Inhomogeneous Magnetic Order Research Articles

Vertical inhomogeneous magnetic order in FeRh film

FeRh films, known as antiferromagnetic materials at low temperatures, exhibit unexpected ferromagnetic (FM) characteristics, unlike the bulk. The detailed temperature-dependent physical properties of FeRh films were examined to elucidate the causes. Temperature-dependent magnetic and electrical measurements showed that the coexistence of antiferromagnetic and residual FM states induces thermomagnetic irreversibility (e.g., spin-glass-like behavior) and negative magnetoresistance. Polarized neutron reflectometry (PNR) revealed the presence of non-uniform FM states at the interfaces. Additional temperature-dependent PNR profile and room temperature structural and spectroscopic analyses confirmed that the bottom FM region is correlated with structural distortion. In contrast, the top FM region is temperature-dependent and originates from non-stoichiometry.

Vertical Inhomogeneous Magnetic Order in Ferh Film

Vertical Inhomogeneous Magnetic Order in Ferh Film

Impurity-induced magnetic ordering in Sr2RuO4

Ti substituting Ru in Sr$_2$RuO$_4$ in small concentrations induces incommensurate spin density wave order with a wave vector $\boldsymbol{Q} \simeq (2 \pi /3, 2 \pi /3)$ corresponding to the nesting vector of two out of three Fermi surface sheets. We consider a microscopic model for these two bands and analyze the correlation effects leading to magnetic order through non-magnetic Ti-doping. For this purpose we use a position dependent mean field approximation for the microscopic model and a phenomenological Ginzburg-Landau approach, which both deliver consistent results and allow us to examine the inhomogeneous magnetic order. Spin-orbit coupling additionally leads to spin currents around each impurity, which in combination with the magnetic polarization produce a charge current pattern. This is also discussed within a gauge field theory in both charge and spin channel. This spin-orbit coupling effect causes an interesting modification of the magnetic structure, if currents run through the system. Our findings allow a more detailed analysis of the experimental data for Sr$_{2}$Ru$_{1-x}$Ti$_{x}$O$_{4}$. In particular, we find that the available measurements are consistent with our theoretical predictions.

Exactly solvable Kondo lattice model in the anisotropic limit

In this paper we introduce an exactly solvable Kondo lattice model without any fine-tuning local gauge symmetry. This model describes itinerant electrons interplaying with a localized magnetic moment via only longitudinal Kondo exchange. Its solvability results from conservation of the localized moment at each site, and is valid for arbitrary lattice geometry and electron filling. A case study on square lattice shows that the ground state is a N\'{e}el antiferromagnetic insulator at half-filling. At finite temperature, paramagnetic phases including a Mott insulator and correlated metal are found. The former is a melting antiferromagnetic insulator with a strong short-range magnetic fluctuation, while the latter corresponds to a Fermi liquid-like metal. Monte Carlo simulation and theoretical analysis demonstrate that the transition from paramagnetic phases into the antiferromagnetic insulator is a continuous $2D$ Ising transition. Away from half-filling, patterns of spin stripes (inhomogeneous magnetic order) at weak coupling, and phase separation at strong coupling are predicted. With established Ising antiferromagnetism and spin stripe orders, our model may be relevant to a heavy fermion compound CeCo(In$_{1-x}$Hg$_{x}$)$_{5}$ and novel quantum liquid-crystal order in a hidden order compound URu$_{2}$Si$_{2}$.

Chemical and hydrostatic-pressure effects on the Kitaev honeycomb material Na2IrO3

The low-temperature magnetic properties of \tcr{polycrystalline} Na$_2$IrO$_3$, a candidate material for the realization of a quantum spin-liquid state, were investigated by means of muon-spin relaxation and nuclear magnetic resonance methods under chemical and hydrostatic pressure. The Li-for-Na chemical substitution promotes an inhomogeneous magnetic order, whereas hydrostatic pressure (up to 3.9\,GPa) results in an enhancement of the ordering temperature $T_\mathrm{N}$. In the first case, the inhomogeneous magnetic order suggests either short- or long-range correlations of broadly distributed $j=\,$\textonehalf\ Ir$^{4+}$ magnetic moments, reflecting local disorder. The increase of $T_\mathrm{N}$ under applied pressure points at an increased strength of three dimensional interactions arising from interlayer compression.

