Freezing Of Moments Research Articles

Magnetic capacity in manganese sulfides with rare earth substitution Mn<sub>1–x</sub>Re<sub>x</sub>S

Polycrystalline samples Mn1–xGdxS and Mn1–xYbxS with a concentration x = 0.2, near the concentration of ion flow through the fcc lattice, are studied in order to determine fluctuations in the valence of the ytterbium ion on dielectric properties. Dielectric constant and dielectric losses were determined from measurements of capacitance and loss tangent in the frequency range 102–106 Hz at temperatures of 80–500 K without a magnetic field and in a magnetic field. The magnetic capacity and dielectric losses in the magnetic field of the sample were determined from the relative change in the real and imaginary parts of the dielectric constant of the sample in a magnetic field H = 12 kOe applied parallel to the capacitor plates. A temperature range with a sharp increase in dielectric constant and with a maximum dielectric loss has been discovered, which shifts with increasing frequency and magnetic field. An increase in dielectric constant and dielectric losses in a magnetic field above 170 K was found in Mn1-xYbxS. The increase in dielectric losses is explained by an increase in relaxation time, as a result of local deformations near ytterbium ions during valence fluctuations. The mechanism for reducing reactance in a magnetic field in Mn1–xYbxS at low frequencies due to capacitance, and at high frequencies due to inductance, has been determined. In the Mn0.8Gd0.2S compound, the imaginary part of the dielectric constant has two maxima. The low-temperature maximum shifts in a magnetic field towards high temperatures and is described in the model of localized electrons with freezing of dipole moments. Dielectric losses decrease in a magnetic field. The magnetic capacity decreases by an order of magnitude in Mn0.8Gd0.2S compared to Mn0.8Yb0.2S. The dielectric constant in both compounds is described in the Debye model with the activation dependence of the relaxation time on temperature, where the activation energies differ for ytterbium and gadolinium ions.

Magnetic properties of the bulk CoFe2O4 polycrystalline under the pressure

Ground state magnetic properties of bulk polycrystalline CoFe2O4 (CFO) were determined based on the competition among inter-grain magnetic dipolar interaction (MDI), exchange coupling (EC), and anisotropy. The present study aims to investigate effects of pressure on magnetic interactions and macroscopic magnetic properties in CFO. Results of zero-field cooling magnetization MZFC measured under different pressures and applied magnetic field of 100 Oe indicated that: (1) pressure effect is temperature-dependent. In low temperature region below 179 K, strong MDI determines the reversal of magnetic moments of particles, and moments are frozen in the direction of random anisotropy. The pressure leads to an increase in magnetization M (ΔM > 0) in low (high) temperature region below (above) 179 K, while reduces M (ΔM < 0) in the intermediate temperature region at around 179 K. The value of ΔM in high temperature region is greater than that in low temperature region. ΔM increases significantly as the pressure increases to 2.21 GPa with ΔM > 0 in the whole temperature range due to pressure-enhanced EC; its value decreases monotonously with increasing temperature. (2) For P = 0 and 0.28 GPa, MZFC changes from positive to negative at 38 and 20 K, respectively. Changes of the anisotropy were caused by magnetic moment freezing, sign conversion of MZFC, and competition of magnetic interactions. (3) For P = 0.12 GPa, remanence ratio increases from 0.73 (P = 0) to 0.80. Moreover, experimental evidence of enhanced EC by pressure was also provided.

Core–shell behavior and exchange bias of Fe-doped CuO nanoparticles

Cu1−xFexO nanoparticles were prepared using a freeze-drying process followed by heat treatment. The particles were then characterized using Mössbauer spectroscopy and magnetization techniques. The results revealed complex magnetic behavior, which can be attributed to the presence of two different magnetic regions: the particle core, which has antiferromagnetic fluctuations, and the particle shell, where uncompensated spins are responsible for their superparamagnetic characteristics. At low temperatures, the moments freeze, revealing a ferromagnetic order for the shells and a dipolar magnetic interaction among the nanoparticles. In addition, an exchange-bias field revealed magnetic interactions between the core and the shell of the nanoparticles. The ferromagnetism observed in this system suggests that antiferromagnetic oxide matrices can be used for diluted magnetic semiconductor applications, if suitably doped.

Deciphering structural and magnetic disorder in the chiral skyrmion host materials Co x Zn y Mn z (x + y+ z = 20).

