Layered Antiferromagnetic Structure Research Articles

Calorimetric Studies of Phase Transformations in the La<sub>1 – x</sub>Y<sub>x</sub>Mn<sub>2</sub>Si<sub>2</sub> System

Differential scanning calorimetry (DSC) is used to determine the magnetic phase transformation temperatures of the La1 – xYxMn2Si2 (x = 0–1) alloys. For the compositions with х from 0 to 0.3, the temperature dependences of DSC signal exhibit λ-like endothermic effects observed near 300 K, which are related to the magnetic phase transition from the ferromagnetic to layered antiferromagnetic structure, and weak anomalies, which are observed in a temperature range of from 458 K for the composition with х = 0 to 323 К for the composition with х = 0.3 upon disordering of the layered antiferromagnetic structure. A clear endothermic peak corresponding to the disordering of interplane antiferromagnetic layered structure was found for the YMn2Si2. The data obtained are used to construct the magnetic phase diagram of the La1–xYxMn2Si2 system in a temperature range of 270–600 К. The differential scanning calorimetry is shown can be successfully used for the determination of temperatures of various magnetic phase transformations in rare-earth intermetallic compounds.

Disclosing the magnetic ground state of PrBaMn2O6

In this work, we report a neutron diffraction study on a single crystal of the layered PrBaMn2O6 perovskite. This compound undergoes two consecutive magnetic transitions with decreasing temperature. At TC ≈ 309 K, a long-range ferromagnetic ordering is established with the Mn magnetic moments oriented along the c axis. Below TN ≈ 240 K, a spin reorientation gives rise to a sudden decrease of the magnetization and the formation of an antiferromagnetic order. This magnetic transition is also accompanied by an electronic localization caused by a structural phase transition from a high-temperature tetragonal phase (P4/mmm) into a low-temperature orthorhombic one (P21am). Our neutron diffraction study at 12 K has unambiguously identified that the ground magnetic structure for this compound is a layered antiferromagnetic structure along the c axis with the Mn moments oriented in the ab plane along one diagonal of the primitive tetragonal cell. No sign of magnetic scattering associated to a CE-type magnetic structure was found in our study, discarding its occurrence in this compound. This result excludes the occurrence of a charge order transition to account for the electronic localization and it is fully consistent with previous structural studies showing a unique crystallographic site for Mn atom in the low-temperature phase. A symmetry analysis was carried out to identify the magnetic space group of the ferromagnetic order above TN and the two possible groups for the antiferromagnetic ordering below TN.

Stoichiometry‐Induced Ferromagnetism in Altermagnetic Candidate MnTe

AbstractThe field of spintronics has seen a surge of interest in altermagnetism due to novel predictions and many possible applications. MnTe is a leading altermagnetic candidate that is of significant interest across spintronics due to its layered antiferromagnetic structure, high Neel temperature (TN ≈ 310 K) and semiconducting properties. The results on molecular beam epitaxy (MBE) grown MnTe/InP(111) films are presented. Here, it is found that the electronic and magnetic properties are driven by the natural stoichiometry of MnTe. Electronic transport and in situ angle‐resolved photoemission spectroscopy show the films are natively metallic with the Fermi level in the valence band and the band structure is in good agreement with first‐principles calculations for altermagnetic spin‐splitting. Neutron diffraction confirms that the film is antiferromagnetic with planar anisotropy and polarized neutron reflectometry indicates weak ferromagnetism, which is linked to a slight Mn‐richness that is intrinsic to the MBE‐grown samples. When combined with the anomalous Hall effect, this work shows that the electronic response is strongly affected by the ferromagnetic moment. Altogether, this highlights potential mechanisms for controlling altermagnetic ordering for diverse spintronic applications.

