Magnetic Peak Research Articles

Magneto-structural coupling through bidirectionally controlling the valence electron concentration in MnCoGe alloy

In this work, bidirectionally controlling the valence electron concentration (e/a) is proposed to tune and realize the coincident magneto-structural transformation in MnCoGe alloy.Withthecommoneffectofe/a and ‘chemical pressure’ produced by the change of chemical composition, the martensitically magneto-structural transformation is achieved from paramagnetic Ni2In-type austenite to ferromagnetic TiNiSi-type martensite in both Mn1−xCoGe1+x and Mn1+yCoGe1−y systems under the same experimental capabilities. It is interesting that a magnetic-field-induced metamagnetic martensitic transformation is observed in Mn1−xCoGe1+x system. Accordingly, large magnetic entropy change peak values of ~31.6 and ~20.1 J kg−1 K−1 are obtained under the applied field change of μ0ΔH = 0–5 T at 276 and 294 K for Mn0.98CoGe1.02 and Mn0.985CoGe1.015 alloys, respectively. Likewise, a moderate magnetic entropy change ~9.13 J kg−1 K−1 is acquired under μ0ΔH = 0–5 T around 320 K for Mn1.03CoGe0.97 alloy. Bidirectionally controlling a physical variable may pave a new and effective way in designing and optimizing compositions to simultaneously realize the magneto-structural transformation in both directions in MnCoGe and other systems.

Itinerancy-dependent noncollinear spin textures in SrFeO3, CaFeO3 , and CaFeO3/SrFeO3 heterostructures probed via resonant x-ray scattering

Non-collinear, multi-q spin textures can give rise to exotic, topologically protected spin structures such as skyrmions, but the reason for their formation over simple single-q structures is not well understood. While lattice frustration and the Dzyaloshinskii-Moriya interaction are known to produce non-collinear spin textures, the role of electron itinerancy in multi-q formation is much less studied. Here we investigated the non-collinear, helical spin structures in epitaxial films of the perovskite oxides SrFeO3 and CaFeO3 using magnetotransport and resonant soft x-ray magnetic diffraction. Metallic SrFeO3 exhibits features in its magnetoresistance that are consistent with its recently proposed multi-q structure. Additionally, the magnetic Bragg peak of SrFeO3 measured at the Fe L edge resonance energy asymmetrically broadens with decreasing temperature in its multi-q state. In contrast, insulating CaFeO3 has a symmetric scattering peak with an intensity 10x weaker than SrFeO3. Enhanced magnetic scattering at O K edge prepeak energies demonstrates the role of a negative charge transfer energy and the resulting oxygen ligand holes in the magnetic ordering of these ferrates. By measuring magnetic diffraction of CaFeO3/SrFeO3 superlattices with thick CaFeO3 layers, we find that the CaFeO3 helical ordering is coherent across 1 unit cell-thick SrFeO3 layers but not 6 unit cell-thick layers. We conclude that insulating CaFeO3 supports only a simple single-q helical structure in contrast to metallic SrFeO3 that hosts multi-q structures. Our results provide important insight into the role of electron itinerancy in the formation of multi-q spin structures.

Search for pressure-induced tricriticality in Cr

The antiferromagnetic ordering of chromium has long been known for its peculiar physical properties. One of them is the observation of the weak first-order character of the N'eel transition that is explained by the lack of a stable fixed point by Bak and Mukamel. Barak et al. predicted that by lowering the symmetry of the order parameter by the application of uniaxial pressure along the [110] direction changes the N'eel transition to second-order. In a previous experiment by Fawcett et al., irreversible broadening of the N\'eel transition was already observed between 160 bar and 300 bar, which could, however, be caused by plastic deformation. Using an improved setup with reduced stress inhomogeneities we succeeded to increase the pressure range until irreversible broadening is observed above 450 bar. Despite the observed tripling of the intensity of the magnetic Bragg peak $[0, 0, 1-\delta]$ at $p_{[110]}\geq450\,$bar, indicating a single $\bf{\text{Q}}_\pm$-domain state of the sample, no hints for a tricritical point were observed.

