Low-temperature Antiferromagnetic State Research Articles

Thickness dependence of Morin transition of Ru-doped α-Fe2O3 films detected by spin Hall magnetoresistance measurements

We conducted spin Hall magnetoresistance (SMR) measurements to investigate the Ru-doping effect on the Morin transition of α-Fe2O3(0001) films, which is the transition from the low-temperature antiferromagnetic state with c-axis magnetic moments to the high-temperature weak ferromagnetic state with c-plane magnetic moments, under a thickness of 6–100 nm. We clarified that the Morin temperature of the 2.6 at. % Ru-doped α-Fe2O3 film was greater than 400 K when the thickness was 100 nm and decreased with a decrease in thickness. Our results demonstrated that SMR measurements are a valid verification method for the Morin transition of very thin α-Fe2O3 films down to several nm.

Anomalous Nernst effect in the topological and magnetic material MnBi4Te7

The recently discovered magnetic topological insulators (MnBi2Te4)(Bi2Te3)n, n = 0–4, are an ideal playground to study the influence of magnetic properties on band topology, giving access to diverse quantum states in a single compound. In the low temperature-antiferromagnetic state and vanishing magnetic field, the n = 1 system is a topological insulator protected by a combination of time reversal and a translation symmetries. It has been argued that, when the antiferromagnetic phase is forced to a the fully spin polarized state by the application of an external magnetic field, this system develops Weyl cones in the conduction band, which become accessible in presence of an intrinsic electronic doping. In this work, we experimentally prove the raising of field-induced Weyl state through the detection of an intrinsic anomalous Nernst effect in a bulk single crystal of MnBi4Te7.

Gapless fermionic excitation in the antiferromagnetic state of ytterbium zigzag chain

The emergence of charge-neutral fermionic excitations in magnetic systems is one of the unresolved issues in recent condensed matter physics. This type of excitations has been observed in various systems, such as low-dimensional quantum spin liquids, Kondo insulators, and antiferromagnetic insulators. Here, we report the presence of a pronounced gapless spin excitation in the low-temperature antiferromagnetic state of YbCuS2 semiconductor, where trivalent ytterbium atoms form a zigzag chain structure. We confirm the presence of this gapless excitations by a combination of experimental probes, namely 63/65Cu-nuclear magnetic resonance and nuclear quadrupole resonance, as well as specific heat measurements, revealing a linear low-temperature behavior of both the nuclear spin-lattice relaxation rate 1/T1 and the specific heat. This system provides a platform to investigate the origin of gapless excitations in spin chains and the relationship between emergent fermionic excitations and frustration.

Simultaneous manifestation of metallic conductivity and single-molecule magnetism in a layered molecule-based compound.

Single-molecule magnets (SMMs) show superparamagnetic behaviour below blocking temperature at the molecular scale, so they exhibit large magnetic density compared to the conventional magnets. Combining SMMs and molecular conductors in one compound will bring about new physical phenomena, however, the synergetic effects between them still remain unexplored. Here we present a layered molecule-based compound, β′′-(BEDO-TTF)4 [Co(pdms)2]·3H2O (BO4), (BEDO-TTF (BO) and H2pdms are bis(ethylenedioxy)tetrathiafulvalene and 1,2-bis(methanesulfonamido)benzene, respectively), which was synthesized by using an electrochemical approach and studied by using crystal X-ray diffraction. This compound simultaneously exhibited metallic conductivity and SMM behaviour up to 11 K for the first time. The highest electrical conductivity was 400–650 S cm−1 at 6.5 K, which is the highest among those reported so far for conducting SMM materials. Furthermore, antiferromagnetic ordering occurred below 6.5 K, along with a decrease in conductivity, and the angle-independent negative magnetoresistance suggested an effective electron correlation between the conducting BO and Co(pdms)2 SMM layers (d–π interactions). The strong magnetic anisotropy and two-dimensional conducting plane play key roles in the low-temperature antiferromagnetic semiconducting state. BO4 is the first compound exhibiting antiferromagnetic ordering among SMMs mediated by π-electrons, demonstrating the synergetic effects between SMMs and molecular conductors.

