Magnetic Field Hysteresis Loops Research Articles

The Topological Hall Effect in CoGd Films Controlled by Hydrogen Migration under Gate Voltage

AbstractThe topological Hall effect (THE) presents hump signals in the Hall resistance versus magnetic field hysteresis loop, showing promise for future spintronics due to its robust chiral magnetic textures. Here, it is shown that solid‐state protonic gating can control possible topological magnetic structures in CoGd thin films. Injecting H+ leads to sizable hump signals in the film. Magneto‐optical Kerr microscopy shows that induced hump signals in transporting measurements do not scale with magnetization, supporting topological magnetism. Successive hydrogen ion extraction completely erases the effect. Thus, topological magnetism manipulations are reversible, nonvolatile, and effective. Ab inito calculations and effective chiral spin models demonstrate that hydrogen injection remarkably enhances the Dzyaloshinskii–Moriya interaction over fourfold, stabilizing chiral structures contributing to the large THE. These findings reveal the vital role of hydrogen ions in topological magnetism and suggest that amorphous ferrimagnetic CoGd thin films are outstanding platforms for realizing controllable topological spintronics at room temperature.

Single-crystal epitaxial europium iron garnet films with strain-induced perpendicular magnetic anisotropy: Structural, strain, magnetic, and spin transport properties

Single-crystal europium iron garnet (EuIG) thin films epitaxially strain-grown on gadolinium gallium garnet (GGG)(100) substrates using off-axis sputtering have strain-induced perpendicular magnetic anisotropy (PMA). By varying the sputtering conditions, we have tuned the europium/iron (Eu/Fe) composition ratios in the films to tailor the film strains. The films exhibited an extremely smooth, particle-free surface with roughness as low as 0.1 nm as observed using atomic force microscopy. High-resolution x-ray diffraction analysis and reciprocal space maps showed in-plane epitaxial film growth, very smooth film/substrate interface, excellent film crystallinity with a small full width at half maximum of 0.012$^{\circ}$ in the rocking curve scans, and an in-plane compressive strain without relaxation. In addition, spherical aberration-corrected scanning transmission electron microscopy showed an atomically abrupt interface between the EuIG film and GGG. The measured squarish out-of-plane magnetization-field hysteresis loops by vibrating sample magnetometry in conjunction with the measurements from angle-dependent x-ray magnetic dichroism demonstrated the PMA in the films. We have tailored the magnetic properties of the EuIG thin films, including saturation magnetization ranging from 71.91 to 124.51 emu/c.c. (increase with the (Eu/Fe) ratios), coercive field from 27 to 157.64 Oe, and the strength of PMA field ($H_\bot$) increasing from 4.21 to 18.87 kOe with the in-plane compressive strain from -0.774 to -1.044%. We have also investigated spin transport in Pt/EuIG bi-layer structure and evaluated the real part of spin mixing conductance to be $3.48\times10^{14} {\Omega}^{-1}m^{-2}$. We demonstrated the current-induced magnetization switching with a low critical switching current density of $3.5\times10^6 A/cm^2$, showing excellent potential for low-dissipation spintronic devices.

Magnetic properties and spin reorientation of hexagonal HoFeO[formula omitted] thin films

We present structural, magnetic, and optical properties of hexagonal HoFeO3/Al2O3 thin films deposited by Magnetron Sputtering. The x-ray diffraction patterns of HoFeO3 thin films show the c-planes of a hexagonal structure. The magnetization data display an antiferromagnetic transition temperature, TN∼120 ± 5 K and the magnetization-field hysteresis loops were measured below 100 K, confirming a weak ferromagnetism arising from a spin canting of the Fe3+ moments. The magnetization data also show an anomaly around ∼40 K due to a spin-reorientation transition caused by the Ho3+- Fe3+ interactions. We observed comparable magnetization along the ab plane and c axis although the spin canting of Fe3+ sites has a preferential component along the c axis, suggesting that the Ho3+- Fe3+ interactions dominate in the low temperature magnetic structures of hexagonal-HoFeO3. The observed electronic excitations at ∼2.29, 2.87, 3.82, 4.79, and 6.53 eV have been assigned to the Fe3+d to d on-site as well as O 2p to Fe 3d, Ho 6s, and 5d charge-transfer excitations, respectively. The room temperature energy band gap of the hexagonal-HoFeO3 thin film was measured to be ∼1.99 ± 0.04 eV.

