Antiferromagnetic Transition Temperature Research Articles

Multiple charge density wave transitions in the antiferromagnetsRNiC2(R=Gd,Tb)

X-ray scattering and electrical resistivity measurements were performed on ${\mathrm{GdNiC}}_{2}$ and ${\mathrm{TbNiC}}_{2}$. We found a set of satellite peaks characterized by ${\mathbit{q}}_{1}=(0.5,\ensuremath{\eta},0)$ below ${T}_{1}$, at which the resistivity shows a sharp inflection, suggesting the charge density wave (CDW) formation. The value of $\ensuremath{\eta}$ decreases with decreasing temperature below ${T}_{1}$, and then a transition to a commensurate phase with ${\mathbit{q}}_{1\mathrm{C}}=(0.5,0.5,0)$ takes place. The diffuse scattering observed above ${T}_{1}$ indicates the presence of soft phonon modes associated with CDW instabilities at ${\mathbit{q}}_{1}$ and ${\mathbit{q}}_{2}=(0.5,0.5,0.5)$. The long-range order given by ${\mathbit{q}}_{2}$ is developed in addition to that given by ${\mathbit{q}}_{1\mathrm{C}}$ in ${\mathrm{TbNiC}}_{2}$, while the short-range correlation with ${\mathbit{q}}_{2}$ persists even at 6 K in ${\mathrm{GdNiC}}_{2}$. The amplitude of the ${\mathbit{q}}_{1\mathrm{C}}$ lattice modulation is anomalously reduced below an antiferromagnetic transition temperature ${T}_{\mathrm{N}}$ in ${\mathrm{GdNiC}}_{2}$. In contrast, the ${\mathbit{q}}_{2}$ order vanishes below ${T}_{\mathrm{N}}$ in ${\mathrm{TbNiC}}_{2}$. We demonstrate that $R{\mathrm{NiC}}_{2}$ ($R$ = rare earth) compounds exhibit similarities with respect to their CDW phenomena, and discuss the effects of magnetic transitions on CDWs. We offer a possible displacement pattern of the modulated structure characterized by ${\mathbit{q}}_{1\mathrm{C}}$ and ${\mathbit{q}}_{2}$ in terms of frustration.

Dielectric properties and specific heat of Bi1–x Sm x FeO3 multiferroics

The specific heat and dielectric permittivity of Bi1–xSmxFeO3 (x = 0–0.30) multiferroics have been studied in the temperature range of 300–800 K. A slight substitution of bismuth with samarium is established to cause a considerable shift in the antiferromagnetic phase transition temperature and to an increase in the specific heat over a wide temperature range. Other anomalies typical of phase transitions have been found in the temperature dependences of the specific heat and dielectric permittivity for the compounds with x = 0.10 and 0.15 at T ≈ 735 and 495 K, respectively. The results of the studies of the specific heat have been discussed together with the data of the structural investigations.

Structural and antiferromagnetic properties ofBa(Fe1−x−yCoxRhy)2As2compounds

We present a systematic investigation of the electrical, structural, and antiferromagnetic properties for the series of Ba(Fe$_{1-x-y}$Co$_{x}$Rh$_{y}$)$_{2}$As$_{2}$ compounds with fixed $x \approx$ 0.027 and $ 0 \leq y \leq 0.035$. We compare our results for the Co-Rh doped Ba(Fe$_{1-x-y}$Co$_{x}$Rh$_{y}$)$_{2}$As$_{2}$ compounds with the Co doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ compounds. We demonstrate that the electrical, structural, antiferromangetic, and superconducting properties of the Co-Rh doped compounds are similar to the properties of the Co doped compounds. We find that the overall behaviors of Ba(Fe$_{1-x-y}$Co$_{x}$Rh$_{y}$)$_{2}$As$_{2}$ and Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ compounds are very similar when the total number of extra electrons per Fe/$TM$ ($TM$ = transition metal) site is considered, which is consistent with the rigid band model. Despite the similarity, we find that the details of the transitions, for example, the temperature difference between the structural and antiferromagnetic transition temperatures and the incommensurability of the antiferromangetic peaks, are different between Ba(Fe$_{1-x-y}$Co$_{x}$Rh$_{y}$)$_{2}$As$_{2}$ and Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ compounds.

