Direct Magnetic Coupling Research Articles

The underlying mechanism of the strain-induced reversible magnetic state transition in MXene nanosheets

MXenes have emerged as one of the most promising members among the two-dimensional (2D) material family owing to their unique physical-chemical properties and outstanding application prospects. Here, we investigated the unique magnetic properties of Ti2C MXenes monolayer by means of the first-principles calculations combining with the Heisenberg spin Hamiltonian approach. A strain-induced reversible magnetic transition for the Ti2C monolayer could be found in our study. When the strain is applied to the Ti2C monolayer, the magnetic configuration of the Ti2C monolayer transforms from the A-type antiferromagnetic (A-AFM) phase to the ferromagnetic (FM) phase with the biaxial tensile strain larger than 6%. Furthermore, the transition from the A-AFM phase to the FM phase was found to be closely related with the change of the Ti and C atomic magnetic moments. The direct magnetic coupling between Ti and C atoms with the increased Ti and C magnetic moments is able to stabilize the structure due to the weakened superexchange interaction between Ti and C atoms under large tensile strain. This is the underlying driving force for the transition from the A-AFM phase to the FM phase. This work provides a promising pathway to better understand the existence of magnetic reversal behavior in the Ti2C MXenes monolayer and opens the door to future magnetic applications of MXenes.

Proximity-Induced Novel Ferromagnetism Accompanied with Resolute Metallicity in NdNiO3 Heterostructure.

Employing X‐ray magnetic circular dichroism (XMCD), angle‐resolved photoemission spectroscopy (ARPES), and momentum‐resolved density fluctuation (MRDF) theory, the magnetic and electronic properties of ultrathin NdNiO3 (NNO) film in proximity to ferromagnetic (FM) La0.67Sr0.33MnO3 (LSMO) layer are investigated. The experimental data shows the direct magnetic coupling between the nickelate film and the manganite layer which causes an unusual ferromagnetic (FM) phase in NNO. Moreover, it is shown the metal–insulator transition in the NNO layer, identified by an abrupt suppression of ARPES spectral weight near the Fermi level (E F), is absent. This observation suggests that the insulating AFM ground state is quenched in proximity to the FM layer. Combining the experimental data (XMCD and AREPS) with the momentum‐resolved density fluctuation calculation (MRDF) reveals a direct link between the MIT and the magnetic orders in NNO systems. This work demonstrates that the proximity layer order can be broadly used to modify physical properties and enrich the phase diagram of RENiO3 (RE = rare‐earth element).

Isolation of a triplet benzene dianion.

Baird's rule predicts that molecules with 4n π electrons should be aromatic in the triplet state, but the realization of simple ring systems with such an electronic ground state has been stymied by these molecules' tendency to distort into structures bearing a large singlet-triplet gap. Here, we show that the elusive benzene diradical dianion can be stabilized through creation of a binucleating ligand that enforces a tightly constrained inverse sandwich structure and direct magnetic exchange coupling. Specifically, we report the compounds [K(18-crown-6)(THF)2]2[M2(BzN6-Mes)] (M = Y, Gd; BzN6-Mes = 1,3,5-tris[2',6'-(N-mesityl)dimethanamino-4'-tert-butylphenyl]benzene), which feature a trigonal ligand that binds one trivalent metal ion on each face of a central benzene dianion. Antiferromagnetic exchange in the Gd3+ compound preferentially stabilizes the triplet state such that it becomes the molecular ground state. Single-crystal X-ray diffraction data and nucleus-independent chemical shift calculations support aromaticity, in agreement with Baird's rule.

Single-Spin Magnetomechanics with Levitated Micromagnets.

We demonstrate a new mechanical transduction platform for individual spin qubits. In our approach, single micromagnets are trapped using a type-II superconductor in proximity of spin qubits, enabling direct magnetic coupling between the two systems. Controlling the distance between the magnet and the superconductor during cooldown, we demonstrate three-dimensional trapping with quality factors around 1×10^{6} and kHz trapping frequencies. We further exploit the large magnetic moment to mass ratio of this mechanical oscillator to couple its motion to the spin degrees of freedom of an individual nitrogen vacancy center in diamond. Our approach provides a new path towards interfacing individual spin qubits with mechanical motion for testing quantum mechanics with mesoscopic objects, realization of quantum networks, and ultrasensitive metrology.

