Short-range Ferromagnetic Correlations Research Articles

Magnetic short-range order and Griffiths-like phase in the S-1/2 spin-dimer ferromagnet BaCu(SeO3)2

The polycrystals of S = 1/2 spin-dimer compound BaCu(SeO3)2 have been synthesized by using solid-state reaction. Without long-range order, the magnetic susceptibility χ(T) curve presents two anomalies at T1 = 5 K and TG = 58 K due to antiferromagnetic (AFM) short-range order and short-range ferromagnetic (FM) correlations, respectively, which is further supported by the specific heat and electron spin resonance data. The high-temperature χ(T) curve follows the Curie-Weiss law with θCW = 11.3 K, showing the dominant intradimer FM interaction. The magnetization M(H) curve at 2 K is characteristic of an AFM-FM transition at 0.4 T, suggesting the weak interdimer AFM couplings. The density function theory calculations and quantum Monte Carlo simulations allow an estimation of the intradimer and interdimer exchanges with J1/kB = 39.09 K, J2/kB = −1.51 K, and J3/kB = −1.68 K. Interestingly, the χ−1(T) curve within θCW <T <TG deviates downward from paramagnetic Curie-Weiss behavior, mimicking the Griffiths-like phase. This is further supported by the ac magnetic susceptibility and magnetic relaxation measurements. The competing intradimer FM and interdimer AFM exchanges with a subtle ratio between them are proposed to be responsible for the formation of Griffiths-like phase.

Anomalous metallic conductivity and short-range ferromagnetic correlation in high-pressure synthesized pyrochlore

Iridates show fascinating properties due to the unpredictable ground states of their Ir cations. Generally, Ir5+(5d 4) systems exhibit insulating nonmagnetic states owing to the strong spin-orbit coupling (SOC). Herein a new pyrochlore iridate Hg2Ir2O7 with an Ir5+ charge state synthesized by high-pressure technique is reported. Hg2Ir2O7 crystallizes in the typical cubic pyrochlore crystal structure. The Ir5+ valence state is evidenced by the XAS spectrum. Surprisingly, Hg2Ir2O7 displays short-range ferromagnetic correlations at low temperatures as evidenced by S-shape field-dependent magnetization curves, positive magnetoresistance, and magnetic excitations in specific heat. Furthermore, it also shows metallic conduction and large electron component of specific heat. These results all indicate that Ir5+ in Hg2Ir2O7 deviates from SOC-dominated insulating nonmagnetic states.

Robust Crystal Phase Separation with Distinct Charge, Orbital, and Spin Orders in AgMn7O12.

An AA'3B4O12-type A-site-ordered quadruple perovskite oxide AgMn7O12 was prepared by high-pressure and high-temperature methods. At room temperature, the compound crystallizes into a cubic Im3̅ symmetry with a charge distribution of AgMn33+Mn43.5+O12. With the temperature decreasing to TCO,OO ≈ 180 K, the compound undergoes a structural phase transition toward a monoclinic C2/m symmetry, giving rise to a B-site charge- and orbital-ordered AgMn33+Mn23+Mn24+O12 phase. Moreover, this charge-/orbital-ordered main phase coexists with the initial cubic AgMn33+Mn43.5+O12 phase in the wide temperature range we measured. The charge-/orbital-ordered phase shows two antiferromagnetic phase transitions near 125 and 90 K, respectively. Short-range ferromagnetic correlations are found to occur for the initial B-site mixed cubic phase around 35 K. Because of the robust phase separation, considerable magnetoresistance effects are observed below TCO,OO in AgMn7O12.

Magnetic properties of a quasi-one-dimensional antiferromagnet Eu2BiS4

We have synthesized c-axis oriented polycrystals of a Eu-based orthorhombic compound Eu2BiS4 and measured transport, magnetic, and thermal properties down to 1.8 K. The crystal structure of Eu2BiS4 was confirmed by powder x-ray diffraction to be orthorhombic CaFe2O4-type with one-dimensional chains of Eu2+ ions along the c-axis. The electrical resistivity ρ increases on cooling from 300 K, which shows that this compound is a semiconductor. Both magnetic susceptibility χ and specific heat C exhibit an obvious peak at TN = 2.5 K, indicating an antiferromagnetic order. Anisotropic behaviors of isothermal magnetization M and χ below TN suggest that the magnetic moments align along the c-axis. Moreover, a tail appears in C(T) in the range of TN < T < 8 K, where χ(T) is reproduced by that calculated with a one-dimensional ferromagnetic Heisenberg model. These behaviors in C(T) and χ(T) are attributed to developed short-range ferromagnetic correlations in the Eu2+ chains.

