Half-Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变简

基于密度泛函理论(DFT),使用局域密度近似(LDA)研究了Heusler合金Cu1-xFex MnSb的电子结构和反铁磁-铁磁相变.研究发现,两种磁状态下的合金晶格常数随掺杂浓度x变化很好地满足Vegard定理.当x0.5时,铁磁态合金的总磁矩很好地符合SP规律,然而当x0.5时,却发生了明显的偏离.由于整个体系存在RKKY和超交换磁耦合的竞争,因而在x=0.25时,我们观察到了独特的反铁磁—铁磁相变.进一步的态密度分析发现,Cu的掺杂浓度可以有效调整铁磁态合金的费米面位置,并且反铁磁态合金由于不同自旋方向的Mn原子的分波态密度相互补偿,总态密度形成了几乎完全对称的自旋向上带和自旋向下带.     

Half-Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变

基于密度泛函理论（DFT）,使用局域密度近似（LDA）研究了Heusler合金Cu1-xFexMnSb的电子结构和反铁磁-铁磁相变。研究发现,两种磁状态下的合金晶格常数随掺杂浓度x变化很好地满足Vegard定理。当x>0.5时,铁磁态合金的总磁矩很好地符合SP规律,然而当x<0.5时,却发生了明显的偏离。由于整个体系存在RKKY和超交换磁耦合的竞争,因而在x=0.25时,我们观察到了独特的反铁磁—铁磁相变。进一步的态密度分析发现,Cu的掺杂浓度可以有效调整铁磁态合金的费米面位置,并且反铁磁态合金由于不同自旋方向的Mn原子的分波态密度相互补偿,总态密度形成了几乎完全对称的自旋向上带和自旋向下带。     

Dynamic magnetic behavior of the mixed spin （2, 5/2） Ising system with antiferromagnetic/antiferromagnetic interactions on a bilayer square lattice

Using the mean-field theory and Glauber-type stochastic dynamics, we study the dynamic magnetic properties of the mixed spin （2, 5/2） Ising system for the antiferromagnetic/antiferromagnetic （AFM/AFM） interactions on the bilayer square lattice under a time varying （sinusoidal） magnetic field. The time dependence of average magnetizations and the thermal variation of the dynamic magnetizations are examined to calculate the dynamic phase diagrams. The dynamic phase diagrams are presented in the reduced temperature and magnetic field amplitude plane and the effects of interlayer coupling interaction on the critical behavior of the system are investigated. We also investigate the influence of the frequency and find that the system displays richer dynamic critical behavior for higher values of frequency than that of the lower values of it. We perform a comparison with the ferromagnetic/ferromagnetic （FM/FM） and AFM/FM interactions in order to see the effects of AFM/AFM interaction and observe that the system displays richer and more interesting dynamic critical behaviors for the AFM/AFM interaction than those for the FM/FM and AFM/FM interactions.     

Influences of Crystal-Field and Interlayer Coupling Interactions on Dynamic Phase Diagrams of a Mixed-Spin(3/2,2) Bilayer System

Influences of crystal-fields(D_A and D_B) and interlayer coupling interactions(J_3) on dynamic magnetic critical behaviors of a mixed-spin(3/2, 2) bilayer system under an oscillating magnetic field are investigated by the Glauber-type stochastic dynamics based on the mean-field theory. For this purpose, dynamic phase diagrams are constructed in the reduced temperature and magnetic field amplitude plane for the ferromagnetic/ferromagnetic(FM/FM),antiferromagnetic/ferromagnetic(AFM/FM) and AFM/AFM interactions in detail. We observe that the influences of D_A, D_B and J_3 interactions parameters on the behavior of the dynamic phase diagrams are very much.     

Size dependence of phase transition temperatures of ferromagnetic, ferroelectric and superconductive nanocrystals

With the miniaturization of devices, size and interface effects become increasingly important for the properties and performances of nanomaterials. Here, we present a thermodynamic approach to the mechanism behind size-induced unusual behavior in the phase stabilities of ferromagnetic (FM), antiferromagnetic (AFM), ferroelectric (FE), and superconductive (SC) nanocrystals, which are different dramatically from their bulk counterparts. This method is based on the Lindemann criterion for melting, Mott’s expression for the vibrational melting entropy, and the Shi model for the size-dependent melting temperature. Simple and unified functions, without any adjustable parameter, are established for the size and interface dependences of thermal and phase stabilities of FM, AFM, FE and SC nanocrystals. According to these analytic functions, as the size of nanocrystals is reduced, the thermal and phase stabilities may strengthen or weaken, depending on the confluence of the surface/volume ratio of nanocrystals and the FM(AFM, FE or SC)/substrate interface situations. The validity of this model is confirmed by a large number of experimental results. This theory will be significant for the choice of materials and the design of devices for practical application.     

