Reentrant Spin Glass Behavior Research Articles

Finite-temperature theory of amorphous magnetic alloys.

A finite-temperature theory of magnetism in amorphous magnetic alloys with strong environment effects is developed on the basis of the functional integral method for thermal spin fluctuations and the distribution-function method for random distribution of local magnetic moments. The theory drastically simplifies numerical calculations by means of the geometrical-mean model for amorphous structure as well as electronic structure, and allows us to investigate the magnetism in amorphous transition-metal alloys with large difference in atomic size via average coordination number which depends on the type of central atom. Numerical example is given of the amorphous Fe-Zr alloys. It is demonstrated that the theory reproduces quantitatively the local densities of states obtained from the first-principles calculations, and describes the magnetic phase diagram, in particular, the itinerant-electron spin glass. It is also shown that the atomic-size effects play an important role in the formation of ferromagnetism in concentrated Fe-Zr amorphous alloys. The reentrant spin-glass behavior around 90 at. % Fe is shown to be due to the thermal spin fluctuations of amplitudes of local magnetic moments.

Magnetic, electrical, and small-angle neutron-scattering studies of possible long-range order in the pyrochloresTl2Mn2O7andIn2Mn2O7

${\mathrm{Tl}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{In}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ show spontaneous magnetization around 121 K indicating magnetic phase transitions. The related pyrochlores ${\mathrm{Y}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{Lu}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ also exhibit spontaneous magnetization at their respective magnetic transition temperatures around 17 K. However, neutron-scattering studies do not provide any evidence for long-range order but suggest reentrant spin-glass behavior. In view of this we have performed small-angle neutron-scattering (SANS) investigations on ${\mathrm{Tl}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{In}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$. SANS results substantiate the long-range magnetic ordering in ${\mathrm{Tl}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ as indicated by the magnetic behavior. Preliminary SANS data of ${\mathrm{In}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$, while more ambiguous, are not inconsistent with the presence of long-range order. Effective paramagnetic and saturation magnetic moments of ${\mathrm{Tl}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ and ${\mathrm{In}}_{2}$${\mathrm{Mn}}_{2}$${\mathrm{O}}_{7}$ indicate the presence of ${\mathrm{Mn}}^{3+}$ ions on the ${\mathrm{Mn}}^{4+}$ sublattice as do the measured lattice constants. The occurrence of ferromagnetic ordering in these materials at high temperatures has been attributed to the double exchange interaction between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ spins.

Spin-glass-like behaviour in U2TSi3 (T identical to Fe, Co, Ni or Cu) intermetallics with disordered AlB2- and alpha -ThSi2-type structures

A new series of ternary uranium intermetallics U2TSi3 (T identical to Fe, Co, Ni or Cu) has been discovered. All these phases crystallize in the hexagonal AlB2-type structure, except for U2CuSi3 which is of the tetragonal alpha -ThSi2 type. The magnetic properties of the new compounds were investigated by means of DC magnetization and AC susceptibility measurements. The results show all the main characteristics of a re-entrant spin-glass behaviour at low temperatures in all the ternaries studied. Owing to some structural properties of these compounds they could be classified as non-magnetic atom disorder spin glasses.

Hyperfine field and reentrant spin glass behavior in amorphous LuFe alloys

Temperature dependence of the differential susceptibility and of Mössbauer spectra has been investigated for high Fe concentrated amorphous LuFe alloys. Amorphous Lu x Fe 100− x ( x = 10.5, 15, 20.7) alloys show cusps at the Curie temperature T c and the spin freezing temperature T f and a broad peak at the spin-canting temperature T g. The average hyperfine field H ̄ hf of the morphous Lu 20.7Fe 79.3 alloy was found to increase with decreasing temperature below T c = 180 K. An additional increase in H ̄ hf was observed below the spin-canting temperature T g= 60 K, being consistent with the results of the differential susceptibility measurements. The Mössbauer spectra were analyzed by both conventional and modified Hess-Rübartsch methods. The results obtained by the latter for the amorphous Lu 20.7Fe 79.3 alloy show no anti-ferromagnetic component in the hyperfine fields.

