ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e765–e767
Frequency-domain magnetic resonance spectroscopy J. van Slagerena,*, S. Vongtragoola, B. Gorshunova,b, A.A. Mukhinb, Martin Dressela 1. Physikalisches Institut, Universitat . Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart, Germany General Physics Institute of Russian Academy of Sciences, 38 Vavilov St., 119991 Moscow, Russia
a b
Abstract A novel method to study high-frequency electron spin resonance is introduced, which sweeps the frequency at fixed magnetic fields. We describe the main features of our frequency-domain spectrometer, which works in the spectral range from 30 GHz to 1.5 THz and at magnetic fields up to 8 T; the temperature can be as low as 0.5 K. Applications include the investigations of paramagnetic ions, in general, and molecular magnets, in particular. r 2003 Elsevier B.V. All rights reserved. PACS: 76.30.v; 75.50.Xx; 76.30.Kg Keywords: High-field EPR; Zero-field splitting; Molecular magnet
The last decade has seen an enormous increase in socalled high-field electron paramagnetic resonance (EPR) research in various fields ranging from materials sciences to medical applications. While conventional high-field EPR operates at fixed frequencies and sweeps the magnetic field, we developed a complementary technique by sweeping the radiation frequency while keeping the field constant. The major improvement compared to previous approaches of frequency-domain EPR spectroscopy [1] is the use of backward-wave oscillators (BWOs) as monochromatic and continuously tunable radiation sources. These are vacuum tubes where electrons are accelerated by a high voltage and create electromagnetic radiation when interacting with a comb-like structure. The range from 1 cm1 to almost 50 cm1 is covered by about a dozen of BWOs, each of them can be tuned over a certain frequency interval. The intensity of the BWOs varies from several hundreds mW at the low-frequency side to 1 mW at the highest frequencies. The radiation is coherent, highly monochromatic and linearly polarized [2]. The frequency resolution depends on the stability of *Corresponding author. Tel.: +49-711-685-4943; fax: +49711-685-4886. E-mail address:
[email protected] (J. van Slageren).
the power supply. Our spectrometer reaches Dn=n up to 106, which makes it possible to study narrow magnetic absorption lines and to investigate the lineshape in detail. Our frequency-domain magnetic spectrometer adopts the design of an optical Mach–Zehnder interferometer, as sketched in Fig. 1, to allow the complex transmission measurements (amplitude and phase, see Fig. 1 and description below). The radiation is generated by a BWO, collimated by a polyethylene lens and finally focused onto the detector. The interferometer is formed by two wire-grid beamsplitters and two metal mirrors. For transmission amplitude measurements only one arm of the interferometer is employed while for the phase measurements the interferometer is utilized to its full extent [2]. The samples are disks with 3–10 mm in diameter; their thicknesses are chosen depending on the absorption of the material; the transmission coefficient should be larger than 104–105 to allow a sufficient signal-to-noise ratio. The sample is inserted into an optical helium bath cryostat (T=1.3 to 300 K) or 3He cryostat (T>0.5 K) for zero-field measurements or into a split-coil superconducting magnet for in-field measurements (T ¼ 1:7–300 K; H ¼ 0–8 T). Both Voigt (wavevector of the electromagnetic radiation perpendicular to the external magnetic field) and Faraday (wavevector parallel to the magnetic field) configurations are used.
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1000
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J. van Slageren et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e765–e767
Fig. 1. Schematic layout of the frequency-swept spectrometer used for high-frequency EPR experiments. The radiation from the backward wave oscillators (with q the wavevector) is polarized and focused onto the sample; the signal is detected by a Golay cell or by a He cooled bolometer; other elements are described in the text. For transmission measurements the reference arm of the interferometer (see dashed line) is not used. To determine the change in phase, the reference arm is adjusted in length by movable mirror 1, so that the beams in both arms interfere destructively. The arrows h, e, and q indicate the direction of the radiation magnetic, and electric fields, and the wavevector, respectively. The radiation propagates perpendicular or parallel to an external magnetic field Hext., corresponding to Voigt or Faraday geometries, respectively.