Anomalous magnetic behavior and complex magnetic structure of proximate LaCrO3—LaFeO3 system

We investigated complex magnetic properties of multifunctional LaCrO3-LaFeO3 system. The magnetic measurements substantiate the presence of competing complex magnetic ordering against temperature, showing paramagnetic to ferrimagnetic transition at ∼300 K, followed by antiferromagnetic (AFM) transition near ∼250 K superimposed on ferrimagnetic phase. The onset of weak ferrimagnetic ordering is attributed to the competing complex interaction between two AFM LaCrO3-LaFeO3 sublattices. The low-temperature AFM ordering is also substantiated by temperature-dependent Raman measurements, where the intensity ratio of 700 cm−1 Raman active mode showed a clear enhancement with lowering the temperature. The non-saturating nature of magnetic moments in LaCrO3-LaFeO6 suggests the predominating AFM ordering in conjunction with ferrimagnetic ordering between 250 K–300 K up to 5 T magnetic field. A complex magnetic structure of LaCrO3-LaFeO3 is constructed, emphasizing the metastable magnetic phase near room temperature and low-temperature antiferromagnetic state.

Influence of Fe and Cr doping on the structural and magnetic properties of ZnV2O4

In this paper, we report the modulation of structural and magnetic properties with Fe and Cr doping at the Zn and V sites in ZnV2O4. Undoped ZnV2O4 exhibits two anomalies in the M–T measurements which are assigned to a structural transition (TS ~ 57 K) followed by a magnetic transition (TN ~ 41 K). With the help of x-ray diffraction and energy dispersive x-ray analysis it is found that the prepared samples are single phase as well as stoichiometric. However, doping at both the cationic sites of ZnV2O4 leads to the disappearance of the structural transition and suppression of the long range magnetic ordering to a cusp like feature below 20 K and the cusps broaden with increasing magnetic field while the position of the cusps remain unaltered with increasing field strength. The obtained results indicate that Zn1−xFexV2O4 and Zn(V1−tCrt)2O4 (x = t = 0, 0.05, 0.1) systems are frustrated cubic normal spinel systems. Moreover, such field independent behavior of cusp positions hints towards the antiferromagnetic ground state at low temperature instead of a spin glass phase. To elucidate this fact, dc aging (magnetic relaxation) and memory effects have been investigated in zero field cooled (ZFC) and field cooled (FC) protocol for the 10% doped samples, which exhibit aging and memory effect. Our study provides an explicit evidence of spin glass like state instead of low temperature antiferromagnetic ground state in the doped compounds.

Two types of magnetic shape-memory effects from twinned microstructure and magneto-structural coupling in Fe1+yTe

A detailed experimental investigation of Fe1+y Te (y = 0.11, 0.12) using pulsed magnetic fields up to 60 T confirms remarkable magnetic shape-memory (MSM) effects. These effects result from magnetoelastic transformation processes in the low-temperature antiferromagnetic state of these materials. The observation of modulated and finely twinned microstructure at the nanoscale through scanning tunneling microscopy establishes a behavior similar to that of thermoelastic martensite. We identified the observed, elegant hierarchical twinning pattern of monoclinic crystallographic domains as an ideal realization of crossing twin bands. The antiferromagnetism of the monoclinic ground state allows for a magnetic-field-induced reorientation of these twin variants by the motion of one type of twin boundaries. At sufficiently high magnetic fields, we observed a second isothermal transformation process with large hysteresis for different directions of applied field. This gives rise to a second MSM effect caused by a phase transition back to the field-polarized tetragonal lattice state.

Magnetic and electrical properties of R5Ir3 (R = Tb, Er)

Magnetic and electrical properties of R5Ir3 (R = Tb, Er) polycrystals with hexagonal structure have been studied by measuring magnetization, dc magnetic susceptibility, and electrical resistivity. Tb5Ir3 shows reentrant magnetism; it possesses ferromagnetic state below TC = 49 K and antiferromagnetic state takes place at Tt = 6.5 K. In the antiferromagnetic state, irreversible magnetic field-induced antiferromagnetic to ferromagnetic transition was observed. Meanwhile, Er5Ir3 shows ferromagnetic property below TC = 10 K; antiferromagnetic state exists below Tt = 2.1 K. In the low temperature antiferromagnetic state, the irreversible antiferromagnetic to ferromagnetic transition was also observed. Paramagnetic susceptibility shows Curie-Weiss behavior in both compounds; the effective magnetic moment is in good agreement with the theoretical values of Tb3+ and Er3+ ions. Electrical resistivity indicated the metallic property; distinct anomalies were observed at the transition temperatures for both compounds.

