Colossal Negative Magnetoresistance Research Articles

Magnetism in the axion insulator candidate Eu5In2Sb6

Eu5In2Sb6 is a member of a family of orthorhombic nonsymmorphic rare-earth intermetallics that combines large localized magnetic moments and itinerant exchange with a low carrier density and perpendicular glide planes. This may result in special topological crystalline (wallpaper fermion) or axion insulating phases. Recent studies of Eu5In2Sb6 single crystals have revealed colossal negative magnetoresistance and multiple magnetic phase transitions. Here, we clarify this ordering process using neutron scattering, resonant elastic x-ray scattering, muon spin-rotation, and magnetometry. The nonsymmorphic and multisite character of Eu5In2Sb6 results in coplanar noncollinear magnetic structures with an Ising-like net magnetization along the a axis. A reordering transition, attributable to competing ferro- and antiferromagnetic couplings, manifests as the onset of a second commensurate Fourier component. In the absence of spatially resolved probes, the experimental evidence for this low-temperature state can be interpreted either as an unusual double-q structure or in a phase separation scenario. The net magnetization produces variable anisotropic hysteretic effects which also couple to charge transport. The implied potential for functional domain physics and topological transport suggests that this structural family may be a promising platform to implement concepts of topological antiferromagnetic spintronics. Published by the American Physical Society 2024

Colossal magnetoresistance in the multiple wave vector charge density wave regime of an antiferromagnetic Dirac semimetal.

Colossal negative magnetoresistance is a well-known phenomenon, notably observed in hole-doped ferromagnetic manganites. It remains a major research topic due to its potential in technological applications. In contrast, topological semimetals show large but positive magnetoresistance, originated from the high-mobility charge carriers. Here, we show that in the highly electron-doped region, the Dirac semimetal CeSbTe demonstrates similar properties as the manganites. CeSb0.11Te1.90 hosts multiple charge density wave modulation vectors and has a complex magnetic phase diagram. We confirm that this compound is an antiferromagnetic Dirac semimetal. Despite having a metallic Fermi surface, the electronic transport properties are semiconductor-like and deviate from known theoretical models. An external magnetic field induces a semiconductor metal-like transition, which results in a colossal negative magnetoresistance. Moreover, signatures of the coupling between the charge density wave and a spin modulation are observed in resistivity. This spin modulation also produces a giant anomalous Hall response.

Colossal negative magnetoresistance in field-induced Weyl semimetal of magnetic half-Heusler compound

The discovery of topological insulators and semimetals triggered enormous interest in exploring emergent electromagnetic responses in solids. Particular attention has been focused on ternary half-Heusler compounds, whose electronic structure bears analogy to the topological zinc-blende compounds while also including magnetic rare-earth ions coupled to conduction electrons. However, most of the research in this system has been in band-inverted zero-gap semiconductors such as GdPtBi, which still does not fully exhaust the large potential of this material class. Here, we report a less-studied member of half-Heusler compounds, HoAuSn, which we show is a trivial semimetal or narrow-gap semiconductor at zero magnetic field but undergoes a field-induced transition to a Weyl semimetal, with a negative magnetoresistance exceeding four orders of magnitude at low temperatures. The combined study of Shubnikov-de Haas oscillations and first-principles calculation suggests that the exchange field from Ho 4f moments reconstructs the band structure to induce Weyl points which play a key role in the strong suppression of large-angle carrier scattering. Our findings demonstrate the unique mechanism of colossal negative magnetoresistance and provide pathways towards realizing topological electronic states in a large class of magnetic half-Heusler compounds.

Coupled magnetic and structural transition in topological antiferromagnet EuAgAs under high pressure

Recently, EuAgAs and its derivatives have emerged as a promising platform for studying the interplay of transports, magnetism and topology. Herein, we report construction of a temperature-pressure phase diagram for antiferromagnetic Dirac semimetal EuAgAs, through electrical transport, synchrotron x-ray diffraction and Raman spectroscopy measurements in diamond anvil cells at high pressures up to 48.6 GPa. At Pc1∼3–5 GPa, we found a pressure-induced magnetic phase transition with accompanying a structural P63/mmc→Pnma transition. In the pristine antiferromagnetic phase, negative colossal magnetoresistance is observed mainly below the Néel temperature TN along with a field-induced metamagnetic transition; in the emergent magnetic phase, however, it is evident both above and below the magnetic critical temperature TC yet with no field-induced metamagnetic transition. These distinct features lead us to argue a ferromagnetic-like state for EuAgAs above Pc1, which is further supported by observation of a T5/3 temperature dependence of low-temperature resistance, a characteristic of marginal Fermi-liquid state expected to exist close to a ferromagnetic instability by spin fluctuations in metallic ferromagnets. At Pc2∼20 GPa, a possible phase transition of electronic origin may take place, which is revealed jointly by change of a T5/3→T5/2 temperature dependence of low-temperature resistance and change in slope of linear-in-T resistance at high temperatures.

