D. Pal

Amorphization of vortex matter and indication of a reentrant peak effect in YBa 2 Cu 3 O 7-δ

The peak effect (PE) has been observed in a twinned crystal of YBa 2 Cu 3 O 7-δ for H||c in the low-field range, close to the zero-field superconducting transition temperature [T c (0)]. A sharp depinning transition succeeds the peak temperature T p of the PE. The PE phenomenon broadens and its internal structure smoothens out as the field is increased or decreased beyond the interval between 250 and 1000 Oe. Moreover, the PE could not be observed above 10 kOe and below 20 Oe. The locus of the Tp(H) values is indicative of the occurrence of a reentrant characteristic with a noselike feature located at T p (H)/T c (0)≈0.99 and H≈100 Oe (where the flux line lattice constant a 0 ≈penetration depth λ). The upper part of the PE curve (0.5 L ≈0.25. The vortex phase diagram near T c (0) determined from the characteristic features of the PE in YBa 2 Cu 3 O 7-δ (H||c) bears close resemblance to that in the 2H-NbSe 2 system, in which a reentrant PE had been distinctly observed earlier.

Disordered type-II superconductors: a universal phase diagram for low-Tc systems

A universal phase diagram for weakly pinned low-Tc type-II superconductors is revisited and extended with new proposals. The low-temperature “Bragg glass” phase is argued to transform first into a disordered, glassy phase upon heating. This glassy phase, a continuation of the high-field equilibrium vortex glass phase, then melts at higher temperatures into a liquid. This proposal provides an explanation for the anomalies observed in the peak effect regime of 2H-NbSe2 and several other low-Tc materials which is independent of the

Flux jumps, second magnetization peak anomaly, and the peak effect phenomenon in single crystals of Y Ni 2 B 2 C and Lu Ni 2 B 2 C

We present magnetization measurements in single crystals of the tetragonal YNi 2 B 2 C compound, which exhibit the phenomenon of peak effect as well as the second magnetization peak anomaly for H>0.5 T (H||c). At the lower field (50 mT 2 B 2 C for H||c in the field region from 2 to 25 mT, which are compatible with the occurrence of a reorientation transition at a lower field in a cleaner crystal of this compound, as compared to those of YNi 2 B 2 C. Vortex phase diagrams drawn for H||c in LuNi 2 B 2 C and YNi 2 B 2 C show that the ordered elastic glass phase spans a larger part of (H,T) space in the former as compared to latter, thereby, reaffirming the difference in the relative purity of the two samples.

Correlation of exchange bias with magneto-structural effects across the compensation temperature of Co(Cr1–xFex)2O4 (x = 0.05 and 0.075)

A small amount of Fe (5% and 7.5%) substitution in the Cr-site of the multiferroic compound CoCr2O4 leads to a magnetization reversal. In these compounds, we report a sign change in the exchange bias across the compensation temperature, accompanied by a non-monotonic change in the local moments across the compensation temperature. Such non-monotonic change in the magnetic moments is triggered by a similar change in the lattice structure. We relate here the sign change of exchange bias with that of the crystalline energy of the lattice and the Zeeman energy term arising from the anti-site disorder.

Possible sign change of Mn}Mn exchange interaction between ThMnAl and ThMnIn

ThMnAl and ThMnIn have the C-15, cubic Laves phase crystal structure. Pronounced thermomagnetic irreversibility is seen in the magnetisation of both compounds. A cusp in the AC susceptibility of ThMnAl near 43 K which shows frequency dependence establishes the spin glass freezing of the Mn moments in this compound. This is further corroborated by the heat capacity data which do not exhibit any anomaly near the cusp temperature. On the other hand, the zero-field-cooled magnetisation of ThMnIn shows a sharp ferromagnetic-like increase around 270 K, a broad peak around 180 K and then decreases monotonically with temperature. Various scenarios based on the sign change of Mn-Mn exchange interaction between ThMnAl and ThMnIn due to the increased Mn-Mn bond length in the latter are discussed. Electrical resistivity is large in both compounds and shows a negative temperature coefficient indicating the disordered nature of both alloys.

Low temperature neutron diffraction studies on Co(Cr1−xFex)2O4 (x = 0.05 and 0.075)

We report the magnetic structures of an Fe substituted cobalt chromite system, Co(Cr1−xFex)2O4 (x = 0.05 and 0.075), determined from the analysis of temperature dependent neutron diffraction measurements. The ferrimagnetic transition temperature (Tc) was found to increase from ∼110 K to ∼118 K with the increase of Fe substitution. Neutron diffraction studies reveal magnetic peaks below the transition temperature, Tc. In addition, we observed a broad hump below the Bragg reflection (111), indicative of diffuse scattering from short-range magnetic interactions coexisting with long range magnetic order below Tc. The length scale of this short range magnetic order shows unusual non-monotonic temperature dependence. The bulk magnetization data reveals that the magneto-structural transition (Ts) remains unaffected with temperature in both the compounds. This magneto-structural transition at Ts gives rise to the magnetic satellite peaks of the conical spin-spiral structure which can be indexed as (220)* and (002)* with a magnetic structure using an incommensurate propagation vector. With a further lowering of the temperature around the lock-in transition, TL (∼10 K), the incommensurate peak of the propagation vector (0.62, 0.62, 0) splits into two peaks with a deviation of 0.02 in Q-value from the centre.

