Low-field Microwave Absorption Signal Research Articles

Understanding low field microwave absorption in Ni-Mn based Heusler alloy films

Low field microwave absorption (LFMA) signal has been observed for the first time in room temperature microwave spectra recorded at 9.44 GHz in polycrystalline Ni-Mn-Sn Heusler alloy films. The observed LFMA spectra is different from conventional ferromagnetic resonance and poses hysteresis near the zero field region of the spectrum. The observed LFMA spectra can be well correlated with the isothermal magnetization (M – H) loops recorded at different film orientations to the applied dc magnetic field from 0° to 90°. Furthermore, with small Fe, Co substitution in Ni-Mn-Sn film, both LFMA signal as well as the M – H loop properties of these films varied in a correlated manner, signifying a strong dependence of LFMA on the magnetization process in these films. The demonstration is useful both for fundamental and application point of view for characterization of magnetic materials and explore their applications in microwave assisted nanoelectronic devices.

First detection of low field microwave absorption in the disordered multiferroic double perovskite BiFe0.5Mn0.5O3

BiFe0.5Mn0.5O3 (BFMO) is an intriguing magnetic double perovskite, only obtainable through high pressure-high temperature synthesis. It shows bulk multiferroic properties, namely the coexistence between a spin canted antiferromagnetic structure superimposed to an externally induced electric polarization at least from 77 K. In particular, the system is characterized by a significant weak ferromagnetic hysteresis loop and by a very rare phenomenon: the spontaneous magnetization reversal (MRV) versus temperature in the low field regime. To clarify the BFMO exotic magnetic phase in the low field regime, the Electron Spin Resonance (ESR) and the low field microwave absorption (LFMA) techniques were used, providing the first observation of LFMA in the bulk BFMO as an additional functionality of this material. A striking feature is that the hysteresis in LFMA signals vanishes above 45 K, while the bulk M-H loop hysteresis, measured in the same field range of LFMA, persists till room temperature. The temperature at which LFMA hysteresis vanishes qualitatively matches the position of the magnetic susceptibility’s second derivative peak, corresponding to the temperature at which the local second order mechanism responsible for MRV is maximum. The line shape of LFMA completely changes above 45 K and the ESR linewidth starts decreasing above this temperature, indicating the role of defect/disorder induced inhomogeneity. The temperature evolution of LFMA hysteresis and line shapes as a measure of the competition between Fe- and Mn-rich clusters suggests a sort of local frustration at the microscopic scale, responsible for the peculiar magnetization reversal of this system.

Novel normal-state low field microwave absorption in SmFeAsO1−xFx iron pnictide superconductors

We report on the systematic normal and superconducting states microwave absorption studies in polycrystalline samples of SmFeAsO1−xFx using an electron spin resonance (ESR) spectrometer. In the normal state, we observed two absorption spectra; at high fields (~3500 G) and low fields (around 0 G). The high field spectrum is in principle a resonance spectrum corresponding to a saturated state. It was observed that this signal broadens and its peak to peak intensity decreases on cooling the sample from room temperature. Ultimately, the signal becomes ESR ‘silent’ in the superconducting state. On the other hand, the low field microwave absorption (LFMA), centered around zero field, was observed for the first time, in a superconducting material. Based on the line-width, hysteresis and peak to peak signal intensity, we have shown that this spectrum is distinctively different from the already known LFMA signal that occurs in the superconducting state. These results present a system that shows LFMA spectrum in both normal and superconducting state thus providing an excellent platform to analyse the elusive LFMA phenomenon.

