Ferromagnetic Coupling Strength Research Articles

Coexistence of Ferroelectricity and Ferromagnetism in Atomically Thin Two-Dimensional Cr2S3/WS2 Vertical Heterostructures.

Two-dimensional (2D) heterostructures with ferromagnetism and ferroelectricity provide a promising avenue to miniaturize the device size, increase computational power, and reduce energy consumption. However, the direct synthesis of such eye-catching heterostructures has yet to be realized up to now. Here, we design a two-step chemical vapor deposition strategy to growth of Cr2S3/WS2 vertical heterostructures with atomically sharp and clean interfaces on sapphire. The interlayer charge transfer and periodic moiré superlattice result in the emergence of room-temperature ferroelectricity in atomically thin Cr2S3/WS2 vertical heterostructures. In parallel, long-range ferromagnetic order is discovered in 2D Cr2S3 via the magneto-optical Kerr effect technique with the Curie temperature approaching 170 K. The charge distribution variation induced by the moiré superlattice changes the ferromagnetic coupling strength and enhances the Curie temperature. The coexistence of ferroelectricity and ferromagnetism in 2D Cr2S3/WS2 vertical heterostructures provides a cornerstone for the further design of logic-in-memory devices to build new computing architectures.

First-principles study of strain effect on magnetic and optical properties in (Ga, Mo)Sb

In recent decades, as a kind of critical material in spintronics field, the diluted magnetic semiconductor with high temperature and intrinsic ferromagnetism has attracted extensive attention. In order to explore the approach to enhancing Curie temperature (<i>T</i><sub>C</sub>), the LDA+<i>U</i> method of the first-principles calculation is adopted to study the effect of strain on electronic structure, magnetic and optical properties in Mo doped GaSb system. The results indicate that the structure of GaSb is stable with strain in a range of –6%—2.5%. Plasticity and toughness of GaSb increase under compressive strains, which is conducive to the improvement of the mechanical properties. The strain affects the electronic structure of Mo<sub>Ga</sub> greatly. In a range from –3% to –1.2%, Mo<sub>Ga</sub> is in the low spin state (LSS) with a 1<inline-formula><tex-math id="M3">\begin{document}${\mu _{{\rm{B}}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20212316_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20212316_M3.png"/></alternatives></inline-formula>local magnetic moment, while in a range of –1.1%—2%, Mo<sub>Ga</sub> is in high spin state (HSS) with a 3<inline-formula><tex-math id="M4">\begin{document}${\mu _{{\rm{B}}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20212316_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="9-20212316_M4.png"/></alternatives></inline-formula>moment. The magnetic interactions between Mo<sub>Ga</sub> and Mo<sub>Ga</sub> are all ferromagnetic for LSS and so is the case for HSS, although they are different in coupling intensity and mechanism. In particular, appropriate compressive strains can improve the strength of ferromagnetic coupling effectively and are favorable for the preparation of the GaSb-based diluted magnetic semiconductors with high Curie temperatures and inherent ferromagnetism. Moreover, we find that Mo can greatly improve the polarization capability of GaSb and play a vital role in forming and separating the electron-hole pairs, and thus further improving the photoelectric conversion capability for long wave photons. The energy required to absorb photons for inter-band transition of electrons decreases because of the impurity levels induced by Mo, which leads the absorption edge to be red-shifted. The optical properties of (Ga,Mo)Sb in infrared region are further enhanced by the tensile strain.

A Metropolis study of the isothermal magnetic entropy change in spin-1/2 Ising model

Magnetic refrigeration is a sound, environment friendly alternative to conventional refrigeration techniques. Isothermal magnetic entropy change, for a magnetic substance, is an important quantity to be investigated for determining its potential for Magnetic refrigeration. We have performed extensive Monte Carlo simulations with Metropolis single spin-flip algorithm on spin-1/2 Ising model on a square Bravais lattice to study the change in the magnetic entropy, ΔSm for five values of the external magnetic field, where spin-field energies are either equal to or greater than the in-plane ferromagnetic coupling strength. Binder’s fourth order magnetization cumulant is employed for the determination of the critical point. We observe the peak of ΔSm to coincide with the critical point or the Curie point. With a gradual increase in the magnitude of the field, the peak value of ΔSm increases. The behaviour of isothermal magnetic entropy change is systematic over the entire range of temperature, under observation. This implies such materials, which obey nearest neighbour Ising interactions, are good candidates for Magnetocaloric applications.

