M.M. Amado

Magnetic properties of MnZnTi and NiZn ferrite films deposited by laser ablation

The magnetic behavior of MnZnTi and NiZn ferrite films, deposited by laser ablation, is studied comparatively to that of the bulk material, using a vibrating-sample magnetometer (VSM), transmission, and conversion electron Mossbauer measurements. Both the VSM and Mossbauer analysis show that the films exhibit the single phase pattern of the target material. Mossbauer hyperfine field values for films are close to those found for the bulk, and this accounts for the same saturation magnetization values determined by VSM. Laser ablated films exhibit a higher coercive field, which is explained in terms of the crystallite size distribution, with average values of 300–600 nm, as estimated from the Mossbauer line broadening.

Critical behaviour of transport and thermal properties in GdAl2 intermetallic compound

Abstract High-accuracy measurements of the electrical resistivity (ϱ), thermo-electric power (S), thermal conductivity (K), and the temperature derivatives dϱ/dT and dS/dT have been carried out in a high purity GdAl2 sample over the temperature range 70–320 K. Our results reveal for the first time the existence of a truly divergent behaviour in transport properties for GdAl2, with very sharp peaks both in dϱ/dT and dS/dT at the Curie point Tc = 164.8 K; a well developed minimum was also observed in K(T) at the transition point. The results are analyzed both within and outside the critical region. Using a high resolution differential thermal-analysis method, a very sharp anomaly was detected at the Curie point, indicative of a well defined λ-anomaly in the specific heat of GdAl2.

Temperature dependence of the electrical resistivity and thermopower of U2Ni2In and Nd2Ni2Sn

Accurate measurements of the electrical resistivity (ρ,dρ/dT) and of the thermoelectric power (S,dS/dT) were performed in the ternary compounds U2Ni2In and Nd2Ni2Sn from 4 K to 250 K, and the results are compared with those previously obtained in U2Ni2Sn. The pair U2Ni2In/U2Ni2Sn enables us to study the influence of the p‐electron character (and other band effects related to the elements In/Sn) on the transport properties, whereas the pair Nd2Ni2Sn/U2Ni2Sn provides direct information on the role played by the 4f/5f electrons. All these compounds order initially in the antiferromagnetic state, exhibiting a characteristic minimum in dρ/dT at TN. In the U2Ni2In compound a drastic reduction is observed in ρ(T) slightly below TN, suggesting the coexistence of the antiferromagnetic state with a coherent Kondo effect when T≤0.8 TN. In Nd2Ni2Sn we observe two distinct phase transitions below TN, of first‐order character and likely associated with order‐order magnetic transitions. The anomalous behavior of ρ(T) in...

Magnetic properties of coarse-grained and nanocrystalline Fe-Cr-Sn alloys

Abstract Magnetic properties of coarse-grained and of nanocrystalline mechanically alloyed Fe–Cr–Sn alloys are investigated. The average magnetic moment, the coercive field and the Curie temperature decrease when the Sn content in both kinds of alloys is increased. Their respective values are found to be smaller in the case of mechanically alloyed samples. This fact is due to the difference between nominal and matrix compositions of coarse-grained alloys.

TRANSPORT AND MAGNETIC PROPERTIES OF GDAG

Abstract Measurements of the electrical resistivity (ρ), thermoelectric power ( S ), temperature derivatives d ρ /d T and d S /d T , thermal conductivity ( K ) and differential thermal analysis, have been made in GdAg single crystals over the temperature range 10–300 K. The electrical resistivity is analysed in terms of impurity, phonon and magnetic contributions. The behaviour of ρ and d ρ /d T near T N = 132 K indicates that the magnetic transition is not driven by an electronic (Fermi level) mechanism. Spin fluctuation effects are observed up to T ∼ 1.5 T N , in the transport, magnetic and thermal properties. However, they affect d ρ /d T and d S /d T differently; whereas d ρ /d T has a maximum at T N , followed by a minimum at T ∼ 1.15 T N , d S /d T has only a minimum at T N . The thermal conductivity exhibits a minimum at T N , resulting from the competition between a decreasing electron mean free path (as T increases) and a linearly increasing electronic specific heat. At low temperatures the transport properties indicate an anomalous behaviour, marked by a sharp decrease in d ρ /d T starting around 35 K, and anomalously large thermopower values going through a maximum around 20 K ( S ∼ 18.5 μ V K −1 ).