Co x Zn y Mn z (x + y + z = 20) compounds crystallizing in the chiral β-Mn crystal structure are known to host skyrmion spin textures even at elevated temperatures. As in other chiral cubic skyrmion hosts, skyrmion lattices in these materials are found at equilibrium in a small pocket just below the magnetic Curie temperature. Remarkably, Co x Zn y Mn z compounds have also been found to host metastable nonequlibrium skyrmion lattices in a broad temperature and field range, including down to zero field and low temperature. This behavior is believed to be related to disorder present in the materials. Here, we use neutron and synchrotron diffraction, density functional theory calculations, and dc and ac magnetic measurements to characterize the atomic and magnetic disorder in these materials. We demonstrate that Co has a strong site preference for the diamondoid 8c site in the crystal structure, while Mn tends to share the geometrically frustrated 12d site with Zn, due to its ability to develop a large local moment on that site. This magnetism-driven site specificity leads to distinct magnetic behavior for the Co-rich 8c sublattice and the Mn on the 12d sublattice. The Co-rich sublattice orders at high temperatures (compositionally tunable between 210 and 470 K) with a moment around 1μ B /atom and maintains this order to low temperature. The Mn-rich sublattice holds larger moments (about 3μ B ) which remain fluctuating below the Co moment ordering temperature. At lower temperature, the fluctuating Mn moments freeze into a reentrant disordered cluster-glass state with no net moment, while the Co moments maintain order. This two-sublattice behavior allows for the observed coexistence of strong magnetic disorder and ordered magnetic states such as helimagnetism and skyrmion lattices.

Scrutinizing the role of size reduction on the exchange bias and dynamic magnetic behavior in NiO nanoparticles

NiO nanoparticles (NPs) with a nominal size range of 2–10 nm, synthesized via high-temperature pyrolysis of a nickel nitrate, have been extensively investigated using neutron diffraction and magnetic (ac and dc) measurements. The magnetic behavior of the NPs changes noticeably when their diameter decreases below 4 nm. For NPs larger than or equal to this size, Rietveld analysis of the room temperature neutron diffraction patterns reveals that there is a reduction in the expected magnetic moment per ion with respect to bulk NiO, which is linked to the existence of a magnetically disordered shell at the NP surface. The presence of two peaks in the temperature dependence of both the dc magnetization after zero-field-cooling and the real part of the ac magnetic susceptibility is explained in terms of a core (antiferromagnetic, AFM)/shell (spin glass, SG) morphology. The high-temperature peak ( K) is associated with collective blocking of the uncompensated magnetic moments inside the AFM core. The low-temperature peak ( K) is a signature of a SG-like freezing of the surface spins. In addition, an exchange bias (EB) effect emerges due to the core/shell magnetic coupling. The cooling field and temperature dependences of the EB effect and the coercive field are discussed in terms of the core size and the effective magnetic anisotropy of the NPs. However, NiO NPs of 2 nm in size no longer show AFM order and the magnetic moments freeze into a SG-like state below K, with no evidence of EB effect.

Synthesis, crystal structure and physical properties of quasi-one-dimensional ACr3As3(A = Rb, Cs)

Recently, new Cr-based superconductors, A2Cr3As3 (A = K, Rb, Cs), have gained intense interest because of their one-dimensional crystal structures and electron correlations. Here we report the crystal structure and physical properties of two related materials ACr3As3 (A = Rb, Cs) which are synthesized via a soft-chemical A+ deintercalation in A2Cr3As3. The new compounds remain one-dimensional (Cr3As3)∞ linear chains, and the interchain distance can be tuned by the incorporation of the alkali-metal cations with different sizes. The physical property measurements indicate a local-moment behavior at high temperatures, and the moments freeze into a cluster spin-glass state below 5–6 K. No superconductivity was observed in both materials. We also found that, with increasing the interchain distance, the Cr effective moments increase monotonically, accompanied with the enhancement of semi-conductivity. Our results shed light on the understanding of occurrence of superconductivity in A2Cr3As3.

Neutron Diffraction Study of Parasitic Nd-Moment Order in the Checkerboard-Type Phase Nd1.3Sr0.7NiO4

The Nd-moment order in the layered nickelate Nd2−xSrxNiO4 (x = 0.7) has been investigated by performing a neutron diffraction experiment using a single crystal sample. First, the checkerboard (CB)-type charge order was confirmed by observing the temperature dependence of the nuclear superlattice peak at \(\boldsymbol{{Q}} = (5,0,0)\) between 1.9 and 300 K, which indicates that the transition temperature of the CB-type charge order is above 300 K. Magnetic superlattice peaks with the propagation vector \(\boldsymbol{{k}} = (1 - \varepsilon ,0,1)\) appear below 67 K, and the value of e was determined to be 0.455 in good agreement with previous studies. The intensity of the magnetic superlattice peaks appearing below 67 K shows a sharp increase below ≈20 K. This behavior indicates that the Nd moments freeze under the influence of the Ni ordering. The CB-type antiferromagnetic (AFM) Ni order in the NiO2 layers is stacked antiferromagnetically in the c-axis direction, while the Nd moments in the Nd/SrO2 layers...