Robustness of Antiferromagnetism and Pyroelectricity in AgCr1–xRhxS2

The effect of the rhodium for chromium substitution in AgCr1–xRhxS2 on the magnetic and dielectric properties has been studied in the 0.0 ≤ x ≤ 0.5 range. It is found that the antiferromagnetic transition temperature TN decreases as x increases by about −0.9 K per %Rh up to x = 0.30. Consequently, the peak temperature in the real part of the dielectric permittivity, associated with TN, is observed to decrease by the same amount. Moreover, the electrical polarization magnitude induced below TN in this pyroelectric sulfide tends to decrease with x increasing. This is consistent with the structural study showing the random character of the substitution, which creates both magnetic dilution and more symmetric local structure. These two factors are expected to hinder the magnetoelastic effect supposed to be responsible for the electrical polarization change in AgCrS2 at TN.

Control of antiferromagnetic domain distribution via polarization-dependent optical annealing.

The absence of net magnetization inside antiferromagnetic domains has made the control of their spatial distribution quite challenging. Here we experimentally demonstrate an optical method for controlling antiferromagnetic domain distributions in MnF2. Reduced crystalline symmetry can couple an order parameter with non-conjugate external stimuli. In the case of MnF2, time-reversal symmetry is macroscopically broken reflecting the different orientations of the two magnetic sublattices. Thus, it exhibits different absorption coefficients between two orthogonal linear polarizations below its antiferromagnetic transition temperature under an external magnetic field. Illumination with linearly polarized laser light under this condition selectively destructs the formation of a particular antiferromagnetic order via heating. As a result, the other antiferromagnetic order is favoured inside the laser spot, achieving spatially localized selection of an antiferromagnetic order. Applications to control of interface states at antiferromagnetic domain boundaries, exchange bias and control of spin currents are expected.

Tight-binding model study of substrate induced pseudo-spin polarization and magnetism in mono-layer graphene

We present here a tight-binding model study of generation of magnetism and pseudo-spin polarization in monolayer graphene arising due to substrate, impurity and Coulomb correlation effects. The model Hamiltonian contains the first-, second- and third-nearest-neighbor hopping integrals for π electrons of graphene besides substrate induced gap, impurity interactions and Coulomb correlation of electrons. The Hubbard type Coulomb interactions present in both the sub-lattices A and B are treated within the mean-field approximation. The electronic Green's functions are calculated by using Zubarev's technique and hence the electron occupancies of both sub-lattices are calculated for up and down spins separately. These four temperature dependent occupancies are calculated numerically and self-consistently. Then we have calculated the temperature dependent pseudo-spin polarization, ferromagnetic and anti-ferromagnetic magnetizations. We observe that there exists pseudo-spin polarization for lower Coulomb energy, u<2.2t1 and pseudo-spin polarization is enhanced with substrate induced gap and impurity effect. For larger Coulomb energy u>2.5t1, there exists pseudo-spin polarization (p); while ferromagnetic (m) and antiferromagnetic (pm) magnetizations exhibit oscillatory behavior. With increase of the substrate induced gap, the ferromagnetic and antiferromagnetic transition temperatures are enhanced with increase of the substrate induced gap; while polarization (p) is enhanced in magnitude only.

Spin dynamics, electronic, and thermal transport properties of two-dimensional CrPS4 single crystal

2-Dimensional (2D) CrPS4 single crystals have been grown by the chemical vapor transport method. The crystallographic, magnetic, electronic, and thermal transport properties of the single crystals were investigated by the room-temperature X-ray diffraction, electrical resistivity ρ(T), specific heat CP(T), and the electronic spin response (ESR) measurements. CrPS4 crystals crystallize into a monoclinic structure. The electrical resistivity ρ(T) shows a semiconducting behavior with an energy gap Ea = 0.166 eV. The antiferromagnetic transition temperature is about TN = 36 K. The spin flipping induced by the applied magnetic field is observed along the c axis. The magnetic phase diagram of CrPS4 single crystal has been discussed. The extracted magnetic entropy at TN is about 10.8 J/mol K, which is consistent with the theoretical value R ln(2S + 1) for S = 3/2 of the Cr3+ ion. Based on the mean-field theory, the magnetic exchange constants J1 and Jc corresponding to the interactions of the intralayer and between layers are about 0.143 meV and −0.955 meV are obtained based on the fitting of the susceptibility above TN, which agree with the results obtained from the ESR measurements. With the help of the strain for tuning the magnetic properties, monolayer CrPS4 may be a promising candidate to explore 2D magnetic semiconductors.

