Formation Of Long-range Magnetic Order Research Articles

Magnetic ordering and the role of superexchange Ni–O–B–O–Ni upon the formation of magnetic order in ludwigite Ni2MnBO5 from first-principal calculations

The energies of various magnetically ordered structures for ludwigite Ni2MnBO5 have been calculated in the framework of the first-principles approach using the Wien2K program package, with the parameters of exchange interactions being determined. Two subsystems can be distinguished in the magnetic system, which are associated with the triads 4-2-4 and 3-1-3. The magnetic moments of the ions in both triads are antiferromagnetically oriented. The analysis of the exchange contribution to the energy shows that there occurs an increase of the magnetic cell relative to the crystallographic one due to antiferromagnetic ordering of the magnetic moments along the c axis in three-legged ladders formed by 4-2-4 triads. However, in three-legged ladders formed by 3-1-3 triads, the magnetic moments of the ions are ordered along the c-axis ferromagnetically. The predicted type of magnetic ordering in Ni2MnBO5 is similar to magnetic ordering in Fe3BO5. Non-collinear ordering of the magnetic moments of the subsystems at different temperatures is also likely to be observed in Ni2MnBO5, as is the case in Fe3BO5. Superexchange (indirect) interactions (No-O-B-O-Ni) influence the orientation of the magnetic moments of two 3-1-3 (4-2-4) triads within the unit cell. It is these interactions that may be responsible for the formation of long-range magnetic order in Co3BO5 and ferrimagnetic-antiferromagnetic spin-reorientation transition in Fe3BO5.

Long-range magnetic order in CePdAl3 enabled by orthorhombic deformation

We investigate the effect of structural deformation on the magnetic properties of orthorhombic CePdAl3 in relation to its tetragonal polymorph. Utilizing x-ray and neutron diffraction, we establish that the crystal structure has the Cmcm space-group symmetry and exhibits pseudotetragonal twinning. According to density functional calculations, the tetragonal-orthorhombic deformation mechanism has its grounds in the relatively small free enthalpy difference between the polymorphs, allowing either phase to be quenched, and fully accounts for the twinned microstructure of the orthorhombic phase. Neutron diffraction measurements show that orthorhombic CePdAl3 establishes long-range magnetic order below TN=5.29(5) K characterized by a collinear, antiferromagnetic arrangement of magnetic moments. Magnetic anisotropies of orthorhombic CePdAl3 arise from strong spin-orbit coupling as evidenced by the crystal-field splitting of the 4f multiplet, fully characterised with neutron spectroscopy. We discuss the potential mechanism of frustration posed by antiferromagnetic interactions between nearest neighbors in the tetragonal phase, which hinders the formation of long-range magnetic order in tetragonal CePdAl3. We propose that orthorhombic deformation releases the frustration and allows for long-range magnetic order. Published by the American Physical Society 2024

Unusual magnetic behavior of cubic double perovskite Ba2YIrO6 with Ir5+(5d 4) ions

Using density functional calculations, we investigate the electronic structure and magnetism of double-perovskite Ba2YIrO6 to explore the origin of its unusual magnetic behavior. Our results show that the breakdown of expected j = 0 nonmagnetic state of Ir5+(5d 4) in Ba2YIrO6 is induced by both bandwidth and exchange interaction. In addition, the bandwidth effect makes the system behave like metal, while the antiferromagnetic interaction between Ir ions should be responsible for the insulator behavior. Moreover, the strong geometrical frustration in Ir triangular unit prevents the formation of long-range magnetic order.

Simulation of exact quantum Ising models with a Mott insulator of paired atoms

Quantum simulation of the XXZ model with a two-component Bose or Fermi Hubbard model based on a Mott insulator background has been widely used in the investigations of quantum magnetism with ultracold neutral atoms. In most cases, the diagonal spin-spin interaction is always accompanied by a large spin-exchange interaction which hinders the formation of long-range magnetic order at low temperature. Here we show that the spin-exchange interaction can be strongly reduced in a Mott insulator of paired atoms, while the diagonal spin-spin interaction remains unaffected. Thus, the effective magnetic model is quite close to an exact classical Ising model in the textbook. And we analysed an experimentally achievable three-component Fermi-Hubbard model of $\mathrm{{}^{6}Li}$ with two hyperfine levels of atoms paired in the lattice. We find the long-range antiferromagnetic order of such a three-component Fermi-Hubbard model can be much stronger than that of a typical two-component Fermi-Hubbard model at low temperature. And we discussed the possiblity of simulating an exact two-dimensional ferromagnetic Ising model in a Mott insulator of paired bosonic atoms. Our results may be useful for experimental investigation of the long-range Ising-like magnetism with ultracold neutral atoms under thermal equilibrium.

