Mn Sublattice Research Articles

Ground-state ferrimagnetism and magneto-caloric effects in Nd2NiMnO6

Extending our earlier investigation of magnetic properties of Nd2NiMnO6, we show that it exhibits a magnetic transition below ∼6 K to a ferrimagnetic state. This behavior is interpreted as arising from a long-range ordering of Nd moments antiferromagnetically coupled to the ferromagnetic Ni–Mn ordered moments. Due to the richness of its multiple magnetic transitions and the easily influenced magnetic state by the application of an external magnetic field, established in our earlier study, it has a remarkable inverse magneto-caloric effect (IMCE) at low temperatures (T < 50 K) together with a significant conventional magneto-caloric effect (CMCE) at the ferromagnetic ordering temperature (Tc ∼ 200 K). IMCE and CMCE correspond to the antiferromagnetic arrangement of Nd and Ni–Mn sublattices and ferromagnetic ordering of Ni–Mn sublattices, respectively. Nd2NiMnO6 with its second order phase transition follows the universal behavior of ∆SM(T); it also shows a power law dependency on the magnetic field as

Effects of oxygen annealing on magnetic properties of epitaxial PrNi0.5Mn0.5O3−δ thin films

A series of epitaxial thin films of PrNi0.5Mn0.5O3 (∼12 nm) were grown on single crystal LSAT [(LaAlO3)0.3(Sr2AlTaO6)0.7] substrate by pulsed laser deposition method. With a purpose to vary oxygen content in the films, the in situ oxygen annealing time was varied (0 to 5 min) during the thin film formation. One film was not annealed after the deposition in order to create high oxygen deficiency. This oxygen-deficient film grew with a different tensile strain. In spite of oxygen variation, all the films of this series show epitaxial growth [00l] on LSAT. Temperature-dependent resistivity and magnetization measurement were performed over a temperature range of 300 K to10 K. All the films show insulating behavior. The temperature-dependent magnetization shows two transitions in the system finally leading to a magnetically frustrated state at low temperatures. These results indicate that Ni and Mn sub-lattices order in antiparallel spin-states in these thin films. A comparative study of magnetization in polycrystalline bulk PrMn0.5Ni0.5O3 and these thin films shows that the epitaxial strain and oxygen content strongly influences the overall magnetic behavior of this system.

Magnetic moment of Mn in disordered Pd2MnGe Heusler alloy as studied by CPA and supercell modelling

Abstract The neutron-scattering measurements and the observation of hyperfine interaction of nuclear moments in Pd2MnGe alloy have prompted a study of disorder in this compound by the density functional theory methods. Two approaches: the coherent-potential approximation (CPA) and supercell modelling were used to gain microscopic insight into the electronic and magnetic properties of disordered Pd2MnGe. The influence of disorder between Pd and Mn sublattices in Pd2MnGe on the magnetic moment of Mn atoms is discussed. The presented CPA studies have shown that the small disorder in Pd2MnGe (between the Pd-Mn sublattices of the L21 structure) does not well describe the experimental evidence of antiparallel alignment of magnetic moment of Mn at positions (A,C) and B positions. However, the appearance of antiparallel moment of Mn at positions A and B was predicted for the Pd2MnGe in the frames of 128-atoms supercell calculations. The generalized gradient approximation (GGA) has been the main basis for the presented supercell first-principles electronic structure calculations. The magnetic moment of Mn(A,C) is found to be small and negative value of −0.177 µB. The values of magnetic moment of Mn(B) are between 3.6 µB and 3.72 µB. The results obtained for the supercell calculations are consistent with available experiments in the literature. The results of presented calculations in the supercell approach have shown that the disordered Pd2MnGe can be predicted as a mixture of several supercells.