Landau-Ginzburg description of anomalous properties of novel room temperature multiferroics Pb(Fe1/2Ta1/2)x(Zr0.53Ti0.47)1-xO3 and Pb(Fe1/2Nb1/2)x(Zr0.53Ti0.47)1−xO3

Landau-Ginzburg thermodynamic formalism is used for the description of the anomalous ferroelectric, ferromagnetic, and magnetoelectric properties of Pb(Fe1/2Ta1/2)x(Zr0.53Ti0.47)1−xO3 and Pb(Fe1/2Nb1/2)x(Zr0.53Ti0.47)1−xO3 micro-ceramics. We calculated temperature, composition, and external field dependences of ferroelectric, ferromagnetic, and antiferromagnetic phases transition temperatures, remanent polarization, magnetization, hysteresis loops, dielectric permittivity, and magnetoelectric coupling. Special attention was paid to the comparison of developed theory with experiments. It appeared possible to describe adequately main experimental results including a reasonable agreement between the shape of calculated and measured hysteresis loops and remnant polarization. Since Landau-Ginzburg thermodynamic formalism appertains to single domain properties of a ferroic, we did not aim to describe quantitatively the coercive field under the presence of realistic poly-domain switching. Information about linear and nonlinear magnetoelectric coupling coefficients was extracted from the experimental data. From the fitting of experimental data with theoretical formula, we obtained the composition dependence of Curie-Weiss constant that is known to be inversely proportional to harmonic (linear) dielectric stiffness, as well as the strong nonlinear dependence of anharmonic parameters in free energy. Keeping in mind the essential influence of these parameters on multiferroic properties, the obtained results open the way to govern practically all the material properties with the help of suitable composition choice. A forecast of the strong enough influence of antiferrodistortive order parameter on the transition temperatures and so on the phase diagrams and properties of multiferroics are made on the basis of the developed theory.

Iridium containing honeycomb Delafossites by topotactic cation exchange.

We report the structure and magnetic properties of two new iridium-based honeycomb Delafossite compounds, Cu3NaIr2O6 and Cu3LiIr2O6, formed by a topotactic cation exchange reaction. The starting materials Na2IrO3 and Li2IrO3, which are based on layers of IrO6 octahedra in a honeycomb lattice separated by layers of alkali ions, are transformed to the title compounds by a topotactic exchange reaction through heating with CuCl below 450 °C; higher temperature reactions cause decomposition. The new compounds display dramatically different magnetic behavior from their parent compounds - Cu3NaIr2O6 has a ferromagnetic like magnetic transition at 10 K, while Cu3LiIr2O6 retains the antiferromagnetic transition temperature of its parent compound but displays significantly stronger dominance of antiferromagnetic coupling between spins. These results reveal that a surprising difference in the magnetic interactions between the magnetic Ir ions has been induced by a change in the non-magnetic interlayer species. A combination of neutron and X-ray powder diffraction is used for the structure refinement of Cu3NaIr2O6 and both compounds are compared to their parent materials.

Thermodynamic properties of superconducting and non-superconducting Pr2Ba4Cu7O[formula omitted] compounds with metallic double chains

To examine the thermodynamic properties of Pr2Ba4Cu7O15-δ compounds with metallic CuO double chains, we measured the specific heats of superconducting and non-superconducting polycrystalline samples at low temperatures (1.8–40 K) under various magnetic fields (up to 9 T). In the as-sintered non-superconducting sample, a λ-like enhancement in the specific heat measurement appeared near the antiferromagnetic transition temperature TN=17 K. In contrast, the reduced superconducting sample with Tcon=26.5 K exhibited no obvious superconducting anomaly in its specific heat data, but a Schottky-like broad maximum appeared at low temperatures. The Schottky-like anomaly was attributed to low-lying quasi-triplet splitting of Pr3+ ions under the crystal field effect.