Inhomogeneous Kondo-lattice in geometrically frustrated Pr2Ir2O7

Magnetic fluctuations induced by geometric frustration of local Ir-spins disturb the formation of long-range magnetic order in the family of pyrochlore iridates. As a consequence, Pr2Ir2O7 lies at a tuning-free antiferromagnetic-to-paramagnetic quantum critical point and exhibits an array of complex phenomena including the Kondo effect, biquadratic band structure, and metallic spin liquid. Using spectroscopic imaging with the scanning tunneling microscope, complemented with machine learning, density functional theory and theoretical modeling, we probe the local electronic states in Pr2Ir2O7 and find an electronic phase separation. Nanoscale regions with a well-defined Kondo resonance are interweaved with a non-magnetic metallic phase with Kondo-destruction. These spatial nanoscale patterns display a fractal geometry with power-law behavior extended over two decades, consistent with being in proximity to a critical point. Our discovery reveals a nanoscale tuning route, viz. using a spatial variation of the electronic potential as a means of adjusting the balance between Kondo entanglement and geometric frustration.

Inelastic light scattering in the spin cluster Mott insulator Cu2OSeO3

Clusters of single spins form the relevant spin entities in the formation of long-range magnetic order in spin cluster Mott insulators. Such type of spin order bears resemblance to molecular crystals, and we therefore may expect a prototypical spin wave spectrum which can be divided into low-energy external and high-energy internal cluster spin wave modes. Here, we study high-energy spin cluster excitations in the spin cluster Mott insulator Cu$_{2}$OSeO$_{3}$ by means of spontaneous Raman scattering. Multiple high-energy optical magnon modes are observed, of which the Raman-activity is shown to originate in the Elliot-Loudon scattering mechanism. Upon crossing the long-range ordering transition temperature the magnetic modes significantly broaden, corresponding to scattering from localized spin excitations within the spin clusters. Different optical phonon modes show a strong temperature dependence, evidencing a strong magnetoelectric coupling between optical phonons and the high-energy spin cluster excitations. Our results support the picture that Cu$_{2}$OSeO$_{3}$ can be regarded as a solid-state molecular crystal of spin nature.

Coupled dynamics of long-range and cluster-internal spin order in the cluster Mott insulator Cu2OSeO3

Cu$_4$ triplet clusters form the relevant spin entity for the formation of long-range magnetic order in the cluster magnet Cu$_2$OSeO$_3$. Using time-resolved Raman spectroscopy, we probed photoinduced spin and lattice dynamics in this Mott insulator. Multiple ps-decade spin-lattice relaxation dynamics is observed, evidencing a separation of the order parameter dynamics into disordering of long-range and internal spin cluster order. Our study exemplifies the double order parameter dynamics of generalized molecular crystals of charge, spin, and orbital nature.

Static and dynamic magnetic properties of two synthetic francisites Cu3La(SeO3)2O2X (X = Br and Cl)

The formation of long-range magnetic order at low temperatures was established in francisite—type compounds Cu3La(SeO3)2O2X (X = Br and Cl) through measurements of magnetic susceptibility, magnetization, specific heat and X-band electron spin resonance. The significantly enhanced critical index p = 1.0 ± 0.1 in Cu3La(SeO3)2Br and p = 0.8 ± 0.1 in Cu3La(SeO3)2Cl in the temperature dependence of the width of ESR signal evidence the reduced dimensionality of the kagome-type francisite’s magnetic subsystem. Under action of external magnetic field, the presumably non-collinear six-sublattices antiferromagnetic structure of these compounds experiences the first-order metamagnetic transformation. The B–T magnetic phase diagrams were established from the positions of singularities in temperature and field dependences of thermodynamic properties. Contrary to pristine mineral Cu3Bi(SeO3)2Cl, no signature of structural phase transition was detected.

Investigation of the GdMn2O5 multiferroic by the μSR method

The GdMn2O5 multiferroic (a ceramic sample and a sample consisting of a large array of randomly oriented single crystals with linear dimensions 2–3 mm) has been studied by the μSR method within the temperature range 10–300 K. Three anomalies in the temperature behavior of the parameters of the muon polarization relaxation function, namely, close to the phase transition driven by the onset of long-range magnetic order in the manganese ion subsystem (T N1 = 40–41 K), near the lock-in transition initiated by an abrupt change of the wave vector of magnetic order (T L = 35 K), and close to the Gd3+ ion ordering temperature (T N2 = 15 K), have been found. An analysis of the time spectra of muon spin precession in the internal magnetic field of the samples has revealed two positions of preferable muon localization sites in samples, which differ in precession frequencies and the character of their behavior with temperature. The lower-frequency precession driven by Mn4+ ions, ferromagnetic Mn4+-Mn4+ + muonium complexes, and Gd3+ions is observed throughout the temperature region T < T N1 and is practically independent of temperature. At temperatures T < T L = 35 K, a higher-frequency precession associated with Mn3+ ions appears also. It is characterized by a temperature dependence ∼(T/T N1)β with the index β = 0.39, which is typical of Heisenberg-type 3D magnets. For T < T N1, a deficiency of the rest total asymmetry is observed. This phenomenon can probably be assigned to formation of muonium, which suggests that charge transfer processes play an important role in formation of long-range magnetic order.

