Antiferromagnetism in 4f-systems with valence instabilities

An extendeds–f model is used for the study of intermediate-valence 4f-systems, which have antiferromagnetic ground states in the normal-valence phase and fluctuate between a magnetic and a nonmagnetic configuration in the intermediate-valence phase. The local magnetic 4f-moment, the sublattice-magnetization as well as the Néel-temperatureT _N are discussed in terms of thes–f hybridizationV and thef-level positionE _f . Reduction of the energy gap betweenE _f and conduction band simulates the influence of external pressure or of alloying with suitable impurities. IncreasingV or decreasing4f-(5d, 6s)-gap intensifies the electronic fluctuations, which have two competitive consequences. They tend to quench the local moments, but simultaneously enhance the coupling between them. This results in a typical behaviour of the Néel-temperature. Conditions for the coexistence of antiferro-magnetism and intermediate valence are derived. Comparison with experimental data for systems like Eu-metal and Eu(Pd_1–x Au _x )₂Si₂ shows qualitative agreement.     

“Influence of valence instabilities on the Curie-temperature of ferromagnetic 4f-systems”

We inspect the stability of ferromagnetism in 4f-systems, which have an insulating, ferromagnetic ground state in the integral valent phase and fluctuate between a magnetic and a nonmagnetic configuration in the intermediate valent phase. It turns out that for large gaps between 4f-level and lower edge of the empty conduction band ferromagnetism is created by a certain interplay between the s-f hybridization, which allows real or virtual electron transitions between f-level and conduction band, and the s-f exchange interaction, which mediates an indirect coupling between the localized 4f-moments. When the gap is reduced by external pressure or by alloying with proper impurities the local f-moment becomes steadily quenched while simultaneously the coupling between the more and more reduced moments is drastically enhanced. These two competitive effects lead to a distinct maximum of the Curie-temperature just in the intermediate valence phase.     

Free carrier absorption in a ferromagnetic semiconductor below Curie temperature

The free carrier absorption (FCA) in a non-degenerate ferromagnetic semiconductor due to electron-two-magnon scattering is calculated below the Curie temperature, assuming a wide conduction band model. It has been found that in EuO FCA due to two-magnon scattering is proportional to λ^2·1 (wavelength of radiation) and it has been noticed that for high conductivity samples, explanation of FCA requires the inclusion of ionized impurity scattering of electrons besides the two-magnon process.     

The Curie temperature of dilute ferromagnetic alloys near the percolation threshold

Upper limit for the Curie temperature T_c(x) of the dilute Heisenberg ferromagnet near percolation threshold is derived. Qualitative arguments are put forward that this limit gives a correct dependence of T_c on the concentration of magnetic atoms x.     

Enhancement of the Curie temperature of ferromagnetic semiconductor(Ga,Mn)As

In this review article,we review the progress made in the past several years mainly regarding the efforts devoted to increasing the Curie temperature(T C) of(Ga,Mn)As,which is most widely considered as the prototype ferromagnetic semiconductor.Heavy Mn doping,nanostructure engineering and post-growth annealing which increase T C are described in detail.     

Thickness dependence of the Curie temperature and critical concentration in diluted ferromagnetic thin films

A thickness-dependent critical concentration of an Ising ferromagnetic thin film is predicted. The calculated values of the Curie temperature are bounded by those known for diluted two- and three-dimensional ferromagnets.     

Field dependence of the thermal conductivity of K2CuCl4 · 2H2O near its ferromagnetic Curie temperature

The thermal conductivity of the ferromagnetic insulator K₂CuCl₄ · 2H₂O has been measured near its Curie temperature T_c. The measurements were made as a function of temperature in constant external magnetic field and as a function of field along isotherms. The results indicate a relaxation rate for magnetic critical scattering of phonons varies as H^?1/2.     

Conditions for noncollinear instabilities of ferromagnetic materials

Two criteria have been identified here which determine whether a magnetic metal orders in a collinear (e.g., ferromagnet) or noncollinear (e.g., spin-spiral) arrangement. These criteria involve the ratio between the strength of the exchange interaction and the width of the electron bands, as well as Fermi-surface nesting between spin-up and spin-down sheets of the Fermi surface. Based on our analysis we predict that even typical ferromagnetic materials (e.g., Fe, Co, and Ni) should be possible to stabilize in a noncollinear magnetic order in, e.g., high pressure experiments.     

