Glassy Magnetic Behavior Research Articles

Low thermal conductivity in a new mixed metal telluride Mn1.8(1)In0.8(1)Si2Te6.

The design of new complex mixed metal tellurides (containing low toxicity cations) with intrinsic ultralow thermal conductivity is of paramount importance in the field of thermoelectrics. Herein, we report the synthesis and characterization of polycrystalline and single crystals of a new mixed-metal quaternary telluride Mn1.8(1)In0.8(1)Si2Te6. The structural aspects and chemical formula of this phase at room temperature have been established using single crystal X-ray diffraction and EDX studies. The trigonal centrosymmetric (space group: P3̄1m) structure of the title phase has cell constants of a = b = 7.0483(7) Å and c = 7.1277(8) Å. The structure has three independent cationic sites, one mixed (In1/Mn1), one partially filled Mn2, and one Si1, which are bonded with Te1 atoms. Each metal atom (In and Mn) in the structure is octahedrally coordinated with six neighboring Te1 atoms. The structure also features dimers of Si atoms, and each Si atom is bonded to three Te1 atoms to form ethane-like Si2Te6 units. The optical absorption study of a polycrystalline Mn1.8In0.8Si2Te6 sample shows a narrow optical bandgap of 0.6(2) eV. Temperature-dependent resistivity and Seebeck coefficient studies confirmed the p-type semiconducting nature of the sample with high values of S (301 μV K-1 to 444 μV K-1). The total thermal conductivity (ktot) study of the polycrystalline sample shows a decreasing trend on heating with an extremely low value of 0.28 W m-1 K-1 at 773 K. Magnetic measurements indicate a glassy magnetic behavior for the sample below 8 K.

Magnetic phase diagram of Ge1–x–y(SnxMny)Te multiferroic semiconductors: Coexistence of ferromagnetic and cluster glass ordering

Noncentrosymmetric IV-VI semiconductors doped with magnetic ions offer exciting possibilities such as carrier mediated ferromagnetism, ferroelectric-controlled spin texture and integration of semiconducting features with ferromagnetic order. Also, coupling between more than one ferroic orders and giant Rashba spin splitting can be studied to develop materials for spintronic applications. In that context, we report the structural and magnetic results of polar α–GeTe doped with Sn and Mn from x = 0.185–0.841 and y = 0.02–0.086, respectively. The magnetic results of Ge1–x–y(SnxMny)Te (GSMT) crystals identify Mn-clustering effect with scaling parameter, R = 0.033 for x ≈ 0.2 and y = 0.06. The excessive Sn ions are assumed to drive the inception of short range ferromagnetic clusters. For the crystals revealing glassy magnetic behavior, the irreversibility temperature, Tirr shifts at high dc magnetic field which is well described by de Almeida-Thouless equation, δTirr ∝ HΦ/2 yielding Φ values of 1.55 and 1.7 that validates the formation of Mn clusters. The spin relaxation time, τ0 ∼ 10–9 s, activation energy, Ea/kB ∼ 5 TF where TF is freezing temperature and Vogel-Fulcher temperature, T0 ∼ TF also signify intracluster interactions. Also, the Mn-hole magnetic exchange constant, Jpd drops from 0.24 eV for the samples with smaller Sn content of x ≈ 0.2 to 0.16 eV for Sn rich alloy with x ≈ 0.8. Consequently, we present a magnetic phase diagram for GSMT bulk crystals as a function of Mn ions.

Study of the synergistic role of multivalence states of 3d cations on the crystal lattice distortion and magnetic behavior of Sr2FeCoO6-δ: A spectroscopic study

In perovskite oxides, the multiple charge states of cations influence their local structure and correlated physical properties. In this report, we studied the role of multivalence cations on the local structure and magnetic behavior of the oxygen-deficient system Sr2FeCoO6-δ (SFCO), prepared by the solid-state synthesis technique. A detailed structural and geometrical analysis confirmed the coexistence of orthorhombic (Brownmillerite) and cubic crystal symmetries and a gesture of the Jahn-Teller (JT) distortion. The intense Raman modes are allowed to be indexed for BO-polyhedra and JT-distortion. Experimental and theoretical analysis of the X-ray photoelectron spectroscopy (XPS) spectra supported the mixed oxidation state of Fe (Fe3+/Fe4+) and Co (Co2+/Co3+), further confirmed by theoretically simulated X-ray absorption spectroscopy (XAS) spectra. The presence of the multivalence states leads to AFM/FM exchange interaction in the sample which further leads to glassy magnetic behavior.

