Ac Magnetic Measurements Research Articles

Reentrant cluster glass and stability of ferromagnetism in the Ga2MnCo Heusler alloy

We present here a detailed investigation into the magnetic ordering of full Heusler alloy Ga$_2$MnCo using dc, ac magnetization measurements, neutron diffraction and neutron depolarization experiments. Crystal structure at room temperature was first confirmed to be L2$_1$ using the highly intense synchrotron X-ray diffraction (XRD) technique. Temperature dependent magnetization reveals that Ga$_2$MnCo enters a ferromagnetic (FM) state at $T_C = $154 K, characterized by a sharp increase in magnetization and a plateau-like region hereafter. As the temperature is decreased further, a sharp drop in magnetization is observed at $T_f$ = 50 K, hinting towards an antiferromagnetic (AFM) phase change. Neutron diffraction (ND) recorded over the range of temperature from 6 to 300 K, provides combined information regarding crystal as well as magnetic structure. Accordingly, an increase in the intensity of the ND pattern is seen at 150 K, signaling onset of long range FM order. However, there is no sign of appearance of superlattice reflections corresponding to the AFM phase, in the patterns recorded below 50 K. An unusual discontinuity in the unit cell volume is seen around $T_f$ indicating a coupling of this second transition with the contraction of the lattice. Attempts to unravel this interesting magnetic behaviour using ac susceptibility measurements lead to the existence of glassy magnetism below $T_f$. Systematic analysis of the susceptibility results along with neutron depolarization measurement, identifies the low temperature phase as a reentrant cluster glass.

Field-Driven Quantum Criticality in the Spinel Magnet ZnCr_{2}Se_{4}.

We report detailed dc and ac magnetic susceptibilities, specific heat, and thermal conductivity measurements on the frustrated magnet ZnCr_{2}Se_{4}. At low temperatures, with an increasing magnetic field, this spinel material goes through a series of spin state transitions from the helix spin state to the spiral spin state and then to the fully polarized state. Our results indicate a direct quantum phase transition from the spiral spin state to the fully polarized state. As the system approaches the quantum criticality, we find strong quantum fluctuations of the spins with behaviors such as an unconventional T^{2}-dependent specific heat and temperature-independent mean free path for the thermal transport. We complete the full phase diagram of ZnCr_{2}Se_{4} under the external magnetic field and propose the possibility of frustrated quantum criticality with extended densities of critical modes to account for the unusual low-energy excitations in the vicinity of the criticality. Our results reveal that ZnCr_{2}Se_{4} is a rare example of a 3D magnet exhibiting a field-driven quantum criticality with unconventional properties.

The Exploration and Analysis of the Magnetic Relaxation Behavior in Three Isostructural Cyano-Bridged 3d–4f Linear Heterotrinuclear Compounds

Three isostructural cyano-bridged 3d–4f linear heterotrinuclear compounds, (H2.5O)4{Ln[TM(CN)5(CNH0.5)]2(HMPA)4} (Ln = YIII, TM = [FeIII]LS (1); Ln = DyIII, TM = [FeIII]LS (2); Ln = DyIII, TM = CoIII (3)), have been synthesized and characterized by single-crystal X-ray diffraction. Due to the steric effect of the HMPA ligands, the central lanthanide ions in these compounds possess a low coordination number, six-coordinate, exhibiting a coordination geometry of an axially elongated octahedron with a perfect D4h symmetry. Four HMPA ligands situate in the equatorial plane around the central lanthanide ions, and two [TM(CN)5(CNH0.5)]2.5− entities occupy the apical positions to form a cyano-bridged 3d–4f linear heterotrinuclear structure. The static magnetic analysis of the three compounds indicated a paramagnetic behavior of compounds 1 and 3, and possible small magnetic interactions between the intramolecular DyIII and [FeIII]LS ions in compound 2. Under zero dc field, the ac magnetic measurements on 2 and 3 revealed the in-phase component (χ′) of the ac susceptibility without frequency dependence and silent out-of-phase component (χ″), which was attributed to the QTM effect induced by the coordination geometry of an axially elongated octahedron for the DyIII ion. Even under a 1 kOe applied dc field, the χ″ components of 2 were revealed frequency dependence without peaks above 2 K. And under a 2 kOe and 3 kOe dc field, the χ″ components of 3 exhibited weak frequency dependence below 4 K with the absence of well-shaped peaks, which confirmed the poor single-ion magnetic relaxation behavior of the six-coordinate DyIII ion excluding any influence from the neighboring [FeIII]LS ions as that in the analogue 2.

