Isothermal Magnetoresistance Research Articles

Isotropic Quantum Griffiths Singularity in Nd_{0.8}Sr_{0.2}NiO_{2} Infinite-Layer Superconducting Thin Films.

This work reports on the emergence of quantum Griffiths singularity (QGS) associated with the magnetic field induced superconductor-metal transition (SMT) in unconventional Nd_{0.8}Sr_{0.2}NiO_{2} infinite layer superconducting thin films. The system manifests isotropic SMT features under both in-plane and perpendicular magnetic fields. Importantly, after scaling analysis of the isothermal magnetoresistance curves, the obtained effective dynamic critical exponents demonstrate divergent behavior when approaching the zero-temperature critical point B_{c}^{*}, identifying the QGS characteristics. Moreover, the quantum fluctuation associated with the QGS can quantitatively explain the upturn of the upper critical field around zero temperature for both the in-plane and perpendicular magnetic fields in the phase boundary of SMT. These properties indicate that the QGS in the Nd_{0.8}Sr_{0.2}NiO_{2} superconducting thin film is isotropic. Moreover, a higher magnetic field gives rise to a metallic state with the resistance-temperature relation R(T) exhibiting lnT dependence among the 2-10K range and T^{2} dependence of resistance below 1.5K, which is significant evidence of Kondo scattering. The interplay between isotropic QGS and Kondo scattering in the unconventional Nd_{0.8}Sr_{0.2}NiO_{2} superconductor can illustrate the important role of rare region in QGS and help to uncover the exotic superconductivity mechanism in this system.

Emergence of quantum Griffiths singularity in disordered TiN thin films

The association of quantum Griffiths singularity (QGS) to the magnetic-field-induced superconductor-metal transition predicts the unconventional diverging behaviour of dynamical critical exponent in low disorder crystalline two-dimensional superconductors. But whether this state exists in the superconducting systems exhibiting superconductor-insulator transition remains elusive. Here, we report the emergence of quantum Griffiths singularity in ultrathin disordered TiN thin films with more than two orders of magnitude variation in their normal state resistance. For both superconductor-metal transition and superconductor-insulator transition types, a diverging critical exponent is observed while approaching the quantum phase transition. Further, the magnetoresistance isotherms obey a direct activated scaling governed by an infinite-randomness fixed critical point. Finally, this work establishes the robustness of the QGS phenomenon towards a wide range of temperature and also towards a wide range of disorder strength as correlated with the normal state resistance.

Quantum phase transition in NbN superconducting thin films

We systematically investigated the low-temperature transport properties of a series of NbN epitaxial films with thickness $t$ ranging from $\sim$2.0 to $\sim$4.0 nm. The films undergo a superconductor-insulator transition (SIT) with decreasing film thickness, and the critical sheet resistance for the SIT is close to the quantum resistance of Cooper pairs $h/4e^2$ (6.45 k$\Omega$). Besides the Berezinski-Koterlitz-Thouless transition, a magnetic-field-driven SIT is observed in those two-dimensional (2D) superconducting films (2.6 nm $\lesssim t \lesssim 4.0$ nm). Interestingly, it is found that the low-temperature magnetoresistance isotherms do not cross at a single fixed point but at a well-distinguished region for these superconducting films. The dynamical critical exponent obtained by analyzing these magnetoresistance isotherms is divergent as the quantum critical point is being approached. The behavior of the dynamical critical exponent, originating from quenched disorder at ultralow temperatures, provides direct evidence for the occurrence of quantum Griffiths singularity in the quantum phase transition process of the films. The field-driven anomalous metal (quantum metal) state does not appear in these films. Our results suggest that the quantum Griffiths singularity not only occurs in the highly crystalline 2D superconductors with superconductor-metal transition but also in those with SIT.

