Antiferromagnetic Host Research Articles

Microstructure Morphology of Chemical and Structural Phase Separation in Thermally Treated KxFe2-ySe2 Superconductor.

The iron-based KxFe2-ySe2 superconductor displays chemical, structural and electronic phase separation at nanoscale to microscale, leading to the coexisting metallic phase embedded in an antiferromagnetic host matrix. The metallic character of the system is believed to arise from a percolative granular network affecting its transport in the normal as well as in the superconducting state. This percolative network can be manipulated and controlled through thermal treatments. In this study, we have used scanning X-ray micro-fluorescence to visualize morphology of the chemical phase separation coupled to the percolation in KxFe2-ySe2, manipulated by two distinct thermal treatments, i. e., fast quenching and slow cooling. We find a differing spatial correlation between Fe and K distributions in the two samples, ascribed to a different degree of Fe vacancy ordering. We have also identified an intermediate phase that acts as an interface between the two main phases. The high temperature quenching produces directionally oriented clustered microstructure in which the percolation threshold is lower and hence a more effective transport networks. Instead, the slow cooling results in larger interfaces around the percolation threshold that seems to affect the superconducting properties of the system. The results provide a quantitative characterization of microstructural morphology of differently grown KxFe2-ySe2 showing potential for the design of electronic devices based on sub-micron scale chemical phase separation, thus opening avenues for further studies of complex heterogeneous functional structures.

Defect induced ferromagnetism in Mn3Ga

Ni-substituted Mn3Ga displays a weak ferromagnetism embedded in an antiferromagnetic (AF) phase. Upon field cooling, the alloy exhibits exchange bias and an open hysteresis loop, signifying kinetic arrest at room temperature. For the first time, a kinetic arrest is seen in a compound due to the first order transition of an embedded defect phase. A systematic study of crystal structure, local structure, and magnetic properties of Mn 3−x Ni x Ga (x = 0, 0.25) alloys reveal the origin of ferromagnetism in Mn2.75Ni0.25Ga is due to the segregation of a Heusler-type environment around Ni in the cubic Mn3Ga matrix. Upon temper annealing at 400 ∘C, these local structural defects around the Ni phase separate into a modulated ferromagnetic (FM) Ni-Mn-Ga Heusler phase. A strong interaction between the AF host and the FM defect phase gives rise to exchange bias. The first-order transition of the defect phase seems to be responsible for the observed kinetic arrest in Mn2.75Ni0.25Ga.

Observation of weak ferromagnetism in the antiferromagnetic host and large electronic heat capacity in β-Mn-type structured Mn3Al alloy

Numerous interesting features of antiferromagnets open up the possibility of designing new antiferromagnetic materials that can be useful for spintronic device applications. Here we present the structural, magnetic, thermal and electrical transport properties of polycrystalline Mn3Al alloys prepared by arc melting. Rietveld refinement of the room temperature X-Ray diffraction patterns reveals that the arc-melted Mn3Al crystallizes in a β Mn phase (space group 213). Detailed analysis of the magnetic data suggests for a dominant antiferromagnetic ordering. However, an in-homogenous magnetic disorder is observed in the system having a weak ferromagnetic phase in the antiferromagnetic host. In spite of the strong spin fluctuations in this system, the signature of spin glass-like nature in these alloys are not observed as probed by the frequency-dependent AC-Susceptibility measurements. Furthermore, the heat capacity was insensitive to the applied field of 3 Tesla with a negligible magnetic contribution suggesting a large electronic contribution to the heat capacity in these alloys. This work provides a useful insight into the tunable physical properties of the Heusler alloys for exploring antiferromagnetic based spintronic applications.

Exchange bias and anomalous Hall effect in a wide temperature range of 5–300 K in non-collinear antiferromagnetic Mn–Cr–Sn alloy

In this paper, noncollinear antiferromagnets Mn3.1Sn0.9 and Mn2.5Cr0.6Sn0.9 with an Ni3Sn-type hexagonal structure has been synthesized. An exchange bias (EB) from 5 K to room temperature has been observed in Mn2.5Cr0.6Sn0.9 with a maximum EB field of 3166 Oe at 5 K, which decreases gradually to 60 Oe at 300 K. It is proposed that the EB is ascribed to the exchange anisotropy between the ferromagnetic component and the antiferromagnetic host due to the magnetic inhomogeneity of the materials. Large anomalous Hall effect ranging from 3.5 to 1 µΩ · cm persists from 5 K to 300 K, which is attributed to the non-vanishing Berry phase arising from the non-collinear spin structure. Both the exchange bias and anomalous Hall effect in a wide temperature range of 5–300 K suggest that the Mn–Cr–Sn alloy has a promising application prospect in antiferromagnetic based spintronics devices.

