Magnetic Order-disorder Transition Research Articles

Magnetic anisotropy in heterogeneous amorphous thin films: insights from thickness- and temperature-driven spin-reorientation

Magnetization orientation in thin films is intricately influenced by multiple anisotropy components, with the dominant anisotropy serving as a key determinant. This complexity becomes particularly intriguing when considering thin films composed of subnanometer-scale heterogeneous amorphous structures. Our investigation builds upon this foundation, specifically focusing on the Fe–Ni–B–Nb alloy system, known for its moderate glass-forming ability and susceptibility to nanocrystallization. In this study, we present thickness- and temperature-driven spin-reorientation (SRT) transition, attributed to competing magnetic anisotropy energies in thin films featuring a heterogeneous amorphous structure. Thermogravimetric investigations unveiled a unique heterogeneous amorphous structure, a revelation unattainable through conventional structural analysis methods. The observed spontaneous perpendicular magnetization in amorphous films, as evidenced by transcritical hysteresis loops and magnetic stripe domains, is ascribed to the pronounced residual stress arising from the substantial magnetostriction of the alloy system. The temperature-driven SRT is correlated to the order-disorder magnetic transition of the heterogeneous amorphous phase, characterized by a Curie temperature of ∼225 K. This transformative magnetic state of the heterogeneous amorphous matrix limits the exchange interaction among the densely distributed α-Fe nuclei regions, ultimately governing the dynamic magnetic responses with varying temperature. This work provides valuable insights into the dynamic magnetic orientation of thin films, especially those with heterogeneous amorphous structures, contributing to the broader understanding of the underlying mechanisms of magnetization reversals.

The multiple magnetic transitions and magnetic entropy change of RGa compounds

<p indent=0mm>Magnetic refrigerant materials working at middle and low temperature range have drawn much attention because they can be used for gas liquefaction such as oxygen, nitrogen, hydrogen and helium. It is continuous goal for researchers to explore and obtain magnetic refrigerant materials with large magnetic entropy change, wide refrigerant temperature span, high refrigeration capacity and large adiabatic temperature change. RGa family is a kind of rare earth based intermetallic compounds with abundant magnetic transitions. This paper reviews the results of low-temperature order-order magnetic transition, high-temperature order-disorder magnetic transition and magnetic entropy change for PrGa, NdGa, SmGa, GdGa, TbGa, DyGa, HoGa, ErGa, TmGa and Er₁₋_{<italic>x</italic>}Gd_{<italic>x</italic>}Ga compounds. The low-temperature order-order magnetic transition includes spin reorientation transition, ferromagnetic to modulated antiferromagnetic transition and cone-ferromagnetic to cone-ferromagnetic transition. There are several peaks on the magnetic entropy change curves which provide ideas for designing magnetic refrigerant materials with large temperature span. The characteristics of magnetic transitions and magnetic entropy change for RGa compounds are analyzed, the dependency relationship together with related physical mechanism are expounded and the possible research interests in the future are discussed.

New ordered phase in geometrically frustrated generalized XY model.

Critical properties of a geometrically frustrated generalized XY model with antiferromagnetic (AFM) and third-order antinematic (AN3) couplings on a triangular lattice are studied by Monte Carlo simulation. It is found that such a generalization leads to a phase diagram consisting of three different quasi-long-range ordered (QLRO) phases. Compared to the model with the second-order antinematic (AN2) coupling, besides the AFM and AN3 phases which appear in the limits of relatively strong AFM and AN3 interactions, respectively, it includes an additional complex canted antiferromagnetic (CAFM) phase. It emerges at lower temperatures, wedged between the AFM and AN3 phases as a result of the competition between the AFM and the AN3 couplings, which is absent in the model with the AN2 coupling. The AFM-CAFM and AN3-CAFM phase transitions are concluded to belong to the weak Ising and weak three-state Potts universality classes, respectively. Additionally, all three QLRO phases also feature true LRO of the standard and generalized chiralities, which both vanish simultaneously at second-order phase transitions with non-Ising critical exponents and the critical temperatures slightly higher than the magnetic and nematic order-disorder transition temperatures.

