Anomalous Hysteresis Loops Research Articles

Crystal growth and magnetic properties of atom K embedded in Na5Co15.5Te6O36

In this paper, we report the structural, magnetic and specific heat properties of a new compound Na2.67K1.33Pb0.67Co15.33Te6O36 (NKPCTO). The compound can be regarded as a new system charactered by the K and Pb atom embedded in the Kagome system Na5Co15.5Te6O36 (NCTO). In the new structure, we find that the disordered occupations of the Na and Co(3) atoms is irrelevant to K atom site, and Pb has same site with K. The ground state is proposed to have an effective S = 1/2 due to the 3d7 Co2+ ion in octahedral coordination and considerable spin–orbit coupling. Magnetic and specific heat data uncover that only an antiferromagnetic phase transition develops in NKPCTO at 49 K. There are also ferromagnetic correlations above the Neel temperature, as well as anomalous hysteresis loops and multiple field-induced magnetic transitions at low temperatures.

X-ray magnetic circular dichroism of Mn4−xGaxN epitaxial thin films confirming ferrimagnetic-ferromagnetic phase transition by nonmagnetic Ga doping

Researchers have focused on anti-perovskite Mn4N-based ferrimagnets as potential candidates for spintronic applications since these films have a small magnetization and exhibit perpendicular magnetic anisotropy. Magnetic compensation (or angular momentum compensation) in impurity-doped Mn4N at room temperature has attracted particular attention because the magnetization switching can be facilitated in the vicinity of such compensation compositions. In this study, we grew Mn4−xGaxN (x = 0.0–1.9) epitaxial thin films on SrTiO3(001) substrates by molecular beam epitaxy and evaluated their crystallinity and surface conditions. The magnetic properties of the grown films were also examined and a sign reversal of anomalous Hall resistivity and hysteresis loops was observed between x = 0.1 and 0.3 at room temperature by magneto-optical Kerr effect microscopy. X-ray magnetic circular dichroism measurement revealed that the magnetic moment of face-centered Mn at x = 0.3 was reversed to be parallel to that of Mn at corner sites, suggesting that the ferrimagnetic-ferromagnetic phase transition occurred with nonmagnetic Ga doping.

A Superconducting Micro-Magnetometer for Quantum Vortex in Superconducting Nanoflakes.

Superconducting quantum interferometer device (SQUID) plays a key role in understanding electromagnetic properties and emergent phenomena in quantum materials. The technological appeal of SQUID is that its detection accuracy for the electromagnetic signal can precisely reach the quantum level of a single magnetic flux. However, conventional SQUID techniques normally can only be applied to a bulky sample and do not have the capability to probe the magnetic properties of micro-scale samples with small magnetic signals. Herein, it is demonstrated that, based on a specially designed superconducting nano-hole array, the contactless detection of magnetic properties and quantized vortices in micro-sized superconducting nanoflakes is realized. An anomalous hysteresis loop and a suppression of Little-Parks oscillation are observed in the detected magnetoresistance signal, which originates from the disordered distribution of the pinned vortices in Bi2 Sr2 CaCu2 O8+δ . Therefore, the density of pinning centers of the quantized vortices on such micro-sized superconducting samples can be quantitatively evaluated, which is technically inaccessible for conventional SQUID detection. The superconducting micro-magnetometer provides a new approach to exploring mesoscopic electromagnetic phenomena of quantum materials.

Anomalous hysteresis loops of ferrimagnetic Gd-Co films of different thickness near the magnetic compensation temperature

Triple hysteresis loops were observed for amorphous ferrimagnetic Gd-Co films near the magnetic compensation temperature, which can be a consequence of both the spin-flop transition and the chemical composition gradient. In the work, an assessment of the possible chemical inhomogeneity of the films based on magnetic measurements was carried out and its relationship with the thickness of the samples was observed. Keywords: amorphous magnetic films, perpendicular magnetic anisotropy, ferrimagnetism, magnetic domain structure, magnetic hysteresis.