Spin canting and hyperfine interactions in the reentrant Ising spin glass Fe0.62Mn0.38TiO3

The reentrant Ising spin glass Fe0.62Mn0.38TiO3 has been studied with Mossbauer spectroscopy as well as with field-cooled and zero-field-cooled magnetization measurements. The Neel and spin glass temperatures obtained are TN=32.8 K and Tg=25.8 K respectively. The 57Fe Mossbauer spectra recorded below 32 K are described in terms of two subspectra emanating from paramagnetic iron atoms and from iron atoms with canted magnetic moments. The canting angle was determined to be theta approximately=45 degrees for all temperatures below TN. The intensity of the paramagnetic subspectrum decreases with decreasing temperature and is typically about 25% at Tg. Longitudinal as well as transverse reentrant spin glass behaviour is observed in magnetization measurements.

Reetrant spin glass behaviour in the replica symmetric solution of the Hopfield neural network model

Numerical and analytical solutions at low temperature are presented for the replica symmetric order parameter equations of the Hopfield neural model with interactions between all the spins. We find that these equations give weak reentrant spin glass behaviour at T<0.023. This is similar behaviour to that found for the replica symmetric solution of the SK spin glass although in that case the reentrant phase is much larger. It is also known from what is believed to be the true solution for the SK spin glass that this reentrant behaviour is unphysical so, by analogy, we believe it is unphysical for neural network models. Even so, the maximum value in replica theory of αc, which measures the storage capacity of the Hopfield model, is to be found at αc(T=0.023)=0.1382. This is slightly higher than the zero temperature value of αc(T=0)=0.1379.

Magnetic and Mössbauer investigations on Cr 75(Fe xMn 1− x) 25 alloys

Magnetic and 57Fe Mössbauer measurements were performed on Cr 75(Fe x Mn 1− x ) 25 alloys. Although quenched from temperatures above the miscibility gap all samples exhibit segregation. Whereas the ordering temperatures strongly depend on x, both the freezing temperature for re-entrant spin glass behaviour and the hyperfine field distribution at 5 K are nearly independent of concentration.

Magnetic Properties and Phase Diagramof Amorphous Fe-Lu Alloys

A magnetic phase diagram of amorphous Fe 100- X Lu X was determined from AC susceptibility and remanent magnetization measurements. The alloys with 30≤ X ≤40 exhibit a spin glass behavior. The alloys with 10.5≤ X ≤25 are ferromagnetic, but show a reentrant spin glass behavior at low temperatures. The reentrant spin glass phase is divided into two phases with weak and strong irreversibility. The ferromagnetic state becomes unstable as X approaches to 8. For X =8, the ferromagnetic state disappears and a direct transition from the paramagnetic to spin glass state is observed. The high field magnetization measurements imply that the instability of ferromagnetism and the appearance of the spin glass state are caused by the development of antiferromagnetic interactions. In the reentrant spin glass state, the freezing of transverse components of spins is confirmed by the analysis of the Mossbauer spectra.

Magnetic field dependences of the characteristic temperatures in the reentrant spin glasses (Fe 0.65Ni 0.35) 1− xMn x

The alloy system (Fe 0.65Ni 0.35) 1− x Mn x shows reentrant spin glass behaviour for 0.04≤ x≤0.16. In this concentration range three characteristic temperatures can be observed: the Curie-temperature T C, the temperature which corresponds to the maximum of the spontaneous magnetisation T m and the third one which is in connection with the onset of strong irreversibility and the transition to a complete disordered spin glass state T im. All these characteristic temperatures are dependent on the external magnetic field which normally is applied to measure the magnetization versus temperature. In this paper we present these dependencies which we investigated using a Faraday balance in order to measure the magnetization. Only approaching the experimentally determined triple point we observe a linear dependence of T m versus the external magnetic field. This is in agreement with the theoretical predictions by Dubiel et al. Therefore our discussion is based upon the contents of the work of Dubiel, Fischer, Sauer and Zinn.

Magnetic properties of La(FexAl1-x)13 amorphous alloys

Several kinds of La(FexAl1-x)13 (0.80<or=x<or=0.95) amorphous alloys have been prepared by high-rate DC sputtering to investigate the magnetic properties. The magnetic phase diagram has been established by DC and AC magnetic field measurements. Above x=0.85, re-entrant spin-glass behaviour has been observed in the concentration range where antiferromagnetic order occurs in the crystalline state. The high-field susceptibility in this concentration range is extremely large and the magnetization curves are not easily saturated. The magnetic moment estimated from the magnetization curves measured up to 380 kOe in pulsed magnetic fields shows a maximum around x approximately 0.85, corresponding to 80 at.% Fe. The Curie temperature in the amorphous state decreases with increasing x and it is higher than the Neel and Curie temperatures in the crystalline state. The spin-wave stiffness constant in the amorphous state is extremely small, suggesting a magnetic instability.

