Long-range Ferromagnetic Order Research Articles

Oxygen vacancies driven size-dependent d0 room temperature ferromagnetism in well-dispersed dopant-free ZnO nanoparticles and density functional theory calculation

This paper reports the intrinsic size-dependent d0 room-temperature ferromagnetism in dopant-free ZnO nanoparticles, which were fabricated by thermal decomposition route. The size of ZnO nanoparticles can be controlled by simply tuning the amount ratio of solvent. Transmission electron microscope shows that the ZnO nanoparticles are well-dispersed without aggregation. Magnetic measurement suggests that the undoped ZnO nanoparticles show room temperature ferromagnetic characteristic, which is sensitive with the particle size. Photoluminescence spectra and X-ray photoelectron spectroscopy measurements elucidate that the relative concentration of oxygen vacancies increase as the particle sizes decrease, leading to long-range ferromagnetic ordering in the undoped ZnO nanoparticles. A correlation among the particle size, ferromagnetism and relative concentration of oxygen vacancies is established, which demonstrates that oxygen vacancies play a key role to induce ferromagnetic ordering. Furthermore, density functional theory calculation demonstrates that the oxygen vacancies in ZnO with positive charge trapping can induce polar-magnetization which is stable states. The correlation between experiments and simulations suggests that the ferromagnetic behavior in diluted magnetic semiconductors can be conveniently manipulated by tuning the surface-volume ratio of nanostructures and surface defects, which is of great significance to understand and explore novel diluted magnetic semiconductors.

Formation of Ruddlesden-Popper Faults and Their Effect on the Magnetic Properties in Pr0.5Sr0.5CoO3 Thin Films.

Epitaxial Pr0.5Sr0.5CoO3 thin films have been grown on single-crystalline (La0.289Sr0.712)(Al0.633Ta0.356)O3(001) substrates by the pulsed laser deposition technique. The magnetic properties and microstructure of these films are investigated. It is found that Ruddlesden-Popper faults (RP faults) can be introduced in the films by changing the laser repetition rate. The segregation of Pr at the RP faults is characterized by atomic-resolution chemical mapping. The formation of the RP faults not only contributes to the epitaxial strain relaxation but also significantly decreases the ferromagnetic long-range order of the films, resulting in lower magnetizations than those of the fault-free films. Our results provide a strategy for tuning the magnetic properties of cobalt-based perovskite films by modifying the microstructure through the film growth process.

Stepping Stone Mechanism: Carrier-Free Long-Range Magnetism Mediated by Magnetized Cation States in Quintuple Layer**Supported by Chinese University of Hong Kong (CUHK) under Grant No 4053084, University Grants Committee of Hong Kong under Grant No 24300814, and the Start-up Funding of CUHK.

The long-range magnetism observed in group-V tellurides quintuple layers is the only working example of carrier-free dilute magnetic semiconductors (DMS), whereas the physical mechanism is unclear, except the speculation on the band topology enhanced van Vleck paramagnetism. Based on DFT calculations, we find a stable long-range ferromagnetic order in a single quintuple layer of Cr-doped Bi2Te3 or Sb2Te3, with the dopant separation more than 9 Å. This configuration is the global energy minimum among all configurations. Different from the conventional super exchange theory, the magnetism is facilitated by the lone pair derived anti-bonding states near the cations. Such anti-bonding states work as stepping stones merged in the electron sea and conduct magnetism. Further, spin orbit coupling induced band inversion is found to be insignificant in the magnetism. Therefore, our findings directly dismiss the common misbelief that band topology is the only factor that enhances the magnetism. We further demonstrate that removal of the lone pair derived states destroys the long-range magnetism. This novel mechanism sheds light on the fundamental understanding of long-range magnetism and may lead to discoveries of new classes of DMS.

