Universal conductance fluctuations in Cu : Mn nanocontacts

Abstract

Universal conductance #uctuations in Cu : Mn nano-contacts of di!erent Mn concentrations c yield the phase coherence length¸P which exhibits a minimum at c+100 ppm. Above this concentration which marks the onset of a spin-glass state below 0.2 K,¸P depends on the magnetic "eld range, rising to nearly the c"0 value for B3 [9.5, 12.5 T] and leveling-o! at a much lower value for B3 [0, 3 T]. These results demonstrate the recovery of phase coherence due to spin-glass correlations and a strong magnetic "eld.2000 Elsevier Science B.V. All rights reserved.Keywords: Mesoscopics; Phase coherent transport; Spin-glasses; Spin scattering Universal conductance #uctuations (UCF) arising from quantum interferences between di!erent electron paths [1] can be used to probe the complex frozen magnetic order in metallic spin-glasses [2,3]. Here we report on UCF in Cu : Mn nanocontacts with a Mn concentration c between 31 and 920 ppm, encompassing the transition from the single-impurity regime to the spin-glass state. We estimate the spin-glass temperature for our thin samples ¹ "0.06, 0.43 and 1.08 K for 31, 315 and 920 ppm Mn, respectively [4].The samples were fabricated by electron-beam lithography followed by #ash-evaporation and lift-o! in a two-lead geometry. The contact width w varies between 60 and 480 nm, and the length¸between 100 and 2000 nm. The Cu : Mn "lms (thickness 10}20 nm) were mounted within the mixing chamber of a He}He dilution refrigerator with a base temperature of ¹+0.05 K.A typical R(B) curve is shown in Fig. 1a. The sample with c"920 ppm exhibits a resistance (R) maximum at B"0. The negative magnetoresistance (MR) arises from the suppression of spin}#ip scattering by the magnetic "eld B. The resistance #uctuations R obtained from the R(B) data by subtracting the negative MR using a spline "t, yield the conductance #uctuations G"! R/R , where R is the resistance of the nanobridge, i.e. the contribution of the contact leads subtracted from R (R "17.7 for the sample of Fig. 1). Fig. 1b shows G whose amplitude increases with the magnetic "eld. G(B) #uctuates on a magnetic-"eld scale B which is determined by the magnetic #ux "B A A P through a phase-coherent region A P where "h/e. A P is bounded by the width w of the sample and the phase coherence length¸P if w(¸P (¸[1]. Therefore, P may be estimated from the correlation "eld B "/w¸P which corresponds to the HWHM of the autocorrelation function of G(B) [1]. This simple relation has been veri"ed for Au and Ag nanowires of widths between 45 and 340 nm [5]. Combining the results of B for samples with the same c but di!erent w, a linear "t B versus w\ gives an estimate for¸P.We calculated a low-"eld (B)3 T) and a high-"eld (B*9.5 T) B for the di!erent samples, yielding¸P (cf. Fig. 2). The large error in¸P is caused by the small number of samples (2}4) for each c as well as an uncertainty in the data analysis due to the restricted "eld range. Furthermore, the simple relation B &¸\ P may be violated by up to 20% in the case of spin}orbit scattering [6].Two reg...