Antiferromagnetic Sublattices Research Articles

Magnetic Order in Li-Mn Spinels

Abstract Magnetic measurements were carried out on different samples of Lithium-Manganese spinel LiMn2O4 , great care having been taken to avoid the presence of spurious magnetic phases, such as Mn3O4 . Susceptibility data, showing deviations from paramagnetic behaviour at about 40 K, were analyzed in terms of local magnetic interactions, taking into account the structural and transport properties of these com-pounds. The magnetic response of pure and stoichiometric samples suggests that the onset of a longrange magnetic ordering is hindered by the topological frustration of the antiferromagnetic octahedral sublattice of the spinel.

Oscillating Antiferromagnetism of Ultrathin EuTe Layers

We study magnetic resonance on EuTe/PbTe superlattices. Analysis of the magnetic dipole anisotropy in the superlattices and of the EPR amplitude of isolated Eu ions in the PbTe wells shows that the diffusion at interfaces is very small. The real thickness of EuTe can be evaluated with an accuracy better than one monolayer. Since for EuTe layers thicker than 2 monolayers there is no difference in the character of the antiferromagnetic resonance observed for even and odd numbers of monolayers, we conclude that there is no static magnetization of the antiferromagnetic sublattices. Nevertheless, long range antiferromagnetic order is clearly evident.

Influence of processing on the Li:Mn ratio in spinel phases of the system Li 1 + xMn 2 − xO 4 − δ

The composition of the spinel phase in the system LiMnO is studied under the influence of a controlled gas atmosphere and temperature/time schedule. The existence of an extended spinel phase of composition Li[Mn 2 − x Li x ]O 4 − δ (0 ≤ x ≤ 1 3 ) is shown. The Li Mn ratio as well as the proportion between Mn 3+ and Mn 4+ is adjustable by the method of preparation. LiMn 2O 4 − δ can be prepared in a cubic and tetragonal modification. According to magnetic and spectroscopic measurement, no simple ionic model can describe the electron structure throughout the entire series of solid solution. A partial charge delocalization takes place resulting in a reduction of the total number of unpaired electrons. At lower temperatures a spin glass behaviour develops due to the inherent frustration of the octahedral antiferromagnetic sublattice. The freezing temperature depends on the Li Mn ratio and decreases with increasing x.

Spin density wave selection in the one-dimensional Hubbard model

The Hartree - Fock ground-state phase diagram of the one-dimensional Hubbard model is calculated, constrained to uniform phases, which have no charge density modulation. The allowed solutions are saturated ferromagnetism (FM), a spiral spin density wave (SSDW) and a double spin density wave (DSDW). The DSDW phase comprises two canted interpenetrating antiferromagnetic sublattices. FM occurs for small filling, SSDW in most of the remainder of the phase diagram, and DSDW in a narrow tongue near quarter (and three-quarter) filling. Itinerant electrons lift the degeneracy with respect to canting angle in the DSDW. The Hartree - Fock states are metallic except at multiples of a quarter filling. Near half filling the uniform SSDW phase is unstable against phase separation into a half-filled antiferromagnetic phase and a hole-rich SSDW phase. The dependence of the ground-state wave number on chemical potential is conjectured to be a staircase. Comparison is made with higher-dimensional Hubbard models and the Heisenberg model.

Weak ferromagnetism and magnetically modulated microwave absorption at low magnetic fields in 1,3,5-triphenyl-6-oxoverdazyl

The magnetic susceptibility and microwave properties of powder samples of 1,3,5-triphenyl-6-oxoverdazyl were investigated between room temperature and 1.8 K. The high-temperature susceptibility follows a Curie-Weiss law with a Curie constant of 0.38(1) emu K mol−1 and a Weiss constant of −12(2) K. At low temperature we find 1D magnetic behaviour evidenced by a broad maximum in the susceptibility. Below 4.9 K weak ferromagnetism is detected which is attributed to a canting of the antiferromagnetic sublattices of about 0.13°. Strong nonresonant microwave absorption appears below 4 K which exhibits a hysteresis when measured in a small scanned external field. Possible origins of the weak ferromagnetism and the nonresonant microwave absorption are discussed.

