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Thermal hysteresis is recognized as one of the main drawbacks for cyclical applications of magnetocaloric and ferromagnetic shape memory materials with first order transformations. As such, the challenge is to develop strategies that improve the compatibility between the phases involved in the transitions and study its influence on thermal hysteresis. With this purpose, we explore the thermal, structural and magnetic properties of the Ni2Mn1-xCuxGa0.84Al0.16 Heusler alloys. The alloys present a thermal hysteresis reduction of ~60% when the Cu content in the compound varies from x = 0.10 to x = 0.25, with a minimum hysteresis width of 6 K being achieved. We applied the geometric non-linear theory of martensite to address the phase compatibility, quantified by the parameter lambda2, the middle eigenvalue of the transformation stretch tensor, and found that the minimum of hysteresis is associated with a better crystallographic compatibility (lambda2 closer to 1) between the austenite and martensite phases. In addition, we show that the valley-like properties of hysteresis found in the Ni2Mn1-xCuxGa0.84Al0.16 compounds is present in several other alloys in the literature. These results provide new pathways to understand as well as to masters the phase compatibility and ultimately achieve a low thermal hysteresis in multifunctional Heusler alloys., 29 pages and 8 figures

In this work we address the interplay between two phenomena which are signatures of the out-of-equilibrium state in phase separated manganites: irreversibility against thermal cycling and aging/rejuvenation process. The sample investigated is La0.5Ca0.5MnO3, a prototypical manganite exhibiting phase separation. Two regimes for isothermal relaxation were observed according to the temperature range: for T > 100 K, aging/rejuvenation effects are observed, while for T < 100 K an irreversible aging was found. Our results show that thermal cycles act as a tool to unveil the dynamical behavior of the phase separated state in manganites, revealing the close interplay between static and dynamic properties of phase separated manganites., 16 pages, 5 figures, accepted in JMMM (2018)

In this work we investigate the critical behavior of two manganite compounds, Pr0.5Sr0.5MnO3 (PSMO) and Pr0.5Sr0.41Ca0.09MnO3 (PSCMO). These are complex magnetic systems, which undergo a paramagnetic to ferromagnetic phase transition on cooling, followed by a first order magnetic transition to a charge ordered antiferromagnetic states at lower temperatures. Magnetization measurements and magnetocaloric calculations were performed in order to determine their critical behavior. As a main result we show that the scaling of the entropy change - Δ S M with magnetization M , instead of the usual scaling with magnetic field H , is a powerful tool to study the critical behavior through magnetocaloric analysis. We use this property, in conjunction with Modified Arrot Plot technique, to determine the basic critical exponents β and γ for each sample. The exponents values indicate that the samples are near to universality class models based in nearest neighbor interactions. Besides intrinsic differences between the Ca-doped and the undoped sample, we confirm the applicability of the magnetocaloric analysis in order to determine critical behavior in complex magnetic systems.

Polycrystalline Heusler compounds Ni2Mn0.75Cu0.25Ga0.84Al0.16 with a martensitic transition between ferromagnetic phases and Ni2Mn0.70Cu0.30Ga0.84Al0.16 with a magnetostructural transformation were investigated by magnetization and thermal measurements, both as a function of temperature and magnetic field. The compound Ni2Mn0.75Cu0.25Ga0.84Al0.16 presents a large magnetocaloric effect among magnetically aligned structures and its causes are explored. In addition, Ni2Mn0.70Cu0.30Ga0.84Al0.16 shows very high, although irreversible, entropy and adiabatic temperature change at room temperature under a magnetic field change 0-1 T. Improved refrigerant capacity is also a highlight of the 30% Cu material when compared to similar Ni2MnGa-based alloys., Comment: 26 pages and 6 figures

Ferromagnetic Ni2MnGa-based alloys play an important role in technological fields, such as smart actuators, magnetic refrigeration and robotics. The possibility of obtaining large non-contact deformation induced by an external perturbation is one of its key strengths for applications. However, the search for materials with low cost, practical fabrication procedures and large signal output under small perturbing fields still poses challenges. In the present study we demonstrate that by judicial choice of substitution on the Mn site, an abrupt magnetostructural transition from a paramagnetic austenite phase to a ferromagnetic martensite one can be tuned to close to room temperature achieving large and reproducible strains. The required magnetic field to induce the strain varies from small values, as low as 0.25 T for 297.4 K and 1.6% of strain, to 8 T for 305 K and 2.6% of strain. Our findings point to encouraging possibilities for application of shape memory alloys in relatively inexpensive, scalable polycrystalline materials., Comment: 26 pages and 7 figures