A silicene-based 4f magnet probed by X-ray photoemission spectroscopy

The honeycomb silicene lattice, the Si counterpart of graphene, provides a building block to form 2D and layered materials with unconventional properties. In particular, silicene-based EuSi2 is a fascinating material morphing from a 3D antifferomagnet to a 2D ferromagnet, from a metal to a semiconductor demonstrating colossal negative magnetoresistance. However, the electronic structure of the material supporting these transformations remains a mystery – the results of computational studies are controversial. Here, we probe the band structure of EuSi2 employing X-ray photoemission spectroscopy. Ultrathin films of EuSi2 are synthesized on silicon by molecular beam epitaxy and characterized by diffraction techniques. Photoemission spectra, including those at the Eu 3d resonance, provide information on both the core levels and the valence band of EuSi2. A key characteristic is the position of the 4f states responsible for EuSi2 magnetism with respect to the Fermi level; its experimental value (about 1.3 eV) resolves the controversy in theoretical studies. The results shed light on the structure of the 4f magnet EuSi2 and form a basis for studies of related materials.

Colossal negative magnetoresistance in spin glass Na(Zn,Mn)Sb

We report the study of magnetic and transport properties of polycrystalline and single crystal Na(Zn,Mn)Sb, a new member of “111” type of diluted magnetic materials. The material crystallizes into Cu2Sb-type structure which is isostructural to “111” type Fe-based superconductors. With suitable carrier and spin doping, the Na(Zn,Mn)Sb establishes spin-glass ordering with freezing temperature (T f ) below 15 K. Despite lack of long-range ferromagnetic ordering, Na(Zn,Mn)Sb single crystal still shows sizeable anomalous Hall effect below T f . Carrier concentration determined by Hall effect measurements is over 1019 cm–3. More significantly, we observe colossal negative magnetoresistance (MR ≡ [ρ(H) − ρ(0)]/ρ(0)) of –94% in the single crystal sample.

Colossal negative magnetoresistance from hopping in insulating ferromagnetic semiconductors

Ferromagnetic semiconductor Ga1–x Mn x As1–y P y thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers. In this regime, we report a colossal negative magnetoresistance (CNMR) coexisting with a saturated magnetic moment, unlike in the traditional magnetic semiconductor Ga1– x Mn x As. By analyzing the temperature dependence of the resistivity at fixed magnetic field, we demonstrate that the CNMR can be consistently described by the field dependence of the localization length, which relates to a field dependent mobility edge. This dependence is likely due to the random environment of Mn atoms in Ga1–x Mn x As1–y P y which causes a random spatial distribution of the mobility that is suppressed by an increasing magnetic field.

Antisite disorder driven cluster glass state and colossal magnetoresistance in MnSb2Se4

Antisite disorder plays a significant role in determining the ground state of a magnetic material. The ternary chalcogenide material ${\mathrm{MnSb}}_{2}{\mathrm{Se}}_{4}$ with $\ensuremath{\sim}26%$ antisite disorder between Mn and Sb sites is known to be an antiferromagnet (${T}_{N}=22.5\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) and a type-II mulitferroic. In this paper, we report the effect of increased antisite disorder ($\ensuremath{\sim}40%$) on the magnetic and electrical transport properties of this compound. Magnetic susceptibility and heat-capacity measurements indicate the absence of long-range magnetic ordering. Furthermore, AC susceptibility measurements unfold its nonequilibrium magnetic dynamics at low temperatures. Data analysis with various dynamic scaling models, namely, power law, Arrhenius law, and Vogel-Fulcher confirm a cluster glass state. Moreover, the presence of magnetic memory effect and magnetic relaxation reveal the nonequilibrium dynamics of the system through many metastable states. In addition, a negative colossal magnetoresistance is observed below the freezing temperature due to competing ferromagnetic and antiferromagnetic interactions induced by increased antisite disorder. These results indicate that ${\mathrm{MnSb}}_{2}{\mathrm{Se}}_{4}$ is a new addition to a very small group of chalcogenides which show negative colossal magnetoresistance.

