Reentrant spin glass behaviour in Nd0.84K0.12MnO3

Polycrystalline Nd_0.84K_0.12MnO₃ was prepared in single phase form with Pbnm space group. The magnetic properties are studied from magnetization, linear and non-linear susceptibility, and thermoremanent magnetization measurements. The sample exhibits paramagnetic to ferromagnetic transition followed by low temperature spin glass like transition. From frequency variation of ac susceptibility measurements, the spin glass transition temperature is found to be 97.6±0.1 K with critical exponents zυ=1.13±0.06. The critical exponent γ corresponding to spin glass transition has been determined from the third harmonic susceptibility analysis and it is found to be 3.09±0.05. The effective number of spins blocked under frustration and their correlation length are determined from the analysis of thermoremanent magnetization.     

Saturation and forced volume magnetostriction of Fe-rich FeZr amorphous alloys

AC susceptibility, saturation and forced volume magnetostriction were studied on iron-rich zirconium amorphous alloys a-Fe_100-xZr_x (8 at% ≤ x ≤ 12 at%). The experimental results are discussed in relation to the re-entrant spin glass state. It is found that the transverse magnetostriction accompanies a remarkable magnetic aftereffect in the spin glass phase, and that the forced volume magnetostriction shows an apparent peak at the ferromagnet to spin glass transition where the ac susceptibility has a cusp.     

Magnetic moment and high-field susceptibility of amorphous FeB Invar-type alloys

The saturation magnetization and high-field susceptibility of amorphous Fe_100?xB_x (11 ? x ? 22) alloys have been measured at 4.2 K, 77 K and room temperature in magnetic fields up to 100 kOe. The concentration dependence of the saturation moment per iron atom shows a maximum around 14% boron, while the high-field susceptibility increases remarkably with decreasing boron content. The features are similar to those of crystalline FeNi Invar alloys.     

Magnetic properties and hydrostatic pressure effect on the curie temperature of (Co,Mn)2B amorphous alloys

The temperature dependence of magnetization and magnetic susceptibility and hydrostatic pressure effect on the Surie temperature (dT_c/dP) are measured for (Co_1-xMn_x)₂B (0?x?0.4) amorphous alloys and the results are compared with those of crystalline compounds with the same composition. The Curie temperature decreases linearly with an increasing Mn content but magnetization shows a maximum around x=0.15. The reciprocal magnetic susceptibility of all the prepared alloys obeys the Curie-Weiss law above T_c. The magnitude of the negative value of dT_c/dP decreases linearly with increasing x from about 1.1 K/kbar (x=0) to zero (x=0.4), the composition dependence of which is opposite to that of the crystalline compound. The composition dependence of the average magnetic moment per transition metal atom and the Curie temperature and dT_c/dP are analysed on the basis of the local environment and the pair order interaction mode, respectively.     

Reentrant magnetism - a low field study

We report low field dc magnetization measurements on (Fe_xMn_1?x)₇₅P₁₆B₆Al₃ alloys at 4 ? T ? 300 K. Reentrant magnetic behavior is observed for x = 0.65, 0.7 and 0.8. By comparing field cooled and zero-field cooled states at low T we separate out the reversible and irreversible contributions to the magnetization M and identify the (field-dependent) temperature for the onset of irreversibility. It is shown that the reversible part of the magnetization can be described by the usual scaling laws for critical behavior in magnetic systems not only at the transition from the paramagnetic to the ferromagnetic phase but also when the latter transforms to a spin glass. We identify the irreversible part of M with a spin glass order parameter.     

Magnetic and electrical properties of amorphous PdCoP alloys

Magnetic susceptibility and electrical resistivity measurements were performed (Pd_100?xCo_x)₈₀P₂₀ alloys where 15 < x < 50. The magnetic properties show that these alloys undergo a ferromagnetic transition between 272 and 399 K as the cobalt concentration increases from 15 to 50 atomic %. Below 20 atomic % Co the short-range exchange interactions which produce the ferromagnetism are unable to establish a long-range magnetic order and a peak in the magnetization shows up at the lowest temperature range under an applied field of 6.0 kOe. The electrical resistivity of these alloys has been measured from 4.2 K up to the vinicity of the melting point (900 K). The electrical resistivity data could be interpreted by the coexistence fo a Kondo-like minimum and ferromagnetism. The minimum becomes less important as the transition metal concentration increases. The coefficients of In T and T² become smaller and concentration dependent. The spin ordering in such alloys can be simulated as either the ordering due to an applied “external field” or as an increase in “internal fields”. These are due to an increase in transition metal concentration. The negative magnetoresistivity is a strong indication of the existence of localized moment.     