Magnetic and electronic properties of NpPdSn

We have studied NpPdSn by means of the heat capacity, electrical resistivity, Seebeck and Hall effect, $^{237}$Np M\"{o}ssbauer spectroscopy, and neutron diffraction measurements in the temperature range 2-300 K and under magnetic fields up to 14 T. NpPdSn orders antiferromagnetically below the N\'eel temperature $T_N$ = 19 K and shows localized magnetism of Np$^{3+}$ ion with a a doubly degenerate ground state. In the magnetic state the electrical resistivity and heat capacity are characterized by electron-magnon scattering with spin-waves spectrum typical of anisotropic antiferromagnets. An enhanced Sommerfeld coefficient and typical behavior of magnetorestistivity, Seebeck and Hall coefficients are all characteristic of systems with strong electronic correlations. The low temperature antiferromagnetic state of NpPdSn is verified by neutron diffraction and $^{237}$Np M\"{o}ssbauer spectroscopy and possible magnetic structures are discussed.

Low-temperature magnetism of alabandite: Crucial role of surface oxidation

Manganese(II) monosulphide crystallizes into three different polymorphs (α-, β-, and γ-MnS). Out of these, α-MnS, also known as mineral alabandite, is considered the most stable and is widespread in terrestrial materials as well as in extraterrestrial objects such as meteorites. In this study, a low-temperature antiferromagnetic state of α-MnS was investigated using macroscopic magnetic measurements as induced and remanent field-cooled (FC) and zero-field-cooled (ZFC) magnetizations and magnetic hysteresis. Both natural alabandite and synthetic samples show: (1) Neel temperatures in a narrow temperature range around 153 K, and (2) a rapid increase of the magnetization around 40 K. An anomalous magnetic behavior taking place at about 40 K was previously ascribed to the magnetic transition from a high-temperature antiferromagnetic to a low-temperature ferromagnetic state documented for non-stoichiometric α-MnS slightly enriched in manganese. However, our detailed microscopic observations and, in particular, oxidation experiments indicate that the anomalous magnetic behavior around 40 K is caused by the presence of an oxide layer of ferrimagnetic hausmannite (Mn3O4) on the surface of α-MnS rather than being an intrinsic property of nearly stoichiometric α-MnS.

Paramagnetic spin excitations in insulating Rb0.8Fe1.6Se2

We use neutron scattering to study temperature dependent spin excitations in insulating antiferromagnetic (AF) Rb$_{0.8}$Fe$_{1.6}$Se$_2$. In the low-temperature AF state, spin waves can be accurately described by a local moment Heisenberg Hamiltonian. On warming to around the Neel temperature of T_N=500$ K, low-energy (E<30meV) paramagnetic spin excitations form Lorentzian-like quasielastic peaks centered at the AF wave vectors associated with spin waves, while high-energy (E>50meV) spin excitations become heavily damped. Upon further warming to above the structural distortion temperature of T_s=524K, the entire paramagnetic excitations become overdamped. These results suggest that AF Rb$_{0.8}$Fe$_{1.6}$Se$_2$ is not a copper-oxide-like Mott insulator, and has less electron correlations compared with metallic iron pnictides and iron chalcogenides.

First-order magnetic phase transition in layered Sr3YCo4O10.5 + δ-type cobaltites

In layered Sr3YCo4O10.5 + δ-type cobaltites with different oxygen contents, we have observed a first order magnetic phase transition from the high-temperature “ferromagnetic” state to the low-temperature antiferromagnetic state. The transition can be induced by an applied magnetic field. It is accompanied by a significant hysteresis in the magnetic field (∼10 T) and temperature (∼10 K). A decrease and an increase in the yttrium content lead to a purely “ferromagnetic” and antiferromagnetic behavior, respectively.

Field induced sign reversal of magnetocaloric effect in Gd2In

We report the magnetocaloric effect in the metamagnetic compound Gd2In obtained from magnetization measurement. Gd2In was previously reported to have two magnetic transitions: (i) a paramagnetic to ferromagnetic transition below 190K and (ii) a ferromagnetic to an antiferromagnetic state below 105K. The low temperature antiferromagnetic state is unstable under an applied magnetic field and undergoes metamagnetic transition to a ferromagnetic like state. We observe conventional positive magnetocaloric effect (the magnetic entropy change, ΔSM<0) around 190K at all applied fields. The magnetocaloric effect is found to be inverse (negative) at low fields around 105K (ΔSM>0), however it turns positive at higher fields (ΔSM<0). The observed anomaly is found to be related to the field induced transition which drives the system from an antiferromagnetic to a ferromagnetic state.