Magnetic properties of Ba 1- xSr xTi 2- ySn yFe 4O 11

BaTi 2Fe 4O 11 shows the reentrant-spin glass behavior. With increasing x in Ba 1- x Sr x Ti 2- y Sn y Fe 4 O 11 the lattice constant becomes smaller, with T f unchanged and T c increasing, while increasing of y makes the lattice constant larger and the reentrant spin glass changes into spin glass. T f in spin glass decreases with increasing lattice constant. T f of reentrant spin glass has a nature different from that of spin glass. Time dependence of dc magnetization was observed for these samples. The time dependence may be produced by the distribution of superparamagnetic clusters. These samples have characteristics of intensively mixed spin glass and ferrimagnetism.

Microstructure and low temperature magnetism of FeNi invar alloys

We report measurements of the low field (≈ 10 G) near zero field quenched (earth's field) SQUID magnetization, M ZFQ, measured on warming in the interval 4.2–250 K for 8 as splat quenched and annealed FeNi alloy samples with 68–35 at % Fe. The samples were characterized by Mössbauer spectroscopy and vibrating sample magnetometry. The as quenched samples showed the predicted M ZFQ behaviour for “strong” ferromagnetic domain wall pinning, which we attribute to quenched-in crystallographic defects. Annealing effectively removed the latter defects, however, in most samples it concomitantly introduced α/γ′-phase precipitation precursors. The 68 at % Fe sample in which precursors were not formed (due to the high temperature of its single phase boundary) showed reentrant spin glass (RSG) behaviour, as seen previously in virgin state samples. The RSG behaviour is believed to arise from pinning due to intrinsic domain wall width scale compositional variations — combined with demagnetization effects and/or normal thermal randomization. No evidence is seen for a true RSG transition or phase occurring in FeNi alloys.

Magnetic properties of Fe-rich amorphous Fe-La alloys

A magnetic phase diagram of Fe-rich amorphous Fe100-xLax alloys has been determined from high-field magnetisation and AC susceptibility measurements. The alloys with x>10 are ferromagnetic, but show a reentrant spin glass behaviour at low temperature. The ferromagnetic state becomes unstable when x approaches 10. For x<or=10, the ferromagnetic state disappears and a direct transition from a paramagnetic to a spin glass state is observed. Extrapolation of the phase diagram to x=0 suggests that pure amorphous Fe is no longer a ferromagnet but a spin glass with a freezing temperature of about 110K. The irreversible process of magnetisation characterising the theoretically proposed spin glass picture has been observed in the low-temperature phases of the alloys.

Mössbauer Study on Fe-Rich Amorphous Fe–La Alloys

Mossbauer spectra have been measured for two Fe-rich amorphous alloys, a -Fe 91.5 La 8.5 with spin glass behavior and a -Fe 82.5 La 17.5 with reentrant spin glass behavior, in the temperature range between 4.2 K and room temperature. These spectra are analyzed using the Hesse and Rubertsch method to obtain the distribution of hyperfine fields. The distribution of hyperfine fields at 4.2 K consists of a main component ranging from 200 kOe to 400 kOe and a sub-component around 150 kOe. The average Fe-moment is estimated to be 1.96µ B for a -Fe 91.5 La 8.5 and 2.00µ B for a -Fe 82.5 La 17.5 . Hyperfine fields in a -Fe 91.5 La 8.5 appear progressively below 150 K, which is apparently higher than the spin glass temperature of 124 K determined by the ac susceptibility measurement. On the other hand, hyperfine fields in a -Fe 82.5 La 17.5 appear just below the ferromagnetic ordering temperature of 267.5 K, but increase anomalously below 70 K. This anomaly can be explained in terms of the freezing of transverse c...