Photoexcited d ‐electron dynamics in transition metal oxide MnO studied by optical pump‐THz probe measurements

AbstractOptical pump‐THz probe (OPTP) measurements allow us to investigate the dynamic response of photoconductivity in materials by observing the transient change of THz transmittance after optical excitation. In this study, the THz response of d ‐electrons in various photoexcited states in MnO was investigated by pump‐energy‐tunable OPTP measurements to elucidate the photoexcited d ‐electron dynamics in a transition metal oxide. At room temperature, photoexcited d ‐electrons induced by a d ‐d transition to the lowest excited d ‐state showed the longest relaxation time. The relaxation time decreased drastically below the antiferromagnetic transition temperature of 120 K. This decrease was accompanied by the emergence of magnetic‐excitation‐assisted photoluminescence from a self‐trapped exciton (STE) state. This suggests that photoexcited d ‐electrons relax to the STE state below the transition temperature. These findings shed new light on the photoexcited d ‐electron dynamics, contrasting with the behavior of photoexcited carriers generated in the upper excited d ‐states that have been shown in a previous study to relax into trap states through carrier scattering. (© 2016 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Magnetic and electronic properties of La0.7Ca0.3MnO3/h-YMnO3 bilayer

La0.7Ca0.3MnO3/h-YMnO3 (h ∼ hexagonal) bilayer thin films are grown using pulsed laser deposition technique and magnetic/band-offset measurements are performed to explore the magnetic/electronic properties. The bilayer displays the signatures of the ferromagnetic (FM) La0.7Ca0.3MnO3 layer and antiferromagnetic (AFM) h-YMnO3 layer. The uncompensated spins at the interface of FM/AFM layers caused exchange bias effect. The vertical and horizontal shift in field cooled isothermal magnetization curves below the AFM transition temperature shows dependence on the sign of the cooling magnetic field. Type-II band alignment at the interface of bilayer is realized through valence band offset and conduction band offset values estimated from the photoemission results.

Field-Induced Structural Transition in the Bond Frustrated Spinel ZnCr2Se4**Supported by the National Basic Research Program of China under Grant No 2011CBA00111, and the National Natural Science Foundation of China under Grant No U1332143.

The effect of an external magnetic field on the structural and magnetic properties of bond frustrated ZnCr2Se4 at low temperatures is investigated using magnetization, dielectric constants and thermal conductivity experiments. With an increase in the magnetic field H, the antiferromagnetic transition temperature TN is observed to shift progressively toward lower temperatures. The corresponding high temperature cubic (Fd3̄m) to low temperature tetragonal (I41amd) structural transition is tuned simultaneously due to the inherent strong spin-lattice coupling. In the antiferromagnetic phase, an anomaly at HC2 defined as a steep downward peak in the derivative of the M–H curve is clearly drawn. It is found that TN versus H and HC2 versus T exhibit a consistent tendency, indicative of a field-induced tetragonal (I41amd) to cubic (Fd3̄m) structural transition. The transition is further substantiated by the field-dependent dielectric constant and thermal conductivity measurements. We modify the T–H phase diagram, highlighting the coexistence of the paramagnetic state and ferromagnetic clusters between 100 K and TN.

Effects of stoichiometry and substitution in quasi-one-dimensional iron chalcogenideBaFe2S3

The effects of off-stoichiometry and elemental substitution on electronic properties of iron-based ladder compound ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$ are investigated. Resistivity and magnetization are revealed to be quite sensitive to the stoichiometry of Fe atoms, and 10% deficiency at Fe sites reduces the antiferromagnetic transition temperature by 40 K. The antiferromagnetic transition temperature decreases even faster and collapses to zero with hole doping through 10% K substitution at the Ba site, while the antiferromagnetic ordering phase remains with electron doping through 20% Co substitution at the Fe site. Such electron-hole asymmetry is opposite to two-dimensional iron-based superconductors, and can be explained on the basis of both itinerant and localized electronic pictures.

