Ferromagnetic Region Research Articles

Structural, magnetic and electrical transport properties of the sol-gel derived La1-xCaxMnO3 (0≤x≤0.3) nanoparticles

Sol-gel (SG) prepared calcium doped La1-xCaxMnO3 (0 ≤ x ≤ 0.3) nanoparticles (>35 nm) show a phase transformation from rhombohedral-R3c to orthorhombic-Pnma. The dopant concentration influences the bond length and lattice parameters and their variations as observed from the rietveld refinement. The phase stability and chemical compositions are confirmed using the Thermogravimetric and differential thermal analysis (TG-DTA) and Fourier transform infrared (FTIR) spectroscopy measurements. The latter confirms the presence of dangling bonds from the changes in the characteristic vibrational frequencies and intensities. Field emission scanning electron microscope-energy dispersive X-ray analysis (FESEM-EDAX) confirms the nano-dimension of the particles and good chemical homogeneity of the samples. The Curie temperature Tc, estimated from the field cooled magnetization curve, shifts towards room temperature (Tc = 160–270 K) with the increase of dopant concentration. The samples exhibit a clear metal to insulator transition, the temperature of which shifts to higher values (169–267 K) with increase in dopant content. With the application of an external magnetic field, local ordering of magnetic moments occurs and the evolution of a ferromagnetic metallic phase suppresses the existing paramagnetic insulating phase, decreasing the value of resistivity. Three different temperature ranges demarked in the resistivity curves are studied using different theoretical models. The observed low temperature resistivity upturn is explained on the basis of the Kondo-like model while the ferromagnetic metallic region is studied using different scattering processes like electron – electron, electron – magnon etc. The variable range and small polaron hopping (VRH and SPH) models are used in the high-temperature region data and with the aid of Holstein's relation the non-adiabatic SPH model it is found to be most suitable to understand the conductivity behavior. The temperature dependent evolution of the Griffith's like phase can explain and correlate the electric transport and magnetic properties exhibited by the samples. This highlights the evolution of Mn3+-O2--Mn4+ FM oriented networks, which form conduction channels with the lowering of temperatures. These channels mediate the enhanced conduction and helps set in long-range FM ordering in the samples/

Hole doping in compositionally complex correlated oxide enables tunable exchange biasing

Magnetic interfaces and the phenomena arising from them drive both the design of modern spintronics and fundamental research. Recently, it was revealed that through designing magnetic frustration in configurationally complex entropy stabilized oxides, exchange bias can occur in structurally single crystal films. This eliminates the need for complex heterostructures and nanocomposites in the design and control of magnetic response phenomena. In this work, we demonstrate through hole doping of a high entropy perovskite oxide that tuning of magnetic responses can be achieved. With detailed magnetometry, we show magnetic coupling exhibiting a variety of magnetic responses including exchange bias and antiferromagnetic spin reversal in the entropy stabilized ABO3 perovskite oxide La1−xSrx(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 family. We find that manipulation of the A-site charge state can be used to balance magnetic phase compositions and coupling responses. This allows for the creation of highly tunable exchange bias responses. In the low Sr doping regime, a spin frustrated region arising at the antiferromagnetic phase boundary is shown to directly couple to the antiferromagnetic moments of the film and emerges as the dominant mechanism, leading to a vertical shift of magnetization loops in response to field biasing. At higher concentrations, direct coupling of antiferromagnetic and ferromagnetic regions is observed. This tunability of magnetic coupling is discussed within the context of these three competing magnetic phases, revealing critical features in designing exchange bias through exploiting spin frustration and disorder in high entropy oxides.

Mirroring Skyrmions in Synthetic Antiferromagnets via Modular Design.

