Vacancy-induced spin-glass behavior of Prussian blue analogue Fe1.1Crx[Cr (CN)6]0.6−x·nH2O nanowires

Prussian blue analogue Fe^II_1.1Cr^II_x[Cr^III(CN)₆]_0.6−x·nH₂O nanowires were synthesized by electrodeposition. The magnetic properties investigation indicates that the nanowires exhibit cluster spin-glass behavior, which undergoes a magnetic transition to a frozen state below about 62 K. Spin disorder arising from reduced coordination and broken exchange bonds between spin centers due to the structural defects may be the reason that causes the spin-glass freezing behavior. The negative magnetization observed at temperature lower than the compensation temperature (T_comp∼43 K) at a field of 10 Oe may be due to the different temperature dependences of the ferromagnetic site Fe-Cr and antiferromagnetic site Cr-Cr.     

Superparamagnetic properties of La1−xBaxCoO3 (0.0<x<0.18)

The static and dynamic magnetic properties of La_1−xBa_xCoO₃ (0<x<0.18) (LBCO) have been investigated by the dc and ac susceptibility measurements systematically. The spin-glass-like characteristics of magnetic behavior, which are always considered as an evidence of spin glasses for La_1−xSr_xCoO₃ (0<x<0.18) (LSCO), have been observed. However, just like that of LSCO, some magnetic behaviors, which are different from the canonical spin-glass characteristics, for example, the difference far above peak temperature T_p of zero-field-cooled curve between field-cooled magnetization (FCM) and zero-FCM (ZFCM) and the very broad peaks in the samples for x>0.10, are also observed simultaneously. Based on the comparisons with canonical spin-glass Au₉₆Fe₄ alloy and conventional superparamagnetic (SPM) Cu₉₇Co₃ alloy, the nature of the magnetic behaviors in LBCO is ascribed to the superparamagnetic cluster rather than spin glass. Though the magnetic properties behave as SPM, LBCO has one critical point different from that of the conventional SPM Cu₉₇Co₃ alloy. The segregated Co nanoparticles in the Cu₉₇Co₃ alloy are isolated by nonmagnetic background while the clusters in LBCO are embedded in an antiferromagnetic matrix. The SPM behavior of LBCO can only exist at low temperature because the antiferromagnetic matrix transfers to paramagnetic phase at Neél temperature T_N and the SPM cluster turns into ferromagnetic (FM) above T_N.     

Large magnetoresistance in low temperature metallic region of manganite compounds (La0.7−2xEux)(Ca0.3Srx)MnO3 (0.05≤x≤0.15)

Magnetoresistance (MR) and magnetization (dc and ac) measurements have been carried out on the manganites, (La_0.7−2xEu_x)(Ca_0.3Sr_x)MnO₃ (0.05≤x≤0.15), in the temperature range of 5-320 K. At 5 K, an unusually large MR of almost 98% is observed in the x=0.15 sample, nearly up to fields of 4-5 T. This large high-field MR occurs in the metallic region, far below the insulator-metal transition temperature, and does not vary linearly with applied field. The unusual magnetoresistance is explained in the light of various possibilities such as phase segregation, cluster spin-glass behavior, etc.     

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.     

Glass formation,magnetic properties and magnetocaloric effect of ternary Ho–Al–Co bulk metallic glass

A ternary Ho–Al–Co system with high glass-forming ability (GFA) was developed and fully glassy rods with diameters up to 1 cm can be produced for the best glass former of Ho₅₅Al_27.5Co_17.5 alloy. The thermal stability and low-temperature magnetic properties of the Ho₅₅Al_27.5Co_17.5 bulk metallic glass (BMG) were studied. The magnetic transition temperature of this alloy is ∼14 K as determined by the thermomagnetic measurement. Two indicators, i.e. isothermal magnetic entropy change (ΔS_M) and the relative cooling power (RCP), were adopted to evaluate the magnetocaloric effect (MCE) of the alloy under a low magnetic field up to 2 T, which can be generated by permanent magnets. The values of |ΔS_M| and RCP are 7.98 J kg⁻¹ K⁻¹ and 191.5 J kg⁻¹, respectively. The Ho₅₅Al_27.5Co_17.5 BMG with good MCE and high GFA provides an attractive candidate for magnetic refrigeration applications, like hydrogen liquefaction and storage.     