Giant exchange bias effect in all-3d-metal Ni38.8Co2.9Mn37.9Ti20.4 thin film

In this work, a giant exchange bias field of over 1 kOe, which is uncommon in film systems, is obtained in all-3d-metal Heusler-type Ni38.8Co2.9Mn37.9Ti20.4 thin films. The thermomagnetic curves show an unambiguous noncoincidence between zero-field-cooled and field-cooled protocols, indicating the presence of magnetically inhomogeneous phases in the film in the low temperature range. Magnetic measurements, including magnetic relaxation and magnetic memory effects, demonstrate that the exchange bias should be attributed to the exchange interaction between ferromagnetic clusters and the antiferromagnetic host in the Ni38.8Co2.9Mn37.9Ti20.4 thin film at low temperature. The local Mn-Co-Mn configuration provides the ferromagnetic contribution, which is formed in the antiferromagnetic Heusler main phase due to the small amount of Co-substitution for Ni. The achievement of the large exchange bias effect in the Ni38.8Co2.9Mn37.9Ti20.4 thin film is a significant addition to the existing multiple magneto-responsive effects in all-3d-metal Ni-Mn-based Heusler systems.

Invar-like Behavior of Antiperovskite Mn3+xNi1–xN Compounds

The antiperovskite Mn3+xNi1–xN compounds have been synthesized and characterized by a variety of experimental techniques. After Mn doping at the Ni site, both ferromagnetic characteristics and an Invar-like effect were observed in the antiferromagnetic host material. The observed Invar-like behavior was assumed to be related to the characteristic magnetic structure induced by the doping. Neutron diffraction results prove that the Mn doping stabilizes the special Γ5g antiferromagnetic phase with strong spin–lattice coupling that can be tuned to achieve Invar-like behavior. The magnetovolume effect (MVE) and significant correlation between spin and lattice were confirmed for the Γ5g magnetic phase by the first-principles calculations. Moreover, Mn 3d electrons were revealed to be the key factor for the MVE from the calculations. Our study presents a new mechanism for precisely controlling the zero thermal expansion of a single compound by achieving the special Γ5g magnetic phase of Mn atoms.

Computational materials design of defect-induced ferrimagnetic MnO

We present a computational materials design for defect-induced ferrimagnetic MnO. The magnetic properties of MnO containing Mn vacancies were investigated using first-principle calculations. For these electronic structure calculations, we employed a pseudo-self-interaction-corrected local density approximation (PSIC-LDA). We used the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) to create a random distribution of atoms at the assigned sites. Having described the magnetic properties with a classic Heisenberg model, we calculated the effective exchange coupling constants by applying the magnetic force theorem to two magnetic sites embedded in the CPA medium. We estimated the Curie temperatures from the calculated exchange interactions. This study found that the Mn vacancies induced ferrimagnetic ground states in MnO, and that the Curie temperature could reach room temperature at Mn vacancy concentrations above 20%. These findings suggest a new route for designing ferrimagnetic materials from anti-ferromagnetic host materials.

Exchange bias and antiferromagnetic interfacial exchange coupling in the mixed-spin oxide Li2/7Ni5/7O

We present a detailed study on magnetic properties of mixed spin oxide Li2/7Ni5/7O, where small ferromagnetic (FM) clusters (with averaged diameter of ∼18 Å) immersed with an antiferromagnetic (AFM) host. The exchange bias (EB) with shifts of the hysteresis loop along both the field and magnetization axes is found, which is due to the interfacial interaction between the FM clusters and the AFM host. An AFM interfacial exchange coupling is deduced from the exchange interactions between Ni ions. The type and strength of this interfacial exchange interaction are discussed in terms of the EB at low temperature.

Electric-field induced ferromagnetic domain changes in exchange biased Co–BiFeO3 composites

We present direct observations of electric-field induced changes in the local ferromagnetic (F) domain structure in films formed from inclusions of F cobalt in an antiferromagnetic host of BiFeO3. Piezoelectric force microscopy was used to demonstrate that the composite Co–BiFeO3 films display ferroelectricity. Strikingly, magnetic contrast changes in magnetic force microscopic images were found with the application of a perpendicular electric field. These are the results of an exchange interaction between Co and BiFeO3.

