Optimization of Pr(0.9)Ca(0.1)MnO(3) thin films and observation of coexisting spin-glass and ferromagnetic phases at low temperature

Optimization of thin films of small bandwidth manganite, Pr(1-x)Ca(x)MnO3 (for x = 0.1), and their magnetic properties are investigated. Using different pulsed laser deposition (PLD) conditions, several films were deposited from the stoichiometric target material on SrTiO3 (001) substrate and their thorough structural and magnetic characterizations were carried out using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), SQUID magnetometry and ac susceptibility measurements. A systematic investigation shows that irrespective of the growth temperature (between 550 and 750?°C), all the as-deposited films have twin boundaries and magnetic double phases. Post-annealing in partial or full oxygen pressure removes the extra phase and the twin boundaries. Zero-field-cooled magnetization data show an antiferromagnetic to paramagnetic transition at around 100 K whereas the field-cooled magnetization data exhibit a paramagnetic to ferromagnetic transition close to 120 K. However, depending on the oxygen treatments, the saturation magnetization and Curie temperature of the films change significantly. Redistribution of oxygen vacancies due to annealing treatments leading to a change in ratio of Mn3+ and Mn4+ in the films is observed from XPS measurements. Low temperature (below 100 K) dc magnetization of these films shows metamagnetic transition, high coercivity and irreversibility magnetizations, indicating the presence of a spin-glass phase at low temperature. The frequency dependent shift in spin-glass freezing temperature from ac susceptibility measurement confirms the coexistence of spin-glass and ferromagnetic phases in these samples at low temperature.     

A Comparative Study on the Magnetic Properties of Arc-Melted and Ball-Milled Fe0.9−x Mn0.1Al x Alloys

Samples of nominal composition Fe_0.9?x Mn_0.1Al _x (0.1 ≤x≤0.5) were prepared both by mechanical alloying and arc-melting. In order to elucidate the effect of the synthesis method upon the magnetic properties of this system, we have carried out a comparative study involving the use of different experimental techniques (Mössbauer, X-ray diffraction, vibrating sample magnetometry and magnetic susceptibility). Results revealed that independently of the employed method and milling time, the samples exhibit ferromagnetism below ～34 at.% Al. Above this concentration, the preparation method became a determinant factor upon the magnetic properties of the system. The differences are attributed, in the case of the mechanically alloyed samples, to Fe contamination arising from jars material. The results of our study are summarized in a magnetic phase diagram including ferromagnetic, paramagnetic, pure spin glass and reentrant spin glass regions.     

Magnetism and magnetocaloric properties of Mn0.95Cr0.05As

In this study the magnetic properties of Mn_0.95Cr_0.05As, prepared by mechanical milling, have been investigated. The results suggest that the presence of strains is very important for the magnetic state of the compound. In the presently studied compound, a combined magnetic and structural transition occurs from paramagnetic MnP phase to ferromagnetic NiAs phase at about 234 K. With further decreasing temperature, at 159 K, a transition from ferromagnetic NiAs phase to helimagnetic (H_a-type) MnP phase is observed, which is accompanied by an inverse magnetocaloric effect. The ferromagnetic phase is recovered when the temperature is increased. At both first-order transitions, at 159 and 234 K, large magnetic-entropy changes are observed.     

High-pressure effect on elastic constants, stacking fault energy and correlation with dislocation properties in MgO and CaO