Exchange bias in structure-phase separated K0.8Fe2−xNixSe2 (x=0.015) single crystal: Interface evidence

Exchange bias behavior is observed in Ni slightly doped K_0.8Fe_2−xNi_xSe₂ (x=0.015) single crystal. Two distinguished phases with epitaxial growth were observed in X-ray diffraction experiments, indicating the structure phase separation in our samples. The magnetic hysteresis loop shifts in both horizontal and vertical directions when the sample was cooled down to 3 K in a magnetic field. The nature of this magnetic anisotropy could be understood as a result of the freezing properties of the local spin disorders in cluster like spin-glass system. Our results suggest that the sample contains short range weak ferromagnetic clusters (phase 2) embedding in the antiferromagnetic backgrounds (phase 1), in which the random distribution of Ni on Fe or Fe-vacancy sites quenched the superconductivity and induced spin disorders.     

Magnetic properties for the Mn2GeTe4 compound

Measurements of magnetic susceptibility χ, in the temperature range from 2 to 300 K, and of magnetization M vs. applied magnetic field B, up to 5 T, at various temperatures were made on polycrystalline samples of the Mn₂GeTe₄ compound. It was found that Mn₂GeTe₄ has a Néel temperature T_N of about 135 K, shows mainly antiferromagnetic behavior with a very weak superimposed ferromagnetic component that is attributed to spin canting. Also, the magnetic results suggest that a possible spin-glass transition takes place at T_f≈45 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory. The M vs. B results indicated that bound magnetic polarons (BMPs) occur in the compound, and that the effects from BMPs disappear at approximately 80 K. The M vs. B curves were well fitted by a Langevin type of equation, and the variation of the fitting parameters determined as a function of temperature. Using a simple spherical model, the radius of the BMP in the material was found to be about 27 Å; this value is similar to the effective Bohr radius for an acceptor in the II-IV-V₂ and I-III-VI₂ ternary semiconductor compounds.     

Cluster glass magnetism in the phase-separated Nd2/3Ca1/3MnO3 perovskite

A detailed study of the low-temperature magnetic state and the relaxation in the phase-separated colossal magnetoresistance Nd_2/3Ca_1/3MnO₃ perovskite has been carried out. Clear experimental evidence of the cluster-glass magnetic behavior of this compound has been revealed. Well defined maxima in the in-phase linear ac susceptibility χ′(T) were observed, indicative of the magnetic glass transition at T_g∼60 K. Strongly divergent zero-field-cooled and field-cooled static magnetizations and frequency dependent ac susceptibility are evident of the glassy-like magnetic state of the compound at low temperatures. The frequency dependence of the cusp temperature T_max of the χ′(T) susceptibility was found to follow the critical slowing down mechanism. The Cole–Cole analysis of the dynamic susceptibility at low temperature has shown extremely broad distribution of relaxation times, indicating that spins are frozen at “macroscopic” time scale. Slow relaxation in the zero-field-cooled magnetization has been experimentally revealed. The obtained results do not agree with a canonical spin-glass state and indicate a cluster glass magnetic state of the compound below T_g, associated with its antiferromagnetic–ferromagnetic nano-phase segregated state. It was found that the relaxation mechanisms below the cluster glass freezing temperature T_g and above it are strongly different. Magnetic field up to about μ₀H∼0.4 T suppresses the glassy magnetic state of the compound.     

Influence of heat treatment on the structure and magnetic properties of the Co/Zr and Co/Hf multilayer amorphous films

The influence of annealing on the structure and magnetic properties of amorphous Co/Zr and Co/Hf multilayer films was studied with particular attention to the dependence of the magnetic properties, thermal stability and crystallization process on layer composition and thickness. The temperature at which crystallization commences increases from 400 to 460 °C as the layer thickness d_Zr or d_Hf increases from 6 to 18 Å, and decreases from 450 to 400 °C as d_Co increases from 12 to 18 Å. Multilayers containing 19–60 at% Zr were studied. The specific magnetization was found to increase even below the temperature at which crystallization commences. Our data are compared with non-multilayer Co–Zr amorphous films and rapidly quenched metallic glasses.     

