Molecular Field Coefficients Research Articles

Magnetic properties of ErY2Co11B4

Abstract The free-powder (monocrystal) magnetization of ErY 2 Co 11 B 4 has been studied at 4.2 K in the Amsterdam High-Field Facility in fields up to 35 T. A metamagnetic transition is found. The magnetization process in this compound can be explained in terms of a two-sublattice model taking into account the Er- and Co-sublattice anisotropy, and the ErCo exchange interaction. The anisotropy constants and the molecular-field coefficient n ErCo are determined by fitting the magnetization curve. The results are in good agreement with the results of analyzing the easy- and hard-magnetization curves measured on magnetically aligned samples.

F-d exchange interaction in the RCo2-type intermetallic compounds with heavy rare earths

Non-collinear magnetic structures were experimentally observed in the Al-substituted (RtY1−t)(Co, Al)2 compounds by magnetization measurements in fields up to 27 T and molecular field coefficients of the f-d exchange interaction, λfd, were determined. The f-d exchange integrals are calculated.

The molecular field theory analysis of RFe 10V 2N x (R = Y, Nd, Sm, Gd, Dy, Er) intermettalic compounds

The temperature of magnetization is analyzed for RFe 10V 2N x via molecular field theory. The molecular field coefficients n FF, n RF and n RR are obtained. The fitted form of H R( T) varying with temperature for RFe 10V 2N x are presented. In addition, the curves of if M R (T) M R (0) and if H Fe (T) H Fe (0) have been plotted, and we found that the feature of Nd was different from other nitrided compounds.

Magnetization measurement of the Co moment induced by the molecular field in Y1-tGdtCo3

The magnetization of polycrystalline Y1-tGdtCo3 with 0<or=t<or=1 has been measured up to 40 T, and the effect of the molecular field from the Gd sublattice on the itinerant metamagnetism of the Co sublattice is studied. The compounds with t<0.25 show a field-induced metamagnetic transition of the Co sublattice. A transition from the collinear magnetic structure to the non-collinear structure is observed in compounds with 0.50<or=t<or=0.55. The spontaneous Co moment, which is evaluated from the magnetization curve, exhibits a stepwise increase at t approximately=0.25 when the Gd concentration is increased. This means that the metamagnetic transition of the Co sublattice is induced by the molecular field from the Gd sublattice in Y1-tGdtCo3. The average intersublattice molecular-field coefficient lambda GdCo, between the Gd and Co sublattices is estimated to be lambda GdCo=40.9+or-0.4 T formula units/ mu B; the corresponding exchange coupling parameter JGdCo is JGdCo=-(1.64+or-0.03)*10-22 J.

Magnetic properties and molecular field theory analysis of RFe10Mo2 alloys

The RFe10Mo2 compounds (R=Y, Nd, Gd, Dy) with ThMn12 type structure have been synthesized, and their magnetizations have been investigated in the temperature range from 2 to 800 K using a SQUID magnetometer. The Curie temperature, saturation moments, and measurements, as well as molecular field analysis of the temperature dependence of the magnetization are reported. Our work demonstrates that the molecular field analysis based on a two-sublattice model is capable of quite accurately describing the temperature dependence of the magnetization for the RFe10Mo2 compounds. The results also suggest that the moments of the R and Fe sublattices are nearly collinear in a large temperature range. According to the calculated molecular field coefficients, we find that the magnetic interactions are dominated by exchange between iron 3d electrons, and, on the other hand, the strength of the R–R interaction has even the same order as that of the R–Fe interaction.

Field-induced magnetic structures in ErGa2

ErGa2 crystallizes in the hexagonal A1B2-type structure and shows a two-step magnetization process along the c-axis below TN = 7 K. The compound has a magnetic structure with a wavevector of Q1 = 12b∗ at H = O, and for the intermediate field range it has a structure with Q1,Q2 = 12a∗, Q3 = −12a∗ + 12b∗ and Q0 = 0. The magnetization process is studied in terms of a wave-like molecular field Hm(u,v,w,) = Σqλm(q)<J(q)#62cos(2πqruvw acting on Er at point ruvw. The analysis involves wavevector-dependent molecular field coefficients λm(q) with q = 0, Q1, Q2 and Q3 and one crystal field coefficient B20. The dipole field due to the dipole-dipole interaction is also involved. Calculations give a reasonable fit with the magnetization data with λ(0) = −1.84 kOe; λm(Q1) = λm(Q2) = λm(Q3) = 0.68 kOeandB20 = − 0.48 K. A possible explanation for the appearance of the magnetic structure in the intermediate field range is presented.

Molecular-field theory analysis of RFe2 intermetallic compounds

On the basis of the two sublattice model the temperature dependence of magnetization for RFe2 compounds (R = Tb, Dy, Ho, Er and Tm) is analyzed. The molecular-field coefficients have been derived by fitting the experimental data, and they are transformed into the spin-exchange parameters. The results show that the spin-exchange interactions in RFe2 are stronger than those in R2Fe17, R6Fe23 and RFe3 compounds, which makes their Curie temperatures the highest ones in the corresponding R-Fe binary compounds.

