Low-field Microwave Absorption Research Articles

Microwave absorption dynamics in Fe3O4nanopowders around Verwey transition

In Fe3O4 nanopowders were carried out microwave absorption measurements at X-band (8.8-9.8 GHz), in the 89-300 K temperature range, by means of the ferromagnetic resonance (FMR), the magnetically modulated microwave absorption spectroscopy (MAMMAS) and the low-field microwave absorption (LFMA). The Verwey transition around Tv ∼ 137 K has been detected employed the FMR parameters: the resonant field (Hres ) and the linewidth (ΔHpp ), as temperature function. MAMMAS response shows features that can also be associated with Verwey transition. Finally, LFMA spectra are associated with a magnetic state in Fe3O4 nanopowders, where a lineal component with positive slope and a hysteresis loop around zero magnetic field are clearly observed, and behavior in temperature of the lineal component also suggests the presence of the Verwey transition.

The Low-Field Microwave Absorption in EMR Spectra for Ni50-xCoxMn35.5In14.5 Ribbons.

This paper contains a detailed study of low-field microwave absorption, which is observed in EMR spectra registered for a series of () Heusler alloys polycrystalline in situ and annealed at 1173 K ribbons. The LFMA spectra for all ribbons were performed at X-band (∼9.5 GHz), at temperatures below Curie temperature. Additionally, for annealed ribbons, the LFMA signal dependencies of the external magnetic field modulation amplitude, modulation frequency, microwave power and microwave magnetic field phase were registered. These results confirm the resonant character of LFMA. To determine the basic EMR parameters, such as linewidth and resonance field, the experimental data were fitted by the Dyson function. The LFMA signal is satisfactorily matched by the two lines, and the variability of the component lines with temperature is remarkable.

Electromagnetic origin of the microwave absorption response of Fe3O4 thin films

Low-field microwave absorption techniques are ultrasensitive, nondestructive methods for probing electric and magnetic properties of solids. Nonresonant low-field microwave absorption techniques such as magnetic field modulated microwave spectroscopy (MFMMS) can easily detect electromagnetic phase transitions in minute and inhomogeneous samples. While this technique can easily and almost selectively identify superconducting transitions, magnetic phase transitions produce more varied responses. Here, we present a technique to investigate the electric and magnetic properties of a sample with complex electromagnetic responses. This technique involves taking a series of magnetic hysteresis loops and magnetoresistance measurements. These can be compared to MFMMS data to identify features having electric or magnetic origin. This approach is applied to magnetite $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$, which possesses an electric, magnetic, and structural phase transition across its Verwey transition. By measuring high-quality ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ thin films in MFMMS and complementary techniques, the previously inscrutable MFMMS signal is analyzed. Furthermore, a model of the MFMMS signal can be calculated from the magnetic and electric data, which reproduces most of the features of the experimentally obtained MFMMS signal. This technique broadens the capabilities of MFMMS beyond the detection of superconductors.

Physical properties of microwave and solid state synthesized La0.7Na0.3MnO3

We report the structural, magnetic, electrical and broadband microwave absorption in La0.7Na0.3MnO3 sample synthesized by microwave (MW) irradiation (Na0.3LMO_MW) and compare them to the sample synthesized by solid-state (SS) reaction method (Na0.3LMO_SS). Single phase Na0.3LMO_MW was synthesized at 800 °C in 30 min, whereas, Na0.3LMO_SS sample was obtained by sintering at 1200 °C for 48 h. Although both these samples show ferromagnetic transition at TC ∼324.8 K, the MW-synthesized sample shows distinct physical properties: broad ferromagnetic transition, smaller saturation magnetization, a large difference between the magnetic ordering and metal-insulator transition temperatures, a large high-field magnetoresistance, a table top-like magnetocaloric effect, and a large low-field microwave absorption compared to the solid state synthesized sample. These differences are suggested to arise from magnetic heterogeneity induced by smaller grain size and surface spin disorder in the MW synthesized La0.7Na0.3MnO3.

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.

Electron magnetic resonance study of the Ni47Co3Mn35.5In14.5 ribbons

In this paper, the electron magnetic resonance (EMR) of Ni47Co3Mn35.5In14.5 alloy in ribbon and powder form are investigated. The EMR spectra for ribbons were registered within the temperature ranges 4 K ⩽T⩽100 K and 140 K ⩽T⩽380 K and for powdered ribbon within the temperature range 140 K ⩽T⩽380 K. Additionally, the magnetic properties of ribbon were also studied by using vibrating sample magnetometer (VSM) in the temperature range of 85 K ⩽T⩽400 K. At high temperature, the EMR signal consist of a single, asymmetric line. The temperature dependence of the inverse integrated intensity exhibits a Curie–Weiss like behaviour and Curie temperature (TC) determined from VSM equals 333 K. Below TC for ribbon sample the low field microwave absorption (LFMA) line was observed. The EMR spectra recorded for the powder sample are of a different shape than those for the ribbon sample. The powdering causes an increase of the effective skin depth, and the high-temperature lines lose asymmetric character. The Curie temperature is shifted relative to Curie temperature determined for the ribbon sample, but the structural phase transition is preserved.

