Thermal expansion property of anomalous magnetic alloy Fe70Al30

D0₃-type Fe₇₀Al₃₀ shows a transition from ferromagnetism to spin glass with anomalous slow spin dynamics below 170 K. Furthermore, it shows a structural transition from D0₃ to body-centered cubic (BCC) at 823 K. In this article, the relationship between the magnetic properties, thermal expansion coefficient (TEC), and crystal structure transition of D0₃-type Fe₇₀Al₃₀ is discussed. Below 170 K, TEC decreases with temperature, accompanied by a decrease in the Fe moments. In the ferromagnetic state between 170 K and the Curie temperature (T_C), TEC increases gradually with temperature. Above T_C, TEC increases rapidly. These temperature variations of TEC could be connected to the high-spin/low-spin transition and thermal spin fluctuations. During transition from D0₃ to disordered BCC at 823 K, TEC shows discontinuous behavior, similar to a first-order transformation. With further increase in temperature, TEC becomes constant. This implies that the phase transition from D0₃ to disordered BCC is accompanied by a change in spin fluctuation.     

Synthesis of heavy fermion CeCoIn5 thin film via pulsed laser deposition

CeCoIn₅ (Co115) thin films have been grown on Al₂O₃ (000l) substrates through the pulsed laser deposition (PLD). The films are grown mainly along the c-axis, with CeIn₃ and In-related alloys. The rock-salt type grains are nucleated, where Co115 grains mixed with excess indium are evenly distributed over the substrate. The electrical resistivity of the films shows a Kondo coherence peak near 47 K and the zero-resistance superconducting state at 1.8 K, which is the first observation in the PLD grown thin films of Co115. The Rietveld refinement of the thin films shows that the c/a ratio (tetragonality) is suppressed to 1.6312 from 1.6374 of single crystals, which is consistent with the linear relationship between the superconducting transition temperature and tetragonality. The good agreement indicates that the PLD could provide an alternative route to tune the 2D character of the critical spin fluctuations to understand the superconducting pairing mechanism of Co115.     

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.     

Magnetic phases of amorphous transition metal-metalloid alloys

Hysteresis loop and ac susceptibility measurements were performed on three series of amorphous alloys: (A_wB_1-w)₇₅P₁₆B₆Al₃, where (A, B) are (Fe, Ni), (Co, Ni) and (Fe, Mn). Upon cooling, low w alloys undergo paramagne t to spin glass transitions. Alloys with higher w first experience a Curie transition to a ferromagnetic state, and then a spin freezing transition to a spin glass state. the T dependence of the width of the ac hysteresis loop is used to determine the spin freezing transition temperature. A magnetic phase diagram is presented for each alloy series and the value of w required for ferromagnetism, w_C, is determined. When measured in the presence of small constant fields, the ac susceptibility of alloys with w just above w_C has maxima near both transition temperatures. The field and temperature dependences of the peaks are explained by scaling arguments, used to determine the critical exponent δ for the Curie transition, and suggest that a similar scaling law holds for the ferromagnet to spin glass transition.     

Mössbauer study of the transition metal phosphorus trichalcogenide FePS3 and spin glass Mn0.5Fe0.5PS3

The transition metal phosphorus trichalcogenides MnPS₃ and the FePS₃ are CdCl₂ type layered compounds, where the transition metal ions form a hexagonal lattice. While these compounds order anti-ferromagnetically at low temperature, the magnetic structures are different. We have reported that these mixtures Mn_0.5Fe_0.5PS₃ is a spin glass with a glass transition temperature T _g=33.7 K. Then, in this work, we report that the results of the temperature variation of the ⁵⁷Fe Mössbauer spectra of FePS₃ and Mn_0.5Fe_0.5PS₃, in detail. In the anti-ferromagnetic state of FePS₃, the hyperfine magnetic field H _int increases with decreasing temperature and the Isomer shift I. S. increases slightly with decreasing temperature. However in Mn_0.5Fe_0.5PS₃, the two broadened peaks are observed and the two peaks became a single peak with decreasing temperature at about 50.0 K, which is higher than T _g=33.7 K. In the spin glass Mn_0.5Fe_0.5PS₃, the Mössbauer spectra suggest that the magnetic interactions exist far above T _g.     

