Terahertz and infrared spectroscopic evidence of phonon-paramagnon coupling in hexagonal piezomagnetic YMnO_{3} (original) (raw)
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Spin lattice coupling in multiferroic hexagonal YMnO3
Pramana, 2008
Aiming to shed light on the possible existence of hybrid phonon-magnon excitations in multiferroic manganites, neutron scattering measurements have been undertaken at LLB and ILL on the particular case of hexagonal YMnO3. Our experiments focused on a transverse acoustic phonon mode polarized along the ferroelectric axis. The neutron data show that below the magnetic transition, this particular phonon mode splits in two different branches. The upper branch is found to coincide with a spin wave mode. This manifestation of a strong spin-lattice coupling is discussed in terms of a possible hybridization between the two types of elementary excitations, the phonon and magnons.
Phonons and magnetic excitation correlations in weak ferromagnetic YCrO3
Journal of Applied Physics, 2014
Here, we report the temperature dependent Raman spectroscopic studies on orthorhombically distorted perovskite YCrO 3 over a temperature range of 20-300 K. Temperature dependence of DC-magnetization measurements under field cooled and zero field cooled protocols confirmed a N eel transition at T N $ 142 K. Magnetization isotherms recorded at 125 K show a clear loop opening without any magnetization saturation up to 20 kOe, indicating a coexistence of antiferromagnetic (AFM) and weak ferromagnetic (WFM) phases. Estimation of exchange constants using mean-field approximation further confirm the presence of a complex magnetic phase below T N . Temperature evolution of Raman line-shape parameters of the selected modes (associated with the octahedral rotation and A(Y)-shift in the unit-cell) reveal an anomalous phonon shift near T N . An additional phonon anomaly was identified at T * $ 60 K, which could possibly be attributed to the change in the spin dynamics. Moreover, the positive and negative shifts in Raman frequencies between T N and T * suggest competing WFM and AFM interactions. A close match between the phonon frequency of B 3g (3)-octahedral rotation mode with the square of sublattice magnetization between T N and T * is indicative of the presence of spin-phonon coupling in multiferroic YCrO 3 . V C 2014 AIP Publishing LLC. [http://dx.
Journal of Magnetism and Magnetic Materials, 2001
The purpose of the present work is a quantitative investigation of the biquadratic exchange interaction effects on the paramagnetic-ferrimagnetic transition arising from two strongly coupled paramagnetic (1-spin) sublattices, of respective moments m and M. The free energy describing the physics of the system is of Landau type. In addition to the quadratic and quartic terms, in both m and M, this free energy involves two mixing interaction terms. The first is a lowest order coupling ÀCmM, where C50 stands for the coupling constant measuring the interaction between the two sublattices. While the second, which is relevant for 1-spin systems and which traduces the dipole-dipole (or biquadratic) interaction, is of type wm 2 M 2 , where w > 0 is the new coupling constant. These two interactions enter in competition, and then, they induce drastic changes of the magnetic behavior of the material. The main change is that, the presence of this high order coupling tends to destroy the ferrimagnetic order of the system. We first show that the introduction of this biquadratic interaction does not affect the values of critical exponents. Also, we find that the compensation temperature (when it exists) and the compensation magnetic field are shifted to their lowest values, in comparison with the w ¼ 0 case. The Arrott-phase-diagram shape is also investigated quantitatively. We show the existence of three regimes depending on the values of w. When the latter is small, we find that the region of competition between the coupling C and the applied magnetic field H becomes more narrow under the effect of w (by competition, we mean the passage from the antiparallel state to the parallel one). While for higher values of w, this competition disappears completely, and then, the system loses its ferrimagnetic character. Kinetics of the phase transition is also examined, when the temperature is lowered from an initial value T i to a final one T f very close to the critical temperature T c. As in the w ¼ 0 case, we find that kinetics is controlled by two kinds of relaxation times t 1 and t 2. The former is the relevant time, and is associated to long-wavelength fluctuations driving the system to undergo a phase transition. The second is a short time, which controls local dynamics. Near T c , we show that, in particular, the longest relaxation time t 1 becomes less important in comparison with that relative to the w ¼ 0 case. Finally, we note that the existence of two relaxation times is consistent with the predictions of a recent experiment, which was concerned with the 1/2-spin compounds Li x Ni 2Àx O 2 , where the composition x is close to 1.
Ferrimagnetic correlations in paramagnetic
Journal of Magnetism and Magnetic Materials, 2007
We have performed X-ray magnetic circular dichroism experiments on ErCo 2 in order to measure the Er and Co net magnetic moments above and below its ferrimagnetic transition. The results demonstrate the occurrence of an unexpected antiparallel alignment of Co and Er sublattices almost 30 K above the magnetic transition. We attribute this antiparallel alignment to magnetic short-range order. We have characterized the temperature dependence of the magnetic correlation length ðxÞ with small angle neutron scattering measurements. We observe the expected divergence of x right above T c and, a wide region well within the paramagnetic phase, where the correlation length has an almost constant value of 7Å. We have further observed a direct correlation between Co magnetic moment and x: at the onset of the long range order both magnitudes experience an abrupt increase, while the temperature range at which x has a constant value coincides with the change of sign of the Co magnetic moment. r
Coupling of phonons and electromagnons in GdMnO3
Physical Review B, 2006
The infrared and Terahertz properties of GdMnO3 have been investigated as function of temperature and magnetic field, with special emphasis on the phase boundary between the incommensurate and the canted antiferromagnetic structures. The heterogeneous incommensurate phase reveals strong magnetodielectric effects, characterized by significant magnetoelectric contributions to the static dielectric permittivity and by the existence of electrically excited magnons (electromagnons). In the commensurate canted antiferromagnetic phase the magnetoelectric contributions to the dielectric constant and electromagnons are suppressed. The corresponding spectral weight is transferred to the lowest lattice vibration demonstrating the strong coupling of phonons with electromagnons.
