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Register a new userCoupling of magnetic and ferroelectric hysteresis by a multi-component magnetic structure in Mn2GeO4
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The olivine compound Mn2GeO4 is shown to feature both a ferroelectric polarization and a ferromagnetic magnetization that are directly coupled and point along the same direction. We show that a spin spiral generates ferroelectricity (FE), and a canted commensurate order leads to weak ferromagnetism (FM). Symmetry suggests that the direct coupling between the FM and FE is mediated by Dzyaloshinskii-Moriya interactions that exist only in the ferroelectric phase, controlling both the sense of the spiral rotation and the canting of the commensurate structure. Our study demonstrates how multi-component magnetic structures found in magnetically-frustrated materials like Mn2GeO4 provide a new route towards functional materials that exhibit coupled FM and FE.
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The interaction between strain and spin has received intensive attention in the scientific community due to its abundant physical phenomena and huge technological impact. Until now, there is no experimental report on ultra-high frequency magnetic resonance through the strain-spin coupling for any technologically relevant perpendicular magnetic material. Here we report the experimental detection of the acoustic strain waves that have a response time on the order of 10 picoseconds in perpendicular magnetic [Co/Pd]n multilayers via a femtosecond laser pulse excitation. Through direct measurements of acoustic strain waves, we observe an ultra-high frequency magnetic resonance up to 60 GHz in [Co/Pd]n multilayers. We further report a theoretical model of the strain-spin interaction. Our model reveals that the energy could be transferred efficiently from the strain to the spins and well explains the existence of a steady resonance state through exciting the spin system. The physical origins of the resonance between strain waves and magnetic precession and the requested conditions for obtaining magnetic resonance within thin magnetic films have also been discussed after thorough analysis. These combined results point out a potential pathway to enable an extremely high frequency (EHF) magnetic resonance through the strain-spin coupling.
Magnetoelectric coupling in ferromagnet/multiferroic systems is often manifested in the exchange bias effect, which may have combined contributions from multiple sources, such as domain walls, chemical defects or strain. In this study we magnetically fingerprint the coupling behavior of CoFe grown on epitaxial BiFeO3 (BFO) thin films by magnetometry and first-order-reversal-curves (FORC). The contribution to exchange bias from 71{deg}, 109{deg} and charged ferroelectric domain walls (DWs) was elucidated by the FORC distribution. CoFe samples grown on BFO with 71{deg} DWs only exhibit an enhancement of the coercivity, but little exchange bias. Samples grown on BFO with 109{deg} DWs and mosaic DWs exhibit a much larger exchange bias, with the main enhancement attributed to 109{deg} and charged DWs. Based on the Malozemoff random field model, a varying-anisotropy model is proposed to account for the exchange bias enhancement. This work sheds light on the relationship between the exchange bias effect of the CoFe/BFO heterointerface and the ferroelectric DWs, and provides a path for multiferroic device analysis and design.
Ti-substituted perovskites, La0.7Sr0.3Mn1-xTixO3, with x between 0 to 0.20, were investigated by neutron diffraction, magnetization, electric resistivity, and magnetoresistance (MR) measurements. All samples show a rhombohedral structure (space group R3c) from 10 K to room temperature. At room temperature, the cell parameters a, c and the unit cell volume increase with increasing Ti content. However, at 10 K, the cell parameter a has a maximum value for x = 0.10, and decreases for x greater than 0.10, while the unit cell volume remains nearly constant for x greater than 0.10. The average (Mn,Ti)-O bond length increases up to x=0.15, and the (Mn,Ti)-O-(Mn,Ti) bond angle decreases with increasing Ti content to its minimum value at x=0.15 at room temperature. Below the Curie temperature T_C, the resistance exhibits metallic behavior for the x _ 0.05 samples. A metal (semiconductor) to insulator transition is observed for the x_ 0.10 samples. A peak in resistivity appears below T_C for all samples, and shifts to a lower temperature as x increases. The substitution of Mn by Ti decreases the 2p-3d hybridization between O and Mn ions, reduces the bandwidth W, and increases the electron-phonon coupling. Therefore, the TC shifts to a lower temperature and the resistivity increases with increasing Ti content. A field-induced shift of the resistivity maximum occurs at x less than or equal to 0.10. The maximum MR effect is about 70% for La0.7Sr0.3Mn0.8Ti0.2O3. The separation of TC and the resistivity maximum temperature Tmax enhances the MR effect in these compounds due to the weak coupling between the magnetic ordering and the resistivity as compared with La0.7Sr0.3MnO3.
We investigate the ferroelectric phase transition and domain formation in a periodic superlattice consisting of alternate ferroelectric (FE) and paraelectric (PE) layers of nanometric thickness. We find that the polarization domains formed in the different FE layers can interact with each other via the PE layers. By coupling the electrostatic equations with those obtained by minimizing the Ginzburg-Landau functional we calculate the critical temperature of transition Tc as a function of the FE/PE superlattice wavelength and quantitatively explain the recent experimental observation of a thickness dependence of the ferroelectric transition temperature in KTaO3/KNbO3 strained-layer superlattices.
This paper presents results of a recent study of multiferroic CCO by means of single crystal neutron diffraction. This system has two close magnetic phase transitions at $T sub{N1}=24.2$ K and $T sub{N2}=23.6$ K. The low temperature magnetic structure below $T sub{N2}$ is unambiguously determined to be a fully 3-dimensional proper screw. Between $T sub{N1}$ and $T sub{N2}$ antiferromagnetic order is found that is essentially 2-dimensional. In this narrow temperature range, magnetic near neighbor correlations are still long range in the ($H,K$) plane, whereas nearest neighbors along the $L$-direction are uncorrelated. Thus, the multiferroic state is realized only in the low-temperature 3-dimensional state and not in the 2-dimensional state.
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