Skyrmions are promising for the next generation of spintronic devices, which involves the production and transfer of skyrmions. The creation of skyrmions can be realized by a magnetic field, electric field, or electric current while the controllable transfer of skyrmions is hindered by the skyrmion Hall effect. Here, we propose utilizing the interlayer exchange coupling induced by the Ruderman-Kittel-Kasuya-Yoshida interactions to create skyrmions through hybrid ferromagnet/synthetic antiferromagnet structures. An initial skyrmion in ferromagnetic regions could create a mirroring skyrmion with an opposite topological charge in antiferromagnetic regions driven by the current. Furthermore, the created skyrmions could be transferred in synthetic antiferromagnets without deviations away from the main trajectories due to the suppression of the skyrmion Hall effect in comparison to the transfer of the skyrmion in ferromagnets. The interlayer exchange coupling can be tuned, and the mirrored skyrmions can be separated when they reach the desired locations. Using this approach, the antiferromagnetic coupled skyrmions can be repeatedly created in hybrid ferromagnet/synthetic antiferromagnet structures. Our work not only supplies a highly efficient approach to create isolated skyrmions and correct the errors in the process of skyrmion transport, but also paves the way to a vital information writing technique based on the motion of skyrmions for skyrmion-based data storage and logic devices.

Chaotic dynamics from coupled magnetic monodomain and Josephson current.

The ordinary (superconductor-insulator-superconductor) Josephson junction cannot exhibit chaos in the absence of an external ac drive, whereas in the superconductor-ferromagnet-superconductor Josephson junction, known as the φ_{0} junction, the magnetic layer effectively provides two extra degrees of freedom that can facilitate chaotic dynamics in the resulting four-dimensional autonomous system. In this work, we use the Landau-Lifshitz-Gilbert model for the magnetic moment of the ferromagnetic weak link, while the Josephson junction is described by the resistively capacitively shunted-junction model. We study the chaotic dynamics of the system for parameters surrounding the ferromagnetic resonance region, i.e., for which the Josephson frequency is reasonably close to the ferromagnetic frequency. We show that, due to the conservation of magnetic moment magnitude, two of the numerically computed full spectrum Lyapunov characteristic exponents are trivially zero. One-parameter bifurcation diagrams are used to investigate various transitions that occur between quasiperiodic, chaotic, and regular regions as the dc-bias current through the junction, I, is varied. We also compute two-dimensional bifurcation diagrams, which are similar to traditional isospike diagrams, to display the different periodicities and synchronization properties in the I-G parameter space, where G is the ratio between the Josephson energy and the magnetic anisotropy energy. We find that as I is reduced the onset of chaos occurs shortly before the transition to the superconducting state. This onset of chaos is signaled by a rapid rise in supercurrent (I_{S}⟶I) which corresponds, dynamically, to increasing anharmonicity in phase rotations of the junction.

Ferromagnetic forces acting on the EU-DEMO divertor

This paper presents an analysis of the mechanical loads produced by the ferromagnetic effects in the divertor of the European DEMO (EU-DEMO) reactor. The exact assessment of this force is an important prerequisite for a reliable structural design, for instance for a correct choice of the fixing supports. To this end, a 3D magnetostatic model has been built with a simplified geometry corresponding to a 22.5° angular sector of the tokamak, exploiting the toroidal symmetry and imposing periodic boundary conditions. The problem has been solved by using CARIDDI code, that implements an integral formulation and provides a significant simplification of the numerical model, since the mesh can be limited to the magnetic materials only. In our case, the main components of the model (divertor and breeding blankets) are made of a ferromagnetic material, namely, EUROFER97 steel. In presence of external static magnetic fields, these components would behave like an "electromagnet" being subjected to forces. The model considers all the sources of such magnetizing fields, namely: (i) the external toroidal field produced by the currents circulating in the external toroidal field coils, (ii) the internal field induced by the toroidal plasma current. Expected static field in the considered components ranges from 3.4 T to 8.6 T. Forces and torques have been evaluated by Kelvin method, starting from the known external magnetic fields and the computed magnetization in the ferromagnetic regions. The contribution of the static magnetic field associated to the equilibrium plasma current has been shown to be negligible.

