Field-induced valence transition in EuPtP<Subscript>1−x</Subscript>As<Subscript>x</Subscript>

The ternary compound EuPtP exhibits two valence transitions at T ₁ = 235 K and T ₂ = 190 K. In order to examine a field-induced valence transition of Eu, we synthesized EuPtP_1−x As _x compounds with 0.05 ≤ x ≤ 0.5 and studied the magnetic and valence behavior. The substitution of As for P increases the lattice volume linearly and decreases both valence transition temperatures, T ₁ and T ₂, in contrast to the behavior under external pressures. The magnetization process in a pulsed magnetic field revealed that EuPtP_0.5As_0.5 exhibits an onset of metamagnetic transition above 50 T with a large hysteresis, which evidences a first-order field-induced valence transition. The analysis of the magnetization curves of x = 0.5 at various temperatures has demonstrated that the field-induced transition is essentially the same as the transition induced by temperature at T ₁.     

Effect of substitution of Ce on superconducting properties of Bi1.7Pb0.3Sr2Ca2−x Ce x Cu3O10+δ system

We have investigated the superconducting properties of the Bi_1.7 Pb_0.3Sr₂Ca_2−xCe _x Cu₃O_10+δ system with x=0.00, 0.02, 0.04, 0.08 and 0.1 by X-ray diffraction and magnetic susceptibility. The substitution of Ce for Ca has been found to drastically change the superconducting properties of the system. X-ray diffraction studies on these compounds indicate decrease in the c-parameter with increased substitution of Ce at Ca site and volume fraction of high T _c (2 : 2 : 2 : 3) phase decreases and low T _c phase increases. The magnetic susceptibility of this compound shows that the diamagnetic on set superconducting transition temperature (onset) varies from 109 K to 51 K for x=0.00, 0.02, 0.04, 0.08 and 0.1. These results suggest the possible existence of Ce in a tetravalent state rather than a trivalent state in this system; that is, Ca²⁺ → Ce⁴⁺ replacement changes the hole carrier concentration. Hole filling is the cause of lowering T _c of the system.     

Magnetic and magnetocaloric properties of Pr0.6−xEuxSr0.4MnO3 manganese oxides

Studies of the structural, magnetic and magnetocaloric properties of polycrystalline Pr_0.6−xEu_xSr_0.4MnO₃ (0≤x≤0.15) perovskite manganites were carried out. Substitution for praseodymium with europium, with smaller ionic radius, induces local distortion in the 〈Mn–O–Mn〉 bonds and consequently causes a random distribution in the magnetic exchange interactions. The competition between magnetic interactions leads to the appearance of an inhomogeneous magnetic state in our samples. Pr_0.6−xEu_xSr_0.4MnO₃ (x=0, 0.05, 0.1 and 0.15) polycrystalline samples were prepared using the solid–solid reaction method at high temperature. The compounds yielded are single phase and crystallize in the orthorhombic system with the Pnma space group. The substitution of Eu for Pr leads to a decrease of the Curie temperature T_C from 303 K for x=0.00 to 260 K for x=0.15. All of our compounds exhibit a large magnetic entropy change with a maximum around 2.2 J/kg K under a magnetic applied field change of 2 T for all compounds.     

Structural and magnetic properties of Pr3(Fe1−xCox)27.5Ti1.5 (x=0.0, 0.1, 0.2, 0.3)

A novel class of Co-substituted 3 : 29 materials, Pr₃(Fe_1−xCo_x)_27.5Ti_1.5 (x=0, 0.1, 0.2, 0.3) have been synthesized. Rietveld analysis of X-ray powder diffraction patterns for the x=0, 0.1 and 0.2 compositions showed that nearly all of the compounds are formed in monoclinic symmetry, with A2/m space group with traces of α-Fe, whereas, in x=0.3, additional traces of a (Co/Fe)–Ti (1 : 12) phase are also seen. The saturation magnetization increases with Co concentration both at 5 and 300 K and is explained on the basis of a rigid band model. A magnetic transition is observed for x=0.1 near 240 K. A large increase in Curie temperature, of about 180 K for x=0.1 and about 110 K for the other concentrations, is discussed on the basis of the strengthening of TM–TM exchange by the preferential occupation of Co in some of the Fe sites originally participating in antiferromagnetic bonds.     