The mixed-valent homometallic ludwigite $({\mathrm{Co}}_{2}^{2+}{\mathrm{Co}}^{3+}){\mathrm{O}}_{2}{\mathrm{BO}}_{3}$ is investigated above the ferrimagnetic ordering temperature ${T}_{C}=43\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ through structural, thermal, magnetic, electric, and spectroscopic probes. X-ray absorption at the Co ${L}_{2,3}$ edges is consistent with the coexistence of ${\mathrm{Co}}^{2+}$ and ${\mathrm{Co}}^{3+}$ ions, as expected by the sample stoichiometry. Magnetic susceptibility shows a relatively large net paramagnetic moment per Co ion above room temperature, $p=4.87$, indicating that the ${\mathrm{Co}}^{3+}$ ions are not in a pure low-spin configuration at high temperatures, also showing a non-Curie-Weiss behavior below 300 K. Electrical conductivity and differential scanning calorimetry measurements on single crystals indicate two phase transitions at $\ensuremath{\sim}475$ and $\ensuremath{\sim}495\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. X-ray powder diffraction shows substantial lattice parameter anomalies below 500 K. These results indicate phase transitions associated with changes in the Co oxidation state in each of its four crystallographic sites. Such transitions are possibly dictated by a competition between (i) an ordered ground state with all ${\mathrm{Co}}^{3+}$ ions occupying the same crystallographic site and (ii) either partially or totally charge-disordered states that are favored at high temperatures due to their higher entropy.

We study the effect of proton irradiation on Ba(Fe$_{0.96}$Co$_{0.04}$)$_2$As$_2$ superconducting single crystals from combined magnetisation and magnetoresistivity measurements. The study allows the extraction of the values of the apparent pinning energy $U_0$ of the samples prior to and after irradiation, as well as comparison of the values of $U_0$ obtained from the flux-flow reversible region with those from the flux-creep irreversible region. Irradiation reduces $T_c$ modestly, but significantly reduces $U_0$ in both regimes: the critical current density $J_c$ is modified, most strikingly by the disappearance of the second magnetisation peak after irradiation. Analysis of the functional form of the pinning force and of the temperature dependence of $J_c$ for zero field, indicates that proton irradiation in this case has not changed the pinning regime, but has introduced a high density of shallow point-like defects. By consideration of a model that takes into account the effect of disorder on the irreversibility line, the data suggests that irradiation produced a considerable reduction in the average effective disorder overall, consistent with the changes observed in $U_0$ and $J_c$., 5 pages 7 figures

We investigate the nature of ultrasharp metamagnetic transitions in La5/8-yNdyCa3/8MnO3 manganites. These compounds change from a low temperature ferromagnetic metallic state at low Nd doping to a charge-ordered antiferromagnetic insulator for high Nd content. At an intermediate doping a phase-separated state is established. At low temperatures (∼2 K), an avalanche-like field-induced metamagnetic transition is observed, where the entire compound changes abruptly from one phase to the other. The behavior of this ultrasharp transition is investigated using magnetization and specific heat measurements. We found a first order transition in the specific heat associated with discontinuous jumps in the magnetization. A strong increase of the sample temperature is simultaneously observed. The results are interpreted in terms of latent heat released from the field-induced enhancement of the ferromagnetic fraction, triggering the avalanche process.

Using a combination of theoretical (first-principles total-energy and electronic structure calculations) as well as experimental (structural, thermodynamics) techniques, we systematically investigated the influence of B incorporation on the structural, electronic and thermodynamic properties of a series of technologically-important B-containing AlNi matrix. Special attention was paid to calculating the energy cost of placing B at various sites within the cubic unit cell. The most energetically favorable defects were identified to be, depending on initial stoichiometry, substitutional B at Al site (B Al ), Ni vacancy ( V Ni ), or Ni antisite (Ni Al ). We show that the induced variation in the lattice parameters can be correlated with the type and concentration of the involved defects: e.g. the surge of V Ni defects leads to a stronger lattice-parameter reduction, that of Ni Al ones to a relatively weaker reduction while that of B Al defects to a much weaker influence. Both electronic band structure calculations as well as thermodynamics measurements indicate that the 3 d bands of Ni are fully occupied and magnetically unpolarized and that the resulting N ( E F ) is very small: all studied compounds are normal conductors with no trace of superconductivity or magnetic polarization.

A series of high quality BaFe$_{2-x}$Ni$_x$As$_2$ pnictide superconductors were studied using magnetic relaxation and isothermal magnetic measurements in order to study the second magnetization peak (SMP) and critical current behaviour in Ni-doped 122 family. The temperature dependence of the magnetic relaxation rate suggests a pinning crossover, whereas, it's magnetic field dependence hints a vortex-lattice structural phase-transition. The activation energy ($U$) estimated using the magnetic relaxation data was analyzed in detail for slightly-underdoped, slightly-overdoped and an overdoped samples, using Maley's method and collective creep theory. Our results confirm that the SMP in these samples is due to the collective (elastic) to plastic creep crossover as has been observed for the other members of 122-family. In addition, we also investigated the doping dependence of the critical current density ($J_c$) and the vortex-pinning behaviour in these compounds. The observed $J_c$ is higher than the threshold limit (10$^5$ A/cm$^2$) considered for the technological potential and even greater than 1 MA/cm$^2$ for slightly underdoped Ni-content, x = 0.092 sample. The pinning characteristics were analyzed in terms of the models developed by Dew-Hughes and Griessen $et$ $al$, which suggest the dominant role of $\delta l$-type pinning., Comment: 26 pages, 13 figures, Accepted for publication in ACS Applied Electronic Materials 2019

