Antiferromagnetic Phase Research Articles

Exchange Bias Demonstrated in Bulk Nanocomposites Processed by High-Pressure Torsion.

Ferromagnetic (Fe or Fe20Ni80) and antiferromagnetic (NiO) phases were deformed by high-pressure torsion, a severe plastic deformation technique, to manufacture bulk-sized nanocomposites and demonstrate an exchange bias, which has been reported predominantly for bilayer thin films. High-pressure torsion deformation at elevated temperatures proved to be the key to obtaining homogeneous bulk nanocomposites. X-ray diffraction investigations detected nanocrystallinity of the ferromagnetic and antiferromagnetic phases. Furthermore, an additional phase was identified by X-ray diffraction, which formed during deformation at elevated temperatures through the reduction of NiO by Fe. Depending on the initial powder composition of Fe50NiO50 or Fe10Ni40NiO50 the new phase was magnetite or maghemite, respectively. Magnetometry measurements demonstrated an exchange bias in high-pressure torsion-processed bulk nanocomposites. Additionally, the tailoring of magnetic parameters was demonstrated by the application of different strains or post-process annealing. A correlation between the amount of applied strain and exchange bias was found. The increase of exchange bias through applied strain was related to the microstructural refinement of the nanocomposite. The nanocrystalline maghemite was considered to have a crucial impact on the observed changes of exchange bias through applied strain.

Generation of modulated magnetic structures based on cluster multipole expansion: Application to α -Mn and CoM3S6

We present a systematic method to automatically generate symmetry-adapted magnetic structures for given crystal structure and general propagation vector k as an efficient approach of the analysis of complex modulated magnetic structures. The method is developed as an extension of the generation scheme based on multipole expansion, which was demonstrated only for the propagation vector k=0 [M.-T. Suzuki et al., Phys. Rev. B 99, 174407 (2019)]. The symmetry-adapted magnetic structures characterized with an ordering vector k are obtained by mapping the multipole magnetic alignments on a virtual cluster to the periodic crystal structure with the phase factor for the wave vector k. This method provides all magnetic bases compatible with irreducible representations under k-group for given crystal structure and wave vector k. Multiple-k magnetic structures are derived from superposition of single-k magnetic bases related with space group symmetry. We apply the scheme to deduce the magnetic structures of $\alpha$-Mn and Co$M_{3}$S$_6$ (M = Nb, Ta), in which large anomalous Hall effect has recently been observed in antiferromagnetic phases, and identify the magnetic structures inducing anomalous Hall effect without net magnetization. The physical phenomena originating from emergent multipoles in the ordered phases are also discussed based on the Landau theory.

Room-temperature weak collinear ferrimagnet with symmetry-driven large intrinsic magneto-optic signatures

Here we present a magnetic thin film with a weak ferrimagnetic (FIM) phase above the N\'eel temperature (${T}_{N}=240\phantom{\rule{0.28em}{0ex}}\mathrm{K}$) and a noncollinear antiferromagnetic (AFM) phase below, exhibiting a small net magnetization due to strain-associated canting of the magnetic moments. A long-range ordered FIM phase has been predicted in related materials, but without symmetry analysis. We now perform this analysis and use it to calculate the magneto-optical Kerr effect (MOKE) spectra in the AFM and FIM phases. From the good agreement between the form of the measured and predicted MOKE spectra, we propose the AFM and FIM phases share the magnetic space group $C{2}^{\ensuremath{'}}/{m}^{\ensuremath{'}}$ and that the symmetry-driven magneto-optic and magneto-transport properties are maximized at room temperature in the FIM phase due to the nonzero intrinsic Berry phase contribution present in these materials. A room temperature FIM with large optical and transport signatures, as well as sensitivity to lattice strain and magnetic field, has useful prospects for high-speed spintronic applications.