Low-field microwave absorption in Zn1−x(Mn:Fe(Ni))xO (x = 0.02) system: hysteresis, line shapes and powdering effects

We studied the low-field microwave absorption (LFMA) in polycrystalline pellet and powdered (0.1–0.2 µm particle size) samples with a nominal composition of Zn1 − x(Mn:Fe(Ni))xO (x = 0.02). LFMA signals are stronger in the case of Mn:Fe co-doped ZnO, as compared to that of Mn:Ni co-doped ZnO. While the bulk samples show hysteresis, it disappears in the case of powdered samples. Further the line shapes of LFMA were modified with powdering, leading to small saturation fields for LFMA. This indicates that the microwave absorption in these powders is more sensitive than in the pellet form. We interpret these results in terms of interparticle–interfacial pinning. In this work, we clearly establish the low-field tunability of microwave absorption in the Zn1 − x(Mn:Fe(Ni))xO (x = 0.02) system, which is good for the applications as microwave absorbers with small field tunability as a functionality. This means LFMA can give inputs for the right choice of material for field-tunable microwave absorber design.

Low field microwave absorption in iron nanoparticles embedded polyaniline nanofibers composite

Low field microwave absorption (LFMA) in conducting polymers is completely different as compared to magnetic systems and is very intriguing. To understand LFMA phenomenon in the conducting polymers, we studied LFMA in the conducting polyaniline (PANI) polymer and iron nanoparticles (NPs) embedded polyaniline nanofibres (PANI-Fe) composite systems. PANI-Fe composite nanofibers were prepared by rapid mixing chemical polymerization of ANI monomer using FeCl3 as an oxidant and followed by the reduction of Fe2+/Fe3+ ions for the deposition of Fe NPs onto the PANI nanofibers matrix. Formation of PANI and deposition of Fe NPs were confirmed by HR-TEM, XRD and ATR-FTIR analyses, respectively. The study showed that in spite of PANI being non-degenerate conducting polymer, LFMA is absent in pure PANI. In contrary, LFMA signals are observed in Fe NPs embedded PANI nanofibers and bare Fe NPs systems. Presence of minor hysteresis in LFMA, as compared to the substantial magnetization hysteresis and the fact that LFMA derivative peaks not having any correlation with the M−H loop anisotropy and coercive fields rules out the conventional interpretation of LFMA arising from the low field spin magnetization processes as reported in the literature. Further it is perceived that microwave absorption in PANI-Fe system has a maximum at zero field and decreases with increasing applied magnetic field. This is totally in contradiction with the predicted behaviour of polaron-bipolaron based mechanism of LFMA origin, where the microwave absorption has a minimum at zero field and increases with the field in the case of non-degenerate conducting polymers, as reported in literature. However, LFMA correlates well with the magnetoresistance data for the PANI-Fe composite nanofibers system, indicating LFMA in this system is actually governed by magneto transport and not exactly by the low field magnetization process itself.

ESR studies of transition from ferromagnetism to superparamagnetism in nano-ferromagnet La0.8Sr0.2MnO3

Electron Spin resonance spectroscopy (ESR) was used to determine the magnetic properties of nanocrystalline samples of La0.8Sr0.2MnO3 having an average crystallite size ranging from 9 to 57 nm. The control of the size is performed by adopting the autocombustion method with two-step synthesis process. The main objective of this study is to determine the effect of crystallite size on the magnetic behavior of materials. As well as the domains of the sizes correspond to the superparamagnetic state and to single-domain and multi-domain ferromagnetic states. However, we have noticed that Electron Magnetic Resonance (EMR) and Low Field Microwave Absorption (LFMA) spectra are highly crystallites size dependent. Significant changes in the line shape, LFMA signal, resonance field, and linewidth (ΔHpp) can be used as indices of magnetic state transitions. Moreover, the discontinuity observed in ΔHpp and g-factor is attributed to the changes of magnetic states. Magnetic measurements showed a good agreement with ESR results. Therefore, samples having crystallite size less than 24.5 nm are in a superparamagnetic state and those between 24.5 and 32 nm are single-domain ferromagnetic. The multi-domains ferromagnetic are manifested for sizes greater than 32 nm. Above 24.5 nm in accordance with the core-shell structure, the increase in g-factor with crystallites size is attributed to the increase in core size magnetic. In the superparamagnetic zone, the practically constant value of the g-factor assumes that the volume of the core magnetic is not affected by increasing crystallites size. This contradictory observation with the core-shell model was explained by the phenomenon of phase separation often reported for manganites perovskite. Thus, the low size crystallites consist of several small ferromagnetic volumes distributed in a paramagnetic matrix, that we called crystallite multi-core superparamagnetic.

