First-order antiferro-ferromagnetic transition in Fe(49)(Rh(0.93)Pd(0.07))(51) under simultaneous application of magnetic field and external pressure

A magnetic field-pressure-temperature (H-P-T) phase diagram for first-order antiferromagnetic (AFM) to ferromagnetic (FM) transitions in Fe(49)(Rh(0.93)Pd(0.07))(51) has been constructed using resistivity measurements under simultaneous application of magnetic field (up to 8 T) and pressure (up to 20 kbar). The temperature dependence of resistivity (ρ-T) shows that the width of the transition and the extent of hysteresis decreases with pressure and increases with magnetic field. By exploiting opposing trends of dT(N)/dP and dT(N)/dH (where T(N) is the first-order transition temperature), the relative effects of temperature, magnetic field and pressure on disorder-broadened first-order transitions has been studied. For this, a set of H and P values are chosen for which T(N)(H(1),P(1)) = T(N)(H(2),P(2)). Measurements for such combinations of H and P show that the temperature dependence of resistivity is similar, i.e. the broadening (in temperature) of transition as well as the extent of hysteresis remains independent of H and P. Isothermal magnetoresistance measurements under various constant pressures show that even though the critical field required for AFM-FM transition depends on applied pressure, the extent of hysteresis as well as transition width (in magnetic field) remains constant with varying pressure.     

Possible link of a structurally driven spin flip transition and the insulator-metal transition in the perovskite La(1-x)Ba(x)CoO3

The nature of the magnetic ground state near the insulator-metal transition (IMT) in La(1-x)Ba(x)CoO3 was investigated via neutron scattering. Below the critical concentration, x(c)～0.22, a commensurate antiferromagnetic (AFM) phase appears initially. Upon approaching x(c), the AFM component weakens and a ferromagnetic (FM) ordered phase sets in while in the rhombohedral lattice. At x(c), a spin flip to a new FM structure occurs at the same time as the crystal symmetry transforms to orthorhombic (Pnma). The Pnma phase may be the driving force for the IMT.     

Magnetic properties and magnetocaloric effects in NaZn13-type La（Fe, Al）13-based compounds

In this article,our recent progress concerning the effects of atomic substitution,magnetic field,and temperature on the magnetic and magnetocaloric properties of the LaFe13-xAlx compounds are reviewed.With an increase of the aluminum content,the compounds exhibit successively an antiferromagnetic(AFM) state,a ferromagnetic(FM) state,and a mictomagnetic state.Furthermore,the AFM coupling of LaFe 13-xAlx can be converted to an FM one by substituting Si for Al,Co for Fe,and magnetic rare-earth R for La,or introducing interstitial C or H atoms.However,low doping levels lead to FM clusters embedded in an AFM matrix,and the resultant compounds can undergo,under appropriate applied fields,first an AFM-FM and then an FM-AFM phase transition while heated,with significant magnetic relaxation in the vicinity of the transition temperature.The Curie temperature of LaFe13-xAlx can be shifted to room temperature by choosing appropriate contents of Co,C,or H,and a strong magnetocaloric effect can be obtained around the transition temperature.For example,for the LaFe 11.5Al1.5C0.2H1.0 compound,the maximal entropy change reaches 13.8 J·kg-1 ·K-1 for a field change of 0-5 T,occurring around room temperature.It is 42% higher than that of Gd,and therefore,this compound is a promising room-temperature magnetic refrigerant.     

Irreversible metamagnetic transition and magnetic memory in small-bandwidth manganite Pr(1-x)Ca(x)MnO3 (x = 0.0-0.5)

The present paper reports detailed structural and magnetic characterization of the low-bandwidth manganite Pr(1-x)Ca(x)MnO(3) (with x = 0.0-0.5) (PCMO) polycrystalline samples. With increasing Ca content, reduction of the unit cell volume and improvement in perovskite structure symmetry was observed at room temperature. Magnetic characterization shows the signature of coexisting AFM-FM ordering and spin-glass phase at the low doping range (x = 0.0-0.2) while increased hole doping (x = 0.3-0.5) leads to charge ordering, training effect and an irreversible metamagnetic phenomenon. The large irreversible metamagnetism in the CO phase of PCMO and the corresponding spin memory effect is a direct consequence of hysteretic first-order phase transition arising from the weakening of the CO state under the external magnetic field and trapping of the spins due to a strong pinning potential in the material.     

