Critical Field Hcr Research Articles

Systematic analysis of metamagnetic transitions in A-site disordered Sm0.5(Ca0.5−ySry)MnO3 compounds: A combined experimental and model Hamiltonian study

We investigate the ultra-sharp jump in the isothermal magnetization and the resistivity in the polycrystalline Sm0.5(Ca0.5−ySry)MnO3(y = 0, 0.1, 0.2, 0.25, 0.3, 0.5) compounds. The critical field (Hcr), required for the ultra-sharp jump, decreases with the increase of ‘Sr’ concentration, i.e. with an increase of average A-site ionic radius 〈rA〉. In addition, The magneto-transport data indicate that the phase separation increases with the increase of 〈rA〉, i.e. with ’y’. Moreover, the dependency of Hcr with magnetic field sweep rate reveals that the ultra-sharp jump from antiferromagnetic (AFM) state to the ferromagnetic (FM) state is of martensitic in nature. In parallel with experiments, we have carried out numerical calculations to understand the role of A-site disorder in the metamagnetic transition. Our two-band double exchange model Hamiltonian calculations show that the ‘Sr’ doping induces the ferromagnetic clusters in the antiferromagnetic insulating phase and in turn reduces the critical field. Overall, our combined study comprising of several materials with different composition of A-site reveal the fact that how and why certain A-type disorder produces prominent metamagnetic transition.

In-situ studies of magnetostriction in TbxHo1-xFe1.9Mn0.1 Laves compounds

The magnetic domain structure and magnetostrictive properties of TbxHo1-xFe1.9Mn0.1compounds have been investigated by in-situ Lorentz transmission electron microscopy (LTEM). The magnetocrystalline anisotropy compensation has been realized to be around x = 0.12 based on the easy magnetization direction (EMD) and by evaluating magnetocrystalline anisotropy constant K1, magnetization and magnetostriction. The EMD at room temperature rotates from the 〈1 0 0〉 axis for x ≤ 0.10 to 〈1 1 1〉 axis for x ≥ 0.12, which is detected directly by electronic holography technique. The magnetic domain wall movements were observed by in-situ magnetic field LTEM, and the critical magnetic field Hcr decreases continuously from 100 Oe for the Tb-rich side of x = 0.16 to 60 Oe for the Tb-poor side of x = 0.12, owing to the decrease in magnetocrystalline anisotropy subjected by anisotropy compensation. A high low-field magnetostriction (λa ~ 200 ppm at 1 kOe) is achieved at the critical magnetocrystalline anisotropy compensation point of x = 0.12. This work helps to understand the correlation between magnetocrystalline anisotropy, magnetization process, magnetic domain wall motion and magnetostriction in the C15 Laves phase system.

The Rule Rather than the Exception: Structural Flexibility of [Ni(cyclam)]2+-Based Cyano-Bridged Magnetic Networks

Changes in structure and magnetic properties accompanying guest removal, inclusion, or exchange in two CN-bridged 2D networks of honeycomb topology: {[Ni(cyclam)]3[Fe(CN)6]2}n (1) and {[Ni(cyclam)]3[Cr(CN)6]2}n (2) (cyclam = 1,4,8,11-tetraazacyclotetradecane) were studied by PXRD and magnetic measurements. For each compound four pseudopolymorphic forms differing in structure and magnetic characteristics were identified: fully hydrated form stable in water, partly hydrated form stable in the air at ambient conditions, anhydrous form, and MeOH-modified form. All forms can be reversibly transformed into one another by several interconversion pathways, which fully correspond between Fe and Cr compounds. All forms of 1 and 2 are metamagnetic-like with varied Tc and critical field Hcr. For several forms, differently shaped magnetic hysteresis loops can be observed. For the partly hydrated and MeOH modified forms structure models are proposed on the basis of PXRD data. Correlations between structural features an...

Critical end point of the first-order ferromagnetic transition in a Sm0.55(Sr0.5Ca0.5)0.45MnO3 single crystal

We report on the magnetic field (H) and hydrostatic pressure (P) dependence of the order of the ferromagnetic (FM) to paramagnetic phase transition in a Sm0.55(Sr0.5Ca0.5)0.45MnO3 single crystal. At ambient condition, the system exhibits a first-order FM transition at TC≈82K (in heating cycle) with strong thermal hysteresis (~13K). The application of external H and P increases TC, suppresses the hysteresis width, and thus weakens the first-order nature of the transition. Our analysis reveals that the hysteresis vanishes and the first-order FM transition becomes a crossover above a critical magnetic field Hcr≈11.5T. The value of Hcr is highest among the manganite family, although the first-order nature of the FM transition is believed to be strongest in Eu1−xSrxMnO3(x≈0.45).