Field-driven Modulation of Spin Hall Magnetoresistance in Nanoscale-layered Pt/IrMn₃/NiFe Heterostructures

Spin Hall magnetoresistance (SMR) emerges from both spin Hall effect (SHE) and inverse spin Hall effect (ISHE), which is usually investigated in a nonmagnetic heavy metal (NM) in contact with a ferromagnet (FM). Depending on the orientation of FM magnetization and spin polarization of NM layer, the transmission or reflection of the spin current at the FM\NM interface could occur. SMR was investigated in 1.5 nm NiFe\3.0 nm IrMn₃\3.0 nm Pt magnetic heterostructures. IrMn₃ thin film grown on ferromagnetic NiFe layer exhibits the glassy magnetic behavior and displays spin reorientation transition in the magnetic field-temperature space. Below the Néel temperature of IrMn₃, the angle dependence of MR measurement in the xy and xz plane showed a normal positive SMR. However, a sign change in SMR was observed in the magnetic field applied in the yz plane. The switching from negative to positive SMR occurs at the field above a critical value of 6 kG. Our results provide a potential advantage for the source of spin flow in the signal in multilayer heterostructures and highlight the importance of magnetic structure (Néel vector) in AF, which opens the possible way to manipulate the spin current transportation and AF memory devices.

Spin glass behavior and magnetic boson peak in a structural glass of a magnetic ionic liquid

Glassy magnetic behavior has been observed in a wide range of crystalline magnetic materials called spin glass. Here, we report spin glass behavior in a structural glass of a magnetic ionic liquid, C4mimFeCl4. Magnetization measurements demonstrate that an antiferromagnetic ordering occurs at TN = 2.3 K in the crystalline state, while a spin glass transition occurs at TSG = 0.4 K in the structural glass state. In addition, localized magnetic excitations were found in the spin glass state by inelastic neutron scattering, in contrast to spin-wave excitations in the ordered phase of the crystalline sample. The localized excitation was scaled by the Bose population factor below TSG and gradually disappeared above TSG. This feature is highly reminiscent of boson peaks commonly observed in structural glasses. We suggest the “magnetic” boson peak to be one of the inherent dynamics of a spin glass state.

Growth of ordered and disordered epitaxial thin films of Lu2MnNiO6 using (0 0 1)-LaAlO3 and (0 0 1)-SrTiO3 substrates: Observation of six monoclinic in-plane twin variants and glassy magnetic behaviour

It is well known that in double perovskites, the degree of cation ordering plays a very important role in determining the intrinsic magnetic properties. Therefore, in order to distinguish between the intrinsic properties and the disordered cation driven magnetic properties, we have grown ordered and disordered epitaxial thin films of Lu2MnNiO6 (LMNO) using two different cubic substrates (001)-LaAlO3 and (001)-SrTiO3 (LAO and STO respectively) by pulsed laser deposition (PLD) method. It is found that both LMNO films grow in monoclinic crystal symmetry and possess two out-of-plane domains (110) and (001). Moreover, various in-plane rotations of these domains give rise to a total of six in-plane twin variants. The films show dominant ferromagnetic nature. Due to good lattice matching with LAO, LMNO/LAO films are well-ordered and exhibit a sharp ferromagnetic to paramagnetic transition at TC = 38 K, whereas, the large lattice mismatch with STO resulted in disordered LMNO/STO films and TC is lowered to 34 K. The value of maximum magnetization (MS, at 70 kOe) in disordered LMNO/STO (MS = 2.4 μB/f.u.) is almost half of that in the ordered LMNO/LAO (MS = 4.0 μB/f.u.). Interestingly, the well-ordered LMNO/LAO films show memory effect and relaxation of magnetization in the entire temperature range below TC. This is in conformance to the intrinsic glassy magnetic nature of LMNO.

Coupling between phase transitions and glassy magnetic behaviour in Heusler alloy Ni50Mn34In8Ga8

The transition sequence in the Heusler alloy Ni50Mn34In8Ga8 has been determined from measurements of elasticity, heat flow and magnetism to be paramagnetic austenite → paramagnetic martensite → ferromagnetic martensite at ∼335 and ∼260 K, respectively, during cooling. The overall pattern of elastic stiffening/softening and acoustic loss is typical of a system with bilinear coupling between symmetry breaking strain and the driving structural/electronic order parameter, and a temperature interval below the transition point in which ferroelastic twin walls remain mobile under the influence of external stress. Divergence between zero-field-cooling and field-cooling determinations of DC magnetisation below ∼220 K indicates that a frustrated magnetic glass develops in the ferromagnetic martensite. An AC magnetic anomaly which shows Vogel–Fulcher dynamics in the vicinity of ∼160 K is evidence of a further glassy freezing process. This coincides with an acoustic loss peak and slight elastic stiffening that is typical of the outcome of freezing of ferroelastic twin walls. The results suggest that local strain variations associated with the ferroelastic twin walls couple with local moments to induce glassy magnetic behaviour.

