Low-temperature Magnetic Moment Research Articles

Photo-induced high-temperature ferromagnetism in YTiO3

In quantum materials, degeneracies and frustrated interactions can have a profound impact on the emergence of long-range order, often driving strong fluctuations that suppress functionally relevant electronic or magnetic phases1–7. Engineering the atomic structure in the bulk or at heterointerfaces has been an important research strategy to lift these degeneracies, but these equilibrium methods are limited by thermodynamic, elastic and chemical constraints8. Here we show that all-optical, mode-selective manipulation of the crystal lattice can be used to enhance and stabilize high-temperature ferromagnetism in YTiO3, a material that shows only partial orbital polarization, an unsaturated low-temperature magnetic moment and a suppressed Curie temperature, Tc = 27 K (refs. 9–13). The enhancement is largest when exciting a 9 THz oxygen rotation mode, for which complete magnetic saturation is achieved at low temperatures and transient ferromagnetism is realized up to Tneq > 80 K, nearly three times the thermodynamic transition temperature. We interpret these effects as a consequence of the light-induced dynamical changes to the quasi-degenerate Ti t2g orbitals, which affect the magnetic phase competition and fluctuations found in the equilibrium state14–20. Notably, the light-induced high-temperature ferromagnetism discovered in our work is metastable over many nanoseconds, underscoring the ability to dynamically engineer practically useful non-equilibrium functionalities.

Experimental study of the structural, magnetic, electrical, and mechanical properties of possible half-metallic Co[formula omitted]V[formula omitted]FeGe Heusler alloys

In this study, we experimentally investigated quaternary Heusler alloys Co2−xVxFeGe with 0≤x≤1 prepared by arc melting and annealing, and showed that they are promising candidates for spintronics applications. Single phase microstructures were observed for V compositions from x=0.25 to x=0.625. Other V concentrations were multi-phased. All single phase samples had a face centered cubic crystal structure with a lattice constant that increased linearly with the V concentration. The low-temperature saturation magnetic moments were shown to obey the Slater–Pauling rule of thumb for half-metals, which is a prerequisite for half metallicity. All alloys had high Curie temperatures, which scaled linearly with the saturation magnetic moment, thereby facilitating applications at room temperature and above. Electrical transport measurements were performed to elucidate the electronic structures of the alloys. The temperature dependence of the electrical resistivity was analyzed and discussed in the framework of the two-current conduction model by considering the existence of an energy gap in the electronic spectrum around the Fermi level of the spin down sub-band. High mechanical hardness values were also observed.

Effect of mixing the low-valence transition metal atoms Y = Co, Fe, Mn, Cr, V, Ti, or Sc on the properties of quaternary Heusler compounds Co2−xYxFeSi ( 0≤x≤1 )

In this paper we report an experimental study of structural, magnetic, and mechanical properties of quaternary Heusler alloys ${\mathrm{Co}}_{2\ensuremath{-}x}{Y}_{x}\mathrm{FeSi}$ ($Y$ = Co, Fe, Mn, Cr, V, Ti, or Sc, $0\ensuremath{\le}x\ensuremath{\le}1$) and the experimental findings are supported by ab initio electronic structure calculations. The alloys were synthesized using an arc-melting technique. Single phase microstructures are observed for all alloys substituted with low-valence transition metals $Y$ except Sc. X-ray powder diffraction patterns at room temperature show the presence of Heusler-like face-centered cubic crystal structure in all single phase specimens. The low-temperature saturation magnetic moments, as determined from magnetization measurements, agree fairly well with our theoretical results and also follow the Slater-Pauling rule of thumb for half-metals, a prerequisite for half-metallicity. The alloys are predicted to exhibit half-metallic ferromagnetism by ab initio electronic structure calculations using the $\mathrm{GGA}+U$ approach. All stable compounds are observed to have high Curie temperatures with linear dependence with the valence electrons concentration in the alloys. Relatively high hardness values are also measured, approaching 15.7 GPa for Ti-substituted material, highest among the values reported for Heuslers so far. All these properties strongly suggest the alloys are promising for the spintronic applications at room temperature and above.

