AC Magnetization Curves Research Articles

Quantitative explanation of the difference in AC magnetization curves between suspended and immobilized magnetic nanoparticles for biomedical application

We study the difference in AC magnetization (M−H) curves between suspended and immobilized magnetic nanoparticles (MNPs). We use three commercial MNP samples that are often used for biomedical application. First, we measure the hysteresis area of the AC M−H curve, A, when the amplitude of the excitation field, Hac, is changed. The A vs Hac curve is compared with previously obtained analytical results by taking account of the MNP size distribution in the sample. We show that they quantitatively agree for both suspended and immobilized samples. From the comparison, we clarify the mechanism that determines the AC M−H curve of a suspended sample. For high Hac, the Néel relaxation becomes dominant in the suspended sample, and alignment of easy axes caused by the AC field increases the hysteresis loss compared to the immobilized case. The effect of Brownian relaxation in the suspended sample increases with decreasing Hac and gives additional loss at low field. The portion of Néel-relaxation- and Brownian-relaxation-dominant MNPs in the sample is quantitatively evaluated. We also clarify the difference in harmonic signals between suspended and immobilized samples. Finally, we discuss the condition for the MNP parameters and excitation field that determines the dominant relaxation mechanism in a suspended sample.

Difference in AC magnetization between suspended and immobilized magnetic nanoparticles in Néel-relaxation dominant case: Effect of easy axis alignment in suspended nanoparticles

Magnetic nanoparticles (MNPs) have been widely studied for bio-sensing applications, where suspended and immobilized MNPs can be magnetically distinguished using their different magnetic properties. We study magnetic properties of suspended and immobilized MNPs when the Néel relaxation time is much shorter than the Brownian. We show in both numerical simulation and experiment that they have different magnetic properties such as AC magnetization curves and harmonic spectra even though the dynamic behavior of both MNP types is primarily dominated by Néel relaxation. This difference is caused by the partial alignment of the easy axes in suspended MNPs when an AC magnetic field is applied. We introduce a distribution function for the angle of easy axis alignment. We also show a method to evaluate the distribution function from the measured AC magnetization curve and clarify the relationship between easy axis alignment and the AC field strength. Using the distribution function, we can quantitatively discuss the effect of easy axis alignment on the magnetic properties of suspended MNPs. The obtained results provide a basis for using MNPs in bio-sensing.

High intrinsic loss power of multicore magnetic nanoparticles with blood-pooling property for hyperthermia

Magnetic hyperthermia is a promising application of magnetic nanoparticles (MNPs) in cancer therapy. It is important to consider and optimize the parameters that affect heat dissipation, such as particle diameters, structures, and surface coatings. In this study, we measured the magnetic properties of two superparamagnetic nanoparticles under DC and AC magnetic fields. Resovist is approved to be used as a magnetic resonance imaging contrast agent. CMEADM-033-02, with the blood-pooling property and biocompatibility, exhibits high magnetization. The blood-pooling property makes it easier for MNPs to accumulate in tumors and tissue. While preparing samples, we aligned the easy axis of the samples using a DC magnetic field to enhance heat dissipation. We discussed the magnetic property in terms of magnetic relaxation associated with anisotropy energy. We observed that the peak frequency of Néel relaxation was considerably shifted owing to effectively changed anisotropy by the alignment of the easy axis. However, the change in the peak frequency of Néel relaxation could not be directly confirmed. Furthermore, we calculated the intrinsic loss power (ILP) and specific loss power (SLP) for heat dissipation from the areas of AC magnetization curves and estimated the SLP at 1 MHz to compare with the high heating characteristic of ILP that has been reported in a conventional study. We achieved equivalent ILP for heat dissipation as that reported in the study by aligning the easy axis of the MNPs with the blood-pooling property under a therapeutic condition.

