The magnetostriction of Invar alloys

The principles of the theory of a phase transition into a magnetically ordered state are formulated for Invar alloys and other similar inhomogeneous ferromagnets, for which the concept of a local Curie temperature distribution corresponding to the experimentally observed broadened temperature interval of the transition into the ferromagnetic state has existed for 10 years. A method is proposed for obtaining information about the local temperature distribution from experimental data on the change in the properties of magnets in response to a change in temperature. For iron-nickel-chromium alloys it is shown how to obtain the temperature dependence of the magnetostrictional susceptibility of the paraprocess from data on the magnetic contribution to the thermal expansion coefficient. This confirms the important role of the local Curie temperature distribution, and it also indicates a need for new analysis of experimental data on temperature-broadened magnetic ordering phase transitions. Zh. éksp. Teor. Fiz. 113, 213–227 (January 1998)     

About the local Curie temperature distribution of Invar alloys

Strongly disordered ferromagnetic alloys which exhibit magnetic inhomogeneities are described in the frame of a generalized Ginzburg-Landau theory by averaging the Ginzburg-Landau energy of a subsystem with a definite Curie temperature over the Curie temperature of all subsystems of the inhomogeneous magnetic system. It is shown that this leads to the rough scaling of a smooth phase transition. Furthermore the distribution function of the Curie temperature is derived from the experimental data of the specific heat, the volume thermal expansion coefficient and the compressibility.     

Correlation between reentrant spin glass behavior and the magnetic order-disorder transition of the martensite phase in Ni-Co-Mn-Sb Heusler alloys

We have performed ac susceptibility and dc magnetization measurements in Ni(50-x)Co(x)Mn(38)Sb(12) Heusler alloys. From the ac susceptibility measurements, the existence of reentrant spin glass (RSG) state is observed for x=0-5. It is found that the signature of RSG behavior diminishes with increase in x. This behavior is in contrast to the fact that the exchange bias field increases with x, which reveals that the origins of RSG and exchange bias are different in the present system. It is found that the system enters a frustrated ferromagnetic state just below the Curie temperature of the martensite phase (T(M)(C)) and then the RSG state at low temperature. The strength of the RSG state is critically dependent on the sharpness of the magnetic transition at (T(M)(C)). This proposition is further supported by the thermo-remanent magnetization and low field thermomagnetic measurements.     

Magnetic cluster expansion simulation and experimental study of high temperature magnetic properties of Fe-Cr alloys

We present a combined experimental and computational study of high temperature magnetic properties of Fe-Cr alloys with chromium content up to about 20?at.%. The magnetic cluster expansion method is applied to model the magnetic properties of random Fe-Cr alloys, and in particular the Curie transition temperature, as a function of alloy composition. We find that at low (3-6?at.%) Cr content the Curie temperature increases with the increase of Cr concentration. It is maximum at approximately 6?at.% Cr and then decreases for higher Cr content. The same feature is found in thermo-magnetic measurements performed on model Fe-Cr alloys, where a 5?at.% Cr alloy has a higher Curie temperature than pure Fe. The Curie temperatures of 10 and 15?at.% Cr alloys are found to be lower than the Curie temperature of pure Fe.     

Investigation of the magnetic properties of the Gd1−xErx alloy system in the x < 0.62 composition range

The magnetic properties of the Gd---Er system (with erbium concentrations ranging from 0 to 62 at%) were investigated at temperatures between 4.2 and 350 K under external magnetic fields up to 1 T. The results show that, similar to other rare-earth systems, simple ferromagnetism prevails for concentrations with a de Gennes factor G higher than about 11.5 (≈ 30 at % Er) while complex magnetic structures are present for higher Er concentrations (G ≤ 11.5. The magnetic transition temperature decreases with increasing Er content, showing good agreement with phenomenological expectations. In the paramagnetic region, the Curie constant of the alloy corresponds to the linear combination of the constituent elements' contribution, thus proving that both Gd and Er maintain their individual magnetic moments. From magnetization data below the transition, the change in magnetic entropy for a 1 T field was calculated and this showed distinct differences between ferromagnetic-paramagnetic transitions and antiferromagnetic transitions. The Landau theory of second-order phase transitions can be well applied for evaluating the thermodynamic transition temperature and the specific heat anomaly in the ferromagnetic alloys but it fails for the more complex structures if the applied field is not far higher than the critical field. The magnetic entropy change displays well defined peaks at the transition temperature for both ferromagnetic and antiferromagnetic alloys. There is an indication that the spin-reorientation transition occurring in alloys with higher Er concentrations is also coupled with significant magnetic entropy changes.     

