Observation of coherent acoustic phonons and magnons in an antiferromagnet NiO

Abstract

Ultrafast lattice and spin dynamics in an antiferromagnet NiO were studied via reflection and transmission experiments using polarization spectroscopy along with the pump-probe technique. The damped oscillations observed in the reflection experiment are not of spin origin but can be attributed to an interference effect caused by the probe pulse reflected by a propagating wave packet of acoustic phonons. Terahertz oscillations of coherent magnons were observed in the transmission experiment. Three antiferromagnetic modes were observed at low temperatures. As the temperature increases, the two higher modes degenerate above ∼250 K and then soften toward the Néel temperature (523 K).Ultrafast spin dynamics has recently been one of the most attractive topics in condensed matter physics, and understanding ultrafast spin dynamics from fundamental and technological viewpoints is greatly desired. Ultrafast spin dynamics and optical spin control have potential applications for the development of high-speed memory devices, spintronics, quantum computing, and optical control of correlated spin systems. As the frequency of antiferromagnetic resonance is considerably higher than that of ferromagnetic resonance, antiferromagnetic spin dynamics has been studied extensively with the aim of realizing faster optical spin control.NiO is a typical transition-metal oxide that exhibits antiferromagnetism. It is a paramagnetic insulator with a cubic rock-salt structure above the Néel temperature T N = 523 K. In the antiferromagnetic phase below T N , the Ni 2+ spins align ferromagnetically in (111) planes and antiferromagnetically between adjacent (111) planes, 1 and a slight lattice deformation from a cubic structure to a rhombohedral structure occurs along the (111) axes owing to the nearest-neighbor and nextnearest-neighbor interactions. Experiments on NiO have been conducted; these experiments have demonstrated terahertz radiation from coherent antiferromagnetic magnons excited by femtosecond laser pulses 2-4 and magnon oscillations induced by circularly polarized light. 5,6 A damped oscillation signal with an oscillation period of 19 ps was observed in NiO through the pump-probe experiment on reflected second harmonic generation (SHG). 7-9 The origin of this signal was interpreted to be the coherent oscillations of Ni 2+ spins between hard-and easy-axis states owing to the change in magnetic anisotropy. In the present study, we investigated the ultrafast lattice and spin dynamics in an antiferromagnet NiO via reflection and transmission experiments using polarization spectroscopy along with the pump-probe technique. In the reflection experiment, damped oscillations of linear birefringence, whose period is ∼20 ps, were observed. The observed dependences on the probe wavelength and the pump power indicate that the observed oscillations are not of spin origin but originate from the coherent acoustic phonons owing to the propagation of strain pulse.In the transmission experiment, terahertz oscillations of circular birefringence were ob...

Controlling coherent and incoherent spin dynamics by steering the photoinduced energy flow

“…Measuring transient MLB allows us to detect simultaneously both the transversal and the longitudinal dynamics of L, due to the contributions to the dielectric permittivity tensor proportional to L z L y and L 2 z , assuming L directed along theẑ axis. In contrast, in earlier investigations of laser-induced spin dynamics in compensated antiferromagnets [6,7,25], the transient magneto-optical Faraday rotation was measured. Since in the ground state the magnetization is zero, the Faraday rotation vanishes.…”

“…We can draw this conclusion because, contrary to Refs. [6,7,25], we monitored simultaneously both the transversal and the longitudinal spin dynamics by measuring variations of MLB. Given the nonresonant nature of the coupling between light and spins in the ISRS process, the frequency range of the excitations accessible with this technique is intrinsically much broader.…”

Coherent acoustic perturbation of second-harmonic generation in NiO

“…Here we also assume that the temperature changes have no direct effect on . Values of ∂n/∂η 33 = 0 have previously been observed in NiO films as well as in (001)-cut bulk NiO using reflectivity and ellipticity measurements [11,12]. A modulation of the linear refractive index already implies possible consequences for the SHG response arising from changes in the SHG coherence length.…”

“…For NiO in particular, Bosco et al [11] observed oscillations in time resolved linear reflectivity measurements and Takahara et al [12] recently remarked at the close resemblance between frequencies of these coherent oscillations and the higher-frequency oscillations reported for the SHG response. Dynamics attributed to coherent acoustic phonons have also been observed in the SHG response of a Fe/AlGaAs heterostructure [13].…”

“…094304-10 )/n 0 matches experimental observations [12] without taking into account the observed wavelength dependence [11]. The magnitude of ( ∂χ (2) ∂η 33 )/χ (2) 0 was chosen to best fit the data and corresponds to local changes in χ (2) (z) of several percent.…”