On the magnetic properties of transition metal substituted MnAs

The properties of Mn_1?tT_t As phases (T: V, Cr, Fe or Co; 0<t$\text{\$}\text{?}$= 0.10) are studied in external magnetic fields up to 250 kOe, and temperatures between 80 and 400 K. The results are presented and discussed in terms of phase diagrams comprising concentration, temperature, and magnetic field axes.     

Lattice distortion effects on the magnetostructural phase transition of MnAs

We present a systematic experimental and theoretical study of the first-order phase transition of epitaxially grown MnAs thin films under biaxial tensile stress. Our results give direct information on the dependence of the phase-transition temperature of MnAs films on the lattice parameters. We demonstrate that an increase of the lattice constant in the hexagonal plane raises the phase-transition temperature (T(p)), while an increase of the perpendicular lattice constant lowers T(p). The results of calculations based on density functional theory are in good agreement with the experimental ones. Our findings open exciting prospects for magneto-mechanical devices, where the critical temperature for ferromagnetism can be engineered by external stress.     

Change in the polarizability of MBBA molecules upon the nematic-isotropic liquid transition and physical consequences

The Lorentz tensor components L _j and the polarizability tensor components γ _j of 4-methoxyben-zylidene-4′-butylaniline (MBBA) liquid crystal molecules in nematic and isotropic phases have been determined experimentally. The quadratic dependences of the mean value $\bar \gamma $ (S) and anisotropy Δγ(S) of the polarizability in the nematic phase on the orientational order parameter S of molecules have been established. Within the phenomenological approach, the dependences $\bar \gamma $ (S) and Δγ(S) have been derived, and their features for MBBA and other known objects have been interpreted. The relation of the dependences $\bar \gamma $ (S) and Δγ(S) to the features of the nematic-isotropic liquid phase transition has been found.     

斯格明子相关的螺旋磁有序体系的临界行为

介绍了与斯格明子相关的螺旋磁有序体系的临界行为.首先阐述了连续相变中的临界现象、临界指数、标度律、普适性等概念;随后介绍了磁相变体系中几种临界指数的获得方法,包括直流磁性迭代法、磁熵变法;进而,分析了几类与斯格明子相关的螺旋磁有序体系的临界行为.MnSi是典型的斯格明子材料,临界指数显示其磁性行为符合三重临界行为.MnSi的临界行为揭示:外磁场可以抑制这一体系在零场下的一级相变,使其转变为二级相变,从而在螺旋磁有序、锥形磁有序、顺磁相的三相交汇点形成三重临界点.斯格明子体系FeGe和Cu_2OSeO_3的临界行为符合三维海森伯相互作用,表明它们的磁性行为主要是由近邻的各向同性的自旋耦合作用所决定;而Fe_(1-x)Co_xSi和新发现的斯格明子体系Fe_(1.5-x)Co_xRh_(0.5)MoN的临界行为显示Co掺杂可以有效地调制其中的磁性耦合.对螺旋磁有序体系的临界行为研究表明,尽管这些体系都表现出类似的斯格明子态,但是它们的磁性耦合机制却大不相同,并且其耦合机制可以受到外界手段的调制.最后,根据普适性原理和标度方程,阐述了一种构建磁场诱导相变体系在临界温度附近H-T相图的方法.     

Resonance features of the dynamic magnetic susceptibility in hexaferrite BaFe12O19 upon the transition from the magnetically ordered state to the paramagnetic state

This paper presents the results of the experimental study on the dynamic magnetic susceptibility of barium hexaferrite crystals in the temperature range of the transition from the magnetically ordered state to the paramagnetic state. The observed features of the magnetic susceptibility have been interpreted as those characteristic of the ultralow-frequency natural ferromagnetic resonance of the spin subsystem of the Bloch domain walls. The results agree well with the concepts of features of the critical spin dynamics near the Curie temperature.     

Spin-dependent localization effects in GaAs:Mn/MnAs granular paramagnetic–ferromagnetic hybrids at low temperatures

We compare the magneto-transport in paramagnetic–ferromagnetic GaAs:Mn/MnAs granular hybrids and paramagnetic GaAs:Mn reference samples. The differences in the hole transport between the two systems at low temperatures arise due to carrier localization effects at the cluster–matrix interface in the hybrids. The localization is caused by a Schottky barrier formation at the interface as well as spin-dependent shifts of the hole bands caused by the stray field of the ferromagnetic clusters. The application of an external magnetic field leads to a delocalization of the carriers and thus a negative magneto-resistance effect. These effects can be simulated using a network model approach.     

On the theory of spin fluctuations in paramagnetic transition metals

We use KKR-bandstructure data and apply recently developed Brillouin-zone integration techniques to evaluate the wavevector dependent spin susceptibilities of Pd and V within the RPA and the local spin density functional approach. Additionally we calculate the electron-spin fluctuation coupling function and show-in agreement with our previous work-that this kind of theory yields reasonable values for the electron mass enhancement.     

Superconducting transition temperature of a paramagnetic material close to magnetic ordering

An explicit expression for the superconducting transition temperatureT _c in a paramagnetic material is derived, when the transition occurs just before a possible magnetic ordering. As first noted by Uspenskii, such a transition may arise from electronic mechanism itself, without the necessary role played by the usual phonon-exchange mechanism. The result is discussed in terms of some recent experimental observations on the binary alloy Y₉Co₇.     