Core-shell model of the vacancy concentration and magnetic behavior for antiferromagnetic nanoparticle

The local defect structure of NiO nanoparticles was determined by extended x-ray absorption fine structure method. By using the bulk and surface sensitive characterization techniques, we are able to show that the vacancies mostly reside on the surface of the particles and the distribution of vacancies can be considered within a core-shell model. We argue that these observations can give a suitable explanation for the venerable problem of observed magnetic behaviors of antiferromagnetic nanoparticles and other functional properties of the material. The observed magnetic moment has been attributed to the interacting vacancies inside the antiferromagnetic host and the distribution of vacancies over the particle volume determines behaviors such as size-dependent different nature of spin-glass freezing and exchange bias.

Electronic structure and magnetic properties of dilute Cr alloys with transition-metal impurities

By using full-potential linearized augmented plane wave (FLAPW) method based on the density function theory (DFT), we have performed ab initio calculations of electronic structure and magnetic properties for dilute alloys of Cr with transition metals. Here, we present our results for the local magnetic properties of $3d$ (Sc\char21{}Ni) and $4d$ (Y\char21{}Pd) impurities in Cr, as well as changes in macroscopic properties such as total magnetization and specific heat of Cr alloys, arising from impurity-induced host perturbations. Results obtained for nonmagnetic local density of states is analyzed within Stoner model, and the condition for local moment formation is found to be satisfied only for Fe, Co, and Ni. For all other cases including Mn, the impurity moments are predicted to be induced by Cr host. Systematic trends for the impurity local moments and hyperfine fields in antiferromagnetic Cr host have been established. The trends show remarkable similarity with those observed in ferromagnetic Fe, Co, and Ni hosts. While the impurity moments for Sc, Ti, V, and Mn are found to be antiparallel with respect to the near neighbor Cr moment, self-consistent solutions for Fe, Co, and Ni show ferromagnetic coupling. For $4d$ impurities, except for Rh and Pd, the exchange interaction remain antiferromagnetic. The calculated changes of the total moment due to host perturbations show qualitative agreements with data from magnetization measurements. A detailed comparison is made for the impurity-induced changes in the total density of states at Fermi energy ${E}_{F}$ and experimentally measured electronic specific heat of Cr alloys. The results presented in this work provide comprehensive and improved understanding of electronic and magnetic properties of dilute Cr alloys. They are also useful for a common understanding of local magnetic properties of transition metal impurities in ferromagnetic and antiferromagnetic hosts.

Intrinsic interface exchange coupling of ferromagnetic nanodomains in a charge ordered manganite

We present a detailed magnetic study of the Pr1/3Ca2/3MnO3 manganite, where we observe the presence of small ferromagnetic (FM) domains (diameter ~ 10A) immersed within the charge-ordered antiferromagnetic (AFM) host. Due to the interaction of the FM nanodroplets with a disordered AFM shell, the low-temperature magnetization loops present exchange bias (EB) under cooling in an applied magnetic field. Our analysis of the cooling field dependence of the EB yields an antiferromagnetic interface exchange coupling comparable to the bulk exchange constant of the AFM phase. We also observe training effect of the EB, which is successfully described in terms of a preexisting relaxation model developed for other classical EB systems. This work provides the first evidence of intrinsic interface exchange coupling in phase separated manganites.

Inhomogeneous magnetic behavior of Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO3

Unlike La0.7Ca0.3CoO2.97, Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO2.95 do not show distinct ferromagnetic transitions, but instead they exhibit very low magnetic moments down to 50K. A detail study of magnetic properties of Pr0.7Ca0.3CoO3 and Nd0.7Ca0.3CoO2.95 shows that the materials are inhomogeneous, exhibiting properties similar to those of frustrated magnetic systems. In both these cobaltates, small ferromagnetic clusters seem to be present in an antiferromagnetic host.

Phase-separated states in antiferromagnetic semiconductors with a polarizable lattice

The possibility of the slab or stripe phase separation (alternating ferromagnetic highly- conductive and insulating antiferromagnetic layers) is proved for isotropic degenerate antiferromagnetic semiconductors. This type of phase separation competes with the droplet phase separation (ferromagnetic droplets in the antiferromagnetic host or vice versa). The interaction of electrons with optical phonons alone cannot cause phase-separated state with alternating highly-conductive and insulating regions but it stabilizes the magnetic phase separation. The magnetostriction deformation of the lattice in the phase-separated state is investigated.

