Novel magnetization processes in ferrimagnetic multilayers with random anisotropy

We carried out a numerical study of magnetization processes in ferrimagnetic multilayers in the presence of pinning forces acting on one of the magnetic components. The multilayers consist of an alternation of two ferromagnetic materials (M and T) which are assumed antiferromagnetically coupled through the interfaces (as in Fe/Tb multilayers for instance). A random anisotropy field acts on the magnetization of material T. The reversal of the magnetization of the multilayers when the applied field is swept from positive saturation to negative saturation is characterized by a competition between coherent rotation and nucleation-propagation mechanisms. In this competition the rotational hysteresis induced by the random anisotropy on the magnetization of material T plays a crucial role. Very original features have been observed in the magnetization processes of these systems such as crossed hysteresis loops with a negative remanent magnetization. An overview of these magnetization processes below and above the compensation temperature is given in this paper.     

Magnetically induced phase transitions in LaCoO<Subscript>3</Subscript> in fields of up to 500 T

The differential magnetization of LaCoO₃ in magnetic fields of up to 500 T has been measured at a temperature of 4.2 K. The magnetization curve reveals several features which suggest a complex pattern of the transition of LaCoO₃ from the low-spin state to the high-spin state. The magnetic moment starts to grow in fields above 50 T to reach a plateau in the 130–240-T region, after which the magnetic moment continues to rise up to saturation in fields ∼500 T.     

Magnetization of single-crystal terbium at very low temperatures by nuclear orientation

We report an investigation of the magnetization of high-purity single crystal terbium below 50 mK, using the nuclear orientation of¹⁶⁰Tb, which substitutionally replaces naturally-occurring, monoisotopic¹⁵⁹Tb. The nuclear alignment is determined as a function of applied magnetic field, and can be related to the macroscopic magnetization of the host crystal by a suitable model. In the easy direction (b-axis), a very rapid rise is seen in the first 7 mT of applied field; this is followed by a pleateau region up to 0.1 T, then a slow saturation, completed at ca. 0.4 T. Along an a-axis, a similar rapid increase to about 50% of saturation is observed below 10 mT, followed by a slow, nearly linear increase which agrees with that calculated for domain rotation using the measured crystalline anisotropy constants.     

Investigation of the pressure and temperature dependence of the intrinsic magnetic properties of YCo4Ge

The YCo₄Ge intrinsic magnetic properties; such as the saturation magnetization and the magnetocrystalline anisotropy parameters; are determined at 5, 100, 200 and 300 K from a fit of the isothermal magnetization curves measured in applied field up to 10 T. Ge for Co substitution induces a reduction of the first order anisotropy parameter as well as of the uniaxial anisotropy field. At 4 K an anisotropy field of 5.8 T is obtained, whereas a value of that is observed at room temperature. The magnetocrystalline anisotropy found for YCo₄Ge is significantly reduced upon heating from 4 to 300 K. The investigation of the magnetic properties of YCo₄Ge in both the ordered and the paramagnetic phases shows that this YCo₄Ge compound exhibits a typical itinerant ferromagnetic behavior. The effect of applied pressure on the isothermal magnetization has been investigated showing that the volume has an important influence on the magnetization of YCo₄Ge. The influence of the Ge for Co substitution on the Co-Co exchange interaction is also discussed. The results are compared and commented on in the light of earlier reports for YCo₅ and other Co containing phases.     

Superconducting spin valve effect of a v layer coupled to an antiferromagnetic [Fe/V] superlattice

We studied superconducting V layers deposited on an antiferromagnetically coupled [Fe(2)V(11)](20) superlattice. The parallel upper critical magnetic field exhibits an anomalous T dependence up to the ferromagnetic saturation field of the superlattice, indicating that the superconducting transition temperature T(S) decreases when rotating the relative sublattice magnetization directions of the superlattice from antiparallel to parallel. This proves that the pair breaking effect of a Fe2 layer is reduced if at a distance of 1.5 nm a second Fe2 layer with antiparallel spin orientation exists.     