Measurements of interlayer exchange coupling of Pt in Py|Pt|Py system

Ferromagnetic coupling strength through Pt is experimentally determined using ferromagnetic resonance studies of Py|Pt(dPt)|[Py|Fe] for Pt thicknesses, dPt, between 0.5 and 2.2 nm. The coupling strength decreases exponentially with the Pt thickness from 4.5 mJ/m2 for dPt = 0.5 nm and reduced to less than 0.02 mJ/m2 for dPt = 2.2 nm. The mechanism mediating exchange coupling is assumed to originate from the induced magnetization of Pt due to its proximity to ferromagnetic Py. The fitting thickness dependence of coupling with this model yields a characteristic coupling length scale of ξ = 0.31 ± 0.01 nm. Additionally, the molar susceptibility of proximity induced Pt is found to be 1.4 × 10−7 ± 0.2 × 10−7 m3/mol, an enhancement of ∼100 times as compared to bulk Pt. Ruderman-Kittel-Kasuya-Yosida type oscillations with a period of ∼0.8 nm are also observed as a small contribution of the total coupling.

Room-Temperature Ordered Spin Structures in Cluster-Assembled Single V@Si12 Sheets

Since most of the existing pristine two-dimensional (2D) materials are either intrinsically nonmagnetic or magnetic with small magnetic moment per unit cell and weak strength of magnetic coupling, introducing transition metal atoms in various nanosheets has been widely used for achieving a desired 2D magnetic material. However, the problem of surface clustering for the doped transition metal atoms is still challenging. Here we demonstrate via first-principles calculations that the recently experimentally characterized endohedral silicon cage V@Si12 clusters can construct two types of single cluster sheets exhibiting hexagonal porous or honeycomb-like framework with regularly and separately distributed V atoms. For the ground state of these two sheets, the preferred magnetic coupling is found to be ferromagnetic due to a free-electron-mediated mechanism. By using external strain, the magnetic moments and strength of magnetic coupling for these two sheets can be deliberately tuned, which would be propitious to their advanced applications. More attractively, our first-principles molecular dynamics simulations show that both the structure and strength of ferromagnetic coupling for the pristine porous sheet are stable enough to survive at room temperature. The insights obtained in this work highlight a new avenue to achieve 2D silicon-based spintronics nanomaterials.

Optical spectra of the colloidal Fe-doped manganate CaMn1−x Fe x O3 (x = 0, 0.01, 0.03, 0.05)

We report the optical behaviors of the Fe-doped CaMnO3 family of compounds at low doping concentrations x ≤ 5%. The study aims at assisting the evaluation of the competition between ferroand antiferromagnetic orderings, which is believed to be a cause of many interesting properties of this class of compounds, including the magnetization reversal effect recently discovered. The structural characterization showed a predominant orthorhombic phase with slightly increased cell constants due to doping. The Raman spectra revealed changes associated with the Mn sites, and the IR absorption spectrum showed a characteristic Fe band at 1.2 eV, which should be accompanied by a change of spin. The analysis of the magnetization data allowed us to predict that while the doping reduced the ferromagnetic coupling strength, and therefore the TC, the maximal doping concentration for the effective exchange to be zero was around 14%.

Superexchange-Mediated Ferromagnetic Coupling in Two-Dimensional Ni-TCNQ Networks on Metal Surfaces

We investigate the magnetic coupling of Ni centers embedded in two-dimensional metal-coordination networks self-assembled from 7,7,8,8-tetracyanoquinodimethane (TCNQ) molecules on Ag(100) and Au(111) surfaces. X-ray magnetic circular dichroism measurements show that single Ni adatom impurities assume a spin-quenched configuration on both surfaces, while Ni atoms coordinating to TCNQ ligands recover their magnetic moment and exhibit ferromagnetic coupling. The valence state and the ferromagnetic coupling strength of the Ni coordination centers depend crucially on the underlying substrate due to the different charge state of the TCNQ ligands on the two surfaces. The results suggest a superexchange coupling mechanism via the TCNQ ligands.