Consecutive topological phase transitions and colossal magnetoresistance in a magnetic topological semimetal

The combination of magnetic symmetries and electronic band topology provides a promising route for realizing topologically nontrivial quasiparticles, and the manipulation of magnetic structures may enable the switching between topological phases, with the potential for achieving functional physical properties. Here, we report measurements of the electrical resistivity of EuCd2As2 under pressure, which show an intriguing insulating dome at pressures between pc1 ~ 1.0 GPa and pc2 ~ 2.0 GPa, situated between two regimes with metallic transport. The insulating state can be fully suppressed by a small magnetic field, leading to a colossal negative magnetoresistance on the order of 105%, accessible via a modest field of ~ 0.2 T. First-principles calculations reveal that the dramatic evolution of the resistivity under pressure can be attributed to consecutive transitions of EuCd2As2 from a magnetic topological insulator to a trivial insulator, and then to a Weyl semimetal, with the latter resulting from a pressure-induced change in the magnetic ground state. Similarly, the colossal magnetoresistance results from a field-induced polarization of the magnetic moments, transforming EuCd2As2 from a trivial insulator to a Weyl semimetal. These findings underscore weak exchange couplings and weak magnetic anisotropy as ingredients for discovering tunable magnetic topological materials with desirable functionalities.

Effects of phosphorus doping on the physical properties of axion insulator candidate EuIn2As2

We report an investigation on the single crystal growth, magnetic and transport properties of EuIn2(As1−x P x )2 (0 ≤ x ≤ 1). The physical properties of axion insulator candidate EuIn2As2 can be effectively tuned by P-doping. With increasing x, the c-axis lattice parameter decreases linearly, the magnetic transition temperature gradually increases and ferromagnetic interactions are enhanced. This is similar to the previously reported high pressure effect on EuIn2As2. For x = 0.40, a spin glass state at T g = 10 K emerges together with the observations of a butter-fly shaped magnetic hysteresis and slow magnetic behavior. Besides, magnetic transition has great influence on the charge carriers in this system and negative colossal magnetoresistance is observed for all P-doped samples. Our findings suggest that EuIn2(As1−x P x )2 is a promising material playground for exploring novel topological states.

Current-Driven Spin Precession in Bulk Nd0.6Sr0.4MnO3

Perovskite manganites are known to exhibit colossal negative magnetoresistance (MR) for direct current. Here, we show that the magnetoresistance of a bulk Nd0.6Sr0.4MnO3 sample carrying radio frequ...

Colossal Negative Magnetoresistance Effect in a La1.37Sr1.63Mn2O7 Single Crystal Grown by Laser-Diode-Heated Floating-Zone Technique

We have grown La 1.37 Sr 1.63 Mn 2 O 7 single crystals with a laser-diode-heated floating-zone furnace and studied the crystallinity, structure, and magnetoresistance (MR) effect by in-house X-ray Laue diffraction, X-ray powder diffraction, and resistance measurements. The La 1.37 Sr 1.63 Mn 2 O 7 single crystal crystallizes into a tetragonal structure with space group I4/mmm at room temperature. At 0 T, the maximum resistance centers around ∼166.9 K. Below ∼35.8 K, it displays an insulating character with an increase in resistance upon cooling. An applied magnetic field of B = 7 T strongly suppresses the resistance indicative of a negative MR effect. The minimum MR value equals −91.23% at 7 T and 128.7 K. The magnetic-field-dependent resistance shows distinct features at 1.67, 140, and 322 K, from which we calculated the corresponding MR values. At 14 T and 140 K, the colossal negative MR value is down to −94.04(5)%. We schematically fit the MR values with different models for an ideal describing of the interesting features of the MR value versus B curves.