脉冲磁化条件下非晶磁芯的损耗特性

基于工频或高频磁化条件下磁芯的测试数据不能准确反映磁芯在单次脉冲磁化下的性能,给出了一种脉冲磁化条件下磁芯性能的测试方法和数据处理方法,实验研究了快脉冲磁化条件下非晶态合金磁芯的损耗特性,磁芯最短饱和时间67 ns,最大磁化速率达到40 T/s。通过数据处理,给出了磁芯损耗与磁化速率的关系曲线,获得了不同磁化速率下磁芯的损耗数据。分析了脉冲磁化条件下涡流损耗和磁滞损耗所占的比例。研究结果表明:脉冲磁化条件下非晶态合金磁芯损耗与磁化速率关系符合饱和波模型,磁芯损耗随磁化速率增大而线性增大。     

Commensurate spin density wave in LaFeAsO: a local probe study

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1-xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57Fe M?ssbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5)muB at the iron site below T(N)=138 K, well separated from a structural phase transition at T(S)=156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below T(N) are reproduced.     

Anomalous magnetization of CsFeCl3 appearing at the applied magnetic fields exceeding 33 T

The magnetization of a singlet-ground-state magnet CsFeCl₃ at 4.2 and 1.3 K is measured by the pulsed magnetic field up to 40 T applied parallel to the crystal c-axis. In both temperatures, a large linear increase in the magnetization appears between 4 and 12 T, which is consistent with the magnetization observed using the static magnetic fields. Above 12 T, the magnetization increases gradually up to 33 T. A metamagnetic transition appears around 34 T suggesting a new magnetic structure. The possible models are discussed.     

Variation of the saturation magnetization through a neon implanted YIG film

The saturation magnetization of a YIG film implanted with 5*10¹⁴ neon ions/cm² at an energy of 450 keV was studied. By removing very thin layers from the ion implanted part of the film the magnetization was found to change. As a result of the analysis of these changes in the saturation magnetization it was possible to establish a profile for the magnetization as a function of depth through the ion implanted part of the film. The profile is asymmetric and shows a decrease in the magnetization of up to 35 % of the initial value at a depth coinciding with the depth of maximum strain.     

Magnetostriction of Fe–Sn polycrystalline alloys

In the present work we study the magnetostriction of Fe₉₁Sn₉ and Fe₈₀Sn₂₀ polycrystalline samples produced by arc melting and heat treated at temperatures of 1153 K for 6 h and 1023 K for 24 h, looking for high values of magnetostriction as in Fe–Ga alloys. Magnetostriction, as well as saturation magnetization measurements, was carried out at temperatures close to 203 K in the magnetic field interval 0 to 1.5 T. Results of magnetostriction on sample Fe₉₁Sn₉, which has almost pure -phase, show magnitude and behavior similar to pure Fe. The two additional Fe₈₀Sn₂₀ samples have a combination of -phase plus either Fe₅Sn₃ or Fe₃Sn₂ and show a peculiar behavior of the magnetostriction. For μ₀H<0.3 T the magnetostriction grows from zero to saturation of the -phase. Following, for μ₀H>0.3 T, the magnetostriction starts again to grow linearly with the field, but saturation was not observed up to 5 T. This behavior was attributed to the presence of Fe₅Sn₃ or Fe₃Sn₂ phases in these samples that are also ferromagnetic as the -phase is.     

Magnetization enhancement at low temperature due to surface ordering in Fe–Ni–B amorphous nanoparticles

We report magnetization measurements up to 5 T as a function of temperature from 5 to 300 K in an amorphous Fe–Ni–B system nanoparticle (average diameter 3 nm). These nanoparticles were dispersed in a non-magnetic polymer matrix to minimize interparticle interactions and the results were contrasted with a concentrated powder sample. The saturation magnetization shows a sudden increase below 40 K. This behavior is observed in both the dispersed and the concentrated systems, being noticeably more pronounced in the first. This result can be explained considering that the large magnetic contribution at low T is due to the magnetic ordering of the particle surface.     