Possible half-metallic behavior of Co2−xCrxFeGe Heusler alloys: Theory and experiment

This paper reports a combined experimental and theoretical study of structural, electronic, magnetic, and mechanical properties of quaternary Heusler alloys ${\mathrm{Co}}_{2\ensuremath{-}x}{\mathrm{Cr}}_{x}\mathrm{FeGe}$ prepared by arc-melting with Cr concentrations $0\ensuremath{\le}x\ensuremath{\le}1$. Single-phase microstructures are observed for Cr compositions from $x=0.25$ to $x=1$. Lower Cr concentrations are multiphased. X-ray diffraction patterns at room temperature reveal a face-centered cubic crystal structure in all single-phase samples. The low-temperature saturation magnetic moments, as determined from magnetization measurements, agree fairly well with our theoretical results and also obey the Slater-Pauling rule for half-metals, a prerequisite for half-metallicity. All alloys are observed to have high Curie temperatures that scale linearly with the saturation magnetic moments. Relatively high mechanical hardness values are also observed. First-principles calculations also predict a finite band gap in the minority spin channel of the alloys, increasing in size with increasing Cr concentration. Cr substitution brings the Fermi level toward the center of this gap while also increasing the majority spin density of states near the Fermi level. As a whole, ${\mathrm{Co}}_{2\ensuremath{-}x}{\mathrm{Cr}}_{x}\mathrm{FeGe}$ shows great promise as a half-metal with 100% spin polarization.

Uncovering pseudo-doublet ground states innate to U(IV) complexes through magnetism

In the June issue of <i>Chem,</i> Chilton, Liddle, and colleagues reported the synthesis and characterization of uranium(IV)-oxo and -imido complexes that exhibit anomalously high low-temperature magnetic moments as a result of nearly doubly degenerate spin-orbit ground states. Traditional crystal-field arguments prove paramount in the elucidation of the electronic configuration of the uranium(IV) molecules.

Anomalous magnetism of uranium(IV)-oxo and -imido complexes reveals unusual doubly degenerate electronic ground states

Anomalous magnetism of uranium(IV)-oxo and -imido complexes reveals unusual doubly degenerate electronic ground states

Influence of Cr-substitution on the structural, magnetic, electron transport, and mechanical properties of Fe3−xCrxGe Heusler alloys

We performed combined experimental and theoretical studies of the effect of Cr substitution for Fe on the structural, magnetic, transport, electronic, and mechanical properties of Fe3-xCrxGe (0⩽x⩽1) intermetallic alloys. Single phase microstructures are observed for x⩽0.70. Higher Cr concentrations x>0.70 are multi-phased. A hexagonal D019 structure is found for all Cr concentrations, with the lattice parameters increasing systematically with an increasing Cr content. All the alloys in the series are found to be ferromagnets with large magnetization values of about 6 μB/f.u. and high Curie temperature above room temperature. The low-temperature saturation magnetic moments agree fairly well with our theoretical results and also obey the Slater-Pauling rule. The density functional theory calculation reveals that Cr substitution energetically favours one of the Fe sites in Fe3Ge. The electrical resistivity measured over the temperature range from 5 K to 400 K shows metallic behavior, with a residual resistivity ratio that decreases with Cr content. Vicker’s hardness values are observed to increase with increasing Cr content to approximately 5 GPa.

Magnetic structure and properties of Ca, Mn-doped bismuth ferrites near the polar/nonpolar phase boundary

Neutron diffraction and magnetization measurements of the Bi0.85Ca0.15Fe1-xMnxO3+δ (x = 0.4, 0.5) compounds have been performed at room and low temperatures to disclose the effect of the mixed-valence Mn substitution on the magnetic structure and properties of the Ca-doped bismuth ferrites near the polar/nonpolar phase boundary. It has been confirmed that the Mn substitution results in the filling of anion vacancies produced by the aliovalent replacement of Bi3+ by Ca2+. The Bi0.85Ca0.15Fe0.6Mn0.4O3+δ compound has the acentric structure specific to the pure BiFeO3 (space group R3c) and displays a G-type antiferromagnetic order at room temperature (m300K = 1.35(2) μB). The magnetic moments localized on the Fe/Mn ions are directed along the polar axis. The spin-reorientation transition from the c to a axis takes place with decreasing temperature. An increase in the Mn concentration gives rise to the polar → nonpolar (R3c → Pnma) structural phase transformation. The nonpolar (x = 0.5) compound has a G-type antiferromagnet structure (TN = 210 K) with spins aligned along the orthorhombic b axis. The low-temperature magnetic moments (m5K = 2.67(2) μB and m5K = 1.80(3) μB for the samples with x = 0.4 and x = 0.5, respectively) are considerably smaller than those predicted for complete spin ordering of the interacting ions of Fe3+, Mn3+ and Mn4+ (>4 μB). While the neutron diffraction measurements reveal no contribution associated with a long-range ferromagnetic order at T = 5 K, a significant increase in the magnetization of the samples, suggesting the formation of a glassy phase, is observed with decreasing temperature.