Magnetic behaviors in melt spun Fe52−xMn23+xGa25 (x = 0–3) ribbons

The effect of volume fraction of B2-type ferromagnetic (FM) phase on magnetic behavior of melt-spun Fe52−xMn23+xGa25 (x = 0, 1, 2, and 3) ribbons has been investigated. X-ray diffraction (XRD) results show that the volume fraction of the B2 phase is decreased, accompanied by the increased antiferromagnetic (AFM) fcc phase, with increasing x. The magnetization isotherms of these ribbons at different temperatures demonstrate that there exists a critical field HCR from AFM to FM state, and the HCR decreases and vanishes finally with increasing temperature. High exchange bias field (HEB) at 10 K through a cooling field at HCR from 300 K could be found and correlated to the volume fraction of B2-type FM phase. Both HCR and HEB are increased with decreasing the amount of B2-type FM phase. The HCR is increased from 20 kOe for x = 0 to 50 kOe for x = 3, and HEB is increased from 0.9 kOe for x = 0 to 2.5 kOe for x = 2. The latter phenomenon might be attributed to the change of exchange interactions at FM/AFM interfaces due to the change of proportion of FM and AFM phase at low temperature.

Field-induced magnetic phase transition in Pr3+ doped Sm0.5Sr0.5MnO3 manganites

Magnetic properties of Pr3+ doped Sm0.5Sr0.5MnO3 manganite with the variation of particle size down to 30 nm have been investigated in detail. We have critically examined the magnetic field (H) dependence of the order of the ferromagnetic (FM) to paramagnetic (PM) phase transition in this Sm0.35Pr0.15Sr0.5MnO3 manganite. It has been shown that all the particle sizes (bulk to nano) exhibit first order FM → PM phase transition under low magnetic field accompanied by magnetization with thermal hysteresis in the field cooled cooling and warming cycle. However, the samples exhibit a second order magnetic phase transition above a critical field HCR. Again with decreasing the particle size, the ferromagnetic transition temperature, the thermal hysteresis width in the magnetizations, and the critical field HCR significantly decrease, which indicate that the ferromagnetism is weakened and the first-order magnetic phase transition is softened. We have also used the Banerjee criteria to distinguish the first-order magnetic phase transition from the second-order one. A detailed analysis of the magnetization measurements of this manganite reveal the disorder-induced softening of the first order phase transition in this phase separated manganite.

Magnetization study of single-crystalline ErFe5Al7

The field and temperature dependence of the magnetization of a ErFe5Al7 single crystal (tetragonal crystal structure of the ThMn12-type) has been studied in magnetic fields up to 14T. The compound is a ferrimagnet with Curie temperature TC=201K and a compensation point Tcomp=34K. ErFe5Al7 displays easy-plane anisotropy. Anisotropy is also present in the basal plane, the easy-magnetization direction is the [100] axis with the spontaneous magnetic moment Ms=1.28 μB/f.u. at T=2K. The compound exhibits field-induced magnetic transitions along the [100] and [110] axes. The transition along [100] is observed at T<40K and its critical field Hcr,100 displays complex non-monotonous temperature dependence. Along [110] magnetization jumps occur in a very narrow temperature interval T=31–40K. Hcr,110 is a very sharp function of temperature. Much higher fields are required to induce the transition at T<31K. At low temperatures strong hysteresis is observed, the coercivity is as high as Hc≈2.5T along the [100] and [110] axes at T=2K.

Magnetic Properties of a HoFe&lt;sub&gt;5&lt;/sub&gt;Al&lt;sub&gt;7&lt;/sub&gt; Single Crystal

A single crystal of HoFe5Al7 (tetragonal ThMn12 structure) has been studied. HoFe5Al7 orders ferrimagnetically at TC = 227 K and has a compensation of the Ho and Fe sublattices at Tcomp = 65 K. The compound exhibits high easy-plane anisotropy. Strong anisotropy is also present within the basal plane, with the [110] axis being the easy magnetization direction. A high coercivity with Hc values attaining 1.6 T is found at 2 K. In the temperature range 40-80 K, HoFe5Al7 displays a field-induced magnetic transition along the easy [110] direction. Temperature dependence of the critical field Hcr of the transition is very strong, Hcr exceeds 14 T below 40 K.