Observation of superparamagnetism in coexistence with quantum anomalous Hall C\u2009=\u2009\xb11 and C\u2009=\u20090 Chern states

Simultaneous transport and scanning nanoSQUID-on-tip magnetic imaging studies in Cr-(Bi,Sb)2Te3 modulation-doped films reveal the presence of superparamagnetic order within the quantum anomalous Hall regime. In contrast to the expectation that a long-range ferromagnetic order is required for establishing the quantum anomalous Hall state, superparamagnetic dynamics of weakly interacting nanoscale magnetic islands is observed both in the plateau transition regions, as well as within the fully quantized C = ±1 Chern plateaus. Modulation doping of the topological insulator films is found to give rise to significantly larger superparamagnetic islands as compared to uniform magnetic doping, evidently leading to enhanced robustness of the quantum anomalous Hall effect. Nonetheless, even in this more robust quantum state, attaining full quantization of transport coefficients requires magnetic alignment of at least 95% of the superparamagnetic islands. The superparamagnetic order is also found within the incipient C = 0 zero Hall plateau, which may host an axion state if the top and bottom magnetic layers are magnetized in opposite directions. In this regime, however, a significantly lower level of island alignment is found in our samples, hindering the formation of the axion state. Comprehension and control of superparamagnetic dynamics is thus a key factor in apprehending the fragility of the quantum anomalous Hall state and in enhancing the endurance of the different quantized states to higher temperatures for utilization of robust topological protection in novel devices.

A two-dimensional Fe-doped SnS2 magnetic semiconductor

Magnetic two-dimensional materials have attracted considerable attention for their significant potential application in spintronics. In this study, we present a high-quality Fe-doped SnS2 monolayer exfoliated using a micromechanical cleavage method. Fe atoms were doped at the Sn atom sites, and the Fe contents are ∼2.1%, 1.5%, and 1.1%. The field-effect transistors based on the Fe0.021Sn0.979S2 monolayer show n-type behavior and exhibit high optoelectronic performance. Magnetic measurements show that pure SnS2 is diamagnetic, whereas Fe0.021Sn0.979S2 exhibits ferromagnetic behavior with a perpendicular anisotropy at 2 K and a Curie temperature of ~31 K. Density functional theory calculations show that long-range ferromagnetic ordering in the Fe-doped SnS2 monolayer is energetically stable, and the estimated Curie temperature agrees well with the results of our experiment. The results suggest that Fe-doped SnS2 has significant potential in future nanoelectronic, magnetic, and optoelectronic applications.

A Coexistence of Short- and Long-Range Ferromagnetic Interactions in La1–xKxMnO3 Compounds

In this paper, we present a detailed study on the critical behavior around the ferromagnetic (FM)–paramagnetic phase transition in La1− x K x MnO3 (LKMO) compounds with $x = 0.05 - 0.20$ . Our results pointed out the easiness in controlling the value of $T_{C}$ from 212 to 306 K by increasing K-doping concentration (x) from 0.05 to 0.20. Analyzing temperature and magnetic field dependences of magnetization M(T, H) at temperatures around $T_{C}$ reveals all the samples undergoing the second-order phase transition. Using the modified Arrott plots, the Kouvel–Fisher, and the critical isotherm analysis methods, we have found that the values of exponent $\beta $ in the range of 0.435–0.493, which deviate from the mean-field theory (MFT) for the long-range FM interactions, indicating a coexistence of short- and long-range FM orders. We also pointed out that the critical exponents ( $\beta $ and $\delta)$ depend strongly on K-concentration, shifting from values approaching those of the MFT ( $\beta = 0.5$ and $\delta = 3$ ) to values approaching those of the 3-D-Heisenberg model ( $\beta = 0.365$ and $\delta = 4.80$ ). In contrast, the value of the exponent $\gamma (=1.222 - 1.249)$ is almost independent of K-concentration. It suggests that K-doping favors establishing a short-range FM order in LKMO compounds.