Observation of Griffith like phase and large magnetocaloric effect in nanocrystalline La0.7Ag0.2Bi0.1MnO3

In this paper, we present structural, magnetic, magnetocaloric, and critical study of perovskite La0.7Ag0.2Bi0.1MnO3 (LABMO) nanocrystalline compound synthesized by the sol–gel method. Temperature dependent magnetization measurements reveal the significant suppression of ferromagnetism in the LABMO sample upon Bi-doping on a La-site. The downturn in inverse magnetic susceptibility (χ−1) observed just above TC (236 K) in the paramagnetic regime corroborates the presence of short-range ferromagnetic correlations, which is the characteristic of the Griffith like phase below 270 K. The deviation from linear paramagnetic behavior in χ−1 implies the strong Griffith singularity. Furthermore, we have employed an integrated Maxwell's thermodynamic relation numerically and used isothermal magnetization data to determine the change in magnetic entropy at various magnetic fields. For a magnetic field change of 5 T, the value of maximum magnetic entropy change is found to be ∼6 J kg−1 K−1. We have also explored the critical behavior of the LABMO sample at transition temperatures using different theoretical models. The value of exponents β, γ, and δ does not fall into any known universality class. Despite this, the scaling relations show that interactions are renormalized around the Curie temperature (TC). The exponent n ≤ 2 extracted from field dependency on the magnetic entropy change confirms the second-order phase transition in LABMO.

Magnetic phase diagram and cluster glasslike properties of stage-1 graphite-intercalated FeCl3

We present a comprehensive investigation of the magnetic properties of stage-1 graphite intercalated FeCl$_3$ using a combination of DC and AC magnetic susceptibility, thermoremanent magnetization and field dependent magnetization measurements. This van der Waals system, with a centrosymmetric honeycomb lattice, combines frustration and disorder, due to intercalation, and may be hosting topologically non-trivial magnetic phases. Our study identifies two magnetic phase transitions at $T_{f1} \approx $ 4.2 K and at $T_{f2} \approx $ 2.7 K. We find that the paramagnetic state, for $T > T_{f1}$, is dominated by short-range ferromagnetic correlations. These build up well above $T_{f1}$ and lead to a significant change in magnetic entropy, which reaches $\Delta S_M^{Pk} = $ -5.52 J kg$^{-1}$ K$^{-1}$ at 7 T. Between $T_{f1}$ and $T_{f2}$ we observe slow spin dynamics characteristic of a cluster glass-like state, whereas for $T$ $ < $ $T_{f2}$ our results indicate the onset of a low temperature long range ordered state. The analysis of the experimental results leads to a complex phase diagram, which may serve as a reference for future investigations searching for topological non-trivial phases in this system.

Magnetic, magnetocaloric and magnetoresistive properties of Tb2Co3Ge5 compound

In this work, the magnetic, magnetocaloric, magnetoresistance, thermodynamic, and transport properties of Tb2Co3Ge5 compound have been studied in detail. The magnetic state of the compound is antiferromagnetic with successive transitions at T = 31.5 K, 19.4 K, and 11 K. Besides, it exhibits the features of metamagnetic transitions. Due to the influence of metamagnetic transitions, a cross over between positive and negative magnetoresistance takes place. At high temperatures, the electrical resistivity obeys the Bloch-Grüniesen-Mott relation denoting the metallicity of the compound. The compound show both conventional and inverse magnetocaloric behaviors. Additionally, an enhanced magnetocaloric effect is evidenced in the paramagnetic region due to the presence of short-range ferromagnetic correlations which is subsequently verified in the electrical resistivity studies.