Field-induced noncollinear magnetic structures in Al-stabilized RCo 2 Laves phases : Study of the Lu 1-yTm y(Co 0.88Al 0.12) 2 system

The magnetization of Lu 1- y Tm y (Co 1- x Al x ) 2 cubic Laves-phase ferrimagnets, in which magnetism of the itinerant d-subsystem is stabilized by partial Al-substitution, has been measured in magnetic fields up to 38T. Field-induced noncollinear magnetic structures are observed for H c1⩽ H⩽ H c2 in compounds with Tm concentrations y⩽0.6. The intersublattice molecular-field coefficient, λ fd, is found to be equal to -(13.5±1) Tf.u./μ B, and to be independent of the Tm concentration. At 4.2 K, the experimentally determined μ 0 H-y phase diagram can be described well, if we take into account the field dependence of the Tm moment.

Intersublattice f-d exchange interaction in the intermetallic Laves phase compounds Y 1−tGd t(Co 1−xAl x) 2

Systems Y 1− t ( Co 1− x Al x ) 2 with x = 0.05, 0.07, 0.085, 0.105 and 0 ≤ t ≤ 0.2 are studied to ascertain the influence of the intersublattice f-d exchange interaction on the magnetic behaviour of itinerant metamagnets Y( Co 1− x Al x ) 2. The field of the metamagnetic transition decreases when the Gd-concentration is increased for the compounds with x ≥ 0.07, and then a ferrimagnetic state is stable for more Gd-substitutions. Results obtained are explained in the molecular field approximation taking into consideration the itinerant metamagnetism of the d-electron subsystem. Intersublattice molecular field coefficient λ fd is determined from both Gd-concentration dependence on the critical field of the metamagnetic transition and values of the critical field of the rise of a non-collinear magnetic structure in high external magnetic fields. The same value of λ fd = 640 kOe/ μ B f. u. is obtained by different methods.

Magnetic coupling in amorphous Fe80−xRxB12Si8(R=Er, Gd) alloys

We have prepared amorphous Fe-Er-B-Si and Fe-Gd-B-Si alloys and magnetization studies in fields up to 35 T at 4.2 K have been carried out on some selected samples. The results have been analyzed on the existing model proposed by Verhoef et al. for crystalline Fe and Co compounds with rare earths. The magnetic moments of Fe, Er, and Gd, as well as other parameters such as the intersublattice molecular-field coefficient (nRT), the strength of the exchange interaction etc., have been determined.

Molecular field theory analysis of R 2Fe 17C x (R = Sm, Er) compounds

Magnetization measurements on the Sm 2Fe 17C x and Er 2Fe 17C x ( x = 0, 0.5, 0.8 or 1.0) intermetallic compounds are reported at different temperatures. On the basis of a two-sublattice model the data have been analyzed using the molecular-field theory. The molecular-field coefficients n FF, n RF, n RR and spin-exchange parameters A FF, A RF, A RR for these two series have been derived. The results of the analysis showed that with increase of x, the pronounced enhancement of n FF or A FF leads to the increase of the Curie temperature ( T c) of the compounds. The data were compared with previous ones and discussed.

Magnetic properties of Cr-Mn austenitic stainless steels

The magnetic susceptibility x of three Cr-Mn austenitic stainless steels was measured as a function of temperature in the range 5–400 K. All specimens showed a characteristic susceptibility maximum. The temperature of the maximum and especially the curve shape depend strongly on specimen composition and metallurgical conditions (as-quenched, deformed). Because no significant field dependence appeared, the susceptibility maximum was identified as the antiferromagnetic Néel temperature. x;( T) measurements above T N were fitted to a modified Curie-Weiss equation. Comparison between measurements and generalized-molecular-field-theory predictions allowed us to identify the magnetic structure as that of a first-type antiferromagnet with fcc crystal structure. The atomic magnetic moment and the molecular-field coefficients depend strongly not only on composition, but also on metallurgical prehistory, that is, on the degree of the applied mechanical deformation and heat treatment. Mainly, manganese affected the antiferromagnetic interactions, while chromium affected the ferromagnetic. Mn and Fe contributed the most to the effective atomic moment. Measurements on mechanically deformed specimens show a structure sensitivity of the molecular-field constants. This could be interpreted consistently in terms of lattice-parameter changes. The apparent structure sensitivity of the effective atomic moment can be attributed to changes in matrix composition caused by precipitation.

Field-induced canting of the ferrimagnetic structure in Gd 2Co 7

Magnetization measurements have been performed at 4.2 K on single crystalline Gd 2Co 7 in fields up to 35 T applied along the main hexagonal directions. The hard axis magnetization curve is characteristic for a high-field magnetization process with a field-induced non-collinear magnetic structure. The spontaneous moment, measured along the hexagonal axis, amounts to 3.45μ B/f.u. Values for the intersublattice molecular-field coefficient between the 3d and 4f sublattices of 14.4 T f.u./μ B and for the intrinsic cobalt-sublattice anisotropy of +51 K/f.u. have been deduced.