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.     

Unambiguous determining the Curie point in perovskite manganite with second-order phase transition by scaling method

The accurate determination of the Curie temperature (T_C) is particularly important in describing the magnetic behavior close to the paramagnetic-ferromagnetic (PM-FM) phase transition. In this paper, we studied the magnetic phase transition and accurately predicted the Curie point of perovskite manganite La_0.825Sr_0.175MnO₃. We find the compound shows a second-order PM-FM transition and has a large magnetic entropy change (MEC) in vicinity of phase transition region. Based on the scaling law and the correlation between magnetic field and MEC, the precise and magnetic-independent Curie temperature was determined to be 281.7 K, obviously lower than 285.4 K decided from the magnetization versus temperature. The reliability of new Curie temperature can be well confirmed by the modified Arrott plot together with the critical exponents. Our results not only open up a new pathway for precise definition of Curie point but also facilitate the efficient exploitation of spontaneous magnetic bubbles in perovskite manganite.     

Magnetostructural phase transitions in manganese arsenide single crystals

The effect of an external magnetic field with a strength up to 140 kOe on the phase transitions in manganese arsenide single crystals has been investigated. The existence of unstable magnetic and crystal structures at temperatures above the Curie temperature T _C = 308 K has been established. The displacements of manganese and arsenic atoms during the magnetostructural phase transition and the shift in the temperature of the first-order magnetostructural phase transition in a magnetic field have been determined. It has been shown that the magnetocaloric effect in a magnetic field of 140 kOe near the Curie temperature T _C is equal to ??T ?? 13 K. A model of the superparamagnetic state in MnAs above the temperature T _C has been proposed using the data on the magnetic properties and structural transformation in the region of the first-order magnetostructural phase transition. It has been demonstrated that, at temperatures close to T _C, apart from the contribution to the change in the entropy from the change in the magnetization there is a significant contribution from the transformation of the crystal lattice due to the magnetostructural phase transition.     

Pressure-induced antiferromagnetism in ferromagnetic Fe51.5Rh48.5 alloy

The pressure-temperature magnetic phase diagram based on electrical resistivity measurements was determined for Fe-Rh alloys, ferromagnetic down to 4.2 K, from room temperature to the Curie point (750 K) and for pressure up to 100 kbar. A pressure-induced first order ferromagnetic-antiferromagnetic phase transition line was found with an inhomogeneous, mixed phase existing at pressures lower than 50 kbar. A new, qualitative model is proposed to explain the phase transitions, the absence of magnetic moment on Rh atoms in the AF state and the shape of the p-T diagram. The model is based on the excitonic antiferromagnetism of semimetallic Fe-Rh and it is connected with the pecularities of the electroni structure and the shape of the Fermi surface.     

Magnetic properties of Europium-graphite intercalation compound C6Eu

An ordered state with a complex magnetic structure has been observed below 4CK. The magnetization for H ⊥ c (c; the c-axis) shows a field-induced phase transition from a small magnetic moment state ( ～ 0.6μ_B) at lower fields to a larger moment one ( 2.2 - 2.5μ_B) above 25kG. The transition field depends on temperature and is found to be 5 - 2kG. The magnetization for H//c has only a sublinear field dependence. Above 40K the magnetic susceptibility obeys the Curie-Weiss law with θ = +1.3K and the molar Curie constant C_m = 6.78, which suggests that Eu ion is divalent in C₆Eu.     