Nanoscale Metastable ε-Fe3N Ferromagnetic Materials by Self-Sustained Reactions.

A single-step method for the preparation of metastable ε-Fe3N nanoparticles by combustion of reactive gels containing iron nitrate (Fe(NO3)3) and hexamethylenetetramine (C6H12N4) in an inert atmosphere is reported. The results of Fourier transform infrared spectroscopy (FTIR) and thermal analysis coupled with dynamic mass spectrometry revealed that the exothermic decomposition of a coordination complex formed between Fe(NO3)3 and HMTA is responsible for the formation of ε-Fe3N nanoscale particles with sizes of 5-15 nm. The magnetic properties between 5 and 350 K are characterized using a superconducting quantum interference device (SQUID) magnetometer, revealing a ferromagnetic behavior with a low-temperature magnetic moment of 1.09 μB/Fe, high room temperature saturation magnetization (∼80 emu/g), and low remanent magnetization (∼15 emu/g). The obtained value for the Curie temperature of ∼522 K is close to that (∼575 K) for bulk ε-Fe3N reported in the literature.

FCC-L12 ordering transformation in equimolar FeCoNiV multi-principal element alloy

The ordered L12 phase generally appears as dispersed precipitates in FCC multi-principal element alloys (MPEAs). In this work, a single L12 ordered intermetallic is obtained in the annealed equimolar FeCoNiV alloy. Experiments show that the FCC to L12 ordering transformation occurs below 778±2 °C, accompanied by lattice contraction, anti-phase boundaries (APBs) formation and low-temperature magnetic moment decrease. A strong linear strain hardening rate up to 5.0 GPa is obtained during the tensile deformation for L12 ordered alloy at room temperature. The ab initio calculations based on the exact muffin‑tin orbitals in combination with coherent potential approximation predict that V atoms prefer to occupy the cubic corner positions of L12 sublattice, and the FCC phase becomes more stable with respect to L12 phase above 849 °C considering the competition between enthalpy and entropy. The ab initio molecular dynamic simulations indicate that V atoms prefer to bond with the unlike atoms (Fe, Co and Ni) in liquid FeCoNiV, which is consistent with the cubic corner site occupation of V in L12 ordered structure. The present work demonstrates a new design strategy of MPEAs which focus on single phase ductile intermetallic compounds.

Octahedral tilt independent magnetism in confined GdTiO3 films

Low temperature polarized neutron reflectometry measurements are presented, exploring the evolution of ferrimagnetism in thin GdTiO3 films embedded within a SrTiO3 matrix. In GdTiO3 films thinner than ∼4 nm, the TiO6 octahedral tilts endemic to GdTiO3 coherently relax toward the undistorted, cubic phase of SrTiO3. Our measurements indicate that the ferrimagnetic state within the GdTiO3 layers survives as these TiO6 octahedral tilts are suppressed. Furthermore, our data suggest that layers of suppressed magnetization (i.e., magnetic dead layers) develop within the GdTiO3 layer at each GdTiO3/SrTiO3 interface and explain the apparent magnetization suppression observed in thin GdTiO3 films when using volume-averaged techniques. Our data show that the low temperature magnetic moment inherent to the core GdTiO3 layers is only weakly impacted as the octahedral tilt angles are suppressed by more than 50% and the t2g bandwidth is dramatically renormalized.

The low-temperature magnetostructure and magnetic field response of Pr0.9Ca0.1MnO3: the roles of Pr spins and magnetic phase separation

With the goal of elucidating the background of photoinduced ferromagnetism phenomena observed in the perovskite structured (Pr,Ca) manganites, the low-temperature magnetostructure of the material was revised using cold neutron powder diffraction, SQUID magnetometry and ab initio calculations. Particular emphasis was placed on determining the presence of nanoscale magnetic phase separation. Previously published results of a canted A-AFM average ground state were reproduced to a good precision both experimentally and theoretically, and complemented by investigating the effects of an applied magnetic field of 2.7 T on the magnetostructure. Explicit evidence of nanoscale magnetic clusters in the material was obtained based on high-resolution neutron diffractograms. Along with several supporting arguments, we present this finding as a justification for extending the nanoscale magnetic phase separation model of manganites to the material under discussion despite its very low Ca doping level in the context of the model. In the light of the new data, we also conclude that the low temperature magnetic moment of Pr must be ca. 300% larger than previously thought in this material, close to the high spin value of per formula unit.