Size dependence of phase transition temperatures of ferromagnetic ,ferroelectric and superconductive nanocrystals

With the miniaturization of devices, size and interface effects become increasingly important for the properties and performances of nanomaterials. Here, we present a thermodynamic approach to the mechanism behind size-induced unusual behavior in the phase stabilities of ferromagnetic (FM), antiferromagnetic (AFM), ferroelectric (FE), and superconductive (SC) nanocrystals, which are different dramatically from their bulk counterparts. This method is based on the Lindemann criterion for melting, Mott’s expression for the vibrational melting entropy, and the Shi model for the size-dependent melting temperature. Simple and unified functions, without any adjustable parameter, are established for the size and interface dependences of thermal and phase stabilities of FM, AFM, FE and SC nanocrystals. According to these analytic functions, as the size of nanocrystals is reduced, the thermal and phase stabilities may strengthen or weaken, depending on the confluence of the surface/volume ratio of nanocrystals and the FM(AFM, FE or SC)/substrate interface situations. The validity of this model is confirmed by a large number of experimental results. This theory will be significant for the choice of materials and the design of devices for practical application.      

Successive inverse and normal magnetocaloric effects in the Mn-vacancy compound Mn_(0.95)Co_(0.75)Cu_(0.25)Ge

Ferromagnetic-structural transformation has been studied widely in MnCoGe-based materials. However, the magnetostructural transition(MST) from antiferromagnetic(AFM) orthorhombic phase to ferromagnetic(FM) hexagonal phase, which may lead to a large inverse magnetocaloric effect(MCE), has rarely been reported. Here, the introduction of Mn vacancy lowers the structural transition temperature while retains the AFM state in the orthorhombic phase, thus successfully realizing the AFM-FM MST in Mn_(0.95)Co_(0.75)Cu_(0.25)Ge. Moreover, successive inverse and normal MCEs are observed around the first-order AFM-FM MST and the second-order FM-paramagnetic(PM) transition, respectively. A thermostat is proposed based on this special feature, which could release heat above the critical temperature while absorb heat below the critical temperature by simply applying the same magnetization/demagnetization cycles. This thermostat can be very useful in many applications where a constant temperature is required, such as cryostats and incubators.     

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.     

First-Principles Studies for the Electronic Structures of Diluted Magnetic Semiconductors (Ga, Fe)As

First-principles LMTO-ASA band calculations are performed for Ga1-xFexAs (x = 1, 1/4, 1/8) by assuming supercell structures. It is found that the antiferromagnetic (AFM) state is stable for x = 1/4. For x = 1/8, ferromagnetic(FM) state is more stable than AFM state, and no stable magnetic state exists for x = 1. In both the cases the magneticmoments of As and Ga atoms are parallel to those of the nearest Fe atoms due to the p-d hybridization. Further, theband structure shows rather localized Fe 3d state in the gap, and the parallel polarization is confined rather in thevicinity of Fe site.     

First-order transition from a Kondo insulator to a ferromagnetic metal in single crystalline FeSi(1-x)Ge(x)

The phase diagram of FeSi(1-x)Ge(x), obtained from magnetic, thermal, and transport measurements on single crystals, shows a discontinuous transition from Kondo insulator to ferromagnetic metal with x at a critical concentration, x(c) approximately 0.25. The gap of the insulating phase strongly decreases with x. The specific heat gamma coefficient appears to track the density of states of a Kondo insulator. The phase diagram is consistent with an insulator-metal transition induced by a reduction of the hybridization with x in conjunction with disorder on the Si/Ge ligand site.     

Interplay between Fe 3d and Ce 4f magnetism and Kondo interaction in CeFeAs(1-x)P(x)O probed by 75As and 31P NMR

A detailed (31)P (I = 1/2) and (75)As (I = 3/2) NMR study on polycrystalline CeFeAs(1-x)P(x)O alloys is presented. The magnetism of CeFeAsO changes drastically upon P substitution on the As site. CeFePO is a heavy fermion system without long-range order whereas CeFeAsO exhibits an Fe 3d SDW type of ordering accompanied by a structural transition from tetragonal (TT) to orthorhombic (OT) structure. Furthermore, Ce 4f(1) orders antiferromagnetically (AFM) at low temperature. At the critical concentration where the Fe magnetism is diminished the Ce-Ce interaction changes to a ferromagnetic (FM) type of ordering. Three representative samples of the CeFeAs(1-x)P(x)O (x = 0.05, 0.3 and 0.9) series are systematically investigated. (1) For the x = 0.05 alloy a drastic change of the linewidth at 130 K indicates the AFM-SDW type of ordering of Fe and the structural change from the TT to the OT phase. The linewidth roughly measures the internal field in the ordered state and the transition is most likely first order. The small and nearly constant shift from (31)P and (75)As NMR suggests the presence of competing hyperfine interactions between the nuclear spins and the 4f and 3d ions of Ce and Fe. (2) For the x = 0.3 alloy, the evolution of the Fe-SDW type of order takes place at around 70 K corroborating the results of bulk measurement and μSR. Here we found evidence for phase separation of paramagnetic and magnetic SDW phases. (3) In contrast to the heavy fermion CeFePO for the x = 0.9 alloy a phase transition is found at 2 K. The field-dependent NMR shift gives evidence of FM ordering. Above the ordering the spin-lattice relaxation rate (31)(1/T(1)) shows unconventional, non-Korringa-like behaviour which indicates a complex interplay of Kondo and FM fluctuations.     

