Reentrant Spin Glass Transition Research Articles

Temperature dependence of the spin lattice relaxation time of proton nmr in the mixed antiferromagnets A1−x B x Cl2·2H2O

The mixed transition metal dichloride dihydrates A1−xBxCl2·2H2O (A, B=Co, Mn, Ni) have been prepared in order to understand the specificity of the Mn spins in the mixtures. The temperature dependences of the spin lattice relaxation times T1 of proton nuclear magnetic resonance (NMR) on these mixed crystals have been measured. In Ni1−xBxCl2·2H2O (B=Co, Mn), Mn substitution increased the relaxation rates 1/T1 more than Co substitution, even when the amounts of substitution were almost the same. In Co1−xBxCl2·2H2O (B=Mn, Ni), Mn has a significant impact on the relaxation rates in comparison with Ni. In Mn1−xBxCl2·2H2O (B=Ni, Co), the relaxation rates are much increased by a slight substitution of Co and exhibit a minimum in the temperature range of observation. This appearance of a minimum in the relaxation rates at low temperatures is believed to reflect an instability due to the occurrence of a reentrant spin-glass transition. A similar trend is seen at low temperatures in the case of Ni substitution, too. A Co1−xFexCl2·2H2O (x = 0.1) sample has been prepared, too. In this sample, a minimum of the relaxation rate is seen in the temperature range of observation. This may reflect an instability due to the occurrence of an oblique antiferromagnetic transition.

Magnetic transitions and spin-glass reentrance in two-dimensional [MnII(TCNE)(NCMe)2]X (X = PF6,AsF6,SbF6) molecular magnets

The structural, spectroscopic and magnetic properties of the two-dimensional (2D) molecule-based magnets of [MnII(TCNE)(NCMe)2]X (X = PF6, AsF6, SbF6; TCNE = tetracyanoethylene, NCMe = acetonitrile) composition are reported. It is shown that the alteration of the interlayer distance by increasing the anion size has little effect on the critical magnetic ordering temperature, Tc, suggesting that it depends predominantly on the intra-plane magnetic exchange. The observed field-induced irreversibility in static magnetization, a slow decay of isothermal remanence below Tc, and the dynamic susceptibility data are in accord with a re-entrant spin-glass nature of the ground state of all materials. In contrast to the isostructural Fe-based magnets, in which strong magnetocrystalline anisotropy facilitates the finite temperature magnetic ordering with the magnetization easy axis perpendicular to the μ4-TCNE•− plane, in the studied Mn-based magnets the easy axis is canted away from the normal direction, due to a small magnetocrystalline anisotropy. The two magnetic transitions observed on cooling are assigned to the ferrimagnetic long-range ordering of the normal magnetization component followed by the re-entrant spin-glass type transition resulting from a random freezing of the in-plane magnetization component.

Preparation, Dielectric, and Magnetic Characteristics of LuFe2O4 Ceramics

Preparation of LuFe2O4 ceramics under vacuum environment was investigated together with the structural, dielectric, and magnetic characterization. Single‐phase LuFe2O4 could be obtained by the present process and the crystal structure was identified to be rhombohedral in space group . An obvious dielectric relaxation with activation energy of 0.29 eV was observed between 175 and 275 K. The Néel temperature of the present ceramics was ~250 K, and a reentrant spin glass transition was indicated at ~216 K. Two anomalies were observed in the DSC curve, which were relative with the ferrimagnetic transition and 2D–3D charge ordering transition of LuFe2O4. Meanwhile, a remarkable decrease appeared in the dielectric constant of the as‐magnetized LuFe2O4 sample, implying the magnetodielectric effect in the present ceramics.

The evolution of the magnetic phases of Sb-doped Mn5Sn3 compounds

The structural and magnetic properties of Mn5(Sn1−xSbx)3 compounds (x = 0-0.4) were investigated by means of X-ray diffraction and magnetization measurements. All samples crystallize in Ni2In-type structure (P63/mmc) with the lattice parameters changing anisotropically with the increase of Sb concentration. Sb doping leads to the merging of the ferrimagnetic transition temperature Tc and the re-entrant spin-glass transition temperature Tf, indicating a direct transition from spin-glass state to paramagnetic state. The M(H) curve at 5 K evolves with x from a soft-magnet type to a linear-like shape. We proposed a basic explanation for the evolution of the magnetic properties in terms of crystal structure and magnetic interactions, which can be applied to other related compounds.