Coupling of magnetoresistance switching and glassy magnetic state at the metal–insulator transition in Ruddlesden-Popper manganite Ca4Mn3O10

Electronic phase separation is a frequent ingredient of the physics of manganites for which the colossal magnetoresistance (CMR) phenomenon is at debate. We investigate the switchable magnetoresistance (MR) effect of the Ruddelsden-Popper (RP) oxide Ca4Mn3O10 (CMO), which shows a metal-to-insulator transition (MIT) at 70 K (TMIT). The static (DC) and dynamic (AC) magnetic susceptibilities of CMO indicate an unusual magnetic phase coexistence below the Néel temperature (TN) i.e., at TMIT, which we describe as a glassy magnetic behavior. Furthermore, by means of synchrotron X-ray diffraction, we observed a structural anomaly directly associated with the MIT. Hence, the onset of glassiness correlates with a slight structural distortion. Interestingly, CMO displays a large positive magneto-resistance (PMR) effect (up to 550% at 4 K) in the metallic state below TMIT, while a small negative magneto-resistance (NMR) effect (-3.5% at 112 K) in the insulating region above TMIT. At TMIT, the PMR switches to NMR behavior. PMR is probably induced via enhancing the exchange interaction by localization of itinerant moment resulting in formation of magnetic polarons, whereas NMR is possibly due to reduced electron-spin scattering. This appears to be a special type of magnetoresistance, different from regular observations, and may open a new avenue in searching for exotic phenomena in manganese-based CMR materials.

A comparative study in different rare earth ions in multiferroic nanocomposites: Low temperature resistivity minima and low field magnetoresistance

Low temperature magnetoresistance and resistivity properties are systematically studied in rare earth based multiferroic nanocomposites La0.7Sr0.3MnO3-TMnO3 (T = Er,Ho) under magnetic fields up to 5 Tesla. Resistivity with temperature exhibits metal-insulator transition. At very low temperature, the sharp upturn of resistivity is observed, which is found to be suppressed by field. The upturn of resistivity is characterized with spin fluctuation of the system. The reduction of the upturn of resistivity with magnetic field ensures the progress of the electrons tunneling. Effective reduction of charging energy is obtained upon an applied magnetic field. Temperature dependent resistivity shows minima at low temperature under different applied magnetic fields. The resistivity fit at low temperature is made with consideration of equally electron-electron interactions concerning (temperature)1/2 term and the Kondo-identical spin dependent scattering for ln(temperature) dependence. Lessening of the spin fluctuation by an applied magnetic field plays a dynamical role in defining the magnetoresistive behavior in samples. Experimental data are analyzed and demonstrated through a simple proposed phenomenological model to provide an insight on the physical mechanism behind the low temperature transport phenomena. The derived surface spin susceptibility is accordance with the temperature dependence of magnetoresistance involving an intergranular surface magnetization. Temperature dependent field cooled and zero field cooled dc magnetization curve having a large bifurcation between them, is a suggestion of a glassy like phase that may be originated from the challenging magnetic exchange interactions. The low temperature glassy magnetic behavior is endorsed to a spin-glass identical frozen state for the surface of the nanoparticles. Magnetic field dependent magnetization study shows an increase in magnetization without saturation in decreasing temperature. The multiferroic nanocomposites exhibit the magnetoelectric coupling, which arises due to the strain mediated magnetostriction behavior of the piezomagnetic phase (La0.7Sr0.3MnO3) in the nanocomposites.

Glassy Magnetic Behavior and Correlation Length in Nanogranular Fe-Oxide and Au/Fe-Oxide Samples.