Spin-spin correlation function of the two-dimensionalXYmodel with weak site or bond dilution

The spin-spin correlation function of the 2D XY model decays as a power law at all temperatures below the Berezinskii-Kosterlitz-Thouless transition point with a temperature dependent exponent $\eta=\eta(T/J)$ ($J$ is the ferromagnetic coupling strength). It is known from computer experiments that in the 2D XY model with site or bond dilution this exponent depends on concentration $p$ of removed sites/bonds as well. Knowing the slope $\partial\eta/\partial p$ at point $p=0$, one can predict the value of the exponent for small dilution concentrations: $\eta(p)\simeq\eta(0)+p(\partial\eta/\partial p)|_{p=0}$. As it is shown in this paper, the spin-wave Hamiltonian allows to obtain exact results for this slope: $(\partial\eta/\partial p)|_{p=0} = T/(2J) + O((T/J)^2)$ and $T/(\pi J) + O((T/J)^2)$ for site and for bond dilution, respectively.

A kind of ferromagnetic-antiferromagnetic alternating spin chain equivalent to the mixed spin Heisenberg chain

By using the method of exact diagonalization, the properties of the spin-1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain are investigated. The spin-spin correlation, the sublattice magnetizations, the spin excitation spectrum and the magnetization curve of the model are calculated and compared with the corresponding properties of the mixed spin chain. The results show that the behaviors of the alternating Heisenberg chain are equivalent to those of the mixed spin chain when the ratio of antiferromagnetic to ferromagnetic coupling strength is very small. So, the properties of the mixed spin chain can be analyzed by using the spin-1/2 ferromagnetic-antiferromagnetic-antiferromagnetic alternating Heisenberg chain.

Kondo screening regimes of a quantum dot with a single Mn ion

We study the Kondo and transport properties of a quantum dot with a single magnetic Mn ion connected to metallic leads. By employing a numerical renormalization group technique we show that depending on the value of ferromagnetic coupling strength between the local electronic spin and the magnetic moment of the Mn, two distinct Kondo regimes exist. In the weak coupling limit, the system can be found in a completely screened Kondo state describing a local magnetic moment decoupled from the rest of the system. In contrast, in the strong coupling regime the quantum dot spin and the local magnetic moment form a single large-spin entity partially Kondo screened. A crossover between these two regimes can be suitably tuned by varying the tunnel coupling between the quantum dot electron and the leads. The model investigated here is also suitable to study magnetic molecules adsorbed on a metallic surface. The rich phenomenology of these systems is reflected in the conductance across the system.

전이금속이 치환된 반도체 물질의 자기적 특성 연구

This is the study of magnetic properties of transition metal doped diluted magnetic semiconductors(DMSs). The wurtzite structure samples were synthesized by the sol-gel method. The thermodynamic characteristics and magnetic properties of single phase was investigated for different doping concentration (x = 0%, 1%, 2%, 3%, 4%, 5%, 10%, 15%). The property of diluted magnetic semiconductors has been comfirmed by X-ray diffraction (XRD), Scanning Electronic Microscope (SEM) and Vibrating sample magnetometer (VSM). The magnetic properties of pure is found to be dominated by the ferromagnetic interaction between doped transition metal ions, where by the ferromagnetic coupling strength is simply increased with the concentration(>5%) of the doped transition metal.

Ferromagnetic properties of Cu-doped ZnS: A density functional theory study

Using plane-wave pseudopotential (PWPP) method, the magnetism and spin-resolved electronic properties of Cu-doped ZnS system are studied. Our calculations indicate that ferromagnetic (FM) state is ground state in Cu-doped ZnS. The FM coupling strength in ZnS doping with Cu fluctuates with the variation of distance between two dopants and the fluctuation gets larger with increase in distance. Room temperature ferromagnetism can be observed in Cu-doped ZnS with high dopant concentration. Formation energy calculation implies that the clustering effect is not obvious in Cu-doped ZnS. Thus, Cu-doped ZnS can be a promising dilute magnetic semiconductor (DMS), which promises to be free of magnetic precipitates.

