Small Magnet Research Articles

Manipulating the magnetic and electric transport properties of La0.7Sr0.3MnO3 epitaxial thin film through ionic liquid-gated technology

Reversible manipulation of magnetic and electric transport properties by electrical means in perovskite transition metal oxides is of paramount importance in modern condensed matter physics and generates promising application prospects in electronic devices. However, the conventional method of field-effect transistor structure presents a huge limitation due to its poor ability to modulate the exceedingly high carrier density of about 1014−1015 cm−2. In this study, La0.7Sr0.3MnO3 (LSMO) epitaxial thin films with tensile and compressive strain states were obtained by growth on different single-crystal substrates. Compared with the LSMO thin film under tensile strain, the compressed LSMO thin film possesses more oxygen vacancies, a higher resistance and a smaller magnetization. In addition, the structural phases, magnetic and electric transport properties in both LSMO thin films were also manipulated by the ionic liquid-gated method. The LSMO thin film under compressive strain shows easier access to the reversible control of the structural phase and magnetic & electrical transport properties than the LSMO thin film under tensile strain. The synergy of the above capabilities proves an effective and reversible route to manipulate the properties of perovskite transition metal oxides, providing tremendous potential in the design of multiple-state memories, neuromorphic transistors, and other multifunctional devices.

Interfacial properties of [Pt/Co/Pt] trilayers probed through magnetometry

The magnetic and interface properties of [Pt/Co/Pt] were investigated. First, the magnetic properties were determined from the magnetic dead layer plots, in which the Co layer was considered as two distinct parts representing different magnetic properties. The two parts with low and high tCo ranges are close to and away from the top interface (Co/Pt), respectively. The part close to the top interface shows a smaller magnetization (M) value and nonlinear behavior. However, the other part shows a higher M value closer to the bulk value and a linear behavior. The nonlinear behavior of the M values of the low tCo range was converted to an impurity level using simple assumptions. The results showed the effect of the top Pt layer on the magnetic properties of the Co layer. The results clearly demonstrate that magnetometry could be utilized as a means to understand the interface quality of magnetic multilayer systems.

Active magnetic regenerative cooling with smaller magnets

• Improved alignment of magnetic field and fluid flow raises efficiency. • Experiments prove the insensitivity of device performance to magnetic field profile. • Overall, results show efficiency can be conserved while halving magnet volume, cost. Magnetocaloric heat pumping near room temperature, a.k.a. magnetic cooling, relies on the active regenerator cycle to achieve functional temperature spans and realize economic and societal benefits promised by this emerging solid-state cooling technology. The cycle itself depends upon synchronizing oscillating flow of a heat transfer fluid through a solid porous active material matrix, or the refrigerant, with periods when the refrigerant is in the highest available (field-on) and in nearly zero (field-off) magnetic fields to accomplish heat transfer. With this in mind, we analyze varying flow and magnetic field wave forms and the timing between when the fluid is pumped and when the magnetic field is turned on and off. We demonstrate that the volume and the cost of permanent magnet generating the field changes can be cut nearly in half with little to no effect on the device temperature span and cooling power normalized by the refrigerant mass.

Electrical and magnetic properties of ohmic contacts of the quantum hall resistance devices

Quantum Hall effect is used to realize resistance standard which is in terms of the Planck constant [Formula: see text] and elementary charge [Formula: see text] in metrology. The quantum Hall resistance devices are fabricated based on two-dimensional electron gas formed in [Formula: see text] as heterostructures. Low-resistance Ohmic contact to the 2DEG is crucial in fabricating quantum Hall devices to obtain reproducible and accurate results. Annealed Au/Ge/Ni multilayer is widely used for Ohmic contacts to GaAs. In this paper, electrical and magnetic properties of optimized Ohmic contacts of quantum Hall devices are studied. For the electrical property evaluation, contact resistances are measured by three-terminal method in the quantum Hall [Formula: see text] plateau regime. Most of the contacts were Ohmic with values less than 2.4 [Formula: see text] and a good yield of better than 99% is obtained. In the magnetic property study, the annealed Au/Ge/Ni multilayer shows much smaller magnetization than the as-deposited one. TEM-EDS study shows the pure Ni layer diffused into the GaAs and AlGaAs layer and converts into the compounds closed to Ni2GeAs and NiAs phase. Density functional theory is used to calculate the atomic moments of pure face-centered cubic Ni, hexagonal Ni2GeAs and NiAs. The calculation results explained the magnetization drop of annealed samples well.

