Dc Magnetization Curves Research Articles

Characterization of Field Penetration in Superconducting Multilayers Samples

The best rf accelerating cavities are made of bulk niobium. This technology is now close to it full development, with any further improvement limited by the superconducting properties of Nb itself. In the best performing cavities the rf equatorial magnetic field H is close to the Nb thermodynamic critical field (HC ≈ 200 mT). In 2006 Gurevich proposed the use of nanoscale layers of superconducting materials having HC higher than Nb. This multilayer structure is expected to shield bulk niobium and therefore to increase the cavity breakdown field. In order to explore this pioneering idea, we have studied the penetration of the magnetic field by mean of nanometric sized multi layer structures deposited on flat single crystal substrates. We have deposited high quality model samples by dc magnetron reactive sputtering on R-plane cut sapphire substrates. A 250 nm layer of niobium represents the bulk material as in rf cavities. Such Nb layers were coated with a single or multiple stacks of NbN layers (25 nm or 12 nm) separated by 15 nm MgO barriers, and characterized by X-ray reflectivity and dc transport measurements, dc magnetization curves have been measured by conventional super conducting quantum interference device (SQUID) to determine the first penetration field BC1. For comparison, BC1 was also mea sured with a local probe method based on 3rd harmonic analysis. The Nb samples coated with NbN multi-layers clearly exhibit a higher first penetration field.

Calibration of ac and dc magnetometers with a Dy2O3 standard

The ac susceptibility and magnetization curves of a glued Dy(2)O(3) powder sample are measured by an ac susceptometer and a dc superconducting quantum interference device magnetometer, both of which have been calibrated previously. It is shown that the magnetic moment of the paramagnetic sample as a function of field and temperature may be accurately expressed by a combination of the Curie-Weiss law and the Langevin function at T > 45 K with three adjusting parameters, so that the dc magnetization curves and the magnitude and phase of ac susceptibility at different values of dc bias field measured by any magnetometer can be calibrated by using Dy(2)O(3) as a standard. The expressions are empirical and cannot be justified in the entire field and temperature range by existing theories of paramagnetism. Below 10 K, indication of approaching a possible phase transition is found. It is shown that pure Dy(2)O(3) powder may be used as a primary standard, with susceptibility [13.28(T + 17)](-1) emu/Oe/g at T > 50 K and H < 10 kOe, in consistency with the Curie-Weiss law and the quantum mechanical theory of paramagnetism.

Magnetic anisotropy and spin-glass behavior in single crystallineU2PdSi3

We present the magnetic and transport properties of single crystallineU2PdSi3 measured with themagnetic field (H) (ormeasuring current, I) applied along two typical crystallographic directions, i.e. -axis and -axis (or -axis and -axis). For both directions, a spin-glass state is confirmed toform at low temperature with the same spin freezing temperatureTf (= 11.5 K), initialfrequency shift δTf (= 0.023)and activation energy Ea/kB (= 90.15 K) in zero dc field. Strong anisotropy in magnetic and transport behavior is found to be a significantfeature of U2PdSi3. The unusual ferromagnetic-like anomaly in ac susceptibility and dc magnetization curves aroundTm = 71 K is observed in the case of -axis but not in the cases of -axis. The characteristic temperatureTir, below which evident irreversible magnetism originated from random spin freezing can beobserved, shows much stronger field dependence for -axis than for -axis. Moreover, an unusual finding is that the electrical resistivity measurements indicatethe formation of magnetic Brillouin-zone boundary gaps and much larger magneticscattering for -axis, while the coherent-Kondo-effect-like behavior is obvious for -axis. We also emphasize that no resistivity minimum can be detected down to 2.5 K for eitherdirection. The observed magnetic and transport behaviors are compared with those in polycrystallineU2PdSi3 and other 2:1:3 intermetallic compounds.

Characterization of superconducting nanometric multilayer samples for superconducting rf applications: First evidence of magnetic screening effect

The best rf bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field $H\ensuremath{\approx}200\text{ }\text{ }\mathrm{mT}$ close to the thermodynamic critical field ${H}_{c}$. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of ${H}_{c}&gt;H_{c}{}^{\mathrm{Nb}}$ for magnetic shielding of bulk niobium to increase the breakdown magnetic field of superconducting rf cavities. Depositing good quality layers inside a whole cavity is rather difficult, so as a first step, characterization of single layer coating and multilayers was conducted on high quality sputtered samples by applying the technique used for the preparation of superconducting electronics circuits. The samples were characterized by x-ray reflectivity, dc resistivity (PPMS), and dc magnetization (SQUID) measurements. Dc magnetization curves of a 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating exhibit different behaviors and clearly show an enhancement of the magnetic penetration field. Because SQUID measurements are influenced by edge and shape effects, we propose to develop a specific local magnetic measurement of ${H}_{C1}$ based on ac third harmonic analysis in order to reveal the true screening effect of multilayers.

