Kuwahara Hideki

The metal-insulator (M-I) phase transitions relevant to charge ordering (GO) have been investigated for perovskite-type (Nd1-ySmy)(1/2)Sr1/2MnO3 (0 less than or equal to y less than or equal to 1) crystals, in which the one-electron bandwidth (W) is systematically controlled by varying the averaged ionic radius of the A site and by application of quasihydrostatic pressure (P). Competition between the ferromagnetic double exchange and the antiferromagnetic CO interactions give rise to complex M-I phase diagrams with temperature (T) and W (y and/or P) as the parameters. The M-I phase boundaries are associated with critically Rr-and T-dependent hystereses, which result in unique appearance of the metastable state. We have demonstrated the pressure-induced phase transition from the metastable ferromagnetic metal to the thermodynamically stable charge-ordered insulator for the y = 0.875 crystal locating near the critical M-I phase boundary. With decrease of W, the CO instability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature (T-c) and enhances the electron-lattice coupling. Consequently, the lattice-coupled first-order I-M transition is observed at T-c in the small-W region of y greater than or equal to 0.5. It was found that application of magnetic field also induces the phase transition from the insulator with antiferromagnetic spin correlations to the ferromagnetic metal, which is accompanied by lattice-structural change.

Effects of hydrostatic pressure on the ferromagnetic phase transition are systematically investigated for doped manganites, R1-xSrxMnO3 (R = La and Nd) by controlling the doping level (x). In both the systems, the positive pressure coefficient (dln T-C/dP) for the Curie temperature increases significantly with the decrease of x (or T-C). We have derived an empirical relation between the coefficient and T-C, and will discuss its origin.

The competition between ferromagnetic metallic state due to double exchange and antiferromagnetic nonmetallic state due to an ordering of Mn3+/Mn4+ with 1/1 (charge ordering) has been studied with use of single crystals of perovskite-type manganese oxides prepared by the floating zone method. The charge-ordered state can be altered to a ferromagnetic metallic state by application of a magnetic field, which accompanies a change in resistivity by several orders of magnitude. This is one of the major origin of the colossal magnetoresistance (CMR) in these manganese oxides.

High-resolution photoemission spectra are measured by changing the sample temperature from 290 K down to 11-15 K under the off- and on-resonance excitations for the Mn 2p, O 1s and Nd 3d states. Three Mn 3d-derived valence band features are clearly revealed in the on-resonance spectrum at room temperature for the Mn 2p core excitation. Among them, the step-like structure just below the Fermi level (E-F), which is thought to be the e(g) state, is suppressed at low temperatures. Observed energy shifts and spectral changes of the whole valence band upon changing the temperature are qualitatively discussed.

Magnetoresistive devices (based on, for example, magnetic multilayers(1)) exhibit large changes in electrical resistance in response to a magnetic field, which has led to dramatic improvements in the data density and reading speed of magnetic recording systems. Manganese oxides having a perovskite structure (the so-called manganites) can exhibit a magnetoresistive response that is many orders of magnitude larger than that found for other materials, and there is therefore hope that these compounds might similarly be exploited for recording applications(2-11). Here we show that the switching of resistive states in the manganites can be achieved not only by a magnetic field, but also by an electric field. For manganites of the form Pr1-xCaxMnO3, we find that an electrical current (and by implication a static electric field) triggers the collapse of the low-temperature, electrically insulating charge-ordered state to a metallic ferromagnetic state. We suggest that such a phenomenon could be exploited to pattern conducting ferromagnetic domains within an insulating antiferromagnetic matrix, and so provide a route for fabricating micrometre- or nanometre-scale electromagnets.

To explore the optimized colossal magnetoresistance (MR), i.e., higher MR with lower field, magnetotransport properties of single-crystalline perovskite manganites have systematically been investigated. Near x=1/2 with intrinsic instability of charge ordering (a 1/1 ordering of Mn3+/Mn4+), the one-electron bandwidth (W) is varied by reducing the radius of R-site cation in R1-xSrxMnO3. For R=Nd, the MR behavior is rather canonical, while for R=Sm, the field-induced nonmetal-to-metal transition of the first order shows up accompanying a change in resistivity by several orders of magnitude as a result of an enhanced antiferromagnetic interaction. © 1997 American Institute of Physics.