Mössbauer spectroscopic study of spin reorientation in Mn-substituted yttrium orthoferrite

The first-order spin-reorientation transition in the Mn-substituted yttrium orthoferrites, YFe1−xMnxO3 (x = 0.1, 0.15 and 0.2), has been investigated using 57Fe Mössbauer spectroscopy. Owing to its large anisotropy, substitution of Mn3+ ions in YFeO3 induces a spin-reorientation transition from the low-temperature antiferromagnetic state to a high-temperature weak ferromagnetic state. With increasing x, the spin-reorientation transition temperature (TSR) increases whereas the Néel temperature (TN) decreases. Analysis of the Mössbauer spectra unambiguously confirms the occurrence of spin reorientation relative to crystal axes. At a given temperature, the mean hyperfine field decreases with the increasing Mn concentration. The variation of canting angle with temperature for YFe0.85Mn0.15O3 has been estimated.

Hall effect of FeTe and Fe(Se1–xTex) thin films

The Hall effect is investigated in thin-film samples of iron–chalcogenide superconductors in detail. The Hall coefficient (RH) of FeTe and Fe(Se1–xTex) exhibits a similar positive value around 300K, indicating that the high-temperature normal state is dominated by hole-channel transport. FeTe exhibits a sign reversal from positive to negative across the transition to the low-temperature antiferromagnetic state, indicating the occurrence of drastic reconstruction in the band structure. The mobility analysis using the carrier density theoretically calculated reveals that the mobility of holes is strongly suppressed to zero, and hence the electric transport looks to be dominated by electrons. The Se substitution to Te suppresses the antiferromagnetic long-range order and induces superconductivity instead. The similar mobility analysis for Fe(Se0.4Te0.6) and Fe(Se0.5Te0.5) thin films shows that the mobility of electrons increases with decreasing temperature even in the paramagnetic state, and keeps sufficiently high values down to the superconducting transition temperature. From the comparison between FeTe and Fe(Se1–xTex), it is suggested that the coexistence of ‘itinerant’ carriers both in electron and hole channels is indispensable for the occurrence of superconductivity.

Rearrangement of the antiferromagnetic ordering at high magnetic fields in SmFeAsO and SmFeAsO0.9F0.1single crystals

The low-temperature antiferromagnetic state of the Sm-ions in both nonsuperconducting SmFeAsO and superconducting SmFeAsO$_{0.9}$F$_{0.1}$ single crystals was studied by magnetic torque, magnetization, and magnetoresistance measurements in magnetic fields up to 60~T and temperatures down to 0.6~K. We uncover in both compounds a distinct rearrangement of the antiferromagnetically ordered Sm-moments near $35-40$~T. This is seen in both, static and pulsed magnetic fields, as a sharp change in the sign of the magnetic torque, which is sensitive to the magnetic anisotropy and hence to the magnetic moment in the $ab$-plane, ({\it i.e.} the FeAs-layers), and as a jump in the magnetization for magnetic fields perpendicular to the conducting planes. This rearrangement of magnetic ordering in $35-40$~T is essentially temperature independent and points towards a canted or a partially polarized magnetic state in high magnetic fields. However, the observed value for the saturation moment above this rearrangement, suggests that the complete suppression of the antiferromagnetism related to the Sm-moments would require fields in excess of 60~T. Such a large field value is particularly remarkable when compared to the relatively small N\'{e}el temperature $T_{\rm N}\simeq5$~K, suggesting very anisotropic magnetic exchange couplings. At the transition, magnetoresistivity measurements show a crossover from positive to negative field-dependence, indicating that the charge carriers in the FeAs planes are sensitive to the magnetic configuration of the rare-earth elements. This is indicates a finite magnetic/electronic coupling between the SmO and the FeAs layers which are likely to mediate the exchange interactions leading to the long range antiferromagnetic order of the Sm ions.

Mobility Analysis of FeTe Thin Films

The Hall effect is investigated in detail for nonsuperconducting and superconducting FeTe thin films. The Hall coefficient commonly exhibits a sign reversal from positive in a high-temperature paramagnetic state to negative in a low-temperature antiferromagnetic state. Phenomenological analysis by a simple two-band Drude model indicates that hole mobility is significantly suppressed in the antiferromagnetic state. When suppression of the hole mobility is insufficient, superconductivity shows up in FeTe. This result strongly suggests that the itinerancy in both hole and electron channels is the essential factor for the occurrence of superconductivity in iron chalcogenide superconductors.

