Partial magnetic ordering and crystal structure of the ludwigites<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>Co</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mtext>FeO</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mtext>BO</mml:mtext></mml:mrow><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>Ni</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mtext>FeO</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mtext>BO</mml:mtext></mml:mrow><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>

We present an extensive study of the structural, magnetic, and thermodynamic properties of the two heterometallic oxyborates: ${\text{Co}}_{2}{\text{FeO}}_{2}{\text{BO}}_{3}$ and ${\text{Ni}}_{2}{\text{FeO}}_{2}{\text{BO}}_{3}$. This has been carried out through x-ray diffraction at room temperature (RT) and 150 K, dc and ac magnetic susceptibilities, and specific-heat experiments in single crystals above 2 K. The magnetic properties of these iron ludwigites are discussed in comparison with those of the other two known homometallic ludwigites: ${\text{Fe}}_{3}{\text{O}}_{2}{\text{BO}}_{3}$ and ${\text{Co}}_{3}{\text{O}}_{2}{\text{BO}}_{3}$. In both ludwigites now studied we have found that the magnetic ordering of the ${\text{Fe}}^{3+}$ ions occurs at temperatures very near to which they order in ${\text{Fe}}_{3}{\text{O}}_{2}{\text{BO}}_{3}$. A freezing of the divalent ions (Co and Ni) is observed at lower temperatures. Our x-ray diffraction study of both ludwigites at RT and 150 K showed very small ionic disorder in apparent contrast with the freezing of the divalent ion spins. The structural transition that occurs in homometallic ${\text{Fe}}_{3}{\text{O}}_{2}{\text{BO}}_{3}$ has not been found in the present mixed ludwigites in the temperature range investigated.