Electrical and magnetic features in the perovskite-type system Y(Co, Mn)O3

Electrical and magnetic properties of the perovskite-type solid solution YCo_xMn_1−xO₃ (x=0.20-0.60) have been studied at different temperatures and magnetic fields. Electrical conductivity measurements show a semi-conducting behaviour throughout the solid solution. The room-temperature conductivity increases with the Co content up to 33 at.%, and then decreases. The effective moment in the paramagnetic state shows a non-monotonic decrease, when the Co content increases. In the ordered state, the behaviour at low or null magnetic fields corresponds to a spin-glass or antiferromagnetic system, with a transition temperature, which raises with the Co content (up to 50 at.% Co), and then decreases. At high fields, all the solid solutions show a ferromagnetic behaviour, although there is a marked difference in their ferromagnetic cycles, at a threshold value of 33 at.% Co.     

The magnetic susceptibility of hydrogen-doped partially crystalline (Zr76Ni24)1−xHx metallic glasses

The magnetizations of Zr₇₆Ni₂₄ metallic glass and hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses have been measured in the temperature range 10-300 K and magnetic fields up to 2 T for various dopant concentrations (x=0, 0.024, 0.043, 0.054). It is found that the samples are paramagnetic and magnetic susceptibility at room temperature, χ(300 K), shows a nonmonotonic behaviour upon hydrogenation. The values of χ(300 K) of the hydrogen-doped partially crystalline (Zr₇₆Ni₂₄)_1−xH_x metallic glasses are reduced with increase in hydrogen content up to x=0.043, whereas for x=0.054, an enhancement of χ(300 K) has been revealed. The magnetic susceptibility is weakly temperature dependent down to 110 K, below which an increase is observed. A shallow minimum exists between 90 and 120 K. The form and magnitude of the observed temperature dependence of the magnetic susceptibility are well accounted for by the sum of the quantum corrections to the magnetic susceptibility. Hydrogen reduces the electronic diffusion constant and influences strongly the quantum interference at defects, slowing down the spin diffusion and enhancing the magnetic susceptibility in the temperature range from 110 down to 10 K.     

Glass formation and magnetic properties of Fe-C-Si-B-P-(Cr-Al-Co) bulk metallic glasses fabricated using industrial raw materials

Formation of Fe-C-Si-B-P-(Cr-Al-Co) bulk metallic glasses with enhanced glass-forming ability (GFA) and magnetic properties fabricated using industrial pig-iron and Fe-P alloys as raw materials has been studied. It was found that GFA of the pig-iron (Fe_79.5C_18.0Si_2.3P_0.2, at%) could be improved by proper additions of only metalloids C, Si, P and B, and fully glassy structure was obtained in the Fe_75.5C_7.0Si_3.3B_5.5P_8.7 alloy with a critical diameter of 1 mm. With suitable additions of metallic elements Cr, Co and Al, the GFA is further increased and fully amorphous rods in 4 mm diameter can be prepared for the Fe_68.2C_7.0Si_3.3B_5.5P_8.7Cr_2.3Al_2.0Co_3.0 alloy that shows a unique combination of good soft-magnetic properties (M_s=1.03 T, H_c=9.39 A/m), high GFA and high fracture strength. These low-cost Fe-based bulk metallic glasses fabricated using industrial raw materials have a potential to be utilized as engineering materials.     

Spin glass like behavior in Cd0.42Mn0.58In2S4

The magnetic behavior of the diluted magnetic semiconductor Cd_0.42Mn_0.58In₂S₄ has been study by dc magnetization and ac susceptibility experiments. Zero field cooled and field cooled measurements reveal irreversibility below T_irr=2.60±0.15 K. Ac susceptibility data, performed as a function of the temperature and the frequency, confirm the spin-glass like behavior of the material with T_f=2.75±0.15 K. High temperature susceptibility data follow a typical Curie-Weiss law with θ=−74±1 K which suggests predominant antiferromagnetic interactions. The randomness of the magnetic ions, necessary to explain the magnetic behavior of the material, has been determined by X-ray powder diffraction experiments.     