Colossal anisotropy of the magnetic properties of doped lithium nitrodometalates

We present a first-principles investigation of the electronic structure and physical properties of doped lithium nitridometalates ${\mathrm{Li}}_{2}$(${\mathrm{Li}}_{1\ensuremath{-}x}{\mathrm{M}}_{x}$)N (LiMN) with M = Cr, Mn, Fe, Co, and Ni. The diverse properties include the equilibrium magnetic moments, magneto-crystalline anisotropy, magneto-optical Kerr spectra, and x-ray magnetic circular dichroism. We explain the colossal magnetic anisotropy in LiFeN by its unique electronic structure which ultimately leads to a series of unusual physical properties. The most unique property is a complete suppression of relativistic effects and freezing of orbital moments for in-plane orientation of the magnetization. This leads to the colossal spatial anisotropy of many magnetic properties including energy, Kerr, and dichroism effects. LiFeN is identified as an ultimate single-ion anisotropy system where a nearly insulating state can be produced by a spin orbital coupling alone. A very nontrivial strongly fluctuating and sign changing character of the magnetic anisotropy with electronic $3d$-atomic doping is predicted theoretically. A large and highly anisotropic Kerr effect due to the interband transitions between atomic-like Fe $3d$ bands is found for LiFeN. A giant anisotropy of the x-ray magnetic circular dichroism for the Fe $K$ spectrum and a very weak one for the Fe ${L}_{2,3}$ spectra in LiFeN are also predicted.

A μSR study of the magnetic properties of Tb2CuIn3

Tb2CuIn3 shows exceptional magnetic behavior within the series of R2CuIn3 intermetallics (R denoting one of the rare earth elements), where long-range magnetic order is absent in favor of spin liquid magnetic ground states as a consequence of frustration by competing exchange. In contrast, neutron studies suggested for the magnetic ground state of Tb2CuIn3 the coexistence of long-range antiferromagnetic and short-range order. The present μSR (muon spin rotation/relaxation) data show that at ∼45 K all Tb magnetic moments freeze into a spin-glass state. At TN = 33 K partial long-range order sets in, its volume fraction being ∼30%. On cooling to 10 K, the antiferromagnetic fraction reaches the saturation value of 70% and hence remains coexistent with the spin-glass state for T → 0. The antiferromagnetic state is characterized by considerable short-range spin disorder. At low temperatures the Tb moments in the two coexisting states approach the static limit. This absence of persistent moment fluctuations together with partial formation of long-range order indicates considerably lower frustration in Tb2CuIn3 compared to the rest of the R2CuIn3 compounds.

Low-temperature spin dynamics of a valence bond glass in Ba2YMoO6

We carried out ac magnetic susceptibility measurements and muon spin relaxation spectroscopy on the cubic double perovskite Ba2YMoO6, down to 50 mK. Below ∼1 K the muon relaxation is typical of a magnetic insulator with a spin-liquid type ground state, i.e. without broken symmetries or frozen moments. However, the ac susceptibility revealed a dilute-spin-glass-like transition below ∼1 K. Antiferromagnetically coupled Mo5+ 4d1 electrons in triply degenerate t2g orbitals are in this material arranged in a geometrically frustrated fcc lattice. Bulk magnetic susceptibility data has previously been interpreted in terms of a freezing to a heterogeneous state with non-magnetic sites where 4d1 electrons have paired in spin-singlets dimers, and residual unpaired Mo5+ 4d1 electron spins. Based on the magnetic heat capacity data it has been suggested that this heterogeneity is the result of kinetic constraints intrinsic to the physics of the pure system (possibly due to topological overprotection) leading to a self-induced glass of valence bonds between neighbouring 4d1 electrons. The muon spin relaxation (μSR) unambiguously points to a heterogeneous state with a static arrangement of unpaired electrons in a background of (valence bond) dimers between the majority of Mo5+ 4d electrons. The ac susceptibility data indicate that the residual magnetic moments freeze into a dilute-spin-glass-like state. This is in apparent contradiction with the muon-spin decoupling at 50 mK in fields up to 200 mT, which indicates that, remarkably, the time scale of the field fluctuations from the residual moments is ∼5 ns. Comparable behaviour has been observed in other geometrically frustrated magnets with spin-liquid-like behaviour and the implications of our observations on Ba2YMoO6 are discussed in this context.