Observation of a New Ordered Phase in the Kondo Semiconductor CeOs4Sb12

We report the results of high-resolution dilatometry and dc magnetization measurements of a small single crystal of CeOs4Sb12, which exhibits an anomalous ordered state. Both linear thermal expansion coefficient and magnetization results reveal the presence of a new phase (phase A) transition at TA = 0.87 K, slightly below the antiferromagnetic (AFM) transition temperature TN = 1.05 K. In addition, the magnetostriction and magnetization results provide unambiguous thermodynamic evidence of the presence of a field-induced phase transition to a high-field ordered phase (phase B), as previously suggested by neutron scattering experiments. The resulting magnetic field–temperature (H–T) phase diagram of CeOs4Sb12 consists of three ordered phases: A, B, and AFM phases. From a thermodynamic point of view, it is likely that the AFM and high-field ordered states coexist within the A phase, and that the B phase is of nonmagnetic origin.

Observation of ferroelectricity and magnetoelectric coupling in Mn-doped orthochromite DyCr0.5Mn0.5O3

We investigate the structural, magnetic, and magnetoelectric properties of DyCr0.5Mn0.5O3. DyCr0.5Mn0.5O3 exhibits ferroelectricity at the paramagnetic–antiferromagnetic transition temperature, i.e., TN = 74 K, demonstrating the polarization is driven by magnetic order. Furthermore, the ferroelectric polarization can be substantially influenced by applied magnetic fields indicating a strong magnetoelectric effect in DyCr0.5Mn0.5O3. It is demonstrated that the ferroelectricity in DyCr0.5Mn0.5O3 may stem from the spin-phonon coupling of Dy and Mn/Cr ions as evidenced by the temperature dependence of Raman phonon modes. The results manifest that DyCr0.5Mn0.5O3 is a potential candidate for magnetoelectric device applications.

Electronic structure and lattice dynamics of LixCoO2 single crystals

Spectroscopic ellipsometry and Raman scattering measurements of single-crystal LixCoO2 (x = 0.33, 0.43, 0.50, 0.53, 0.72, and 0.87) are reported. The room temperature optical absorption spectra for x values in the range of 0.33–0.72 exhibit three bands near 1.60, 3.35, and 5.20 eV. On the basis of first-principles calculations, the observed optical excitations were appropriately assigned. The charge-transfer absorption bands shift to higher energies in Li0.87CoO2 because of symmetry-breaking-induced distortions of the hybridized Co-O orbitals with shifted oxygen 2p states. Furthermore, two Raman-active phonon modes, which display Eg and A1g symmetry, are sensitive to lithium doping. Upon cooling across 200 K, which is the antiferromagnetic phase transition temperature of Li0.50CoO2, large splitting of the Eg mode and a discontinuous change in the frequency of the A1g mode were observed. These results highlight the importance of spin-phonon coupling in Li0.50CoO2.

Magnetic and dielectric properties of chain cuprate Li

AbstractAbstractauthoren We propose a microscopic model in order to study the multiferroic properties of LiZrCuO at low temperatures taking into account the competing nearest and next‐nearest magnetic interactions, frustration, and a linear magnetoelectric coupling. To understand the experimental observation an anti‐ferroelectric interaction between the CuO chains is included. The magnetization exhibits a peak at the antiferromagnetic transition temperature , whereas the dielectric function offers no anomaly at that temperature, however can be governed by an external magnetic field . It decreases with increasing magnetic field . Our analysis provides no indication about a glass‐like order of Li(1) discussed controversially.

Multiple magnetic transitions, dynamical magnetic liquid and magnetic glass in La1−x−yPryCaxMnO3 (x≈0.42, y≈0.40) thin films: A thickness dependent study