Antiferromagnetic ordering and crystal-field splittings of the Ho3+ ions in HoBa2Cu4O8

The antiferromagnetic structure of the high-temperature superconductor HoBa2Cu4O8 with Tc=77 K was determined by means of neutron diffraction at T=9 mK. Imperfect three-dimensional antiferromagnetic ordering of the Ho3+ ions corresponding to propagation vector K=(0, 1/2 , 0) is found below TN=100 mK with magnetic moments of magnitude 1.5 mu B at saturation and oriented parallel to the a-axis. The antiferromagnetic sublattice is characterized by stacking faults corresponding to a finite correlation length xi c=18 AA along the c-axis. The majority of the 17 crystal-field level within the ground-state J-multiplet of the Ho3+ ions is established by inelastic neutron scattering, which allowed an unambiguous determination of the nine assignable crystal-field parameters. The observed temperature dependence of the order parameter is found to be in excellent agreement with mean-field calculations including crystal-field, exchange and hyperfine interactions.

Magnetic specific heat and critical magnetic susceptibility of the diluted antiferromagnet CopMg1−pO

For CopMg1−pO, a prototype randomly diluted fcc antiferromagnet, measurements of the specific heat versus temperature show that the size of the λ anomaly at TN decreases as p decreases. These results show that with increasing dilution, a progressively larger fraction of the magnetic entropy is removed by short-range order processes. The critical susceptibility χc at TN increases as p decreases, supporting the idea that near Mg sites, the balance of the two antiferromagnetic sublattices is destroyed. After correcting for single ion contribution, χc(p)/χc(1) is found to vary as (p−pc)−τ for p<0.47 (pc=0.136). The experimental value of τ≊0.7 is in reasonable agreement with the numerical estimate of τ=0.5±0.2 by Harris et al. for cubic systems.

Theory of optical absorption by a localized carrier in an antiferromagnetic insulator

We calculate the optical absorption of a spin-1/2 carrier bound to a defect site in an insulating antiferromagnet. We show that when the defect potential is symmetric under interchange of the antiferromagnetic sublattices (as is believed to be the case in most doped high-${\mathit{T}}_{\mathit{c}}$ parent compounds) the interaction of the carrier with the antiferromagnetic background leads to a low-frequency absorption feature near the zone-boundary magnon energy. This feature, however, is not consistent in peak position or total oscillator strength with the low-frequency absorption feature observed in lightly doped high-${\mathit{T}}_{\mathit{c}}$ parent compounds. We suggest the observed feature is due to electron-phonon coupling.

Nuclear resonance scattering of synchrotron radiation by two non-equivalent crystal sublattices in57Fe3BO6

Pure nuclear Bragg reflections off a single crystal of57Fe3BO6 consisting of two antiferromagnetic sublattices were studied using Mossbauer and synchrotron radiation. Energy and time spectra were measured of different reflections (h00), when either only one sublattice was reflecting, (300), or both sublattices at comparable strength either in antiphase, (500), or in phase, (700). Characteristic line shapes and quantum beat modulations revealed the interference of the scattering by the two sublattices.

Magnetic properties of the dense Kondo compounds CePdSn and YbNiSn

The temperature dependence of the electrical resistivity of Ce 0.2La 0.8PdSn suggest that CePdSn is a dense Kondo compound. The experimental Γ value is moderately large, 60 mJ/mol K 2. The magnetic structure of CePdSn determined by neutron diffraction is of a simple spiral one with the magnetic moment of (1±0.3)μ B. The dense Kondo compound YbNiSn is a weak ferromagnet, which originates from the canting of antiferromagnetic sublattice within the ac-plane.

Magnetic Properties ofthe Dense Kondo Compound YbNiSn

Specific heat measurements revealed that YbNiSn is a dense Kondo compound with a large electronic specific heat coefficient γ of 300 mJ/mol K 2 . This compound orders magnetically at 5.5 K, below which the ferromagnetic moment appears along the c -axis. The magnetization along the a -axis vs external magnetic field H curve shows three anomalies at H C1 , H C2 and H C3 , whereas the magnetization along the b -axis increases gradually from zero without anomalies. These results suggest that YbNiSn is the weak ferromagnet with the ferromagnetic moment along the c -axis, which is caused by the canting of antiferromagnetic sublattice within the ac -plane. The magnetic field dependences of the electrical resistivity of YbNiSn are strongly correlated with the magnetizations.