Observation of a new magnetic anomaly below the ferromagnetic Curie temperature in Yb14MnSb11

Yb14MnSb11 is an unusual ferromagnet with a Curie temperature of 52 +/- 1 K. Recent optical, Hall, magnetic, and thermodynamic measurements indicate that Yb14MnSb11 may be a rare example of an underscreened Kondo lattice. We report the first experimental observation of a new magnetic anomaly in this system at around 47 K, a few degrees below T(c). Systematic investigations of the ac and dc susceptibilities of Yb14MnSb11 single crystals reveal features associated with possible spin reorientation at this temperature. This new anomaly is extremely sensitive to the applied measurement field and is absent in temperature-dependent dc magnetization data for fields above 50 Oe. The origin of this could be due to decoupling of two distinct magnetic sublattices associated with MnSb4 tetrahedra.     

Phonon induced instabilities in intermediate valence compounds

The coupling of 4f electrons and longitudinal optical phonons and its implications on different types of structural phase transitions in intermediate valence compounds is discussed within the framework of the periodic Anderson model. Two types of interactions are considered. First, the usual density type of coupling of 4f electrons and phonons leads to a weakly temperature dependent renormalization of the positionE ₀ of the 4f level with respect to the 5d band. Secondly, phonon induced 4f–5d interband transitions lead to a renormalization of the hybridization energyV of 4f and conduction electrons. For appropriate parameter values the latter effect gives rise to discontinuous changes of the occupation of the 4f state.The tendency towards a ferromagnetic or antiferromagnetic instability is essentially suppressed by the presence of phonons.The dependence of the susceptibility on temperature and onE ₀ is calculated and a jump is found whenever a discontinuous change of the occupation of the 4f state occurs.Work performed within the research program of the Sonderforschungsbereich 125, Aachen-Jülich-Köln     

Defect-induced valence instabilities in rare-earth semiconducting compounds

We investigate a possible mechanism for local valence instabilities in a NaCl-type rare earth compound (semiconducting SmS) containing p-substitutional anionic impurities (A=O, Se, Te, P…) or sulphur vacancies (A=υ). Using group theory analysis we discuss the nature of the defect levels from considering the whole cluster Sm₆A formed by a point defect A and the six neighbouring Sm atoms. Phase diagrams for valence transitions are obtained in terms of the defect potential matrix elements from the competition between the promotion energy of an f electron into the conduction bands and the available band structure energy. Especially, possible solutions are presented for A=O, Se, Te.     

Conduction electrons in ferromagnetic semiconductors near the Curie point

Near the Curie point the conduction electron energy in a nondegenerate ferromagnetic semiconductor is determined by the short range magnetic ordering. This prohibits formation of the magnetic polaron and leads to an anomalous temperature shift of the conduction band bottom in the paramagnetic region.     

Single-phase epitaxial InFeSb layers with a Curie temperature above room temperature

Physics of the Solid State - Epitaxial InFeSb/GaAs heterostructures were obtained by laser deposition in vacuum. Investigations by high-resolution transmission electron microscopy and...     

Effect of pressure on the Curie temperature of CoCr2O4

The pressure effect on the Curie temperature T_c of CoCr₂O₄ was measured under pressures up to 11.5 kbar. The pressure derivative of T_c is found to be +0.25 K/kbar. On the basis of this result, the relationship between the magnetic transition temperature and volume is discussed.     

利用RL交流电桥测量磁性材料的居里温度

利用RL交流电桥测量了磁性材料的居里温度,并讨论了工作频率和工作电压的变化对测量结果的影响.     

The Curie temperature of ZrZn2

The curie of temperature of ZrZn₂ is determined from the stoner theory of itinerant electron magnetism by renormalising the first coefficient of the Landau expansion thus taking spin fluctuations into account. These fluctuations are found to be saturated well below T_c so that the behaviour of ZrZn₂ around T_c is of stoner type. The calculated and measured values of T_c are in good agreement.     

Curie temperature in the Hubbard model with alloy disorder

Magnetic and electric properties of the Hubbard model with binary alloy disorder are studied within the dynamical mean-field theory. A paramagnet-ferromagnet phase transition and a Mott-Hubbard metal-insulator transition are observed upon varying the alloy concentration. A disorder induced enhancement of the Curie temperature is demonstrated and explained by the effects of band splitting and subband filling.