Tuning structural, electrical, dielectric and magnetic properties of Mg–Cu–Co ferrites via dysprosium (Dy3+) doping

In current research work, Dy3+ substituted Mg0.5Cu0.25Co0.25Fe2‒xDyxO4 (0.0 ≤ x ≤ 0.04 with the step interval of 0.01) soft ferrites were synthesized by the sol–gel auto combustion method. The prepared samples were characterized by the techniques using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared (FTIR) spectroscopy, current–voltage (I–V) measurement, LCR meter, vibrating sample magnetometer (VSM) and Raman. XRD data reveal that the average crystallite size is 49.71 nm and the lattice constant is 0.83703 nm for sample x = 0.03. The non-uniform grain growth was demonstrated by micrographs and impurity-free elemental composition was observed from EDX analysis. The DC resistivity has an increasing and decreasing trend in ferromagnetic and paramagnetic regions with an increase in temperature. Moreover, the high resistivity is observed with the order of 1010 Ω·cm, and the activation energy is 0.944 eV for samples x = 0.03. The dielectric parameters including dielectric constant, dielectric losses, and impedance gradually decrease with the increase in frequency from 8 Hz to 8 MHz. The minimum dielectric loss at high frequency is found for sample x = 0.03. The coercivity (Hc) and saturation magnetization (Ms) are found in the ranges of 520.82–544.02 Oe and 20.5841–21.1473 emu/g, respectively. These observations confirm that dysprosium (x = 0.03) doped MCC-soft ferrites may be applicable in transformer cores, microwave absorbance, and telecommunication devices.

Intrinsic giant magnetoresistance due to exchange-bias-type effects at the surface of single-crystalline NiS2 nanoflakes.

The coexistence of different properties in the same material often results in exciting physical effects. At low temperatures, the pyrite transition-metal disulphide NiS2 hosts both antiferromagnetic and weak ferromagnetic orders, along with surface metallicity dominating its electronic transport. The interplay between such a complex magnetic structure and surface-dominated conduction in NiS2, however, is still not understood. A possible reason for this limited understanding is that NiS2 has been available primarily in bulk single-crystal form, which makes it difficult to perform studies combining magnetometry and transport measurements with high spatial resolution. Here, NiS2 nanoflakes are produced via mechanical cleaving and exfoliation of NiS2 single crystals and their properties are studied on a local (micron-size) scale. Strongly field-asymmetric magnetotransport features are found at low temperatures, which resemble those of more complex magnetic thin film heterostructures. Using nitrogen vacancy magnetometry, these magnetotransport features are related to exchange-bias-type effects between ferromagnetic and antiferromagnetic regions forming near step edges at the nanoflake surface. Nanoflakes with bigger steps exhibit giant magnetoresistance, which suggests a strong influence of magnetic spin textures at the NiS2 surface on its electronic transport. These findings pave the way for the application of NiS2 nanoflakes in van der Waals heterostructures for low-temperature spintronics and superconducting spintronics.

Ta thickness effect on field-free switching and spin–orbit torque efficiency in a ferromagnetically coupled Co/Ta/CoFeB trilayer

Current induced spin–orbit torque (SOT) switching of magnetization is a promising technology for nonvolatile spintronic memory and logic applications. In this work, we systematically investigated the effect of Ta thickness on the magnetic properties, field-free switching and SOT efficiency in a ferromagnetically coupled Co/Ta/CoFeB trilayer with perpendicular magnetic anisotropy. We found that both the anisotropy field and coercivity increase with increasing Ta thickness from 0.15 nm to 0.4 nm. With further increase of Ta thickness to 0.5 nm, two-step switching is observed, indicating that the two magnetic layers are magnetically decoupled. Measurements of pulse-current induced magnetization switching and harmonic Hall voltages show that the critical switching current density increases while the field-free switching ratio and SOT efficiency decrease with increasing Ta thickness. Both the enhanced spin memory loss and reduced interlayer exchange coupling might be responsible for the β DL decrease as the Ta spacer thickness increases. The studied structure with the incorporation of a CoFeB layer is able to realize field-free switching in the strong ferromagnetic coupling region, which may contribute to the further development of magnetic tunnel junctions for better memory applications.