Research on the comprehensive performance of PrFeB magnets for synchrotron radiation and free electron laser

PrFeB magnets, which possess excellent magnetic properties at low temperatures, have important application value as cryogenic permanent magnet undulators for synchrotron radiation sources and free electron lasers. In this research study, several high-performance PrFeB magnets (P42H, P48SH, and P48UH) were prepared, and their performance was comprehensively examined, including evaluations of their magnetic properties, microstructures, uniformity, and stability. Next, their application prospects were analyzed and discussed. In China, the first cryogenic permanent magnet undulators (CPMUs) with P48SH magnets with 18 mm cycle lengths have been developed. When the temperature is 80 K and the gap is 6.0 mm, the magnetic field measurement results of the CPMU showed that the effective magnetic field peak was approximately 0.92 T, yielding an increase of 11.76% relative to operation at 300 K, with an RMS phase error of about 4.99°.

Electromagnetic Fields Associated With the M‐Component Mode of Charge Transfer

AbstractIn upward flashes, charge transfer to ground largely takes place during the initial continuous current (ICC) and its superimposed pulses (ICC pulses). ICC pulses can be associated with either M‐component or leader/return‐stroke‐like modes of charge transfer to ground. In the latter case, the downward leader/return stroke process is believed to take place in a decayed branch or a newly created channel connected to the ICC‐carrying channel at relatively short distance from the tower top, resulting in the so‐called mixed mode of charge transfer to ground. In this paper, we study the electromagnetic fields associated with the M‐component charge transfer mode using simultaneous records of electric fields and currents associated with upward flashes initiated from the Säntis Tower. The effect of the mountainous terrain on the propagation of electromagnetic fields associated with the M‐component charge transfer mode (including classical M‐component pulses and M‐component‐type pulses superimposed on the initial continuous current) is analyzed and compared with its effect on the fields associated with the return stroke (occurring after the extinction of the ICC) and mixed charge transfer modes. For the analysis, we use a 2‐Dimentional Finite‐Difference Time Domain method, in which the M‐component is modeled by the superposition of a downward current wave and an upward current wave resulting from the reflection at the bottom of the lightning channel (Rakov et al., 1995, https://doi.org/10.1029/95JD01924 model) and the return stroke and mixed mode are modeled adopting the MTLE (Modified Transmission Line with Exponential Current Decay with Height) model. The finite ground conductivity and the mountainous propagation terrain between the Säntis Tower and the field sensor located 15 km away at Herisau are taken into account. The effects of the mountainous path on the electromagnetic fields are examined for classical M‐component and M‐component‐type ICC pulses. Use is made of the propagation factors defined as the ratio of the electric or magnetic field peak evaluated along the mountainous terrain to the field peak evaluated for a flat terrain. The velocity of the M‐component pulse is found to have a significant effect on the risetime of the electromagnetic fields. A faster traveling wave speed results in larger peaks for the magnetic field. However, the peak of the electric field appears to be insensitive to the M‐component wave speed. This can be explained by the fact that at 15 km, the electric field is still dominated by the static component, which mainly depends on the overall transferred charge. The contribution of the radiation component to the M‐component fields at 100 km accounts for about 77% of the peak electric field and 81% of the peak magnetic field, considerably lower compared to the contribution of the radiation component to the return stroke fields at the same distance. The simulation results show that neither the electric nor the magnetic field propagation factors are very sensitive to the risetimes of the current pulses. However, the results indicate a high variability of the propagation factors as a function of the branch‐to‐channel junction point height. For junction point heights of about 1 km, the propagation factors reach a value of about 1.6 for the E‐field and 1.9 for the H‐field. For a junction height greater than 6 km, the E‐field factor becomes slightly lower than 1. The obtained results are consistent with the findings of Li, Azadifar, Rachidi, Rubinstein, Paolone, et al. (2016, https://doi.org/10.1109/TEMC.2015.2483018) in which an electric field propagation factor of 1.8 was inferred for return strokes and mixed‐mode pulses, considering that junction points lower than 1 km or so would result in a mixed mode of charge transfer, in which a downward leader/return‐stroke‐like process is believed to take place. It is also found that the field enhancement (propagation factor) for return stroke mode is higher for larger ground conductivities. Furthermore, the enhancement effect tends to decrease with increasing current risetime, except for very short risetimes (less than 2.5 μs or so) for which the tendency reverses. Finally, model‐predicted fields associated with different charge transfer modes, namely, return stroke, mixed‐mode, classical M‐component, and M‐component‐type ICC pulse are compared with experimental observations at the Säntis Tower. It is found that the vertical electric field waveforms computed considering the mountainous terrain are in very good agreement with the observed data. The adopted parameters of the models that provide the best match with the measured field waveforms were consistent with observations. The values for the current decay height constant adopted in the return stroke and mixed‐mode models (1.0 km for the return stroke and 0.8 km for the mixed‐mode pulse) are lower than the value of 2.0 km typically used in the literature.

