αij with nine nonzero components. Additionally, a magnetically induced electric polarization as large as 98.5 μC/m2 is observed along the [110] direction, with a ME coefficient of 13.5 ps/m at 10 K for a magnetic field of 9 T applied along the [110] direction. Importantly, we discuss our experiments based on symmetry and microscopic analysis, thereby suggesting that the spin-dependent p-d hybridization mechanism plays an important role in the emergence of magnetic-field-induced ferroelectricity. Hence, our findings provide insights for exploring the ME coupling in vdW materials. [ABSTRACT FROM AUTHOR]

Polycrystalline YMn2-xTixO5 (x=0.00-0.05) samples with partial substitution of Mn by Ti are prepared in order to modulate the multiferroicity of YMn2O5. It is revealed that the substituting Ti ions are in the Ti4+ valence state and prefer to occupy the Mn4+ sites. The lattice structure has slight volume expansion upon the Ti substitution, although the symmetry remains unchanged. A proper Ti4+ substitution surprisingly allows significant enhancement of ferroelectric polarization and appearance of weak ferromagnetism. It is suggested that the substitution induced Ti4+O6 octahedra and modulated exchange-striction are possibly responsible for the enhanced ferroelectricity and improved ferromagnetism. [ABSTRACT FROM AUTHOR]

The structure, ionic valences, magnetism, and magneto-transport behaviors of mixed valence oxides La1–xCaxMn1–xRuxO3 are systematically investigated. The simultaneous substitutions of La3+ and Mn3+ ions by Ca2+ and Ru4+, respectively, are confirmed by the structural and ionic valence characterizations, excluding the presence of Mn4+ and Ru3+ ions. The enhanced ferromagnetism, induced metal-insulator transition, and remarkable magnetoresistance effect are demonstrated when the substitution level x is lower than ~0.6, in spite of the absence of the Mn3+-Ru4+ eg-orbital double-exchange. These anomalous magnetotransport effects are discussed based on the competing multifold interactions associated with the Mn3+-Ru4+ super-exchange and strong Ru4+-Ru4+ hopping, while the origins for the metal-insulator transition and magnetoresistance effect remain to be clarified. [ABSTRACT FROM AUTHOR]

Magnetic orders in (LaFeO3)n-(LaCrO3)n superlattices and in the corresponding LaFe0.5Cr0.5O3 are studied by Monte Carlo simulations. Because of the different exchange couplings of Fe-O-Fe, Cr-O-Cr, and Fe-O-Cr, the superlattices show nontrivial magnetic modulations with the stack periods. Our simulations not only reproduce the experimental observation of strong ferromagnetism in the n = 1 superlattice, but also predict other complex antiferromagnetic or ferrimagnetic orders in thicker cases. The possible chemical phase separation in LaFe0.5Cr0.5O3 bulk is also revealed in our simulation. [ABSTRACT FROM AUTHOR]

A direct inter-conversion between conduction electron spin-current and spin wave spin-current has been experimentally realized in Pt/Y3Fe5O12 and Pt/Y3Fe5O12/Pt heterostructures [Y. Kajiwara et al., Nature 464, 262 (2010)]. We investigate the dynamics of this inter-conversion in an in-plane spiral magnet with the nearest-neighboring interaction and Dzyaloshinskii-Moriya interaction, using the modified Landau-Lifshitz-Gilbert equation. The calculations indicate that an unattenuated conical spin wave in the spiral magnet can be excited by a spin-torque representing a conduction-electron spin-current. The substantial role of the Dzyaloshinskii-Moriya interaction in stabilizing such unattentuated spin wave transmission is revealed. In addition, the influences of the spin-torque term and damping coefficient on the stability of the spin wave transmission are discussed. [ABSTRACT FROM AUTHOR]

The influence of B-site nonmagnetic substitution on the spiral spin ordering in multiferroic manganites is investigated by Monte Carlo simulation within the framework of classical Heisenberg model. It is revealed that the nonmagnetic substitution significantly suppressed the multiferroic phase transitions, consistent with experimental results. The boundary between ab-cycloidal phase and bc-cycloidal phase in the temperature-substitution phase diagram becomes faint gradually with increasing substitution level. A coexistence of two cycloidal spin phases is identified when the substitution level surpassed a threshold. The physical origin for the two-phase coexistence induced by the nonmagnetic B-site substitution is discussed. [ABSTRACT FROM AUTHOR]