Reentrant spin-glass behavior induced by the frustration of Fe-Fe interactions in Laves phase Nb1−xHfxFe2 alloys

We carried out magnetization and specific-heat measurements of metallic compounds Nb1−xHfxFe2 (x = 0 and 0.65), which exhibit reentrant spin-glass freezing. The frequency dependence of ac susceptibility and the bifurcation between zero-field-cooled and field-cooled magnetization curves suggest the complex coexistence of magnetic phases at low temperatures. A long-time relaxation of magnetization is possibly due to the cluster-type and traditional reentrant spin-glass transition, which may be induced by the frustration of long-range Fe-Fe interactions. We did not observe any discontinuity indicating long-range magnetic phase transition in the data of specific heat versus temperature, but a broad hump characteristic of spin-glass freezing. The difference between the behaviors of two samples proves the existence of two types of reentrant spin-glass in alloys. The moment arrangements are briefly discussed.

Magnetic ordering in the ilmenite‐hematite solid solution: A computational study of the low‐temperature spin glass region

Magnetic ordering in the ilmenite‐hematite solid solution (Fe2‐xTixO3) has been investigated using Monte Carlo simulations, with particular emphasis on the low‐temperature spin glass region of the phase diagram. Complex magnetic behavior is observed due to the presence of two competing magnetic order parameters: a “hematite‐like” ordering with a two‐layer repeat (Q2) and an “ilmenite‐like” ordering with a four‐layer repeat (Q4). The susceptibility and degree of magnetic order were calculated from the Fourier transform of the layer‐averaged spin distribution, allowing long‐range and short‐range contributions from Q2 and Q4 to be analyzed separately. For x < 0.8, the ferrimagnetic (FM) phase remains stable down to 0 K. For x ≥ 0.8 a heterogeneous FM phase (HFM) followed by a modulated FM phase (MFM) develops. There is an increasing contribution from Q4 with increasing x, and a pronounced cusp in both Q2 and Q4 susceptibilities develops at 30 K. The superposition of Q2 and Q4 leads to frustrated layers containing dynamically disordered spins. Freezing of this spin disorder below 30 K is responsible for the cusp in susceptibility, which can be classified as a reentrant spin glass (RSG) transition. A gradual loss of long‐range FM order occurs as the percolation threshold is approached, resulting in a conventional spin glass (CSG) with no long‐range order below 30 K for x ≥ 0.92. For x > 0.95, a transition to an antiferromagnetic (AF) phase occurs at 40–55 K, followed by an RSG transition at 20–30 K. Changes to the phase diagram caused by chemical clustering are determined using a preannealing algorithm. Clustering expands the AF field to x > 0.9 and the HFM field to x ≥ 0.55. The topology of the simulated phase diagram compares favorably with experiments but suggests that the nature of some phase boundaries should be reexamined from both experimental and computational perspectives.

Spin-glass-like behaviour in IrSr 2RECu 2O 8 ( RE=Sm and Eu)

We report the results of magnetic and specific heat measurements on the 1212-type compounds IrSr 2 RECu 2O 8 with RE=Sm and Eu, prepared by high-pressure and high-temperature synthesis. The magnetic susceptibility of these compounds shows a large difference in the temperature dependence of the magnetization measured under zero-field-cooled and field-cooled conditions below 87 and 71 K, respectively, and upon further cooling below ∼10 K substantial maxima are observed too. Further AC susceptibility measurements support a glassy behaviour in lower magnetic transitions whereas the specific heat measurements do not show the typical long-range ordering commonly displayed in ferro, ferri or antiferromagnetic transitions. Hysteresis loops suggest the presence of magnetic clusters in the otherwise paramagnetic zone, indicating that these compounds probably display a reentrant spin-glass transition. Results are presented and discussed.

