Low Field Ac Susceptibility Measurements Research Articles

Cluster spin glass ordering in diluted spinel Zn 0.5Co 0.5FeCrO 4

The disordered spinel system, Zn 0.5Co 0.5FeCrO 4 has been investigated using the low field magnetization and ac susceptibility measurements. From the present results it appears that this system orders into a cluster spin glass state with the magnetic moments of the ferrimagnetic clusters randomly frozen. Compared to the Ni and Co zinc ferrites with the same magnetic dilution, introduction of Cr into the B sites appears to increase the frustration and disorder dramatically. The predicted phase diagrams for the ordering in diluted magnetic spinels do not describe the magnetic behaviour of this system, presumably due to the disorder in the B sublattice in addition to the dilution in the A sublattice.

Magnetism and atomic short-range order in Ni-Rh alloys

Low-field ac susceptibility measurements of Ni-Rh samples of various concentrations are presented. Giant effects of the metallurgical state on the magnetic ordering temperature are associated with changes in the degree of atomic short-range order. By careful control of this degree of short-range order, it is possible to demonstrate the existence of a spin-glass state in Ni-Rh alloys.

High field high pressure magnetic and superconducting properties of Y9Co7

The effects of both high magnetic field and high pressure on the magnetic and superconducting properties of the compound Y9Co7 have been studied via magnetization and low field ac susceptibility measurements. The results imply that the magnetic state at ambient pressure is that of nearly ferromagnetic Fermi systems and most likely is quasi-low-dimensional in character. We find that the dependence of the magnetization on the field has the unusual form M(H)=M* exp−(H*/H),1/4 where M* and H* are temperature dependent parameters. Application of pressure rapidly suppresses the magnetic interactions whereas it enhances the superconducting transition temperature. This suggests coexistence of magnetism and superconductivity.

Ferromagnetism of Pd-Fe (abstract)

We present new low field ac susceptibility measurements on Pd1−xFex alloys (0.002≤X<0.01). The Curie temperature TC, determined from these measurements, are significantly lower than those previously obtained in higher magnetic fields [G. J. Nieuwenhuys, Adv. Phys. 24, 515 (1975)]. We also found that for a given sample, TC depends very sensitively on its heat treatment. As an example, for an alloy with 0.4 at. % Fe, TC can be varied between 4 and 10 K. In other alloys, like PdNi or RhNi similar changes in TC are due to changes in the degree of atomic short-range order [S. Crane, D. W. Carnegie, Jr., and H. Claus, J. Appl. Phys. 53, 2179 (1982)]. However, for PdFe we show evidence that the changes in TC are due to absorption of small amounts of oxygen, the samples with the highest amount of oxygen having the highest TC. It thus seems that oxygen has the opposite effect from hydrogen on the exchange enhanced susceptibility of Pd [J. A. Mydosh, Phys. Rev. Lett. 33, 1562 (1974)].

Magnetic behavior of RRh 3B 2 (R = La to Gd) ternary borides

Magnetization of the rare earth ternary borides RRh 3B 2 (R = La to Gd), which crystallize in the hexagonal CeCo 3B 2 type structure, has been measured in the temperature range 4.2 to 300 K. The Curie temperatures have been determined from the low field ac susceptibility measurements. All except LaRh 3B 2 order magnetically. The Gd and Nd compounds have Curie temperatures ( T c) of 91 K and < 20 K, respectively; the effective paramagnetic moments are slightly larger than the corresponding free ion values for trivalent rare earths. The Ce, Sm and Eu compounds show unusual behavior. From unit cell volume considerations, Eu is trivalent (ground state 4f 6; 7F 0) and cerium has a valence greater than 3. In spite of this, CeRh 3B 2 and EuRh 3B 2 exhibit magnetic ordering with T c's of 108 and 40 K and saturation moments of 0.38 and 0.55μ B at 4.2 K, respectively. The magnetism in these two compounds is presumed to be of the itinerant type. The compound SmRh 3B 2 has a very small moment at 4.2 K in 20 kOe field (0.046μ B) and exhibits a peculiar temperature dependence of magnetization.