The elastic properties and the generalized-stacking-fault-energy (GSFE) in MeO (Me = Mg, Ca) under different pressures have been calculated using the first principle calculations. In the anisotropic elasticity theory approximation, by using the Foreman’s method, the core structure and Peierls stress of $\tfrac{1} {2}\left\langle {110} \right\rangle \left\{ {110} \right\}We report structural, DC magnetization, detailed linear/non-linear AC susceptibility, (with applied frequency and amplitude) isothermal and thermoremanent magnetization (TRM) behavior for RuSr₂Y_1.5Ce_0.5Cu₂O₁₀ (YRu-1222) magneto-superconductor to understand its complex magnetism. Studied sample is synthesized through the novel solid state high pressure (6?GPa) high temperature (1450?°C) (HPHT) technique. The compound is crystallized in tetragonal structure with space group I4/mmm (No.?139). DC magnetic susceptibility shows that studied YRu-1222 is magneto-superconducting with Ru spins magnetic ordering at around 110?K and superconductivity (SC) in the Cu-O₂ planes below ?~?30?K. Frequency and field dependent detailed AC magnetic susceptibility measurements confirms the spin-glass (SG) behavior with homogeneous/non-homogeneous ferromagnetic (FM) clusters in this system. Variation of cusp position with applied AC frequency follows the famous Vogel-Fulcher law, which is commonly accepted feature for spin-glass (SG) system with homogeneous/non-homogeneous ferromagnetic clusters embedded in spin-glass (SG) matrix. Above the freezing temperature (T _f ), first and third harmonics AC susceptibility analysis indicated possibility of the co-existence of spin cluster ferromagnetism with superparamagnetism (SPM). The M-H loops at low temperature exhibit the ferromagnetic behavior with rather small coercive field (H _c ) and remnant magnetization (M _r ). Summarily, the magnetic (DC and AC) susceptibility measurements and their analysis have enabled us to unearth the complex magnetism in terms of successive SG-FM-SPM transitions with temperature.     

Fluctuations of the order parameter in R 0.55Sr0.45MnO3 manganites near the metal-insulator phase transition

The magnetic phase transformations induced by changes of the composition, external magnetic field strength, and temperature in manganites with a nearly half-filled conduction band in the vicinity of the metal-insulator phase transition have been investigated experimentally. It has been found that the substitution of rare-earth ions (Sm) for Nd ions with a larger ionic radius in R _0.55Sr_0.45MnO₃ manganites leads to a linear decrease in the Curie temperature T _C from 270 to 130 K and a transformation of the second-order ferromagnetic (FM) phase transition into a first-order phase transition. The results of measurements of the alternating-current (ac) magnetic susceptibility in the (Nd_{1 ? y} Sm _y )_0.55Sr_0.45MnO₃ system indicate the existence of a Griffiths-like phase in samples with a samarium concentration y > 0.5 in the temperature range T _C < T < T* (where T* ～ 220 K). For samples with y > 0.5, the magnetization isotherms at temperatures above T _C exhibit specific features in the form of reversible metamagnetic phase transitions associated with strong fluctuations of the short-range ferromagnetic order in the system of Mn spins in the high-temperature Griffiths phase consisting of ferromagnetic clusters. According to the results of measurements of the ac magnetic susceptibility in the (Sm_{1 ? y} Gd _y )_0.55Sr_0.45MnO₃ system for a gadolinium concentration y = 0.5, there is an antiferromagnetic (AFM) phase with an unusually low critical temperature of the spin ordering T _N ? 48.5 K. An increase in the external static magnetic field at 4.2 K leads to an irreversible induction of the ferromagnetic phase, which is stable in the temperature range 4.2–60 K. In the temperature range 60 K < T < 150 K, there exists a high-temperature Griffiths-like phase consisting of clusters (correlations) with a local charge/orbital ordering. The metastable antiferromagnetic structure is retained in samples with gadolinium concentrations y = 0.6 and 0.7, but it is destroyed with a further increase in the gadolinium concentration upon the transition to the spin-glass state. The magnetization isotherm obtained with variations in the external static magnetic field in the field range ±70 kOe at 4.2 K and the temperature dependence of the ac-magnetic susceptibility χ suggest that, in the Gd_0.55Sr_0.45MnO₃ ceramics, there is a mixed two-phase low-temperature state consisting of the quantum Griffiths phase with a characteristic divergence of χ(T) near T = 0, which was embedded in the spin-glass matrix with the spin “freezing” temperature T _G ? 42 K. The low-temperature state with quantum fluctuations exists in the (Sm_{1 ? y} Gd _y )_0.55Sr_0.45MnO₃ system for y ≥ 0.5.     