Size-dependent ferromagnetic phase transition in Sm.5Sr.5MnO3 nanoparticles

Magnetic properties of Sm0.5Sr0.5MnO3 nanoparticles with different particle sizes are investigated. It is found that all the nanoparticles show a first-order ferromagnetic transition under low magnetic fields, but a second-order one above a critical field HCR. As the particle size decreases, the ferromagnetic transition temperature, the thermal hysteresis width in the magnetizations, and HCR exhibit a significant decrease, which indicates that the ferromagnetism is weakened and the first-order transition is softened. A detailed analysis on the magnetic susceptibilities of these nanoparticles reveals that this weakening and softening are attributed to the weakened double-exchange interactions and the strongly suppressed charge-ordered antiferromagnetic state by the size reduction, respectively.

Ferromagnetic resonance, magnetic susceptibility, and transformation of domain structure in CoFeB film with growth induced anisotropy

Field dependence of magnetic susceptibility in nanocrystalline CoFeB film was studied in a wide frequency range from 500 kHz to 15 GHz. Anomalies of the susceptibility were detected exciting CoFeB film with a solenoidal coil, microwave strip line, in the tunable microwave cavity as well as employing magneto-optical domains imaging. Critical spin fluctuations in the form of “soft” modes were observed in a whole range of orientations of magnetic field perpendicular to the “easy” magnetic axis. A sequence of domain structure transformations was extensively examined in a “hard” direction in in-plane magnetic field reduced below the field of uniaxial anisotropy Hp = 535 Oe. At first, uniformly magnetized state in CoFeB film transforms to stripe domains separated by low angle Néel domain walls (DWs) parallel to the “hard”-axis. Then, at critical field Hcr = 232 Oe, Néel DWs gradually convert to the“easy”-axis oriented Bloch DWs loaded with vertical Bloch lines (VBLs). After field reversal at H = −Hcr, backward conversion of VBL-loaded Bloch DWs to Néel DWs results in instantaneous energy release and sharp anomaly of magnetic susceptibility. Appearance of critical spin fluctuations accomplishes domains transformation to the uniformly magnetized state at H = −535 Oe.

First-order ferromagnetic to helimagnetic transition in MgMn6Ge6

The magnetic and magnetocaloric properties of the new HfFe6Ge6-type (P6/mmm) MgMn6Ge6 compound have been studied by magnetic measurements and powder neutron diffraction experiments. MgMn6Ge6 magnetically orders at TC∼345 K in a ferromagnetic easy-plane arrangement. At TAF∼235 K, it undergoes a first-order transition to a helical structure, characterized by a temperature dependent propagation vector k=⟨0,0,qz⟩ (qz∼0.116 reciprocal lattice units at 1.4 K), without structural modification. The Mn atoms carry a magnetic moment of about 2 μB. A metamagnetic transition with a low critical field (Hcr&amp;lt;1 T) is observed below TAF. At low temperature, both magnetic and magnetocaloric data strongly suggest that some kinds of intermediate noncollinear ferromagnetic arrangements are stabilized above the critical field while the pure ferromagnetic state is obtained for significantly higher magnetic fields (H&amp;gt;5 T). The magnitude of the magnetic entropy change at TC (−ΔSM∼20.5 mJ cm−3 K−1 for ΔH=5 T) is about 25% that of Gd metal. The magnetocaloric effect at the order-order transition is of opposite sign and of lower magnitude.

Perminvar Effect in Amorphous Hitperm Alloy

Influence of the directional ordering on magnetization processes in the amorphous Fe44.5Co44.5Zr7B4 alloy was investigated in the temperature region of 27 to 300◦C for stabilization times of up to 1000 min by means of the Perminvar effect. The time-temperature dependent Perminvar effect was found to be very sensitive on the stage of the domain structure stabilization. The Perminvar critical fields HCR were shifted to higher values with increasing stabilization temperatures and times. At lower temperatures, the susceptibility increases over wide range of the measuring fields. Stabilization at higher temperatures causes that the susceptibility abruptly increases in the narrow field range. The record critical field HCR = 246 Am −1 was obtained.

