Weak Ferromagnetic Component Research Articles

Rise and fall of ferromagnetism in O-irradiated Al2O3 single crystals

In dilute magnetic semiconductors studies, sapphire was usually used as non-magnetic substrate for films. We observed weak ferromagnetic component in Al2O3 single crystal substrate, and excluded the possibility of ferromagnetic contaminations carefully by inductively coupled plasma mass spectrometry and X-ray photoelectron spectroscopy. The ferromagnetism rise and fall during the process of annealing-oxygen irradiation-annealing of the sapphire. The ferromagnetic changes are consistent with Al-vacancy related defects detected by positron annihilation spectroscopy. With first-principle calculations, we confirm that Al-vacancy can introduce magnetic moment for 3 μB in Al2O3 crystal and form stable VAl-VAl ferromagnetic coupling at room temperature.

The magnetic structures and transitions of a potential multiferroic orthoferrite ErFeO3

Rare-earth orthoferrites are very interesting due to their appealing optical and multiferroic properties. In this study, the magnetic structures and transitions of a typical rare-earth orthoferrite, ErFeO3, have been reinvestigated in detail. The spin-reorientation transition of the Fe3+ magnetic phase and the low-temperature magnetic ordering of Er3+ were observed by neutron powder diffraction. The corresponding magnetic structures have been solved anew by symmetry analysis and refinement of the diffraction results. The magnetic moments of Fe3+ align in an antiferromagnetic way along the c axis with a weak ferromagnetic component along the b axis below the Neel temperature and above the spin-reorientation transition. Below the spin-reorientation transition, the Fe3+ moments rotate into an antiferromagnetic ordering state along the b axis with weak ferromagnetic alignment along the c axis. The spin-reorientation takes place in the bc plane. The Er3+ moments align antiferromagnetically with a Cy mode below...

Ba3 v2 s4 o3 : a mott insulating frustrated quasi-one-dimensional s=1 magnet.

Through a solid-state reaction, a practically phase pure powder of Ba3 V2 S4 O3 was obtained. The crystal structure was confirmed by X-ray single-crystal and synchrotron X-ray powder diffraction (P63 , a=10.1620(2), c=5.93212(1) Å). X-ray absorption spectroscopy, in conjunction with multiplet calculations, clearly describes the vanadium in charge-disproportionated V(III) S6 and V(V) SO3 coordinations. The compound is shown to be a strongly correlated Mott insulator, which contradicts previous predictions. Magnetic and specific heat measurements suggest dominant antiferromagnetic spin interactions concomitant with a weak residual ferromagnetic component, and that intrinsic geometric frustration prevents long-range order from evolving.

Giant magnetization on Mn3Ga ultra-thin films grown by magnetron sputtering on SiO2/Si(001)

Mn3Ga films were grown by rf-sputtering on SiO2/Si(001) substrates at 300 °C. Ultra-thin films with 2 nm of thickness are composed of small grains with an in-plane diameter ∼27 nm. The film exhibits an out-of-plane saturation magnetization Ms⊥ = 1690 emu/cm3 (15 times the bulk magnetization value) and in-plane Ms// = 216 emu/cm3, corresponding to 3.3 and 0.43μB/Mn at. Giant magnetization is attributed to uncompensated Mn moments located at the intergrain boundaries. No surface anisotropy is expected due to the high anisotropy constant of Mn3Ga. Ms decreases as the film thickness increases due to grain growth and subsequent decrease in intergrain boundary surface area. Magnetic M(H) loops remain anhysteretic for thickness ≤10 nm, corresponding to an average grain size 10 nm exhibit coercivity between 9 and 19 kOe and Ms from 25 to 72 emu/cm3 with a residual weak ferromagnetic component. Hysteretic M(H) loops are the signat...