Electromagnetic Haptic Feedback System for Use With a Graphical Display Using Flat Coils and Sensor Array

We have developed a haptic interaction system which wirelessly generates 3D forces onto a small magnet which can be fixed to a user's fingertip or a handheld stylus. Magnet position sensing and haptic force generation can both be performed through a thin screen, and the actuation and sensing components of the system are sufficiently thin and compact to be easily mounted behind the screen in the same enclosure. In this manner, the system is particularly well suited for use for co-located haptic and graphic display, with 3D force feedback added to an interactive display. The purpose of the work is to describe, analyze, and validate a method and device to generate haptic force feedback on a magnet held near a display screen using thin flat arrays of coils and sensors. The location of the magnet during haptic interaction is obtained by an array of Hall effect sensors. Flat rectangular coils are used to generate Lorentz forces on the interaction magnet and can act in combination to produce 3D forces in any direction, both parallel and normal to the screen surface. The active area of the haptic interaction system prototype is approximately 120 × 120 mm, with effective force generation and position sensing from the screen surface to an elevation of approximately 15 mm. The localization methods do not depend on the size, shape, or magnetization strength of the magnet, so that larger or smaller magnets may be used for greater forces or more ease of manipulation, without modifying the software other than the force actuation model. The design of the system is presented, including modeling and analysis of the sensing and actuation methods. Experimental results are given for field sensing, magnet localization, and haptic interaction with simulated objects. Forces up to 0.3 N are demonstrated to be generated in different specific desired directions while sensing planar magnet position to an accuracy within approximately 2.0 mm.

Observation of Magnetic Antiskyrmions in the Low Magnetization Ferrimagnet Mn2Rh0.95Ir0.05Sn.

Recently, magnetic antiskyrmions were discovered in Mn1.4Pt0.9Pd0.1Sn, an inverse tetragonal Heusler compound that is nominally a ferrimagnet, but which can only be formed with substantial Mn vacancies. The vacancies reduce considerably the compensation of the moments between the two expected antiferromagnetically coupled Mn sub-lattices so that the overall magnetization is very high and the compound is almost a “ferromagnet”. Here, we report the observation of antiskyrmions in a second inverse tetragonal Heusler compound, Mn2Rh0.95Ir0.05Sn, which can be formed stoichiometrically without any Mn vacancies and which thus exhibits a much smaller magnetization. Individual and lattices of antiskyrmions can be stabilized over a wide range of temperature from near room temperature to 100 K, the base temperature of the Lorentz transmission electron microscope used to image them. In low magnetic fields helical spin textures are found which evolve into antiskyrmion structures in the presence of small magnetic fields. A weaker Dzyaloshinskii-Moriya interaction (DMI), that stabilizes the antiskyrmions, is expected for the 4d element Rh as compared to the 5d element Pt, so that the observation of antiskyrmions in Mn2Rh0.95Ir0.05Sn establishes the intrinsic stability of antiskyrmions in these Heusler compounds. Moreover, the finding of antiskyrmions with substantially lower magnetization promises, via chemical tuning, even zero moment antiskyrmions with important technological import.