Electronic and Magnetic Structure and Elastic and Thermal Properties of Mn2-Based Full Heusler Alloys

Magnetism, electronic structure, elastic and thermal properties of Mn2YAl (with Y = Cr, V) have been investigated. The optimized lattice parameters, bulk modulus, and cohesive energy have been obtained. These alloys have the ferrimagnetic state as the most stable magnetic configuration, since the calculations showed a strong Mn-V antiferromagnetic coupling leading to the ferromagnetism of the Mn sublattices. A small and itinerant magnetic moment of Mn at the A site is found, which is antiparallel to the moment of Y at the B position in Mn2YAl (with Y = Cr, V) compounds. The calculated total spin moments are integral values and increase from − 2 μB/f.u. for Mn2VAl to – 1 μB/f.u. for Mn2CrAl with increasing the number of valence electrons. Band structure and total and partial density of states could be calculated via applying the modified Becke Johnson approximation (mBJ). Based on these results, Mn2YAl (with Y = Cr, V) are half-metallic ferrimagnets with the energy gap lies in the majority spin direction and a high-spin polarization (100%). The main difference between these two compounds is that the band gap is increased by 48% (0.210 eV for Mn2CrAl and 0.401 eV Mn2VAl). Elastic anisotropies, brittleness, and thermodynamic properties are determined for the Mn2YAl (with Y = Cr, V). The slight difference in the spatial distributions of Young’s moduli of Mn2YAl (with Y = Cr, V) reflects the small differences for the elastic anisotropies of the alloys under consideration. The mechanical stability of Mn2YAl (with Y = Cr, V) alloys are studied based on the elastic constants. The thermal properties are studied and investigated using the quasi-harmonic model, in addition, the temperature effect on heat capacities at constant pressure and volume, entropy, and thermal expansion are analyzed and discussed.

Temperature-dependent resistivity and anomalous Hall effect in NiMnSb from first principles

We present implementation of the alloy analogy model within fully relativistic density-functional theory with the coherent potential approximation for a treatment of nonzero temperatures. We calculate contributions of phonons and magnetic and chemical disorder to the temperature-dependent resistivity, anomalous Hall conductivity (AHC), and spin-resolved conductivity in ferromagnetic half-Heusler NiMnSb. Our electrical transport calculations with combined scattering effects agree well with experimental literature for Ni-rich NiMnSb with 1--2% Ni impurities on Mn sublattice. The calculated AHC is dominated by the Fermi surface term in the Kubo-Bastin formula. Moreover, the AHC as a function of longitudinal conductivity consists of two linear parts in the Ni-rich alloy, while it is nonmonotonic for Mn impurities. We obtain the spin polarization of the electrical current $P&gt;90%$ at room temperature and we show that $P$ may be tuned by chemical composition. The presented results demonstrate the applicability of an efficient first-principles scheme to calculate temperature dependence of linear transport coefficients in multisublattice bulk magnetic alloys.

Possible Mott transition in layeredSr2Mn3As2O2single crystals

Single crystals of ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{2}$ have been grown for the first time, for which we show a possible layer-selective Mott insulator behavior. This compound stands out as a hybrid structure of ${\mathrm{MnO}}_{2}$ and MnAs layers, analogously to the active ${\mathrm{CuO}}_{2}$ and FeAs layers, respectively, in the cuprate and iron-based high-temperature superconductors. Electrical transport, neutron diffraction measurements, together with density functional theory calculations on ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{2}$ single crystals converge toward a picture of independent magnetic order at ${T}_{1}\ensuremath{\sim}79$ K and ${T}_{2}\ensuremath{\sim}360$ K for the two Mn sublattices, with insulating behavior at odds with the metallic behavior predicted by calculations. Furthermore, our inelastic neutron-scattering studies of spin-wave dispersions for the Mn(1) sublattice reveal an effective magnetic exchange coupling of $SJ\ensuremath{\sim}3.7$ meV. This is much smaller than those for the Mn(2) sublattice.

