Phase separation in the itinerant metamagnetic transition of Sr4Ru3O10

We have investigated the electronic transport properties of the triple-layered ruthenate Sr4Ru3O10. We observed surprising anomalous features near its itinerant metamagnetic transition, including ultrasharp magnetoresistivity steps, a nonmetallic temperature dependence in resistivity for upward field sweeps, and a resistivity drop in temperature dependence for downward field sweeps. These features suggest that the metamagnetic transition of Sr4Ru3O10 occurs via an electronic phase separation process with magnetic domain formation.     

Magnetization anomaly in metamagnetic superconductors

The magnetization in the tetragonal c-plane was measured below T_N using a single crystal of bct ErRh₄B₄. The magnetization decreases with increasing magnetic field in a field range just below the metamagnetic transition. This magnetization anomaly is explained by the fact that the entrance of the vortices is suppressed by an increase of the vortex core energy due to the precursor metamagnetic transition in the core around metamagnetic transition field.     

de Haas-van Alphen effect across the metamagnetic transition in Sr3Ru2O7

We report a study of the de Haas-van Alphen (dHvA) effect on the itinerant metamagnet Sr3Ru2O7. Extremely high sample purity allows the observation of dHvA oscillations both above and below the metamagnetic transition field of 7.9 T. The quasiparticle masses are fairly large away from the transition, and are enhanced by up to an extra factor of 3 as the transition is approached, but the Fermi surface topography change is quite small. The results are qualitatively consistent with a field-induced Stoner transition in which the mass enhancement is the result of critical fluctuations.     

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 differentmagnetic anisotropies, a positive ΔS as large as 4.1 J·kg^-1·K^-1 under a field change of 0–0.9 T was first observed at martensitic transition temperature T_M ~ 197 K. Through adjusting the Ni:Mn:Ga ratio to affect valence electron concentration e/a, T_M was successfully tuned to room temperature, and a large negative ΔS was observed in a single crystal. The -ΔS attained 18.0 J·kg-1·K-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 T_M 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(Co_1-xFe_x)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 T_M in Ni₅₁Mn_49-xIn_x metamagnetic systems, which must be related to different nucleation mechanisms of structural transition under different external perturbations.     

Magnetic properties and magnetoresistance of HfFe2Ge2-type Dy1-xGdxMn6Ge6 (x=0.1－0.6) compounds

The magnetic properties and magnetoresistance effects of Dy_{1-x}Gd_xMn_6Ge_6 (x=0.1－0.6) compounds have been studied by magnetic properties and resistivity measurements in applied magnetic fields up to 5T. The compounds with x=0.1, 0.2, 0.4 and 0.5 order antiferromagnetically at 425, 428, 430 and 432K, respectively, and there are second magnetic phase transitions below 100K. The compound with x=0.6 exhibits a transition from ferrimagnetic to antiferromagnetic, then to ferrimagnetic state again with decreasing temperature. Furthermore, it displays a field-induced metamagnetic transition, and its threshold field decreases with increasing temperature. The magnetoresistance curve of the compound with x=0.6 in applied magnetic fields up to 5T is presented and the magnetoresistance effects are related to the metamagnetic transitions.     

Metamagnetic phase transitions induced by static and pulsed fields in (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics

It has been found that the magnetic susceptibility of (Sm_0.5Gd_0.5)_0.55Sr_0.45MnO₃ ceramic samples in zero external magnetic field exhibits a sharp peak near the temperature of 48.5 K with a small temperature hysteresis that does not depend on the frequency of measurements and is characteristic of the phase transition to an antiferromagnetic state with a long-range charge orbital ordering, which is accompanied by an increase in the magnetic susceptibility with a decrease in the temperature. The magnetization isotherms in static and pulsed magnetic fields at temperatures below 60 K demonstrate the occurrence of an irreversible metamagnetic transition to a homogeneous ferromagnetic state with a critical transition field independent of the measurement temperature, which, apparently, is associated with the destruction of the insulating state with a long-range charge ordering. In the temperature range 60 K ?? T ?? 150 K, the ceramic samples undergo a magnetic-field-induced reversible phase transition to the ferromagnetic state, which is similar to the metamagnetic transition in the low-temperature phase and is caused by the destruction of local charge/orbital correlations. With an increase in the temperature, the critical transition fields increase almost linearly and the field hysteresis disappears. Near the critical fields of magnetic phase transitions, small ultra-narrow magnetization steps have been revealed in pulsed fields with a high rate of change in the magnetic field of ??400 kOe/??s.     

