Magnetic to nonmagnetic transition of cerium impurities in (La,Th) alloys

The initial depression of the superconducting temperature of ($\text{La, Th}$) Ce alloys has been measured. It has been found that there is a steep maximum in the initial depression as a function of La concentration in the (La, Th) matrix, This effect is compared with the nonmonotonic behaviour of the critical temperature of $\text{La}$Ce under pressure.     

The magnetic susceptibility of AgPd alloys with Mn and Fe impurities

Measurements of the magnetic susceptibility of AgPd alloys with Fe and Mn impurities have been performed at temperatures from 1.4 K to 300 K. Alloys with 5 and 10 at.% Pd, 2 and 6 at. ppm Fe and with Mn concentrations in the range 10 to 7,000 at. ppm were investigated. The temperature variation of the impurity susceptibility was analysed according to a Curie-Weiss law. The effective moment for Fe varies with the Pd concentration, which is interpreted as due to interactions between near neighbour Fe and Pd atoms. These interactions tend to lower the characteristic temperature of single Fe impurities and induce a spin on the Pd atom ferromagnetically coupled to the Fe spin. InAgPdMn one finds no such induced moment on the Pd atoms. The effective momentµ _eff =(5.36±0.10)µ _B per Mn atom and the Curie-Weiss temperature=(0.08±0.09) K are independent of Pd as well as Mn concentrations.µ _eff is slightly higher than for Mn in very diluteAgMn, which may be due to a different polarisation of the conduction electron gas around the impurities.     

Possibility of a magnetic transition in Ce alloys

The fact that divalent rare earth impurities have a strong effect in lowering the α-γ transition temperature in Ce, allows the γ phase to become stable at low temperature without much change in its physical properties. The assumption that in this way an ordering (Néel) temperature can be reached, plus a thermodynamical analysis of the electronic transformation that takes place at the α-γ transition, leads to the prediction of a phase diagram for these alloys similar to that found in Cr doped V₂O₃.     

Noncollinear magnetic structures in binary alloys of transition elements

Concepts [1–4] are developed on these structures for A–B alloys having antiferromagnetic interaction between the individual local magnetic moments . The following cases are considered: 1) the exchange integrals I_AA > 0, I_BB < 0, I_AB > 0, while _A and _B are dependent on the composition; 2) I_AA > 0, I_BB < 0, I_AB < 0; _A, _B = const. Models are used to calculate the mean atomic magnetic moment for Co-Cr, Co-Mn, and Ni-Cr alloys. The agreement with experiment is satisfactory.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, Vol.12, No. 1, pp. 96–102, January, 1969.     

Theory of volume anomalies in magnetic transition metal alloys

The electronic contribution to the pressures which determines the equilibrium volume of transition metals is parametrized by use of the interacting Anderson model. It is discussed that there are two mechanisms which can explain the volume anomalies in the Invar alloys. The Fe-Fe atomic interaction which seems to be important in the Invar effect is also discussed.     

Investigation of a magnetic phase transition in fcc iron-nickel alloys

A magnetic phase transition in carbon-doped (0.1 and 0.7 at. %) Fe₇₀Ni₃₀ Invar alloys was investigated by the method of depolarization of a transmitted neutron beam and by small-angle scattering of polarized neutrons. It is shown that for both alloys, two characteristic length scales of magnetic correlations coexist above T _c. Small-angle scattering by critical correlations with radius R _c is described well by the Ornstein-Zernike (OZ) expression. The longer-scale (second) correlations, whose size can be estimated from depolarization data, are not described by the OZ expression, and hypothetically can be modeled by a squared OZ expression, which in coordinate space corresponds to the relation 〈M(r)M(0)〉∝exp(−r/R _d), where R _d is the correlation length of the second scale. The temperature dependence of the correlation radius R _c was obtained: R _c ∝ ((T−T _c)/Tc)^−ν , where ν≈2/3 is the critical exponent for ferromagnets, over a wide temperature range up to T _c ^exp , at which the correlation radius becomes constant and equals its maximum value R _c(T _c)=R _c ^max . The maximum correlation radius established (R _c ^max =140 Å and 230 Å for the first and second alloys, respectively) characterizes the length-scale of the fluctuation for which the appearance of critical correlations first results in the formation of a ferromagnetic phase, and the phenomenon itself exhibits a “disruption” of the second-order phase transition at T=T _c ^exp , as a result of which a first-order transition arises. Temperature hysteresis was also detected in the measured polarization of the transmitted beam and intensity of small-angle neutron scattering in the alloy above T _c, confirming the character of this magnetic transition as a first-order transition close to a second-order transition. Zh. éksp. Teor. Fiz. 112, 2134–2155 (December 1997)     

Formation andstability of magnetic moment on isolated Rh impurities in dilute PtFe alloys

The local susceptibilities (T) of isolated Rh impurities in paramagnetic dilute Pt(Fe) alloys have been measured using the TDPAC method. The results show existence of 4d magnetic moment for Rh with reduced spin fluctuation temperatures (T _sf). It is suggested that moment stability scaled byT _sf is governed mainly by the interatomic d-d exchange interaction caused by induced ferromagnetic polarization of host d band electrons.     