Ac susceptibility studies of multiferroicBiMnO3 and solidsolutions between BiMnO3 and BiScO3

The real and imaginary parts of the ac susceptibilities of powderBiMnO3 and BiMn1−xScxO3 (x = 0.1,0.2, 0.3, 0.4, 0.5, and 0.7) samples have been measured as functions of temperature, differentdriving ac and applied dc magnetic fields, and different ac magnetic field frequencies. Bothχ′ versus T and χ′′ versus T showed strong dependence on the parameters of the ac magnetic field for ferromagnetic samplesBiMnO3 and BiMn1−xScxO3 (x = 0.1,0.2, and 0.3), indicating that the ac field interacted mainly with the domain structure. InBiMn1−xScxO3 (x = 0.1, 0.2, and 0.3), a re-entrant spin-glass transition emerged at lowtemperatures. The re-entrant spin-glass transition temperatureTRSG increased withincreasing x (TRSG = 4 Kfor x = 0.1, TRSG = 5 Kfor x = 0.2,and TRSG = 9 K for x = 0.3). For BiMn1−xScxO3 (x = 0.4,0.5, and 0.7), only the classical spin-glass transition was found (the freezing temperatureTf = 24 Kfor x = 0.4, Tf = 18 Kfor x = 0.5,and Tf = 7 K for x = 0.7) with characteristic frequency dependence of broad maxima on theχ′ versus T and χ′′ versus T curves and no dependence on the driving ac field.

Distinguishable reentrant spin-glass behavior induced by the frustration of exchange interaction in phase-separatedSm0.5(Ca,Sr)0.5MnO3systems

In order to understand the relationship between the stability of the charge-ordered state and the low-temperature phase competition, the magnetic and electrical properties are systemically investigated for half-doped ${\mathrm{Sm}}_{0.5}{A}_{0.5}\mathrm{Mn}{\mathrm{O}}_{3}$ systems with a dissimilar average $A$-site cationic radius of Ca and Sr, which induces a complete different magnetic ordering and the complex coexistence of magnetic phases at the lower temperature. Some abnormal phenomena are observed in both systems. The frequency dependence of ac magnetization and the bifurcation between zero-field-cooled and field-cooled magnetization curves indicate the reentrant spin-glass behavior in both oxides, which may be induced by the frustration of the ferromagnetic double-exchange interaction and antiferromagnetic superexchange interaction. A long-time relaxation of magnetization is possibly due to the cluster-type and traditional reentrant spin-glass transition. The difference between the behaviors of these two series proves the existence of two types of reentrant spin glass.

Fluctuating clusters in a reentrant spin-glass system

We theoretically examined the reentrant spin-glass transition in a dilute ferromagnetic (FM) system. Results show that clusters of fluctuating spins exist in the FM phase. As the temperature is decreased, the spins in those clusters freeze in conjunction with all other spins in the system. This freezing breaks up the FM order into ferromagnetic clusters. These results are compatible with those of a phenomenological random field model that was proposed for experimentation.

Reentrant spin-glass transition in a dilute Heisenberg ferromagnet

We perform a large-scale Monte Carlo simulation of a dilute ferromagnetic Heisenberg model with antiferromagnetic next-nearest neighbor interactions. We find that the model reproduces a reentrant spin-glass (RSG) transition. That is, as the temperature is decreased, the magnetization M once increases rapidly below a certain temperature, reaches a maximum value, and then disappears at some lower temperature. The low-temperature phase is suggested to be a spin-glass (SG) phase that is characterized by ferromagnetic clusters. We present a possible mechanism responsible for the RSG transition.

Re-entrant spin glass behavior in Mn-rich YMnO3

We use magnetism and specific heat measurements to investigate the hexagonal Mn-rich YMnO3. It is found that upon cooling from a high temperature, the compound first orders antiferromagnetically at TN∼72K and then undergoes a re-entrant spin glass (RSG) transition at TSG∼42K. This RSG behavior results from the competition between the ferromagnetic interaction and the antiferromagnetic interaction, which is related to the intrinsic geometric magnetic frustration in this system.

