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15 May 2003

Volume 93, Issue 10, pp. 5855-8792

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Anisotropy and Ising-type transition of the S=5/2 two-dimensional Heisenberg antiferromagnet Mn-formate di-Urea

Alessandro Cuccoli, Tommaso Roscilde, Valerio Tognetti, Ruggero Vaia, and Paola Verrucchi

J. Appl. Phys. 93, 7637 (2003); http://dx.doi.org/10.1063/1.1538176 (3 pages) | Cited 2 times

Online Publication Date: 9 May 2003

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Recently reported measurements of specific heat on the compound Mn-formate di-Urea by Takeda et al. [Phys. Rev. B 63, 024425 (2001)] are considered. As a model to describe the overall thermodynamic behavior of such a compound, the easy-axis two-dimensional Heisenberg antiferromagnet is proposed and studied by means of the pure quantum self-consistent harmonic approximation. In particular, it is shown that when the temperature decreases, the compound exhibits a crossover from two-dimensional (2D) Heisenberg to 2D-Ising behavior, followed by a 2D-Ising-type phase transition, whose location allows one to get a reliable estimate of the easy-axis anisotropy driving the transition itself. Below the critical temperature TN=3.77 K, the specific heat is well described by the two-dimensional easy-axis model down to a temperature T∗=1.57 K, where a T3 law sets in, possibly marking a low-temperature crossover of magnetic fluctuations from two to three dimensions. © 2003 American Institute of Physics.
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75.30.Wx Spin crossover
75.50.Ee Antiferromagnetics
75.30.Gw Magnetic anisotropy
75.10.Jm Quantized spin models, including quantum spin frustration
75.10.Hk Classical spin models
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Quantum Monte Carlo simulation of two-dimensional S=1/2 antiferromagnets with very weak easy-plane anisotropy

Alessandro Cuccoli, Tommaso Roscilde, Valerio Tognetti, Ruggero Vaia, and Paola Verrucchi

J. Appl. Phys. 93, 7640 (2003); http://dx.doi.org/10.1063/1.1537698 (3 pages) | Cited 2 times

Online Publication Date: 9 May 2003

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We study the critical and thermodynamic properties of an S=1/2 two-dimensional quantum XXZ antiferromagnet with weak easy-plane anisotropy using the continuous-time quantum Monte Carlo method based on the loop-cluster algorithm. A detailed finite-size scaling analysis reveals that the model displays a finite temperature Berezinskii–Kosterlitz–Thouless (BKT) transition for anisotropy as small as 10−3, the dominant isotropic coupling. For larger anisotropy (10−2) we single out relevant features of anisotropic behavior that anticipate the BKT transition. Implications for experimental observation of a BKT phase in real magnets are discussed. © 2003 American Institute of Physics.
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75.10.Jm Quantized spin models, including quantum spin frustration
75.40.Mg Numerical simulation studies
02.70.Ss Quantum Monte Carlo methods
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Gw Magnetic anisotropy

Monte Carlo simulations of phase transitions in Rb2MnF4

H. K. Lee, D. P. Landau, and T. C. Schulthess

J. Appl. Phys. 93, 7643 (2003); http://dx.doi.org/10.1063/1.1541644 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2003

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Rb2MnF4 is believed to consist of very weakly coupled magnetic layers with isotropic superexchange between nearest neighbor Mn2+ ions in the same plane. To test this conjecture, Monte Carlo simulations were performed on a theoretical model consisting of stacked, nearest neighbor Heisenberg square lattices coupled by dipolar interactions. Simulation parameters such as the lattice constants and the superexchange constant were taken from experimental measurements, and periodic boundary conditions were used in all directions. We succeeded in achieving an algorithmic speed up from O(N2) to O(N ln N) through the combination of fast Fourier transforms for the evaluation of the dipolar interactions and global updates in the Monte Carlo steps. Our simulations reproduced very well the behavior observed in experiments with Rb2MnF4 crystals. © 2003 American Institute of Physics.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.40.Mg Numerical simulation studies
75.50.Ee Antiferromagnetics
75.30.Et Exchange and superexchange interactions
75.10.Jm Quantized spin models, including quantum spin frustration
75.30.Gw Magnetic anisotropy

Tricritical points in La-based ferromagnetic manganites

V. S. Amaral, J. P. Araújo, Yu. G. Pogorelov, J. B. Sousa, P. B. Tavares, J. M. Vieira, P. A. Algarabel, and M. R. Ibarra