Extended “orbital molecules” and magnetic phase separation in Bi0.68Ca0.32MnO3

The low-temperature structure of ${\mathrm{Bi}}_{0.68}{\mathrm{Ca}}_{0.32}{\mathrm{MnO}}_{3}$ has been solved from electron and neutron diffraction data. The quantitative simultaneous refinement indicates an ordering of the Mn cations in a ``stripe/chess''-like pattern. The ordering is accompanied by the formation of short Mn---Mn distances and the rearrangement of the Mn---O bonds indicating the development of complex extended ``orbital molecules.'' The primary order parameter breaks inversion symmetry and allows the generation of a spontaneous electrical polarization as the secondary order parameter. The neutron data at low temperature indicate the coexistence of a pseudo-CE long-range-ordered structure with a strongly reduced moment and short-range ferromagnetic correlations. These results indicate an intricate competition between the charge, orbital, and magnetic degrees of freedom and the ${\mathrm{Bi}}^{3+}$ stereoactivity in this manganite system.

Magnetocaloric and magnetoresistance properties of reentrant spin glass Tb2Ni0.94Si3.2 alloy

Magnetic, magnetocaloric, and magnetoresistance properties of polycrystalline Tb2Ni0.94Si3.2 alloy have been investigated. Magnetic susceptibilities (AC and DC), remanent magnetization and the heat capacity studies of Tb2Ni0.94Si3.2 provide evidence for the spin glass behavior below the temperature, Tf = 5.2 K. At the Neel temperature, TN = 12.7 K, Tb2Ni0.94Si3.2 alloy orders antiferromagnetically. The presence of metamagnetic transition is observed in isothermal magnetization and magnetoresistance studies. Also, a magnetoresistance of 22% is exhibited by the alloy at temperature T = 4 K in an applied magnetic field of 9 T. A magnetic entropy change of 12 J/kg K with the relative cooling power of 504 J/kg for a magnetic field change of 9 T is observed in the studied alloy. Influence of spin fluctuations and short-range ferromagnetic correlations is reflected in magnetoresistance and relative cooling power. The magnetic and magnetoresistance properties make this alloy as a good magnetocaloric material with moderate magnetoresistance.

Electron spin resonance and ferromagnetic resonance spectroscopy in the high-field phase of the van der Waals magnet CrCl3

We report a comprehensive high-field/high-frequency electron spin resonance (ESR) study on single crystals of the van der Waals magnet CrCl$_3$. This material, although being known for quite a while, has received recent significant attention in a context of the use of van der Waals magnets in novel spintronic devices. Temperature-dependent measurements of the resonance fields were performed between 4 and 175 K and with the external magnetic field applied parallel and perpendicular to the honeycomb planes of the crystal structure. These investigations reveal that the resonance line shifts from the paramagnetic resonance position already at temperatures well above the transition into a magnetically ordered state. Thereby the existence of ferromagnetic short-range correlations above the transition is established and the intrinsically two-dimensional nature of the magnetism in the title compound is proven. To study details of the magnetic anisotropies in the field-induced effectively ferromagnetic state at low temperatures, frequency-dependent ferromagnetic resonance (FMR) measurements were conducted at 4 K. The observed anisotropy between the two magnetic-field orientations is analyzed by means of numerical simulations based on a phenomenological theory of FMR. These simulations are in excellent agreement with measured data if the shape anisotropy of the studied crystal is taken into account, while the magnetocrystalline anisotropy is found to be negligible in CrCl$_3$. The absence of a significant intrinsic anisotropy thus renders this material as a practically ideal isotropic Heisenberg magnet.