Effect of Zn doping in hole-type 1111 phase (Pr, Sr)FeAsO

There is an anomalous broad hump in the normal state resistivity in hole-type 1111 phase FeAs-based superconductors and its origin is an open issue. We study the effect of Zn doping on this anomaly in order to determine whether it is associated with the residual structural/antiferromagnetic (AFM) phase transition as in the parent compounds. A series of Zn doped Pr?.?Sr?.?FeAsO samples are prepared and their resistivity, magnetoresistance, Hall effect and specific heat are measured. Zn doping should not introduce extra charge carriers, and instead it can suppress the structural/AFM transition efficiently in the parent LaFeAsO system. The hump in resistivity remains unchanged with 6% Zn doping in Pr?.?Sr?.?FeAsO. The measurements of magnetoresistance reveal that the magnetoresistance is negligible in the Zn doped Pr?.?Sr?.?FeAsO samples, in contrast to the large positive magnetoresistance below the temperature of structure/AFM phase transition in the parent compound PrFeAsO. The results indicate that the anomalous broad hump in resistivity does not originate from the structural/AFM transition. The Hall effect and specific heat data are also consistent with this conclusion.     

High-temperature heat capacity of Gd2CuO4

The heat capacity of Gd₂CuO₄ has been studied by differential scanning calorimetry in the temperature range 362–958 K. It has been found that the temperature dependence of the molar heat capacity has an extremum near 590 K; the extremum is related to a phase transition from the high-temperature tetragonal phase to the low-temperature orthorhombic phase.     

Structure of ferromagnetic CrAs epilayers grown on GaAs(001)

Magnetic and structural properties of CrAs epilayers grown on GaAs(001) by molecular beam epitaxy have been studied. CrAs epilayers are orthorhombic for all thicknesses investigated but show a structural transition from a metastable phase for very thin films, to the usual bulk MnP-type orthorhombic phase at higher thicknesses. At intermediate thicknesses, there is a predominance of the new phase, although a contribution from the usual CrAs bulk phase remains clearly present. These results strongly suggest that the ferromagnetic signal measured at room temperature comes from the new metastable orthorhombic structure with an expanded b-axis induced by the substrate strain.     

Neutron powder diffraction investigation on the crystal and magnetic structure of (Ho(0.50+x)Ca(0.50-x))(Mn(1-x)Cr(x))O3

The crystal and magnetic structure of (Ho(0.50+x)Ca(0.50-x))(Mn(1-x)Cr(x))O(3) (x = 0.00, 0.01, 0.02, 0.03) has been investigated between 5 and 300 K by means of neutron powder diffraction followed by Rietveld refinement and dc magnetic measurements. During cooling an orthorhombic to monoclinic phase transition occurs on account of the charge and orbital ordering taking place in the Mn sub-lattice; at low temperature phase separation takes place and the main monoclinic phase coexists with a secondary orthorhombic phase, whose amount slightly increases with the increase of Cr content. Cr(3+) is not involved in orbital ordering or superexchange interactions. The charge and magnetic ordering are decoupled: the Mn moments order according to a CE-type structure in all samples.     

First-principles prediction of spin-density-reflection symmetry driven magnetic transition of CsCl-type FeSe

Based on results of density functional theory (DFT) calculations with the local spin density approximation (LSDA) and the generalized gradient approximation (GGA), we propose a new magnetic material, CsCl-type FeSe. The calculations reveal the existence of ferromagnetic (FM) and antiferromagnetic (AFM) states over a wide range of lattice constants. At 3.12 Å in the GGA, the equilibrium state is found to be AFM with a local Fe magnetic moment of . A metastable FM state with Fe and Se local magnetic moments of 2.00 and , respectively, lies 171.7 meV above the AFM state. Its equilibrium lattice constant is ∼2% smaller than that of the AFM state, implying that when the system undergoes a phase transition from the AFM state to the FM one, the transition is accompanied by volume contraction. Such an AFM-FM transition is attributed to spin-density z-reflection symmetry; the symmetry driven AFM-FM transition is not altered by spin-orbit coupling. The relative stability of different magnetic phases is discussed in terms of the local density of states. We find that CsCl-type FeSe is mechanically stable, but the magnetic states are expected to be brittle.     

Structural properties of complex (dusty) plasma upon crystallization and melting

A change in the local order of a bounded complex (dusty) plasma in the process of its crystallization and melting has been examined by molecular dynamics simulations. The dynamics of microparticles is considered in the framework of a Langevin thermostat, the pair interaction between charged particles is described by a screened Coulomb potential (Yukawa potential) with the hard wall potential as a confinement. It has been shown that the beginning of the crystallization of such a system is accompanied by the formation of clusters with the hexagonal close packed (hcp) structure; a noticeable number of these clusters are then transformed to the face centered cubic (fcc) phase. A plasma crystal formed after crystallization consists of the metastable hcp phase, fcc clusters, and a small number of clusters with a body centered cubic (bcc) crystal lattice. Beginning with a certain threshold value of the thermostat temperature, the number of fcc/bcc clusters decreases sharply with increasing temperature, which is an important signature of the beginning of the melting of the plasma crystal.     