Low-temperature magnetic ordering in the perovskites Pr1−xAxCoO3 (A=Ca,Sr)

The magnetic and electrical properties of polycrystalline Pr_1?xA_xCoO₃ cobaltites with A=Ca, Sr and 0≤x≤0.5 were studied in the temperature range 4 K≤T≤1000 K and field up to 7 T. The X-ray analyses show the presence of only one phase having monoclinic or orthorhombic symmetry. The magnetic measurements indicate that the Ca-doped samples have at low temperatures, similar properties to the frustrated magnetic materials. PrCoO₃ is a paramagnetic insulator in the range from 4 to 1000 K. The Sr-doped cobaltites exhibit two phase transitions: a paramagnetic–ferromagnetic (or magnetic phase separated state) phase transition at about 240 K and a second one at about 100 K. The magnetic measurements suggest the presence of magnetic clusters and a change in the nature of magnetic coupling between Co ions at low temperatures. A semiconducting type behavior and high negative magnetoresistance was found for the Ca-doped samples, while the Sr-doped ones were metallic and with negligible magnetoresistance. The results are analyzed in the frame of a phase separation scenario in the presence of the spin-state transitions of Co ions.     

The magnetic and superconducting phase diagram of Gd-doped lanthanumsulfides

The system La_3-x( )_xS₄ [where ( ) denotes a vacancy] containing up to 25 at.% Gd is shown to be a model system in which magnetic interactions in metals can be studied. By measuring the temperature dependence of the low field a.c. susceptibility, the superconducting, spin-glass and ferromagnetic transitions can be determined as a function of the Gd concentration. The depression of the superconducting transition temperature mainly follows the theory of Abrikosov and Gor'kov, with a possibility for coexistence of superconductivity and spin-glass magnetic order near a critical concentration of 3 at.% Gd. The transition from spin-glass to ferromagnet is well-defined with the percolation limit for the long range ferromagnetic order at 14 at.% Gd.     

Zn1-xMnxSe的低温磁化率和自旋玻璃现象

用超导量子干涉器(SQUID)磁强计对稀释磁性半导体Zn_1-xMn_xSe(0.1≤x≤0.50)的低温低场直流磁化率作了测量,测量温度从4.2K到30K,测量磁场为15Oe。当x≥0.30时,从磁化率-温度曲线的浑圆峰值,观察到了自旋玻璃的转变。自旋玻璃的转变温度T_f,对x=0.30,0.40,0.50,分别为10.5K,16K,19.5K。给出了顺磁相和自旋玻璃相的相图。比较了Zn_1-xMn_xSe和Cd_1-xMn_xSe的自旋玻璃转变温度,发现对同样的Mn离子浓度,Zn_1-xMn_xSe的T_f高于Cd_1-xMn_xSe的T_f,用交换作用的理论作了讨论。 关键词：     

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.     

Raman scattering from magnetic excitations in Cd1−xMnxTe semiconductor alloys

We have observed antisymmetric Raman scattering from low frequency magnetic excitations in Cd_1?xMn_xTe alloys with 0.1 ≤ × ≤ 0.7. Crystals with x = 0.65 and 0.7 exhibit a magnon line in the antiferromagnetic phase, centered at 10.5 and 12.0 cm^-1 respectively, at 5 K. The frequency and the intensity of these lines decrease markedly with increasing temperature, finally disappearing in the paramagnetic phase. Crystals with 0.4 ≤ × ≤ 0.6 exhibit a similar feature at 5 K over the frequency range 4.5 – 5.5 cm^-1. These are attributed to spin waves originating from antiferromagnetically ordered clusters within their spin-glass phase. All the alloys show a distinct inelastic wing in the paramagnetic phase, which may be related to the magnetic excitations within residual clusters of coupled Mn⁺⁺ spins.     

