The low-temperature phase of the MnBi alloy has a coercivity μ0Hc of 2.0 T at 400 K and exhibits a positive temperature coefficient from 0 to 400 K. In the higher temperature range it shows a much higher coercivity than that of the NdFeB magnets, which suggests that it has considerable potential as a permanent magnet for use at high temperatures. In the temperature range from 30 to 150 K, the Mn atom is found to change its spin direction from a perpendicular to a parallel orientation with respect to the c axis. The anisotropy field increases with increasing temperature which gives rise to a higher coercivity at the higher temperatures. The maximum energy product (BH)max of the magnet is 7.7 and 4.6 MG Oe at room temperature and 400 K, respectively. The electronic structure of MnBi indicates that the Mn atom possesses a magnetic moment of 3.6μB, and that the Bi atom has a magnetic moment of −0.15μB which is due to the s–d and p–d hybridization between Bi and Mn atoms. We have also investigated the volume dependence of the magnetic moments of Mn and Bi. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume expansion leads to a large magnetic moment for the Mn atom. The Mn magnetic moment attains a value of 4.6μB at a volume expansion rate of ΔV/V ≈ 100%.

No abstract

No abstract

Magnetic hysteresis and flux creep measurements in single crystal samples of YBa 2 Cu 3 O 7−␦ ͑YBCO͒ are presented for a wide range of B,T phase space. Some of these samples can be described as weakly or collectively pinned. For these, over a large portion of this phase space, the flux creep can be described in terms of thermally activated single-fluxoid motion. A simple model based on maximizing the pinning energy of a fluxoid segment provides a good, semiquantitative picture of the low-temperature data, where the experimentally measured critical current density j is proportional to 1 / T and the activation barrier height is proportional to j − , where = 1. In this model individual fluxoids are pinned by stochastic fluctuations in defect concentration, and are driven over the pinning barriers by critical currents and thermal activation. Incorporating flux lattice elasticity into this simple model leads to new predictions for the low-temperature data and allows the simple model to be extended to higher temperature. There are two distinct effects, both of which can be put in the form of effective current densities. One effective current density j s arises from direct fluxoid-fluxoid repulsion, and the second effective current density j r arises from fluxoid relaxation. In YBCO at 7 K and 2 T, where the measured critical current density is j = 8.9ϫ 10 9 A / m 2 , we find j s = 0.57ϫ 10 9 A / m 2 ͑6%͒ and j r = −2.1ϫ 10 9 A / m 2 ͑−20% ͒. We present a discussion of their origin that leads to plausible temperature and field dependences. The model accounts for the rapid drop of j͑T͒ with increasing temperature, the peak effect in j͑B͒ at high temperature, and the temperature and field dependence of the "critical exponent" . Thermal fluxoid vibrations play an important role in the pinning, and we find effects consistent with calculations in the literature. The model postulates that fluxoid motion takes place by hopping in segments on a characteristic length scale l model . In the model we find l model = 104 nm at 7 K and 2 T. A completely independent measurement from the creep-derived four-volume VX yields a length l VX = 102Ϯ 5 nm at the same temperature and field. Excellent agreement between the two independently determined lengths persists over a wide range of temperatures. A failure of these two lengths to agree marks the boundary for single-fluxoid hopping, and we present a diagram of the pinning regimes in B,T phase space. From the measured prefactor of thermally activated creep at 10 K and 2 K we infer a value for the attempt frequency f a = 8.5ϫ 10 10 s −1 . This value is in reasonable agreement with a published theoretical calculation of the relaxation frequency for overdamped fluxoids in an Abrikosov lattice. Finally, based on these data we estimate the mass per unit length of a YBCO fluxoid segment, and compare our result with Suhl's theory to obtain a quasiparticle effective mass of 30 free electron masses.

The superconducting ruthenocuprates RESr2Cu2RuO8 (where RE is a rare earth or Y) have been interpreted as ferromagnetic superconductors, based on a strong response to applied field at the Ru Néel temperature (around 130K). However, neutron diffraction measurements on the Gd compound by Lynn et al. [Phys. Rev. B 61, 1214964 (2000)] (using a separated isotope) show evidence only of antiferromagnetic order, while very limited data by Takagiwa et al. [J. Phys. Soc. Jpn. 70, 333 (2001)] on the Y compound show one antiferromagnetic Bragg peak and possible coexisting ferromagnetism. We have studied a sample of YSr2Cu2.1Ru0.9O7.9 prepared by a high pressure-high temperature route. Temperature dependent neutron powder diffraction shows antiferromagnetic order, manifested by the observation of the 12, 12, 12 and 12, 12, 32 reflections, which appear at 135K [as suggested by the superconducting quantum interference device (SQUID) data]. Their intensities at the lowest temperature are consistent with a c-axis orientation of the Ru magnetic moments. Although the lowest order nuclear Bragg peak shows a small intensity increase as the temperature is lowered, this is attributed to magnetic ordering of the Cu planes and not to the presence of a ferromagnetic component, in agreement with the polarized neutron data of Lynn et al. [Phys. Rev. B. 61, 1214964 (2000)] Magnetic resonance and SQUID data on the same sample (to be reported elsewhere) show a complex picture, associated with Cu magnetic ordering.