Low-temperature Magnetic Phase Transition Research Articles

Magnetic properties of polycrystalline bulk crystals of MnGeP2

Room temperature ferromagnetism observed in polycrystalline MnGeP2 bulk samples was investigated by different experimental techniques. The magnetization measurements show a high temperature ferromagnetic phase with a critical temperature TC = 294.0 K and a low temperature magnetic phase transition at TN = 40.5 K. As these parameters (TC,TN) correspond to those of MnP, a ferromagnetic half-metal, whose composition of the samples was further characterized by x-ray diffraction and Rutherford backscattering measurements. They confirmed the presence of orthorhombic MnP precipitates. Electron paramagnetic resonance (EPR) and ferromagnetic resonance (FMR) measurements in the temperature range of T = 4 K to T = 380 K confirm the attribution of the magnetic phase to MnP and show no additional EPR or FMR signal associated with MnGeP2. We conclude that the magnetic properties of the polycrystalline MnGeP2 bulk samples grown by the horizontal gradient freeze technique are dominated by MnP inclusions and that phase pure chalcopyrite MnGeP2 is not ferromagnetic in contrary to previous reports.

Magnetocaloric Materials for Low-Temperature Magnetic Cooling

State of research in the study of magnetocaloric materials based on rare-earth metals that are promising for application in the technology of low-temperature magnetic cooling is reviewed. Physical principles and characteristics of the magnetocaloric effect in materials based on rare-earth metals with low-temperature magnetic phase transitions are presented.

Synthesis and magnetic properties of ε-AlxFe2-xO3 nanoparticles

ε-AlxFe2-xO3 (x = 0, 0.125, 0.167, 0.222 and 0.4) nanoparticles were manufactured by a new kind of mesoporous template-compacted fumed silica. The effect of Al3+ composition on the purity and magnetic performances of the ε-AlxFe2-xO3 samples were investigated. The structural and magnetic performances of the samples were characterized by X-ray diffraction, transmission electron microscope and SQUID magnetometer. The results indicate that the doping of Al3+ can decrease the dimension of ε-Fe2O3 nanoparticles and effectively reduce the formation of α-Fe2O3 impurity in the samples. And the magnetic phase transition temperature of the ε-AlxFe2-xO3 samples decreases with the increase of Al3+. And the low temperature magnetic phase transition finally vanishes at x = 0.222. The ε-AlxFe2-xO3 samples exhibit large coercivity. With the increase of Al3+ content, the coercivity of ε-AlxFe2-xO3 increases firstly to a maximum and then decreases.

Influence of Ga-substitution to the structural and magnetic properties of (Mn,Fe)-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- bixbyite

To study the dependence of the properties of ternary oxides (Mn,Fe,Ga)2O3 with the bixbyite structure on the composition, the temperature dependences of the magnetization and ac magnetic susceptibility of two single-crystal samples of different compositions obtained using the flux method were analyzed. A detailed study of the structure was carried out using single-crystal X-ray diffraction analysis, and the changes in structural parameters depending on the composition were analyzed. The dc magnetization and ac magnetic susceptibility of Fe1.1Mn0.76Ga0.14O3 and Fe0.65Mn1.1Ga0.26O3 bixbyites have been studied. Despite the qualitatively similar behavior of the magnetic properties, significant differences were also found, despite a small difference in the Mn/Fe/Ga ratio in the samples under study. It is shown that both compounds experience two successive low-temperature magnetic phase transitions from the paramagnetic phase at T=20-32 K as the temperature is lowered. Calculations of the Mydosh parameter for the detected phase transitions showed different degrees of ordering in the compounds under study. Keywords: transition metal oxides, magnetic phase transitions, spin glass state, bixbyite.

Влияние Ga-замещения на структурные и магнитные свойства биксбиита (Mn,Fe)-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-

To study the dependence of the properties of ternary oxides (Mn,Fe,Ga)2O3 with the bixbyite structure on the composition, the temperature dependences of the magnetization and ac magnetic susceptibility of two single-crystal samples of different compositions obtained using the flux method were analyzed. A detailed study of the structure was carried out using single-crystal X-ray diffraction analysis, and the changes in structural parameters depending on the composition were analyzed. The dc magnetization and ac magnetic susceptibility of Fe1.1Mn0.76Ga0.14O3 and Fe0.65Mn1.1Ga0.26O3 bixbyites have been studied. Despite the qualitatively similar behavior of the magnetic properties, significant differences were also found, despite a small difference in the Mn/Fe/Ga ratio in the samples under study. It is shown that both compounds experience two successive low-temperature magnetic phase transitions from the paramagnetic phase at T = 20–32 K as the temperature is lowered. Calculations of the Mydosh parameter for the detected phase transitions showed different degrees of ordering in the compounds under study.