Effects of Zr Doping on Magnetic and Structural Properties of DyBa2Cu3O <inline-formula> <tex-math notation="LaTeX">$_{7-\delta }$</tex-math> </inline-formula> Thin Films

DyBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> -Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> thin films (with x = 0.00, 0.02, 0.04, and 0.06) were synthetized using the chemical solution deposition method on single crystalline LaAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrates. Effects resulting from Zr doping were investigated by means of magnetic and structural measurements. X-ray diffraction analysis demonstrated a strong c-axis orientation in the DyBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Zr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-δ</sub> thin films, with only minor reflections due to the presence of Dy <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and BaZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> phases. Narrow transitions with ΔTc ranging from 1 to 2 K for samples with x = 0.00 and 0.06, respectively, were observed in ac magnetic susceptibility curves, where the nondoped sample demonstrated the highest T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">onset</sup> of 91.7 K compared to the Zr-doped samples. Critical current densities J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the thin films were obtained from magnetization loop measurements with applied magnetic fields up to 6 T and by employing the Bean model. The sampled doped with 4% Zr exhibited the highest J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> value (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 3.5 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , self-field, 77 K) in the low field range and we, in general, observed that Zr-doped films demonstrated higher values compared to the pure sample. Pinning force plots (F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> versus B) reveal a significant improvement over the magnetic field range investigated of the maximum pinning force for Zr-doped samples. We found F <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Max</sup> = 1.7 GN/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and 3.8 GN/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for pure and 6% Zr samples, respectively. Analysis based on the Dew-Hughes model shows that normal surface pinning is the dominating pinning mechanism.

Hybrid nanoparticles for magnetic and plasmonic hyperthermia.

The present manuscript reports the use of hybrid magneto-plasmonic nanoparticles (HMPNPs) based on iron oxide nanoparticles and Au nanorods as colloidal nanoheaters. The individual synthesis of the magnetic and plasmonic components allowed optimizing their features for heating performance separately, before they were hybridized. Besides, a detailed characterization and finite element simulations were carried out to explain the interaction effects observed between the phases of the HMPNPs. The study also analyzed the heating power of these nanostructures when they were excited with infrared light and AC magnetic fields, and compared this with the heating power of their plasmonic and magnetic components. In the latter case, the AC magnetization curves revealed that the magnetic dipolar interactions increase the amount of heat released by the hybrid nanostructures.

Effects of the Third Harmonic Demagnetization Field on the ac Losses of a Granular FeSeTe Superconductor in ac Field

The first harmonics of the magnetic response, of inter- and intragranular volume fractions of a FeSe0.5Te0.5 granular superconducting sample in an alternate magnetic field, have been numerically extracted from the measured magnetization of the whole system by using a model which considers the effects of the fundamental demagnetization fields generated by the magnetization itself. This allowed us to obtain the temperature and field behaviors of the inter- and intragranular ac losses during an ac cycle. These parameters have been found sensitive to the existence of the third harmonic components of the demagnetization field produced by the corresponding magnetization harmonics of the sample. This confirms that the effects of the magnetic response third harmonic components on the sample magnetization, through the demagnetization field, should be taken into account for a more detailed analysis of the measured ac magnetic curves.

Core loss and magnetic susceptibility of superparamagnetic Fe nanoparticle assembly

Toroidal-shaped high-density Fe nanoparticle assemblies (FNAs) were fabricated by molding different sized Fe nanoparticles (NPs), and the effect of the magnetic behavior of the FNAs on the core loss and the magnetic susceptibility was investigated. An FNA with 4.3 nm diameter Fe NPs exhibits superparamagnetism at room temperature while an FNA with 6.4 nm diameter Fe NPs doesn’t exhibit superparamagnetism at room temperature. AC magnetization curves at 1, 10 and 100 kHz were measured to evaluate the core loss of the toroidal-shaped FNAs. Both FNAs exhibited no significant eddy current loss, which suggests that surfactants on the NP surface effectively act to electrically insulate the NPs, and the NPs are not sintered together when the FNAs are molded. The AC magnetization curves had no hysteresis for the FNA with 4.3 nm diameter Fe NPs, i.e., the core loss was minimal for the superparamagnetic FNA. The magnetic susceptibility of the superparamagnetic FNA with 4.3 nm Fe NPs was 12 times higher than that estimated from Langevin theory due to the effect of strong magnetic dipole interaction. These results suggest that the superparamagnetic FNA has potential as a magnetic core material that exhibits low core loss and high magnetic susceptibility, even at high frequency.