Sign reversal of magnetization and tunable exchange bias field in NdCr1−xFexO3 (x=0.05–0.2)

Abstract Magnetization reversal and tunable exchange bias behavior are observed in NdCr 1− x Fe x O 3 compounds for x =0.05–0.20. The magnetic compensation temperature ( T comp ) is found to increase with increase in Fe concentration and its maximum value is 198 K for x =0.15 sample. The observed magnetization reversal is explained by considering the competition between the weak ferromagnetic component of Cr 3+ ions and the paramagnetic moments of Nd 3+ and Fe 3+ ions under the influence of negative internal magnetic field. The exchange anisotropy between the above two components of magnetic moments give rise to tunable positive and negative exchange bias fields. The sign reversal of exchange bias field also coincides with T comp . Bipolar switching of magnetization is demonstrated at T comp by just varying the positive magnetic field.

New phase of MnSb2O6 prepared by ion exchange: structural, magnetic, and thermodynamic properties.

A new layered trigonal (P3̅1m) form of MnSb2O6, isostructural with MSb2O6 (M = Cd, Ca, Sr, Pb, and Ba) and MAs2O6 (M = Mn, Co, Ni, and Pd), was prepared by ion-exchange reaction between ilmenite-type NaSbO3 and MnSO4-KCl-KBr melt at 470 °C. It is characterized by Rietveld analysis of the X-ray diffraction pattern, electron microprobe analysis, magnetic susceptibility, specific heat, and ESR measurements as well as by density functional theory calculations. MnSb2O6 is very similar to MnAs2O6 in the temperature dependence of their magnetic susceptibility and spin exchange interactions. The magnetic susceptibility and specific heat data show that MnSb2O6 undergoes a long-range antiferromagnetic order with Néel temperature TN = 8.5(5) K. In addition, a weak ferromagnetic component appears below T1 = 41.5(5) K. DFT+U implies that the main spin exchange interactions are antiferromagnetic, thereby forming spin-frustrated triangles. The long-range ordered magnetic structure of MnSb2O6 is predicted to be incommensurate as found for MnAs2O6. On heating, the new phase transforms to the stable P321 form via its intermediate disordered variant.

Multiferroic Properties of Cu3(Mo,W)2O9

We report the substitution effects of W6+ ions on multiferroic material Cu3Mo2O9 having a dis- torted tetrahedral quasi-one-dimensional spin structure. In Cu3Mo2O9, the weak ferromagnetic component of the spin moments and the ferroelectricity coexist below TN = 7.9K. In case of Cu3(Mo,W)2O9, the nonmagnetic Mo6+ ions are replaced with the nonmagnetic W6+ ions. We study the specific heat, magnetization, and dielectric constants, and show the magnetic-field- temperature phase diagrams in the single crystals with various W concentrations. We compare the phase diagram of Cu3(Mo,W)2O9 to that of (Cu,Zn)3Mo2O9 and discuss the similarity and the difference between the direct and the indirect substitution effects. The efficiency of the W substitution is much weaker than that of the Zn substitution.

Brownmillerite Ca2Co2O5: Synthesis, Stability, and Re-entrant Single Crystal to Single Crystal Structural Transitions

Ca2Co2O5 in the brownmillerite form was synthesized using a high-pressure optical-image floating zone furnace, and single crystals with dimensions up to 1.4x0.8x0.5 mm3 were obtained. At room temperature, Ca2Co2O5 crystallizes as a fully ordered brownmillerite variant in the orthorhombic space group Pcmb (No. 57) with unit cell parameters a=5.28960(10) {\AA}, b=14.9240(2) {\AA}, and c=10.9547(2) {\AA}. With decreasing temperature, it undergoes a re-entrant sequence of first-order structural phase transitions (Pcmb to P2/c11 to P121/m1 to Pcmb) that is unprecedented among brownmillerites, broadening the family of space groups available to these materials and challenging current approaches for sorting the myriad variants of brownmillerite structures. Magnetic susceptibility data indicate antiferromagnetic ordering in Ca2Co2O5 occurs near 240 K, corroborated by neutron powder diffraction. Below 140 K, the specimen exhibits a weak ferromagnetic component directed primarily along the b axis that shows a pronounced thermal and magnetic history dependence.