Synthesis and Magnetic Study of μ1,1‐Azido‐Bridged Dinuclear Manganese(II) Complexes Based on Tripyridyl Ligands

AbstractThree azido‐bridged MnII complexes of formulas [Mn2(N3)4(ttp)2] (1), [Mn2(N3)4(ttp‐N3)2] (2) and [Mn2(N3)4(ttp‐N3)2]3[MnIII(ttp‐N3)(N3)3]2 (3), where ttp and ttp‐N3 represent 4′‐p‐tolyl‐2,2′:6′,2″‐terpyridine and 4′‐p‐azidomethylphenyl‐2,2′:6′,2″‐terpyridine, were synthesized and characterized by single‐crystal X‐ray diffraction analysis and magnetic studies. The Mn ions in complexes 1 and 2 are coordinated by three N atoms of the ttp or ttp‐N3 ligands, and they are connected by double end‐on (EO) azide ligands; this forms a dinuclear MnII system with Mn–N–Mn bridging angles of 103.5 and 103.1°. The Br atoms of the –CH2Br ligands were replaced by azido groups during the formation of complexes 2 and 3. The structure of complex 3 comprises two structurally similar MnII dimers with double end‐on bridging azide groups and one mononuclear MnIII structure. The bridging Mn–N–Mn angles in 3 are 104.2, 105.1, and 106.73°. Magnetic studies indicate the presence of intramolecular ferromagnetic superexchange. The strength of ferromagnetic coupling within the Mn2 cores in 1–3 is dependent on the Mn–N–Mn bridging angles. The magnetic coupling constants for intermolecular exchange are 2.46(4), 2.25(2), and 1.92(4) cm–1 for 1, 2, and 3, respectively, on the basis ofHamiltonian Ĥ = –2JŜ1Ŝ2.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Analysis for toggling magnetic random access memories with low writing field using four ferromagnetic layers for free layer stack

We calculate writing characteristics of two kinds of low-current magnetic random access memory (MRAM) devices using a single domain model. A four-layer synthetic antiferromagnet (SAF) has four antiferromagnetically coupled ferromagnetic (FM) layers, and a two-SM-layer-FC SAF has a two-FM-layer SAF with ferromagnetically coupled (FC) two soft magnetic (SM) layers. The threshold field of these free layers can be calculated analytically. The spin flop field of four-FM-layer SAF film is independent of the strength of antiferromagnetic coupling (AFC) between inner films. The saturation field increases with increasing the AFC strength. The spin flop field of the two-SM-layer-FC SAF film decreases, and the saturation field increases with decreasing ferromagnetic coupling strength. However, activation energy analysis demonstrates that both types of films exhibit a decrease in activation energy near the spin flop field. Considering the activation energy decrease during the toggling write sequence and a sufficient writing margin, MRAM with four ferromagnetic layers decreases writing current by 40%.

Ferromagnetic interlayer coupling and switching process of exchange coupled composite media

The ferromagnetic coupling effects in the exchange coupled composite (ECC) magnetic recording media were studied by varying the interlayer thickness and the saturation magnetization (Ms) of the soft layer. A minimum coercivity was observed at a certain interlayer thickness for the ECC media with a low Ms soft layer. When the Ms of the soft layer was increased (700emu∕cm3 in this work), no interlayer was needed to get the lowest coercivity for the ECC media, which makes the ECC media practical for a manufacturing process. A method to characterize the ferromagnetic coupling strength for the ECC media was quantitatively proposed. The switching process of the ECC media was observed to consist of the reversible switching of the soft layer and the irreversible switching of the hard layer.

