The simplified bandstructure introduced by Mazurenko et al to model FeSi is used to analyze the singlet semiconductor to ferromagnetic metal transition in the isoelectronic isostructural alloys, FeSi$_{1-x}$Ge$_x$. The complex bandstructure of the alloy is replaced by an alternating chain of doubly and singly degenerate atoms to represent Fe and Si/Ge respectively. The former(latter) form narrow(broad) bands with a substantial hybridization between them. A substantial onsite repulsion including a Hund's rule coupling is introduced on the Fe sites. The mean field phase diagram contains a first order phase transition from the singlet semiconductor to a ferromagnetic metal with increasing temperature and interaction strength similar to the alloys. The analysis also reproduces the rapid rise of the spin susceptibility in the semiconductor with a crossover to a Curie-Weiss form at higher temperatures. Good agreement is found at zero temperature between the mean field and accurate DMRG calculations.
The effects of various transition metal dopants on the electrical and thermal transport properties of Fe1-xMxSi alloys (M= Co, Ir, Os) are reported. The maximum thermoelectric figure of merit ZTmax is improved from 0.007 at 60 K for pure FeSi to ZT = 0.08 at 100 K for 4% Ir doping. A comparison of the thermal conductivity data among Os, Ir and Co doped alloys indicates strong electron-phonon coupling in this compound. Because of this interaction, the common approximation of dividing the total thermal conductivity into independent electronic and lattice components (κTotal = κelectronic + κlatti…
The phase diagram of FeSi1-xGex, obtained from magnetic, thermal and transport measurements on single crystals, shows a first-order transition from a correlated electron semiconductor to a ferromagnetic metal at a critical concentration, x ~ 0.25. The gap of the insulating phase strongly decreases with x. The specific heat coefficient appears to track the density of states of a Kondo insulator. The phase diagram is consistent with a correlation induced insulator-metal transition in conjunction with disorder on the Si/Ge ligand site.
A chiral compound FeSi is a nonmagnetic narrow-gap insulator, exhibiting peculiar charge and spin dynamics beyond a simple band-structure picture. Those unusual features have been attracting renewed attention from topological aspects. Although a signature of surface conduction was indicated according to size-dependent resistivity in bulk crystals, its existence and topological properties remain elusive. Here we demonstrate an inherent surface ferromagnetic-metal state of FeSi thin films and its strong spin-orbit-coupling (SOC) properties through multiple characterizations of the two-dimensiona…
We propose FeSb2 to be a nearly ferromagnetic small gap semiconductor, hence a direct analog of FeSi. We find that despite different compositions and crystal structures, in the local density approximation with on-site Coulomb repulsion correction (LDA+U) method magnetic and semiconducting solutions for U=2.6 eV are energetically degenerate similar to the case of FeSi. For both FeSb2 and FeSi (FeSi1-xGex alloys) the underlying transition mechanism allows one to switch from a small gap semiconductor to a ferromagnetic metal with magnetic moment 1 muB per Fe ion with external magnetic field.