Spin ordering and physical properties of NaPrFeWO6 and NaSmFeWO6 with polar double perovskite structure

NaPrFeWO6 and NaSmFeWO6 have been synthesized as polycrystalline powders by means of a sol-gel route. Both samples adopt a noncentrosymmetric structure derived from the simple perovskite ABO3 and characterized by a layered ordering of A-site atoms (Na and rare earth) and a rock-salt ordering of B-site atoms (Fe and W) as deduced from x-ray and neutron powder diffraction techniques. Magnetization measurements revealed antiferromagnetic ordering below TN ≈23 K in both compounds but weak spontaneous magnetization is observed in the hysteresis loops of the samples above and below TN revealing a parasitic ferromagnetic component that comes from minor ferromagnetic impurities. The magnetic structure of NaPrFeWO6 is formed by two antiferromagnetic sublattices (rare earth and Fe) that follow the same propagation vector k = (½, 0, ½). The magnetic structure is purely collinear with the moments oriented along the b-axis. It presents competitive magnetic interactions and reduced ordered moments at low temperatures. The rare earth moments in the zig-zag chains running along a (parallel to the ab plane) are ordered following a sequence typical of an E-type arrangement (up-up-down-down). Each Fe2+ atom has four nearest Pr3+ neighbors and it is coupled antiferromagnetically to three of them and ferromagnetically to the fourth one. Strong neutron absorption from Sm atoms hinders obtaining reliable neutron patterns of NaSmFeWO6 for Rietveld analysis but the comparison of neutron patterns above and below TN detected the same magnetic peaks indicating a similar magnetic structure. Dielectric measurements are compatible with a paraelectric system with a strong Maxwell-Wagner contribution from depletion layers and electrical properties are dominated by leakage currents.

Two-dimensional ordering and collective magnetic excitations in the dilute ferromagnetic topological insulator (Bi0.95Mn0.05)2Te3

Employing elastic and inelastic neutron scattering (INS) techniques, we report on detailed microscopic properties of the ferromagnetism in he magnetic topological insulator (Bi$_{0.95}$Mn$_{0.05}$)$_{2}$Te$_{3}$. Neutron diffraction of polycrystalline samples show the ferromagnetic (FM) ordering is long-range within the basal plane, and mainly 2D in character with short-range correlations between layers below $T_{\mathrm{C}} \approx 13$ K. Despite the random distribution of the dliute Mn atoms, we find that the 2D-like magnetic peaks are commensurate with the chemical structure, and the absence of (00L) magnetic peaks denote that the Mn$^{2+}$ magnetic moments are normal to the basal planes. Surprisingly, we observed collective magnetic excitations, in this dilute magnetic system. Despite the dilute nature, the excitations are typical of quasi-2D FM systems, albeit are severely broadened at short wavelengths, likely due to the random spatial distribution of Mn atoms in the Bi planes. Detailed analysis of the INS provide energy scales of the exchange couplings and the single ion anisotropy.

