Magnetovolume effects in transition metal alloys near the critical composition for ferromagnetism

Results of magnetovolume studies are presented on intermetallic compounds and binary alloys with weakly ferromagnetic properties of the following compositions: Ni₃Al, ( )₃Ga, Ti(Fe, Co), (Ti, Al)Co, Ni-Pt and Pd-Ni. Data for the pressure dependence of the spontaneous magnetization at 4.2 K and the shift of the Curie temperature with pressure follow roughly the realtions that can be derived in the model for weak itinerant ferromagnetism in most of the investigated systems. Invar-type of anomalies in the thermal expansion have already been observed in several cases. Experimental values for the magnetovolume parameter are derived and compared with theoretical predictions in the Stoner model.     

Magnetic properties of ferromagnetic VFe alloys near the critical concentration for ferromagnetism

Magnetic measurements on a series of FeV alloys in the composition range 29–35 at .% Fe have been interpreted in terms of a local environment model. Values for the moment and density of magnetic clusters have been determined. Good agreement is obtained between values obtained from Curie constants, and those estimated from the critical coefficients. The results may be understood in terms of a random distribution of solute atoms.     

Diffuse scattering of neutrons near the critical composition in AuCr alloys

Magnetic diffuse scattering of neutrons was observed in an AuCr alloy with the critical composition of the formation of the long range magnetic order. The line shape has an insotropy with respect to the propagation direction of the antiferromagnetic order.     

Specific heat experiments and critical concentration for ferromagnetism in nickel-platinum alloys

We have studied the electronic specific heat of various nickel-platinum alloys near the critical concentration for the appearance of ferromagnetism. The electronic specific heat coefficient has a maximum for an atomic concentration of 42.5 ± 0.5% nickel. This maximum coincides with that of the high field susceptibility. The results obtained are discussed qualitatively.     

The specific heat anomaly of alloys with a critical concentration for ferromagnetism

The concentration dependence of the low temperature electronic heat coefficient of binary nickel alloys having a critical concentration is calculated on the basis of Miedema's scheme and the effect of band splitting for the ferromagnetic alloys. The results are compared especially with measurements on Ni-Pt.     

Local atomic ordering in nickel based Ir and Rh alloys

Experimental measurements of the diffuse X-ray scattering are performed on alloys of Ni with Rh and Ir. The atomic short range order (SRO) parameters α_i are calculated from the measured intensity. The existence of SRO is established in the two systems. The values of α₁ are observed to have anomalously large negative values for all the samples.The experimental data so obtained is interpreted theoretically by calculating the interaction energies on the basis of electronic theory of ordering. Theoretically calculated values of interaction energies are found to be in agreement with the experimentally determined type of order in these alloys.     

Local critical properties near an isolated impurity

The effect of an isolated impurity on local static critical properties is examined by extrapolating exact high-temperature series. In accord with results of alternative methods no change in the critical exponents is found. The amplitudes of the singularities change in general, but a special choice of the nature of the impurity may even leave the amplitudes unchanged. This provides a convenient situation for numerical tests.     

Thermal conductivity of superconducting lead-indium alloys near the upper critical field

The thermal conductivity of a series of lead/indium alloys was measured in the normal, superconducting and mixed states. The electronic part of the thermal conductivity near the upper critical field was compared with the theory of Caroli and Cyrot. There is good agreement in the dirty limit but large deviations are observed as the indium concentration is reduced.     

Resistivity variation near the critical temperature in some Ni-Fe base amorphous alloys

Accurate measurements of the electrical resistivity of three Ni_80?xFe_xP₁₄B₆ alloys (with x=10, 20 and 30) were performed in the temperature interval 1.5–500 K. Resistivity (ρ) and dρ/dT behaviours near T_c for the alloys with different x values are mutually very different. In particular $\text{dρ}\text{dT}$ variation near T_c for the alloys with x=10 and 20 is Ornstein-Zernicke like while for the alloy with x=30 is of more usual energy like type. A tentative explanation of this behaviour in terms of the influence of the percolation limit on the nature of the magnetic transition is offered.     

On the ferromagnetism of liquid gold-cobalt alloys

The magnetizationM of the eutectic AuCo alloy was measured in various fields above the melting pointT _s. There is strong evidence for spontaneous magnetization in the temperature range some 17 degrees aboveT _s.T _s was examined by thermal analysis during the measurement ofM.     

Canted ferromagnetism in PdMn and PdMnFe alloys

From nuclear orientation measurements on Pd 3% Mn and Pd 5% Mn 0.35% Fe we find that at low temperatures the alloys behave as canted ferromagnets. We can estimate the probability distribution for the individual spin orientations and can observe the reduction in canting in strong applied fields. In the PdMnFe sample the Fe spins are much more highly aligned than the Mn spins.     

Atomic ordering and ferromagnetism in binary alloys

Atomic and magnetic ordering in binary alloys are studied within the mean field approximation. The influence of magnetism on the spatial order-disorder critical temperature and, conversely, the influence of chemical order on the Curie temperature are analyzed in detail. Phase diagrams for various cases are presented. In particular we find that the interplay of the two order parameters yields in some cases unusual sequences of phases as a function of temperature. For example we obtain a possible sequence with increasing temperature of spatially ordered ferromagnet → spatially ordered paramagnet → spatially disordered ferromagnet → disordered paramagnet.     

Interface double-exchange ferromagnetism in the Mn-Zn-O system: new class of biphase magnetism

In this Letter, we experimentally show that the room temperature ferromagnetism in the Mn-Zn-O system recently observed is associated with the coexistence of Mn(3+) and Mn(4+) via a double-exchange mechanism. The presence of the ZnO around MnO(2) modifies the kinetics of MnO(2)-->Mn(2)O(3) reduction and favors the coexistence of both Mn oxidation states. The ferromagnetic phase is associated with the interface formed at the Zn diffusion front into Mn oxide, corroborated by preparing thin film multilayers that exhibit saturation magnetization 2 orders of magnitude higher than bulk samples.     

Transition from ferromagnetism to paramagnetism in Ni-Pt alloys

Magnetization measurements on Ni-Pt alloys in the concentration range 50–75 at.% Pt show that a transition from ferromagnetism to paramagnetism occurs near 59 at.% Pt. The results show that these alloys may be tentatively described as weak itinerant homogeneous ferromagnets.     

Local and itinerant magnetism and superconductivity in R Rh2si2 (R = rare earth)

Magnetization and Mossbauer studies reveal that R Rh₂Si₂ (R = rare earth) have two magnetic phase transitions, one corresponding to the ordering of the rare earth (T^N = 27?130K) and the other to the itinerant electron ordering of the Rh sublattice (T^M= 5?17K). LaRh₂si₂ has also been studied by resistivity, specific heat and a.c. susceptibility measurements. All studies indicate that LaRh₂Si₂ orders magnetically at T^M= 7K and becomes superconducting, type II, at T^c= 3.8±0.2K.     

Structural disorder in Fe−Si alloys near Fe3Si composition

Ordered and slightly disordered Fe_1?xSi_x alloys near X=0.25 are studied by means of Mössbauer spectroscopy. Quantitative analysis of the spectra after correction for the thickness effect is presented. The short range order as well as the long range order parameters are calculated from Mössbauer spectra. Distribution of atoms among different sites is determined. A slight disorder for x<0.252 is mainly due to an interchange of Fe atoms from A sites with Si atoms residing at D sites. However, at larger x the B disorder prevails.