Phase transformations and magnetotransport properties of the Pr0.5Sr0.5Co1 ? xMnxO3 system

The structural, magnetic, and magnetotransport properties of Pr_0.5Sr_0.5Co_{1 − x} Mn _x O₃ (x < 0.65) perovskites are studied by magnetization and electrical conductivity measurements in magnetic fields up to 14 T and by neutron diffraction. In the manganese concentration range x < 0.5 and T = 300 K, the crystal structure is described by monoclinic space group I2/a; at x > 0.5, it is described by orthorhombic space group Imma. When the temperature decreases, a structural transformation without changing the symmetry takes place in all compounds. This transformation is caused by an active role of the inner shells of the praseodymium ion in chemical bond formation. The substitution of manganese for cobalt breaks a long-range ferromagnetic order near x ≈ 0.25, and a metal-dielectric transition occurs at x ≈ 0.15. The negative magnetoresistance is found to be maximal near a critical manganese concentration, where a long-range magnetic order is broken; it reaches 95% in a field of 14 T at T = 10 K for x = 0.2. An unusual dielectric magnetic state with a small spontaneous magnetic moment and a sharp transition into a paramagnetic state at T > 200 K is revealed in the concentration range 0.30 ≤ x ≤ 0.65 in spite of the absence of coherent magnetic neutron scattering. A model is proposed to explain the behavior of the magnetic properties in this phase.     

Microstructure and magnetocaloric effect in cast LaFe11.5Si1.5Bx (x=0.5, 1.0)

Phase formation, structure, and the magnetocaloric effect (MCE) in as-cast LaFe_11.5Si_1.5B_x (x=0.5, 1.0) compounds have been studied. The Curie temperatures, T_C, are ∼211 and 230 K for x=0.5 and 1.0, respectively, which are higher than that of annealed LaFe_11.5Si_1.5 (T_C=183 K), while the maximum magnetic entropy changes at the respective T_C under a magnetic field change of 0-5 T are 7.8 and 5.8 J/(kg K). Wavelength dispersive spectrometry (WDS) analysis shows that only a small fraction of boron atoms is dissolved in the NaZn₁₃-type structure phase, and that the compositions of the as-cast LaFe_11.5Si_1.5B_x (x=0.5, 1.0) alloys are much different from the intended nominal compositions. These as-cast alloys exhibit second-order magnetic phase transitions and low MCEs. However, based on the relative cooling power, the as-cast LaFe_11.5Si_1.5B_x alloys are promising candidates for magnetic refrigerants over a wide temperature range.     

Metal-insulator transition in electron-doped Ba1−x La x MnO3 compounds

Electron-doped (Ba_1−x La _x )MnO₃ compounds were prepared for x=0−0.5. Measurements of X-ray diffraction (XRD) at room temperature and temperature variation of dc electrical resistivity down to 20 K were carried out. Samples with x=0.2–0.5 exhibit metal-insulator (M-I) transition. The maximum M-I transition temperature (T _c) of 289 K was observed for 30% of La doping (x=0.3). XRD patterns of these samples (x=0.2−0.5) were analyzed using Rietveld refinement. These samples are found to be mostly in single-phase form with orthorhombic symmetry (space group Pbnm). We have found strong correlation between Mn-O-Mn bond angles and T _c of M-I transition. The resistivity data below T _c could be fitted to the expression ρ=ρ ₁+ρ ₂ T ² and this shows that double exchange interaction plays a major role even in Mn⁴⁺-rich compound. Above T _c the resistivity data were fitted to variable range hopping and small polaron models.     

Magnetic phase transitions induced by strong magnetic fields in Zn1-x Ga0.667x Cr2se4

Powders of the spinels Zn_1-x Ga_0.667x Cr₂Se₄ for x = 0.0, 0.1, 0.2, 0.3, 0.5 have been obtained. For these spinels magnetization measurements were carried out by induction in strong pulsed magnetic fields up to 20T, in the temperature range 2.9 K to 30 K.

For x = 0.0 (ZnCr₂Se₄) three phase transitions were observed, whilst for concentrations with x = 0.1, 0.2 and 0.3 there were only two and for x = 0.5 only one. A hypothesis on the magnetic structure of the various phases separated by these transition is put forward.     