Atomic reconfiguration among tri-state transition at ferroelectric/antiferroelectric phase boundaries in Pb(Zr,Ti)O3

Phase boundary provides a fertile ground for exploring emergent phenomena and understanding order parameters couplings in condensed-matter physics. In Pb(Zr1-xTix)O3, there are two types of composition-dependent phase boundary with both technological and scientific importance, i.e. morphotropic phase boundary (MPB) separating polar regimes into different symmetry and ferroelectric/antiferroelectric (FE/AFE) phase boundary dividing polar and antipolar dipole configurations. In contrast with extensive studies on MPB, FE/AFE phase boundary is far less explored. Here, we apply atomic-scale imaging and Rietveld refinement to directly demonstrate the intermediate phase at FE/AFE phase boundary exhibits a rare multipolar Pb-cations ordering, i.e. coexistence of antipolar or polar displacement, which manifests itself in both periodically gradient lattice spacing and anomalous initial hysteresis loop. In-situ electron/neutron diffraction reveals that the same parent intermediate phase can transform into either FE or AFE state depending on suppression of antipolar or polar displacement, coupling with the evolution of long-/short-range oxygen octahedra tilts. First-principle calculations further show that the transition between AFE and FE phase can occur in a low-energy pathway via the intermediate phase. These findings enrich the structural understanding of FE/AFE phase boundary in perovskite oxides.

Epitaxial Growth and Structural Characterizations of MnBi2Te4 Thin Films in Nanoscale

The intrinsic magnetic topological insulator MnBi2Te4 has attracted much attention due to its special magnetic and topological properties. To date, most reports have focused on bulk or flake samples. For material integration and device applications, the epitaxial growth of MnBi2Te4 film in nanoscale is more important but challenging. Here, we report the growth of self-regulated MnBi2Te4 films by the molecular beam epitaxy. By tuning the substrate temperature to the optimal temperature for the growth surface, the stoichiometry of MnBi2Te4 becomes sensitive to the Mn/Bi flux ratio. Excessive and deficient Mn resulted in the formation of a MnTe and Bi2Te3 phase, respectively. The magnetic measurement of the 7 SL MnBi2Te4 film probed by the superconducting quantum interference device (SQUID) shows that the antiferromagnetic order occurring at the Néel temperature 22 K is accompanied by an anomalous magnetic hysteresis loop along the c-axis. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) at 80 K reveals a Dirac-like surface state, which indicates that MnBi2Te4 has topological insulator properties in the paramagnetic phase. Our work demonstrates the key growth parameters for the design and optimization of the synthesis of nanoscale MnBi2Te4 films, which are of great significance for fundamental research and device applications involving antiferromagnetic topological insulators.

Improvement of magnetostriction performance by doping Mg in spinel MnV2O4

By a combination of magnetization M(T), M(H) and strain ΔL/L700K, ΔL/L0Oe, the positive magnetostrictions up to ∼5000 ppm originating from the rearrangement of tetragonal domains were observed in spinel Mn0.85Mg0.15V2O4, which exhibits two successive magnetic transitions at TC ∼ 42 K and T* ∼ 28 K. An anomalous magnetic hysteresis loop under a high field occurs below T*, caused by the rearrangement of tetragonal domains. We found that the Mg-doping at the Mn site can effectively promote the positive magnetostriction in Mn1-xMgxV2O4 systems. Moreover, the remnant strain can be significantly lowered by Mg-doping due to the enhancement of magnetic anisotropy. These results provide a possible approach for further optimizing the performance of magnetic shape-memory materials.

Anomalous Loss and Hysteresis Loop in Electrical Steel Sheet

This article presents an algorithm for identification of anomalous magnetic field and loss in silicon steel sheet (SSS). Since the magnetic field in an SSS consists of static hysteresis, eddy current, and anomalous magnetic fields, the anomalous magnetic field is extracted from the measured hysteresis field by excluding the static hysteresis and eddy current fields. In order to predict the anomalous magnetic field for an arbitrary B-waveform, an artificial neural network model with deep learning algorithm is suggested. Through comparisons with experimental measurements over a non-oriented electrical steel sheet, 35PN440, the proposed algorithm is validated.