Magnetic phase diagram of ZnCr2xGa2-2xO4spinels: an antiferromagnetic frustrated lattice

The magnetic phase diagram of the system ZnCr2xGa2-2xO4 has been experimentally determined by using low-field DC and AC susceptibility measurements, neutron diffraction experiments and Mossbauer spectroscopy. A spin glass behaviour has been found in the concentration range 0.40<or=x<0.85; this behaviour is a result of the frustration inherent in the octahedral spinel sublattice with antiferromagnetic interactions only.

Spin glass behaviour in chromium spinels

Experimental evidence of spin glass behaviour (Tf = 2.50 K at v = 17 Hz) is given for ZnCr1.6Ga0.4O4 spinel by low field dc and ac susceptibility measurements, as well as by neutron diffraction measurements and Mossbauer spectra.

Phase diagrams of two dilute insulating systems with competing interactions: CdCr2xIn2-2xS4and ZnCr2xAl2-2xS4

Spin glass behaviour has been observed in a very large concentration range in both CdCr2xIn2-2xS4 and ZnCr2xAl2-24 spinel systems. It is a result of the presence in these systems of competing interactions, ferromagnetic between first-nearest neighbours and antiferromagnetic between higher-order neighbours. The ratios between the interactions are such that the non-dilute x=1 compounds are ferromagnetic for the first system and helimagnetic for the second. The entire phase diagrams of both solutions have been determined by very low field AC and DC susceptibility measurements. The results for the ferromagnetic system are compared with theoretical predictions derived from a bond-diluted Ising model using the replica method and a second-order variational approximation.

Very low field susceptibility of the highly frustrated CsMnFeF6 and CsNiFeF6 compounds

Low field ac and dc susceptibility measurements of the highly frustrated compounds CsNiFeF6 and CsMnFeF6 reveal fundamental differences with the classical spin glass behaviour, and do not allow to conclude on the low temperature state of these compounds.

Kondo effect and spin-glass freezing of the magnetic impurities Cr, Mn, and Fe in superconducting palladium hydride

Through low-field ac susceptibility measurements we have determined the depression of the superconducting transition temperature T/sub c/ in palladium hydride (T/sub c/0 = 9.3 K) as a function of impurity concentration x for Cr, Mn, and Fe. For Cr and Fe similar values for the initial T/sub c/ depression were found, i.e., -150 K/at. % Cr and -145 K/at. % Fe. From resistivity experiments we are able to estimate the Kondo temperatures T/sub K/, i.e., T/sub K/approx. =10 K for Cr and T/sub K/approx. =5 K for Fe. Since i.e., T/sub K/approx. =10 K for Cr and T/sub K/approx. =5 K for Fe. Since systems exhibits an enhanced pair breaking as described by the theory of Mueller-Hartmann and Zittartz. In contrast, for Mn the initial T/sub c/ depression is -21 K/at. % and T/sub K/<

Observation of a spin-glass behavior of the susceptibility in a dilute chromium spinel

Low field dc and ac susceptibility measurements have been performed on several samples of the CdCr2xIn2x−2S4 spinel system. Spin-glass like behavior has been found for x ≤ 0.85. From the results, a tentative phase diagram is proposed.

Ferromagnetic vs. spin glass behavior in amorphous Fe-Ni based alloys

Our recent measurements on amorphous (FexNi1−x)75P16B6Al3 alloys yielded the magnetic phase diagram (T vs. x). For x&amp;lt;xc (xc≲0.17) paramagnet (PM) to spin glass (SG) transitions occur and for 0.4≳x≳xc, cooling produces a PM to ferromagnet transition followed, at lower T, by a FM-SG transition. We report low field ac susceptibility measurements on alloys for x in the neighborhood of xc. Application of a constant magnetic field (Ba) produces two peaks in χac(T) which mark the PM-FM and FM-SG transition temperatures. We obtain phase diagrams in the T vs. Ba plane, compare them to our numerical calculations based on recent theory, and infer the presence of large magnetic clusters for x≃xc.