Giant magnetoresistance in Y0.9La0.1Mn6Sn6 compound

Magnetic transitions and magnetoresistance effect of the HfFe_6Ge_6-type Y_{0.9}La_{0.1}Mn_6Sn_6 compound have been investigated in the temperature range of 5－380K. The sample displays antiferromagnetic behaviour in the whole temperature range below Néel temperature T_N=309K. The metamagnetic transition from antiferromagnetism to ferromagnetism can be induced by an applied field. The metamagnetic transition field decreases monotonically from 2T at 5K to 0.4T at 300K. The giant magnetoresistance effect is observed with the metamagnetic behaviour, such as -10.4% at 245K under a field of 5T.     

High field magnetoresistance of CeAl2 at low temperatures

We have measured the magnetoresistance ρ(H,T₀) of the magnetic Kondo lattice CeAl₂ at temperatures T₀ in the range 0.035 – 1.3 K, well below the Néel temperature T_N=3.8 K, in magnetic fields H up to 145 k0e. The ρ vs H curve exhibits a rapid decrease between H=45–65 kOe corresponding to the metamagnetic transition at H=H_M. The resistivity then levels out to a value which depends only weakly on field and temperature. For HM a small positive magnetoresistance was observed with the derivative dρ/dH increasing as the temperature is lowered. The present results are compared with the ρ(H,T₀) data obtained for the CePb₃ magnetic Kondo lattice, where the decrease in the resistivity at HH_M was considered earlier to be evidence of field induced superconductivity.     

Physical properties and anisotropies of the RNiGe3 series (R = Y, Ce-Nd, Sm, Gd-Lu)

We have studied RNiGe₃ (R=Y, Ce-Nd, Sm, Gd-Lu) single crystals by measuring crystal structure and stoichiometry, magnetic susceptibility, magnetization, electrical resistivity, magnetoresistance, and specific heat. Clear anisotropies as well as antiferromagnetic ordering in the RNiGe₃ series (R=Ce-Nd, Sm, Gd-Tm) have been observed above 1.8 K from the magnetic susceptibility. A metamagnetic transition in this family (except for R=Sm) was detected at 2 K for applied magnetic fields below 70 kOe. The electrical resistivity of this series follows metallic behavior in the high temperature region. Below the antiferromagnetic ordering temperature a significant anisotropy is exhibited in the resistivity and magnetoresistance for different current directions. The anisotropic magnetic, transport, and thermal properties of RNiGe₃ compounds are discussed in terms of Ni site occupancy as well as a combination of the effect of formation of a magnetic superzone gap and the crystalline electric field.     

Metamagnetism near <Emphasis Type="Italic">T</Emphasis><Subscript>C</Subscript> in Mn-substituted chalcopyrite Cd<Subscript>0.90</Subscript>Mn<Subscript>0.10</Subscript>GeAs<Subscript>2</Subscript>

The magnetic and electric properties of a Cd_0.90Mn_0.10GeAs₂ solid solution with the chalcopyrite structure have been investigated in wide temperature and field ranges. It has been found that a metamagnetic transition from a low-magnetization state to a high-magnetization one is initiated in Cd_0.90Mn_0.10GeAs₂ near the magnetic ordering temperature. This transition is accompanied by the hysteresis of magnetic properties. An external magnetic field at temperatures above T _C also induces the metamagnetic transition. When the temperature increases above T _C, the magnetization jump decreases, whereas fields inducing the metamagnetic transition increase. The band character of magnetism and metamagnetism in the effective magnetic field is assumed on the basis of the behavior of magnetization in the metamagnetic transition and analysis of the band structure of the solid solution of cadmium-germanium diarsenide with manganese.     

