Photoinduced phase transition accompanied with the changes in magnetic properties

Recently, many studies have been started in search for materials which show a photoinduced phase transition (PIPT). In this work, we review two systems as typical examples of PIPT accompanied with changes in magnetic characteristics; (1) organo-metal complex [Fe(2-pic)₃]Cl₂ EtOH (2-pic = 2-amino-methyl-pyridine) and (2) III-V based magnetic semiconductors (In_1-x , Mn _x )As. In the former case, we show several nonlinear characteristics in dynamical process of photoinduced spin state transition from low-spin to high-spin states. In the latter one, photocarrier-induced ferromagnetic order has been observed by both magnetic and transport measurements.     

The magnetic phase transition in Cu-doped ZnO: From bulk to nanocluster

Using the first-principles calculations based on the density functional theory, we have investigated the magnetic properties of Cu-doped ZnO both in bulk and nanocluster. The calculated results show that the substitutional Cu ions are spin polarized and have a tendency to assemble. It is found that the ground state has shown a change from ferromagnetic phase to antiferromagnetic phase as the size for the doping system decreases from bulk to nanocluster. In bulk ZnO, the ferromagnetism is attributed to the strong hybridization between Cu 3d and O 2p states. In ZnO nanocluster, however, the antiferromagnetic exchange interaction is dominant because of the very close Cu–Cu distance.     

Rashba coupling induced spin susceptibility and magnetic phase transition of conduction electrons in monolayer graphene

Isolated graphene cannot be obtained by the known synthesis processes and it should be placed on a substrate. This substrate introduces a new type of spin–orbit interaction known as Rashba coupling. Using the Kubo formalism, the magnetic properties of the system in the linear regime have been investigated. Mainly the effect of non-magnetic substrate on the spin susceptibility is calculated. Results show that the Rashba coupling has a central role in the magnetic response function of the system and it is really remarkable since this type of spin orbit coupling can be effectively controlled by an external gate voltage. Most importantly, it was shown that, in the presence of the Rashba interaction a magnetic phase transition could be observed. This magnetic phase corresponds to a magnetic order of conduction electrons that takes place at some special frequencies of external magnetic field.     

Crystallographic and magnetic phase transition in TlMnCl3

The results of our NMR, EPR and magnetic susceptibility measurements in the paramagnetic state of TlMnCl₃ are reported here. The NMR paramagnetic shift of thallium is found to be small but positive. Mn²⁺ EPR line is exchange narrowed. The susceptibility measurements indicate an antiferromagnetic transition. The heat of crystallographic phase transition ΔH, in TlMnCl₃ has been measured using differential scanning calorimetry. The crystallographic phase transition appears to be first order and ΔH is unusually low viz. 10 cal mole⁻¹. In the case of KMnF₃ Δ_H, which is reported here for the first time, is determined to be 2 cal mole⁻¹.     

X-ray diffraction and magnetic susceptibility of sodium fullerides NaxC60

The X-ray diffraction (XRD), magnetic susceptibility and electron spin resonance (ESR) measurements have been carried out for Na_xC₆₀. The XRD profiles with x<4 can be assigned to a face-centered cubic (fcc) lattice, while those with 4≦x to a hexagonal one. The temperature dependence of magnetic susceptibility χ for Na_xC₆₀ using SQUID was fitted to the Curie law, and estimated temperature-independent component χ₀. The composition x dependence of the χ₀ for Na_xC₆₀ shows two maxima at around x=3 and x=10, and minimum at x=6. The absence of Pauli contribution at x=6 was confirmed using ESR. A trace of superconducting transition at 14 K has been found for some Na_xC₆₀ specimens with 8<x<9.     

Soft mode and magnetic phase transition in PrNi

The spectrum of magnetic excitations in a single crystal of intermetallic compound PrNi was studied by inelastic neutron scattering. Experiment showed the substantial softening of some collective magnetic excitation modes near the ferromagnetic ordering temperature T _c≈20 K. The result is analyzed within the framework of a model that describes the magnetic phase transition in systems with induced magnetic moment.     