We studied three distorted perovskite manganites, Pr1/2Sr1/2MnO3 and Nd1-xSrxMnO3 with x = 1/2 and 0.55 by the neutron diffraction technique. Two samples with x = 1/2 exhibit a transition from a ferromagnetic (FM) metal to an antiferromagnetic (AFM) nonmetal. We demonstrate that, in the low temperature phase, Nd1/2Sr1/2MnO3 has a CE-type AFM structure with charge ordering, while Pr1/2Sr1/2MnO3 and Nd0.45Sr0.55MnO3 exhibit an A-type layered AFM structure, but show no clear sign of charge ordering. From the present results, we suggest a possible anisotropy of transport as well as magnetic properties in the A-type AFM structure near x similar to 1/2.

We have investigated magnetoresistance (MR) phenomena relevant to the charge ordering (CO), namely, the real space ordering of both Mn3+ and Mn4+, in single crystals of (Nd1-ySmy)(1/2)Sr1/2MnO3, in which the one-electron bandwidth (W) is systematically controlled by varying the average ionic radius of the A site. The low-field colossal MR is observed for the small-W region of y greater than or equal to 0.5; e.g., rho(0)/rho(H) > 10(3) in a field of 0.3 T at 115 K for the y = 0.94 crystal. This is viewed as a first-order insulator-to-metal phase transition induced by a magnetic field, which accompanies a lattice-structural change. In the small-W region, the CO instability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature T-c. The competition between the ferromagnetic double-exchange and antiferromagnetic CO interactions gives rise to such a lattice-coupled first-order phase transition induced by a relatively low magnetic field. (C) 1997 American Institute of Physics.

Effects of hydrostatic pressure on the magnetic and transport properties of R(1-x)Sr(x)MnO(3) have been investigated with systematic variation of the one-electron bandwidth (W) of a conduction electron, as well as of the doping level (x). The positive pressure coefficient (dlnT(C)/dP) of the Curie temperature is observed for all the compounds, and its absolute value increases with decrease of W, or equivalently, T-C. We have derived a nearly x-independent empirical relation between the coefficient and T-C in the lightly doped region (x less than or equal to 0.4). The coefficients at x approximate to 1/2, however, are found to deviate from the above relation due to competing antiferromagnetic and/or charge-ordering interaction.

Effects of chemical and external pressures have been investigated on the two types of charge-ordering (GO) systems of perovskite manganites with the use of single-crystal specimens: One is Nd1-xSrxMnO3 with moderate CO instability occurring only near x=1/2 and the other is Pr1-xCaxMnO3 with stronger CO instability extending over a wide x region 0.3 less than or equal to x less than or equal to 0.7. We have partially substituted the Nd ions of Nd1/2Sr1/2MnO3 with larger La ions or applied external pressure on them with the aim of destabilizing the CO state via an increase of the 3d-electron hopping interaction. An electronic phase diagram relevant to the CO transition was derived for (Nd1-zLaz)(1/2)Sr1/2MnO3 by such a control of the one-electron bandwidth (W). With an increase of W, the enhanced ferromagnetic double-exchange interaction increases the Curie temperature (T-C) and suppresses the charge-ordered state with a concomitant antiferromagnetic charge-exchange-type spin ordering (AF-CE). In a narrow window of z (0.4 less than or equal to z less than or equal to 0.6) or in the pressurized state for z = 0.4, another type of antiferromagnetic (perhaps the A type) phase replaces the AF-CE state. Application of external pressure and resultant enhanced carrier itineracy suppresses the CO transitions also for Pr1-xCaxMnO3. For the x = 0.30 crystal application of pressure induces a metallic phase from the low-temperature side in the charge-ordered insulating phase. The pressure-temperature phase diagrams relating to the CO transition or the concurrent insulator-to-metal transition were shown to scale well with the magnetic-field-temperature phase diagrams.

The stability of the charge-ordered state against variations of temperature(T) and external magnetic field (H) has been investigated for Pr-0.65/Ca1-ySry)(0.35)MnO3 crystals with controlled one-electron bandwidth. For y = 0.3. the charge- and orbital-ordering appears below 200 K as in the y < 0.3 crystals, but collapses into the metallic state below 100 K. Under a magnetic field, such a reentrant feature is observed in a wider region of y (< 0.3). A variation of the H-T phase diagrun for the charge-ordered state with the one-electron bandwidth is presented in view of the origin of the colossal magnetoresistance.