Effects of substrate temperature on physical properties of microrod-type multiferroic composite thin films fabricated by metal organic chemical vapor deposition

Microrod-type CoFe2O4(CFO)/Bi3.25Nd0.65Eu0.10Ti3O12(00ℓ) (BNEuT) composite thin films were fabricated by a combination of high-temperature sputtering, reactive ion etching, and metal organic chemical vapor deposition (MOCVD) on Pt(100)/MgO(100) substrates. The substrate temperature for MOCVD was varied from 450 °C to 600 °C to examine its effect on the structural, magnetic, and ferroelectric properties. The substrate temperature affects the compressive stress at the interface between the CFO and BNEuT. The surface morphology changed drastically above 550 °C. The room temperature magnetization–magnetic field hysteresis loops for the films showed clear ferromagnetic hysteresis loop and magnetic shape anisotropy. The room temperature polarization–electric field (P−E) hysteresis loops for the films showed a clear ferroelectric hysteresis loop, and slightly leaky P−E hysteresis loop. The coercive field increased slightly with increasing substrate temperature. Judging from the structural, ferromagnetic, and ferroelectric properties, the film deposited at 550 °C has potential as an excellent multiferroic material.

Fabrication and characterization of micropillar-type multiferroic composite thin films by metal organic chemical vapor deposition using a ferroelectric microplate structure

Using a a-axis-oriented epitaxial (Bi3.25Nd0.65Eu0.10)Ti3O12 (BNEuT) thin films with a microplate-like shape as a ferroelectric pillar material, micropillar-type CoFe2O4 (CFO)/BNEuT(h00)/Nb:TiO2(101) composite multiferroic films were fabricated by metal organic chemical vapor deposition. The deposition time was varied from 90–150 min to examine its effect on the structural, ferroelectric, and ferromagnetic characteristics of the films. All the films exhibit single-phase cobalt ferrite with a cubic inverse-spinel structure. The deposited CFO films have a similar columnar structure with a comparatively good step coverage of 37%–66%. The room-temperature magnetization−magnetic field hysteresis loop of the films showed a clear ferromagnetic hysteresis loop, and coercivity decreased from 1.1 to 0.9 kOe with increasing the deposition time. The room-temperature polarization−electric field hysteresis loop of the films showed a ferroelectric hysteresis loop, and the effect of the leakage current was the smallest for a deposition time of 120 min.

A detailed study of magnetization and magnetoelectric effect in P(VDF-TrFe) based SmFeO3 nanocomposites

In this study, we investigate magnetoelectric (ME) properties in nanocomposite films (P(VDF-TrFe))/SmFeO3 consist of ferroelectric copolymer, poly(vinylindene fluoride-trifluoroethylene) (P(VDF-TrFe)) and magnetic SmFeO3 nanoparticles. Comparative magnetization vs magnetic field hysteresis loops for un-poled and electric field poled (P(VDF-TrFe))/SmFeO3 nanocomposite films demonstrate strong electric filed control of magnetization. The nanocomposite films exhibit large exchange biasing (EB) effects in magnetic measurements near spin switching temperature (~ 100 K) of SmFeO3 and the EB effects were significantly modified under electric field (E) poled conditions (at E = 200 kV/cm), indicating ME coupling. Remarkably, we observe electric field poling induced self biased converse ME effect in the (P(VDF-TrFe))/SmFeO3 nanocomposite films. The maximum magnetoelectric output voltage (α) obtained is 55 mV/cm/Oe at Hbias = 1 kOe and 25 mV/cm/Oe at Hbias = 0, after electric field poling. The discovery of self biasing phenomenon with converse ME effects at room temperature in flexible (P(VDF-TrFe))/SmFeO3 nanocomposite films is highly desirable for device applications such as electrically controlled thin memory devices and ME sensors etc.