Phase diagram and dynamics of the SU(N) symmetric Kondo lattice model

In heavy-fermion systems, the competition between the local Kondo physics and intersite magnetic fluctuations results in unconventional quantum critical phenomena which are frequently addressed within the Kondo lattice model (KLM). Here we study this interplay in the SU($N$) symmetric generalization of the two-dimensional half-filled KLM by quantum Monte Carlo simulations with $N$ up to 8. While the long-range antiferromagnetic (AF) order in SU($N$) quantum spin systems typically gives way to spin-singlet ground states with spontaneously broken lattice symmetry, we find that the SU($N$) KLM is unique in that for each finite $N$ its ground-state phase diagram hosts only two phases -- AF order and the Kondo-screened phase. The absence of any intermediate phase between the $N=2$ and large-$N$ cases establishes adiabatic correspondence between both limits and confirms that the large-$N$ theory is a correct saddle point of the KLM fermionic path integral and a good starting point to include quantum fluctuations. In addition, we determine the evolution of the single-particle gap, quasiparticle residue of the doped hole at momentum $(\pi,\pi)$, and spin gap across the magnetic order-disorder transition. Our results indicate that increasing $N$ modifies the behavior of the coherence temperature: while it evolves smoothly across the magnetic transition at $N=2$ it develops an abrupt jump -- of up to an order of magnitude -- at larger but finite $N$. We discuss the magnetic order-disorder transition from a quantum-field-theoretic perspective and comment on implications of our findings for the interpretation of experiments on quantum critical heavy-fermion compounds.

Magnetism and Magnetocaloric Efffects of R3Pd4 (R = Nd and Pr) Compounds

The magnetic properties and magnetocaloric effects of the intermetallic compounds R3Pd4 (R = Nd and Pr) were investigated. The results show that the ground state of the Nd3Pd4 compound is weakly antiferromagnetic below the Neel temperature TN of 9.4 K, which can be induced to be ferromagnetic state with a small applied field. For the Pr3Pd4 compound, it does not show any magnetic order-disorder transition down to 5 K. In the paramagnetic region, the reciprocal magnetic susceptibilities χ−1 for both compounds obey the Curie-Weiss law. The values of the effective magnetic moment for R3Pd4 (R = Nd and Pr) are determined to be 3.83 μB/Nd3+ and 3.51 μB/Pr3+, respectively, and the corresponding paramagnetic Curie temperatures were determined to be 6.5 K and − 17 K. For a magnetic field change of 0–5 T, the maximum magnetic entropy changes (-ΔSM) for Nd3Pd4 and Pr3Pd4 are found to be 7.8 J/kg K and 6.9 J/kg K, respectively. The corresponding value of refrigerant capacity (RC) for the Nd3Pd4 compound is estimated to be 74 J/kg.

Thermodynamic and magnetic properties of magnesium-gallium ferrite ceramics

A comparative study of the thermodynamic and magnetic properties of the Ga-substituted magnesium ferrite polycrystalline samples Mg(Fe1-xGax)2O4 with x = 0.2 and x = 0.4 was carried out. The low-temperature heat capacity of the sample with x = 0.2 was measured by adiabatic calorimetry. Based on the smoothed heat capacity data Cp,m°(T) for Mg(Fe0.8Ga0.2)2O4, the standard thermodynamic functions were determined in a temperature range 2–800K. A broad heat capacity anomaly below 60K, which can be attributable to the magnetic order-disorder transition, was observed for both samples. The field-dependence of magnetizations in an applied magnetic field up to ± 50kOe revealed the ferrimagnetic behaviour of the samples. The values of Curie temperature (TC) and the blocking temperature (TB) were determined from ZFC-FC magnetization measurements. Based on the results of heat capacity and magnetic measurements, the low temperature state in the Mg(Fe0.6Ga0.4)2O4 sample may be attributed to a spin glass-like state.

Magnetic order-disorder transitions on a one-third-depleted square lattice

Quantum Monte Carlo simulations are used to study the magnetic and transport properties of the Hubbard Model, and its strong coupling Heisenberg limit, on a one-third depleted square lattice. This is the geometry occupied, after charge ordering, by the spin-$\frac{1}{2}$ Ni$^{1+}$ atoms in a single layer of the nickelate materials La$_4$Ni$_3$O$_8$ and (predicted) La$_3$Ni$_2$O$_6$. Our model is also a description of strained graphene, where a honeycomb lattice has bond strengths which are inequivalent. For the Heisenberg case, we determine the location of the quantum critical point (QCP) where there is an onset of long range antiferromagnetic order (LRAFO), and the magnitude of the order parameter, and then compare with results of spin wave theory. An ordered phase also exists when electrons are itinerant. In this case, the growth in the antiferromagnetic structure factor coincides with the transition from band insulator to metal in the absence of interactions.

