Concentrated Spin Glasses Research Articles

From FeNi-Invar to FeNiMn reentrant spin-glasses

Thirty years of Mossbauer spectroscopy correspond roughly to twenty-five years of FeNi-invar research using this method. The first systematic study of hyperfine fields in FeNi alloys, in both bcc and fcc phases, appeared in 1963, the first spectra were published probably in 1964. A very important fact is that fcc iron is an antiferromagnet; however, this iron phase is unstable at low temperature. In 1963, the possibility of performing low-temperature Mossbauer spectroscopy experiments on fcc iron precipitates in copper was shown. We shall demonstrate that the low temperature57Fe Mossbauer spectra reveal a spatially inhomogeneous magnetic structure in FeNi-invar alloys. This structure corresponds to magnetic invar anomalies. From the viewpoint of basic research in magnetism, it was opportune to enhance these anomalies. One possible way to do this was the alloying of manganese into the invar matrix. In this contribution, we limited our examples to the study of the classical Fe0.65Ni0.35 and the new (Fe0.65Ni0.35)1−x.Mnx alloys, and refer to the literature with respect to other transition metal systems with similar properties. We will show the goal of Mossbauer spectrometry investigating the spin structure in disordered alloy systems via the vector nature of the hyperfine fields. Additionally, the combination of Mossbauer spectrometry with global techniques, i.e. DC magnetization and AC susceptibility allowed us to study the changes in magnetic properties of our disordered 3d transition metal alloys from a nearly collinear ferromagnet (pure FeNi-invar) over the reentrant spin-glasses and “pure” concentrated spinglasses towards the antiferromagnetic behaviour by varying the manganese concentration in small steps.

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.

Magnetic phase diagrams of Fe1-xAlx concentrated spin glass alloys - interpretation in terms of short range chemical order and clustering

The details of the magnetic phase diagram of Fe1-xAlx alloys are discussed within a model exactly, describing the effects of short range chemical order and clustering. A good fitting to experimental data is obtained assuming that in the range of low concentrations of Al the clustering dominates while for other concentrations the partial short range chemical ordering exists. It is also explained why the temperature range of the re-entrant spin glass phenomenon is so narrow.

Field induced magnetization and energy gap in reentrant magnets and concentrated spin glasses

We present the temperature dependence of the field induced magnetization M(H,T) of a reentrant magnet and two concentrated spin glasses of the system Fe 80-xRu xB 20. In the former M (50k0e, T) reduces as T 3 2 at high T and as T below 40K. In the latter, as well as in the reentrant magnet at 7 k0e, M is roughly constant at low T and drops linearly with T at high T. These findings confirm the T dependence of M found by Liao and Bhagat from magnetic resonance data (on other alloys) and are interpreted in terms of their model.

Energy gap in concentrated spin glasses

Using magnetic resonance data at several frequencies, we have obtained the temperature dependence of the field-induced magnetization (M) in three concentrated spin glasses. At every frequency, M is independent of T for T≲TSG and drops linearly with T at higher temperatures. [M(0)−M(T)]/M(0) is closely represented by the function [exp(Δ/T)−1]−1, where Δ is a characteristic gap energy with values close to the corresponding Curie–Weiss temperatures θp but much larger than the respective spin-glass transition temperatures TSG.

Elementary excitations in reentrant magnets and concentrated spin glasses

We present a systematic study of the temperature dependence of the field induced magnetization, (M), derived from magnetic resonance data, in reentrant magnets and concentrated spin glasses. In the former, M reduces as T at low T and as T 3 2 at high T. In the latter, M is roughly constant at low T and drops linearly with T at high T. An attempt is made to understand the observations by introducing an energy gap in the magnetic excitation spectrum along with a non-zero density of states at the gap energy.