The physical properties of the semiconductor FeSi with very narrow band gap, anomalous behavior of the magnetic susceptibility and metal-insulator transition at elevated temperatures attract gross interest due to the still controversial theoretical understanding of their origin. On one side the purely band like mechanism of the gap formation in FeSi at low temperature is well established, on other side a number of experiments and their theoretical interpretation suggest a rich physics of strong correlations at finite temperature. In this work we use an ab-initio scheme based on the Random Phas…
Regularities and features in the behavior of electrical and magnetic properties of Co$_2$FeZ (Z = Al, Si, Ga, Ge, Sn, Sb) half-metallic ferromagnetic Heusler alloys
本文测量 Co$_2$FeZ 系列 Heusler 合金的电阻率、磁化和 Hall 效应，揭示其随 Z 元素原子序数变化的电磁关联及不同温区的电阻散射规律。
In this work, we report the structural, magnetic and electrical and thermal transport properties of the Heusler-type alloy Ru2NbAl. From the detailed analysis of magnetization data, we infer the presence of superparamagnetically interacting clusters with a Pauli paramagnetic background, while short-range ferromagnetic interaction is developed among the clusters below 5 K. The presence of this ferromagnetic interaction is confirmed through heat capacity measurements. The relatively small value of electronic contribution to specific heat, gamma (~2.7 mJ/mol-K2), as well as the linear nature of t…
Low temperature magnetic and thermal (C_m) properties of the ferromagnetic (FM) alloys Ce_2.15 (Pd_1-x Rh_x)_1.95 In_0.9 were investigated in order to explore the possibility for tuning a quantum critical point (QCP) by doping Pd with Rh. As expected, the magnetic transition observed at T = 4.1K in the parent alloy decreases with increasing Rh concentration. Nevertheless it splits into two transitions, the upper being antiferromagnetic (AF) whereas the lower FM. The AF phase boundary extrapolates to T_N = 0 for x_cr ~ 0.65 whereas the first order FM transition vanishes at x ~ 0.3. The QC chara…
We report the effects of electron doping on the ground state of a diamagnetic semiconductor FeGa$_{3}$ with a band gap of 0.5 eV. By means of electrical resistivity, magnetization and specific heat measurements we have found that gradual substitution of Ge for Ga in FeGa$_{3-y}$Ge$_{y}$ yields metallic conduction at a very small level of $y = 0.006$, then induces weak ferromagnetic (FM) order at $y = 0.13$ with a spontaneous moment of 0.1 $μ_{B}$/Fe and a Curie temperature $T_{C}= 3.3$ K, which continues increasing to $T_{C} = 75$ K as doping reaches $y = 0.41$. The emergence of the FM state i…
We study ferromagnetic transition in three-dimensional double-exchange model containing impurities. The influence of both spin fluctuations and impurity potential on conduction electrons is described in coherent potential approximation. In the framework of thermodynamic approach we construct Landau functional for the system "electrons (in disordered environment) + core spins". Analyzing the Landau functional we calculate the temperature of ferromagnetic transition $T_C$ and paramagnetic susceptibility $χ$. For $T_C$, we thus extend the result obtained by Furukawa in the framework of the Dynami…
We have investigated the evolution of ferromagnetic order in the correlated metal URhGe (Curie temperature $T_{\rm C} = $9.5 K) by chemical substitution of Ru, Co and Si. Polycrystalline samples URh$_{1-x}$Ru$_x$Ge ($x \leq $0.6), URh$_{1-x}$Co$_x$Ge ($x \leq $0.9) and URhGe$_{1-x}$Si$_x$ ($x \leq $0.2) have been prepared and the magnetic properties have been investigated by magnetization and transport experiments. In the case of Ru doping, $T_{\rm C}$ initially increases, but then decreases linearly as a function of $x$ and is completely suppressed for $x_{\rm cr} \approx 0.38$. The Curie tem…
Magnetic semiconductors have attracted interest because of the question of how a magnetic metal can be derived from a paramagnetic insulator. Here our approach is to carrier dope insulating FeSi and we show that the magnetic half-metal which emerges has unprecedented optical properties, unlike those of other low carrier density magnetic metals. All traces of the semiconducting gap of FeSi are obliterated and the material is unique in being less reflective in the ferromagnetic than in the paramagnetic state, corresponding to larger rather than smaller electron scattering in the ordered phase.
