Dc Magnetoresistance Research Articles

Large Low-Magnetic-Field Magnetocapacitance Effect and Spin Accumulation in Graphene Oxide

We report large magnetocapacitance (MC) effect and spin accumulation in graphene oxide (GO) studied at low magnetic fields and low frequencies, synthesized using modified Hummer’s method. Raman spectroscopy, atomic force microscopy (AFM), vibrating sample magnetometry (VSM), impedance spectroscopy, and dc transport measurements are used to investigate the origin of spin accumulation in GO. It was observed that synthesized GO was flake-like in morphology with prominent <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$G$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D$ </tex-math></inline-formula> -bands of vibrations, with large defect density. It was found to have ferromagnetism with coercivity of the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 134$ </tex-math></inline-formula> Oe. The GO showed MC of 20% at around 200 Hz and 100 Oe, where capacitive coupling is higher and 2.2% at 1.1 MHz where inductive coupling is higher in the capacitor under study. The relaxation process in GO was modeled using modified Havriliak–Negami (H–N) approach for inhomogeneous medium, which predicted the spin accumulation length of the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$187~\mu \text{m}$ </tex-math></inline-formula> . The spin accumulation in GO here is proposed to be due to weak localization and exchange interaction at lower fields below coercivity and above it is interface mediated and dipolar in nature. The results are supported with dc–magnetoresistance (MR) study showing the depletive nature of the defects dominating the magnetic scattering process. The observations in GO are compared with reduced GO (rGO) which did not show significant spin accumulation. The comparison of data yields a proof of defect-mediated MC in GO.

Image of Dynamic Local Exchange Interactions in the dc Magnetoresistance of Spin-Polarized Current through a Dopant.

We predict strong, dynamical effects in the dc magnetoresistance of current flowing from a spin-polarized electrical contact through a magnetic dopant in a nonmagnetic host. Using the stochastic Liouville formalism we calculate clearly defined resonances in the dc magnetoresistance when the applied magnetic field matches the exchange interaction with a nearby spin. At these resonances spin precession in the applied magnetic field is canceled by spin evolution in the exchange field, preserving a dynamic bottleneck for spin transport through the dopant. Similar features emerge when the dopant spin is coupled to nearby nuclei through the hyperfine interaction. These features provide a precise means of measuring exchange or hyperfine couplings between localized spins near a surface using spin-polarized scanning tunneling microscopy, without any ac electric or magnetic fields, even when the exchange or hyperfine energy is orders of magnitude smaller than the thermal energy.

Lifshitz transition underlying the metamagnetic transition of UPt3

Comparing quantum oscillation measurements, dc magnetoresistance measurements, and Fermi surfaces obtained from LDA calculations, we argue that the metamagnetic transition of UPt3, which occurs at an applied field μ ◦ H M ∼ 20 T, coincides with a Lifshitz transition at which an open orbit on the band 2 hole-like Fermi surface becomes closed for one spin direction. At low field, proximity of the Fermi energy to this particular van Hove singularity may have implications for the superconducting pairing potential of UPt3. In our picture the magnetization comes from non-linear spin-splitting of the heavy fermion bands. In support of this, we show that the non-linear field dependence of a particular quantum oscillation frequency can be fitted by assuming that the corresponding extremal Fermi surface area is proportional to the magnetization. In addition, below H M , we find in our LDA calculations a new, non-central orbit on band 1, whose non-linear behaviour explains a field-dependent frequency recently observed in magnetoacoustic quantum oscillation measurements.