Effects of Ca substitution on the magnetic properties in La1-xCaxMn0.90Cu0.10O3 (x=0.05–0.30) manganites

Systematic AC and DC magnetic measurements were carried out to investigate the magnetic properties of La1-xCaxMn0.90Cu0.10O3 (x=0.05–0.30) system. A striking paramagnetism (PM) to ferromagnetism (FM) transition is observed at the FM ordering temperature TC. Below TC, the samples with x≤.0.20 show a visible unexpected drop in zero-field cooled (ZFC) DC magnetization curves near 100 K, which is believed to be associated with the domain wall pinning effects. As for the sample with x=0.30, there is no anomalous behavior near 100 K, while another different magnetic phase transition is shown up below 50 K in both FC and ZFC curves. Detailed experimental results show that the phenomenon is resulted from the phase competition rather than solely magnetic interactions.

Coexistence of ferromagnetism and cluster glass state in superconducting ferromagnet RuSr2Eu1.5Ce0.5Cu2O10−δ

Systematic dc and ac magnetic susceptibility measurements performed on RuSr2Eu1.5Ce0.5Cu2O10−δ (Ru1222) demonstrate a paramagnetic to ferromagnetic transition around 95 K. The third harmonic of the ac susceptibility reveals that the system undergoes a spin glass transition below 75 K. The features of the zero-field cooled and field cooled dc magnetization curves of Ru1222 material resemble those of a cluster glass state, rather than those of a canonical spin glass state. The magnetization versus applied field loops do not saturate, even at very high applied fields, resulting in the short range magnetic order in the system, which facilitates the formation of clusters that freeze at low temperature. The temperature dependence of the second and third harmonic ac susceptibilities further confirms the coexistence of a cluster glass state and ferromagnetic order in the Ru1222 system.

Nano-sized Domain Wall Pinning Effects in Dilute Cu-Doped Perovskite LaMn1-xCuxO3 Manganites

Magnetic properties of Cu-doped LaMn1-xCuxO3 (x=0.05-0.30) systems are carefully studied in the temperature range of 2–300 K. A visible unexpected drop is observed in the ac susceptibility and the zero-field cooled dc magnetization curves for the dilute x ≤0.10 near 100K, which depends on the measuring frequency and magnetic field. Measurements on frequency dependence of ac susceptibility, observation of magnetic relaxation, and the existence of critical field indicate that the anomaly can be attributed to the domain wall pinning effects. This is directly proven by the results of ball milled nano-sized powder counterparts compared with the bulk materials.

Magnetic and hyperfine properties of chemically synthesized nanocomposites of (Al 2O 3) x(Ni 0.2Zn 0.6Cu 0.2Fe 2O 4) (1− x) ( x=0.15,0.30,0.45)

(Al 2O 3) x (Ni 0.2Zn 0.6Cu 0.2Fe 2O 4) (1− x) ( x=0.15,0.30,0.45) nanocomposites were synthesized by coprecipitation and sol–gel methods. The as-prepared samples were annealed at 400 ∘C to obtain the desired nanosystems. Particle sizes and crystallographic phases of the magnetic nanoparticles were evaluated from XRD and TEM observations. TEM observations revealed the formation of nanocrystalline phase in the amorphous matrix of aluminium oxide. High resolution TEM also hints that some nanoparticles were coated with aluminium oxides. Heat-treated samples were characterized by DC magnetization, AC hysteresis loops at 37 Hz and Mössbauer effect measurements. Mössbauer spectra of all the samples indicate the presence of relaxation in all the samples. DC magnetization curves have the typical nature of superparamagnetic particles which are also in good agreement with that of Mössbauer study. Coercive fields obtained from the observed AC hysteresis loops are quite low in the present samples which would be suitable for soft magnetic device applications.