Field- and Current-Driven Magnetization Reversal and Dynamic Properties of CoFeB-MgO-Based Perpendicular Magnetic Tunnel Junctions* *Supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300803), the Open Research Fund of Jiangsu Provincial Key Laboratory for Nanotechnology, the National Natural Science Foundation of China (Grant Nos. 11774150 and 11874135), and the Natural Science Foundation of Jiangsu Province of

We report a perpendicular magnetic tunnel junction (pMTJ) cell with a tunnel magnetoresistance (TMR) ratio of nearly 200% at room temperature based on CoFeB/Ta/CoFeB as the free layer (FL) and a synthetic antiferromagnetic (SAF) multilayer [Pt/Co]/Ru/[Pt/Co]/Ta/CoFeB as the reference layer (RL). The field-driven magnetization switching measurements show that the pMTJs exhibit an anomalous TMR hysteresis loop. The spin-polarized layer CoFeB of SAF-RL has a lower critical switching field than that of FL. The reason is related to the interlayer exchange coupling (IEC) through a moderately thick Ta spacer layer among SAF-RLs, which generates a moderate and negative bias magnetic field on CoFeB of RL. However, the IEC among RLs has a negligible influence on the current-driven magnetization switching of FL and its magnetization dynamics.

Dynamical Strain-Driven Phase Separation in Flexible CoFe2O4/CoO Exchange Coupling System.

Epitaxial CoFe2O4(CFO)/CoO bilayers were fabricated by pulsed laser deposition on flexible muscovite mica substrate. Samples with different CFO thicknesses were employed to study the phenomenon of exchange bias involving strongly anisotropic ferromagnet. Magnetic measurements exhibited great enhancement in the features of exchange bias. Raman and X-ray absorption spectroscopies indicated that a new phase emerged within the CFO layer because of the cation charge redistribution in CFO layer under bending, which in turn gave rise to anomalous hysteresis loops exhibited in the bent bilayers. These results provide a fundamental understanding about the mechanisms of exchange bias prevailing in these bilayers and call attention to the implementation of spintronic devices using flexible heterostructures such as the present CFO/CoO bilayers.

Unusual Field Dependence of the Anomalous Hall Effect in Ta/Tb−Fe−Co

Experimental studies of anomalous Hall effect are performed for thin filmed Ta/TbFeCo in a wide range of temperatures and magnetic fields up to 3 T. While far from the compensation temperature (TM=277 K) the field dependence has a conventional shape of a single hysteresis loop, just below the compensation point the dependence is anomalous having the shape of a triple hysteresis. To understand this behavior, we experimentally reveal the magnetic phase diagram and theoretically analyze it in terms of spin-reorientation phase transitions. We show that one should expect anomalous hysteresis loops below the compensation point if in the vicinity of it the magnetic anisotropy is dominated by FeCo sublattice due to interaction with Ta.

H−T phase diagram of rare-earth–transition-metal alloys in the vicinity of the compensation point

Anomalous hysteresis loops of ferrimagnetic amorphous alloys in high magnetic field and in the vicinity of the compensation temperature have so far been explained by sample inhomogeneities. We obtain H-T magnetic phase diagram for ferrimagnetic GdFeCo alloy using a two-sublattice model in the paramagnetic rare-earth ion approximation and taking into account rare-earth (Gd) magnetic anisotropy. It is shown that if the magnetic anisotropy of the $f$-sublattice is larger than that of the $d$-sublattice, the tricritical point can be at higher temperature than the compensation point. The obtained phase diagram explains the observed anomalous hysteresis loops as a result of high-field magnetic phase transition, the order of which changes with temperature. It also implies that in the vicinity of the magnetic compensation point the shape of magnetic hysteresis loop is strongly temperature dependent.

Field-cooling induced giant vertical magnetization shift in frustrated low dimensional spin-chain Sr3NiIrO6 system

In this study, low-dimensional and magnetically-frustrated Sr3NiIrO6 (SNIO) compound was prepared by solid-state reaction method under argon (Ar) gas atmosphere. The sample was crystallized in a rhombohedral structure with a space group of R-3c. XPS measurement of SNIO studies revealed Ni2+ and Ir4+ valence states. A systematic study of the temperature-dependent magnetization data revealed distinct magnetic anomalies: one was partially-disordered antiferromagnetic (PDAF) at TN1 ∼ 74 K and the second was a completely frozen state of PDAF at TN2 ∼ 18 K. Isothermal sweeping field, and field-cooling induced alteration of the PDAF state are responsible for the anomalous magnetic hysteresis loop in the vicinity of TN2. Isothermal field-cooling caused significant vertical magnetization shift in the M-H loop is possibly related to the exchange bias like phenomena with field-cooled induced soft ferromagnetic phase in the PDAF phase and also attributed to the frozen state of partially disordered antiferromagnetism. This spin pinning mechanism is stable and robust against the sweeping field cycle, field-cooling and isothermal field-sweeping.