Magnetic and magnetocaloric properties of TbMnO3 and Tb0.67R0.33MnO3 (R=Dy, Y, and Ho) bulk powders

Abstract Orthorhombically distorted perovskite rare-earth manganites with smaller rare-earth ions, such as TbMnO3, have mostly been explored for their multiferroic properties. In this work, magnetic and magnetocaloric properties in both TbMnO3 and A-site substituted Tb0.67R0.33MnO3 (R=Dy, Y, Ho) powder samples, prepared via solution route, were investigated. A weak ferromagnetic component was observed in the dc magnetic data of Dy- and Ho-substituted TbMnO3 that may be due to the change in average ionic radius at the A-site (Tb-site) as well as the larger magnetic moment of the substituting ion (~10.6 μB for Dy3+ and Ho3+ as compared to 9.7 μB for Tb3+). The magnetic entropy was found to decrease with the dilution of the Tb3+ ordering by the introduction of both magnetic and non-magnetic dopants at the A-site. Magnetocaloric properties of these samples were evaluated and for pure TbMnO3, a maximum relative cooling power of ~103 J/kg with 5 T applied field was observed that shows its potential for magnetic refrigeration.

Doping dependent magnetism and exchange bias in CaMn1−xWxO3 manganites

Magnetic properties of CaMn1−xWxO3 (0 ≤ x ≤ 0.1) have been investigated, and the research was focused on the exchange bias (EB) phenomenon in CaMn0.93W0.07O3. Magnetic ground state was found to be dependent on tungsten doping level and the following states were distinguished: (i) G-type antiferromagnetic (AFM) state with a weak ferromagnetic (FM) component at x = 0 and 0.04; (ii) mostly orbitally ordered C-type AFM at x = 0.07 and 0.1. For the studied manganites, spontaneous magnetization increases sharply with increasing doping level reaching M0 ≈ 9.5 emu/g at T = 10 K for x = 0.04, and then decreases rapidly reaching zero for x = 0.1. Exchange bias effect, manifested by vertical and horizontal shifts in the hysteresis loop for field cooled sample, has been observed in CaMn0.93W0.07O3. Exchange bias field, coercivity, remanence asymmetry, and magnetic coercivity depend strongly on temperature, cooling field, and maximal measuring field. Horizontal and vertical shifts of magnetization loop sharply decrease with increasing temperature and vanish above 70 K, whereas coercivity and magnetic coercivity disappear only above 100 K (temperature of transition to the G-type AFM state). The exchange bias field HEB increases with increasing cooling field Hcool and goes through a broad maximum at 40 kOe, while the remanence asymmetry increases monotonously in the entire investigated cooling field range, up to 50 kOe. For the compound with x = 0.07, the size of the FM regions ≈2 nm was estimated from the dependence of exchange bias field HEB upon Hcool. It is suggested that the exchange bias originates from interface exchange coupling between small FM clusters and the G-type AFM phase inside the primary C-type orbitally ordered AFM phase.

Ferro and antiferromagnetism of ultrafine‐grained hematite

AbstractAluminum‐substituted hematite (here referred to as Al‐Hm) is an important magnetic mineral for paleo and environmental magnetism. However, the magnetic properties of nanosized Al‐Hm are poorly known. In this study, a series of Al‐Hm samples (fourteen samples) and their nonsubstituted counterparts (referred to as pure‐Hm) (seven samples) were synthesized with particle sizes ranging from 14 to 124 nm. With decreasing particle size, coercive force (Bc), magnetic remanence (Mr) acquired in a 5 T field at room temperature, and the maximum blocking temperature (Tb) of Al‐Hm descend gradually; Tb drops abruptly from 800 to 200 K at 17.2 ± 3.7 nm. This size defines the superparamagnetic (SP)/single‐domain (SD) threshold of Al‐Hm which is significantly lower than that of pure‐Hm (27.5 ± 1.5 nm). The antiferromagnetic high‐field susceptibility (χanti) and the saturation magnetization (Ms) for the weak ferromagnetic component in hematite are both correlated negatively with temperature between 20 and 300 K, and grain size in the vicinity of the SP threshold. For very small grains (below 16–17 nm), Tb increases with increasing particle size. However, beyond this size, it descends with increasing particle size up to 19 nm before increasing with grain size again. These trends are attributed to the competition of surface anisotropy and bulk anisotropy (elastic anisotropy). This new study substantially improves our understanding of the complex magnetic properties of fine‐grained Al‐Hm and pure‐Hm in natural sample, which is significant for the paleoenvironmental and climatic studies of natural samples, e.g., red beds/soils.

