Reentrant Magnetic Behavior Research Articles

Reentrant magnetic behavior and other oddities related to an intragranular ordered mesh of vacancies in magnesium-deficient MgB 2

The low-temperature magnetization of high-quality samples of Mg 1− x B 2 ( x<∼0.15) reveals a reentrant behavior as the deficiency concentration x is increased. This uncommon result is related to the percolative nature of superconductivity in this system, which presents a superconducting phase, poor in Mg-deficiency, coexisting with an insulating phase, vacancy-rich. This contribution reports on the magnetic response of a number of samples of this system. Results are consistent with the idea that Mg-vacancies cluster to form insulating regions. Particularly enhanced in Mg 0.975B 2, the effect on the magnetic response resembles that of a columnar-like structure that percolates throughout the length of the sample, a feature that could hardly be detected by direct observation techniques.

Reentrant behavior in the temperature dependence of metamagnetic transitions in single crystal Nd/sub 6/Fe/sub 13-x/Al/sub 1+x/

Nd/sub 6/Fe/sub 13-x/Al/sub 1+x/, (/spl delta/) is a complex antiferromagnet with a Neel temperature that ranges from /spl sim/250 K to /spl sim/350 K. Studies of polycrystalline samples of the Nd compound have shown that the material undergoes a series of metamagnetic transitions as a function of magnetic field. In the present study, single crystals of /spl delta/ have been grown and characterized. Low temperature magnetization studies demonstrate that there are a number of competing magnetic interactions in the material, which have different temperature dependencies. As a result, for a narrow range of fields, the /spl delta/ crystals exhibit unique reentrant magnetization versus temperature behavior.

The magnetic properties of a magnetic detector using oxidized amorphous Co 95− xFe 5(BSi) x alloys

A comparative oxidation study of several amorphous Co 75− x Fe 5(BSi) 20+ x alloys was carried out. Reentrant magnetization behavior and field-induced anisotropy which are of a critical importance for a magnetic detector were obtained after oxidation of the amorphous Co-rich ribbons. During this oxidation, the ribbons develop surface oxides which are primarily nonmagnetic borosilicate or a combination of borosilicate and magnetic oxides such CoO or FeO. Beneath this lies a 100–1000 Å thick Co-rich magnetic alloy which may be either HCP or FCC in its crystal structure. The thickness of the Co-crystallized layer is determined by the type of the surface oxides. The oxidation products such as appear to affect the reentrant magnetization behavior of Co-rich amorphous alloys significantly. We have determined the amount of metalloids (a critical concentration of B and Si) which is necessary to form a continuous layer of the most thermodynamically stable oxide, in our case borosilicate, on the surface. We also observed that there is a good correlation between reentrant magnetization and the thickness of Co layer. The best reentrant M– H loop for the magnetic detector was obtained in ribbons with a surface borate-rich borosilicate since it ensures conditions such as (1) metalloid depletion in the substrate and (2) formation of oxygen impurity faults in Co grains that are required for strong reentrant magnetization behavior.

Spin glass and percolation in MnCr 2−2 xIn 2 xS 4 system

DC magnetization measurements in low magnetic fields revealed the spin-glass-like and reentrant spin-glass behavior at low temperatures in crystals of the MnCr 2−2 x In 2 x S 4 system in the substitution range 0 ⩽ x ⩽ 0.5. Reentrant magnetic behavior was observed for 0 ⩽ x ⩽ 0.10. Magnetization curves for MnCr 1.4In 0.6S 4 and MnCrInS 4 spinels were measured in magnetic fields up to 50 kOe in order to determine the effects of random anisotropy. The experimental results are discussed in terms of the existing models.

Magnetic properties of disordered binary Heisenberg alloy

The magnetic properties of a disordered binary magnetic Heisenberg alloy (A p B 1− p ) with both ferromagnetic and antiferromagnetic exchange interactions are studied theoretically using the cluster variation method. The approximate free energy is evaluated by treating the interactions within the clusters of all possible configurations exactly and replacing the rest of the interaction by variational effective field. Depending on p and the exchange parameters, the Heisenberg alloy exhibits ferromagnetic phase with incomplete magnetic saturation and the re-entrant magnetic behaviour. These results are compared with the corresponding results of an Ising system.

The effect of oxidation on magnetic properties of amorphous Co 75.26−xFe 4.74(BSi) 20 + x alloys

A comparative oxidation study of several amorphous Co 75.25−xFe 4.74(BSi) 20+x alloys was carried out, A reentrant magnetization behavior and field-induced anhotropy were obtained after oxidation of the amorphous Co-rich ribbons. During this oxidation, the ribbons were found to develop surface oxides which are primarily non-magnetic borosilicate or a combination of borosilicate and magnetic oxides such CoO or FeO. Beneath this lies about 100–1000 Å thick Co-rich magnetic alloy which may be either hcp or fcc in its crystal structure. The thickness of Co crystallized layer is determined by the type of the surface oxides. The oxidation products such as surface and internal oxides (oxygen impurity faults) appear to affect magnetization behavior of Co rich amorphous alloys significantly. We have determined the amount of metalloids (a critical concentration) which is necessary to form a continuous layer of the most thermodynamically stable oxide, in our case borosilicate, on the surface. We also observed that there is a good correlation between reentrant magnetization and thickness of Co layer. The best reentrant M-H loop was obtained in ribbons with a surface borate-rich borosilicate since this ensures the conditions such as (1) metalloid depletion in the substrate and (2) formation of oxygen impurity faults in Co grains, that are required for strong reentrant magnetization behavior.

