The low temperature specific heat of Gd-Cu-Y metallic glasses

The specific heat of a series of amorphous aloys of composition Gd_xCu_0.37Y_0.63−x with 0 x 0.63 has been measured in the temperature range 2 to 50 K. A magnetic contribution to the specific heat is apparent in all the samples and has been determined with fair accuracy by subtracting the specific heat of corresponding nonmagnetic amorphous samples prepared with Lu rather than Gd. A qualitative study of the low field ac susceptibility of the magnetic samples has also been made. Samples with less than 35 at% Gd appear to be spin glasses, the results being qualitatively in accord with the predictions of the cluster mean field theory of concentrated spin glasses. As the Gd concentration is increased above 35 at% the samples behave increasingly like ferromagnets. However additional evidence from magnetisation measurements shows that even the sample with the highest concentration of Gd is not a simple ferromagnet.     

Influence of hydrogen on the low temperature heat capacity of lutetium-rich lutetium-hydrogen alloys

The low temperature (1–20K) heat capacity results of lutetium containing up to 15.5 at.%H in solid solution were measured. In alloys containing between 0.1 and <3.0 at.%H a low temperature anomaly was observed in the C/T vs. T² plots below 2.5K, but alloys containing 3.0 or more at.% H were linear down to the lowest temperature measured. The electronic specific heat constant, γ, initially increased by ～35% when 1.5 at.% H was added, and then γ decreased at higher hydrogen concentrations. The Debye temperature, θ_D, may also show a similar variation. These anomalous behaviors are thought to be due to hydrogen tunneling.     

Low temperature specific heat and magnetic properties of V/FE and V/FE/H alloys

Low temperature specific heats of V_1?xFe_x (0?x?0.34) and V_1?xFe_xH_n (x = 0.02; 0.05; 0.1; 0.15 a 0 ? n ? 0.98) were investigated between 1.5 and 16 K.In V/Fe alloys temperature independent band paramagnetism was observed at iron concentrations below 20at%; at higher iron contents local magnetic moments occur and contribute magnetic terms to the low temperature specific heat. According to Schröder [14] these contributions can be explained by thermal agitation of superparamagnetic clusters of the Fe atoms with local moments. In the temperature range of the specific heat measurements the magnetic terms are dependent on temperature (contrary to Schröder) and can be described by Planck-Einstein functions. A separation of the magnetic contributions from the lattice and electronic term of heat capacity could be obtained in a satisfying way by least squares fitting with three Planck-Einstein functions.In “nonmagnetic” V/Fe alloys with less than 20at% Fe absorption of hydrogen generates local magnetic moments and superparamagnetic contributions. These are, therefore, connected primarily more with the band electron concentration n_e, than with the iron content X_Fe At the highest n_e, values, n_e >0, large negative deviations from the regular course of the heat capacity plot are observed; these can be related to mictomagnetic behavior (freezing temperature of spin-glass state above the range of measurements, T_F > 16 K).After correction for the superparamagnetic contributions there remains in the Fe containing alloys an appreciable enhancement of the electronic heat coefficient γ, up to a factor of about 2 when compared with the iron-free V/H samples. This effect is interpreted as resulting from mass enhancements of the itinerant electrons by electron-magnon (and-paramagnon) interactions.     

The low temperature specific heat of Lu-Cu-Y metallic glasses

The specific heat of a series of amorphous metallic alloys of the form Lu_xCu_0.37Y_0.63-x (x = 0, 0.1, 0.3 and 0.4) has been measured between 2 and 50 K, primarily in order to be able to determine the non-magnetic contributions of the specific heat in magnetic RE-Cu-Y amorphous alloys. The data at low temperature fit the simple form C_p = γT + βT³ from which values of γ and θ_D(0) have been determined. Consideration is given to the error that arises if Y is used rather than Lu or La in forming non-magnetic rare earth intermetallics for purposes of determining the non-magnetic contributions to the specific heat of magnetic samples. A simple procedure is described that allows a useful improvement in accuracy in estimating non-magnetic contributions below 20 K if Y is used. The method may also be useful if only a restricted range of compositions using Lu is possible.     