Spin-driven ordering of Cr in the equiatomic high entropy alloy NiFeCrCo

Spin-driven ordering of Cr in an equiatomic fcc NiFeCrCo high entropy alloy (HEA) was predicted by first-principles calculations. Ordering of Cr is driven by the reduction in energy realized by surrounding anti-ferromagnetic Cr with ferromagnetic Ni, Fe, and Co in an alloyed L12 structure. The fully Cr-ordered alloyed L12 phase was predicted to have a magnetic moment that is 36% of that for the magnetically frustrated random solid solution. Three samples were synthesized by milling or casting/annealing. The cast/annealed sample was found to have a low temperature magnetic moment that is 44% of the moment in the milled sample, which is consistent with theoretical predictions for ordering. Scanning transmission electron microscopy measurements were performed and the presence of ordered nano-domains in cast/annealed samples throughout the equiatomic NiFeCrCo HEA was identified.

Thiocyanate complexes of uranium in multiple oxidation states: a combined structural, magnetic, spectroscopic, spectroelectrochemical, and theoretical study.

A comprehensive study of the complexes A4[U(NCS)8] (A = Cs, Et4N, (n)Bu4N) and A3[UO2(NCS)5] (A = Cs, Et4N) is described, with the crystal structures of [(n)Bu4N]4[U(NCS)8]·2MeCN and Cs3[UO2(NCS)5]·O0.5 reported. The magnetic properties of square antiprismatic Cs4[U(NCS)8] and cubic [Et4N]4[U(NCS)8] have been probed by SQUID magnetometry. The geometry has an important impact on the low-temperature magnetic moments: at 2 K, μeff = 1.21 μB and 0.53 μB, respectively. Electronic absorption and photoluminescence spectra of the uranium(IV) compounds have been measured. The redox chemistry of [Et4N]4[U(NCS)8] has been explored using IR and UV-vis spectroelectrochemical methods. Reversible 1-electron oxidation of one of the coordinated thiocyanate ligands occurs at +0.22 V vs Fc/Fc(+), followed by an irreversible oxidation to form dithiocyanogen (NCS)2 which upon back reduction regenerates thiocyanate anions coordinating to UO2(2+). NBO calculations agree with the experimental spectra, suggesting that the initial electron loss of [U(NCS)8](4-) is delocalized over all NCS(-) ligands. Reduction of the uranyl(VI) complex [Et4N]3[UO2(NCS)5] to uranyl(V) is accompanied by immediate disproportionation and has only been studied by DFT methods. The bonding in [An(NCS)8](4-) (An = Th, U) and [UO2(NCS)5](3-) has been explored by a combination of DFT and QTAIM analysis, and the U-N bonds are predominantly ionic, with the uranyl(V) species more ionic that the uranyl(VI) ion. Additionally, the U(IV)-NCS ion is more ionic than what was found for U(IV)-Cl complexes.

Magnetic structures of Mn3−xFexSn2: An experimental and theoretical study

We investigate the magnetic structure of Mn${}_{3\ensuremath{-}x}$Fe${}_{x}$Sn${}_{2}$ using neutron powder diffraction experiments and electronic structure calculations. These alloys crystallize in the orthorhombic Ni${}_{3}$Sn${}_{2}$ type of structure ($Pnma$) and comprise two inequivalent sites for the transition metal atoms (4$c$ and 8$d$) and two Sn sites (4$c$ and 4$c$). The neutron data show that the substituting Fe atoms predominantly occupy the 4$c$ transition metal site and carry a lower magnetic moment than Mn atoms. Four kinds of magnetic structures are encountered as a function of temperature and composition: two simple ferromagnetic structures (with the magnetic moments pointing along the $b$ or $c$ axis) and two canted ferromagnetic arrangements (with the ferromagnetic component pointing along the $b$ or $c$ axis). Electronic structure calculations results agree well with the low-temperature experimental magnetic moments and canting angles throughout the series. Comparisons between collinear and noncollinear computations show that the canted state is stabilized by a band mechanism through the opening of a hybridization gap. Synchrotron powder diffraction experiments on Mn${}_{3}$Sn${}_{2}$ reveal a weak monoclinic distortion at low temperature ($\ensuremath{\beta}\ensuremath{\sim}90.08$${}^{\ensuremath{\circ}}$ at 175 K). This lowering of symmetry could explain the stabilization of the $c$-axis canted ferromagnetic structure, which mixes two orthorhombic magnetic space groups, a circumstance that would otherwise require unusually large high-order terms in the spin Hamiltonian.