Broadband dielectric spectroscopy of La0.65Sr0.35MnO3@TiO2 core–shell nanocomposites

Core–shell composites of ferromagnetic conducting nanoparticles La0.65Sr0.35MnO3 (LSMO) embedded in an insulating matrix of TiO2 (LSMO@TiO2) have been processed, structurally and magnetically characterized, and their DC magnetoresistivity and complex dielectric response measured and fitted from Hz up to the infrared (IR) range (1014 Hz). XRD indicates that the TiO2 shells are amorphous. Modelling of the IR spectra using standard models based on the effective medium approximation has it confirmed and has characterized the effective phonon modes of the LSMO nanoceramics and LSMO@TiO2 composite. Modelling of the lower-frequency spectra has shown that TiO2 shell thicknesses are rather non-uniform down to thin nm values, which leads to giant low-frequency permittivity values and non-negligible free-carrier tunnelling among the LSMO cores. Two main dielectric dispersion regions were observed and shown to be due to the inhomogeneous conductivity—the one occuring in the 1011–1012 Hz range relates to nonmagnetic less-conducting dead layers on the surface of LSMO nanocrystallites and the broad second one below the 1010 Hz range is due to the non-uniform thicknesses of the dielectric TiO2 shells. In the IR range, effective phonon modes of the LSMO nanoceramics and LSMO@TiO2 composite were characterized from the reflectivity spectra.

AC magnetoresistance and its frequency-dependence of Fe-doped Bi2Se3 topological insulator

Bismuth selenide (Bi2Se3) is one of the most important base materials in the study of topological insulators. Doping with iron is a direct method to manipulate the magnetic order in Bi2Se3. In this paper, we first report the measurements of magnetoresistance of Fe-doped Bi2Se3 with alternating current. Unlike the DC magnetoresistance, skin effect plays an important role in AC magnetoresistance. Another interesting finding is that resonance occurs at a certain frequency, which implies an internal relaxation mechanism. The relations between resistance, magnetic field and current frequency are discussed.

Microwave magnetoimpedance and ferromagnetic resonance in Pr0.6Sr0.4MnO3.

We report the magnetic field dependence of electrical impedance (magnetoimpedance) of a ferromagnetic Pr0.6Sr0.4MnO3 sample carrying alternating current (ac) of frequency f = 1 MHz to 3 GHz measured using an impedance analyzer and broad band ferromagnetic resonance (f = 2 to 18 GHz) measured using a coplanar wave guide based spectrometer. Ac magnetoresistance is much larger than dc magnetoresistance and its sign at low magnetic fields changes from negative to positive with increasing frequency of the ac current. The field dependence of ac magnetoresistance shows a peak around Hdc = 0 for low frequencies but a double peak feature emerges at Hdc = ±Hp at higher frequencies and it shifts to higher magnetic field as the frequency of ac current increases. The field derivative of microwave power absorption measured by the broad band spectrometer shows features of ferromagnetic resonance and the resonance field increases with increasing frequency of microwave radiation following Kittel's equation for ferromagnetic resonance. A close correlation is found between the ferromagnetic resonance line shape and the positive peak in the ac magnetoresistance, which suggests the possibility of electrical detection of ferromagnetic resonance using high frequency current injected into a conducting magnetic sample.

Role of CoFeB thickness in electric field controlled sub-100 nm sized magnetic tunnel junctions

We report a comprehensive study on the role of the free layer thickness (tF) in electric-field controlled nanoscale perpendicular magnetic tunnel junctions (MTJs), comprising of free layer structure Ta/Co40Fe40B20/MgO, by using dc magnetoresistance and ultra-short magnetization switching measurements. Focusing on MTJs that exhibits positive effective device anisotropy (Keff), we observe that both the voltage-controlled magnetic anisotropy (ξ) and voltage modulation of coercivity show strong dependence on tF. We found that ξ varies dramatically and unexpectedly from ∼−3 fJ/V-m to ∼−41 fJ/V-m with increasing tF. We discuss the possibilities of electric-field tuning of the effective surface anisotropy term, KS as well as an additional interfacial magnetoelastic anisotropy term, K3 that scales with 1/tF2. Voltage pulse induced 180° magnetization reversal is also demonstrated in our MTJs. Unipolar switching and oscillatory function of switching probability vs. pulse duration can be observed at higher tF, and agrees well with the two key device parameters — Keff and ξ.