Frustration driven magnetic states of A-site spinels probed by μSR

The normal A-site spinels MnAl2O4, FeAl2O4, CoAl2O4, as well as related mixed (Mn0.5Fe0.5Al2O4) and partially inverted (Fe1.4Al1.6O4) spinels have been studied by μSR. The magnetic ions are subject to magnetic frustration by competing interactions. In all materials and at all temperatures the μSR spectra consist of two signals suggesting a bimodal distribution of the fluctuation rates of magnetic moments. A characteristic temperature TM is found in each compound, representing either a magnetic phase transition into a long-range ordered state (MnAl2O4, Fe1.4Al1.6O4) or the formation of a spin liquid phase (FeAl2O4, CoAl2O4, Mn0.5Fe0.5Al2O4). The magnetic ground state of MnAl2O4 shows coexistence of antiferromagnetic and spin liquid phases. In FeAl2O4 and CoAl2O4 long-range order is suppressed altogether, the ground state can be characterized as a fast relaxing spin liquid coexisting with a small fraction of paramagnetic spins. The partial replacement of Mn by Fe in Mn0.5Fe0.5Al2O4 prevents long-range order and leads to a spin liquid state in the low temperature limit. The partial occupancy of B-sites by magnetic ions in Fe1.4Al1.6O4 strengthens the exchange coupling, allowing the formation of long-range magnetic order at a rather high temperature (~100 K). Magnetic phase diagrams are presented demonstrating that for the studied compounds the magnetic properties are determined by the degree of frustration.

Realization of the Nersesyan-Tsvelik model in(NO)[Cu(NO3)3]

The topology of the magnetic interactions of the copper spins in the nitrosonium nitratocuprate $(\text{NO})[\text{Cu}{({\text{NO}}_{3})}_{3}]$ suggests that it could be a realization of the Nersesyan-Tsvelik model [A. A. Nersesyan and A. M. Tsvelik, Phys. Rev. B 67, 024422 (2003)], whose ground state was argued to be either a resonating valence-bond state or a valence-bond crystal. The measurement of thermodynamic and magnetic resonance properties reveals a behavior inherent to low-dimensional spin $S=\frac{1}{2}$ systems and provides indeed no evidence for the formation of long-range magnetic order down to 1.8 K.

RKKY Interaction in Coupled Quantum Dots

In artificial nanostructures the control of localized, single spin is of great technological importance for nanospintronic applications. In that respect quantum dots (QD), as the building blocks of nanodevices, seem to be most perspective. Quantum dots are electronic systems which confine a well-defined number of electrons. The total spin of a dot can be integer or half-integer (the latter stands for odd number of confined electrons). Typical nanodevices are composed of a few QD interconnected by leads. Electrons (from a lead or metallic substrate) scattered off QD spins induce an effective spin exchange interaction, the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction [1]. Magnetic correlations of QDs spins arising from the RKKY interaction are very important as they affect conductivity, an effect of primary importance for spintronics. Magnetic moments of QDs are Kondo (partially) screened, so the strength of the RKKY coupling depends both on the QDs separation and the Kondo screening. The Kondo screening can be tuned by the gate voltage; this effect offers an opportunity to control the strength of the effective RKKY exchange. Such a tuned RKKY interaction between QD moments is deemed as a possible way to couple QD based qubits in construction of universal quantum logic gates. We focus our attention on the study of pure RKKY coupling between two dots joined by a lead, leaving the Kondo contribution aside. Two distinct configurations of interacting QDs are considered; the interaction via electrons of a substrate and the other one via electrons of metallic lead which joins adjacent dots. The problem of formation of long-range magnetic order within a quasi-1D ferromagnetic lead is discussed independently.