Pressure-induced ferromagnetism in (In,Mn)Sb dilute magnetic semiconductor

Recent advances in III(1-x)Mn(x)V ferromagnetic semiconductors (for example in Ga(1-x)Mn(x)As) have demonstrated that electrical control of their spin properties can be used for manipulation and detection of magnetic signals. The Mn(2+) ions in these alloys provide magnetic moments, and at the same time act as a source of valence-band holes that mediate the Mn(2+)-Mn(2+) interactions. This coupling results in the ferromagnetic phase. In earlier workit was shown that the ferromagnetic state can be enhanced or suppressed by varying the carrier density. Here we demonstrate that, by using hydrostatic pressure to continuously tune the wavefunction overlap, one can control the strength of ferromagnetic coupling without any change in the carrier concentration. Tuning the exchange coupling by this process increases the magnetization spectacularly, and can even induce the ferromagnetic phase in an initially paramagnetic alloy. These results may open new directions for strain-engineering of nanodevices.

Ferromagnetic coupling strength and electron-doping effects in double perovskites

We review experiments and results on ferromagnetic and metallic A 2FeMoO 6 double perovskites that made it possible to obtain a detailed understanding of the nature of the ferromagnetic coupling and paved the way for further enhancement of the Curie temperature. We show that appropriate chemical substitutions, combined with detailed structural, magnetotransport and spectroscopic data allow us to map quite a complete picture of the properties of these oxides.

Magnetoresistance study of Ni 80Fe 20/Co1/CuAgAu/Co2 asymmetric sandwiches

The (Ni 80Fe 20/Co1)/CuAgAu/Co2 asymmetric sandwiches have been examined for different sublayer thicknesses. Magnetoresistance, magnetooptical Kerr effect and alternating gradient magnetometry measurements showed that for 1.2 nm⩽ d CuAgAu⩽1.6 nm our trilayers are antiferromagnetically coupled whereas for d CuAgAu⩾1.9 nm the coupling between Ni 80Fe 20/Co1 and Co2 is ferromagnetic. The ferromagnetic coupling strength oscillates with CuAgAu thickness with a period of 1 nm. The appearance of the ferromagnetic interlayer coupling for d CuAgAu⩾1.9 nm and its oscillatory character result from the interplay between RKKY-like exchange coupling and the magnetostatic orange-peel coupling. The values of the ferromagnetic coupling field between Ni 80Fe 20/Co1 and Co2 for a constant CuAgAu thickness can be either enhanced or reduced depending on the ratio between Co1, Co2 and Ni 80Fe 20 thicknesses. In spite of oscillatory ferromagnetic coupling strength for d CuAgAu⩾1.9 nm our trilayers show the GMR amplitude of about 10% and the GMR field sensitivity of about 5%/Oe at room temperature being very promising from the application point of view.

Magnetization Plateaus in Antiferromagnetic-(Ferromagnetic)nPolymerizedS=1/2XXZ Chains

The plateau-non-plateau transition in the antiferromagnetic-(ferromagnetic)$_{n}$ polymerized $S=1/2$ XXZ chains under the magnetic field is investigated. The universality class of this transition belongs to the Brezinskii-Kosterlitz-Thouless (BKT) type. The critical points are determined by level spectroscopy analysis of the numerical diagonalization data for $4 \leq p \leq 13$ where $p(\equiv n+1)$ is the size of a unit cell. It is found that the critical strength of ferromagnetic coupling decreases with $p$ for small $p$ but increases for larger enough $p$. It is also found that the plateau for large $p$ is wide enough for moderate values of exchange coupling so that it should be easily observed experimentally. This is in contrast to the plateaus for $p = 3$ chains which are narrow for a wide range of exchange coupling even away from the critical point.

Magnetism and orbital ordering in La 7/8Sr 1/8MnO 3

Muon relaxation techniques have been used to investigate the spin dynamics and magnetic transitions of the La 7/8Sr 1/8MnO 3 manganite. Competing orbital states cause changes in the ferromagnetic coupling strength and mechanism. The data suggest that diffusive magnetic polarons compete with the particular orbital ordering of the Jahn–Teller distorted phase in the temperature interval 0.8< T/ T C<1.1.