Experimental Observation of Magnetic Surface Polaritons in FeF2by Attenuated Total Reflection

Abstract

We present the first direct experimental evidence for the existence of magnetic surface polaritons in a uniaxial antiferromagnet.The modes are excited in FeF2 by attenuated total reflection with the uniaxis along an applied magnetic field Ho. The resonance is observed as an attenuation of the reflected intensity as the frequency of the far-infrared beam is scanned. We find nonreciprocity between the +Ho and -Ho spectra as a consequence of the nonreciprocity of the surface polariton modes when Ho is introduced. We also observe broad dips due to the excitation of bulk modes. PACS numbers: 75.50.Ee, 75.30.Pd, 76.50.+g, 78.20.Ci Surface polaritons are now well established as a sensitive probe in surface analysis. The study of surface plasmon polaritons, for example, provides valuable information about material parameters associated with surface plasma oscillations such as the effective masses of charge carriers and surface charge densities. This technique has been applied to studies of metals [1] and semiconductors and semiconductor superlattices [2 -5].In ferromagnets, the study of magnetostatic surface spin waves by Brillouin light scattering has yielded important information on interface exchange constants, bulk and surface anisotropies, surface magnetization, and spinreorientation transitions for thin films and superlattices [6 -9]. In contrast, the experimental study of surface modes and the surface structure of antiferromagnets has been almost completely neglected. This is despite the fact the magnetic surface structure in antiferromagnets has recently been emphasized in a number of fundamental works [10], and also has technological importance in the exchange biasing of ferromagnetic films [11].We report the first direct experimental observation of magnetic surface polaritons on the uniaxial antiferromagnet FeF2 by attenuated total reIIection (ATR). The present development of ATR for magnetic surface excitations opens up the possibility of a wide range of experimental studies of magnetic surfaces in the far-infrared (FIR) spectral range. This technique should be applicable to studying surface modes and parameters for a wide variety of other magnetic materials. These include ferrimagnets, easy-plane antiferromagnets [12], rare earth magnets with helical orderings [13,14], and rare earth and antiferromagnetic superlattices [15].Magnetic surface polaritons (MSPs) were theoretically discussed first for ferromagnets [16] and later for antiferromagnets [17,18]. Subsequently a large theoretical literature has appeared [15] but no direct measurements of MSP have yet been reported. The experiments are challenging because it is necessary to establish ATR with high-frequency resolution at low temperatures.One of the key features to emerge from the theoretical studies is that MSP propagation is nonreciprocal, i.e. , reversing the direction of propagation (or reversing the applied field) changes the ATR rellectively. In our experimental results we indeed find a pronounced nonreciprocal reAectively. In addition, when th...

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Spin canting induced nonreciprocal Goos-Hänchen shifts

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