Lattice distortion modified anisotropic magnetoresistance in epitaxial La0.67Sr0.33MnO3 thin films

Effects of lattice distortion on the anisotropic magnetoresistance (AMR) of magnetron sputtered epitaxial La0.67Sr0.33MnO3 (LSMO) thin films grown on (001)-oriented SrTiO3 and MgO substrates were investigated. Lattice mismatch at the film/substrate interface gives rise to weak and strong lattice distortion of the LSMO/SrTiO3 and LSMO/MgO films, respectively. The Jahn-Teller electron-phonon coupling in LSMO leads to significant localization of the eg electrons, causing smaller magnetization, lower transition temperatures (TC, TMI) and larger resistivity. Moreover, AMR of the LSMO films show strong correlations with the substrates. Only two-fold symmetric AMR can be observed in the LSMO/SrTiO3 films. However, the AMR curves of the LSMO/MgO system show clear transitions between two- and four-fold symmetry. This behavior is ascribed to the lattice distortion in the LSMO films which enhances the magnetocrystalline anisotropy.

Spatially Resolved Large Magnetization in Ultrathin BiFeO3.

Here, a quantitative magnetic depth profile across the planar interfaces in BiFeO3 /La0.7 Sr0.3 MnO3 (BFO/LSMO) superlattices using polarized neutron reflectometry is obtained. An enhanced magnetization of 1.83 ± 0.16 μB /Fe in BFO layers is observed when they are interleaved between two manganite layers. The enhanced magnetic order in BFO persists up to 200 K. The depth dependence of magnetic moments in BFO/LSMO superlattices as a function of the BFO layer thickness is also explored. The results show the enhanced net magnetic moment in BFO from the LSMO/BFO interface extends 3-4 unit cells into BFO. The interior part of a thicker BFO layer has a much smaller magnetization, suggesting it still keeps the small canted AFM state. The results exclude charge transfer, intermixing, epitaxial strain, and octahedral rotations/tilts as dominating mechanisms for the large net magnetization in BFO. An explanation-one suggested by others previously and consistent with the observations-attributes the temperature dependence of the net magnetization of BFO to strong orbital hybridization between Fe and Mn across the interfaces. Such orbital reconstruction would establish an upper temperature limit for magnetic ordering of BFO.

Effect of structural disorder on the ground state properties of Co2CrAl Heusler alloy

In order to discuss the difference between the available theoretical and experimental values of the total magnetic moment of Co2CrAl Heusler alloy, in this paper we studied the effects of a structural disorder on the magnetic and electronic ground state properties of the alloy studied by means of ab initio and Monte Carlo methods. On the one hand, it is shown that a calculated magnetic ground state of the austenite L21 structure is ferromagnetic, and the alloy demonstrates half-metallic behavior. However, the equilibrium lattice parameter and magnetic moment calculated for ferrimagnetic state (where the Cr atoms are ordered antiferromagnetically) are in better agreement with the available experimental data than the ferromagnetic one. On the other hand, an account of a structural disorder results in a decrease in the magnetic moment to a value close to the experimental. However, systems with a structural disorder are energetically unfavorable in comparison with the ordered L21 structure at zero temperature. Using the calculated exchange coupling parameters in the Heisenberg Hamiltonian, the temperature dependences of magnetization, specific heat, magnetic part of internal energy as well as Helmholtz energy are simulated in the framework of Monte Carlo technique for both ordered and disordered cases. Eventually, it is shown that the disordered structure with smaller magnetization is more stable at higher temperatures. This indicates that the experimental compound might be disordered.

Structural, morphological, ferromagnetic and photoluminescence properties of Fe-doped ZnO, prepared by hydrothermal route