Ferroelectricity driven by soft phonon and spin order in multiferroic BiMn3Cr4O12

AbstractBiMn3Cr4O12 shows an unusual joint multiferroicity, which facilitates the coexistence of considerable ferroelectric polarization and remarkable magnetoelectric coupling in a single‐phase multiferroic material. Based on first‐principles calculations, we investigate the two different types of ferroelectric phase transitions in the BiMn3Cr4O12 material. Our results show that the first ferroelectric phase transition is driven by soft mode and leads BiMn3Cr4O12 into the Cm space group. The predicted ferroelectric polarization in single crystal is about ~9.8 μC/cm2. With the emergence of spin order on both Mn and Cr sublattices, it is the polar Cm structure that triggers the exchange striction mechanism and therefore results in a large type‐II multiferroicity (~1.1 μC/cm2). In addition, the intrinsic direction of the spin‐driven ferroelectric polarization is always opposite to that of the existing Cm phase structure. Our results imply a feasible strategy in searching/designing novel type‐II multiferroics with large ferroelectric polarization.

Simultaneous magnetic and structural transitions in Nd0.15Ca0.85MnO3 manganite: Magnetization and neutron diffraction studies

The manganite Nd0.15Ca0.85MnO3 crystallizes in GdFeO3-type, orthorhombic structure (space group Pnma) at 300 K and orders antiferromagnetically at 112 K (TN). Magnetization-field (M-H) isotherms indicate metamagnetic transition at temperatures below TN. However, metamagnetism does not lead to saturation in magnetization. The magnetization value at 2 K in 140 kOe field is only 0.4 μB/f.u. The isothermal magnetic entropy change (ΔSm) has been calculated using the M-H data and inverse magnetocaloric effect is observed around TN and the maximum value of ΔSm is ∼12.2 J kg−1K−1 at 107.5 K for 140 kOe field change. Powder neutron diffraction study in zero applied field reveals monoclinic distortion of Nd0.15Ca0.85MnO3 crystal lattice and antiferromagnetic ordering of Mn sublattice in ac-plane with wave vector K = [1/2, 0, 1/2] at 125 K. From the neutron data, the Mn magnetic moment value at 3 K is found to be 2.54 μB. Electrical resistivity increases tremendously below TN to an insulating state and shows thermal hysteresis near TN in zero field. Large magnetic fields drive insulator to metal-like transition and negative colossal magnetoresistance of about 70% is realized at 50 K in 70 kOe field and that could be due to the field-induced order.

Imaging Domains in a Zero-Moment Half Metal

We have a choice of methods for examining domains at the surface of a ferromagnet that depend on probing the stray field distribution, but these methods do not work in antiferromagnets or compensated ferrimagnets, which produce no stray field. The discovery of compensated ferrimagnetic half metals allows for the local magnetization state to be observed directly with polarized light. The example considered here, Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ru <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga, has two inequivalent but oppositely aligned Mn sublattices with equal and opposite moments, but only one of them contributes spin-polarized conduction electrons at the Fermi energy. The material looks like an antiferromagnet from the outside, but from the point of view of the electronic structure, it resembles a spin-polarized ferromagnetic metal. The anisotropy axis is perpendicular to the film plane, which allows domains to be imaged directly by polar magneto-optic Kerr effect. The domain structure in a film with a composition of Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ru <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.4</sub> Ga has been imaged in a Kerr microscope and hysteresis loops have been traced. Domains have dimensions of order 20 μm with meandering domain walls and a fractal dimension D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</sub> = 1.85. Our results open new direct imaging possibilities of magnetically ordered materials with no net moment.

Magnetic coupling in 3D-hierarchical MnO2 microsphere

The development of new routes for the synthesis of the magnetic nanostructures with low cost, simple and rapid growth has inspired as alternative materials in a wide range of applications. In this report, 3D-hierarchical MnO2 microspheres with an ultrathin nanosheet structure have been synthesized by microwave heating method. X-ray diffraction analysis exhibit the microcrystalline nature of microspheres and (211) is the major pattern with the highest intensity. High purity and surface adsorbed oxygen species have been detected by valance state of microspheres. The magnetic properties of microsphere α-MnO2 investigated experimentally and combined with density functional theory (DFT) calculations, suggest the coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) interaction in their nanostructures. The observed FM state is arising from noncollinear magnetic field that tilts two AFM Mn spins consequently, which make a distortion through intermediate oxygen ion in the crystal structure. While, the canted angle between two Mn sublattices lead them to a weak FM state. Similarly, the superexchange interaction is also responsible for magnetic ordering in the adjacent O ion, that can help to exchange magnetic information by direct electron hops of Mn–Mn chain. This is further confirmed from our DFT calculations. They may be found the strong potential for alternative materials other than partially oxidized metal nanoparticles.

