Magnetic-field induced competition of two multiferroic orders in a triangular-lattice helimagnet MnI2

Magnetic and dielectric properties with varying magnitude and direction of magnetic field H have been investigated for a triangular lattice helimagnet MnI2. The in-plane electric polarization P emerges in the proper screw magnetic ground state below 3.5 K, showing the rearrangement of six possible multiferroic domains as controlled by the in-plane H. With every 60-degree rotation of H around the [001]-axis, discontinuous 120-degree flop of P-vector is observed as a result of the flop of magnetic modulation vector q. With increasing the in-plane H above 3 T, however, the stable q-direction changes from q|| to q||, leading to a change of P-flop patterns under rotating H. At the critical field region (~3 T), due to the phase competition and resultant enhanced q-flexibility, P-vector smoothly rotates clockwise twice while H-vector rotates counter-clockwise once.Comment: 4 pages, 3 figures. Accepted in Physical Review Letter

Cupric chloride CuCl2 as an S=1/2 chain multiferroic

Magnetoelectric properties were investigated for an S=1/2 chain antiferromagnet CuCl2, which turns out to be the first example of non-chalcogen based spiral-spin induced multiferroics. Upon the onset of helimagnetic order propagating along the b-axis under zero magnetic field (H), we found emergence of ferroelectric polarization along the c-axis. Application of H along the b-axis leads to spin-flop transition coupled with drastic suppression of ferroelectricity, and rotation of H around the b-axis induces the rotation of spin-spiral plane and associated polarization direction. These behaviors are explained well within the framework of the inverse Dzyaloshinskii-Moriya model, suggesting the robustness of this magnetoelectric coupling mechanism even under the strong quantum fluctuation.Comment: 5 pages, 5 figure

Direct observation of cycloidal spin modulation and field-induced transition in N\'eel-type skyrmion-hosting VOSe2_2O5_5

We investigate the spin rotational structure of magnetic skyrmions in a tetragonal polar magnet VOSe2O5 via polarized small-angle neutron scattering (SANS). Spin polarization analysis of the scattered neutrons provides consistent evidence for the cycloidal spin modulation in all the incommensurate phases at zero and non-zero magnetic field along the c axis, including the triangular skyrmion-lattice phase. In the vicinity of the skyrmion phase, we performed extensive SANS measurements to unravel a field-induced incommensurate phase (IC-2 state). We discuss the possibility of anisotropic double-q state as an alternative spin structure to provisional square skyrmion-lattice state.Comment: 27 pages, 5 figure

Signatures of Ultrafast Reversal of Excitonic Order in Ta₂NiSe₅

In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta₂NiSe₅ and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta₂NiSe₅ based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our Letter expands the field of ultrafast order parameter control beyond spin and charge ordered materials

Advances in ab-initio theory of Multiferroics. Materials and mechanisms: modelling and understanding

Within the broad class of multiferroics (compounds showing a coexistence of magnetism and ferroelectricity), we focus on the subclass of "improper electronic ferroelectrics", i.e. correlated materials where electronic degrees of freedom (such as spin, charge or orbital) drive ferroelectricity. In particular, in spin-induced ferroelectrics, there is not only a {\em coexistence} of the two intriguing magnetic and dipolar orders; rather, there is such an intimate link that one drives the other, suggesting a giant magnetoelectric coupling. Via first-principles approaches based on density functional theory, we review the microscopic mechanisms at the basis of multiferroicity in several compounds, ranging from transition metal oxides to organic multiferroics (MFs) to organic-inorganic hybrids (i.e. metal-organic frameworks, MOFs)Comment: 22 pages, 9 figure

Creation and annihilation of topological meron pairs in in-plane magnetized films

Merons which are topologically equivalent to one-half of skyrmions can exist only in pairs or groups in two-dimensional (2D) ferromagnetic (FM) systems. The recent discovery of meron lattice in chiral magnet Co8Zn9Mn3 raises the immediate challenging question that whether a single meron pair, which is the most fundamental topological structure in any 2D meron systems, can be created and stabilized in a continuous FM film? Utilizing winding number conservation, we develop a new method to create and stabilize a single pair of merons in a continuous Py film by local vortex imprinting from a Co disk. By observing the created meron pair directly within a magnetic field, we determine its topological structure unambiguously and explore the topological effect in its creation and annihilation processes. Our work opens a pathway towards developing and controlling topological structures in general magnetic systems without the restriction of perpendicular anisotropy and Dzyaloshinskii-Moriya interaction

Néel-Type Skyrmions Detected through Polarized Small-Angle Neutron Scattering

Magnetic skyrmions are two-dimensional, vortex-like spin states that carry a topological number. Due to their particle nature and emergent electromagnetic properties, skyrmions are viewed as promising candidates for information transport in future spintronics devices. A skyrmion can be described by the internal degrees of freedom of the spin configuration, termed helicity and vorticity. The two main types of skyrmions, Bloch and Néel-type, are characterized by the helicity , which is defined by the angle between the spin-modulation vector q and the spin-rotation plane [1]. Bloch-type skyrmions are characterized by a proper-screw type spin configuration with = ±/2 (Fig. 1(a) and (b)) while Néel-type skyrmions have a cycloidal spin modulation with = 0 or (Fig. 1(c) and (d)). Recent studies have shown that this internal spin rotational form plays a key role in the current-induced control of skyrmions through spin-induced torques. To harness the spintronic functionalities of skyrmions, developing experimental techniques to determine the helicity is fundamentally important