Magnetic properties of Y2Fe15Cr2 single crystal

A Y₂Fe₁₅Cr₂ single crystal with the Th₂Ni₁₇-type structure has been prepared by the Czochralski method and investigated by means of Laue back-reflection, metallographic observation, X-ray diffraction, the singular point detection technique and magnetic measurements. A magnetohistory effect has been observed at a low temperature. Magnetization curves have been measured along the easy and hard directions in fields up to 6.5 T. The saturation magnetization and magnetocrystalline anisotropy field decrease with increasing temperature. The experimental magnetocrystalline anisotropy constant is in good agreement with the calculation results on first approximation.     

Magnetic order of FeCrP

⁵⁷Fe Mössbauer spectroscopic studies of the orthorhombic compound FeCrP, complemented with magnetization measurements, have been undertaken. The results indicate that FeCrP is an antiferromagnet with a Néel temperature of 265 K. The saturation magnetic hyperfine field present at the Fe nuclei is found to be only 1.0 T. Possible antiferromagnetic structures are discussed as well as different contributions to the magnetic interaction.     

Synthesis of multiwall carbon nanotubes by chemical vapor deposition of ferrocene alone

Multiwall carbon nanotubes (MWNTs) filled with Fe nanoparticles (NPs) have been synthesized by thermal chemical vapor deposition of ferrocene alone as the precursor. The MWNTs were grown at different temperatures: 980 and 800 ^°C. Characterization of as-prepared MWNTs was done by scanning and transmission electron microscopy, and X-ray diffraction. The transmission electron microscopy study revealed that Fe NPs encapsulated in MWNTs grown at 980 and 800 ^°C are spherical and rod shaped, respectively. Room-temperature vibrating sample magnetometer studies were done on the two samples up to a field of 1 T. The magnetization versus magnetic field loop reveals that the saturation magnetization for the two samples varies considerably, almost by a factor of 4.6. This indicates that Fe is present in different amounts in the MWNTs grown at the two different temperatures.     

Effects of minor Cu addition on glass-forming ability and magnetic properties of FePCBCu alloys with high saturation magnetization

The effects of minor addition of Cu on glass-forming ability (GFA), thermal stability and magnetic properties of (Fe_0.8P_0.09C_0.09B_0.02)_{100? x} Cu _x (x?=?0–1) alloys were investigated. The introduction of Cu slightly increases the GFA and efficiently improves the primary crystallization of α-Fe nanoparticles. The alloy with 0.3% Cu addition shows the best GFA and the fully glassy rods can be produced up to 2?mm in diameter. The saturation magnetization of the glassy sample with 0.7% Cu addition can be enhanced from 1.44 to 1.60?T after proper heat treatment due to the precipitation of α-Fe nanoparticles in the glassy matrix. The combination of large GFA and high saturation magnetization as well as low cost makes the FePCBCu alloys as a kind of promising soft magnetic materials for industrial applications.     