Importance of frequency dependent magnetoresistance measurements in analysing the intrinsicality of magnetodielectric effect: A case study

Magnetodielectric (MD) materials have attracted considerable attention due to their intriguing physics and potential future applications. However, the intrinsicality of the MD effect is always a major concern in such materials as the MD effect may arise also due to the MR (magnetoresistance) effect. In the present case study, we report an experimental approach to analyse and separate the intrinsic and MR dominated contributions of the MD phenomenon. For this purpose, polycrystalline samples of LaGa1-xAxO3 (A = Mn/Fe) have been prepared by solid state reaction method. The purity of their structural phase (orthorhombic) has been validated by refining the X-ray diffraction data. The RTMD (room temperature MD) response has been recorded over a frequency range of 20 Hz to 10 MHz. In order to analyse the intrinsicality of the MD effect, FDMR (frequency dependent MR) by means of IS (impedance spectroscopy) and dc MR measurements in four probe geometry have been carried out at RT. A significant RTMD effect has been observed in selected Mn/Fe doped LaGaO3 (LGO) compositions. The mechanism of MR free/intrinsic MD effect, observed in Mn/Fe doped LGO, has been understood speculatively in terms of modified cell volume associated with the reorientation/retransformation of spin-coupled Mn/Fe orbitals due to the application of magnetic field. The present analysis suggests that in order to justify the intrinsic/resistive origin of the MD phenomenon, FDMR measurements are more useful than measuring only dc MR or analysing the trends of magnetic field dependent change in the dielectric constant and tanδ. On the basis of the present case study, we propose that IS (FDMR) alone can be used as an effective experimental tool to detect and analyse the resistive and intrinsic parts contributing to the MD phenomenon.

Magnetic behavior of metallic kagome lattices, Tb3Ru4Al12 and Er3Ru4Al12

We report the magnetic behavior of two intermetallic-based kagome lattices, Tb3Ru4Al12 and Er3Ru4Al12, crystallizing in the Gd3Ru4Al2-type hexagonal crystal structure, by measurements in the range 1.8–300 K with bulk experimental techniques (ac and dc magnetization, heat capacity, and magnetoresistance). The main finding is that the Tb compound, known to order antiferromagnetically below (TN=) 22 K, shows glassy characteristics at lower temperatures (15 K), thus characterizing this compound as a re-entrant spin-glass. The data reveal that the glassy phase is quite complex and is of a cluster type. Since glassy behavior was not seen for the Gd analog in the past literature, this finding on the Tb compound emphasizes that this kagome family could provide an opportunity to explore the role of higher-order interactions (such as quadrupole) in bringing out magnetic frustration. Additional findings reported here for this compound are: (i) The plots of temperature dependence of magnetic susceptibility and electrical resistivity data in the range 12–20 K, just below TN, are found to be hysteretic leading to a magnetic phase in this intermediate temperature range, mimicking disorder-broadened first-order magnetic phase transitions; (ii) features attributable to an interesting magnetic phase co-existence phenomenon in the isothermal magnetoresistance in zero field, after travelling across metamagnetic transition fields, are observed. With respect to the Er compound, we do not find any evidence for long-range magnetic ordering down to 2 K, but this compound appears to be on the verge of magnetic order at 2 K.

Room-Temperature Magneto-dielectric Effect in LaGa0.7Fe0.3O3+γ; Origin and Impact of Excess Oxygen.