Size-dependent magnetic anisotropy of PEG coated Fe3O4 nanoparticles; comparing two magnetization methods

Understanding the size dependent magnetic anisotropy of iron oxide nanoparticles is essential for the successful application of these nanoparticles in several technological and medical fields. PEG-coated iron oxide (Fe3O4) nanoparticles with core diameters of 12 nm, 15 nm, and 16 nm were synthesized by the usual co-precipitation method. The morphology and structure of the nanoparticles were investigated using transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and X-ray diffraction (XRD). Magnetic measurements were conducted using a SQUID. The effective magnetic anisotropy was calculated using two methods from the magnetization measurements. In the first method the zero-field-cooled magnetization versus temperature measurements were used at several applied magnetic fields. In the second method we used the temperature-dependent coercivity curves obtained from the zero-field-cooled magnetization versus magnetic field hysteresis loops. The role of the applied magnetic field on the effective magnetic anisotropy, calculated form the zero-field-cooled magnetization versus temperature measurements, was revealed. The size dependence of the effective magnetic anisotropy constant Keff obtained by the two methods are compared and discussed.

Enhanced magnetic and dielectric properties of Ti‐doped YFeO3 ceramics

AbstractPolycrystalline YFeO3 (YFO) and YFe1−(4/3)xTixO3(YFTO) ceramics were prepared using the powder synthesized from the sol‐gel route. X‐ray diffraction analyses of the polycrystalline ceramics revealed the crystallization of the phase in orthorhombic crystal structure associated with the space group Pnma. The magnetization versus magnetic field hysteresis loops were obtained at room temperature for YFO and YFTO ceramics. The magnetic property changes from weak ferromagnetic in YFO to ferromagnetic in YFTO ceramics. The dielectric constant recorded at room temperature for YFTO ceramics was six times higher than that of YFO, whereas the dielectric loss gets reduced to 0.06 from 0.3 for YFO at 1 kHz. Impedance spectroscopy study carried out on YFO and YFTO ceramics confirmed the existence of non‐Debye‐type relaxation. Observed single semicircle in Z′ vs −Z′′ plot established the incidence of intrinsic (bulk) effect and ruled out any grain boundary or electrode effects. The mechanism for the dielectric relaxation and electrical conduction process observed in YFO and YFTO ceramics was discussed by invoking electric modulus formalisms. Activation energy obtained by ac conductivity study suggested that the conduction process in YFO was linked up with the existence of the polaron and oxygen vacancies, whereas only oxygen vacancies contribute to the conduction process in YFTO ceramics.

Surface inhomogeneities and the electronic phase separated states in thin films of La0.35Pr0.35Ca0.3MnO3

We have studied the effects of film thickness and surface inhomogeneities on the fluid and static phase separated states of La0.35Pr0.35Ca0.3MnO3 films grown on LaAlO3 and SrTiO3 substrates which provide compressive and tensile strain, respectively. The shapes and areas of the resistance versus magnetic field hysteresis loops were used to identify the type of phase separated state in the films. Atomic force microscopy revealed an increase of the relative surface roughness of the films with a decreasing thickness. The resultant distribution of the strain produces a dramatic enhancement of the inhomogeneous electronic phase separation as well as large changes in the dependence of the resistivity on magnetic field in the thinnest films deposited on both types of substrates. In contrast to thick films, the static phase separated state in the thinnest films is more inhomogeneous than the fluid phase separated one.