We compare different approximation schemes for investigating ferromagnetism in the periodic Anderson model. The use of several approximations allows for a detailed analysis of the implications of the respective methods, and also of the mechanisms driving the ferromagnetic transition. For the Kondo limit, our results confirm a previously proposed mechanism leading to ferromagnetic order, namely an RKKY exchange mediated via the formation of Kondo screening clouds in the conduction band. The contrary case is found in the intermediate-valence regime. Here, the bandshift correction ensuring a corr…
Magnetic and thermal properties of Ferromagnetic (FM) Ce_{2.15}(Pd_{1-x}Ag_x)_{1.95}In_{0.9} alloys were studied in order to determine the Quantum Critical Point (QCP) at T_C => 0. The increase of band electrons produced by Pd/Ag substitution depresses T_C(x) from 4.1K down to T_C(x=0.5)=1.1K, with a QCP extrapolated to x_{QCP}~ 0.6. Magnetic susceptibility from T>30K indicates an effective moment slightly decreasing from μ_{eff}=2.56μ_B to 2.4μ_B at x=0.5. These values and the paramagnetic temperature θ_P~ -10K exclude significant Kondo screening effects. The T_C(x) reduction is accompanied b…
Zero (ZF) and longitudinal field (LF) muon spin relaxation data of the {\it d}-metal alloy Ni$_{1-x}$V$_{x}$ are presented at several vanadium concentrations $x$ below and above the critical $x_c \approx 11$% where long-range ferromagnetic (FM) order is suppressed. The clear single precession frequency observed for Ni, as expected for a homogeneous FM, changes to rather damped oscillation with small V substitution at $x=4$%, confirming magnetic inhomogeneities caused by the less magnetic V environments in the magnetic Ni matrix. Furthermore, local fields and spatial field distributions can be …
Very sharp magnetization step is observed across the field induced antiferromagnetic to ferromagnetic transition in various doped-CeFe$_2$ alloys, when the measurement is performed below 5K. In the higher temperature regime (T$>$5K) this transition is quite smooth in nature. Comparing with the recently observed similar behaviour in manganites showing colossal magnetoresistance and magnetocaloric material Gd$_5$Ge$_4$ we argue that such magnetization step is a generalized feature of a disorder influenced first order phase transition.
We report results of magnetization, zero field resistivity and magnetoresistance measurements in ferromagnetic Heusler alloy Co$_2$TiSn. There is a striking change in the character of electron transport as the system undergoes the paramagnetic to ferromagnetic transition. In the paramagnetic state the nature of the electron transport is like that of a semiconductor and this changes abruptly to metallic behaviour at the onset of ferromagnetic ordering. Application of external magnetic field tends to suppress this semiconducting like transport leading to a negative magnetoresistance which reache…
A brief survey of experimental and theoretical studies of half-metallic ferromagnets (HMFs) and spin gapless semiconductors is given, the possible candidates being the X$_2$YZ (X = Mn, Fe, Co; Y = Ti, V, Cr, Mn, Fe, Co, Ni; Z = Al, Si, Ga, Ge, In, Sn, Sb) Heusler alloys. The data on the electrical resistivity, normal and anomalous Hall Effect, and magnetic properties are presented. It is shown that the Co$_2$FeZ alloys demonstrate properties of conventional ferromagnets, the HMF properties being also manifested at the variation of the Z-component. The Fe$_2$YAl and Mn$_2$YAl alloys show at the…
We present small angle neutron scattering (SANS) data collected on polycrystalline Ni$_{1-x}$V$_x$ samples with $x\geq0.10$ with confirmed random atomic distribution. We aim to determine the relevant length scales of magnetic correlations in ferromagnetic samples with low critical temperatures $T_c$ that show signs of magnetic inhomogeneities in magnetization and $μ$SR data. The SANS study reveals signatures of long-range order and coexistence of short-range magnetic correlations in this randomly disordered ferromagnetic alloy. We show the advantages of a polarization analysis in identifying t…
A sigma-phase Fe65.9V34.1 alloy was investigated with the AC magnetic susceptibility as a function of temperature, frequency and external magnetic field. An evidence was found that its magnetism shows features characteristic of a reentrant behavior viz. two transitions: first at TC ca.312K from the paramagnetic state into the collinear ferromagnetic one, and second at Tf ca.302K to a mixed state (sometimes termed as a ferromagnetic re-entrant spin glass) which, finally, at a lower temperature (TRSG ca.60K) transforms to a state where replica symmetry is broken. The frequency dependence of Tf i…
