Transitions between spin-glass and ferromagnetic (or related) states in disordered Ni-Mn alloys

Magnetic measurements reveal that the alloy Ni₇₈Mn₂₂ undergoes a field-induced spin-glass-to-ferromagnetic transition at a critical field that decreases from ≈ 140 Oe to zero as the temperature is raised from 4.2 to 40 K, its ferromagnetic Curie point being ≈ 270 K. The alloy Ni₇₄Mn₂₆ is found to have a related state of infinite initial susceptibility but no spontaneous magnetization between ≈ 80 and ≈ 155 K. Below 40 and 80 K, respectively, Ni₇₈Mn₂₂ and Ni₇₈Mn₂₂ exhibit the thermoremanent properties of a spin glass.     

Magnetic properties of (Mn1 − x Fe x )1.68Sn solid solutions

Solid solutions in the (Mn_{1 ? x} Fe _x )_1.68Sn system (x ≤ 0.5) with a Ni₂In-type structure are synthesized by the solid-phase reaction method in a stepwise temperature regime. The unit cell parameters a and c decrease with an increase in the iron concentration in the alloys and become equal to a = 0.430 nm and c = 0.538 nm for the (Mn_0.5Fe_0.5)_1.68Sn alloy. A superstructure with the unit cell parameters a _ss = 3a and c _ss = c is revealed in alloys of the system under investigation. The specific magnetization of the alloys increases nonlinearly from 53 G cm³ g^?1 in the Mn_1.68Sn alloy to 72 G cm³ g^?1 in the (Mn_0.5Fe_0.5)_1.68Sn solid solution. The Curie temperature changes from 270 K in the initial alloy of the composition Mn_1.68Sn to 365 K in the alloy of the composition (Mn_0.5Fe_0.5)_1.68Sn. All solid solutions in the (Mn_{1 ? x} Fe _x )_1.68Sn (x ≤ 0.5) system exhibit metallic conductivity in the temperature range from 77 to 450 K.     

Order-disorder phase transition in Ni4W alloy

The order-disorder phase transition in Ni₄W alloy with a D1 _a superlattice was studied experimentally by x-ray structural analysis and theoretically by the Green function method. An experimental temperature dependence was obtained for the equilibrium parameter of long-range order for Ni₄W alloy. It was established that the order-disorder phase transition in the alloy is a first-order transition; the transition occurs through the two-phase region D1_a + A1. The Green function technique, with account for the pair-correlation functions, was used to describe the order-disorder transition for the D1 _a superlattice. The theoretical temperature dependence obtained for the equilibrium parameter of long-range order is in satisfactory agreement with the experimental curve.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 92–97, April, 1978.     

Concerning the two magnetic correlation lengths above T c in an Invar iron-nickel alloy

The magnetic phase transition in the Invar alloy Fe₇₀Ni₃₀ is investigated by means of small-angle neutron scattering over a wide range of momentum transfer. This method was used to measure two magnetic correlation lengths R _c 1 and R _c 2 which coexist in the alloy above the phase transition temperature T _c . The critical correlations with correlation length R _c 1 are described well by an Ornstein-Zernicke expression, and the critical correlations with the second correlation length, an order of magnitude larger than R _c 2, are described well by a squared Ornstein-Zernicke expression. The temperature dependences obtained for the correlation lengths R _c 1 and R _c 2 satisfy the power law R _c ∼((T−T _c )/T _c ) ^−υ with critical exponents υ₁=0.65±0.05 and υ₂=1.3±0.1 for the shorter and longer scales, respectively. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 1, 53–57 (10 July 1997)     

Hyperfine interaction in Ni79Mn21 and Ni76Mn24

We report on PAC measurements revealing very different magnetic behaviour in two nearby alloys. The Ni₇₉Mn₂₁ alloy orders at T_C=280(5) K, but a two field sites interpretation is needed. Both hyperfine fields (hff) are in the ratio 12 and their distributions have the same absolute width, the relative abundance of the high field site is 30%. A slope change in the hff (T) curve at low temperature may indicate the transition into the mixed state. In the Ni₇₆Mn₂₄ alloy we observe magnetic ordering at T=95(3) K followed by a smooth increase of the effective hff upon cooling but with an anomalous minimum around T=20 K.     

