First-order Magnetization Process Research Articles

Investigation of the intrinsic magnetic properties of GdCo4B single crystal: determination of the magnetocrystalline anisotropy from the first-order magnetization processes

We report on the intrinsic magnetic properties of a GdCo4B single crystal as derived from magnetization measurements. The occurrence of a first-order magnetization process (FOMP) in a magnetic field applied perpendicularly to the easy magnetization direction provides an opportunity to determine the anisotropy parameters K1 and K2. To this end, the theoretical approach proposed previously for easy-plane magnets has been adapted for the case of easy-axis compounds exhibiting a FOMP. The obtained anisotropy parameters of GdCo4B are compared with the values deduced by other classical techniques. The presence of a compensation point in the thermal dependence of the spontaneous magnetization has enabled the determination of the exchange field on Gd, Bex = 131 T, which is in good agreement with inelastic neutron scattering results published earlier. Influence of the applied pressure on the anisotropy parameters is quantified using the pressure dependence of the FOMP, revealing a significant sensitivity of the anisotropy parameters to pressure.

Elastic response of U3Cu4Ge4 to spontaneous and field-induced phase transitions

${\mathrm{U}}_{3}{\mathrm{Cu}}_{4}{\mathrm{Ge}}_{4}$ is a uniaxial ferromagnet that displays a first-order magnetization process (FOMP) at 25 T for field applied along the hard $b$ axis. Here, we report on ultrasound and magnetostriction measurements of ${\mathrm{U}}_{3}{\mathrm{Cu}}_{4}{\mathrm{Ge}}_{4}$ in static and pulsed magnetic fields up to 40 T. The FOMP causes stepwise anomalies in the magnetoelastic properties. ${\mathrm{U}}_{3}{\mathrm{Cu}}_{4}{\mathrm{Ge}}_{4}$ elongates along the $a$ and $c$ axes and shrinks along the $b$ axis, leading to an almost zero volume effect. The sound velocities of the longitudinal and transverse acoustic waves decrease sharply at the FOMP, whereas the sound attenuation shows pronounced peaks. An analysis of the ultrasound data using a mean-field theory suggests the existence of quadrupolar interactions and crystal-electric-field effects. The $5f$ electronic states are between itinerant and localized, typical of uranium-based intermetallic compounds.

Statistical and analytical approaches to finite-temperature magnetic properties of the compound SmFe12

To investigate the magnetic properties of $\mathrm{Sm}{\mathrm{Fe}}_{12}$, we construct an effective spin model, where magnetic moments, crystal-field (CF) parameters, and exchange fields at 0 K are determined by first-principles calculations. Finite-temperature magnetic properties are investigated by using this model. We further develop an analytical method with strong mixing of states with a different quantum number of angular momentum $J$ ($J$-mixing), which is caused by a strong exchange field acting on the spin component of $4f$ electrons. Comparing our analytical results with those calculated by Boltzmann statistics, we clarify that the previous analytical studies for Sm transition-metal compounds overestimate the $J$-mixing effects. The present method enables us to perform a quantitative analysis of the temperature dependence of magnetic anisotropy (MA) with high reliability. The analytical method with model approximations reveals that the $J$-mixing caused by the exchange field increases the spin angular momentum, which enhances the absolute value of the orbital angular momentum and MA constants via spin-orbit interaction. It is also clarified that these $J$-mixing effects remain even above room temperature. Magnetization of $\mathrm{Sm}{\mathrm{Fe}}_{12}$ shows a peculiar field dependence known as the first-order magnetization process (FOMP), where the magnetization shows an abrupt change at a certain magnetic field. The result of the analysis shows that the origin of FOMP is attributed to competitive MA constants between positive ${K}_{1}$ and negative ${K}_{2}$. The sign of ${K}_{1(2)}$ appears due to an increase in the CF potential denoted by the parameter ${A}_{2}^{0}\ensuremath{\langle}{r}^{2}\ensuremath{\rangle}$ (${A}_{4}^{0}\ensuremath{\langle}{r}^{4}\ensuremath{\rangle}$) caused by hybridization between $3d$-electrons of Fe on the $8i$ ($8j$) site and $5d$ and $6p$ valence electrons on the Sm site. It is verified that the requirement for the appearance of FOMP is given as $\ensuremath{-}{K}_{2}<{K}_{1}<\ensuremath{-}6{K}_{2}$.