Reentrant magnetism, antiferromagnetism, and domain wall pinning in nominally ferromagnetic Fe-Ni invar

We report measurements of the low field ( ≈ 10 Oe) near zero field quenched dc magnetization, M ZFQ, measured on warming in the interval 4.2–250 K for three Fe-Ni Invar alloys of nominal compositions, Fe 66 Ni 34, Fe 65 Ni 35, and Fe 64 Ni 36. The samples were characterized by chemical analysis, X-ray diffraction, Mössbauer spectroscopy, and high field magnetization, prior to and subsequent to various annealing and aging treatments which affected the measured M ZFQ curves. These curves showed two distinct behaviours: typical reentrant spin glass (RSG) behaviour was seen in virgin-state samples whereas a new behaviour involving linear in temperature dependencies was seen in all samples which contained crystallographic precipitates. Both ferromagnetic α-phase (bcc, T C≈1000 K) and antiferromagnetic low-spin γ'-phase (fcc, T N≈20 K) precipitates were identified. Their occurrence and production by heat treatment and/or aging were studied in detail and their amounts, sizes, and magnetic properties were inferred from their influence on M ZFQ. Both the RSG behaviour and the new behaviour are understood in terms of thermally activated depinning of domain walls, as is shown more fully in an accompanying paper. There, it is also shown that the new behaviour is predicted to occur under precisely the same conditions which lead to a logarithmic time dependence in the magnetic viscosity. It should therefore be termed normal viscous (NV) behaviour. A complete review of the evidence for low temperature antiferromagnetism in Fe-Ni Invar is given.

Mössbauer effect spectroscopy on the reentrant spin glass FeNiMn

We have performed 57Fe Mössbauer effect measurements on 16 alloys of the ternary (Fe 0.65Ni 0.35) 1- x Mn x system with and without an external magnetic field in the temperature range 4 K ⩽ T ⩽ 300 K varying χ in small steps between 0 ⩽ χ ⩽ 0.245. We have determined the mean hyperfine field H m using a novel mathematical method which delivers the intensity ratios of the Mössbauer lines, too. In a limited concentration range from χ = 0.039 to 0.126 a sharp kink in the course of H m( T) occurs in the low temperature region, which is indicating the onset of reentrant spin glass behaviour. We present a model in which the appearance of this kink is related to the freezing of frustrated spins. The determination of the Mössbauer line intensity ratio from in-field measurements shows that below the kink temperatures T k the spin structure of the alloys is changing from a nearly collinear to a random spin-glass-like arrangement. Our measurements result in a phase diagram for the alloy system investigated here containing distorted ferromagnetism, reentrant spin glasses, concentrated spin glasses and distorted antiferromagnetic phases. Additionally we make an attempt to determine the relative shares of ferromagnetic and antiferromagnetic coupled and the frustrated spins.

Carrier-Concentration-Dependent Magnetic Properties of the Diluted Magnetic Semiconductor SnMnte

ABSTRACTIn this paper experimental evidence is presented for the carrier concentration dependence of the magnetic properties of Sn0.97Mn0.03Te, yielding a critical concentration above which ferromagnetic interactions are dominant. The observed behavior can be fairly well explained within a modified RKKY-model. Preliminary experiments on the low temperature magnetic phases indicate re-entrant spinglass behavior, which is qualitatively described with the spinglass model of Sherrington and Kirkpatrick.

EFFECTS OF ALLOYING ON THE MAGNETIC AND STRUCTURAL BEHAVIOUR OF CeFe2

EFFECTS OF ALLOYING ON THE MAGNETIC AND STRUCTURAL BEHAVIOUR OF CeFe₂

Phase diagram and reentrant spin-glass behavior in a random mixture of two Ising-antiferromagnets Fe xMn 1−xTiO 3

Low-field DC susceptibility measurements have been made on a random-mixture of two Ising-antiferromagnets Fe xMn 1−xTiO 3. A spin-glass state appears in a wide range of intermediate concentration. In samples with certain x in the Fe-rich side or the Mn-rich side, the reentrant transition from the antiferromagnetic state or the random-antiferromagnetic state is found to occur. The reentrant behavior appears drastically, which we attribute to the strong frustrations existing in a magnetically non-diluted short-range system. The concentration versus temperature phase diagram is also shown.

Low-temperature spin waves in amorphous Fe90−xNixZr10

Inelastic neutron scattering measurements have been performed on rapid-quenched ribbons of Fe90−xNixZr10 (x=1,5,10) to measure the small wave-vector spin-wave spectrum. At all three concentrations a quadratic dispersion relation typical of an isotropic ferromagnet is observed, but with anomalous properties at low temperatures. For x=5 and 10 the excitations are found to broaden considerably for temperatures ≤0.3 Tc. For x=1 the spin waves not only broaden for T≤0.5Tc, but the spin-wave energies decrease monotonically with decreasing temperature while a resolution-limited quasielastic component of the scattering develops. These results are consistent with a picture of competing ferromagnetic and antiferromagnetic interactions, with reentrant spin-glass behavior evident if the frustration is sufficiently strong.