In nanoscale magnetic systems, the possible coexistence of structural disorder and competing magnetic interactions may determine the appearance of a glassy magnetic behavior, implying the onset of a low-temperature disordered collective state of frozen magnetic moments. This phenomenology is the object of an intense research activity, stimulated by a fundamental scientific interest and by the need to clarify how disordered magnetism effects may affect the performance of magnetic devices (e.g., sensors and data storage media). We report the results of a magnetic study that aims to broaden the basic knowledge of glassy magnetic systems and concerns the comparison between two samples, prepared by a polyol method. The first can be described as a nanogranular spinel Fe-oxide phase composed of ultrafine nanocrystallites (size of the order of 1 nm); in the second, the Fe-oxide phase incorporated non-magnetic Au nanoparticles (10–20 nm in size). In both samples, the Fe-oxide phase exhibits a glassy magnetic behavior and the nanocrystallite moments undergo a very similar freezing process. However, in the frozen regime, the Au/Fe-oxide composite sample is magnetically softer. This effect is explained by considering that the Au nanoparticles constitute physical constraints that limit the length of magnetic correlation between the frozen Fe-oxide moments.

Magnetic phases of skyrmion-hostingGaV4S8−ySey(y=0,2,4,8)probed with muon spectroscopy

We present the results of a muon-spin spectroscopy investigation of ${\mathrm{GaV}}_{4}{\mathrm{S}}_{8\ensuremath{-}y}{\mathrm{Se}}_{y}$ with $y=0,2,4$, and 8. Zero-field measurements suggest that ${\mathrm{GaV}}_{4}{\mathrm{Se}}_{8}$ and ${\mathrm{GaV}}_{4}{\mathrm{S}}_{8}$ have distinct magnetic ground states, with the latter material showing an anomalous temperature dependence of the local magnetic field. It is not possible to evolve the magnetic state continuously between these two systems, with the intermediate $y=2$ and 4 materials showing glassy magnetic behavior at low temperature. The skyrmion lattice (SkL) phase is evident in the $y=0$ and 8 materials through an enhanced response of the muon-spin relaxation to the emergent dynamics that accompany the SkL. For our polycrystalline samples of ${\mathrm{GaV}}_{4}{\mathrm{Se}}_{8}$, this enhanced dynamic response is confined to a smaller region of the magnetic field-temperature phase diagram than the previous reports of the SkL in single crystals.

Unusual magnetic properties of the polar orthorhombic BiFe0.5Sc0.5O3 perovskite

Magnetic properties of new multiferroic BiFe0.5Sc0.5O3 perovskite which exists in two different orthorhombic polymorphs, the antipolar Pnma and the polar Ima2, were studied using a SQUID-magnetometer technique in magnetic fields up to 50 kOe in the temperature range of 5–300 K. Both polymorphs show a weak ferromagnetic component below the Néel temperature, TN∼220 K. A wide cusp-like anomaly of the temperature dependent magnetic susceptibility was revealed below 200 K. The anomaly is dependent on the magnetic field applied and can be associated with the glassy magnetic behavior of the compound. The best fit of the initial magnetization curves taken at each temperature can be obtained as a sum of two contributions, namely the nonlinear in field and the linear one. The nonlinear contribution appeared to be temperature and magnetic field dependent, which is in a good agreement with the magnetization experimentally obtained as a function of temperature in different magnetic fields applied. Increase of external magnetic field leads to a decrease of the nonlinear contribution to the total magnetic moment.

Size-dependent exchange bias in single phase Mn3O4 nanoparticles**Project supported by the National Natural Science Foundation of China (Grant No. 11464007), the Natural Science Foundation of Guangxi, China (Grant Nos. 2012GXNSFGA060002 and 2014GXNSFBA118241), the Guangxi Key Laboratory of Information Material Foundation, China (Grant No. 131021-Z), and the Guangxi Department of Education Foundation, China (Grant Nos. YB2014120 and KY2015YB104).

Glassy magnetic behavior and exchange bias phenomena are observed in single phase Mn3O4 nanoparticles. Dynamics scaling analysis of the ac susceptibility and the Henkel plot indicate that the observed glassy behavior at low temperature can be understood by taking into account the intrinsic behavior of the individual particles consisting of a ferrimagnetic (FIM) core and a spin-glass surface layer. Field-cooled magnetization hysteresis loops display both horizontal and vertical shifts. Dependence of the exchange bias field (HE) on the cooling field shows an almost undamped feature up to 70 kOe, indicating the stable exchange bias state in Mn3O4. HE increases as the particle size decreases due to the higher surface/volume ratio. The occurrence of the exchange bias can be attributed to the pinning effect of the frozen spin-glass surface layer upon the FIM core.