Fe doped ZnO particles of flower-like shape, hexagonal prisms and hybrid structures flower-prisms have been synthesized by hydrothermal technique, and their luminescence and magnetic properties have been investigated as a function of the morphology changes due to iron dopant (0–0.03 at.% Fe3+). The X-ray diffractograms of Zn1-xFexO powders indicated a hexagonal wurtzite polycrystalline structure. SEM images reveal the change of Zn1-xFexO grains shape from flower-like to hexagonal prisms as the Fe concentration (x) increases from 0 to 3 at.% Fe. Undoped ZnO shown weak room temperatures ferromagnetism, with high coercivity (Hc = 107 Oe) and saturation magnetization Ms of 1.5·10−3 emu/g. ZnO doped with 1 and 3 at.% Fe presented a significant increase of the magnetization in comparison with the undoped ZnO. For ZnO doped with 3 at.% Fe, Ms = 32.5·10−3 emu/g and Mrem = 0.78·10−3 emu/g. Compared with other reports on magnetic properties of undoped and Fe doped ZnO, these results indicated higher coercivity and smaller magnetizations. The drop in the intensity of characteristic green-yellow photoluminescence band of ZnO at about 550–600 nm was attributed to the decrease of the number of oxygen vacancies and interstitial oxygen. By increasing the Fe concentration, the electron paramagnetic resonance (EPR) signal of undoped ZnO decreases due to the decrease of defects concentration.

Influence of Cu Doping on Martensitic and Magnetic Transitions in Ni-Mn-Sn Alloys

In this work, the fourth element Cu was introduced to substitute Ni in polycrystalline Ni-Mn-Sn alloys. It was shown that Cu doping did not change the crystal structure of the martensite in Ni50-xCuxMn39Sn11 (x=0, 1, ......,7) alloys, but resulted in the decrease of martensitic transformation temperatures. Due to the higher atomic radius of Cu with respect to that of Ni, the lattice volume of martensite unit cell increases with the gradual substitution of Ni by Cu. For the alloys with the Cu content of 0-4%, the martensitic transformation is from weak magnetic (paramagnetic) austenite to weak magnetic (paramagnetic) martensite. When the Cu content is higher than 4%, the paramagnetic to ferromagnetic transition of austenite was introduced. The temperature interval between magnetic transition and structural transformation was enlarged with the increase of Cu content. Due to the relatively smaller magnetization difference between austenite and martensite, the field induced inverse martensitic transformation behavior is not significant in the present Cu-doped alloys.

Scaling Limits on All-Spin Logic

Scaling limits on all-spin logic (ASL) are theoretically studied using the spin circuit theory and the stochastic Landau–Lifshitz–Gilbert equation under the macrospin approximation. It is found that as ASL circuits are scaled, the device delay significantly increases due to a stronger dipole coupling between the input and the output magnets. The effect of the dipole interaction can be mitigated by increasing the input current and by using smaller magnets with stronger material anisotropy and weaker saturation magnetization. Furthermore, the presence of the leakage current modifies the device delay. Finally, both delay and energy of ASL dramatically increase as the shunt path is shortened. Possible solutions to eliminate the leakage current and the shunt path are discussed.

Sustained magnetization oscillations in polyaniline-Fe3O4 nanocomposites.

We report experiments with polyaniline-Fe3O4 (PANI-Fe3O4) nanocomposites synthesized under several different conditions. With a reaction carried out at room temperature and assisted by intense ultra-violet (UV) irradiation, we observe sustained oscillations in the magnetization with a period of about 25 min. The oscillations are interpreted as the result of an oscillatory chemical reaction in which part of the Fe(+2) ions of magnetite, Fe3O4, are oxidized by the UV irradiation to form Fe(+3) so that a fraction of the magnetite content transforms into maghemite, γ-Fe2O3. Then, Fe(+3) ions at the nanoparticle surfaces are reduced and transformed back into Fe(+2), when acting as an oxidizing agent for polyaniline in the polymerization process. Since maghemite has smaller magnetization than magnetite, the oscillating chemical reaction results in the oscillatory magnetization. The observations are interpreted with the Lotka-Volterra nonlinear coupled equations with parameters that can be adjusted to fit very well the experimental data.

The extrinsic hysteresis behavior of dilute binary ferrofluids.