We present magnetization studies as a function of time, temperature and magnetic field for $H$ $\parallel$ c-axis, in a hole-doped pnictide superconductor, La$_{0.34}$Na$_{0.66}$Fe$_2$As$_2$, with, $T_c$ $\approx$ 27 K. The obtained vortex phase-diagram shows that the magnetic irreversibility line is very close to the mean-field superconducting transition line, similar to the low $T_c$ superconductors, evidencing a strong pinning behavior. The irreversibility line does not follow a power law behavior with ($T_c$-$T$), however, it is well described using an expression developed in the literature considering the effect of disorder in the system. The critical current density estimated using the Bean's critical-state model is found to be of the order of 10$^5$ A/cm$^2$ below 12 K in the limit of zero magnetic field. A plot of the normalized pinning force density as a function of the reduced magnetic field at different temperatures shows a good scaling and the analysis suggests that the vortex pinning is due to normal point like pinning centers. The temperature dependence of the critical current density suggests that the pinning due to the variation in charge carrier mean free path alone is not sufficient to explain the experimental data. Magnetic relaxation rate as a function of temperature and magnetic field is also studied., Comment: 19 pages, 6 Figures, Accepted for Publication in the "Journal of Applied Physics 2019"

Controlling magnetism with electric field directly or through strain-driven piezoelectric coupling remains a key goal of spintronics. Here, we demonstrate that giant piezomagnetism, a linear magneto-mechanic coupling effect, is manifest in antiperovskite Mn3NiN, facilitated by its geometrically frustrated antiferromagnetism opening the possibility of new memory device concepts. Films of Mn3NiN with intrinsic biaxial strains of ±0.25% result in Néel transition shifts up to 60 K and magnetization changes consistent with theory. Films grown on BaTiO3 display a striking magnetization jump in response to uniaxial strain from the intrinsic BaTiO3 structural transition, with an inferred 44% strain coupling efficiency and a magnetoelectric coefficient α (where α = dB/dE) of 0.018 G cm/V. The latter agrees with the 1000-fold increase over Cr2O3 predicted by theory. Overall, our observations pave the way for further research into the broader family of Mn-based antiperovskites where yet larger piezomagnetic effects are predicted to occur at room temperature.

We investigate the effects of A-site cation disorder on the magnetic transition in doped Sr2YRuO6 insulating double perovskites. Disorder is induced by partially substituting Sr for Ca, Ba and La. The relative concentration of the doping ions was carefully chosen in order to warrant a constant average ionic radius equal to the one found in Sr2+. The variance, σ2 = − 2, where rA is the ionic radius of each type of ion occupying the A-site, is the parameter used to quantify disorder. Unexpectedly, in spite of the fact that the magnetic ground state in Sr2YRuO6 is the result of a delicate balance between competitive ferro- and antiferromagnetic interactions with bound lengths and angles playing a relevant role, the magnetic transition temperature is virtually insensitive to disorder, in sharp contrast to the behavior observed for metallic perovskites. This result supports the idea that the leading mechanism suppressing the magnetic transition in Sr2YRuO6 is the deviation from planarity of the RuO2 layers.

In the present work we investigate the alloys (Fe 0.8 Al 0.2 ) 100− x B x , for x > 2.0 up to x = 5.0. A maximum of the total magnetostriction was found for the alloy (Fe 0.80 Al 0.20 ) 97 B 3 . In this sample we measured the boron contents using WDS and we did not find boron in the matrix, while the precipitates formed after annealing of as cast samples are essentially the phase Fe 2 B. By a detailed XRD study of this sample using Mo Kα radiation, the presence of the phases Fe 3 Al (D03) and α (A2) was determined. The coexistence of both phases was determined to be the origin of the increase of the magnetostriction of this material. The value of λ long = 80 × 10 −6 achieved in the as cast sample with x = 3 indicates that the introduction of texturing to the material should provide even higher enhancement of the magnetostriction. In addition to high magnetostriction values, the alloy can be machined, welded and is also a low cost material, therefore the alloy (Fe 0.80 Al 0.20 ) 97 B 3 reveals a great promise as a material to be used on sensors and actuators assembly.

We investigate the magnetostriction of polycrystalline Fe 100− x V x alloys as a function of vanadium content, with values of x varying from 5.2% to 40.7% (at.%V). The lattice parameter curve behavior as a function of vanadium deviates from a linear behavior at x ≈ 10.2 and x ≈ 30.7. Close to x = 10.2 is possible to observe a small lattice contraction and close to x = 30.7 an expansion. The magnetostriction curve as a function of vanadium content shows two maxima at x = 11.8 and 30.7 and has values of 3/2 λ s of 68.8 ppm and 49.3, respectively. The alloys microstructures investigations do not offer enough elements to connect it with magnetostriction. Therefore first-principles calculations were performed and resulted that the lattice contraction at ∼10.2% of V is due to the formation of strengthened Fe–V interactions associated to stronger hybridization between minority spin d subbands, while the lattice expansion observed in the range 25.5–30.7% of V is associated to the weakening of the Fe–V interaction. As for the case of simulating V–V pairs for the alloy Fe 12 V 4 , representative of x = 25.0, we associated the experimental result of the lattice expansion observed in the range among 25.5–30.7% of V to the weakening of the Fe–V interaction. These calculated electronic properties explain the lattice contraction (at x = 10.2) and expansion ( x = 25.5–30.7) and are expected to be the sources of the magnetostrictive behavior of these Fe–V alloys.