Tetragonal CuMnAs alloy: Role of defects

The antiferromagnetic (AFM) CuMnAs alloy with tetragonal structure is a promising material for the AFM spintronics. The resistivity measurements indicate the presence of defects about whose types and concentrations is more speculated as known. We confirmed vacancies on Mn or Cu sublattices and Mn$_{\rm Cu}$ and Cu$_{\rm Mn}$ antisites as most probable defects in CuMnAs by our new ab initio total energy calculations. We have estimated resistivities of possible defect types as well as resistivities of samples for which the X-ray structural analysis is available. In the latter case we have found that samples with Cu- and Mn-vacancies with low formation energies have also resistivities which agree well with the experiment. Finally, we have also calculated exchange interactions and estimated the N\'eel temperatures by using the Monte Carlo approach. A good agreement with experiment was obtained.

Structural and magnetic properties of the new Yb1-xScxMn6Sn6 solid solution

We have investigated the crystal and magnetic properties of the new Yb1-xScxMn6Sn6 compounds (0.00 ≤ x ≤ 1.00). X-ray diffraction experiments show that all samples are isotypic with HfFe6Ge6 and the lattice constants shrink upon increasing the Sc content. The lack of anomaly in the composition dependence of the cell parameters indicates that Yb remains strongly intermediate valent (ʋYb ∼ 2.6–2.7) as in the YbMn6Sn6 parent compound (ʋYb ∼ 2.6). As a result, Yb does not magnetically order and the magnetism arises exclusively from the Mn sublattice. The Sc-doping yields a moderate increase of the ordering temperatures (<380 K). While YbMn6Sn6 is a simple ferromagnet (TC = 284 K), a low-temperature antiferromagnetic-like state, whose temperature extent increases with the Sc content, is stabilized. It manifests through a sign change of the magnetocaloric effect. The alloys with x = 0.90 and 1.00 are antiferromagnetic-like over the whole ordered temperature range. Low-field metamagnetism is evidenced in the antiferromagnetic-like state. The results are discussed and compared with previously published data.

Magnetic structures and magnetic phase transitions in RMn2Si2

Magnetic properties of the LaMn2Si2 and La0.6R0.4Mn2Si2 (R = Sm, Tb, Gd) compounds with layered ThCr2Si2-type structure have been studied using quasi-single-crystalline and polycrystalline samples. Substitution of the R atoms for La decreases the interatomic distances and changes the ordering of the Mn sublattice from ferromagnetic to antiferromagnetic. Magnetic structure of the substituted compounds has been found to depend on the anisotropy of R ions. The Sm moments are oriented in the basal plane and weakly coupled with the Mn sublattice. For R = Tb, frustrations prevent the formation of long-range magnetic ordering in Tb sublattice. For the system with Gd, the antiferromagnetic Mn-Mn and Gd-Mn interactions lead to a change of magnetic anisotropy from the easy-axis to the easy-plane type.

