No abstract

Nature © Macmillan Publishers Ltd 1998 8 letters to nature 144 NATURE | VOL 396 | 12 NOVEMBER 1998 | www.nature.comThe line¯ux from the SNR can be translated into an ejected mass of 44 Ti, if we know the age and distance of the SNR. Recent measurements of the 44 Ti lifetime (see refs 18, 19, and references therein) were used to derive a weighted mean of 90:4 6 1:3 years. We note that the effective 44 Ti lifetime in SNRs could be larger, depending on the degree of ionization of the 44 Ti and its Lorentz factor. The derived value of the ejected 44 Ti mass is mainly sensitive to the actual value of the lifetime and is less critically dependent on the distance to, and the age of, the SNR.The parameters (age and distance) are not available from the gray measurements alone. Fortunately however, a possible counterpart of the newly discovered SNR was recently (independently) detected in Rosat data 20 : an extended feature of ,28 diameter centred at Galactic longitude l 266:38, latitude b 1:28 only 0.48 away from the 44 Ti excess, well within the measurement uncertainties.By combining the g-ray line¯ux and the X-ray diameter 20 with an assumed typical 44 Ti yield of ,5 3 10 2 5 M ( for supernovae of different types 6±9 , and taking as representative an expansion velocity of ,5,000 km s -1 (ref. 21) for the supernova ejecta, we derive a distance of ,200 pc, and an age of the SNR of ,680 yr. For larger 44 Ti yields and larger expansion velocities, the distance estimate becomes larger and the age estimate becomes less. We note that the SNR expansion velocity, when evaluated from the SNR X-ray spectrum 20 , has the same value of ,5,000 km s -1 .We can only speculate about the reasons why this supernova was not observed ,700 years ago: we can consider the possible existence both of optically subluminous supernovae 22 and of absorbing material in front of the supernova. In addition, the celestial position and the time of the event might have been unfavourable for an observation. Information about the existence and type of the compact stellar-like remnant of the supernova, and the elemental abundances of the SNR, will have to await future optical, radio, Xray and g-ray measurements. M

We report a magnetization measurement technique which allows quantitative studies of thermodynamic properties of individual submicron superconducting and ferromagnetic particles.

We have used ballistic Hall micromagnetometry to study the magnetization of individual submicrometer nickel disks ͑80 nm high, 0.1-1.0 m diameter͒. At low temperatures, hysteresis loops of the disks no longer show inversion symmetry in a magnetic field, as if the time reversal symmetry were broken. Furthermore, the magnetization of the smallest disks can be ''frozen'' in two possible states that are characterized by hysteresis loops which are each other's inverse. At temperatures below 19.5 K a magnetic field as high as 2 T cannot switch between the states, proving that it is extremely difficult to fully polarize a small ferromagnetic particle. On the other hand, at slightly higher temperatures ͑only TϾ19.8 K), a field as low as 0.1 T appears to be enough to fully polarize the disks. We attribute this extraordinary behavior to the glass-liquid transition experienced by spins at the particle surface. ͓S0163-1829͑98͒06542-4͔

No abstract