Electronic, structural, and magnetic properties of the half-metallic ferromagnetic quaternary Heusler compounds CoFeMnZ(Z=Al, Ga, Si, Ge)

The quaternary intermetallic Heusler compounds CoFeMn$Z$ ($Z=\text{Al}$, Ga, Si, or Ge) with $1:1:1:1$ stoichiometry were predicted to exhibit half-metallic ferromagnetism by ab initio electronic structure calculations. The compounds were synthesized using an arc-melting technique and the crystal structures were analyzed using x-ray powder diffraction. The electronic properties were investigated using hard x-ray photoelectron spectroscopy. The low-temperature magnetic moments, as determined from magnetization measurements, follow the Slater-Pauling rule, confirming the proposed high spin polarizations. All compounds have high Curie temperatures, allowing for applications at room temperature and above.

Electronic and crystallographic structure, hard x-ray photoemission, and mechanical and transport properties of the half-metallic Heusler compound Co2MnGe

This work reports on the electronic and crystalline structure and the mechanical, magnetic, and transport properties of the polycrystalline Heusler compound Co${}_{2}$MnGe. The crystalline structure was examined in detail by extended x-ray absorption fine-structure spectroscopy and anomalous x-ray diffraction. The compound exhibits a well-ordered $L{2}_{1}$ structure as is typical for Heusler compounds with 2:1:1 stoichiometry. The low-temperature magnetic moment agrees well with the Slater-Pauling rule and indicates a half-metallic ferromagnetic state of the compound, as is predicted by ab initiocalculations. Transport measurements and hard x-ray photoelectron spectroscopy were performed to explain the electronic structure of the compound. The obtained valence band spectra exhibit small energy shifts that are the result of the photoexcitation process, whereas electron-electron correlation in the ground state is negligible. The vibration and mechanical properties of the compound were calculated. The observed hardness values are consistent to a covalent-like bonding of Co${}_{2}$MnGe.

Synthesis, structural and magnetic study of polycrystalline LaNi 1− xMn xO 3 films

Polycrystalline double perovskite LaNi 1− x Mn x O 3 ( x=0.3, 0.4, 0.5 and 0.7, which is defined as Mn03, Mn04, Mn05 and Mn07, respectively) thin films are successfully deposited on Si (1 0 0) substrates via chemical solution deposition method. Their structural and magnetic properties are measured. All the thin films are of single phase. Raman spectra indicate that relative intensity of Mn05 is stronger than that of others that can be attributed to the higher degree of B-site ordering. The low temperature magnetic moment of Mn05 is about 500 emu/cm 3, which is obviously larger than that of Mn03 and Mn07 because of the long-range ordering of Mn and Ni ions in Mn05.

Structural and magnetic modulations inCaCuxMn7 − xO12

Low temperature atomic position modulations and magnetic moment modulations are reported forCaCuxMn7 − xO12 (x = 0.0, 0.1and 0.23) using neutron powder diffraction. Both modulations are described with propagation vectors(0, 0, q) parallelto the c-axis in the hexagonal setting. The present neutron diffraction studies confirmthe quantitative model describing the atomic position modulations inCaCuxMn7 − xO12 (x = 0.0 and 0.1) as derived from synchrotron based powder x-ray diffractionstudies (Sławiński et al 2009 Acta Crystallogr. B 65 535). Neutron diffractionstudies confirm the relation between the atomic position modulation lengthLp and the magnetic modulationlength Lm = 2Lp between 50 K andthe Néel temperature TN. CaCuxMn7 − xO12 (x = 0.1 and 0.23) shows a magnetic phase transition near 50 K associated with considerable changes of themagnetic modulation length and the magnetic coherence length, similar to that observed in the parentCaMn7O12.

Shape-controlled solution synthesis of ferromagnetic copper chromium selenide (CuCr 2Se 4) crystallites

Ferromagnetic crystallites of chalcospinel CuCr 2Se 4 have been synthesized in solution via a one-pot thermal decomposition of metal–acetylacetonate precursors in tri- n-octylamine. The reaction is carried out at temperatures of 300–350 °C in the presence of the cation precursor H 2Se, formed in situ by the reduction of Se powder in β-sitosterol. Good control on the growth rate and the shape of the crystallites is obtained by varying the reaction time and temperature, and the concentration of Se. The crystallites are ferromagnetic at room temperature and under optimal synthesis conditions exhibit a low temperature magnetic moment of about 2.3 μ B per Cr, which is close to reported value for the bulk.