Revealing spin-valve-like and exchange bias effect in Co3Sn1.9In0.1S2 crystal

We report on sharp spin-valve-like magnetoresistance (MR) below ferromagnetic order (TC), which has not been discussed in the pristine as well as doped crystals. In addition, typical manifestation of exchange bias (EB) effect is also observed by systematic shifts of the loop observed in MR driven by the cooling field. The EB effect appears below a characteristic temperature (TA), below which ferromagnetic and antiferromagnetic phases have been suggested to coexist. As a result of In doping, the sharp spin-valve-like evidence and EB effect over a wide temperature range, which are observed even at much higher temperatures than the liquid nitrogen temperature, suggest the system as a potential candidate for application in topological spintronics.

Exchange bias effect and inhomogeneous magnetism in 6H Ba3CoFeRuO9: Role of structural site disorder

Structural, magnetic, electrical and thermal investigations on ternary barium ruthenate Ba3CoFeRuO9 (BCFRO) reveal the role of the two different octahedral B-site in creating locally varying competing antiferromagnetic and ferromagnetic interactions. BCFRO forms in 6H hexagonal structure in the P63/mmc space group. Neutron diffraction and specific heat capacity measurements affirmed the absence of any long-range ordering and structural transition in the compound. DC thermomagnetic studies evidences spin glass magnetism with Tg ∼ 50 K. A large exchange bias (EB) field of 3.48 kOe is obtained at 3 K under a cooling field of 50 kOe. Careful analysis indicates, EB in our system stems from inhomogeneous magnetism owing to spatially varying chemical composition. Competing ferromagnetic and antiferromagnetic phases giving rise to spin glass behaviour is the manifestation of site disorder induced by mixed valences of magnetic ions. The exchange bias training effect in our system follows spin relaxation model of rotatable and frozen spins which are exchange coupled at the interface. Strong electron localisation occurs by virtue of the specific 6H structure with corner shared octahedral 2a site and face shared octahedral 4f site. The electrical and thermal transport in BCFRO follows Efros-Shklovskii variable range hopping (ES VRH) model below 150 K. The high resistivity of BCFRO at low temperatures is also reflected in the low thermal conductivity and huge Seebeck coefficient.

Low-temperature theory of inversion and quantum oscillations in Kondo insulators

The half-filled Kondo lattice model is studied at low temperatures on a simple cubic lattice using the self-consistent theory developed by Ram and Kumar [Phys. Rev. B 96, 075115 (2017)]. It is found to have three distinct insulating phases in the temperature-hopping plane, namely, the strong-coupling Kondo singlet (KS) phase, the inverted Kondo singlet (iKS) phase distinguished from the KS by inversion, and the antiferromagnetic (AFM) phase. The quasiparticle density of states across the inversion transition is noted to exhibit a dimensional reduction, which can differentiate between the KS and iKS insulators in experiments. Magnetic quantum oscillations obtained in the iKS and AFM phases are found to show a Lifshitz-Kosevich-like behavior with temperature as well as quantum fluctuations induced by the Kondo interaction.

Can the ferroelectric soft mode trigger an antiferromagnetic phase transition?

Type-II multiferroics, where spin interactions induce a ferroelectric polarization, are interesting for new device functionalities due to large magnetoelectric coupling. We report on a new type of multiferroicity in the quadruple-perovskite BiMn3Cr4O12, where an antiferromagnetic phase is induced by the structural change at the ferroelectric phase transition. The displacive nature of the ferroelectric phase transition at 125 K, with a crossover to an order-disorder mechanism, is evidenced by a polar soft phonon in the THz range and a central mode. Dielectric and pyroelectric studies show that the ferroelectric critical temperature corresponds to the previously reported Néel temperature of the Cr3+ spins. An increase in ferroelectric polarization is observed below 48 K, coinciding with the Néel temperature of the Mn3+ spins. This increase in polarization is attributed to an enhanced magnetoelectric coupling, as no change in the crystal symmetry below 48 K is detected from infrared and Raman spectra.