Magnetic properties of La0.70Sr0.30MnO2.85 anion-deficient manganite under hydrostatic pressure

The magnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite are studied experimentally under hydrostatic pressure. The results show that, in the whole pressure range under investigation (0–1 GPa), the sample is a spin glass with a smeared phase transition to the paramagnetic state. It is found that the spin glass state arises from the frustration of the exchange coupling of the ferromagnetic clusters embedded in the antiferromagnetic matrix. The fraction of the sample volume occupied by the ferromagnetic phase is found to be V _fer ～ 13%. Under hydrostatic pressure, the freezing temperature T _f of the magnetic moments of the ferromagnetic clusters increases at a rate of 4.30 K/GPa and the magnetic ordering temperature T _MO increases at a rate of 12.90 K/GPa. In addition, the ferromagnetic part of the sample increases by ΔV _fer ～ 5%. The enhancement of the ferromagnetic properties of La_0.70Sr_0.30MnO_2.85 anion-deficient manganite under hydrostatic pressure is explained by the redistribution of oxygen vacancies and a decrease in the unit-cell parameters.     

Optical and magnetic properties of laser-deposited Co-doped ZnO thin films

We report the optical and magnetic properties of laser-deposited Zn_1−xCo_xO (x=0.06-0.3) thin films with no intentional electrical carrier doping. The analysis of the high-temperature magnetization data provides an unambiguous evidence that antiferromagnetic superexchange interaction is the dominant mechanism of the exchange coupling between Co ions in Zn_1−xCo_xO alloy, yielding the value of the effective exchange integral J₁/k_B to be about −27 K. The low-temperature magnetization data reveals a spin glass transition in Zn_1−xCo_xO alloy for the Co content x>0.15, giving the value of the spin freezing temperature T_f to be ∼8 and ∼12 K for x=0.2 and 0.25, respectively. Optical spectra analysis shows a linear increase of the band gap E_g with the increase of the Co content following E_g=3.231+1.144x eV.     

Mössbauer study on the magnetic field-induced insulator-to-metal transition in perovskite Eu0.6Sr0.4MnO3

We have investigated the spin dynamics of a distorted perovskite Eu_0.6Sr_0.4MnO₃ by means of Mössbauer spectroscopy. Below 70 K the exchange interaction grows gradually, and below 42 K the spins turn into a cluster glass state. The magnetic field-induced insulator-to-metal (IM) transition at low temperature is a transition from cluster glass to ferromagnet. The induced metallic phase seems to be still in non-uniform electronic state. On the other hand, at 80 K, just above T _c of the induced ferromagnet, a metamagnetic transition was observed.     

Effect of Co or Mn addition on the soft magnetic properties of amorphous Fe89−xZr11Bx (x=5, 10) alloy ribbons

The temperature and field dependent magnetic properties of melt-spun amorphous Fe_89−x−yZr₁₁B_x(Co,Mn)_y (x=5, 10 and 0≤y≤10) alloys in the temperature range 5-1200 K are reported. The Curie temperature and saturation magnetization at room temperature increase (decrease) almost linearly with Co (Mn) addition. With increasing Co concentration, the room temperature coercivity increases at the rate of 2.26 (0.28) A/m per at% for the x=5 (10) samples. The high-field magnetic susceptibility and local magnetic anisotropy decrease (increases) rapidly with increasing Co (Mn) concentration. The thermomagnetic curves show a marked increase in magnetization above 850 K corresponding to the crystallization of α-FeCo (α-Fe) phase in samples containing Co (Mn). The Curie temperature of the crystalline phase increases (remains same) with increasing Co (Mn) concentration with the formation of α-FeCo (α-Fe). Addition of Co up to 10 at% in Fe-Zr-B improves the room temperature saturation magnetization from 0.56 to 1.2 T, and Curie temperature from 315 to 476 K. Also, the coercivity increases with Co addition from 1.27 to 23.88 A/m for x=5 and from 7.64 to 10.35 A/m for x=10 alloy. The non-collinear spin structures that characterize Fe rich Fe-Zr-B amorphous alloys have been used to describe the observed results.     