Evidence for cluster spin glass phase with precursor short-range antiferromagnetic correlations in the B -site disordered Ca(Fe1/2Nb1/2)O3 perovskite

The origin of the spin glass (SG) phase in the well-known multiferroic $\mathrm{Pb}(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ compound remains controversial due to the complications introduced by the coexistence of SG and long-range ordered (LRO) antiferromagnetic (AFM) phases. We have addressed this controversy through a comprehensive study on a Pb-free system $\mathrm{Ca}(\mathrm{F}{\mathrm{e}}_{1/2}\mathrm{N}{\mathrm{b}}_{1/2}){\mathrm{O}}_{3}$ (CFN) which does not exhibit LRO AFM transition. The SG transition in CFN is confirmed by the appearance of a cusp in the temperature dependence of dc magnetization $M(T)$ with a SG freezing temperature ${T}_{\mathrm{f}}\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and bifurcation of the zero-field-cooled and field-cooled magnetization $M(T)$ below the irreversibility temperature ${T}_{\mathrm{irr}}\ensuremath{\sim}27\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Using ac susceptibility $[\ensuremath{\chi}(\ensuremath{\omega},T)]$ measurements, we show that the spin dynamics follows power/Vogel-Fulcher law-type critical dynamics which diverges at ${T}_{\mathrm{SG}}\ensuremath{\sim}24\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with an attempt time ${\ensuremath{\tau}}_{\mathrm{o}}\ensuremath{\sim}{10}^{\ensuremath{-}6}\phantom{\rule{0.28em}{0ex}}\mathrm{s}$ suggesting cluster spin glass (CSG) behavior. The field dependence of ${T}_{\mathrm{f}}(H)$ and ${T}_{\mathrm{irr}}(H)$ is shown to follow the de Almeida--Thouless line which separates the ergodic and nonergodic phases in the $H\ensuremath{-}T$ plane and gives ${T}_{\mathrm{f}}(H=0)\ensuremath{\sim}25\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, which is in close agreement with ${T}_{\mathrm{SG}}$ obtained from $\ensuremath{\chi}(\ensuremath{\omega},T)$. The existence of the glassy phase below ${T}_{\mathrm{SG}}$ is further confirmed by the observation of slow nonexponential decay of thermoremanent magnetization with time, memory and rejuvenation effects, and unidirectional exchange-bias effect in the $M\ensuremath{-}H$ hysteresis loop of field-cooled samples. The neutron powder-diffraction patterns reveal the absence of any magnetic peak due to LRO AFM phase but show a broad diffuse peak due to the presence of $\ensuremath{\sim}2\ensuremath{-}\mathrm{nm}$-size AFM spin clusters which are responsible for the CSG freezing in CFN.

Dielectric relaxation and magnetic resonance in microwave absorption performance of CoxFe3-xO4 (0≤x≤1) nanocrystals

Cobalt ferrites (Co x Fe 3-x O 4 , 0 ≤ x ≤ 1) may possess large magnetocrystalline anisotropy and coercivity at certain cobalt/iron (Co/Fe) ratios, while further explorations on their microwave absorption mechanisms are not adequate so far. In this study, a series of Co x Fe 3–x O 4 nanocrystals were synthesized by a developed oxidation-precipitation method, and a combination of dielectric relaxation and magnetic resonance was revealed in electromagnetic studies. Dielectric relaxation peaks were originated from orientation polarization and affected by oxygen vacancy densities. Magnetic resonance peaks were shifted to higher frequency due to the increased magnetocrystalline anisotropy at higher Co/Fe ratios. The excellent microwave absorption performance of as-prepared Co x Fe 3–x O 4 were also obtained, which should be attributed to the electromagnetic matching of dielectric relaxation and magnetic resonance at higher frequency ranges.

Synthesis, Structure, and Compositional Tuning of the Layered Oxide Tellurides Sr2MnO2Cu2- xTe2 and Sr2CoO2Cu2Te2.