The structures and positive magnetoresistance of metallic Sr2CrWO6 epitaxial thin film

Double perovskite Sr 2 CrWO 6 films have been prepared on SrTiO 3 (111) substrates by pulsed laser deposition in high vacuum (10 −5 Pa). X-ray diffraction patterns indicate that the films are (111)-oriented. Both atomic force microscopy (AFM) and cross-section transmission electron microscopy (TEM) images prove that the films have very smooth surfaces. Detailed microstructures given by high resolution transmission electron microscopy (HRTEM) further confirm that the films are epitaxial with sharp and coherent substrate/film interface. Well saturated magnetization–magnetic field hysteresis loop is observed with the saturation magnetization of 1.2 μ B /formula unit at 10 K. The films show metallic transport behavior and large positive magnetoresistance (∼180% at 10 K). The structure–property relationship is discussed in detail.

The Effect of Heat Treatment on Structural and Multiferroic Properties of Phase-Pure BiFeO3

Phase-pure multiferroic BiFeO3 (BFO) was prepared by the coprecipitation technique using diverse precursors bismuth oxide at temperature as low as 400°C. The dependence of structural, microstructural, thermal, electrical (AC and DC), and magnetic properties on sintering temperature was systematically investigated. Uniaxially pressed samples (O8 mm) were sintered in air at 500°C to 800°C for 4 h. X-ray diffraction analysis was used to determine the amorphous and perovskite nature of as-synthesized and calcined/sintered samples, respectively. The crystallite size of sintered powders increased from 47 nm to 67 nm. Scanning electron microscopy showed grain growth during sintering, which improved intergranular connectivity and decreased porosity in the samples. The ferroelectric to paraelectric Curie transition temperature (TC) of pure BFO powder was detected by differential scanning calorimetry analysis and found to be 820°C ± 1°C. The samples exhibited high AC resistivity and dielectric constant, and low loss tangent values. The samples exhibited weak ferromagnetic behavior with an unsaturated magnetization versus magnetic field hysteresis loop at room temperature. Ferroelectric behavior and variation in remnant polarization and coercivity were observed from polarization versus electric field loops. Enhanced capacitance in the magnetic field revealed the magnetoelectric effect in the samples.

Domain wall dynamics and Hall effect in eddy current loop in amorphous ferromagnetic wire with small helical anisotropy

Small helical anisotropy was induced in amorphous ferromagnetic Co 68.2Fe 4.3Si 12.5B 15 wire by current annealing and simultaneous application of tensile stress and torsion. Presence of helical anisotropy was confirmed by measurement and analysis of the circular magnetic flux versus axial magnetic field hysteresis loops. These measurements also showed that a single domain wall between circular domains can be created by placing the wire in a sufficiently high inhomogeneous magnetic field generated by Helmholtz coils with opposite currents. The domain wall velocity versus axial driving field was measured. The results show that the basic dynamic properties (magnitude of the wall mobility, field interval in which linear dependencies between velocity and field are observed, accelerated increase of the velocity for higher fields) are very similar to those obtained for the domain wall between circular domains driven by a constant circular field. The Hall effect was detected in the eddy current loop generated by the moving domain wall.

Dielectrical and Magnetic Properties of CoFe2O4-BaTiO3Composite Films Prepared by a Combined Method of Sol-Gel and Electrophoretic Deposition

CoFe2O4–BaTiO3 composite thick films were prepared using a combined method of sol–gel and electrophoretic deposition. Phase compositions and microstructures of the films were examined using X-ray diffraction (XRD) and scanning electron microscopic techniques, respectively. The XRD results indicated that the films consisted of both spinel CoFe2O4 and BaTiO3 perovskite phase. The leakage current density of the films is in the order of about 10−8 A/cm2 at zero electric field. Polarization– electric field (P–E) hysteresis loops and magnetization–magnetic field hysteresis loops of the thick films were investigated. At room temperature, the composite thick films show good ferroelectric and ferromagnetic properties.