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 characterization of Bi2FeMnO6 film grown on (100) SrTiO3 substrate

Single phase Bi₂FeMnO₆ films on (100) SrTiO₃ substrate were fabricated using a pulsed laser deposition method through optimization of the preparation conditions. The magnetic moment is 0.30μ_B at 5 K in the magnetic field of 1 T, indicating that B site cations of Fe and Mn are disordered in the sample. The zero‐field‐cooling (ZFC) and field‐cooling (FC) magnetization curves measured from 2 K to 400 K coincide at 360 K. This is consistent with the observation that hysteresis disappears at 360 K, revealing the antiferromagnetic transition at this temperature. A spin‐glass‐like behaviour was observed at low temperature (～100 K) with a cusp of 25 K. Mn shows multiple valence states in the film. It is possibly because Mn²⁺ and Mn⁴⁺ could decrease the Jahn–Teller effect from Mn³⁺ in the film which results in less lattice distortion. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

μSR Investigation of magnetic phases in R1−xSrxCoO3 oxides (R=Pr, Nd)

We report results of our muon spin relaxation measurements in the series of polycrystalline compounds Pr_1−xSr_xCoO₃ (x=0.3, 0.4 and 0.5) and Nd_1−xSr_xCoO₃ (x=0.3 and 0.5). For the Pr-based samples our data clearly indicate the existence of two magnetic transitions, as also inferred from macroscopic measurements. While the high temperature transition is typical for cobaltites (∼200 K), the low temperature one is unusual. In our experiments it occurs below about 120 K and it manifests itself as a change in the slope of the temperature variation of the muon spin depolarization rate λ(T). For the Nd-based samples we found an increase of the muon spin depolarization rate below 45 K, temperature at which the sample is ferrimagnetic, when the Nd sublattice coupled antiparallel to the Co lattice. No phase separation could be evidenced in our samples by μSR experiments.     

Measurement of complex susceptibility on a metallic spin glass with broad relaxation spectrum

AC susceptibility measurements (0.625 H_z?_v?625 H_z) have been performed in a SQUID magnetometer on the amorphous metallic spin glass (Fe_0.06Ni_0.94)₇₅P₁₆B₆Al₃. The in-phase component of the susceptibility (x') shows a sharp peak at the spin glass freezing temperature T_g≈8 K with a frequency dependence of ?T_g/T_g≈0.015 per decade of frequency increase. The out-of-phase component (x″) exhibits a small anomaly at the spin glass transition with the magnitude of the anomaly almost independent of frequency. In particular we find the relation x″=(π;/2)?x'/? lnω to be valid in the vicinity of T_g. This relation and a frequency independence of x″ are fundamental characteristics of a spin glass having a broad spectrum of relaxation times. We argue that similar results from ac susceptibility measurements should be found in many spin glass systems, metallic as well as non-metallic.     

Mössbauer spectroscopic study of the thermal spin crossover in [Fe(II)(isoxazole)6](ClO4)2

The ⁵⁷Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)₆](ClO₄)₂ has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with T_C(↓)=223 and T_C(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below T_C. The present study reveals an increase in high-spin fraction upon heating in the temperature range below T_C, and an explanation is provided.     

Spin glass behavior and critical analysis across the magnetic phase transition in Gd2CoMnO6: dc magnetization and ac susceptibility study

We report here on critical analysis across magnetic phase transition and spin dynamics in Gd₂CoMnO₆. We found that this material behaves differently below and above the applied magnetic field of 20 kOe. The magnetic phase transition switches from nearly mean-field type to unusual class and T_c shifts towards the high temperature above 20 kOe field. The nature of the magnetic phase transition is explored by carrying out critical analysis at low as well as at high magnetic field. The critical exponents obtained at low field using Kouvel-Fisher method are β = 0.65 (2) γ = 0.90 (2), δ = 2.43 and T_c = 120 K. Apparently, these values of critical exponents appear close to mean-field model. For high field the critical exponents are β = 1.24 (2) γ = 0.64 (5), δ = 1.51 (3) and T_c = 128 K. The critical exponents show significant deviation from any universal class. This switchover in the nature of the magnetic phase transition is unique and not seen in many compounds. The formation of non-Griffiths-like clusters in this compound can be a reason for such unique behavior. Further, ac susceptibility has been measured to understand the spin dynamics in detail. The dispersion of frequency-dependent χ_ac below T_c confirms a spin glass state in this material. The observed value of τ_o and T_o indicate the slow dynamic spin which is caused by co-existence of Co/Mn spin magnetic moments. The magneto-caloric effect is also presented for Gd₂CoMnO₆ in this study. The magnetic study and critical analysis across the phase transition reveal a switchover in the nature of phase transition in this material. A non-Griffiths like cluster formation above T_c is found and dynamic susceptibility study reveals a spin glass state below T_c in Gd₂CoMnO₆.     