Magnetic and thermodynamic properties of [formula omitted

We here report the magnetization, M, and specific heat, C, of Nd3NiGe2, which crystallizes in the orthorhombic Gd3NiSi2-type structure. Nd ions occupy three nonequivalent sites in a unit cell. Upon cooling, magnetization divided by magnetic field, M/B, increased sharply at the Curie temperature, TC, of 87K and below 40K. The former result indicates that the increase in M/B observed at TC is due to the long-range ferromagnetic order. The increase below 40K is derived from a short-range correlation because of the absence of clear anomaly in C(T). At 10K and 2K, the values of M undergo metamagnetic transitions. The value of magnetic specific heat divided by temperature shows a shoulder-like anomaly at around 20K, which is attributed to antiferromagnetic behavior. Furthermore, two peaks in C(T) were observed at 4.5K and 3.8K, and these peaks occurred at lower temperatures in the presence of a magnetic field. This behavior is typical of materials with antiferromagnetic order. These observations are attributed to the competition between ferromagnetic and antiferromagnetic interactions, which is a result of the three nonequivalent Nd sites.

Critical exponent analysis of lightly germanium-doped La0.7Ca0.3Mn1-xGexO3 (x = 0.05 and x = 0.07)

We have used a critical behavior study of La0.7Ca0.3MnO3 (LCMO) manganite perovskites whose Mn sites have been doped with Ge to explore magnetic interactions. Light Ge doping of 5 or 7 percent tended to produce LCMOs with second order magnetic transitions. The critical parameters of 5- and 7-percent Ge-doped LCMO were determined to be TC = 185 K, β = 0.331 ± 0.019, and γ = 1.15 ± 0.017; and TC = 153 K, β = 0.496 ± 0.011, and γ = 1.03 ± 0.046, respectively, via the modified Arrott plot method. Isothermal magnetization data collected near the Curie temperature (TC) was split into a universal function with two branches M(H,ε) = |ε|βf±(H/|ε|β+γ), where ε=(T–TC)/TC is the reduced temperature. f+ is used when T>TC, while f̲ is used when T<TC. Such results demonstrate the existence of ferromagnetic short-range order in lightly Ge-doped LCMOs. Notably, doping with Ge at high concentrations tends to generate long-range ferromagnetic order.

Skyrmions and multisublattice helical states in a frustrated chiral magnet

We investigate the existence and stability of skyrmions in a frustrated chiral ferromagnet by considering the competition between ferromagnetic (FM) nearest-neighbour (NN) interaction ($J_1$) and antiferromagnetic (AFM) next-nearest-neighbour (NNN) interaction ($J_2$). Contrary to the general wisdom that long-range ferromagnetic order is not energy preferable under frustration, the skyrmion lattice not only exists but is even stable for a large field range when $J_2 \leq J_1$ compared with frustration-free systems. We defend that the enlargement of stability window of skyrmions is a consequence of the reduced effective exchange interaction caused by the frustration. A multi-sublattice helical state is found below the skyrmion phase, which results from the competition between AFM coupling that favors a two-sublattice N\'{e}el state and the chiral interaction that prefers a helix. As a byproduct, the hysteresis loop of the frustrated chiral system shrinks as the magnetization goes to zero and then opens up again, known as wasp-waist hysteresis loop. The critical field that separates the narrow and wide part of the wasp-waist loop depends exponentially on the strength of NNN coupling. By measuring the critical field, it is possible to determine the strength of NNN coupling.

Effects of Li doping and point defect on the magnetism of ZnO

The magnetism sources and magnetic mechanism of Li doping and point defect, which coexist in the presence of hexagonal wurtzite ZnO, are controversial. To solve these problems, the effects of Li doping and point defect on the magnetism of ZnO were studied using geometry optimization and energy calculation based on the first-principle generalized gradient approximation + U (GGA + U) method of density functional theory. Results showed that the coexistence of Li doping and Zn vacancy can achieve ferromagnetic long-range order, and the Curie temperature of the doping system can achieve room temperature. In addition, results showed that magnetic moments are significantly different when the structures of the systems are also different with the same doping amount and doping method, which are advantageous for the enhancement of the magnetic properties of dilute magnetic semiconductors. The magnetism source of Zn14LiO16 is the hybrid coupling electron exchange effect between O-2p and Zn-3d orbits. With the coexistence of Li replacing Zn and Zn vacancy, the closest relative distance between doping and vacancy leads to the lowest formation energy and highest stability. In the condition of the highest stability of the ground state, all the doping systems of Li replacing Zn and O vacancy, doping system of interstitial Li and Zn vacancy, and doping system with the coexistence of Li replacing Zn, interstitial Li, and Zn vacancy are non-magnetic, which are considered worthless in the design and preparation of diluted magnetic semiconductors (DMSs).