Structural and magnetic investigations in the vicinity of first-order transformations in Ni–Mn–Ga–Co ferromagnetic shape memory alloys

Among the series of alloys derived from Ni₅₀Mn₂₉Ga₂₁ on selective substitution of Co for Ni and Mn, two alloys Ni_49.8Mn_27.2Ga_21.2Co_1.8 and Ni_46.9Mn_28.8Ga₂₁Co_3.3 referred to as CoMn-1.8 and CoNi-3.3, respectively, are found to exhibit an additional first-order transformation below their martensitic transformation temperatures. Systematic studies on temperature and field dependence of magnetic properties of these alloys are carried out, through the transformations, to understand their origin. An examination of these results in conjunction with those from structural investigations reveals that the transformation in the CoMn-1.8 alloy is an intermartensitic transformation and has a structural origin, while that in the CoNi-3.3 alloy is not of the structural origin and is attributed to local spin inversion of Co moments, which is of the magnetic origin.     

Martensitic transformation and electrical properties of a Ni<Subscript>2.14</Subscript>Mn<Subscript>0.81</Subscript>Fe<Subscript>0.05</Subscript>Ga alloy in its different structural states

Phase transformations in a Ni_2.14Mn_0.81Fe_0.05Ga alloy in different structural states are studied from the temperature dependences of its electrical resistivity. The dependences obtained indicate that, in the coarse-grained state, this alloy undergoes two structural phase transformations: intermartensitic modulation transformation and martensite-austenite transformation. In the nanocrystalline state, these transformations are absent. The recrystallization of a nanocrystalline sample at 773 K for 30 min results in the martensite-austenite transformation; however, the phase transformation related to a change in the martensite modulation period does not occur in this state. The resistivity is shown to depend on the structural state of the alloy.     

Crystallographic features of the structure of a martensite packet of the NiMn intermetallic compound

Optical microscopy, scanning electron microscopy, and X-ray diffraction are used to show that a pseudosingle crystal forms upon cooling of an alloy Ni₄₉Mn₅₁ single crystal below the temperature of the β→θ (bcc → fct) transformation. At room temperature, this pseudosingle crystal has the structure of tetragonal L1₀ martensite with parameters a = 0.3732 nm and c = 0.3537 nm and a tetragonality c/a = 0.94775. The temperatures of the forward and reverse B2 → L1₀ transformations are determined. The crystallographic features of martensite packet formation are analyzed. As shown by EBSD, neighboring martensite packets always have three kinds of tetragonal martensite plates, which are in a twin position and have different tetragonality axis directions. Repeated heating and quenching of the pseudosingle crystal result in recrystallization with the formation of coarse grains. The packet structure of the tetragonal martensite is retained in this case, and the sizes of the packets formed within a grain decrease by a factor of 2–3 as compared to the initial pseudosingle crystal.     

Giant entropy change at the co-occurrence of structural and magnetic transitions in the Ni Mn Ga Heusler alloy

We have studied the isothermal entropy change around a first-order structural transformation and in correspondence to the second-order Curie transition in the ferromagnetic Heusler alloy Ni_2.15Mn_0.85Ga. The results have been compared with those obtained for the composition Ni_2.19Mn_0.81Ga, in which the martensitic structural transformation and the magnetic transition occur simultaneously. With a magnetic field span from 0 to 1.6 T, the magnetic entropy change reaches the value of 20 J/kg K when transitions are co-occurring, while 5 J/kg K is found when the only structural transition occurs. Received 27 September 2002 / Received in final form 17 February 2003 Published online 11 April 2003 RID="a" ID="a"e-mail: solzi@fis.unipr.it     

Martensitic transformation temperature hysteresis narrowing and magnetocaloric effect in ferromagnetic shape memory alloys Ni-Mn-Ga

The hysteresis narrowing and magnetocaloric effect (MCE) were studied in the Ni-Mn-Ga alloys. Ni₅₄Mn₂₄Ga₂₂ and Ni_51.9Mn₂₇Ga_21.1 were selected in order to obtain specific characteristic temperatures of alloys and to study an influence of several factors on the hysteresis width (δ) of basic austenite-martensite (AM) transformation. The effect of martensite crystal lattice softening due to intermartensitic transition (IT) taking place immediately after AM transformation on δ was examined in alloy 1. MCE was measured in alloy 2, in which Curie temperature (T_C) and austenitic start temperature (A_S) coincides. It was demonstrated that the hysteresis width δ reaches about 2 K, if IT is enough close to AM transition. MCE was found to have a maximum at T_C≈A_S.     