Superparamagnetic blocking and superspin-glass freezing in ultra small δ-(Fe0.67Mn0.33)OOH particles

The magnetic properties of ultra-small (~2 nm) δ-(Fe(0.67)Mn(0.33))OOH nanoparticles prepared by a microemulsion technique have been investigated by magnetization and ac susceptibility measurements at variable frequency. The results provide evidence of two different magnetic regimes whose onset is identified by two maxima in the zero-field-cooled susceptibility: a large one, centered at ~150 K (T(mh)), and a narrow one at ~30 K (T(ml)). The two temperatures exhibit a different frequency dependence: T(mh) follows a Vogel-Fulcher law τ = τ(0)exp[(E(a)/k(B))/(T-T(0))], indicating a blocking of weakly interacting nanoparticle moments, whereas T(ml) follows a power law τ = τ(0)(T(g)/T(mν)-T(g))(α), suggesting a collective freezing of nanoparticle moments (superspin-glass state). This picture is coherent with the field dependence of T(ml) and T(mh) and with the temperature dependence of the coercivity, strongly increasing below 30 K.

Low temperature noncollinear behavior in ultrathin Fe/Al multilayer structures

We report the low temperature noncollinear magnetic behavior of electron beam evaporated ultrathin Fe/Al multilayer (ML) structures. Investigations have been carried out with ML samples with Fe-layer thicknesses in a range of 10–40 Å and Al-layer thickness of 10 Å. The structural studies show that heavy atomic interdiffusion between Fe and Al layers occurs at the interface, resulting MLs of different complicated structures according to different sublayer thicknesses Brajpuriya, et al.., [Eur. Phys. J. B 51, 131 (2006)]. The magnetic measurements show that these films are “re-entrant” systems and not ordinary ferromagnets as often assumed. The obtained results, interestingly, indicate a transition from ferromagnetic state to a low temperature disordered state where a collective frozen magnetic state with grain moments oriented randomly occurs. We interpret the observed low temperature noncollinear magnetic behavior to be due to random freezing of grain moments.

Impurity-Induced Magnetic Order in Low-Dimensional Spin-Gapped Materials

We have studied the effect of nonmagnetic Zn impurities in the coupled spin ladder Bi(Cu_{1-x}Zn_{x})_{2}PO_{6} using ;{31}P NMR, muon spin resonance (microSR), and quantum Monte Carlo simulations. Our results show that the impurities induce in their vicinity antiferromagnetic polarizations, extending over a few unit cells. At low temperature, these extended moments freeze in a process which is found universal among various other spin-gapped compounds: isolated ladders, Haldane, or spin-Peierls chains. This allows us to propose a simple common framework to explain the generic low-temperature impurity-induced freezings observed in low-dimensional spin-gapped materials.

Spin-glass order induced by dynamic frustration

Disorder and geometric frustration usually lead to magnetic spins that point in random directions, as in a spin glass. So how can spin-glass behaviour emerge in a well-ordered system without static frustration? The presence of ‘dynamic frustration’ may explain the situation. Spin glasses are systems whose magnetic moments freeze at low temperature into random orientations without long-range order1. It is generally accepted that both frustration and disorder are essential ingredients in all spin glasses, so it was surprising that PrAu2Si2, a stoichiometric compound with a well-ordered crystal structure, was reported2 to show spin-glass freezing. Here, we report on inelastic neutron scattering measurements of crystal-field excitations, which show that PrAu2Si2 has a singlet ground state and that the exchange coupling is very close to the critical value to induce magnetic order. We propose that spin-glass freezing results from dynamic fluctuations of the crystal-field levels that destabilize the induced moments and frustrate the development of long-range magnetic correlations. This novel mechanism for producing a frustrated ground state could provide a method of testing the concept of ‘avoided criticality’ in glassy systems.

Synthesis of Ordered Mesoporous NiO with Crystalline Walls and a Bimodal Pore Size Distribution

A mesoporous solid with crystalline walls and an ordered pore structure exhibiting a bimodal pore size distribution (3.3 and 11 nm diameter pores) has been synthesized. Previous attempts to synthesize solids with large ordered mesopores by hard templating focused on the preparation of templates with thick walls (the thick walls become the pores in the target materials), something that has proved difficult to achieve. Here the large pores (11 nm) do not depend on the synthesis of a template with thick walls but instead on controlling the microporous bridging between the two sets of mesopores in the KIT-6 template. Such control determines the relative proportion of the two pore sizes. The wall thickness of the 3D cubic NiO mesopore has also been varied. Preliminary magnetic characterization indicates the freezing of uncompensated moments or blocking of superparamagnetism.