Temperature Dependence Low-Field Microwave Absorption in a Powder Sample of SmFeAs(O,F) Iron Pnictide Superconductor

The effect of temperature variation on low-field microwave absorption (LFMA) was investigated on the SmFeAs(O,F) powder sample of average particle size of ∼3μm in the superconducting region (40K). The two peaks (broad and narrow) which were reported on a pellet of the same sample (Onyancha et al., J. Supercond. Nov. Magn. 28, 2927 (2015)) are observed on the LFMA line shape signal of this powdered sample. It evidently indicates that the peaks are not necessary confined to pellet sample only. Also, the LFMA intensity is found to evolve as a function of temperature. This temperature dependence of the LFMA intensity is interpreted on the framework of effective medium theory in which coupling and decoupling of Josephson junction is considered. Furthermore, we observed an anomalous hysteresis (the LFMA signal in the forward DC field sweep is above the backward DC field sweep) which is consistent with the prediction of the two-level critical state model.

Observation of low field microwave absorption in co-doped ZnO system

Abstract Room temperature low field microwave absorption (LFMA) in magnetic materials find application in microwave absorbers and low field sensors. However not all the magnetic materials show LFMA and the phenomenon is not fully understood. We report on the observation of low field microwave absorption (LFMA) or the non-resonant microwave absorption (NRMA) in the transition metal (TM) co-doped ZnO samples of the composition Zn 1− x (TM:TM) x O synthesized by solid state reaction technique. LFMA peaks and hysteresis matches very well with that of the magnetization hysteresis loop and the anisotropy fields at room temperature similar to the reports in the literature for other magnetic systems. However we show through our careful experiments that such a correlation between LFMA and the magnetization does not survive at low temperatures and particularly at 10 K the LFMA hysteresis collapses in our TM co-doped ZnO system; whereas the magnetization hysteresis loop becomes very big and anisotropy field becomes bigger in the range of kOe. We interpret the LFMA as field dependent surface impedance or eddy current losses, in terms of a possible role of anomalous hall resistivity that follows magnetization and the ordinary hall resistivity that only follows the applied field. We then argue that LFMA accordingly follows magnetization or applied field when AHE or OHE dominates respectively. Also we confirm the absence of LFMA signals in the rare earth co-doped ZnO system.

