Frequency Dependence Of Tf Research Articles

Signatures of nearly compensated magnetism and spin glass behavior in highly frustrated β-Mn-type Mn50Fe25+xAl25-x Heusler alloys.

The present study examines the effect of Fe/Al concentration on the structural and magnetic properties of Mn-rich Mn50Fe25+xAl25-x (x = 5, 10, 15) Heusler alloys through x-ray diffraction, temperature- and field-dependent DC magnetization, thermoremanent magnetization, magnetic memory effect, AC susceptibility measurements, and DFT calculations. The samples crystallize in a cubic β-Mn structure. The trend shows a reduction in lattice parameters (unit cell volume) with the increasing Fe proportion. These alloys exhibit strong antiferromagnetic interactions with large frustration parameters, indicating the presence of competing magnetic interactions. The DC magnetization data reveal spin glass-like features with a peak at spin glass freezing temperature (Tf). The observation of bifurcation in temperature-dependent zero-field-cooled and field-cooled magnetization curves, exponential dependence of the temperature variation of remanence and coercivity, magnetic relaxation, and magnetic memory effect below Tf support the spin-glass character of these alloys. The frequency dependence of Tf is also examined in the context of dynamic scaling laws, such as the Vogel-Fulcher law and critical slowing down model, which further supports the presence of spin glass behavior. In the theoretical DFT calculations, the electronic structure is found to be metallic and similar for both spin projections. Moreover, the antiferromagnetic arrangement of the magnetic moments, in line with the experimental observations, is stabilized by exchange interactions, resulting in an almost compensated total magnetic moment of 0.02-0.38µB/f.u. This is probably caused by the frustrated structure and non-stoichiometric compositions of Mn50Fe25+xAl25-x.

Cluster glass behaviour in MnMn0.25Sb

Magnetic properties of intermetallic MnMn0.25Sb crystallizing in the NiAs-type hexagonal structure are reported. A spin-glass transition at Tf ≈ 10.7 K was identified from Ac susceptibility measurements. The value of the Mydosh parameter (φ≈0.06). was further indicative of cluster spin glass like state. The frequency dependence of Tf was investigated by critical slowing-down model and Vogel-Fulcher law. The characteristic time constant of τ0∗≈0.18×10−7s was analysed using power-law for a single spin-flip and a critical exponent of zv=4.37±0.14. The characteristic time constant was obtained from the Vogel-Fulcher law for a single spin-flip τ0≈7.96×10−8s, and T0=6.49±0.21K. The cluster spin-glass behaviour in the current system is due to the existence of both competing interactions and atomic disorder at 2d site.

Magnetic properties of nickel carbide nanoparticles with enhanced coercivity

Rhombohedral nickel carbide (Ni3Cx, x=0.7, 1.2 and 1.5) nanoparticles (∼ 110 nm) with enhanced magnetic coercivity (HC up to 1.3 kOe) at temperature below the spin-glass freezing (Tf) have been demonstrated. The presence of spin-glass state, which is seen at ∼17 K, is evident by the field dependence of the freezing temperature following the de Almeida–Thouless (AT) relationship and frequency dependence of Tf. Moreover, the spin-glass state is irreversible to the sweeping applied field and results in high HC at 10 K. With increases of the carbon content, we have found a gradual increasing trend in the saturation magnetization (MS: 6.6 to 9.2 emu/g) and coercivity (350 Oe to 1.3kOe) at 10 K. This is attributed to the increase of spin-glass contribution and the weak ferromagnetic phase.

An AC magnetic susceptibility study of a sigma-phase Fe65.9V34.1 alloy

A sigma-phase Fe65.9V34.1 alloy was investigated with the AC magnetic susceptibility as a function of temperature, frequency and external magnetic field. Evidence was found that its magnetism shows features characteristic of a reentrant behavior viz. two transitions: first at TC ∼312 K from the paramagnetic state into a collinear ferromagnetic one, and second at Tf ∼302 K to a mixed state (sometimes referred to as a ferromagnetic or re-entrant spin glass) which, finally, at a lower temperature (TRSG ∼60 K) transforms to a state where replica symmetry is broken. The frequency dependence of Tf is lower than that of canonical spin glasses, a feature that in the light of a high concentration of magnetic carriers can be understood in terms of a weak coupling between magnetic clusters.

