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تسجيل مستخدم جديدGinzburg-Landau theory for the conical cycloid state in multiferroics: applications to CoCr$_2$O$_4$
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We show that the cycloidal magnetic order of a multiferroic can arise in the absence of spin and lattice anisotropies, for e.g., in a cubic material, and this explains the occurrence of such a state in CoCr$_2$O$_4$. We discuss the case when this order coexists with ferromagnetism in a so called `conical cycloid state, and show that a direct transition to this state from the ferromagnet is necessarily first order. On quite general grounds, the reversal of the direction of the uniform magnetization in this state can lead to the reversal of the electric polarization as well, without the need to invoke `toroidal moment as the order parameter.
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Magnetoelectric materials have generated wide technological and scientific interest because of the rich phenomena these materials exhibit, including the coexistence of magnetic and ferroelectric orders, magnetodielectric behavior, and exotic hybrid e
xcitations such as electromagnons. The multiferroic spinel material, CoCr$_2$O$_4$, is a particularly interesting example of a multiferroic material, because evidence for magnetoelectric behavior in the ferrimagnetic phase seems to conflict with traditional noncollinear-spin-driven mechanisms for inducing a macroscopic polarization. This paper reports an inelastic light scattering study of the magnon and phonon spectrum of CoCr$_2$O$_4$ as simultaneous functions of temperature, pressure, and magnetic field. Below the Curie temperature ($T_C sim 94$ K) of CoCr$_2$O$_4$ we observe a $omega sim 16 ,text{cm}^{-1}$ $boldsymbol q=0$ magnon having T$_{1g}$-symmetry, which has the transformation properties of an axial vector. The anomalously large Raman intensity of the T$_{1g}$-symmetry magnon is characteristic of materials with a large magneto-optical response and likely arises from large magnetic fluctuations that strongly modulate the dielectric response in CoCr$_2$O$_4$. The Raman susceptibility of the T$_{1g}$-symmetry magnon exhibits a strong magnetic-field dependence that is consistent with the magnetodielectric response observed in CoCr$_2$O$_4$, suggesting that magnetodielectric behavior in CoCr$_2$O$_4$ primarily arises from the field-dependent suppression of magnetic fluctuations that are strongly coupled to long-wavelength phonons. Increasing the magnetic anisotropy in CoCr$_2$O$_4$ with applied pressure decreases the magnetic field-dependence of the T$_{1g}$-symmetry magnon Raman susceptibility in CoCr$_2$O$_4$, suggesting that strain can be used to control the magnetodielectric response in CoCr$_2$O$_4$.
We report on an investigation of optical properties of multiferroic CoCr$_{2}$O$_{4}$ at terahertz frequencies in magnetic fields up to 30 T. Below the ferrimagnetic transition (94 K), the terahertz response of CoCr$_{2}$O$_{4}$ is dominated by a mag
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We report a soft x-ray resonant magnetic scattering study of the spin configuration in multiferroic thin films of Co$_{0.975}$Ge$_{0.025}$Cr$_2$O$_4$ (Ge-CCO) and CoCr$_2$O$_4$ (CCO), under low- and high-magnetic fields, from 0.2 T up to 6.5 T. A cha
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