Magnetic Properties of the Nanocrystalline Nd-Ho-Fe-Co-B Alloy at Low Temperatures: The Influence of Time and Annealing

A study is made of the effects of various factors such as time (7 years), temperature, high magnetic field up to 580 kOe and heat treatment (HT) on the morphological structure and magnetic hysteresis properties of a high-coercive nanocrystalline (Nd0.55Ho0.45)2.7(Fe0.8Co0.2)14B1.2 alloy with a low temperature coefficient of remanence. We find a rather weak time effect on (Nd0.55Ho0.45)2.7(Fe0.8Co0.2)14B1.2. After 7 years, the loss in the maximum magnetic energy product (BH)max is no more than 5%. Annealing of the sample at 250 °C for 30 min decreases the amount of amorphous phase from 7.2 to 1.7%, while the grains’ size of the 2-14-1 phase increases from 83 to 109 nm. For the HT alloy, a magnetization jump is observed at H ~500 kOe. It can be attributed to the first-order magnetization process or a spin-flip magnetic transition. Rectangularity of the hysteresis loop degrades after annealing. In case of the short-time heat treatment, losses in (BH)max are ~10%.

Magnetic properties of the Tm2Fe17−xMnx single-crystals

Magnetization curves of the Tm2Fe17−xMnx (x=0, 0.2, 0.5, 1, 1.5) single crystals have been studied. The low-temperature easy-axis type magnetic anisotropy enhances, the critical field of the first-order magnetization process (FOMP) increases, and the value of magnetization anisotropy decreases in the Tm2Fe17−xMnx compounds with the increase in Mn content. The FOMP disappears in the compositions with x=1, 1.5. Compositional and temperature dependences of the magnetic anisotropy constants were calculated via analyzing the magnetization curves.

First-order magnetization process as a tool of magnetic-anisotropy determination: Application to the uranium-based intermetallicU3Cu4Ge4

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Synthesis and structural/physical properties of U3Fe2Ge7: a single-crystal study.

A single crystal of U3Fe2Ge7 was synthesized by the tin-flux method, and its structural and electronic properties were studied. The compound crystallizes in the orthorhombic crystal structure of La3Co2Sn7 type with two Wyckoff sites for the U atoms. U3Fe2Ge7 displays a ferromagnetic order below TC = 62 K. Magnetization measurements in static (up to 14 T) and pulsed (up to 60 T) magnetic fields revealed a strong two-ion uniaxial magnetic anisotropy. The easy magnetization direction is along the c axis and the spontaneous magnetic moment is 3.3 μB per formula unit at 2 K. The moment per Fe atom is 0.2 μB, as follows from Mössbauer spectroscopy. The magnetic moments are oriented perpendicular to the shortest inter-uranium distances that occur within the zigzag chains in the ab plane, contrary to other U-based isostructural compounds. The magnetization along the a axis reveals a first-order magnetization process that allows for a quantitative description of the magnetic anisotropy in spite of its enormous energetic strength. The strong anisotropy is reflected in the specific heat and electrical resistivity that are affected by a gap in magnon spectrum.

Micromagnetic analysis of spin-reorientation transitions. The role of magnetic domain structure

A method for calculating micromagnetic state of ferro- or ferrimagnetic single-crystals based on the Néel's method of phases is proposed. The standard Néel technique requires different approaches to calculation of micromagnetic state of samples with different anisotropy types. Furthermore, this technique cannot be used to calculate magnetization curves of materials with a complex anisotropy type, in which the first-order magnetization process (FOMP) occurs. On the contrary, the technique proposed in the present work makes it possible to calculate micromagnetic state of a sample within one unified approach. This technique has no limitations in terms of the anisotropy type as well. In case of the FOMP, the simulation methods that we used show results different from conventional calculation methods. The reason is that the conventional methods imply coherent rotation of magnetization in single domain particle (so-called Stoner–Wohlfarth model). We explain this discrepancy by the fact that a magnetic domain structure appears in the region of the FOMP. In the present work we show that magnetization processes do not occur in a jump under the FOMP but gradually pass though nucleation and new high-field phase growing, which substitutes for the low-field phase.