Field-induced quantum critical point in CeCoIn5

The resistivity of the heavy-fermion superconductor CeCoIn5 was measured as a function of temperature, down to 25 mK and in magnetic fields of up to 16 T applied perpendicular to the basal plane. With increasing field, we observe a suppression of the non-Fermi liquid behavior, rho approximately T, and the development of a Fermi liquid state, with its characteristic rho=rho(0)+AT2 dependence. The field dependence of the T2 coefficient shows critical behavior with an exponent of 1.37. This is evidence for a field-induced quantum critical point (QCP), occurring at a critical field which coincides, within experimental accuracy, with the superconducting critical field H(c2). We discuss the relation of this field-tuned QCP to a change in the magnetic state, seen as a change in magnetoresistance from positive to negative, at a crossover line that has a common border with the superconducting region below approximately 1 K.     

Interplay of metamagnetic and structural transitions in Ca2-xSrxRuO4

Metamagnetism in layered ruthenates has been interpreted as a novel kind of quantum critical behavior. In an external magnetic field, Ca_2-xSr_xRuO₄ undergoes a metamagnetic transition accompanied by a pronounced magnetostriction effect. In this paper we present a mean-field study for a microscopic model that naturally reproduces the key features of this system. The phase diagram calculated is equivalent to the experimental T-x phase diagram. The presented model also gives a good basis to discuss the critical metamagnetic behavior measured in the system.     

Low temperature properties of the metallic antiferromagnet ErZn12

The metallic antiferromagnet ErZn₁₂ orders magnetically at 2·76°K. We report measurements of the susceptibility, magnetisation, resistivity and specific heat of polycrystalline specimens of it in the vicinity of the magnetically ordered regime. We find evidence for a metamagnetic or spin-flop transition in fields of 3–7KOe. The resistivity increases monotonically from low temperature to the Néel point and has a power-law dependence on temperature of Tⁿ, where n～6·5. The specific heat shows a lambda anomaly at the Néel point and the divergences which appear above and below it are compared with those reported previously in the resistivity. The magnetic entropy indicates that the ordering process involves only one crystal-field doublet.     

Field induced sign reversal of magnetocaloric effect in Gd2In

We report the magnetocaloric effect in the metamagnetic compound Gd₂In obtained from magnetization measurement. Gd₂In was previously reported to have two magnetic transitions: (i) a paramagnetic to ferromagnetic transition below 190 K and (ii) a ferromagnetic to an antiferromagnetic state below 105 K. The low temperature antiferromagnetic state is unstable under an applied magnetic field and undergoes metamagnetic transition to a ferromagnetic like state. We observe conventional positive magnetocaloric effect (the magnetic entropy change, ΔS_M<0) around 190 K at all applied fields. The magnetocaloric effect is found to be inverse (negative) at low fields around 105 K (ΔS_M>0), however it turns positive at higher fields (ΔS_M<0). The observed anomaly is found to be related to the field induced transition which drives the system from an antiferromagnetic to a ferromagnetic state.     

Electrical properties of nanocrystalline magnetite with large non-stoichiometry, near Verwey transition

DC electrical measurements were carried out on compacted powders of magnetite with an average particle diameter of 50 nm over the temperature range 10-300 K. The non-stoichiometry was estimated from Mossbauer spectroscopy analysis. High-resolution X-ray diffraction studies in the temperature range 93-300 K did not show any phase transition. There was a drastic change in resistivity around 80 K but no discontinuity thereof. Electrical resistivity vs. temperature data were analysed on the basis of Mott's small polaron and variable-range hopping models, respectively. The Verwey temperature as estimated from this analysis was 93 K. From voltage-current characteristics it was concluded that there was a small intrinsic gap at the Fermi level above the transition temperature and the same increased drastically below the transition temperature. This was ascribed to a transition from short-range order to long-range order as the temperature was lowered.     

