Detailed measurements and analysis of the temperature and magnetic field dependence of the magnetization and the magnetic relaxation of Nh films have been carried out. An irreversibility line has been identified, below which the remanent magnetization was found to decay logarithmically in time. The data were quantitatively compared to predictions of flux creep, vortex glass, and vortex-lattice-melting models. Only the vortex-lattice-melting model self-consistently explained the data.PACS numbers: 74.60.Ge, 74.60.EcThe magnetic-field-temperature (H-T) phase diagram for high-temperature superconductors has been the subject of intense interest. An irreversibility line (IL) and logarithmic decay of the remanent magnetization were first seen in ceramic LaSrCuO [l]. There has since been an ongoing debate over whether they are caused by glassy kinetics [2][3][4], flux creep [5-8], or vortex-lattice melting [9][10][11][12][13]. Although not necessarily expected [14], similar ILs and time-dependent magnetic properties have been observed in conventional type II superconductors [15,16]. While it is not clear that these features have the same origin as their counterparts in high-7V materials-which are highly anisotropic and more heavily influenced by thermal fluctuations-conventional materials offer a simpler platform for the examination of the various models. In this Letter we present the results of detailed measurements and comprehensive analysis of the temperature and magnetic field dependence of the magnetization, and the decay of the remanent magnetization of Nb films. The analysis strongly suggests the irreversibility line in Nb films is a signature of vortex-lattice melting.The sample studied in detail was rf sputtered onto an oxide layer on a Si substrate and had dimensions 5000 Ax2 mmx2 mm. The resistivity ratio was 5.9. Fieldcooled (FC) and zero-field-cooled (ZFC) magnetization measurements were made in fields ranging from 10 to 3000 G. The procedure is well known and has been reported elsewhere [1,5,6,15,17]. The measurements were performed using a Quantum Design SQUID susceptometer with the sample aligned perpendicular to the magnetic field. The temperature control parameters of the susceptometer were adjusted to eliminate temperature overshoot at the sample during the ZFC measurement. To improve the precision of the data, temperature steps were limited to 0.05 K, and 16 scans were averaged at each temperature. A typical temperature sweep took 12 h. T C (H) was determined from the onset of diamagnetism, and T*(//), the irreversibility temperature, was defined as the lowest temperature where the difference between the FC and ZFC magnetizations was less than the standard deviation of the mean of the FC and ZFC measurements. Above r* the magnetization was reversible; below T* only the FC magnetization was thermal-history independent. (It has been reported that the FC magnetization of Nb powder can be thermal-history dependent below the irreversibility temperature [18], but no evidence of this was seen in our sample....

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