Lateral double magnetic tunnel junction device with orthogonal polarizer for high-performance magnetoresistive memory

Magnetic tunnel junction (MTJ)-based memory devices have larger switching delay and energy consumption, compared to cache or dynamic random access memory. In order to broaden the applications of the magnetoresistive random access memory, reducing the switching time and energy consumption of the MTJ is required. Here, a novel lateral double MTJ with an orthogonal polarizer is proposed. The proposed device consists of three ferromagnetic regions: the first pinned region (PR1) with perpendicular magnetic anisotropy (PMA), a free region (FR) with PMA, and the second pinned region (PR2) with in-plane magnetic anisotropy (IMA). PR1 and PR2 are placed on top of the oxide barrier, which separates them from the FR, comprising a lateral double MTJ structure. The current pulse through PR2 helps to perturb the magnetization of the FR. Since the angle between PR2 and FR is 90°, the initial torque increases significantly, decreasing switching delay by 4.02 times and energy-delay product by 7.23 times. It is also shown, that the area of the access transistor can be reduced by approximately 10%, while maintaining the same energy-delay product and reducing gate RC delay.

New Gd-based magnetic compoundGdPt2Bwith a chiral crystal structure

Herein, we report the discovery of a novel Gd-based magnetic compound GdPt$_2$B with a chiral crystal structure. X-ray diffraction and chemical composition analyses reveal a CePt$_2$B-type crystal structure (space group: $P6_422$) for GdPt$_2$B. Moreover, we successfully grew single crystals of GdPt$_2$B using the Czochralski method. Magnetization measurements and the Curie$-$Weiss analysis demonstrate that the ferromagnetic interaction is dominant in GdPt$_2$B. A clear transition is observed in the temperature dependence of electrical resistivity, magnetic susceptibility, and specific heat at $T_{\rm O}$ = 87 K. The magnetic phase diagram of GdPt$_2$B, which consists of a field-polarized ferromagnetic region and a magnetically ordered region, resembles those of known chiral helimagnets. Furthermore, magnetic susceptibility measurements reveal a possible spin reorientation within the magnetically ordered phase in magnetic fields perpendicular to the screw axis. The results demonstrate that GdPt$_2$B is a suitable platform for investigating the competing effects of ferromagnetic and antisymmetric exchange interactions in rare-earth-based chiral compounds.

Nonlinear features of the superconductor-ferromagnet-superconductor φ0 Josephson junction in the ferromagnetic resonance region.

We demonstrate the manifestations of nonlinear features in magnetic dynamics and I-V characteristics of a φ0 Josephson junction in the ferromagnetic resonance region. We show that at small values of the system parameters damping, spin-orbit interaction, and Josephson-to-magnetic energy ratio, the magnetic dynamics is reduced to the dynamics of a scalar Duffing oscillator driven by the Josephson oscillations. The role of the increasing superconducting current in the resonance region is clarified. Shifting of the ferromagnetic resonant frequency and the reversal of its damping dependence due to nonlinearity are demonstrated by the full Landau-Lifshitz-Gilbert-Josephson system of equations and in its different approximations. Finally, we demonstrate the negative differential resistance in the I-V characteristics and its correlation with the fold-over effect.

Random magnetic anisotropy in a new compound Sm2AgSi3

We report the experimental study on the structural and magnetic properties of a new ternary intermetallic compound Sm2AgSi3. The properties of the sample were investigated in detail by X-ray diffraction, dc–magnetization, and heat capacity measurements. The polycrystalline compound of Sm2AgSi3 crystallizes in the ThSi2-type tetragonal structure (space group I41/amd). The temperature dependent dc–magnetization and heat capacity results demonstrate that the compound undergoes ferromagnetic behaviour with a Curie temperature (TC) of 14 K. The large coercive field in hysteresis loops and the thermomagnetic irreversibility in the ferromagnetic region revealed that the compound exhibits a large magnetic anisotropy. The magnitude of the applied field and the coercivity obtained from the M-H loops corroborate with the thermomagnetic irreversibility in the magnetization data. The magnetic contribution of the heat capacity reveals a broad Schottky–type anomaly above TC, due to the presence of the crystal electric field effect of Sm3+ in Sm2AgSi3.

Vertical inhomogeneous magnetic order in FeRh film

FeRh films, known as antiferromagnetic materials at low temperatures, exhibit unexpected ferromagnetic (FM) characteristics, unlike the bulk. The detailed temperature-dependent physical properties of FeRh films were examined to elucidate the causes. Temperature-dependent magnetic and electrical measurements showed that the coexistence of antiferromagnetic and residual FM states induces thermomagnetic irreversibility (e.g., spin-glass-like behavior) and negative magnetoresistance. Polarized neutron reflectometry (PNR) revealed the presence of non-uniform FM states at the interfaces. Additional temperature-dependent PNR profile and room temperature structural and spectroscopic analyses confirmed that the bottom FM region is correlated with structural distortion. In contrast, the top FM region is temperature-dependent and originates from non-stoichiometry.