Bose-Glass-Like Phases in Oriented-Twin YBa2Cu3O7-δCrystals

We experimentally study the dynamical response of the superconducting vortex system near the solid-liquid transition by applying forces parallel and perpendicular to a planar random defect array. Our results show that for magnetic fields above a certain critical field H cr≈4 T the solid glassy vortex phase in YBa2Cu3O7−δ crystals with oriented twin planes does not correspond to the so-called Bose-glass phase.

Structural and magnetic properties of RFe9Si2Cx compounds (R=Ce, Pr, Nd, Sm; x=0.5, 1.0, 1.5)

A systematic investigation of crystallographic and intrinsic magnetic properties of the compounds RFe9Si2Cx (R=Ce, Pr, Nd, Sm; x=0.5, 1.0, 1.5) has been carried out. The lattice parameters a, c and the unit cell volumes increase with increasing C contents, resulting in the increases of Curie temperatures by about 100–150 K between x=0.5 and x=1.5. The saturation magnetization of RFe9Si2Cx (R=Pr, Nd, Sm) at 1.5 K passes through a maximum at x=1.0, and then decreases slightly. Meanwhile, the average iron moment μFe at 1.5 K is 1.57μB for x=1.0 and 1.63μB for x=1.5 in CeFe9Si2Cx compounds. SmFe9Si2Cx compounds have uniaxial anisotropy with the anisotropy fields HA estimated around 132 kOe at 1.5 K and 50 kOe at 300 K (x=1.5), respectively. First-order magnetization processes are observed in the NdFe9Si2Cx compounds and the first-order magnetization processes critical field Hcr is found to increase with x. In addition, the critical temperature for the onset of the first-order magnetization processes transition is around 90 K in the case of x=1.0.

Tunnelling in Pb-p-HgCdTe Schottky barriers in longitudinal and transverse magnetic fields

The tunnelling current in reverse-biased Pb-p-Hg1-xCdxTe (x=0.2-0.3) Schottky barriers is investigated in a magnetic field H=0-50 kOe parallel and perpendicular to the electric field E in the temperature range T=20-250 K. The tunnelling current in magnetic fields of both orientations decreases by a factor of 10 and more. For H//E the magnitude of the effects as well as the dependence of the interband tunnelling current on magnetic field, reverse voltage (electric field), temperature and alloy composition is in good agreement with the theory. In contrast to the existing theory, for H perpendicular to E orientation the magnetic-field dependence is similar to the H//E case. No peculiarities are observed near the critical magnetic field Hcr=cE/s (s is the Kane velocity). The experimental results for H perpendicular to E may be explained qualitatively if we take into account a non-resonant scattering of light holes during tunnelling through the barrier. The scattering length estimated from the ratio of the effects in two orientations is proportional to H-1/2 and is of the order of the magnetic length, as would be expected for quantum diffusion in crossed fields.

Domain wall pinning in amorphous CoZrNb films

A series of CoZrNb amorphous films, with thickness t ranging from 200 to 630 nm, has been studied by using several techniques: Kerr imaging, dc and audio frequency hysteresis loops, field dependence of the permeability, torque balance, broadband frequency susceptometer (1 kHz≤f≤500 MHz). Only samples with t≤500 nm have simple domain structures typical of an in-plane anisotropy (with a small dispersion of the easy axis). Although domain walls (DW) are visible, they do not contribute to the initial permeability in the observed frequency range (even in the favorable case where the magnetic field is parallel to the easy axis), but they do play a role on the irreversible magnetization processes as shown by the existence of a critical field Hcr, typical of the onset of irreversible DW motion. These observations are consistant with a model which assumes that the DW are pinned at the film surfaces: the calculations give a negligible DW bulging contribution to the initial permeability, and predict the occurence of a DW unpinning field, thickness dependent as observed.

Magnetization of ferromagnetic amorphous alloys modified by a high-frequency magnetic field (abstract)

