which may be used in the present invention may contain metals such as na k ti v mn fe co ni tc ru sn ce and pb semiconductors such as p sb and te or oxides of these metals in a proportion within the range of 0 to 20 wt the above mentioned ferromagnetic iron oxide and ferromagnetic chromium dioxide should generally have an acicular ratio within the range of about 2 1 to 20 1 preferably an acicular ratio of 5 1 or higher and an average grain length within the range of 0 2 u03bcm to 2 0 u03bcm preferably 0 3 u03bcm to 1 5 u03bcm the ferromagnetic alloy grains which may be used as ferromagnetic fine grains in the present invention should have a metal content of 75 wt or more of which 80 wt or more should be constituted by at least one ferromagnetic metal i e fe co ni fe co fe ni co ni or co ni fe at most 20 wt of the metal content preferably 0 5 to 5 wt thereof may be constituted by al si s sc ti v cr mn cu zn y mo rh pd ag sn sb te ba ta w re au hy pb bi la ce pr nd b p or the like the ferromagnetic alloy grains.