The influence of substrate induced strain and its relaxation on the evolution of the multiple magnetic transitions and ensuing modifications in the degree of phase separation, the nature of the dynamical magnetic liquid, the randomly frozen glass and insulator–metal transitions have been investigated in single crystalline La1−x−yPryCaxMnO3 (x≈0.42, y≈0.40) in t~20–140nm thick films deposited on LaAlO3 (001) substrates. The ferromagnetic (FM) transition temperature (TC) first decreases as the film thickness is increased from t~20nm to t~60nm and then increases with increasing film thickness. In contrast the charge ordering (CO), antiferromagnetic (AFM) and glass transition temperatures shift towards higher values with increasing film thickness. The field cooled cooling (FCC) and field cooled warming (FCW) magnetization (M–T) of films having t≥60nm shows pronounced hysteresis and ΔTC=TCFCW−TCFCC decreases concomitantly from 46K to 35K as the thickness increases from ~60 to ~140nm. The thinnest film shows insulator to metal transitions (IMT) only at magnetic field H>40kOe. Films with t≥TC show sharp hysteretic IMT, with ΔTIM=TIMW−TIMC decreasing from ~70K to ~50K as the thickness increases from ~60nm to ~140nm. Such strong hysteresis is a characteristic of first order phase transition and also a signature of magnetic liquid like phase created by the magnetic frustration created by the delicate balance between FM and AFM/CO phases. The H induced AFM/CO to FM transition reduces ΔTIM and at higher fields the phase transition appears akin to the second order. The observed difference in the magnetic and transport properties have been explained in terms of the substrate induced strain at lower film thickness and its relaxation at higher thickness.

Pr- and La-doping effects on the magnetic anisotropy in the antiferromagnetic phase of Kondo semiconductorCeRu2Al10

We have studied the Pr- and La-doping effects on the magnetic anisotropy in the antiferro-magnetic (AFM) phase of ${\mathrm{CeRu}}_{2}{\mathrm{Al}}_{10}$. The crystalline electric field (CEF) splitting in ${\mathrm{PrRu}}_{2}{\mathrm{Al}}_{10}$ was found to be as large as $\ensuremath{\sim}800$ K with a singlet ground state. In ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ru}}_{2}{\mathrm{Al}}_{10}$, the CEF level scheme of the Pr ion is not changed with $x$. The AFM moment (${m}_{\mathrm{AF}}$) is rotated from $c$ to $b$ axis in both systems at ${x}_{c}^{\mathrm{sr}}\ensuremath{\sim}0.03$ and $\ensuremath{\sim}0.07$ for Ln=Pr and La, respectively. As the ionic radius of La and Pr is larger and smaller than that of Ce, respectively, these results indicate that the chemical pressure effect is not associated with the rotation of ${m}_{\mathrm{AF}}$, but is caused by the suppression of the $c\ensuremath{-}f$ hybridization originating from the decrease of $4f$ electrons of Ce ions by Ce-site substitution. Since a small amount of Pr or La doping changes easily the magnetization easy axis of all the moments on Ce sites, the origin of the magnetic anisotropy is not the local single ion effect but the bandlike effect through the anisotropic $c\ensuremath{-}f$ hybridization. The magnetic phase diagrams of ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Ln}}_{x}{\mathrm{Ru}}_{2}{\mathrm{Al}}_{10}$ indicate that above ${x}_{c}^{\mathrm{sr}}$, the AFM order with ${m}_{\mathrm{AF}}\ensuremath{\parallel}b$ continues to exist up to ${x}_{c}$, which is $\ensuremath{\sim}0.4$ and $\ensuremath{\sim}0.6$ in Ln=Pr and Ln=La, respectively. This indicates that even in the sample with an AFM transition temperature (${T}_{0}$) near ${x}_{c}$, the anisotropic $c\ensuremath{-}f$ hybridization dominates the AFM order. A large positive transverse magnetoresistance is seen below ${T}_{0}$, but a very small one above ${T}_{0}$. Together with the results of Hall resistivity and the observation of Shubnikov--de Haas oscillation, we propose that there exist large Fermi surfaces above ${T}_{0}$ and small ones below ${T}_{0}$. A gap is opened by the AFM order on almost the area of the large Fermi surface, and small Fermi surfaces are constructed below ${T}_{0}$, although we do not know the mechanism, which might be specific to the AFM order in Kondo semiconductors. The largest suppression of the magnetic scattering below ${T}_{0}$ is observed for the current $I\ensuremath{\parallel}a$ and the smallest one for $I\ensuremath{\parallel}b$. This anisotropy may be associated with the anisotropic $c\ensuremath{-}f$ hybridization, which may contribute to the anisotropic magnetic scattering of the conduction electron below ${T}_{0}$.