Effect of Co on magnetic property and phase stability of Ni--Mn--Ga ferromagnetic shape memory alloys: A first-principles study

The effect of Co content on magnetic property and phase stability of Ni50-xMn25Ga25Cox ferromagnetic shape memory alloys has been investigated using first-principles calculations. The total energy difference between paramagnetic and ferromagnetic state of austenite plays an important role in the magnetic transition. The high Curie temperature can be attributed to the stronger Co-Mn exchange interaction as compared to the Ni-Mn one. The phase stability of Ni50-xMn25Ga25Cox austenite increases with increasing Co content, which is discussed based on the electronic structure.     

Metamagnetic phase transitions induced by static and pulsed fields in (Sm0.5Gd0.5)0.55Sr0.45MnO3 ceramics

It has been found that the magnetic susceptibility of (Sm_0.5Gd_0.5)_0.55Sr_0.45MnO₃ ceramic samples in zero external magnetic field exhibits a sharp peak near the temperature of 48.5 K with a small temperature hysteresis that does not depend on the frequency of measurements and is characteristic of the phase transition to an antiferromagnetic state with a long-range charge orbital ordering, which is accompanied by an increase in the magnetic susceptibility with a decrease in the temperature. The magnetization isotherms in static and pulsed magnetic fields at temperatures below 60 K demonstrate the occurrence of an irreversible metamagnetic transition to a homogeneous ferromagnetic state with a critical transition field independent of the measurement temperature, which, apparently, is associated with the destruction of the insulating state with a long-range charge ordering. In the temperature range 60 K ?? T ?? 150 K, the ceramic samples undergo a magnetic-field-induced reversible phase transition to the ferromagnetic state, which is similar to the metamagnetic transition in the low-temperature phase and is caused by the destruction of local charge/orbital correlations. With an increase in the temperature, the critical transition fields increase almost linearly and the field hysteresis disappears. Near the critical fields of magnetic phase transitions, small ultra-narrow magnetization steps have been revealed in pulsed fields with a high rate of change in the magnetic field of ??400 kOe/??s.     

钙钛矿La0.8Ca0.2MnO3多晶样品的制备及低温磁学特性

我们利用固相反应法制备了粉末状La0.8Ca0.2MnO3样品,其空间群为pnma,样品中存在超顺磁性粒子,其截止态和超顺磁态的转变温度为185K,从dσ/dT的负峰得到铁磁-顺磁相变居里温度为210K,在顺磁领域线性拟合而得到顺磁居里温度为216.4K,分子式有效磁矩为6.25μB,低温领域磁化曲线服从布洛赫T32定律,其自旋波劲度系数为49.2meV2.     

Invar-type new ferromagnetic amorphous Fe-B alloys

Measurements of magnetization, electrical resistivity, thermal expansion and differential thermal change were made on amorphous Fe_100-xB_x (9 ≦ X ≦ 21) alloys prepared by rapid quenching from the liquid state.With decreasing boron content in the alloys, the Curie temperature falls remarkably, while the magnetic moment increases sluggishly. The thermal expansion curves exhibit the invar characteristics below the Curie temperature due to a large positive spontaneous volume magnetostriction, and the reduced magnetization curves decrease much more rapidly with increasing temperature than those of other ferromagnetic amorphous alloys.     

Magnetic structure of critical Fe-Ni alloys as derived from Arrott plots and Mössbauer spectra

New evidence of magnetic phase transition brought about by an external magnetic field acting on critical composition bulk Fe-Ni alloys was found by using iron-nickel alloys containing 30 wt% Ni and 31.3 wt% Ni, respectively (balance Fe). The split in the central peak of Mössbauer spectrum of the Fe-Ni alloy containing 29.7 wt% Ni, at its Curie temperature in a weak external magnetic field of 3100 Oe, is a conclusive evidence of its superparamagnetic structure.     

Transition to the quasi-two-dimensional magnetic state in Pd-Fe films

The Mössbauer spectra of Pd-Fe films with different thicknesses and iron concentrations are measured. In these films, the spin fluctuations associated with the transition to a quasi-two-dimensional magnetic state are observed over a wide range of temperatures. The dependences of the magnetic field strength and the Curie temperature on the film thickness are quite consistent with the scaling theory. Owing to the long-range exchange coupling, the films containing 14.4 and 7.8 at. % Fe undergo a transition to the quasi-two-dimensional state at a film thickness of several thousand angströms.     