CoFe/MnIr 박막 재료에서 저자장 마이크로파 흡수 특성 분석

본 연구에서는 교환 결합력을 갖는 CoFe/MnIr 박막 재료에서 강자성 공명 장치를 이용하여 자기장 방향에 따른 저자장 마이크로파 흡수(Low field microwave absorption, LFMA) 및 강자성 공명 신호를 측정하였다. 낮은 자기장 영역에서 나타나는 LFMA 신호는 자구의 회전 밀접하게 관계됨을 자화 곡선으로부터 알 수 있었다. 이러한 LFMA 신호 특성을 분석하기 위하여 강자성 공명 신호로부터 측정한 교환 바이어스(<TEX>$H_{ex}$</TEX> = 58.5 Oe) 및 일축 이방성 자기장(<TEX>$H_k$</TEX> = 30Oe)을 S-W model에 적용하여 자화과정의 횡방향 자화량(transverse magnetization, <TEX>$M_{\tau}$</TEX>) 및 투자율(transverse susceptibility, <TEX>${\mu}_{\tau}$</TEX>)을 계산하였다. 자화 곤란축에서 측정된 LFMA 신호는 <TEX>$M_{\tau}$</TEX> 비례하는 경향을 보였지만, 자화 곤란축과 수직 방향으로 접근하면 <TEX>$M_{\tau}$</TEX> 및 <TEX>${\mu}_{\tau}$</TEX> 모두에 의존하는 특성을 보였다. We measured the low field microwave absorption (LFMA) and ferromagnetic resonance (FMR) signals at various magnetic field angle in exchange biased CoFe/MnIr thin film. The LFMA signals were dominantly related to the magnetization rotation process. In order to analyze the LFMA signal, we calculated transverse magnetization (<TEX>$M_{\tau}$</TEX>) and permeability (<TEX>${\mu}_{\tau}$</TEX>) for CoFe/MnIr thin film by using S-W model, which magnetic parameters of exchange bias (<TEX>$H_{ex}$</TEX> = 58.5 Oe) and uniaxial anisotropy field (<TEX>$H_k$</TEX> = 30Oe) was obtained from FMR signals. The LFMA signal at hard axis showed similar behavior compared with that of <TEX>$M_{\tau}$</TEX>. As the magnetic field angle approach to the perpendicular to hard axis, the LFMA signals were depending on both of <TEX>$M_{\tau}$</TEX> and <TEX>${\mu}_{\tau}$</TEX>.

Microwave absorption behavior in Cr2O3 nanopowders

We have investigated the microwave power absorption at X-band (8.8–9.8GHz) in Cr2O3 nanopowders, for the 294–400K temperature range. For all temperatures, two different kinds of microwave absorptions are observed: the electron magnetic resonance (EMR) at high magnetic field and the low-field microwave absorption (LFMA) around zero field. EMR spectrum can be separated in two contributions associated with two different resonant absorption modes, where this result is interpreted as the combination of two different magnetic phases; corresponding to the core (mode A) and the surface (mode B) of the Cr2O3 nanoparticles. Also, we have detected the onset of the antiferro-paramagnetic transition in resonant mode A, which is characteristic of Cr2O3 samples; being confirmed the magnetic transition by means of the magnetically modulated microwave absorption spectroscopy (MAMMAS). Additionally, the temperature behaviors of the resonant mode B and the LFMA signal are associated with a ferromagnetic state on the surface of the Cr2O3 nanoparticles.

Shape-selective dependence of room temperature ferromagnetism induced by hierarchical ZnO nanostructures.

We report on the room temperature ferromagnetism of various highly crystalline zinc oxide (ZnO) nanostructures, such as hexagonally shaped nanorods, nanocups, nanosamoosas, nanoplatelets, and hierarchical nano "flower-like" structures. These materials were synthesized in a shape-selective manner using simple microwave assisted hydrothermal synthesis. Thermogravimetric analyses demonstrated the as-synthesized ZnO nanostructures to be stable and of high purity. Structural analyses showed that the ZnO nanostructures are polycrystalline and wurtzite in structure, without any secondary phases. Combination of electron paramagnetic resonance, photoluminescence, and X-ray photoelectron spectroscopy studies revealed that the zinc vacancies (VZn) and singly ionized oxygen vacancies (VO(+)) located mainly on the ZnO surface are the primary defects in ZnO structures. A direct link between ferromagnetism and the relative occupancy of the VZn and VO(+) was established, suggesting that both VZn and VO(+) on the ZnO surface plays a vital role in modulating ferromagnetic behavior. An intense structure- and shape-dependent ferromagnetic signal with an effective g-value of >2.0 and a sextet hyperfine structure was shown. Moreover, a novel low field microwave absorption signal was observed and found to increase with an increase in microwave power and temperature.