Observation of Griffiths Phase in Oxygen-Deficient La0.70Sr0.30MnO3−γ by Electron Magnetic Resonance

The oxygen-deficient polycrystalline La0.70Sr0.30MnO3−γ (0 < γ ≤ 0.2) manganites were studied by electron magnetic resonance (EMR). The EMR spectra provide evidence for an unusual and γ-dependent transition of manganites from the ferromagnetic phase to the paramagnetic phase. A Griffiths phase, which reveals itself in the coexistence of ferromagnetic and paramagnetic resonance signals at a temperature significantly higher than the magnetic ordering temperature, was found in samples with 0.1 ≤ γ ≤ 0.2. The formation of this phase, characterized by the presence of ferromagnetic clusters in a paramagnetic matrix, is due to an oxygen deficiency that hinders realization of long-range ferromagnetic ordering of the manganese ions.

Magnetic phase separation and unusual scenario of its temperature evolution in porous carbon-based nanomaterials doped with Au and Co

Two porous glassy carbon-based samples doped with Au and Co were investigated. The magnetization study as well as measurements of the nonlinear longitudinal response to a weak ac field (NLR) and electron magnetic resonance give evidences for a presence of magnetic nanoparticles (MNPs) embedded in paramagnetic/ferromagnetic matrix respectively, both samples being in magnetically phase-separated state at temperatures above 300K. Matrix, forming by paramagnetic centers located in matrix outside the MNPs, reveals exchange interactions providing its ferromagnetic (FM) ordering below TC≈ 210K in Au-doped sample and well above 350K in Co-doped one. For the former, NLR data suggest a percolation character of the matrix long-range FM order, which is mainly caused by a porous amorphous sample structure. Temperature dependence of the magnetization in the Au-doped sample evidences presence of antiferromagnetic (AF) interactions of MNPs with surrounding matrix centers. At magnetic ordering below TC these interactions promote origination of “domains” involving matrix fragment and surrounding MNPs with near opposite orientation of their moments that decreases the magnetostatic energy. On further cooling, the domains exhibit AF ordering below Tcr∼140K<TC, resulting in formation of a peculiar “ferrimagnet”. The porous amorphous structure leads to absence of translational and other symmetry features through the samples that allows canted ordering of magnetic moments in domains and in whole sample providing “canted ferrimagnetism”. At low temperatures Ttr∼3K, “order-oder” transition, evidencing the non-Heisenberg character of this magnetic material, occurs from ordering like “canted ferrimagnet” to FM alignment, which is stimulated by external magnetic field. The data for Co-doped sample imply the similar evolution of magnetic state but at higher temperatures above 350K. This state exhibits more homogeneous arrangement of the FM nanoparticles and the FM matrix. Order-order transition occurs in it at higher Ttr∼10–15K as well and followed by formation of long-range FM ordering found earlier by neutron diffraction. Doping of carbon-based nanomaterials by magnetic metals provides advantages for their possible practical applications as Co-doped sample with higher TC (>350K) and larger remanent magnetization evidences.

Effect of Tb3+ doping in mixed-valence manganites and cobaltites

The magnetic ordering of four Tb3+-doped manganites and cobaltites, La0.7Tb0.1Sr0.2MnO3, La0.7Tb0.1Ca0.2MnO3, La0.7Tb0.1Sr0.2CoO3 and La0.7Tb0.1Ca0.2CoO3, have been studied by means of neutron diffraction and SQUID magnetometry. All the samples were prepared by sintering of sol–gel precursors and their orthorhombic or rhombohedral perovskite structures at room and low temperatures were refined. A long-range ferromagnetic (FM) order was detected at the Mn and Co sites. In addition, a small but significant ordered moment was observed at A sites of studied cobaltites, which was attributed to local Tb3+ moments, aligned by exchange interactions due to FM ordered Co sublattice. No or minor Tb3+ contribution was detected in studied manganites.