Formation of oxygen impurity faults during magnetic annealing of cobalt-rich amorphous alloys

Magnetic annealing effects such as field induced anisotropy and re-entrant magnetization behavior in cobalt-rich amorphous alloys appear to have their origin in structural inhomogeneities produced by internal segregation of oxygen atoms on cobalt 111 lattice planes. Electron diffraction demonstrated that these segregated oxygen atoms form planar oxygen impurity faults in the cobalt lattice normal to 〈111〉 directions. These faults interrupt the long-range ordering of cobalt atoms and lead to the altered cobalt lattice structure. A pair ordering of cobalt atoms may arise from oxygen impurity faults due to breaking of the cubic symmetry. A strong response to magnetic field annealing and the best re-entrant magnetic flux reversal were observed when the annealed ribbon surface consisted of cobalt grains with a high density of oxygen impurity faults.

The domain wall pinning and exchange coupling induced by surface crystallization in field-annealed cobalt-rich amorphous alloys

Attempts were made to correlate magnetic properties of Co-rich amorphous alloys such as domain wall pinning and exchange coupling with microstructural changes during field annealing. It was observed that M-H loops of first annealed material under longitudinal field showed weak exchange anisotropy due to coupling of the amorphous alloy to a semi-hard Co-rich crystalline surface layer. After second annealing without magnetic field, re-entrant magnetization behavior was observed.These alloys show appreciable surface crystallization during annealing. Striking differences in the microstructural morphology were revealed for differnt glass former ratios B/Si. For high B/Si ratios, the surface crystallites are predominantly f.c.c. Co and show a high density of oxygen faults. For low B/Si ratios, the surface crystallites are predominantly h.c.p. Co and almost free of faults.The degree of exchange coupling is proportional to the B/Si ratio and correlates with the presence of oxygen faults in surface crystallites. It is also observed that there is a good correlation between domain wall pinning field and thickness of crystalline Co layer.

Correlation between the magnitude of structural disorder and the frustration of magnetic order in Gd 4Mn 3 metallic glass

The magnetic properties of Gd 4Mn 3 amorphous alloy have been investigated by ac and dc susceptibility measurements in the temperature range from 4.3 to 500 K. The alloy has been found to exhibit a re-entrant magnetic behaviour with paramagnetic to ferromagnetic to spin-glass-like state transitions at T c = 167 K and T f = 72 K, respectively. The radial distribution function for Gd 4Mn 3 glass has been determined; the partial atomic distances and the partial coordination numbers have been estimated on the basis of a detailed structural analysis. The relationship between the magnitude of structural disorder and the frustration of magnetic order in Gd 4Mn 3 glass has been clarified from the comparison of its structural and magnetic properties with those of Gd 4T 3 glasses (T = Al, Cu).

Reentrant magnetic behavior in fcc Co-Cu alloys.

Crystalline ${\mathrm{Co}}_{\mathit{x}}$${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ alloys, which cannot be obtained by equilibrium methods, have been fabricated by magnetron sputtering. The magnetic properties of the alloys, whose structure remains single-phase fcc up to x=0.80, is studied over the entire range of compositions. As in the case of other random magnetic systems such as Fe-Au, the metastable alloy is seen to evolve from a spin glass at low Co concentrations to a reentrant spin glass with evidence for both ferromagnetic and spin-glass ordering, and finally to a ferromagnet for xg0.40. A magnetic phase diagram for fcc Co-Cu is proposed.

Re-entrant magnetic behaviour in amorphous Fe-rich Fe100−x Zr x alloys

We report results of57Fe Mossbauer measurements on the magnetically concentrated re-entrant amorphous Fe100−xZr x alloys (8≤x≤12) which give convincing evidence for a microscopic origin of the low-temperature magnetization anomaly atT f<T c. This is shown by an anomalous increase in the magnetic hyperfine field and in the relative intensities of the (†m=0)-Mossbauer lines belowT f upon cooling. No evidence was found for an antiferromagnetic phase. The shape of the hyperfine-field distribution atT=4.2 K in zero external field and in a field of 3 T is unchanged, indicating homogeneous behaviour of all Fe moments near saturation. Ferromagnetic order is not long-ranged, but determined by exchange-coupled magnetic clusters. The cluster moment nearT c is found to decrease strongly with increasing Fe content and extrapolates to zero atx≈96.