Intercalation induced ferromagnetism in group-V transition metal dichalcogenide bilayer

Two-dimensional (2D) ferromagnetic materials are receiving great attention in recent years. However, owing to strong direct magnetic coupling between different layers, they usually prefer antiferromagnetic coupling between different layers once stacked together. It would be of great interest if one can tune such antiferromagnetism to ferromagnetism, which is preferable for further magnetic information storage, and large magnetic moments can be achieved (proportional to thin-film thickness). In the current work, we theoretically and computationally suggest an effective method to tune the interlayer magnetic coupling between two magnetic materials (VX2, X = S, and Se). We show that intercalating a layer of alkali metals could enhance indirect magnetic exchange, and ferromagnetic interlayer coupling between different VX2 layers can be achieved. Our work provides a new and effective route to control and modulate the magnetic exchange between 2D magnetic materials.

Interfacial magnetic coupling and the confinement effect of spin electrons for τ-MnAl/Ni multilayers

The interface energy and magnetic coupling of τ-MnAl/Ni were studied by first-principles calculations. It was found that τ-MnAl/Ni with Al terminal would be more steady. The magnetic coupling between τ-MnAl film and Ni films oscillated with increasing the thickness of Ni layer. Moreover, the couplings of τ-MnAl/Ni were ferromagnetic and antiferromagnetic when the number of Ni layers were even and odd, respectively. Based on the Fermi surface and energy band analysis, the observed oscillation was attributed to the quantum well states that formed by the confinement of the spin up and spin down band with Δ1 symmetry at Γ‾ point of Ni. The study is valuable for the understanding of direct magnetic coupling among magnetic multilayers and useful for the design of future spintronics devices.

Theoretical Study on the Structures and Magnetic Properties of Metal String Complexes [Ni<sub>3</sub>(L)<sub>4</sub>(NCS)<sub>2</sub>] (L = dpa<sup>-</sup>, mpta<sup>-</sup>, mdpa<sup>-</sup>, mppa<sup>-</sup>)

? ? ?nb ?¤ Density functional theory at the BP86 level and natural bond orbital theory were used to investigate the influence of bridging ligands on the Ni―Ni interactions and magnetic coupling properties of metal string complexes [Ni3(L)4(NCS)2] (L =1: dpa– (dipyridylamine),2: mpta– (4-methylpyridyl-thiazolylamine),3: mdpa– (4-methyl-dipyridylamine),4: mppa– (4-methylpyridyl-3H-pyrrolylamine)) with potential applications in molecular wires. The folowing conclusions can be drawn. (1) The ground states of the complexes are antiferromagnetic (AF) singlet states, which correspond to the quintet state (HS). The energy and structure of HS is similar to AF. There are three-center-four-electron bonds ( ) along the chains. (2) The Ni―Ni and Ni―N distances are unaffected by methyl substituents on the pyridine ring of dpa– ligands. However, substitution of the 3H-pyrrole ring or thiazole ring by the pyridine ring in mdpa– lengthens the N1―N2 and Ni―Ni distances but shortens the Ni2―N2 distance. These effects of the thiazole ring are weaker than those of the 3H-pyrrole ring. Therefore, the strength of the Ni―Ni interaction is1 ≈3 >2 >4. (3) The predictedJab values of3 and4n? are –103 and –88 cm–1, respectively. The AF magnetic coupling effects of the complexes increase with increasing Ni―Ni interaction strength: the stronger the Ni―Ni interaction, the greater the direct magnetic coupling in the orbitals along the chains. In addition, the stronger the Ni2―N2 interaction, the larger the indirect magnetic coupling involving the bridging ligand. The direct magnetic coupling is stronger than the indirect magnetic coupling.