3d-4f exchange interactions in R 2Ni 17

Magnetization isotherms at 4.2 K in fields up to 38 T have been measured on free powder samples R 2Ni 17 (R-Dy, Ho and Er). From the transition-field values the intersublattice molecular-field coefficient n RT and molecular field acting on the R moment have been derived.

Exchange and crystal-field interactions in the antiferromagnets GdRh 2Si 2 and TbRh 2Si 2

Magnetization curves up to 35 T have been measured at 4.2 K on free powder particles of the antiferromagnets GdRh 2Si 2 and TbRh 2Si 2. The magnetization process is analyzed within a two-sublattice model for antiferromagnets. For GdRh 2Si 2 the intersublattice molecular-field coefficient amounts to 7.2 T f.u./μ B. For TbRh 2Si 2 metamagnetic transitions at 8 and 15 T are observed.

Molecular field calculations of the magnetization process in CeBi

The complicated magnetization processes of CeBi have been studied in terms of a molecular field model. A long range oscillatory exchange interaction between Ce ions along the [001]-axis is employed in a wave-like-form molecular field. H m( i) = Σ q λ( q, φ q )〈 J q 〉cos(π qi+φ q ), and strong ferromagnetic interactions within the (001)-planes are included. The wavenumber dependent molecular field coefficient, λ( q, φ q ), is a sensitive function of temperature for 12 K < T ≤ 25 K (= T N), and it takes a maximum at q=1 and φ q = 0 and this stabilizes the magnetic structure with a sequence of + -. For T ≤ 12 K, the λ( q, φ q ) is almost independent of temperature. Calculations reproduce the five-step magnetization process for T ≤ 12 K and the four-step process for 15 K < T<22 K. Numerical results on the wave-like molecular fields and sums of simple harmonic functions for the thermal averages of the total angular momenta, 〈 J q 〉cos(π qi+φ q ), are also presented. A brief discussion is made on the simplified Hamiltonian.

Analysis of temperature-dependent magnetization in Sm-Fe-C compounds using a two-sublattice model

The temperature dependence of magnetization in Sm/sub 3/Fe/sub 20/C/sub x/ (x =0.3, 0.6, 0.8) and Sm/sub 2/Fe/sub 16.5/C/sub 1.0/ intermetallic compounds with rhombohedral structure has been analyzed using molecular field theory. On the basis of a two-sublattice model, the molecular field coefficients are calculated using a numerical fitting method. The Curie temperature and the Fe-Fe, Sm-Fe, Sm-Sm magnetic interaction energies for the compounds are determined from these coefficients. The results show that the increase of the Curie temperature (T/sub c/) with the increase of carbon content x is attributed mainly to the enhancement of the Fe-Fe exchange interaction energy caused by carbon atoms added.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Mean-field analysis on the Macintosh

A brief overview of magnetic ordering in solids is presented and the basic principles of mean-field theory are outlined. An algorithm for determining the molecular field coefficients of ferrimagnetic materials having several magnetic sublattices is developed that reproduces the observed temperature dependence of magnetization. The method is then applied to several ferrimagnetic garnets having very different thermomagnetic properties. The calculations were performed on a Macintosh II computer running MATHEMATICA■.

Exchange and crystal field interactions in RCo4B (R  Gd, Dy, Ho, Er)

The exchange and crystalline-electric field effects in RCo4B compounds with heavy rare earth are analysed using a series of experiments. These experiments include magnetic moment behaviour, Curie temperature, temperature dependence of magnetocrystalline anisotropy and reorientation of spin magnetization in Ho and Dy compounds. The molecular field coefficient nRCo is evaluated from temperature dependence of magnetization in two-sublattice model and from the analysis of the Curie temperature. The latter depends on the nature of the rare earth. The value of the RCo exchange field is discussed. The magnetocrystalline anisotropy behaviour is treated in terms of the exchange and crystal field interactions which may differ at a and b sites. The increase of the crystal effects in comparison with RCo5 compounds results in a reduction of the 4f magnetocrystalline anisotropy especially for the b site.

Molecular Field Analysis of Ce, Ga:DyIG Films

We report here the results of molecular field analysis of the temperature dependence of magnetization of Dy2Ce1Fe4.2Ga0.8O12 sputtered films that displayed large variations in compensation temperature due to different sputtering conditions. A three sublattice model was used to compute the molecular field coefficients and estimate the Ga and Fe2+ site distribution. The model accounted for the reduced moment of Fe2+, known to be present due to the relatively high concentrations of tetravalent Ce measured in the films. Calculations show that upon an increase in the Ga tetrahedral site and Fe2+ octahedral site occupancies, as well as upon increasing the concentration of Ce3+, large increases in the compensation temperature can be expected.