Spin glass behavior and colossal negative magnetoresistance of the p−Zn1−xMnxTe strongly doped with phosphorus

Magnetoresistance, ac and dc magnetization, and photoluminescence (PL) experiments have been performed on the $p$-type ${\mathrm{Zn}}_{0.99}{\mathrm{Mn}}_{0.01}\mathrm{Te}$ and ${\mathrm{Zn}}_{0.97}{\mathrm{Mn}}_{0.03}\mathrm{Te}$ alloys strongly doped with phosphorus (P). The investigated samples exhibit spin glass behavior with ferromagnetic regions near the freezing temperature ${T}_{f}=1.9$ K and colossal negative magnetoresistance (negMR) of $2.2\ifmmode\times\else\texttimes\fi{}{10}^{3}$ times at 4.2 K for $0\ensuremath{\le}B\ensuremath{\le}\ifmmode\pm\else\textpm\fi{}6$ T. The negMR curves contain occurrences of hysteresis providing evidence for the memory effect. Looking for possible ferromagnetic interactions responsible for the spin glass behavior at low hole densities, we observe in PL spectra that the P doping quenches internal recombination in ${\mathrm{Mn}}^{2+}$ ions. This strongly indicates a charge transfer between ${\mathrm{Mn}}^{2+}$ and ${\mathrm{P}}^{2\ensuremath{-}}$ ions leading to the creation of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{P}}^{3\ensuremath{-}}$ ions. The creation of ${\mathrm{Mn}}^{3+}$ is confirmed by an observation of strong PL enhancement at excitation energies lower than the band gap and PL study at various temperatures. The resulting simultaneous existence of the mixed valence ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Mn}}^{3+}$ ions can lead to double exchange mechanism of ferromagnetic interaction. Also, ${\mathrm{Mn}}^{3+}$ ions can exhibit a superexchange interaction leading to local ferromagnetic phases.

Room temperature Co-doped manganite/graphene sensor operating at high pulsed magnetic fields

The demand to increase the sensitivity to magnetic field in a broad magnetic field ranges has led to the research of novel materials for sensor applications. Therefore, the hybrid system consisting of two different magnetoresistive materials – nanostructured Co-doped manganite La1−xSrx(Mn1−yCoy)zO3 and single- and few-layer graphene – were combined and investigated as potential system for magnetic field sensing. The negative colossal magnetoresistance (CMR) of manganite-cobaltite and positive one of graphene gives the possibility to increase the sensitivity to magnetic field of the hybrid sensor. The performed magnetoresistance (MR) measurements of individual few layer (n = 1–5) graphene structures revealed the highest MR values for three-layer graphene (3LG), whereas additional Co-doping increased the MR values of nanostructured manganite films. The connection of 3LG graphene and Co-doped magnanite film in a voltage divider configuration significantly increased the sensitivity of the hybrid sensor at low and intermediate magnetic fields (1–2 T): 70 mV/VT of hybrid sensor in comparison with 56 mV/VT for 3LG and 12 mV/VT for Co-doped magnanite film, respectively, and broadened the magnetic field operation range (0.1–20) T of the produced sensor prototype.

Hybrid graphene-manganite thin film structure for magnetoresistive sensor application

An increasing demand of magnetic field sensors with high sensitivity at room temperatures and spatial resolution at micro-nanoscales has resulted in numerous investigations of physical phenomena in advanced materials, and fabrication of novel magnetoresistive devices. In this study the novel magnetic field sensor based on combination of a single layer graphene (SLG) and thin nanostructured manganite La0.8Sr0.2MnO3 (LSMO) film—hybrid graphene-manganite (GM) structure, is proposed and fabricated. The hybrid GM structure employs the properties of two materials—SLG and LSMO—on the nanoscale level and results in the enhanced sensitivity to magnetic field of the hybrid sensor on the macroscopic level. Such result is achieved by designing the hybrid GM sensor in a Wheatstone half-bridge which enables to employ in the device operation two effects of nanomaterials—large Lorentz force induced positive magnetoresistance of graphene and colossal negative magnetoresistance of nanostructured manganite film, and significantly increase the sensitivity S of the hybrid GM sensor in comparison with the individual SLG and LSMO sensors: S = 5.5 mV T−1 for SLG, 14.5 mV T−1 for LSMO and 20 mV T−1 for hybrid GM at 0.5 T, when supply voltage was 1.249 V. The hybrid GM sensor operates in the range of (0.1–2.3) T and has lower sensitivity to temperature variations in comparison to the manganite sensor. Moreover, it can be applied for position sensing. The ability to control sensor’s characteristics by changing technological conditions of the fabrication of hybrid structure and tuning the nanostructure properties of manganite film is discussed.