Topological magnetic phase transition in Eu-based A -type antiferromagnets

Recently, a colossal magnetoresistance (CMR) was observed in $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{P}}_{2}$, a compound that does not fit the conventional mixed-valence paradigm. Instead, experimental evidence points to a resistance driven by strong magnetic fluctuations within the two-dimensional ferromagnetic planes of the layered antiferromagnetic structure. While the experimental results have not yet been fully understood, a recent theory relates the CMR to a topological vortex-antivortex unbinding, i.e., Berezinskii-Kosterlitz-Thouless (BKT), phase transition. Motivated by these observations, in this work we explore the magnetic phases hosted by a microscopic classical magnetic model for $\mathrm{Eu}{\mathrm{Cd}}_{2}{\mathrm{P}}_{2}$, which easily generalizes to other Eu A-type antiferromagnetic compounds. Using Monte Carlo techniques to probe the specific heat and the helicity modulus, we show that our model can exhibit a vortex-antivortex unbinding phase transition. We find that this phase transition displays the same sensitivity to in-plane magnetization, interlayer coupling, and easy-plane anisotropy that is observed experimentally in the CMR signal, providing qualitative numerical evidence that the effect is related to a magnetic BKT transition.

Spin-polarized imaging of the antiferromagnetic structure and field-tunable bound states in kagome magnet FeSn

Kagome metals are an exciting playground for the explorations of novel phenomena at the intersection of topology, electron correlations and magnetism. The family of FeSn-based kagome magnets in particular attracted a lot of attention for simplicity of the layered crystal structure and tunable topological electronic band structure. Despite a significant progress in understanding their bulk properties, surface electronic and magnetic structures are yet to be fully explored in many of these systems. In this work, we focus on a prototypical kagome metal FeSn. Using a combination of spin-averaged and spin-polarized scanning tunneling microscopy, we provide the first atomic-scale visualization of the layered antiferromagnetic structure at the surface of FeSn. In contrast to the field-tunable electronic structure of cousin material Fe3Sn2 that is a ferromagnet, we find that electronic density-of-states of FeSn is robust to the application of external magnetic field. Interestingly, despite the field insensitive electronic band structure, FeSn exhibits bound states tied to specific impurities with large effective moments that strongly couple to the magnetic field. Our experiments provide microscopic insights necessary for theoretical modeling of FeSn and serve as a spring board for spin-polarized measurements of topological magnets in general.

Magnetic transitions and the magnetocaloric effect in the Pr1−xYxMn2Ge2 system

Layered rare earth compounds in the RMn2X2 series (R = rare‐earth; X = Ge, Si) are of interest for potential cooling applications at lower temperatures as they enable the structural and magnetic behavior to be controlled via substitution of R, Mn, and X atoms on the 2a, 4d, and 4e sites respectively. We continue investigations of the Pr1−xYxMn2Ge2 magnetic phase diagram as functions of both composition and Mn–Mn spacing using X‐ray and neutron diffraction, magnetization and differential scanning calorimetry measurements. Pr1−xYxMn2Ge2 exhibits an extended region of re‐entrant ferromagnetism around x ∼ 0.5 with re‐entrant ferromagnetism at for Pr0.5Y0.5Mn2Ge2. The entropy values −ΔSM around the ferromagnetic transition temperatures from the layered antiferromagnetic AFl structure to the canted ferromagnetic structure Fmc (typically ) have been derived for Pr1−xYxMn2Ge2 with x = 0.0, 0.2, and 0.5 for ΔB = 0–5 T. The changes in magnetic states due to Y substitution for Pr are discussed in terms of chemical pressure, external pressure, and electronic effects.

Spin-dependent tunneling in epitaxial Fe/Cr/MgO/Fe magnetic tunnel junctions with an ultrathin Cr(001) spacer layer