We experimentally study how the magnetic correlations develop in a binary alloy close to the ferromagnetic quantum critical point with small-angle neutron scattering (SANS). Upon alloying the itinerant ferromagnet nickel with vanadium, the ferromagnetic order is continuously suppressed. The critical temperature Tc vanishes when vanadium concentrations reach the critical value of xc=0.116 indicating a quantum critical point separating the ferromagnetic and paramagnetic phases. Earlier magnetization and $μ$SR data have indicated the presence of magnetic inhomogeneities in Ni(1-x)V(x) and, in par…
We report the observation of giant magneto-caloric effect (MCE) in ferromagnetic shape memory alloys (FSMAs) of nominal compositions Ni$_2$Mn$_{1.36}$Sn$_{0.64-x}$Ga$_{x}$ ($x$ =0.24, 0.28 and 0.32). Irrespective of the Ga doping, all the samples undergo long range ferromagnetic ordering below around 330 K. However, the martensitic transition temperature ($T_{MS}$) of the samples varies strongly with Ga concentration. Clear signature of field induced transition around $T_{MS}$ is visible for all the samples. The observed MCE ($ΔS$ = 13.6 J/kg K for field changing from 0 to 50 kOe) is found to …
Magnetism in Ni-Mn-Z (Z = Ga,In,Sn,Sb) Heusler alloys has so far been predominantly attributed to Rudermann-Kittel-Kasuya-Yoshida type interactions between Mn atoms. We investigate magnetic interactions in one such alloy, Ni$_{2}$Mn$_{1.4}$In$_{0.6}$ and attempt to explain the origin of antiferromagnetic (AFM) interactions that coexist with ferromagnetic ones. Through the combination of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD), we find that Ni plays an important role along with Mn in the overall magnetism. A significant hybridization that develops between Mn a…
We report magnetization ($M$) measurements close to the ferromagnetic quantum phase transition of the d-metal alloy Ni$_{1-x}$V$_x$ at a vanadium concentration of $x_c \approx 11.4 %$. In the diluted regime ($x>x_c$), the temperature ($T$) and magnetic field ($H$) dependencies of the magnetization are characterized by nonuniversal power laws and display $H/T$ scaling in a wide temperature and field range. The exponents vary strongly with $x$ and follow the predictions of a quantum Griffiths phase. We also discuss the deviations and limits of the quantum Griffiths phase as well as the phase bou…
We consider, theoretically and experimentally, the effects of structural disorder, quantum and thermal fluctuations in the magnetic and transport properties of certain ferromagnetic alloys.We study the particular case of UCu2Si(2-x)Ge(x). The low temperature resistivity, rho(T,x), exhibits Fermi liquid (FL) behavior as a function of temperature T for all values of x, which can be interpreted as a result of the magnetic scattering of the conduction electrons from the localized U spins. The residual resistivity, rho(0,x), follows the behavior of a disordered binary alloy. The observed non-monoto…
Polycrystalline samples of the isostructural alloys UNi1-xCoxSi2 (0 <= x <= 1) were studied by means of x-ray powder diffraction, magnetization, electrical resistivity and specific heat measurements, at temperatures down to 2 K and in magnetic fields up to 5 T. The experimental data revealed an evolution from strongly anisotropic ferromagnetism with pronounced Kondo effect, observed for the alloys with x < 0.98 and being gradually suppressed with rising Co-content, to spin-glass-like states with dominant spin fluctuations, seen for the sample with x = 0.98. Extrapolation of the value of TC(x) …
The magnetic correlations in the austenite phase and the consequent martensitic transition in inverse magnetocaloric alloys, Ni$_{2}$Mn$_{1+x}$Sn$_{1-x}$, have been a matter of debate for decades. We conclusively establish using {\it ab initio} phonon calculations that the spin alignment of excess Mn at the Sn site (Mn$_{Sn}$) with the existing Mn in the unit cell in the high temperature cubic phase of Ni-Mn-Sn alloy is ferromagnetic (FM), and not ferrimagnetic (FI), resolving a long lasting controversy. Using first principles density functional perturbation theory (DFPT), we observe an instab…
The fascinating multiple magnetic states observed in the Ni-Mn-Sn based metamagnetic shape memory alloy are addressed through a combined muon spin relaxation (muSR) and neutron powder diffraction studies. The material used in the present investigation is an off-stoichiometric alloy of nominal composition, Ni[2.04]Mn[1.4]Sn[0.56]. This prototypical alloy, similar to other members in the Ni-Mn-Sn series, orders ferromagnetically below T[CA] (= 320 K), and undergoes martensitic type structural transition at T[MS] (= 290 K), which is associated with the sudden loss of magnetization. The sample reg…