Electronic structure and magnetic phase transition in MnSi

Temperature variations of the amplitude of zero-point and thermal spin fluctuations in a helicoidal ferromagnetic (MnSi) are characterized using the electronic structure model that follows from ab initio LDA + U + SO calculations. It is found that a drastic reduction in the amplitude of zero-point spin fluctuations at temperature T _S (in the vicinity of the magnetic phase transition) leads to ferromagnetic solution instability (a change in the sign of the intermode interaction parameter). The observed magnetovolume effect and a sharp change in the radius of spin correlations have the same underlying cause. The results of calculation of the volumetric coefficient of thermal expansion agree well with the observed anomaly in the region of the magnetic phase transition.     

On the magnetic phase transition in terbium ferrite-garnet

It is shown that anomalies on the magnetization curves of the samples of terbium ferrite-garnet (Tb₃Fe₅O₁₂) revealed by Tran Khanh Vien and Dormann [1] are due to the first order spin-reorientation phase transition in iron sublattices. This transition is satisfactorily described by the simple phenomenological theory taking into consideration only anisotropic interactions.     

Configurational spin reorientation phase transition in magnetic nanowire arrays

Classical microscopic spin reorientation phase transitions (RPT) are the result of competing magnetocrystalline anisotropies. RPTs can also be observed in discrete macroscopic systems induced by competing shape anisotropies and magnetostatic coupling. Such a configurational RPT was recently observed in series of self-organized hexagonal arrays of 2.5 μm long, 25-60 nm diameter circular permalloy nanowires grown in anodic alumina matrix. This RPT is a crossover transition from a one-dimensional easy axis “wire” behavior of weakly interacting uniaxial nanowires to a two-dimensional behavior of strongly coupled “wire film” having an easy plane anisotropy. It is shown that RPT takes place due to the competition between the intrinsic dipolar forces in individual wires and the external dipolar field of interacting nanowires in the array. The crossover occurs at a volume ratio of 0.38 for 65 nm periodicity. The experimental results are in agreement with the semi-analytical calculations of the dipolar interaction fields for these arrays of circular ferromagnetic nanowires, and are interpreted in terms of the Landau phase transition theory. The conditions for the crossover and the order of the phase transition are established. Based on the contribution to the magnetic energy from the flower state at the ends of the wires, it is concluded that the observed transition is of the first order.     

Molecular dynamics, phase diagrams and electronic properties of fullerenes and alkali fullerides: insights from solid-state nuclear magnetic resonance spectroscopy

Solid-state nuclear magnetic resonance (NMR) measurements have made important contributions to the current understanding of the structural, dynamical, and electronic properties of fullerenes (C₆₀ and C₇₀) and the alkali fullerides A_xC₆₀ (A = alkali metal). These measurements and their interpretation are reviewed. One- and two-dimensional ¹³C NMR lineshapes and spin-lattice relaxation rates provide evidence for rapid molecular rotations and orientational order-disorder transitions in the fullerenes and alkali fullerides. The kinetics of molecular reorientations are determined from the NMR data. ¹³C and alkali metal NMR spectra indicate that the alkali fullerides are stoichiometric compounds. Each stable, stoichiometric phase has distinctive NMR signatures. ¹³C and alkali metal NMR spectra and relaxation measurements provide valuable and unique information about the electronic properties of the metallic, superconducting, and non-metallic phases of the alkali fullerides.     