Quantum Griffiths Phase Inside the Ferromagnetic Phase of Ni_{1-x}V_{x}.

We study by means of bulk and local probes the d-metal alloy Ni_{1-x}V_{x} close to the quantum critical concentration, x_{c}≈11.6%, where the ferromagnetic transition temperature vanishes. The magnetization-field curve in the ferromagnetic phase takes an anomalous power-law form with a nonuniversal exponent that is strongly x dependent and mirrors the behavior in the paramagnetic phase. Muon spin rotation experiments demonstrate inhomogeneous magnetic order and indicate the presence of dynamic fluctuating magnetic clusters. These results provide strong evidence for a quantum Griffiths phase on the ferromagnetic side of the quantum phase transition.

Magnons Heat Transfer and Magnons Scattering in Magnetic Sandwich Lattices: Application to Fe/Gd(5)/Fe System

A model calculation is presented for the coherent magnon transmission and thermal transport at ferromagnetic nanojunction boundaries. The system consists of a Gd ultrathin film sandwiched between two Fe semi-infinite ferromagnetically ordered crystals. The dynamic of the system is analyzed using the equations of motion for the spin precession amplitudes on the lattice sites, valid for the range of temperatures of interest. The coherent transmission and reflection cross sections at the nanojunction boundary are calculated using the matching method. These calculations are presented for arbitrary directions on the boundary, for all accessible frequencies in the propagating bands, at variable temperatures and for a given thicknesses of the ultrathin nanojunction, with no externally applied magnetic field. The model is applied in particular to the Fe/Gd(5)/Fe system with a ferromagnetic Gd nanojunction. Our model yields the total integrated coherent thermal conductivity due to coherent magnons transmission via the sandwiched five Gd spin layers of the nanojunction. It elucidates, in particular, the dependence of the coherent magnons transmission and thermal transport in relation to the spatially inhomogeneous magnetic order of the atomic planes of the nanojunction for a given thickness.

Quantum criticality and inhomogeneous magnetic order in Fe-dopedα−YbAlB4

The intermediate-valent polymorphs $\alpha$- and $\beta$-YbAlB$_4$ exhibit quantum criticality and other novel properties not usually associated with intermediate valence. Iron doping induces quantum criticality in $\alpha$-YbAlB$_4$ and magnetic order in both compounds. We report results of muon spin relaxation ($\mu$SR) experiments in the intermediate-valent alloys $\alpha$-YbAl$_{1-x}$Fe$_x$B$_4$, $x = 0.014$ and 0.25. For $x = 0.014$ we find no evidence for magnetic order down to 25 mK\@. The dynamic muon spin relaxation rate $\lambda_d$ exhibits a power-law temperature dependence $\lambda_d \propto T^{-a}$, $a = 0.40(4)$, in the temperature range 100 mK--2 K, in disagreement with predictions by theories of antiferromagnetic (AFM) or valence quantum critical behavior. For $x = 0.25$, where AFM order develops in the temperature range 7.5--10 K, where we find coexistence of meso- or macroscopically segregated paramagnetic and AFM phases, with considerable disorder in the latter down to 2 K.

Structure and Properties of Nanostructured Vacuum-Deposited Foils of Invar Fe–(35–38 wt%)Ni Alloys

The process of electron beam vacuum deposition of the Fe–(35–38 wt%)Ni alloys at substrate temperatures Ts from 300 to700 °C were used to produce vacuum-deposited foils with the FCC structure, differing by the size of characteristic microstructural elements (grains and subgrains). It was shown that refinement of foil microstructural elements to nanoscale is accompanied by their microhardness increase up to 4–5 GPa. The change of the thermal expansion coefficient (TEC) of the nanostructured (NS) foil of the Fe–35.1Ni alloy within the temperature range from −50 to 150 °C has some deviation from that observed for cast Invar alloy of the same composition. It has been found that the main factors affecting the peculiarities of thermal expansion of the NS foil can be related to the presence of small fraction of BCC-phase in them, high level of crystalline lattice microstrains and inhomogeneous magnetic order in FCC-phase. It was shown that as a result of additional thermal treatment of NS foils their invar properties become similar to that observed for cast Invar alloy but mechanical properties remain on the same level.