Monte Carlo Study of the Magnetic Order-Disorder Layering Transitions in a Polyamidoamine (PAMAM) Dendrimer Nanostructure

In this paper, we study the magnetic order-disorder layering transitions in a polyamidoamine (PAMAM) dendrimer nanostructure, under the effect of a variable crystal field. This PAMAM dendrimer nanostructure is found in many scientific domains. Firstly, we examine the magnetic properties of this model consisting of the spin S=1 Ising ferromagnetic model in real nanostructure. At null temperature\(\thinspace \left (T\mathrm {=0} \right )\), we give and discuss the ground-state phase diagrams in different planes. For non-null temperature values, we applied the Monte Carlo simulations showing interesting results, summarized in different phase diagrams. We also examined the effect of the coupling exchange interactions on the order-disorder layering transitions of the studied system.

Finite-temperature elastic constants of paramagnetic materials within the disordered local moment picture fromab initiomolecular dynamics calculations

We present a theoretical scheme to calculate the elastic constants of magnetic materials in the high-temperature paramagnetic state. Our approach is based on a combination of disordered local moments picture and ab initio molecular dynamics (DLM-MD). Moreover, we investigate a possibility to enhance the efficiency of the simulations of elastic properties using recently introduced method: symmetry imposed force constant temperature dependent effective potential (SIFC-TDEP). We have chosen cubic paramagnetic CrN as a model system. This is done due to its technological importance and its demonstrated strong coupling between magnetic and lattice degrees of freedom. We have studied the temperature dependent single-crystal and polycrystalline elastic constants of paramagentic CrN up to 1200 K. The obtained results at T= 300 K agree well with the experimental values of polycrystalline elastic constants as well as Poisson ratio at room temperature. We observe that the Young's modulus is strongly dependent on temperature, decreasing by ~14% from T=300 K to 1200 K. In addition we have studied the elastic anisotropy of CrN as a function of temperature and we observe that CrN becomes substantially more isotropic as the temperature increases. We demonstrate that the use of Birch law may lead to substantial errors for calculations of temperature induced changes of elastic moduli. The proposed methodology can be used for accurate predictions of mechanical properties of magnetic materials at temperatures above their magnetic order-disorder phase transition.

Origin of metallic behavior in NiCo2O4 ferrimagnet.

Predicting and understanding the cation distribution in spinels has been one of the most interesting problems in materials science. The present work investigates the effect of cation redistribution on the structural, electrical, optical and magnetic properties of mixed-valent inverse spinel NiCo2O4(NCO) thin films. It is observed that the films grown at low temperatures (T < 400 °C) exhibit metallic behavior while that grown at higher temperatures (T > 400 °C) are insulators with lower ferrimagnetic-paramagnetic phase transition temperature. So far, n-type Fe3O4 has been used as a conducting layer for the spinel thin films based devices and the search for a p-type counterpart still remains elusive. The inherent coexistence and coupling of ferrimagnetic order and the metallic nature in p-type NCO makes it a promising candidate for spintronic devices. Detailed X-ray Absorption and X–ray Magnetic Circular Dichroism studies revealed a strong correlation between the mixed-valent cation distribution and the resulting ferrimagnetic-metallic/insulating behavior. Our study clearly demonstrates that it is the concentration of Ni3+ions and the Ni3+–O2−Ni2+ double exchange interaction that is crucial in dictating the metallic behavior in NCO ferrimagnet. The metal-insulator and the associated magnetic order-disorder transitions can be tuned by the degree of cation site disorder via growth conditions.

The evolution of magnetic transitions, negative thermal expansion and unusual electronic transport properties in Mn3AgxMnyN

Abstract The antiperovskite compounds Mn 3 Ag x Mn y N with Ag vacancies and Mn doping at Ag site were synthesized and investigated. The introduction of Ag vacancies has a very small influence on magnetic transitions. However, the magnetic transitions at T N (Neel temperature) and T t (transition at lower temperature) gradually overlap with Mn doping accompanied by broadening of negative thermal expansion behavior. We also observed the nearly zero temperature coefficient of resistivity (NZ-TCR) behavior above magnetic order–disorder transition. The tunable TCR values from positive to negative could be achieved in Mn 3 Ag x Mn y N by reducing the contribution of (electron–phonon) e–p scattering in resistivity. Our results reveal the significance of e–p scattering for the evolution of TCR values, which could enrich the understanding of NZ-TCR behavior in antiperovskite manganese nitrides.