Polar and antipolar polymorphs of metastable perovskiteBiFe0.5Sc0.5O3

A metastable perovskite BiFe0.5Sc0.5O3 synthesized under high-pressure (6 GPa) and high- temperature (1500 K) conditions was obtained in two different polymorphs, antipolar Pnma and polar Ima2, through an irreversible behaviour under a heating/cooling thermal cycling. The Ima2 phase represents an original type of a canted ferroelectric structure where Bi3+ cations exhibit both polar and antipolar displacements along the orthogonal [110]p and [1-10]p pseudocubic directions, respectively, and are combined with antiphase octahedral tilting about the polar axis. Both the Pnma and Ima2 structural modifications exhibit a long-range antiferromagnetic ordering with a weak-ferromagnetic component below TN ~ 220 K. Analysis of the coupling between the dipole, magnetic and elastic order parameters based on a general phenomenological approach revealed that the weak-ferromagnetism in both phases is mainly caused by the presence of the antiphase octahe- dral tilting whose axial nature directly represents the relevant part of Dzyaloshinskii vector. The magnetoelectric contribution to the spontaneous magnetization allowed in the polar Ima2 phase is described by a fifth-degree free-energy invariant and is expected to be small.

Spin-cycloid instability as the origin of weak ferromagnetism in the disordered perovskiteBi0.8La0.2Fe0.5Mn0.5O3

Powder neutron diffraction and magnetometry studies have been conducted to investigate the crystallographic and magnetic structure of ${\mathrm{Bi}}_{0.8}{\mathrm{La}}_{0.2}{\mathrm{Fe}}_{0.5}{\mathrm{Mn}}_{0.5}{\mathrm{O}}_{3}$. The compound stabilizes in the $Imma$ orthorhombic crystal symmetry in the measured temperature range of 5 to 380 K, with a transition to antiferromagnetic order at ${T}_{\mathrm{N}}\ensuremath{\approx}240$ K. The spin cycloid present for BiFeO${}_{3}$ is found to be absent with 50% Mn${}^{3+}$ cation substitution, leading to $G$-type antiferromagnetic order with an enhanced out-of-plane canted ferromagnetic component, evident from measurable weak-ferromagnetic hysteresis. Structural modifications do not solely explain this behavior, indicating that modified electron exchange interactions must be taken into account. A classical spin simulation was developed to investigate the effect of random substitution in a disordered pseudocubic perovskite. The calculations took into account the nearest-neighbor, next-nearest-neighbor, and Dzyaloshinskii-Moriya interactions, along with the local spin anisotropy. Using this framework to extend the established Hamiltonian model for BiFeO${}_{3}$, we show that only certain types of perturbations at a magnetic defect and the surrounding molecular fields trigger a simultaneous collapse of cycloidal order and the emergence of the long-range weak-ferromagnetic component. By adopting values for the Mn molecular fields appropriate for $\mathit{RE}$MnO${}_{3}$ ($\mathit{RE}=$ rare earth), simulations of BiMn${}_{0.5}$Fe${}_{0.5}{\mathrm{O}}_{3}$ exhibit the key magnetic properties of our experimental observations.

Crystal structure and magnetic properties of Tb-doped Ru-1222 ruthenocuprates

The effects of Tb doping on the structural, superconducting, and magnetic properties of (Ru0.9Nb0.1)Sr2(Gd1.34−xTbxCe0.66)Cu2Oz and RuSr2(Eu1.34−xTbxCe0.66)Cu2Oz (x=0, 0.34) systems were investigated. It was found that Tb doping in both systems results in a significant suppression of superconductivity and an enhancement of the magnetic ordering temperature, as well as an enhancement of the weak-ferromagnetic component of field-cooled magnetization. The Rietveld refinements of the X-ray diffraction data at room temperature showed that both a and c lattice parameters decrease and the rotation angle of the RuO6 octahedron around the crystallographic c-axis decreases by Tb doping in both systems. The superconducting and magnetic behaviors induced by the Tb doping are discussed in conjunction with the change in hole concentration and the local structural changes in the Ru sublattice induced by Tb doping.