We report an observation of room-temperature magneto-dielectric (RTMD) effect in LaGa0.7Fe0.3O3+γ compound. The contribution of intrinsic/resistive sources in the presently observed RTMD effect was analyzed by measuring direct-current (dc) magnetoresistance (MR) in four-probe geometry and frequency-dependent MR via impedance spectroscopy (MRIS). Present MRIS analysis reveals that at frequencies corresponding to grain contribution (≥1 × 106 Hz for present sample), the observed MD phenomenon is MR-free/intrinsic, whereas at lower probing frequencies (<1 × 106 Hz), the observed MD coupling appears to be MR-dominated possibly due to oxygen excess, that is, due to coexistence of Fe3+ and Fe4+. The magnetostriction is anticipated as a mechanism responsible for MR-free/intrinsic MD coupling, whereas the MR-dominated part is attributed to hopping charge transport along with Maxwell-Wagner and space charge polarization. The multivalence of Fe ions in LaGa0.7Fe0.3O3+γ was validated through iodometric titration and Fe K-edge X-ray absorption near-edge structure measurements. The excess of oxygen, that is, coexistence of Fe3+ and Fe4+, was understood in terms of stability of Fe4+ by means of "bond-valence-sum" analysis and density functional theory-based first-principles calculations. The cation vacancies at La/Ga site (or at La and Ga both) were proposed as the possible origin of excess oxygen in presently studied compound. Present investigation suggests that, to justify the intrinsic/resistive origin of MD phenomenon, frequency-dependent MR measurements are more useful than measuring only dc MR or comparing the trends of magnetic-field-dependent change in dielectric constant and tan δ. Presently studied Fe-doped LaGaO3 can be a candidate for RTMD applications.

Magnetic-field-driven electron transport in ferromagnetic/ insulator/semiconductor hybrid structures

Extremely large magnetotransport phenomena were found in the simple devices fabricated on base of the Me/SiO2/p-Si hybrid structures (where Me are Mn and Fe). These effects include gigantic magnetoimpedance (MI), dc magnetoresistance (MR) and the lateral magneto-photo-voltaic effect (LMPE). The MI and MR values exceed 106% in magnetic field about 0.2T for Mn/SiO2/p-Si Schottky diode. LMPE observed in Fe/SiO2/p-Si lateral device reaches the value of 104% in a field of 1T. We believe that in case with the Schottky diode MR and MI effects are originate from magnetic field influence on impact ionization process by two different ways. First, the trajectory of the electron is deflected by a magnetic field, which suppresses acquisition of kinetic energy and therefore impact ionization. Second, the magnetic field gives rise to shift of the acceptor energy levels in silicon to a higher energy. As a result, the activation energy for impact ionization significantly increases and consequently threshold voltage rises. Moreover, the second mechanism (acceptor level energy shifting in magnetic field) can be responsible for giant LMPE.

Room temperature magneto-transport properties of La0.7Ba0.3MnO3 manganite

Polycrystalline La0.7Ba0.3MnO3 manganite compound with high homogeneity was prepared by the sol-gel method. This compound shows a metal-semiconductor dc resistivity transition at Tms temperature of 300 K. The resistivity shows a dc electric field dependence relation, where, it increases monotonically with the applied dc electric current. In contrast, the Tms is insensitive to the applied electric field, where, it is kept unchanged. The dramatic increase in the dc resistivity with the applied electric current leads to a positive electroresistance that also increases monotonically with the applied electric current. The ac resistivity and the effect of low frequencies are also studied, the results show the monotonic increase in the ac resistivity with increasing the frequency due to the skin effect. The zero frequency (dc) magnetoresistance is enhanced with the frequency increase, where, the magnetoresistance peak shows the values −2.05, −2.9, −4.5 and −4.8% for dc, 3, 70 and 128 Hz, respectively. The dc magnetization measurement shows the ferromagnetic-paramagnetic transition at 300 K revealing the room temperature magnetocaloric properties for the La0.7Ba0.3MnO3 compound. For example, it shows a magnetic entropy change (ΔS) of 1.3 J kg−1 K−1 with a relative cooling power of 41 J kg−1 at 2 T applied magnetic field. In addition, the experimental data of ΔS were modeled by Landau theory that proves the absence of elastic, magnetoelastic and magnetoelectronic coupling effects in the magnetocaloric properties.

