Spin-glass Freezing Research Articles

Finite size versus surface effects on magnetic properties of antiferromagnetic particles

The observation of finite magnetic moment in antiferromagnetic materials is quite unusual and has been immensely investigated in nanoparticle systems. Here, the structural and magnetic properties of NiO particles are explored by x-ray diffraction, extended x-ray absorption fine structure, and magnetization measurements. Using similar-sized particles with different surface defect structure, we show that the observed magnetic enhancement, which is present even beyond finite-size limit, is due to the surface effects. However, the well known spin glass freezing is found to occur only in nano-regime.

Spin State of Co Ions in Lightly Doped Strontium LaCoO3: Via Study of High-Field Induced Spin State Transition

We have investigated the high field magnetization of La 1- x Sr x CoO 3 for x ≤0.05 up to µ 0 H ext = 67 T. In a dilute limit, about 30 nonmagnetic Co ions of two species (Co I and Co II in LaCoO 3 ) are converted to magnetic Co ions (Co III ) by introducing one Sr 2+ , leaving the remaining Co I and Co II little affected by the Sr doping. This finding shows that a new spin-phase separation with three Co species appears by the Sr doping. The Co III ions form a magnetic cluster with a net magnetization several times smaller than that expected from the ferromagnetic aligned full moment. A spin-glass freezing appears below T g = 10 and 18 K for x = 0.03 and 0.05, respectively, in a weak magnetic field of 2 mT. In µ 0 H ext ≥10 T, however, the magnetization of Co III for 0.01 ≤ x ≤0.05, as a function of temperature and H ext , is almost independent of x after normalization with the Co III population. It means that the magnetic correlation among Co III within the cluster is robust against H ext , but the magnetic interactions among the clusters, which cause the spin glass transition, are decoupled by H ext . The origin of the strong magnetic interaction among Co III in the cluster is considered to arise from the mobile hole within the cluster as proposed in “spin-polaron” model.

Optimization of Pr0.9Ca0.1MnO3 thin films and observation of coexisting spin-glass and ferromagnetic phases at low temperature

Optimization of thin films of small bandwidth manganite, Pr1−xCaxMnO3 (for x = 0.1), and their magnetic properties are investigated. Using different pulsed laser deposition (PLD) conditions, several films were deposited from the stoichiometric target material on SrTiO3 (001) substrate and their thorough structural and magnetic characterizations were carried out using x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy (XPS), SQUID magnetometry and ac susceptibility measurements. A systematic investigation shows that irrespective of the growth temperature (between 550 and 750 °C), all the as-deposited films have twin boundaries and magnetic double phases. Post-annealing in partial or full oxygen pressure removes the extra phase and the twin boundaries. Zero-field-cooled magnetization data show an antiferromagnetic to paramagnetic transition at around 100 K whereas the field-cooled magnetization data exhibit a paramagnetic to ferromagnetic transition close to 120 K. However, depending on the oxygen treatments, the saturation magnetization and Curie temperature of the films change significantly. Redistribution of oxygen vacancies due to annealing treatments leading to a change in ratio of Mn3+ and Mn4+ in the films is observed from XPS measurements. Low temperature (below 100 K) dc magnetization of these films shows metamagnetic transition, high coercivity and irreversibility magnetizations, indicating the presence of a spin-glass phase at low temperature. The frequency dependent shift in spin-glass freezing temperature from ac susceptibility measurement confirms the coexistence of spin-glass and ferromagnetic phases in these samples at low temperature.

Spin-glass behavior of the polyvinyl pyrrolidone-protected Prussian blue analog K1.14Mn[Fe(CN)6]0.88 nanocubes

Prussian Blue analog K1.14Mn[Fe(CN)6]0.88 nanocubes were synthesized by using polyvinyl pyrrolidone (PVP) as a protective matrix. The PVP-protected MnFe PBA nanocubes with face centered cubic structure are well dispersed with a narrow size distribution of around 50nm. A spin-glass behavior (including hysteresis, a peak in the zero-field-cooled magnetization and frequency-dependent AC magnetic susceptibility) is observed in the nanoparticles. A possible origin of this spin-glass freezing is discussed. Spin disorder due to the structural defects may be the reason that causes the spin-glass freezing in the MnFe PBA nanoparticles.