Reentrant Spin-Glass Transition in a Dilute Magnet

We perform a large-scale Monte Carlo simulation of a dilute classical Heisenberg model with ferromagnetic nearest neighbor and antiferromagnetic next-nearest neighbor interactions. We found that the model reproduces a reentrant spin-glass transition. That is, as the temperature is decreased, the magnetization increases rapidly below a certain temperature, reaches a maximum value, and then disappears at some lower temperature. The low temperature phase was suggested to be a spin-glass phase that is characterized by ferromagnetic clusters.

Enhancement of Tc and the re-entrant spin glass transition inLa0.86Ca0.14Mn1−yCryO3 (y = 0,0.1 and 0.2)

We report on structural, frequency dependent ac susceptibility, dc magnetizationand magnetoresistance (MR) measurements on polycrystalline samples ofLa0.86Ca0.14Mn1−yCryO3 (y = 0, 0.1 and 0.2) prepared by the sol–gel technique. Fory = 0, a paramagnetic to ferromagnetic transition was observed atTc = 185 K.Fory = 0.1, the value ofTc = 200 K, an increase of15 K, and for y = 0.2, Tc = 195 K,anincrease of 10 K. The imaginary part of the ac susceptibility for all three samples shows a secondary transitionat Tf<Tc.For y = 0, there is no definite law accounting for the frequency dependence ofTf and it is attributed to a transition arising out of a canted structure. However, in the cases ofy = 0.1 and 0.2, the frequency dependence indicates the presence of a re-entrant spin glass transition atTf. Although all three samples show a semiconducting behaviourbetween 300 and 5 K, a negative MR was observed corresponding toTc and Tf. The value of MR decreased for the Cr substituted samples.

Magnetic Phase Diagram of Three-Dimensional Diluted Ising Antiferromagnet Ni0.8Mg0.2(OH)2

$H$-$T$ diagram of 3D diluted Ising antiferromagnet Ni$_{c}$Mg$_{1-c}$(OH)$_{2}$ with $c$ = 0.8 has been determined from measurements of SQUID DC magnetization and AC magnetic susceptibility. At $H$ = 0, this compound undergoes two magnetic phase transitions: an antiferromagnetic (AF) transition at the N\'{e}el temperature $T_{N}$ (= 20.7 K) and a reentrant spin glass (RSG) transition at $T_{RSG}$ ($\approx$ 6 K). The $H$-$T$ diagram consists of the RSG, spin glass (SG), and AF phases. These phases meet a multicritical point $P_{m}$ ($H_{m}$ = 42 kOe, $T_{m}$ = 5.6 K). The irreversibility of susceptibility defined by $\delta$ (= $\chi_{FC} - \chi_{ZFC}$) shows a negative local minimum for 10 $\leq H \leq$ 35 kOe, suggesting the existence of possible glassy phase in the AF phase. A broad peak in $\delta$ and $\chi^{\prime \prime}$ at $H \geq$ 20 kOe for $T_{N}(c=0.8,H) \leq T \leq T_{N}(c=1,H=0)$ (= 26.4 K) suggests the existence of the Griffiths phase.

Dynamic susceptibility of reentrant Fe-rich inhomogeneous amorphous alloys

Melt-spun amorphous alloys of Fe91Zr9, Fe91Zr7B2, Fe90Zr7B3 and Fe88Zr8B4 have been characterized by AC susceptibility at frequencies between 30 Hz and 10 KHz. The measurements reflect the existence of reentrant spin glass transitions below 40 K for all cases. The transition shift per frequency decade is large compared to the observed in conventional spin-glass alloys. The shift value increases with the boron content and becomes closer to values in fine-particle systems. We find that the relaxation in Fe91Zr9 follows a critical slowing down at the reentrant transition, with exponents \(z\nu = 7.2\) and \(\beta = 0.7\). The non-linear susceptibility displays a peak at the transition in this alloy, but wider than in canonical spin glasses. A Vogel-Fulcher (VF) activation process can explain the frequency variation in all the Fe-Zr-B alloys. The reduction of the ideal glass temperature in the VF approach, found in the higher Boron content alloys, is an indication of a superparamagnetic-like behavior. The behavior shown by these alloys is intermediate between a collective freezing and superparamagnetic-like relaxation. We propose that this is arising due to a heterogeneous spin structure. The inclusion of a reduced amount of B, affects the magnetic spin structure.