Local atomic order and re-entrant magnetic behaviour in Au85Fe15

A perturbed angular correlation study on Au85Fe15 in different metallurgical states and over a wide temperature range is reported. The hyperfine field and its temperature variation reflect a cluster glass behaviour due to chemical clusters of different sizes rather than a magnetic double transition.

Temperature hysteresis and relaxation effects in amorphous Fe-Ni-Zr alloys (abstract)

Amorphous Fe-Ni-Zr alloys in the ‘‘as quenched’’ and irradiated state have been studied with 57Fe Mössbauer spectroscopy and ac and dc susceptibility. Samples were irradiated by 14-MeV neutrons at room temperature to fluences of ≊1017 ns/cm2 corresponding to a dpa of 3×10−4 at the Lawrence Livermore National Laboratory RTNS-II facility. Two samples (Fe89.7Ni0.3Zr10 and Fe70Ni20Zr10) ‘‘as quenched’’ and irradiated, have been studied between room temperature and ≊4 K. ac susceptibility shows reentrant magnetic behavior for both alloys. Temperature hysteresis (no applied fields) is observed initially in the Mössbauer spectra for both alloys, ‘‘as quenched’’ and irradiated. It decreases and virtually disappears after a few thermal cycles between 20 K and room temperature. Relaxation effects are present in the magnetic hyperfine spectra of both alloys, ‘‘as quenched’’ and irradiated. The usually assumed proportionality between the magnetic hyperfine splitting and the magnetization is probably not valid for these alloys.

Local evidence for re-entrant magnetic behaviour in amorphous Fe 90Zr 10 alloys

We report results of 57Fe Mössbauer measurements on the magnetically concentrated amorphous Fe 90Zr 10 alloy which give the first evidence of a microscopic origin for the low temperature magnetization anomaly at T f = 35 K. This is shown by an anomalous increase in the average magnetic hyperfine field B hf (T) below T f, similar to that observed in crystalline and amorphous re-entrant spin-glass alloys in the concentration range near the percolation threshold.

Properties of a spin-1 system with competing interactions and reentrant magnetic behavior.

We study a spin-1 lattice model with anisotropic competing interactions. The phase diagram obtained by use of generalized molecular field treatment exhibits various modulated phases and, in addition, a transition from a high-temperature ferromagnetic to a low-temperature modulated magnetic phase. The obtained reentrant magnetic behavior is compared with experimental results for ${\mathrm{Ce}}_{3}$${\mathrm{Al}}_{11}$ compound.

Large Barkhausen discontinuity in (Fe1−xCox)-Si,B amorphous wire (abstract)

The magnetostrictive amorphous wires made by the in-water quenching technique have been shown to exhibit the large Barkhausen discontinuities,1 which make them attractive for sensor application.1 We reported earlier that the reentrant magnetic behavior of these wires is attributable to the magnetostrictive anisotropy caused by the residual stresses quenched in during solidification in the water.23 For further understanding of the magnetoelastic effect in amorphous wires, we measured the magnetic properties including the magnetic domain patterns for the in-water quenched (Fe1−xCox)-Si,B amorphous wires. The large Barkhausen discontinuity was observed only for wires having the positive magnetostriction. We found that the compositional variations of the width of labyrinthine domain and the critical field for reverse domain nucleation agree with those expected from the magnetostriction change with composition.

Evidence against reentrant magnetic behaviour in AuFe alloys

The temperature and concentration dependence of the transferred hyperfine field in Au 1− x Fe x with 0.15< x<0.28, was measured by perturbed angular correlation spectroscopy. The temperature dependence of the hyperfine field at the 111Cd probe shows a slope change at T/ T c =0.25 for all concentrations below 20 at% Fe, but an almost continuous behaviour for higher concentrations. These results manifest a percolation-like magnetic behaviour but no magnetic double transition.

Spin glass and reentrant behavior in amorphous transition metal alloys Pd 77.5- xCu 5Si 16.5M x

Dc-susceptibility and magnetization measurements indicate that the amorphous alloys Pd 66.5Cu 6Si 16.5Co 11 and Pd 62.5Cu 6Si 16.5Cr 15 show a reentrant magnetic behavior, the former going through a ferromagnetic state before forming a spin glass, the latter going through an antiferromagnetic state. Comparison with similar alloys shows that Co exhibits stronger ferromagnetism than Fe and Cr stronger antiferromagnetism than Mn.

Reentrant magnetism - a low field study

We report low field dc magnetization measurements on (Fe x Mn 1− x ) 75P 16B 6Al 3 alloys at 4 ⪅ T ⪅ 300 K. Reentrant magnetic behavior is observed for x = 0.65, 0.7 and 0.8. By comparing field cooled and zero-field cooled states at low T we separate out the reversible and irreversible contributions to the magnetization M and identify the (field-dependent) temperature for the onset of irreversibility. It is shown that the reversible part of the magnetization can be described by the usual scaling laws for critical behavior in magnetic systems not only at the transition from the paramagnetic to the ferromagnetic phase but also when the latter transforms to a spin glass. We identify the irreversible part of M with a spin glass order parameter.