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Quantized electronic transport in magnetic topological insulators is intertwined with spontaneous magnetic ordering, as magnetization controls band topology through the exchange interaction. We show that considering the exchange gaps at the mean-field level is inadequate to predict phase transitions between electronic states of distinct topology. Thermal spin fluctuations that disturb the magnetization can act as frozen disorders, which strongly scatter electrons and thus reduce the onset temperature of quantized transport appreciably even in the absence of structural impurities. This effect, which has hitherto been overlooked, provides an alternative explanation of recent experiments on magnetic topological insulators.

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2022 Joint MMM-INTERMAG

Spin Fluctuations in Quantized Transport of Magnetic Topological Insulators

The 15th Joint MMM-INTERMAG Conference (2022 Joint) is sponsored jointly by AIP Publishing and the IEEE Magnetics Society. Members of the international scientific and engineering communities interested in recent developments in fundamental and applied magnetism are invited to attend and contribute to the technical sessions. The technical program will include invited and contributed papers in oral and poster sessions, invited symposia, a plenary session, and an evening session, with about 1500 presentations overall. This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

The 2022 Joint Conference will offer both in-person and prerecorded on-demand content.

For tickets to the event, please register at the following link: https://magnetism.org/register

Please register for the Joint MMM-Intermag conference to join the event.

This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

technical paper

Effect of calcium doping on magnetocaloric properties and critical behaviour of La0.55CaxSr0.45-xMnO3 (x = 0.00, 0.05, 0.1, 0.2 and 0.25) prepared by solid state reaction technique has been reported here. XRD measurements reveal rhombohedral structure for all the samples. Lattice parameters as well as Mn-O-Mn bond angle decreases with increasing Ca content. These structural changes control ferromagnetism in the materials via the double-exchange mechanism [1]. Temperature dependent magnetization measurement reveals a reduction of saturation magnetization and decrease in Curie temperature (TC) from 354 K to 311 K with increase in Ca content. A second order ferromagnetic (FM) to paramagnetic (PM) phase transition of all the samples is confirmed by Arrott plots. The magnetocaloric effect is calculated from the isothermal magnetization measurements using Maxwell’s thermodynamic relations. The maximum entropy change ((-ΔSm)max) and relative cooling power (RCP) are observed to decrease while TC shifted toward the room temperature with increase in Ca content. Therefore the sample La0.55CaxSr0.45-xMnO3 could be used as good solid state refrigerant for magnetic refrigerator near room temperature. Critical behaviour around PM-FM phase transition has been investigated using various methods including modified Arrott plots (MAP) [2], Kouvel–Fisher (KF) [3] method and critical isotherm analysis. The M-H data taken around TC was investigated in the framework of the mean field model, the 3D–Heisenberg model, the 3D–Ising model and tricritical mean field model [2]. The well agreement between MAP and KF methods confirms that the calculated values of the critical exponents are reliable. Calculated critical exponents reveal that the samples with x=0.00, 0.05, 0.1 possess a long-range ferromagnetic interaction belonging to the mean-field model, and x=0.2, 0.25 possess ferromagnetic short-range interactions belonging to the 3D Heisenberg model. Chemical pressure due to substitution of larger Sr ions with smaller Ca ions, cooperative Jahn–Teller distortions, and presence of ferromagnetic clusters all have significant influence on the nature of the FM-PM phase transition in the studied samples.

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K-F plot

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Entropy chang vs. T

Effect of Calcium Doping on Magnetocaloric Properties and Critical Behaviour of La0.55CaxSr0.45-xMnO3 around FM-PM Phase Transition

The long-range ordering of two-dimensional magnets with the isotropic short-ranged exchange interaction is destroyed by the fundamentally strong thermal fluctuation of low dimensional systems. We show that the spatial fluctuation, modeled by a random field at the lattice sites, significantly suppresses the long-range ordering as well. By using the random phase approximation along with the quantum statistics of low-energy magnon excitations, we have derived, from the anisotropic Heisenberg model, the magnetization as a function of the temperature in the presence of the random field. We found that the random field effectively decreases the energy gap of the magnon excitations. Since the energy gap is essential for 2D magnetic ordering, the random field would significantly reduce the transition temperature. Another finding is that the phase transition between the ferromagnet and the paramagnet becomes first-order, namely, the magnetization jumps from a finite value in the ferromagnetic phase to nearly zero at the paramagnetic phase. We will present the quantitative relations of above findings with the strength of the random field, and we estimate how these predictions could be experimentally realized. This work is supported by NSF-ECCS-2011331.