High-temperature ferromagnetism in epitaxial (In,Mn)Sb films

We report ferromagnetism in single phase, epitaxial ${\text{In}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{Sb}$ alloy films, with $x\ensuremath{\le}0.035$, grown by metalorganic vapor phase epitaxy. The alloy films exhibit well-defined magnetization versus magnetic field hysteresis loops from 4--298 K. High-field magnetotransport measurements indicated that the Hall resistivity has a nonlinear dependence on magnetic field. Ferromagnetism is supported by observation of an anomalous Hall effect and clear hysteresis in the anomalous Hall resistivity versus magnetic field measurements. Zero field cooled and field cooled magnetization measurements show reversibility indicating absence of second phase precipitates. The temperature dependence of the magnetization is described by a modified Brillouin function with a ${T}_{C}$ of 590 K. The observed magnetic and magnetotransport properties are attributed to carrier mediated ferromagnetism involving Mn and its complexes that form shallow or resonant electronic states through correlated substitution in the semiconductor host.

Structure and Magnetic Properties of Mn-Implanted ZnO Films

Unintentionally doped n-ZnO(0002) films prepared by pulse laser deposition were implanted with various doses of Mn+. Photoluminescence measurement revealed that the incorporation of Mn in the films shifts the near band edge position towards the higher energy side. Magnetism measurements of the Mn-implanted samples showed clear magnetic properties. Magnetization–field hysteresis loops were observed at 300 K and the temperature-dependent magnetization showed magnetic behavior up to 350 K, which is in agreement with the theoretical prediction.

Multiferroic properties of BiFeO 3/Bi 4Ti 3O 12 double-layered thin films fabricated by chemical solution deposition

Multiferroic BiFeO 3/Bi 4Ti 3O 12 (BFO/BTO) double-layered film was fabricated on a Pt(111)/Ti/SiO 2/Si(100) substrate by a chemical solution deposition method. The effect of an interfacial BTO layer on electrical and magnetic properties of BFO was investigated by comparing those of pure BFO and BTO films prepared by the same condition. The X-ray diffraction result showed that no additional phase was formed in the double-layered film, except BFO and BTO phases. The remnant polarization (2 P r) of the double-layered film capacitor was 100 μC/cm 2 at 250 kV/cm, which is much larger than that of the pure BFO film capacitor. The magnetization–magnetic field hysteresis loop revealed weak ferromagnetic response with remnant magnetization (2 M r) of 0.4 kA/m. The values of dielectric constant and dielectric loss of the double-layered film capacitor were 240 and 0.03 at 100 kHz, respectively. Leakage current density measured from the double-layered film capacitor was 6.1 × 10 − 7 A/cm 2 at 50 kV/cm, which is lower than the pure BFO and BTO film capacitors.

Microstructure, electrical and magnetic properties of Ce-doped BiFeO3 thin films

Bi 1 − x Ce x FeO 3 (x=0, 0.05, 0.1, 0.15, and 0.20) (BCFO) thin films were deposited on Pt/TiN/Si3N4/Si and fluorine-doped SnO2 glass substrates by sol-gel technique, respectively. The effect of Ce doping on the microstructure, electrical and magnetic properties of BCFO films was studied. Compared to counterparts of BiFeO3 (BFO) film, the fitted Bi 4f7/2, Bi 4f5/2, Fe 2p3/2, Fe 2p1/2, and O 1s peaks for Bi0.8Ce0.2FeO3 film shift toward higher binding energy regions by amounts of 0.33, 0.29, 0.43, 0.58, and 0.49 eV, respectively. Raman redshifts of 2–4 cm−1 and shorter phonon lifetimes for the Bi0.8Ce0.2FeO3 film might be related to anharmonic interactions among Bi–O, Ce–O, (Bi, Ce)–O, and Fe–O bonds in the distorted oxygen octahedron. Compared to the pure counterparts, the dielectric and ferroelectric properties of the Bi0.8Ce0.2FeO3 film are improved due to the decreased oxygen vacancies by the stabilized oxygen octahedron. Current density values for the BFO and Bi0.8Ce0.2FeO3 film capacitors are 9.89×10−4 and 5.86×10−5 A/cm2 at 10 V, respectively. The current density–applied voltage characteristics indicate that the main conduction mechanism for the BCFO capacitors is the interface-controlled Schottky emission. Both the in-plane and out-of-plane magnetization–magnetic field hysteresis loops reveal that the saturation magnetization values of the BCFO films increase with increasing the Ce concentration. The enhanced magnetic properties for the BCFO films might be attributed to the presence of Fe2+ caused by oxygen vacancies, the suppressed spiral spin structure, and/or the increased canting angle induced by Ce doping.