Antiferromagnetism of the new non-superconducting 2201 stacking polytype BiPbSr2Fe1-xMxO6+z with M = Co, Ni and Morin-like spin-flop-transition in BiPbSr2FeO6+z

The magnetic properties and the spin structure of the BiPbSr2Fe1-xMxO6+z compounds with M = Co, Ni, isotypical with the members of the Bi2(T12)Sr2(Ba2)Can-1CunO2n+4 compositional series with n=1, were studied using magnetic susceptibility measurements and 57Fe Mössbauer spectroscopy. All the compounds investigated (with x=0, xCo=0.5 and xNi=0.25) are basically antiferromagnetic. The highest Néel temperature of 212 K is reached for BiPbSr2FeO6+z, where the hyperfine field at the Fe site has with 48.9 T a value close to the expectation for Fe3+ in a high spin state. A comparison between the values of the quadrupole splitting in the paramagnetic and in the magnetically ordered state shows that a spin-flop transition from the basal plane to the c-axis occurs in the temperature range 120-77 K in BiPbSr2FeO6.1. Such a behaviour is reminiscent of the Morin transition in α-Fe2O3, where the spin direction of the antiferromagnetic sublattices rotates from the plane to the trigonal axis. No spin-flop transition takes place in BiPbSr2Fe0.5Co0.5O6.01.

The crystal structure and the magnetic order of U 14Au 51

The crystal structure of U 14Au 51 at 295, 30 and 11 K and its magnetic structure at 11 K were determined by means of neutron diffraction, using the Rietveld powder method. The crystal structure is of the hexagonal Gd 14Ag 51 type (space group P6/ m) over the whole temperature range from 11 to 295 K. Below the Néel temperature T N = 22 K each set of uranium atoms at the positions 6j and 6k forms an antiferromagnetic sublattice, corresponding to the Shubnikov space group P6′/ m. The ordered magnetic moments μ U1 = 0.5(3) μ B and μ U2 = 1.6(3) μ B at saturation are oriented parallel to the c axis. The U3 atoms in positions 2e do not contribute to the magnetic ordering. The relatively short U3-U3 distance is suggestive of 5f-electron energy-band formation.

Antiferromagnetism and enhancement of superconductivity

Enhancement of superconductivity for a superconductor in coexistence with an antiferromagnetic sublattice is investigated. The approach is based on a generalised Ginzburg-Landau free energy functional for the above system. The exchange interaction mechanism between both phases can either lead to enhancement or suppression of superconductivity. The size of enhancement depends on the density of the antiferromagnetic order parameter and its corresponding correlation length.

Magnetic ordering of monoclinic FeNaP2O7 studied by Mössbauer spectroscopy

Mossbauer spectroscopy in zero applied field performed on powder samples of monoclinic FeNaP2O7 denote spontaneous magnetic ordering at temperatures below 29.2±0.2 K. Preliminar results from susceptibility and neutron diffraction measurements, currently in progress, establish the same P21/c space group as the crystalline structure and an equally sized magnetic cell for the array of spins. The hyperfine fields of the two antiferromagnetic sublattices obtained from Mossbauer spectra display coincident Brillouin-type temperature dependences with an extrapolated OK value of 55.0±0.3 T. Below the Neel temperature the spin orientation does not change relative to the crystalline axes. Mean field theory self-consistent calculations performed in order to estimate the relative magnitudes of the exchange interactions produced the values:JAB=−0.61 cm−1, andJAA=−0.37 cm−1. The antiferromagnetism of the system is likely to originate in the different relative numbers of superexchange pathways between sublattices.

The magnetic structure of alpha -CoC2O4.2D2O investigated by means of neutron diffraction

The linear chain system cobaltous oxalate dihydrate alpha -CoC2O4.2D2O with a broad peak of magnetic susceptibility around 18 K was investigated by means of neutron diffraction in the temperature range from 1.4 to 293 K. By means of profile analysis the parameters of the nuclear and magnetic structure were refined. Below the Neel temperature TN=6.1+or-0.1 K, three-dimensional antiferromagnetic long-range order exists, corresponding to a collinear magnetic structure with k=(0,1/2,0) and magnetic moments aligned parallel to the a axis. The observed magnetic structure may be described as two interpenetrating antiferromagnetic sublattices belonging to the Shubnikov group Pbc. The temperature dependence of zero-field magnetisation indicates an abrupt first-order-like change at temperatures close to TN and may be associated with exchange-induced lowering of the crystal symmetry. The ordered magnetic moment of Co2+ at saturation amounts to (3.18+or-0.07) mu B. An estimation of the orbital and spin components of the magnetic moment yields the values mu CoL2+=0.8 mu B and mu CoS2+=2.3 mu B.

