An electronic (metal-to-insulator) phase transition of the first order, which can be caused by an external magnetic field, was discovered in Nd, , , Sr, , MnO, .A clear hysteresis was observed during the increase and decrease of an external magnetic field at a fixed temperature. The hysteretic field region was observed to depend critically on temperature and to drastically expand with a decrease of temperature, perhaps as a result of suppression of the effect of thermal fluctuations on the first-order phase transition. Although it has seldom been observed, this is thought to be a generic feature of the first-order phase transition at low temperatures near 0 kelvin. I n general, a first-order phase transition is accompanied by hysteresis; that is, a difference of the transition points as a thermodvnamic variable that drives a ohase transition is increased and decreased ( 1 ) . A well-known example is the melting of a solid, which often shows a supercooled (or superheated) state as a metastable state below (or above) the normal melting (or solidification) temperature. Here we describe a first-order phase transition induced by an external magnetic field in one of the man-" ganese oxide compounds with perovskitetype structure, Ndl12Srll~Mn03. On the basis of a peculiar phase diagram in the temperature field plane, a novel reentrant, irreversible behavior of the insulator-tometal transition is demonstrated. Figure 1 shows the temperature dependence of the magnetization, lattice parameters, and resistivity of the Ndl12Srl12Mn03 crystal. As a generic feature of hole-doped perovskites of manganese oxide, the crystal undergoes a ferromagnetic transition at the Curie temperature (T,, 255 K) (2, 3). This is attributable to the so-called double-exchange mechanism (4). which is a ferro-. , magnetic coupling between the local 3d electron (t2 state) spins through kinetic exchange of the itinerant 34 electron (e, state). At temperatures above 160 K, the crvstal shows metallic conductivitv: The resistivity shows an appreciable decrease below T,, owing to reduced magnetic scattering of the charge carriers in the spin-ordered ferromagnetic phase. What is unique in the compound Ndl12Srll,Mn0, is that another distinct phase transition occurs at Tco = 158 K (Fig. 1). (Tco represents the critical temperature for the charge-ordering transition.)When this ~h a s e transition occurs at Tco, the resistivity jumps by more than two orders of magnitude from a typically metallic value (5 X l o p 4 ohm. cm), and the ferromagnetic magnetization disappears, which indicates the simultaneous ferromagnetic-to-antiferromagnetic transition. In accord with these changes in electric and magnetic properties, the lattice parameters in the orthorhombicall distorted perovskite lattice (a -b .-d a P and c -la,; ap relating to the cubic perovskite) show a distinct change, as shown in Fig. 1. According to recent studies ( 5 , 6 ) on the analogous crystal Prl12Srl12Mn03, the transition from ferromagnetic metal to antiferromagnetic insulator can be assigned to t...