The synthesis and structure of two new transition metal oxide tellurides, Sr2MnO2Cu1.82(2)Te2 and Sr2CoO2Cu2Te2, are reported. Sr2CoO2Cu2Te2 with the purely divalent Co2+ ion in the oxide layers has magnetic ordering based on antiferromagnetic interactions between nearest neighbors and appears to be inert to attempted topotactic oxidation by partial removal of the Cu ions. In contrast, the Mn analogue with the more oxidizable transition metal ion has a 9(1)% Cu deficiency in the telluride layer when synthesized at high temperatures, corresponding to a Mn oxidation state of +2.18(2), and neutron powder diffraction revealed the presence of a sole highly asymmetric Warren-type magnetic peak, characteristic of magnetic ordering that is highly two-dimensional and not fully developed over a long range. Topotactic oxidation by the chemical deintercalation of further copper using a solution of I2 in acetonitrile offers control over the Mn oxidation state and, hence, the magnetic ordering: oxidation yielded Sr2MnO2Cu1.58(2)Te2 (Mn oxidation state of +2.42(2)) in which ferromagnetic interactions between Mn ions result from Mn2+/3+ mixed valence, resulting in a long-range-ordered A-type antiferromagnet with ferromagnetic MnO2 layers coupled antiferromagnetically.

Comprehensive study of the magnetic phase transitions in Tb3Co combining thermal, magnetic and neutron diffraction measurements

A comprehensive study of the magnetic phase transitions in Tb3Co has been undertaken combining different techniques. Using single crystal neutron diffraction in the paramagnetic state a weak crystal structure distortion from the room temperature orthorhombic structure of the Fe3C type described with the Pnma space group toward structure with lower symmetry has been observed with cooling below 100 K. At 81 K there is a second order phase transition to an antiferromagnetic incommensurate phase with the propagation vector k = (0.155, 0, 0). As derived from thermal diffusivity measurements, the critical exponents for this transition are very close to the 3D-Heisenberg universality class, proving that the magnetic interactions are short-range but with a deviation from perfect isotropy due to crystal field effects. At T2 ≈ 70 K there is another magnetic phase transition to a ferromagnetic state whose character is shown to be weakly first order. The low temperature magnetic state has a non-coplanar ferromagnetic structure with strong ferromagnetic components of Tb magnetic moments along the crystallographic c-axis. The application of an external magnetic field B = 2 T along the c crystallographic axis suppresses the incommensurate antiferromagnetic phase and gives rise to the ferromagnetic phase. The magnetic entropy peak change as well as the refrigerant capacity indicate that Tb3Co is a competitive magnetocaloric material in this temperature range.

Non-Kitaev spin liquids in Kitaev materials

We point out that the Kitaev materials may not necessarily support Kitaev spin liquid. It is well-known that having a Kitaev term in the spin interaction is not the sufficient condition for the Kitaev spin liquid ground state. Many other spin liquids may be stabilized by the competing spin interactions of the systems. We thus explore the possibilities of non-Kitaev spin liquids in the honeycomb Kitaev materials. We carry out a systematic classification of gapped $\mathbb{Z}_2$ spin liquids using the Schwinger boson representation for the spin variables. The presence of strong spin-orbit coupling in the Kitaev materials brings new ingredients into the projective symmetry group classification of the non-Kitaev spin liquid. We predict the spectroscopic properties of these gapped non-Kitaev spin liquids. Moreover, among the gapped spin liquids that we discover, we identify the spin liquid whose spinon condensation leads to the magnetic Bragg peak structure of the zig-zag magnetic order that was observed in Na$_2$IrO$_3$ and $\alpha$-RuCl$_3$. We further discuss the possibility of gapped $\mathbb{Z}_2$ spin liquid and the deconfined quantum criticality from the zig-zag magnetic order to spin dimerization in pressurized $\alpha$-RuCl$_3$.