Contribution of weak localization, electron-electron interaction, and Zeeman spin-splitting effects in corrective term “mT” of the metallic conductivity in n-type GaAs

We present measurements of the electrical conductivity of barely metallic n-type GaAs that are driven to the metal-insulator transition (MIT) by magnetic field. The experiments were carried out at low temperature in the range (4.2-0.066 K) and in magnetic field up to 4 T. We have determined the magnetic field for which the conductivity changes from the metallic behavior to insulator regime. On the metallic side of the MIT, the electrical conductivity is found to obey σ=σ₀+mT^1/2 down to 66 mK. Physical explanation to the temperature dependence of the conductivity is given in metallic side of the MIT using a competition between different effects involved in the mechanisms of conduction, like electron-electron interaction effect, Zeeman spin-splitting effect, and weak localization effect.     

Signatures of spin-glass freezing in Co/CoO nanospheres and nanodiscs

We present a study of the magnetic properties of Co nanoparticles having a combination of both spherical and disk shapes. The hcp Co nanospheres with an average diameter of 11 nm and nanodiscs of dimensions ∼2.5×15 nm² were prepared by thermal decomposition of di-cobalt octacarbonyl in the presence of an amine surfactant. The as-synthesized nanoparticles were oxidized to grow an antiferromagnetic layer. High resolution transmission electron microscopy showed the presence of a ferromagnet/antiferromagnet (Co/CoO) interface with a 2.2-nm thick CoO shell on the spherical nanoparticles and 0.5 nm thick on nanodiscs. We report the temperature and field dependent DC magnetization, frequency, field, and temperature dependent AC susceptibility, and the radio frequency transverse susceptibility. A low temperature paramagnetic behavior was observed in the DC magnetization at high fields and is assigned to defects in the CoO shell that are not coupled to the antiferromagnetic lattice. Our results support the existence of a low temperature frozen, disordered magnetic state, characterized by a strong exchange coupling between the structurally disordered, spin-glass CoO shell and Co core.     

Effect of oxygen vacancy and dopant concentration on the magnetic properties of high spin Co doped TiO2 nanoparticles

Co doped TiO₂ nanoparticles have been synthesized by a simple sol-gel route taking 7.5, 9.5 and 10.5 mol% of cobalt concentration. Formation of nanoparticles is confirmed by XRD and TEM. Increase in d-spacing occurs for (0 0 4) and (2 0 0) peak with increase in impurity content. Valence states of Co and its presence in the doped material is confirmed by XPS and EDX. The entire vacuum annealed samples show weak ferromagnetism. Increased magnetization is found for 9.5 mol% but this value again decreases for 10.5 mol% due to antiferromagnetic interactions. A blocking temperature of 37.9 K is obtained, which shows shifting to high temperature as the dopant concentration is increased. The air annealed sample shows only paramagnetic behavior. Temperature dependent magnetic measurements for the air annealed sample shows antiferromagnetic behavior with a Curie-Weiss temperature of −16 K. Here we report that oxygen vacancy and cobalt aggregates are a key factor for inducing ferromagnetism-superparamagnetism in the vacuum annealed sample. Appearance of negative Curie-Weiss temperature reveals the presence of antiferromagnetic Co₃O₄, which is the oxidation result of metallic Co or cobalt clusters present on the host TiO₂.     

Structure and magnetic entropy change of suck-cast Gd60Co26Al6Ge8 alloy

The Gd₆₀Co₂₆Al₆Ge₈ alloy has been prepared by the copper-mold suck-casting and its phase component has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). It is shown that this alloy consists of primary crystalline Gd₅Ge₃ phase and amorphous matrix. The glass transition temperature (T_g) and crystallization temperatures (T_x) occur at 292 and 320 °C, respectively. The maximal magnetic entropy change (ΔS_M) under 0-5 T field is about 7.6 J (kg⁻¹ K⁻¹) at 155 K and the refrigeration capacity (RC) is about 768 J kg⁻¹, which makes Gd₆₀Co₂₆Al₆Ge₈ bulk metallic glass matrix composite a promising candidate for magnetic refrigerant.     