Acoustic wave propagation through the boundary between a liquid and the Heusler ferromagnetic alloy

Incidence of an acoustic wave upon a plane boundary between a liquid and a ferromagnetic crystal is considered. The ferromagnet is the Ni_2+x+y Mn_1−x Ga_1−y Heusler alloy with a shape memory, which is in the region of the premartensite or martensite phase transition in temperature. The directions of propagation and polarization and the amplitudes of the reflected and transmitted quasilongitudinal and quasitransverse waves in the (110) plane of the crystal are determined. Starting from a certain critical angle of incidence, a longitudinal wave in the crystal becomes inhomogeneous and gliding along the boundary with an accompanying surface oscillation. In the vicinity of the phase transition point, this wave may be radiated into the crystal bulk. Proceeding from the experimental data by Trivisonno for ultrasonic velocities and absorption in a Ni₂MnGa crystal, numerical estimates are obtained for the aforementioned acoustic effects.     

FeTe合金结构分析及其薄膜制备

利用固态反应法制备了名义成分为FeTe的合金, 采用X射线粉末衍射技术和Rietveld全谱拟合分析方法测定了其相组成和晶体结构. 研究表明,主相为Fe_1.08Te,空间群为P4/nmm,点阵参数 a = 3.8214(3) Å, c = 6.2875(3) Å, Z = 2, Fe原子占据2a和2c晶位, Te原子占据2c晶位. 利用脉冲激光沉积技术制备的FeTe薄膜超导转变起始温度为13.2 K,零电阻温度为9.8 K. 关键词： FeTe Rietveld结构精修 超导薄膜     

哈斯勒合金Ni50Mn35In15的马氏体相变及其磁热效应

利用电弧炉熔炼了Ni₅₀Mn₃₅In₁₅多晶样品,根据磁性测量对其马氏体相变和磁热效应进行了系统研究.结果表明,随着温度的降低,样品在室温附近先后发生了二级磁相变与一级结构相变特征的马氏体相变,导致它的磁化强度产生突变. 同时通过低温下的磁滞回线的测量发现样品存在交换偏置行为,表明低温下马氏体相中铁磁和反铁磁共存. 此外,根据Maxwell方程,计算了样品在马氏体相变温度附近的磁熵变,当温度为309K,磁场改变5 T时,样品的磁熵变可达22.3J/kgK. 关键词： 哈斯勒合金 50Mn₃₅In₁₅')" href="#">Ni₅₀Mn₃₅In₁₅ 马氏体相变 磁热效应     

Investigation of a magnetic phase transition in fcc iron-nickel alloys

A magnetic phase transition in carbon-doped (0.1 and 0.7 at. %) Fe₇₀Ni₃₀ Invar alloys was investigated by the method of depolarization of a transmitted neutron beam and by small-angle scattering of polarized neutrons. It is shown that for both alloys, two characteristic length scales of magnetic correlations coexist above T _c. Small-angle scattering by critical correlations with radius R _c is described well by the Ornstein-Zernike (OZ) expression. The longer-scale (second) correlations, whose size can be estimated from depolarization data, are not described by the OZ expression, and hypothetically can be modeled by a squared OZ expression, which in coordinate space corresponds to the relation 〈M(r)M(0)〉∝exp(−r/R _d), where R _d is the correlation length of the second scale. The temperature dependence of the correlation radius R _c was obtained: R _c ∝ ((T−T _c)/Tc)^−ν , where ν≈2/3 is the critical exponent for ferromagnets, over a wide temperature range up to T _c ^exp , at which the correlation radius becomes constant and equals its maximum value R _c(T _c)=R _c ^max . The maximum correlation radius established (R _c ^max =140 Å and 230 Å for the first and second alloys, respectively) characterizes the length-scale of the fluctuation for which the appearance of critical correlations first results in the formation of a ferromagnetic phase, and the phenomenon itself exhibits a “disruption” of the second-order phase transition at T=T _c ^exp , as a result of which a first-order transition arises. Temperature hysteresis was also detected in the measured polarization of the transmitted beam and intensity of small-angle neutron scattering in the alloy above T _c, confirming the character of this magnetic transition as a first-order transition close to a second-order transition. Zh. éksp. Teor. Fiz. 112, 2134–2155 (December 1997)     

Magnetism of an Fe65Mn35 alloy in the fcc/hcp phase region

Among the three possible phase structures (bcc, fcc and hep) of iron rich Fe₁₀₀-cMn _c alloys (c _Mn <40 at%), the two closed packed structures, fcc and hep, produce poor resolved and similar Mössbauer spectra. Therefore the evaluation of spectra collected on a sample (nominalc _Mn =35 at%) with coexisting fcc and hep phases only was possible with the help of the Afans’ev/Tsymbal method of “Mössbauer lines sharpening” introduced recently/1,2,3/. A comparison of the hyperfine field vs. temperature with an FeNi-Invar alloy (c _Ni =35 at%) shows a surprising similarity.     

