High-temperature Ferromagnetic Phase 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.

Determination of sub-ps lattice dynamics in FeRh thin films

Understanding the ultrashort time scale structural dynamics of the FeRh metamagnetic phase transition is a key element in developing a complete explanation of the mechanism driving the evolution from an antiferromagnetic to ferromagnetic state. Using an X-ray free electron laser we determine, with sub-ps time resolution, the time evolution of the (–101) lattice diffraction peak following excitation using a 35 fs laser pulse. The dynamics at higher laser fluence indicates the existence of a transient lattice state distinct from the high temperature ferromagnetic phase. By extracting the lattice temperature and comparing it with values obtained in a quasi-static diffraction measurement, we estimate the electron–phonon coupling in FeRh thin films as a function of laser excitation fluence. A model is presented which demonstrates that the transient state is paramagnetic and can be reached by a subset of the phonon bands. A complete description of the FeRh structural dynamics requires consideration of coupling strength variation across the phonon frequencies.

Chirality Memory Stored in Magnetic Domain Walls in the Ferromagnetic State of MnP.

Chirality in a helimagnetic structure is determined by the sense of magnetic moment rotation. We found that the chiral information did not disappear even after the phase transition to the high-temperature ferromagnetic phase in a helimagnet MnP. The 2nd harmonic resistivity ρ^{2f}, which reflects the breaking down of mirror symmetry, was found to be almost unchanged after heating the sample above the ferromagnetic transition temperature and cooling it back to the helimagnetic state. The application of a magnetic field along the easy axis in the ferromagnetic state quenched the chirality-induced ρ^{2f}. This indicates that the chirality memory effect originated from the ferromagnetic domain walls.

Critical behavior of the ferromagnets CrI3, CrBr3 , and CrGeTe3 and the antiferromagnet FeCl2 : A detailed first-principles study

We calculate the Curie temperature of layered ferromagnets, chromium tri-iodide (CrI3), chromium tri-bromide (CrBr3), chromium germanium tri-telluride (CrGeTe3), and the Neel temperature of a layered anti-ferromagnet iron di-chloride (FeCl2), using first-principles density functional theory calculations and Monte-Carlo simulations. We develop a computational method to model the magnetic interactions in layered magnetic materials and calculate their critical temperature. We provide a unified method to obtain the magnetic exchange parameters (J) for an effective Heisenberg Hamiltonian from first-principles, taking into account both the magnetic ansiotropy as well as the out-of-plane interactions. We obtain the magnetic phase change behavior, in particular the critical temperature, from the susceptibility and the specific-heat, calculated using the three-dimensional Monte-Carlo (Metropolis) algorithm. The calculated Curie temperatures for ferromagnetic materials (CrI3, CrBr3 and CrGeTe3), match very well with experimental values. We show that the interlayer interaction in bulk CrI3 with R3 stacking is significantly stronger than the C2/m stacking, in line with experimental observations. We show that the strong interlayer interaction in R3 CrI results in a competition between the in-plane and the out-of-plane magnetic easy axis. Finally, we calculate the Neel temperature of FeCl2 to be 47 +- 8 K, and show that the magnetic phase transition in FeCl2 occurs in two steps with a high-temperature intralayer ferromagnetic phase transition, and a low-temperature interlayer anti-ferromagnetic phase transition.

Влияние кристаллической структуры подложки на магнитную анизотропию плeнок Mn-=SUB=-x-=/SUB=-Si-=SUB=-1-x-=/SUB=- (x~0.5)

Thin films of MnxSi1-x (x ≈ 0.5) alloy were synthesized by pulsed laser deposition in a droplet-free mode on sapphire substrates with different crystallographic orientations of the cut plane (c-Al2O3 and r-Al2O3) at various laser energy densities E at the polycrystalline MnSi target. The X-ray structural, as well as static and resonance magnetic properties of the films were studied depending on E and the orientation of the substrate. The films deposited at high E > 6 J/cm2 are found to demonstrate the presence of a ferromagnetic phase with anomalously high Curie temperature TC ~ 300 K which is not typical of MnSi single crystals (TC ≈ 30 K). In the case of the c-Al2O3 sub-strates, the magnetic moment of the films turns out to be somewhat higher than in the case of the r-Al2O3 substrates. Moreover, the films deposited on the c-Al2O3 substrates exhibit noticeably higher values of the effective surface anisotropy field 4πMeff measured by ferromagnetic resonance. The data obtained indicate a significant effect of the substrate structure on the formation and magnetic properties of the high-temperature ferromagnetic phase in the MnxSi1-x films.

