A-type Antiferromagnetic Research Articles

Magnetic properties of cobalt-based oxypnictide SmCoAsO

The magnetic properties of cobalt-based oxypnictides SmCoAsO are investigated by measuring magnetization, magnetoresistance and specific heat. The compound undergoes a ferromagnetic (FM) transition around Tc of 80 K, and a ferromagnetic to antiferromagnetic (AFM) transition below TN1 of about 45 K, and finally an AFM order of Sm ion at TN2 of 5.6 K. The weak FM order should originate from the itinerant 3d electrons of Co ions in the CoAs layers. We propose that the magnetic structure should be A-type AFM, which means that the FM order remains within the CoAs layer and the magnetic coupling between the CoAs layers becomes AFM below TN1 of 45 K. The AFM coupling between the CoAs layers should be very weak. A magnetic field µ0H of about 2 T may cause an AFM-FM metamagnetic transition. A rich magnetic phase diagram is established and the interplay between the 3d electrons of Co ions and 4f electrons of Sm ions is discussed.

4 d electronic and magnetic characteristics in postperovskite CaRuO3

The magnetic and electronic properties of postperovskite CaRuO3 are investigated by performing the first-principles calculation based on generalized gradient approximation plus parameter U method. A Mott–Hubbard insulating ground state with low spin Ru4+ is found, and the A-type antiferromagnetic (AFM) spin ordering is preferred. Ru4+ ions exhibit the spatially anisotropic exchange with the exchange in a-b plane weaker than that along c direction. The large Jahn–Teller distortion lowers the energy of dz2 orbital, leading to the stabilization of low spin Ru4+ and ferromagnetic (FM) orbital ordering. The transitions of metal-insulator and FM-AFM-FM depend on the Hubbard parameter U.

Orbital ordering in undoped manganites via a generalized Peierls instability

We study the ground-state orbital ordering of ${\text{LaMnO}}_{3}$, at weak electron-phonon coupling, when the spin state is A-type antiferromagnet. We determine the orbital ordering by extending to our Jahn-Teller system a recently developed Peierls instability framework for the Holstein model [S. Datta and S. Yarlagadda, Phys. Rev. B 75, 035124 (2007)]. By using two-dimensional dynamic response functions corresponding to a mixed Jahn-Teller mode, we establish that the ${Q}_{2}$ mode determines the orbital order.

Magnetic phase diagram of [formula omitted] single crystals

We have systematically measured resistivity, susceptibility and specific heat under different magnetic fields ( H) in Eu 1 - x La x Fe 2 As 2 single crystals. It is found that a metamagnetic transition from A-type antiferromagnetism to ferromagnetism occurs at a critical field for magnetic sublattice of Eu 2 + ions. The jump of specific heat is suppressed and shifts to low temperature with increasing H up to the critical value, then shifts to high temperature with further increasing H. Such behavior supports the metamagnetic transition. Further, a twofold in-plane magnetization is found in antiferromagnetic state which is similar to spin density wave (SDW) order. In contrast, isotropic in-plane magnetization is presented in both ferromagnetic and paramagnetic states. Moreover, ferromagnetic state becomes more easily established under external field with suppressing SDW order. An anisotropic exchange model is proposed to understand our findings. Our results suggest that metamagnetism of Eu 2 + ions is somewhat related to SDW order. Finally, detailed H– T phase diagrams for x = 0 and 0.15 crystals are given, and possible magnetic structure is proposed.

Double-exchange model study of multiferroic RMnO3 perovskites

In this proceeding, recent theoretical investigations by the authors on the multiferroic $R$MnO$_3$ perovskites are briefly reviewed at first. Using the double-exchange model, the realistic spiral spin order in undoped manganites such as TbMnO$_3$ and DyMnO$_3$ is well reproduced by incorporating a weak next-nearest neighbor superexchange ($\sim10%$ of nearest neighbor superexchange) and moderate Jahn-Teller distortion. The phase transitions from the A-type antiferromagnet (as in LaMnO$_3$), to the spiral phase (as in TbMnO$_3$), and finally to the E-type antiferromagnet (as in HoMnO$_3$), with decreasing size of the $R$ ions, were also explained. Moreover, new results of phase diagram of the three-dimensional lattice are also included. The ferromagnetic tendency recently discovered in the LaMnO$_3$ and TbMnO$_3$ thin films is explained by considering the substrate stress. Finally, the relationship between our double-exchange model and a previously used $J_1$-$J_2$-$J_3$ model is further discussed from the perspective of spin wave excitations.

