High resolution ESR imaging

Physica138B(1986) 261-263 North-Holland,Amsterdam

HIGH RESOLUTION ESR IMAGING

G.G. MARESCH and M. MEHRING Physikalisches lnstitut Teil 2, Universitiit Stuttgart, D-7000 Stuttgart 80, Fed. Rep. Germany

S. EMID Department of Applied Physics, Delft University of Technology, P. 0. Box 5046, 2600 GA Delft, The Netherlands

Received 9 November 1985 We demonstratehigh resolutionESR imagingin solidswith a spatial resolutionof 10 tim by applyinga stepped gradient technique.

NMR imaging in liquids [1-3] and solids [4, 5] has been developed to a point, where a spatial resolution of the order of some 100/.~m has been reached in favourable conditions. The literature on ESR imaging, however, is still very scarce [6-8] and no significant improvement in resolution has been reported to date. The main issue of this letter is to demonstrate that a nominal resolution of less than 10/~m can be obtained by ESR imaging in solids. We have applied the method of Emid and Creyghton [9] where the magnetization is measured at a specific time ts after pulsed excitation while incrementing the magnetic field gradient G = n AG ;

n = O, +-1, +-2,. • • + - N .

(1)

The gradient increment AG and the maximum gradient Gmax -- N 8 G are chosen appropriately. The signal, i.e. the resulting free induction decay (FID), varies as S ( z , tf) = exp(-inO(z), t f ) F ( z , tf).

(2)

for spins with gyromagnetic ratio ~/at position z, where O ( z ) = - y A G z . The spatial resolution obtainable with this technique can be expressed as AZ = 7r/(TGma x - tf),

(3)

whereas the standard procedure with constant gradient G and Fourier transformation yields Az = 2/(TG.

T2)

(4)

if a Lorentzian lineshape of width 1/T 2 is assumed. The principal technique is sketched in Fig. 1 together with the actual interferogram obtained for the sample used here. The experiment was performed at X-band frequencies (9.8 GHz) in two single crystals of (fluoranthenyl) 2AsF6, abbreviated as (FA)EX, a one-dimensional conductor with long decay times [10-12]. The observed ESR signal results in this case from the conduction electrons. The electron spin concentration is about 1 0 - 2 per FA-unit. As shown on the photograph in fig. 2 the two single crystals were mounted on a glass rod, both oriented with their stacking axes perpendicular to the magnetic field gradient to avoid additional decay effects due to spin motion [13] and the influence of g-factor anisotropy. Although their center of gravity is separated by about 100/zm, there is only a minimum separation of 25 gm. The spatial profile obtained by the stepped gradient method (bottom) clearly shows this resolution, whereas the standard technique is not able to resolve this separation completely. The maximum gradient used here w a s G m a x = 500 mT/m which results in

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G.G. Maresch et al. / High resolution ESR imaging

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G / mTm - I Fig. 1. Pulse excitation and detection scheme (top) and corresponding interferogram (bottom). The gradient pulse was stepped in increments up to a maximum value of Gmax = 500 mT/m.

a nominal resolution according to eq. (3) of Az ~ 10/~m with y = 2Ir. 28 MHz/mT. We note that the upper spectrum was obtained at a fixed gradient of G = 30 mT/m. Larger gradients (like Gmax) could not be applied because of dead time problems, which in practice increase with increasing gradients in the standard method. In the high resolution method no dead time exists, when measuring the signal a long time tf after the excitation pulse. This permits the use of high quality probes to optimize the signal to noise ratio in the high resolution method.

Fig. 2. One-dimensional projection of spatially resolved spin density distribution (bottom) of two single crystals of (FA)2AsF6 separated by about 25/~m (top). Two different techniques, namely a fixed gradient (standard method) and a stepped gradient (high resolution method) have been employed as described in the text.

Acknowledgements We thank A. Grupp for discussions and technical support with the pulsed ESR spectrometer. The Stiftung Volkswagenwerk has given a financial support.

G.G. Maresch et al. / High resolution ESR imaging

References [1] P.C. Lauterbur, Nature 242 (1973) 190. [2] P.C. Lauterbur, Bull. Am. Phys. Soc., Ser. II 18 (1972) 86. [3] A. Kumar, D. Welti and R.R. Ernst, J. Magn. Res. 18 (1975) 69. [4] B.H. Suits and D. White, Sol. State Comm 50 (1984) 291. [5] A.N. Garroway, J. Baum, M.G. Munowitz and A. Pines, J. Magn. Res. 60 (1984) 337. [6] M.J.R. Hoch, Bull, Magn. Res. 2 (1981) 415.

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[7] S.S. Eaton and G.R. Eaton, J. Magn. Rev. 59 (1984) 474. [8] K. Ohno, J. Appl. Phys. (Japan) 23 (1984) L224. [9] S. Emid and J.H.N. Creyghton, Physica 128B (1985) 81. [10] G. Sachs, W. St6cklein, B. Bail, E. Dormann and M. Schwoerer, Chem. Phys. Lett. 89 (1982) 179. [11] W. H6ptner, M. Mehring, J.U. yon Schiitz, H.C. Wolf, B.S. Morra, V. Enkelmann and G. Wegner, Chem. Phys. 73 (1982) 253. [12] E. Dormann, Physik. Bliitter 39 (1983) 220. [13] G.G. Maresch, A. Grupp, M. Mehring. J.U. yon Schiitz and H.C. Wolf, J. Phys. (Paris) 46 (1985) 461.