强磁场下NH4Cl水溶液结构的拉曼光谱研究

迄今为止,国内外存在很多关于强磁场处理对水溶液结构影响的争议,并且关于强磁场对无机盐水溶液结构的研究也相对较少。运用拉曼光谱法,测定了在外加强磁场1.8 T强度下,高纯水与质量分数分别为1%,5%,10%,20%和28%的NH₄Cl水溶液在不同磁化时间下的拉曼散射数据,为拉曼光谱法探究强磁场对水溶液结构的影响提供了一个新的可行方式,丰富了拉曼光谱的研究领域。由实验结果可知,随着磁化时间的增加,水分子中氢键的伸缩振动峰值逐渐升高,经过一定的磁化时间后可以达到饱和。高纯水与不同质量分数的NH₄Cl水溶液的饱和效应时间均不同。高纯水与质量分数为1%,5%,10%,20%和28%的NH₄Cl水溶液峰值饱和时间分别为150,120,120,100,80和80 min。随着NH₄Cl水溶液质量分数的增加,达到磁效应饱和的时间呈现减少的趋势。磁场移除后,测定高纯水与不同质量分数NH₄Cl水溶液的去磁记忆时间。高纯水与质量分数为1%,5%,10%,20%和28%的NH₄Cl水溶液的去磁记忆时间分别为30,40,50,60,80和80 min。随着NH₄Cl水溶液质量分数的增加,去磁记忆时间呈现增加的趋势。利用去卷积拟合的方法对磁化2 h后不同质量分数的NH₄Cl水溶液进行处理。由去卷积拟合结果可知,质量分数为20%的NH₄Cl水溶液比质量分数为10%的NH₄Cl水溶液增加了一个N-H峰,该信号峰随着NH₄Cl水溶液质量分数的增大逐渐增强。DDAA型氢键结构整体上随磁化时间增加而减少,磁化时间的增加对四面体水结构具有破坏作用。当达到饱和磁化时间后,DDAA型氢键不再发生变化。实验结果表明,通过拉曼光谱法可以得到1.8 T强磁场对NH₄Cl水溶液结构的影响规律,为外加强磁场条件下其他无机盐水溶液的研究提供了一定的理论基础。     

Simple recipe to synthesize single-domain BaFe12O19 with high saturation magnetization

We report a new synthesis route for preparation of single-domain barium hexaferrite (BaFe₁₂O₁₉) particles with high saturation magnetization. Nitric acid, known as a good oxidizer, is used as a mixing medium during the synthesis. It is shown that formation of BaFe₁₂O₁₉ phase starts at 800 °C, which is considerably lower than the typical ceramic process and develops with increasing temperature. Both magnetization measurements and scanning electron microscope micrographs reveal that the particles are single domain up to 1000 °C at which the highest coercive field of 3.6 kOe was obtained. The best saturation magnetization of ≈60 emu/g at 1.5 T was achieved by sintering for 2 h at 1200 °C. Annealing at temperatures higher than 1000 °C increased the saturation magnetization, on the other hand, decreased the coercive field which was due to the formation of multi-domain particles with larger grain sizes. It is shown that the best sintering to obtain fine particles of BaFe₁₂O₁₉ occurs at temperatures 900-1000 °C. Finally, magnetic interactions between the hard BaFe₁₂O₁₉ phase and impurity phases were investigated using the Stoner-Wohlfarth model.     

Analysis of longitudinal relaxation rate constants from magnetization transfer MR images of human tissues at 0.1 T.

The human calf muscle was examined by using the magnetization transfer MR imaging technique. The time-dependent saturation transfer (TDST) method was applied at low magnetic field 0.1 T in order to measure the mobile water relaxation time T1w, the magnetization transfer rate Rwm from water to solid macromolecules, and the magnetization transfer contrast (MTC) of the human tissue. The magnetization transfer contrast of 0.67 was attained. The transfer rate Rwm was 4.5 sec-1 (+/- 0.3 sec-1) for the anterior tibial muscle and 5.0 sec-1 (+/- 0.4 sec-1) for the gastrocnemius muscles. The values of Rwm are considerably larger than the values of corresponding relaxation rates measured at high fields. The relaxation rate measurements of human tissues in vivo was shown to be possible at 0.1 T even within the framework of normal routine MR imaging. Magnetization transfer MR imaging is a very promising and practical method in order to assess the relaxation processes in heterogeneous human tissues in vivo, and it can improve the tissue characterization possibilities of MR imaging techniques.     

Mössbauer study of very dilute Pd(Fe) alloys at very low temperatures

The magnetic behaviour of very dilute (7 ppm and 0.6 ppm) Pd(Fe) alloys has been studied by Mössbauer spectroscopy in external fields up to 6 T in the temperature range between 0.018 and 300 K. No magnetic ordering was observed even at the lowest temperature. Above 0.067 K the magnetization exhibits free-spin behaviour. The saturation moment was found to be independent of temperature. Magnetic hyperfine patterns observed below 0.067 K were interpreted in terms of paramagnetic hyperfine interactions in the regime of slow electronic relaxation.