Spin freezing and the ferromagnetic and reentrant spin-glass phases in a reentrant ferromagnet

The reentrant spin-glass (RSG) transition and the magnetic nature of the RSG and ferromagnetic (FM) phases of a standard reentrant ferromagnet ${(}^{57}\mathrm{Fe}$-doped) NiMn were investigated using ac susceptibility and M\"ossbauer measurements. The spin-freezing temperature, at the observation time of the ac magnetic method, was determined by a peak in the in-phase component of nonlinear susceptibility. The distribution of the hyperfine field in the zero-field M\"ossbauer data consisted of two peaks in the FM phase but one antisymmetric peak in the RSG phase. The average hyperfine field rapidly increases so as to deviate from the Brillouin function below a certain temperature. The onset of deviation corresponds to spin freezing on the M\"ossbauer time scale. The two peaks in the hyperfine field in the FM phase were assigned to two kinds of spin groups having different relaxation times. The spin-freezing process can be explained based on the concept of ``melting of frustrated spins'' introduced by Saslow and Parker. The local magnetization, deduced based on the in-field M\"ossbauer data, was dependent on the applied field in the same manner as the magnetization in both the RSG and FM phases. This indicates that a spin-glass correlation coexists with ferromagnetic order in the RSG phase in a different way from the mean-field model for vector spin glasses. The application of a magnetic field induced one additional peak in the hyperfine field in the RSG phase. This implies that the long-range spin-glass order becomes unstable by applying a magnetic field in the RSG phase. Based on all the experimental results, we construct a comprehensive picture of a reentrant ferromagnet.

Magnetic properties of the reentrant spin glass FeCr 2 xIn 2−2 xS 4

Mössbauer spectroscopy, and magnetization measurements were performed on FeCr 2 x In 2−2 x S 4 ceramic samples with x=0.1 and 0.3. Mössbauer hyperfine parameters indicate that iron is in a Fe 2+ state, with major population at octahedral B sites and ∼10% of Fe 2+ at A sites of the spinel structure. Low-field DC magnetization data shows a sharp onset of the magnetic order at T C=156(1) K. Below this temperature, field-cooled (FC) and zero-field-cooled (ZFC) curves display strong irreversibility. The irreversibility decreases with increasing applied magnetic field. Reentrant spin-glass transition is observed at T sg=9.0(1) K, which shifts to lower T and broadens with applied field. From AC data at different driving frequencies f, it is found that the frequency shift of T sg follows a Vogel–Fulcher law ln ( f/f 0)=−E a /k B (T sg −T 0) with E a=3.2×10 −22 J and T 0=7.9 K.

Spin Frustration Effect in Magnetic Graphite Intercalation Compounds

The magnetic phase transition of stage-2 CucCo1-cCl2 GIC's has been studied using SQUID AC magnetic susceptibility. For 0.4 ≤ c ≤ 0.9 the system undergoes a phase transition at Tcl and a reentrant spin glass transition at TRSG (<Tcl). For 0.9<c ≤ 0.93 the system undergoes a spin glass transition at TSG from the paramagnetic phase to the spin glass phase. No phase transition is observed for c = 1 at least above 0.3 K. The nature of reentrant spin glass phase and spin glass phase is examined from the frequency dependence of absorption X″.

Dynamical response of vibrating ferromagnets

The resonance frequency of vibrating ferromagnetic reeds in a homogeneous magnetic field can be substantially modified by intrinsic and extrinsic field-related contributions. Searching for the physical reasons of the field-induced resonance frequency change and to study the influence of the spin glass state on it, we have measured the low-temperature magnetoelastic behavior and the dynamical response of vibrating amorphous and polycrystalline ferromagnetic ribbons. We show that the magnetoelastic properties depend strongly on the direction of the applied magnetic field. The influence of the re-entrant spin glass transition on these properties is discussed. We present clear experimental evidence that for applied fields perpendicular to the main area of the samples the behavior of ferromagnetic reeds is rather independent of the material composition and magnetic state, exhibiting a large decrease of the resonance frequency. This effect can be very well explained with a model based on the dynamical response of the reed and the magnetomechanical pole effect within a domain rotation model and is not related to magnetoelasticity.