The effect of impurities on Peierls transition

The impurity concentration dependence of the transition temperature into the Peierls phase is discussed in the low concentration region (i.e. in the linear region), assuming a δ-function type potential. The effect of fluctuation is taken account of by the method of Scalapino, Sears and Ferrell. The decrease of the transition temperature per one per cent of impurities is estimated in the case of TTF—TCNQ but found to be one order of magnitude smaller than that observed recently in an experiment.     

Ab-initio calculations of the electronic structure of impurities and alloys of ferromagnetic transition metals

We review recent theoretical work on the electronic structure and the magnetic properties of ferromagnetic transition-metal alloys. All calculations are based on density-functional theory in the local-spin-density approximation. We report about calculations for dilute alloys using the KKR-Green's function method and for concentrated disordered alloys using the charge-self-consistent KKR-CPA method.     

The alpha-gamma transition of cerium is entropy driven

We emphasize, on the basis of experimental data and theoretical calculations, that the entropic stabilization of the gamma phase is the main driving force of the alpha-gamma transition of cerium in a wide temperature range below the critical point. Using a formulation of the total energy as a functional of the local density and of the f-orbital local Green's functions, we perform dynamical mean-field theory calculations within a new implementation based on the multiple linear muffin tin orbital (LMTO) method, which allows us to include semicore states. Our results are consistent with the experimental energy differences and with the qualitative picture of an entropy-driven transition, while also confirming the appearance of a stabilization energy of the alpha phase as the quasiparticle Kondo resonance develops.     

Reversible structural phase transition in Ni-Mn-Ga alloys in a magnetic field

In Ni_2+x Mn_1?x Ga shape-memory ferromagnetic alloys with coincident magnetic and structural phase transitions, a reversible structural field-induced phase transition was observed at constant temperature and pressure in magnetic fields of about 10 T. Computational results are in qualitative agreement with experiment.     

Electronic structure of magnetic and non-magnetic substitutional 3d transition metal impurities in germanium

The electronic structure around the ideal 3d transition metal impurities at substitutional site of germanium is calculated self-consistently within the local density formalism. Chemical trend of 3d impuritiy levels caused by the strong p-d hybridization, is revealed in the case of non-magnetic state. The magnetic state of the iron impurity is also presented and the local magnetic moment experienced by iron impurity is calculated to be 3.72μ_B.     

Solubility of substitutional impurities in decomposing alloys

Solubility of substitutional impurities in decomposing alloys with a cubic structure is calculated. The effect of elastic lattice vibrations on the decomposition and solubility is taken into account. It is shown that the temperature that corresponds to the peak in the decomposition curve is increased and the range of the biphase states is widened if the phonon effects are taken into consideration. The dependence of solubility on the decomposition parameter is determined; several particular cases of this dependence are analyzed.     

The influence of magnetic impurities in the vortex core dynamics in magnetic nano-disks

In this work we have used spin dynamics simulations to study the gyrotropic frequency behavior in nano-disks of Permalloy with magnetic impurities. We consider the effect of attractive impurity and repulsive impurity placed near the vortex core gyrotropic trajectory. We observed that the gyrotropic frequency is affected by the presence of impurity. The gyrotropic frequency shift depends on the relative position between the impurity and the vortex core gyrotropic trajectory and if impurity is attractive or repulsive. Our results agree with the analytical model and with experimental behavior for the gyrotropic frequency shown in the literature.     

Determination of temperature of phase transition in magnetic alloys by branching theory methods

In this article a transcendental equation is obtained for the temperature T_c of the phase transition of a binary magnetic alloy by means of the cluster-variation method. The dependence of T_c on the exchange and crystallographic parameters, concentration, and coordination number is investigated. Particular cases are analyzed of a thinned and concentrated magnetic alloy. In the present article the cluster-variation method is employed to evaluate the dependence of concentration on the critical temperature of a magnetic binary alloy either with fixed or mobile atoms. A complete system of equations for the critical temperature can be obtained by using the branching theory [1]. When solving the equations, one finds that the equation which is written in terms of a determinant of a high order is the most difficult one. If one takes into account that the parameter of a near order depends on temperature (mobile atoms), then this increases the order of the determinant. The uses of symmetry enables one to lower considerably the order of the determinant in both cases,which makes it finally possible to solve the problem under consideration. The method yields qualitatively different results depending on whether the approximation is of fixed or mobile atoms (as previously noted, e.g., in [2]).Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 107–112, May, 1978.