High temperature crystallographic and low temperature magnetic phase transitions in Fe doped Sr2RuMnO6

Fe doped Sr2RuMnO6 (SRMO) double perovskites (Sr2RuMn1-xFexO6, x = 0, 0.1, 0.2 and 0.3) were prepared by solid-state route. Both x-ray diffraction and Raman spectroscopy were performed to investigate the crystal structure of the synthesized double perovskites. Rietveld refinement of the x-ray diffraction patterns confirmed a phase transition from tetragonal to cubic space group as a function of doping concentration of iron. Raman spectroscopy at room temperature and group theory analysis revealed the phonon modes associated with the space group of the samples. The temperature dependent Raman spectroscopy showed an anharmonic behaviour of the phonon modes of the Fe doped SRMO samples. The temperature evolution of the phononic modes in the range of 300 K–620 K is predominantly influenced by the lattice degrees of freedom. The presence of several oxidation states Mn (2+, 3+ and 4+) and Fe (3+ and 4+) was confirmed by an X-ray photoemission spectroscopy analysis of the highest doped sample (x = 0.3). The magnetic properties measurements showed that the samples were completely paramagnetic at room temperature. The samples exhibit antiferromagnetism at very low temperatures and we conclude that they exhibit ferrimagnetic ground state in the mid temperature region.

Structural and magnetic alteration of Cu2GaBO5 forced by Mn3+ doping

To study the gradual change of the structure and the magnetic state of the ludwigite Cu2GaBO5 upon Mn3+ doping, single crystals of Cu2Ga1−xMnxBO5 (x = 0.55, 0.7, 0.8) with the size up to 3 × 3 × 10 mm3 were grown using the flux technique. The phase homogeneity and crystal structure of the obtained compounds were investigated by the powder and single crystal X-ray diffraction. All the samples possessed the monoclinic-distorted ludwigite structure with the P21/c space group. The study of the actual Cu/Ga/Mn composition by the EDX (energy-dispersive X-ray spectroscopy) technique revealed the lower Mn content in all the samples and the refined formulas were Cu2Ga0.47Mn0.53BO5, Cu1.92Ga0.5Mn0.58BO5 and Cu2Ga0.32Mn0.68BO5, respectively. Despite the high manganese content, the concentration transition (from Cu2GaBO5 to Cu2MnBO5) and change of the monoclinic angle did not occur, but strong Me-O octahedra distortions exceeding those both in the parent ludwigites Cu2GaBO5 and Cu2MnBO5 were found. The study of the thermodynamic and magnetic properties revealed the low-temperature magnetic phase transition inherited from the parent Cu2GaBO5 in all the samples. However, the nature and ordering type for the compounds with different Mn content were different: there was a complex transformation of the magnetic state from the partially ordered AFM (antiferromagnetically) in Cu2Ga0.47Mn0.53BO5, through the spin glass state, to the combined spin glass/ordered state in Cu2Ga0.32Mn0.68BO5 with the appearance of magnetic anisotropy. The evident dependence of Tc (phase transition temperature) on the magnetic field was found as well as its decrease at the nonzero magnetic field in the samples with x = 0.53 and 0.68.

Cryostat for cooling samples in the study of low-temperature structural and magnetic phase transitions by neutron diffraction

A vertical cryostat was developed built around a Sumitomo RDK-101D cold head. The cryostat is intended for investigating the structure and phase transformations of different materials via neutron diffraction experiments on samples under high vacuum of up to 10−5 mbar. A working temperature of the sample close to 2 K was reached. The drift diameter of the head is 54 mm, making it possible to cool samples with a height of 55 mm and 15 mm in diameter. Test measurements were conducted and illustrative spectra were recorded on the example of a sample of HoFeO3. The cryostat was designed and manufactured at the Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research.