Temperature dependence of the propagation vector in Ni3−xCoxV2O8 with x=0.1 and 0.5

We present the susceptibility and neutron diffraction data on multiferroic Ni3−xCoxV2O8 with x=0.1 and 0.5. The temperature dependence of the susceptibility indicates that the magnetic order–disorder transition occurs at THTI=8.5K in both samples. Below THTI, the high-temperature incommensurate magnetic structure is realized, which undergoes a transition to the low-temperature incommensurate phase when the sample is cooled. The temperature evolution of the propagation vector k for x=0.1 is very weak, which confirms previous studies showing that substitution of 3.5% Ni ions by Co ions suppresses the explicit temperature dependence of k that is observed in the parent compound. On the other hand, we found that the vector k for x=0.5 exhibits a definite temperature dependence, which differs from the case of the undoped sample (x=0).

Magnetic order-disorder phase transition in Cr70Fe30 thin films

Dynamic magnetic response of Cr70Fe30 thin films with thickness in the range 11 nm ≤ t ≤ 978 nm and composition above the critical concentration for ferromagnetism was determined by measuring linear (χ1) and nonlinear (χ2 – χ5) ac-magnetic susceptibilities over three decades of frequency (101 – 104 Hz) in the temperature range 2–300 K. An elaborate analysis of χ1(T), measured in the absence or presence of a superposed dc magnetic field, Hdc, reveals that the asymptotic critical behavior of the films with t ≥ 21 nm is that of a three-dimensional (3D) isotropic dipolar ferromagnet. Interplay between long-range dipole–dipole and short-range exchange interactions causes a crossover to the 3D isotropic Heisenberg critical behavior when the temperature is increased above the Curie temperature, TC, outside the asymptotic critical region (ACR). By contrast, in the ACR, the film with t = 11 nm behaves as a 3D Ising ferromagnet. TC decreases with film thickness in accordance with the finite-size scaling with the value for the spin–spin correlation length exponent that corresponds to the 3D isotropic dipolar universality class. In the films with t ≤ 42 nm, besides the macroscopic length scale of the ferromagnetic order in the static limit, there exists a length scale that corresponds to finite spin clusters, whose dynamics is spin glass-like.

Quasi-two-dimensional character of the magnetic order-disorder transition in YMn6Sn6

The magnetic structure of YMn6Sn6 helimagnet has been studied by small-angle neutron scattering in the temperature range from 25 to 360 K. Two peaks with centers at 1.7 and 0 nm−1 have been observed in the dependences of the intensity of neutron scattering on the momentum transfer. An increase in the temperature is accompanied by a decrease in the intensity of the first peak and an increase in the intensity of the second peak. In addition, the shape of the peak with the center at 1.7 nm−1 changes from Gaussian to Lorentzian. This change can be explained by the fact of the magnetic order-disorder transition from the helimagnet to the state with spatial ferromagnetic fluctuations occurs in YMn6Sn6 in the interval of 250–360 K. The anisotropy of the shape along the momentum transfer is observed for the peak with the center at 1.7 nm−1. This, as well as the compatible variation of the FWHM and intensity of the peak with the center at 1.7 nm−1 with the temperature, indicates the quasi-two-dimensional character of the occurring magnetic transition.

Magnetic Dimensionality of Metal Formate $M[(H_2O)_2 (HCOO)_2]$ Compounds ($M={}$Co(II), Cu(II))

The magnetic properties of two metal formate systems with formula M[(H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (HCOO) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ], which are isostructural, are described (M=Co,Cu). These compounds contain two different crystallographic metal centers (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ), in octahedral coordination, which are connected through (HCOO) bridges, leading to the formation of two types of infinite chains: M(1)-HCOO-M(2) and M(1)-HCOO - M(1). The packing of these chains, along with hydrogen bonds, generates a 3-D framework. Cu-sample presents an usual paramagnetic behavior, following Curie law. On the contrary, Co-sample has an order-disorder magnetic transition, further analyzed by means of magnetization and specific heat. These data lead us to conclude an anisotropic environment for Co ions, as expected for an octahedral neighborhood. Critical exponents of these thermodynamic quantities around the thermal transition are in accordance with 3-D model. This is a very important remark, since a previous work had shown that the magnetic phase transition is 2-D.

The Effect of Quantum Transverse Anisotropy on Multilayer Transitions in a Spin-3/2 Blume–Capel Model with RKKY Interaction

We have studied the effect of quantum transverse anisotropies on the multilayer transition and magnetic properties of a spin-3/2 Blume–Capel model of a system formed by two magnetic multilayer materials, of different thicknesses, separated by a nonmagnetic spacer of thickness and RKKY interaction. Using mean-field theory, it is found that the multilayer magnetic order–disorder transition temperature depends strongly on the value of the transverse anisotropy. The multilayer transition temperature decreases when increasing the transverse anisotropy. Furthermore, there exists a critical quantum transverse anisotropy Δ xL beyond which the separate transitions occur in the two magnetic layers. The critical transverse anisotropy Δ xL increases on increasing the magnetic crystal field and/or the thickness of the magnetic multilayers N. On other hand, the Δ xL decreases on increasing the Fermi level and/or the nonmagnetic thickness M.