We fabricated fully epitaxial Fe/Cr/MgO/Fe(001) magnetic tunnel junctions (MTJs) with an atomically flat ultrathin Cr(001) layer grown below the MgO barrier layer and studied the spin-dependent transport to clarify scattering process of tunneling electrons. Because Cr does not have Bloch states with ${\ensuremath{\Delta}}_{1}$ symmetry at the Fermi energy $({E}_{F})$, ${\ensuremath{\Delta}}_{1}$ evanescent states in MgO, which dominantly mediate the tunneling current, cannot couple with Cr Bloch states without a scattering process. The Fe/Cr/MgO/Fe(001) MTJs are therefore a model system for studying nonspecular scattering processes where the orbital symmetry of tunneling states is not conserved. The resistance-area (RA) product of the MTJs was found to not increase exponentially as a function of the Cr thickness $({t}_{\text{Cr}})$, indicating that the Cr layer does not act as a perfect tunnel barrier despite of the absence of ${\ensuremath{\Delta}}_{1}$ states at ${E}_{F}$. Moreover, the magnetoresistance ratio of the MTJs was seen to oscillate as a function of ${t}_{\text{Cr}}$ with a period of 2 monolayers, reflecting the layered antiferromagnetic structure of Cr(001). Surprisingly, the MR ratio showed local maxima at the odd numbers of Cr monolayers and local minima at the even numbers of Cr monolayers, indicating that the tunneling current is oppositely spin polarized with respect to the interface magnetization. These results suggest that nonspecular scatterings mediate the coupling between evanescent states in MgO and certain non-${\ensuremath{\Delta}}_{1}$ Bloch states in Cr that have negative spin polarization, thereby inducing nonspecular tunneling current even at a low temperature and a small bias voltage. We also investigated, as a reference sample, Fe/MgO/Cr/Fe MTJs with a less-oxidized Cr/MgO interface by growing the Cr(001) layer on the MgO barrier layer and found that their RA product increased much more rapidly with increasing ${t}_{\text{Cr}}$. This indicates that partial oxidation of interface Cr atoms in the Fe/Cr/MgO/Fe MTJs is one of the major origins of nonspecular scatterings. Both an increase in temperature and the application of bias voltage were found to greatly enhance the electron scattering that gives rise to tunneling conductance. While the temperature dependence of the antiparallel conductance due to magnon scatterings follows the Bloch ${T}^{3/2}$ law, the conductance due to nonspecular scatterings deviates from the Bloch ${T}^{3/2}$ law. The present experimental results give us important clues to a clearer understanding of the tunneling process in MgO-based MTJs.

Structure and magnetism of near-stoichiometric FePd nanoparticles

We investigate from first principles the energetic order of single crystalline L1 0-ordered and multiply twinned morphologies of FePd nanoparticles close to the stoichiometric composition considering up to 561 atoms. The results are related to previous analogous calculations of FePt and CoPt nanoparticles. We find that compared to the isoelectronic FePt alloy, multiply twinned structures are slightly favored in energy, while the latent tendencies to form a layered antiferromagnetic structure in the L1 0 phase are less pronounced.

Tailoring the spin density waves in Fe/Cr multilayers by selective inclusion of Sn, V and Mn

The possibility of tailoring the spin density waves in Fe/Cr(001) multilayers through the selective inclusion of Sn, V and Mn monolayers is investigated with the density functional tight-binding linear muffin-tin orbital method in the generalized gradient approximation of the exchange and correlation potential. Despite the non-magnetic character of Sn and V when substituting Cr atoms located at the nodes, the modifications induced on the spin density waves are important due to the strong hybridization. In general we find that V modifies drastically the global features of the spin density waves leading to the onset of a magnetically dead region in the Cr spacer whereas Mn inserted at the nodes rather destroys the density wave working in favor of stabilizing the layered antiferromagnetic structure. The trends obtained are consistent with experimental data when available. Since both the magnetic profile and the position of the nodes at the spacer can be modified, the present results are relevant in the context of the spin-dependent transport through magnetic multilayers in which the magnetoresistance will vary if the scattering regions across the transport direction are modified.

On the spin wave spectrum in a layered antiferromagnetic structure of the LaMnO3 compound

Theoretical expressions are obtained for the energy of spin waves in a layered antiferromagnetic structure (A type) of the perovskite-like compound LaMnO3. The Heisenberg exchange interactions of the central spin moment of the trivalent manganese ion (spin S = 2) with its eighteen neighbors from five coordination shells and the single-ion magnetic anisotropy energy are taken into account. The analysis is performed using six parameters: five exchange integrals and one anisotropy constant. Three of these five integrals determine the interactions in the two-dimensional ferromagnetic planes and two integrals characterize the interplanar interaction. The expressions derived for the magnon energy are valid for any direction and any magnitude of the quasi-momentum in the first Brillouin zone. For the three principal crystallographic directions, the theoretical results are compared with the experimental data for the dispersion curves obtained from inelastic neutron scattering studies of the LaMnO3 compound. It is shown that the in-plane ferromagnetic interaction can be more than sixteen times as great as the interplanar antiferromagnetic interaction. The conclusions drawn in this study are compared with the results obtained by other authors.