A comparative study of the magnetic properties of the1/1 approximantAg50In36Gd14 and the icosahedralquasicrystal Ag50In36Gd14

We report on measurements of the dc and ac magnetic susceptibility, 155Gd Mössbauer spectra, andspecific heat of the 1/1 approximant Ag50In36Gd14, and of the ac magnetic susceptibility of the icosahedral quasicrystalAg50In36Gd14. These alloys are shown to be spin glasses. For the icosahedral quasicrystalAg50In36Gd14, spin freezing occurs atTf = 4.3 K, and the frequencydependence of Tf is well accounted for by the Vogel–Fulcher and power laws. Spin freezing in the1/1 approximantAg50In36Gd14 occurs in twostages: at Tf1 = 3.7 K,Gd spins develop short-range correlations but continue to fluctuate, and then long-range freezing is achieved atTf2 = 2.4 K. The frequencydependences of Tf1 and Tf2 can be accounted for by means of the Vogel–Fulcher law and the criticalslowing down dynamics. It is shown that the spin freezing in both alloys is anonequilibrium phenomenon rather than a true equilibrium phase transition. The 155Gd Mössbauerspectra of the 1/1 approximant Ag50In36Gd14 confirm that the Gd spins are frozen at 1.5 K and are fluctuating at 4.6 K. Themagnetic specific heat exhibits a maximum at a temperature that is 30% larger thanTf1, but the temperature derivative of the magnetic entropy peaks atTf1. The Debyetemperature of the 1/1 approximant Ag50In36Gd14 is 199(1) K as determined from the Mössbauer data, and 205(2) K as determined from thespecific heat data.

Dynamic susceptibility measurements at the spin-glass transition in theZn1−xMnxIn2Se4 semiconductor

Measurements of the magnetic properties of the magnetic semiconductorZn1−xMnxIn2Se4 for two Mncompositions (x = 0.87 and 1.00) are presented in this work. The localized Mn ions in the layered rhombohedral structureundergo a spin-freezing transition below 3.5 K. The frequency-dependent freezing temperatureTf variesas ΔTf/(TfΔlogω)≈0.022, indicating a spin-glass behaviour. From the frequency dependence ofTf, the validity of the critical slowing down associated with a true phase transition is tested. ForT>Tc, thedynamic scaling of the imaginary component of ac susceptibility data yields the following set of criticalparameters: zν = 9.7 ± 1.0, β = 1.18 ± 0.20 andTc = 2.61 ± 0.06 Kfor x = 0.87 and zν = 10.7 ± 0.9, β = 1.20 ± 0.20 andTc = 3.43 ± 0.01 Kfor x = 1.00. These values are in good agreement with reported values for other spin glasses.

Glassy ferromagnetism and frustration in La0.7Ba0.3Mn0.7Ti0.3O3

Glassy ferromagnetism and the magnetic frustration effect in La0.7Ba0.3Mn0.7Ti0.3O3 were investigated by means of dc magnetization, ac susceptibility, and electron-spin-resonance (ESR) measurements. As a result, the spin-glass (SG) phase-transition temperature, Tg, was observed at 27.6 K. The in-phase component of the ac susceptibility χ′(T) indicated that the SG freezing temperature, Tf, shifted to low temperatures with lowering frequency. Dynamic processes on the frequency dependence of Tf have been analyzed by the slowing-down scaling law, τ∕τ0∝(Tf∕Tg−1)−zv, where the characteristic time τ0 and an exponent zv obtained were about 3.7×10−16s and 9.66, respectively. The microscopic picture characterizing phase separation and competing interactions in the compound were observed by ESR.

Enhancement of Tc and the re-entrant spin glass transition inLa0.86Ca0.14Mn1−yCryO3 (y = 0,0.1 and 0.2)

We report on structural, frequency dependent ac susceptibility, dc magnetizationand magnetoresistance (MR) measurements on polycrystalline samples ofLa0.86Ca0.14Mn1−yCryO3 (y = 0, 0.1 and 0.2) prepared by the sol–gel technique. Fory = 0, a paramagnetic to ferromagnetic transition was observed atTc = 185 K.Fory = 0.1, the value ofTc = 200 K, an increase of15 K, and for y = 0.2, Tc = 195 K,anincrease of 10 K. The imaginary part of the ac susceptibility for all three samples shows a secondary transitionat Tf<Tc.For y = 0, there is no definite law accounting for the frequency dependence ofTf and it is attributed to a transition arising out of a canted structure. However, in the cases ofy = 0.1 and 0.2, the frequency dependence indicates the presence of a re-entrant spin glass transition atTf. Although all three samples show a semiconducting behaviourbetween 300 and 5 K, a negative MR was observed corresponding toTc and Tf. The value of MR decreased for the Cr substituted samples.