The effect of dipole-dipole interactions on coercivity, anisotropy constant, and blocking temperature of MnFe2O4 nanoparticles

Superparamagnetic manganese ferrite nanoparticles with mean size of 〈D〉 = 6.5(±1.5) nm were synthesized through a solvothermal method using Tri-ethylene glycol as a solvent. The peak temperature of zero field cooled measurements of magnetization and AC magnetic susceptibility curves shifted toward higher temperatures by applying different pressures from 0 to 1 kbar and increasing the powders compaction. The frequency dependence of AC susceptibility measurements indicated the presence of weak dipole-dipole interactions between nanoparticles. By increasing the powders compaction and interactions strength, the coercive field (Hc) increased and squareness (Mr/Ms) decreased. The obtained effective anisotropy constant (Keff), by susceptibility measurements, was from 1.72 × 106 to 2.36 × 106 ergs/cm3 for pressure of 0 to 1 kbar. These values are larger than those obtained from hysteresis loops at 5 K (0.14 × 106 to 0.34 × 106 erg/cm3). Also, the Keff was two orders of magnitude greater than that of bulk MnFe2O4. Size, surface effects, and total energy barrier between equilibrium states were reported as the main causes of large anisotropy. Below 75 K, a signature of weak surface spin glass was observed. However, memory effect experiment indicated that there is no collective superspin glass state in the samples. This study suggests the role of powders compaction on properties of a magnetic nanoparticles system. Furthermore, the coercivity, the anisotropy constant, and the blocking temperature are affected by changing nanoparticles compaction.

Approximation and Prediction of AC Magnetization Curves for Power Transformer Core Analysis

Based on the latest experimental results of two different modern graded grain-oriented electrical steels measured up to 2 T under ac, the existing 10 popular analytical approximation functions for ac magnetization curves are critically evaluated. None of them are capable of making a satisfactory approximation over the full useful range (0-2 T). Therefore, a new simple explicit empirical function is proposed that is simple to use (six parameters) and sufficiently accurate (adjusted R2 ≥ 0.9992) to represent the ac magnetization curve. The empirical function is also shown to be suitable (adjusted R2 ≥ 0.9913) for extrapolating the ac magnetization curve into the high-flux-density region (1.80-2 T) when only using the available experimental data in the region 0-1.80 T from steel manufacturers.

Characterization of the magnetic moment distribution in low-concentration solutions of iron oxide nanoparticles by a high-Tc superconducting quantum interference device magnetometer

We developed a highly sensitive AC/DC magnetometer using a high-temperature superconductor superconducting quantum interference device for the evaluation of magnetic nanoparticles in solutions. Using the developed system, we investigated the distribution of magnetic moments of iron oxide multi-core particles of 100 nm at various iron concentrations that are lower than 96 μg/ml by analyzing the measured magnetization curves. Singular value decomposition and non-regularized non-negative least-squares methods were used during the reconstruction of the distribution. Similar distributions were obtained for all concentrations, and the iron concentration could be determined from the measured magnetization curves. The measured harmonics upon the excitation of AC and DC magnetic fields curves agreed well with the harmonics simulated based on the reconstructed magnetization curves, implying that the magnetization curves of magnetic nanoparticles were successfully obtained as we will show in the article. We compared the magnetization curves between multi-core particles of 100 nm and 130 nm, composed of 12-nm iron oxide nanoparticles. A distinctive magnetic property between the 100 nm and 130 nm particles in low-concentration solutions was successfully characterized. The distribution characteristic of magnetic moments suggests that the net magnetic moment in a multi-core particle is affected by the size of the magnetic cores and their degree of aggregation. Exploration of magnetic properties with high sensitivity can be expected using the developed system.

Spin-Glass Behavior, Magnetic, and IR Spectroscopy Analysis of Multimetallic Compound Ni0.25Mn1.25[Fe(CN)6]·6.1H2O

Multimetallic Prussian blue compound Ni0.25Mn1.25[Fe(CN)6]·6.1H2O has been prepared by coprecipitation. The temperature-dependent magnetic susceptibilities show the magnet transition for the compound at 8.5 K. According to DC variable-temperature magnetic susceptibility paramagnetic Curie temperatureθis −9.32 K. The observed value of coercive field (Hc) and the remanent magnetization (Mr) for the compound are 0.32 KOe and 0.36 μB. According to study of zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves and AC magnetization curves, there exists a spin-glass behaviour in the compound, which exhibits freezing temperatureTg=7.76 K, below magnetic transitionTC=8.5 K; that glass behavior is termed “reentrant” spin glass.