Magnetic behavior of Eu(3)Ni(4)Ga(4): antiferromagnetic order and large magnetoresistance

The results of the magnetic susceptibility, isothermal magnetization, heat capacity, electrical resistivity and magnetoresistance measurements on polycrystalline Eu(3)Ni(4)Ga(4) are presented. Eu(3)Ni(4)Ga(4) forms in Na(3)Pt(4)Ge(4)-type cubic crystal structure (space group [Formula: see text]). The temperature dependence of the magnetic susceptibility of Eu(3)Ni(4)Ga(4) confirms the divalent state (Eu(2+)) of Eu ions with an effective magnetic moment μ(eff)?=?7.98?μ(B). At low fields, e.g.?at 0.01?T, a magnetic phase transition to an antiferromagnetically ordered state occurs at T(N)?=?10.9?K, which is further confirmed by the temperature dependence of the heat capacity and electrical resistivity. The field dependence of isothermal magnetization at 2?K reveals the presence of two field induced metamagnetic transitions at H(c1) and H(c2)?=?0.55 and 1.2?T, respectively and a polarized phase above H(PO)?=?1.7?T. The reduced jump in the heat capacity at the transition temperature, ΔC|(T(N))?=?13.48?J/mol-Eu?K would indicate an amplitude modulated (AM) antiferromagnetic structure. An interesting feature is that a large negative magnetoresistance, MR?=?[ρ(H)?-?ρ(0)]/ρ(0), is observed in the vicinity of magnetic transition even up to 2T(N). Similar large magnetoresistance has been observed in the paramagnetic state in some Gd and Eu based alloys and has been attributed to the magneto-polaronic effect.     

Density-of-state effects in the low temperature properties of TiBe2

The importance of exact theoretical and experimental determination of the band structure near E_F for an understanding of the properties of TiBe₂ is pointed out. Precise density-of-state data, obtained from ab-initio self-consistent band calculations, are used to determine low temperature variations in the susceptibility χ and specific heat γ. The influence on the field and temperature variations on χ is striking and qualitative agreement with experiment is possible without invoking spin-fluctuations.The field and temperature variations of γ are important enough to be considered, but are smaller than for χ and considerably smaller than observed experimentally by Stewart et al. Thus evidence of spin-fluctuations are more convincing when extracted from γ-data than from χ-data.     

Low-temperature heat capacity and magnetization of ferromagnets

The change in the magnetization of a ferromagnet due to disordering in the system of atomic magnetic moments with increasing temperature is put in correspondence with the change in the entropy of the magnetic subsystem. A relationship is obtained for calculating the magnetic component of the heat capacity of the ferromagnet, which satisfactorily describes the temperature dependence of the heat capacity for Fe, Co, TbB₂, and DyB₂ at low temperatures.     

Magnetic susceptibility of iron pyrite (FeS2) between 4.2 and 620 K

The magnetic susceptibilities (χ) of several natural single crystals of iron pyrite (FeS₂) have been measured between 4.2 and 620 K with a force magnetometer. The measured susceptibilities are shown to related to the magnetic impurities present in the samples. For a pure sample, χ = 1.7 × 10^?7 emu/g at room temperature, with a small monotonic increase of χ with temperature. The results for a pure sample are interpreted in terms of contributions from diamagnetism and Van Vleck paramagnetism.     

The hyperfine magnetic field in Fe-Hf amorphous alloys at low temperatures

The time-differential perturbed angular correlation (TDPAC) technique was applied to study the hyperfine magnetic field in amorphous Fe-Hf alloys with different Hf contents at low temperatures. The Hf content was changed from 20 to 40 at% andthe TDPAC measurements were carried out in the temperature range from room temperature down to 18 K. Very strong perturbations were observed for the specimen of low Hf content at low temperatures due to a strong hyperfine magnetic field. The characteristic feature observed by the present study is that the temperature dependence of the perturbation for the Fe-30 at% Hf source is quite different from that for other sources, indicating a maximum around 80 K. Discussions have been given on the explanation of the experimental results in the light of magnetic properties of these alloys.     

Possible observation of the coexistence of superconductivity and long-range magnetic order in NdRh4B4

The ternary rare earth compound NdRh₄B₄ has been studied by means of critical field, low temperature heat capacity, and static magnetic susceptibility measurements. Features in the upper critical field and heat capacity data at 1.31 K and 0.89 K suggest the occurrence of long-range magnetic order in the superconducting state. The temperature dependence of the static magnetic susceptibility follows a Curie-Weiss law with an effective magnetic moment μ_eff = 3.58 ± 0.05 μ_B and a Curie-Weiss temperature θ_p = ?6.2 ± 1.0 K between 20 K and room temperature. However,, magnetization vs. applied magnetic field isotherms suggest the development of a ferromagnetic component in the Nd³⁺ magnetization at low temperatures.     