First-order pinning interaction in type-II superconductors with Ginzburg–Landau descriptions

The spontaneous magneto-elastic (ME) interaction dictates the coupling between vortex arrays and crystal lattice, triggered by the spontaneous vortex nucleation in mixed state. In Ginzburg–Landau theory, the pinning interaction is ascribed to the interaction energy of first-order defect strain field and spontaneous elastic field. The pinning force is therefore estimated as the force on a dislocation imposed by the spontaneous stress field. Subsequently, the first-order pinning is compared with the weak collective pinning quantitively. The magnetization contribution by the first-order pinning is a parabolic dependence with field H , while the magnetization of the collective pinning shows a severe suppression at high fields as H −3 . The difference in magnetization is attributed to the energy characterization in pinning mechanisms. As for the critical current density, the combination of first-order and collective pinning generally reproduces the experimental data of Ba0.75 K0.25 Fe2 As2 sample where the second magnetization peak (SMP) disappears. In this view, the elasticity effect in mixed state is sufficient to cause the vanishment of SMP, especially in samples with high pressure dependence of T c .

Second Magnetization Peak Effect in a Fe(Se,Te) iron based superconductor

The iron based superconductor FeSe0.5Te0.5 has been investigated by means of DC magnetic measurements as a function of magnetic field (H). By considering the superconducting m(H) hysteresis loops at different temperatures, the sample shows a strong superconducting signal together with the presence of a peak effect that causes an anomalous increase in the field dependence of the critical current density Jc(H). The presence of the peak effect has been studied by means of the Jc(T) obtained at different magnetic fields starting from the Jc(H) curves. The analysis of the Jc(T) curves shows that the peak effect is due to a crossover from a weak pinning regime to a strong pinning regime.

Temperature effect on the magnetic oscillations in 2D materials

We study the magnetic oscillations (MO) in 2D materials with a buckled honeycomb lattice, considering a perpendicular electric and magnetic field. At zero temperature the MO consist of the sum of four sawtooth oscillations, with two unique frequencies and phases. The values of these frequencies depend on the Fermi energy and electric field, which in turn determine the condition for a beating phenomenon in the MO. We analyse the temperature effect in the MO by considering its local corrections over each magnetization peak, given by Fermi–Dirac like functions. We show that the width of these functions is related to the minimum temperature necessary to observe the spin and valley properties in the MO. In particular, we find that in order to observe the spin splitting, the width must be lower than the MO phase difference. Likewise, in order to observe valley mixing effects, the width must be lower than the MO period. We also show that at high temperatures, all the maxima and minima in the MO shift to a constant value, in which case we obtain a simple expression for the MO and its envelope. The results obtained show unique features in the MO in 2D materials, given by the interplay between the valley and spin.

Cubic B20 helimagnets with quenched disorder in magnetic field

We theoretically address the problem of cubic B20 helimagnets with a small concentration $c\ensuremath{\ll}1$ of defect bonds in external magnetic field $\mathbf{H}$, which is relevant to mixed B20 compounds at small dopant concentrations. We assume that the Dzyaloshinskii-Moriya interaction and the exchange coupling constant are changed on imperfect bonds which leads to distortion of the conical spiral ordering. In a one-impurity problem we find that the distortion of the spiral pitch is long ranged and it is governed by the Poisson equation for an electric dipole. The variation of the cone angle is described by the screened Poisson equation for two electric charges with the screening length being of the order of the spiral period. We calculate corrections to the spiral vector and to the cone angle at finite $c$. The correction to the spiral vector is shown to be independent of $H$. We demonstrate that diffuse neutron scattering caused by disorder appears in the elastic cross section as power-law decaying tails centered at magnetic Bragg peaks.

Spin ordering induced by lattice distortions in classical Heisenberg antiferromagnets on the breathing pyrochlore lattice