Effect of dipolar and exchange interactions on magnetic blocking of maghemite nanoparticles

Magnetic interparticle interactions compete with the magnetic blocking of ultrafine magnetic nanoparticles. We have prepared maghemite (γ-Fe 2O 3) nanoparticles by microwave plasma synthesis as a loose powder and in compacted form. In ZFC/FC measurements, blocking temperature of the compacted sample C is larger than that of the powder sample P. The frequency dependence of AC susceptibility of the sample C shows a large shift of blocking temperature with increasing frequency. Vogel–Fulcher law gives a large value of T 0 for the sample C. To get evidence of a possible spin-glass freezing in both samples, scaling law fitting is applied to the AC susceptibility data. The value of the exponent ( zv) of the critical slowing down dynamics fits to the spin-glass regime for both samples. For the sample P, spin-glass freezing occurs on the surface of individual nanoparticles, while in the sample C surface spin-glass freezing is concomitant with a superspin-glass formation as a consequence of coupling between particles. The sample C also shows an enhancement of coercivity due to dipolar interactions among the nanoparticles. Exchange interactions are attributed only to touching nanoparticles across their interfaces. All these measurements indicate the presence of strong interparticle dipolar interactions in the compacted sample C.

Distinguishing magnetic blocking and surface spin-glass freezing in nickel ferrite nanoparticles

Nickel ferrite nanoparticles dispersed in SiO2 matrix have been synthesized by sol-gel method. Structural analysis has been performed by using x-ray diffraction and transmission electron microscopy. Magnetic properties have been investigated by using superconducting quantum interference device magnetometry. In addition to the average blocking temperature peak at TB=120 K measured by a zero field cooled temperature scan of the dc susceptibility, an additional hump near 15 K is observed. Temperature dependent out-of-phase ac susceptibility shows the same features: one broad peak at high temperature and a second narrow peak at low temperature. The high temperature peak corresponds to magnetic blocking of individual nanoparticles, while the low temperature peak is attributed to surface spin-glass freezing which becomes dominant for decreasing particle diameter. To prove the dynamics of the spin (dis)order in both regimes of freezing and blocking, the frequency dependent ac susceptibility is investigated under a biasing dc field. The frequency shift in the “frozen” low-temperature ac susceptibility peak is fitted to a dynamic scaling law with a critical exponent zv=7.5, which indicates a spin-glass phase. Exchange bias is turned on at low temperature which signifies the existence of a strong core-shell interaction. Aging and memory effects are further unique fingerprints of a spin-glass freezing on the surface of isolated magnetic nanoparticles.

Magnetic properties of Mn-oxide nanoparticles dispersed in an amorphous SiO2 matrix

Samples of Mn-oxide nanoparticles dispersed in an amorphous SiO2 matrix with manganese concentration 0.7 and 3at% have been synthesized by a sol–gel method. Transmission electron microscopy analysis has shown that the samples contain agglomerates of amorphous silica particles 10–20nm in size. In silica matrix two types of Mn-rich particles are dispersed, smaller nanoparticles with dimensions between 3 and 10nm, and larger crystalline areas consisting of aggregates of the smaller nanoparticles. High-temperature magnetic susceptibility study reveals that dominant magnetic phase at higher temperatures is λ-MnO2. At temperatures below TC=43K strong ferrimagnetism originating from the minor Mn3O4 phase masks the relatively weak magnetism of λ-MnO2 with antiferromagnetic interactions. Magnetic field dependence of the maximum in the zero-field-cooled magnetization for both the samples in the vicinity of 40K, and a frequency shift of the real component of the ac magnetic susceptibility in the sample with 3at% Mn suggest that the magnetic moments of the smaller Mn3O4 nanoparticles with dimensions below 10nm are exposed to thermally activated blocking process just below the Curie temperature TC. Appearance of a maximum in the zero-field-cooled magnetization for both the samples below 10K indicates possible spin glass freezing of the magnetic moments at low temperatures which might occur in the geometrically frustrated Mn sublattice of the λ-MnO2 crystal structure.