Effects of Random Fields on the Phase Transition of a Two-Dimensional Ferromagnet

Strong correlations in metals are known to yield a rich variety of quantum phases [1]. Whether and how they also drive electronic topology is of considerable current interest and, yet, a largely open question. A promising setting arises in heavy fermion systems, which may feature not only strong correlations but also a large spin-orbit coupling. Recently, a Weyl-Kondo semimetal phase has been concurrently discovered in theoretical [2] and experimental [3] studies. The theoretical work was carried out in a Kondo lattice model that is time-reversal invariant but inversion-symmetry breaking. In this talk, I will i) describe the basic characteristics of the Weyl-Kondo semimetal phase, with a particular emphasis on how strong correlations not only produce the Weyl nodes but also pin them near to the Fermi energy, and its experimental signatures, as has been advanced in the material Ce3Bi4Pd3; ii) discuss the extreme topological control of the correlation-driven Weyl nodes [4,5]; and iii) present a design principle – via the cooperation of strong correlations with space-group symmetry--for new Weyl-Kondo semimetal materials [6]. Finally, I will discuss the enrichment of our results for the strong correlation physics, in the form of a global phase diagram, of heavy fermion metals [1]. *This work was done with Sarah Grefe, Hsin-Hua Lai, Lei Chen, Chandan Setty and Haoyu Hu at Rice University and with Silke Paschen, Sami Dzsaber, Andrey Prokofiev, Jennifer Cano and Maia G. Vergniory. It was supported by the Air Force Office of Scientific Research Grant No. FA9550-21-1-0356, the National Science Foundation Grant No. DMR-1920740, and the Robert A. Welch Foundation Grant No. C-1411.

Weyl-Kondo Semimetal – How Strong Correlations Intersect with Electronic Topology

An experimental realization of the Haldane conjecture of a gapped spin liquid is of great interested among condensed matter scientists. The conjecture is implemented in the chains of Heisenberg integer spins bounded by identical intrachain exchange interactions J and could be realized in uniform quasi-one-dimensional integer spin compounds [1]. The Haldane phase has to compete with magnetic anisotropy and long-range ordered phases, depending on the ratio of interchain J' and intrachain J exchange interactions and uniaxial D and rhombic E single-ion anisotropy. The critical ratios J'/J and D/J for Haldane phase, easy axis long ordered AFM and easy plane long ordered XY-AFM were first discovered by Sakai and Takahashi [2]. Many compounds, both organic and inorganic, have been studied to verify this idea, some of them have been identified as belonging to the Haldane sector of the Sakai-Takahashi phase diagram. In this work, the position on Sakai-Takahashi phase diagram was determined for the novel integer spin S = 1 chain compound, strontium nickel selenite chloride Sr2Ni(SeO3)2Cl2. XRD measurements showed that the crystal structure of the compound contains the NiO4Cl2-octahedra bridged by the SeO3-pyramids into uniform spin chains. These chains are well separated by strontium cations and cavities of selenium’s lone pair of electrons that could lead to 1D-dimensionality of magnetic subsystem. In magnetic measurements it was determined that strontium nickel selenite chloride, Sr2Ni(SeO3)2Cl2, is a spin S = 1 chain system which passes through a correlations regime at Tmax ~ 12 K to long-range order at TN = 6 K. The values of J'/J = 0.083 and D/J = 0.357 place this compound into a hitherto unoccupied XY-AFM sector of the Sakai-Takahashi phase diagram. The results was recently published in Scientific reports journal [3]. Support by the P220 program of Government of Russia through the project 075-15-2021-604 is acknowledged. We also acknowledge the support of the HLD-HZDR, member of the European Magnetic Field Laboratory (EMFL).