Electrical and thermoelectric transport properties of RZn and RCd compounds (R = Tb, Gd, Nd, Pr)

Abstract We present the results of an extended investigation of the transport properties of the CsCl structure intermetallic compounds GdZn, TbZn, GdCd, TbCd, PrCd and NdCd (all single crystals), using high resolution measurements of the electrical resistivity (ϱ, dϱ dT ) and the thermoelectric power (S, dS dT ) , from 4.2 up to 300 K. Both the resistivity and the thermoelectric power are separated in their magnetic and nonmagnetic contributions, and relevant information and parameters are obtained for the different scattering mechanisms. A detailed analysis of the data is presented in the vicinity of the different phase transitions present along the series: of magnetic ( T c , T N ), structural ( T ∗ , T ∗ L ) and spin reorientation types ( T sr ). A through comparison of the data with the available theoretical treatment to describe transport properties in magnetic metals is presented. For the case of the thermopower we show that the existing theories are inadequate to describe quantitatively, and in some cases even qualitatively, the main features associated with the magnetic transitions. Finally, new experimental evidence is given on the CsCl lattice instability in RCd intermetallic compounds.

Aspects on the critical behaviour of transport properties in ferromagnetic metals

Abstract New experimental results are presented to extend information into less well known aspects concerning the behaviour of transport properties in ferromagnetic systems. We study in particular the critical behaviour of the temperature derivative of the electrical resistivity (dϱ/d T ) in the vicinity of a spin-reorientation transition ( T s ). In addition, the extreme sensitivity of dϱ/d T is used to investigate the role of magnetic domains in the vicinity of a ferro-paramagnetic phase transition ( T c ).

Universal behaviour of thermal and electrical transport coefficients near the néel temperature of Cr99.94Al0.06

Abstract Measurements of electrical and thermal transport coefficients in the antiferromagnetic alloy Cr 99.94 Al 0.06 reveal an anomaly near the Neel temperature T N . Temperature derivatives of the electrical resistivity and thermo-power are analysed and are shown to be proportional to each other above and below T N in the reduced temperature range 6 × 10 −2 ⩾| t |⩾ 4 × 10 −4 . This is consistent with the theoretical prediction of a universal critical behaviour of all transport coefficients.

Conditions for metastable thermal and magnetic domain structures and for observable oscillations in dρ/dT near an electrothermal instability

Experimental conditions for observing a new type of oscillations in the temperature derivative of the electrical resistivity near magnetic transition temperatures are discussed. An electrothermal instability arises due to a nonlinear electrical current‐voltage drop characteristics as in a ballast resistor. Inhomogeneous temperature distributions are found for various experimental conditions.

Magnetic and Transport Studies of the α—σ Transformation in Fe54Cr46 and Fe52Cr46Sn2 Alloys: Kinetics of σ Phase Formation

The kinetics of the α-σ isothermal transformation in Fe 54 Cr 46 and Fe 52 Cr 46 Sn 2 (at%) alloys was studied by magnetization (M) and electrical resistivity (?) measurements at constant temperature, T = 973 K. The variations of the resistivity with temperature during heating or cooling are interpreted in terms of the magnetic and structural evolutions of the alloys. The time dependence of the isothermal increase of? and of the decrease of M follows Johnson-Mehl-type equations with similar kinetic coefficients. Magnetization and resistivity studies, and also in-situ transmission electron microscopy investigations, confirm that the presence of Sn retards the α-σ transformation by preventing the σ phase nucleation at grain boundaries.