Comparison of the Magnetic Ordering in Sr2Yb1Ru0.9Cu0.1O6 and Ru1Sr2Gd1Cu2O8 Superconductors

Ru1Sr2Gd1Cu2O8 and Sr2Yb1Ru0.9Cu0.1O6 were synthesized using a solid-state reaction. X-ray diffraction showed that Ru1Sr2Gd1Cu2O8 crystallized into the tetragonal Ru-1212 structure (space group P4/mmm), whereas the Sr2Yb1Ru0.9Cu0.1O6 had a monoclynic structure (space group P21/n). Sr2Yb1Ru0.9Cu0.1O6 showed a possible magnetic transition ( T m a g ) at 44 K with a superconducting transition temperature T c at 30 K. In contrast, Ru1Sr2Gd1Cu2O8 showed a magnetic transition at 140 K with a T c at 25 K. The magnetization data for Sr2Yb1Ru0.9Cu0.1O6 suggests that this sample exhibits antiferromagnetic ordering in zero-field-cooled (ZFC) magnetization, whereas it reveals a weak ferromagnetic component during field-cooled (FC) cycles. In addition, larger diamagnetic signals were observed during FC cycle compared to the ZFC. These results suggest that the valence state of the Yb ion have a significant effect on the magnetic state of the Sr2Yb1Ru0.9Cu0.1O6 superconductor.

Epitaxial strain effect on theJeff= 1/2 moment orientation in Sr2IrO4thin films

We have grown Sr${}_{2}$IrO${}_{4}$ (SIO) epitaxial thin films on SrTiO${}_{3}$ (STO) and NdGaO${}_{3}$ (NGO) substrates by a pulsed laser deposition method and characterized their structures and magnetic properties. We find that SIO films grown on STO substrates display tetragonal structure with a tensile strain of 0.13%, while SIO films grown on NGO substrates exhibit slightly orthorhombic structure with anisotropic biaxial tensile strains of 0.39% and 0.51% along the in-plane crystallographic axes. Although both films display insulating properties as bulk SIO does, their magnetic properties are distinct from that of bulk SIO. The ferromagnetic (FM) component of the ${J}_{\mathrm{eff}}$ = 1/2 canted antiferromagnetic order, which emerges below \ensuremath{\sim}240 K in bulk SIO, is significantly weakened in both films, with a greater weakening appearing in the SIO/NGO film. From structural and magnetoresistance anisotropy analyses for both films, we reveal that the weak FM component in SIO films is dependent on the epitaxial strain. The greater tensile strain leads to a smaller octahedral rotation: The rotation angle is \ensuremath{\sim}9.7(1)\ifmmode^\circ\else\textdegree\fi{} for the SIO/NGO film and \ensuremath{\sim}10.7(2)\ifmmode^\circ\else\textdegree\fi{} for the SIO/STO film. These findings indicate that the ${J}_{\mathrm{eff}}$ = 1/2 moment orientation in SIO follows the IrO${}_{6}$ octahedral rotation due to strong spin-orbit interaction.

Structures and Magnetism of the Rare-Earth Orthochromite Perovskite Solid Solution LaxSm1–xCrO3

A new mixed rare-earth orthochromite series, LaxSm1-xCrO3, prepared through single-step hydrothermal synthesis is reported. Solid solutions (x = 0, 0.25, 0.5, 0.625, 0.75, 0.875, and 1.0) were prepared by the hydrothermal treatment of amorphous mixed-metal hydroxides at 370 °C for 48 h. Transmission electron microscopy (TEM) reveals the formation of highly crystalline particles with dendritic-like morphologies. Rietveld refinements against high-resolution powder X-ray diffraction (PXRD) data show that the distorted perovskite structures are described by the orthorhombic space group Pnma over the full composition range. Unit cell volumes and Cr-O-Cr bond angles decrease monotonically with increasing samarium content, consistent with the presence of the smaller lanthanide in the structure. Raman spectroscopy confirms the formation of solid solutions, the degree of their structural distortion. With the aid of shell-model calculations the complex mixing of Raman modes below 250 cm(-1) is clarified. Magnetometry as a function of temperature reveals the onset of low-temperature antiferromagnetic ordering of Cr(3+) spins with weak ferromagnetic component at Néel temperatures (TN) that scale linearly with unit cell volume and structural distortion. Coupling effects between Cr(3+) and Sm(3+) ions are examined with enhanced susceptibilities below TN due to polarization of Sm(3+) moments. At low temperatures the Cr(3+) sublattice is shown to undergo a second-order spin reorientation observed as a rapid decrease of susceptibility.