Here, we study the temporal evolution of the magnetic field-driven paramagnetic to ferromagnetic transition in the La(Fe,Mn,Si) 13 material family. Three compositions are chosen that show varying strengths of the first-order character of the transition, as determined by the relative magnitude of their magnetic hysteresis and temperature separation between the zero-field transition temperature T c and the temperature T crit , where the transition becomes continuous. Systematic variations in the fixed field, isothermal rate of relaxation are observed as a function of temperature and as a function of the degree of first-order character. The relaxation rate is reduced in more weakly first-order compositions and is also reduced as the temperature is increased towards T crit . At temperatures above T crit , the metastability of the transition vanishes along with its associated temporal dynamics. This article is part of the themed issue ‘Taking the temperature of phase transitions in cool materials’.

The paramagnetic Meissner effect (PME) is related to the appearance of a positive magnetization when a superconducting specimen is field cooled through its critical temperature. In this work we report on the PME and ac magnetization in roll-bonded Cu–Nb (RB/Cu–Nb) layered composites. We present typical DC magnetization loops obtained in the normal magnetic field configuration that show the PME. In addition, we present ac magnetization measurements that reveal a crossover behavior at a characteristic field value. We show evidence that such a crossover behavior, attributed to activation processes of vortices, is probably related to the disappearance of the PME in the RB/Cu–Nb layered composites.

The Fex(Cr2O3)1−x system, with 0.10 ≤ X ≤ 0.80, was mechanically processed for 24 h in a high-energy ball-mill. In order to examine the possible formation of iron–chromium oxides and alloys, the milled samples were, later, thermally annealed in inert (argon) and reducing (hydrogen) atmospheres. The as-milled and annealed products were characterized by X-ray diffraction, Mossbauer spectroscopy, transmission electron microscopy and magnetization. The as-milled samples showed the formation of an Fe1+YCr2−YO4−δ nanostructured and disordered spinel phase, the α1-Fe(Cr) and α2-Cr(Fe) solid solutions and the presence of non-exhausted precursors. For the samples annealed in inert atmosphere, the chromite (FeCr2O4) formation and the recrystallization of the precursors were verified. The hydrogen treated samples revealed the reduction of the spinel phase, with the phase separation of the chromia phase and retention of the Fe–Cr solid solutions. All the samples, either as-milled or annealed, presented the magnetization versus applied field curves typical for superparamagnetic systems.

In this work we focus on the microstructural and magnetic characterization of wires with different architectures (design and reinforcement). The microstructural characterization was performed using scanning electron microscopy. AC magnetic susceptibility was measured with field applied both parallel and perpendicular to the wire axis. The heat treatment performed to form the A-15 superconducting phase leads to partial spheroidization followed by coarsening of the Nb filaments in the reinforcement material. The differences concerning the microstructure of the reinforcement material among the investigated wires were reflected in the broadening of the superconducting transition of Nb, more evident for a field applied parallel to the wire axis. From the magnetic data the wires were also compared in terms of the superconducting volume fraction.

The influence of Co substitution on the structural and magnetic properties of La(Fe 1− x Co x ) 11.4 SP 1.6 compounds (SP=Al or Si) was investigated by X-ray diffraction, Mossbauer spectroscopy and magnetization measurements. The studied compounds were crystallized in the NaZn 13 structure, with a fraction of α-Fe(Co,SP) phase. Partial substitution of Fe by Co atoms changes (i) the magnetic ordering temperature (200 K T x = 0 to about 0.5 T for x = 0.04 . For x > 0.04 , the compounds are ferromagnetic as the ground state. On the other hand, the Si compounds have ferromagnetic ground state for all Co compositions studied. The field-induced (5 T) maximum magnetic entropy variation remains almost constant at about −5 J/kg K, for x values up to 0.12, in the Al compound. However, this entropy variation reduces from −18 to −5 J/kg K (∼72%), when Co substitution is increased from 0 to 8 at%. in the Si compounds. This effect may be associated with the disappearing of the first-order transition with the addition of Co. Metamagnetic transition of the Co free Al compound has entropy variation of 13 J/kg K at about 11 K.

We report on the influence of the microstructure on the AC and DC magnetic properties of Cu–3.5% Nb nanofilamentary wires. Samples obtained from a single Cu–3.5% Nb wire were subsequently submitted to different plastic strain levels via drawing so that their microstructure was altered. Noticeable differences are observed in their isothermal DC magnetization curves that present a double-peak structure. The first peak, which occurs at low magnetic fields, is attributed to superconductivity induced in the Cu matrix due to the proximity effect. It is argued that the second peak is related exclusively to niobium. The dependence of these two distinct peaks on the characteristic nanometre length scales of the samples, i.e. dimension of the Nb filaments and interfilamentary spacing, are discussed.