Neutron diffraction investigation of the magnetic structure and magnetoelastic effects inNdMnO3

We investigated the evolution of the magnetic structure and magnetoelastic effects ofNdMnO3 by neutron powder diffraction. We confirmed the A-type antiferromagnetic (AF) structure ofNdMnO3 belowTN≈82 K with magneticmoments parallel to the b axis of the orthorhombic crystal structure (space groupPbnm). We found that the magnetic moments of Nd order below aboutTN(Nd)≈20 K in a ferromagnetic structure with moments parallel to thec axis.At the same temperature the magnetic moments of Mn develop a ferromagnetic component parallel tothe c axis. We found strong magnetoelastic effects associated with the AF transition atTN≈82 K. The effect is veryprominent for the b lattice parameter. All three lattice parameters and therefore the unit cell volume contracts atTN≈82 K. We treated quantitatively the magnetoelastic effect of theb lattice parameter and found that the extra change in the lattice parameterΔb due to the magnetoelastic effect is proportional to the magnetic moment of the Mnion. We also determined the critical exponent of the AF phase transition to beβ = 0.296 ± 0.008.

Magnetic phase coexistence in Tb +3- and Sr +2-doped La 0.7Ca 0.3MnO 3 manganite:A temperature-dependent neutron diffraction study

We report the results of the temperature-dependent neutron diffraction measurements on the nearly half-doped (La 0.325Tb 0.125)(Ca 0.3Sr 0.25)MnO 3 manganite sample. The simultaneous doping of magnetic Tb 3+ and divalent Sr 2+ in the La 0.7Ca 0.3MnO 3 system results into a large A-site size disorder. Rietveld refinement of neutron diffraction data reveal that the single phase sample crystallizes in a distorted orthorhombic structure. Increased 〈 r A〉 value affects the transport behavior that results into an insulating-like behavior of the sample. Under application of 1 T field sample exhibit insulating-like behavior while insulator–metal transition ( T IM) is exhibited under 5 and 8 T fields. Variable range hoping (VRH) mechanism of charge carriers is exhibited in the insulating region. Field cooled and zero field cooled magnetization measurement shows the Curie temperature ( T C)~47 K. The refinement of the ND data collected at various temperatures below 300 K shows that there is no structural phase transition in the compound. Around 100 K, a magnetic peak appears at lower angle that can be ascribed to the presence of the A-type antiferromagnetic (AFM) phase. Two more peaks are observed around 50 K at lower angles that can be fitted in CE-type antiferromagnetic phase. Splitting of the peaks at lower temperatures is the signature of orbital ordering in the presently studied nearly half-doped manganite system. Results of the detailed structural analysis of the temperature-dependent ND measurements on (LaTb) 0.45(CaSr) 0.55MnO 3 sample has been discussed in the light of coexisting A-type and CE-type antiferromagnetic phases present in the sample at low temperature.

Long-Range Magnetic Interactions Induced by the Lattice Distortions and the Origin of the E-Type Antiferromagnetic Phase in the Undoped Orthorhombic Manganites

With the increase of the lattice distortion, the orthorhombic manganites R MnO 3 ( R = La, Pr, Nd, Tb, and Ho) are known to undergo the phase transition from the layered A-type antiferromagnetic (AFM) state to the zigzag E-type AFM state. We consider the microscopic origin of this transition. Our approach consists of the two parts. First, we construct an effective low-energy model for the manganese 3 d -bands and derive parameters of this model from the first-principles electronic structure calculations. Then, we solve this model in the Hartree–Fock approximation (HFA) and analyze the behavior of the interatomic magnetic interactions. We argue that the nearest-neighbor interactions decrease with the increase of the distortion and at certain stage start to compete with the longer range (particularly, second- and third-neighbor) AFM interactions in the orthorhombic ab -plane, which trigger the formation of the E-phase. The origin of these interactions is closely related to the orbital ordering, which takes place in the distorted orthorhombic structure. The model is able to capture the main experimental trends and explain why LaMnO 3 develops the A-type AFM order and why it tends to transform to the E-type AFM order in the more distorted compounds. Nevertheless, the quantitative agreement with the experimental data crucially depends on other factors, such as the magnetic polarization of the oxygen sites as well as the correlation interactions beyond HFA.