Laser synthesis of thin MnxSi1-x films (x ~ 0.5) on c- and r-Al2O3 substrates at different laser energy densities at the target

Abstract Thin films of MnxSi1-x (x ~ 0.5) were obtained by pulsed laser deposition in droplet-free mode on c- and r-Al2O3 substrates at different laser energy densities E at the target. Their structural, electrical, and magnetic properties were studied depending on the value of E and the orientation of the substrate. The films are X-ray amorphous on the substrates of both types at E 5.5 J/cm2 the high-temperature ferromagnetic phase shows in the films, and at E ≅ 4 - 5 J/cm2, the low-temperature ferromagnetic phase predominates and there is no influence of the sapphire substrate orientation. The highest Curie temperature TC reached was 330 K at E ≅ 7.4 J/cm2 for the MnxSi1-x films obtained on c- and r-Al2O3. The correlated behavior of the magnetization and the signal of diffuse X-ray radiation scattering confirms the existence of e-MnSi nanocrystallites.

Evolution of the Ferromagnetism of MnxSi1 –x (x ≈ 0.5) Films Produced by Laser Synthesis on c- and r-Cut Sapphire Substrates as the Laser Energy Density on the Target Changes

MnxSi1 – x (x ≈ 0.5) thin films have been produced on c- and r-cut sapphire substrates by the droplet-free pulsed laser deposition method at various laser energy densities E on the target. Their magnetic, electric, and X-ray diffraction properties as a function of E and substrate orientation have been investigated. It has been established that the high-temperature ferromagnetic phase in the films at E ≥ 6 J cm–2 is more pronounced than that at E ≈ 4–5 J cm–2, when the low-temperature ferromagnetic phase dominates and there is no influence of the sapphire substrate orientation. The attained Curie temperature TC was 330 K at E ≈ 7.4 J cm–2 for the MnxSi1 – x films produced on c- and r-cut sapphire substrates. The magnetization of the MnxSi1 – x films produced at E ≥ 6 J cm–2 on c-cut sapphire is higher than that on r-cut sapphire and, conversely, lower when E ≤ 5.5 J cm–2. A change in the ratio of the diffuse signal amplitudes in the X-ray spectra for the films grown on different substrates is also observed under these conditions. Such a correlated behavior with magnetization is explained by the existence of e-MnSi nanocrystallites of optimal sizes, which, on the one hand, are responsible for the emergence of a diffuse signal in the X-ray spectra and, on the other hand, determine the high-temperature ferromagnetism of the films. The concentration of such nanocrystallites and the distribution of defects in the films are controlled by the type of substrate and the energy density on the target.

Change in magnetic properties of La2MnCoO6 in composite with CaCu3Ti4O12

The magnetic properties of double perovskite La2MnCoO6 (LCMO) by making a composite with ordered double perovskite CaCu3Ti4O12 (CCTO) have been investigated. The composites were prepared by mixing the prior synthesized pure phases of LCMO and CCTO at different mole ratio. We observed that a new low temperature ferromagnetic phase (TC ~ 110 K) with lower magnetization value evolved at the expense of a high temperature ferromagnetic phase (TC ~ 228 K). The latter ferromagnetism is associated with ordering of Mn4+ and Co2+ ions. The new magnetic phase could be correlated to vibronic superexchange interaction between Mn3+–O–Co3+ cations, which would establish a ferromagnetic state. The genesis of this magnetic phase can be attributed to the local structural modulation of LCMO across the grain boundary in contact with CCTO. The lower magnetic moments and low temperature transitions have been attributed to the atomic disorder antiferromagnetic interactions (Mn4+–O–Mn4+ and Co2+–O–Co2+) akin to parent LCMO.