Magnetization Reversal and Glassy Behavior in Tb0.7Sr0.3MnO3

Magnetization reversal and glassy phase in Tb0.7Sr0.3MnO3 are reported. Polycrystalline samples of Tb0.7Sr0.3MnO3 are prepared by solid-state synthesis. This material crystallizes in orthorhombic Pbnm space group with lattice parameters: a = 5.471(1)A, b = 7.608(2)A, and c = 5.398(2)A. Field-cooled/zero field-cooled magnetization measurements show two magnetic transitions, one at 40 K and another at 8 K. The transition at 40 K is due to magnetic ordering of Mn while that at 8 K arises due to Tb ordering. Low field-cooled measurements reveal magnetization reversal at low temperature. The temperature at which this occurs increases with decrease in field strength. The observed magnetization reversal is attributed to the competition between interacting Mn (3 d) and Tb (4 f) magnetic sublattices which makes the system ferrimagnetic. The glassy magnetic behavior below 50 K and absence of long-range order in Tb0.7Sr0.3MnO3 is confirmed through ac susceptibility studies.

Interface adjustment and exchange coupling in the IrMn/NiFe system

The exchange bias effect was investigated, in the 5–300K temperature range, in samples of IrMn [100Å]/NiFe [50Å] (set A) and in samples with inverted layer-stacking sequence (set B), produced at room temperature by DC magnetron sputtering in a static magnetic field of 400Oe. The samples of each set differ for the nominal thickness (tCu) of a Cu spacer, grown at the interface between the antiferromagnetic and ferromagnetic layers, which was varied between 0 and 2Å. It has been found out that the Cu insertion reduces the values of the exchange field and of the coercivity and can also affect their thermal evolution, depending on the stack configuration. Indeed, the latter also determines a peculiar variation of the exchange bias properties with time, shown and discussed with reference to the samples without Cu of the two sets. The results have been explained considering that, in this system, the exchange coupling mechanism is ruled by the glassy magnetic behavior of the IrMn spins located at the interface with the NiFe layer. Varying the stack configuration and tCu results in a modulation of the structural and magnetic features of the interface, which ultimately affects the spins dynamics of the glassy IrMn interfacial component.

Modifications in the frustrated magnetism, oxidation state of Co and magnetoelectric coupling effects induced by a partial replacement of Ca by Gd in the spin-chain compound Ca3Co2O6

We have systematically investigated the influence of the gradual replacement of Ca by Gd on the magnetic and complex dielectric properties of the well-known geometrically frustrated spin-chain system Ca3Co2O6 (TN = 24 K with additional magnetic transitions below 12 K), by studying the series Ca3−xGdxCo2O6 (x ≤ 0.7), down to 1.8 K. Heat-capacity measurements establish that the reduction of TN with Gd substitution is much less compared to that by Y substitution. The magnetic moment data reveal that there are changes in the oxidation state of Co as well, unlike for Y substitution, beyond x = 0.2. Thus, despite being isovalent, both these substitutions interestingly differ in changing these magnetic properties in these oxides. We propose that the valence electrons of Y and those of R ions play different roles in deciding the magnetic characteristics of these mixed oxides. It is observed that a small amount (x = 0.3) of Gd substitution for Ca is enough to suppress glassy ac magnetic susceptibility behavior for the peak around 12 K. An additional low-temperature magnetic anomaly close to 5 K gets more prominent with increasing Gd concentration as revealed by heat-capacity data. Trends in temperature dependence of complex dielectric behavior were also tracked with varying composition and a frequency dependence is observed, not only for the transition in the region around 10 K (for some compositions), but also for the 5 K transition which is well resolved for a higher concentration of Gd. Thus, the Gd-substituted Ca3Co2O6 series is shown to reveal interesting magnetic and dielectric behaviors of this family of oxides.

Glassy magnetic behavior in the metamagnetic DyAlO3 doped with Cr

Magnetic properties of DyAl0.926Cr0.074O3 and DyAlO3 were studied. We found that both the compounds are antiferromagnetic with a low Néel transition temperature. At higher temperatures the magnetic characteristics show a Curie–Weiss dependence. Néel temperature disappears when a field of about 2 T is applied, the system changes from an antiferromagnetic to a weak ferromagnetic behavior due to a metamagnetic transition. Furthermore, AC magnetic measurements in the Cr-doped compound, at different frequencies, show a spin glass-like behavior. These transitions were studied and corroborated by specific heat measurements. We found the presence of metamagnetism and spin glass in the compound doped with chromium, determining that the small addition of chromium atoms modifies the magnetic properties of the compound DyAlO3, resulting in new features such as the spin glass-like behavior.