Ferromagnetic polarons in dilute La-doped Ca 2RuO 4

Ca 2RuO 4 is an antiferromagnetic insulator with a Neel temperature at T N =110 K and a Mott transition at T MI =357 K . Slight substitution of trivalent La for divalent Ca in Ca 2RuO 4 not only drastically reduces T MI but also simultaneously precipitate robust ferromagnetism in the antiferromagnetic host. It is believed that the ferromagnetic behavior is closely associated with the kinetic energy of the electron added from the La ion that modifies the magnetic structure, and as a result, forming ferromagnetic polarons in the antiferromagnetic background. These dramatic changes underline the subtlety of the competition between the antiferromagnetic and ferromagnetic coupling in Ca 2RuO 4.

Percolative phase separation induced by nonuniformly distributed excess oxygen in low-dopedLa1−xCaxMnO3+δ(x<~0.2)

Zero-field ${}^{139}\mathrm{La}$ and ${}^{55}\mathrm{Mn}$ nuclear magnetic resonances were studied in ${\mathrm{La}}_{0.8}{\mathrm{Ca}}_{0.2}{\mathrm{MnO}}_{3+\ensuremath{\delta}}$ with different oxygen stoichiometries \ensuremath{\delta}. The signal intensity, peak frequency, and line broadening of the ${}^{139}\mathrm{La}$ NMR spectrum show that excess oxygen atoms have a tendency to concentrate and establish local ferromagnetic ordering around themselves. These local orderings connect ferromagnetic clusters embedded in the antiferromagnetic host to form percolative ferromagnetic conduction paths. This phase separation is not a charge segregation type, but an electroneutral type. The magnetoresistance peak at the temperature where percolative paths start to form provides direct evidence that phase separation is one source of the colossal magnetoresistance effect.

Spin-charge coupling for dilute La-dopedCa3Ru2O7

${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ is a Mott-like system with a narrow gap (\ensuremath{\approx}0.1 eV). Very dilute doping of trivalent La (\ensuremath{\leqslant}5%) for divalent Ca prompts rapid and simultaneous changes in both the magnetic and transport properties, unambiguously illustrating the strong spin-charge coupling that characterizes ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}.$ The antiferromagnetic metallic phase unique to ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ persists into $({\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{)}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ with $x=0.04,$ which is followed by a fully metallic ground state with a quenching of the magnetic moment at $x=0.05.$ A large sudden drop in the magnetoresistivity up to 20 T defines a transition that is highly hysteretic and becomes irreversible at low temperatures and with La doping. The impact of La doping on ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ is much more drastic than that of Sr doping, implying the importance of the added electron from the La ion. Unlike ${\mathrm{Ca}}_{2}{\mathrm{RuO}}_{4}$ where ferromagnetism is instantly induced in the antiferromagnetic host by La doping, ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ shows no ferromagnetic ordering when La doped, but readily becomes a ferromagnet in modest magnetic fields.

Possible signatures of magnetic phase segregation in electron-doped antiferromagneticCaMnO3

Specimens of the form ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{MnO}}_{3}$ $(0l~xl~0.2)$ are utilized to study the effect of electron doping on the magnetic saturation moment ${M}_{\mathrm{sat}}(5\mathrm{K}).$ The systematic behavior of ${M}_{\mathrm{sat}}(5\mathrm{K})$ vs x is understood through a phenomenological model which suggests the existence of local ferromagnetic regions within the antiferromagnetic host for $0.03lxl0.08.$

Electron Paramagnetic Resonance Study of Magnetic Ordering in MnPS3, Mn0.79PS3(4,4′-bipy)0.42 and Mn0.84PS3(1,10-Phen)0.64 Compounds

We report Electron Paramagnetic Resonance (EPR) studies on intercalation ferromagnets Mn0.79PS3(4,4′-bipy)0.42 and Mn0.84PS3(1,10-phen)0.64 and their antiferromagnetic host MnPS3, in the temperature range 300–15 K. The nature of the organic guest cation appears to have a strong influence on the magnetic behaviour of this inorganic-organic hybrid system. The continued appearance of the exchange-narrowed sharp line, characteristic of the paramagnetic phase, even at 15 K suggests that the bulk of the intercalate ferromagnets in this system do not attain a completely ordered state.

Breakdown of the Kondo effect in critical antiferromagnets

The breakdown of the Kondo effect may be the origin of the anomalous properties of the heavy-fermion compounds at low temperatures. We study the dynamics of one impurity embedded in an antiferromagnetic host at the quantum critical point and show that the impurity is not screened and develops a power-law correlation function. This suggests that the breakdown of the Kondo effect can simply be a consequence of the system's proximity to the quantum critical point.