We propose a novel practicable self-control scheme to suppress chaos in a spin-torque nano-oscillator in the presence of spin-polarized dc and ac. The magnetization dynamics is investigated by performing micromagnetic simulation. A complete chaos control diagram is obtained, indicating that employment of this proper self-control scheme over a broad frequency range of the ac can greatly reduce the degree of chaoticity in the oscillator. [ABSTRACT FROM AUTHOR]

The conduction relaxations of polycrystalline La0.5Ca0.5Mn0.95Fe0.05O3 upon imposing rectangle electric current pulses (with minute time scale) are studied in details. We observe clear “overshooting” features of the conduction relaxation at both the front and rear edges of the current pulse so long as the pulse is sufficiently high. The Joule heating effect and the intrinsic stabilization of the ferromagnetic metal phase by the current pulse are linked in understanding our observations, and these effects may be applicable for resistance switching function. [ABSTRACT FROM AUTHOR]

We simulate by the Monte Carlo method the dynamic hysteresis and hysteresis scaling of two-dimensional uniaxially anisotropic Heisenberg model submitted to a time-oscillating magnetic field h of frequency f and amplitude h0. It is revealed that the hysteresis area as a function of f and h0 exhibits well-defined power law behaviors over the low-f and high-f regimes as long as h0 is high enough. The power law exponents for the low-f regime depend on the uniaxially anisotropic factor K, but the exponents for the high-f regime are universal. The single-peaked hysteresis dispersions at different h0 follow both the empirical scaling approach and the single-variable scaling hypothesis, respectively, demonstrating the existence of a single characteristic time for spin reversal mode given the amplitude h0. However, the scaling may be broken when h0 is low, due to the coexistence of spin reversal and spin tilting resonances. [ABSTRACT FROM AUTHOR]

Unlike spin ice on pyrochlore lattice, the spin ice structure on kagome lattice retains net magnetic charge, indicating non-negligible dipolar interaction in modulating the spin ice states. While it is predicted that the dipolar spin ice on kagome lattice exhibits a ground state with magnetic charge order and √3x√3 spin order, our work focuses on the magnetization plateau of this system. By employing the Wang-Landau algorithm, it is revealed that the lattice exhibits the fantastic three-step magnetization in response to magnetic field h along the [10] and [01] directions, respectively. For the h//[1 0] case, an additional √3/6Ms step, where Ms is the saturated magnetization, is observed in a specific temperature range, corresponding to a new state with charge order and short-range spin order. [ABSTRACT FROM AUTHOR]

We synthesize a series of orthorhombic polycrystalline YMn1-xCrxO3 (o-YMCO, x < 0.3) samples using high-pressure sintering, and investigate the multiferroicity by various characterization techniques. It is identified that Cr doping remarkably enhances the ferromagnetism, and the ferroelectric polarization is also significantly modulated. The enhanced ferromagnetism is argued to originate from the ferromagnetic interactions between neighbouring Cr3+ spins and Mn3+ spins. While the microscopic mechanism for polarization generation remains to be clarified, it may be argued, based on the response of polarization against external magnetic field, that Cr doping induces a partial transition from the symmetric exchange striction mechanism in o-YMnO3 to the asymmetric exchange striction mechanism in o-YMCO for ferroelectricity generation [ABSTRACT FROM AUTHOR]

In this proceeding, recent theoretical investigations by the authors on the multiferroic RMnO3 perovskites are briefly reviewed at first. Using the double-exchange model, the realistic spiral spin order in undoped manganites such as TbMnO3 and DyMnO3 is well reproduced by incorporating a weak next-nearest neighbor superexchange (~10% of nearest neighbor superexchange) and moderate Jahn-Teller distortion. The phase transitions from the A-type antiferromagnet (as in LaMnO3), to the spiral phase (as in TbMnO3), and finally to the E-type antiferromagnet (as in HoMnO3), with decreasing size of the R ions, were also explained. Moreover, new results of phase diagram of the three-dimensional lattice are also included. The ferromagnetic tendency recently discovered in the LaMnO3 and TbMnO3 thin films is explained by considering the substrate stress. Finally, the relationship between our double-exchange model and a previously used J1-J2-J3 model is further discussed from the perspective of spin wave excitations. [ABSTRACT FROM AUTHOR]

Nanosized YMnO3 powders of grain size below 100 nm were prepared and characterized by various techniques to understand the size effect of the microstructures and the stoichiometry of the as-prepared powders. It is revealed that the grain surface oxidization state depends on grain size, with a lower oxidization state for the smaller grains. Raman and infrared spectroscopy studies show a remarkable broadening of the reflection peaks with decreasing grain size, identifying the significant surface structure relaxation effect for small-size powders. The ferromagnetic–antiferromagnetic transition around 65–68 °C is determined by the specific-heat measurement, also indicating the remarkable size dependence. [ABSTRACT FROM AUTHOR]