Superspin Glass State in Mgfe2o4 Nanoparticles

MgFe2O4 nanoparticles were prepared at different annealing temperature of 400, 450 and 500°C. Transmission electron microscope image show that powders consist of ∼ 8nm particles. Scanning electron microscope image show aggregation of particles and formation of large particles with size of 20-100nm. Obtained nanoparticles were superparamagnetic at room temperature. A broad peak (TP) was observed in temperature dependant AC magnetic susceptibility curves. The TP increased by increasing the annealing temperature, as a result of increase in particles size. The effective anisotropy constant estimated about 2 (to 3) × 105 erg/cm3. The aggregation caused high interparticles interactions and consequently the total energy barrier between equilibrium states increased. Analysis of frequency dependence AC magnetic susceptibility showed presence of superspin glass state at low temperatures.

Magnetic properties of some opal-based nanocomposites

Nanoparticles of complex titanium, cobalt, and manganese oxides with ilmenite and spinel structure have been synthesized in pores of an opal. The particle composition has been determined by X-ray diffraction analysis. The magnetic properties of the obtained nanocomposites with different particles embedded in pores have been studied. The temperature dependences of the dc and ac magnetizations in the range from 2 to 300 K have been measured. It has been shown that the magnetic ordering in all the nanocomposites studied emerges at temperatures above 150 K, which not in all cases can be related directly to the properties of the materials identified by X-ray diffraction. The appearance of peaks in the ZFC susceptibility and ac magnetization curves below 50 K is assigned to disordering and frustration in nanoparticles of titanates of the type of CoTiO3, NiTiO3, and Co2TiO4.

Glassy ferromagnetism and phase separation in Pr0.5Ca0.5CoO3

The dc and ac susceptibilities, magnetization curves and long-time relaxation were measured on a stoichiometric polycrystalline sample of Pr0.5Ca0.5CoO3. The compound shows a metal-insulator transition at TM-I=76 K, associated with combined spin-state and valency transformation of cobalt ions. Below 10 K the onset of a glassy ferromagnetism (Tf≈6.5 K) is observed, characterized by a frequency dependent susceptibility, S-type character of the virgin curve and ageing effect. An anomalously constricted hysteresis loop at T=Tf accompanied by a minimum in the temperature dependence of the coercivity was ascribed to an interaction between the glassy ferromagnetic and additional minor FM phase.

Characterization of Magnetic Markers for Liquid-Phase Immunoassays Using Brownian Relaxation

We characterized the magnetic markers used in biological immunoassays based on Brownian relaxation. Because the markers are composed of aggregated nanoparticles, i.e., magnetic nanoclusters, we first clarified their magnetic properties using AC susceptibility measurements, magnetization (M–H) curves, and magnetic relaxation properties. Analyzing the experimental results, we obtained the key parameters for the immunoassay, i.e., hydrodynamic diameter dh, magnetic moment mB, and anisotropy energy EB of the markers. Because these parameters were distributed in practical samples, we took their distribution into account in the analysis. Next, we showed the relationship between these parameters obtained from different samples. It was shown that mB increased approximately in proportion to dh. On the other hand, no clear correlation between mB and EB was obtained. These results were very different from those expected from single-domain nanoparticles and must be taken into account when magnetic markers are used in immunoassays based on Brownian relaxation.