Room temperature ferroelectric–ferromagnetic Bi3.25Sm0.75Ti2.98V0.02O12/La0.67 Sr0.33 MnO3 double layer heterostructure

A ferroelectric Bi3.25Sm0.75Ti2.98V0.02O12 (BSVT) film and colossal magnetoresistive La0.67Sr0.33MnO3 (LSMO) film heterostructure on Si with excellent performances of both ferroelectric and ferromagnetic properties at room temperature was fabricated by the pulsed laser deposition method. Saturated magnetization–magnetic field hysteresis loops and polarization–electric field loops proved the materials compatibilities in keeping their own intrinsic properties of LSMO and BSVT in the heterostructure. The remanent polarization (2Pr) of the BSVT is 56 µC cm−2 at an applied electrical voltage of 11 V. Fatigue measurements show a 25% decrease in remanent polarization after 106 read/write cycles but no further changes afterwards.

Thermal hysteresis in the magnetization of the layered III-VI diluted magnetic semiconductor In1−xMnxSe

Magnetic properties of single-crystalline In1−xMnxSe (x=0.10) have been measured. A prominent thermal hysteresis in the magnetization is observed between 90 and 290K. The magnetization is reversible (deviating by only ∼0.8%) from 400 down to ∼120K along the upper branch of the hysteresis. In contrast, the lower branch magnetization is irreversible from 5 up to 290K and deviates by 30% of the 0.010emu∕g hysteresis splitting at 140K. Magnetic-field hysteresis loops at 200K between −7 and +7T demonstrate that changing the magnetic field does not allow movement between the upper and lower branches of the thermal hysteresis. This magnetic behavior is consistent with a charge-density wave. However, the observed ΔT=200K hysteresis in In1−xMnxSe is roughly an order of magnitude larger than other previously reported values of ΔT.

Origin of the fast magnetization tunneling in tetranuclear nickel single-molecule magnets

Abstract Several tetranuclear nickel(II) single-molecule magnets (SMMs) have been prepared with the general composition of [Ni(hmp)(ROH)X]4 · S, where hmp− is the monoanion of 2-hydroxymethylpyridine, X− is either Cl− or Br− and S is the solvate molecule. Magnetization versus magnetic field hysteresis loops for these Ni4 SMMs show that there is a relatively fast rate of magnetization tunneling (small coercive field) and, in certain cases, an exchange bias present. Detailed measurements have been carried out in order to determine the origin of the fast magnetization tunneling. High-field electron paramagnetic resonance (HFEPR) data were collected on a single crystal of [Zn(hmp)(dmb)Cl]4 doped with a small amount of Ni(II), where, dmb is 3,3-dimethyl-1-butanol. These variable-frequency/temperature data give values of the single-ion zero-field splitting parameters Di and Ei, and the orientations of these interactions, for the single NiII ions in a Zn3Ni complex doped into a Zn4 crystal. HFEPR data were also obtained at many frequencies and temperatures for a single crystal of isostructural [Ni(hmp)(dmb)Cl]4. Rotation of the single crystal such that the external field is positioned in the hard plane clearly establishes that the transverse zero-field interaction B 4 4 is the cause of the fast magnetization tunneling in the S = 4 ground state of this SMM. The magnitude of B 4 4 and the Ni4 D value can be related to the directionality and magnitude of the Di and Ei interactions at the individual NiII ions, determined for the doped crystal. The microenvironments and ligand dynamics were probed by means of a single-crystal X-ray structure at 12 K and by heat capacity data.