The metal-insulator (Formula presented)-(Formula presented) phase transitions relevant to charge ordering (CO) have been investigated for perovskite-type (Formula presented) crystals, in which the one-electron bandwidth (Formula presented) is systematically controlled by varying the averaged ionic radius of the (Formula presented) site and by application of quasihydrostatic pressure (Formula presented). Competition between the ferromagnetic double exchange and the antiferromagnetic CO interactions give rise to complex (Formula presented) phase diagrams with temperature (Formula presented) and (Formula presented) and/or (Formula presented) as the parameters. The (Formula presented) phase boundaries are associated with critically (Formula presented)- and (Formula presented)-dependent hystereses, which result in unique appearance of the metastable state. We have demonstrated the pressure-induced phase transition from the metastable ferromagnetic metal to the thermodynamically stable charge-ordered insulator for the (Formula presented)=0.875 crystal locating near the critical (Formula presented) phase boundary. With decrease of (Formula presented), the CO instability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature (Formula presented) and enhances the electron-lattice coupling. Consequently, the lattice-coupled first-order (Formula presented) transition is observed at (Formula presented) in the small-(Formula presented) region of (Formula presented). It was found that application of magnetic field also induces the phase transition from the insulator with antiferromagnetic spin correlations to the ferromagnetic metal, which is accompanied by lattice-structural change. © 1997 The American Physical Society.

Magnetotransport properties were investigated for Sm1-xSrxMnO3 films that were prepared by the pulsed laser deposition technique. Application of a magnetic field causes a huge change in the resistivity for x=0.45, 0.50, and 0.55, in some cases exceeding five orders of magnitude. Measurement of the magnetization curve at low temperature which is removed after loading of a high magnetic field (5T) suggests the existence of the antiferromagnetic correlations. In accord with this, the magnetoresistance shows an irreversible behavior against the field sweep. The results imply that the colossal isothermal magnetoresistance in the thin films, of which the hole concentration is of a commensurate value of around, x= 1/2 has the same origin as the magnetic-field-induced first-order transition reported for the single crystals that is the magnetic-field-induced melting of the charge-ordered stale. (C) 1996 American Institute of Physics.

The current-perpendicular-to-plane magnetoresistance (CPP-MR) has been investigated for the layered manganite, La2-2xSr1-2xMn2O7 (x = 0,3), which is composed of the ferromagnetic-metallic MnO2 bilayers separated by nonmagnetic insulating block layers, The CPP-MR is extremely large (10(4) percent at 50 kilo-oersted) al temperatures near above the three-dimensional ordering temperature (T-c approximate to 90 kelvin) because of the field-induced coherent motion between planes oi the spin-polarized electrons. Below T-c, the interplane magnetic domain boundary on the insulating block layer serves as the charge-transport barrier, but it can be removed by a low saturation field which gives rise to the low-field tunneling MR as large as 240 percent.

Phenomena of colossal magnetoresistance (MR) or magnetic-field-induced insulator-metal (I-M) transitions have been investigated for single crystals of perovskite-type manganese oxides with controlled carrier density and one-electron bandwidth. In addition to the canonical MR behaviors near the Curie temperature. the first-order phase transition accompanying a several orders of magnitude change in resistivity has been observed under an external magnetic field for many of the composition-controlled crystals as an intrinsic bulk phenomenon. It was proved by the systematic experimental investigations that the collapse of the charge-ordered state under a magnetic field, which accompanies the lattice-structural as well as metamagnetic transition, is a major origin of such a colossal MR. Versatile MR phenomena and I-M phase diagrams in the T-H plane are presented with their interpretations.

Magnetotransport characteristics and their nominal hole concentration (x) dependence were investigated in Nd1-xSrxMnO3 films. Thermal hysteresis in resistivity indicating the occurrence of a first-order phase transition was observed for x = 0.35 and 0.50, but essentially not for x = 0.40 and 0.45. The x = 0.35 and 0.50 films also show hysteresis in magnetoresistance. Once the resistivity is decreased by an order of magnitude by application of a magnetic field, the low-resistivity state is maintained even after removal of the field below a critical temperature. Observed hysteretic behavior suggests the presence of instabilities which compete with double-exchange interaction in specific hole concentration regions, which may be one of the origins of the colossal magnetoresistance.

Magnetoresistance resulting in a drop in resistivity of more than three orders of magnitude that is strongly coupled to lattice striction has been observed under a relatively low magnetic field (0.4 tesla at 115 kelvin) for a single crystal of perovskite-type manganese oxide with finely controlled ionic radii of the A sites, (Nd,Sm)(1/2)Sr1/2MnO3. The colossal magnetoresistance phenomena are viewed as a first-order insulator-to-metal phase transition induced by a magnetic field, which accompanies a metamagnetic (antiferromagnetic-to-ferromagnetic) transition and a structural change in the lattice. Clear hystereses and abrupt changes in magnetization, striction, and resistivity were observed in increasing and decreasing magnetic fields at temperatures (113 to 150 kelvin) just above the Curie temperature.