The magnetic properties of BaCo0.5Ni0.5F4

The family of BaMF4 with M of magnetic 3d transition metal ions is the typical multiferroic material. Pure phase solid solution of BaCoF4 and BaNiF4 with molar ratio of 1:1 (BaCo0.5Ni0.5F4) is prepared by solid state reaction, which has been confirmed by X ray diffraction patterns. Field dependent magnetization measurements only show the linear curve with temperature down to 5 K, indicating the antiferromagnetic nature. Compared with BaCoF4 and BaNiF4, no significant enhancement of magnetization is observed, indicating the absence of ferrimagnetism and the random distribution of Co and Ni ions. The low temperature magnetic anomalies are studied by zero field cooled (ZFC) and field cooled (FC) temperature dependent magnetization (M-T) measurements. A bifurcation between FC and ZFC M-T curves happens at 118 K, indicating the onset of 2-dimensional antiferromagnetism. The magnetization maximum at 87 K is attributed to the 2-dimensional antiferromagnetic clusters, followed by the drastic decrease of magnetization, which is due to the onset of 3-dimensional antiferromagnetism. A dip is observed in FC M-T curve at 40 K, which is attributed to the 3-dimensional antiferromagnetic clusters. A drastic increase of magnetization is observed at 9 K, which is due to the uncompensated isolated spins. Exchange bias is clearly observed, with blocking temperature of 90 K. The contribution from surface spin glass has been excluded by the AC magnetization measurements, and the mechanism has been explained by the exchange coupling between the two antiferromagnetic phases.

Nature of magnetic properties in R3Co1.87In4 where R = Ho, Er and Tm

ABSTRACTCrystal structure and magnetic properties of the R3Co1.87In4 compounds (R = Ho, Er, Tm) have been investigated by means of X-ray diffraction as well as dc and ac magnetic measurements. The compounds crystallize in the hexagonal Lu3Co1.87In4-type structure which is closely related to the ZrNiAl-type. Magnetic data indicate different properties at low temperatures, namely: ferromagnetism below about 10 K followed by a like spin glass behavior below 5.8 K in R = Ho, antiferromagnetic order with ferromagnetic component below 5.2 K (R = Er) and pure antiferromagnetism below 2.8 K (R = Tm).

AC magnetic field measurement using a small flip coil system for rapid cycling AC magnets at the China Spallation Neutron Source (CSNS)

The China Spallation Neutron Source (CSNS) has two major accelerator systems, a linear accelerator and a rapid cycling synchrotron (RCS). The RCS accelerator is used to accumulate and accelerate protons from the energy of 80 MeV to the design energy of 1.6 GeV at the repetition rate of 25 Hz, and extract the high energy beam to the target. The main magnets of the RCS accelerator are excited by AC current with DC bias. The magnetic field quality is very important for the RCS accelerator operation, since it should guarantee and focus a circulating beam. In order to characterize the AC magnets, a small flip coil measurement system has been developed and one of each type of AC magnets has been studied. The measurement system and selected measurement results are presented in this paper.

Magnetic Characteristics of CuCr2S4Nanospinels Obtained by Mechanical Alloying and Heat Treatment

Chalcogenide bulk spinels with general formula CuCr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> X <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> (X = S, Se, and Te) are ferromagnetic and p-type metallic materials with the thermoelectric figure of merit of 0.15. They can be used to dope or alloy with related semiconducting spinels. Therefore, it is expected that their nanosized crystallites display also unique properties and new potential applications. This paper presents the results of dc and ac magnetic measurements, including the higher harmonics of ac magnetic susceptibility as well as electrical conductivity and thermoelectric power of the CuCr2S4 nanospinels. These studies showed that decreasing the size of crystallites to nanometer scale leads to a dramatic change in their physical properties.

Design of PCB search coils for AC magnetic flux density measurement

This paper presents single-layer, double-layer and ten-layer planar square search coils designed for AC magnetic flux density amplitude measurement up to 1 T in the low frequency range in a 10 mm air gap. The printed-circuit-board (PCB) method was used for producing the search coils. Special attention is given to a full characterization of the PCB search coils including a comparison between the detailed analytical design method and the finite integration technique method (FIT) on the one hand, and experimental results on the other. The results show very good agreement in the resistance, inductance and search coil constant values (the area turns) and also in the frequency dependence of the search coil constant.