MAGNETIC BRAKING OF STELLAR CORES IN RED GIANTS AND SUPERGIANTS

Magnetic configurations, stable on the long term, appear to exist in various evolutionary phases, from Main-Sequence stars to white dwarfs and neutron stars. The large scale ordered nature of these fields, often approximately dipolar, and their scaling according to the flux conservation scenario favor the model of a fossil field (Duez et al. 2010). We make some first estimates of the magnetic coupling between the stellar cores and the outer layers in red giants and supergiants. Analytical expressions of the truncation radius of the field coupling are established for a convective envelope and for a rotating radiative zone with horizontal turbulence. The timescales of the internal exchanges of angular momentum are considered. Numerical estimates are made on the basis of recent model grids. The direct magnetic coupling of the core to the extended convective envelope of red giants and supergiants appears unlikely. However, we find that the intermediate radiative zone is fully coupled to the core during the He-burning and later phases. This coupling is able to produce a strong spin down of the core of red giants and supergiants, also leading to relatively slowly rotating stellar remnants, like white dwarfs and pulsars. Some angular momentum is also transferred to the outer convective envelope of red giants and supergiants during the He-burning phase and later.

Circuit-quantum electrodynamics with direct magnetic coupling to single-atom spin qubits in isotopically enriched 28Si

Recent advances in silicon nanofabrication have allowed the manipulation of spin qubits that are extremely isolated from noise sources, being therefore the semiconductor equivalent of single atoms in vacuum. We investigate the possibility of directly coupling an electron spin qubit to a superconducting resonator magnetic vacuum field. By using resonators modified to increase the vacuum magnetic field at the qubit location, and isotopically purified 28Si substrates, it is possible to achieve coupling rates faster than the single spin dephasing. This opens up new avenues for circuit-quantum electrodynamics with spins, and provides a pathway for dispersive read-out of spin qubits via superconducting resonators.

Effect of Ball Milling on Structure and Magnetic Properties of Fe-Nb-B-Tb Bulk Nanocrystalline Alloys

It was shown that pulverization of the examined alloys cause a decrease of mean diameters of nanocrystallites, a change of the phase composition and affects magnetic properties. For the alloy with x=0.08 magnetization remanence at T=300 K increases about two times with remaining constant Hc while for x=0.12 Hc decreases from 1.16 t to 0.36 T. The observed effects were attributed to the phase structure changes and breaking of direct magnetic intergrain coupling

Design and Fabrication of Directly-Coupled HTS-SQUID Magnetometer With a Multi-Turn Input Coil

To improve the field resolution of a magnetometer for use in the Earth's field, we have developed a new magnetometer using a hybrid superconducting quantum interference device (SQUID). A multi-turn input coil (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> turns) is inserted between a directly-coupled SQUID and a pickup coil in series in the hybrid SQUID. Screening current couples with the SQUID by both direct current injection and magnetic coupling. To avoid flux trapping, which occurs during cooling in the Earth's field, the width of the SQUID is restricted to be 5 μm. Therefore, the input coil is not stacked on the SQUID inductor like a conventional integrated SQUID but formed around the SQUID. There is an optimal N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> as a result of the trade-off relationship between the increase of the coupling strength and the decrease of the screening current with increasing N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> . The magnetometer using the hybrid SQUID with a slit hole inductor showed an effective area of 0.45 mm>; <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at the optimal N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> of 3. The magnetic coupling was enhanced by using a square hole inductor that had a larger flux capturing area. The obtained A <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> (~ 0.6 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) was almost doubled from that of the conventional directly coupled magnetometer, resulting in a field noise as low as 18 fT/Hz <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1/2</sup> (white).

Novel compact low temperature cofired ceramic ultrawideband SIR bandpass filter using different coupling schemes

AbstractA new ultrawideband quarter‐wavelength SIR filter using different coupling schemes is presented. In the proposed filter, weak coupling coefficient is realized using conventional edge‐coupled structure, while great coefficient is obtained by applying direct magnetic coupling between two resonators or overlapping resonators in different low temperature cofired ceramic layers. Based on the proposed strong coupling structures, the bandwidth is extended, while the size is largely reduced. Besides, spur lines are adopted to improve the stopband characteristic. In this article, a four‐pole filter of dimension 0.11λg × 0.18λg, with a 3‐dB bandwidth of 74.5% and a wide stopband up to 12 GHz, is designed and fabricated. Good agreement between measured and simulated results is observed. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:967–970, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26679