Spin-Dependent Tunneling in Magnetic Tunnel Junctions with a Layered Antiferromagnetic Cr(001) Spacer: Role of Band Structure and Interface Scattering

The tunnel magnetoresistance effect (TMR), which is intrinsically determined by the interface monolayer of an electrode, was realized by using magnetic tunnel junctions (MTJs) with a single-crystal Cr(001) layer inserted between a tunnel barrier and an electrode. The MTJs showed an oscillation of the TMR ratio as a function of the thickness of the Cr(001) layer with a period of 2 monatomic layers, which corresponds to the layered antiferromagnetic structure of Cr(001). These oscillations originate from electron scattering at the interface, due to the mismatching of the symmetry of the wave functions and band structure in Cr(001).

Peculiarities of the magnetic, galvanomagnetic, elastic, and magnetoelastic properties of Sm1−x SrxMnO3 manganites

Manganites of the Sm1−xSrxMnO3 system (x=0.33, 0.4, and 0.45) possess giant negative values of the magnetoresistance Δρ/ρ and the volume magnetostriction ω near the Curie temperature TC. In the compound with x=0.33, the isotherms of Δρ/ρ, ω, and magnetization σ exhibit smooth variation and do not reach saturation up to maximum magnetic field strengths (120 kOe) studied (according to the neutron diffraction data, this substance comprises a ferromagnetic (FM) matrix with distributed clusters of a layered antiferromagnetic (AFM) structure of the A type). In the compounds with x=0.4 and 0.45 containing, besides the FM matrix and A-type AFM phase, a charge-ordered AFM phase of the CE type (thermally stable to higher temperatures as compared to the A-type AFM and the FM phases), the same isotherms measured at T ≥ TC show a jumplike increase in the interval of field strengths between Hc1 and Hc2 and then reach saturation. In the interval Hc1 > H > Hc2, the σ, ω, and Δρ/ρ values exhibit a metastable behavior. At temperatures above TC, the anisotropic magnetostriction changes sign, which is indicative of rearrangements in the crystal structure. The giant values of ω and Δρ/ρ observed at T ≥ TC for all compounds, together with excess (relative to the linear) thermal expansion and a maximum on the ρ(T) curve, are explained by the phenomenon of electron phase separation caused by a strong s-d exchange. The giant values of magnetoresistance and volume magnetostriction (with ω reaching ∼10−3) are attributed to an increase in the volume of the FM phase induced by the applied magnetic field. In the compound with x=0.33, this increase proceeds smoothly as the FM phase grows through the FM layers in the A-type AFM phase. In the compounds with x=0.4 and 0.45, the FM phase volume increases at the expense of the charge-ordered CE-type AFM structure (in which spins of the neighboring manganese ions possess an AFM order). The jumps observed on the σ(H) curves, whereby the magnetization σ reaches ∼70% of the value at T=1.5 K, are indicative of a threshold character of the charge-ordered phase transition to the FM state. Thus, the giant values of ω and Δρ/ρ are inherent in the FM state, appearing as a result of the magnetic-field-induced transition of the charge-ordered phase to the FM state, rather than being caused by melting of this phase.

Surface magnetism during the early stages of oxygen-assisted growth of Cr on Fe(0 0 1): A SPMDS study

The surface magnetic properties of ultrathin Cr films grown on O(1×1)-Fe(0 0 1) have been studied by means of spin-polarised metastable de-excitation spectroscopy (SPMDS). The spin-selected surface density of states extracted from the experimental SPMDS data provides information on both composition and magnetic properties of the topmost layer. We find that oxygen floats on the surface of the growing Cr film and that Cr/Fe intermixing is restricted to the first Cr layer. Concerning the magnetic properties, a strong polarisation of Cr 3d electrons is found in agreement with theoretical calculations. Oxygen 2p states are polarised too and their polarisation is opposite to that of Cr 3d states. Surface magnetisation as a function of the Cr film thickness shows an oscillatory behaviour induced by the layered antiferromagnetic structure of Cr. At variance with the results of other experimental investigation in this case the layered antiferromagnetic structure of Cr is already present in the 3 ML thick film.

Structural and magnetic properties of η-phase manganese nitride films grown by molecular-beam epitaxy

Face-centered tetragonal (fct) η-phase manganese nitride films have been grown on magnesium oxide (001) substrates by molecular-beam epitaxy. For growth conditions described here, reflection high energy electron diffraction and neutron scattering show primarily two types of domains rotated by 90° to each other with their c axes in the surface plane. Scanning tunneling microscopy images reveal surface domains consisting of row structures which correspond directly to the bulk domains. Neutron diffraction data confirm that the Mn moments are aligned in a layered antiferromagnetic structure. The data are consistent with the fct model of G. Kreiner and H. Jacobs for bulk Mn3N2 [J. Alloys Compd. 183, 345 (1992)].