Size dependence study of the ordering temperature in the Fast Monte Carlo method

Based on the framework of the Fast Monte Carlo approach, we study the diameter dependence of the ordering temperature in magnetic nanostructures of cylindrical shape. For the purposes of this study, Fe cylindrical-shaped samples of different sizes (20 nm height, 30–100 nm in diameter) have been chosen, and their magnetic properties have been computed as functions of the scaled temperature. Two main set of results are concluded: (a) the ordering temperature of nanostructures follows a linear scaling relationship as a function of the scaling factor x, for all the studied sizes. This finding rules out a scaling relation T′c = x3ηTc (where η is a scaling exponent, and T′c and Tc are the scaled and true ordering temperatures) that has been proposed in the literature, and suggests that temperature should scale linearly with the scaling factor x. (b) For the nanostructures, there are three different order–disorder magnetic transition modes depending on the system’s size, in very good agreement with previous experimental reports.

Correlation between reentrant spin glass behavior and the magnetic order–disorder transition of the martensite phase in Ni–Co–Mn–Sb Heusler alloys

We have performed ac susceptibility and dc magnetization measurements in Ni50−xCoxMn38Sb12 Heusler alloys. From the ac susceptibility measurements, the existence of reentrant spin glass (RSG) state is observed for x = 0–5. It is found that the signature of RSG behavior diminishes with increase in x. This behavior is in contrast to the fact that the exchange bias field increases with x, which reveals that the origins of RSG and exchange bias are different in the present system. It is found that the system enters a frustrated ferromagnetic state just below the Curie temperature of the martensite phase () and then the RSG state at low temperature. The strength of the RSG state is critically dependent on the sharpness of the magnetic transition at . This proposition is further supported by the thermo-remanent magnetization and low field thermomagnetic measurements.

Temperature dependence of the Fe2+ Mossbauer parameters in triphylite (LiFePO4)

The ferrous Mossbauer parameters are determined for synthetic triphylite (LiFePO 4 ) over a wide temperature range between 8 and 900 K. A sharp magnetic order-disorder transition is observed at (52.5 ± 0.5) K. In the paramagnetic state, the Mossbauer spectra are adequately analyzed by one Fe 2+ quadrupole doublet with a narrow line width of 0.285 mm/s at room temperature. In the magnetic state, the spectra show up to eight absorption lines, which is typical for Fe 2+ . These spectra are analyzed through diagonalization of the full nuclear-interaction hamiltonian. The hyperfine field B hf is found to be collinear to the local principal axis of the electric field gradient (EFG) tensor. The saturation value for B hf is 125.5 kOe. For temperatures below 52 K, the adjusted value for the asymmetry parameter of the EFG is η ≈ 0.8 and for the quadrupole splitting Δ E Q = 3.06 mm/s. This latter value is perfectly in line with the value at 80 K (paramagnetic regime). The temperature dependence of the quadrupole splitting is interpreted within the 5 D orbital energy level scheme of Fe 2+ by a crystal field calculation based on the point symmetry of the Fe 2+ site in triphylite. The variation of the hyperfine field with temperature is interpreted within the molecular field approximation, however, taking exchange magnetostriction into account. From the temperature dependence of the isomer shift the characteristic Mossbauer temperature Θ M is determined to be 418 K and the intrinsic isomer shift δ I = 1.46 mm/s. From these, the Mossbauer fraction at room temperature is calculated to be 0.785.

Multilayer transition in a spin-1 Blume—Capel model with RKKY interaction and quantum transverse anisotropy

Using mean-field theory, we have studied the effect of quantum transverse anisotropies with RKKY interaction on the multi-layer transition and magnetic properties of the spin-1 Blume—Capel model of a system formed by two magnetic multi-layer materials, of different thicknesses, separated by a non-magnetic spacer of thickness M. It is found that the multilayer magnetic order—disorder transition temperature depends strongly on the value of the transverse anisotropy. The multilayer transition temperature decreases when increasing the transverse anisotropy. Furthermore, there exists a critical quantum transverse anisotropy ΔxL beyond which the separate transitions occur in the two magnetic layers. The critical transverse anisotropy ΔxL decreases (increases) on increasing the non-magnetic spacer of thickness M (on increasing the crystal field), and ΔxL undergoes oscillations as a function of the Fermi level.