Influence of Fe doping on electrical properties of LCMO

The polycrystalline samples La_0.67Ca_0.33Mn_(1?x)Fe _x O₃ (x?=?0.00,?0.01,?0.03, and 0.1) have been grown in single phase by solid state route. The analysis of the reaction has been done by thermogravimetry and differential thermal analysis measurements. DC electrical resistivity measurements have been carried out down to 15?K. The samples with x?=?0.00, 0.01, and 0.03 exhibit metal–insulator (MI) transition at temperatures 221.5?K, 217?K, and 215?K respectively, whereas the sample with x?=?0.1 is insulating in nature for entire temperature range. Interestingly, the electric transport properties of these samples are not consistent with their magnetic phase transitions and the samples show MI transition at a temperature, T _MI, which is significantly lower than the paramagnetic to ferromagnetic transition temperature (T _c). The resistivity data below T _MI has been analyzed using the empirical relation ρ?=?ρ₀?+?ρ₁ T ⁿ and the data above this temperature has been analyzed using two existing models, Mott's variable range hopping model and spin polaronic conduction model.     

非晶态Fe13Ni67.2P4.5B15.3合金的临界现象

本文报道非晶态Fe₁₃Ni_67.2P_4.5B_15.3合金的磁化强度与温度和磁场关系的测量结果。在居里温度附近样品的磁特性符合二级相变规律,得到临界指数β=0.39±0.02,γ=1.56±0.06,δ=5.20±0.1,样品的居里温度T_c=(180.4±0.2)K。在实验误差范围内,临界指数β,γ,δ满足γ=β(δ-1)关系,在168—192K温度范围,实验数据满足二级相变的磁状态方程。当T>270K时,样品顺磁磁化率服从居里-外斯定律,由居里-外斯常数c计算出有效顺磁磁矩P_eff=3.19 μ_B。 关键词：     

Magnetic properties of a cermet on the base of Al2O3

The zero-field-cooled and field-cooled magnetizations, magnetization versus field, and remanent magnetization were measured for a cermet on the base of Al₂O₃ using a SQUID magnetometer in the temperature range of 2–360 K. It was shown that magnetic properties of the cermet are determined by independent ferromagnetic, paramagnetic and spin-glass contributions. The spin-glass behavior was studied.     

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.     

Characterization of magnetic phases in the FexMn0.65−xAl0.35 disordered alloys

Melted alloys of the Fe_xMn_0.65−xAl_0.35 disordered system, 0.25?x?0.65, were experimentally studied by Mössbauer spectrometry, vibrating sample magnetometry and AC magnetic susceptibility. All the alloys exhibit the BCC structure with a nearly constant lattice parameter (2.92 Å). Mössbauer studies at room temperature (RT) show that in the 0.25 ?x?0.45 range the alloys are paramagnetic (P) while in the 0.50?x?0.65 range, they are ferromagnetic. At 77 K, Mössbauer studies show that the alloy with x=0.25$x=0.25$ presents weak magnetic character that is consistent with an antiferromagnetic (AF) behavior due to the high Mn content, while those with 0.30?x?0.40 are paramagnetic, and those in the 0.45?x  ?0.65 range are ferromagnetic (F) with a mean field increasing with the Fe content. Hysteresis cycles at RT prove the paramagnetic character of the alloys between x=0.25$x=0.25$ and 0.40 and the ferromagnetic character for x?0.45$x?0.45$. Complementary measurements using AC magnetic susceptibility permit a magnetic phase diagram to be proposed, with the P phase for high temperature and all the compositions, the AF phase for low Fe content and at low temperature, the F phase for high Fe content above RT and the spin glass phase for all the compositions and at temperatures lower than 46 K. In addition, the mean field renormalization group (MFRG) method, applied to a random competitive and site dilute Ising model with nearest-neighbor, gives rise to magnetic phase diagram, which fairly agrees with previous experimental one.     