Low-field microwave absorption in Ga-doped polycrystalline La0.6Sr0.4MnO3 in the frequency range from 0.1 to 4 GHz

We investigated the magnetic-field-dependent microwave absorption in polycrystalline La0.6Sr0.4Mn1-xGaxO3 samples (x = 0.0–0.3) over a wide frequency spectrum (f = 0.1–4 GHz) and up to a magnetic field of Hdc= 2.5 kOe at room temperature. The field dependence of microwave power absorption (ΔP) in each sample exhibits a peak at a critical value of Hdc and the peak moves towards higher fields as frequency increases. The shift is at a faster rate for x = 0.2 and 0.3 compared to x = 0.01 and 0.1. Based on the line shape analysis, we attribute the observed features to ferromagnetic resonance for x ≤ 0.1 and a combined effect of electron spin resonance and superparamagnetic resonance for x ≥ 0.2. Our results are also verified by magnetic resonance spectra recorded using a coplanar waveguide spectrometer. We extracted the gyromagnetic ratio, saturation magnetization, and anisotropy field in our samples. Frequency-tunable microwave absorption at resonance found in these materials will be of interest for high-frequency device applications.

Effect of annealing on the magnetic ordering and electron magnetic resonance of melt-spun Ni-Mn-In ribbons

In this study, the electron magnetic resonance (EMR) spectra of annealed Ni50Mn35.5In14.5 Heusler alloy ribbons within the temperature range 240 K⩽T⩽360 K are investigated. In the paramagnetic region, the temperature dependence of the EMR linewidth is governed by the Korringa relaxation process. The g value exhibits a positive shift, which indicates a non-bottleneck regime. The temperature dependence of inverse integrated intensity exhibits a Curie-Weiss like behaviour with TC equals 298 K. The effects of annealing on the magnetic and structural ordering are discussed. Annealing increases the Curie temperature and changes the first-order phase transition temperature. Additionally, the low-field microwave absorption peak appears below the magnetic ordering temperature.

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.

Non-Resonant Microwave Absorption in Nano Nickel Added YBCO Superconducting Powders: Line Shape and Hysteresis

Nonresonant Microwave Absorption (NRMA) or the ‘Low field microwave absorption’ (LFMA) measurements on high quality Ni nanoparticle added YBCO powder (Ni-YBCO) were carried as a function of temperature. The NRMA signal has structure in its line shape. As the Ni-YBCO powder sample is warmed up from 20K towards Tc, a phase reversal of the signal occurred around 60K, indicating a cross over from ‘normal’ to ‘anomalous’ type of absorption. We believe that the phase reversal could be due to the π-junction formation because of the nano nickle particles in the weaklinks between particles. Further, we observed strong low field hysteresis in NRMA. The low field hysteresis can be deduced from the boundary current response using Dulcic model. Our studies establish that, the addition of Nano Ni , can tune the microwave absorption properties and the low field hysteresis of NRMA

Low Field Microwave Absorption in Mn:Ni Co-Doped ZnO μm Size Powders

Low Field Microwave Absorption in Mn:Ni Co-Doped ZnO μm Size Powders

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.

Non-Resonant Microwave Absorption in SmFeAsO 0.80 F 0.20: Line Shape and Structure Evolution with Temperature

‘Non-resonant Microwave Absorption’ (NRMA) or the ‘Low field microwave absorption’ (LFMA) measurements on high-quality polycrystalline SmFeAsO0.80F0.20 superconducting sample were carried as functions of temperature and microwave power. The LFMA line shape is complex with two peaks namely; broad peak 1 and narrow peak 2 akin to one reported in SmFeAsO0.88F0.12 as reported by Onyancha et al (Supercond. Nov. Magn. 28, 2927–2934, 2015). This unquestionably illustrates that these peaks are a common feature in F-doped SmFeAsO. The LFMA signal as a function of temperature reveals that T c − T ∗ = 1K in SmFeAsO0.80F0.20 compared to 4 K in SmFeAsO0.88F0.12 (T ∗ is the characteristic temperature at which the narrow peak appears as we cool down the sample below T c); hence inferring that the narrow peak is fluorine doping dependent. Furthermore, LFMA signal evolution with microwave power does not show phase reversal (anomalous absorption) at 2.227 mW which is a stark contrast to what was observed in SmFeAsO0.88F0.12 as reported by Onyancha et al (Physica C: Supercond. Appl. 533:49–52, 2017). The absence of phase reversal within measured microwave power indicates presence of hysteretic Josephson junction. These findings establish few non-superconducting inclusions in SmFeAsO0.80F0.20 system.

Low-Field Microwave Absorption in a Graphite-Sulfur Composite: Indication of Microscopic Superconductivity

Low-Field Microwave Absorption in a Graphite-Sulfur Composite: Indication of Microscopic Superconductivity

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.

Non-resonant Microwave Absorption in Terbium Powders

The non-resonant microwave absorption is reported in terbium powders at X-band (8.8–9.8 GHz) in the 160–300 K temperature region. Two microwave absorption-type responses are observed as follows: the magnetically modulated microwave absorption spectroscopy (MAMMAS) and the low-field microwave absorption (LFMA). MAMMAS response detects the paramagnetic to antiferromagnetic-helical and to ferromagnetic transition at T N = 227 K and T C = 214 K, respectively, being confirmed by means of magnetic measurements. Additionally, LFMA response is only observed at temperatures below ∼230 K, and it is associated with the magnetic order (antiferromagnetic-helical/ferromagnetic) at low temperature in terbium powders.