Magnetic coupling in Gd(Fe6−x Cr x )Al6: A57Fe Mössbauer study

⁵⁷Fe Mössbauer measurements have been made on the ternary ThMn₁₂-type intermetallic compounds Gd(Fe_6–x Cr _x )Al₆ withx=0, 0.5, 1.0, 1.5 and 2.0, at temperatures of 4.2 and 77 K. The principal effect of the Cr substitution is to reduce the⁵⁷Fe magnetic hyperfine field at 4.2 K in this series. The analysis of the⁵⁷Fe Mössbauer spectra is consistent with a ferromagnetic coupling between the Gd and Cr magnetic moments. These results are in agreement with previous studies by Felner et al. on GdCr₆Al₆, in which a ferromagnetic ordering withT _C=170 K was observed.On leave from Applied Acoustics Institute, Shaanxi Teachers University, Xian, PR China.     

Spin-glass and reentrant spin-glass states in iron sulfospinels having dilute A and B sublattices

The magnetic and electrical properties of new compounds having spinel structure Fe_1−x Cr_2(1−x)Sn_2x S₄ (0.1⩽x⩽0.33) (system 1), Fe_0.67 [Fe_0.165CrSn_0.835]S₄, and Fe_0.67 [Fe_0.33Cr_0.67Sn]S₄ has been studied. These compounds are p-type semiconductors with magnetic properties characteristic of the following magnetic-order types: ferrimagnetic (the x=0.1 composition of system 1), spin glass (the x=0.33 composition of system 1 and Fe_0.67 [Fe_0.165CrSn_0.835]S₄), and reentrant spin glass (the x=0.2 composition of system 1 and Fe_0.67 [Fe_0.33Cr_0.67Sn]S₄). For the spin-glass compositions, the dependence of the freezing temperature T _f defined as the temperature of the maximum of initial magnetic susceptibility, on temperature and magnetic field obeys the Almeida-Thouless relation, and the dependence of T _f on magnetic-field frequency is a power-law function. For the spin-glass and reentrant spin-glass compositions, a large peak in the absolute value of negative isotropic magnetoresistance was found near T _f , which becomes as high as 15% in spin glasses and 30% in reentrant spin-glass compositions. In compositions with reentrant behavior, the activation energy of conductivity in the region of T _f was found to change by about two orders of magnitude. This experimental evidence suggests that the spin-glass-paramagnet (in spin glasses) and spin-glass-long-range magnetic-order transformations are actually phase transitions, and that the spin-glass region contains ferron-type ferromagnetic clusters. These are the first spin-glasses among the chalcospinels with magnetically active ions on the tetrahedral and octahedral sublattices. Fiz. Tverd. Tela (St. Petersburg) 41, 84–90 (January 1999)     

Low temperature specific heat of bi-layered manganites La2−2xSr1+2xMn2O7 (x=0.3 and 0.5)

The specific heat (C) of bi-layered manganites La_2−2xSr_1+2xMn₂O₇ (x=0.3 and 0.5) is investigated for the ground state of low temperature excitations. A T^3/2 dependent term in the low temperature specific heat (LTSH) is identified at zero magnetic field and suppressed by magnetic fields for x=0.3 sample, which is consistent with a ferromagnetic metallic ground state. For x=0.5 sample, a T² term is observed and is consistent with a two-dimensional (2D) antiferromagnetic insulator. However, it is almost independent of magnetic field within the range of measured temperature (0.6-10 K) and magnetic field (6 T).     

Magnetocaloric effects in (Gd1−xTbx)Co2

The structures and magnetocaloric effects of (Gd_1−xTb_x)Co₂ (x=0, 0.25, 0.4, 0.5, 0.6, 0.7, 0.8, and 1) pseudobinary compounds were investigated by X-ray powder diffraction and magnetic properties measurement. The results show that the T_c of the alloy is near room temperature when X=0.6. The magnetic entropy changes of the compounds increase from 1.7 to 3.6 J/kg K with increasing the content of Tb under an applied field up to 2 T. All the compounds exhibit second order magnetic change. As a result, the values of their ΔS_M are lower than that of some large magnetocaloric effect materials.     