Anomalous frequency dependence of spin relaxation in the concentrated spin glass, a − (Fe 0.16Ni 0.84) 75P 16B 6Aℓ 3

Systematic studies of the frequency and temperature dependence of magnetic resonance in the most concentrated spin glass studied so far, a − (Fe 0.16Ni 0.84) 75P 16B 6Aℓ 3, show the low T linewidth rise observed previously in randomized spin systems. The parameter describing this rise not only has the resonant anomaly seen earlier in other concentrated spin glasses but also exhibits a dramatic increase below 3.5 GHz. The results are not adequately explained by any existing model.

Magnetic response near the critical concentration for ferromagnetism: Linear and nonlinear susceptibility in concentrated spin glasses

Low-field magnetization measurements are reported for a-Fe12Ni63G25 and a-Fe7Ni73G20. These are highly concentrated spin glasses since the critical Fe concentrations (xc) for onset of ferromagnetism are 12.4 and 7.7, respectively. Whereas the nonlinear susceptibility shows the expected rapid rise near Tg, it is remarkable that the close proximity to xc reveals itself via a similar sharp increase in the linear susceptibility. It turns out that all the data can be represented by a single empirical form: χ(t)=[χ(Tg)t−γ]/[Aχ(Tg)+t−γ].

Amorphous magnetic alloys near critical concentration: Low field reversible dc magnetization

Measurements of the reversible magnetization at low dc fields have been used to investigate the magnetic response near the multicritical point ( x c) of two sets of amorphous alloys. In both cases, the ferromagnetic (FM) to spin glass (SG) transition line is found to be non-monotonic. The collapse of the magnetization as x→ x + c and the rapid increase in the susceptibility as x→ x - c are suggestive of a percolation transition in the magnetic network at x= x c. From a study of the non-linear susceptibility in the most concentrated spin glass alloy in each system, we obtain scaling exponents in agreement with previous reports providing further support for a thermodynamic phase transition at the spin glass temperature. For the first time we find a divergence in the linear susceptibility in these samples similar to that expected for the non-linear susceptibility and attribute it to their proximity to the FM phase. Dramatic changes in the transition temperatures and a perceptible shift in x c are observed when normal boron is replaced by enriched boron (≈100% 11B) in one series of alloys.

Magnetic Phase Diagrams of Concentrated Spin Glass Systems - Application to Fe1-xAlx Alloys

The magnetic phase diagram of Fe1-xAlx alloys has been calculated. The short range comositional order was included. Two different models were used. The reentrant spin glass phenomenon has been obtained for the better of them. The effects of clustering were also discussed.

ESR study of the concentrated spin glass a-MnSi.

ESR study of the concentrated spin glass a-MnSi.

Spin relaxation in random magnets

The low-temperature spin resonance measurements in a variety of metallic and non-metallic spin glasses and reentrant alloys, over an extended frequency range (2- 35GHz), reveal a universal behavior of the resonance linewidth and the existence of a 'resonant' anomaly in concentrated metallic spin glasses. The results clearly point to the absence of time-scale for spin dynamics in random magnets.

The low temperature specific heat of Gd-Cu-Y metallic glasses

The specific heat of a series of amorphous aloys of composition Gd x Cu 0.37Y 0.63− x with 0 ⩽ x ⩽ 0.63 has been measured in the temperature range 2 to 50 K. A magnetic contribution to the specific heat is apparent in all the samples and has been determined with fair accuracy by subtracting the specific heat of corresponding nonmagnetic amorphous samples prepared with Lu rather than Gd. A qualitative study of the low field ac susceptibility of the magnetic samples has also been made. Samples with less than 35 at% Gd appear to be spin glasses, the results being qualitatively in accord with the predictions of the cluster mean field theory of concentrated spin glasses. As the Gd concentration is increased above 35 at% the samples behave increasingly like ferromagnets. However additional evidence from magnetisation measurements shows that even the sample with the highest concentration of Gd is not a simple ferromagnet.