Structural and magnetic properties of epitaxial In1–xMnxSb semiconductor alloys with x > 0.08

High temperature ferromagnetic In1−xMnxSb semiconductor alloys with a Curie temperature (TC) above 400 K were investigated. Alloys with x ranging from 0.08 to 0.22 deposited by metalorganic vapor phase epitaxy were examined. X-ray diffraction indicated alloys are primarily two phase consisting of a zinc blende InMnSb solid solution and hexagonal MnSb precipitates. Transmission electron microscopy analysis confirmed the presence of hex-MnSb nanoprecipitates as well as the presence of the additional minority phases Mn3Sb, metallic Mn, and MnAs1−xSbx. Magnetization measurements indicate that the alloy films are ferromagnetic, showing clear hysteresis in field dependent measurements from 5 to 400 K. Magnetization values as high as 47 emu/cm3 for an alloy with x = 0.22 were measured at room temperature. Irreversibility is observed between field-cooled and zero-field-cooled magnetization curves that is attributed to inhomogeneous magnetic order arising from randomly distributed ferromagnetic nanoprecipitates. Temperature dependent magnetization indicates at least two magnetic phases are present, one with a nominal TC of 300 K that is attributed to MnAs1−xSbx nanoprecipitates which form at the GaAs substrate interface, and a second with a TC > 400 K that is attributed to hex-MnSb nanoprecipitates and to the InMnSb matrix. The majority of magnetization arises from the high TC phase, where fitting the temperature dependent magnetization curve with a Brillouin function indicated a TC of 570 K. Magnetization measurements indicate that there is an interparticle magnetic interaction. Large saturation magnetization at 300 K and TC of 570 K make these multiphase InMnSb epitaxial films excellent candidates for ferromagnetic layers in semiconductor spintronic devices that operate at room temperature.

Ferromagnetic Order from p-Electrons in Rubidium Oxide

Magnetic dioxygen molecules can be used as building blocks of model systems to investigate spin-polarization that arises from unpaired p-electrons, the scientific potential of which is evidenced by phenomena such as spin-polarized transport in graphene. In solid elemental oxygen and all of the known ionic salts comprised of magnetic dioxygen anions and alkali metal cations, the dominant magnetic interactions are antiferromagnetic. We have induced novel ferromagnetic interactions by introducing oxygen deficiency in rubidium superoxide (RbO(2)). The anion vacancies in the resulting phase with composition RbO(1.72) provide greater structural flexibility compared to RbO(2) and facilitate a Jahn-Teller-driven order-disorder transition involving the anion orientations at similar to 230 K, below which their axes become confined to a plane. This reorganization gives rise to short-range ferromagnetic ordering below similar to 50 K. A ferromagnetic cluster-glass state then forms below similar to 20 K, embedded in an antiferromagnetic matrix that orders at similar to 5 K. We attribute this inhomogeneous magnetic order to either subtly different anion geometries within different structural nanodomains or to the presence of clusters in which double exchange takes place between the anions, which are mixed-valence in nature. We thus demonstrate that nonstoichiometry can be employed as a new route to induce ferromagnetism in alkali metal oxides.

Inhomogeneous magnetic order in Th-doped UPt 3 detected by μSR

We report on a μSR study of U 1− x Th x Pt 3 ( 0 ⩽ x ⩽ 0.05 ) conducted to investigate the onset of “large-moment antiferromagnetism” (LMAF). At low Th content ( x ⩽ 0.002 ) magnetic ordering on the time scale of the μSR experiment (10 −8 s) is not detected. For x = 0.005 a weak magnetic signal appears below T = 2 K , while for 0.006 ⩽ x ⩽ 0.05 , spontaneous oscillations in the μSR spectra signal the presence of the LMAF phase. Analysis of the μSR spectra with a three-component depolarization function reveals that the magnetic phase transition is quite broad. The broadening may be the effect of disorder on the fluctuation spectrum of the small-moment antiferromagnetic state reported for UPt 3 below T N ∼ 6 K .