Room temperature giant magnetoimpedance in polycrystalline La0.75Ba0.25MnO3

We report the magnetic field dependence of electrical impedance Z in polycrystalline La0.75Ba0.25MnO3 a function of frequency from f = 1 MHz to 1 GHz at room temperature. The magnetoimpedance [MI = ΔZ/Z(0) where ΔZ = Z(H)-Z(0)] is −37.5 % (-58.5%) at 1(10) MHz for H = 1.2 kOe, which far exceeds mere -1.15% dc magnetoresistance. As f increases above 10 MHz, MI decreases in magnitude and changes sign from negative to positive. The change in sign of MI results from the transition of a single peak at H = 0 to two peaks at H = ± Hp accompanied by a minimum at the origin. Hp shifts towards higher field with increasing frequency. The occurrence of giant radio-frequency magnetoimpedance is promising for low-field sensor applications.

Anomalous Magnetic and Electrical Transport Behavior in Intermetallic Co&lt;sub&gt;58.5&lt;/sub&gt;Ga&lt;sub&gt;41.5&lt;/sub&gt;

Equi-atomic intermetallic Co-Ga compound exhibits unusual properties, such as, non-metallic electrical transport and weakly magnetic behavior. Electronic band structure calculations suggest the presence of deep psuedogap in the density of states near Fermi level. As the cobalt concentration varies over 50–60 at% it presents an interesting magnetic phase diagram along with drastic variation in electronic properties across a critical concentration (x∼58) in Co x Ga 100-x alloy system. The magnetic characteristics observed in these alloy were attributed to antisite defects [1, 2]. Despite several works on this alloy no clear consensus emerged on the nature of magnetic phase transitions observed and the magnetic ground state. A detailed analysis of thermal and magnetic field (H) dependence of magnetization (M) can throw light on magnetic ground state and the origin of anomalous behavior at critical composition. In this work we present dc magnetization, magnetoresistance (MR) and specific heat study of Co 58.5 Ga 41.5 composition prepared under different process conditions to elucidate magnetic ground state.

Preparation of Anomalous Magnetoresistance and Transport Properties of Itinerant Ferromagnet Fe1–xCoxSi

DC magnetization, electrical transport, and magnetoresistance (MR) measurements of Fe1– x Co x Si single crystals with $x \approx $ 0.7–0.3 are reported. Fe1– x Co x Si series presents a typical Stoner band excitation characteristic in the field-dependent magnetization profile. Co-doping ( x > 0.3) in the Fe1– x Co x Si compound shows a semiconductor–to–metal transition. Enhanced positive MR has been observed when $\sim$ 1/3 of the Fe was substituted with Co, which suggests that the spin density of states in the minor band (semiconductor characteristic) dominates the electronic properties.

Bias-voltage-controlled ac and dc magnetotransport phenomena in hybrid structures

We report some ac and dc magnetotransport phenomena in silicon-based hybrid structures. The giant impedance change under an applied magnetic field has been experimentally found in the metal/insulator/semiconductor (MIS) diode with the Schottky barrier based on the Fe/SiO2/p-Si and Fe/SiO2/n-Si structures. The maximum effect is found to observe at temperatures of 10–30K in the frequency range 10Hz–1MHz. Below 1kHz the magnetoresistance can be controlled in a wide range by applying a bias to the device. A photoinduced dc magnetoresistance of over 104% has been found in the Fe/SiO2/p-Si back-to-back Schottky diode. The observed magnetic-field-dependent effects are caused by the interface states localized in the insula-tor/semiconductor interface.

Low-Field Ac Magnetotransport in La0.6Bi0.1Sr0.3MnO3

We report temperature and field dependence of ac resistance (Z') and reactance (Z") of La0.6Bi0.1Sr0.3MnO3 in the radio frequency range (f = 0.1-5 MHz) under low dc magnetic fields (H {less than or equal to} 1 kOe). While the Z' in zero field shows a weak peak around the ferromagnetic-paramagnetic transition (TC) for f {less than or equal to} 100 kHz, it shows an abrupt jump around the TC for f = 5 MHz. In contrast, the Z" shows an abrupt increase around the TC for all frequencies. We found a maximum ac magnetoresistance of -24.2% at f = 5 MHz and magnetoreactance of -19.6% at f = 0.1 MHz for ΔH = 500 Oe. In comparison to ac magnetoresistance, the dc magnetoresistance shows similar value for ΔH = 50 kOe. It is suggested that the huge ac magnetoresistance arises from the suppression of ac transverse permeability which results in enhanced electromagnetic skin depth under a magnetic field.