Glass-formation, phase relations and magnetic properties of the splat-quenched system of laser-melted (Fe, Mn)2O3-(Bi, B)2O3

Presintered rods of oxide mixtures in the system (Fe, Mn)2O3-(Bi, B)2O3 were melted crucible-free with the help of a laser while suspended in a rhodium-wire microheater. The molten hanging drop was then quenched by pressing between two small copper plates mounted at the ends of tweezers inside a globular reflector. The crystalline phases of Fe2O3, Fe4Bi2O9 and FeBiO3 gradually disappeared when the cooling rate was increased from about 102 to an estimated maximum of 104 deg s−1. Substitution of B instead of Bi, and Mn instead of Fe, substantially changes the phase relations and facilitates glass-formation. The glasses exhibit magnetic inleractions of antiferromagnetic type; only FeBiO3 provides magnetic measurements explainable on the basis of weak ferromagnetism, probably caused by canted spins of the antiferromagnetic sublattice.

A study of the nuclear spin-lattice relaxation time T1for54Mn in the antiferromagnetic dielectric crystal MnCl2.4H2O

A systematic study of the dependence of the nuclear spin-lattice relaxation of 54Mn present as a dilute nuclear impurity in MnCl2.4H2O has been made below 1 K, using the technique of low-temperature nuclear orientation. Variation of T1 for the two antiferromagnetic sublattices is presented as a function of applied field and of temperature, in the region of the antiferromagnetic-spin-flop transition. The differing applied field dependences of T1 for the two magnetic sublattices whose electronic moments are respectively parallel and antiparallel (up and down) to the external field have been interpreted in terms of unequal transverse fluctuations of the electronic spin and spin-wave damping. For the temperature dependence, it is found that 1/T1 increases as Tn with n=1.40(2) and n=1.10(1) for the down and up sublattices respectively. It is shown that a simple spin-wave approach is inadequate to provide a quantitative description of the relaxation phenomena near the antiferromagnetic-spin-flop transition.

Muon spin rotation and magnetic order in the heavy-fermion compound URu2Si2.

Muon spin rotation and relaxation experiments have been carried out in the paramagnetic and magnetically ordered states of the magnetic heavy-fermion superconductor URu/sub 2/Si/sub 2/. The positive-muon (..mu../sup +/) linewidth ..sigma../sub ..mu../ in zero and low applied fields (approx. <100 Oe) was consistent with nuclear dipolar broadening at temperatures above the ordering temperature T/sub N/, but increased below T/sub N/ to a value of )similarreverse arrowto)0.1 ..mu..s/sup -1/. This is definite evidence for magnetic ordering. However, the value of ..sigma../sub ..mu../ corresponds to a rms local field of only )similarreverse arrowto)1 Oe, which is more than an order of magnitude smaller than expected from an antiferromagnetic (AF) array of U moments ..mu..)similarreverse arrowto)0.03..mu../sub B/, as obtained from neutron diffraction. Either the ..mu../sup +/ site is approximately symmetrically placed between the AF sublattices, or the muon suppresses AF ordering in its vicinity. The isotropic component of the observed ..mu../sup +/ frequency shift is small, and the anisotropic shift is of the order of the calculated dipolar contribution for several candidate ..mu../sup +/ sites (including the symmetric one)

Effect of uniaxial stresses on the character of the first-order antiferromagnetic phase transition in an Mn-8 at. % Ge alloy

The method of neutron diffraction is used to construct temperature dependences of the intensity of 110 antiferromagnetic reflection under the influence of compressive stresses σ = 0–150 MPa along [001]. Application of the stresses leads to an increase in the transition temperature, the transition being delayed in proportion to the increase in the load. On the other hand, the loading leads to a reduction in the magnetization jump for the antiferromagnetic sublattices. Thus, at σ > 100 MPa, the antiferromagnetic transformation takes on features of a second-order phase transformation. The transition which takes place in the stressed state is characterized by smoother rearrangment of short-order antiferromagnetic order into long-range order, as is evidenced by erosion of the transition boundary. The model proposed here — which does not ascribe the usual effect to uniaxial stresses on the antiferromagnetic transition in γ-Mn alloys — is based on the anomalous temperature dependence of the elastic constants.