Impact of magnetic phase coexistence on magnetotransport in polycrystalline Nd0.51Sr0.49MnO3 thin film

We report the magnetic and transport properties of Nd0.51Sr0.49MnO3 polycrystalline thin films (thickness ∼ 100 nm) prepared by dc magnetron sputtering on single crystal Y-stabilized ZrO2(1 0 0) substrates. The paramagnetic (PM) to ferromagnetic (FM) transition at TC = 260 K is followed by A-type antiferromagnetic (AFM) ordering at and charge ordered (CO) phase having the CE type spin order at . The strong CO-AFM transition observed at TCO ∼ 150 K at half doping is softened by a small increase in the Nd concentration and the disorder due to the polycrystalline nature of these films. The multiple magnetic transitions are suggestive of strong phase coexistence. In this study the insulator-like to metal-like transition temperature TIM ∼ 140 K is considerably lower than the TC and remains independent of the external magnetic field up to 70 kOe. This suggests that electrical transport is dominated by grain boundaries and has percolation type characteristics due to the existence of metallic (FM + A-AFM) and insulating (CO-AFM + GB) paths. The films show large magnetoresistance at low as well as high magnetic fields.

Metamagnetic transition in EuFe2As2 single crystals

We report the measurements of the anisotropic magnetization and magnetoresistance (MR) on single crystals of EuFe2As2, a parent compound of ferro-arsenide high-temperature superconductor. Apart from the antiferromagnetic (AFM) spin density wave (SDW) transition at 186 K associated with Fe moments, the compound undergoes another magnetic phase transition at 19 K due to AFM ordering of Eu2+ spins (J=S=7/2). The latter AFM state exhibits metamagnetic (MM) transition under magnetic fields. Upon applying magnetic field with H∥c at 2 K, the magnetization increases linearly to 7.0 μB f.u.−1 at μ0H=1.7 T and then remains at this value for saturated Eu2+ moments under higher fields. In the case of H∥ab, the magnetization increases step-like to 6.6 μB f.u.−1 with small magnetic hysteresis. An MM phase was identified with the saturated moments of 4.4 μB f.u.−1. The MM transition accompanies negative in-plane MR, reflecting the influence of Eu2+ moments ordering on the electrical conduction of FeAs layers. These results were explained in terms of spin-reorientation and spin-reversal based on an A-type AFM structure for Eu2+ spins. The magnetic phase diagram has been established.

Crystal and magnetic structure of single-crystal La1-x Sr x MnO 3 (x ≈ 1/8)

A neutron powder diffraction (NPD) study on the crystal and magnetic structure of a crushed La1-xSrxMnO3 (x ≈ 1/8) single crystal has been performed. The sample belongs to orthorhombic (Pnma, O) above the Jahn-Teller (JT) transition temperature (TJT) and monoclinic (P121/c1, M′) in the JT regime. We have also refined the NPD data below the charge/orbital ordering (CO/OO) temperature (TCO/OO) with a monoclinic (P121/c1, M′′) model because the experimental resolution was insufficient to clearly identify a triclinic $(P\bar {1})$ structure. The refined lattice parameters show an obvious breathing-mode distortion between TCO/OO and TJT, accompanied by a large deviation of the monoclinic angle β from 90°, signifying a very strong cooperative JT distortion. A ferromagnetic (FM) moment of 3.43(5)μ B/Mn besides an A-type antiferromagnetic (A-AFM) moment of 0.54(2) μ B/Mn is directed mainly along the b axis in P121/c1 symmetry at 5 K. With increasing temperature, the A-AFM domains transform into FM ones above ~100 K and the FM spin orientation turns from the b to the c axis in crystallographic b-c plane below Tc = 187(1) K. The magnetization measurements show typical anomalies around TCO/OO and TJT. The measured saturation moment of 3.9(1)μ B/Mn at 70 kOe and 5 K is well consistent with the sum 3.97(5)μ B/Mn of the refined FM and A-AFM moments at 5 K, implying the A-AFM spins are aligned in field direction at 70 kOe. The applied magnetic field can affect the paramagnetic insulating (PMI) state in the range of magnetic polarons. Based on the size of JT distortion and the bond-valence sums (BVS’s), the CO/OO phenomenon is being discussed.

Anisotropic spin correlations in bilayered La 1.1Sr 1.9Mn 2O 7 investigated by electron spin resonance

Electron spin resonance (ESR) and magnetization measurements were performed on a bilayered single crystal of La 1.1Sr 1.9Mn 2O 7. A coexistence of anisotropic paramagnetic resonance (PMR) line and ferromagnetic resonance (FMR) line was observed in a wide temperature range. Temperature dependence of the ESR intensity reveals the competition between local ferromagnetic (FM) correlations and antiferromagnetic (AFM) correlations in the PM state. The antiferromagnetic ordering transition was identified from temperature dependence of resonance field and linewidth of the PMR line in both the ab plane and c -axis directions. The observed FMR line suggests the existence of local intralayer FM clusters. Below 200 K, the FMR line is found to be split and an optical mode appears in addition to the main acoustic mode, indicating the appearance of AFM correlation across MnO 2 bilayers below the A-type AFM ordering temperature T N .

