Sun Ji-Rong

Resistance switching in oxides with inhomogeneous conductivity

Electric-field-induced resistance switching (RS) phenomena have been studied for over 60 years in metal/dielectrics/metal structures. In these experiments a wide range of dielectrics have been studied including binary transition metal oxides, perovskite oxides, chalcogenides, carbon- and silicon-based materials, as well as organic materials. RS phenomena can be used to store information and offer an attractive performance, which encompasses fast switching speeds, high scalability, and the desirable compatibility with Si-based complementary metal—oxide—semiconductor fabrication. This is promising for nonvolatile memory technology, i.e., resistance random access memory (RRAM). However, a comprehensive understanding of the underlying mechanism is still lacking. This impedes faster product development as well as accurate assessment of the device performance potential. Generally speaking, RS occurs not in the entire dielectric but only in a small, confined region, which results from the local variation of conductivity in dielectrics. In this review, we focus on the RS in oxides with such an inhomogeneous conductivity. According to the origin of the conductivity inhomogeneity, the RS phenomena and their working mechanism are reviewed by dividing them into two aspects: interface RS, based on the change of contact resistance at metal/oxide interface due to the change of Schottky barrier and interface chemical layer, and bulk RS, realized by the formation, connection, and disconnection of conductive channels in the oxides. Finally the current challenges of RS investigation and the potential improvement of the RS performance for the nonvolatile memories are discussed.

Great magnetic entropy change in La(Fe, M)13 (M=Si, Al) with Co doping*

Very large magnetic entropy change ?SM, which originates from a fully reversible second-order transition at Curie temperature TC, has been discovered in compounds La(Fe, Si)13, La(Fe, Al)13 and those with Co doping. The maximum change ?SM ? 19 J?kg-1?K-1, achieved in LaFe11.4Si1.6 at 209K upon a 5T magnetic field change, exceeds that of Gd by more than a factor of 2. The TC of the Co-doped compounds shifts to higher temperatures. ?SM still has a considerable large magnitude near room temperature. The phenomena of very large ?SM, convenience of adjustment of TC, and also the superiority of low cost, strongly suggest that the compounds La(Fe, M)13 (M=Si, Al) with Co doping are suitable candidates for magnetic refrigerants at high temperatures.

Large magnetic entropy change near room temperature in the LaFe11.5Si1.5H1.3 interstitial compound

The LaFe11.5Si1.5H1.3 interstitial compound has been prepared. Its Curie temperature TC (288 K) has been adjusted to around room temperature and the maximal magnetic entropy change (|?S|~17.0 J?kg-1?K-1 at TC) is larger than that of Gd (|?S|~9.8 J?kg-1?K-1 at TC=293 K) by ~73.5% under a magnetic change from 0 to 5 T. The origin of the large magnetic entropy change is attributed to the first-order field-induced itinerant-electron metamagnetic transition. Moreover, the magnetic hysteresis of LaFe11.5Si1.5H1.3 under the increase and decrease of the field is very small, which is favourable to magnetic refrigeration application. The present study suggests that the LaFe11.5Si1.5H1.3 compound is a promising candidate as a room-temperature magnetic refrigerant.

Effect of R substitution on magnetic properties and magnetocaloric effects of La1-xRxFe11.5Si1.5 compounds with R=Ce, Pr and Nd

Magnetic properties and magnetocaloric effects of La1−xRxFe11.5Si1.5 (R = Pr, (0 ≤ x ≤ 0.5); R = Ce and Nd, (0 ≤ x ≤ 0.3)) compounds are investigated. Partially replacing La with R = Ce, Pr and Nd in La1−xRxFe11.5Si1.5 leads to a reduction in Curie temperature due to the lattice contraction. The substitution of R for La causes an enhancement in field-induced itinerant electron metamagnetic transition, which leads to a remarkable increase in magnetic entropy change ΔSm and also in hysteresis loss. However, a high effective refrigerant capacity RCeff is still maintained in La1−xRxFe11.5Si1.5. In the present samples, a large ΔSm and a high RCeff have been achieved simultaneously.

Magnetic entropy change involving martensitic transition in NiMn-based Heusler alloys