Layer-controlled laws of electron transport in two-dimensional ferromagnets

Recent discoveries of intrinsic two-dimensional ferromagnetism in layered compounds drive intense research on their fundamental properties and spintronic applications in atomically thin materials. In these materials, ferromagnetism engenders remarkable transport and optical phenomena and can be controlled electrically. However, coupling of carriers with magnetic moments in lateral charge transport is yet to be explored. Here, we report layer-dependent electron transport in two-dimensional rare-earth ferromagnets MSi 2 , formed by silicene, evolving from an antiferromagnetic metal in the bulk to a ferromagnetic semiconductor in one monolayer. Small integer numbers of monolayers (1, 2, etc. ) result in qualitatively different transport regimes following simple analytical laws. Most remarkably, a gap opening in the monolayer limit gives rise to a colossal negative magnetoresistance, depending exponentially on the magnetic field. The results call for applications in nanoelectronics and stimulate studies of fundamental spin behaviors.

Simultaneous magnetic and structural transitions in Nd0.15Ca0.85MnO3 manganite: Magnetization and neutron diffraction studies

The manganite Nd0.15Ca0.85MnO3 crystallizes in GdFeO3-type, orthorhombic structure (space group Pnma) at 300 K and orders antiferromagnetically at 112 K (TN). Magnetization-field (M-H) isotherms indicate metamagnetic transition at temperatures below TN. However, metamagnetism does not lead to saturation in magnetization. The magnetization value at 2 K in 140 kOe field is only 0.4 μB/f.u. The isothermal magnetic entropy change (ΔSm) has been calculated using the M-H data and inverse magnetocaloric effect is observed around TN and the maximum value of ΔSm is ∼12.2 J kg−1K−1 at 107.5 K for 140 kOe field change. Powder neutron diffraction study in zero applied field reveals monoclinic distortion of Nd0.15Ca0.85MnO3 crystal lattice and antiferromagnetic ordering of Mn sublattice in ac-plane with wave vector K = [1/2, 0, 1/2] at 125 K. From the neutron data, the Mn magnetic moment value at 3 K is found to be 2.54 μB. Electrical resistivity increases tremendously below TN to an insulating state and shows thermal hysteresis near TN in zero field. Large magnetic fields drive insulator to metal-like transition and negative colossal magnetoresistance of about 70% is realized at 50 K in 70 kOe field and that could be due to the field-induced order.

Electronic phase separation in insulating (Ga, Mn) As with low compensation: super-paramagnetism and hopping conduction

In the present work, low compensated insulating (Ga,Mn)As with 0.7% Mn is obtained by ion implantation combined with pulsed laser melting. The sample shows variable-range hopping transport behavior with a Coulomb gap in the vicinity of the Fermi energy, and the activation energy is reduced by an external magnetic field. A blocking super-paramagnetism is observed rather than ferromagnetism. Below the blocking temperature, the sample exhibits a colossal negative magnetoresistance. Our studies confirm that the disorder-induced electronic phase separation occurs in (Ga,Mn)As samples with a Mn concentration in the insulator–metal transition regime, and it can account for the observed superparamagnetism and the colossal magnetoresistance.

Negative Colossal Magnetoresistance Driven by Carrier Type in the Ferromagnetic Mott Insulator GaV4S8

We report here a study on the evolution of structural and electronic properties of the lacunar spinel compounds GaV4S8 with charge doping. In this ferromagnetic (FM) Mott insulator, the heterovalent substitutions of Ga3+ by Zn2+ or Ge4+ allow induction of charge doping either by holes or by electrons. We show that electron-doped GaV4S8 displays a bulk, negative, and colossal magnetoresistance (CMR) with a relative drop of resistivity reaching −80% at 7 T in the vicinity of the Curie temperature. Conversely, hole-doped GaV4S8 does not display any negative CMR but a classical positive magnetoresistance. This asymmetric electron–hole doping effect challenges the common view stating that CMR effects in doped FM Mott insulators depends only on the density of carrier and not on their electron/hole nature. We show that a simple model based on multiorbital effects and Hund’s rule is able to capture the presence (absence) of negative CMR in electron- (hole-) doped GaV4S8.

Nonlinear transport in two-dimensional electron gas exhibiting colossal negative magnetoresistance

We report on nonlinear transport measurements in a GaAs/AlGaAs quantum well exhibiting a colossal negative magnetoresistance effect. Under applied dc bias, the magnetoresistance becomes nonmonotonic, exhibiting distinct extrema that move to higher magnetic fields with increasing current. In the range of magnetic fields corresponding to the resistivity minimum at zero bias, the resistivity increases linearly with current and the rate of this increase scales with the inverse magnetic field. The latter observation is consistent with the theory, proposed more than 35 years ago, considering classical memory effects in the presence of strong, dilute scatterers.