Competition between the ferromagnetic double-exchange interaction and the antiferromagnetic charge-ordering instability gives rise to thermally as well as magnetic field (H) induced insulator-metal transitions in distorted perovskites (Nd1-ySmy)(1/2)Sr1/2MnO3 with a controlled one-electron bandwidth. A systematic study on the metal-insulator phase diagram in the T-H plane with varying y as well as a high-pressure study has revealed the metastable nature of the undercooled ferromagnetic metallic state.

Phenomena of colossal magnetoresistance (MR) or magnetic field induced insulator-metal (I-M) transitions have been investigated for single crystals of perovskite-type manganese oxides with controlled carrier density and one-electron bandwidth. In addition to the canonical MR behavior near the Curie temperature, the first-order phase transition accompanying several orders of magnitude change in resistivity has been observed under an external magnetic field for many of the composition-controlled crystals as an intrinsic bulk phenomenon. It was proved by the systematic experimental investigations that the field-induced destruction of the charge-ordered state accompanying the lattice structural as well as metamagnetic transition is a major origin of such a colossal MR. Versatile MR phenomena and I-M phase diagrams in the T-H plane are presented with their interpretation. (C) 1996 American Institute of Physics.

Manganese oxides with the cubic perovskite structure (typified by LaMnO3) have stimulated considerable interest because of their magnetoresistive properties(1-9); they exhibit extremely large changes in electrical resistance in response to applied magnetic fields, a property that is of technological relevance for the development of magnetic memory and switching devices. But for such applications to be viable, great improvements will be needed in both the sensitivity and temperature dependence of the magnetoresistive response. One approach under consideration for optimizing these properties is chemical substitution(10). Here we demonstrate an alternative strategy, in which we synthesize layered variants of the cubic perovskite parent compounds that have a controlled number of MnO2 sheets per unit cell. This strategy is structurally analogous to that employed for the systematic exploration of the high-transition-temperature copper oxide superconductors(11). We find that the magnetoresistive properties of these materials depend sensitively on the dimensionality of the manganese oxide lattice. Although the properties of our materials are still far from optimal, further exploration of this series of layered perovskites may prove fruitful.

Field-induced insulator-to-metal transitions have been found in (Formula presented) single crystals, which are accompanied by a melting of the insulating charge-ordered (i.e., (Formula presented)/(Formula presented) ordered) state. The transition is of the first order with a hysteresis and is even irreversible at low temperatures. The (Formula presented)-dependent metal-insulator phase diagrams in the (Formula presented) plane indicate that a deviation of (Formula presented) from 0.5 modifies the robustness of the charge-ordering state, which is argued in terms of the effect of discommensuration of the charge concentration on the charge-ordered state. © 1996 The American Physical Society.

Field-induced insulator-to-metal transitions have been found in Pr1-xCaxMnO3 (0.3 less than or equal to x less than or equal to 0.5) single crystals, which are accompanied by a melting of the insulating charge-ordered (i.e., Mn3+/Mn4+ ordered) state. The transition is of the first order with a hysteresis and is even irreversible at low temperatures. The x-dependent metal-insulator phase diagrams in the H-T plane indicate that a deviation of x from 0.5 modifies the robustness of the charge-ordering state, which is argued in terms of the effect of discommensuration of-the charge concentration on the charge-ordered state.

An electronic (metal-to-insulator) phase transition of the first order, which can be caused by an external magnetic field, was discovered in Nd1/2Sr1/2MnO3. 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.

Zn-sustitution effect for Cu was studied in Bi2Sr2Ca(Cu1-xZnx)(2)O-y single crystals (0 less than or equal to x less than or equal to 0.01). We found the increase of the out-of-plane conductivity with Zn substitution and the abrupt reduction of the anisotropic parameter gamma from 200 similar to 250 for the unsubstituted sample to 30 similar to 50 for the 0.98% Zn substituted one.

We have measured temperature dependence of sigma-xx and sigma-xy for the Tl2Ba2CaCu2Oy samples with various carrier number and found that the relation sigma-xx proportional-to tau and sigma-xy- proportional-to tau2 expected from Boltzmann theory hold only for the samples with about maximal Tc, where tau was relaxation time.