In contrast to conventional EPR experiments which place the samples inside microwave cavities or waveguides we use a single-path optical arrangement which has the advantage of simplicity. The size of samples should be two to three times the radiation wavelength. If no single crystals of suitable size are available, we prepare large enough mosaics assembled from single crystals [3]; powder samples are pressed in pellets [4]. Recently, we have applied our novel technique to investigate the zero-field splitting in the complex [Ni(Cl)2(PPh3)2] [5] and the anisotropy of the g-factor in the magnetic cluster V15 [6] which allowed a precise determination of the spin Hamiltonian parameters. The advantage of frequency-domain magnetic spectroscopy becomes obvious in the study of lineshapes and their dependence on magnetic field. A nice example are crystal-field transitions in single crystals of Mn12Ac molecular magnets. For H ¼ 0 T and zero-field-cooled state the MS ¼ 710 to 79 transitions are observed close to 10 cm1. On applying a magnetic field along the easy c-axis the absorption line splits due to the Zeeman interaction (Fig. 2). In good approximation, the up and down shift of the lines is symmetric with respect to the zero field line position; n0 ðn7 ¼ n0 7ðgmB =hÞHz Þ: Note that the intensities of the split modes practically remain equal at low fields, because the populations of the MS ¼ 710 states remain unchanged in spite of the fact that the MS ¼ 10 state becomes metastable for Hz > 0: When increasing the field further, the intensity of the lowfrequency mode, corresponding to the transition from
Fig. 2. Minima in transmission coefficient of a plane Mn12Ac sample corresponding to |10S - |79S crystal field transitions at different magnetic fields. The sample was cooled below the blocking temperature of 3.3 K in zero field, followed by the application of a magnetic field (Voigt geometry).
the metastable state, decreases dramatically and eventually disappears because the relaxation is enhanced due to the reduced energy barrier in the field and resonance tunnelling between corresponding energy levels. If the crystal is cooled in the presence of a magnetic field, the absorption lines become asymmetric even in the absence of an external magnetic field (H ¼ 0 T). This dependence upon the history of the magnetic state is quantitatively explained by the combined effect of inhomogeneous broadening due to the distribution of inter- and intramolecular magnetic interactions and non-diagonal components of magnetic susceptibility in transverse magnetic media. In conclusion, the reviewed frequency-swept quasioptical EPR spectroscopy based on backward-wave oscillators opens new possibilities to study magnetic properties and spin excitations, not only in molecular magnets, but also in many other magnetic materials. This work was support from the Deutsche Forschungs-gemeinschaft (DFG) and by RFBR (No. 0202-16597).
References [1] R.R. Joyce, P. Richards, Phys. Rev. 179 (1969) 375; G.C. Backett, P.I. Richards, W.S. Caughey, J. Chem Phys. 54 (1971) 4383. [2] G.V. Kozlov, A.A. Volkov, in: G. Gruner . (Ed.), Coherent Source Submillimeter Wave Spectroscopy, Spinger, Berlin, 1998. [3] S. Vongtragool, A. Mukhin, B. Gorshunov, M. Dressel, to be published.
ARTICLE IN PRESS J. van Slageren et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e765–e767 [4] A.A. Mukhin, B. Gorshunov, M. Dressel, C. Sangregorio, D. Gatteschi, Phys. Rev. B 63 (2001) 214411; M. Dressel, B. Gorshunov, K. Rajagopal, S. Vongtragool, A. Mukhin, Phys. Rev. B 67 (2003) 060405.
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[5] S. Vongtragool, B. Gorshunov, M. Dressel, J. Krzystek, D.M. Eichhorn, J. Telser, Inorg. Chem. 42 (2003) 1788. [6] S. Vongtragool, B. Gorshunov, A.A. Mukhin, J. van Slageren, M. Dressel, A. M.uller, Phys. Chem. Chem. Phys. 5 (2003) 2278.