Magnetic properties of amorphous rare-earth—Iron alloys

The magnetic properties of various amorphous alloys of the type R_1-xFe_x (R = Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) have been determined in the concentration range 0.3 ? x ? 0.5. With the exception of the Gd and Lu alloys pronounced thermomagnetic history effects were observed in the temperature dependence of the magnetization. These effects are due to a strong temperature dependence of the coercive force (H_c) which is found to obey an exponential law of the form H_c ∞ exp(-αT). ⁵⁷Fe Mössbauer spectra were obtained on the alloys of a composition close to 40 at% Fe. From the combined results of the Mössbauer spectroscopy and magnetic measurements it is derived that the Fe moment decreases if one passes through the rare-earth series. It is postulated that this decrease is due to small differences in the compositional short-range order in the amorphous alloys caused by the heat of mixing becoming more negative in the same sense.     

Molecular dynamics simulations of the melting of Al–Ni nanowires

Molecular dynamics (MD) simulations were performed to investigate the influence of nickel (Ni) composition and nanowire thickness on the thermal properties of Al-x%Ni (at%) nanowires using the embedded atom model (EAM) potential. The melting of the nanowire was characterised by studying the temperature dependence of the cohesive energy and mean square displacement. The effect of the nanowire thickness on the cohesive energy, melting temperature, heat capacity as well as latent heat was studied in canonical ensemble. Moreover, the crystal stability of Al, Al-20%Ni, Al-40%Ni, Al-60%Ni, Al-80%Ni, Al₃Ni, Ni₃Al and Ni nanowires was studied at different temperatures using mean square displacement and cohesive energy.     

射频溅射功率对FeZrBCu软磁合金薄膜巨磁阻抗效应的影响

用射频溅射法制备了FeZrBCu软磁合金薄膜.研究了不同溅射功率对FeZrBCu薄膜软磁特性和巨磁阻抗效应的影响.用电子探针显微镜测量发现，当溅射功率为240W时，薄膜样品中Fe的原子含量为8732%，Cu的原子含量为29%.这种样品的矫顽力最小，为68A/m，饱和磁化强度约为111×105⁵A/m，软磁性能最佳，巨磁阻抗效应最大，溅态膜在 13MHz最大巨磁阻抗比纵向为17%，横向为11%.重点分析了阻抗的电阻、电感分量及横向有 效磁导率随频率的变化，得到在低频下主要是磁电感 关键词： 铁基合金 薄膜 巨磁阻抗效应     

Influence of high magnetic fields on the low temperature heat capacity of the itinerant electron ferromagnet Sc3In

The low temperature (1.3–20 K) heat capacity of the weak itinerant electron ferromagnet Sc₃In was measured in magnetic fields up to 10 T. The measurements were made on three different samples containing 24.1, 24.3 and 24.4 at.% In with Curie temperatures at H = 0 of 5.5, 6.0 and 6.3 K, respectively. The heat capacity peak at T_c becomes smaller with increasing fields and at 9.98 T the magnetic entropies are only 11 to 19% of the zero field value. Above T_c the spin fluctuation contribution to heat capacity, which is enhanced by the magnetic field at low fields (<≈ 5 T), is quenched at high fields ( ≈ 5 T). Our results show that the spin fluctuations in Sc₃In are almost completely quenched by the magnetic fields of ≈ 10 T, and that the characteristics spin fluctuation temperature, T_s, of this itinerant electron ferromagnet is about the same as T_c.     

Magnetization study of ITER-type internal-Sn Nb3Sn superconducting wire

Through magnetization measurement with a SQUID magnetometer the heat treatment optimization of an international thermonuclear experimental reactor （ITER）-type internal-Sn Nb3Sn superconducting wire has been investigated. The irreversibility temperature T^＊ （H）, which is mainly dependent on A15 phase composition, was obtained by a warming and cooling cycle at a fixed field. The hysteresis width △M（H） which reflects the flux pinning situation of the A15 phase is determined by the sweeping of magnetic field at a constant temperature. The results obtained from differently heat-treated samples show that the combination of T^＊ （H） with AM（H） measurement is very effective for optimizing the heat reaction process. The heat treatment condition of the ITER-type wire is optimized at 675℃/128 h, which results in a composition closer to stoichiometric Nb3Sn and a state with best flux pinning.     