Magnetic and Transport Anomalies and Large Magnetocaloric Effect in Cubic R4PtAl (R = Ho and Er)

We report the electronic properties of R4PtAl (R = Ho, and Er), which contains three sites for R, by the measurements of magnetization (ac and dc), heat-capacity, transport, and magnetoresistance (MR). Dc magnetization data reveal antiferromagnetic order below 19 K and 12 K in Ho and Er compounds, respectively. Additional features observed at lower temperatures (12 K for Ho4PtAl and 5 K for Er4PtAl) are akin to the cluster spin-glass phase. Resistivity data exhibit a weak minimum at a temperature marginally higher than their respective Néel temperature (TN), which is unusual for such rare-earths with well-localized 4f states. Isothermal magnetization and magnetoresistance data well below TN exhibit signatures of a subtle field-induced magnetic transition for a small magnetic field (<10 kOe). Notably, the isothermal entropy change at TN has the largest peak value within this rare-earth family; for a field change from zero to 50 kOe, the entropy change is ~14.5 J/kg K (Ho4PtAl) and ~21.5 J/kg K (Er4PtAl) suggesting a role of anisotropy of 4f orbital in determining this large value. The results provide some clues for the advancement of the field of magnetocaloric effect. The magnetocaloric property of Er4PtAl is nonhysteretic, meeting a challenge to find materials with reversible magnetocaloric effect.

Study of metal–insulator like transition in Co[formula omitted]Fe[formula omitted]S[formula omitted

The metal–insulator like transition of the ferromagnetic (FM) state in Co1−xFexS2 (0 ≤x≤ 0.7) with varying x is investigated using X-ray diffraction, Mössbauer spectroscopy, and magnetoresistance measurements. The linear decrease of unit cell parameter with x confirmed Fe substitution at the Co site. The room temperature Mössbauer study showed the absence of magnetic moment on Fe and both isomer shift and quadrupole splitting were found to vary linearly with x. The temperature-coefficient-of-resistivity (TCR) in the FM state became negative for x≥ 0.15. For intermediate compositions i.e., x= 0.05 and 0.1, TCR changed from positive to negative (T ≤ TC) to positive with an increase in temperature. By studying the temperature-dependent and isothermal magnetoresistance data around the transition region, it was shown that resultant behavior could be arising due to changes in spin polarization over a broad temperature range.

Spin fluctuations and superconductivity in Y5Rh6Sn18 doped with Pd and Co: Evidence of peak effect in the superconducting mixed state

We performed a systematic study of the nonmagnetic skutterudite-related ${\mathrm{Y}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$ superconductor, where the lattice disorder on the coherence length scale $\ensuremath{\xi}$ additionally generates a nonhomogeneous, high-temperature superconducting phase with disorder-enhanced critical temperature ${T}_{c}^{★}$. We have previously discussed various possibilities of local atomic disorder; one of the possibilities is doping. Our present studies focus on the series of ${\mathrm{Y}}_{5\ensuremath{-}\ensuremath{\delta}}({\mathrm{Rh}}_{5.5}{M}_{0.5}){\mathrm{Sn}}_{18}$ compounds ($\ensuremath{\delta}\ensuremath{\ll}1$), where the dopants $M=$ Co, Ir, Ru, and Pd, when they are smaller (Co) or larger (Pd) than Rh, generate the peak effect in fields smaller than the critical field ${H}_{c2}$. This phenomenon manifests itself as a weak peak in the real and imaginary parts of ac susceptibility and is more distinct in magnetoresistance. Using a simple theoretical model, we demonstrate that the effectiveness of this mechanism depends not only on the magnitude of the difference between the size of the dopant and the host atom, but also on whether the dopant is smaller or larger. The agreement between this prediction and our experimental data strongly supports the impurity-based scenario of the observed peak effect. Magnetoresistance isotherms of the remaining samples ($M=$ Ir and Ru) with the radius of $M$ being very similar to that of Rh show, however, a weak peak-effect-like behavior, which is mainly due to vacancies $\ensuremath{\delta}$ at Y sites, while the corresponding ac susceptibility isotherms exhibit a distinct peak at $H\ensuremath{\sim}{H}_{c2}$. This field-dependent ${\ensuremath{\chi}}_{\mathrm{ac}}$ anomaly appears to be similar in nature to the peak effect; however, it cannot be attributed to pinning and appears to be an equilibrium property of the system. We also report the coexistence of spin fluctuations and superconductivity for ${\mathrm{Y}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$ doped with Pd and Co.