Real-space observation of individual skyrmions in helimagnetic nanostripes

Controllable formation and manipulation of domain walls in one-dimensional(1D) nanostripes underpins a promising type of emergent spintronic device. Magnetic skyrmion is topologically stable whirlpool-like spin texture and is expected to replace familiar domain wall phenomena to build such devices, owing to its prominent features including small size,topological stability and the small critical current required to move it. It is thus essential to understand skyrmions' properties in such a nanostructured element. In this paper, we mainly give fundamental insight into this issue. Experimental achievements in the formation and stability of individual skyrmions in the nanostripe are outlined in detail.     

Effect of interfacial defects on the electronic and magnetic properties of epitaxial CrAs/InAs and CrAs/CdSe half-metallic multilayers

We present an extended study of single impurity atoms at the interface between the half-metallic ferromagnetic zinc-blende CrAs compound and the zinc-blende binary InAs and CdSe semiconductors in the form of very thin multilayers. Contrary to the case of impurities in the perfect bulk CrAs studied in Galanakis and Pouliasis [J. Magn. Magn. Mater. 321 (2009) 1084] defects at the interfaces do not alter in general the half-metallic character of the perfect systems. The only exception are Void impurities at Cr or In(Cd) sites which lead, due to the lower-dimensionality of the interfaces with respect to the bulk CrAs, to a shift of the p bands of the nearest neighboring As(Se) atom to higher energies and thus to the loss of the half-metallicity. But Void impurities are Schottky-type and should exhibit high formation energies and thus we expect the interfaces in the case of thin multilayers to exhibit a robust half-metallic character.     

The ferromagnetic to paramagnetic phase transition of Fe studied by x-ray photoelectron spectroscopy

Valence band x-ray photoelectron spectra from Fe(100) have been measured as a function of temperature to above the Curie temperature, T_c. The room temperature data can be reconciled with the theoretical one-particle density of states (DOS). At T = 1.034T_c, the data do not resemble the paramgnetic DOS of Fe as calculated in the disordered-local-moment limit.     

First order character of the B transition in liquid 3He

Measurements have been made of the discontinuous changes in entropy and static magnetization of liquid ³He at the B transition. These changes are related to previous measurement of the magnetic field dependence of this phase transition.     

Change in the chemical potential of niobium at transition to the superconducting state

A temperature-modulation technique was applied to investigate change in the chemical potential of niobium film at transition to the superconducting state. The first derivative of the chemical potential is continuous at T _c whereas the second derivative shows a jump. Received 10 April 2000     

Phase transition studies of paramagnetic crystals at low temperatures

Magnetic, thermal, electrical and optical properties of a series of paramagnetic compounds of general formula ABF₆, 6H₂/6D₂O and A(ClO₄)₂, 6H₂O where A=Co, Na, Zn, Hg and B=Si, Ti, Zr, showing structural transition from room temperature hexagonal with one molecule in the unit cell to low temperature monoclinic with two molecules in the unit cell, are reviewed.     

Strain engineering of the magnetocaloric effect in MnAs epilayers

By using heteroepitaxy on two different GaAs templates, we have investigated the impact of anisotropic strain on the magnetocaloric effect (MCE) of MnAs. The temperature range, spread around room temperature, and the maximal MCE position are markedly different in the two epitaxial systems. Simulated MCE curves, obtained from a model based on the mean-field approximation, are in good agreement with the experimental data, indicating that the entropy variation is magnetic in origin. These results illustrate how strain can be used to tune the MCE in materials with coupled structural and magnetic phase transition and suggest that the MCE of MnAs may find applications in microelectronic circuitry.     

Orbital switching and the first-order insulator-metal transition in paramagnetic V2O3

The first-order metal-insulator transition (MIT) in paramagnetic V2O3 is studied within the ab initio scheme LDA+DMFT, which merges the local density approximation (LDA) with dynamical mean field theory (DMFT). With a fixed value of the Coulomb U=6.0 eV, we show how the abrupt pressure driven MIT is understood in a new picture: a pressure-induced decrease of the trigonal distortion within the strong correlation scenario (which is not obtained within LDA). We find good quantitative agreement with (i) switch of the orbital occupation of (a(1g),e(pi)(g1),e(pi)(g2)) and the spin state S=1 across the MIT, (ii) thermodynamics and dc resistivity, and (iii) the one-electron spectral function, within this new scenario.     

Spin treatment-based approach for electronic transport in paramagnetic liquid transition metals

A novel concept is proposed to calculate both the electrical resistivity and thermoelectric power (TEP) of liquid transition metals (Mn, Fe, Co and Ni) characterized by a paramagnetic state in the liquid phase. By contrast to a previous work (PRB64, 094202 (2001)), where the resistivity was calculated by treating separately the interactions between spin up and spin down using the Matthiessen rule, our current approach is based on two types of muffin tin potentials in the t-matrix, namely spin up and spin down. The resistivity is treated as the result of the interference of the two kinds of spin states of electrons including a cross-contribution. The calculated resistivity values agree reasonably well with the available experimental ones for all the metals considered. Moreover, the calculated TEP, as deduced from the slope of resistivity vs. energy, has been found to be positive for Mn and Fe but negative for Co and Ni. Besides that, this formalism for resistivity calculation may be generalized to a system that may exist in different atomic states. It is worth mentioning that this concept is analogous to the one used in the process of neutron scattering on a metal composed of multiple isotopes.