Low-pressure-driven barocaloric effects at colinear-to-triangular antiferromagnetic transitions in Mn3-xPt1+x

A large driving pressure is required for barocaloric effects (BCEs) in intermetallics, usually above 100 MPa. Here, we report barocaloric effects in Mn3-xPt1+xalloys saturated at about 60 MPa, the lowest pressure reported in intermetallics to date. A first-order phase transition occurs from the colinear antiferromagnetic phase to the triangular antiferromagnetic phase as temperature decreases. The transition temperature strongly depends on the composition x, ranging from 331 K for x = 0.18 to 384 K for x = 0.04, and is sensitive to pressure, with dTt/dP up to 139 K/GPa. However, the maximum pressure-induced entropy changes are as small as 13.79 J kg-1 K-1, attributed to the mutual cancellation of lattice and magnetic entropy changes. The small driving pressure and total entropy changes are due to the special magnetic geometric frustration.

Magnetic anisotropy as an evidence of local magnetic moments in paramagnetic phase of Cr-V alloys

Chromium-rich Cr-V alloys exhibit three phases. The paramagnetic phase (P) above Néel temperature (TN), a transverse polarization spin-density-wave (SDW) phase (AF1) below TN and above the spin–flip temperature (TSF), when the longitudinal polarization SDW phase (AF2) is formed. In the paramagnetic phase, Cr exhibit a Pauli susceptibility with weak temperature dependence. Otherwise, the introduction of small amounts of V in Cr not only decreases TN continuously, but also induces a Curie-Weiss behavior (CW) that we associate with local magnetic moments. This behavior is limited up to 0.67%V and magnetic fields of 15 kOe and was also observed for different Cr alloys. The origin of local magnetic moments has been associated with the establishment of local spin-density waves (LSDW) around V impurities. In this work, we presented an investigation of the effects of local magnetic moments in antiferromagnetic phases in Cr-V alloys. The magnetic susceptibility measurements around the spin–flip transition suggest that the local-SDW induces the strong magnetic anisotropy in the long-range order of the spin-density wave as it is revealed in the thermomagnetic results.

Probing antiferromagnetism in exfoliated Fe3GeTe2 using magneto-transport measurements.

Among the material class of van der Waals magnets, Fe3GeTe2 (FGT) has emerged as one of the most studied owing to features such as its relatively high Curie temperature, metallic nature, and large spin polarization. Though most studies only investigate its explicitly ferromagnetic properties, FGT is also predicted to have an antiferromagnetic phase in the out-of-plane direction emerging at temperatures below 150 K, leading to a blend of ferromagnetic and antiferromagnetic ordering. Here, we explore the emergence of this phase and its effects in FGT/h-BN heterostructures using magneto-transport measurements. The devices' anomalous Hall and magnetoresistance responses exhibit a complex trend with temperature that is consistent with multiple magnetic phases. In addition to the usual out-of-plane sensing, we also rotate the applied field to the in-plane direction and observe behavior resembling the planar topological Hall effect. Intriguingly, this response follows a similar temperature trend to the out-of-plane response. We also use the out-of-plane anomalous Hall response to show that, at sufficiently low temperatures, both positive and negative field-cooling results in an increased saturation Hall resistance. Such a field-cooling divergence is consistent with antiferromagnetic ordering resulting in a spin-glass like state in the sample. In addition to providing insight into one of the most exciting candidate materials for 2D magnetic devices, our work demonstrates the power of magneto-transport measurements to probe complex behavior in vdW magnets where common magnetometry techniques used on bulk samples may not be viable.

Antiferromagnetic spin-1 large-D phase in organic spin-chain crystals

This work experimentally demonstrates that the β-phase iron(ii) phthalocyanine (β-FePc) single crystal is a new antiferromagnetic spin-1 large-D phase material, expanding the scope of low-dimensional magnetism research in organic molecular crystals.