Magnetic field induced transition from a vortex liquid to Bose metal in ultrathin a -MoGe thin film

We identify a magnetic field induced transition from a vortex liquid to Bose metal in a 2-dimensional amorphous superconductor, a-MoGe, using a combination of magnetotransport and scanning tunnelling spectroscopy (STS). Below the superconducting transition, Tc ~ 1.36 K, the magnetoresistance isotherms cross at a nearly temperature independent magnetic field, H_c^*~ 36 kOe. Above this field, the temperature coefficient of resistance is weakly negative, but the resistance remains finite as T --> 0, as expected in a bad metal. From STS conductance maps at 450 mK we observe a very disordered vortex lattice at very low fields that melts into a vortex liquid above 3 kOe. Up to H_c^* the tunnelling spectra display superconducting gap and coherence peak over a broad background caused by electron-electron interactions, as expected in a vortex liquid. However, above H_c^* the tunnelling spectra continue to display the gap but the coherence peak gets completely suppressed, suggesting that Cooper pairs lose their phase coherence. We conclude that H_c^* demarcates a transition from a vortex liquid to Bose metal, that eventually transforms to a regular metal at a higher field H* where the gap vanishes in the electronic spectrum.

Effect of substrate and Fe/Rh stoichiometry on first order antiferromagnetic–ferromagnetic transition in FeRh thin films

The effect of substrate induced strain and Fe/Rh stoichiometry on the antiferromagnetic (AFM)-ferromagnetic (FM) first order transition in sputter deposited FeRh thin films has been studied. Fe1−xRhx films of about 50 nm thickness and with x = 0.45, 0.50, 0.55 were deposited using magnetron sputtering on MgO (001), Al2O3 (0001) (AlO) and quartz (Qtz) substrates. These films were found to be epitaxial under compressive strain on MgO and polycrystalline under tensile strain on AlO. Films with x = 0.50 and 0.55 showed a first order AFM-FM transition near or above 300K. Our study shows that the transition temperature (Tt) is dictated by strain as it was found to vary linearly with out-of-plane strain for films on MgO and AlO. In the case of films on Qtz substrate, an open loop in isothermal magnetoresistance (MR) with a large negative MR was observed at room temperature.

Magnetic monopole density and antiferromagnetic domain control in spin-ice iridates

Magnetically frustrated systems provide fertile ground for complex behaviour, including unconventional ground states with emergent symmetries, topological properties, and exotic excitations. A canonical example is the emergence of magnetic-charge-carrying quasiparticles in spin-ice compounds. Despite extensive work, a reliable experimental indicator of the density of these magnetic monopoles is yet to be found. Using measurements on single crystals of Ho2Ir2O7 combined with dipolar Monte Carlo simulations, we show that the isothermal magnetoresistance is highly sensitive to the monopole density. Moreover, we uncover an unexpected and strong coupling between the monopoles on the holmium sublattice and the antiferromagnetically ordered iridium ions. These results pave the way towards a quantitative experimental measure of monopole density and demonstrate the ability to control antiferromagnetic domain walls using a uniform external magnetic field, a key goal in the design of next-generation spintronic devices.