Magnetic phase diagram of layered manganites in the highly doped regime

The naturally layered colossal magnetoresistive manganites La2−2xSr1+2xMn2O7 exhibit an extremely varied range of magnetic and electronic behavior over a very narrow composition range between x=0.3 and x=0.5. The successful synthesis in our laboratories of compounds with nominally greater than 50%Mn4+ concentration has now allowed the study of this region of the phase diagram. Here we present detailed neutron diffraction measurements of these compounds with doping levels 0.5<x<1.0. As predicted by simple theories, the type-A layered antiferromagnetic (AF) structure is found at x∼0.5 and the type-G “rocksalt” AF structure at x=1.0. Between these two extremes is found a C-type structure (ferromagnetic rods parallel to b coupled antiferromagnetically to all neighboring rods) stabilized by orbital ordering of y2 states. Also in this Mn4+-rich regime is found a region in which no long-range magnetic order is observed. We discuss how semiempirical models can explain the variety of magnetic structures and how structural trends as a function of doping corroborate the unifying notion of a shift from in-plane to axial orbital occupation as the Mn4+ concentration is decreased.

Study of the growth and the magnetism of ultrathin films of Cr on Fe

The surface magnetic properties of ultrathin Cr films grown on an O/Fe/Ag(100) substrate have been studied by means of spin polarized metastable de-excitation spectroscopy. The presence of oxygen on top of the Fe film inhibits the segregation of Ag and strongly reduces Fe/Cr intermixing. The magnetization of the outermost Cr layer is zero immediately after its deposition, whereas, upon annealing up to 500 K, a net magnetization is observed which reverse its direction as a function of the film thickness already from a 3 ML thick film. The magnetization reversal is in agreement with the layered antiferromagnetic structure of Cr clearly observed for thicker films.

Role of electron-lattice interactions in determining the magnetic structure of insulating manganites

Together with the strong electron-electron interactions, the role of the lattice distorsions turns out to be relevant in determining the specific magnetic structure of some insulating manganites. In particular, while a substantial Jahn-Teller distorsion is needed to account for the layered antiferromagnetic structure of LaMnO 3 , the observed collective shear of alternate ions in La 0.5 Ca 0.5 MnO 3 is required to stabilize the CE magnetic structure of this compounds. The specific action of the lattice in stabilizing these peculiar magnetic orderings is thus clarified.

Magnetic dichroism and spin-resolved photoemission from rough interfaces

The magnetic structure of ultrathin Cr films on Fe is analyzed by taking into account the roughness of the surface and of the interface. An algorithm for the epitaxial growth of the film simulates the near-surface structure, and the magnetic moment distribution is calculated self-consistently in a periodic Anderson model. For rough interfaces we find that the layered antiferromagnetic structure of the Cr adlayer is quenched. Within this model we determine the spin polarization and the value of the magnetic linear dichroism to be expected in angle-resolved photoemission. Comparing with experimental data it is concluded that Cr grows on the Fe(001) in a Stranski-Kastranoff mode. We propose an explanation based on the confinement of the itinerant electrons within the Cr islands on the Fe substrate.

Layered antiferromagnetic structure in vanadium slabs

Layered antiferromagnetic structure is obtained for unsupported vanadium films. The magnetism is described in the unrestricted Hartree-Fock approximation of the Hubbard Hamiltonian for d electrons. Within the recursion method we show that the onset of antiferromagnetism depends strongly on the crystallographic faces investigated.

Thickness dependence of the spin polarization in V(001) and Pd(001) slabs

We report the electronic structure of free-standing V(001) slabs and of Pd layers grown on Ag(001) through self-consistent real space description of the Hubbard Hamiltonian. The layered antiferromagnetic structure of V(001) slabs with different thicknesses does not display a great difference. The ferromagnetic structure of an n layers slab of Pd grown on Ag(001) displays ferromagnetism for an intermediate value of n whereas paramagnetism is present for n = 1 and n ⪢ 3.