Effect of noncollinear spin structure on magnetic entropy change and its field dependence in Fe90Sc10 amorphous alloy

Temperature-dependent magnetic entropy change (ΔSm) in Fe90Sc10 shows a sign conversion due to field-driven metamagnetic transition from spin-glass-like to ferromagnetic state. It further reveals a clear correlation between spin-glass structure and field-dependent −ΔSm. The exponent n as in |−ΔSm|=αHn reaches a minimum value of 1.01 at spin-glass freezing onset temperature, followed by an increase with further decreasing temperature. The coefficient α shows an opposite temperature dependence with respect to the n. Variations of high-field susceptibility with temperature evidence association of the n with existence of the noncollinear spin structure, which in turn affects the field-dependent behavior of −ΔSm.

Low-temperature magnetic properties of Heusler compoundsRu2−xFexCrSi(x=0.1,0.3,and0.5)

We carried out magnetization $M(T)$ and specific-heat ${C}_{P}(T)$ measurements of the new Heusler compounds ${\text{Ru}}_{2\ensuremath{-}x}{\text{Fe}}_{x}\text{CrSi}$ $(x=0.1,\text{ }0.3,\text{ }\text{and}\text{ }0.5)$, which exhibit spin-glass freezing. $M(T)$ has peak- and irreversibility-type anomalies. The temperatures at which these anomalies appear have magnetic field dependence described by the Gabay-Toulouse and de Almeida-Thouless lines in the low-field range. For the magnetic specific heat ${C}_{m}(T)$, we did not observe a discontinuity indicating long-range magnetic phase transition but a broad hump characteristic of spin-glass freezing. ${C}_{m}(T)$ in the low-temperature range is described as a combination of linear- and quadratic-$T$ terms. The quadratic-$T$ dependence of ${C}_{m}(T)$ is probably associated with excitation of the Ising component of the freezing spins.

Vacancy-induced spin-glass behavior of Prussian blue analogue Fe II1.1Cr IIx[Cr III (CN) 6] 0.6− x· nH 2O nanowires

Prussian blue analogue Fe II 1.1Cr II x [Cr III(CN) 6] 0.6− x · nH 2O nanowires were synthesized by electrodeposition. The magnetic properties investigation indicates that the nanowires exhibit cluster spin-glass behavior, which undergoes a magnetic transition to a frozen state below about 62 K. Spin disorder arising from reduced coordination and broken exchange bonds between spin centers due to the structural defects may be the reason that causes the spin-glass freezing behavior. The negative magnetization observed at temperature lower than the compensation temperature ( T comp ∼43 K) at a field of 10 Oe may be due to the different temperature dependences of the ferromagnetic site Fe–Cr and antiferromagnetic site Cr–Cr.

Spin-glass and novel magnetic behavior in the spinel-type [formula omitted

A dual non-magnetic substitution system on A- and B-sites in the spinel structure has been studied. The mother compound is a ferromagnet CuCr 2 S 4 with the Curie temperature T c ≃ 380 K . A system of Cu 1 ‐ x Ag x CrSnS 4 , which is the same notation as ( Cu 1 ‐ x Ag x ) ( Cr 0.50 Sn 0.50 ) 2 S 4 , has been prepared over the entire range of 0.00 ≤ x ≤ 1.00 although the Cr-Sn sublattice is unchanged in the fixed composition of 0.50 on B-sites. All these compounds exhibit the spin-glass phase with the freezing temperature T g approximately at 16 K in 100 Oe. Since only Cr ions have the magnetic moment on the B-sites, the substitution of Ag for Cu on the A-sites does not influence strongly the spin-glass freezing behavior over the whole composition range. Nevertheless, the magnetization of Cu 1 ‐ x Ag x CrSnS 4 with x = 0.50 and 0.55 cause a broad upturn hump over 30–130 K where the spin-glass phase is broken. Strong magnetic field dependence of this hump anomaly has been observed with an irreversibility between zero-field-cooled (ZFC) and field-cooled (FC) magnetizations even though above T g . The hump is suppressed in higher fields and collapsed down at approximately 1.0 kOe with a tiny trace quantity of the anomaly where the difference between the ZFC and FC processes disappears. The specimen with x = 0.45 shows a small hump anomaly in low field < 20 Oe which corresponds to a precursor of the huge anomaly for x = 0.50 . The hump anomaly could be attributed to a formation of the cluster-glass. The spin-clusters are embedded in the matrix of spin-glass elements in high degree of disorder without long-range order. All the spins eventually are frozen below T g . The strange magnetic freezing originates from the delicate dual substitutions. The mechanism of the anomaly is far from a complete picture and remains enigmatic.