Giant spontaneous resistivity jumps in La0.825Ce0.175Fe12B6

We report the observation of spontaneous and very sharp resistivity jumps at low temperatures in the antiferromagnetic itinerant-electron system La0.825Ce0.175Fe12B6. This intermetallic compound undergoes multiple magnetic transformations, antiferromagnetic–ferromagnetic (AFM–FM) and ferromagnetic–paramagnetic (FM–PM), triggered by changes in both the temperature and the applied magnetic field. The magnetoresistance isotherms display irreversible abrupt steps at T ≤ 4 K, whereas the field dependence of the resistivity becomes smooth above 4 K. Meanwhile, the evolution with time of the electrical resistivity exhibits a huge spontaneous jump after an incubation time when both the applied magnetic field and temperature are constant. A giant negative magnetoresistance (MR = −71%) is discovered associated with the magnetic-field-induced first-order AFM-FM transition.

Kinetics of topological phase transitions and topological phases in the Josephson medium of granular high-temperature superconductors under the action of temperature, magnetic field and transport current

The paper we presented is essentially the logical conclusion of a large-scale work - from developing a strategy for detecting topological BKT phase transitions in a Josephson medium of granular high-temperature superconductors to establishing the identity of topological phases arising from two fundamentally different types of external influence. The work is devoted to the study of the topological phases arising in the two-dimensional Josephson medium of granular high-temperature superconductors YBa2Cu3O7-δ during the Berezinskii-Kosterlits-Thouless phase transitions (BKT transition) under the influence of temperature, external magnetic field and transport current. The features of the behavior of magnetoresistance isotherms R(Нext)T=const and current – voltage characteristics (CVCs) E(j)T=const in the vicinity of the temperatures of the BKT phase transitions have been discovered. It has been experimentally established that granular HTSC YBa2Cu3O7-δ is a unique object for studying the influence of external actions (magnetic field, transport current) on topological phase transitions and topological phases, since in its crystal structure there are no sources of an internal magnetic field.

Multiple crossing points and possible quantum criticality in the magnetoresistance of thin TiN films

We have measured $R(T,B)$ of a TiN thin-film very close to the disorder-driven superconductor-insulator transition but still superconducting at zero field and low temperatures. In a magnetic field we find three distinct crossing point of the magnetoresistance isotherms occur at magnetic fields $B_{cX}$ in three different temperature regions. Each crossing point in $R(T,B)$ corresponds to a plateau in $R(T,B_{cX})$. We systematically study the evolution of these crossing point near the disorder-induced superconductor/insulator transition, identify the most promising candidate for a quantum phase transition, and provide estimates for the two critical exponents $z$ and $\nu$.

Magnetocaloric and magnetoresistance properties of reentrant spin glass Tb2Ni0.94Si3.2 alloy

Magnetic, magnetocaloric, and magnetoresistance properties of polycrystalline Tb2Ni0.94Si3.2 alloy have been investigated. Magnetic susceptibilities (AC and DC), remanent magnetization and the heat capacity studies of Tb2Ni0.94Si3.2 provide evidence for the spin glass behavior below the temperature, Tf = 5.2 K. At the Neel temperature, TN = 12.7 K, Tb2Ni0.94Si3.2 alloy orders antiferromagnetically. The presence of metamagnetic transition is observed in isothermal magnetization and magnetoresistance studies. Also, a magnetoresistance of 22% is exhibited by the alloy at temperature T = 4 K in an applied magnetic field of 9 T. A magnetic entropy change of 12 J/kg K with the relative cooling power of 504 J/kg for a magnetic field change of 9 T is observed in the studied alloy. Influence of spin fluctuations and short-range ferromagnetic correlations is reflected in magnetoresistance and relative cooling power. The magnetic and magnetoresistance properties make this alloy as a good magnetocaloric material with moderate magnetoresistance.

Nonreciprocal superconducting NbSe2 antenna

The rise of two-dimensional (2D) crystalline superconductors has opened a new frontier of investigating unconventional quantum phenomena in low dimensions. However, despite the enormous advances achieved towards understanding the underlying physics, practical device applications like sensors and detectors using 2D superconductors are still lacking. Here, we demonstrate nonreciprocal antenna devices based on atomically thin NbSe2. Reversible nonreciprocal charge transport is unveiled in 2D NbSe2 through multi-reversal antisymmetric second harmonic magnetoresistance isotherms. Based on this nonreciprocity, our NbSe2 antenna devices exhibit a reversible nonreciprocal sensitivity to externally alternating current (AC) electromagnetic waves, which is attributed to the vortex flow in asymmetric pinning potentials driven by the AC driving force. More importantly, a successful control of the nonreciprocal sensitivity of the antenna devices has been achieved by applying electromagnetic waves with different frequencies and amplitudes. The device’s response increases with increasing electromagnetic wave amplitude and exhibits prominent broadband sensing from 5 to 900 MHz.