The Effects of Crystallinity on Charge Transport and the Structure of Sequentially Processed F4TCNQ‐Doped Conjugated Polymer Films

AbstractThe properties of molecularly doped films of conjugated polymers are explored as the crystallinity of the polymer is systematically varied. Solution sequential processing (SqP) was used to introduce 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F4TCNQ) into poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) while preserving the pristine polymer's degree of crystallinity. X‐ray data suggest that F4TCNQ anions reside primarily in the amorphous regions of the film as well as in the P3HT lamellae between the side chains, but do not π‐stack within the polymer crystallites. Optical spectroscopy shows that the polaron absorption redshifts with increasing polymer crystallinity and increases in cross section. Theoretical modeling suggests that the polaron spectrum is inhomogeneously broadened by the presence of the anions, which reside on average 6–8 Å from the polymer backbone. Electrical measurements show that the conductivity of P3HT films doped by F4TCNQ via SqP can be improved by increasing the polymer crystallinity. AC magnetic field Hall measurements show that the increased conductivity results from improved mobility of the carriers with increasing crystallinity, reaching over 0.1 cm2 V−1 s−1 in the most crystalline P3HT samples. Temperature‐dependent conductivity measurements show that polaron mobility in SqP‐doped P3HT is still dominated by hopping transport, but that more crystalline samples are on the edge of a transition to diffusive transport at room temperature.

Cobalt(II) Ions Connecting [CoII4] Helicates into a 2-D Coordination Polymer Showing Slow Relaxation of the Magnetization.

The reactions of cobalt(II) perchlorate with a diazine tetratopic helicand, H4L, in the presence of sodium carbonate afford two coordination polymers constructed from tetranuclear anionic helicates as building blocks: ∞3[Co4L3Na4(H2O)4]·4H2O (1) and ∞2[Co5L3Na2(H2O)9]·2.7H2O·DMF (2). The tetranuclear triple-stranded helicates, {CoII4L3}4-, are connected in 1 by sodium(I) ions and in 2 by sodium(I) and cobalt(II) ions (H4L results from the condensation reaction between 3-formylsalicylic acid and hydrazine). The crystal structures of the two compounds have been solved. In both compounds the anionic helicates interact with the assembling cations through the carboxylato oxygen atoms. Compound 2 features chains resulting from connecting the tetranuclear helicates through cobalt(II) ions. The analysis of the magnetic properties of compounds 1 and 2 evidenced a dominant antiferromagnetic coupling for 1, resulting in a diamagnetic ground state. In contrast, the magnetic behavior of 2 is dominated at low temperature by the CoII ion which connects the antiferromagnetically coupled {CoII4} helical moieties. The ac magnetic measurements for 2 reveal the occurrence of slow relaxation of the magnetization that is due to the single, uncorrelated cobalt(II) ions, which are diluted in an essentially diamagnetic matrix of {CoII4} moieties (ΔEeff = 26.7 ± 0.3 cm-1 with τ0 = (2.3 ± 0.2) × 10-6 s).

Magnetic phase diagram of Co(Cr1−xAlx)2O4 (x = 0.0–1.0)

We report the role of Al substitution in the magnetic properties of spinel CoCr2O4 by means of temperature dependent dc and ac magnetization and heat capacity measurements. Various compositions (0.0 ≤ x ≤ 1.0) of polycrystalline Co(Cr1-xAlx)2O4 samples have been prepared by sol-gel processing and their crystal structure was investigated by X-ray diffraction which was found to crystallize in the normal cubic spinel structure. For x ≤ 0.1, the system exhibits multiple magnetic orderings (long range ferrimagnetic ordering TC, spin-spiral ordering TS, and lock-in transition TL), similar to that of the parent compound, CoCr2O4. However, all the compositions between x = 0.1 and 0.5 exhibit long range ferrimagnetic ordering below TC and also a short range order at low temperature. Spin-glass like ordering was noticed between x = 0.6 and 0.8 due to the diluted B-site occupancy, whereas the end compound CoAl2O4 (x = 1.0) shows antiferromagnetic behavior. On the basis of these results, we propose a magnetic phase diagram for the Co(Cr1-xAlx)2O4 series as a function of the Al content (x) and measuring temperature (T).