Abnormal magnetic phenomenon at low temperature in Zn doped <inline-formula><tex-math id="M1">\begin{document}$ \left[{(\mathbf{C}\mathbf{H}}_{3}{)}_{2}\mathbf{N}{\mathbf{H}}_{2}\right]{\mathbf{C}\mathbf{o}}_{\mathit{x}}{\mathbf{Z}\mathbf{n}}_{1-\mathit{x}}{\left[\mathbf{H}\mathbf{C}\mathbf{O}\mathbf{O}\right]}_{3} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221761_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221761_M1.png"/></alternatives></inline-formula> frameworks

Metal-organic framework (MOF) is a new type of inorganic-organic hybrid porous material composed of organic ligands bridging metal ions, and it has the characteristics of tunable functions, good stability and porosity. In this study, Zn doped Co-based metal organic frame works single-crystal samples<inline-formula><tex-math id="M2">\begin{document}$\left[{(\rm{C}\rm{H}}_{3}{)}_{2}\rm{N}{\rm{H}}_{2}\right]{\rm{C}\rm{o}}_{1-x}{\rm{Z}\rm{n}}_{x} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221761_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221761_M2.png"/></alternatives></inline-formula><inline-formula><tex-math id="M2-1">\begin{document}$ {\left[\rm{H}\rm{C}\rm{O}\rm{O}\right]}_{3}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221761_M2-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20221761_M2-1.png"/></alternatives></inline-formula>are synthesized by the solvothermal method with normal ratio <i>x</i> = 0, 0.1, 0.2, 0.3, 0.4, 0.5. Single crystal diffraction, scanning electron microscope and energy dispersive X-ray spectroscopy results show that Zn ions are uniformly doped into Co-based MOFs crystals. The field cooling curves show that antiferromagnetic phase transition temperature of Co-based MOFs decreases from 15 K for pure Co-MOF <i>x</i> = 0 to 12.8 K for <i>x</i> = 0.2. Abnormal large magnetic hysteresis is obtained for Zn doped crystals with large coercive field 3600 Oe (<i>x</i> = 0.3) compared with 450 Oe coercive field for pure Co-MOF (<i>x</i> = 0), and the hysteresis area of Zinc-doped sample is more than 3 times that of pure cobalt sample. On the other hand, we find a series of steps on the hysteresis loop of DMCo<sub>0.9</sub>Zn<sub>0.1</sub>F sample, which gradually disappears with the increase of temperature, similar to the quantum tunneling phenomenon of a single molecule magnet. Previous studies have shown that the long range magnetic interaction and the magnetic single-ion behavior competition coexist in these systems. It is believed that the doping of non-magnetic zinc ions weakens the long-range interaction between Co ions and makes Co ions show the step effect caused by quantum tunneling at low temperature.

Successive short- and long-range magnetic ordering in rosiaite-type CoGeTeO6 prepared by ion-exchange reaction.

The missing member of the rosiaite family, CoGeTeO6, was synthesized by mild ion-exchange reactions and characterized by magnetization M and specific heat Cp measurements. It exhibits a successive short- and long-range magnetic ordering at Tshort-range ≈ 45 K and TN = 15 K, respectively. Based on these measurements, the magnetic H-T phase diagram was established, showing two antiferromagnetic phases separated by a spin-flop transition. The reason why the pronounced short-range correlation occurs at a temperature nearly three times higher than TN was found by evaluating the Co-O⋯O-Co exchange interactions using energy-mapping analysis. Although CoGeTeO6 has a layered structure, its magnetic structure consists of three-dimensional antiferromagnetic lattices made up of rhombic boxes of Co2+ ions. The experimental data obtained at high temperatures agree well with the computational results by treating the Co2+ ions of CoGeTeO6 as S = 3/2 ions, but the heat capacity and magnetization data were obtained at low temperatures by treating the Co2+ ion as a Jeff = 1/2 ion.