Influence on magnetic properties of substitution of Co for Fe in TbFe11.3Nb0.7

The magnetic properties of Tb(Fe_1−xCo_x)_11.3Nb_0.7 compounds with x=0, 0.05, 0.1, 0.15, 0.2 and 0.3 have been investigated. All compounds studied crystallize in the ThMn₁₂-type of structure. Substitution of Co for Fe leads to a contraction of the unit-cell volume. The Curie temperature clearly increases with increasing Co content from 551 K for x=0 to 831 K for x=0.3. The magnetic moment of the transition-metal sublattice increases with increasing Co content from 22.2 μ_B/f.u. for x=0 to 23.1 μ_B/f.u. for x=0.3. As the temperature increases, a spin reorientation from easy-plane to easy-cone is found in all compounds investigated. The spin-reorientation temperatures T_sr have been derived from the temperature dependence of the magnetization in a low field and decrease monotonously with increasing Co content. The easy magnetization direction at room temperature has been determined by X-ray diffraction on magnetically-aligned powder samples. The influence of the substitution of Co for Fe on the magnetic anisotropy is discussed in terms of crystal-field theory.     

Magnetic and electrical properties of the ZnGeAs<Subscript>2</Subscript>: Mn chalcopyrite

Doping of the ZnGeAs₂ semiconductor with manganese has produced compositions with spontaneous magnetization and high Curie temperatures of up to 367 K for the composition 3.5 wt% Mn. Their magnetic properties are characteristic of spin glasses at temperatures T < T _S and magnetic fields H < 11 kOe. In stronger fields, the spin glass state transforms into a phase with a spontaneous magnetization 4–5 times weaker than that to be expected under ferromagnetic ordering of all Mn ions. This is obviously a singly-connected ferromagnetic phase containing regions with frustrated bonds. The frustrated regions and the spin glass phase have inclusions of noninteracting ferromagnetic clusters, because these regions and the spin glass phase at low temperatures exhibit a strong increase in the magnetization M, with the dependence M(T) being described by the Langevin function. Measurements of the electrical resistivity ρ and the Hall effect have revealed that, for T < 30 K, the resistivity ρ of compositions with 1.5 and 3.5 wt % Mn is higher that at 30 K, which makes superexchange dominant and gives rise to the onset of the spin glass state. The nonuniform distribution of Mn ions in the spin glass phase accounts for the existence of isolated ferromagnetic clusters, their ferromagnetism being generated by carrier-mediated exchange. As the temperature increases still more, the increase in the mobility occurs faster than the decrease in the concentration, thus promoting an enhancement of the carrier-mediated exchange and growth of the ferromagnetic clusters in size, which at T = T _S come in contact. This signifies a transition from a multiply-to a singly-connected ferromagnetic phase, which contains microregions with frustrated bonds.     

Magnetic properties of half-metallic semi Heusler Co1−xCuxMnSb compounds

A study of the half-metallic character of the semi Heusler alloys Co_1−xCu_xMnSb (0?x?0.9) is presented. We investigated the saturation magnetization M_S at temperatures from 5 K to room temperature and the temperature dependence of the DC magnetic susceptibility χ above Curie temperature T_C. The magnetic moments at 5 K, for most compositions are very close to the quantized value of 4 μ_B for Mn³⁺ ion, the compound with 90% Co substituted by Cu is still ferromagnetic with M_S (5 K)=3.78 μ_B/f.u. These results emphasize the role of Co atoms in maintaining the ferromagnetic order in the material. The Curie temperature is decreased from 476 K to about 300 K as the Cu content increases from 0% to 90%. Above T_C, the χ⁻¹ vs T curves follow very well the Curie–Weiss law. The effective moment μ_eff and paramagnetic Curie temperature θ are derived. A comparison between the values of M_S at 5 K and μ_eff shows a transition from localized to itinerant spin system in these compounds.     

Magnetic and transport property studies of nanocrystalline ZnxFe3−xO4

The magnetic and transport properties of nanocrystalline Zn_xFe_3−xO₄ with x=0.0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0, respectively, fabricated by the sol-gel method have been investigated. Large magnetoresistance (MR) was observed and found to be originated both from the tunneling of the spin-polarized electrons across the adjacent ferromagnetic grains and the scattering by the canted spins at the grain surface near the grain boundaries. It has been revealed that the MR for the Zn_xFe_3−xO₄ samples (x=0, 0.5 and 1.0) increases with the temperature decreasing from room temperature until a maximum is reached at around 55 K. Then a sharp drop occurs with the further decrease in temperature, regarded as a spin (cluster) glass transition. For the samples studied, a biggest low field (0.5 T) MR value of about 20% for x=0 at 55 K has been obtained. The mechanism of the MR behavior of the materials was discussed.