RESUMEN
An electromagnon in the magnetoelectric (ME) hexaferrite Ba_{0.5}Sr_{2.5}Co_{2}Fe_{24}O_{41} (Co_{2}Z-type) single crystal is identified by time-domain terahertz (THz) spectroscopy. The associated THz resonance is active on the electric field (E^{ω}) of the THz light parallel to the c axis (⥠[001]), whose spectral weight develops at a markedly high temperature, coinciding with a transverse conical magnetic order below 410 K. The resonance frequency of 1.03 THz at 20 K changes -8.7% and +5.8% under external magnetic field (H) of 2 kOe along [001] and [120], respectively. A model Hamiltonian describing the conical magnetic order elucidates that the dynamical ME effect arises from antiphase motion of spins which are coupled with modulating electric dipoles through the exchange striction mechanism. Moreover, the calculated frequency shift points to the key role of the Dzyaloshinskii-Moriya interaction that is altered by static electric polarization change under different H.
RESUMEN
Ultrafast optical manipulation of magnetic phenomena is an exciting achievement of mankind, expanding one's horizon of knowledge toward the functional nonequilibrium states. The dynamics acting on an extremely short timescale push the detection limits that reveal fascinating light-matter interactions for nonthermal creation of effective magnetic fields. While some cases are benchmarked by emergent transient behaviors, otherwise identifying the nonthermal effects remains challenging. Here, a femtosecond time-resolved resonant magnetic X-ray diffraction experiment is introduced, which uses an X-ray free-electron laser (XFEL) to distinguish between the effective field and the photoinduced thermal effect. It is observed that a multiferroic Y-type hexaferrite exhibits magnetic Bragg peak intensity oscillations manifesting entangled antiferromagnetic (AFM) and ferromagnetic (FM) Fourier components of a coherent AFM magnon. The magnon trajectory constructed in 3D space and time domains is decisive to evince ultrafast field formation preceding the lattice thermalization. A remarkable impact of photoexcitation across the electronic bandgap is directly unraveled, amplifying the photomagnetic coupling that is one of the highest among AFM dielectrics. Leveraging the above-bandgap photoexcitation, this energy-efficient optical process further suggests a novel photomagnetic control of ferroelectricity in multiferroics.
RESUMEN
The Fe3+ and Co2+ NMR spectra for Ba0.3Sr1.7Co2(Fe0.96Al0.04)12O22 (BSCFAO) and Ba0.3Sr1.7Co2Fe12O22 (BSCFO) were obtained in a zero magnetic field at a low temperature. We observed change in the enhancement effect of the NMR signals depending on the setting field, which was varied when applied along the b-axis and then turned off before the measurement was taken. The experimental results indicate that the magnetic structure changes from an alternating longitudinal cone to a transverse cone when the setting field is 250 mT. They also show that the spins of Co2+ ions together with those of Fe3+ ions constitute a part of the overall magnetic structure and that the substitution of Al3+ for Fe3+ weakens the magnetic anisotropy within the easy plane. From a comparison of the enhancement factors of the Fe3+ NMR obtained with the RF pulse applied along the a-axis and the c-axis, we found that the magnetic easy plane anisotropy is approximately 16 times greater than the anisotropy within the easy plane. No changes of the NMR spectra were observed under an electric field of 1.2 MV m-1.
RESUMEN
The magnetocaloric effect (MCE) associated with the spin transitions of alternating longitudinal conical (ALC)-mixed conical (MC) and MC-ferrimagnetic (FIM) states in a Ba0.3Sr1.7Co2Fe12O22 single crystal has been investigated. For magnetic field directions applied along either the [120] or [001] directions, the crystal is found to exhibit the conventional and inverse MCE near the ALC-MC (T(N1) = 235 K) and MC-FIM (T(N2) = 348 K) states, respectively. The dependence of the magnetic entropy on the magnetic field also exhibits such sign change behaviors in the MCE, which is attributed to the magnetic field induced gradual collapse of heliconical magnetic order.