We theoretically investigate effects of lattice distortions on the spin ordering of classical Heisenberg antiferromagnets on the breathing pyrochlore lattice. In the model, local lattice distortions originating from the site vibration are taken into account to yield effective spin interactions via the spin-lattice coupling (SLC). The breathing lattice-alternation is characterized by the ratio of the nearest-neighbor interaction for large tetrahedra to that for small ones, J'/J. It is found by Monte Carlo simulations that the system exhibits a first-order transition into four different types of collinear magnetic ordered states. In the uniform case (J'/J=1), the state realized at stronger SLC is cubic-symmetric characterized by the (1/2,1/2,1/2) magnetic Bragg peaks, while the one at weaker SLC is tetragonal-symmetric characterized by the (1,1,0) ones. With increasing the breathing distortion (decreasing J'/J), the ordering pattern of the (1/2,1/2,1/2) state becomes non-cubic with its magnetic Bragg reflections almost unchanged, while the (1,1,0) state is robust. The non-cubic state peculiar to the breathing pyrochlores is further categorized into two types. We demonstrate that these two non-cubic orderings result in the massive degeneracy of the ground state and that the associated residual entropy per spin takes characteristic values of k_B ln(4)/16 and k_B ln(6)/16. Experimental implications of the results are discussed.

Pinning energy and anisotropy properties of a Fe(Se, Te) iron based superconductor

The measurements of DC magnetization M as a function of magnetic field (H) and time (t) have been performed in order to study the superconducting and pinning properties of a Fe(Se, Te) iron based superconductor fabricated by means of the Bridgman technique. By performing the superconducting hysteresis loops M(H) at different temperatures in the case of perpendicular and parallel field, the critical current density Jc(H) has been extracted in the framework of the Bean critical state model for both configurations. The Jc(H) curves have shown the presence of the second magnetization peak effect that causes an anomalous increase in the field dependence of the critical current density. In order to obtain the Jc anisotropy of the sample, we have performed the ratio between perpendicular and parallel critical current density values and compared its values with the literature ones. The information regarding the pinning energy U have been extracted by means of the relaxation of the irreversible magnetization M(t) in the case H∣∣c. In particular, performing relaxation measurements at different temperatures and magnetic fields, the temperature dependence of the pinning energy U(T) at different magnetic fields has been obtained showing an anomalous temperature scaling of the curves. The presence of a maximum in the U(T) curves suggests a pinning crossover at a given field and temperature Hcr(T). The Hcr(T) values have been fitted with the equation Hcr(T) = Hcr(0) (1 − T/T*)n whose results confirm the correlation between the elastic/plastic crossover and the end of the peak effect phenomenon.

Evolution of the Magnetization Peak–Hill Features in La0.15Sm0.85MnO3+δ Near to Transition Temperature to the Superconducting State, TC ≈ 41 K, Caused by Increasing of an External Magnetic Field

Evolution of the Magnetization Peak–Hill Features in La0.15Sm0.85MnO3+δ Near to Transition Temperature to the Superconducting State, TC ≈ 41 K, Caused by Increasing of an External Magnetic Field

Surface barriers and field direction dependent vortex phase diagrams of YBa2Cu3−xAlxOδ for H c

We report the observation of an unusual field direction dependent second magnetisation peak (SMP) anomaly at very low fields, close to the field of full penetration Hp, in an aluminium-doped single crystal of YBa2Cu3−xAlxOδ, for H c. Such an anomaly is observed at different field values in the field ramp-up direction (H) compared to the field-down direction (H), and it is also found to persist to different temperatures. For the field-up direction, it is observed in the range of 8 K to 30 K (upper range extendible to 40 K depending on the crystal), while for the field-down direction, it is observed between 8 K to 65 K (upper range extendible to 70 K depending on the crystal). The temperature dependence of the fields of full penetration Hp, the onset field of the SMP anomaly, as well as the central magnetisation peak, seem to be affected by surface barriers at low temperatures, where they are seen to rise anomalously. An exponential fit to the low temperature penetration field and central magnetisation peak width, as well as an asymmetric magnetisation towards the irreversibility field, illustrate the effect of surface barriers at low temperatures. At higher temperatures, the onset field of the SMP anomaly is described well by a dislocation mediated order–disorder transition. The temperature dependence of the peak field of the second magnetisation anomaly, however, does not seem to be affected by the surface barrier and is well described by a power law. An oxygen deficient aluminium cluster model is used to explain the differences in the SMP anomalies in the two quadrants. Two vortex phase diagrams, one for each field direction, have been made describing the various phases.