Engineering of high-temperature ferromagnetic Si1–xMnx (x ≈ 0.5) alloyed films by pulsed laser deposition: Effect of laser fluence

Thin films of Si1−xMnx were deposited on Al2O3 substrate using pulsed laser deposition(PLD)technique employing time-of-flight separation of the ejected particles with different laser fluence (E = 3.8–7.6 J/cm2). Magnetic properties of these films reveal that the variation in the laser fluence has engineered several magnetic phases in Si1−xMnx films: high temperature (HT) ferromagnetic (FM) phase Si1−xMnx(x = 0.51–0.52) with uniformly distributed Si vacancies, and the low-temperature (LT) FM phases MnSi and Mn4Si7 with Curie temperature ≤40 K. At small fluence, the mesoscopic size MnSi and Mn4Si7 crystallites are formed in the upper layer of Si1−xMnx films with Si excess (x ≈ 0.4–0.5), yielding to inhomogeneous magnetic phase distribution along the film growth direction, while the high and optimal fluence has resulted in uniform distribution of the HT FM phase during the film growth. The XPS, magnetotransport and ferromagnetic resonance (FMR) measurements confirm the presence of several FM phases in the samples and the amplifying role of LT FM for films grown at E ≤ 4.4 J/cm2.

Magnetic states of MnP: muon-spin rotation studies

Muon-spin rotation data collected at ambient pressure (p) and at p = 2.42 GPa in MnP were analyzed to check their consistency with various low- and high-pressure magnetic structures reported in the literature. Our analysis confirms that in MnP the low-temperature and low-pressure helimagnetic phase is characterised by an increased value of the average magnetic moment compared to the high-temperature ferromagnetic phase. An elliptical double-helical structure with a propagation vector , an a-axis moment elongated by approximately 18% and an additional tilt of the rotation plane towards c-direction by –8° leads to a good agreement between the theory and the experiment. The analysis of the high-pressure μSR data reveals that the new magnetic order appearing for pressures exceeding 1.5 GPa can not be described by keeping the propagation vector . Even the extreme case—decoupling the double-helical structure into four individual helices—remains inconsistent with the experiment. It is shown that the high-pressure magnetic phase which is a precursor of superconductivity is an incommensurate helical state with .

Investigation of magneto-structural phase transition in FeRh by reflectivity and transmittance measurements in visible and near-infrared spectral region

Magneto-structural phase transition in FeRh epitaxial layers was studied optically. It is shown that the transition between the low-temperature antiferromagnetic phase and the high-temperature ferromagnetic phase is accompanied by a rather large change of the optical response in the visible and near-infrared spectral ranges. This change is consistent with ab initio calculations of reflectivity and transmittance. Phase transition temperatures in a series of FeRh films with thicknesses ranging from 6 to 100 nm is measured thereby demonstrating the utility of the method to quickly characterise samples. Spatially resolved imaging of their magnetic properties with a micrometer resolution shows that the phase transition occurs at different temperatures in different parts of the sample.

A pathway to optimize the properties of magnetocaloric Mn2-xFexP1-yGey for magnetic refrigeration

Magnetocaloric materials can be useful in magnetic refrigeration applications, but to be practical the magneto-refrigerant needs to have a very large magnetocaloric effect (MCE) near room temperature for modest applied fields (<2 T) with small hysteresis and magnetostriction, and should have a complete magnetic transition, and environmentally friendly. One system that may fulfill these requirements is Mn2-xFexP1-yGey, where a combined first-order structural and magnetic transition occurs between the high temperature paramagnetic and low temperature ferromagnetic phase. We have used neutron diffraction, differential scanning calorimetry, and magnetization measurements to study the effects of Mn and Ge location in the structure on the ordered magnetic moment, MCE, and hysteresis for a series of compositions of the system near optimal doping. The diffraction results indicate that the Mn ions located on the 3f site enhance the desirable properties, while those located on the 3 g sites are detrimental. The phase fraction that transforms, hysteresis of the transition, and entropy change can be affected greatly by both the compositional homogeneity and the particle size, and an annealing procedure has been developed that substantially improves the performance of all three properties of the material. We also establish a correlation between applied magnetic field to complete the transition and the temperature range of coexistence of the PM and FM phase. On the basis of these results we have identified a pathway to understand the nature and to optimize the MCE properties of this system for magnetic refrigeration applications.