Identification of the inverse melting line in the vortex phase diagram of a low Tc superconductor, Ca3Rh4Sn13

We present the vortex phase diagram in a weakly pinned single crystal of a low Tc superconductor, Ca3Rh4Sn13 (Tc ~ 8.37 K). We believe that a well ordered Bragg glass phase is sandwiched between the multi-domain vortex glass and an amorphous vortex state in a portion of the field–temperature phase space. From an exploration of the thermal variation of the onset of the non-monotonic variation of the critical current density as fingerprinted by the second magnetization peak (SMP) in the magnetization hysteresis loops, we have sketched a phase boundary akin to the inverse melting phenomenon reported earlier only in samples of high Tc superconductors. We reveal here a step change in the equilibrium magnetization across the SMP and peak effect regions, which is a suggestive feature of the first order phase transition. We have determined a lower field limit (like a spinodal line) of the transition to the vortex glass phase via the identification of the SMP anomaly in the ac susceptibility measurements, where shaking of the vortex matter can facilitate the accession of the underlying equilibrium phase.

Comparison of the history effects in magnetization in weakly pinned crystals of high Tc and low Tc superconductors

A comparison of the history effects in weakly pinned single crystals of a high Tc YBa2Cu3O7−δ (for H || c) and a low Tc Ca3Rh4Sn13, which show anomalous variations in critical current density Jc(H), is presented via the tracings of the minor magnetization hysteresis loops using a vibrating sample magnetometer. The sample histories focussed are: (i) the field cooled (FC), (ii) the zero field cooled (ZFC) and (iii) an isothermal reversal of field from the normal state. An understanding of the results in terms of the modulation in the plastic deformation of the elastic vortex solid and supercooling across the order–disorder transitions is sought.

Modulation in the non-uniformity of the vortex state across the peak effect: evidence via the magnetic quadrupole moment in a very clean crystal of 2H-NbSe2 for H∥c

Abstract The non-uniform magnetic character of the mixed (vortex) state is known to result in a magnetic quadrupolar response of a type-II superconductor. The nucleation of stronger pinned regions coexisting with weaker pinned regions across the peak effect (PE) in a weakly pinned superconductor has been demonstrated using ac scanning Hall bar microscopy by Marchevsky et al. [Nature 409 (2001) 591]. Coexistence of this nature could produce an imprint in the measured quadrupolar response of such samples. We present successful detection of such an imprint across the PE region of a very clean crystal of 2H–NbSe 2 for H ∥ c via a well-established procedure of preferentially recording a signal arising from the non-uniformity in the magnetization of the sample in a vibrating sample magnetometer.

Tuning of magnetic structure and its effect on magnetic properties in Co(Cr1–xMnx)2O4 (x = 0–0.3)

We report here the magnetic properties of Co(Cr1–xMnx)2O4 (x = 0.00–0.30) through bulk-magnetization measurements and temperature variable neutron powder diffraction experiments. We have noticed the compensation temperature (Tcomp) and then the onset of negative magnetization at T = 60 K and 83 K only for Co(Cr0.73Mn0.27)2O4, i.e., x = 0.27, and Co(Cr0.70Mn0.30)2O4, i.e., x = 0.30 samples. Low temperature neutron diffraction experiments on these polycrystalline samples (i.e., x = 0.27 and 0.30) are performed to explore the understanding of observed negative magnetization, complex magnetic properties, and correlation between chemical and magnetic structures. A broad hump has been observed around the (111) Bragg reflection which gives the signature of diffuse scattering from short-range magnetic interactions coexisting with long range magnetic order below ordering temperature. The variation of bond lengths, bond angles, and magnetic moments with temperature is correlated, which clearly indicates magneto-elastic coupling around compensation temperature in these compounds. The presence of this magneto-elastic coupling results in the sign change of the magneto-caloric effect at compensation temperature.We report here the magnetic properties of Co(Cr1–xMnx)2O4 (x = 0.00–0.30) through bulk-magnetization measurements and temperature variable neutron powder diffraction experiments. We have noticed the compensation temperature (Tcomp) and then the onset of negative magnetization at T = 60 K and 83 K only for Co(Cr0.73Mn0.27)2O4, i.e., x = 0.27, and Co(Cr0.70Mn0.30)2O4, i.e., x = 0.30 samples. Low temperature neutron diffraction experiments on these polycrystalline samples (i.e., x = 0.27 and 0.30) are performed to explore the understanding of observed negative magnetization, complex magnetic properties, and correlation between chemical and magnetic structures. A broad hump has been observed around the (111) Bragg reflection which gives the signature of diffuse scattering from short-range magnetic interactions coexisting with long range magnetic order below ordering temperature. The variation of bond lengths, bond angles, and magnetic moments with temperature is correlated, which clearly indicates magneto-ela...