Magnetic-Field-Controlled Quantum Critical Points in the Triangular Antiferromagnetic Cs2CuCl4-xBrx Mixed System

The triangular antiferromagnetic Cs₂CuCl_4-xBr_x mixed system is studied by neutron single-crystal diffraction in magnetic field. It shows a rich magnetic phase diagram consisting of four regimes depending on the Br concentration and is characterized by different exchange coupling mechanisms. For the investigated compositions from regime I (0 < x ≤ 1.5), a critical magnetic field B_c is found for a Br concentration x = 0.8 at B_c = 8.10(1) T and for x = 1.1 at B_c = 7.73(1) T and from regime IV (3.2 < x < 4) for x = 3.3 at B_c = 0.99(3) T. For magnetic fields larger than the respective B_c, magnetic superlattice reflections of these compounds are not found. The incommensurate magnetic wave vector q = (0, 0.470, 0) appears below the ordering temperature T_N = 0.51(1) K for Cs₂CuCl_3.2Br_0.8, and q = (0, 0.418, 0) below T_N = 1.00(6) K for Cs₂CuCl_0.3Br_3.7. Neutron diffraction experiments at around 60 mK for x = 3.7 in a magnetic field show the critical magnetic field at B_c = 7.94(16) T and the formation of the second magnetic phase at around 8.5 T depending on the temperature. Inelastic neutron scattering experiments for the compound from regime III (2 < x ≤ 3.2) with x = 2.2 show dynamical correlations at a temperature around 50 mK giving evidence for a spin liquid phase.     

A non-magnetic Fe probe of multiple magnetic phase transitions in RMn2Si2−x Ge x,R=rare earth

Summary Dilute⁵⁷Fe M?ssbauer-spectroscopy studies of RMn₂X₂ (X=Si and/or Ge, R=La, Ce, Pr, Nd, Sm, Eu and Gd) at 4.2 to 650 K yield the following results: Fe in RMn₂X₂ does not carry a magnetic moment. It reveals the magnetic order in the Mn and R sublattices through transferred hyperfine fields (∼100 kOe). The compounds LaMn₂Si₂, LaMn₂Ge₂, CeMn₂Ge₂, PrMn₂Ge₂, NdMn₂Ge₂ and SmMn₂Ge₂, known to be ferromagnets withT _c=300–350 K, are antiferromagnetically ordered above their correspondingT _c. TherT _N values extend from 385 K (SmMn₂Ge₂) to 470 K (LaMn₂Si₂). At the ferromagnetic-antiferromagnetic phase transition, a sharp reorientation of the Mn magnetic moments relative to the crystalline axes occurs. In RMn₂Si_2−x Ge _x with intermediatex values, the Ge is much more dominant in determining the magnetic properties of the Mn sublattice. While PrMn₂Si₂ is an antiferromagnet (T _N=365 K) and PrMn₂Ge₂ is a ferromagnetantiferromagnet (T _c=328 K,T _N=415 K), we find that in PrMn₂SiGe, the magnetic behaviour is similar to that in pure PrMn₂Ge₂,T _c=305 K andT _N=395 K. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Phase diagram of LaTiO x : from 2D layered ferroelectric insulator to 3D weak ferromagnetic semiconductor

LaTiO _x compounds are structurally related to perovskites and there are two known phases. The first,x=3.50, is a 2D layered-type ferroelectric. The second,x=3.00, is a weak ferromagnet with a 3D orthorhombic distorted perovskite structure. 20 samples with varying oxygen stoichiometry between these end members were prepared by floating zone melting, and then characterized by means of X-ray powder diffraction, electron microscopy, thermogravimetric analysis, resistivity and magnetic measurement. A phase diagram is established which displays the following physical and structural properties. A structural phase boundary atx=3.20 separates a new series of 2D layered structures from the 3D orthorhombic one. The former series represents the first conducting titanium oxides with a 2D layered structure to be reported. Atx=3.10 a phase boundary exists between a metallic and a weak ferromagnetic state where the magnetic transition temperatureT _c can be sensitively tuned by the oxygen stoichiometryx. Samples withT _c between 100 K and 130 K exhibit a metal-semiconductor transition whereas samples with higherT _c , up to 149 K, are semiconductors between room temperature and 4.2 K.     