Novel Magnetic Chiral Structures and Unusual Temperature Hysteresis in the Metallic Helimagnet MnP

We have reinvestigated the magnetic properties of the classical metallic helimagnet MnP by magnetization and neutron scattering experiments. Our neutron scattering results indicate that the previously reported magnetic structure in the low-temperature (LT) helimagnetic phase (T < 47K) should be modified to an alternately tilted helimagnetic structure pro- duced by the Dzyaloshinsky-Moriya interaction. In the intermediate temperature (IT) range between the LT helimagnetic phase and the high-temperature (HT) ferromagnetic phase along the c-axis, 47K < T < 282K, we have found a weak ferromagnetic behavior along the b-axis. Surprisingly, the IT weak ferromagnetic phase has two different states, namely, the large magnetization (LM) and small magnetization (SM) states. The SM state emerges with cooling from the paramagnetic phase above 292 K via the HT ferromagnetic phase and LM state emerges with warming from the LT helimagnetic phase. The weak ferromagnetism along the b-axis and the unusual temperature hysteresis in the IT phase can be understood by assuming a spontaneous formation of the stripe structure consisting of alternately arranged HT ferromagnetic and LT helimagnetic domains.

Tunable resistivity in magnetic glass phase of Gd1−xCaxBaCo2O5.5

We present magnetization and resistance measurements carried out on pristine and Ca-doped Gd1−xCaxBaCo2O5.5 (x = 0.02) samples using the cooling and heating in unequal field (CHUF) protocol. The measurements reveal that the high temperature ferromagnetic phase is kinetically arrested at low temperature when the sample is cooled in a magnetic field. The volume fraction of this arrested phase increases upon Ca substitution and also by increasing the field in which the sample is cooled. Since the ferromagnetic phase is less resistive when compared to the low temperature antiferromagnetic phase, a tunable resistance is achieved in the sample by cooling in different magnetic fields. By cooling in magnetic fields of 9 T a reduction in resistivity by an order of magnitude is achieved. These results are consistent with the coexistence of the low temperature equilibrium antiferromagnetic phase with kinetically arrested high temperature ferromagnetic phase in the system.

Determination of the normal and anomalous hall effect coefficients in ferromagnetic Ni50Mn35In15 − x Si x Heusler alloys at the martensitic transformation

The magnetization, the electrical resistivity, the magnetoresistance, and the Hall resistivity of Ni50Mn35In15 − xSix (x = 1.0, 3.0, 4.0) Heusler alloys are studied at T = 80-320 K. The martensitic transformation in these alloys occurs at T = 220–280 K from the high-temperature ferromagnetic austenite phase into the low-temperature martensite phase having a substantially lower magnetization. A method is proposed to determine the normal and anomalous Hall effect coefficients in the presence of magnetoresistance and a possible magnetization dependence of these coefficients. The resistivity of the alloys increases jumpwise during the martensitic transformation, reaches 150–200 μΩ cm, and is almost temperature-independent. The normal Hall effect coefficient is negative, is higher than that of nickel by an order of magnitude at T = 80 K, decreases monotonically with increasing temperature, approaches zero in austenite, and does not undergo sharp changes in the vicinity of the martensitic transformation. At x = 3, a normal Hall effect nonlinear in magnetization is detected in the immediate vicinity of the martensitic transformation. The temperature dependences of the anomalous Hall effect coefficient in both martensite and austenite and, especially, in the vicinity of the martensitic transformation cannot be described in terms of the skew scattering, the side jump, and the Karplus-Lutinger mechanisms from the anomalous Hall effect theory. The possible causes of this behavior of the magnetotransport properties in Heusler alloys are discussed.