Bifurcation structure and chaos in dynamics of nanomagnet coupled to Josephson junction.

Irregular easy axis reorientation features are observed in numerical simulations of the nanomagnet coupled to the Josephson junction. We study magnetization bifurcations and chaos that appear in this system due to the interplay of superconductivity and magnetism. The bifurcation structure of magnetization under the variation of Josephson to magnetic energy ratio as a control parameter demonstrates several precessional motions that are related to chaotic behavior and orbits with different periodicities in the ferromagnetic resonance region. The effect of an external periodic signal on the bifurcation structure is also investigated. The results demonstrate high-frequency modes of a periodic motion and a chaotic response near resonance. Far from the ferromagnetic resonance, we observe a quasiperiodic behavior. The obtained results explain the irregular reorientation of the easy axis and the transitions between different types of motion.

Resonance behavior of embedded and freestanding microscale ferromagnets

The ferromagnetic resonance of a disordered A2 Fe60Al40 ferromagnetic stripe, of dimensions 5 µm × 1 µm × 32 nm, has been observed in two vastly differing surroundings: in the first case, the ferromagnetic region was surrounded by ordered B2 Fe60Al40, and in the second case it was free standing, adhering only to the oxide substrate. The embedded ferromagnet possesses a periodic magnetic domain structure, which transforms to a single domain structure in the freestanding case. The two cases differ in their dynamic response, for instance, the resonance field for the uniform (k = 0) mode at ~ 14 GHz excitation displays a shift from 209 to 194 mT, respectively for the embedded and freestanding cases, with the external magnetic field applied along the long axis. The resonant behavior of a microscopic ferromagnet can thus be finely tailored via control of its near-interfacial surrounding.

Metamagnetism and tricritical behavior in the Kondo lattice model

The phase diagram of the Kondo lattice model is explored around the ferromagnetic region, including an external magnetic field, and allowing the coexistence of ferromagnetism and Kondo effect ($\mathrm{FM}+\mathrm{K}$). Considering the combined effect of temperature and pressure, the evolution of the $\mathrm{FM}+\mathrm{K}$ phase yields a wing structure phase diagram with a tricritical point and a ferromagnetic quantum critical end point. The evolution of the spin-selective Kondo insulator phase, inside the coexistence region, introduces another critical end point. The results show a correspondence with the experimental phase diagram of heavy-fermion compounds, in particular, the ferromagnetic superconductor ${\mathrm{UGe}}_{2}$.

Suppression of ferromagnetism governed by a critical lattice parameter in CeTiGe3 with hydrostatic pressure or V substitution

We combine structural and magnetic measurements to compare the different magnetic phase diagrams between the pressure and substitution studies in ${\mathrm{CeTiGe}}_{3}$. We report on the structural, magnetic, and electrical transport properties of single crystals of ${\mathrm{CeTi}}_{1\ensuremath{-}x}{\mathrm{V}}_{x}{\mathrm{Ge}}_{3}$ ($x=0$, 0.1, 0.2, 0.3, 0.4, 0.9, and 1), and of polycrystalline samples ($x=0.5$, 0.6, 0.7, 0.8), as well as structural properties of ${\mathrm{CeTiGe}}_{3}$ under pressure up to $9\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. The ferromagnetic ordering in ${\mathrm{CeTiGe}}_{3}$ is suppressed with V doping in ${\mathrm{CeTi}}_{1\ensuremath{-}x}{\mathrm{V}}_{x}{\mathrm{Ge}}_{3}$, and suggests a possible ferromagnetic quantum critical point near $x=0.45$. We perform a detailed crystalline electric field (CEF) analysis, and the magnetic susceptibility data in pure ${\mathrm{CeTiGe}}_{3}$ and ${\mathrm{CeVGe}}_{3}$ can be well explained by the CEF model. The proposed CEF energy levels suggest that there is a gradual change of the ground state from $|\ifmmode\pm\else\textpm\fi{}5/2\ensuremath{\rangle}$ in ${\mathrm{CeTiGe}}_{3}$ to $|\ifmmode\pm\else\textpm\fi{}1/2\ensuremath{\rangle}$ in ${\mathrm{CeVGe}}_{3}$, and a suppression of CEF splitting energies near the quantum critical region. When hydrostatic pressure is used instead of chemical substitution, the quantum critical point is avoided by the appearance of magnetic phases above around $4.1\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. In the substitution study, the ferromagnetic and antiferromagnetic regions are well separated, whereas they touch in the pressure study. We observe a different trend in the temperature dependence of the resistivity maximum in both studies, suggesting that the CEF splitting energy is suppressed by V substitution but enhanced by pressure. We also observe different responses in lattice constants between the two studies, highlighting the fact that substitution effects cannot be reduced to chemical pressure effects only. Nevertheless, when the magnetic phase diagrams of both hydrostatic pressure and substitution are compared, we find a common critical lattice constant $c=5.78\phantom{\rule{0.16em}{0ex}}\AA{}$ where the ferromagnetic ordering is suppressed in both studies.