Hydrogen bond dynamics in dodecanoic acid studied by QNS and NMR

The dynamics of hydrogen atoms in the hydrogen bonds of molecular dimers in dodecanoic acid (c phase) have been studied by quasi-elastic neutron scattering and pulsed nuclear magnetic resonance. Q-dependence measurements of the intensity of the quasi-elastic peak have established that the hydrogen atoms move along a line connecting the two oxygen atoms in the hydrogen bond. The correlation time for this motion has been studied by temperature dependence measurements of the width of the quasi-elastic line and of the proton spin-lattice relaxation time,T ₁. These studies reveal the quantum mechanical nature of the dynamics in the low temperature region. The dynamical parameters which characterise the motion have been determined by fitting the data to a model which invokes phonon assisted tunnelling. The frequency dependence ofT ₁ at low temperature is anomalous because the gradient of the ln(T ₁) vs 1/T curve is dependent on the applied magnetic field.     

Metastable compositionally and magnetically modulated state of Fe-Ni Invar and the associated super-moment dynamics from Mössbauer spectroscopy

The presence of magnetic and chemical clustering, in Fe₆₅Ni₃₅ Invar, had previously been inferred from various measurements which we review. We independently confirm both types of clustering and measure the associated supermoment dynamics for the first time. The temperature dependence of the fluctuation frequency is understood in terms of inter-cluster surface exchange energy and leads to a spherical cluster radius of 7.2 Å. The observed super-moment excitations are sufficient to explain the anomalous temperature dependence of the magnetization and are slow enough, compared to lattice response times, to cause the magnetovolume anomalies. The magnetic clusters are seen to persist far above T_c - at least up to 600 K. Using a two-cluster-type approximation, the mean compositional morphology is obtained as: 39 at% Fe₈₅Ni₁₅/61 at% Fe₅₃Ni₄₇. Also, the alloy which is most often referred to as “Invar”, is found to contain ⪅ 2 at% of α-Fe precipitate. A change of + 1 at% in the amount of this precipitate is found to cause a change of ≃ + 50 K in the T_c measured on first heating. The three most common models for Invar - itinerate model, two γ-state model, and mixed exchange model - are discussed in terms of both data available in the literature and our own results. The mixed exchange approach is shown to be the most reasonable. Its crudeness is argued to arise from two factors: (i) since detailed structural data were not available, it has assumed the wrong fine scale schemical morphology, and (ii) it uses only three values of J_ij whereas we expect a broad distribution of exchange energies to arise from a Slater-Bethe-Bozorth-type interaction whose strength is a function of local composition.     

Spin-glass-like distribution of interaction fields in a nearly ferromagnetic Ni-Cu alloy

The magnetic specific heat (C_m) of a Ni-Cu alloy of 40 at% Ni was determined from low-temperature heat capacity measurements in various magnetic fields and is shown to derive from a dilute concentration of giant-moment clusters, consistent with previous magnetization measurements. From the field and temperature dependences of C_m, it is also deduced that the distribution of interaction (exchange and anisotropy) fields experienced by the clusters extends continuously from large positive fields to large negative fields, relative to the applied field.     

Structural and magnetic characterization of Co67Fe4Ni2Si15B12

 Soft magnetic properties of Co-based amorphous alloy of the composition Co₆₇Fe₄Ni₂Si₁₅B₁₂ have been investigated by isothermal heat treatment up to the conventional crystallization temperature. In the as-cast condition the Curie temperature of the sample is 272 °C and saturation magnetization is 74 emu/g. Magnetic properties undergo variation depending on the heat treatment temperature. For the heat treatment temperatures of around 420 and 490 °C, superior soft magnetic properties are obtained. For both the temperatures initial permeability, μ′ reaches value up to ten times the value of permeability in the as-received samples. Annealing effect on giant magneto-impedance has been observed for the current-driving frequencies of 4.5 and 6 MHz. Field dependence of magneto-impedance shows hysteresis at low field, which is related to the changes in the magnetization process of the sample.     

Electronic heat capacity in disordered graphene systems

We analyzed the electronic heat capacity of graphene systems in the presence of disorder. We consider the case of strong scatterers, working in the unitary limit. The temperature dependence of the electronic heat capacity is analyzed. Close to the clean limit we obtained the quadratic temperature dependence, corrected with a temperature and disorder dependent factor which slightly enhance the heat capacity. At very low temperatures, and in the presence of disorder, we obtained a linear temperature dependence of the electronic heat capacity. We also analyzed the temperature dependence of the electronic heat capacity in the case of extrinsic graphene.