Influence of Fe addition on the microstructure, magnetic properties and magnetocaloric behavior of Gd5Si1.7Ge2.3

In the present investigation, the effect of iron addition on the structural, magnetic and magnetocaloric properties of Gd5Si1.7Ge2.3-xFex alloys with x = 0–0.4 has been carried out. Structural analysis reveal that Gd5Si1.7Ge2.3-xFex compounds crystallize in a combination of monoclinic and orthorhombic structures at room temperature. With increase in Fe substitution up to x ≤ 0.3, the compounds stabilize in orthorhombic structure with Pbnm space group. First-order and second-order transitions are observed for low Fe concentrations. For x = 0.4, only a second-order transition is evident which is due to the stabilization of the orthorhombic structure. At lower fields, the presence of short-range ferromagnetic correlations indicate competing intra-layer and inter-layer magnetic interactions. The Curie temperature increases from 243 K to 287 K with increase in Fe concentration. The entropy change (-ΔSM) is found to be 10.1 J/kg K at 247 K, 11.3 J/kg K at 253 K, 7.9 J/kg K at 273 K and 7 J/kg K at 287 K for x = 0.1, 0.2, 0.3 and 0.4 respectively, for a field change of 50 kOe. Universal curve analysis corroborates the transition from first-order to second-order as evidenced by magnetization measurements. Tuning the transition temperature to near room temperature through the proper substitution of transition elements paves an interesting route towards the development of materials for room temperature refrigeration applications.

Tuning the Structural Transitions and Magnetocaloric Response in Gd5Si1.7Ge2.3 Alloys Through Cobalt Addition

Materials with a large magnetocaloric effect are of great significance for applications in thermomagnetic power generation and magnetic refrigeration. We report the effect of cobalt addition on the structural, magnetic, and magnetocaloric properties of Gd5 Si1.7 Ge2.3− x Co x with $x = 0$ , 0.1, 0.2, 0.3, and 0.4. Rietveld refinement of the X-ray diffraction patterns reveals that the Gd5 Si1.7 Ge2.3− x Co x compounds with $x\le 0.3$ crystallize in a mixed state of monoclinic and orthorhombic structures. With the increase in Co concentration, the compounds stabilize in orthorhombic structure with the Pbnm space group. Magnetization measurements indicate first-order and second-order transitions for lower Co concentrations ( $x\le 0.3$ ), while for the composition with $x=0.4$ , only a second-order transition is evident, which is due to the stabilization of the orthorhombic structure for higher compositions. At lower fields, short-range ferromagnetic correlations are apparent, which is attributed to the presence of competing intra-layer and inter-layer magnetic interactions. An enhancement in Curie temperature has been observed from 243 to 283 K with an entropy change ( $-\Delta S_{M}$ ) of 12.8 at 243 K, 14.5 at 262 K, 10.4 at 270 K, and 5.8 J/kg $\cdot \text{K}$ at 283 K for $x = 0.1$ , 0.2, 0.3, and 0.4, respectively, for a field change of 5 T. Universal curve analysis corroborates the transition of first order to second order as evidenced by magnetization measurements. The ability to tune the transition temperature to near room temperature through the proper addition of transition elements could provide an interesting pathway toward the development of materials for room temperature refrigeration applications.

Spin Seebeck effect in the layered ferromagnetic insulators CrSiTe3 and CrGeTe3

We have studied the longitudinal spin Seebeck effect (LSSE) in the layered ferromagnetic insulators CrSiTe$_3$ and CrGeTe$_3$ covered by Pt films in the measurement configuration where spin current traverses the ferromagnetic Cr layers. The LSSE response is clearly observed in the ferromagnetic phase and, in contrast to a standard LSSE magnet Y$_3$Fe$_5$O$_{12}$, persists above the critical temperatures in both CrSiTe$_3$/Pt and CrGeTe$_3$/Pt samples. With the help of a numerical calculation, we attribute the LSSE signals observed in the paramagnetic regime to exchange-dominated interlayer transport of in-plane paramagnetic moments reinforced by short-range ferromagnetic correlations and strong Zeeman effects.

Large magnetocaloric effect in ErCoSn driven by metamagnetic phase transition and short-range ferromagnetic correlations

Abstract Here, we report the results of structural, magnetic, thermal and magnetocaloric properties of ErCoSn compound. The compound undergoes an antiferromagnetic (AFM) ordering around TN = 4.6 K and magnetic correlation is observed above TN, up to T = 11 K. Below TN, a spin-glass state was observed for ErCoSn due to an apparent coexistence of ferromagnetic (FM) interaction with long-range AFM order. External applied magnetic field induces a first-order-like metamagnetic phase transition from AFM to FM state that is responsible for the large values of the maximum isothermal entropy change (− Δ M max = 16.5 J/kgK@50 kOe) and adiabatic temperature change (ΔTad = 8.2 K). Furthermore, short-range ferromagnetic correlation above TN contributes to the widening of −ΔSM and ΔTad peaks as well as the increase of the ErCoSn compound refrigerant capacity. The results indicate attractive properties of this compound for magnetic refrigeration at cryogenic temperatures.