The magnetic properties of the superconducting ruthenate-cuprate RuSr2Eu 1.4Ce0.6Cu2O10-δ (Ru-1222) have been studied by a scaling analysis of the dc magnetization and ac susceptibility measurements. Non-linear M(H) curves reveal the presence of nano-size clusters with a net magnetic moment of ∼102 μB at 180 K, near the deviation from a Curie-Weiss behavior. On cooling, no scaling was observed down to 90 K, discarding the possibility of magnetic phase separation of collinear ferromagnetic particles. We explain this result in terms of a variable number of contributing particles, with a temperature dependent net magnetic moment. For 70 K ≤T≤ 90 K the scaling plots evidence the emergence of a system of non-interacting particles, which couple on further cooling. The observed cluster-glass features are preserved down to the lowest measured temperature (10 K); no signature of long-range order was detected. The frequency shift of the peak in the real part of the ac susceptibility does not follow the Vogel-Fulcher type dependence, as previously reported. The puzzling temperature dependence of the coercive field, HC(T), is correlated with the changes in the scaling factors.

The present work presents the steps for the manufacture of an Nb?Ti artificial pinning centre (APC) multifilament wire for AC applications. The wire production starts from a Ti bar inserted in a Nb tube. The superconductor Nb?Ti alloy was obtained using heat treatments during the wire production steps through the growth of the diffusion layer between the pure Nb and Ti regions. The final wire is composed of many filaments dispersed in a Cu?Ni matrix. The transport critical current density was measured in samples for different conditions of heat treatment, followed by wire diameter reduction. Magnetization hysteresis loops were measured to extract the hysteretic losses in wires with filament diameters of 76 and 92??m and compared to the results in commercial wires with comparable filament diameters. We obtained an Nb?Ti APC wire with hysteretic losses of the same order of magnitude as the commercial wires. However, critical current densities Jc are higher for fields lower than 2?T, where AC applications are most important. The Jc results confirmed a previous result that the decrease of normal region sizes is a less important pinning source in this material than the pinning from variations in the composition of the diffusion layer.

We report on the microstructure evolution in Cu-15% Nb multifilamentary wires upon annealing and the corresponding effects on their electrical and magnetic properties. During annealing at temperatures higher than 800degC thermal instability mechanisms take place in the microstructure of the Cu-15%Nb composite, leading to spheroidization, followed by coarsening of niobium filaments. Such microstructural changes exert considerable influence on the magnitude of the normal electrical resistivity for the Cu-Nb composite. In addition, the annealing process leads to a double-peak structure in the isothermal DC magnetization curves of the composite, which is strongly dependent on the annealing time. This behavior suggests that the spheroidization kinetics can be obtained from M(H) curves

We report on the microstructural evolution in Cu15%Nb multifilamentary wires upon annealing and the corresponding effects on their magnetic properties. During annealing at temperatures higher than 800 °C thermal instability mechanisms take place in the microstructure of the Cu15%Nb composite, leading to the spheroidization of niobium filaments. Another important consequence of annealing is the recrystallization of Nb, that has a straightforward influence on the magnetization data. When compared to the as-drawn composite, in the annealed Cu15%Nb samples the bulk upper critical field Hc 2(T) is reduced significantly, while the lower critical field Hc 1(T) is increased as a function of the annealing temperature and time. This indicates that we may employ this process as an efficient method to modify the basic superconducting parameters of Nb, namely the coherence length ξ(T) and the penetration depth λ(T).

We studied electric and magnetic properties of polycrystalline La 0.325 Pr 0.300 Ca 0.375 MnO 3 , a manganese oxide-based compound exhibiting intrinsic multiphase coexistence of submicrometric ferromagnetic, charge-ordered and charge-disordered regions. Time relaxation in both resistivity and magnetization were observed in a broad temperature range, signaling that coexisting phases are out of equilibrium. Different regimes of instability are presented and discussed.

We have studied a non-volatile memory effect in the mixed valent compound La 0.5 Ca 0.5 MnO 3 induced by magnetic field ( H ). In a previous work (Phys. Rev. B 65 (2002) 104403), it has been shown that the response of this system upon application of H strongly depends on the temperature range, related to three well-differentiated regimes of phase separation occurring below 220 K. In this work we compare memory capabilities of the compound, determined following two different experimental procedures for applying H , namely zero-field cooling and field cooling the sample. These results are analyzed and discussed within the scenario of phase separation.

We report on the annealing effects on the microstructure, electrical, and magnetic properties of jelly-rolled Cu –xvol %Nb (x = 25, 33, 50, 63) composite wires, a promising material to be used in high-field magnets. During annealing at temperatures as high as 850 °C, noticeable changes take place in the microstructure, including recovery and recrystallization of copper and niobium, followed by partial spheroidization and further coarsening of niobium filaments. The influence of these microstructural changes on the electrical and magnetic properties of jelly-rolled Cu–Nb composite wires is discussed.