Magnetic behavior of Fe0.2Mg0.8TiO3 and Fe0.3Mg0.7TiO3 probed by Mössbauer timescale

Magnetization and Mössbauer measurements have been made on a diluted antiferromagnets Fe_xMg_1?xTiO₃ with x=0.2, 0.3 and 0.7. It has been demonstrated that this system exhibits a spin glass or a reentrant spin glass like behavior in the samples with x around the percolation concenrration x_c≈0.25. It has been shown that various sizes of magnetic clusters are formed well above the transition temperatures determined by the magnetization measurements in the samples with x near x_c. The behavior of Fe_0.2Mg_0.8TiO₃ is discussed as a typical cluster glass.     

Magnetic dynamics of charge ordered Nd0.80Na0.20MnO3 compound

The magnetic dynamics of charge ordered Nd_0.8Na_0.2MnO₃ compound was studied by measuring the temperature variation of magnetization for different magnetic fields up to 7 T and, the field variation of magnetization at different temperatures down to 5 K. This sample exhibits a charge-ordering transition at 180 K, followed by a weak ferromagnetic (FM) transition at around 100 K and a spin glass like transition below 40 K. Suppression of charge-ordering and spin glass like transition and increase in FM T_C were observed with an increase in magnetic field. A reversible metamagnetic transition above a threshold field (H_f) of 4.5 T was observed at 130 K, followed by a saturation magnetization of 3.2 μ_B/f.u. However at 5 K, an irreversible field induced first order phase transition from charge ordered state to FM state was observed at H_f=5 T. For comparison, the temperature and field variations of magnetization were studied on a FM compound from the same series with the composition Nd_0.90Na_0.10MnO₃. A clear FM transition with a T_C of 113 K and a saturation magnetization of 4.3 μ_B/f.u was observed.     

Magnetic clusters in Fe3−x V x Al alloys with DO3 type structure forx≤1.1

Polycrystalline Fe_3?x V _x Al alloys with the DO₃ type structure were investigated by magnetostatic and Mössbauer methods. In alloys with 0.2<x<0.5 the ferro- and superpara-magnetic phases coexist. For alloys withx≥0.5 these investigations showed that in spite of the lack of magnetic order, they are characterized by strong nonlinear variations of the magnetization in a magnetic field over a wide temperature range. This indicates the presence of magnetic clusters of Fe atoms with very large magnetic moments. Analysis of the magnetization isotherms indicates that in these alloys a distribution of magnetic moments of clusters (μ_c) from 10 μ_B to 10⁴ μ_B exists.     

Weakferromagnetism in FexNi1−xS2 (x = 0−0.075)

The effects of nonmagnetic impurities (Fe²⁺ in a low spin state) on the weakferromagnetism in NiS₂ were investigated. The weak-ferromagnetic moment decreases rapidly with Fe content (x) and almost vanishes at about x = 0.05. On the other hand, T_c remains quite constant. Above T_c, two anomalies in the temperature dependence of the susceptibility were observed except for samples with x = 0 and 0.006. The effective magnetic determined from the susceptibility above T_c is about 3 μ_B/Ni, and is independent of x. Discussions are given on the origin of weakferromagnetism and on the hysteresis of the virgin magnetization curve.     

Tunable exchange bias in La1.5Sr0.5CoMnO6 double perovskite doped with nonmagnetic Ga ions

The crystal structure, electronic structure, and magnetic behaviors of nonmagnetic Ga ions doped double perovskite La_1.5Sr_0.5CoMnO₆ single phase crystals have been investigated. Different from the traditional magnetic dilution effect of nonmagnetic doping, Ga doping in La_1.5Sr_0.5CoMnO₆ enhances the ferromagnetic (FM) exchange interaction of Co³⁺-O-Mn³⁺. Moreover, both conventional and spontaneous exchange bias (EB) effects can be tuned by modulating the Ga doping content, which is accompanied by the variation of the Co^3+/4+ and Mn^3+/4+ and the effective magnetic moment. The EB field and magnetization can be improved by nonmagnetic Ga³⁺ doping with content lower than 0.2. The evolution of conventional and spontaneous EB effects in La_1.5Sr_0.5Co_1-xGa_xMnO₆ can be understood in terms of the unidirectional interface anisotropic coupling between FM/anti-FM, and/or FM/spin glass, which is affected by antisite disorder, spin glass, and the uncompensated coupling between Co and Mn.     