Effect of Sn-doping on the structural,magnetic and magnetocaloric properties of La0.67Ba0.33Mn1−xSnxO3 compounds

Magnetocaloric effect (MCE) in fine-grained perovskite manganites of the type La_0.67Ba_0.33Mn_1−xSn_xO₃ (x=0.05, 0.1 and 0.15) were prepared by the solid-state method. The prepared samples remain single phase and exhibit paramagnetic to ferromagnetic phase transition (T_C) at 340, 325 and 288 K for x=0.05, 0.1 and 0.15, respectively. From the measured magnetization data of La_0.67Ba_0.33Mn_1−xSn_xO₃ compounds as a function of field (2 T), the associated magnetic entropy change close to their respective Curie temperatures and the relative cooling power (RCP) have been determined. Large MCE has been obtained in all samples and |ΔS_M|_max reached the highest value of 2.49 J/kg K at T_C (288 K) for the sample x=0.15, with H=2 T.     

Effect of pressure and magnetic field on bilayer La1.25Sr1.75Mn2O7 single crystal

We report the temperature dependence of susceptibility for various pressures, magnetic fields and constant magnetic field of 5 T with various pressures on La_2−2xSr_1+2xMn₂O₇ single crystal to understand the effectiveness of pressure and magnetic field in altering the magnetic properties. We find that the Curie temperature, T_c, increases under pressure (dT_c/dP=10.9 K/GPa) and it indicates the enhancement of ferromagnetic phase under pressure up to 2 GPa. The magnetic field dependence of T_c is about 26 K for 3 T. The combined effect of pressure and constant magnetic field (5 T) shows dT_c/dP=11.3 K/GPa and the peak structure is suppressed and broadened. The application of magnetic field of 5 T realizes 3D spin ordered state below T_c at atmospheric pressure. Both peak structure in χ_c and 3D spin ordered state are suppressed, and changes to 2D-like spin ordered state by increase of pressure. These results reveal that the pressure and the magnetic field are more competitive in altering the magnetic properties of bilayer manganite La_1.25Sr_1.75Mn₂O₇ single crystal.     

Magnetic ordering in the perovskites Eu1−x CaxMnO3 (0⩻x⩻0.5)

The magnetic properties of Eu_1−x Ca_xMnO₃ have been investigated. As the calcium content increased up to x=0.2, the magnetization and the blocking temperature of the magnetic moments of clusters increased and the magnetic anisotropy decreased. As the calcium content increased further, the magnetization decreased, while the “freezing” temperature of the magnetic moments increased. Anomalies of the magnetic properties were observed in compositions with x=0.4 and 0.5 at T=40 K; these anomalies are attributed to a transition to the antiferromagnetic state in the charge-ordered phase. Fiz. Tverd. Tela (St. Petersburg) 39, 117–120 (January 1997)     

Magnetic entropy change and magnetic phase transition of LaFe11.4Al1.6Cx （x=0-0.8） compounds

The unit cell volume and phase transition temperature of LaFe11.4Al1.6Cx compounds have been studied. The magnetic entropy change, refrigerant capacity and the type of magnetic phase transition are investigated in detail for LaFe11.4Al1.6Cx with x=0.1, All the LaFe11.4Al1.6Cx （x=0-0.8） compounds have the cubic NaZn13-type structure. The addition of carbon atoms brings about a considerable increase in the lattice parameter. The bulk expansion results in the change of phase transition temperature （Tc）, Tc increases from 187K to 269 K with x varying from 0.1 to 0.8, Meanwhile an increase in the lattice parameter can also cause a change of the magnetic ground state from antiferromagnetic to ferromagnetic. Large magnetic entropy change IASI is found over a large temperature range around Tc and the refrigerant capacity is about 322J/kg for LaFe11.4Al1.6C0.1. The magnetic phase transition belongs in weakly first-order one for x=0.1.     

Magnetic ordering of Fe atoms in YBa2(Cu1−x Fe x )3O3 in superand semiconducting states

Magnetic behavior of Fe atoms in YBa₂(Cu_1−x Fe _x )₃O _y system in superconducting (SC) and non-SC states is studied. In SC region one magnetic ordering point T_m1 is observed for all Fe atoms in Cu1 sites: T_m1=10 K and 16 K for x=0.05 and x=0.10, respectively. In the non-SC state for x=0.05 Cu1 and Cu2 sublattices are magnetically independent and two magnetic ordering points are found: T_m1=20 K and T_m2=405K. For x=0.10 a strong magnetic interaction between Cu1 and Cu2 sublattices appears and only one ordering point T_m2=435 K is observed.