Magnetocaloric effect and magnetothermopower in the room temperature ferromagnet Pr0.6Sr0.4MnO3

We have investigated magnetization (M), magnetocaloric effect (MCE), and magnetothermopower (MTEP) in polycrystalline Pr0.6Sr0.4MnO3, which shows a second-order paramagnetic to ferromagnetic transition near room temperature (TC = 305 K). However, field-cooled M(T) within the long range ferromagnetic state shows an abrupt decrease at TS = 86 K for μ0H &amp;lt; 3 T. The low temperature transition is first-order in nature as suggested by the hysteresis in M(T) and exothermic/endothermic peaks in differential thermal analysis for cooling and warming cycles. The anomaly at TS is attributed to a structural transition from orthorhombic to monoclinic phase. The magnetic entropy change (ΔSm = Sm(μ0 H)-Sm(0)) shows a negative peak at TC (normal MCE) and a positive spike (inverse MCE) at TS. ΔSm = −2.185 J/kg K (−3.416 J/kg K) with refrigeration capacity RC = 43.4 J/kg (103.324 J/kg) for field change of μ0ΔH = 1.5 T (3 T) at TC = 304 K is one of the largest values reported in manganites near room temperature. Thermopower (Q) is negative from 350 K to 20 K, which shows a rapid decrease at TC and a small cusp around TS in zero field. The MTEP [ΔQ/Q(0)] reaches a maximum value of 25% for μ0ΔH = 3 T around TC, which is much higher than 15% dc magnetoresistance for the same field change. A linear relation between MTEP and magnetoresistance and between ΔSm and ΔQ are found near TC. Further, ac magnetotransport in low dc magnetic fields (μ0 H ≤ 0.1 T), critical analysis of the paramagnetic to ferromagnetic transition, and scaling behavior of ΔSm versus a reduced temperature under different magnetic fields are also reported. Coexistence of large magnetic entropy change and magnetothermopower around room temperature makes this compound interesting for applications.

Low-Field Giant Magnetoimpedance and AC Huge Magnetoresistance for La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Sol–Gel Manganites

The sol-gel La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> shows a very small dc magnetoresistance (Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = -3.5%) under H=10 kOe at room temperature. However, a giant magnetoimpedance (GMI effect) of Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Z</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = -21.3% at 100 kHz and a huge ac magnetoresistance of Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = -52% at 1 MHz could be obtained for La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> sol-gel manganite under a low field of H=500 Oe. For La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> sol-gel manganites, the thicker sample prefers to have a stronger magnetoimpedance at low frequencies. The GMI and huge ac magnetoresistance at frequencies with strong skin effect are mainly connected with the field induced variation of transverse permeability via the penetration depth.

Anomalous galvanomagnetism, cyclotron resonance, and microwave spectroscopy of topological insulators

The surface quantum Hall state, magneto-electric phenomena and their connection to axion electrodynamics have been studied intensively for topological insulators. One of the obstacles for observing such effects comes from nonzero conductivity of the bulk. To overcome this obstacle we propose to use an external magnetic field to suppress the conductivity of the bulk carriers. The magnetic field dependence of galvanomagnetic and electromagnetic responses of the whole system shows anomalies due to broken time-reversal symmetry of the surface quantum Hall state, which can be used for its detection. In particular, we find linear bulk dc magnetoresistivity and a quadratic field dependence of the Hall angle, shifted rf cyclotron resonance, nonanalytic microwave transmission coefficient and saturation of the Faraday rotation angle with increasing magnetic field or wave frequency.