The influence of magnetic and electric coupling properties on the magnetocaloric effect in quantum paraelectric EuTiO3

We report on the magnetic and magnetocaloric effect calculations in antiferromagnetic perovskite-type EuTiO₃. From the isothermal magnetic entropy change calculated upon low magnetic field changes (below 1 T) several results were predicted: inverse magnetocaloric effect, latent heat associated to spin AFM-FM reorientation transition and a temperature interval (controlled by magnetic field) where the EuTiO₃ does not change heat in an isothermic process. The magnetocaloric effect described through magnetic entropy change was correlated with magnetocapacitance formula. The theoretical investigation was carried out using a Heisenberg Hamiltonian considering the G-type antiferromagnetic structure with exchange interactions, in mean field approximation, between nearest-neighbor and next-nearest-neighbor magnetic Eu⁺² ions.     

Exchange-coupling-induced fourfold magnetic anisotropy in CoFeB/FeRh bilayer grown on SrTiO3(001)

Exchange coupling across the interface between a ferromagnetic (FM) layer and an antiferromagnetic (AFM) or another FM layer may induce a unidirectional magnetic anisotropy and/or a uniaxial magnetic anisotropy, which has been extensively studied due to the important application in magnetic materials and devices. In this work, we observed a fourfold magnetic anisotropy in amorphous CoFeB layer when exchange coupling to an adjacent FeRh layer which is epitaxially grown on an SrTiO₃(001) substrate. As the temperature rises from 300 K to 400 K, FeRh film undergoes a phase transition from AFM to FM phase, the induced fourfold magnetic anisotropy in the CoFeB layer switches the orientation from the FeRh$\langle 110\rangle $ to FeRh$\langle 100\rangle $ directions and the strength is obviously reduced. In addition, the effective magnetic damping as well as the two-magnon scattering of the CoFeB/FeRh bilayer also remarkably increase with the occurrence of magnetic phase transition of FeRh. No exchange bias is observed in the bilayer even when FeRh is in the nominal AFM state, which is probably because the residual FM FeRh moments located at the interface can well separate the exchange coupling between the below pinned FeRh moments and the CoFeB moments.     

Ba2+掺杂钙钛矿Bi0.5Ca0.5MnO3多晶材料的结构、磁性和输运性质研究

本文系统地研究了具有电荷有序(CO)和C型反铁磁(AFM)磁结构的钙钛矿锰氧化物Bi0.5Ca0.5-xBaxMnO3(x=0,0.03,0.05,0.07 and 0.10)的结构、磁性和电输运性质.系统晶格对称性保持Pnma空间群正交结构不变.随着Ba掺杂浓度的提高,电荷有序转变温度和反铁磁转变温度明显降低,表明掺...     

Retention of the tetragonal to orthorhombic structural transition in F-substituted SmFeAsO: a new phase diagram for SmFeAs(O(1-x)F(x))

In this Letter we propose a new phase diagram for the SmFeAs(O(1-x)F(x)) system, based on careful analysis of synchrotron powder diffraction data, SQUID, and muon spin rotation measurements. The tetragonal to orthorhombic structural transition is slightly affected by F content and is retained for the superconducting samples, even at optimal doping. These findings relate the AFM transition on a different ground with respect to the structural one and suggests that orbital ordering could be the driving force for symmetry breaking.     

Melting temperature and enthalpy variations of phase change materials (PCMs): a differential scanning calorimetry (DSC) analysis

Differential scanning calorimetry (DSC) analysis is a standard thermal analysis technique used to determine the phase transition temperature, enthalpy, heat of fusion, specific heat and activation energy of phase change materials (PCMs). To determine the appropriate heating rate and sample mass, various DSC measurements were carried out using two kinds of PCMs, namely N-octadecane paraffin and calcium chloride hexahydrate. The variations in phase transition temperature, enthalpy, heat of fusion, specific heat and activation energy were observed within applicable heating rates and sample masses. It was found that the phase transition temperature range increased with increasing heating rate and sample mass; while the heat of fusion varied without any established pattern. The specific heat decreased with the increase of heating rate and sample mass. For accuracy purpose, it is recommended that for PCMs with high thermal conductivity (e.g. hydrated salt) the focus will be on heating rate rather than sample mass.     