Charge conduction mechanisms and MR behaviour of sol–gel-grown nanostructured La0.6Nd0.1Sr0.3MnO3 manganites

In this communication, structure and charge, conduction mechanisms have been understood for sol–gel-grown nanostructured La0.6Nd0.1Sr0.3MnO3 (LNSMO) manganites prepared at different process temperatures under air and oxygen environments. Obtained X-ray diffraction patterns of all the samples were analysed using Rietveld refinements and obtained structural lattice parameters have been discussed in correlation with resistivity behaviour of the samples. Observed low temperature resistivity upturn behaviour has been examined in the context of electron–electron scattering mechanism. Metallic and insulating/semiconducting behaviours of all the nanostructured LNSMO manganites have been understood by using various models and mechanisms. Magnetoresistance isotherms have also been theoretically fitted and separate grain and grain boundary contributions have been studied for LNSMO manganite samples. All obtained fitting parameters have been discussed in the context of role of applied magnetic field, process temperature and annealing environment.

Magnetotransport and magnetic properties of the layered noncollinear antiferromagnetic Cr2Se3 single crystals

We report on the growth of high-quality stoichiometric layered Cr2Se3 single crystals with metallic and noncollinear antiferromagnetic ground state using the chemical vapor transport (CVT) method. The crystals show weak ferromagnetism in the in-plane and out-of-plane directions below the Neél temperature (TN), however, the field-cooled out-of-plane magnetization at 500 Oe and 10 K (∼0.24 μB/f.u.) is approximately 15 times larger than that of the in-plane one, indicating strong c-axis easy uniaxial magnetic anisotropy, which is further supported by the in-plane and out-of-plane isothermal anisotropic magnetic hysteresis loops and the angular dependent magnetoresistance (MR). The latter also reveals a decrease of the coercive field of the crystal upon the tilting of the weak ferromagnetic easy axis away from the direction of the magnetic field. Further, the out-of-plane isothermal MR are negative below TN and show butterfly shapes for T < 10 K and couple with the magnetic hysteresis M(H) loop. These results may help researchers better understand the interplay between the weak ferromagnetism and the magnetotransport properties of 2D itinerant noncollinear antiferromagnetic systems.

Scaling analysis of anomalous Hall resistivity in the Co2TiAl Heusler alloy

A comprehensive magnetotransport study including resistivity (ρxx), isothermal magnetoresistance, Hall resistivity (ρxy) and magnetization have been carried out at different temperatures on the Co2TiAl Heusler alloy. Co2TiAl alloy shows a paramagnetic to ferromagnetic (FM) transition below the Curie temperature (TC) ∼ 125 K. In the FM region, resistivity and magnetoresistance reveal a spin flip electron–magnon scattering and the Hall resistivity unveils the anomalous Hall resistivity. Scaling of anomalous Hall resistivity with resistivity establishes the extrinsic scattering process responsible for the anomalous Hall resistivity; however skew scattering is the dominant mechanism compared to the side-jump contribution. A one to one correspondence between magnetoresistance and side-jump contribution to anomalous Hall resistivity verifies the electron–magnon scattering being the source of side-jump contribution to the anomalous Hall resistivity.