Single crystals of Pb(Fe 1/2 Nb 1/2 )O 3 (PFN) are grown from high temperature solution with PbO as flux. Mössbauer study on the powder samples of Pb(Fe 1/2 Nb 1/2 )O 3 single crystals has been made. It proves that the ferromagnetic phase transition temperature is below 150 K. The temperature dependence of the quadrupole splitting of the PFN samples shows the iron ions in PFN single crystals is in Fe 3+ high spin state. The calculation proved that the B site niobium ions and iron ions are randomly distributed. [ABSTRACT FROM AUTHOR]

BiFe1-4x/3TixO3 (x = 0-0.2) ceramics with Fe vacancies created by nonstioichiometric compositions were synthesized to study their crystal structures, dielectric, and ferromagnetic behaviors. X-ray diffraction and Raman spectroscopy analysis reveal a structure transition from rhombohedral to orthorhombic phases at x = 0.2. Dielectric measurement shows that the dielectric loss is significantly reduced in the Ti-doped BiFeO3 samples. With increasing x concentration, the remanent magnetization (Mr) first increases and then decreases. The maximal Mr of 0.13 emμ/g is obtained at x = 0.05. Furthermore, theoretical calculations based on the density-functional theory prove that the Ti-doping does enhance the lattice constants, band gap, and magnetization. These results show that the Ti-doped BiFeO3 with Fe vacancies could enhance resistivity and magnetism, implying a possible improvement in multiferroic behavior. [ABSTRACT FROM AUTHOR]

Polycrystalline CuCr1-xNixO2 is synthesized and its multiferrocity is characterized in order to enhance the ferromagnetism and ferroelectricity of CuCrO2-based multiferroics. At the optimized doping level x=0.05, we observe not only an enhancement of one order of magnitude in magnetization but also a remarkable increasing of polarization up to ∼50 μC/m2 from ∼35 μC/m2 of polycrystalline CuCrO2. It is argued that the Ni-doping may modulate the antiferromagnetic interactions between Cr3+ ions and probably induce the conical-like spin component responsible for the enhanced ferromagnetism. [ABSTRACT FROM AUTHOR]

Perovskite-type polycrystalline La1/3Sr2/3FeO3 particles with different sizes (80–2000 nm) were prepared using a simple sol-gel technique. In samples of nanoparticles with a diameter of less than 300 nm, weak ferromagnetism was observed at room temperature, which was attributed to the lattice distortion. The magnetic and specific heat measurements suggest that the charge ordering state was largely suppressed due to the lowering of the particle size, but the charge ordering temperature remained unaffected. [ABSTRACT FROM AUTHOR]

La0.4Ca0.6MnO3 nanoparticles of grain size as small as ∼20 nm are prepared and their magnetic behaviors are investigated in order to understand the size effect of the charge ordering in manganites. The highly stable charge-ordered state can be significantly suppressed upon reduction of the grain size down to nanometer scale, while the ferromagnetism is enhanced. The magnetic phase separation due to the competition between ferromagnetic state and charge-ordered state as well as the surface spin disordering is responsible for the spin-glass-like state at low temperature. [ABSTRACT FROM AUTHOR]

BiFeO3 films prepared by pulsed laser deposition on Pt/TiO2/SiO2/Si substrates were studied as photoelectrode for water splitting. Under visible-light irradiation, the photocurrent intensity of the polycrystalline BiFeO3 film was found to double that of the amorphous one in a three-electrode cell. The incident photon to current conversion efficiency for the polycrystalline BiFeO3 electrode was approximately 16% at 350 nm and 7% at 530 nm at 1.5 V (versus saturated calomel electrode). The ferromagnetism of the amorphous BiFeO3 film was an order of magnitude weaker than that of the polycrystalline one, supporting the “size effect” explanation for magnetic origin. [ABSTRACT FROM AUTHOR]

Recent experiments showed that the robust charge ordering in manganites can be weakened by reducing the grain size down to nanoscale. Weak ferromagnetism was evidenced in both nanoparticles and nanowires of charge-ordered manganites. To explain these observations, a phenomenological model based on surface phase separation is proposed. The relaxation of superexchange interaction on the surface layer allows formation of a ferromagnetic shell, whose thickness increases with decreasing grain size. Possible exchange bias and softening of the ferromagnetic transition in nanosized charge-ordered manganites are predicted. [ABSTRACT FROM AUTHOR]