Surface Spin-glass Freezing and Blocking in NiFe2O4 Nanoparticles

AbstractWe prepared single-phase nickel ferrite nanoparticles separated by silicon dioxide using sol-gel method with tetraethyl orthosilicate (TEOS) as a precursor for SiO2. The magnetic properties are investigated by using SQUID-magnetometry over a broad temperature range (4.2 – 350 K), magnetic field (2–70,000 Oe) and frequency (0.1 – 1000 Hz) range. The particle size is in the range 8 – 12 nm. Exchange bias and spin disorder appear at the core-shell interface due to broken bonds on the surface. Disorder and core-shell interaction induces spin-glass freezing which is manifested by a low temperature peak in the AC susceptibility well separated from magnetic blocking peak. This low temperature peak is assigned to spin-glass freezing. The proof of spin-glass freezing is managed by zero field cooled/field cooled (ZFC/FC), frequency and DC field dependence of AC susceptibility, low temperature hysteresis loop and time dependent thermoremanent magnetization at different temperatures. All the measurements stated above signify blocking/unblocking at higher temperatures and surface spin-glass freezing at low temperatures. The aim of our work is to contribute to a better understanding of “spin-frozen” magnetic ferrite nanoparticles at diameters 8 – 12 nm which could be important in future for stabilizing the magnetic state of “core-shell”-structured nanomagnets.

Magnetic field dependence of the magnetization of a 29 nm thick AuFe spin glass film

We performed polarized neutron reflectometry (PNR) experiments on a 29 nm thick Au93Fe7 film. The measurements were done in a temperature range from 300 K to 2 K in a magnetic field up to 6 T applied in the sample's plane, at 2 K we took PNR data up to 11 T. The magnetization as determined by PNR can be described with a Brillouin function from 295 K down to 50 K assuming the magnetic moment of isolated Fe atoms, i.e. 4 |μB per Fe atom. Below 50 K the onset of the spin-glass freezing was observed as a strong reduction of the magnetic moment compared to the isolated Fe atom.

Core-shell model of the vacancy concentration and magnetic behavior for antiferromagnetic nanoparticle

The local defect structure of NiO nanoparticles was determined by extended x-ray absorption fine structure method. By using the bulk and surface sensitive characterization techniques, we are able to show that the vacancies mostly reside on the surface of the particles and the distribution of vacancies can be considered within a core-shell model. We argue that these observations can give a suitable explanation for the venerable problem of observed magnetic behaviors of antiferromagnetic nanoparticles and other functional properties of the material. The observed magnetic moment has been attributed to the interacting vacancies inside the antiferromagnetic host and the distribution of vacancies over the particle volume determines behaviors such as size-dependent different nature of spin-glass freezing and exchange bias.

ChemInform Abstract: Magnetism in Quasicrystals

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Electronic Transport and Magnetic Properties in Y$_{0.125}$Ca${\rm }_{0.875}$MnO$_{3}$ Manganite

A rare-earth-like manganite Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.125</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.875</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> was systemically studied by the structure, electric and magnetic measurement. The results show that there exists a considerable magnetoresistance effect in this system. And two transitions are found on electrical transport corresponding to conventional metal-insulator (M-I) transition and Kondo-like behavior. From the magnetization in the zero-field-cooled (ZFC) and field-cooled (FC), a large irreversibility was observed with a cusp at 108 K about considered as spin-glass freezing temperature. Below 108 K, a coexistence of ferromagnetic spin-glass and ferromagnetic ordering phase appears, which indicates a long-range ferromagnetic behavior in spin-glass state. These suggest an intrinsic ferromagnetic spin-glass state in the Y <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.125</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.875</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> system at low temperatures and may be suppressed with applied higher field of about 8.0 Tesla.