Effect of Mn-doping on the low temperature magnetic phase transitions of BiFeO3

In the present work, we have carried out a comparative study of the low temperature phase transitions using dc magnetization (M (T)) and ac susceptibility (χ (ω, T)) measurements on undoped and 0.3 wt% MnO2 doped BiFeO3 samples in the 2–300 K temperature range. It is shown that MnO2 doping increases the resistivity and decreases the dielectric loss as a result of reduced oxygen vacancy concentration as confirmed by iodometry and x-ray photoelectron spectroscopy (XPS) studies. A comparative study of the dc M (T) and ac χ (ω, T) results on two types of samples reveal that the transitions around 25 K, 110 to 150 K and 260 K are intrinsic to BiFeO3. The widely reported transition at 50 K is argued to be defect induced, as it is absent in the doped samples. We also show that the spin-glass transition temperature (TSG) ∼20 K, determined from an analysis of χ (ω, T), is less than the spin-glass freezing temperature (Tf) ∼25 K in marked contrast to TSG > Tf reported by earlier workers.

Lattice disorder effect on magnetic ordering of iron arsenides

This study investigates magnetic ordering temperature in nano- and mesoscale structural features in an iron arsenide. Although magnetic ground states in quantum materials can be theoretically predicted from known crystal structures and chemical compositions, the ordering temperature is harder to pinpoint due to potential local lattice variations that calculations may not account for. In this work we find surprisingly that a locally disordered material can exhibit a significantly larger Néel temperature (TN) than an ordered material of precisely the same chemical stoichiometry. Here, a EuFe2As2 crystal, which is a ‘122’ parent of iron arsenide superconductors, is found through synthesis to have ordering below TN = 195 K (for the locally disordered crystal) or TN = 175 K (for the ordered crystal). In the higher TN crystals, there are shorter planar Fe-Fe bonds [2.7692(2) Å vs. 2.7745(3) Å], a randomized in-plane defect structure, and diffuse scattering along the [00 L] crystallographic direction that manifests as a rather broad specific heat peak. For the lower TN crystals, the a-lattice parameter is larger and the in-plane microscopic structure shows defect ordering along the antiphase boundaries, giving a larger TN and a higher superconducting temperature (Tc) upon the application of pressure. First-principles calculations find a strong interaction between c-axis strain and interlayer magnetic coupling, but little impact of planar strain on the magnetic order. Neutron single-crystal diffraction shows that the low-temperature magnetic phase transition due to localized Eu moments is not lattice or disorder sensitive, unlike the higher-temperature Fe sublattice ordering. This study demonstrates a higher magnetic ordering point arising from local disorder in 122.

Characterization of LiFePO4 samples obtained by pulse combustion under various conditions of synthesis

Lithium iron phosphate (LiFePO4, LFP) is one of the widely used cathode materials for rechargeable lithium ion batteries. LFP batteries are widely used for electric vehicles and backup power due to their important advantages such as low cost, lifetime, efficiency, and reliability. There are still several technical challenges that need to be addressed: the increase of energy density or further reduction of their final cost. This paper concerned with the characterization of carbon coated LiFePO4 nanopowder cathode materials produced under different conditions by pulse combustion for providing energy to the reactor for the synthesis. The reactor was built according to the principles of the thermoacoustic burner on the basis of the Helmholtz resonator. The investigated nanopowders are synthesized by complete and incomplete combustion and calcined at 700 °C. The obtained samples were characterized by X-ray diffraction, Fourier transform infrared, Raman, and Mössbauer spectroscopy. Observed low-temperature magnetic phase transitions definitively identified the crystal phases. The morphology of samples was controlled by scanning electron microscopy. The aim of this work is to show that it is possible to achieve a desired crystal phase by pulse combustion in a relatively cheap and fast way. The extremely rapid synthesis of almost pure phase material is possible due to the reduction in size of interacting particles and to an enormous number of collisions between them as a result of strong turbulent flow associated with explosive combustion.

Monte Carlo study of a geometrically frustrated rare-earth magnetic compound: SrGd2O4

We study the low-temperature magnetic phase transitions and phase diagram of one member of the lanthanide family of frustrated compounds, ${\text{SrGd}}_{2}{\text{O}}_{4}$, using Monte Carlo simulation techniques. A detailed experimental study of two members of the ${\text{SrLn}}_{2}{\text{O}}_{4}$ family of compounds, ${\text{SrHo}}_{2}{\text{O}}_{4}$ and ${\text{SrGd}}_{2}{\text{O}}_{4}$, has recently been carried out by Young et al. [O. Young, Ph.D. thesis, University of Warwick, 2013; O. Young, G. Balakrishnan, M. R. Lees, and O. A. Petrenko, Phys. Rev. B 90, 094421 (2014)]. A phase diagram for ${\text{SrGd}}_{2}{\text{O}}_{4}$ was mapped out in the field-temperature ($H\ensuremath{-}T$) plane from magnetization, susceptibility, and specific-heat measurements and several ordered phases were identified. However, the detailed nature of these phases was not determined. We have used a model of classical Heisenberg spins to investigate the low-temperature behavior. Our simulation results qualitatively agree with the experimental findings. By collecting data from all measured thermodynamic quantities, a $H\ensuremath{-}T$ phase diagram is constructed. It reveals four separate regions of phases with unique magnetic ordering. We have identified the nature of the order in each of these phases.