Charge and orbital ordered phases ofLa2−2xSr1+2xMn2O7−δ

Our studies have significantly modified the conventionally held view of the phase diagram of ${\text{La}}_{2\ensuremath{-}2x}{\text{Sr}}_{1+2x}{\text{Mn}}_{2}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ for two compositions exhibiting charge (and orbital) order (CO), i.e., at hole-doping levels, $h=x\ensuremath{-}\ensuremath{\delta}$, of $\ensuremath{\sim}0.5$ and $\ensuremath{\sim}0.6$. These CO states are stable over very narrow doping ranges $(\ensuremath{\Delta}h\ensuremath{\sim}\ifmmode\pm\else\textpm\fi{}0.005)$ at the lowest temperatures, but those ranges increase at higher temperatures (to $\ensuremath{\Delta}h\ensuremath{\sim}\ifmmode\pm\else\textpm\fi{}0.02$) in a manner consistent with simple entropy considerations. Such narrow ranges dictate the crucial need for crystal homogeneity. Attesting to such homogeneity is a conductivity ratio of $g{10}^{10}$ upon crossing the first-order phase boundary from CO at $h=0.60$ to $\text{A}$-type antiferromagnetic (AAFM) at $h\ensuremath{\sim}0.59$ or $h\ensuremath{\sim}0.61$ plus two findings that were missed in the existing literature: that these CO phases are the ground state at the lowest temperatures, and for $h\ensuremath{\sim}0.5$, that coexistence of the CO and AAFM phase is absent at any temperature.

Electron spin resonance study of spin correlations in charge-ordered La2−2xSr1+2xMn2O7(x=0.6)

Electron spin resonance (ESR) studies have been performed on a bilayered manganite La2−2xSr1+2xMn2O7(x=0.6) single crystal. The charge ordering (CO) and antiferromagnetic (AFM) transitions can be well identified from the ESR spectra in both the ab plane and c axis directions. A coexistence of paramagnetic resonance signal and low-field AFM resonance signal was observed below the CO temperature TCO. The analysis of the ESR parameters as a function of temperature suggests the existence of weak ferromagnetic (FM) correlations in the paramagnetic state and AFM correlations below TCO. The competition of FM and AFM correlations results in a broad peak in ESR intensity. Our results also show that the intensity of AFM correlations increases with decreasing temperature as the system enters from the CO state into the A-type AFM ground state. Therefore, the evolution and anisotropy of spin correlations can be fully mapped by ESR study.

Versatile and competing spin-charge-orbital orders in the bilayered manganite systemPr(Sr1−yCay)2Mn2O7

Electronic-phase competition has been investigated for single crystals of half-doped bilayer-structure manganites, $\mathrm{Pr}{({\mathrm{Sr}}_{1\ensuremath{-}y}{\mathrm{Ca}}_{y})}_{2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ $(0\ensuremath{\leqslant}y\ensuremath{\leqslant}1.0)$, with controlled one-electron bandwidth $(W)$. For $y\ensuremath{\leqslant}0.4$, the A-type antiferromagnetic (AF) state with ${x}^{2}\ensuremath{-}{y}^{2}$ orbital order shows up as the ground state while accompanying the short-range vertical charge order with the periodicity of $\ensuremath{\approx}4{a}_{0}$ $({a}_{0}\ensuremath{\approx}0.4\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$. For $y>0.4$, with reduced $W$, the CE-type AF state emerges with the ordered staggered orbitals $(3{x}^{2}\ensuremath{-}{r}^{2}∕3{y}^{2}\ensuremath{-}{r}^{2})$ whose diagonal stripes undergo the 90\ifmmode^\circ\else\textdegree\fi{} rotation with decreasing temperature, perhaps coupled with the evolution of spin correlation.