The role of iron in the formation of the magnetic structure and related properties of La0.8Sr0.2Co1−xFexO3 (x=0.15, 0.2, 0.3)

La_0.8Sr_0.2Co_1−xFe_xO₃ (x=0.15, 0.2, 0.3) samples were studied by means of AC magnetic susceptibility, magnetization, magnetoresistance and ⁵⁷Fe Mössbauer spectrometry. Iron was found to take on a high spin 3d^5−α electronic state in each of the samples, where α refers to a partly delocalized 3d electron. The compounds were found to exhibit a spin-cluster glass transition with a common transition temperature of ∼53 K. The spin-cluster glass transition is visualized in the ⁵⁷Fe Mössbauer spectra as the slowing down of magnetic relaxation below ∼70 K, thereby showing that iron takes part in the formation of the glassy magnetic phase. The paramagnetic-like phase found at higher temperatures is identified below T_c≈195 K as being composed of weakly interacting, magnetically ordered nanosized clusters of magnetic ions in part with a magnetic moment oriented opposite to the net magnetic moment of the cluster. For each of the samples a considerable low-temperature negative magnetoresistance was found, whose magnitude in the studied range decreases with increasing iron concentration. The observed results obtained on the present compounds are qualitatively explained assuming that the absolute strengths of magnetic exchange interactions are subject to the relation ∣J_Co–Co∣<∣J_Fe–Co∣<∣J_Fe–Fe∣.     

Evidence of glassy ferromagnetic phase and kinetic arrest of electronic phase in Sm0.35Pr0.15Sr0.5MnO3 manganites

The effect of doping of rare earth Pr³⁺ ion as a replacement of Sm³⁺ in Sm_0.5Sr_0.5MnO₃ is investigated. Temperature dependent dc and ac magnetic susceptibility, resistivity, magnetoresistance measurements on chemically synthesized (Sm_0.5−xPr_x)Sr_0.5MnO₃ show various unusual features with doping level x=0.15. The frequency independent ferromagnetic to paramagnetic transition at higher temperature (∼191 K) followed by a frequency dependent reentrant magnetic transition at lower temperature (∼31 K) has been observed. The nature of this frequency dependent reentrant magnetic transition is described by a critical slowing down model of spin glasses. From non-linear ac susceptibility measurements it has been confirmed that the finite size ferromagnetic clusters are formed as a consequence of intrinsic phase separation, and undergo spin glass-like freezing below a certain temperature. There is an unusual observation of a 2nd harmonic peak in the non-linear ac susceptibility around this reentrant magnetic transition at low temperature (∼31 K). Arrott plots at 10 and 30 K confirm the existence of glassy ferromagnetism below this low temperature reentrant transition. Electronic- and magneto-transport measurements show a strong magnetic field—temperature history dependence and strong irreversibility with respect to the sweeping of magnetic field. These results are attributed to the effect of phase separation and kinetic arrest of the electronic phase in this phase separated manganite at low temperatures.     

Observation of mixed-phase behavior in the Mn-doped cobaltite La0.7Sr0.3Co1−xMnxO3 (x=0-0.5)

Studies on La_0.7Sr_0.3Co_1−xMn_xO₃ (x=0-0.5) compounds evidence that the interaction between Mn and Co ions in this system is antiferromagnetic super-exchange and not ferromagnetic (FM) double-exchange (DE). As a result, antiferromagnetism and magnetic glassiness develop steadily with increasing Mn content and the system becomes a spin glass at x∼0.1. Analyses of high-field magnetization data indicate that the system consists of two major phases: a metallic FM phase which magnetically saturates in rather low field, and an insulating non-FM phase which has a linear dependence of magnetization on magnetic field. In the low doping regime, the fraction of the non-FM component expands with temperature at the expense of the FM phase and becomes maximal at T_C. Ferromagnetism reappears in highly doped (x≥0.2) compounds due to the presence of DE interaction between the Mn ions. The small volume fraction of the FM phase derived from the M(H) data in high-field region supports the coexistence of insulating and FM behaviors in the highly doped samples.     