Investigation of a magnetic phase transition in fcc iron-nickel alloys

A magnetic phase transition in carbon-doped (0.1 and 0.7 at. %) Fe₇₀Ni₃₀ Invar alloys was investigated by the method of depolarization of a transmitted neutron beam and by small-angle scattering of polarized neutrons. It is shown that for both alloys, two characteristic length scales of magnetic correlations coexist above T _c. Small-angle scattering by critical correlations with radius R _c is described well by the Ornstein-Zernike (OZ) expression. The longer-scale (second) correlations, whose size can be estimated from depolarization data, are not described by the OZ expression, and hypothetically can be modeled by a squared OZ expression, which in coordinate space corresponds to the relation 〈M(r)M(0)〉∝exp(−r/R _d), where R _d is the correlation length of the second scale. The temperature dependence of the correlation radius R _c was obtained: R _c ∝ ((T−T _c)/Tc)^−ν , where ν≈2/3 is the critical exponent for ferromagnets, over a wide temperature range up to T _c ^exp , at which the correlation radius becomes constant and equals its maximum value R _c(T _c)=R _c ^max . The maximum correlation radius established (R _c ^max =140 Å and 230 Å for the first and second alloys, respectively) characterizes the length-scale of the fluctuation for which the appearance of critical correlations first results in the formation of a ferromagnetic phase, and the phenomenon itself exhibits a “disruption” of the second-order phase transition at T=T _c ^exp , as a result of which a first-order transition arises. Temperature hysteresis was also detected in the measured polarization of the transmitted beam and intensity of small-angle neutron scattering in the alloy above T _c, confirming the character of this magnetic transition as a first-order transition close to a second-order transition. Zh. éksp. Teor. Fiz. 112, 2134–2155 (December 1997)     

Spin glass behavior and critical analysis across the magnetic phase transition in Gd2CoMnO6: dc magnetization and ac susceptibility study

We report here on critical analysis across magnetic phase transition and spin dynamics in Gd₂CoMnO₆. We found that this material behaves differently below and above the applied magnetic field of 20 kOe. The magnetic phase transition switches from nearly mean-field type to unusual class and T_c shifts towards the high temperature above 20 kOe field. The nature of the magnetic phase transition is explored by carrying out critical analysis at low as well as at high magnetic field. The critical exponents obtained at low field using Kouvel-Fisher method are β = 0.65 (2) γ = 0.90 (2), δ = 2.43 and T_c = 120 K. Apparently, these values of critical exponents appear close to mean-field model. For high field the critical exponents are β = 1.24 (2) γ = 0.64 (5), δ = 1.51 (3) and T_c = 128 K. The critical exponents show significant deviation from any universal class. This switchover in the nature of the magnetic phase transition is unique and not seen in many compounds. The formation of non-Griffiths-like clusters in this compound can be a reason for such unique behavior. Further, ac susceptibility has been measured to understand the spin dynamics in detail. The dispersion of frequency-dependent χ_ac below T_c confirms a spin glass state in this material. The observed value of τ_o and T_o indicate the slow dynamic spin which is caused by co-existence of Co/Mn spin magnetic moments. The magneto-caloric effect is also presented for Gd₂CoMnO₆ in this study. The magnetic study and critical analysis across the phase transition reveal a switchover in the nature of phase transition in this material. A non-Griffiths like cluster formation above T_c is found and dynamic susceptibility study reveals a spin glass state below T_c in Gd₂CoMnO₆.     

Investigations of the first order magnetic phase transition in dysprosium

The first order magnetic phase transition in polycrystalline sample of dysprosium (Dy) has been investigated in detail by dc-resistance vs. temperature (R-T) measurements. Distinct signature of coexistence of two magnetically ordered phases (i.e. ferromagnetic and antiferromagnetic), originating as a result of the metastable behavior associated with the first order phase transition (FOPT) is found near 90 K. By successfully recording the minor hysteresis loops within the hysteretic R-T regime of Dy, a varying degree of phase coexistence in the FOPT is demonstrated.     

First order magnetic phase transition in GeCo2O4

The antiferromagnetic compound GeCo₂O₄ exhibits a magnetic phase transition characterized by thermal hysteresis of the susceptibility versus temperature curve and by a diffuse neutron scattering with a small correlation length. The data are compared to the expected first-order phase transition of the n ≥ 4 component vector models.