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• Curie and Weiss constants vary with composition regularly but not in a mean-field way. • The location of a susceptibility maximum varies modestly until nickel content is high. • The magnetization and its hysteresis evolves regularly with composition. • The location of a metamagnetic transition varies regularly with composition. • The thermoremanent magnetization is largest at the composition extremes. Effects of disorder, frustration and low dimensional character occur in mixed magnetic Co 1−x Ni x Cl 2 ·H 2 O, examined by dc magnetization and susceptibility measurements across the composition range. The pure components are quasi-one-dimensional antiferromagnets ordering at 15.0 K (Co) and 5.6 5 K (Ni), with susceptibility maxima at 16.2 and 8.4 K respectively; a spin glass transition also appears in the cobalt system near 8 K. In each compound weaker antiferromagnetic interchain exchange interactions supplement dominant ferromagnetic exchange along MCl 2 MCl 2 M... chemical and structural chains. High temperature magnetic susceptibilities are analyzed to yield Curie and Weiss constants in χ M = C/(T − θ). Regular but not mean-field composition dependences appear for C and θ. The former is not linear and the latter displays a minimum near x = 0.30. Notable is a susceptibility maxima varying in location only weakly from x = 0.10 to x = 0.70, decreasing markedly only for higher x. Magnetization isotherms evolve regularly as x varies, with systematic composition dependences of a metamagnetic transition field and hysteresis through x = 0.70. A qualitative change in the isotherms occurs for x = 0.90, but substantial curvature still appears with smaller but definite hysteresis. A thermoremanent magnetization is seen in all mixtures, and is largest for the composition extremes. Thus, nonequilibrium magnetic behavior characteristic of the cobalt component, in contrast to the more conventional nickel component, occurs even when the latter is present in strong majority. [ABSTRACT FROM AUTHOR]

The magnetic properties of little examined CuCl 2 ·2D 2 O are studied and compared with those of CuCl 2 ·2H 2 O. New CuBr 2 ·6H 2 O is also examined. Susceptibility maxima appear for chlorides at 5.35 and 5.50 K, in the above order, with estimated antiferromagnetic ordering at 4.15 and 4.25 K. Curie-Weiss fits yield g of 2.210 and 2.205, and Weiss θ of −6.0 and −4.7 K, respectively, in χ M = C/(T − θ). One-dimensional Heisenberg model fits to susceptibilities, including interchain exchange in a mean-field approximation, are performed. Interchain exchange is significant but much weaker than intrachain. The bromide hexahydrate strongly differs magnetically from any chloride hydrate, but exhibits notable similarities and differences compared to previously studied CuBr 2 . A broad susceptibility maximum occurs near 218 K, only 4% lower than for CuBr 2 , but with almost twice the magnitude. Powder X-ray diffraction data for CuBr 2 ·6H 2 O may be best accounted for by a monoclinic unit cell that is metrically orthorhombic. The volume per formula unit is consistent with trends in metal ionic radii. However, an alternative monoclinic cell with 5% smaller volume more readily rationalizes the magnetism. [ABSTRACT FROM AUTHOR]

The magnetic properties of little examined CoBr 2 •2H 2 O and new CoBr 2 •H 2 O and CoBr 2 •D 2 O are studied. Curie-Weiss fits, χ M =C/(T-θ), yield θ of −9.9, 9.4 and 10.0 K, respectively, over a 30–80 K linear range for each. Higher temperature data are fit assuming two moderately separated low lying Kramers doublets, with exchange accounted for in a mean-field approximation. Susceptibility maxima appear at 9.5, 15.4 and 15.5 K, with χ max of 0.163, 0.375 and 0.435 emu/mol, respectively. Antiferromagnetic ordering is estimated to occur at 9.0, 13.7 and 13.8 K, in the same order. The ratio T c /T max is 0.95, 0.89 and 0.89, respectively, suggesting little low dimensional magnetic character in singly hydrated systems. Data at lower temperatures for the dihydrate are fit with an antiferromagnetic 3D-Ising model. For singly hydrated systems the large size of χ max prevents this; weakened interchain antiferromagnetic interactions yield enhanced susceptibility maxima. Magnetization data exhibit field induced transitions near 13.5 kG for the dihydrate, and near 6.5 kG for singly hydrated systems with enhanced hysteresis. These transitions are interpreted as metamagnetic in nature. [ABSTRACT FROM AUTHOR]

Magnetic measurements on variously hydrated nickel chlorides and bromides, including deuterated forms, are reported. Results include locations and sizes of susceptibility maxima, T max and χ max , ordering temperatures T c , Curie constants and Weiss theta in the paramagnetic regime, and primary and secondary exchange interactions from analysis of low temperature data. For the latter a 2D Heisenberg model augmented by interlayer exchange in a mean-field approximation is applied. Magnetization data to 16 kG as a function of temperature show curvature and hysteresis characteristics quite system dependent. For four materials high field magnetization data to 70 kG at 2.00 K are also obtained. Comparison is made with theoretical relations for spin-1 models. Trends are apparent, primarily that T max of each bromide hydrate is less than for the corresponding chloride, and that for a given halide n D 2 O ( n =1 or 2) deuterates exhibit lesser T max than do nH 2 O hydrates. A monoclinic unit cell determined from powder X-ray diffraction data on NiBr 2 ·2D 2 O is different from and slightly larger than that of NiBr 2 ·2H 2 O. This provides some rationale for the difference in magnetic properties between these. [ABSTRACT FROM AUTHOR]