Magnetocrystalline anisotropy of Er2(Fe1 − x V x )17 compounds

The magnetic properties and magnetic anisotropy of the Er2(Fe1−xVx)17 compounds (with x = 0–0.05) have been studied. The Curie temperature (TC) of the compounds has been found to increase as the vanadium concentration increases. It has been shown that the Er2(Fe1 − xVx)17 compounds have the easy-plane anisotropy in the temperature range from, 77 K to TC. Magnetization curves have been measured and the temperature dependences of the K1 and K2 anisotropy constants have been calculated. Contributions from the Er and Fe magnetic sublattices to the magnetic anisotropy constants were distinguished. An analysis of the obtained data allowed us to conclude that the cause for the existence of first-order magnetization processes in Er2Fe17 at low temperatures is a positive contribution from the Er sublattice to K1 and a negative contribution from it to K2.

On the orientation of the magnetization vector and the associated entropy in Nd2Fe14B

Abstract We have used the methods of classical statistical mechanics in calculating the temperature and field-dependence of specific magnetic and magnetothermal properties of Nd2Fe14B. For example, we correlate the orientation of the magnetization vector, relative to a specific crystallographic axis, to the angular coordinates of the most probable location in a probability landscape of this anisotropic system. Further, we correlate the probability distribution to the associated magnetic entropy. The field dependence of the canting angle, either off the c-axis or off the basal plane, is also discussed in particular for critical magnetic fields at which first-order magnetization process takes place. Specific features of magnetization curves, at certain temperatures, are discussed in the light of results based on models involving crystal field and/or exchange interaction effects.

Phase transitions of anisotropic and exchange origins inTmFe5Al7

Magnetization and sound propagation reveal a number of unusual spontaneous and field-induced transformations in ferrimagnetic ${\mathrm{TmFe}}_{5}{\mathrm{Al}}_{7}$ (${T}_{\mathrm{C}}$ = 193 K). The rare-earth sublattice was found to provide a uniaxial magnetic anisotropy, whereas the iron sublattice favors an easy-plane anisotropy. A competition between them results in a first-order spin-reorientation transition at 64 K as the magnetic moments rotate from the $c$ axis to the basal plane of a tetragonal structure. The transition is preceded by a first-order magnetization process of type II along the hard axis. Remarkably, the intersublattice Tm-Fe exchange interaction is weakened at the spin-reorientation transition. Concomitantly, the spontaneous magnetic moment disappears, and the ferrimagnetic state changes to antiferromagnetic. With increasing temperature, the strength of the Tm-Fe exchange is recovered, and the ferrimagnetism is restored at 82 K through another first-order phase transformation. Below 40 K, a first-order field-induced transition occurs for a magnetic field applied along the easy [001] axis. It reflects a rotation of the magnetic moments towards the forced ferromagnetic state observed above 30 T. Along the hard [100] axis the ferromagnetic saturation is not reached even at 60 T.

Magnetostriction and thermal expansion of HoFe11−xCoxTi intermetallic compounds

The thermal expansion and magnetostriction of the HoFe11−xCoxTi (0≤x≤11) alloys have been investigated, using the strain gauge technique in the temperature range 77–600K. Both thermal expansion and thermal expansion coefficient exhibit an anomalous behaviour and Invar effect below Curie temperature (TC=516K) in sample with x=0. The increase of x in HoFe11−xCoxTi system leads to disappearing of the Invar effect and increasing of the average thermal expansion coefficient from 6.370×10−6K−1 in x=0 to 10.735×10−6K−1 in x=11 below room temperature. In addition, the spontaneous volumemagnetostriction decreases with Co content. The magnetostriction compensation point is observed in the anisotropic magnetostriction curve of all samples. The maximum value of anisotropic magnetostriction (Δλ≈50×10−6) at room temperature is observed in sample with x=9. The saturation behaviour only appears in samples with x=5, 7 and 9. In samples with x=3, 5 and 7, a noticeable volume magnetostriction (ΔV/V) is observed in low temperature due to first-order magnetisation process. Moreover, ΔV/V exhibits a large anomaly about 45×10−6 and 20×10−6 around room temperature related to spin reorientation transition in samples with x=9 and 11, respectively. The results are discussed based on the local magnetic moment model and irreducible magnetoelastic coupling modes.