Multiple magnetic transitions, Griffiths-like phase, and magnetoresistance in La2CrMnO6

DC and AC magnetic measurements indicate the presence of multiple magnetic transitions arising from the competing magnetic interaction between Cr and Mn in the perovskite La2CrMnO6. Ferromagnetic and spin glass transitions are observed in La2CrMnO6. The material also has a Griffith-like phase with the occurrence of ferromagnetic short range correlations above TC. The system ultimately turns into paramagnetic at the Griffiths temperature 180 K. A combination of variable range hopping, and the nearest neighbor small polaron hopping governs the conduction mechanism in the material. A negative magnetoresistance of 22% at 105 K is observed for the material at 90 kOe which increases to 29% near 110 K and reduces gradually to zero on further increase in the temperature.

Exchange bias and spin glass transition in quaternary MnCuNiSn Heusler alloy

We have systematically investigated the exchange bias (EB) effect and low-temperature magnetic state transition in Mn42Cu6Ni42Sn10 Heusler alloy by means of AC and DC magnetization measurements. Experimental results suggest that the martensitic phase is in the state of superparamagnetic (SPM), following that the particles turn into the ferromagnetic (FM) state at the martensitic Curie temperature, and the moments are frozen as the temperature is below 150K. In addition, EB effect was observed in the phase separated martensitic state, which can achieves about 536Oe under the field of 500Oe after field-cooled (FC) to 10K. First-principle investigations indicate that such a small value is originated from nonmagnetic Cu atoms substitution. The appearance of EB effect can be ascribed to the competition between the FM martensitic phase and spin glass (SG) phase due to the impact of Mn atoms, whereas the nonmagnetic Cu atoms doping would hinder the magnetic aggregation, resulting the small EB value eventually.

Synthesis, structure and magnetic properties of nanostructured La1−xAxFe0.5Mn0.5O3 (A = Ca, Sr and Pb; x = 0 & 0.25) perovskites

The effect of hole doping on magnetic properties of LaFe0.5Mn0.5O3 have been investigated. All the ceramics samples La1−xAxFe0.5Mn0.5O3 (A=Ca, Sr and Pb; x=0 & 0.25) were synthesized at 500°C by sol-gel method and the particles size were found to be in nanodimension. The samples were characterized by X-ray and electron diffraction, HRTEM and both dc and ac-magnetization measurements. The X-ray and electron diffraction patterns were indexed by cubic Pm-3m space group. The particle size of the LaFe0.5Mn0.5O3 is ∼100nm, whereas the Pb-doped sample is ∼50nm and for Ca or Sr doped samples the size is ∼10–30nm. Both dc and ac-susceptibility measurements suggest that the effect of hole doping and A-site cationic radius in LaFe0.5Mn0.5O3 have no significant role on magnetic properties. However, the particle size plays an important role on magnetic property due to the development of surface ferromagnetic cluster at nanoscale. The competing interactions lead to magnetic phase separation where local ferromagnetic clusters coexist within the antiferromagentic matrix in all the samples.

Large exchange bias effect in the super spin glass state of Mn50Ni38Al12 alloy

A large exchange bias field (HEB) of 5.3 kOe which so far, is the largest value found in MnNi-based alloys, was observed in Mn50Ni38Al12 ribbon. DC and AC magnetic measurements show super spin glass (SSG) behavior below 130 K and superparamagnetic behavior above this temperature. Different from other systems, the pinning and pinned phases are both SSG. Due to the high content of Mn and low content of Al there exists considerable disorder in Mn50Ni38Al12 which is responsible for the large HEB.

Griffith-like phase in Crednerite CuMnO2

Ac and dc magnetic measurements have been performed on CuMnO2. From both the magnetic data the existence of Griffith-like phase established. Both ac and dc magnetic studies demonstrate aspects of Griffith phase-like behavior. The onset of Griffith like phase is associated with an effective slowing down. The presence of short range ferromagnetic ordering is observed above the antiferromagnetic transition temperature. Different directions in the triangular ab plane in CuMnO2 become inequivalent: there exist for each Mn two short and four long MnMn distances in the plane. The exchange interaction at short bonds is stronger, which, with the uniaxial magnetic anisotropy of Mn3+(spins are oriented predominantly along the long MnO bonds), leaves the system frustrated. This frustration may induce the Griffith-like phase.

Spectroscopic and magnetic studies of highly dispersible superparamagnetic silica coated magnetite nanoparticles

Superparamagnetic behavior in aqueously well dispersible magnetite core-shell Fe3O4@SiO2 nanoparticles is presented. The magnetic properties of core-shell nanoparticles were measured with use of the DC, AC magnetometry and EPR spectroscopy. Particles where characterized by HR-TEM and Raman spectroscopy, showing a crystalline magnetic core of 11.5±0.12nm and an amorphous silica shell of 22±1.5nm in thickness. The DC, AC magnetic measurements confirmed the superparamagnetic nature of nanoparticles, additionally the EPR studies performed at much higher frequency than DC, AC magnetometry (9GHz) have confirmed the paramagnetic nature of the nanoparticles. Our results show the excellent magnetic behavior of the particles with a clear magnetite structure, which are desirable properties for environmental remediation and biomedical applications.