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Sakai-Takahashi phase diagram for uniform spin-1 chain compounds, novel compound Sr2Ni(SeO3)2Cl2 is also marked.

Magnetic properties of S = 1 spin chain in Sr2Ni(SeO3)2Cl2: XY-antiferromagnet at Sakai-Takahashi phase diagram

Spin-crossover (SCO) complexes form an interesting class of switchable materials that can exist in two different spin states, known as the low-spin (LS) and high-spin (HS) states. These complexes may be switched between the two spin states under various stimuli including temperature, pressure, light, magnetic and electrical fields [1]. Among these stimuli, pressure is one of the most appealing thermodynamic parameters used to probe the spin transition properties of spin-crossover materials since it allows for tuning both the crystal field as well as the interactions between SCO centers. Since the pioneering works of Drickamer [2], Konig [3] and Gutlich[4], the pressure effect has continuously attracted the interest of the scientific community, being intensively studied until nowadays. In general, the application of an external pressure induces a transformation of the SCO complex to the low-spin state, i.e. to the state of lower molecular volume, and has the effect of shifting the switching temperatures towards higher values. However, the interpretation of the pressure effect on molecular SCO phenomena has been, in some cases, compromised by the relatively poor or incomplete information resulted from experimental difficulties related to the need for hydrostatic conditions at low temperatures. In this work I will talk on both hydrostatic [5, 6] and non-hydrostatic [7] pressure effects on the spin transition properties of molecular spin crossover complexes in terms of major thermal hysteresis loop and FORC diagram methods. We have shown that for high pressure gradients, the thermal hysteresis loop becomes asymmetric. Remarkably, the FORC diagram method has been proved to be much more sensitive to local changes than the “bulk” parameters, which characterize the major hysteresis loop. This makes the FORC diagram method a suitable technique to characterize the hydrostaticity degree of a pressure cell. Acknowledgements. This work was partially funded by UEFISCDI, project number PN-III-P1-1.1-TE-2019-2194 (Contract No: TE 123/2020)

Pressure effect on spin-crossover materials: Experiment vs theory

Recently, 5d and 4d oxides and halides with honeycomb lattice structures are of great interest [1-2] for exhibiting an interplay between an isotropic Heisenberg-type exchange interaction (J) and an anisotropic Kitaev-type exchange interaction (K), see Fig. 1a. We predict that such an interplay can lead to topological edge states with chiral conduction when the anisotropic interaction is at least twice the strength of the isotropic interaction (i.e. K ≥ 2J), see Figs. 1b-c. We show that these interactions give rise to new types of antiferromagnetic orders, which determine the chirality of the edge atomic chain. We show calculations for sodium iridate and alpha-ruthenium chloride, both known to exhibit a zigzag antiferromagnetic order at low temperatures, and discuss the chiral edge conduction in the context of the relative strength between J and K. The edge conduction in the presence of a zigzag antiferromagnetic order will be manifested as a quantized Hall conductance when the number of zigzag atomic chains is odd (see Fig. 1b) since both edges will have the same magnetic orders. On the other hand, the Hall conductance will be zero when the number of zigzag atomic chains is even (see Fig. 1c) since the edges will have opposite magnetic orders. These signatures of the exchange interaction induced chiral edge conductions in odd and even numbers of two-dimensional atomic chains are similar to the well-known signatures of a quantum anomalous Hall insulator and an axion insulator states typically observed in a three-dimensional magnetic topological insulator [3-4], respectively. We further use a widely used model for two-dimensional topological materials to analyze the underlying mechanisms of the predicted phenomena.

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(a) Honeycomb lattice with isotropic (J) and anisotropic (K) exchange interactions. Hall conductance Gxy = Vx / Iy is calculated when the number of zigzag atomic chains is (b) odd and (c) even, as a function of K / J.

Chiral edge conduction induced by large anisotropic exchange interactions.