55Mn NMR study of the A‐type antiferromagnetic and metallic manganites La1–xSrxMnO3 (0.5 ≤ x ≤ 0.6)

AbstractThe microscopic magnetic and electronic properties of La1‐xSrxMnO3 (0.5 ≤ x ≤ 0.6) were investigated using 55Mn nuclear magnetic resonance (NMR). The NMR spectral intensity showed a large decrement as x increases above the boundary (x ∼ 0.52) of ferromagnetic (FM) and A‐type antiferromagnetic (AFM) phases. The spectrum of the samples shows a coexistence of two different phases, i.e., a metallic major phase and an insulating minor phase. The spin‐spin relaxation time strongly depends on the resonance frequency, indicating that the Shul‐Nakamura process by spin‐wave excitation is the dominant relaxation mechanism. This type of frequency dependence of the spin‐spin relaxation time was not observed in the CE‐type AFM manganite Nd0.5Sr0.5MnO3. The A‐type AFM spin structure and the Shul‐Nakamura relaxation process appears to be a unique characteristics of La1‐xSrxMnO3 (0.54 ≤ x ≤ 0.6) that is related to the 2D‐like spin coupling in a nearly tetragonal crystal structure. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Domain structure and magnetization reversal of antiferromagnetically coupled perpendicular anisotropy films

We describe experimental and theoretical investigations of the magnetic domain formation and the field reversal behavior in antiferromagnetically coupled perpendicular anisotropy multilayers that mimic A-type antiferromagnet (AF) structures. The samples are sputter deposited Co/Pt multilayers with perpendicular anisotropy that are periodically interleaved with Ru to mediate an antiferromagnetic interlayer exchange. This structure allows precise tuning of the different magnetic energy terms involved. Using various magnetometry and magnetic imaging techniques as well as resonant soft X-ray scattering we provide a comprehensive study of the remanent and demagnetized configurations as well as the corresponding reversal mechanisms. We find that adding AF exchange to perpendicular anisotropy system alters the typical energy balance that controls magnetic stripe domain formation, thus resulting in two competing reversal modes for the composite system. In the AF-exchange dominated regime the magnetization is ferromagnetically ordered within the film plane with the magnetization of adjacent layers anti-parallel thus minimizing the interlayer AF exchange energy. In the dipolar dominated regime the magnetization pattern forms ferromagnetic (FM) stripe domains where adjacent layers are vertically correlated, but laterally anti-correlated thus minimizing the dipolar energy at the expense of the AF interlayer coupling. By tuning the layer thickness or applying a magnetic field, we observed the co-existence of AF domains and FM stripe domains. We find that a FM phase exists at AF domain boundaries, causing complex mesoscopic domain patterns with surprising reversibility during minor loop field cycling.

Control of the ferroelectricity in a metastable state for orthorhombicRMnO3 crystals

We have investigated orthorhombicRMnO3 (R = (Gd1−yTby)) crystals near the phase boundary between the paraelectric A-type-antiferromagnetic (AF) phase(PA) and the ferroelectric transverse-spiral-AF one (FS). The spiral AF structure breaks inversionsymmetry and induces the ferroelectric polarization through the inverse Dzyaloshinskii–Moriya(DM) interaction. We have found that the PA–FS phase boundary is located at0.15<y<0.2. Inthe y = 0.15 compound, PA and FS phases appear in the cooling scan, while FS is notobserved in the warming scan. This result suggests that FS observed iny = 0.15 is a metastable state arising from the competition between PA and FS. Furthermore, wedemonstrate the phase control between these competing phases by the application of theexternal magnetic field and external quasihydrostatic pressure.

High-pressure effects on the crystal and magnetic structure of managanites

Nuetron diffraction studies of high-pressure effects on the crystal and magnetic structure of A1-x A ′ MnO3 manganites (A = Pr, La; A′ = Sr, Ca, Na) are reviewed. High pressure leads to various changes in the magnetic structure of manganites: the appearance of a new A-type antiferromagnetic (AFM) state in compounds with the initial ferromagnetic (FM) or pseudo-CE type AFM state, such as La1 − x Ca x MnO3 (x = 0.25, 0.33), Pr1 − x Sr x MnO3 (x = 0.48), Pr0.7Ca0.3Mn1 − y Fe y O3 (y = 0, 0.1), Pr1 − x Na x MnO3 (x = 0.2, 0.25); and the appearance of a new C-type AFM state in the Pr0.44Sr0.56MnO3 compound with the initial A-type AFM state. The observed changes in the magnetic structure and the behavior of the transition temperature to the FM state under high pressure are discussed in the framework of the current theoretical concepts.

Orbital and angle dependency of the superexchange interaction: [formula omitted] (A=Se,Te)

We have investigated electronic and magnetic properties of SeCuO 3 and TeCuO 3 . We have confirmed the ferromagnetic (FM) and the A-type antiferromagnetic (AFM) ground state for SeCuO 3 and TeCuO 3 , respectively, and demonstrated the orbital ordering in these systems. We have also performed the analytic calculation on the angle dependency of the superexchange interaction based on the calculated orbital structure, and explained the transformation from the FM state for SeCuO 3 to the AFM state for TeCuO 3 .