Griffiths phase-like behavior and origin of spin-phonon interaction in Eu0.75Y0.25MnO3

Abstract The magnetic state and the spin-phonon coupling in the Eu0.75Y0.25MnO3 are investigated using temperature dependent magnetization, Raman spectroscopy and Mossbauer spectroscopy measurements. The magnetization data showed bifurcation in zero-field cooled and field-cooled warming curves and indicate two antiferromagnetic transitions at 47 K and 22 K. Short range ferromagnetic correlations are clearly observed well above the antiferromagnetic transition temperatures, which are attributed to the presence of Griffiths-like phase. The temperature dependent Raman spectroscopy measurements showed anomalous softening of phonon modes well above the antiferromagnetic ordering temperature of ∼47 K and also showed signatures of both the ordering temperatures. The Mossbauer spectra showed anomalous changes in hyperfine field as a function of temperature which match with the temperature at which B2g phonons showed softening.

Cluster-glass dynamics of the Griffiths phase in Tb5−xLaxSi2Ge2

The static magnetization and dynamic susceptibility responses of the cluster system within a Griffiths phase of the magnetocaloric compound ${\mathrm{Tb}}_{5\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Si}}_{2}{\mathrm{Ge}}_{2}$ ($x=0.075$) have been investigated. A novel cluster-glass state within the Griffiths phase is formed at a characteristic freezing temperature where short-range ferromagnetic correlations set in the paramagnetic regime. Ferromagneticlike correlations are built up at around 155 K, which suddenly become frozen at a lower temperature $\ensuremath{\sim}140\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, thus in analogy with a reentrant spin glass behavior. The ac susceptibility near the freezing temperature follows a critical slowing down process characterized by ${\ensuremath{\tau}}_{0}={10}^{\ensuremath{-}13}s$ and dynamic exponents $\mathrm{z}\ensuremath{\nu}\ensuremath{\sim}6$ and $\ensuremath{\beta}\ensuremath{\sim}0.4$, similar to well-known spin glass systems. The nonlinear ac susceptibility analysis shows clearly the existence of a transition associated to the reentrant behavior. The origin of the intermediate cluster-glass phase inside the Griffiths phase is proposed to be the result of a combination of short-ranged RKKY intralayer positive exchange interactions between rare-earth ${\mathrm{Tb}}^{3+}$ ions and antiferromagnetic exchange between adjacent interlayers involving Si and Ge atoms in connection to the ${\mathrm{Tb}}^{3+}$ atoms.

Magnetization sign reversal, exchange bias, and Griffiths-like phase in orthorhombic perovskite Pr2FeMnO6

Abstract The perovskite Pr2FeMnO6 has been synthesized by citrate-gel combustion method. The material has an orthorhombic crystal structure with Pbnm space group. The presence of Fe3+ and Mn3+ cations in the material are confirmed by XPS analysis. A magnetization sign reversal is observed for the material at low-temperatures in low applied magnetic fields. When the applied field or temperature is increased, the negative magnetization got diminished. Such a sign reversal of magnetization is observed for the material as a result of the interaction between components of paramagnetic Pr3+ and Mn3+ moments opposite to the component of the ferromagnetic Fe3+ moment or the anti-aligned Fe-rich regions to that of Mn-rich regions containing with Pr3+. Negative exchange bias is also exhibited by Pr2FeMnO6 below 200 K which can be ascribed to the Dzyaloshinskii-Moriya interaction and magnetic exchange anisotropy. A high-temperature magnetic transition at TC = 576 K is identified for the material from the thermomagnetic data. From the detailed analysis of the thermomagnetic data, the presence of ferromagnetic short-range correlations is observed far above the TC, which suggests a Griffiths-like phase with Griffiths temperature at TG = 681 K.

