The combination of the track etching method and atomic force microscopy allows us to search for weakly interacting massive particles (WIMPs) in our Galaxy. A survey of 80720 p, m of 0.5 Gyr old muscovite mica found no evidence of WIMP-recoil tracks. This enables us to set limits on WIMPs which are about an order of magnitude weaker than the best spin-dependent WIMP limits. Unlike other detectors, however, the mica method is, at present, not background limited. We argue that a background may not appear until we have pushed our current limits down by several orders of magnitude.PACS nombers: 95.35.+d, 14.80.Ly, 29.40.Ym, 61.72.Ff Much research is being devoted to the questions of the nature and detectability of the dark matter that comprises more than 90% of the mass of the Universe [1]. One of the most promising candidates is a weakly interacting massive particle (WIMP) which is being sought with instruments capable of detecting the -keV/amu recoiling ions which would be produced in elastic collisions between WIMPs and nuclei [1]. The best limits on the mass and scattering cross section of WIMPs trapped in the Galactic halo result from the use of natural Ge, NaI, and CaF detectors [2]. These limits, however, fall short, by several orders of magnitude, of ruling out one of the favored WIMP candidates, the neutralino [3]. We show here that the natural mica crystals, with an integration time of -10 yr, can record and store the tracks of recoil nuclei struck by WIMPs, and that these tracks can be measured with an atomic force microscope (AFM). Our approach is an extension of the etching method for studying ancient tracks in minerals [4,5]. With it, we report a new limit that is about an order of magnitude weaker than the best spin-dependent limits from NaI and CaF detectors, but show that we have the potential to push these limits down by several orders of magnitude.As with the Ge, NaI, and CaF detectors, mica serves both as the target and as the detector. Muscovite mica is primarily composed of 'H (I = 2), '60 (I = 0), 27A1 (I = 2), Si (I = 0), and K (I = 2). The range of one of these nuclei with a typical recoil energy of -keV/amu is only a few hundred angstroms [6] and the etched depth is even smaller.Although such etched tracks cannot be studied with an optical microscope, we have shown that their dimensions can be accurately measured with an AFM [7]. As shown in Fig. 1, the technique is to cleave open a mica crystal, etch the freshly exposed surfaces, and use an AFM to scan and measure the tracks crossing the cleavage plane. For each area scanned (typically 40 p.m X 40 p, m) a 256 x 256 grid of heights is obtained and fitted line by line (to remove the effect of the piezo motion on the heights) with a fourth order polynomial using a robust fitting algorithm [8]. New, fiattened heights are calculated from the difference between the old height (a) WIMP (b) FIG. 1. An illustration of the etching technique. (a) If WIMPs exist they would cause the constituent atoms of muscovite mica, mainly ' 0, Al, Si, and K, to r...