Effect of Mn substitution on the cation distribution and temperature dependence of magnetic anisotropy constant in Co1−xMnxFe2O4 (0.0≤x≤0.4) ferrites

Abstract The effect of Mn substitution on temperature dependent magnetic properties of Mn substituted cobalt ferrite, i.e., Co1−xMnxFe2O4 (x=0.0–0.4), prepared by a ceramic method has been investigated. X-ray diffraction (XRD) analysis reveals that all samples posses a single phase cubic spinel structure. The lattice constant determined from XRD increases with Mn substitution whereas the bulk density of the samples decreases. Mossbauer results reveal that Co, Fe and Mn ions are distributed over the tetrahedral (A) and octahedral (B) sites for the prepared samples. Hysteresis loops yield a saturation magnetization (Ms) and coercive field (Hc) that vary significantly with temperature and Mn content (x). The temperature dependence of the magnetization obtained for μoH=5 T presents a maximum at 175 K which is also dependent on the value of x. The high field regimes of the hysteresis loops are modeled using the Law of Approach to Saturation (LAS) to determine the first-order cubic anisotropy coefficient (K1). It has been found that the anisotropy of these materials increases significantly with decreasing temperature. However, below 175 K, the shape of the anisotropy energy function changes significantly causing a first-order magnetization process (FOMP) at higher fields, which also prevents the magnetization to saturate even under a maximum applied field of 5 T. In general, the anisotropy coefficient decreases with increasing Mn substitution at a given temperature, which could be explained in terms of the site occupancy of the Mn2+ substituent in the cubic spinel lattice.

Structural and Magnetic Properties of Ho&lt;sub&gt;3&lt;/sub&gt;Fe&lt;sub&gt;29-x&lt;/sub&gt;&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;x&lt;/sub&gt; (&lt;i&gt;T&lt;/i&gt;=V and Cr) Compounds

The crystallographic structural characteristics and magnetic properties of Ho3Fe29-xTx (T=V and Cr) compounds have been investigated by using Rietveld refinement analysis of X-ray diffraction (XRD) pattern and magnetic measurements. The calculated results indicate that among the 11 different kinds of Fe sites in these Ho-Fe compounds the preferential sites of the stabilizing elements V and Cr are quite different. The refined lattice parameters of these compounds are in good agreement with the experimental data. Spin reorientations of easy magnetization direction (EMD) are observed at around 150 K for Ho3Fe27V2 and Ho3Fe25.5Cr3.5. At the around 1.7 T critical fields (HCR) first order magnetization process (FOMP) occurs in magnetization curves at 4.2 K for the magnetically aligned samples of Ho3Fe27V2 and Ho3Fe25.5Cr3.5.

Field-induced magnetic transition in cobalt-ferrite

We present magnetostriction and magnetization measurements of a cobalt ferrite (Co0.8Fe2.2O4) single crystal. We observe unusual behavior in the magnetic hard axis of the single crystal which manifests in a jump of the magnetization curve at a critical field. This first order magnetization process (FOMP), which is explained as an anisotropy-driven transition, is visible at temperatures lower than 150 K. By analyzing the anisotropy constants, we found that the higher order anisotropy constant K2 dominates the anisotropy energy. In the magnetostriction measurements, the FOMP is clearly visible as a huge jump in the [111] direction, which can be explained by means of a geometric model.