Theory of Electron Spin Resonance in Ferromagnetically Correlated Heavy Fermion Compounds

We studied the electron spin resonance (ESR) line width for localized moments within the framework of the Kondo lattice model. Only for a sufficiently small Kondo temperature can an ESR signal be observed for a Kondo impurity. On the other hand, for a Kondo lattice representing a heavy fermion compound, short-range ferromagnetic correlations (FM) between the localized moments are crucial to observe a signal. The spin relaxation rate (line width) and the static magnetic susceptibility are inversely proportional to each other. The FM enhance the susceptibility and hence reduce the line width. For most of the heavy fermion systems displaying an ESR signal, the FM order arises in the ab-plane from the strong lattice anisotropy. CeB6 is a heavy fermion compound with cubic symmetry having a &Gamma;8 ground-quartet. Four transitions are expected for individual Ce ions with a &Gamma;8 ground-multiplet, but only one has been observed. Antiferro-quadrupolar order (AFQ) arises below 4 K due to the orbital content of the &Gamma;8-quartet. We addressed the effects of the interplay of AFQ and FM on the ESR line width and the phase diagram. It is usually difficult to distinguish among ESR resonances due to localized moments and conducting heavy electron spins, especially for anisotropic Ce and Yb compounds. However, for CeB6, an itinerant picture within the AFQ phase is necessary to explain the electron spin resonances. The longitudinal magnetic susceptibility has a quasi-elastic central peak of line width 1/T1 and inelastic peaks for the absorption/emission of excitations. The latter are measured via inelastic neutron scattering (INS) and provide insights into the magnetic order. We briefly summarize some of the INS results for CeB6 in the context of the picture that emerged from the ESR experiments.

Strong spin-orbit effects in transition metal oxides with tetrahedral coordination

To prove that spin-orbit coupling can play a relevant role in determining the magnetic structure of transition metal oxides with tetrahedral coordination, we investigate the d1 Mott insulator KOsO4, combining density functional theory calculations and the exact diagonalization approach. We find that the interplay between crystal field, strong spin-orbit coupling, electronic correlations and structural distortions brings the system towards an antiferromagnetic phase, characterized by a non-vanishing orbital angular momentum and anisotropy among the in-plane and the out-of-plane antiferromagnetic correlations. We also show that, due to the peculiar interplay between spin-orbit coupling, Hund's coupling and hopping connectivity the system is on the verge of developing short range ferromagnetic correlations marked by strong directionality.

Pentavalent iridium pyrochlore Cd2Ir2O7 : A prototype material system for competing crystalline field and spin-orbit coupling

A new pyrochlore oxide $\mathrm{C}{\mathrm{d}}_{2}\mathrm{I}{\mathrm{r}}_{2}{\mathrm{O}}_{7}$ with an $\mathrm{I}{\mathrm{r}}^{5+}$ charge state was prepared by high-pressure techniques. Although strong spin-orbit coupling (SOC) dominates the electronic states in most iridates so that a SOC-Mott state is proposed in $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ir}{\mathrm{O}}_{4}$ in the assumption of an undistorted $\mathrm{Ir}{\mathrm{O}}_{6}$ octahedral crystalline field, the strongly distorted one in the current $\mathrm{C}{\mathrm{d}}_{2}\mathrm{I}{\mathrm{r}}_{2}{\mathrm{O}}_{7}$ exhibits a competing interaction with the SOC. Unexpected from a strong SOC limit, $\mathrm{C}{\mathrm{d}}_{2}\mathrm{I}{\mathrm{r}}_{2}{\mathrm{O}}_{7}$ deviates from a nonmagnetic and insulating $J=0$ ground state. It displays short-range ferromagnetic correlations and metallic electrical transport properties. First-principles calculations well reproduce the experimental observation, revealing the large mixture between the ${j}_{\mathrm{eff}}=1/2$ and ${j}_{\mathrm{eff}}=3/2$ bands near the Fermi surface due to the significant distortion of $\mathrm{Ir}{\mathrm{O}}_{6}$ octahedra. This work sheds light on the critical role of a noncubic crystalline field in electronic properties which has been ignored in past studies of $5d$-electron systems.