Reentrant phases in the ± J spin glass

We consider an enlarged phase space of the ±J spin glass which includes the dilute Ising model and the frustrated system. The three orthogonal axes in this space are: (i) The fraction of ferro- to antiferro-magnetic bonds, p; (ii) the ratio of the strengths of the antiferro- to ferromagnetic interacions, q; and (iii) the temperature, T. Within this phase space we observe extended regions of the low-temperature spin-glass phase which is characterized by a unique distribution of the local-order parameter. We observe reentrant phase transitions: for fixed p and q with varying T the distribution of the local order parameter shows paramagnetic, ferromagnetic and then spin-glass phases; for fixed p and T and varying q the distribution shows ferromagnetic to paramagnetic and then spin-glass phases.     

Room temperature ferromagnetism and magnetotransport properties of the layered manganite system La1.2Ba1.8Mn2−xRuxO7 (0≤x≤1.0)

The dc magnetization and ac susceptibility measurements on two dimensional layered manganite La_1.2Ba_1.8Mn₂O₇ samples reveal the occurrence of ferromagnetism above room temperature with ferromagnetic (FM) to paramagnetic (PM) transitions at 338 K. The bifurcation temperatures shown by the zero-field cooled (ZFC) and field cooled (FC) dc magnetization curves at high temperatures shift towards lower temperatures as the applied field is increased from 100 to 2500 Oe. The data are suggestive of a large magnetic anisotropy due to the strong competing ferromagnetic and antiferromagnetic interactions resulting in a spin-glass-like state. Ru doping is found to enhance the ferromagnetism and metallicity of the system in a remarkable way. The magnetoresistance (MR) values obtained are very high and about 40% even at 260 K for the undoped sample.     

Spin frozen in the Kondo-lattice compound: CeCuSi2

In this paper, we present the experimental results of X-ray powder diffraction, electrical resistivity, magnetic susceptibility, and specific-heat measurements as well as Ce-L_III-edge X-ray absorption spectrum of the Ce-based intermetallic compound CeCuSi₂. The results revealed that CeCuSi₂ is a Kondo-lattice compound with no superconducting or magnetic-phase transition above 0.4 K. In addition, we found spin-glass behavior in the DC susceptibility measurements. The AC susceptibility measurements and the magnetic entropy calculation also confirm the presence of the spin-glass phase. The possible formation mechanism for the spin-frozen state is also discussed in this paper.     

Ferromagnetic correlations in both the α and γ-phases of paramagnetic iron

Paramagnetic scattering experiments using polarised neutrons and polarisation analysis in the paramagnetic phase of iron are reported. The measurements have been performed at 1120 K in the α-bcc phase and 1320 K in the γ-fcc phase. Iron moments to respectively 1.3 (0.1)μ_B and 0.9 (0.1)μ_B have been shown to persist at these temperatures. In both phases, ferromagnetic correlations are present and are especially strong in the α-phase. The ferromagnetic correlations also determine the magnetic character of iron in the γ-phase, contrary to previous suggestions of antiferromagnetic behaviour postulated in the literature on the basis of indirect experimental arguments.     

Entropy changes accompanying the magnetic phase transitions in low Si-doped Ce2Fe17−xSix Alloy

A summary of the results of ac susceptibility and isothermal magnetization measurements on polycrystalline samples of Ce₂Fe_17−xSi_x with nominal composition of x=0.0, 0.1, 0.2 and 1.0 is presented. These data reveal that the substitution of small amounts of Si for Fe produce a significant increase in temperature at which ferromagnetism appears, to the extent that, at x=1, characteristics of the anti ferromagnetic to paramagnetic transition (at temperature T_N) have disappeared completely. The nature of the various magnetic phase transitions — identified through the use of Arrott plots — and the accompanying magnetic entropy change, ΔSm, are both affected significantly by small amounts of Si substitution. In particular, while the peak entropy change is modest (occurring at x=0.1), the temperature interval over which a substantial entropy change occures is significant, approaching 150 K, an important criterion for improving the overall effectiveness of such materials for magnetic refrigeration.