Low-temperature magnetic phase transitions in the multiferroic Nd0.9Dy0.1Fe3(BO3)4. Part 1. Transitions induced by magnetic fields directed along the trigonal symmetry axis. Spontaneous transitions with temperature changes

The elastic and magnetic characteristics of single-crystal Nd0.9Dy0.1Fe3(BO3)4 are studied at low temperatures in zero magnetic field and in external fields H||C3. The temperature dependences of the acoustic mode velocities and the magnetic susceptibility manifest a transition of the magnetic subsystem into a magnetically ordered state and two successive, spontaneous spin-reorientation phase transitions. The possibility of a spontaneous transition into an incommensurate (spiral) magnetic phase in the crystal is discussed. It is shown that an external magnetic field directed along the trigonal axis of the crystal induces a sequence of spin-reorientation phase transitions. An H-T phase diagram (H||C3) is constructed for this compound.

Competing magnetic interactions and low temperature magnetic phase transitions in composite multiferroics

Novel magnetic properties and magnetic interactions in composite multiferroic oxides Pb[(Zr0.52Ti0.48)0.60(Fe0.67W0.33).40]O3]0.80–[CoFe2O4]0.20 (PZTFW–CFO) have been studied from 50 to 1000 Oe field cooled (FC) and zero field cooled (ZFC) probing conditions, and over a wide range of temperatures (4–350 K). Crystal structure analysis, surface morphology, and high resolution transmission electron microscopy images revealed the presence of two distinct phases, where micro- and nano-size spinel CFO were embedded in tetragonal PZTFW matrix and applied a significant built-in compressive strain (∼0.4–0.8%). Three distinct magnetic phase transitions were observed with the subtle effect of CFO magnetic phase on PZTFW magnetic phase transitions below the blocking temperature (TB). Temperature dependence magnetic property m(T) shows a clear evidence of spin freezing in magnetic order with lowering in thermal vibration. Chemical inhomogeneity and confinement of nanoscale ferrimagnetic phase in paramagnetic/antiferromagnetic matrix restrict the long range interaction of spin which in turn develop a giant spin frustration. A large divergence in the FC and ZFC data and broad hump in ZFC data near 200 (±10) K were observed which suggests that large magnetic anisotropy and short range order magnetic dipoles lead to the development of superparamagnetic states in composite.

Low-temperature properties of theS=12spin systemYb3Ru4Al12with a distorted kagome lattice structure

We have synthesized single crystals of ternary intermetallic ${\mathrm{Yb}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$ with a distorted kagome lattice structure, and investigated the low-temperature resistivity, specific heat, magnetization, and magnetic phase transitions. ${\mathrm{Yb}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$ is the first $4f$ system that has a ${\mathrm{Gd}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$-type crystal structure where antiferromagnetic interaction acts on the spin. The crystal electric field (CEF) ground state of this compound is determined as a well isolated twofold degenerate state that is subjected to a strong easy-plane-type magnetic anisotropy. In the present study, the spin system of ${\mathrm{Yb}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$ is regarded as an AFM $XY$ model of $S=1/2$. This compound undergoes successive magnetic phase transitions at 1.5 and 1.6 K, and the resistivity exhibits ${T}^{2}$ behavior below 1 K. The ratio of the coefficient of the ${T}^{2}$ term in the resistivity $A$, and that of the electronic specific heat coefficient ${\ensuremath{\gamma}}_{0}$, deviates from the Kadowaki-Woods (KW) law. The successive phase transitions and low-temperature properties of ${\mathrm{Yb}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Al}}_{12}$ where geometrical frustration and heavy fermion behavior occur are discussed.