La2-xBaxCuO4单晶反常的正常态输运性质研究

测量了La2-xBaxCuO4系列单晶样品的电阻率和热电势,我们发现,当＝0．125（x＝1／8）时,La2-xBaxCuO4的截流子是所有样品中局域化最强的,但数据分析结果显示,它又是弱域化行为,文中我们讨论了Tc的被压制的原因,得出这可能与低温下LTO到LTT的结构相变,空穴与自旋的静态条纹有序有关,热电势结果观察不到任何声子曳引的痕迹,表明在LBCO体系中电声子的相互作用很弱。     

Electrical transport properties of CuS single crystals

Electrical resistivity, transverse magnetoresistance and thermoelectric power measurements were performed on CuS high quality single crystals in the range 1.2-300 K and under fields of up to 16 T. The zero field resistivity data are well described below 55 K by a quasi-2D model, consistent with a carrier confinement at lower temperatures, before the transition to the superconducting state. The transverse magnetoresistance develops mainly below 30 K and attains values as large as 470% for a 16 T field at 5 K, this behaviour being ascribed to a band effect mechanism, with a possible magnetic field induced DOS change at the Fermi level. The transverse magnetoresistance shows no signs of saturation, following a power law with field Δρ/ρ(0) ∝ H(1.4), suggesting the existence of open orbits for carriers at the Fermi surface. The thermoelectric power shows an unusual temperature dependence, probably as a result of the complex band structure of CuS.     

Field-induced paramagnons at the metamagnetic transition of Ca1.8Sr0.2RuO4

The magnetic excitations in Ca1.8Sr0.2RuO4 were studied across the metamagnetic transition and as a function of temperature using inelastic neutron scattering. At low temperature and low magnetic field the magnetic response is dominated by a complex superposition of incommensurate antiferromagnetic fluctuations. Upon increasing the magnetic field across the metamagnetic transition, paramagnon and finally well-defined magnon scattering is induced, partially suppressing the incommensurate signals. The high-field phase in Ca1.8Sr0.2RuO4, therefore, has to be considered as an intrinsically ferromagnetic state stabilized by the magnetic field.     

Large magnetocaloric effect in HoFeO3 single crystal

Magnetocaloric properties of HoFeO₃ single crystal are investigated along the direction [100]. Magnetic field dependent magnetization isotherms at different temperatures undergo a metamagnetic transition, entropy change as large as 19.2 J/kg K and 15.8 J/kg K are obtained at 7 T in the vicinity of antiferromagnetic ordering temperature of Ho³⁺ and the metamagnetic transition, respectively. The coupling of Ho and Fe spins generates the compensation behavior at 6.5 K, separating the two large magnetic entropy change. Its refrigeration capacity (RC) value, as high as 220 J/kg, is appreciable and can be considered as a promising magnetic refrigerant. New evidence for spin reorientation of Fe³⁺ in HoFeO₃ is also provided by the change of magnetic entropy.     

Electrically controllable metal–insulator-like transition in nanoparticle compacts

We report on the observations of tunneling transport in nanocompacts, where nanoparticles are packed into compact units using selective mass compositions and packing densities. An insulator-like thermal behavior in electron transport is seen in a very loosely packed 6-nm Ag nanocompact, whereas the densely packed 4.5-nm Au nanocompact displays a metal-like thermal behavior. Metal–insulator-like transitions, with the transition temperature can readily be tuned by controlling the bias voltage, are observed in the nanocompact consists of mixtures of 2.4 nm Ag and 4.8 nm core/shell Cu/Cu₂O nanoparticles. The resistivity across the metal–insulator transition is found to change by more than four orders-of-magnitude. At low bias voltages or small excitation currents, the metal–insulator transition occurs at ~190 K. The transition temperature can be tuned to reach the ambient temperature when a higher bias voltage or a larger current is allowed. Possible mechanisms that may produce the observed transport characteristics in the nanocompacts are discussed.