Enhancement of ferromagnetic correlations on multiferroic TbMnO3 by replacing Mn with Co

The structural, electronic and magnetic properties of TbMn1−xCoxO3 (0.1 ≤ x ≤ 0.9) compounds are reported. The samples are isostructural to TbMnO3 adopting the orthorhombic distorted perovskite structure (Pbnm), except for x = 0.4, 0.5 and 0.6, where an ordered double perovskite structure (P21/n) is found. X-ray absorption spectra at the Mn and Co K edges show an incomplete charge transfer between Mn and Co atoms yielding a mixed valence state Mn3+/Mn4+ and Co3+/Co2+ for the whole series. Neutron powder diffraction measurements show the development of a ferromagnetic ground state for the intermediate compositions (0.3 ≤ x ≤ 0.6) indicating that the ferromagnetic superexchange Mn4+–O–Co2+ interaction is the strongest among a wide set of competitive interactions. The ferromagnetic ordering is, however, not fully achieved and coexists with glassy magnetic properties. With increasing concentration of Co (x ≥ 0.7) the long range ordering vanishes and only a glassy magnetic behavior with slow dynamics is found. These properties could be related to the existence of magnetically inhomogeneous small clusters arising from competitive magnetic interactions.

Glassy magnetic phase driven by short-range charge and magnetic ordering in nanocrystalline La1/3Sr2/3FeO3−δ: Magnetization, Mössbauer, and polarized neutron studies

The charge ordered La$_{1/3}$Sr$_{2/3}$FeO$_{3-\delta}$ (LSFO) in bulk and nanocrystalline forms are investigated using ac and dc magnetization, M\"{o}ssbauer, and polarised neutron studies. A complex scenario of short range charge and magnetic ordering is realized from the polarised neutron studies in nanocrystalline specimen. This short range ordering does not involve any change in spin state and modification in the charge disproportion between Fe$^{3+}$ and Fe$^{5+}$ compared to bulk counterpart as evident in the M\"{o}ssbauer results. The refinement of magnetic diffraction peaks provides magnetic moments of Fe$^{3+}$ and Fe$^{5+}$ are about 3.15$\mu_B$ and 1.57$\mu_B$ for bulk, and 2.7$\mu_B$ and 0.53$\mu_B$ for nanocrystalline specimen, respectively. The destabilization of charge ordering leads to magnetic phase separation, giving rise to the robust exchange bias (EB) effect. Strikingly, EB field at 5 K attains a value as high as 4.4 kOe for average size $\sim$ 70 nm, which is zero for the bulk counterpart. A strong frequency dependence of ac susceptibility reveals cluster-glass like transition around $\sim$ 65 K, below which EB appears. Overall results propose that finite size effect directs the complex glassy magnetic behavior driven by unconventional short range charge and magnetic ordering, and magnetic phase separation appears in nanocrystalline LSFO.

The low‐temperature magnetic signature of Fe‐rich serpentine in CM2 chondrites: Comparison with terrestrial cronstedtite and evolution with the degree of alteration

We have studied the low temperature magnetic properties of one CI and five CM2 chondrites, well characterized in the literature, and of cronstedtite, the Fe end‐member of the (Fe,Mg)‐serpentine solid solution. Cronstedtite is highly anisotropic and its relaxation properties below ≈10 K suggest a glassy magnetic behavior. Frustration of the magnetic interactions in octahedral layers would account for the observed magnetic freezing, with a possible role of the random distribution of ferric iron in tetrahedral layers. Based on the comparison between powders and crystals characterized by SCXRD and EMPA, we show that grain size and substitution of octahedral Fe by Mg induce changes in the position of the low‐Tmagnetic susceptibility peak. A low‐Tpeak is also found in each of the CM2 chondrites, but not in the CI chondrite. It is interpreted as the signature of Fe‐serpentines, which are considered by most authors to have mostly formed during asteroïdal aqueous alteration events. The low‐Tmagnetic properties of the less altered meteorites compare well with those of cronstedtite, and the position of the susceptibility peak varies with the published degree of alteration. By comparing magnetic data on reference cronstedtite, magnetic and published mineralogical data on CM2 chondrites, we suggest that the evolution of the low‐Tmagnetic signature of the latter reflects bulk changes in Fe‐serpentines' chemical composition, rather than only mixing between Fe‐rich and Mg‐rich serpentines. This is analogous to previous observations on the changes in Fe incorporation to Mg‐serpentines during progressive serpentinization of abyssal peridotites.