Ultrafast Spin Dynamics in the Iron Borate Easy-Plane Weak Ferromagnet
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ast Spin Dynamics in the Iron Borate Easy-Plane Weak Ferromagnet A. K. Zvezdina,*, A. V. Kimelb, D. I. Plokhova,**, and K. A. Zvezdina a Prokhorov
General Physics Institute of the Russian Academy of Sciences, Moscow, 119991 Russia for Molecular Materials, Radbout University, AJ Nijmegen, 6525 The Netherlands *e-mail: [email protected] **e-mail: [email protected]
b Institute
Received February 15, 2020; revised March 30, 2020; accepted April 9, 2020
Abstract—Ultrafast processes of the spin dynamics in iron borate FeBO3 are considered theoretically; the mechanisms responsible for excitation of quasi-ferromagnetic as well as quasi-antiferromagnetic spin resonance modes by a one-period terahertz pulse are indicated. In full agreement with experimental observations [27], the excitation of the high-frequency quasi-antiferromagnetic mode is resonant by nature, and its amplitude is a linear function of the electric field of the terahertz pulse. The amplitude of the low-frequency quasiferromagnetic mode is a quadratic function of the electric field of the pulse, and the excitation of this mode is governed by the mechanism of the inverse Cotton–Mouton effect. DOI: 10.1134/S1063776120070195
1. INTRODUCTION Trigonal (rhombohedral) antiferromagnet iron borate (FeBO3) is nowadays among most actively investigated antiferromagnets. Keen interest in this material is due to not only its comparatively high Néel temperature (TN = 348 K), but also a number of its interesting physical (magnetic, optical, acoustic, etc.) properties. Recent advances in generation of high-intensity ultrashort laser pulses have paved the way for new approaches to studying nonlinear optical regimes, in which the strong coupling of electromagnetic radiation with a material can ensure a large number of basically new possibilities for changing the electromagnetic wave frequency [1–3] and for controlling over properties of the material by high-frequency electromagnetic radiation [4–6]. In magnetism, this resulted in the development of a new field of investigations such as ultrafast magnetism [7–9] and the possibility of using the potentialities of femtosecond optical pulses for the most rapid and energy-effective recording of magnetic memory data [10]. Even the first investigations of nonlinear optical effects in magnets [2, 11] revealed that electromagnetic radiation can act on a material as an effective magnetic field [12, 13] by ordering spins and, hence, producing magnetization [13]. The vector product of two orthogonal electric field components of circularly polarized light has the same symmetry as that of a magnetic field directed along the electromagnetic wave vector. Linearly polarized light can produce
magnetic anisotropy in a magnetically ordered material. These effects can be described phenomenologically as the inverse Faraday effect [14, 15] and the inverse Cotton–Mouton effect [13]. It has been established quite recently that pulses of the far infrared range (with a characteristic frequency range from 300 GHz to 3 THz) are most energ
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