Non-zero Spontaneous Magnetization Research Articles

Nd3+ induced twofold continuous spin reorientation transition and magnetization along the b-axis in a Dy0.9Nd0.1FeO3 single crystal

An intriguing hybrid magnetic configuration Γ4+3 with non-zero spontaneous magnetization along b-axis was found in DNFO91 single crystal. Contributions of R3+ were distinguished and the relative strength can be manipulated with magnetic field.

Presence of the weak ferromagnetism in pure antiferromagnetic systems with octahedral structure: Effective field theory analysis

The recently reported (Jurčišinová E. and Jurčišin M., J. Magn. & Magn. Mater., 513 (2020) 167085) possibility of the existence of the weak ferromagnetism with small but nonzero spontaneous magnetization in pure antiferromagnetic systems with the octahedral structure is confirmed using the six-site effective-field theory cluster approximation of the antiferromagnetic spin- Ising model on the octahedral lattice. The sublattice magnetizations, the total spontaneous magnetization, as well as the Néel temperature of the model are found and compared to those obtained in the framework of the recursive lattice approach. The performed analysis allows one to conclude that the existence of the weak ferromagnetism in pure antiferromagnetic systems with the octahedral structure is directly related to their intrinsic geometry. The spontaneous magnetization and the Curie temperature of the ferromagnetic model are also determined and briefly discussed.

Ground state with nonzero spontaneous magnetization of the two-dimensional spin-1/2 Heisenberg antiferromagnet with frustration

The S = 1/2 Heisenberg antiferromagnet on the two-dimensional pyramid lattice is studied by the numerical-diagonalization method. This lattice is obtained by the combination of the Lieb lattice and the square lattice. It is known that when interaction on the square lattice is increased from the ferrimagnetic limit of strong interaction on the Lieb lattice, this system shows gradual decrease and disappearance of spontaneous magnetization in the ground state. The present study treats the region near the case of the square-lattice antiferromagnet accompanied by isolated spins by numerical-diagonalization calculations of finite-size clusters with the maximum size of 39 sites. Our numerical results suggest the existence of a new phase with small but nonzero spontaneous magnetization between two zero-spontaneous-magnetization phases.

Wetting transition in the two-dimensional Blume-Capel model: A Monte Carlo study

The wetting transition of the Blume-Capel model is studied by a finite-size scaling analysis of L×M lattices where competing boundary fields ±H_{1} act on the first or last row of the L rows in the strip, respectively. We show that using the appropriate anisotropic version of finite-size scaling, critical wetting in d=2 is equivalent to a "bulk" critical phenomenon with exponents α=-1, β=0, and γ=3. These concepts are also verified for the Ising model. For the Blume-Capel model, it is found that the field strength H_{1c}(T) where critical wetting occurs goes to zero when the bulk second-order transition is approached, while H_{1c}(T) stays nonzero in the region where in the bulk a first-order transition from the ordered phase, with nonzero spontaneous magnetization, to the disordered phase occurs. Interfaces between coexisting phases then show interfacial enrichment of a layer of the disordered phase which exhibits in the second-order case a finite thickness only. A tentative discussion of the scaling behavior of the wetting phase diagram near the tricritical point is also given.

The generalization of the Mermin–Wagner theorem and the possibility of long-range order in the isotropic discrete one-dimensional quantum Heisenberg model

The problem of existence of long-range order in the isotropic quantum Heisenberg model on the D=1 lattice is reconsidered in view of the possibility of sufficiently slow decaying exchange interaction with infinite effective radius. It is shown that the macrosopic arguments given by Landau and Lifshitz and then supported microscopically by Mermin and Wagner fail for this case so that the non-zero spontaneous magnetization may yet exist. This result was anticipated by Thouless on the grounds of phenomenological analysis, and we give its microscopic foundation, which amounts to the generalization of Mermin–Wagner theorem for the case of the infinite second moment of the exchange interaction. Two well known in lattice statistics models – i.e., Kac-I and Kac-II – illustrate our results.