Transition metal doping and co-doping effect on electronic and magnetic properties of zb-ScN

The ab-initio study of electronic and magnetic properties and Curie temperature of transition metal (TM) doped ScN diluted magnetic semiconductor (DMS) are calculated using generalized gradient approximation (GGA) implemented in AKAI-KKR-CPA package. The total and partial density of states (DOS) of pure zinc blend ScN as well as TM doped compounds are plotted for different concentrations of dopants. Our results shows that the Sc substituted by TM materials induces a half-metallic character in the system for different concentrations except for Sc1−xNixN (x > 8%). Also, We found that the Sc1−xVxN, Sc1−xCrxN, Sc1−xMnxN compounds are stable in ferromagnetic states, while Sc1−xCoxN and Sc1−xNixN compounds prefer antiferromagnetic phase. In addition, the minority-spin bands depicted a half-metallic ferromagnetic (HMF) gap and half-metallic (HM) gap. To improve this latest result, doping ScN with double impurities (Co, Ni) is also investigated. As a result, our results show a stable antiferromagnetic state in the majority of cases. The mechanism of exchange interaction is also discussed for all doped systems and the high curie temperature are found for most concentrations. The present results suggest TM doped ScN as potential candidate for spintronic devices.

Electronic and thermal transport properties of the metallic antiferromagnet MnSn2.

We report the structure, magnetic and electrical/thermal transport properties of the antiferromagnet MnSn2. Importantly, the existence of the two antiferromagnetic states below TN2 (∼320 K) is confirmed by magnetism and electrical transport measurements. An unsaturated positive magnetoresistance up to 150% at ∼9 T was observed at 5 K, whereas the magnetoresistance becomes negative in the whole range at high temperatures (T > 74 K). Systematic investigations of the Hall transport and thermoelectric properties reveal that the hole-type carriers are the majority carriers in MnSn2. The kink around 320 K in the Seebeck coefficient originates from the effect of the antiferromagnetic phase on the band structure, while the pronounced peak around 231 K is attributed to the phonon-drag effect. The results suggest that the spin arrangement plays a vital role in the magnetic, electrical, and thermal transport properties in MnSn2.

A new method to enhance the magnetocaloric effect in (Sc,Ti)Fe2 via magnetic phase separation

Magnetic refrigeration based on the magnetocaloric effect (MCE) is a novel refrigeration technology that will replace traditional vapor-compression refrigeration in the future. Improvement in the performance of MCE materials is crucial for the development of magnetic refrigeration technology. This study presents a new method that enhances the MCE performance of (Sc,Ti)Fe2 via magnetic phase separation. The maximum magnetic entropy change induced by the coexistence of an in-plane ferromagnetic phase (FMab) and a canting antiferromagnetic phase (CAFM) in Sc0.3Ti0.7Fe2 is twice that found in other (Sc,Ti)Fe2 compounds. Variable-temperature neutron diffraction experiments directly reveal that the large magnetic entropy change in Sc0.3Ti0.7Fe2 is dominated by the transformation from a highly ordered FMab state to a CAFM state with a lower magnetic order. The magnetic phase separation is a direct transition from a higher-ordered state with a larger lattice to a lower-ordered state with a smaller lattice that induces a large magnetic order change and lattice contraction. The combination of the metamagnetic transition and negative thermal expansion leads to enhanced MCE. This study suggests the possibility that magnetic phase separation can be an effective approach to achieving and controlling a large MCE in magnetic materials.

Evidence of exchange striction and charge disproportionation in the magnetoelectric material Ni3TeO6

The chiral magneto-electric compound Ni3TeO6 is investigated through temperature-dependent synchrotron-based powder x-ray diffraction and x-ray absorption spectroscopy between 15 to 300 K. Our work provides direct evidence for the exchange-striction in the material around the concomitant onset point of collinear antiferromagnetic and magneto-electric phases. The x-ray absorption near edge spectra and x-ray photoelectron spectra show that the sample consists of both Ni2+ and Ni3+ ions in the lattice. The ionic state of Ni is found to be quite robust, and it is largely independent of the preparation route. Additionally, the minority Te4+ state is found to coexist with the majority Te6+ state, which may arise from the charge disproportionation between Ni and Te ions (Ni2+ + Te6+ --> Ni3+ + Te4+). The observed mixed valency of Ni is also confirmed by the total paramagnetic moment per Ni atom in the system. This mixed valency in the metal ions and the exchange-striction may be attributed to the observed magneto-electric effect in the system.