Magnetocaloric effect in Gd-based Gd60FexCo30−xAl10 metallic glasses

Magnetocaloric effect and refrigerant capacity of Gd-based Gd₆₀Fe_xCo_30−xAl₁₀ metallic glasses are investigated for x = 0, 10 , 20 and 30. It is found that the non-linearity of saturation magnetization in crystalline Co-Fe binary alloys can be transferred to the quaternary metallic glass. Whereas the magnetocaloric specific values of Gd₆₀Co₃₀Al₁₀ are comparable in magnitude with those of other Gd-based metallic glasses, Fe addition leads to an increase of the saturation magnetization and refrigerator capacity with a maximum for x = 20. Simultaneously, the temperature of maximum isothermal change of magnetic entropy T_ΔSmax increases from 145 to 200 K with increasing Fe-content and also the halfwidth ΔT_Smax/2 of the ΔS-T-curve is considerably broadened. Furthermore, the effect of thermal treatment slightly above the first crystallization event on the magnetocaloric effect are investigated, showing a lowering of the working temperature in the first place.     

Magnetic properties for the Cu2MnSnSe4 and Cu2FeSnSe4 compounds

Magnetic susceptibility χ measurements in the range from 2 to 300 K were carried out on samples of the Cu₂FeSnSe₄ and Cu₂MnSnSe₄ compounds. It was found that Cu₂FeSnSe₄ was antiferromagnetic showing ideal Curie-Weiss behavior with a Néel temperature T_N of about 19 K and Curie-Weiss temperature θ=−200 K, while for Cu₂MnSnSe₄ the behavior was spin-glass with a freezing temperature T_f of about 22 K and Curie-Weiss temperature θ=−25 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory.     

The effect of different temperature annealing on phase relation of LaFe11.5Si1.5 and the magnetocaloric effects of La0.8Ce0.2Fe11.5−xCoxSi1.5 alloys

The phase relation of LaFe_11.5Si_1.5 alloys annealed at different high-temperature from 1223 K (5 h) to 1673 K (0.5 h) has been studied. The powder X-ray diffraction (XRD) patterns show that large amount of 1:13 phase begins to form in the matrix alloy consisting of α-Fe and LaFeSi phases when the annealing temperature is 1423 K. In the temperature range from 1423  to 1523 K, α-Fe and LaFeSi phases rapidly decrease to form 1:13 phase, and LaFeSi phase is rarely observed in the XRD pattern of LaFe_11.5Si_1.5 alloy annealed at 1523 K. With annealing temperature increasing from 1573  to 1673 K, the LaFeSi phase is detected again in the LaFe_11.5Si_1.5 alloy, and there is La₅Si₃ phase when the annealing temperature reaches 1673 K. There almost is no change in the XRD patterns of LaFe_11.5Si_1.5 alloys annealed at 1523 K for 3-5 h. According to this result, the La_0.8Ce_0.2Fe_11.5−xCo_xSi_1.5 (0≤×≤0.7) alloys are annealed at 1523 K (3 h). The analysis of XRD patterns shows that La_0.8Ce_0.2Fe_11.5xCo_xSi_1.5 alloys consist of the NaZn₁₃-type main phase and α-Fe impurity phase. With the increase of Co content from x=0 to 0.7, the Curie temperature T_C increases from 180 to 266 K. Because the increase of Co content can weaken the itinerant electron metamagnetic transition, the order of the magnetic transition at T_C changes from first to second-order between x=0.3 and 0.5. Although the magnetic entropy change decreases from 34.9 to 6.8 J/kg K with increasing Co concentration at a low magnetic field of 0-2 T, the thermal and magnetic hysteresis loss reduces remarkably, which is very important for the magnetic refrigerant near room temperature.