Influence of temperature on magnetic properties of silicon steel lamination

In this paper, we studied the influence of thermal effect on the iron loss components by DC and AC magnetic measurement. The measured result shows that iron loss of nonoriented silicon steel is more influenced by temperature than grain oriented one. Based on loss separation model, we have found a suitable iron loss expression for nonoriented and grain oriented steels. Then a temperature dependent iron loss model is proposed, where temperature coefficient k is introduced to consider thermal effect on dynamic loss. The iron loss model is validated by all series of silicon steel stripe made by WISCO. The relative error of the model is about 11% in a wide range of 20∼400Hz, 20∼200°C, 0∼2T. The proposed model can be applicable to other types of magnetic materials as long as their resistivity rate exhibits approximately linear thermal dependence within a temperature range of 20∼200°C.

Structures and Magnetostructural Correlation Analyses for Two Novel Hexanuclear Complexes Based on a Pentadentate Schiff Base Ligand

Abstract]have been synthesized, structurally characterized and magnetically studied. X‐ray diffraction analyses indicated that the potentially pentadentate symmetric ligand affords variable coordination conformations. The copper ions are bound into hexameric moieties with helix‐like arrangement whereas cobalts with four face‐sharing monovacant distorted cubanes. Magnetic investigations revealed that within the hexanuclear unit, compound 1 exhibits antiferromagnetic interactions with the best fit parameters of J1=‐17.96 cm−1, J2=‐22.26 cm−1, zJ=‐0.56 cm−1 and R=1.1 × 10−5, while compound 2 affords ferromagnetic intramolecular couplings with the best simulation parameters of J1=+0.10 cm−1; J2=+0.15 cm−1; Jx=Jy =0.1 cm−1; Jz=0.02 cm−1; D1=‐0.50; D2=‐0.45; g1=2.67; g2=2.57. Furthermore, the compound 2 displays slow magnetic relaxation behaviour detected by ac magnetic measurement under a 600 Oe field.

Syntheses, structures, and magnetic properties of a family of end-on azido-bridged CuII-LnIII complexes.

The reaction of Cu(valdmpn) (H2valdmpn = N,N'-bis-(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane), LnCl3·nH2O (Ln = La(1), Ce (2), Pr(3), Nd(4), Sm(5), Eu(6), Gd(7), Tb(8), Dy(9), Ho(10), Er(11), Tm(12), Yb(13), Lu(14)) and NaN3 in methanol led to a whole series of heterometallic CuII-LnIII complexes. The basic structural components of these complexes are the [CuII(valdmpn)LnIII] units with a compartmental Schiff-base ligand. These [CuIILnIII] units were then further connected by the end-on (EO) azides between the Ln3+ centers to form tetranuclear {[CuLn](μ-N3)n[LnCu]} motifs, where n is 3 for the complexes of larger radii lanthanides (1CuLa to 5CuSm) and 2 for the complexes of smaller radii lanthanides (6CuEu to 14CuLu). While compounds 6CuEu to 14CuLu remain isolated tetranuclear clusters, compounds 1CuLa to 5CuSm are one-dimensional chains where tetranuclear motifs are further connected by end-to-end (EE) azides between CuII and LnIII centers. Direct current (dc) magnetic susceptibilities have been measured in the range of 1.8-300 K for all complexes, revealing the different nature of magnetic interactions between the spin centers. Among them, dominant ferromagnetic interactions were found in compounds 7CuGd, 8CuTb and 9CuDy. Alternating current (ac) magnetic measurements performed on 8CuTb, 9CuDy, and 10CuHo demonstrated the presence of slow magnetic relaxation under zero applied dc field. The spin relaxation barrier Ueff was determined to be 19.54 cm-1 (28.12 K) and 10.34 cm-1 (14.87 K) for compounds 8CuTb and 9CuDy. The SMM behavior of 8CuTb was further confirmed by the presence of magnetic hysteresis loops; while no hysteresis loop was observed for 9CuDy and 10CuHo. These results demonstrated that azido-bridged copper(ii)-lanthanide(iii) complexes could be synthesized successfully despite the generally weak azido-lanthanide interaction.