Materials exhibiting high degree of spin polarization such as half-metals or spin-gapless semiconductors are in demand for spin-transport-based devices. Especially, half metallic Heusler compounds have attracted much attention because they have tunable magnetic properties and exhibit high Curie temperature much above room temperature. We have synthesized one such predicted half metal, FeCrVAl, using arc melting and annealing. The vacuum annealing was done at 600oC for 48 hours. In addition, we also studied the effect of Mn substitution on the structural, magnetic, and electronic band properties of FeCrVAl synthesizing samples with compositions FeCr0.5Mn0.5VAl and FeCrV0.5Mn0.5Al. The room temperature, x-ray diffraction patterns indicate that FeCrVAl and FeCrV0.5Mn0.5Al are cubic in structure with A2 type disorder, whereas FeCr0.5Mn0.5Val is more ordered as evidenced by the presence of superlattice peaks. All samples show small saturation magnetizations at room temperature and the thermomagnetic curves M(T) are similar to that of a paramagnetic material. However, as shown in Fig.1 below, the M(T) of FeCr0.5Mn0.5VAl is different and shows a slow magnetic transitionat 800 K similar to that of a ferrimagnetic material. In this presentation, we will discuss the structural, magnetic and electron band properties of the synthesized materials and the effect of disorder on the properties of FeCrVAl. This research is supported by the National Science Foundation (NSF) under Grant Numbers 2003828 and 2003856 via DMR and EPSCoR.

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Figure 1: Thermomagnetic curve M(T) of FeCr0.5Mn0.5VAl alloy. The inset shows the M (H) curve recorded at 300 K.

Experimental and theoretical investigation of FeCrVAl and related compounds

Heusler alloys have been investigated intensively as a potential spin source with 100% spin polarisation at room temperature [1,2]. Theey can be used in magnetoresistive junctions for a next generation read head and a magnetic memory cell [3]. We have recently reduced their crystallisation temperature by growing on the (110) plane [4]. This can be achieved by growing on a body-centred cubic seed layer [5],[6]. In this study, we systematically investigate the effect of seed and capping layers on Co2FeAl0.5Si0.5 (CFAS) Heusler-alloy films. Heusler alloy films were grown on a thermally oxidised Si substrate in the form of Cr (0 and 3)/W (10)/CF(A)S (10)/X (5)/Ru (3) (thickness in nm, X: W, Ta, V, Ag and Pt) using a high target utilisation sputtering system at room temperature. These films were annealed at 300~350°C for 1~3 hours. The structural and magnetic properties were characterised by X-ray diffraction (XRD) and vibrating sample magnetometry. The magnetisation reversal process was also imaged using magnetic force microscope. Figure 1(a) confirms all films show W crystallisation along the (110) surface orientation, promoting the CFAS crystallisation as confirmed by the CFAS(220) peak. As shown in Fig. 1(b), all CFAS films exhibit perpendicular magnetic anisotropy but with clear deviations under the influence of top-interfaces. The magnetisation curve of the W cap exhibits the highest squareness. In-plane magnetisation measurements also confirm the CFAS/W top interface shows single-step reversal. This means that the sample consisting of a W/CFAS/W trilayer shows the ideal magnetic properties for junction applications. As listed in Fig. 1(d), all films show MS within the range of 650~850 emu/cm3, which is almost 30% smaller than the bulk value. This work was partially supported by EPSRC (EP/M02458X/1 and EP/V007211/1) and JST CREST (No. JPMJCR17J5).

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(a) XRD θ-2θ scans and (b) out-of-plane and (c) in-plane magnetisation curves of Si/SiO2//W (10)/CFAS (10)/Y (5)/Ru (3) (thickness in nm, Y: W, Ta, V, Ag and Pt). (d) Saturation magnetisation of these films calculated from (c).

