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Register a new userMagnetic field dependent cycloidal rotation in pristine and Ge doped CoCr$_2$O$_4$
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Nazaret Ortiz Hernandez
Publication date
2020
fields
Physics
and research's language is
English
Authors
N Ortiz Hernandez
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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 characterization of Ge-CCO at a low magnetic field is performed and the results are compared to those of pure CCO. The ferrimagnetic phase transition temperature $T_C approx 95$ K and the multiferroic transition temperature $T_S approx 27$ K in Ge-CCO are comparable to those observed in CCO. In Ge-CCO, the ordering wave vector $textit{(qq0)}$ observed below $T_S$ is slightly larger compared to that of CCO, and, unlike CCO, the diffraction intensity consists of two contributions that show a dissimilar x-ray polarization dependence. In Ge-CCO, the coercive field observed at low temperatures was larger than the one reported for CCO. In both compounds, an unexpected reversal of the spiral helicity and therefore the electric polarization was observed on simply magnetic field cooling. In addition, we find a change in the helicity as a function of momentum transfer in the magnetic diffraction peak of Ge-CCO, indicative of the presence of multiple magnetic spirals.
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We study $S=1/2$ dimer excitation in a coupled chain and dimer compound Cu$_2$Fe$_2$Ge$_4$O$_{13} by inelastic neutron scattering technique. The Zeeman split of the dimer triplet by a staggered field is observed at low temperature. With the increase of temperature the effect of random field is detected by a drastic broadening of the triplet excitation. Basic dynamics of dimer in the staggered and random fields are experimentally identified in Cu$_2$Fe$_2$Ge$_4$O$_{13}.
Cu$_2$Fe$_2$Ge$_4$O$_{13}$ is a bicomponent compound that consists of Cu dimers and Fe chains with separate energy scale. By inelastic neutron scattering technique with high-energy resolution we observed the indirect Fe - Fe exchange coupling by way of the Cu dimers. The obtained parameters of the effective indirect interaction and related superexchange interactions are consistent with those estimated semi-statically. The consistency reveals that the Cu dimers play the role of nonmagnetic media in the indirect magnetic interaction.
We report the results of a $^{45}$Sc nuclear magnetic resonance (NMR) study on the quasi-one-dimensional compound Cu$_2$Sc$_2$Ge$_4$O$_{13}$ at temperatures between 4 and 300 K. This material has been a subject of current interest due to indications of spin gap behavior. The temperature-dependent NMR shift exhibits a character of low-dimensional magnetism with a negative broad maximum at $T_{max}$ $simeq $ 170 K. Below $% T_{max}$, the NMR shifts and spin lattice relaxation rates clearly indicate activated responses, confirming the existence of a spin gap in Cu$_2$Sc$_2$Ge% $_4$O$_{13}$. The experimental NMR data can be well fitted to the spin dimer model, yielding a spin gap value of about 275 K which is close to the 25 meV peak found in the inelastic neutron scattering measurement. A detailed analysis further points out that the nearly isolated dimer picture is proper for the understanding of spin gap nature in Cu$_2$Sc$_2$Ge$_4$O$_{13}$.
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 excitations 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$_4$.
The unusual magnetic properties of a novel low-dimensional quantum ferrimagnet Cu$_2$Fe$_2$Ge$_4$O$_{13}$ are studied using bulk methods, neutron diffraction and inelastic neutron scattering. It is shown that this material can be described in terms of two low-dimensional quantum spin subsystems, one gapped and the other gapless, characterized by two distinct energy scales. Long-range magnetic ordering observed at low temperatures is a cooperative phenomenon caused by weak coupling of these two spin networks.
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