Low-field microwave absorption in pulse laser deposited FeSi thin film

Low field microwave absorption (LFMA) measurements at 9.4GHz (X-band), were carried out on pulse laser deposited (PLD) polycrystalline B20 cubic structure FeSi thin film grown on Si (111) substrate. The LFMA properties of the films were investigated as a function of DC field, temperature, microwave power and the orientation of DC field with respect to the film surface. The LFMA signal is very strong when the DC field is parallel to the film surface and vanishes at higher angles. The LFMA signal strength increases as the microwave power is increased. The LFMA signal disappears around 340K, which can be attributed to the disappearance of ferromagnetic state well above room temperature in these films. We believe that domain structure evolution in low fields, which in turn modifies the low field permeability as well as the anisotropy, could be the origin of the LFMA observed in these films. The observation of LFMA opens the possibility of the FeSi films to be used as low magnetic field sensors in the microwave and rf frequency regions.

ChemInform Abstract: Low-Field Microwave Absorptions of Silver-Doped YBa2Cu3O7-δ Superconductors.

Superconducting and microstructural properties of silver-doped YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} composites have been studied as a function of silver content. An increase by {approximately}8 K in the superconducting transition temperature is observed by resistance measurement above 4.2 wt % silver. Silver seems to act as a flux to propagate the formation of oblong-shaped grains. The grain size increases with silver doping, and at high concentrations of silver it also induces grain alignment. Silver is not observed to substitute for Cu in YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}. Low-field microwave absorption (LFMA) shows different hysteresis and power dependence behavior between YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} and silver-doped samples. The LFMA signal intensity decreases with increasing silver content, and the maximum response shifts to higher temperature. The LFMA signal is explained by magnetic flux trapping at pinning sites, inter- and intragranular currents through Josepheson junctions, and variation in microwave penetration depth with temperature and silver content.

Microwave absorption studies of MgB2 superconductor

Microwave absorption studies have been carried out on MgB2 superconductor using a standard X-band EPR spectrometer. The modulated low-field microwave absorption signals recorded for polycrystalline (grain size ∼ 10 µm) samples suggested the absence of weak-link character. The field dependent direct microwave absorption has been found to obey a ✓H dependence with two different slopes, which indicated a transition from strongly pinned lattice to flux flow regime.

Superconductivity of a θ-type et salt prepared using CuCN AND PH 4 PN(CN) 2 as electrolyte

A θ-type ET salt θ-(ET) 2 Cu 2 (CN)[N(CN) 2 ] 2 has been obtained by electrocystallization in PhCN, using Ph 4 N[N(CN) 2 ] and CuCN as electrolytes. Some of them showed anomalous behaviors for Low-Field-Microwave-Absorption (LFMA) measurements, which were one of the characteristics of superconductivity. However, the others did not show the LFMA signals, indicating that there are two phases in this compound having different electronic states at low temperature.

Magnetic field dependent microwave absorption studies on a MgB2superconductor

Polycrystalline MgB2, synthesized using a conventionalsolid-state reaction route, has been investigated for its magnetic fielddependent microwave absorption using a standard X-band EPR spectrometer. Thechanges in microwave absorption as a function of temperature and/or magneticfield were recorded by following the microwave reflectivity of a sample-loadedcavity. The modulated low-field microwave absorption signal was found to besimilar to that observed in high-temperature superconducting materials. Thefield dependent direct microwave absorption in the 0-10 kG range was used todetermine Hc1 values at different temperatures, and the value ofHc1(0) was found to be 250 G. The absorbed microwave power has beenfound to obey a (H)1/2 dependence with a change of slope indicating atransition from a strongly pinned flux lattice to the flux flow regime.

Microwave absorption studies of diluted high-temperature superconductors: delineation of superconductor-insulator-superconductor and superconductor-normal-superconductor junctions

Abstract With a view to investigating the nature of weak links in granular superconductors at very dilute level, we have studied the low-field microwave absorption signal (below about 50 G) in the electron paramagnetic resonance mode of detection in samples of Tl2BaCa2Cu2O8+δ and YBa2Cu3O7-δ at different dilution levels in the Al2O3 matrix. The signal has been monitored as a function of temperature as well as concentration. The temperature dependence of the signal intensity has been understood by applying a model in which microwave absorption is assumed to be caused by the field- and tempera ture-dependent currents in the Josephson links. It is shown that the parameter ηo quantifying the strength of coupling between superconducting grains can be calculated from the value of temperature at which the intensity I pp(T) (the subscript pp indicates a peak-to-peak value), goes through a maximum. Using the ηo value so obtained, the fitting for I pp(T) fhroughout the temperature range, was used to distinguish sup...