Low-energy properties of two-dimensionalmagnetic nanostructures: interparticle interactions anddisorder effects

The low-energy properties of two-dimensional ensembles ofdipole-coupled magnetic nanoparticles are studied as a function of structural disorder and particle coverage. Already smalldeviations from a square particle arrangement lift thedegeneracies of the microvortex (MV) magnetic configuration andresult in a strongly inhomogeneous magnetic order of the particle ensemble. The energy distribution of metastable statesis determined. For a low degree of disorder a stronglyasymmetric shape with a pronounced peak of the ground-stateenergy results. In contrast, for a strong disorder aGaussian-like distribution is obtained. The average dipoleenergy Ēdip decreases with increasingstructural disorder. Above a coverage-dependent degree ofdisorder Ēdip resembles the average dipole energy of a random particle set-up, for which a simplescaling behaviour is derived. The role of vacancies has beenstudied for a square particle array by determining the angulardistribution of the preferred MV angle as a function ofthe vacancy concentration. Preferred angles along the axial aswell as along the diagonal directions of the square array areobtained. A corresponding investigation for disturbed squarearrays yields preferred MV angles only along the axialdirections. The effect of dipole-quadrupole correctionsresulting from the finite size of the particles is quantified.

Inhomogeneous magnetism in electron doped cuprates

We performed μ +SR experiments on polycrystalline specimens of Nd 2− y Ce y CuO 4 and Pr 2− y Ce y CuO 4 with Ce content y= 1 8 . From the viewpoint of μ +SR, both compounds show nearly the same behaviour. Long range magnetic order has been observed below 90 K, which is reflected in the onset of a spontaneous muon spin precession in zero magnetic field. The onset of a broad frequency distribution at 30 K is ascribed to the appearance of inhomogeneous magnetic order in the CuO 2 planes below this temperature. The comparison of the two samples proves that the Nd 3+ magnetic moment is not involved on this anomaly.

High field Mössbauer spectroscopy on Mn-substituted Fe3N

High field Mössbauer experiments are reported for (Fe1−xMnx)3N and compared with results of ASW bandstructure calculations. Depending on the local (FeMn) environment ferro- as well as antiferromagnetic exchange interactions are present, leading to a strongly inhomogeneous magnetic order.

Superconducting gap function and upper critical field in the presence of inhomogeneous magnetic order

Rare earth ternary superconductors are known to exhibit oscillatory magnetic orders below their superconducting transition temperatures. The study of behaviour of superconducting electrons in an inhomogeneous magnetic field is therefore important for such systems. We report here the results of our theoretical study of superconducting gap function Δ(T) and upper critical fieldH c2 (T). The results are applied to analyse and explain the variation of Δ(T) andH c2 (T) in case of NdRh4B4.

Low-temperature magnetic susceptibility of Fe-B amorphous alloys (abstract)

Magnetic properties of amorphous iron based alloys have been extensively studied, however, the mechanism of thermal demagnetization is not well understood and temperature dependence of the static magnetic susceptibility is not well known. In order to address some of these problems we performed an accurate magnetization study of Fe100−xBx metallic glasses (x=20,18,16,14) at low temperatures (4.2–160 K) and in several magnetic fields up to 15 kOe. Magnetization measurements were carried out using VSM with an accuracy of 10−4. At a fixed temperature, the magnetization was measured as a function of an external magnetic field. The demagnetization field was subtracted and the saturation magnetization was determined by linear extrapolation to zero internal field. The saturation susceptibility was obtained from the slope of this curve. The temperature dependence of the saturation magnetization is best described by a T3/2 term, however, the saturation susceptibility is a nonmonotonous function of temperature and cannot be described by spin wave contribution. It exhibits a minimum at low temperature suggesting that some atoms are in zero effective internal fields induced by the mixture of ferro- and antiferromagnetic interactions. The temperature of the susceptibility minimum is lower for higher iron concentration in the alloys. The influence of magnetovolume effects on experimental data is addressed and the results are discussed in terms of inhomogeneous magnetic order.