Magnetic after-effect in Ni–Mn–Sb Heusler alloy

The ferromagnetic Heusler-type alloy _Ni50Mn35Sb15${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{35}{\mathrm{Sb}}_{15}$ exhibits well defined shape memory behaviour. We have investigated the transport and magnetic properties of this alloy across the martensitic transformation. Pronounced thermo-magnetic irreversibility between zero-field-cooled and field-cooled susceptibility data was observed below the martensitic transition temperature. We observe significant magnetic after-effect in magnetisation in both austenite and martensitic phases. However, a clear change in the nature of relaxation is observed as the sample is cooled across the martensitic transition temperature. These observations can be explained on the basis of complex domain dynamics in presence of rich micro-structure formation in the martensite.     

Large successive magnetocaloric effects around room temperature in Ni<Subscript>50</Subscript>Mn<Subscript>34</Subscript>In<Subscript>15</Subscript>Al alloy

Two successive magnetocaloric effects consisting of inverse magnetocaloric effect around martensitic transition and negative magnetocaloric effect around magnetic transition of austenitic phase have been observed in Ni₅₀Mn₃₄In₁₅Al alloy. Large inverse magnetic entropy change ΔS_m ( ~ 21.3 J kg^?1 K^?1), small thermal and magnetic hysteresis of martensitic transition give rise of large net refrigerant capacity ( ~ 152.3 J kg^?1) under a magnetic field of 50 kOe, which is comparable with that ( ~ 157.9 J kg^?1) of second-order transition. The large combined magnetocaloric effects make the Ni₅₀Mn₃₄In₁₅Al alloy as a promising candidate material for room temperature magnetic refrigeration.     

Comparison of superconducting parameters of A 15- and σ-phases of Nb−Pt

The superconducting properties of A 15- and σ-phases of the system Nb−Pt were investigated. Alloys of compositions Nb₃Pt, Nb₂Pt, and Nb₆₂Pt₃₈ were prepared by arc melting and subjected to optical microscopic and x-ray analysis for structure determination. The transition temperatures were measured inductively as well as resistively. The temperature dependences of critical fields were measured up to 35 kG. The A 15-phase has aT _c value of 8.97K and a zero temperature upper critical fieldH _c2(0) of 100 kG as compared to aT _c value of 2.14K and anH _c2(0) value of 14kG for the σ-phase. A comparison with theoretical upper critical field values seems to indicate that a spin-orbit coupling mechanism is responsible for the higher value ofH _c2(0) in the case of the A 15-phase alloy. The temperature dependent resistance up to 300 K is also measured and discussed for both the phases.     

Magnetic,thermal, and electrical properties of an Ni<Subscript>45.37</Subscript>Mn<Subscript>40.91</Subscript>In<Subscript>13.72</Subscript> Heusler alloy

The magnetization, the electrical resistivity, the specific heat, the thermal conductivity, and the thermal diffusion of a polycrystalline Heusler alloy Ni_45.37Mn_40.91In_13.72 sample are studied. Anomalies, which are related to the coexistence of martensite and austenite phases and the change in their ratio induced by a magnetic field and temperature, are revealed and interpreted. The behavior of the properties of the alloy near Curie temperature T_C also demonstrates signs of a structural transition, which suggests that the detected transition is a first-order magnetostructural phase transition. The nontrivial behavior of specific heat detected near the martensite transformation temperatures is partly related to a change in the electron density of states near the Fermi level. The peculiar peak of phonon thermal conductivity near the martensitic transformation is interpreted as a consequence of the appearance of additional soft phonon modes, which contribute to the specific heat and the thermal conductivity.     

Phase transitions in the ferromagnetic alloys Ni2+x Mn1−x Ga

The phase diagram of the cubic ferromagnetic shape-memory alloy Ni_2+x Mn_1−x Ga is constructed theoretically for the case when the Curie temperature is close to the structural transition temperature. This diagram agrees well with the experimental data obtained from resistance and magnetic susceptibility measurements. It is shown that the transition from the paramagnetic cubic phase to the ferromagnetic tetragonal phase can be second-order or first-order. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 212–216 (10 February 1998)