One pot synthesis of nickel ferrite-graphitic layers nanocomposite with inverted magnetic hysteresis

Abstract For the first time, we report the synthesis of nickel ferrite-graphitic layers nanocomposite (C-NiFe2O4) via a straightforward approach using iron-nickel cyanide powder (Fe[Ni(CN)4]·3H2O). Firstly, iron-nickel cyanide powder was prepared by mixing equal molar amount of iron salt and potassium tetracyanonickelate (II) hydrate. Secondly, the as-synthesized iron-nickel cyanide powder was thermally treated in air at a temperature as low as 400 °C. The ignition starts at one point of high energy like the edges and then propagates like a wave through all the material. The ignition process was accompanied by huge expansion finally forming C-NiFe2O4 nanocomposite. All samples were characterized by the standard characterization techniques such as field emission scanning electron microscope (FE-SEM), high-resolution transmission electron microscope (HRTEM), Raman spectroscopy, X-ray diffraction (XRD) and thermogravimetric analysis (TGA) as well as vibrating sample magnetometer (VSM) to probe the magnetic properties at room temperature. HRTEM investigation reveals the presence of the graphitic layer on the nickel ferrite nanoparticles. Iron-nickel cyanide powder which has sheet-like morphology was directly converted into C-NiFe2O4 nanocomposite during the thermal treatment in air. The VSM magnetic investigations revealed unusual inverted hysteresis behavior of the as-prepared C-NiFe2O4 nanoparticles. While traditional hysteresis loop was obtained in the case of iron-nickel cyanide with specific magnetization saturated at 0.142 emu/g, anomalous hysteresis loop was observed in the case of C-NiFe2O4 nanocomposite with specific magnetization saturated at 28.668 emu/g. The mechanism of the C-NiFe2O4 nanocomposite formation has been proposed based on the obtained experimental data. The present approach for the C-NiFe2O4 nanocomposite synthesis is straightforward, surfactant free, simple and scalable as well as it can be applied to the other ferrites.

Partially-disordered to frozen-state crossover induced magnetocaloric properties of the antiferromagnetic one-dimensional spin-chain Sr3CoIrO6

Low-dimensional and geometrically frustrated spin-chain Sr3(Co/Ni)IrO6 compounds exhibit interesting physical phenomena due to the exotic anisotropy associated with the spin–orbit entangled Ir4+ state. Temperature dependent magnetic susceptibility of polycrystalline Sr3CoIrO6 characterizes the partially-disordered antiferromagnetic with TN1 ∼ 90 K and the frozen state of this antiferromagnetic order below TN2 ∼ 30 K. The fitting of χT versus T (K) data to the Ising model reveal that the intra-chain exchange interactions are ferromagnetic, whereas the inter-chain exchange correlations are antiferromagnetic. Field-induced alteration of the partially-disordered antiferromagnetic state in the temperature regime of TN1 ≤ T ≤ TN2 is responsible for the anomalous magnetic hysteresis loop. Further, a large value of isothermal magnetic-entropy change (ΔSM ∼ 22 J kg−1K−1 for 7 T), absence of thermal hysteresis in magnetization and highly insulating nature consents Sr3CoIrO6 to be classified as a good magnetocaloric material for the low-temperature refrigeration.

Anti-Lenz supercurrents in superconducting spin valves

Here, we present a study on Si(111)/\-Ta($150$\AA )/\-IrMn($150$\AA )/\-NiFe($50$\AA )/\-Nb($x$)/\-NiFe($50$\AA )/\-Ta($50$\AA ) and Si(111)/\-Ta($150$\AA )/\-NiFe($50$\AA )/\-Nb($x$)/\-NiFe($50$\AA )/\-IrMn($150$\AA )/\-Ta($50$\AA ) spin-valves with $x=100$ to $500$\AA . For both sample families, above a specific critical thickness of the Nb-layer and below $T_c$, the superconducting Nb-layer contributes strongly to the magnetization. These systems show an anomalous hysteresis loop in the magnetization of the superconducting layer; the hysteresis loop is similar to what is generally expected from hard superconductors and many superconductor/ferromagnet hybrid systems, but the direction of the hysteresis loop is inverted, compared to what is generally observed (paramagnetic for up sweeping fields and diamagnetic for down sweeping fields). This means that the respective samples exhibit a magnetization, which is contrary to what should be expected from the Lenz' rule.