LFMA and irreversibility line studies on Vanadium doped Bi(2223) superconducting system

Suresh Babu et al [7] have reported the stabilization of Bi high T c (105 K) phase by vanadium oxide doped Bi(2223) superconducting system. Stabilization of Bi high T c phase in the absence of Pb makes this system interesting. In this paper we report low field microwave absorption (LFMA) studies of Bi 2−xV x Sr 2 Ca 2 Cu 3 O y (x=0 and 0.4 superconducting samples. Temperature dependent LFMA signal amplitude of x=0 sample showed definite peaks at 97 K and 70 K which may correspond to the 2223 and 2212 phases. In the case of x=0.4 sample, it showed a single peak at 90 K corresponding to 2223 phase. The thermodynamical critical magnetic field H Cl ∗ and average Josephson loop diameter was estimated for these samples from the LFMA signals. In addition we have also determined the irreversibility line from ZFC and FC susceptibility measurements. The irreversibility line followed the universal law H ∗= H o(1- T ∗/ T c) a . n and H o were foud to be 2.8 and 4075 Oe respectively.

Electron spin resonance and low-field microwave absorption of alkali-metal-doped fullerene C60 superconductors

Alkali-metal-doped C[sub 60] superconductors formed by alkali-metal azide decomposition show a time evolution for the development of the superconducting phase after initial sample preparation which extends over about 15 h for storage at room temperature. This is studied quantitatively for K[sub 3]C[sub 60] and Rb[sub 3]C[sub 60], and differences are seen between these two systems which are consistent with redistribution of the alkali metal in the C[sub 60] matrix accounting for the time evolution observed. This is supported by separate experiments in which the storage temperature after sample preparation is decreased to 77 K. It is interesting that electron spin resonance (ESR) signals are seen in all of these samples in both the superconducting and nonsuperconducting temperature regions and that they show time evolutions after sample preparation that are qualitatively similar to the low-field microwave absorption (LFMA) signals which characterize the development of the superconducting phase. However, a detailed comparison shows differences between the ESR signal changes with time and the development of the superconducting phase which suggests that the origins of the ESR and LFMA signals are independent. It is interesting that similar ESR changes with storage time are observed in nonsuperconducting Cs[sub 3]C[sub 60] as are observed inmore » superconducting K[sub 3]C[sub 60] and Rb[sub 3]C[sub 60]. 10 refs., 10 figs.« less

A new preparation method for superconducting alkali-metal-doped fullerenes: comparison of ac susceptibility and low-field microwave absorption characterization

Alkali-metal-doped fullerenes are prepared by a rapid, new method employing alkali-metal azides. By means of ac susceptibility and low-field microwave absorption (LFMA), the authors have investigated the superconducting phases in K- and Rb-doped C. With Rb doping, the transition to the superconducting state takes place at T{sub c} = 38 K when observed by LFMA. The LFMA signal shows significant hysteresis behavior. During a day following the doping process, the LFMA for nominal composition Rb{sub 3}C{sub 60} is found to be drastically time dependent. Measurements by ac susceptibility, on the same sample, show an onset at 34 K which is lower than the T{sub c} obtained by LFMA but higher than the highest value reported for Rb{sub 3}C{sub 60} by ac susceptibility. This could indicate the presence of another superconducting phase. At 12 K the superconducting fraction is found to be about 95% by ac susceptibility so the material formed by this new preparation method is almost entirely superconducting. 22 refs., 4 figs., 1 tab.