Exchange-induced spin reorientation in La1-xGdxMn2Si2

Magnetic properties of the La1-xGdxMn2Si2 (0 ≤ x ≤ 1) compounds with layered ThCr2Si2-type structure have been studied using quasi-single-crystalline and polycrystalline samples. Concentration dependences of critical temperatures of magnetic phase transitions, effective magnetic moments and paramagnetic Curie temperatures have been determined for different compositions. Using the obtained results, we plotted the concentration magnetic phase diagram. The type of the interlayer Mn-Mn magnetic ordering is found to depend on the intralayer Mn-Mn distance. With increasing the Gd concentration, the ordering of the Mn sublattice changes from ferromagnetic (F) to antiferromagnetic (AF) at x ≈ 0.2–0.27. We found that the concentration F-AF magnetic phase transition is accompanied by a change of magnetic anisotropy from the easy-axis to the easy-plane type at low temperatures when the Gd moments are magnetically ordered. The transition can hardly be expected since the Mn sublattice has a strong easy-axis type anisotropy in LaMn2Si2, while the Gd has zero orbital momentum and typically weakly contribute to the magnetic anisotropy. The spin reorientation in the La1-xGdxMn2Si2 has been attributed to a competition between the Gd-Mn and Mn-Mn exchange interactions.

Magnetic Structures and Magnetic Phase Transitions in Rare-Earth RMn2Si2 Intermetallic Compounds (R = Sm, Tb)

The magnetic structures that form in La1–xRxMn2Si2 (R = Sm, Tb) layered compounds with various concentrations x have been determined by magnetic neutron diffraction and magnetic measurements, and the magnetic phase diagrams have been built. It is shown that the formation of the magnetic structures is dependent not only on exchange interactions, but also on the type of the magnetic anisotropy of a rare-earth atom. It is found that, in La1–xTbxMn2Si2 compounds with 0.2 < x < 0.5, the competition of the Tb–Mn and Mn–Mn interlayer exchange interactions and the existence of a strong uniaxial magnetic anisotropy in the Mn and Tb sublattices leads to the frustrated magnetic state and prevents the formation of the long-range magnetic order in the Tb sublattice.

Mn2(Fe0.8Mo0.2)MoO6: A Double Perovskite with Multiple Transition Metal Sublattice Magnetic Effects

Transition-metal-only perovskite oxides can introduce additional magnetic functionality with robust magnetoelectric properties but are rare. In this work we prepared a new transition-metal-only perovskite Mn2(Fe0.8Mo0.2)MoO6 at high pressure and temperature. Uniquely, Mn2(Fe0.8Mo0.2)MoO6 was discovered as a line phase upon composition modulation that was motivated from the above-room-temperature multiferroic Mn2FeMoO6 corundum phase. It exhibits ferrimagnetic Fe–Mo sublattice (TC = 194 K) and Mn sublattice antiferromagnetic (Tm ∼ 45 K) transitions. Below Tm the two sublattice orderings are coupled and give rise to canted components in both. A first-order field-induced transition is also observed below 45 K. Mn2(Fe0.8Mo0.2)MoO6 is a Mott variable range hopping semiconductor. These findings for the first time show that either an exotic perovskite or a corundum phase can be achieved by composition modulation besides the pressure effect.

Canted antiferromagnetism in phase-pure CuMnSb

We report the low-temperature properties of phase-pure single crystals of the half-Heusler compound CuMnSb grown by means of optical float-zoning. The magnetization, specific heat, electrical resistivity, and Hall effect of our single crystals exhibit an antiferromagnetic transition at $T_{\mathrm{N}} = 55~\mathrm{K}$ and a second anomaly at a temperature $T^{*} \approx 34~\mathrm{K}$. Powder and single-crystal neutron diffraction establish an ordered magnetic moment of $(3.9\pm0.1)~\mu_{\mathrm{B}}/\mathrm{f.u.}$, consistent with the effective moment inferred from the Curie-Weiss dependence of the susceptibility. Below $T_{\mathrm{N}}$, the Mn sublattice displays commensurate type-II antiferromagnetic order with propagation vectors and magnetic moments along $\langle111\rangle$ (magnetic space group $R[I]3c$). Surprisingly, below $T^{*}$, the moments tilt away from $\langle111\rangle$ by a finite angle $\delta \approx 11^{\circ}$, forming a canted antiferromagnetic structure without uniform magnetization consistent with magnetic space group $C[B]c$. Our results establish that type-II antiferromagnetism is not the zero-temperature magnetic ground state of CuMnSb as may be expected of the face-centered cubic Mn sublattice.