NMR Study of Spin State in Geometrically Frustrated Compound Co2Cl(OH)3

NMR studies of a geometrically frustrated system, Co 2 Cl(OH) 3 , are carried out at low temperature. The NMR results of 59 Co and 1 H nuclei show that long-range ordered ferromagnetic moments on the triangular lattice plane coexist microscopically with disordered moments on the kagome lattice plane. The moment direction is not ordered. Although the disordered moments freeze within the NMR observation time (10 -4 s) at low temperature, the frozen moments begin to fluctuate with increasing temperature. Up to 4.2 K (0.4 T C ), the magnitude of the ferromagnetically ordered moments is independent of temperature.

Muon Spin Relaxation Measurements on a Spin–Ladder Material Na2Co2(C2O4)3(H2O)2

Positive muon spin-relaxation (µ + SR) measurements on a spin–ladder material Na 2 Co 2 (C 2 O 4 ) 3 (H 2 O) 2 are reported. We find that the relaxation rate, λ, under a longitudinal field of 3950 Oe increases with decreasing temperature, T , and is a maximum at T peak ≃18 K. Below T peak , λ decreases with decreasing T . This finding implies the observation of a spin gap formation by µ + SR technique. The approach of the exponent β in the relaxation function to 0.5 with decreasing T suggests a freezing of dilute moments. Below about 5 K, we observe a sinusoidal oscillation in the time spectrum in zero applied field, which indicates an onset of long-range magnetic order. All these observations give microscopic evidence that the spin–ladder material is close to a quantum critical point.

Low temperature noncollinear behavior in FePt nanogranular thin film system

We report the low temperature noncollinear magnetic behavior of direct current (DC) sputtered FePt thin films investigated by performing DC magnetization, thermoremanence, magnetic relaxation, and electrical transport measurements down to 1.8 K. The obtained results, interestingly, indicate a transition from ferromagnetic state to a low temperature disordered state where a collective frozen magnetic state with grain moments oriented randomly occurs. The magnetic relaxation and electrical resistivity measurements at low temperature support the spin-glass like phase, which diminishes and finally disappears with an applied field of moderate strength. We interpret the observed low temperature noncollinear magnetic behavior to be due to random freezing of grain moments.

Magnetic and Electrotransport Properties of the Anion-Deficient Manganites with Perovskite Structure

The chemical phase content and microstructure as well as magnetic and magnetotransport properties for the doped anion-deficient La3+1-xSr2+ x Mn3+O2−3-x/2 orthomanganites have been experimentally studied. The high-quality anion-deficient ceramic with controlled chemical phase content and microstructure has been obtained from the parent stoichiometric one at the 800 C in vacuum. The average grain size is ~5 μm. It is established that the samples in the region of 0 ⩽ x ⩽ 0.125 are O′-orthorhombic perovskites whereas in the 0.175 ⩽ x ⩽ 0.30—rhombohedric. As doping level increases along with the oxygen vacancies number the samples in ground state undergo a transition from the antiferromagnet A-type (x=0) through the inhomogeneous magnetic state (0 < x ⩽ 0.175 to the cluster spin glass one (0.175 < x ⩽ 0.30. The temperature of magnetic moment freezing is ~ 45 K. All the samples are semiconductors and show considerable magnetoresistance over a wide temperature range with peak for x = 0.175 only. Fitting of the electrical resistivity to T−1 and T−1/4 realized. The concentration dependences of the activation energy, spontaneous magnetization, coercivity as well as magnetic transition temperatures for the anion-deficient La3+1-xSr2+ x Mn3+O2−3-x/2 ortomanganites have been established. Obtained experimental results are interpreted in terms of the oxygen vacancies, isotropic superexchange Mn3+-O–Mn3+ interactions and phase separation models.

Possible ferromagnetic interactions between IS Co3+ ions in Nb doped cobaltites

AbstractMagnetic and electrotransport properties studies have been performed for La1–xSrxCo1–x/2Nbx/2O3 solid solutions containing cobalt ions in three valence electronic state. It was shown that these solid solutions are spin glasses with temperature of magnetic moment freezing (Tf) of about 25 K. The ferromagnetic component is the most pronounced in the sample with x = 0.15. This result can be explained by a model according to which Co ions near to Sr ones are in intermediate spin state and the Co3+–O–Co3+ superexchange interactions are ferromagnetic due to local dynamic orbital correlations. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)