We report on the magnetization behavior of dilute binary ferrofluids based on γ-Fe(2)O(3)/Ni(2)O(3) composite nanoparticles (A particles), with diameter about 11 nm, and ferrihydrite (Fe(5)O(7)(OH) ・4H2O) nanoparticles (B particles), with diameter about 6 nm. The results show that for the binary ferrofluids with A-particle volume fraction φ(A) = 0.2% and B-particle volume fractions φ(B) = 0.1% and φ(B) = 0.6%, the magnetization curves exhibit quasi-magnetic hysteresis behavior. The demagnetizing curves coincide with the magnetizing curves at high fields. However, for single γ-Fe(2)O(3)/Ni(2)O(3) ferrofluids with φ(A) = 0.2% and binary ferrofluids with φ(A) = 0.2% and φ(B) = 1.0%, the magnetization curves do not behave in this way. Additionally, at high field (750 kA/m), the binary ferrofluid with φ(B) = 1.0% has the smallest magnetization. From the model-of-chain theory, the extrinsic hysteresis behavior of these samples is attributed to the field-induced effects of pre-existing A particle chains, which involve both Brownian rotation of the chains'moments and a Néel rotation of the particles' moments in the chains. The loss of magnetization for the ferrofluids with φ(B) = 1.0% is attributed to pre-existing ring-like A-particle aggregates. These magnetization behaviors of the dilute binary ferrofluids not only depend on features of the strongly magnetic A-particle system, but also modifications of the weaker magnetic B-particle system.

Magnetic Foreign Body Injuries: A Large Pediatric Hospital Experience

To examine trends in magnet-related injuries and hypothesize that changes are a result of new neodymium-iron-boron magnets that are smaller, stronger, and commonly sold in sets. In this retrospective chart review, we searched our institution's electronic patient record for patients less than 18 years old who were diagnosed with magnetic foreign body ingestion between 2002 and 2012. Cases were analyzed for patient, magnetic foreign body, and management characteristics. Incidence rates and case characteristics were compared between the first 8 years of the study period and the last 3. We identified 94 patients who met our search criteria. Of confirmed ingestions, the median age was 4.5 years and 65% were male. The incidence of visits increased between the 2002-2009 period and the 2010-2012 period by a factor of 2.94 (95% CI, 1.84-4.70), whereas the incidence of injuries involving multiple magnets increased by a factor of 8.40 (95% CI, 3.44-20.56). The volume of the magnets decreased from 878.6 mm3 to 259.8 mm3. Six cases required surgical removal of the magnets because of intra-abdominal sepsis or concern for imminent bowel perforation. Since 2002, there has been a significant increase in the incidence of magnetic foreign body injuries. These injuries have increasingly involved multiple, smaller magnets and required operative intervention.

Novel Magnetic Chiral Structures and Unusual Temperature Hysteresis in the Metallic Helimagnet MnP

We have reinvestigated the magnetic properties of the classical metallic helimagnet MnP by magnetization and neutron scattering experiments. Our neutron scattering results indicate that the previously reported magnetic structure in the low-temperature (LT) helimagnetic phase (T < 47K) should be modified to an alternately tilted helimagnetic structure pro- duced by the Dzyaloshinsky-Moriya interaction. In the intermediate temperature (IT) range between the LT helimagnetic phase and the high-temperature (HT) ferromagnetic phase along the c-axis, 47K < T < 282K, we have found a weak ferromagnetic behavior along the b-axis. Surprisingly, the IT weak ferromagnetic phase has two different states, namely, the large magnetization (LM) and small magnetization (SM) states. The SM state emerges with cooling from the paramagnetic phase above 292 K via the HT ferromagnetic phase and LM state emerges with warming from the LT helimagnetic phase. The weak ferromagnetism along the b-axis and the unusual temperature hysteresis in the IT phase can be understood by assuming a spontaneous formation of the stripe structure consisting of alternately arranged HT ferromagnetic and LT helimagnetic domains.