The critical magnetic field Hcr for the Barkhausen effect of ferromagnetic amorphous alloys can be modified and reduced by applying ac high-frequency magnetic fields. Both straight and toroidal specimens of amorphous ribbons Fe78B13Si9 (Allied 2605S2) and Fe40Ni38Mo4B18 (Allied 2826MB) were used in our experiment. The waveforms of ac magnetic fields were sine, triangular, or square waves, and the directions of the applied ac magnetic field were either parallel or perpendicular to the length direction of the specimen. For as-cast Fe78B13Si9, applying ac magnetic fields of 100 Hz–1 kHz and magnitudes 0.1–0.3 Oe, the critical magnetic field of the specimen could be reduced from 0.075 to 0.02 Oe under the same exciting field and magnetic induction. For as-cast Fe40Ni38Mo4B18, applying ac magnetic field of 100 Hz–1 kHz and magnitudes 0.1–0.3 Oe, could reduce the Hcr from 0.06 to ≤0.01 Oe under the same exciting field and magnetic induction. If the specimens were heat treated first by ac Joule heating and then applying the same ac magnetic fields when measuring the B-H curves, the critical magnetic fields could be reduced to lower than 0.001 Oe under the same exciting magnetic field and magnetic induction. By using a suitable frequency and magnitude of the ac magnetic field, we observed a magnetic behavior similar to superparamagnetism (i.e., coercive force Hc=0).

Temperature dependence of the critical magnetic field in polycrystalline ferrites

The critical magnetic field Hcr which separates the low-field reversible magnetization range from the irreversible, hysteretic part of the ferromagnetic B-H diagram is conveniently described by the Globus model, by attributing a reversible bowing mechanism of magnetic domain walls to the former range, and a depinning and displacement for the latter one. A simple calculation led Globus to expressions for Hcr and initial (reversible) magnetic permeability μ. In this paper, we show that the thermal dependence of the critical magnetic field can be given in terms of the saturation magnetization and the initial magnetic permeability. We use experimental results of saturation magnetization and initial permeability of NiFe2O4 and compare to experimental results of critical magnetic field as a funciton of temperature. A remarkable agreement is observed when we use a more exact approximation for the initial permeability expression than Globus’.

Spin reorientation transitions in DyFeO3induced by magnetic fields

Mossbauer spectrometry was used to study spin reorientation transitions in single crystals samples of DyFeO3. In agreement with magnetisation data of Belov (1974), a Morin transition was observed at 35+or-3K where the coupled Fe3+ ions reorient from a magnetic structure having an antiferromagnetic vector parallel to the crystal a axis and weak ferromagnetism along the crystal c axis ( Gamma 4) to a pure antiferromagnetic structure along the crystal b axis ( Gamma 1). Spin reorientations were induced with an external field. At 270, 130 and 77K fields applied parallel to the a axis caused continuous rotation of the Fe3+ antiferromagnetic vector from the a to the c axis ( Gamma 4 to Gamma 2) the rotation being complete as fields of 7.5, 6.7 and 5.5 T respectively. At 4.2K a field parallel to the b axis induced a transition Gamma 1 to Gamma 4 at a critical field Hcr=0.75+or-0.1 T. Reorientations were also observed at 4.2K in which the antiferromagnetic vector rotated in a plane perpendicular to the applied field: a field parallel to the c axis caused a rotation of the antiferromagnetic axis from b to a ( Gamma 1 to Gamma 4 with Hcr=0.7+or-0.1 T) and a field parallel to the a axis caused the antiferromagnetic axis to rotate from b to c ( Gamma 1 to Gamma 2 with Hcr=1.6+or-0.4 T). A model incorporating Dy3+-Fe3+ interactions is discussed to explain the reorientations.

Size of bloch wall and parameters of the magnetic susceptibility in ferrimagnetic spinels and garnets

In this paper we investigate the influence of the granular structure on the curve of magnetic susceptibility as a function of an alternating field, for different compositions of spinel and garnet materials. An ideal curve is defined which presents when H increases first a flat part below H = Hcr, then a part showing an almost linear increase of (μ'— 1)c and finally a peak of (μ' — 1)c. Five characteristic parameters of the curve are set: initial susceptibility, maximum susceptibility, critical field hcr, field Hm corresponding to the maximum value of (μ' — 1)c, slope of the curve beyond hcr. It is shown, for a given composition and measurement temperature, that all these parameters are related to the mean grain diameter of the sample. The differences between experimental and ideal curves are explained by means of a loss of the quality of the elementary crystallites. The role of the porosity is particularly investigated: the grain porosity acts strongly only on the initial susceptibility and becomes inoperative beyond hcr. The total porosity has no determining influence on the curves. The results are explained by extending a model we have already published, which assumes a wall in a spherical shaped grain. The surface of the wall is defined by its intersection with the grain boundary. For H < hcr, the wall is deformed by curvature being fixed along a great circle to the grain boundary. For H > hcr, the wall moves about inside the grain and reaches a smaller diameter d, determined by an equilibrium between the magnetic strength (proportional to d2) and the catching strength (proportional to d).