The frequency dependence of the AC susceptibility of rare-earth-doped scandium spin glasses

Measurements of the AC susceptibility chi '(f) over a large frequency range (10-4<f<104 Hz) on anisotropic ScRE(ER=Gd, Dy, Er) spin glasses are reported. The authors have focused in the frequency dependence of Tf, the cusp temperature of chi '. Tf(f) is analysed using an Arrhenius, a Vogel-Fulcher, and a power law for a finite transition temperature. The authors also tried the power law for a 0 K transition. As usual in spin glasses, the Vogel-Fulcher law fits the observed behaviour better than the Arrhenius law. The two power laws fit the data of the three samples equally well. However, the comparison of the values of the exponents vz with those of other spin glasses slightly favours the existence of a transition at a finite temperature in ScGd5%. Here, vz approximately=6, in rough agreement with measurements on classical spin glass systems and theoretical predictions. For the Ising spin glass ScEr 10% and the XY system ScDy, the data are more compatible with a T=0 transition. In this case vz approximately=4-5, which is surprisingly close to the value obtained by simulations in two dimensions.

Magnetic properties of quench-condensed CuFe films

Random substitutional alloys of CuFe with Fe concentrations 0.8-8 at.% Fe are produced by quenching from the vapour phase onto a cold substrate. The paramagnetic susceptibility chi is measured in situ using AC magnetic fields of 2.2-11.5 mT at frequencies between 33 and 5500 Hz and temperatures from 2.5 to 80K. The paramagnetic Curie temperature theta , the effective magnetic moment peff and the temperature Tf of the susceptibility maximum depend on Fe concentration and annealing. Alloys annealed at 80K do not show a frequency dependence of Tf. A reduction of chi near Tf in small DC fields is found only after annealing at 300K. A comparison with the properties of bulk CuFe alloys shows that the Tf values are nearly the same whereas the theta and peff values of the alloy films are much smaller than for the corresponding bulk alloys.

Spin glass transition and hyperfine parameters in FeO-Al2O3-SiO2glasses

AC susceptibility and 57Fe Mossbauer spectroscopy measurements are reported for Fe2+ aluminosilicate glasses. The frequency dependence of Tf (over a range from 10 to 108 Hz) is described by a Fulcher law. This indicates a progressive spin freezing at Tf, comparable with results reported in Co and Mn aluminosilicate glasses. At T<<Tf the Fe2+ glasses order speromagnetically, without angular correlation between Fe2+ moments and local-field-gradient axes. A linear correlation between isomer shift and quadrupole splitting distributions is established. This is tentatively attributed to a changing covalency of bonding of six-coordinated Fe2+ ions. The wide distribution of quadrupole splitting cannot be interpreted unequivocally but indicates a large distribution of crystal-field parameters.

Frequency dependence of Tf and short time remanence for Eu0.4Sr0.6S spin glass

We report on the frequency variation of the freezing temperature and on the time variation of the magnetization of the insulating spin-glass Eu0.4Sr0.6S. Faraday rotation was used to monitor optically the susceptibility or the magnetization and is well adapted to measurements in a large frequency range (10−2–107 Hz) or at various time scale (down to 25 ns). The temperature Tf(ν) associated with the maximum of the susceptibility peak exhibits a linear dependence with Log(ν) in the range 102–107 Hz. At lower frequencies (down to 10−2 Hz) it approaches a ’’static’’ limit equal to 1.55 K. This result cannot be explained by a model of superparamagnetic clouds. The frequency shift of the freezing temperature is experimentally shown to be related to the occurrence of a remanent magnetization during the time associated with the measuring frequency. Finally the time dependent behavior of the magnetization was investigated down to 25 ns after cutting a weak magnetic field (∼2 Oe). The main part of the magnetization is shown to relax more quickly than 25 ns even at 1.26 K. The only change when lowering the temperature below Tf is the emergence of the remanent magnetization; its time dependence is briefly discussed.

New magnetic properties of a dilute spin-glass

The insulating Sr1−xEuxS system exhibits spin-glass properties for x≳0.05 due to competing ferro and antiferromagnetic interactions. A scaling law x/Tf=f (ln ν) represents the frequency (ν) dependence of Tf, the temperature of the maximum of χa.c. (or spin-glass freezing temperature). This frequency dependence of Tf is at least an order of magnitude bigger than in a canonical spin-glass such as AuFe. The dipolar interaction is responsible for the anisotropy inside magnetic clusters. For x&amp;lt;0.05 the a.c. susceptibility, χa.c., doesn’t exhibit a single maximum, but after an initial maximum or plateau around Tp it increases to a second rounded maximum, at much lower temperatures. Below Tp, which is concentration independent, a remanent magnetization appears proportional to x2. Furthermore, at this temperature deviations from a Curie law of χa.c. increase more rapidly than x. Moreover, the frequency dependence of Tp permits a determination of a blocking energy. This energy is in agreement with the anisotropy barrier due to the dipolar interaction for a ferromagnetic pair of first neighbors. All these results support the idea of short range order for individual pairs, and small magnetic clusters which are blocked below Tp on random anisotropy directions determined by the intra-cluster dipolar interaction. A similar change in the magnetic properties of other spin-glasses is expected.