Critical properties around the ferromagnetic-paramagnetic phase transition in La0.7Ca0.3-xAxMnO3 compounds (A = Sr, Ba and x = 0, 0.15, 0.3)

Influence of alkaline earth element doping in the A-site on the structure, magnetic and magnetocaloric properties of La0.7Ca0.3-xAxMnO3 (A = Sr and Ba; x = 0, 0.15 and 0.3) perovskite manganites have been investigated. X-ray diffraction data indicates that samples x = 0 and 0.15 crystallize in the orthorhombic structure (space group Pnma), whereas samples x = 0.3 belong to the rhombohedral structure (space group R-3c). Substitution of Sr or Ba for Ca plays an important role in the increase of Curie temperature (TC) and modification the nature of the magnetic phase transition in materials. Sample x = 0 exhibits the first-order phase transition (FOPT) with a large magnetic entropy change (ΔSm), whereas samples x = 0.15 and 0.3 exhibit the second-order phase transition (SOPT) characterizations and the moderate values of ΔSm. Using the modified Arrott plots method, the critical behaviors around TC for SOPT samples have been investigated through the values of critical parameters (TC, β, γ, and δ). We pointed out that Ba-doping favors establishing long-range ferromagnetic order, contrary to the case of Sr-doping.

Quasi-two-dimensional magnetism in Cr-based MAX phases

The MAX phase is one possible candidate for quasi-two-dimensional (Q2D) magnets owing to its layered structure. We report itinerant electron ferromagnetic and antiferromagnetic long-range orders in (Cr1−xMnx)2GeC and Cr2GaN, respectively. Because Cr2GaN orders in a spin-density-wave state, the Q2D character is highly plausible. However, the Q2D nature is not straightforward in (Cr1−xMnx)2GeC because of the absence of useful criteria to judge the dimensionality in the itinerant electron ferromagnet. In this article, we attempt to reveal the Q2D character of ferromagnetic (Cr1−xMnx)2GeC by analyzing magnetic data in terms of the spin fluctuation theory for the Q2D system.

Magnetic proximity effect in graphene coupled to a BiFeO3 nanoplate

Graphene, a very intriguing two-dimensional Dirac electronic system with high carrier mobility, is promising for spintronics. However, the long-range ferromagnetic order is always absent in pristine graphene. Here we report the fabrication and transport properties of graphene-$\mathrm{BiFe}{\mathrm{O}}_{3}$ heterostructures. It is found that the magnetic proximity effect results in a strong Zeeman splitting in graphene with the exchange field up to hundreds of tesla. The $\ensuremath{\nu}=0$ quantum Hall state of graphene is further transformed into a ferromagnetic state or a canted antiferromagnetic state in the presence of a perpendicular magnetic field. Our findings in graphene/$\mathrm{BiFe}{\mathrm{O}}_{3}$ heterostructure are therefore promising for future spintronics.

Discovery of intrinsic ferromagnetism in two-dimensional van der Waals crystals.

The realization of long-range ferromagnetic order in two-dimensional van der Waals crystals, combined with their rich electronic and optical properties, could lead to new magnetic, magnetoelectric and magneto-optic applications. In two-dimensional systems, the long-range magnetic order is strongly suppressed by thermal fluctuations, according to the Mermin-Wagner theorem; however, these thermal fluctuations can be counteracted by magnetic anisotropy. Previous efforts, based on defect and composition engineering, or the proximity effect, introduced magnetic responses only locally or extrinsically. Here we report intrinsic long-range ferromagnetic order in pristine Cr2Ge2Te6 atomic layers, as revealed by scanning magneto-optic Kerr microscopy. In this magnetically soft, two-dimensional van der Waals ferromagnet, we achieve unprecedented control of the transition temperature (between ferromagnetic and paramagnetic states) using very small fields (smaller than 0.3 tesla). This result is in contrast to the insensitivity of the transition temperature to magnetic fields in the three-dimensional regime. We found that the small applied field leads to an effective anisotropy that is much greater than the near-zero magnetocrystalline anisotropy, opening up a large spin-wave excitation gap. We explain the observed phenomenon using renormalized spin-wave theory and conclude that the unusual field dependence of the transition temperature is a hallmark of soft, two-dimensional ferromagnetic van der Waals crystals. Cr2Ge2Te6 is a nearly ideal two-dimensional Heisenberg ferromagnet and so will be useful for studying fundamental spin behaviours, opening the door to exploring new applications such as ultra-compact spintronics.