Phase transition and thermodynamic properties of BiFeO3 from first-principles calculations

The first-principles projector-augmented wave method employing the quasi-harmonic Debye model,is applied to investigate the thermodynamic properties and the phase transition between the trigonal R3c structure and the orthorhombic Pnma structure.It is found that at ambient temperature,the phase transition from the trigonal R3c phase to the orthorhombic Pnma phase is a first-order antiferromagnetic-nonmagnetic and insulator-metal transition,and occurs at 10.56 GPa,which is in good agreement with experimental data.With increasing temperature,the transition pressure decreases almost linearly.Moreover,the thermodynamic properties including Grneisen parameter,heat capacity,entropy,and the dependences of thermal expansion coefficient on temperature and pressure are also obtained.     

Electron diffraction and electron microscopic study of BaYCuO superconducting materials

Electron microscopy and electron diffraction have been applied to show that the orthorhombic phase in the compound Ba₂YCu₃O_7−δ is responsible for the high superconducting transition temperature. A positive correlation is found between the volume fration of the orthorhombic phase and the superconducting transition temperature. By means of an “in-situ” heating experiment it is found that the orthorhombic phase is formed on cooling from a high temperature tetragonal phase with disordered vacancies. It is suggested that the low temperature tetragonal phase that occurs in the same specimens as the orthorhombic phase also contains an ordered arrangement of vacancies different from that present in the orthorhombic phase. The order-disorder transition associated with the structural vacancies is shown to be reversible, provided there has been no oxygen loss.     

Phase coexistence in NaZn13-type LaFe11.4Al1.6C0.02 compound

In NaZn₁₃-type LaFe_11.4Al_1.6C_0.02 compound, a signature of weak ferromagnetism is observed at ∼100 K under a low field by ac magnetic-susceptibility and electrical-resistivity measurements, implying the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases. The hysteresis in isofield magnetization curves and large magnetic relaxation demonstrate the metastability of the magnetic state in the AFM-FM transition region. The variations of magnetization with temperature, time and field show distinct step-like behaviors, which is probably attributed to the discontinuous growth of ferromagnetic cluster in antiferromagnetic matrix.     

Metastability and inverse magnetocaloric effect in doped manganite (Nd(0.25)Sm(0.25)Sr(0.5)MnO(3)) and ferromagnetic shape memory alloy (Ni(2)Mn(1.36)Sn(0.64)): a comparison

The manganite Nd(0.25)Sm(0.25)Sr(0.5)MnO(3) (NSSMO) shows a first-order metal to insulator transition on cooling, which is concomitant with a magnetic transition from the ferromagnetic to antiferromagnetic state. In some respect the sample shows a striking similarity with Ni-Mn-Sn based ferromagnetic shape memory alloys (FSMAs) undergoing a first-order magneto-structural transition, and efforts have been made to highlight the similarities and dissimilarities of the studied manganite with one such FSMA of composition Ni(2)Mn(1.36)Sn(0.64). From our transport and magnetic investigations, the region of transition in the NSSMO is found to be highly metastable, with a clear indication of a magnetically arrested state which persists even when the sample is cooled down to the lowest temperature of measurement. Interestingly, the studied manganite shows an inverse magnetocaloric effect similar to the FSMA. However, a striking difference between the two compositions is evident in the low-temperature magneto-transport behavior: while a clear signature of tunneling magnetoresistance is present in NSSMO due to the coexisting metallic and insulating clusters of nanometer dimension, the studied FSMA do not show such behavior due to the absence of any insulating phase in the intermetallic alloy.     

Antiferromagnetism of nuclear matter in the model with effective Gogny interaction

The possibility of ferromagnetic (FM) and antiferromagnetic (AFM) phase transitions in symmetric nuclear matter is analyzed within the framework of a Fermi liquid theory with effective Gogny interaction. It is shown that, at some critical density, nuclear matter with the D1S effective force undergoes a phase transition to the AFM spin state (opposite directions of neutron and proton spins). The self-consistent equations of spin-polarized nuclear matter with the D1S force have no solutions corresponding to FM spin ordering (the same direction of neutron and proton spins) and, hence, the FM transition does not appear. The AFM spin polarization parameter is found for zero and finite temperature. It is shown that the AFM spin polarization parameter of partially polarized nuclear matter at low enough temperatures increases with temperature. The entropy of the AFM spin state for some temperature range is larger than the entropy of the normal state. Nevertheless, the free energy of the AFM spin state is always less than the free energy of the normal state, and the AFM spin-polarized state is preferable for all temperatures below the critical temperature. The text was submitted by the authors in English.