Frequency dependence of magnetic resonance in concentrated metallic spin glasses

We report ESR measurements on two concentrated metallic spin glasses; a-Fe 7Ni 13P 14B 6 and a-Mn 48B 52 encompassing wide ranges of frequency (2–35 GHz) and temperature (4–300 K). The temperature dependence of the linewidth ( T) falls into three regimes; (i) T ≈ T 0, independent of T for T around 2–3 T SG, (ii) ( T- T 0)= - a + bT for T ⪆ 3 T SG; (iii) ( T- T 0) α T exp(- T/ T 0) for T ⪅2 T SG. The resonance fields yield a magnetisation varying as M( T, H)=[ M ∈( T) - C'/√ H] with M ∈( T) reducing linearly with T 3/2 for T ⪆ 20 K. At low T an anisotropy field becomes evident and varies with T as (1 - T/2 T SG). Comparison with a recent model based on coupling to two level systems is formally suggestive but quantitatively unsatisfactory.

ESR in concentrated spin glasses: Frequency/temperature dependence

ESR in Fe 7Ni 73P 14B 6 and Mn 48B 52 was studied over wide ranges of frequency and temperature. As T is lowered linewidth first reduces linearly, goes through a broad minimum, rises and finally has a maximum (or plateau) at ≈ 0.5 T SG. Resonance fields yield magnetization, M( H, T)=( M ∞- c/√ H), M ∞=M 0(1-BT 3 2 ) , and internal field H an ≅ H 0[1-( T/2 T SG )].

Resonant frequency dependence of spin relaxation in concentrated metallic spin glasses.

Resonant frequency dependence of spin relaxation in concentrated metallic spin glasses.

Partial thermoremanent magnetization and Bc( T) phase-diagram of a concentrated PtMn spin glass

From measurements of the remanence behavior of a Pt+10 at% Mn spin glass we determine critical temperatures T crit and fields B c in four different ways. A double logarithmic plot of B c = B 0(1- T/ T f(0)) β results in B 0 = (4.3±0.3) T and the critical exponent β = 2.1±0.2, as compared to β = 1.5 from theory.

Hyperfine tails and exchange-field distributions in amorphous magnetic spin glasses.

Almost nothing is yet known about the nature of the ``frozen disorder'' of magnetic moments in concentrated magnetic spin glasses (or ``speromagnets'') such as (amorphous) a-YIG (yttrium iron garnet), a-${\mathrm{FeF}}_{3}$, and a-${\mathrm{NaFeF}}_{4}$, beyond the qualitative description of ``randomlike.'' This paper probes the nature of the nearest-neighbor spin correlations in such systems by analysis of hyperfine-field distributions as measured by 4.2-K M\ossbauer spectroscopy. Low-field tails are found to exist on the hyperfine distribution spectra of amorphous speromagnets but not amorphous ferromagnets. They indicate the presence of low-energy exchange-field states in the former caused by the cancellation of nearest-neighbor contributions within the frustrated spin network. Their interpretation has been used to determine the detailed exchange distribution function for a-YIG, a-${\mathrm{FeF}}_{3}$, and a-${\mathrm{NaFeF}}_{4}$ and, by comparison of the results to the expectation for a set of randomly oriented spins, to monitor the nature of nearest-neighbor spin frustration in those amorphous speromagnetic systems.

Spin correlations in a concentrated metallic spin-glass.

We have performed polarized- and unpolarized-neutron scattering measurements on amorphous MnSi. The results are consistent with bulk susceptibility data for the same sample, and show that a-MnSi behaves like the classical, dilute-moment spin-glass, CuMn. Relative to the spin-glass transition temperature, the energy scale for spin fluctuations is much larger in this material than in other concentrated spin-glasses.

A New Oxide Spin Glass System of (1-x) FeTiO3–xFe2O3. I. Magnetic Properties

Magnetic properties of (1- x )Fe 2 TiO 3 – x Fe 2 O 3 with x near the percolation threshold x ∼0.2 have been studied by using single crystals. It has been found that 0.9FeTiO 3 –0.1Fe 2 O 3 exhibits typical characteristics of the cluster spin glass, while 0.79FeTiO 3 –0.21Fe 2 O 3 can be identified with the re-entrant spin glass. Referring to the neutron scattering data obtained for those compounds, the magnetic phase diagram has been determined. The results have been discussed based on the current theories on the concentrated spin glass systems.