Nature of short-range order in the paramagnetic state of manganites
Journal of Alloys and Compounds, 2013
We study the nature of short-range magnetic interactions observed in the paramagnetic phase of colossal magnetoresistance compounds. Our results reveal that ferromagnetic-like interaction between Mn ions cannot be explained by the conventional double exchange mechanism. The results show evidence that the e g electrons are localized in Mn 3+ ions regardless the introduction of holes leading to ferromagnetic/antiferromagnetic superexchange-like interactions.
Coupling of Ferromagnetic and Antiferromagnetic Spin Dynamics in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline">mml:mrowmml:msubmml:mrowmml:miMnmml:mrowmml:mn2mml:miAumml:mo/mml:miNiF...
Physical Review Letters, 2023
We investigate magnetization dynamics of Mn 2 Au/Py (Ni 80 Fe 20 ) thin film bilayers using broadband ferromagnetic resonance (FMR) and Brillouin light scattering spectroscopy. Our bilayers exhibit two resonant modes with zero-field frequencies up to almost 40 GHz, far above the singlelayer Py FMR. Our model calculations attribute these modes to the coupling of the Py FMR and the two antiferromagnetic resonance (AFMR) modes of Mn 2 Au. The coupling-strength is in the order of 1.6 T nm at room temperature for nm-thick Py. Our model reveals the dependence of the hybrid modes on the AFMR frequencies and interfacial coupling as well as the evanescent character of the spin waves that extend across the Mn 2 Au/Py interface. Ferromagnets have a net magnetic moment and uniform spin dynamics in the GHz range . In contrast, collinear antiferromagnets have two equal but opposite sublattice magnetizations with vanishing net magnetic moment [2, 3] and spin dynamics that can reach the THz range . In addition to THz spin dynamics, antiferromagnets can exhibit significant stability of their magnetic moments against external magnetic perturbations [6]. The difference in the magnetization dynamics of ferromagnetic (FM) and antiferromagnetic (AFM) materials could potentially be exploited in applications that integrate AFM materials in high-frequency spintronic devices. A promising approach to enhance the FM spin dynamics frequencies and control FM spin-wave dispersions might be the combination of FM and AFM thin-film layers with interfacial exchange coupling. As a result of interfacial exchange coupling, a pronounced increase in the coercivity of the FM layer and exchange bias can be observed. Interfacial coupling also modifies GHz spin dynamics in FM/FM, chiral FM/FM and AFM/FM heterostructures, in particular the magnetic damping and anisotropy . However, the role of the THz frequency spin dynamics of AFMs for the hybrid spin dynamics in AFM/FM bilayers has not been revealed so far. Consequently, AFM/FM bilayers have so far not been leveraged to control hybrid mode frequencies or study AFM dynamics without requiring THz spectroscopy tools. Here we show that by coupling AFM and FM modes, we can make the AFM dynamics visible in the GHz range and use the exchange-enhancement of AFM modes to elevate the FM spin dynamics frequencies. The existence of such a coupling between FM and AFM spin dynamics opens new possibilities to design next generation magneto-electronic devices that exploit AFM materials beyond exchange bias. The Mn 2 Au/Py system
Physical Review B, 2006
A Raman spectroscopic study of the high-frequency optical phonons in single crystals of the multiferroic system RMn 2 O 5 ͑R = Bi, Eu, Dy͒ was performed. All studied materials show anomalous phonon shifts, below a new characteristic temperature for these materials, T * ϳ 60-65 K. The sign and magnitude of such shifts appear to be correlated with the ionic radius of R, envolving from softenings for R = Bi to hardenings for R = Dy and showing an intermediary behavior for R = Eu. Additional phonon anomalies were identified below ϳT N ϳ 40-43 K, reflecting the onset of long-range ferroelectric and/or magnetic order of the Mn sublattice. Complementary dc-magnetic susceptibility ͓͑T͔͒ measurements for BiMn 2 O 5 up to 800 K yield a Curie-Weiss temperature CW = −253͑3͒ K, revealing a fairly large frustration ratio ͉͑ CW ͉ / T N = 6.3͒. Deviations of ͑T͒ from a Curie-Weiss paramagnetic behavior due to magnetic correlations were observed below temperatures of the order of ͉ CW ͉, with the inverse susceptibility showing inflection points at ϳ160 K and ϳT * . Supported by ͑T͒ data, the anomalous Raman phonon shifts below T * are interpreted in terms of the spinphonon coupling, in a scenario of strong magnetic correlations. Overall, these results support significant magnetic frustration, introduce a new characteristic temperature ͑T * ͒, and suggest a surprisingly rich behavior for the magnetic correlations in the paramagnetic phase of this system. FIG. 1. ͑Color online͒ A view of the crystal structure of RMn 2 O 5 ͑R =Bi,Dy,Eu͒ along the z-axis.