Superradiance-induced multistability in one-dimensional driven Rydberg lattice gases

We study steady state phases of a one-dimensional array of Rydberg atoms coupled by a microwave (MW) field where the higher energy Rydberg state decays to the lower energy one via single-body and collective (superradiant) decay. Using mean-field approaches, we examine the interplay among the MW coupling, intra-state van der Waals (vdW) interaction, and single-body and collective dissipation between Rydberg states. A linear stability analysis reveals that a series of phases, including uniform, antiferromagnetic, oscillatory, and bistable and multistable phases can be obtained. Without the vdW interaction, only uniform phases are found. In the presence of the vdW interaction, multistable solutions are enhanced when increasing the strength of the superradiant decay rate. Our numerical simulations show that the bistable and multistable phases are stabilized by superradiance in a long chain. The critical point between the uniform and multistable phases and its scaling with the atom number is obtained. Through numerically solving the master equation of a finite chain, we show that the mean-field multistable phase could be characterized by expectation values of Rydberg populations and two-body correlations between Rydberg atoms in different sites.

Quantum anomalous layer Hall effect in the topological magnet MnBi2Te4

Recently, a type of Hall effect due to an unusual layer-locked Berry curvature called the layer Hall effect (LHE) has been reported in the even-layered two-dimensional antiferromagnetic (AFM) MnBi2Te4 [A. Gao et.al, Nature 595, 521 (2021)]. In this work, we report that the quantization of LHE, which we call the quantum anomalous layer Hall effect (QALHE), can be realized in MnBi2Te4. The QALHE originates from kicking a layer-locked Berry-curvature monopole out of the Fermi sea by a vertielectric cal field. Remarkably, we demonstrate that the electric-field reversal can switch the sign of the quantized Hall conductance of QALHE in the even-layered AFM phase. The QALHE can also be realized in the ferromagnetic phase. These results provide a promising way toward the electric engineering of the Berry curvature monopoles and quantized-layered transport in topological magnets.

Hexagonal RFeO3 (R = Dy, Er, and Lu) Films Grown on Glass Substrates with Both Magnetic and Ferroelectric Orders

Hexagonal rare-earth ferrites (h-RFeO3) are promising multiferroic materials owing to the spontaneous magnetization, high ferroelectric transition temperature, and fascinating features provided by strong magnetoelectric coupling. However, h-RFeO3 is metastable, and its synthetic methods are limited, particularly when ionic size of R is large. One of useful synthesis methods is epitaxial thin-film growth on single-crystal substrates, because the metastable phase is stabilized by the lattice match between h-RFeO3 and the substrate. Compared to single-crystal substrates, glass substrates are more cost-effective. However, films on glass tend to consist of a thermodynamically stable phase owing to the amorphous nature of glass substrates. Herein, we report that a h-RFeO3 pure phase can be obtained as thin film even on glass substrates. The metastable phase was formed owing to the nonequilibrium synthesis process of pulsed laser deposition. The obtained h-RFeO3 thin films on glass with R = Dy, Er, and Lu exhibited weak ferromagnetism with Néel temperatures of 120–140 K. Furthermore, a clear magnetic phase transition from weak ferromagnetic to antiferromagnetic phase was observed in the h-ErFeO3 films. Additionally, piezoelectric force microscopy measurements exhibited the ferroelectricity of the film at room temperature. These results provide a convenient synthesis method of a pure phase multiferroic h-RFeO3 system.