Perpendicular Anisotropy Controlled by Seed and Capping Layers of Heusler Alloy Films

Co-based Heusler alloys are currently exploited in several interesting spintronics applications, such as magnetic electrodes in spin-valve and magnetic tunnel junction devices, due to their half-metallic ferromagnetic properties [1]. Their magnetic and electronic transport behavior at room temperature are intensely studied and tuned to meet several applications demands. In particular, the electronic transport properties display an unusual trend of Seebeck coefficient and electrical conductivity proving the half-metallic ferromagnetic behavior of these compounds [2]. In this work, Co2HfSn and Co2ZrSn bulk alloys are synthetized by arc-furnace in which the melt contains pure elements in the appropriate stoichiometric quantities; subsequent, annealing at 800 °C for 6 days is performed to release stress and reach the equilibrium phase. Magnetic properties were investigated in the wide temperature range from 2 to 500 K by SQUID and VSM magnetometers; isothermal hysteresis loops M(H) and magnetization as a function of temperature curves M(T) have been measured. Magnetic results are in excellent agreement with the mean-field model of itinerant ferromagnetism (Stoner model) in the limit of very weak ferromagnetism. The linear relationship between M2 and H/M (i.e. Arrott plot) is maintained at all studied temperatures, both well below and above the Curie temperature (see Fig. 1a). The weak itinerant ferromagnetism model, by means of its temperature-dependent coefficients, is successfully used to determine the Curie temperature resulting at 432 K and 458 K for the Co2HfSn and Co2ZrSn alloy, respectively. In addition, the extrapolation of the experimental M(T) curve at T = 0 K reveals a magnetic moment of 2.0 μB/f.u. 1.81 μB/f.u. for the Co2HfSn and Co2ZrSn samples, respectively. Electric and magnetic experimental results are compared with the ab-initio calculation, which provides the spin-polarized density of states (DOS) confirming that Co2HfSn and Co2ZrSn alloys are half-metallic compounds with a majority-spin channel across the Fermi surface, whereas the minority-spin channel shows a small band gap (see Fig. 1b).

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a) Arrott plot for the Co2HfSn sample; b) DOS by ab-initio calculation for the Co2ZrSn sample

Weak ferromagnetism and electronic properties of half-metallic Co2HfSn and Co2ZrSn Heusler alloys

Exceptional tunability in the electronic and magnetic properties makes Heusler compounds very promising from both the fundamental and applied perspectives. Half-metallic ferromagnetism, half-metallic fully-compensated ferrimagnetism, spin-gapless semiconducting behavior, and fully-compensated ferrimagnetic spin-gapless semiconducting behavior have been predicted or demonstrated experimentally in Heusler compounds which makes them suitable for applications in a number of novel spintronic devices1,2,3,4,5. Half-metallic Co-based Heusler alloys are among the most attractive systems due to their high Curie temperatures, high spin polarization and the structural similarity to binary semiconductors6,7,8. Spin-gapless semiconductors can be regarded as a combination of gapless semiconductors and half-metallic ferromagnets where the conducting electrons or holes are not only 100% spin polarized but also easily excited. The realization of spin-gapless semiconducting behavior in Heusler alloys is expected to fulfill the needs for semiconductor spintronics. In this work, we have introduced the substitution of low valence transition metal atoms Y = Ti, V, Cr, Mn, and Fe to Co atoms in the parent Co2FeGa system, and examined the properties of quaternary Heusler compounds Co2-xYxFeGa (x = 0.50), both experimentally and theoretically to get a global overview of the electronic, magnetic, electron transport and mechanical properties. All single-phase alloys exhibit Heusler like L21 ordering, as corroborated by X-ray diffraction. The low-temperature saturation magnetic moments agree fairly well with the values expected from a Slater-Pauling rule for half metals (Fig.1). Electrical transport measurements are performed to explain the electronic structure of the alloys. The temperature dependence of electrical resistivity for Mn substituted alloy shows semiconducting nature in the complete temperature regime (5-400)K (Fig.2). Ab initio calculations are also performed to understand the experimental findings.

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The variation of saturation magnetic moment with valence electron counts per formula units with element Y.

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Temperature dependence of electrical resistivity in Co1.50Mn0.50FeGa at zero magnetic field showing semiconducting behavior.

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Effect of Calcium Doping on Magnetocaloric Properties and Critical Behaviour of La0.55CaxSr0.45-xMnO3 around FM-PM Phase Transition

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Next from 2022 Joint MMM-INTERMAG

Effect of Calcium Doping on Magnetocaloric Properties and Critical Behaviour of La0.55CaxSr0.45-xMnO3 around FM-PM Phase Transition

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