Magnetotransport in Tb2Fe17 single crystals

We have performed measurements of the Hall effect, electrical resistivity, and magnetization on Tb2Fe17 single crystals in the 5 to 300 K temperature range, and in magnetic fields of up to 5 T. We find that the anomalous Hall effect of this ferromagnet depends strongly on the magnetization direction relative to the crystal axes. The AHE resistivity, measured with an applied magnetic field H perpendicular to the c-axis, is very large and varies linearly with the longitudinal resistivity ρ, whereas the AHE resistivity for H along the hard magnetization direction is much smaller and increases as ρ2. For the latter configuration, the electrical resistivity shows a sharp decrease at a field-induced first-order magnetization process (FOMP) which is observed in H ∼ 2.7 T up to a temperature of 250 K.

First-order Field-Induced Magnetization Processes and Magnetostriction in Tb2Co12Fe5

We report the results of magnetization and magnetostriction measurements performed by pulsed-magnetic-field techniques on the intermetallic compound Tb2Co12Fe5. The temperature dependence of the critical field at which the first-order magnetization process takes place was measured by a singular point detection technique on aligned powders. Magnetostriction measurements were performed by a strain-gauge-based technique at different temperatures. The obtained results are compared with observations recently made for the closely related compound Pr2Co12Fe5, allowing to clarify the crucial role of both the rare-earth and the transition-metal sublattices in the peculiar behaviour of the latter.

Determining anisotropy constants from a first-order magnetization process inTb2Fe17

The critical parameters of a first-order magnetization process, ${H}_{\mathrm{cr}}$ and ${m}_{\mathrm{cr}}$, are used to determine the anisotropy constants ${K}_{1}$ and ${K}_{2}$ of ${\mathrm{Tb}}_{2}{\mathrm{Fe}}_{17}$ in the temperature range between 5 and $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Both anisotropy constants are proportional to the critical field ${H}_{\mathrm{cr}}$ (times spontaneous magnetization), whereas their ratio is independent of ${H}_{\mathrm{cr}}$ and depends solely on the dimensionless reduced critical magnetization ${m}_{\mathrm{cr}}$. The employed method of determining ${K}_{1}$ and ${K}_{2}$ is complementary to the classical Sucksmith--Thompson technique, which is inapplicable in the case of a first-order magnetization process.

On the pressure and temperature dependence of the magnetization and anisotropy behavior ofYCo4B

The YCo4B exhibits peculiar behavior of the magnetization curves related to the occurrence of a spinreorientation phenomenon at low temperature and of a first-order magnetization process.We present here a detailed investigation of the pressure dependence of this anomaly in awide temperature range 5–300 K. The evolution of the saturation magnetizationversus pressure is also presented on both sides of the spin reorientation transition.Studies were performed on single-crystalline samples under hydrostatic pressureup to 1 GPa pressure and in a magnetic field up to 5 T. Remarkable pressureeffects on the magnetocrystalline anisotropy parameter and on the critical field atwhich the first-order magnetization process occurs are analyzed and discussed.

Spin reorientation and first-order magnetisation process in HoFe 11−xCo xTi compounds

The influence of Co substitution for Fe on the lattice parameters, ordering temperature, saturation magnetisation, spin-reorientation temperature, and first-order magnetisation process (FOMP) of HoFe 11− x Co x Ti ( x=0.3, 0.7, 1, 3, 5, 7, 9, 11) compounds is investigated. The tetragonal crystal structure of the parent HoFe 11Ti does not change upon Co substitution, but the lattice parameters a and c decrease monotonically with the increase of Co content. The increase of x in HoFe 11− x Co x Ti compounds has a beneficial effect in improving the Curie temperature from 513 K in HoFe 11Ti to 983 K in HoCo 11Ti compound. The highest saturation magnetisation was observed in the compound with x=3 at all measured temperatures. In HoFe 11− x Co xTi compounds, with x=0, 1, 3, 5, 7 a FOMP of type II occurs at fields less than 2 T and below 100 K. In HoFe 2Co 9Ti and HoCo 11Ti compounds spin reorientations are observed at T SR=260 and 273 K, respectively.