Disorder effect on the magnetic behavior of mechanically alloyedFe1−xAlx(0.2⩽x⩽0.4)

We report on the magnetic properties of Fe1�xAlx alloys 0.2x 0.4 produced by mechanical alloying by milling pure element powders for t = 12, 24, and 36 h. The alloys present a bcc lattice with compositional disorder and are ferromagnetic at room temperature, independently of the milling time. The lattice parameter of the x = 0.2 sample presents a small decrease with t, whereas those of the x = 0.3 and 0.4 samples remain constant independently of the milling time. The magnetic properties of the alloys with x = 0.2 and 0.3 do not show important variations with t, while those of x = 0.4 are strongly dependent on the milling time. For this latter alloy it was found that: i despite being the most diluted of the series, it presents a well developed ferromagnetic order at room temperature as the Mossbauer and hysteretic data have shown; ii the temperature dependence of the ac susceptibility and the Mossbauer spectra recorded at different temperatures evidence the occurrence of reentrant spin-glass and superparamagnetic phenomena. The enhancement of the ferromagnetic behavior and the presence of reentrant spin-glass freezing temperature and of a superparamagnetic blocking process are interpreted in terms of a simple localized model based on the disorder present in that alloy and on the occurrence of competitive interactions, namely, the ferromagnetic nearest-neighbor Fe-Fe interactions and the antiferromagnetic near-nearest-neighbor Fe-Fe ones. Taken together, these results evidence that the stabilization of the magnetic order takes place in the x = 0.4 sample exclusively through the induction of compositional disorder and without any contribution from the lattice expansion.

Magnetic properties of the Zn-doped Haldane-gap material NENB

Experimental studies of the spin-1 Haldane-chain material [Ni(C2H8N2)2NO2](BF4) (NENB) doped with diamagnetic Zn(II) ions in a range up to nominally 5% by means of the magnetic-susceptibility and the electron-spin resonance (ESR) techniques are reported. The presence of fractional S = 1/2 chain-end states, revealed by ESR and susceptibility measurements is found to be responsible for spin-glass freezing effects. It is suggested that a higher doping with Zn ions suppresses the spin-glass behaviour by creating shorter and isolated chain fragments.

Complex magnetic order in quasi-one-dimensional compound Ca3CoIrO6

In the spin-chain compound Ca 3 CoIrO 6 muon spin precession and relaxation spectra at temperatures above 100 K exhibit non-exponential behavior characteristic of a system of dynamic, disordered magnetic moments. On cooling below about 100 K the onset of spontaneous magnetic order is revealed via the presence of coherent oscillations in muon spin relaxation spectra taken in zero external field. This magnetic transition is attributed to the intra-chain ferromagnetic order. There is one more change in the μ SR signal associated with the spin-glass like freezing transition at T f = 30 K previously established by bulk susceptibility and neutron scattering data.

Spin-glass order induced by dynamic frustration

Disorder and geometric frustration usually lead to magnetic spins that point in random directions, as in a spin glass. So how can spin-glass behaviour emerge in a well-ordered system without static frustration? The presence of ‘dynamic frustration’ may explain the situation. Spin glasses are systems whose magnetic moments freeze at low temperature into random orientations without long-range order1. It is generally accepted that both frustration and disorder are essential ingredients in all spin glasses, so it was surprising that PrAu2Si2, a stoichiometric compound with a well-ordered crystal structure, was reported2 to show spin-glass freezing. Here, we report on inelastic neutron scattering measurements of crystal-field excitations, which show that PrAu2Si2 has a singlet ground state and that the exchange coupling is very close to the critical value to induce magnetic order. We propose that spin-glass freezing results from dynamic fluctuations of the crystal-field levels that destabilize the induced moments and frustrate the development of long-range magnetic correlations. This novel mechanism for producing a frustrated ground state could provide a method of testing the concept of ‘avoided criticality’ in glassy systems.