Neutron diffraction studies of the magnetic properties of Pr0.5Sr0.5Co1 − x Mn x O3 solid solutions

The crystal structure and magnetic properties of a system of Pr0.5Sr0.5Co1 − xMnxO3 solid solutions were studied by neutron diffraction and magnetization measurements. It is shown that, at a low manganese concentration, the structure can be described by the I/2a monoclinic space group; with increasing substitution level x the structure becomes orthorhombic. For x > 0.9 the crystal structure is tetragonal at high temperatures and the symmetry is lowered to orthorhombic with lowering the temperature. The substitution of cobalt for manganese leads to the destruction of long-range ferromagnetic order near x ∼ 0.25. A transition from the high-temperature ferromagnetic phase to the A-type low-temperature antiferromagnetic phase is observed at x ∼ 0.93 in the temperature range 110–160 K.

Spin disorder induced reentrant ferromagnetism in iron-based nanocomposites

The low-temperature magnetic behavior of pure iron nanocrystallites dispersed in MgO matrix has been investigated by dc magnetization and ac susceptibility measurements. Irreversibilities in the magnetization data at low field are observed at low temperatures, indicating a passage from a high temperature ferromagnetic phase to a low-temperature disordered, frozen state. Quenching of these irreversibilities is seen when the external magnetic field is increased. The variation of the in-phase susceptibility with temperature showed marked frequency dependence whereas the out-of-phase susceptibility component passes through a maximum. The origin of this behavior is traced to the spin disorder at the grain boundaries, which exhibit a co-operative freezing below a certain temperature.

Magnetic phases in Pr0.5Sr0.5Mn1 − x Co x O3 (x ≤ 0.5) solid solutions

The magnetic and magnetotransport properties of Pr0.5Sr0.5Mn1 − xCoxO3 (x ≤ 0.5) solid solutions have been investigated using neutron diffraction methods. The magnetization and electrical conductivity have been measured in magnetic fields up to 140 kOe. It has been established that, during cooling in the temperature range from 160 to 110 K, the compounds of compositions with a cobalt content x ≤ 0.07 undergo a structural phase transition from the high-temperature ferromagnetic phase to the antiferromagnetic phase. A further substitution of cobalt for manganese leads to a stabilization of the inhomogeneous dielectric ferromagnetic state, whereas a state of the cluster spin-glass type has been revealed in compositions with x = 0.15 and 0.20. At x ≥ 0.25, a new magnetic phase with a Curie temperature up to 210 K is formed as a result of the magnetic interaction between manganese and cobalt ions. A magnetic phase diagram of the system under investigation has been constructed.

Glassy dynamics in magnetization across the first order ferromagnetic to antiferromagnetic transitionin Fe0.955Ni0.045Rh

We present the results of magnetization relaxation measurementsacross the ferromagnetic to antiferromagnetic transition inFe0.955Ni0.045Rh. The transition from the high temperature ferromagnetic phase to the low temperatureantiferromagnetic phase seems to be arrested by increasing the applied magnetic field. Thecrossover from crystallization-like dynamics to glassy dynamics can be tracked bymeasuring isothermal time dependent magnetization at various constant temperatureswhile cooling across this ferromagnetic to antiferromagnetic transition. The initialconversion from the ferromagnetic to antiferromagnetic phase as a function of time athigher temperatures follows a distinct power law relaxation. The transition is incomplete atlow temperatures with the stretched exponential relaxation behaviour dominating over thepower law, which is indicative of glassy dynamics or the arrest of the kinetics of the phasetransition. In the intermediate temperature regime, the magnetic relaxation can beexplained as a combination of both the power law and stretched exponential. Thetemperature dependence of the time constant of the stretched exponential follows theArrhenius law which is usually observed in the case of strong glass-forming liquids.

Kinetics of changes in the saturation magnetization during annealings of the Fe-36% Ni invar alloy subjected to severe plastic deformation

Changes in the saturation magnetization and microhardness of the Fe-36% Ni invar alloy with a nanocrystalline structure, which were caused by annealings at different temperatures, are studied. The nanocrystalline structure was produced by severe plastic deformation. A high-temperature ferromagnetic phase was found to result from the formation of such a structure. The temperature of annealing after which the content of the phase is maximum was determined. When analyzing the kinetics of phase-composition changes, an increase in the diffusive mobility of atoms during the formation of the nanocrystalline state in the material was taken into account.