Superconductivty and magnetic properties of <TEX>$(Ru_{1-x}Nb_x)Sr_2(Sm_{1.4}Ce_{0.6})Cu_2O_z$</TEX>

We investigated the effect of Nb substitution for Ru on the structural and magnetic properties of <TEX>$(Ru_{1-x}Nb_x)\;Sr_2(Sm_{1.4}Ce_{0.6})Cu_2O_z$</TEX> Samples. X-ray diffraction measurements indicated that nearly single-phase samples are formed in the range from x = 0 to 1.0. The superconducting transition temperature determined from the inflection in the field-cooled magnetic susceptibility decreased only slightly from <TEX>$T_c$</TEX> = 25 K for x = 0 to <TEX>$T_c$</TEX> = 22 K for x = 1.0, in consistent with the change in room temperature thermopower of the samples. However, the Nb substitution for Ru above x = 0.25 significantly suppressed the weak ferromagnetic component of the field-cooled magnetic susceptibility. It was also found that the Nb substitution for Ru results in an enhanced diamagnetic susceptibility with Nb content above x = 0.5 in both zero field-cooled and field-cooled magnetization measurements, in contrast to the behavior of the samples with <TEX>$x{\leq}0.5$</TEX> in which the diamagnetic susceptibility decreases as the Nb content increases.

Weak ferromagnetic component on the bulk ZnFe2O4 compound

Magnetization data on the bulk ZnFe2O4 antiferromagnetic compound (TN≈10K) obtained via solid state reaction at different synthesis temperatures show one weak ferromagnetic component at room temperature. We have related it with the cationic disorder effect present on spinel structure of our bulk samples which comes from the magnetic interaction between iron ions sit on both octahedral and tetrahedral sites. The magnetization measurements show to all samples a clear peak around 10K consistent with the antiferromagnetic phase transition. On the other hand, after extracted the paramagnetic component, the hysteresis loops measured at room temperature display one weak ferromagnetic component. Once the T-dependence of magnetization does not fit to a Curie–Weiss law to temperatures well above the magnetic transition we have used a combination of the Curie–Weiss law (paramagnetic spins) and a typical temperature dependence of M0, M0(T)=M0(0)[1−(T/TC)2]0.5 (ordered ferromagnetic spins). We note an increase of the M0(0) as function of the synthesis temperature. This reinforce our supposition of a cationic disorder effect driving the system to present two kinds of magnetic interactions between iron ions on A and B sites.

Weak ferromagnetism and domain effects in multiferroic LiNbO3-type MnTiO3-II

Magnetic order consistent with multiferroism has been observed in the acentric LiNbO${}_{3}$-type, high pressure form II of MnTiO${}_{3}$ using neutron diffraction and magnetization measurements. Spin order below the 28 K magnetic transition has propagation vector (0 0 0), and spins lie in the $ab$ plane. Representation symmetry analysis shows that the antiferromagnetic Mn${}^{2+}$ spin component observed by neutron scattering, of magnitude 3.9(1) \ensuremath{\mu}${}_{B}$ at 2 K, coexists with a weak ferromagnetic component of magnitude 0.0014 \ensuremath{\mu}${}_{B}$. This magnetization is perpendicular to the electrical polarization resulting from cation displacements in this acentric structure, permitting coupled switches of the two ferroic orders. The spin order is stable to fields of at least 5 T; however, facile domain reorientation within the $ab$ plane enhances the antiferromagnetic susceptibility and a constant magnetization/field ($M$/$H$) is observed in field strengths greater than \ensuremath{\sim}1.5 T. Suppression of the {101} neutron intensity with field follows the same Brillouin-dependence as $M$/$H$ and enables the antiferromagnetic easy axes to be identified as parallel to the $ab$ plane hexagonal axes.