High pressure magnetic measurements on strongly correlated electron systems with a miniature ceramic anvil high pressure cell

We have designed a miniature ceramic anvil high pressure cell (mCAC) for magnetic measurements at pressures up to 12.6 GPa in a commercial superconducting quantum interference (SQUID) magnetometer [1-3]. The simplified mCAC without anvil alignment mechanism is easy-to-use for researchers who are not familiar with high-pressure technology and magnetic measurements can be done above 10 GPa. Here, we show one additional modification in the mCAC. This modification enables more precise magnetic measurements on samples with smaller magnetization. We confirm the spontaneous dc magnetization in the pressure-induced ferromagnetic phase in YbCu2Si2 by high pressure magnetic measurements with the mCAC. The pressure-induced phase has the strong uniaxial Ising-type anisotropy.

Combining positive and negative magnetophoreses to separate particles of different magnetic properties

A new separation method that combines both positive and negative magnetophoreses based on ferrofluids is used to separate mixtures of particles with different magnetic properties. Ferrofluids are stable magnetic nanoparticles suspensions. Under external magnetic field gradients, particles with a larger magnetization within the ferrofluids are attracted to a magnet while the ones with a smaller magnetization are pushed away from it. Based on this principle, we report the design, modeling, fabrication, and characterization of the separation device and use it to separate magnetic and non-magnetic particles, as well as particles with different magnetizations. This scheme is simple, cost-effective, and label-free compared to other existing techniques.

Critical current densities and microstructures in rod-in-tube and tube type Nb3Sn strands—present status and prospects for improvement

In this work, the transport and magnetization properties of distributed barrier rod-in-tube (RIT) strands and tube type strands are studied. While tube type strands had smaller magnetizations and thus better stabilities in the low-field region, their 12 T non-Cu Jc values were somewhat smaller than those of the RIT strands. Microstructures were investigated in order to find out the reasons for the difference in non-Cu Jc values. Their grain size and stoichiometry were found to be comparable, leading to similar layer Jc values. Accordingly it was determined that the lower A15 area fraction rather than the quality of the A15 layer was the cause of the discrepancy in non-Cu Jc. Subsequently, the area utilizations of subelements were investigated. While for a RIT strand the fine grain (FG) A15 area occupies ∼60% of a subelement, for a tube type strand it is no more than 40%. Further analysis indicates that the low FG area fraction in a tube type strand is attributed to its much larger unreacted Nb area fraction. Finally, a simple change in strand architecture is proposed to reduce the unreacted Nb area fraction.

Nanocasting of Mesoporous In‐TM (TM = Co, Fe, Mn) Oxides: Towards 3D Diluted‐Oxide Magnetic Semiconductor Architectures

AbstractTransition metal (Co, Fe, Mn)‐doped In2O3−y mesoporous oxides are synthesized by nanocasting using mesoporous silica as hard templates. 3D ordered mesoporous replicas are obtained after silica removal in the case of the In‐Co and In‐Fe oxide powders. During the conversion of metal nitrates into the target mixed oxides, Co, Fe, and Mn ions enter the lattice of the In2O3 bixbyite phase via isovalent or heterovalent cation substitution, leading to a reduction in the cell parameter. In turn, non‐negligible amounts of oxygen vacancies are also present, as evidenced from Rietveld refinements of the X‐ray diffraction patterns. In addition to (In1−xTMx)2O3−y, minor amounts of Co3O4, α‐Fe2O3, and MnxOy phases are also detected, which originate from the remaining TM cations not forming part of the bixbyite lattice. The resulting TM‐doped In2O3−y mesoporous materials show a ferromagnetic response at room temperature, superimposed on a paramagnetic background. Conversely, undoped In2O3−y exhibits a mixed diamagnetic‐ferromagnetic behavior with much smaller magnetization. The influence of the oxygen vacancies and the doping elements on the magnetic properties of these materials is discussed. Due to their 3D mesostructural geometrical arrangement and their room‐temperature ferromagnetic behavior, mesoporous oxide‐diluted magnetic semiconductors may become smart materials for the implementation of advanced components in spintronic nanodevices.