Our recent progress on magnetic entropy change (ΔS) involving martensitic transition in both conventional and metamagnetic NiMn-based Heusler alloys is reviewed. For the conventional alloys, where both martensite and austenite exhibit ferromagnetic (FM) behavior but show different magnetic anisotropies, a positive ΔS as large as 4.1 Jkg−1 K−1 under a field change of 0–0.9 T was first observed at martensitic transition temperature TM ~ 197 K. Through adjusting the Ni:Mn:Ga ratio to affect valence electron concentration e/a, TM was successfully tuned to room temperature, and a large negative ΔS was observed in a single crystal. The −ΔS attained 18.0 Jkg−1K−1 under a field change of 0–5 T. We also focused on the metamagnetic alloys that show mechanisms different from the conventional ones. It was found that post-annealing in suitable conditions or introducing interstitial H atoms can shift the TM across a wide temperature range while retaining the strong metamagnetic behavior, and hence, retaining large magnetocaloric effect (MCE) and magnetoresistance (MR). The melt-spun technique can disorder atoms and make the ribbons display a B2 structure, but the metamagnetic behavior, as well as the MCE, becomes weak due to the enhanced saturated magnetization of martensites. We also studied the effect of Fe/Co co-doping in Ni45(Co1−xFex)5Mn36.6In13.4 metamagnetic alloys. Introduction of Fe atoms can assist the conversion of the Mn—Mn coupling from antiferromagnetic to ferromagnetic, thus maintaining the strong metamagnetic behavior and large MCE and MR. Furthermore, a small thermal hysteresis but significant magnetic hysteresis was observed around TM in Ni51Mn49−xInx metamagnetic systems, which must be related to different nucleation mechanisms of structural transition under different external perturbations.

Large magnetic entropy change and magnetic properties in La (Fe1-xMnx)11.7Si1.3Hy compounds

Magnetic properties and magnetic entropy change in La (Fe1-xMnx)11.7Si1.3Hy compounds have been investigated. A significant increase of the Curie temperature TC and a small increase of the saturation magnetizations µS have been observed after the introduction of interstitial H, which caused a slight volume expansion. The first-order field-induced itinerant-electron metamagnetic (IEM) transition remains and brings about a large magnetic entropy change around room temperatures for the compounds. The maximal magnetic entropy change is about 23.4, 17.7 and 15.9J/kgK under a magnetic field change from 0 to 5T for x=0.01, 0.02 and 0.03, respectively. Therefore, the compounds appear to be potential candidates for magnetic refrigerants around room temperatures.

Order of magnetic transition and large magnetocaloric effect in Er3Co

We have studied the magnetic and magnetocaloric properties of the Er3Co compound, which undergoes ferromagnetic ordering below the Curie temperature TC = 13 K. It is found by fitting the isothermal magnetization curves that the Landau model is appropriate to describe the Er3Co compound. The giant magnetocaloric effect (MCE) without hysteresis loss around TC is found to result from the second-order ferromagnetic-to-paramagnetic transition. The maximal value of magnetic entropy change is 24.5 J/kgK with a refrigerant capacity (RC) value of 476 J/kg for a field change of 0–5 T. Large reversible MEC and RC indicate the potentiality of Er3Co as a candidate magnetic refrigerant at low temperatures.

Magnetocaloric effect in Gd6Co1.67Si3 compound with a second-order phase transition

The magnetic properties and the magnetic entropy change ΔS have been investigated for Gd6Co1.67Si3 compounds with a second-order phase transition. The saturation moment at 5 K and the Curie temperature TC are 38.1 μB and 298 K, respectively. The ΔS originates from a reversible second-order magnetic transition around TC and its value reaches 5.2J/kgK for a magnetic field change from 0 to 5T. The refrigerant capacity (RC) of Gd6Co1.67Si3 are calculated by using the methods given in Refs.[12] and [21], respectively, for a field change of 0-5T and its values are 310 and 440 J/kg, which is larger than those of some magnetocaloric materials with a first-order phase transition.

Magnetic properties and magnetocaloric effect in TbCo2-xFex compounds

Magnetic properties and magnetocaloric effect in TbCo2 xFex compounds are studied by DC magnetic measure- ment. With increasing content of Fe, the entropy changes decrease slightly, though the Curie temperature is tuned from 231 K (x = 0) to 303 K (x = 0.1). Magnetic entropies of TbCo2 compound are calculated by using mean field approximation (MFA). Results estimated by using Maxwell relation are consistent with that of MFA calculation. It is shown that the entropy changes are mainly derived from the magnetic entropy changes. The lattice has almost no contribution to the entropy change in the vicinity of phase transition.

Magnetic properties and magnetic entropy changes of La1-xPrxFe11.5Si1.5 compounds with 0 ≤ x ≤ 0.5

Magnetic properties and magnetic entropy changes in LaFe11.5Si1.5 have been investigated by partially substituting Pr by La. It is found that La1-xPrxFe11.5Si1.5 compounds remain cubic NaZn13-type structures even when the Pr content is increased to 0.5, i.e. x = 0.5. Substitution of Pr for La leads to a reduction in both the crystal constant and the Curie temperature. A stepwise magnetic behaviour in the isothermal magnetization curves is observed, indicating that the characteristic of the itinerant electron metamagnetic (IEM) transition above TC becomes more prominent with the Pr content increasing. As a result, the magnetic entropy change is remarkably enhanced from 23.0 to 29.4 J/kgK as the field changes from 0 to 5 T, with the value of x increasing from 0 to 0.5. It is more attractive that the magnetic entropy changes for all samples are shaped into high plateaus in a wide range of temperature, which is highly favourable for Ericsson-type magnetic refrigeration.