替代掺杂的MnNiGe1-xGax合金中马氏体相变和磁-结构耦合特性

研究了MnNiGe_1-xGa_x (x=0–0.30) 系列合金中成分、结构、马氏体相变性质和磁性的相互关系. 在较小的成分范围内, Ga取代Ge元素可有效地将马氏体相变温度降低近400 K. Ga的引入削弱了体系中的共价成键作用, 马氏体相显示出磁交换作用的增强. 相图显示, 掺杂使马氏体相变先后穿过T_N 和T_C 两个磁有序温度, 居里温度窗口效应在体系有存在的可能, 磁性对相变温度的成分关系有所影响. 实验观察到合金变磁转变的特性及相变行为对制备方法的敏感性. 这些特性的发现, 有利于进一步优化这类材料的磁结构和相变特性, 获得具有应用价值的新材料. 关键词： MM’X合金 马氏体相变 磁有序温度 变磁转变     

Evidence of Clustering in Heusler like Ferromagnetic Alloys

Two Heusler like alloys Fe₂CrAl, and Mn₂CoSn had been prepared and studied using the Mössbauer effect. Both these alloys were ferromagnetic in nature. Their Mössbauer spectra showed the co-existence of a paramagnetic portion along with magnetic hyperfine part upto temperatures well below their Curie temperatures. Low temperature Mössbauer spectra for both these alloys showed a steady variation in the intensity of the paramagnetic portion. X-ray diffraction studies made on these samples ruled out the possibilities of a separate phase. Explanation of the observed phenomena is given by clustering around the magnetic ions present in these samples.     

Studies of the surface magnetic state of the Sr-M hexagonal ferrites near the phase transition at the Curie temperature

A study is reported of the temperature dependences of the hyperfine (HF) interaction parameters in a ～200-nm thick surface layer and in the bulk of macroscopic hexagonal ferrite crystals of the Sr-M type (SrFe₁₂O₁₉ and SrFe_10.2Al_1.8O₁₉). The method used for the measurements is Mössbauer spectroscopy with simultaneous detection of gamma quanta, characteristic x-ray emission, and electrons, which permits direct comparison of the HF parameters in the bulk and the near-surface layers of a sample. As follows from the experimentally determined temperature dependences of the effective magnetic fields, the fields at the nuclei of the iron ions located in a ～200-nm thick near-surface layer decrease with increasing temperature faster than those of the ions in the bulk. The transition to paramagnetic state in a ～200-nm thick surface layer was found to occur 3° below the bulk Curie temperature. This offers the first experimental evidence for the transition to paramagnetic state in a surface layer of macroscopic ferromagnets to take place below the Curie temperature T _c for the bulk of the crystal. It has been established that the transition temperature T _c (L) of a thin layer at a depth L from the surface of a crystal increases as one moves away from the surface to reach T _c at the inner boundary of the surface layer called critical. In the vicinity of T _c one observes a nonuniform state, with the crystal being magnetically ordered in the bulk but disordered on the surface. The experimental data obtained were used to construct a phase diagram of surface and bulk states for macroscopic magnets near the Curie (or Néel) temperature.     

Scaling analysis of the magnetocaloric effect in Gd5Si2Ge1.9X0.1 (X=Al, Cu, Ga, Mn, Fe, Co)

The field dependence of the magnetic entropy change has been studied for a series of doped Gd₅Si₂Ge₂ alloys, which possess a magnetic phase transition that is either entirely second order or a combination of primarily second-order mixed to a very minor degree with a first-order transition arising from a magneto-structural phase change. By analyzing the field scaling of the refrigerant capacity as well as of the reference temperatures used for constructing a universal scaling curve, a procedure for estimating the values of the critical exponents for the alloys was developed. For the cases where the transition is entirely second order, the results obtained from this procedure are comparable to the values obtained from the Kouvel–Fisher method. For the case of Fe-doped alloys which partially possess a first-order phase change, the Kouvel–Fisher method is inapplicable. However, their critical exponents determined by our developed procedure can be used to estimate the Curie temperature of the orthorhombic majority phase.