Polarization rotation in copper doped zinc oxide (ZnO:Cu) thin films studied by Piezoresponse Force Microscopy (PFM) techniques

In this study, the polarization rotation with approximately 180⁰, i.e., the phenomenon that polarization changes its angle by ∼180°, in copper doped ZnO (ZnO:Cu) films under an applied electrical field are characterized by using Piezoresponse Force Microscopy (PFM) based techniques. First, the Band Excitation PFM (BE-PFM) measurements show that the polarizations in ZnO:Cu sample prefer the upward direction when the copper concentration is higher (above 8 at.%). Second, it is found that the piezoresponse property is enhanced with increasing the copper concentration. Third, by conducting PFM in spectroscopy mode (PFS), it reveals that the anomalous hysteresis loops occurs at certain locations and can be tuned to symmetry by increasing the bias window. Furthermore, it is found the built-in field during the DC poling processes are contributed by the oxygen vacancy in the films. The PFS results from as-grown sample and samples annealed in air and vacuum also verify that the oxygen vacancy plays an important role in the polarization rotation. Finally, by using the switching spectroscopy PFM (SS-PFM) technique, it is confirmed that the local polarization rotation is mediated by copper concentration. The parameters determined from the PFS measurements, including positive and negative coercive bias, Vp and Vn, coercive bias, Ec, maximum and remanent switchable responses, Rs and R0, all increase with the copper concentration.

Oscillatory Interlayer Exchange Coupling in Amorphous CoSiB/Pt/CoSiB Structure

Oscillatory interlayer exchange coupling (IEC) is found in amorphous CoSiB(1.5 nm)/Pt( $t_{\rm {Pt}})$ /CoSiB(1.5 nm) structure as a function of the Pt layer thickness, $t_{\rm {Pt}}$ , which is varied from 1.4 to 3.0 nm. We prepared the samples using dc magnetron sputtering method, and measured anomalous Hall resistance and magnetization hysteresis loops. All the samples show out-of-plane easy axes with perpendicular magnetic anisotropy energies. Surprisingly, strong antiferromagnetic IEC is found when $t_{\rm {Pt}}=2.2-2.7$ nm, otherwise they are simple ferromagnets like multilayer systems of Co/Pt and Fe/Pt. The Pt layer is known as near ferromagnet, because it is easily polarized with an adjacent ferromagnetic layer or by a small amount of ferromagnetic impurities. The interlayer coupling via Pt has always been observed to be ferromagnetic so far. We report the antiferromagnetic IEC in amorphous CoSiB/Pt/CoSiB trilayer structure and propose its possible physical origin based on quantum well theory and negative spin polarization of Pt layer.

Transport characteristics of focused beam deposited nanostructures

AbstractWe review the transport properties of different nanostructures produced by ion- and electron-beam deposition, as prepared as well as after certain treatments. In general, the available literature indicates that the transport properties are determined by conduction processes typical for disordered metallic grains embedded in a carbon-rich matrix, including intergrain tunneling and variable range hopping mechanisms. Special emphasis is given to the superconducting behavior found in certain Tungsten-Carbide nanostructures that, in a certain field and temperature range, is compatible with that of granular superconductivity. This granular superconductivity leads to phenomena like magnetic field oscillations as well as anomalous hysteresis loops in the magnetoresistance.

Morphology-dependent multi-step ferromagnetic reversal processes in Co thin films on crescent-shaped antidot arrays

We report on the anomalous hysteresis loops and isolation of nanodomains in Co films deposited on hexagonally ordered antidot arrays produced by anodization of alumina templates. Using the Magneto-optic Kerr effect and vibrating sample magnetometry, we analyzed how the magnetization varied with depth from the antidot surface. Our results show that the multi-step reversal of the hysteresis loops was associated both with the feature size of the antidot array and with the growth morphology. Magnetic force microscopy showed that as the pore diameter increased, the magnetic domains became more distinct and separate, following the topography of the hexagonal array. The porous regions exhibited highly stable vortex-like magnetic domains, and the perpendicular magnetic moments observed around the circumference of magnetic nanotubes. These domain configurations indicate that the three-dimensional magnetic profile was caused by the growth morphology.