An Investigation of Magnetic Field Influence in Underground High Voltage Cable Shields

Magnetic fields and the shielding efficiency of the shields of underground high voltage cables are studied in this paper regarding several shielding configurations and materials. Shielding efficiency and magnetic fields are computed for shields with the same mesh but from different shielding materials, such as aluminum, ferrite, metal, and steel. In order to get the best shield configuration depending on the source characteristics and the material, a conducting ferromagnetic region with various thickness values is considered as shielding. A finite element model is introduced to investigate the influence of the parameters of magnetic fields and the shielding efficiency of underground high voltage cables. Furthermore, the reduction of the magnetic fields with or without shieldings is also presented. The developed method is performed with the magnetic vector potential formulations and validated on a practical problem.

Magnetic rare-earth ion mediated 4f-3d interlayer coupling and giant exchange bias in single layered Ruddlesden-Popper perovskites SrLnCo0.5Mn0.5O4 (Ln = Pr, Nd)

We report an in-depth study on exotic magnetic nature including giant exchange bias in single layered Ruddlesden-Popper structure polycrystalline perovskites SrRCo0.5Mn0.5O4 (R = Pr, and Nd). X-ray diffraction, neutron diffraction, and Raman spectroscopic studies on both compounds confirm tetragonal crystal structure with I4/mmm space group. Temperature evolution of dc magnetization and ac susceptibility show multiple magnetic phase transitions in both the compounds, i.e., high temperature ferromagnetic transition (Tc = 178/172 K) followed by antiferromagnetic transition at an intermediate temperature (TN = 90/100 K) and finally entering to a glassy magnetic state at lower temperatures (Tg = 35/25 K). Neutron powder diffraction confirms the absence of long-range magnetic order in both compounds. Atomically disordered (Co/Mn)O2 layers give rise to intralayer short range 3d-3d magnetic interactions resulting in co-existence of ferromagnetic and antiferromagnetic regions producing frozen magnetic state below Tg. Present experimental results and analysis show giant exchange bias of − 3.6/− 1.7 kOe (Pr/Nd systems) under cooling field of 50 kOe at 5 K, which stems from the inherent magnetic inhomogeneity. Our results reveal that magnetic rare-earth ion (Pr/Nd) induces a pathway of interlayer 4f-3d magnetic interactions leading to antiferromagnetic interaction between adjacent (Co/Mn)O2 layers, and this has a significant influence on the exchange bias values at low temperature.

The Structural and Electrical Studies on Cu-Co Ferrites

The system Cobalt substituted copperferritesampleswere synthesized by a conventional double sintering ceramic method.A series of doped Copper ferrites withcomposithion Cu1-xCoxFe2O4 (x = 0.2, 0.4, and 0.6 ) are synthesized. The X-raydiffraction pattern at room temperature show that the sample exists in single phase with a spinel structure.Copper-Cobalt ferrites seriesaretakenand their electrical conductivityproperty are studied with a variation of composition and temperature. Log (σT) vs 103/T is almost linear with a variation close to the temperature of Curie.In general, the paramagnetic region activation energy is greater than that of the ferromagnetic region’s.