Comparison of the DC resistivity, magnetisation, 55 Mn NMR, AC susceptibility and microwave impedance of single crystalline and bulk polycrystalline samples of La 0.9 Ca 0.1 MnO 3 reveals the complexity of these materials. The observation of a motionally narrowed peak in the NMR data is indicative of double exchange ferromagnetism existing in isolated metallic regions, whilst combined magnetisation and DC resistivity suggest that at macroscopic scales the material is a super-exchange mediated ferromagnetic insulator. High-frequency conductivity measurements are used to search for evidence of non-percolative metallic clusters. The microwave signature associated with the metal insulator transition in micron sized isolated particulates of La 0.67 Ca 0.33 MnO 3 were studied in order to establish how metallic cluster formation might be identified.

In this paper, the annealing effects on the magnetic properties of a multifilamentary Cu–15vol%Nb composite were investigated. During vacuum annealing, noticeable changes take place in the microstructure, mostly the partial spheroidization and further coarsening of the niobium filaments. Results show that spheroidization becomes noticeable at about 700 °C and, even after annealing at severe conditions, e.g. 1050 °C for 32 h, the continuity of the niobium-conducting path is partially preserved. The influence of these microstructural changes on the magnetic properties of the multifilamentary Cu–Nb composite conductor is discussed.

Made available in DSpace on 2018-12-11T17:11:37Z (GMT). No. of bitstreams: 0 Previous issue date: 2017-04-10 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) The oxyborate Fe3O2BO3 presents a charge density wave (CDW) transition close to room temperature. As we show here, this is associated with a well defined anomaly in the specific heat. Below this transition, when applying in a single crystal of Fe3O2BO3 a DC voltage above a temperature dependent threshold, a high current is liberated in this material. We study the conduction in single crystals of Fe3O2BO3 with voltage applied parallel and perpendicular to the crystallographic c axis direction. The observed currents are attributed to the depinning of charge ordered domains above a threshold voltage V T2 that gives rise to a collective conduction due to coherent domains. Compliance limited DC data shows that above a lower threshold voltage depinning is smooth and follows a power law scaling. Similar depinning with power law scaling is also revealed in the AC conductivity. Departamento de Física IGCE Universidade Estadual Paulista UNESP - C.P. 178 Instituto de Física Universidade Federal Fluminense Campus da Praia Vermelha Instituto de Física Universidade Federal Do Rio de Janeiro, Caixa Postal 68528 Centro Brasileiro de Pesquisas Físicas Rua Dr. Xavier Sigaud, 150 - Urca Departamento de Física IGCE Universidade Estadual Paulista UNESP - C.P. 178

The exchange interactions between copper ions in (l-tryptophyl-glycinato)copper(II), C13H13CuN3O3 (named Cu(II)Trp-Gly) are determined by ac magnetic susceptibility and dc magnetization measurements in powder samples, and by EPR measurements in single-crystal samples. The copper ions in this compound are arranged in two symmetry-related types of chains along the b axis. Neighbor coppers in a chain at 5.14 A are connected by equatorial syn−anti carboxylate bridges, which transmit the intrachain exchange interactions. Indole rings of neighbor tryptophan residues at apical positions of the copper ions provide cation−π contacts that give rise to pathways supporting exchange interactions between the chains. The susceptibility and magnetization data obtained in the temperature range 1.8 < T < 100 K, with applied dc magnetic fields (B) up to 9 T are explained assuming one-dimensional spin chains with ferromagnetic intrachain coupling 2J/k = 3.9 ± 0.1 K, calculated from the zero-field susceptibility. The magnetiz...

The existence of superconductivity in samples of PrBa 2 Cu 3 O 7−δ (Pr-123) is still a polemic subject. To further study this problem, we have prepared high-quality polycrystalline Pr-123 samples by following a sol-gel method, associated with heat treatment in inert atmosphere. In this paper we report the structural, thermal and magnetic properties of superconducting sintered-polycrystalline Pr-123 and give a descriptive explanation about its unique dependence on the preparation procedure. The list of evidences of superconductivity in Pr-123 is further expanded with new data on a superconducting thin film. The present results confirming the occurrence of superconductivity in Pr-123 revitalizes the hypothesis that superconductivity originates outside the cuprate-plane.