Ferrimagnetism of MnV2O4 spinel

The spinel MnV2O4 is a two-sublattice ferrimagnet, with site A occupied by theMn2+ ion and siteB by the V3+ ion. The magnon of the system, the transversal fluctuation of the total magnetization, is acomplicated mixture of the sublattice A and B transversal magnetic fluctuations. As aresult, the magnons’ fluctuations suppress in a different way the manganese andvanadium sublattice magnetization and one obtains two phases. At low temperature(0,T*) the sublattice Mn magnetization and sublattice V magnetization contributeto the magnetization of the system, while at a high temperature(T*,TN), the vanadium sublattice magnetization is suppressed by magnon fluctuations,and only the manganese ions have non-zero spontaneous magnetization. Amodified spin-wave theory is developed to describe the two phases and tocalculate the magnetization as a function of temperature. The anomalousM(T) curve reproduces the experimentally obtained zero-field-cooled (ZFC) magnetization.

Anomalous Hall effect in IV-VI magnetic semiconductors

Topological contribution to the anomalous Hall effect in IV-VI narrow-gap semiconductors with a nonzero spontaneous magnetization due to magnetic impurities is considered theoretically. The off-diagonal conductivity is calculated in the relativistic model of the IV-VI semiconductors. Spin-orbit interaction in these compounds is strong and cannot be treated perturbatively. Therefore, it is included in the Hamiltonian of a clean (defect-free) system. Geometrical interpretation of the topological contribution to the Hall conductivity is also briefly discussed.

Towards the theory of ferrimagnetism

A two-sublattice ferrimagnet, withspin-s1 operatorsS1i at the sublatticeA site and spin-s2 operators S2i at the sublattice B site, is considered. The magnon of the system, the transversal fluctuation ofthe total magnetization, is a complicate mixture of the transversal fluctuations of thesublattice A and B spins. As a result, the magnons’ fluctuations suppress in a different way themagnetic orders of the A and B sublattices and one obtains two phases. At low temperature(0,T*) the magnetic orders of the A and B spins contribute to the magnetization of the system, while at hightemperature (T*,TN), the magnetic order of the spins with a weaker intra-sublattice exchangeis suppressed by magnon fluctuations, and only the spins with strongerintra-sublattice exchange have non-zero spontaneous magnetization. TheT* transition is a transition between two spin-ordered phases in contrastto the transition from the spin-ordered state to the disordered state(TN-transition). There is no additional symmetry breaking, and the Goldstoneboson has a ferromagnetic dispersion in both phases. A modified spin-wavetheory is developed to describe the two phases. All known Neel’s anomalousM(T) curves are reproduced, in particular that with ‘compensation point’. Thetheoretical curves are compared with experimental ones for sulfo-spinelMnCr2S4−xSex and rare earth iron garnets.

High Temperature Ferromagnetism and Nano-Scale Phase Separations in Diluted Magnetic Semiconductors and Oxides

In this paper the present understanding of the origin of ferromagnetic response that has been detected in a number of diluted magnetic semiconductors and diluted magnetic oxides at room temperature is outlined. It is argued that in these systems, owing to a typically small solubility of magnetic ions, crystallographic or chemical phase separation into regions with a large and a small concentration of magnetic component takes place. The ferromagnetic signatures then come from the regions with a large concentration of magnetic ions, which show non-zero spontaneous magnetization up to the blocking temperature, whose magnitude is proportional to the nanocrystal volume and magnetic anisotropy. Novel methods enabling a control of nano-assembling of magnetic nanocrystals in non-conducting matrices as well as possible functionalities of these spatially modulated magnetic systems are described. We also discuss phase separations into paramagnetic and ferromagnetic regions, which are driven by the Anderson–Mott localization and/or competing ferromagnetic and antiferromagnetic interactions. Finally, the question whether the high temperature ferromagnetism is possible in materials without magnetic ions is addressed.

