From the second magnetization peak to peak effect. A study of superconducting properties in Nb films and MgB₂bulk samples

Abstract

We report on magnetic and magnetoresistance measurements in two categories of superconducting Nb films grown via magnetron sputtering and MgB−2 bulk samples. In the first category, films of Tc = 9.25 K were produced by annealing during deposition. In these films, the magnetic measurements exhibited the so-called "second magnetization peak" ("SMP"), which is accompanied by thermomagnetic instabilities (TMI). The characteristic field H fj , where the first flux jump occurs, has been studied as a function of the sweep rate of the magnetic field. Interestingly, in the regime T < 6.4 K, the respective line H fj (T ) is constant, H fj (T< 6.4 K)= 40 Oe. A comparison to TMI observed in MgB2 bulk samples is also performed. Our experimental findings can't be described accurately by current theories on TMI. In the second category, films of Tc = 8.3 K were produced without annealing during deposition. In such films, we observed a peak effect (PE). In high magnetic fields the PE is accompanied by a sharp drop and a narrow hysteretic behavior (∆T < 20 mK) in the measured magnetoresistance. In contrast to experimental works presented in the past, the comparison of our magnetic measurements with the magnetoresistance data suggests that rather the appearance of surface superconductivity than the melting transition of vortex matter, is the cause of the observed behavior.

Relaxation measurements in the regime of the second magnetization peak in Nb films

“…In addition, the upper-critical fields H c2 (T ) (squares) are presented. The upper-critical points were determined by comparative magnetic and magnetoresistance measurements (not shown here) [3]. As we see the H fp (T ), H fj (T ) and H′′ smp ′′ (T ) lines connect at a characteristic point (T o , H o ) ≈ (7.2 K, 80 Oe).…”

“…Details on the preparation of the films are presented elsewhere [3]. The critical temperature of the film under discussion is T c = 9.25 K, equal to the one of high purity single crystals [20].…”

“…These studies concluded that for the case of Nb films the "SMP" is probably motivated by thermomagnetic instabilities (TMIs) that occur in the low-temperature regime, far below the uppercritical field [1,2,3]. From the theoretical point of view, the understanding of the fundamental differences between the "SMP" observed in Nb films and in high-T c superconductors is of great importance.…”

Surface superconductivity, positive field cooled magnetization, and peak-effect phenomenon observed in a spherical single crystal of niobium

“…Historically, reports on peakeffect phenomenon and surface superconductivity in highpurity thin wires of Nb have existed in literature for a long time. [13][14][15][16][17][18][19] In addition, there have been numerous reports of positive magnetization signal on field cooling superconducting specimen of different variety. [20][21][22][23] In the context of high T c cuprate superconductors 20,21,24 ͑HTSC͒, the Meissner-type diamagnetic response on the application of a magnetic field is typically found only in the zero-field cooled ͑ZFC͒ state; on field cooling ͑FC͒, a HTSC sample often shows paramagnetic magnetization at low fields.…”

Ferromagnetic-superconducting hybrid films and their possible applications: A direct study in a model combinatorial film

“…Nb and CoPt FNs were chosen as the ingredients of the CFs since their respective superconducting and magnetic properties are well studied and can be modified in a controlled way by altering the preparation conditions during sputtering and subsequent annealing. 7,8,17,18,19 The FNs employed in this work are anisotropic with their easy-axiŝ a e of magnetization normal to the surface of the film. In the constructed CFs the FNs are preferably embedded in only half of a high quality Nb layer of thickness d = 200 nm (see inset of Fig.2(b), below).…”