Effect of the 16O → 18O isotopic substitution and a magnetic field on the spin dynamics in the (La0.25Pr0.75)0.7Ca0.3MnO3 manganite according to the 139La NMR data

139La nuclear magnetic resonance spectra have been recorded and the spin-spin relaxation times in the paramagnetic region in external magnetic fields of 5 and 9.4 T have been measured for two types of manganites: initial (La0.25Pr0.75)0.7Ca0.3Mn16O3 and enriched with the 18O isotope ((La0.25Pr0.75)0.7Ca0.3Mn18O3). The manganite enriched with the heavier oxygen isotope exhibits disappearance of a signal in the charge-ordering region (T < T co) in an external field of 5 T. This effect is related to the anomalous increase in the spin-spin relaxation rate. With an increase in the external magnetic field to 9.4 T, the difference in the behavior of the relaxation rate for the two samples in the charge-ordering region remains pronounced, although the magnitude of this effect becomes much smaller. The observed giant isotope effect is a bright evidence of the important role of oxygen motion in the formation of long-range magnetic order in the manganites under consideration.

Magnetic short-range order in the new ternary phase Mn 8Pd 15Si 7

A new compound, Mn 8Pd 15Si 7, is reported to crystallize in a face centered cubic unit cell of dimension a = 12.0141(2) Å, space group F m 3 ¯ m , and can thus be classified as a G-phase. The crystal structure was studied by single crystal X-ray diffraction, X-ray and neutron powder diffraction and electron diffraction. A filled Mg 6Cu 16Si 7 type structure was found, corresponding to the Sc 11Ir 4 type structure. The magnetic properties were investigated by magnetization measurements and Reverse Monte Carlo modeling of low temperature magnetic short-range order (SRO). Dominating near neighbor antiferromagnetic correlations were found between the Mn atoms and geometric frustration in combination with random magnetic interactions via metal sites with partial Mn occupancy were suggested to hinder formation of long-range magnetic order.

On the relationship between crystal structure and magnetism in U2Pd2Sn and U2Ni2In (abstract)

Isostructural U2T2X compounds (T=transmission metal, X=p-electron element) offer a new perspective on the relationship between magnetic anisotropy and the geometrical arrangement of the elements, because (unlike most other uranium compounds) two approximately equal shortest interuranium distances are found. Our recent neutron diffraction indicate that the moment arrangement in magnetically ordered U2T2X compounds is extremely sensitive to the choice of the constituents.1 In order to get a deeper understanding of what finally determines the moment configuration in U2T2X compounds, we have studied two members of this large family, namely U2Ni2In and U2Pd2Sn. Both compounds have the shortest U–U links along the c axis and order antiferromagnetically at low temperatures. The exchange coupling along the shortest U–U link is antiferromagnetic in U2Ni2In, while it is ferromagnetic in U2Pd2Sn, though both compounds exhibit a noncollinear in-plane configuration of the U moments. Here, we present the results of our neutron-diffraction studies performed at various temperatures between 12 and 300 K, for temperatures above and below TN. Our analysis reveals that neither U2Ni2In nor U2Pd2Sn exhibit “normal” thermal-expansion behavior (linear contraction of the lattice with decreasing temperature). However, while deviations from the normal behavior in U2Pd2Sn occur only below TN (and may be attributed to magnetically driven distortions of the crystal lattice), unusual thermal expansion for U2Ni2In is found already at much higher temperatures (far above TN). We discuss, whether the unusual thermal expansion in U2Ni2In supports the formation of long-range magnetic order or whether short-range magnetic fluctuations above TN are responsible for the expansion behavior in this compound.

Investigation of new lanthanum-, cerium- and uranium-based ternary intermetallics

While searching for new Ce- and U-based “heavy fermion” systems, we have investigated the following intermetallic compounds: CeCu 2Ge 2, CeCu 2Sn 2, CeAg 2Ge 2, CeZn 2Al 2 (tetragonal ThCr 2Si 2-structure and modifications thereof); CeRu 3Si 2, URu 3Si 2 (hexagonal LaRu 3Si 2-structure); Ce 2Cu 7Al 10, U 2Cu 7Al 10 (trigonal Th 2Mn 17-structure); CeCu 4Al 8, CeAg 4Al 8, UCu 4Al 8 (tetragonal CeMn 4Al 8-structure); LaCu 13− x Al x with x = 3, 5.5, and 6.5, as well as CeCu 6.5Al 6.5 (cubic NaZn 13-structure). Resistive and inductive measurements were undertaken between T = 30 mK and room temperature, and two new superconductors were found: LaCu 6.5Al 6.5 ( T c = 0.65 K) and CeRu 3Si 2 ( T c = 1.2 K). The specific heat and thermopower of two of these systems have also been measured. CeCu 4Al 8 shows interesting features that are presumably related to fine structure in the “heavy-fermion” density of states near the Fermi level. CeCu 6.5A1 6.5 shows a magnetic transition at T = 0.5 K which resembles a “spin-glass freezing” rather than a formation of long-range magnetic order.