The room-temperature weak ferromagnetism of amorphous HfAlOx thin films deposited by pulsed laser deposition on various substrates in oxygen-defective ambient is demonstrated. The magnetization is independent of film thickness, but depends on substrates and deposition temperatures. A magnetic moment of ∼0.26μB per HfAlOx f.u. is recorded for HfAlOx films deposited under optimized conditions [deposited at 600 °C on (001) sapphire in high vacuum]. It is argued that interfacial defects are one of the possible sources of the weak ferromagnetism. [ABSTRACT FROM AUTHOR]

Single-phase Bi1-xNdxFeO3 (BNFOx) (x=0–0.2) multiferroic ceramics were prepared to study the effects of Nd substitution on their crystal structure and ferroelectromagnetic behavior. Rietveld refinement of x-ray diffraction data showed a continual transformation of crystal structure from the rhombohedral structure of BNFOx=0 (BiFeO3) to a triclinic structure in BNFOx=0.05–0.15 and a pseudotetragonal structure in BNFOx=0.175–0.2. Ferroelectromagnetic measurements revealed the existence of ferroelectricity with remnant polarization of ∼9 μC/cm2 in BNFOx=0–0.175, paraelectricity in BNFOx=0.2, and weak ferromagnetism with remnant magnetizations of 0.07–0.227 emu/g in BNFOx=0.15–0.2. Magnetoelectric coupling was obvious in BNFOx=0.15–0.175 near the Néel temperature of ∼380 °C. [ABSTRACT FROM AUTHOR]

We study the magnetic phase diagram of a classical Heisenberg spin model on the Shastry-Sutherland lattice using the Monte Carlo method. The simulated results indicate that the particular collinear phase region can be enlarged due to the implementation of easy-axis anisotropy, leading to the broadening of the magnetization plateau in the classical Heisenberg model. Also, the random exchange term is taken into account to study the effect of inhomogeneity on the steplike magnetization feature. It is indicated that the multi-step magnetic behaviors in the low temperature range remain observable when the inhomogeneity is in the appropriate range. [ABSTRACT FROM AUTHOR]

With the discovery of two-dimensional (2D) ferroelectricity and ferromagnetism in van der Waals (vdW) materials, there has been significant interest in 2D multiferroics. Herein, we report the occurrence of weak ferromagnetism and magnetoelectricity in vdW antiferromagnet MnPSe3 single crystals. Our results demonstrate that MnPSe3 undergoes an antiferromagnetic transition at the Néel temperature TN = 70 K with weak ferromagnetism along the [1–10] direction. Detailed magnetoelectric (ME) data show that MnPSe3 exhibits a linear ME tensor αij with nine nonzero components. Additionally, a magnetically induced electric polarization as large as 98.5 μC/m2 is observed along the [110] direction, with a ME coefficient of 13.5 ps/m at 10 K for a magnetic field of 9 T applied along the [110] direction. Importantly, we discuss our experiments based on symmetry and microscopic analysis, thereby suggesting that the spin-dependent p-d hybridization mechanism plays an important role in the emergence of magnetic-field-induced ferroelectricity. Hence, our findings provide insights for exploring the ME coupling in vdW materials. [ABSTRACT FROM AUTHOR]

The eg-orbital double-exchange mechanism as the core of physics of colossal magnetoresistance (CMR) manganites is well known, which usually covers up the role of super-exchange at the t2g-orbitals. The role of the double-exchange mechanism is maximized in La0.7Ca0.3MnO3, leading to the concurrent metal-insulator transition and ferromagnetic transition as well as CMR effect. In this work, by a set of synchronous Ru-substitution and Ca-substitution experiments on La0.7-yCa0.3+yMn1-yRuyO3, we demonstrate that the optimal ferromagnetism in La0.7Ca0.3MnO3 can be further enhanced. It is also found that the metal-insulator transition and magnetic transition can be separately modulated. By well-designed experimental schemes with which the Mn3+-Mn4+ double-exchange is damaged as weakly as possible, it is revealed that this ferromagnetism enhancement is attributed to the Mn-Ru t2g ferromagnetic super-exchange. The present work allows a platform on which the electro-transport and magnetism of rare-earth manganites can be controlled by means of the t2g-orbital physics of strongly correlated transition metal oxides. [ABSTRACT FROM AUTHOR]