Anomalous magnetic behavior for Mn-site doped La0.7Ca0.3MnO3: internal magnetic interactions and extrinsic inhomogeneity

Magnetic properties of Mn-site doped La0.7Ca0.3Mn0.94TM0.06O3+Δ (TM=Cu, Zn) were experimentally and theoretically studied. The low-temperature magnetization and magnetic phase transition temperature TC of the samples combining with simulations of 2D doped Ising model suggest that Cu2+ ions have an antiferromagnetic interaction with their nearest neighboring Mn ions. Interestingly, an anomalous magnetic behavior, a “step-like” magnetic phase transition, is observed. The existence and magnitude of this step vary with different doping ions and oxygen ratio, which is experimentally suggested to be relevant to the interaction between magnetic ions and oxygen content. A superposition between two independent magnetic systems with different values of an exchange integral J well explains the anomalous magnetic phase transition, which suggests that this step-like behavior results from extrinsic inhomogeneity and negligible coupling between grains in polycrystalline.

Low temperature magnetic phase transition and interlayer coupling in double-wall carbon nanotubes

The magnetic properties of double wall carbon nanotubes (DWCNTs) were investigated using electron spin resonance (ESR) spectroscopy. An asymmetric resonance line of low intensity was identified and analyzed by the superimposition of a narrow and a broad metallic lineshape, attributed to the distinct contributions of defect spins located on the inner and outer DWCNTs shells. The spin susceptibilities of both ESR components revealed a ferromagnetic phase transition at low temperatures (T < 10 K) with small variation in the corresponding Curie-Weiss temperatures, approaching closely that of metallic single wall carbon nanotubes. Interlayer coupling between the DWCNT layers is suggested to effectively reduce the difference between the transition temperatures for the inner and outer shells and enhance spin-spin interactions between defect spins via the RKKY-type interaction of localized spins with conduction electrons.

Evidence for lithium superoxide-like species in the discharge product of a Li–O2 battery

We report on the use of a petroleum coke-based activated carbon (AC) with very high surface area for a Li-O(2) battery cathode without the use of any additional metal catalysts. Electrochemical measurement in a tetra(ethylene) glycol dimethyl ether-lithium triflate (TEGDME-LiCF(3)SO(3)) electrolyte results in two voltage plateaus during charging at 3.2-3.5 and 4.2-4.3 V versus Li(+)/Li. Herein we present evidence from Raman and magnetic measurements that the lower plateau corresponds to a form of lithium peroxide with superoxide-like properties characterized by a low temperature magnetic phase transition and a high O-O stretching frequency (1125 cm(-1)). The magnetic phase transition and the high O-O stretching frequency disappear when charged to above 3.7 V. Theoretical calculations indicate that a surface superoxide structure on lithium peroxide clusters and some lithium peroxide surfaces have an unpaired electron and a high O-O stretching frequency that help explain the observations. These results provide evidence that the form of the lithium peroxide discharge product is important to obtaining a low charge overpotential, and thus improving the round-trip efficiency between discharge and charge.

Synthesis of magnetic 1D dichalcogenide nanostructures

We report the first synthesis of magnetic one-dimensional transition metal dichalcogenides. Pure and Mn-doped tantalum disulfide nanotapes were synthesized using a catalyst-free single-step growth process. The sample yield of nanostructures was nearly 100 %. The nanotapes were composed of a number of smaller nanowires which appear to be open-ended nanotubes. The smaller nanowires have diameters ranging from about 30 to 300 nm. The Mn dopants were incorporated within intercalation sites between TaS2 layers. Interactions between these dopant ions induced low temperature magnetic phase transitions similar to those seen in macroscopic Mn x TaS2 compounds. The magnetic properties do not exactly correspond to their bulk counterparts, as exemplified by a higher than expected ferromagnetic transition temperature in 1D nanostructures. This indicates the magnetic properties are likely influenced by quantum size effects. The simplicity of this one-step synthesis method should allow for this technique to be extended to other dichalcogenide systems and/or magnetic dopants, opening up a new class of 1D magnetic nanostructures.

Effects of substitution on low-temperature physical properties of LuFe2Ge2

The low-temperature magnetic phase transition in LuFe2Ge2 is thought to be associated with itinerant magnetism. The effects of Y and Sc substitutions on the Lu site, as well as Ru and Co substitutions on the Fe site, on the low-temperature magnetic phase transition of the LuFe2Ge2 compound have been studied in single crystals via microscopic, thermodynamic and transport measurements. On one hand, Co substitution suppresses the transition below our base temperature of 2 K even at our lowest substitution level. On the other hand, Sc substitution enhances the transition temperature, and Y or Ru substitution suppresses the transition to lower temperature. Phase diagrams for Y, Sc and Ru substitutions have been constructed and the possibility of a unifying, composite diagram is discussed.