Quantum bicriticality in Mn1 − x Fe x Si solid solutions: Exchange and percolation effects

The T-x magnetic phase diagram of Mn_{1 ? x} Fe _x Si solid solutions is probed by magnetic susceptibility, magnetization and resistivity measurements. The boundary limiting phase with short-range magnetic order (analogue of the chiral liquid) is defined experimentally and described analytically within simple model accounting both classical and quantum magnetic fluctuations together with effects of disorder. It is shown that Mn_{1 ? x} Fe _x Si system undergoes a sequence of two quantum phase transitions. The first “underlying” quantum critical (QC) point x* ～ 0.11 corresponds to disappearance of the long-range magnetic order. This quantum phase transition is masked by short-range order phase, however, it manifests itself at finite temperatures by crossover between classical and quantum fluctuations, which is predicted and observed in the paramagnetic phase. The second QC point x _c ～ 0.24 may have topological nature and corresponds to percolation threshold in the magnetic subsystem of Mn_{1 ? x} Fe _x Si. Above x _c the short-range ordered phase is suppressed and magnetic subsystem becomes separated into spin clusters resulting in observation of the disorder-driven QC Griffiths-type phase characterized by an anomalously divergent magnetic susceptibility χ ～ 1/T ^ξ with the exponents ξ ～ 0.5–0.6.     

Spin glass phase transition in Hg1−kMnkTe semimagnetic semiconductors

Low field interband Faraday rotation and a.c. magnetic susceptibility are investigated in Hg_1?kMn_kTe and Hg_1?k?xMn_kTe semimagnetic semiconductors (k?0.5). The spin glass phase transition is experimentally manifested by characteristic cusps in χ(T) and kinks in θ(T). The transition temperatures obtained from both types of experiments are well correlated which enable us to determine the phase diagram for Hg_1?kMn_kTe system. The spin glass phase transition occurs at low temperature in quaternary alloys than in Hg_1?kMn_kTe alloys with identical Mn composition. This implies that the antiferromagnetic interaction between Mn²⁺ ions due to the virtual valence to conduction band transitions plays a significant role in the mechanism responsible for the spin glass formation.     

Variation of the magnetic inhomogeneity in glassy FexNi80-xB18Si2 alloys

A careful analysis of the magnetization measurements of amorphous Fe_xNi_80-xB₁₈Si₂ alloys (3 ⩽ x ⩽ 15) above the percolation threshold (x_c = 2.55) has been performed. The non-monotonic variation of the effective critical exponent of the initial magnetic susceptibility, γ¹, with temperature is found to become less pronounced with increasing x - x_c, and probably disappears around x = 20. The tendency towards a homogeneous ferromagnetism around that concentration is also obtained from the analysis of the Arrott-Belov-Kouvel plots. The results are compared with those for similar glassy ferromagnetic systems and a possible origin of the magnetic inhomogeneity is discussed.     

Effects of divalent alkaline earth ions on the magnetic and transport features of La0.65A0.35Mn0.95Fe0.05O3 (A = Ca, Sr, Pb, Ba) compounds

We have studied the magnetic and transport properties of Fe doped La_0.65A_0.35Mn_0.95Fe_0.05O₃ (A = Ca, Sr, Pb, Ba) manganites. All the compositions show ferromagnetic/metal to paramagnetic/insulator transition (T_C) except the Pb doped sample which is insulating and ferromagnetic (FM) in the entire temperature range. The magnetization and T_C are decreased by decreasing the cation size on La site. The transition temperature and magnetic moment at 77 K is a maximum for Sr doped sample and is decreasing if we increase or decrease the cation size from Sr size. The maximum value of T_C and magnetic moment for Sr based sample is most likely due to the closer ionic sizes of La and Sr as compared to the other dopants (Ca, Pb, and Ba). We observed a spin freezing type effect in the Pb doped sample below 120 K in resistivity, ac susceptibility and in magnetization. This suggests that the AFM interactions introduced by the Fe are most effective in the Pb doped composition leading to increased competition between the FM and AFM interactions. This FM and AFM interaction generates some degree of frustration leading to the appearance of spin glass like phase whose typical magnetic behavior is studied for small ion when the metallic like behavior is lost.     

High field susceptibility of Zn1−xCuxCr2Se4

We present the results concerning field dependences of magnetization for Zn_1-xCu_xCr₂Se₄ mixed spinels studied at low temperatures with the use of both high magnetic stationary fields up to 14 T and high pulsed fields up to 25 T. From the isotherms of static magnetization we have obtained the incremental high field susceptibility and from pulsed measurents the differential one. The temperature and concentration (x) dependences of the calculated high field susceptibility χ_hf are interpreted based on the magnetic properties of the spinels under study presented in refs. [4–7]. The greatest changes of χ_hf were observed in the intermediate concentration range exhibiting a conical spin structure, in which the appearance of the double-exchange interaction between Cr³⁺ and Cr⁴⁺ ions plays a main role. It drives the transition from a noncollinear magnetic structure to a ferromagnetic one with increasing copper concentration.