J. Appl. Phys. 93, 7646 (2003); http://dx.doi.org/10.1063/1.1558271 (3 pages) | Cited 5 times

Online Publication Date: 9 May 2003

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A detailed study of the magnetization M(H,T) of manganites near TC is presented. Analysis, in the framework of Landau theory of phase transitions (G=G0+1/2AM2+1/4BM4+1/6CM6−MH) reveals that for La0.67Ca0.33MnO3 (TC=267 K), La0.8MnO3 (TC=250 K), and La0.60Y0.07Ca0.33MnO3 (TC=150 K) the phase transition is first-order and the B coefficient is temperature dependent, negative near TC. In field (H<HC∗) and temperature (TC<T<TC∗) ranges, below the critical point of the first-order phase transition, clear features are found, with hysteresis. For La0.60Y0.07Ca0.33MnO3, HC∗∼20 kOe, ΔT=TC∗−TC∼20 K. For La0.67Ca0.33MnO3, on the other hand, HC∗ is only 250 Oe and ΔT<1 K. These effects are related with electronic and elastic energy contributions coupled to the magnetic order parameter. The analysis of the vanishing of B above TC suggests that a tricritical point should occur at T∼310 K, in agreement with the temperature at which the magnetic and structural transitions in La0.7(Ca1−ySry)0.3MnO3 coincide. © 2003 American Institute of Physics.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.80.+q Magnetomechanical effects, magnetostriction
75.50.Dd Nonmetallic ferromagnetic materials

Study of the enhancement of the magnetic properties of Fe70Al30 in the order-disorder transition

E. Apiñaniz, F. Plazaola, J. S. Garitaonandia, D. Martín, and J. A. Jimenez

J. Appl. Phys. 93, 7649 (2003); http://dx.doi.org/10.1063/1.1544504 (3 pages) | Cited 15 times

Online Publication Date: 9 May 2003

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The order-disorder transition produced by ball milling in the intermetallic Fe70Al30 has been systematically studied by x-ray diffraction, calorimetry, and Mössbauer experiments. These techniques show a monotonous transformation that ends after 6 h of milling. In the transition, the lattice parameter increase amounts to 0.7% and there is a large enhancement of the alloy’s magnetism. © 2003 American Institute of Physics.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
81.05.Bx Metals, semimetals, and alloys
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
75.40.-s Critical-point effects, specific heats, short-range order
75.50.Bb Fe and its alloys
61.50.Ks Crystallographic aspects of phase transformations; pressure effects
64.70.K- Solid-solid transitions
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep
76.80.+y Mössbauer effect; other γ-ray spectroscopy
61.66.Dk Alloys
71.70.Jp Nuclear states and interactions

Atomistic modeling of nanocrystalline ferromagnets

Guang-Ping Zheng, Dirk Gross, and Mo Li

J. Appl. Phys. 93, 7652 (2003); http://dx.doi.org/10.1063/1.1537699 (3 pages) | Cited 2 times

Online Publication Date: 9 May 2003

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A model nanocrystalline ferromagnet is investigated at the atomistic level using numerical simulations. The nanocrystalline microstructure is constructed using methods of stochastic optimization and Voronoi construction. The atomic structure of the grains is obtained by molecular dynamics. Nanostructures obtained in this way are characterized by the log–normal grain size distribution and the coordination number of defects at grain boundaries. An Ising model is employed to model the magnetic properties of nanostructured cobalt films with strong perpendicular anisotropy. Particular attention is paid to the effects of the grain boundary structure. We consider J∼exp(−αr) on the grain boundary, where α is a parameter that represents grain boundary effects. The dependences of the Curie temperature, coercivity, and hysteresis loop on the grain-isolated parameter α are investigated. © 2003 American Institute of Physics.
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75.40.Mg Numerical simulation studies
75.50.Tt Fine-particle systems; nanocrystalline materials
75.70.Ak Magnetic properties of monolayers and thin films
61.46.-w Structure of nanoscale materials
75.50.Cc Other ferromagnetic metals and alloys
81.07.Bc Nanocrystalline materials
02.70.Ns Molecular dynamics and particle methods
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
07.05.Tp Computer modeling and simulation
05.10.Gg Stochastic analysis methods (Fokker-Planck, Langevin, etc.)
02.60.Pn Numerical optimization
75.10.Hk Classical spin models
75.30.Gw Magnetic anisotropy
61.72.Mm Grain and twin boundaries
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