We present magnetization ($M$) data of the $d$-metal alloy Ni$_{1-x}$V$_x$ at vanadium concentrations close to $x_c \approx 11.4%$ where the onset of long-range ferromagnetic (FM) order is suppressed to zero temperature. Above $x_c$, the temperature ($T$) and magnetic field ($H$) dependencies of the magnetization are best described by simple nonuniversal power laws. The exponents of $M/H \sim T^{-γ}$ and $M \sim H^α$ are related by $1-γ=α$ for wide temperature ($10K < T \leq 300K$) and field ($H \leq 5T$) ranges. $γ$ is strongly $x$ dependent, decreasing from 1 at $x\approx x_c$ to $γ< 0.1$ fo…
We investigate the structural and critical properties of CrTe1-xSbx with 0.0 \leq x \leq 0.2. The XRD patterns revealed that Sb-substitution resulted in a pure NiAs-hexagonal structure with P63/mmc (194) space-group. Lattice refinement of the structure revealed little changes in the a-lattice parameter, along with a more pronounced reduction in the c-axis. The critical behavior in CrTe1-xSbx has been investigated using the magnetization isotherms near the ferromagnetic transition. The obtained critical exponents ( β, γ,and δ) revealed that all samples (with 0.0 \leq x \leq 0.2) closely follow …
The ground state properties of the ferromagnetic shape memory alloy of nominal composition Ni2Mn1.36Sn0.64 have been studied by dc magnetization and ac susceptibility measurements. Like few other Ni-Mn based alloys, this sample exhibits exchange bias phenomenon. The observed exchange bias pinning was found to originate right from the temperature where a step-like anomaly is present in the zero-field-cooled magnetization data. The ac susceptibility study indicates the onset of spin glass freezing near this step-like anomaly with clear frequency shift. The sample can be identified as a reentrant…
We have synthesized a nominal composition of Ni50Mn30Sn20 alloy using arc melting technique. Rietveld refinement confirms the austenite L21 structure in Fm-3m space group. Electrical resistivity has been found to clearly exhibiting two different phenomena viz. a magnetic transition from paramagnetic to ferromagnetic and a structural transition from austenite to martensitic phase. Thermo-magnetization measurements M(T) confirms ferromagnetic transition temperature TC at 222 K and martensitic transition starting at 127 K(MS). Magnetization measurement M(H) at 10 K confirms the ferromagnetic stat…
Effects of the Co substitution have been observed on the structural, magnetic and magneto-transport properties of Fe2-xCoxMnSi alloy. Curie temperature (TC) and saturation magnetization (MS) of these alloys increased linearly with the Co substitution. Competitive magnetic interaction between ferromagnetic (FM) and anti-ferromagnetic (AFM) phases exists in Fe2-xCoxMnSi for x less than 0.2, AFM phase is completely disappears for x greater than or equal to 0.2. The value of Rhodes-Wohlfarth ratio pc/ps is greater than one for these alloys which is the characteristics of iterant magnetism present …
We present results of dc magnetisation study showing that the low temperature antiferromagnetic state in various CeFe$_2$-based pseudobinary alloys can be transformed into ferromagnetic state through a magnetic field induced phase transition. We highlight the presence of hysteresis and phase coexistence across this metamagnetic transition and argue that the observed phase transition is of first order in nature.
We present evidence for the existence of magnetic clusters of approximately 20 Åin the strongly correlated alloy system CeNi$_{1-x}$Cu$_{x}$ (0.7 $\le$ x $\le$ 0.2) based on small angle neutron scattering experiments as well as the occurrence of staircase-like hysteresis cycles during very low temperature (100 mK) magnetization measurements. An unusual feature is the observation of long-range ferromagnetic order below the cluster-glass transition without any indication of a sharp transition at a Curie temperature. These observations strongly support a phenomenological model where a percolative…
We present combined spin model and first principles electronic structure calculations to study the weak ferromagnetism in bulk Mn$_3$Z (Z=Sn, Ge, Ga) compounds. The spin model parameters were determined from a spin-cluster expansion technique based on the relativistic disordered local moment formalism implemented in the screened Korringa--Kohn--Rostoker method. We describe the magnetic ground state of the system within a three-sublattice model and investigate the formation of the weak ferromagnetic states in terms of the relevant model parameters. First, we give a group-theoretical argument ho…