A systematic study of phase separation effects in polycrystalline La_0.5 Ca_0.5 Mn O_3 obtained under different thermal treatments is reported. Samples with average grain size ranging from 200 to 1300 nm were studied. Magnetic and electrical measurements show quantitative differences among samples in their low temperature behavior. indicating that the fraction of the ferromagnetic (FM) phase gradually decreases as the grain size increase. Percolation of the FM phase in samples with a small fraction of this phase suggests that grain boundaries play a distinctive role in the spatial distribution of coexisting phases. The defective structure at the grain surface could explain the local inhibition of the antiferromagnetic charge ordered phase, an effect that is gradually removed with increasing grain size. Qualitative agreement of the data with this description is found. This effect is also found to be highly dependent on the oxygen content of the samples and its spatial distribution., To be published, Phys.Rev. B; 5 pages, 6 figures

The influence of the Y doping on the magnetic properties of La 0.7− x Y x Ca 0.3 MnO 3 compounds ( x =0; 0.07; 0.1; 0.15) and on the low-field and high-field magnetoresistance responses are investigated by means of transport, susceptibility and magnetisation measurements. The evolution of the magnetic state with yttrium doping is first described and then correlated with the low-temperature magnetoresistance effects. Y doping does not influence in a monotonic way the coercive field or the initial susceptibility of the samples, but modifies the magnetic state from a ferromagnetic ( x =0) to a cluster glass ( x =0.15). The key observation is that by relatively low temperatures (50 K) the resistivity change in low magnetic fields is controlled by the bulk magnetisation value only, independent of the Y content. It is further shown that the high-field behaviour of resistivity depends on the increased spin disorder resulting directly from the presence of yttrium.

Differential thermal analysis is used to measure the temperature change (ΔT) due to the application of an external magnetic field in a manganite with phase separation. The temperature dependence of ΔT is compared with the expected from magnetization measurements. A phenomenological model for the enthalpy change is used to obtain ΔT during the metamagnetic transition.

In the present work we study the magnetostriction of Fe 91 Sn 9 and Fe 80 Sn 20 polycrystalline samples produced by arc melting and heat treated at temperatures of 1153 K for 6 h and 1023 K for 24 h, looking for high values of magnetostriction as in Fe–Ga alloys. Magnetostriction, as well as saturation magnetization measurements, was carried out at temperatures close to 203 K in the magnetic field interval 0 to 1.5 T. Results of magnetostriction on sample Fe 91 Sn 9 , which has almost pure α-phase, show magnitude and behavior similar to pure Fe. The two additional Fe 80 Sn 20 samples have a combination of α-phase plus either Fe 5 Sn 3 or Fe 3 Sn 2 and show a peculiar behavior of the magnetostriction. For μ 0 H μ 0 H >0.3 T, the magnetostriction starts again to grow linearly with the field, but saturation was not observed up to 5 T. This behavior was attributed to the presence of Fe 5 Sn 3 or Fe 3 Sn 2 phases in these samples that are also ferromagnetic as the α-phase is.

We have studied experimentally the electrical conductivity and specific heat near the superconducting transition of granular samples of YBa 2 Cu 3 O 7− δ , YBa 2 (Cu 2.98 Zn 0.02 )O 7− δ and GdBa 2 Cu 3 O 7− δ . The results show that the transition proceeds in two stages. Careful analysis of the conductivity in the regime of approach to the zero resistance state reveals the occurrence of a coherence transition, which is related to the connective nature of the granular samples. This transition occurs when the fluctuating phases of the order parameter in individual grains become long-range ordered. We obtain the exponent for fluctuation conductivity and the relevant critical temperature, T co , which is close to the point where resistivity vanishes. The specific heat results, when analyzed as d C /d T , show a weak but reproducible cusp-like anomaly at T co . This finding gives strong support to the interpretation of the coherence transition as a genuine critical phenomenon.

Magnetization measurements in La0.225Pr0.40Ca0.375MnO3 allowed us to construct an H–T phase diagram of this prototype phase-separated manganite. Magnetic relaxation results clearly demonstrate the dynamic nature of the phase-separated state. Our results showed the existence of large relaxation effects, and the relative fraction of the coexisting phases rapidly changes as a function of time. Through a thermodynamic phenomenological model, it is possible to interpret the experimental results, both static and dynamic, of the phase-separated system. The phase diagram resulting from this model is presented, including the static and dynamic properties characteristics of the phase-separated state in manganites.

We present an experimental method based on magnetoresistance data to evaluate the lower critical field H c1 in the ruthenate–cuprate RuSr 2 Gd 1.5 Ce 0.5 Cu 2 O 10− δ (Ru-1222), in which superconducting order and a ferromagnetic component coexist. The method is based on the effects of the flux trapped in the grains on the intergranular junctions. In contrast to the behavior of typical high-temperature superconductors, H c1 monotonically increases on cooling in Ru-1222. At T / T C = 0.2, H c1 = 3.0 kOe, a value nearly four times larger in comparison to YBa 2 Cu 3 O 7 . Previously reported rough assessments greatly underestimate this parameter, due to the strong Ru-ferromagnetic background. Our results can be understood in terms of a model based on the theory of superconducting/ferromagnetic multilayers.