The conduction relaxations of polycrystalline La0.5Ca0.5Mn0.95Fe0.05O3 upon imposing rectangle electric current pulses (with minute time scale) are studied in details. We observe clear “overshooting” features of the conduction relaxation at both the front and rear edges of the current pulse so long as the pulse is sufficiently high. The Joule heating effect and the intrinsic stabilization of the ferromagnetic metal phase by the current pulse are linked in understanding our observations, and these effects may be applicable for resistance switching function. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the quantum and classical pairwise correlations in the one-dimensional anisotropic XY spin chain with a single defect. The rigorous analysis and numerical results reveal that the quantum discord and classical correlation in different quantum phases present different responses to the defect. In particular, we show that the non-local long-range quantum discord can be modified by a local control of the defect under ferromagnetic phase in such system. [Copyright &y& Elsevier]

Abstract: The phase diagram of a ferromagnetic XY model with a nematic coupling (coupling strength x) on a triangular lattice is studied by means of Monte Carlo simulation. The algebraic-magnetic order associated with Kosterlitz–Thouless (KT) transition is observed over the whole x range. In the large x region, the phase transition from the algebraic-magnetic order to the algebraic-nematic order occurs at T I. In addition, this phase transition can be scaled with the two-dimensional Ising critical exponents, demonstrating that the present system belongs to the universality class of Ising transition at T I. [Copyright &y& Elsevier]

We synthesize a series of orthorhombic polycrystalline YMn1-xCrxO3 (o-YMCO, x < 0.3) samples using high-pressure sintering, and investigate the multiferroicity by various characterization techniques. It is identified that Cr doping remarkably enhances the ferromagnetism, and the ferroelectric polarization is also significantly modulated. The enhanced ferromagnetism is argued to originate from the ferromagnetic interactions between neighbouring Cr3+ spins and Mn3+ spins. While the microscopic mechanism for polarization generation remains to be clarified, it may be argued, based on the response of polarization against external magnetic field, that Cr doping induces a partial transition from the symmetric exchange striction mechanism in o-YMnO3 to the asymmetric exchange striction mechanism in o-YMCO for ferroelectricity generation [ABSTRACT FROM AUTHOR]

Abstract: Multiferroic DyFeO3 thin films deposited on (001) Nb-doped SrTiO3 (NSTO) substrates are prepared by pulsed laser deposition. We demonstrate the epitaxial deposition of DyFeO3 thin films on the substrates with the DyFeO3(001)//NSTO(001) out-of-plane and DyFeO3[100]//NSTO[110] in-plane epitaxial relationship. In addition, the weak ferromagnetism and significant magnetodielectric coupling effect at low temperature are revealed. It is indicated that the DyFeO3 thin films have very different properties from the DyFeO3 single crystals in term of multiferroicity. [ABSTRACT FROM AUTHOR]

Abstract: The thin films of Ni-doped dielectric LaAlO3 (LAO) by a co-ablation of magnetic metal Ni and dielectric LAO on silicon-based substrates have been prepared by pulsed laser deposition. A significant enhancement of dielectric constant of LAO upon a Ni doping is observed. Furthermore, the dielectric modulation by applying a magnetic field to the samples is verified, obviously due to the ferromagnetism of Ni metal clusters embedded in the LAO thin films. A series of microstructural and dielectric characterizations on the as-prepared thin films has been performed and the mechanism underlying the dielectric enhancement upon the Ni doping is discussed. [Copyright &y& Elsevier]

Polycrystalline CuCr1-xNixO2 is synthesized and its multiferrocity is characterized in order to enhance the ferromagnetism and ferroelectricity of CuCrO2-based multiferroics. At the optimized doping level x=0.05, we observe not only an enhancement of one order of magnitude in magnetization but also a remarkable increasing of polarization up to ∼50 μC/m2 from ∼35 μC/m2 of polycrystalline CuCrO2. It is argued that the Ni-doping may modulate the antiferromagnetic interactions between Cr3+ ions and probably induce the conical-like spin component responsible for the enhanced ferromagnetism. [ABSTRACT FROM AUTHOR]

Abstract: We prepare polycrystalline CuCrO2 and CuCr0.98Ni0.02O2 samples by solid state reaction and investigate their multiferroicity. It is revealed that the Ni-doping can not only enhance slightly magnetization but also improve ferroelectricity. It is argued that the Ni3+-doping destabilizes the antiferromagnetic order of Cr3+ ions and modulates the spin configuration, leading to the weak ferromagnetism and enhanced ferroelectric polarization. The coupling between the magnetic order and ferroelectric order is also characterized. [ABSTRACT FROM AUTHOR]