On the first-order phase transition in MnFeP0.5As0.4Si0.1

O. Tegus, E. Brück, W. Dagula, X. W. Li, L. Zhang, K. H. J. Buschow, and F. R. de Boer

J. Appl. Phys. 93, 7655 (2003); http://dx.doi.org/10.1063/1.1544506 (3 pages) | Cited 10 times

Online Publication Date: 9 May 2003

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We have studied the first-order magnetic phase transition and related physical properties of MnFeP0.5As0.4Si0.1 by means of x-ray diffraction, magnetization, thermal expansion, and electrical resistance measurements. The compound crystallizes in the hexagonal structure with Fe2P-type. The temperature dependence of magnetization shows a first-order magnetic phase transition around 299 K. There is no crystallographic structural transition associated with the magnetic phase transition. The linear thermal expansion shows that this magnetic phase transition is accompanied by a negative volume change. The electrical resistance shows an anomalous behavior at the transition, which is probably caused by the change in c/a ratio. © 2003 American Institute of Physics.
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75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.30.Sg Magnetocaloric effect, magnetic cooling
75.50.Dd Nonmetallic ferromagnetic materials
65.40.De Thermal expansion; thermomechanical effects
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.80.+q Magnetomechanical effects, magnetostriction

Magnetic properties of hydrogen-induced amorphous YFe2

K. Suzuki, K. Ishikawa, M. Dilixiati, J. M. Cadogan, and K. Aoki

J. Appl. Phys. 93, 7658 (2003); http://dx.doi.org/10.1063/1.1539075 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2003

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In order to clarify the origin of the absence of a spin glass state from hydrogen-induced amorphous (HIA) YFe2(H) alloys, the magnetic properties of HIA YFe2Hx (x=0.9, 1.9, and 3.6) alloys were investigated by 57Fe Mössbauer spectroscopy, ac susceptometry, and thermogravimetric magnetic analysis. Both the mean hyperfine field (〈Bhf〉) at 0 K and the magnetic transition temperature in the HIA YFe2Hx alloys show a tendency to decrease with a decrease in x. The exchange integral (J) estimated from the temperature dependence of 〈Bhf〉 is also reduced dramatically with a decrease in x. The absence of a spin glass state in HIA YFe2(H) is understood well by the enhancement of J through the volume expansion induced by the absorption of hydrogen. © 2003 American Institute of Physics.
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75.50.Kj Amorphous and quasicrystalline magnetic materials
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)
75.30.Et Exchange and superexchange interactions
76.80.+y Mössbauer effect; other γ-ray spectroscopy
75.50.Bb Fe and its alloys
75.30.Cr Saturation moments and magnetic susceptibilities
71.70.Jp Nuclear states and interactions

Numerical studies of the two- and three-dimensional gauge glass at low temperature

Helmut G. Katzgraber

J. Appl. Phys. 93, 7661 (2003); http://dx.doi.org/10.1063/1.1538177 (3 pages) | Cited 1 time

Online Publication Date: 9 May 2003

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We report results from Monte Carlo simulations of the two- and three-dimensional gauge glass at low temperature using parallel tempering Monte Carlo. In two dimensions, we find strong evidence for a zero-temperature transition. By means of finite-size scaling, we determine the stiffness exponent θ=−0.39±0.03. In three dimensions, where a finite-temperature transition is well established, we find θ=0.27±0.01, compatible with recent results from domain-wall renormalization group studies. © 2003 American Institute of Physics.
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75.10.Nr Spin-glass and other random models
75.40.Mg Numerical simulation studies
75.40.Cx Static properties (order parameter, static susceptibility, heat capacities, critical exponents, etc.)

Magnetism of the trigonal B12 cluster compound REB17CN (RE=Er, Ho)

Takao Mori and Andreas Leithe-Jasper

J. Appl. Phys. 93, 7664 (2003); http://dx.doi.org/10.1063/1.1543853 (3 pages) | Cited 12 times

Online Publication Date: 9 May 2003

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Magnetic properties of trigonal B12 cluster compounds REB17CN (RE=Er, Ho) were investigated. The structure (space group P-3m1) is composed of a three dimensional network based on interconnected B12 icosahedra and B6 octahedra. The rare earth atoms reside in voids of the boron framework, while C–B–C chains and nitrogen atoms bridge the icosahedra. Peaks in the zero field cooled magnetic susceptibility are observed at 29 and 6 K for HoB17CN and ErB17CN, respectively. Divergences are observed between the zero field cooled and field cooled susceptibilities, while the isothermal remanent magnetization shows a time dependent relaxation over several decades of time. The results are indicative of spin glass-like behavior. The spin glass-like behavior is thought to be caused by disorder from partial occupancy of the rare earth atomic sites and also possible frustration of magnetic interactions. These results are also of interest in the context of the ongoing investigation into the magnetism discovered in nonmetallic B12 icosahedral compounds of which it has been found that the B12 clusters are indicated to mediate the interaction. © 2003 American Institute of Physics.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.50.Lk Spin glasses and other random magnets
75.30.Cr Saturation moments and magnetic susceptibilities
61.46.-w Structure of nanoscale materials
61.72.Qq Microscopic defects (voids, inclusions, etc.)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.30.Et Exchange and superexchange interactions
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