First-Principles Studies for the Electronic Structures of Diluted Magnetic Semiconductors （Ga, Fe）As

First-principles LMTO-ASA band calculations are performed for Ga1-xFezAs (x = 1, 1/4, 1/8) by assuming supercell structures. It is found that the antiferromagnetic (AFM) state is stable for x = 1/4. For x = 1/8, ferromagnetic (FM) state is more stable than AFM state, and no stable magnetic state exists for x = 1. In both the cases the magnetic moments of As and Ga atoms are parallel to those of the nearest Fe atoms due to the p-d hybridization. Fhrther, the band structure shows rather localized Fe 3d state in the gap, and the parallel polarization is confined rather in the vicinity of Fe site.     

Effect of electron doping on the magnetic correlations in the bilayered brownmillerite compound Ca(2.5-x)La(x)Sr(0.5)GaMn2O8: a neutron diffraction study

The effect of electron doping on the magnetic properties of the brownmillerite type bilayered compounds has been investigated by neutron powder diffraction in La substituted Ca(2.5-x)La(x)Sr(0.5)GaMn(2)O(8) compounds (x = 0.05 and 0.1), in comparison with the undoped compound (x = 0). In all compounds, a long-range three-dimensional collinear antiferromagnetic (AFM) structure is found below the Néel temperature T(N) of the respective compound, whereas, well above T(N), three-dimensional short-range magnetic ordering is observed. In the intermediate temperature range just above T(N), a strong effect of electron doping (La substitution) on the magnetic correlations has been observed. Here, a short-range AFM correlation with a possible dimensionality of three has been found for substituted compounds (x = 0.05 and 0.1) as compared to the reported two-dimensional long-range AFM ordering in the parent compound. With increasing electron doping, a decrease in T(N) is also observed. The short-range magnetic correlations set in over a large temperature range above T(N). A magnetic phase diagram in the x-T plane is proposed from these results.     

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.     

Order-disorder transition on Si(001)

A phase transition between c(4x2) and 2x1 structures on the Si(001) surface has been observed at 200 K by low-energy electron diffraction. This transition is a second order order-disorder transition of the asymmetric dimer configuration. The streak pattern remains up to well above the transition temperature. The temperature dependence of the width and the length of the streak can be described in terms of the effects of a strong anisotropic coupling between adjacent asymmetric dimers.     

First order magnetic transition in doped CeFe2 alloys: phase coexistence and metastability

First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in doped CeFe2 alloys is studied with the micro-Hall probe technique. Clear visual evidence of magnetic phase coexistence on micrometer scales and the evolution of this phase coexistence as a function of temperature, magnetic field, and time across the first order FM-AFM transition is presented. Such phase coexistence and metastability arise as a natural consequence of an intrinsic disorder-influenced first order transition. The generality of these phenomena involving other classes of materials is discussed.     

Mn位W掺杂对La0.3Ca0.7MnO3体系磁结构的影响

通过对La_0.3Ca_0.7Mn_1-xW_xO₃(x=0.00，0.04，0.08，0.12，0.15)多晶样品M-T曲线、M-H曲线及ESR谱的测量，研究了Mn位W掺杂对电荷有序体系La_0.3Ca_0.7MnO₃磁结构的影响.结果表明，当掺杂量为0.00≤x≤0.08时，体系存在电荷有序(CO)相，AFM/CO态共存于相变温度以下，电荷有序温度T_CO随着W掺杂量的增加而增加；x＝0.04时，样品在低温下为FM相与AFM/CO相共存，在CO相建立前、后均有FM从PM中分离出来；当x≥0.12时，CO态融化，在极低温度下存在顺磁-铁磁(PM-FM)相变. 关键词： 磁结构 电荷有序 融化 Mn位掺杂     

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.     

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.     

(75)As NMR study of antiferromagnetic fluctuations in Ba(Fe(1-x)Ru(x))(2)As(2)

The evolution of (75)As NMR parameters with composition and temperature was probed in the Ba(Fe(1-x)Ru(x))(2)As(2) system where Fe is replaced by isovalent Ru. While the Ru end member was found to be a conventional Fermi liquid, the composition (x = 0.5) corresponding to the highest T(c) (20 K) in this system shows an upturn in the (75)As [Formula: see text] below about 80 K, evidencing the presence of antiferromagnetic (AFM) fluctuations. These results are similar to those obtained in another system with isovalent substitution, BaFe(2)(As(1-x)P(x))(2) (Nakai et al 2010 Phys. Rev. Lett. 105 107003) and point to a possible role of AFM fluctuations in driving superconductivity.