Magnetic structure of Ho0.5Y0.5Mn6Sn6 compound studied by powder neutron diffraction

The crystallographic and magnetic structures of the HfFe6Ge6-type compound Ho0.5Y0.5Mn6Sn6 have been studied by powder neutron diffraction and in-situ Lorentz transmission electron microscopy. Besides the nonlinear thermal expansion of lattice parameters, an incommensurate conical spiral magnetic structure was determined in the temperature interval of 2–340 K. A spin reorientation transition has been observed from 50 to 300 K, where the alignment of the c-axis component of magnetic moments of the Ho sublattice and the Mn sublattice transfers from ferrimagnetic to ferromagnetic.

Strain-induced tetragonal distortions and multiferroic properties in polycrystalline Sr1−xBaxMnO3 ( x=0.43−0.45 ) perovskites

We report the structure-property phase diagram of unique single-ion type-1 multiferroic pseudocubic ${\mathrm{Sr}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ perovskites. Employing a specially designed multistep reduction-oxidation synthesis technique, we have synthesized ${\mathrm{Sr}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ compositions in their polycrystalline form with a significantly extended Ba solubility limit that is only rivaled by a very limited number of crystals and thin films grown under nonequilibrium conditions. Understanding the multiferroic interplay with structure in ${\mathrm{Sr}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ is of great importance as it opens the door wide to the development of newer materials from the parent $(A{A}^{\ensuremath{'}})(B{B}^{\ensuremath{'}}){\mathrm{O}}_{3}$ system with enhanced properties. To this end, using a combination of time-of-flight neutron and synchrotron x-ray scattering techniques, we determined the exact structures and quantified the Mn and oxygen polar distortions above and below the ferroelectric Curie temperature ${T}_{\mathrm{C}}$ and the N\'eel temperature ${T}_{\mathrm{N}}$. In its ferroelectric state, the system crystalizes in the noncentrosymmetric tetragonal $P4mm$ space group, which gives rise to a large electric dipole moment ${P}_{\mathrm{s}}$, in the $z$ direction, of 18.4 and $29.5\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2}$ for $x=0.43$ and 0.45, respectively. The two independently driven ferroelectric and magnetic order parameters are single-handedly accommodated by the Mn sublattice leading to a novel strain-assisted multiferroic behavior in agreement with many theoretical predictions. Our neutron diffraction results demonstrate the large and tunable suppression of the ferroelectric order at the onset of AFM ordering and confirm the coexistence and strong coupling of the two ferroic orders below ${T}_{\mathrm{N}}$. The refined magnetic moments confirm the strong covalent bonding between Mn and the oxygen anions, which is necessary for stabilizing the ferroelectric phase.

Plastic deformation inducing ferromagnetism in Fe2MnAl: Probing Fe magnetism

Structural and magnetic properties of the disordered and ordered Fe2MnAl Heusler alloys have systematically been investigated by local and bulk experimental methods. Plastic deformation, induced by cold-work, leads to a structural phase transformation from the ordered L21 to a disordered A2-structure, concomitantly with a change from paramagnetic PM to ferromagnetic FM state; an effect attributed to the strain-induced ferromagnetism. This transition is explained assuming a gradual transformation of the L21-structure to the anti-phase boundary APB tubes, which result in Fe-cluster formation. Plastic deformed ribbon annealed at low temperatures (Tan < 673 K) favors stabilization of L21-structure with (4 0 0) preferential orientation and magnetic ordering temperature of about 120 K. Considering that Fe atoms order magnetically at 120 K, as seen by Mössbauer, and the measured reduction of total magnetization for temperatures below 50 K, one proposes that Mn and Fe sublattices interact magnetically in a non-collinear magnetic structure that is established due to Fe-Fe, Mn-Mn and Fe-Mn frustrated magnetic couplings. High temperature annealing (Tan > 673 K) favors a strong Mn segregation, concomitantly with formation of disordered Fe-Mn-Al alloys that also have magnetic ordering temperature approaching to 300 K.