Crystal structure and magnetic properties of Al2O3 nanoparticles by 27Al NMR data

The magnetization of a series of Al2O3 with different particle sizes and their 27Al NMR spectra have been studied at room temperature. The field dependence of the magnetization demonstrated the existence of a long-range ferromagnetic order in a small part of the sample at room temperature; however, the relative volume of this contribution was very small (less than 1%), and this seems likely due to an impurity phase. The NMR spectra did not contain any lines of metallic aluminum the existence of which in these nanooxides was assumed before in a surface layer of the nanoparticles, according to the data of other techniques. The data on the phase composition and the charge distribution in different phases of the Al2O3 nanoparticles have been obtained. The change in the mean particle size (by a factor of almost three) only insignificantly changed their phase composition.

Long-range ferromagnetism and magnetocaloric effects in rapidly quenched Ni 50−x Co x Mn 50−y Al y ribbons

Abstract Ni50−xCoxMn50−yAly (x = 7 and 9; y = 17, 18 and 19) alloy ribbons were prepared by melt-spinning with a tangential velocity of copper wheel of 40 m s−1. X-ray diffraction patterns reveal multi-crystalline phase behavior in the fabricated ribbons. The shape of thermomagnetization curves clearly depends on Co and Al concentrations. The Curie temperatures (TC) of the alloy ribbons strongly increase with increasing the Co concentration and slightly decrease with increasing the Al concentration. The martensitic-austenitic phase transition in the alloy ribbons can be manipulated by tuning Co and Al concentrations. The maximum magnetic entropy change |ΔSm|max of about 0.75 J kg−1 K−1 for a field change of 12 kOe at TC ≈ 364 K was achieved for the Ni43Co7Mn32Al18 ribbon. Critical analysis using the Arrott-Noaks and Kouvel–Fisher methods demonstrates the existence of a long-range ferromagnetic order in this ribbon.

Effect of Cu substitution on magnetocaloric and critical behavior in Ni47Mn40Sn13−Cu alloys

Abstract A series of Heusler phases Ni47Mn40Sn13−xCux (x = 0, 0.5, 1, 1.25), which are magnetic shape-memory alloys, has been synthesized by mechanical alloying (MA) and characterized. The martensite-austenite phase transition was investigated by X-ray diffraction, differential scanning calorimetry, and DC magnetization measurements. With greater Cu content, the martensitic transition temperature TM increases while the magnetic transition temperature of austenite T c A remains almost unchanged. The maximum inverse magnetic entropy changes Δ S M for the x = 0.5 and x = 1 samples are nearly twice as high as for the undoped sample. Furthermore, the martensitic transformation of the doped samples occurs around room temperature, which is important for applications. Effective refrigerant capacities (RCeff) reduced due to the higher hysteresis losses in the x = 0.5 and x = 1 samples compare to the undoped one. Refrigerant capacities (RC) were examined in T c A . The largest value of RC = 207 J/kg was measured for the undoped sample. Critical properties near the ferromagnetic–paramagnetic (FM–PM) phase transition were analyzed in detailed. The critical parameters β, γ, δ and T c A of the x = 0 and x = 0.5 samples, which were determined through use of modified Arrott plots, the Kouvel-Fisher method, and the Widom scaling relation, differed from the mean-field values. The critical behavior indicates that long-range ferromagnetic order dominates in these Cu-doped samples.