Magnetic behavior in Ni–Fe–Ga martensitic phase

The magnetic behavior of a polycrystalline Ni 53.5Fe 19.5Ga 27 (at.%) alloy in the low-temperature martensitic phase has been analyzed. The alloy undergoes a martensitic transformation (MT) below room temperature from austenite to a mixture of five (10M) and seven (14M) layered phases. The order degree of the alloy modifies not only some characteristics of the MT but also the magnetic behavior in the martensite phase temperature range. The real component of the AC magnetic susceptibility displays a broad peak in that region that shifts towards higher temperatures as disorder increases. The peak disappears under application of a 500 Oe bias field. In turn, the DC magnetization curves under different applied constant fields in the range of 0–500 Oe show similar maxima at corresponding temperatures, while for higher magnetic fields the peak disappears. No discontinuities in the saturation magnetization (measured at 6 T) have been observed in this martensitic region. Structural changes such as modifications in the volume fraction of martensitic phases seem to be involved in the magnetic behavior of the alloy below the MT.

Room temperature ferromagnetism and magnetotransport properties of the layered manganite system La 1.2Ba 1.8Mn 2− xRu xO 7 (0≤ x≤1.0)

The dc magnetization and ac susceptibility measurements on two dimensional layered manganite La 1.2Ba 1.8Mn 2O 7 samples reveal the occurrence of ferromagnetism above room temperature with ferromagnetic (FM) to paramagnetic (PM) transitions at 338 K. The bifurcation temperatures shown by the zero-field cooled (ZFC) and field cooled (FC) dc magnetization curves at high temperatures shift towards lower temperatures as the applied field is increased from 100 to 2500 Oe. The data are suggestive of a large magnetic anisotropy due to the strong competing ferromagnetic and antiferromagnetic interactions resulting in a spin-glass-like state. Ru doping is found to enhance the ferromagnetism and metallicity of the system in a remarkable way. The magnetoresistance (MR) values obtained are very high and about 40% even at 260 K for the undoped sample.

Coexistence of ferromagnetism and cluster-glass behavior in YFe 5.5Al 6.5 and YFe 5.8Al 6.2

YFe x Al 12− x ( x = 5.5 and 5.8) polycrystalline samples were prepared by induction melting and found to crystallize in the tetragonal ThMn 12-type structure. Mössbauer spectroscopy, magnetization, and AC-susceptibility measurements show a reentrant ferromagnetic behavior. Above room temperature, there is evidence of uncorrelated iron-rich nanoclusters with short-range ferro- or ferrimagnetic order. At T c approx. 230 K for YFe 5.5Al 6.5 and 270 K for YFe 5.8Al 6.2 a paramagnetic ferromagnetic-like transition takes place. At lower temperatures fall-offs of the DC-magnetization curves are observed. Analysis of the frequency and field dependence of the linear AC-susceptibility as well as of the higher harmonics reveal a cluster-glass behavior below 70 and 50 K for YFe 5.5Al 6.5 and YFe 5.8Al 6.2, respectively. Comparing with previous results T c as well as the ferromagnetic character of YFe x Al 12− x are found to increase with increasing Fe concentration.

Size effects on the magnetic properties of Cu–Nb nanofilamentary wires processed by severeplastic deformation

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

Spheroidization Effects on the Electrical and Magnetic Properties of Cu-Nb Composites

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 800 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ C$</tex> 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.

Exchange anisotropy in Fe-MnF/sub 2/ nanogranular films

This paper reports on the magnetic properties of granular films consisting of iron, a ferromagnetic metal, and an insulator MnF/sub 2/, which is antiferromagnetic below the Neel temperature T/sub N/ = 67 K. Films were prepared by co-evaporation in vacuum and deposited onto polyimide films. DC magnetization curves recorded by a PPMS magnetometer from 5 to 300 K. Magnetization measurements for the sample with x = 0.25 showed superparamagnetic behavior at 300 K. The sample with x = 0.50 was soft ferromagnetic with the coercive field H/sub c/ = 20 Oe at 300 K. X-ray diffraction and Mossbauer spectroscopy were utilized for the characterization of the samples. Above T/sub N/, strong ferromagnetic exchange coupling between Fe and MnF/sub 2/ is dominant. Below T/sub N/, each iron particle is magnetized in a random direction as determined by the magnetic anisotropy.