Insulating spin-glass systemEuxSr1−xS

A systematic experimental study of the magnetic properties in insulating ${\mathrm{Eu}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}\mathrm{S}$ is reported, giving evidence of spin-glass behavior for concentrations $0.13l~xl~0.5$ due to short-ranged exchange interactions with opposite sign. The magnetic properties below the spin-glass transition temperature ${T}_{f}$ are explored by low-field magnetization, ac susceptibility, and neutron-diffraction measurements. The magnetization and remanent magnetization are found to depend on the thermomagnetic history. An unusually pronounced maximum is observed in the thermoremanent magnetization as function of the previously applied field. The saturation values of the remanent magnetizations remain only a few percent of the saturation magnetizations, even for the high Eu concentrations. Their temperature, field, and time dependence are studied. Comparison is made with metallic spin-glass samples. A detailed experimental study of the susceptibility at the spin-glass transition ${T}_{f}$ in (Eu,Sr)S is presented exhibiting a strong sensitivity of $\ensuremath{\chi}({T}_{f})$ and ${T}_{f}$ to small external magnetic fields and different measuring frequencies. In an attempt to analyze the data, the distinction between the spin-glass phenomenon and superparamagnetism is stressed. Near the percolation threshold of the exchange interactions, ${x}_{p}=0.13$, a transition from spin-glass behavior to pure superparamagnetism is clearly observed in ${\mathrm{Eu}}_{x}{\mathrm{Sr}}_{1\ensuremath{-}x}\mathrm{S}$.

A calorimetric investigation of an yttrium-dysprosium spin glass

In an effort to compare the spin glass characteristics of yttrium–rare earth alloys with those of the noble-metal spin glasses, the susceptibility and heat capacity of Y0.98Dy0.02 have been measured in the temperature range 2.5–40 K. The low-field ac susceptibility measurement shows the characteristic cusp-like peak at 7.64 K. The magnetic specific heat of the same sample shows a peak at 7.0 K and may be qualitatively described as a semi-cusp. The magnetic entropy change from absolute zero to 7 K is approximately 0.52 of cR ln(2J+1). These results are qualitatively different than previous calorimetric results on the archetypal spin glasses, AuFe and CuMn, where rounded maxima are observed at temperatures above the spin glass transition temperatures.

Transport Properties of Thulium Single Crystals

Single crystals of thulium have been grown by the strain-anneal technique. The electrical resistivity, thermal conductivity, and Seebeck coefficients were measured along the $〈11\overline{2}0〉$ ($a$-axis), $〈10\overline{1}0〉$ ($b$-axis), and $〈0001〉$ ($c$-axis) crystallographic directions as a function of temperature from 5 to 300\ifmmode^\circ\else\textdegree\fi{}K. All measurements indicated a magnetic transition near 57\ifmmode^\circ\else\textdegree\fi{}K, in good agreement with a N\'eel temperature of 56\ifmmode^\circ\else\textdegree\fi{}K as determined in earlier work by the Oak Ridge National Laboratory neutron diffraction group. The Seebeck coefficient measured along the $c$ axis exhibited a second anomaly at 32\ifmmode^\circ\else\textdegree\fi{}K, in agreement with low-field ac susceptibility measurements; but this anomaly did not appear in the neutron-diffraction measurements. The anomalies observed in the electrical resistivity and thermal conductivity at the N\'eel temperature are interpreted in terms of magnetic superzone planes. All transport properties exhibited a significant anisotropy between the $a$- or $b$-axis and $c$-axis directions. The high-temperature anisotropy of the electrical resistivity and the thermal conductivity is interpreted in terms of the anisotropy of the Fermi surface.

Magnetic-Field Penetration and Breakdown of Surface Superconductivity I

Low-frequency low-field ac susceptibility measurements on type II and certain type I superconductors have shown complete diamagnetism, until the transition at some surface upper critical field ${\mathrm{H}}_{c3}$. In this paper we describe measurements in a swept dc field which show that at sufficiently large sweep rates the ac field can be made to penetrate the superconductor, and the real part of the ac susceptibility will be $\frac{\mathrm{dM}}{\mathrm{dH}}$ along the magnetization curve, with no observable transition at ${\mathrm{H}}_{c3}$. At intermediate sweep rates, the value of the susceptibility becomes more diamagnetic and a partial transition is observed at ${\mathrm{H}}_{c3}$. For the slowest sweeps and highest frequencies, essentially the full diamagnetic-susceptibility characteristic of the point-by-point measurements is obtained, along with a transition at ${\mathrm{H}}_{c3}$. These results are of particular significance since they indicate that an ac field of small amplitude can be made to penetrate a type II superconductor in the mixed state, and can therefore be used to investigate the bulk properties. Two models which we have considered to explain our results are briefly discussed.