Superconducting properties of tungsten nanowires fabricated using focussed ion beam technique

We report superconducting properties of tungsten meander structures fabricated using the focussed ion beam (FIB) induced technique. Three meander structures with individual line widths of ∼70, ∼300 and ∼450 nm were fabricated for evaluation and comparison of the superconducting properties. The resistance-temperature characteristics of the meanders were measured and analysed down to a temperature of 100 mK. The superconducting properties such as critical temperature (TC) and upper-critical field (HC2) of these wires are in comparison to the reported values of FIB deposited tungsten available in literature. While the normal state resistance increases sharply as the width of the wire decreases, the superconducting transition temperature registered a slight decrease. Significant amount of residual resistance (3.8% of normal state value at 100 mK) was observed for the sample with the lowest width (70 nm). The residual resistance trails as function of temperature was analysed invoking theoretical models governing the phase slip induced dissipations in superconducting nanowires. The results indicated signature of phase slips as the width of the wire decreases: thermally activated phase slips dominant near to the TC and quantum phase slip (QPS) near to TC as well as much below to the TC. The magneto-resistance isotherms indicated quantum phase transitions (QPT); typical of a superconductor-to-insulator transition (SIT) driven by magnetic field. The SIT transition which originates from the intrinsic disorder present in the sample can be tuned by an external parameter such as magnetic field, and can be modelled by standard theories of QPT for quasi 2D or (2 + 1) D XY models. The successful fabrication of meander structures of W using FIB and the demonstration of superconductivity suggest that FIB deposited W can be exploited for many of the technological applications of superconducting nanowires such as superconducting nanowire single photon detectors, bolometers, transition edge sensors and even for quantum current standard employing the QPS phenomenon.

Reentrant spin-glass and transport behavior of Gd4PtAl, a compound with three sites for Gd

We report temperature (T) dependence (2-330 K) of DC and AC magnetization (M), isothermal remnant magnetization (M_IRM), heat capacity (C), electrical resistivity (rho), and magnetoresistance (MR) of a ternary intermetallic compound, Gd4PtAl, crystallizing in a cubic (space group F-43m) structure. In this structure, there are three sites for the rare-earth. The magnetization data reveal that, in addition to a magnetic transition at 64 K, there is another magnetic feature below 20 K. The C(T) data reveal an upturn below 64 K, shifting to a lower temperature with increasing field, which establishes that the onset of magnetic order is of an antiferromagnetic type. However, there is no worthwhile feature near 20 K in the C(T) curve. AC susceptibility peak undergoes an observable change with frequency and, in particular, the peak around 20 K gets suppressed with the application of a dc magnetic field; in addition, M_IRM undergoes a slow decay with time and isothermal M exhibits low-field hysteresis below 20 K only, which is typical of spin-glasses. The results overall suggest that this compound is a reentrant spin-glass in zero-field. There are experimental signatures pointing to the existence of both antiferromagnetic and ferromagnetic components, competing with the variation of temperature and magnetic field, as a result of which electrical and magnetoresistance behaviors are peculiar. The results overall suggest this compound exhibits interesting magnetic and transport properties.

Giant negative magnetoresistance and kinetic arrest of first-order ferrimagnetic-antiferromagnetic transition in Ge doped Mn2Sb

The effect of partial substitution of Ge for Sb on the first-order ferrimagnetic (FRI)–antiferromagnetic (AFM) transition in Mn2Sb has been studied. It shows that the transition temperature (Tt) can be tuned between 119 K and 271 K by substituting 2.5%–10% Ge at Sb sites in Mn2Sb. The variation of density of state at the Fermi level N(Ef) with Ge substitution shows that dN(E)/dE is positive at Ef in the AFM state. With the application of a magnetic field, Tt shifts to low temperature, which results in a giant negative magnetoresistance (MR) reaching a value of 70% for 2.5% substitution. Our results show that FRI to AFM transformation during cooling stops around 35 K, even though it remains incomplete. This, along with the nonmonotonic variation of lower critical field, open loop in isothermal MR, and increasing difference in zero field cooled warming and field cooled warming resistivity with increasing magnetic field, shows that the FRI to AFM transition is kinetically arrested in the case of 2.5% Ge substitution.