The magnetic properties of previously unexamined MnBr 2 ·2H 2 O, MnBr 2 ·H 2 O, MnBr 2 ·2D 2 O and MnBr 2 ·D 2 O are studied. Curie–Weiss fits to high temperature data yield θ of −13.1, −3.9, −8.2 and −5.0 K, respectively, in χ M = C /( T − θ ). The net antiferromagnetic exchange yields susceptibility maxima at 6.34, 3.20, 2.10, and 3.40 K, with χ max of 0.197, 0.357, 0.465 and 0.348 emu/mol, respectively. Noteworthy is the contrast between dideuterate and dihydrate, the largest deuteration effect observed for hydrated transition metal halides. Antiferromagnetic ordering is estimated to occur at 5.91, 2.65, 2.00 and 2.50 K, respectively. The ratio T c / T max is 0.93, 0.83, 0.95 and 0.74 in the same order, implying low dimensional magnetism for monohydrate and monodeuterate. Heisenberg model fits to susceptibilities yield primary and secondary exchange interactions. Magnetization data at moderate fields and different temperatures are presented for each substance, and high field data to 70 kG at 2.00 K. Spin-flop transitions are estimated to occur at 45, 33 and 30 kG, respectively, for dihydrate, monohydrate and monodeuterate, but are not observable for MnBr 2 ·2D 2 O. The results are analyzed from various perspectives. A different monoclinic unit cell is determined for MnBr 2 ·2D 2 O than for MnBr 2 ·2H 2 O, with 1.3% larger volume, providing some rationale for the difference in magnetic properties. [ABSTRACT FROM AUTHOR]

The properties of ternary mixed magnetic Co 1− x Mn y Ni x − y Cl 2 ·2H 2 O are examined by dc magnetization and susceptibility measurements, from 1.8 to 300 K as a function of composition. This is only the second ternary magnetic insulator so studied. The three transition metal chloride dihydrate components are known to differ in the degree of spin anisotropy and in the distribution of ferromagnetic and antiferromagnetic exchange interactions within and between strongly coupled chemical and structural chains. The Curie and Weiss constants, in χ M = C /( T − θ ) fits to high temperature susceptibilities, are compared with weighted averages of pure component values. The observed Weiss constant is almost uniformly less negative than calculated. Maxima in low temperature susceptibilities vary widely in presence and location with composition. Some compositions exhibit no susceptibility maximum, many exhibit one maximum, and three exhibit two maxima. A T ( x , y ) diagram is constructed. Magnetization vs field isotherms exhibit different shapes as a function of composition, with hysteresis markedly composition dependent. For three mixtures hysteresis loops are studied as a function of temperature. An activation process model does not describe the temperature dependence well. [ABSTRACT FROM AUTHOR]

Abstract: A new mixed magnet, Mn1−x Ni x Cl2·H2O, is examined by dc magnetization and susceptibility measurements across the entire composition range. The pure components are quasi-one-dimensional Heisenberg antiferromagnets ordering at 2.17K (Mn) and 5.65 K (Ni) due to weaker interchain exchange supplementing the dominant exchange along MCl2MCl2M… chemical and structural chains. High temperature magnetic susceptibilities yield Curie and Weiss constants in χ M=C/(T−θ). C(x) is linear but θ(x) displays curvature, which is analyzed to show that unlike-ion exchange is ferromagnetic and similar in size to like-ion. Most notable is the absence of antiferromagnetic susceptibility maxima down to 1.6K from x=0.10 to 0.95. For x=0.05 a susceptibility maximum appears, with T max almost 20% lower than in the pure Mn component but T c reduced by 2%. The size of the susceptibility is enhanced by admixture, the effect of disrupted antiferromagnetic tendencies. Magnetization isotherms evolve with composition. Larger values of magnetization, under the same measuring conditions, occur for mixtures than for pure components, consistent with frustration, which weakens antiferromagnetic alignment tendencies. The competing ferromagnetic (Ni) and antiferromagnetic (Mn) intrachain interactions, along with disorder and low dimensional characteristics, presumably lead to the absence of magnetic order over a remarkably broad composition range. [Copyright &y& Elsevier]

Abstract: The magnetic properties of Mn1− x Ni x Cl2·2H2O are examined by DC magnetization and susceptibility measurements on a wide range of compositions. The pure components are three-dimensional Heisenberg antiferromagnets, ordering at 6.7K (MnCl2·2H2O) and at 7.25 K (NiCl2·2H2O). Each contains MC12MC12M… chemical (and structural) chains, with intrachain exchange ferromagnetic and antiferromagnetic in the Ni and Mn materials, respectively. Interchain exchange is predominantly antiferromagnetic in each. The Curie and Weiss constants, in fits to high temperature data, vary regularly with composition. C(x) is essentially linear, and exhibits systematic curvature which is analyzed to show that unlike-ion exchange is antiferromagnetic and weaker than like-ion interactions. The form of the susceptibility evolves with composition: maxima in ) broaden and shift to lower temperature with admixture of either component, but beyond about 20% minority component such maxima are absent, with upturns appearing instead. T c descends rapidly with admixture from either composition extreme, but varies only weakly between and 0.70. Magnetization isotherms display a subtle composition dependence which suggests that the ordered state differs for intermediate compositions from those for the composition extremes. Although frustration in mixtures is likely, from competing (sign) intrachain interactions, it is not strong enough to yield magnetic irreversibilities or time dependences signaling a spin glass phase. [Copyright &y& Elsevier]