Origin of experimental order-disorder transition line curvature in theLa2−xSrxCuO4single crystal vortex system at low temperatures

We investigated the dc magnetization curves of overdoped ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ single crystals in the low-temperature $T$ range, with the external magnetic field $H$ oriented along the $c$ axis. It was found that the onset field for the second magnetization peak, associated with the order-disorder transition in the vortex system, increases significantly with decreasing $T$ in the low-$T$ region, where the superconductor parameters are independent of $T$. The upward curvature of the order-disorder transition line determined in standard magnetization measurements at low $T$ is explained by considering the reduction of the actual pinning energy due to the macroscopic currents induced in the sample. A simple energy balance equation for the dynamic conditions generated in the widely performed magnetization studies is proposed.

AC susceptibility study of the bilayered cyano-bridged Cu [sbnd]W and Cu [sbnd]Mo ferromagnets

AC susceptibility, χ AC , for ferromagnetic Cu II[W V(CN) 8]- and Cu II[Mo V(CN) 8]-based molecular magnets with T C ≈ 30 K of the unique bilayered structure is investigated. χ AC ( T ) is measured with different frequency and amplitude of the oscillating field for various magnitudes of static applied field. Cu II[W V(CN) 8]-based compounds show a sharp anomaly at the transition and extremely low effect below T C ; second harmonic of χ AC and DC magnetization curves suggest some weak antiferromagnetic interactions while critical behaviour points to the Ising anisotropy in these samples. Susceptibility of these compounds is strongly enhanced by applying DC field of about 50 Oe. For the Cu II[Mo V(CN) 8]-based compounds χ AC is more typical and the critical exponent γ is close to that of 3D Heisenberg magnets. It is suggested that the different behaviour of these isostructural and isospin compounds is related to the more diffuse 5d shell of tungsten centres that may enhance the possible antiferromagnetic exchange through the CN-bridges. At H DC = 0 there is almost no frequency dependence of χ AC . Weak glass-like character of tungstate compounds which appears in the applied field is probably due to random weak interactions through the hydrogen bonds disordered in the space between the bilayers and random anisotropy coming from the countercations.

Patterned bilayer films as a new class of superconducting wire networks

We present results of AC and DC magnetic moment measurements on a Pb(50 nm)/Cu(10 nm) bilayer film with a triangular antidot lattice of spacing 866 nm and diameter 350 nm. The sharp peaking of matching anomalies in the AC and DC magnetization curves indicates a proximity coupling of the Pb and Cu layers, which is also supported by an asymmetric splitting of matching peaks in AC moment data. This work shows superconducting wire network behavior can be created and controlled in a patterned bilayer film without the complex fabrication of a Josephson tunnel junction array, or very thin superconducting wires of diameter ≈ coherence length.

Magnetic study on Co3O4 nanoparticles

Magnetization measurements for the Co3O4 nanoparticles were performed. From the X-ray diffraction patterns Co3O4 nanoparticles surrounded by amorphous SiO2 were expected for the samples annealed between 623 and 873 K. Their diameters were estimated to be 2 or 3 nm. DC magnetization curves of zero-field cooled and field-cooled bifurcated around Tt=10 K, while the bulk Co3O4 crystal exhibits antiferromagnetic transition temperature at TN=33 K. Characteristic properties of extremely small Co3O4 nanoparticles were confirmed.

Interaction evaluation in particulate media using the integral generalized ΔM plot

This paper presents a method of estimating the parameters of the generalized moving Preisach model by simple dc measurements. Two points on the integral generalized ΔM (IGDM) plot are used, which are very sensitive to choice of the parameters describing the interactions. The technique allows a refinement of parameters once that the coercive field distribution is approximated by using the magnetic hysteresis loop (MHL), dc magnetization curve (DCD), and the isothermal remanent magnetization curve starting from the dc demagnetized state of the system (IRMdc). The method may also be used to test the parameters provided by different identification techniques by considering the two points on the IGDM curve as check points for the simulations.

Study of important factors affecting AC losses of multifilamentary Bi-2223 tapes

We measured DC critical currents and magnetization curves on samples of Bi-2223/Ag multifilamentary tape with 55 untwisted filaments and on individual filaments extracted from the tape at frequencies from few mHz up to 150 mHz at 77 K. Hall probe technique was used for magnetization measurements on tape as well as filament samples. A calibration method based on the properties of the virgin curve is described. We also measured resistivity between a filament and Ag matrix for an individual filaments extracted from the tape. To detect filament intergrowths we measured I– V curves on tape sample with current flowing in the tape perpendicular to its axis. Consequences of the observed effects for the interpretation of AC loss behavior of multifilamentary tapes are discussed.