Electric Field Switching of the Magnetic Anisotropy of a Ferromagnetic Layer Exchange Coupled to the Multiferroic CompoundBiFeO3

We report here that a Permalloy layer deposited on top of a multiferroic BiFeO3 single crystal acquires an easy magnetic direction along the propagation vector of the cycloidal arrangement of antiferromagnetic moments in BiFeO3. This anisotropy originates from a direct magnetic coupling with the canted spins forming the cycloid. Moreover, we show that an electric field-induced change of electric polarization is able to toggle the direction of anisotropy in the ferromagnet through the magnetoelectric effect, which links the antiferromagnetic spins to the local polarization in BiFeO3.

Controlling magnetic switching properties of EuS for constructing double spin filter magnetic tunnel junctions

EuS is a well established spin filter (SF) material and its magnetic properties are shown to be tunable with deposition temperature. In tunnel junctions consisting of double EuS SF barriers, different coercive fields of the two adjacent SFs are achieved by depositing one EuS layer at room temperature and quench condensing the other at liquid nitrogen temperature. A thin Al2O3 spacer layer is introduced in between to prevent direct magnetic coupling. Magnetoresistance with well defined spin-parallel and spin-antiparallel states is obtained this way, and the resistance change originates completely within the SF/I/SF composite tunnel barrier, with no ferromagnetic electrodes involved.

Detection of pinholes in magnetic tunnel junctions by magnetic coupling

Pinholes in tunnel barriers are detrimental to the performance of magnetic tunnel junctions (MTJs) since they create direct magnetic exchange coupling between the free and pinned magnetic films and may act as current short circuits. A simple and straightforward technique which enables observation of pinholes and distinguishes pinhole coupling from orange-peel coupling would aid greatly in optimizing the performance of MTJs. However, the existing methods for this determination are quite complex and destructive and do not work on complete structures. We have developed a simpler, nondestructive method that works on full MTJ structures which is able to identify whether an observed coupling arises primarily from magnetic exchange coupling through pinholes or from orange-peel coupling. The method is based on the shift in the free layer hysteresis loop at low temperatures. It is well known that the shift in the pinned layer loop at low temperatures is due to the sharp increase of the IrMn pinning strength. If pinholes exist, the free layer loop will also exhibit a shift due to direct exchange coupling. If there are no pinholes, no shift will be observed since orange-peel coupling is magnetostatic and cobalt has essentially no increase in magnetization below 300K. In this way, a quick diagnosis can be made of whether or not pinholes exist in the MTJ.

Dynamics of baryons from string theory and vector dominance

We consider a holographic model of QCD from string theory, a la Sakai and Sugimoto, and study baryons. In this model, mesons are collectively realized as a five-dimensional \$U(N_F)=U(1)\times SU(N_F)$ Yang-Mills field and baryons are classically identified as $SU(N_F)$ solitons with a unit Pontryagin number and $N_c$ electric charges. The soliton is shown to be very small in the large 't Hooft coupling limit, allowing us to introduce an effective field ${\cal B}$. Its coupling to the mesons are dictated by the soliton structure, and consists of a direct magnetic coupling to the $SU(N_F)$ field strength as well as a minimal coupling to the $U(N_F)$ gauge field. Upon the dimensional reduction, this effective action reproduces all interaction terms between nucleons and an infinite tower of mesons in a manner consistent with the large $N_c$ expansion. We further find that all electromagnetic interactions, as inferred from the same effective action via a holographic prescription, are mediated by an infinite tower of vector mesons, rendering the baryon electromagnetic form factors completely vector-dominated as well. We estimate nucleon-meson couplings and also the anomalous magnetic moments, which compare well with nature.