Helimagnetic order in the re-entrant ferromagnet Ce2Fe15.3Mn1.7

Substitution of Mn for Fe in Ce2Fe17 strongly suppresses the ferromagnetic ground state, changing it into an antiferromagnetic one. However, a state of spontaneous magnetization is re-entered in the Ce2Fe17−xMnx compounds when the Mn content reaches x=1.3. Magnetic structures and magnetic interactions in the re-entrant ferromagnetic Ce2Fe15.3Mn1.7 compound have been studied using both microscopic (neutron powder diffraction in magnetic fields of up to 0.7T) and macroscopic (ac susceptibility and magnetization measurements under high pressures of up to 8kbar in magnetic fields of up to 5T) techniques. A spiral magnetic structure distorted by the presence of an in-plane ferromagnetic component is found below TN=182K. Enhancement of the ferromagnetic interactions leads to a steep transformation of the magnetic spiral to a magnetic structure with nonzero spontaneous magnetization below ΘT=59K. This structure is described by a distorted elliptical spiral model with an incommensurate propagation vector along the c axis. Magnetic field significantly expands the range of stability of this re-entrant ferromagnetic phase. In contrast to the magnetic field, application of pressure does not change ΘT within the achieved pressure range. It leads to a moderate reduction of the Néel temperature, dTN∕dP=−2.3K∕kbar. The preferential placement of Mn on some crystallographic sites in Ce2Fe17−xMnx and the related changes of volume and internal magnetic field will be discussed as possible reasons of the peculiar magnetic behavior of this compound.

Origin of weak ferromagnetism in TDAE-C 60

A mechanism leading to the coexistence of weak ferromagnetism and spin glass behavior in the weak organic ferromagnet TDAE-C 60 is proposed. In view of the non-metallic character of TDAE-C 60, the C 60 −-C 60 − intra-chain coupling along the c-axis as well as the inter-chain C 60 −-C 60 − assumed to be of antiferromagnetic nature. If the spin density on the TDAE + ion is inhomogeneously distributed the TDAE + mediated super-exchange leads to a ferromagnetic coupling between C 60 − ions on different chains. This leads to a Dzyaloshinsky-Moriya type coupling responsible for the spin-canting and a non-zero spontaneous magnetization. Other models like three dimensional ferromagnetic orbital ordering of Jahn-Teller distorted C 60 molecules are discussed as well.

High field and temperature dependent magnetisation measurements on some RE/sub 2/Co/sub 17/ single crystals

Single crystals of Pr/sub 2/CO/sub 17/ and Nd/sub 2/Co/sub 17/ have been grown using a tri-arc Czochralski method. High-field magnetization measurements up to 35 T have been performed on Pr/sub 2/Co/sub 17/ and Nd/sub 2/Co/sub 17/ at 4.2 K. The experimental curves show very large anisotropy; for fields along the c axis in Pr/sub 2/Co/sub 17/ and along the a axis in Nd/sub 2/Co/sub 17/ the magnetization does not reach the saturation value within this field range. Furthermore, the hard axis shows in both compounds a nonzero spontaneous magnetization indicating a cone structure with cone angles of about 26 degrees and 7 degrees for Pr/sub 2/Co/sub 17/ and Nd/sub 2/Co/sub 17/, respectively. Low-field (up to 6 T) temperature-dependent magnetization measurements (4.2-300 K) show that in both compounds the cone angle remains visible up to 300 K, but for Pr/sub 2/Co/sub 17/ the cone angle is drastically reduced above 150 K. These two RE/sub 2/TM/sub 17/ compounds (RE stands for a rare-earth metal and TM stands for a transition metal) with a cone structure are compared with other RE-TM compounds with a cone structure.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Rigorous upper and lower bounds on the critical temperature in Ising models with random, quenched, broken-bond disorder

Upper and lower bounds for Tc(p) are rigorously established for the Ising model with random, quenched, broken-bond disorder. Tc is the critical temperature, defined by the onset of a non-zero spontaneous magnetization, and p is the fraction of unbroken bonds. Bounds are also found for other quantities, such as the two-point correlation functions. These bounds may be applied at any temperature and any value of p with 0 pc and pc is the lowest value of p above which the system has a non-zero spontaneous magnetization at T=O.

On non-existence of spontaneous magnetization in a one -dimensional Heisenberg ferromagnet

It is pointed out that Bogoliubov's inequality may be used to rule out the existence of non-zero spontaneous magnetization for the one-dimensional Heisenberg isotropic model provided that lim n →∞ J( n) n 2 is finite.

Absence of spontaneous magnetization in isotropic partially finite systems

It is pointed out that BOgoliubov's inequality may be used to rule out the existence of non-zero spontaneous magnetization in partially finite three-dimensional Heisenberg models (isotropic models) with finite range interactions at temperature T ≠ 0°K.