Detailed specific heat measurements from 4.5 to 300 K are reported on a series of polycrystalline La1−xCaxMnO3−δ samples, with x=0.10, 0.33 and 0.62. The samples were also investigated using electrical transport, magnetic measurements and low temperature NMR. The x=0.10 composition does not show a well defined transition in the specific heat at the ferromagnetic ordering temperature, but the low temperature specific heat shows a ferromagnetic T3/2 spin wave term, and no electronic linear term as expected for an insulator. The x=0.33 composition undergoes a transition to a ferromagnetic metal with a well defined peak in the specific heat, suggestive of an underlying first order transition at Tc; at low temperatures a linear electronic specific heat term is observed but no spin wave ferromagnetic term. High temperature specific heat data on the x=0.62 sample shows only one anomaly, suggesting simultaneous onset of charge ordering and antiferromagnetism. Low temperature data indicates that the magnetic phase is an A-type antiferromagnetic insulator with no electronic contribution. The NMR data at 4.2 K on this compound shows a motionally narrowed peak indicating short range ferromagnetic interactions or possibly magnetic polarons at 4.2 K.

We report a detailed study of the electric transport and magnetic properties of the LaNdCaMnO manganite system. Substitution of LaIII by smaller NdIII ions, reduces the mean ionic radius of the A site ion. We have studied samples in the entire range between rich La and rich Nd compounds. Results of DC magnetization and resistivity show that doping destabilize the FM character of the pure La compound and triggers the formation of a phase separated state at intermediate doping. We have also found evidence of a dynamical behaviour within the phase separated state. A phase diagram is constructed, summarizing the effect of chemical substitution on the system., Comment: 15 pages, 8 figures

Structural, magnetic, calorimetric and magnetoresistive studies on the solid solution Ce(Ni 1- x Co x ) 2 B 2 C demonstrate that the Co-substitution decreases the unit-cell volume, the Sommerfeld constant γ, and the Pauli susceptibility χ 0 . It increases the Debye temperature and the strength of the crystalline electric field. The structural modification can be related to the variation of the tetrahedral angle subtended by the B-(Ni, Co)-B bonds: a decrease in the tetrahedral angle contracts the parameter a and elongates the length c and vice versa. The variation in the magnetic and electronic properties can be traced down to the shift of E F (predominantly due to the less filling of the 3d-band) away from the peak maximum of the density of state and the consequent reduction of N ( E F ). The influence of the variation of the band filling and the f-d hybridization on the magnetic behavior of CeT 2 B 2 C will be considered.

We report on low-temperature thermal expansion and specific heat measurements in a series of non-random composites YBCO/Ag x wt% (x = 0, 6, 11 and 23). A reference YBCO polycrystalline sample is also investigated. At the lowest temperatures, the expansivities and specific heat are analysed as sums of a extrinsic T-linear term and a -Debye contribution. The Gruneisen parameters are determined and shown to be almost constant for . The values of the Gruneisen parameters increase with the silver content in the composites.

The present work reports the procedure used for the production and characterization of Nb–Ti superconductor wires with Cu as artificial pinning centers (APC) for two different pin geometries: island and barrier. Cold swaging, bundling, drawing processing in addition to controlled heat treatments were used to reduce the pinning centers size. Superconducting properties characterization was made by means of transport and magnetization measurements. We obtained J c values at 4.2 K of 124 A/mm 2 (island) and 101 A/mm 2 (barrier) at 5 T.

We report on measurements of the resistive transition in several samples of polycrystalline YBa 2 Cu 3 O 7− y and Bi 2 Sr 2 Ca 2 Cu 3 O 10+ x , and single crystal Bi 2 Sr 2 CaCu 2 O 8+ x . The results are analyzed in terms of the temperature derivative of the resistivity, d ϱ /d T . In the ceramic samples, we systematically find that the transition proceeds as a two-stage process. Initially, superconductivity establishes in some disconnected regions of the homogeneous phase. Long range ordering is obtained at lower temperatures as a consequence of a paracoherent-coherent transition involving the granular array. A new, reproducible and general feature is also revealed in our results. The principal maximum of d ϱ /d T , which denotes the intragranular transition, shows a two-peak splitting. This characteristic, which is observed in all investigated samples strongly suggests the occurrence of a double bulk superconducting transition, which is in itself a signature of unconventional order parameter symmetry.

We report on magnetic irreversibility and Meissner flux expulsion data of YBa2Cu3O7−δ/Ag nonrandom composites, having 0, 6, 11 and 23 wt.% Ag. While Tc remains practically unaltered, we observed a strong decrease in the fractional flux expulsion upon silver addition. On the contrary, the ZFC-FC magnetic irreversibility line scarcely changes in the composites when compared to that of the pure compound. This indicates that the irreversibility line depends only weakly on the granularity at the crystallite level.

We report a heat dynamics analysis of the electrocaloric effect (ECE) in commercial multilayer capacitors based on BaTiO3 dielectric, a promising candidate for applications as a solid state cooling device. Direct measurements of the time evolution of the sample's temperature changes under different applied voltages allow us to decouple the contributions from Joule heating and from the ECE. Heat balance equations were used to model the thermal coupling between different parts of the system. Fingerprints of Joule heating and the ECE could be resolved at different time scales. We argue that Joule heating and the thermal coupling of the device to the environment must be carefully taken in to account in future developments of refrigeration technologies employing the ECE., Comment: Acepted to be published in Applied Phys. Letters (2011)