Self-Heating Property of Magnetite Nanoparticles Dispersed in Solution

The magnetic characterization of polyethyleneimine (PEI)-coated magnetite (Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) nanoparticles having diameters of 20-30 nm that were dispersed in a solution was performed, and their self-heating property was investigated. The hydrodynamic diameter of PEI-coated Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> nanoparticles was approximately 155 nm on an average. To investigate the self-heating property, the increase in the temperature of a sample heated by an applied ac magnetic field was measured, and the specific loss power (SLP) was calculated. The effect of magnetic relaxation loss was estimated by determining the dependence of the SLP on the frequency of an applied magnetic field. For the magnetic characterization of the nanoparticles, dc and ac magnetization curves of the sample were measured and compared with each other to elucidate the heating mechanism. Uncoated Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> nanoparticles having diameters of 20-30 nm exhibited ferromagnetic characteristics in dry condition. However, the PEI-coated Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> nanoparticles dispersed in a solution did not exhibit hysteresis of the dc magnetization curve because the particles could easily rotate with the changing magnetic field. The magnetization curve measured under an ac magnetic field had a large area compared to the dc magnetization curve. The results indicate that Brownian relaxation is dominant during magnetization reversal.

Calibration of ac and dc magnetometers with a Dy2O3 standard

The ac susceptibility and magnetization curves of a glued Dy(2)O(3) powder sample are measured by an ac susceptometer and a dc superconducting quantum interference device magnetometer, both of which have been calibrated previously. It is shown that the magnetic moment of the paramagnetic sample as a function of field and temperature may be accurately expressed by a combination of the Curie-Weiss law and the Langevin function at T > 45 K with three adjusting parameters, so that the dc magnetization curves and the magnitude and phase of ac susceptibility at different values of dc bias field measured by any magnetometer can be calibrated by using Dy(2)O(3) as a standard. The expressions are empirical and cannot be justified in the entire field and temperature range by existing theories of paramagnetism. Below 10 K, indication of approaching a possible phase transition is found. It is shown that pure Dy(2)O(3) powder may be used as a primary standard, with susceptibility [13.28(T + 17)](-1) emu/Oe/g at T > 50 K and H < 10 kOe, in consistency with the Curie-Weiss law and the quantum mechanical theory of paramagnetism.

High-pressure study of the magnetic phase transition in MnSi

Measurements of ac magnetic susceptibility and dc resistivity of a high-quality single-crystal MnSi were carried out at high pressure making use of helium as a pressure medium. The form of the ac magnetic susceptibility curves at the magnetic phase transition suddenly changes upon helium solidification. This implies strong sensitivity of magnetic properties of MnSi to nonhydrostatic stresses and suggests that the early claims on the existence of a tricritical point at the phase-transition line are probably a result of misinterpretation of the experimental data. At the same time resistivity behavior at the phase transition does not show such a significant influence of helium solidification. The sharp peak at the temperature derivative of resistivity, signifying the first-order nature of the phase transition in MnSi successfully survived helium crystallization and continued the same way to the highest pressure.

Structure and Physical Properties of Ternary Uranium Transition‐Metal Antimonides U3MSb5 (M: Zr, Hf, Nb).

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Structure and physical properties of ternary uranium transition-metal antimonides U 3MSb 5 (M = Zr, Hf, Nb)

The ternary uranium transition-metal antimonides U 3MSb 5 (M = Zr, Hf, Nb) were prepared by arc-melting reactions followed by annealing at 800 °C, or by use of a Sn flux. These compounds extend the previously known series U 3MSb 5 (M = Ti, V, Cr, Mn) and RE 3MSb 5 (RE = La, Ce, Pr, Nd, Sm; M = Ti, Zr, Hf, Nb). The crystal structures of U 3MSb 5 were determined by single-crystal X-ray diffraction data (Pearson symbol hP18, hexagonal, space group P6 3/ mcm, Z = 2; U 3ZrSb 5, a = 9.2223(3) Å, c = 6.1690(2) Å; U 3HfSb 5, a = 9.2084(4) Å, c = 6.1629(3) Å; U 3NbSb 5, a = 9.1378(4) Å, c = 6.0909(6) Å). U 3TaSb 5 has also been identified in microcrystalline form ( a = 9.233(3) Å, c = 6.142(3) Å). Four-probe electrical resistivity measurements on single crystals and dc magnetic susceptibility measurements on powders indicated prominent transitions that are attributed to ferromagnetic ordering. The Curie temperatures, T C, located from ac magnetic susceptibility curves, are 135 K for U 3ZrSb 5, 141 K for U 3HfSb 5, and 107 K for U 3NbSb 5.