Martensitic features in Si doped CeFe 2 revealed by magnetization and transport study

We report the effect of Si doping on the magnetization and transport behavior in CeFe 2. It is found that Si doping is able to stabilize the low temperature antiferromagnetic state in CeFe 2, resulting in a first order ferromagnetic to antiferromagnetic transition. The strong magnetostructural coupling in this system is found to give rise to martensitic features, which manifest in the magnetization and transport data. We show clear evidence of this influence through the multi-step magnetization behavior, unusual relaxation effect, thermal and magnetic history dependencies. The non-equilibrium magnetic state at low temperatures is also revealed by the fact that the magnetization steps can be induced with the help of an appropriate measurement protocol. Detailed magnetization relaxation studies have been carried out to understand the dynamics of magnetic phase transition. Martensitic scenario is also found to cause multi-step behavior in the magnetoresistance isotherms. By compiling all the distinct features in this system with those of other related systems, we conclude that this can be a model system that may contribute to understand the rich physics of phase separation and the martensitic scenario, both theoretically and experimentally.

Antiferromagnetic resonance in methylaminated potassium fulleride(CH3NH2)K3C60

High-frequency magnetic resonance measurements $({\ensuremath{\nu}}_{L}=9.6--420\phantom{\rule{0.3em}{0ex}}\mathrm{GHz})$ were employed to investigate the low-temperature antiferromagnetic ground state of the $(\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{2}){\mathrm{K}}_{3}{\mathrm{C}}_{60}$ fulleride. The frequency and temperature dependence of the intensity, linewidth, and center of the resonance signal detected below ${T}_{N}$ are characteristic of antiferromagnetic resonance (AFMR). The AFMR intensity is consistent with an ordered magnetic moment of ${\ensuremath{\mu}}_{\mathit{eff}}=0.7(1){\ensuremath{\mu}}_{B}∕{\mathrm{C}}_{60}$, while the narrowing of the AFMR signal with increasing resonance frequency can be modeled with a spin-flop field of ${H}_{\mathit{sf}}=840(80)\phantom{\rule{0.3em}{0ex}}\mathrm{G}$ and a $g$-factor anisotropy of $\ensuremath{\delta}\ensuremath{\gamma}=710(50)\phantom{\rule{0.3em}{0ex}}\mathrm{ppm}$. We stress that the spin-flop field is reduced compared to the ammoniated analog $(\mathrm{N}{\mathrm{H}}_{3}){\mathrm{K}}_{3}{\mathrm{C}}_{60}$ on the account of reduced $\mathrm{C}_{60}{}^{3\ensuremath{-}}\text{\ensuremath{-}}\mathrm{C}_{60}{}^{3\ensuremath{-}}$ exchange interactions. Differences in the level of the anisotropic expansion between $\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{2}$ and $\mathrm{N}{\mathrm{H}}_{3}$ cointercalated fullerides are likely to be responsible for the differences in the electronic structure between the two systems and ultimately may account for the reduced N\'eel temperature in $(\mathrm{C}{\mathrm{H}}_{3}\mathrm{N}{\mathrm{H}}_{2}){\mathrm{K}}_{3}{\mathrm{C}}_{60}$.

Magnetic phase inLaFe11.4Al1.6with very low interstitial carbon content

${\mathrm{NaZn}}_{13}$-type ${\mathrm{LaFe}}_{11.4}{\mathrm{Al}}_{1.6}{\mathrm{C}}_{\ensuremath{\delta}}$ $(\ensuremath{\delta}=0\ensuremath{-}0.08)$ compounds were prepared by arc melting. At low temperature, the ground state is antiferromagnetic and ferromagnetic for $\ensuremath{\delta}=0$ and 0.04, respectively. Although the ground state remains antiferromagnetic for $\ensuremath{\delta}=0.02$, the ferromagnetic state can be induced by a magnetic field at about $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. During the transition between antiferromagnetic and ferromagnetic states, the magnetization exhibits a sharp discontinuity, indicating the formation of a homogeneous phase by intersitial carbon in spite of the very small $\ensuremath{\delta}$. The transition from low-temperature antiferromagnetic state (AFI) to ferromagnetic state induced by a magnetic field is irreversible. On the contrary, the transition from high-temperature antiferromagnetic state (AFII) to ferromagnetic state is reversible. Considering the magnetization behaviors and the variation of heat capacity with temperature, we can deduce that the AFI is different from the AFII.