Magnetic exchange force microscopy with atomic resolution

The ordering of neighbouring atomic magnetic moments (spins) leads to important collective phenomena such as ferromagnetism and antiferromagnetism. A full understanding of magnetism on the nanometre scale therefore calls for information on the arrangement of spins in real space and with atomic resolution. Spin-polarized scanning tunnelling microscopy accomplishes this but can probe only conducting materials. Force microscopy can be used on any sample independent of its conductivity. In particular, magnetic force microscopy is well suited to exploring ferromagnetic domain structures. However, atomic resolution cannot be achieved because data acquisition involves the sensing of long-range magnetostatic forces between tip and sample. Magnetic exchange force microscopy has been proposed for overcoming this limitation: by using an atomic force microscope with a magnetic tip, it should be possible to detect the short-range magnetic exchange force between tip and sample spins. Here we show for a prototypical antiferromagnetic insulator, the (001) surface of nickel oxide, that magnetic exchange force microscopy can indeed reveal the arrangement of both surface atoms and their spins simultaneously. In contrast with previous attempts to implement this method, we use an external magnetic field to align the magnetic polarization at the tip apex so as to optimize the interaction between tip and sample spins. This allows us to observe the direct magnetic exchange coupling between the spins of the tip atom and sample atom that are closest to each other, and thereby demonstrate the potential of magnetic exchange force microscopy for investigations of inter-spin interactions at the atomic level.

PrF3Van Vleck paramagnet as a promising material for the nuclear dynamic polarization of 3He

We suggest using insulating Van Vleck paramagnet PrF3 as a solid substrate for the dynamic polarization of 3He nuclei at high magnetic fields. The exploring of solid effect for this purpose assumes the knowledge of the optimal conditions for transfer of polarization from the Pr3+ electron shell in PrF3 to 141Pr nuclear spins as well as the existence of effective channel for magnetization transfer from 141Pr nuclear spins to nuclear spins of liquid 3He (so-called magnetic coupling phenomenon). To study solid effect in PrF3 the magnetic field dependencies of the Stark energy levels of 3H4 ground state multiple were calculated for high magnetic fields up to 40 T using the set of crystal-field parameters obtained early from magnetization measurements and exchange charges model. Also the results of searching for a direct magnetic coupling between nuclei of the liquid 3He and 141Pr nuclei in the system "PrF3 powder – liquid 3He" by pulse NMR method are represented. Also the surface effects in the PrF3 crystal were studied by SQUID and optical microscopy measurements. Other aspects of the nuclear polarization transfer are discussed.

Force‐Free Magnetosphere of an Accretion Disk–Black Hole System. II. Kerr Geometry

We consider a stationary axisymmetric force-free degenerate magnetosphere of a rotating Kerr black hole surrounded by a thin, Keplerian, infinitely conducting accretion disk. We focus on the closed-field geometry characterized by a direct magnetic coupling between the disk and the hole's event horizon. We first argue that the hole's rotation necessarily limits the radial extent of the force-free link on the disk surface: the faster the hole rotates, the smaller the magnetically connected inner part of the disk has to be. We then show that this is indeed the case by solving numerically the Grad-Shafranov equation, the main differential equation describing the structure of the magnetosphere. An important element in our approach is the use of the regularity condition at the inner light cylinder to fix the poloidal current as a function of the poloidal magnetic flux. As an outcome of our computations, we are able to chart out the maximum allowable size of the portion of the disk that is magnetically connected to the hole as a function of the black hole spin. We also calculate the angular momentum and energy transfer between the hole and the disk that takes place via the direct magnetic link. We find that both of these quantities grow rapidly and that their deposition becomes highly concentrated near the inner edge of the disk as the black hole spin is increased.

Detection of Pinholes in Ultrathin Films by Magnetic Coupling

ABSTRACTWhen two magnetic films are separated by a nonmagnetic film, pinholes in the nonmagnetic film can allow direct contact and, thereby, direct magnetic exchange coupling between the two magnetic films. We have studied this coupling by having one of the magnetic films pinned and leaving the other free to switch at low field. The pinning is accomplished with test structures based on exchange bias and synthetic antiferromagnetic layers. Since the pinning strength increases sharply at low temperatures but orange-peel coupling does not, low-temperature (77 K) measurements appear to identify whether an observed coupling arises primarily from magnetic coupling through pinholes or primarily from orange-peel roughness. Our measurements appear to indicate that the observed coupling arises primarily from magnetic coupling through pinholes for Cu films less than 2.1 nm thick and for Al2O3films less than 0.6 nm thick but primarily from roughness-induced (orange-peel) magnetostatic coupling for larger thicknesses.